Challenges in Grease Selection Without Full Bearing Specifications

Bearing manufacturers have provided a significant amount of detailed advice for lubricant selection, application, and replenishment.  Formulas used by the machine designers incorporate details that are typically not readily available to the maintenance practitioner, namely load rating and ratio, grease L₁₀ lifecycle, and specific bearing dimensions. 

 The bearing diameters (OD, ID) may be satisfactorily estimated, but there are multiple bearing models for a bearing type that will share a bore dimension.  Without the correct bore and outer diameter, it is impossible to arrive at an exact replacement volume.

The Role of Grease Viability in Replacement Frequency

 Grease viability drives replacement frequency. Grease Viability is best determined by testing.  Per DIN 51825, greases can be evaluated under laboratory conditions to deliver a provisional expected lifecycle, with results reported in either L₁₀ or L₅₀ values. The grease’s L₁₀ and L₅₀ values depict operating hours to 90% and 50% viability. The frictional measurement of a loaded bearing in the FE8 test stand determines grease viability. 

Without grease viability data, replacement frequency is always an educated guess.

 When the grease can no longer separate and protect surfaces during test conditions, it is evidenced by an increase in friction beyond a threshold. At this point, the test hours are noted, and the grease is assigned a value in hours for grease viability. These values are not often published for customer use. Without these discrete pieces of information for the greased bearing, estimating the best replacement frequency is challenging.

Engineering Principles for Reliability-Centric Grease Relubrication

Suppose the machine owner wants to use machine lubrication as a leading practice to improve machine reliability. In that case, the machine owner must invest time to fully define the bearing and lubricant details to calculate appropriate volumes and intervals to deliver reliability-centric lubrication practices.

 This article presents the engineering principles for bearing grease relubrication, including consideration for open, single, and double shielded bearing configurations, and will include both theoretical methods and general advice useful to calculate both volumes and intervals when the exact details are not available.   The calculations presented here are also used to define volume and frequency for operating conditions.

Even without full specs, sound engineering principles can guide precise grease relubrication.

 Bearing manufacturers have provided detailed advice for selecting lubricant type, volume, and frequency requirements. They intend to assist the user with placing the optimum volume of a lubricant product with viscometric properties and surface performance (AW and EP) additives that precisely address the operating parameters (heat, load, vibration, moisture, contaminant, process chemical challenges).

Once accomplished, the user can expect the grease to feed oil to the race incrementally between the current date and the planned replenishment date so that the replacement practice provides a seamless flow of lubricant to the load zone, as depicted in Figure 1.

Either too much or too little grease, and/or inappropriately high or low oil viscosity causes viscous drag and/or destruction of the bearing surfaces and lubricant within the bearing. 

In moderate and high-speed bearings (nDm > 150K), even slight variations in consistency of replenishment and fill volume produce effects including dry surfaces and elevated high-frequency vibration (inadequate feed), elevated temperatures and increased energy consumption (overfeed).

The faster the shaft speed, and the higher the load, the more pronounced the deficiencies. As the shaft speed decreases, the negative impact (churning, overheating, and energy losses) declines, but is still evident. The first part of this multi-part document addresses lubricant viscosity and NLGI selection. 

The second part addresses volume and frequency. The third part addresses sealed and shielded bearings and electric motor configurations.

Lubricant Selection: Viscosity, Additives, and NLGI Grade

Understanding the Impact of Viscosity on Bearing Performance

Viscosity changes with temperature and pressure. As temperature increases, viscosity decreases, and as pressure increases, viscosity increases. These factors are interdependent on one another. The central questions for selecting the correct lubricant grade for a given brand and product are:

1. What is the minimum acceptable viscosity for a given bearing?
2. What is the optimum viscosity for the bearing at operating temperature?
3. What is the viscosity of the current lubricant at the normalized bearing (machine) operating temperature?

Determining the minimum allowable viscosity to sustain element and race separation (EHD film formation) is a simple calculation, as follows:

V_min= 27,878 * RPM ^-0.7114  * Dm ^-0.52

Where:

V_min   = minimum allowable viscosity

RPM = shaft rotational speed

Dm    = bearing mean diameter

For example, assuming the bearings on a 254-frame-size motor are operating at 2400 RPM, and contain single row deep groove ball bearings with a bore diameter (ID) of 45 mm and an outer diameter (OD) of 85 mm, then the pitch diameter is 65 mm. The minimum allowable oil thickness for EHD film formation would be 12.505 centistokes at operating temperature.  The optimum operating viscosity will be three to five times this value, or 36 to 60 centistokes.

Once determined, this should be compared to the viscosity supplied by the selected lubricant.  Assuming the grease contains a 100 centistoke (ISO VG 100) oil, and the bearing is operating at 50°C, one can use a commonly available viscosity/temperature chart to determine the acceptability of the operating viscosity of the product in use.  Figure 2 illustrates this process.

Matching operating viscosity to bearing needs is the cornerstone of reliable lubrication.

As can be seen in the example, the suggested product would fulfill the optimum viscosity, delivering 60 centistokes at the stated temperature.  The product would function with a margin up to 65°C, and deliver the minimum allowable viscosity to 95°C. 

As long as the dynamic (operating) viscosity is above the minimum allowable viscosity, the use of EP agents is discouraged.  This example reflects why many electric motor lubricants are filled with wear resistance (AW) rather than seizure resistance (EP) agents and contain ISO 100 viscosity oils.

Viscosity selection for other bearing types and speeds follows this pattern.  The bearing’s maximum allowable operating speed and the limiting speed for grease lubrication (the point at which any given bearing should be oil lubricated) is determined by the bearing Pitch Line Velocity (PLV = mean bearing diameter times shaft speed = n*dM). 

Spherical and thrust bearings approaching a PLV of 150K, and ball and roller bearings approaching PVL values of 350K must be qualified for reliable operation with grease.

Choosing the Right NLGI Grade for Application Conditions

Grease stiffness influences grease performance in the bearing cavity.  The stiffer, or harder, the grease is, the less it will move within the housing once initial movement and settling have occurred.  There are nine grades of stiffness, as defined by the NLGI (National Lubricating Grease Institute).  The stiffness grades, and a parallel to a commonly recognized product, are shown in Figure 3.

Stiffness is a reflection of the amount of shear resistance that the grease presents to a weighted cone that is allowed to settle into a grease sample, as shown in Figure 4.  The rod that connects the cone to the instrument is also attached to a dial indicator at the top of the instrument. 

As the cone settles into the cup, the dial moves clockwise until movement stops. The number indicated by the dial is assigned to the grease as its stiffness value. The value correlates to the range of values on the NLGI Grade chart.

Assuming that the selection process has properly addressed the viscosity and additive type, selection of the grease grade (NLGI #1, #3, etc.) depends on bearing speed, temperature, vibration, shaft orientation, and application method.  Some general rules to follow:

1. Use #0, #1 for: Automatic systems with long distances, narrow feed lines, cold feed lines, significant number of 90°
2. Use #1 for: Outdoor single-point (low-pressure) applicators
3. Use #3 for: Vertical shaft axis applications.
4. Use #3 for: Very large bearings, high vibration conditions, very high-speed conditions, very high temperature conditions.
5. Use #2 for: Manual, battery powered, or air powered grease gun applications, moderate to low speeds, low vibration rates, and low heat load.

The majority of grease-fed components can be successfully serviced with #2 grade greases.   However, some circumstances warrant a change.  If the selected grease tends to show puddles of oil on the grease surface of unopened containers, then a step up in NLGI grade is appropriate.

If a bearing housing proves consistently difficult to purge, then consider moving to a softer grade.  If the bearing is subject to grease dilution or removal from frequent exposure to water, or wash down activities, consider a stiffer grade.

Calculating Initial Grease Fill and Replenishment Volumes

When an element bearing is first placed into service, the initial fill volume in the housing (if space permits) should be based on the volume needed to fill the base of the housing up to the bottom edge of an element sitting at the 6:00 position in the race.

If it is not feasible to observe the internal spaces in the housing, then a fill volume equal to 3X the replenishment volume of bearing for low-speed bearings, and 1X for high-speed bearings. In this instance, ultrasonic methods should be used to validate a proper oil film within 4 hours of initial operation.

Practical Formulas for Estimating Bearing Replenishment Volumes

There are two options for calculating the bearing net capacity and replenishment value. Schaeffler FAG bearings company provides an option to determine this as follows:

V = ((Pi/4) * W * (OD² – ID²) * 10^-9 – G/7800)*10⁶, where

V = volume in cubic centimeters,

OD = Bearing Outer Diameter, mm

ID = Bore Diameter, mm

W = Bearing Width, mm

G = Bearing weight, Kg

SKF bearing company provides an option to determine this volume as follows:

V = W * OD  * .005, where

V = volume in grams

OD = Bearing Outer Diameter, mm

W = Bearing Width, mm

From a practical perspective, the SKF approach offers greater flexibility in asset assessment when the exact bearing number (required for weight in Kg) is unavailable, making it the preferred method in LubeCoach calculations.

In addition to the grease introduced into the element spaces, enough grease should be placed into the housing to bring the grease level up to the lip of the outer race of the bearing.  When the excess from the initial fill is pushed away from the elements, it accumulates on the grease shelf at the race. It becomes a reservoir to continuously serve oil back to the raceway without crowding the elements.

A well-filled housing isn’t guesswork – it’s precision that feeds reliability.

The engineer/practitioner making these decisions has to know precisely which bearing by manufacturer number is in use to provide all the required values. Bearing manufacturer numbers are readily available at the time of initial installation and/or bearing replacement, so enough information is available for a correct initial fill.

Replenishment volumes: The bearing number details become fuzzy as repairs occur, CMMS systems are upgraded, and data is lost, and as the details from the original installation fade from memory. Therefore, it is necessary to have a more user-friendly approach to estimate replacement volumes for ‘in-situ’ applications.

One should consider both feed volume and feed interval since the two are interrelated. The formula shown in Figure 5 gives volumes in both grams (for metric dimensions) and ounces (for English dimensions) for three different interval ranges. 

Where actual bearing dimensions are not known, a close proximity to the actual suggested value could be estimated by using housing dimensions and factoring again by one-third {(D * B * .114) *.33}.

CAUTION: This provides only an approximation. For critical applications, the actual bearing make and model should be determined.

Excessive lubricant volume applied to bearings with labyrinth style seals and low pitch line velocity bearings (PLV ≤ 50,000 for ball and cylindrical roller, ≤ 30,000 for spherical and thrust roller) is not considered to be as problematic to the grease or bearing as it would be at higher speeds.

Excess grease dissipates readily, and any grease remaining in the working area has adequate transport time and space.  However, the same bearings with shields and plugged relief ports can accumulate grease residue. Over time, the residue can crowd the housing and cause churning and overheating. 

It is best to identify the precise bearing details for all relubrication volume and frequency calculations, and use the precise values to make well-defined decisions.

Grease Volume Guidelines for High-Speed Bearing Applications

The replacement volume for high pitch line velocity (PLV ≥ 330,000 for radial ball type; ≥ 150,000 for spherical roller and thrust type) element bearings requires thoughtful consideration due to shearing and heat produced by overfilling.  All bearings operating at high speeds benefit from more frequent but lower volume doses, emulating continuous replenishment that occurs with oil-lubricated elements.

At high speeds, it’s not about more grease – it’s about smaller, smarter doses.

For instance, the volume calculated for the short interval, Gq-Weekly, would ideally be uniformly distributed into the number of working hours for the period and applied accordingly. This technique would require automatic application, incorporating the use of timers and low-volume injectors or quality single-point lubricators.

Determining Optimal Grease Relubrication Intervals

The most dependable calculation for relubrication interval will be based on a combination of machine operating conditions and the expected grease service life for those conditions. Grease lifecycles can be predicted empirically.

Much like a bearing L₁₀ lifecycle value that indicates an operating interval for which 10% of a given bearing population would fail under identical operating conditions, the grease F_10Real value projects an operating interval for grease lifecycles and, consequently, relubrication intervals.

The F10 grease prediction model, as shown in Figure 6, is based on known grease degradation performance under test conditions, such as the FAG FE9 Tester (DIN 51821, Part 2), or similar test methods (SKF ROF Tester, DIN 51806).

The (theoretical) F_10Real formula for grease replenishment intervals, in hours, is shown in Figure 6.

Factor F₃ pertains to the actual operating temperature (given under T), and Factor F₄ pertains to the bearing load factor (given under P). Similar to the earlier comment about grease fill volumes, this approach works well when specific greases are being tested for specific applications during design considerations, but is difficult for the plant lubrication technician to apply to in-service components when the specific data points aren’t available.

When FE9 test data and F_10Real values for specific lubricant products are not available (it is typically not reported in OEM performance data), a modified approach can provide the reliability practitioner with a well-defined starting point. 

This empirically derived approach (formula shown in Figure 7) assumes applications where the actual load is a low percentage of net capacity, and where bearings are operating below the rated speed limits (Pitch line values are ≤ 300K for ball and roller type elements, ≤ 140K for spherical and thrust type elements).

In this approach, ‘K’ is the product of machine operating condition parameters, shown in Figure 8. The F₁₀ value is modified (hours to failure value is reduced) to allow equipment owners to factor in plant conditions.  Each of several factors becomes a judgment call, but with time and experience, results similar to the DIN 81825 calculation for net relubrication frequencies are achieved.

Where,

T_f = Time in hours between grease replenishment events

K = Product of environmental correction factors

N = Shaft speed

D = Bearing bore in millimeters

The correction factor, K, shown in Figure 8, allows the engineer to adjust frequencies based on machine operating and environmental considerations. The six provided conditions reflect practical issues that degrade bearing life and grease performance.

Figure 8 includes the correction factors for a 90 mm bore spherical roller bearing operating at 1200 rpm (PLV = 160,800) in direct exposure to rain and in a dusty environment, such as near an unpaved roadway and directly exposed to the weather. The calculated interval amounts to 18 days between relubrication events.

Multiple Bearing OEM Lubrication Guideline publications provide alternate quantitative approaches that are also valid and could be considered as a strong reference starting point.

Lubrication Practices for Single and Double Shielded Bearings

Key Differences Between Shields and Seals

Seals and shields perform similar functions in supporting an effective bearing lifecycle.  Shielded bearings may be used where no routine relubrication for the life of the machine is the design objective, but are typically used in housings where replenishment can be accomplished.  The key difference between sealed and shielded bearings is that shields are in contact with only one race, and seals contact both.

Grease Entry Paths in General Service Bearings

In general service bearing applications (pillow block, flange mount) grease may enter the raceway either from the face (axial feed) or from the outer perimeter of the bearing (radial feed).  Bearings are identified as radially fed in the OEM equipment catalog if they are serviced in this manner.

For instance, SKF identifies radial feed bearings with the W33 designation in the bearing number. Other bearing suppliers may use this or other nomenclature to differentiate between styles.  For bearings that are large enough that the housing is retained and only the element is replaced during a repair, the bearing will have an outer seal (lip or labyrinth type) at the outer periphery of the housing cavity.

Without a shield, gravity takes over – and so does premature grease failure.

It may or may not be equipped with a shield on the element itself.  The shield serves the function of metering grease and keeping contaminants out of the element area.  If the shield is missing from the element, then the grease slumps by gravity around the lower lip of the bearing and is drawn into the element gradually. This approach doesn’t prevent grease churning and premature loss of usefulness.

Configurations where the bearing and housing are replaced as a unit should contain shields on both faces.  Grease may enter axially or radially into the element pathway, and the shield in these instances is intended to vent pressure and prevent contamination entry.

Shield Orientation and Its Effect on Grease Flow

Electric motor bearing construction is highly user-specific.  If the user requests a shield or seal, then it can be supplied.  If the user doesn’t specify either, then it is the motor rebuilder’s or OEM’s prerogative to follow their advice. Unless the user specifically asks the question, he/she may not know.

Shield orientation is also user-driven.  The shield may face out or away from the windings. In these configurations, the annulus gap between the inner race and the shield performs a metering function, allowing grease to enter the raceway through the gap while in operation.

Shield direction shapes grease flow – and determines what stays cool and clean.

The grease also provides a baffle to prevent churning and heating of the grease away from the movement of the elements. It may also be configured with the shield facing toward the windings.  In these instances, the shield is thought to minimize the risk that the grease will enter the windings.

In both configurations, the gap between the lip of the shield and the inner face of the bearing ring is sufficiently open that fresh, viable grease is drawn into the raceway easily. The shield and gap can be seen in Figures 9 and 10.  Different installation arrangements can be seen in Figure 11.

Figure 10 provides a cross-sectional view of the element and races, and illustrates the gap in more detail.  The annulus is between 125 and 375 microns (0.005” and 0.015”). The shield also provides restraint of bulk contaminant flow into the raceway, but does not eliminate contamination problems.

Given that the dynamic element to race clearances ranges between 0.5 and 1.5 microns, it is clear that particulates that can corrupt the dynamic oil film can readily pass into the race area.

Installation Considerations for Shielded Bearings

Figure 11 (below) demonstrates accepted mounting techniques for shielded bearings in electric motor housings.  (Original Graphic Ref., Heinz Bloch, “Practical Lubrication for Industrial Facilities”). Single shield bearings may be installed such that the shield is facing the grease supply, or is on the opposite side of the bearing receiving grease supply.

When installed facing the flow of grease, the shield can behave as a baffle to limit the flow of grease, if the grease volume is not overpowering, and minimize the risk of churning.  Unfortunately, if grease is supplied under too much force (high pressure or volume), the shield may collapse into the raceway and compromise the bearing.  It is important to know which configuration exists, if possible, before proceeding with the lubrication event. 

There is no single position taken by bearing manufacturers for the use of shields and seals (single or double shield configurations).  Machine manufacturers select seals and shields when contamination from the environment is expected. Shields are also prevalent on electric motor applications. 

The shield is beneficial to prevent grease churning in the housing, but does not prevent the movement of the grease toward the center of the motor. The motor owner should be aware of the options provided by the builder and should publish and provide technical specifications according to what is believed to be best for the production site.

Best Practices for Initial Bearing Grease Fills

The initial fill for a single shielded bearing should conform to the advice provided above under open face bearings.  OEMs do not differentiate between fill and replenishment practices based on the bearing component or seal configuration. 

The quantity of grease to be placed into the bearing at the time of installation is governed by the vacant space within the bearing.  The quantity of grease for the housing is not definable because a bearing can be fitted to multiple bearing housings.  Bearings are shipped with a quantity of grease that serves as both a corrosion inhibitor and an initial charge for operation.

Any addition of grease via hand-packing before mounting the bearing should be conducted under clean room conditions with dust-free/lint-free gloves.  Even slight handling of element bearings can induce corrosion.

As noted previously, when a bearing is placed into a housing, it is necessary to create a grease floor in the housing that is flush with the outer race lip at the bottom of the housing.  This will allow any new grease to slump to the area at the bottom of the shield/open face and provide a renewing reservoir. 

Guidelines for Relubricating Shielded Bearings

The volume for replenishment is determined by the formulas provided above. The advice is based on bearing size and speed, grease longevity, and operating conditions.  Technicians should be aware of the use of shielded bearings and whether the shield faces the grease flow or is on the opposite side of the bearing.

Shielded bearings should be lubricated while the bearing is running to prevent overpressurization of the seal and possible collapse into the bearing pathway.  Movement of the elements during lubrication will cause the grease to draw into the element pathway for maximum flushing and distribution effectiveness.

Greasing on the run keeps pressure down and distribution up.

Bearings should not be greased while idle if possible.  Where this is necessary, the equipment owner must determine the minimal acceptable amount of grease for the installation and its operating conditions, and restrain grease addition to this value only to avoid collapsing the shield.

Short of physical observation of the immediate area at the bearing (which is not possible without disassembly of the housing/machine), it is not possible to know the pathway that the grease follows once it is in the housing. 

Understanding and Maintaining Sealed-for-Life Bearings

Within the last few years, there has been a marked increase in the dependence on sealed for life bearings for a wide variety of commercial, residential, and even some industrial machines.  The concept ‘sealed for life’ reflects the design goal, not the expected operational period.  ‘Sealed for life’ is also not a guarantee of operational performance. Sealed for life bearing applications have grown from the traditional deep groove ball bearing to include all shapes, sizes, and design parameters.

Equipment manufacturers’ primary determining factor for whether to choose a seal (not to be replenished while in use), a shield, or neither is driven by machine lifecycle cost and duration requirements.  For typical components where sealed bearings are widely or singularly used, the component supplier has concluded that the likelihood of achieving the required lifecycle is better if the component is not relubricated.

Figure 12. Sealed Bearing Configuration

Sealed bearing lifecycles are greatly influenced by the in-use grease condition, which is itself influenced by the seal condition (leakage and contaminant exclusion). The significant improvements seen in both grease and seal materials have enabled machine manufacturers to design for and achieve longer lifecycles with sealed bearings in progressively more challenging conditions.

Favorable conditions for sealed bearings could include:

• Small bearing dimensions
• Low shaft rotational speeds
• Low shaft circumferential speeds
• Low loads
• Clean conditions (no moisture, no dust)
• Low heat
• Short expected lifecycles

As the relative load, surface contact speed, temperature, and contaminant load increase dependence on shielded or open face relubricatable bearings increases. Sealed bearings are not intended to be relubricated during the machine’s expected lifecycle.

However, shielded bearings are configured for and are expected to be replenished at some interval.  Elastomeric radial lip seals are designed primarily to retain the lubricant and are only marginally expected to prevent external contaminant ingression.

Seals are capable of containing both liquids and semi-solids, are capable of operating in bearing sumps varying from -60 to 200°C, can operate with peripheral speeds up to 20 m/s, and support pressures between 20 and 100 kPa (2.9 to 14.5 PSI).  Seal radial loading is determined by the types of elastomers used, the contact area of the seal on the race surface, internal pressure from the fluid, and spring tension.

Every turn of the shaft turns the seal into a precision fluid pump.

As the shaft turns, the movement of the shaft causes the seal to flex. This provides a subtle pumping motion that serves to push the fluid toward its reservoir area. The fluid creates a film barrier between 0.125 mm and 1.25 mm wide. Lip contact load is a key performance factor.

Contact load ranges between 0.05 and 0.12 N/mm (0.3 to 0.7 lb/in) of circumference. As the lip load (spring tension) increases, the surface temperature rises in relation to shaft speed. Since temperature is a prime cause of seal failure, lip loads should be as low as possible and still maintain a seal. Figure 12 provides a look at the key features of a lip seal.

Key Takeaways for Effective Grease Relubrication

Grease relubrication practices should be handled with care.  Precise grease volumes and carefully calculated intervals will help the reliability professional reduce outages, reduce costs, improve machine performance, and enjoy a less stressful career.   The formulas provided above are either directly or indirectly associated with bearing supplier recommendations.

The LubeCoach recommendations reflect the principles noted in the formulas provided. These may be programmed into a worksheet with minimal effort.  The LubeCoach is designed to offer insights without requiring complex spreadsheet construction. Learn more about LubeCoach Circular Bearing Lubrication Calculators.

References

Con GMBH, Bearing lubrication Calculation Worksheet, FAG Bearings, German Society of Tribology, others.

FAG Bearings Limited, Roller Bearing Lubrication Guide, Publication Number WL 81 115/4 EC/ED

LubCon USA, LubCon GMBH, Bearing Lubrication Calculation Worksheet,

FAG Roller Bearing Lubrication Guideline WL81115E.

Machinery Lubrication magazine

Web Reference X.X – Timken Bearing Company

Web Reference X.X – SKF Bearing Company. https://mapro.skf.com.

Snyder, D.R “Sealed-for-Life Bearings: To Relubricate or Not?” Tribology and Lubrication Technology, December 2004. Pages 33 to 40.

Booser, R.E., Tribology Data Handbook, Chapter 14, Dynamic Seals.  CRC Press

Hodowanec, M.M., “Evaluation of Anti-Friction Bearing Lubrication Methods on Motor Life Cycle Cost”. Siemens Industry and Automation Incorporated. 0-7803-4785-4/98.  IEEE. 

During inspection and maintenance activities, it is common to identify possible sources of lubrication-related failures, and one of the most recurrent is the constant leakage of oil in grease-lubricated bearings. This problem, often underestimated, can cause dryness and premature hardening of the grease, compromising the reliability and performance of the assets.

What Is Exudation and How Does It Affect Lubrication?

A exudation It is a condition where lubricating oil separates from the thickener (the structure that holds the oil), resulting in a hardening of the grease. Small releases of oil are normal because the thickener works like a sponge, releasing oil in a controlled manner to lubricate. However, when oil separation is excessive, grease hardens prematurely and loses its ability to protect contact surfaces, resulting in accelerated failures and increased maintenance requirements

How Does Temperature Impact Grease Exudation and Hardening?

High temperature is one of the critical factors for separating oil from grease. In extreme conditions, the oil can reach its dropping point, which causes it to lose viscosity and separate more easily from the thickener. This phenomenon can occur due to:

1. Excess Grease: When there is excessive lubrication, the bearing needs to disperse excess lubricant, which generates heat and accelerates grease degradation.
2. Long Periods Without Relubrication: The absence of purging and the lack of relubrication at appropriate intervals make it difficult to remove old grease, which can oxidize and harden over time, reducing the efficiency of the lubricant.
3. Lack of Purge During Lubrication: In bearings without relief valves or drains, excess pressure caused by the application of new grease pushes the oil, separating it from the thickener and contributing to premature hardening.
4. Centrifugal Forces and High Speeds: In high-speed bearings, centrifugal forces act on the oil, forcing it to separate from the thickener, especially in greases not suitable for this type of operation.
5. Incompatible Lubricant: The incorrect choice of lubricant for the type of operation, temperature and load can also favor oil separation, compromising the performance of the asset.

Other Factors Contributing to Hardening and Premature Failure

In addition to exudation and the impact of temperature, cross-contamination between different lubricants can negatively influence the stability of the grease. For example, using a higher viscosity grease can result in an increased shear strength, which generates additional heat and accelerates oil separation, especially in high-speed bearings.

In addition, the lack of contamination control, such as the entry of particles and moisture, degrades the thickener, increasing friction and operating temperature, which leads to hardening and decreased lubricant life.

Practices and Procedures to Minimize Hardening and Improve Grease Life

Implementing a structured routine based on operating conditions is critical to reducing the risk of hardening and premature failure. Best practices include:

1. Check the DN and NDM Factor of the Lubricant: These factors indicate the suitability of the grease for high-speed operations. Lubricants for high-speed bearings must have low viscosity and shear strength, avoiding premature separation.
2. Consider Operating Conditions: It is essential to evaluate whether the grease can withstand high centrifugal forces, high loads, and extreme temperatures. Periodic analysis of the condition of the asset is essential to ensure that the lubricant used is adequate.
3. Adopt Appropriate Relubrication Intervals: Establish relubrication periods adjusted to the actual conditions of the equipment, considering factors such as operating speed, load, and temperature. This approach helps prevent grease degradation and oxidation.
4. Prevent Cavity Formation When Handling Grease: When grease is removed from the packaging, avoid cavities that create low-pressure zones, which makes it easier for the oil to drain out. This simple care can prevent premature grease degradation.
5. Ultrasonic Lubrication: Ultrasound tools allow real-time monitoring of the amount of lubricant required. This prevents overheating and reduces pressure on the system, helping to maintain application efficiency and prevent unnecessary heating.
6. Contamination Control: Use old grease sealing and purging techniques to minimize the ingress of solid and liquid contaminants. A clean environment with low presence of particles increases the useful life of the grease.

Benefits of a Condition-Based Lubrication Program

Implementing a lubrication program that is based on operating conditions (conditioned lubrication) maximizes efficiency and reduces unplanned downtime. By adopting a plan with continuous monitoring and techniques such as ultrasound, it is possible to identify relubrication needs with greater precision, adjusting the frequency and quantity of application according to the actual wear and tear of the asset.

Case Study: Excess Pressure and Equipment Damage

A real case illustrates well the importance of a controlled lubrication program: in one factory, the lack of purging and the application of excessive pressure during lubrication resulted in the rupture of the seals, causing grease to accumulate inside the electric motor. This buildup caused heating and bearing failures, resulting in irreversible damage to the engine and impacting plant productivity.

Maximize Asset Life with Best Lubrication Practices

By following these practices, it is possible to achieve significant operational gains, minimizing lubrication failures and promoting asset reliability. The adoption of a structured lubrication plan, combined with monitoring tools, allows not only to reduce maintenance costs, but also to extend the useful life of components and avoid unexpected downtime.

In most industrial facilities, over 50% of all lubrication points are lubricated with grease instead of oil. Don’t believe me? Just count the number of electric motors in your plant. Unlike most oil-lubricated assets, where OEM recommendations provide at least a starting point for lubricant selection, greased lubricated components often don’t have simple guidelines for choosing the proper lubricant.

This lack of information frequently leads to many “experiential” grease selections (red grease worked well the last time…) instead of solid lubrication engineering. In my experience, incorrect grease selection is one of the most common failures in many lubrication programs.

What Is Grease?

Greases are comprised of three main “ingredients”: base oil, additives, and a thickener. The amount of each will vary based on grease type and NLGI grade but is usually in the range of 60-75% base oil, 5-20% additives, and 5-25% thickener. Just like lubricating oils, they are formulated for specific applications by choosing the appropriate base oil, additives, and, in the case of grease, thickener chemistry for optimum performance.

Base Oil Basics: Building Blocks of Lubricating Grease

Just like lubricating oil, the purpose of the base oil is to provide oil with sufficient viscosity to create an oil film capable of maintaining the complete separation of moving parts in the load zone of the bearing or other lubricated component. Base oils used in grease conform to the same API base stock categories as lubricating oil:

API Group I

Base oils derived from refined crude oil containing relatively high concentrations of unsaturated and aromatic compounds and sulfur. These oils provide good natural solvency and help enhance additive solubility but are prone to oxidative breakdown.

API Group II

More highly refined base oils with lower levels of unsaturated aromatics and sulfur compounds exhibiting better oxidation resistance and higher viscosity indices than Group I oils.

API Group III

Hydrocarbon base oil that has undergone significant chemical treatment and processing to remove almost all undesirable impurities, resulting in better oxidative stability and a higher viscosity index than either API Group I or II.

API Group IV

These oils are synthetic hydrocarbons, with the overwhelming majority being poly alpha olefin (PAO) based. Most greases labeled “synthetic” are manufactured using API Group IV or highly refined API Group III base oils.

API Group V

Oils in Group V comprise a host of different synthetic fluids, such as die-esters, polyol esters, poly glycols, and fluorocarbons. Greases containing API Group V base oils are far less common and are typically only found in highly specialized lubricants, such as those used for very high temperatures, extreme service life, or radiological applications.

Just like lubricating oils, the viscosity of the base oil within the grease can usually be found in the Product Datasheet (PDS) and is one of the most important properties when deciding the most appropriate grease for any given application.

Understanding Grease Additives: Roles and Benefits

The additives found in greases are similar in form and function to oil additives. The most common being antioxidants and rust and corrosion inhibitors.

Common Additives and Their Functions

Antioxidants help to prevent the base oil from degrading through a combination of free radical scavenging and breaking the chain reaction of oxidation. Rust and corrosion inhibitors coat metal surfaces to prevent rust and other corrosive processes, and they serve to neutralize corrosive materials through an acid-base reaction.

In addition, many greases contain anti-wear (AW) and/or EP additives (EP). These additives protect components under mixed-film or boundary lubrication conditions by providing a heat-activated layer on the surface of the component under elevated loads and temperatures.

AW and EP additives found in grease are similar in function to AW and EP additives found in lubricating oils. They are comprised of organo-sulfur and phosphorus compounds such as zinc-dialkyl-dithiophosphate (ZDDP).

The one class of additives that can be slightly different in lubricating greases is the incorporation of solid additives, such as molybdenum disulphide, often referred to simply as “Moly.” While some oils contain solid additives, their use is far more prevalent in lubricating grease since the grease matrix can more readily hold these additives in suspension.

Greases that contain moly or other solid additives are often used in slow-turning, heavily loaded applications, particularly where shock loading is anticipated. Moly is similar in crystalline structure to graphite, existing in mineral “plains.” While very strong in supporting radial loads, the weekly bonded crystals plains allow for a reduction in the coefficient of sliding friction under “stick-slip” conditions.

The Role of Thickeners in Grease Performance

Grease thickeners have a minimal bearing (pun intended!) on how well a grease lubricates.

Types of Grease Thickeners Explained

While some thickening agents, such as calcium, have been shown to have some mild EP properties, the number one role of the thickener is as a carrier of the base oil and additives. For this reason, grease thickeners are sometimes referred to as “sponges,” much like dish sponges that carry water (base oil) and dish detergent (additives) to clean dirty dishes.

Grease thickeners are usually made from one of three types of compounds. Simple and Complex soaps of inorganic elements such as lithium, calcium, and aluminum are the most common.

Soap-Based vs. Non-Soap Thickeners

The term “soap” refers to the chemical nature of these compounds and how they are made through a saponification reaction. Simple soaps are made by reacting an organic long-chain acid, such as stearic acid derivatives, with an alkali metal to form an organic salt.

Complex soaps use a shorter (smaller) compounding organic acid in addition to the long-chain fatty acid to “bind” the thickener together. The net result is a thickening agent with a more “entwined” network of fibers that helps with better grease stability properties such as bleed rate, dropping point, and shear stability.

Non-soap thickeners are most commonly a class of thickeners called “polyurea.” Polyurea thickeners are made from the reaction of a diisocyanate with a mono or di-amine. Because polyurea thickeners are “ashless” (do not contain any metals), they have better oxidative and structural stability, which is why they are sometimes preferred for electric motor greases and other applications with longer regrease intervals.

High-Temperature Applications and Clay Thickeners

Clay thickeners are used for high-temperature applications (for example, lubricating bearings in a kiln or oven). Most commonly, bentonite clay is purified and milled to an exacting particle size before being blended with base oil and additives. Clay works in a similar fashion to adding corn starch to a gravy or sauce, binding together other constituents into a thick slurry (grease). Like corn starch, the more thickener you add, the “thicker” the grease.

As stated earlier, the primary role of the thickener is as a carrier for the base oil and additives. As such, grease performance properties shown on a grease’s Product Datasheet (PDS) include a series of tests determining how the grease thickener will work during application and in-service.

Essentially, these seek to answer the questions, “How easy will it be to get the grease from the grease gun (or automatic greasing system) to the bearing, and how long will it retain its chemical and structural integrity during prolonged use?”?

Grease Performance Properties

Consistency

Consistency is one of grease’s most important properties. It is measured using the ASTM D217 cone penetration test (Figure 1).

Prior to the test, grease is forced through a mechanical grease worker to apply a shearing force to it. After working for 60 double strokes, a metal cone is dropped into the grease sample, and the depth of penetration is measured in tenths of a millimeter. The result is used to assign the grease’s NLGI grade, referenced when greases are labeled AW1 or EP2, for example.

Consistency, sometimes called thickness, should not be confused with viscosity. The viscosity of grease is the viscosity of the base oil contained within the grease, not how “thick” or “thin” the grease appears when it is pumped from the grease gun. In fact, grease consistency and grease viscosity often trend in opposite directions.

For example, grease that is to be used for a high-speed open bearing needs to be thick enough that it will not get displaced by centrifugal force but will be formulated with a low base oil viscosity due to the high operating speed. We might choose a “thick” NLGI Grade 3 grease with a low viscosity ISO VG 100 cSt base oil.

Grease consistency and viscosity often trend in opposite directions.

By contrast, if we elect to use grease for a small gear reducer to prevent shaft seal leakage, we need a base oil with a higher ISO VG grade due to the output shaft’s slow speed. Still, we would likely choose a “thin” grease (NLGI 00 or 000) so that the rotating gears can properly distribute the lubricant throughout the gear case.

In some cases, where grease is to be applied in cold-temperature conditions or through long lines in a centralized lubrication system, it is advantageous to use a very low consistency (“thin”) grease such as an NLGI grade 0 or 1. Compared to the equivalent NLGI grades 2 or 3, these greases may appear to be thinner.

However, as stated previously, consistency is unrelated to lubrication performance once the grease enters the load zone. Table 1 shows the relevant performance properties for four commonly used multipurpose greases from a major lubricant supplier.

As can be seen from the product specifications, viscosity and viscosity index, as well as load-carrying tests, show identical results for all four grades. As such, each of these greases would provide the same lubrication performance, assuming again that the difference in grease consistencies does not prevent the grease from getting to and staying at the load zone of the bearing.

Dropping Point (ASTM D2265)

The dropping point of a grease refers to the lowest temperature at which the grease will start to liquify and is a measure of how well a grease can resist elevated temperatures. The test grease is slowly heated until the grease is observed to have liquefied, as signified by a “drop” of liquid falling from the bottom of the test grease.

Grease should never be used close to its rated dropping point. As a rule of thumb, a grease’s maximum sustained operating temperature should be 100-150 °F (50-80 °C) below the dropping point. Typically, complex metal soap thickeners such as calcium complex, lithium complex, and aluminum complex exhibit the highest dropping point compared to other commonly used greases (Table 2).

Other Performance Tests

Other tests are used to ensure the chemical and mechanical stability of grease of different thickener types, depending on the application. These include water washout (ASTM D1264), grease bleed rate (the rate at which the base oil separates from the thickener) (ASTM D1742), roll stability (ASTM D1831), wheel bearing leakage (ASTM D1263), and water spray test (ASTM D4049).

Compatibility between grease types can make or break lubrication success.

When selecting a grease for a specific application, it is important to choose the correct thickener type and consistency. This should include the anticipated operating temperature and ambient stressing conditions. Table 3 compares different thickener types and their performance under different conditions.

Because thickeners are made from such a wide array of chemistries, compatibility must always be considered when intentionally or accidentally mixing different greases. Whenever two greases are incompatible, the consistency of the mixed greases will either increase to the point of solidifying or decrease and liquefy; neither condition is desirable.

A quick internet search for “grease compatibility” will yield any number of compatibility charts identifying which grease thickener types might be compatible with another type. These charts are very misleading and are often inaccurate.

While it is true to say that when mixing two different greases, there is a 30-50% chance of incompatibility, it is almost impossible to predict how two greases will react based on their chemical constituents.

For this reason, whenever switching from one grease to another, the question of compatibility must always be addressed. Most lubricant manufacturers can provide evidence of compatibility testing for commonly used greases with their own and their competitors’ products. However, if no data exists or is ambiguous, proper compatibility testing prior to switching greases is always recommended.

To do this, binary mixtures of grease A and B should be made in the ratios of 75:25 and 25:75 and key thickener performance properties, mainly worked penetration and dropping point compared to pure samples of both A and B.

Any significant deviation in thickener performance between the mixtures and the pure greases should be considered evidence of an incompatibility requiring careful cleaning and flushing before switching greases.

Multipurpose vs. All-purpose Grease: Understanding the Difference

While most grease manufacturers make dozens of different greases, by far (50-75%) of lubricated bearings are greased with multipurpose EP2 grease. Most of these greases use a simple lithium or lithium complex soap, though calcium complex can also be found in some formulations.

Aluminum complex is also used, particularly in food-grade NSF H1 greases, since lithium is not considered safe for incidental food contact. Most multipurpose greases have a base oil viscosity in the 150-320 cSt range, are ideal for pillow block bearings and other moderate-speed applications, and are usually formulated with chemically active AW or EP additives.

While “multipurpose” suggests a wide variety of applications, multipurpose is not the same as “all-purpose.”

Common Misapplications of Multipurpose Grease

The base oil viscosity of most multipurpose greases is too high for electric motors – particularly higher HP or with rotational speeds >1800 RM – or directly coupled bearings. This can give rise to excess internal (fluid) friction, which can increase internal temperatures, resulting in premature lubrication degradation and damage to the motor’s winding insulation.

The base oil viscosity of most multipurpose greases is too high for electric motors.

A better choice would be a designated “electric motor grease,” likely to have a base oil viscosity in the 100-120 cSt range and a more shear-stable thickener.

Another common misapplication of multipurpose grease is its use in grid and gear couplings. Larger couplings exert significant torque, which must be accounted for when formulating a grease for a lubricated coupling. Most coupling grease is formulated with a shear-stable lithium complex thickener, together with a high (700-3500 cSt @ 40°C) base oil viscosity to support the load.

Most are NLGI Grade 1 to permit ease of dispersion and are formulated with tackiness agents to help the grease stay on the coupling under high-speed centrifugal force. The specific gravity of the thickener is also carefully balanced to be similar to the base oil to prevent excess base oil bleed-out under operating conditions. Whenever greasing a coupling, always use a designated coupling grease rated for the coupling type (CG-1, CG-2, or CG-3).

Grease Selection: Matching Performance to Application Needs

One of the biggest mistakes I see from my 28 years of field experience is incorrect grease selection. Unlike oil-lubricated assets, where OEM recommendations are often a great starting point, there is a lot of “tribal knowledge” and “smoke and mirrors” around grease selection.

I recall dozens of times when I was told that red grease is better than blue or green beats purple. For the record – grease color has no relevance to how it lubricates. The color is simply a die the manufacturer puts into the lubricant for branding and identification purposes.

Understanding Application-Specific Requirements

Like any other application, grease selection starts with the viscosity of the base oil contained within the grease. This can be found on the grease Product Data Sheet (PDS), which is readily available online. For rolling element bearing lubrication, the viscosity should be chosen to yield a Kappa value (the ratio of the actual viscosity of the base oil relative to the bearing minimum viscosity) of 2-4.

For a primer on Kappa and viscosity selection for element bearings, refer to my article in the April 2023 edition of Precision Lubrication Magazine.

We must decide the best thickener once the correct viscosity has been selected.

Evaluating Delivery Mechanisms and Environmental Factors

This requires understanding the ambient conditions (speed, moisture, ambient temperature, machine temperature, etc.) and delivery mechanism. We need to choose not just the correct thickener chemistry but also the preferred consistency.

In doing so, we need to know, “Will the regrease be applied to a zerk fitting on the bearing housing or through 20 feet of grease line? Is this for a large multi-point centralized grease system, or will we simply pump grease via a grease gun? How cold will it get in winter?”

Lastly, we need to decide what other properties do we need. Do we need AW or EP additives? Is synthetic grease better due to higher or lower temperatures or extended grease intervals?

Grease selection is as critical, if not more critical, than oil selection. So, think carefully and don’t get caught up in the hype of the latest “silver bullet grease” that’s “slicker” than everyone else’s and as sticky as molasses!

Correct grease selection requires careful consideration of bearing operating conditions such as load, speed, size, and type, along with some basic knowledge of ambient stressing conditions and delivery mechanisms. That is all that might stand between successful lubrication and a failed bearing!

All machines with moving parts have bearings of some type. The bearings may be as simple as flat surfaces working against other flat surfaces, such as the slideway in a machine tool, or they may have sophisticated geometries like a ball screw. Plain and element bearings supporting rotating shafts are found in nearly every machine type you might imagine.

Machine designers use sophisticated tools and techniques to create element and plain bearings that provide a specific function for the machine within which they are found.

This article provides reliability engineers and managers with simple but dependable guidelines for selecting lubricants with the correct chemical and viscometric qualities necessary for long-term reliable operation.

Different Bearing Types and Their Roles in Machine Reliability

Element bearings could be categorized by the function they are designed to provide. Assuming that the shaft’s axis is parallel to the ground, some bearings offer support for shafts with only radial (up and clown directional) load, while others provide support for both radial and axial (side-to-side directional) load.

Figure 1 shows the types and shapes of five common element bearings.

Roller bearings and deep groove ball bearings have elements shaped to support a load that is primarily in one direction perpendicular to the axis of the shaft.

Comparing the shape of the inner and outer rings (races) for the deep groove ball bearings to that of the thrust ball bearings, one can see from the difference in curvature of the race contact area around the ball (element) that the thrust ball type of bearing should be capable of with­ standing more axial force than the deep groove bearing.

Figure 2 shows the orientation of the force relative to the shape of the race in a radial thrust bearing. All bearings are designed by the designer to accommodate a given type (direction) and amount of thrust.

The OEM is responsible for installing the bearing into the machine in the proper orientation. In some instances the machine designer may choose to specify two thrust ball bearings with their respective thrust orientation turned opposite to one another to support a shaft with a load that moves back and forth.

The machine builder may select bearings designed to accommodate force in both directions for machines that operate with a significant amount of directional force along one or more shafts.

An increase in element surface area increases the load potential for the element bearing.

Cylindrical and spherical roller bearings are designed to support shafts with both radial and axial (directional) forces. Again, comparing the differences in the shapes of the elements and the races, one can see how the shape of the element conforms to the shape of the race, accommodating thrust from one or both directions.

One may also observe a large difference in the amount of contact surface areas between the races and elements of the deep groove ball bearing and the thrust and roller bearings. An increase in element surface area increases the load potential for the element bearing.

How Oil Films Protect and Extend Bearing Life

The type of oil film produced by mechanical components in dynamic interaction is dictated by the kind of surface interaction that the components experience.

Element bearings exhibit rolling contact characteristics and are characterized as forming and operating in EHD film conditions. EHD films are exceedingly thin, ranging between one-half and one-and-one-half microns thick. To put this into perspective, there are 25.4 microns in 1/1,000th of an inch.

Figure 3 provides an array of commonly recognized items categorized by their respective micron dimension. The dimensions of the oil film thicknesses representative of the EHD film are on par with the dimensions of a bacteria.

Despite the thinness of the EHD film condition, a properly established oil firm demonstrates remarkable durability when the lubricant is maintained in a healthy, clean, dry state.

The dynamic thickness of the EHD film is influenced by many design parameters, including:

1. Bearing material hardness
2. Material pliability
3. Dynamic loading
4. Dynamic temperature
5. Contact surface area
6. The lubricant pressure viscosity response (pressure viscosity coefficient)
7. The initial oil viscosity
8. Other parameters

A properly established oil film demonstrates remarkable durability when the lubricant is maintained in a healthy, clean, dry state.

In the selection process, the reliability engineer is keenly interested in ensuring that the selected lubricant meets the minimum acceptable operating limit to achieve an EHD film condition. The reliability engineer can do so by systematically following a few key principles and establishing an acceptable operating viscosity.

How to Select the Optimal Viscosity for Element Bearings

The most critical characteristic to establish is the lubricant’s viscosity at operating temperature. Viscosity changes with temperature and pressure. As temperature increases, viscosity decreases, and viscosity increases as pressure increases. These factors are interdependent.

The most critical characteristic to establish is the lubricant’s viscosity at operating temperature.

The pressure viscosity relationship depends on the type of raw materials used to construct the lubricant. For any given lubricant selection, the reliability engineer cannot change this characteristic, so we will focus on selecting the correct oil thickness regardless of the type of lubricant.

The central questions for selecting the correct lubricant grade for a given brand and product type are:

• What will the lubricant’s viscosity be at the normalized machine operating temperature?
• What are the allowable, minimum, and optimum viscosities for a given element bearing regardless of operating temperature?

The first question begs for knowledge of the machine’s operating state. Machine speed, load, process temperatures, oil viscosity, and frictional conditions at the element contact area influence temperature. If the machine is already in operation, then the answer may be evident from machine observation and measurement. If not, the reliability engineer must consult with the OEM and production personnel and collect sufficient information to project a safe answer.

For this exercise, assume that the temperature is known; we’ll use a figure of 154°F (70°C). We will also assume the shaft speed is 2,000 RPM, and the bearing has been properly selected for the application.

An exact number can be produced if every incremental de­ tail is known (speeds, loads, forces, material compositions and strengths, VP responses, etc.). As most plant circumstances afford only estimates of these details, this article provides a model that can be followed by plant personnel to appropriately answer questions without the requirement for a full set of exact details and a computer-aided design program.

Step 1

Locate a viscosity selection reference chart for element bearings. Fortunately, most bearing manufacturers provide suitable tables and charts in their lubrication reference guidebooks. Figure 4 is provided by FAG Bearings and is appropriate for this task¹.

Step 2

Use the following formula to estimate bearing pitch diameter, d111, where:

D_m = (OD + ID)/2
OD = Bearing Outer Diameter
ID = Bearing Bore

Assuming you wish to lubricate the bearings in a 254-frame-size motor containing bearings with a bore diameter (ID) of 45 mm and an outer diameter (OD) of 85 mm, the pitch diameter is 65 mm, locate this value on the chart’s x-axis (bottom of the chart), and plot a vertical line from this point to the top of the chart.

This line is referenced on the chart as item I.

Step 3

Determine shaft rotation speed (noted above as 2,000 RPM). Locate the diagonal line labeled with this value on the chart.

Step 4

Using a chart similar to the one in Figure 4, locate the intersection of the pitch diameter and shaft speed lines.

Step 5

Draw a line from this intersecting point to the left side of the chart, to the y-axis, to read the mini­ mum allowable viscosity in centistokes (mm2/sec).

Following these instructions, these points coincide on the y-axis at approximately 12 centistokes. This value represents the bearing manufacturer’s projected minimum operating viscosity or the required oil thickness at the normal machine operating temperature. It is advisable to try to provide three to four times this value as a target operating viscosity.

The practitioner must still determine which of the available grade options will deliver this result.

Step 6

Determine the correct starting viscosity (always measured at 40°C), as noted above. After observing the following steps, the practitioner may use Figure 5 to determine the viscosity starting point (viscosity value at 40°C)2.

Step 6 (a)

Determine the target viscosity (three times the required viscosity= 12 cSt * 3 = 36 cSt). Locate this viscosity value on the y-axis. Plot a line parallel to the x-axis (left to right) from this point.

Step 6 (b)

Locate the machine operating temperature on the x-axis. From this point, plot a line parallel to the y-axis (bottom to top).

Step 6(c)

Note where the two lines intersect. If the value is not at a normal ISO code specification, select the viscosity grade representing the next highest category. This chart represents paraffinic mineral oils with a viscosity index of around 100.

The black arrows in Figure 5 represent the minimum recommended operating viscosity parameters, and the red lines provide parameters to meet the preferred operating viscosity. A lubricant with a viscosity grade above ISO 100 and below ISO 150 would be appropriate.

Given that it is an electric motor bearing on a small frame-size motor that is nearly always grease lubricated, one should look for grease constructed with a viscosity grade at or slightly above 100 centistokes.

Choosing the Right Additives for Bearing Lubricants

The minimum allowable viscosity estimated for the conditions is expected to maintain a ‘fat’ (EHD) oil film in an element bearing. EHD conditions provide for complete separation of interfacing surfaces, but the separation ranges from a paltry 0.5 to 1.5 microns for ball and roller-type bearings.

Within this range (one-time minimum allowable limit), manufacturers suggest the use of rust and oxidation-fortified (R&O) mineral oils and greases containing these types of fortified oils.

Some bearing manufacturers and recognized authorities propose that wear-resistant (AW) and seizure-resistant (EP) additives should be incorporated to protect surfaces³ if the film ratio falls below the minimum allowable level.

It is possible to estimate whether a given bearing in a given set of conditions requires an EP-fortified oil or grease through a film thickness ratio K, or Kappa factor. This is the ratio of the proposed viscosity (at operating temperature) divided by the bearing manufacturers’ required minimum viscosity (at operating temperature).

A ratio of 3 is optimum, so a value of three times the allowable minimum was recommended during Step 6(a). Figure 6 shows a viscosity ratio range for which EP additives are highly recommended.

Choosing the correct oil viscosity can significantly impact bearing life and overall machine reliability.

A manufacturer’s general-purpose (GP) greases commonly have viscosities between 100 cSt and 220 cSt, even though most element-bearing applications carry minimum requirements in the 12 to 22 cSt range.

For very slow-moving and heavily loaded element bearings, it is appropriate to select even higher-viscosity oils and greases and incorporate solid-film agents for enhanced protection against shock loading and loss of EHD condition. Remember that thicker oils and grease consistencies tend to churn, generate heat, and consume energy, particularly in moderate to high-speed applications.

Element bearings are manufactured in a variety of sizes and configurations. Ball bearings have lower contact areas than thrust bearings, but bearings with higher contact areas can support greater loads. Element bearings have definable minimum allowable viscosity limits.

A reliability engineer may use a relatively simple approach to verify that the correct viscosities have been selected. Viscosities should be optimized to a level at least three times greater than the allowable minimum. Bearings that operate with viscosities below the recommended mini­ mum limit should incorporate wear and seizure-resistant additives (AW/EP).

References

1. FAG Roller Bearing Lubrication Guideline WL81115E.
2. SKF Corp. Bearing Maintenance and Installation Guide, p. 29. February 1992.
3. Moller, Boor. Lubricants in Operation, p. 116.

The lowliest and most ubiquitous device in the world of lubrication recently celebrated its centennial anniversary. Designed as a simple yet efficient mechanical gateway device, the grease nipple has only one job: to connect to a manual grease-delivery gun and provide protected one-way access to the bearing cavity for filling purposes.

Some Lubrication-World History

Before Arthur Gulborg invented the grease gun in 1918, a bearing was lubricated individually using a grease cup device. The grease cup, aptly named, resembled a large thimble-sized reservoir sitting above the bearing entry point. The reservoir, or grease cup, had a screw-in lid that, upon removal, allowed grease to be manually paddled in place.

Before Arthur Gulborg invented the grease gun in 1918, a bearing was lubricated individually using a grease cup device.

Once filled, the lid was partially screwed back in place. In doing so, the screw-in action would mechanically force new grease into the bearing.

As the machine continued to run, the operator or designated lubricator would, at regular intervals, screw in each cup lid another few turns to re-lubricate the bearing with more grease. This time-consuming effort would repeat until the cup was empty and required manual refilling.

While working for the Alemite Die Casting and Manufacturing Co. in Chicago, IL, Gulborg ensured all machine-lubrication cups were operated correctly and regularly filled.

Recognizing an opportunity for increased efficiency, he designed the first-ever pressurized grease gun that could be directly coupled to the grease fitting, making the grease cup obsolete.

He named his design the “Alemite High Pressure Grease System.” When it was adopted in 1918 as the greasing standard for the U.S. Army, the company made a fortune.

By 1922, Gulborg’s grease-gun design had evolved into a Push/Pump-style device that could be placed against his newly designed button-head, compression grease fitting. This type of fitting opened under the grease-gun pressure, allowing the lubricant to flow through to the bearing point.

But, as it did not afford a positive “lock on” connection, the grease-gun operator had to use both hands to keep the end of the gun positioned square to the grease fitting’s face. (That was the only way to prevent grease leakage from occurring.)

At the same time, in Kenosha, WI, an eccentric Hungarian immigrant named Oscar Ulysses Zerk was busy inventing a simple nipple-shaped, spring-activated, high-pressure grease fitting. The result of Zerk’s efforts was destined to take the lubrication world by storm.

All the while, Gulborg and the Alemite Co. continued developing their high-pressure grease system. They soon introduced a hand-trigger-operated grease gun that connected to the first positive-lock grease nipple (referred to as a “pin-type grease fitting.”)

By 1922, Alemite’s new system was successfully being promoted and used on mass-produced automobiles. (Alemite’s pin-type grease fitting is still used wherever a guaranteed high-pressure leak-proof seal is required.)

Oscar Zerk’s smaller, less expensive, more compact snap on-off ball lock design proved a worthy competitor to the more cumbersome Alemite pin-lock design.

In 1923, Oscar Zerk received his patent for the Zerk grease fitting and began marketing it through the Allyn-Zerk Co. of Cleveland, OH. His smaller, less expensive, more compact snap on-off ball lock design proved a worthy competitor to the more cumbersome Alemite pin-lock design.

So much so that, in 1924, the Alemite Co. purchased the Allyn-Zerk Co. outright and proceeded to standardize the Zerk grease fitting for the high-pressure Alemite grease-gun systems that we know (and continue to use) today.

As a testament to its original design, the grease nipple has changed little over the past century. Ask anyone to describe a manual lubrication system, and most will immediately visualize a trigger- or lever-action grease gun connected to a Zerk grease nipple. Beware, though: Not all of today’s grease fittings are the same. Successfully employing or replacing grease fittings requires some investigation.

Choose The Right Grease Nipple for Your Needs

Grease nipples are straightforward devices. Incorporating a shaped and threaded housing, ball, and retaining spring, these fittings rarely fail. Difficulty pumping grease into the nipple usually indicates a hydraulic lock situation caused by a problem with the bearing itself.

That said, grease nipples can corrode or be physically damaged when struck. Therefore, they may need to be replaced from time to time. When this is the case, a maintainer or planner should be aware of the multiple available choices.

Material Choice

The most popular and least expensive grease nipple material is mild steel. In wet, humid, and corrosive environments, choosing a nipple made from zinc-plated steel, aluminum, stainless steel, or brass may make for a better decision. Fittings made from monel are recommended in highly corrosive or caustic environments.

Thread Choice

Depending on where the machinery was made or specified, grease nipple threads may be imperial or metric. Depending on the size and application, the thread may be a straight (parallel) or taper thread, British Pipe, National Pipe, or Unified thread.

When in doubt, use a grease fitting thread gauge or a set of thread pitch gauges to determine the correct thread size. Fitting an incorrectly threaded nipple can cross-thread the bearing and cause grease leakage.

Note: for light duty applications, non-threaded drive fittings are available

Style Choice

Depending on access to the grease nipple, the fitting style may need to be considered. Most grease nipples are straight (directly lined with the bearing entrance). For poor-access areas, angled grease nipples may prove advantageous. Grease nipples can be purchased in 90-, 60-, and 45-degree angles.

Extended-barrel nipples can be purchased when reach is a problem. If protrusions of any type interfere, non-protruding or flush grease nipples can be employed.

Specialty Fittings

A large button-head fitting (sometimes called a DIN fitting) is available for heavy, high-powered machinery. This is a positive-lock design that will require its own grease-gun applicator device.

Where absolutely no leakage is allowed, the original Alemite pin-lock fitting is still available. Like the button-head fitting, though, it will require a specialized grease-gun applicator device.

An air-vent, or breather-style, grease fitting can be purchased for closed gearboxes, transmissions, etc.

Pressure-relief grease fittings relieve internal bearing pressure where continued positive internal pressure can cause problems.

Hydraulic-shut-off grease fittings are designed to restrict grease flow at a designed pressure to protect the bearing seal from rupture during the filling operation.

When manual greasing is your preferred lubrication method, choosing and using the correct grease nipple is very much a RAM (reliability, maintenance, availability) best practice.

So, don’t just think of the lowly nipple as something that’s been doing the same job the world over for 100 years: Think of this fitting as a critical, engineered choice that can ensure a bearing is always ready to receive lubricant.

Initially published in The RAM Review.

Grease has been used since ancient times, and new technologies and equipment design require us to improve our understanding and perception of it. These advancements enable those working with grease to recognize better its impact, effective properties, and the proper methods for testing grease samples, shedding new light on its applications and benefits.

The Importance of Monitoring Grease

Through these continuous technological advancements, the formulations of grease have significantly expanded, making it a crucial component to monitor when maintaining and enhancing the performance of modern equipment across various industries.

Greased equipment in mobile and industrial industries is getting more scrutinized as predictive and proactive maintenance are becoming the standard. To this end, greased components need to be viewed as crucial as any lubrication program, and any downtime resulting from failed greased components should be thoroughly investigated to determine the cause of the failure. Was it environmental conditions, over or under lubrication, incorrect grease, or exceeding the equipment design capacity?

The Benefits of Grease Analysis

By testing grease components, analyzing and recognizing wear trends, and determining lubricant properties, we can increase the capacity to react to potential equipment failures. These failures can lead to a reduction in production and compromised safety. While many industry sectors view greased components as replaceable or run-to-failure parts, testing these components allows us to become more informed.

Technological advancements now allow for precise determination of wear concentration and lubricant conditions. With routine testing, we can identify and provide the information to better schedule lubrication intervals, plan equipment repair, and determine the best time to replace components, thus increasing uptime and productivity. With this knowledge, industries can effectively conduct Root Cause Analysis (RCA) to prevent future failures.

Today, failure can be prevented with as little as 2 grams of grease. ATSM D7718-11 Standard Practice for Obtaining In-Service Samples of Lubricating Grease was created to make the process more accessible. This standard describes the method to obtain in-service grease samples that can be tested for trending purposes.

The basic tests, which include Ferrous Density, FTIR, color, and water, are used as a screening tool.

In addition, more complete evaluations of grease include testing for Total Water, Remaining Useful Life (RUL) Antioxidants Levels, Microbial, Elemental Metals, and Extrusion Values. These tests’ key component is for each to be compared to a known provided baseline sample.

The Wear that Grease Testing Identifies

Data obtained from grease evaluations can assist in identifying not only the characteristics of the grease itself but also the quality of the base oil it provides for lubrication.

Effective grease should lead to minimal wear metals. Elevated levels of antioxidants with extended Remaining Useful Life (RUL) across several samples may indicate a need to reassess current relubrication schedules. Proactive monitoring of these factors can reduce lubrication expenses and ensure consistent reliability.

On the other hand, a high wear metal concentration could be a sign of an increase in lubrication, as this would lead to the base oil not providing the correct fluid film for protection.

The National Lubricating Grease Institute (NLGI) grade measures the hardness of grease in relation to pumpability and soap structure, and the ISO Viscosity grade provides the proper fluid lubricant film protection. In addition, tools such as the Analytical Ferrogram give insight into the type of wear being generated and are a great aid in the RCA process.

For example, when analyzing a recent grease sample with severe levels of Ferrous Debris for a crane wheel bearing, it was recommended that an analytical ferrogram be performed. The amount and type of wear observed indicated insufficient lubrication was occurring, yet regreasing was being conducted at regular intervals.

There was no indication that the bearing was in failure mode. However, an abundance of fresh wear was generated. And, since the lubrication regime was boundary, the base oil viscosity plays an important role. Usually, with a slow-moving crane bearing, a base oil Viscosity of ISO 320 or 460 is standard.

This grease being tested had a base oil Viscosity of ISO 220, which caused an increase in wear as the lubricant film was insufficient for the heavy loading occurring. A recommendation was made to contact the crane manufacturer for further guidance on the proper grease for that operating condition.

Looking through this new lens of analyzing grease, adding regular testing on grease components can help reduce unnecessary downtime and increase overall safety.

Grease guns seem simple enough. But how well do you know yours? Do you know how to correctly cartridge and bulk load your gun? Do you know how to expel trapped air from it? Do you know its shot size in cc or cu in (e.g., in cubic centimeters or cubic inches)?

Do you know how much pressure your gun can develop? If you don’t, you’re not alone.

In reality, many grease-gun operators have never been adequately trained to correctly fill a grease gun or determine its output or pressure.

Since most of these devices are now being manufactured offshore, they rarely come with specification sheets or quality instructions. Let’s examine some best-practice user fundamentals associated with these lubrication workhorses.

Grease-Cartridge Loading

Most of the grease guns that are purchased these days use grease cartridges. To load a new cartridge correctly:

1. Wipe the grease gun clean with a lint-free rag.
2. Unscrew the grease gun head from the barrel and place it on a clean surface or paper towel.
3. Firmly hold the grease barrel in one hand and pull back the rod handle at the end of the barrel with the other hand until it can go no further and lock into position. NOTE: Depending on the rod style, it may have a friction-lever lock built into the end of the barrel that automatically holds the rod in place when extended. Or, the barrel end may have a slotted hole that requires the extended rod to be positioned across the slot into the locked position.
4. Carefully remove the spent cartridge from the open end of the barrel, taking care not to cut a finger with the sharp open edge of the spent cartridge.
5. Place the barrel next to the head on a clean surface.
6. Ensure the new grease cartridge is filled with the same grease as the old cartridge. If not, the grease gun must be thoroughly degreased and cleaned to ensure bearings are not cross-contaminated with two different greases.
7. Pull the plastic end cap off the grease cartridge and insert the cartridge into the grease-gun barrel open end first. Once fully inserted, remove the pull-tab foil end from the cartridge.
8. Fully screw the grease gun head back on the barrel and back off (loosen) one turn.
9. Release the rod handle by pushing the friction-lock lever or returning the lever rod back across the slot to its center position, then slowly push the rod back in place in the barrel as far as it will go.
10. Pull the trigger or pull/push the lever until grease begins to dispense, then securely tighten the gun head.
11. If no grease flows, an airlock is likely to blame. To release trapped air, pump the grease gun a couple of times, then, if fitted on the grease head, push the air-release valve and re-pump the grease gun. If grease still doesn’t appear, repeat the process. On smaller grease guns with no air-release valves, back off (loosen) the barrel a couple of turns, and pump until grease appears, then retighten the barrel,
12. Wipe the grease gun with a lint-free cloth, then place all spent materials in a contaminated waste container for removal.

Bulk-Loading Grease

Some grease guns have a dual-fill feature, whereby the barrel can accommodate a standard grease cartridge or be bulk-loaded. For such guns and those designed specifically for bulk loading:

1. Wipe the grease gun clean with a lint-free rag,
2. Unscrew the grease-gun head from the barrel and place it on a clean surface or paper towel.
3. Immerse the open end of the grease barrel into the bulk-grease container and slowly plunge it into the grease while drawing back the rod handle until it is fully extended and the grease barrel is full.
4. Alternatively, the barrel can be hand-packed from the open end with the rod extended and locked. NOTE: This fill type can be messy and prone to dirt and air inclusion.
5. Fully screw the grease-gun head back on the barrel and back off (loosen) one turn.
6. Release the rod handle by pushing the friction-lock lever or returning the lever rod back across the slot to its center position, then push the rod back in place in the barrel as far as it will go.
7. Pull the trigger or push/pull the lever until grease begins to dispense, then securely tighten the gun head.
8. If no grease flows, an airlock is likely to blame. To release trapped air, pump the grease gun a couple of times, then, if fitted on the grease head, push the air-release valve and re-pump the grease gun. If grease still doesn’t appear, repeat the process. On smaller grease guns with no air-release valves, back off (loosen) the barrel a couple of turns, pump until grease appears, and then securely retighten the barrel.
9. Wipe the grease gun with a lint-free cloth, then place all spent materials in a contaminated waste container for removal.

Output and Delivery Matters

A grease gun is a simple design based on basic hydraulic pump principles. Depending on your device’s internal design, a few strokes of the trigger or lever can produce an astounding delivery pressure (ranging from 2500 to 15,000 psi).

With that type of output pressure and lack of grease-delivery knowledge and discipline, it can be easy for an untrained grease gun operator to destroy bearing seals and over-lubricate bearings, which, in turn, can result in premature bearing failure and downtime.

Sadly, grease-gun output pressure is rarely stamped on these devices. A simple pressure-test rig can be constructed using a fixed 20,000-psi hydraulic gauge connected to a grease fitting. Connect the grease pump to the fitting and perform a “dead head” pump to see and record the generated pressure.

To measure the grease-pump-delivery output, purchase a test tube marked in cubic centimeters or cubic inches and pump in 10 shots of grease (One complete lever or trigger cycle equals one shot). Divide the total amount shown in the test tube by 10 to arrive at the actual shot size.

For example, if the test tube showed 27 centimeters of grease, the actual shot size would be 27/10 = 2.7 cc. (Pneumatic- and battery-operated grease-gun shot size can be calibrated the same way or by hooking these types of devices up to a flow-metering device.)

Note: Grease guns aren’t all built to the same design specifications. Therefore, their displacement-output volumes, or “shot” sizes, will likely differ. This poses enormous problems for a plant when a PM task calls for two shots of grease and the guns used at the site haven’t been standardized.

Remember that two shots of a 3-cc-displacement gun will deliver six times more lubricant than two shots of a 1/2- cc-displacement gun.

Some grease-gun manufacturers confuse the delivery-output issue by marketing grease-gun-reservoir capacities and “shot” displacement by weight (in grams and/or ounces).

On the other hand, grease manufacturers use different ingredients and formulations for each grease type, resulting in different specific gravity ratings and weights. This means that similar volumes of grease can have different weights.

For example, greases can be marketed in a standard grease-gun cartridge with the same volume, but one cartridge might weigh 300 grams while another weighs 400 grams. Bearing-fill cavities are measured in volume, not weight. Therefore, always use volume displacement as your grease unit of measure.

Finally, once a grease gun’s pressure and shot size are known, it’s essential to print out a tag with that information and attach it to the gun.

Cleaning and Storage Practices

After a grease gun has been used, it should be cleaned and made ready for its next assignment. Be sure to store it vertically in a grease caddy. These types of caddies can be magnetized to your toolbox or metal cupboard or screwed to a wall.

This type of vertical storage will ensure your lubrication workhorse remains free of damage, dirt, and, importantly, cross-contamination from other grease guns.

Years ago, I had my first encounter with one of the predictive maintenance technologies. Since then, I have seen its potential, especially when combined with other PdM tools.

Airborne ultrasound for bearing inspection is probably one of the simplest, most versatile tools, and the results can be seen almost immediately.

Depending on the source and author, ultrasound is usually at the beginning of the P-F curve ahead of other tools. While this is debatable, and there will be cases where it is and others where it is not, we agree that we cannot deny its value and the benefit it has in determining the condition of the bearing.

In the worst-case scenario, if no remedy is applied, the inaudible ultrasound will tend to be audible, and the bearing will be very close to functional failure.

In a very simplistic way, the theory governing this tool says that ultrasound (dB) results from friction between the rolling element, the inner race, the cage, the lubricant, and other elements, such as external contamination and internally generated wear.

To keep the ultrasound in a normal range, the lubricant and grease for the present study’s analysis are added to minimize friction between the bearing components. In this way, contact and wear are reduced, and the service life of the bearing is extended to achieve the expected life.

Around this concept and with the booming entry of Industry 4.0 into the PdM sector, sensors have been designed that automatically add grease, a precise volume so as not to overload the tiny space that exists in the bearing with grease, and whenever they are detected abnormal ultrasound values.

Up to this point, everything is going smoothly and in such a way that the lubrication of bearings is a before and after this technology.

But what happens to the grease that is in service? If we were to interrogate said grease, what would it tell us? What value would it bring us?

 What Can Grease Tell Us?

The first grease analysis report in service in an electric motor bearing I had in my hands dated back to 1994. Year of the Soccer World Cup, while I was watching the opening match that pitted the almighty German team against 11 Bolivian warriors, someone under the midsummer sun was taking a grease sample from a bearing that had failed, with the intention of knowing if that sample could provide any clue about the failure.

At that time, oil analysis had already been providing benefits for some decades, but grease… who would think of analyzing grease?

Now we know how much grease analysis can contribute to maintenance and reliability; at least some of us are clear about it, and we know how to get the most out of it.

A grease sample obtained properly shows the condition of the bearing; it can reveal if there is contamination, what type, how much contamination, if there is current passage, if there was a mixture of greases, if that mixture is incompatible and a long etcetera, that is too much to ask, at least from a non-Newtonian fluid.

Grease analysis is so vital that in the wind generation industry, for example, an appropriate analysis can warn of a functional failure of the main bearing, thus avoiding incurring a high preventable maintenance cost.

What Do Ultrasound and Grease Analysis Contribute Together?

As we have seen, both tools are incredible in isolation and provide many benefits if used properly.

But could they be used together?

I had been asking myself this question for years. I had been able to analyze about 30 bearings, obtaining grease samples and ultrasound, but specifically, without achieving adequate traceability, at least two points are needed to draw a straight line, and with only one point, I had no more than assumptions and no coherent results.

In a petrochemical plant, I quenched that thirst for knowledge. For two years, I collected grease and ultrasound samples in a group of around 60 assets of different characteristics and conditions.

The results and conclusions presented in this article cannot be generalized since they depend significantly on factors such as the type of bearing, speed, operating conditions, operating environment, types of grease used in the plant, and the human factor. Regarding the variability of manual regreasing, we cannot use a sensor in 100% of the assets, at least for now.

Although the measurement obtained by ultrasound is unique and is expressed in dB, the result of the grease analysis is divided into the following variables

• Wear ≤ 7 microns
• Wear ≥ 5 microns
• Cage wear (taking into account that there were no polyamide cages)
• Grease consistency
• Incompatibility between thickeners
• Different oil viscosities

Of these six variables, the results shown in this short article focus only on the first two, bearing wear. On the other hand, only results for the coupling side bearing are shown.

After two years, around 500 samples of each technology were obtained using a color system. In isolation, the results are as follows:

Broadly speaking, most bearings behave similarly depending on wear and decibel level, and an analysis of this type can yield quite a few conclusions.

We will take as an example the coupling side bearing corresponding to the JBX – 102A unit.

This bearing had to be replaced the month after the last analysis because the friction produced internally could be heard, and subsequent analysis showed irreparable damage to the internal race.

• Greasing is done within the concept of preventive maintenance based on frequency
• Bearings are changed for corrective maintenance; they operate until failure
• The regreasing frequency is bimonthly, marked with a box in Figure 2
• In the months when regreasing is not required, only information is collected
• The volume of grease added does not exceed the calculated volume

Initial results

Following the timeline, we can see some remarkable results.

• The ultrasound baseline for this bearing is 38dB, and the technology alerts that this limit has been exceeded after the third month (Jun-18).! Point for ultrasound! The grease analysis does not show any abnormal wear.
• Moving forward in time and after the usual regreasing, the ultrasound returns to normal values, and when everything seems to be okay, the wear measured by particles less than 7m exceeds the established limit (Sep-18).! Point for grease analysis! Ultrasound analysis is within normal values.
• After the Nov-18 regreasing, the ultrasound is within normal levels, but both types of wear have exceeded their permissible limits.
• From this point, we enter a zone of no return with increasing trends for the three variables. Due to human error, the regreasing of Feb-19 was not carried out, and to balance accounts with the management system maintenance, it was regreased two months in a row, Mar-19 and Apr-19.
• Past this point, the dB does not dramatically increase as wear does until it ends in functional failure of the bearing.

Conclusions: They May Help

Going back to the P-F curve, who anticipates the functional failure of the bearing in this case? It’s hard to say who, but the truth is that we can learn interesting things from this situation:

•  Ultrasound, at least in this situation, is an excellent indicator of the qualitative outcome of the bearing condition. Yes, it gives us a value in dB, and this result is quantifiable, but it does not tell us what the wear is like.
• Grease analysis, at least in this situation, is an excellent indicator of the quantitative outcome of the bearing condition. Yes, it tells us how much it wears, but it does not tell us the friction between the bearing elements.
• Bearing component wear is a result of friction, and this, in turn, generates more wear.
• The added value of working with both technologies lies precisely in determining the friction and wear.
• Unfortunately, grease does not flow easily and tends to retain wear particles generated by friction.
• Many bearing configurations do not have a purge point to adequately remove grease and particles included, so wear, in many cases, is part of the system.
• Going from preventive maintenance, where greasing is based on frequency, to predictive maintenance, where greasing is based on condition, requires an almost perfect monitoring system. Friction—wear knows no limits.
• Limits must be respected. Whether it is 80 km/h, 38dB, or 50ppm, iron has been set for some reason.
• Corrective maintenance actions must be appropriate to the results obtained. You may not have to replace the bearing at 44dB/40ppm Fe, but it will be necessary to define what should be done; otherwise, the functional failure will decide for you.

Onsite oil analysis is an effective tool to analyze samples and optimize maintenance activities quickly. As part of a comprehensive condition-based maintenance program (CBM), oil analysis effectively complements other diagnostic technologies like vibration analysis, infrared thermography, and ultrasound technology.

However, this critical lubrication monitoring step is often overlooked in grease-lubricated equipment. SKF states that 80% of the world’s bearings are grease-lubricated, leaving a vast opportunity to incorporate grease analysis techniques into the overall CBM strategy.

The Electric Power Research Institute (EPRI) suggests that nearly 50% of bearing failures are related to poor lubrication and contamination¹. The development of grease condition monitoring standards, ASTM D7718 and ASTM D7918 have laid the foundation for a consistent methodology to sample and test grease to implement condition monitoring strategies.

By monitoring key data points, such as wear, oxidation, and additive health, the asset manager can transition from calendar-based to condition-based changeouts. This has the potential to save hundreds of thousands of dollars per year for an owner of a large fleet.

The wind, rail, and automotive robotics industries are implementing these strategies into their programs and potentially avoiding thousands in maintenance costs by predicting failures and extending greasing intervals³.

Historically, incorporating any CBM strategy into grease-lubricated components has been challenging.

The small quantity of grease typically available on an in-service component and the limited testing available for small amounts of grease often present barriers to routine grease sampling and analysis.

To address these challenges, grease sampling tools can capture a representative sample from bearings and gears requiring as little as one gram of grease. Onsite analysis tools are available to assess the wear and physical properties of the grease.

This simple sampling technique can be used in various industries, including but not limited to wind, rail, robotics, mining, and nuclear, to sample a large number of grease-lubricated components and evaluate the next actions based on the criticality of the data.

Periodic sampling and analysis of the grease from these components can give asset owners a clearer picture of equipment health, determine grease conditions for optimal change-out periods, and pinpoint latent issues that can be addressed before failure.

Grease Sampling

In most circumstances, procedures for obtaining grease samples from bearing housings and gears are not consistent and likely do not represent the actual condition of the “active” grease near the lubricated surface.

Therefore, the challenge in optimizing a grease analysis program is developing test methodologies that measure in-service grease conditions utilizing a small amount of grease and a sampling process that enables representative grease samples to be taken without disassembly of the component².

While it is a reality that the user may need to scoop and scrap to the best of their ability to get a sample, there is a standard method that exists for taking in-service grease samples: ASTM D7718 Standard Practice for Obtaining In-Service Samples of Lubricating Grease.

This standard shows several standard methods to take a representative grease sample and utilizes the Grease Thief® sampling device to sample the grease from a bearing, valve, or gearbox.

Grease Thief

Depending on the instrumentation being used, some scoops or tubes may be available with the instrumentation. Speak with your instrument manufacturer and ask which tools are available to sample and measure grease within the instrument. But know, there are standard methods out there to be utilized.

Grease Analysis as a Screening Tool for Fleet Analysis

Routine grease analysis is common in high-value fleet applications such as locomotives, automotive robotics, and wind turbines, where a relatively straightforward set of screening tests for wear and oxidation can guide grease relubrication frequency, cases of mixup, and monitoring wear levels.

Bearing or joint failures could result in millions of dollars in lost product or power, or even worse, could threaten the safety of employees and customers. Asset owners must be able to look at a large quantity of data and pinpoint latent issues where they can focus their resources and prioritize accordingly.

Compared to other diagnostic technologies, grease analysis can detect issues earlier on the P-F interval than vibration analysis, allowing asset owners more time to address the problems and avoid potential downtime.

Once a representative sample is taken, onsite monitoring of the grease sample can be done using existing onsite equipment that was likely purchased for oil analysis.

Onsite Tools for Grease Analysis

Ferrous monitoring tools, handheld infrared spectrometers, and metals spectrometers can be used to measure ferrous debris, physical properties, and contaminants present in the grease. These onsite tools allow for quick monitoring of many samples so immediate action can be taken.

Ametek Spectro Scientific offers a variety of onsite tools like the Ferrocheck, FluidScan, and Spectroil M or 100 that can be used for both onsite oil and grease analysis:

Ferrous Debris Monitoring with the FerroCheck

Ferrous debris monitoring is the most common and cost-effective way to trend wear issues on bearings, gearboxes, or valves. The FerroCheck is a magnetometer that senses the disruption of the magnetic field due to the presence of magnetic particles in the grease.

The particles’ disruption can be directly correlated to the amount of ferrous debris in the grease. The FerroCheck provides a quick, straightforward, non-destructive solution for measuring the ppm of Iron in grease⁷.

Ferrocheck

Based on the criticality of the component, the sampling frequency can be determined so wear trend analysis and alarm limits can be determined. It’s important to understand that wear particulates in grease are cumulative and, unlike oil, wear particles will remain in the grease until deliberately purged or flushed from the component.

With the ability to detect up to 15% ferrous wear, the FerroCheck is an effective tool for trend analysis and can identify outliers in a fleet application.

Infrared Spectroscopy with the FluidScan

Utilizing the FluidScan (compliant with ASTM D7889), Infrared Spectroscopy is a powerful tool that can be used onsite to measure grease oxidation and identify potential contaminants like moisture or mixtures with other greases.

Monitoring these parameters via trending and direct property analysis makes it possible to inform the user when the grease has completed its useful service. Using a comparison library (over 800 oils and greases), FluidScan can compare the grease sample to the reference to identify potential grease mixing.

If possible, mixing greases is a practice that should be avoided. Mixing of greases can lead to changes in the rheological properties of the grease and eventual separation of the oil from the thickener. If considering mixing two greases, it is best to perform a compatibility study (ASTM D6185) to determine if mixing is acceptable.

Moisture and oxidation can also be determined on the FluidScan. A typical moisture peak can appear on the IR Spectrum around 3400cm-1. As these greases age and oxidize, the buildup of oxidation products may be monitored by the FluidScan infrared analysis.

This is translated into an oxidation value and water index. It’s important to note that some polyurea-thickened greases also have a peak on the IR spectrum in this region, and care should be taken not to mistake this peak for moisture.

The polyurea peak at 3400cm-1 will be a short, small peak versus a moisture peak, which will be a larger, broader peak. These same IR masking issues can also occur in greases formulated with an ester-based synthetic base oil.

These greases will show a peak at 1750cm-1, where oxidation also appears. It’s essential to understand when these greases are being used and note this could impact the oxidation and moisture trend values.

As with any effective CBM program, it is important to establish trends and focus on how the grease deviates from the trend. Any significant deviations from the trend would require action. Over time, alarm limits can be established for particular components based on equipment load, runtime, and environmental conditions.

Spectroil M and 100 Series Spectrometer

Using a Rotating Disc Electrode (RDE) Spectrometer, the concentration of metals in the grease can be compared to the new grease to identify significant differences in additive metals that could point toward grease mixing.

Also, the presence of wear metals (Iron, Lead, Tin, and Copper) can be determined. RDE Spectroscopy has been a common laboratory and field method for quick grease analysis over the last 15 years. Sample preparation is important; however, it differs based on early adopter experiences.

The two most common preparation methods are dilution (slurry) and wet smear. In the case of dilution (slurry), the grease sample is diluted with a solvent to create a low-viscosity slurry that may be placed in the sample cup and excited as normal.

A second method is the smear (wet) approach whereby a rotrode is rolled in the grease sample to create a coating on the disk edges, and then it is mounted on the shaft, and a sample cup of base oil is used. Either method relies on the consistency of the operator and an understanding of the goals.

The tests and instrumentation listed here for in-service grease testing are not exhaustive.

For a complete list of recommended tests, consider reviewing ASTM D7918 Standard Test Method for Measurement of Flow Properties and Evaluation of Wear, Contaminants, and Oxidative Properties of Lubricating Grease by Die Extrusion Method and Preparation, which discusses additional points to monitor like moisture, color, and consistency. For more information on adding those tests to your grease analysis program, contact MRG Labs.

Acknowledgments & References

Special thanks to Rich Wurzbach, MRG Labs, for sharing his grease analysis knowledge with me over many years.

[1] E. (2001, October). EPRI_Lubrication Guide 1003085. Retrieved from https://www.scribd.com/doc/128347108/EPRI-Lubrication-Guide-1003085

[2] Williams, L. A., & Wurzbach, R. N. (2016). Managing the Health and High Costs of Robotics Using Grease Sampling and Analysis (Tech.). Hot Springs, VA: NLGI National Meeting.

[3] Kowalik, G., & Janosky, R. (2017). Advancements in Grease Sampling and Analysis Using Simple Screening Techniques (Tech.). York, PA: MRG Labs.

[4] Williams, L., Wurzbach, R., & Alarcon, J. (2016). Integrating Grease Sampling and Analysis into Wind Turbine Maintenance Programs (Tech.). Bilbao Spain: LUBMAT.

[5] https://www.awea.org/wind-energy-facts-at-a-glance

[6] McKenna, P., Subramanian, M., Berwyn, B., Kusnetz, N., Jr., J. H., Lavelle, M., . . . Spiegel, J. E. (2017, June 01). U.S. Wind Energy Installations Surge: A New Turbine Rises Every 2.4 Hours. Retrieved from https://insideclimatenews.org/news/03052017/wind-power-rising-clean-energy-jobs

[7] Oil Analysis Handbook (3rd ed.). (2017). Chelmsford, MA: Spectro Scientific.

They are used to lift and pull heavy containers in Ship-To-Shore Cranes (STS), Rubber Tyre Gantry (RTG), and Straddle Carriers. I can also see them on my way to work in different construction machines, and I feel their usefulness when I use the elevator of the building where I live.

But they are not only used in handling heavy loads! They also serve as structural support cables in large static structures like The Mohammed VI Bridge, the longest cable-stayed bridge in Africa, linking the capital city of Rabat to the city of Salé.

Getting optimum life out of dynamic steel wire rope is one of the key strategies within a maintenance program. A simple discussion with maintenance folks in the field will reveal that wire ropes are a big issue. In this article, I will not dwell on dimensional characteristics but focus on the lubrication side of dynamic steel wire ropes.

I will explore how to keep dynamic steel wire ropes youthful through relubrication, starting by shedding light on the importance of relubricating dynamic steel wire ropes, then reviewing the types of lubricants used. This article concludes with different possible methods to relubricate dynamic steel wire ropes and the best practices for effective dynamic steel wire rope relubrication.

Why Is Lubricating Dynamic Steel Wire Rope Required?

Adequate and proper lubrication is vital to keeping dynamic steel wire rope running at its best. DIN 15020 standard explains that lubricants in wire rope diminish friction and corrosion between the groove and the wire rope and between the individual wires.

The same standard explains that shorter rope service life should be expected if the wire rope lubrication is stopped for operational reasons. Thus, lubrication is essential in the steel wire rope life cycle.

It considerably impacts its operability and aims to support the integrity of the wire ropes with continued safe and effective operations. Lubrication could eliminate or minimize most of the causes of wire rope failures. We can segregate between 3 tiers for steel wire rope lubrication:

Lubrication Of Dynamic Steel Wire Rope During The Manufacturing Process

Individual steel wires are subjected to extensive bending, twisting, and tension during their manufacturing process to form the rope. These actions generate friction between the individual wires and strands, leading to wear and potential damage.

The lubricant helps the wires move smoothly through manufacturing machines. Also, it helps create a protective layer between the wires to allow slight movement between them and minimize the risk of wear and corrosion. ISO 4346 Standard specifies the nature, properties, and basic requirements of lubricants used to manufacture wire ropes for general purposes [2].

Lubrication Of Dynamic Steel Wire Rope During Storage

Dynamic steel wire ropes depart the manufacturing plant saturated in lubricant. During their storage, wire ropes can be exposed to several factors affecting their performance and longevity after deployment.

Direct exposure to rain or snow, humidity, temperature changes, and chemicals increases the risk of corrosion before deployment. Proper lubrication is essential in protecting their structural integrity and readiness for use.

Regular wire rope inspections during storage can help determine when relubrication is necessary. Signs of wear, corrosion, or excess dirt and debris may show the need for relubrication.

Additionally, wire ropes stored for an extended period without use may require relubrication to ensure they are still in optimal condition before installation. If necessary, apply a suitable preservative or lubricant compatible with the rope manufacturing lubricant [3].

Lubrication Of Dynamic Steel Wire Rope In Service

During operations, a steel wire rope is bent considerably. The wires and the strands move against each other. Relative movements also occur between the wires in stranded ropes changing the tensile forces by friction. There are also movements between wire ropes and sheaves [4].

On the other hand, steel wire ropes must withstand demanding working conditions (heavy loads, constant friction) and harsh environments (exposure to dirt, chemicals, moisture from rain or high humidity, and other contaminants). These factors can cause the wire rope to deteriorate over time, leading to loss of strength, wear, and eventual failure. Lubrication helps to minimize the effects of these factors by:

• Reducing internal (metal-to-metal contact between wires and strands) and external friction (wire rope exterior surfaces with grooves).
• Enhancing flexibility.
• Mitigating fatigue failure.
• Protecting from external contaminants.
• Preventing Corrosion.

What Is the Right Lubricant For Dynamic Steel Wire Rope Relubrication?

To make dynamic steel wire rope conduct its duty smoothly while being protected from external contamination and corrosion, consider two categories or types of lubricants: penetrating and surface coating lubricants.

Penetrating Lubricant

Dynamic steel wire ropes may fail from the inside with regular use. Penetrating lubricants are intended to reach the steel wires in the strands and rope core to reduce friction and wear during normal use and provide maximum protection against corrosion. Desired properties for penetrating lubricants are:

• In grease, suitable consistency, drop point, thickener, and base oil viscosity with friction modifiers additives (AW & EP).
• In the case of oil, suitable base oil viscosity with desired additives (AW & EP).

In both cases, the lubricant should withstand the elevated temperatures generated during wire rope operation and have excellent water and oxidation resistance.

Coating Lubricant

The coating lubricant should act as a barrier and seal to protect the outside of the wire rope from moisture and contaminants. Desired properties of coating lubricant are:

• Corrosion prevention
• Resistance to hot temperatures and water/wash
• Sufficient adhesive strength to allow the lubricant to remain on the rope

It is advisable to avoid asphaltic compounds because they dry into a dark hardened surface that makes inspection and cleaning difficult.

Several lubricating products can be used to relubricate dynamic steel wire ropes, including greases, oils, and dry lubricants. The choice of lubricant will depend on the application, environment, and operating conditions.

When selecting a lubricant for dynamic steel wire rope, make sure it complies with the recommendation of the steel wire rope manufacturer.

What Is the Right Quantity of Lubricant To Use While Relubricating Dynamic Steel Wire Rope?

The amount of lubricant applied is critical. Over-lubrication can lead to excess buildup and attract dirt and debris, while under-lubrication will not adequately protect the wire rope and lead to increased wear. Following the manufacturer’s recommendations, applying the right lubricant amount can help ensure optimal lubrication and wire rope protection.

Relubrication Intervals for Wire Ropes

After the wire rope is put into service, the original lubricant is lost gradually with normal use. Regular relubrication is essential to ensure the wire rope’s longevity. However, the relubrication frequency is difficult to manage as it depends on various factors, such as the application and operating conditions, including load, speed, and environment.

With wire rope relubrication, we should differentiate between coating lubrication and penetrating lubrication. The coating lubrication frequency follows the inspection frequency. Before visual or magnetic inspection, the wire rope should be cleaned (remove the coating lubricant). After the wire rope inspection, the coating lubricant should be re-applied.

However, as a rule, dynamic steel wire rope relubrication intervals should be scheduled per the wire rope manufacturer’s recommendations and the equipment manufacturer where it is installed. Without such instructions, lubricate wire ropes for benign operating conditions at least once a year.

More frequent lubrication would be necessary for severe operating conditions and environments to maintain optimal performance and prevent wear and corrosion.

Methods to Relubricate a Wire Rope

Relubricating wire ropes should be preceded all the time by cleaning. Any residual coating lubricant and contaminants should be removed first before applying any new lubricant. There are three common methods for wire ropes lubrication: [6]

Manual Lubrication

Manual lubrication involves applying the lubricant to the wire rope by painting, Swabbing, or brushing. It is typically used only for coating outer wires and is suitable for wire ropes not exposed to elevated temperatures. When using oil, a spray can be used for the wire rope relubrication.

Manual Lubrication (painting) of wire ropes is time-consuming and poses safety and environmental issues. Technicians come into close contact with lubricants and may have some safety concerns due to broken strands and intensive labor.

Additionally, manual lubrication will use more lubricant than other methods, probably resulting in poor lubricant penetration. The more lubricant quantity used, the greater the lubricant cost and risk of environmental impact.

Semi-Automatic Technique

The drip, spray, or trough method can apply the lubricant. These methods apply the lubricant at a single point and use the rope’s movement to spread the lubricant over the entire length of the system.

Automatic Technique

Pressurized lubricant application is the best relubrication method for wire ropes. This approach utilizes a pump, the force of the pressurized lubricant between the wires and into the wire rope’s small vacant spaces.

This mode provides maximum penetration of the lubricant into the gaps of the wire rope, enables the lubricant to adhere to each wire, and offers the best protection against corrosion. Side benefits from the pressurized application include less safety risk to technicians and less chance of environmental impact from routine care.

Best Practices for Wire Rope Lubrication

To ensure adequate dynamic steel wire rope lubrication, follow these best practices:

Do’s

Lubricant Compliance

Use the right lubricant suitable for the wire rope material and application. Consider the factors mentioned above and follow recommendations from the wire rope manufacturer to ensure compatibility and effectiveness.

Cleaning Before Relubrication

Before applying the lubricant, clean the wire rope thoroughly to remove any dirt, debris, or any other contaminant from the outer strands and in the valleys of the wire rope. A wire rope cleaner could be used. This will enable a proper penetration of the new lubricant and enhance the corrosion protection to optimize wire rope lifespan.

In case of dirty wire rope, hardened lubricant, or accumulated layers of the old lubricant or other contaminants, the wire rope should be cleaned with a wire brush and petroleum solvent, compressed air, or steam cleaner before relubrication. For this, use only compatible cleaning fluids, which will not impair the original rope lubricant nor affect the rope-associated equipment.

Lubrication Method

Use the wire rope lubrication proper method, which will facilitate the application of the lubricant evenly over the wire rope full length.

If the lubricant contains a diluent that evaporates after exposure to the atmosphere, the lubricant should be given 4 to 8 hours to ‘cure’ before the device is returned to service.

Safety

All mandatory Personal Protective Equipment (PPE) shall be worn in addition to the relevant PPE if needed. If cleaning by brush, eye protectors must be worn. If using fluids, understand that some products are highly inflammable. Wear a respirator if cleaning with a pressurized spray system.

Don’ts

Lubrication Method

Over-lubrication can cause the lubricant to accumulate and attract dirt and debris, leading to wire rope wear.

Safety

Don’t discount the potential safety hazards of working on a moving rope.

Please don’t attempt to clean and lubricate the wire rope while it is suspending a load unless otherwise stated in the OEM’s instruction manual or other relevant documents.

Proper lubrication is essential for protecting the dynamic steel wire rope against corrosion and contaminants and for protection from wear and other potential damage. Select the right lubricants, following the most suitable application method, and lubricate at the optimum frequency.

A helpful starting point would be a review of the lubrication recommendations and any specific requirements specified by the wire rope manufacturer and OEM of the equipment.

Precision lubrication is vital for extending wire rope lifespan but is not the only parameter to address. Proper storage conditions, handling, precision installation, and regular inspection can help prolong the service life of the wire rope and reduce maintenance costs.

[1] DIN 15020: Lifting Appliances; Principles Relating to Rope Drives; Calculation and Construction.

[2] ISO 4346: Steel wire ropes for general purposes — Lubricants — Basic requirements.

[3] ISO 4309: Cranes — Wire ropes — Care and maintenance, inspection, and discard.

[4] Wire Ropes; Tension, Endurance, Reliability; Second Edition; Klaus Feyrer. pp 31 to 33.

[5] Lubrication and Reliability Handbook; M.J. NEALE pp A10.1 from to A10.3.

[6] Handbook of Lubrication and Tribology; Volume I: Application & Maintenance; Second Edition; George E. Totten. pp from 15-1 to 15-6.