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. http://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.

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.

Whether we realize this or not, oil refineries are critical in our everyday lives. They convert crude products into diesel, gasoline, LPG, and plastics. The equipment in this plant must withstand very high temperatures, sometimes over 500°C (in the distillation unit).

The lubrication systems in these plants also must withstand very harsh environmental conditions. Quite often, the bearing temperatures in critical compressors may increase to over 100°C, threatening a trip or shutdown. Unplanned downtime reduces refinery output and may cost a refinery up to Euro 1.2 M/day. All of this can be avoided by maintaining the quality of the lubricants.

Why Varnish Threatens Refinery Operations

Varnish, lubricant-derived system deposits occur in most types of equipment. It plates out as deposits on the insides of the equipment, which will act as a thick layer of insulator, preventing heat from escaping. One of the main functions of oil is to provide cooling to the equipment by transferring heat away from the internals of the equipment. However, with the formation of varnish, this function of the oil is eliminated.

Essentially, varnish can be described as polar compounds that have been formed as a result of the degradation of oil. There are various ways in which oil can degrade to produce deposits. As such, varnish can have varying characteristics depending on the system’s conditions, the formulation of the oil, and any contaminants that may be present in the system.

Despite the way varnish was created, some aspects remain the same. The presence of varnish can cause the sticking of valves or impact the efficiency of heat exchangers. Given the small clearances for hydraulic or other precision equipment (such as turbines or in bearings), any deposits inside these components can affect oil flow.

This results in elevated temperatures, which further increase thermal stress on the oil, thereby establishing a continuous feedback loop.

Varnish buildup creates a feedback loop of rising temperatures and increased stress on oil.

According to the varnish lifecycle illustrated in Figure 1, varnish can precipitate in and out of the solution even after oxidation has occurred.

From this lifecycle, it is clear that the double arrows are used between the solubility and varnish formation stages. As such, even if varnish is formed and deposited using the right technologies, it is possible that it can be redissolved into the oil.

In compressor applications, its bearings are the most critical place for varnish to form. This occurs in the minimum film thickness zone, as seen in Figure 2.

As per (Jang, Khonsari, Soto, & Livingstone, 2024), one can predict the effect of varnish on bearing performance by solving the Reynolds equation for pressure distributions with the mass conservation algorithm coupled with the energy equation through viscosity. However, this method was not utilized at the refinery.

Based on the study by (Jang, Khonsari, Soto, & Livingstone, 2024), the maximum pressure decreases when the varnish size extends circumferentially at a given varnish thickness, but the temperature remains relatively constant.

Case Study: How TÜPRAŞ Tackled Varnish Issues

Tüpraş is Turkey’s largest oil refiner, and it is located in western Turkey. It manufactures LPG, gasoline, jet fuel, and diesel fuel. They experienced trips on their compressor in the Kırıkkale Refinery, which has a mid-level complexity by Mediterranean Standards, including hydrocracker, isomerization, diesel sulphurization, and CCR reformer units.

The Kırıkkale Refinery has an annual crude oil processing capacity of 5.4 M tonnes, and its supply is carried by the BOTAȘ’ Ceyhan Terminal and Ceyhan- Kırıkkale pipeline. It was established in 1986 to meet the petroleum demands of the Ankara, Central Anatolia, Eastern Mediterranean, and Eastern Black Sea regions.

The K1151 compressor (RB5B Thermodyne compressor) in the Kırıkkale Refinery is critical to the Crude oil processing and isomerization unit. Every time this asset trips, the facility undergoes maintenance and repairs, typically lasting seven business days. This issue may occur at least 4 times annually, and each trip costs roughly around USD274k per event.

A trip usually occurs when the bearing temperature goes above 115°C, however, based on the history of the machine, the temperatures typically see a spike to 100°C followed by a rapid increase to 115°C.

For this compressor, it was also noticed that there were sawtooth-like temperature patterns where the values fluctuated between 76-86°C, as shown in Figure 3 below. The “safe zone” of operation for the bearings is less than 89°C.

This sawtooth temperature pattern is typically seen in instances where there is varnish buildup on bearings or shafts. Usually, the varnish builds up layer by layer, acting as an insulator, which causes the temperatures to increase.

Eventually, the buildup will get to a point where the shaft wipes away the varnish, resulting in a plunge in temperature, forming this sawtooth pattern. This repeats constantly until the varnish is either removed from the system or shut down so it can be removed.

After dismantling the NDE (Non-Drive End) bearing, varnish was found along the shaft and the actual bearing, as shown in Figures 4 and 5 below.

Interestingly enough, the bearings gave different temperature readings for this component. TI 633 showed slightly different readings compared to TI 632 mainly because of where they are positioned on the bearing, as these pads had varying levels of varnish on them, as shown below in Figure 6.

The refinery elected to add Fluitec’s DECON to the system during operation to provide immediate temperature relief to the bearing. DECON enhances the solubility of the oil, which does two things:

1. It dissolves varnish throughout the system, and in this case, specifically in the bearings and shaft.
2. It prevents future varnish from forming again.

Solubility enhancers are soluble in the oil and can react with already degraded products (deposits or varnish) to make them soluble in the oil. In this way, the degraded products are not allowed to agglomerate into the layers of deposits and remain in the oil as inert, harmless products.

After Fluitec’s DECON was added to the system, the compressor saw an immediate decline in temperature from 101°C to 93°C and continued to decline afterward. Currently, this bearing is operating in the temperature range of 63°C. The highest temperature experienced by the system is 73°C, which is significantly below their “safe-zone” temperatures of 89°C.

After adding DECON, compressor bearing temperatures dropped from 101°C to 63°C.

Since adding DECON to their system, they have not had any other trips due to rapid temperature increases, and their bearings now operate below the maximum threshold temperature range.

Tüpraş is committed to sustainability and aims to be carbon-neutral by 2050. The company focuses on innovative technologies and digital transformation to enhance operational efficiency and reduce carbon emissions. DECON is consistent with their organization’s objectives as it significantly improves compressor efficiency while increasing operational reliability.

Using Fluitec’s Value Impact Calculator, it is estimated that 32 tons of CO2e will be reduced from their operation over 5 years simply by optimizing the life and performance of their lubricant.

References

Jang, J. Y., Khonsari, M. M., Soto, C., & Livingstone, G. (2024). Effect of varnish on the performance and stability of journal bearings. Tribology International, Volume 198.

In a Middle Eastern refinery, a newly commissioned fire water pump, crucial for emergency services, experienced repeated high temperatures at the non-drive end (NDE) bearing during periodic test runs. Over time, these temperature spikes eventually led to a premature bearing failure, sparking a detailed investigation that revealed a lubrication issue.

The root cause? Oil starvation, despite the constant level oiler showing a seemingly adequate oil level.

Uncovering the Lubrication Problem

The investigation raised a critical question: Why did the bearing lack lubrication even when the sight glass of the constant level oiler indicated the correct oil level? The answer pointed to a fundamental issue – incorrect orientation of the piping connected to the constant level oiler during the construction phase.

Understanding Constant Level Oilers and Installation Errors

Constant level oilers are designed to automatically maintain the correct oil level in the bearings to prevent both over and under-lubrication. Their effectiveness, however, relies entirely on precise installation. In this case, despite the oiler indicating a full level, the incorrect orientation of the piping prevented oil from reaching the bearing, ultimately causing overheating and failure.

The Importance of Proper Installation

Any failure can have catastrophic consequences for critical equipment like fire water pumps, which serve an emergency function. If this issue had gone unnoticed, the fire water pump could have failed during an actual emergency, putting the entire refinery at risk of a major fire.

The root cause investigation clearly highlighted that the construction contractor did not follow the vendor’s recommendation regarding the orientation of the constant level oiler piping. Manufacturers provide precise instructions for a reason – deviations from these instructions can result in operational issues and critical equipment failure in cases like this.

Once the constant level oiler piping was corrected and aligned per the manufacturer’s drawings, the pump operated smoothly without further bearing temperature issues. The problem was resolved, but it emphasized the importance of attention to detail during critical equipment’s construction and commissioning phases, which is often overlooked.

Lessons Learned

This incident highlighted several key lessons to improve reliability and avoid similar failures in the future:

1. Strict Adherence to Vendor Guidelines: Construction contractors must follow detailed instructions from equipment manufacturers for correctly installing equipment and associated piping. This attention to detail ensures the equipment performs as intended without introducing avoidable risks. Deviations from these guidelines can lead to operational failures and increased risks
2. Peer Review and Construction Audits: Conduct peer reviews and audits during installation to ensure all equipment and piping are installed correctly according to the vendor’s recommendations. Construction audits, focusing on equipment and piping per these vendor guidelines, can catch mistakes early and prevent costly failures later.
3. Proactive Inspections: After this incident, the refinery proactively inspected all the fire water pumps to verify the correct installation of their oilers. This proactive approach is necessary to avoid similar failures and ensure emergency equipment is ready to perform when needed.

This case is a reminder that even minor oversights in equipment installation can lead to major consequences in high-stakes environments. The incorrect piping orientation might seem like a small error, but it can seriously affect critical systems. Ensuring that every component is correctly installed is essential to maintaining reliability and safety.

The lessons learned emphasize the importance of precision, strict adherence to standards, and thorough inspections, particularly during the early phases of the project, to ensure long-term reliable operation – principles that every engineer should practice in the field of lubrication and reliability.

It’s very much a lubrication issue. To prevent two metal surfaces from welding due to excess friction, keep them apart when expected to interact. A suitable film of lubricating oil accomplishes that.

Some 50 years ago, MIT Mechanical Engineering Professor Emeritus Ernest Rabinowicz was experimenting on the tribological effects of metal-to-metal surface interaction. In his research, Dr. Rabionwicz discovered that 70% of bearings lost their usefulness due to (avoidable) surface degradation, 20% due to corrosion, and a whopping 50% due to mechanical wear.

In calculating the situation, Rabinowicz determined that 6% of the U.S. GDP (Gross Domestic Product) was lost every year through mechanical wear. In today’s terms, that equates to trillions of dollars lost to avoidable wear. We now refer to this equation/finding as “The Rabinowicz Law.”

Simply put, surface degradation can be directly and indirectly attributed to ineffective lubrication practices.

These include under- and over-application of lubricant, use of mixed lubricants, incorrect lubricant choice (viscosity and additive package), particle and moisture contamination, and neglect.

Adopting a 5R Lubrication Approach by applying the Right lubricant, in the right place, in the Right amount, at the right time, with the Right level of cleanliness, surface degradation can be minimized to acceptable and often negligible levels.

Once established, the level of lubrication protection is dependent on the lubricant film, the thickness of which affects and controls the degree of interacting surface degradation and resulting wear. These varying thickness states are called lubricant film regimes.

Lubricant Film Regimes

There are five lubricant film regimes. Each describes a different relationship between two interacting surfaces as they slide over one another.

HDL—Hydrodynamic Lubrication is often described as “Full Film” lubrication, in which the moving surfaces are completely separated by the lubricant.

Viewing bearing surfaces under a microscope reveals that even finely machined surfaces are anything but flat. They’re more likely to resemble a series of craggy, rubbed-down peaks and valleys.

A lubricant must first fill those cavities to separate and ensure optimal interaction between two moving metal surfaces. In sliding friction bearings, HDL is the most desirable lubrication state. It’s often referred to as thick-film lubrication, wherein any friction is entirely due to the fluid friction between the viscous planes shearing in the lubricant.

EHDL—Elastohydrodynamic Lubrication is unique to rolling-friction surfaces seen in ball—and roller-style bearings and in combination with sliding- and rolling-friction circumstances found in the mating of gear teeth as they pass over one another.

When a ball is rolling in a race and comes under full load, the mating surfaces will momentarily deform, trapping the lubricant in the deformed area known as the Hertzian contact area. Under deformation pressure, lubricant viscosity rapidly rises, and the lubricant changes state from a liquid to a solid, thus providing complete protection to the rolling surfaces.

The lubricant returns to its original viscosity as the ball moves out of the load zone. Because rolling-surface contact is in a line and not over an entire surface area, far less lubricant is required to achieve full-film lubrication. Viewing a vehicle’s tire in motion can easily demonstrate this action.

A properly inflated tire always appears round except for the deformed or flattened portion, taking the load and providing traction with the ground. As the tire rotates, it elastically comes back to its rounded form.

MF – Mixed Film Lubrication is classified as an Intermediate lubrication regime when lubricant is present between two sliding surfaces but not enough to fully separate the surface, allowing intermittent contact between the highest points of the surface peaks. This is known as an “unstable” regime.

Extended time spent in this regime will result in the surface high points shearing off and, in turn, create additional cutting wear to the bearing surface due to asperities rubbing both surfaces. MF conditions are generally caused by insufficient lubricant, heavy loads at rest, or using a lubricant with too low viscosity.

In the 1800s, steam engines were designed with large-diameter Babbit (very soft material) crankshaft bearings. These bearings were lubricated with two different manual-oil-lubrication-delivery systems used in sequence.

The first was a manual pressure pump used to hand-pump oil into the bearing cavity and hydraulically float the bearing before startup. Once running with the shaft at a steady 70- to 100-rpm working speed, lubrication delivery was switched to a gravity-feed oiler that ensured continuous flow into the bearing to achieve full-film lubrication.

This two-part process moved a bearing’s lubrication state from HSL to HDL while minimizing/eliminating the problematic effects of MF and BL (see below) lubrication states.

BL – Boundary Layer Lubrication, or thin-film lubrication, is the least desirable regime offering the least frictional protection. Although minimal lubricant is present, the sliding surfaces are in full contact with one another at rest. With heavy loads and slow-moving machinery, a BL to MF regime may be the best condition achievable. This situation, though, requires a lubricant with EP (Extreme Pressure) and AW (Anti Wear) additives to offset the extreme bearing-surface working condition.

If insufficient lubricant or an incorrect viscosity is used, a normally loaded bearing can stay in a boundary-layer state when in full motion. In that case, the surfaces will interfere and cause rapid wear.

In 1902, Professor Richard Stribeck graphically described the coefficient of friction changes for bearings under different lubrication regimes. His work resulted in what we know as the Stribeck Curve (see figure). The Stribeck Curve demonstrates that hydrodynamic film regimes of the correct viscosity and Lambda thickness lead to the lowest coefficient of friction and least wear.

The Stribeck Curve describes the coefficient of friction changes for bearings experiencing different lubrication regimes.

A typical example of Stribeck’s findings is found in normally loaded sliding friction bearings, such as those in a shaft and sleeve bearing setup. At rest, the bearing surfaces will begin in a boundary-layer or mixed-film state before startup or shutdown.

As the shaft ramps up speed, it will start to centrifugally center and move through a mixed film regime to a full film HDL regime at operating speed. Load, speed, and lubricant viscosity changes can also affect the regime state.

Full-film lubrication has a typical thickness between 1 and 5 microns. To put that into perspective, consider that bacteria are 2 microns in size; a red blood cell is 8 microns; a human hair is 75 microns or 0.003 of an inch. In contrast, silt or dirt comes in at around 5 microns (possibly larger), which could easily create wear surface damage if allowed into the bearing surface area.

Humans only begin to see objects at 40 microns in size. Therefore, we must be diligent in understanding and ensuring that the 5 Rights of Lubrication are always followed. Doing so will ensure a bearing operates in its designed lubrication regime, avoids unnecessary wear, and lives a long life. In the end, a bearing’s fate is always in the hands of the maintenance department.

First 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.

All bearings are destined to fail eventually. While ineffective lubrication practices are arguably the greatest contributing factor in such failures, the root cause(s) are rarely investigated due to production and maintenance turnaround demands.

It’s through understanding the bearing lifecycle and associated built-in and application failures that maintenance and planning groups are able to develop effective premature-failure strategies.

As a bearing transitions from the drawing board to its final intended working environment, many elements can affect it. Design, storage, installation, and setup play essential roles in a bearing’s early life.

However, effective lubrication throughout a bearing’s working life will significantly impact the component’s longevity, performance, and equipment uptime.

Awareness of a bearing’s needs throughout its life stages will determine maintainability requirements and assure maximized reliability. The following overview of these five stages highlights influences and elements that can affect the reliability of your bearings.

Stage 1: Manufacture

Reputable bearings are manufactured to the highest tolerances, assembled, and packaged in a contamination-free, environmentally-controlled process, often called “white Room” manufacturing. This ensures that the bearing leaves the factory as contamination-free as possible.

The first step to ensuring a long lifecycle for ball bearings is to only specify and use precision bearings. Precision bearings are easily identified by their ABEC (Annular Bearing Engineering Rating) classification rating, which ranges from ABEC 1 to ABEC 9. The higher the rating, the closer the bearing tolerance, making it more accurate and efficient.

The type of steel used to make the bearing will also affect its lifecycle. All rolling-element bearings are assigned an L10 rating that states their expected reliability in service. If the bearing has an L10a rating, the service life will be longer than a normal L10 bearing due to the use of a cleaner type of steel manufactured in an electric arc furnace.

Before packaging for distribution and sale, bearings can be pre-lubricated with a light assembly lube, a standard bearing grease or oil, or with a customer-specified lubricant. If pre-lubed with grease, bearings will rarely be shipped with more than a 30% cavity fill.

Stage 2: Application Design

Original equipment manufacturers (OEMs) generally design machines in two different ways. Suppose the machine will be mass-produced and marketed to many other industries simultaneously. In that case, the OEM must base its design and bearing choices on a perceived set of generic operating conditions, i.e., load, temperature, speed, etc.

The second scenario is based on the end-user purchasing a “custom” designed and built machine. In this situation, the customer is in a better position to assure maximum bearing lifecycle, as the OEM can adjust the machine design and bearing choices to accurately accommodate a known set of operating and ambient conditions found in the machine’s intended workplace.

In turn, bearing type, style, and size can be more accurately engineered based on actual operating speed, design load, temperature, and working condition factors. These factors and the customer’s financial considerations determine which bearings will be oil or grease-lubricated.

At this point, the machine budget must be extended to accommodate centralized and automated lubrication systems. Compared to manual lubrication practices, these systems (when set up correctly) have been known to almost triple bearing life while reducing machine energy consumption.

The maintenance department must be included in machine-design-specification discussions during the application-design stage.

Maintenance can provide standardized lists of preferred lubrication-delivery systems (to reduce internal spares) and preferred lubricants, allowing bearings and reservoirs to be pre-filled and delivered with customer-standardized lubricant types and viscosity. This will reduce any chance of initial fill lubricant cross-contamination and could lessen the bearing lifecycle.

At this point, the maintenance department can also request that all bearings on the machine be tagged with a number and identified on an OEM schematic that locates and lists all lubricated bearings by type, style, bearing identification number, purchase lead time, and ABEC rating. This will go a long way in facilitating regular maintenance in service.

Stage 3: Inventory Storage & Handling

When bearings eventually reach their end-of-life, they will require replacement. Most maintenance-planning departments will already have a bearing-replacement strategy in place.

This usually includes ad-hoc purchases from third-party suppliers, when needed, or purchases and in-house inventory stocking bearings, especially those with long lead times. If the second option is chosen, the maintenance department will require a storage strategy to ensure that the bearings are opened and used as if just received fresh from the factory.

New bearings must always be handled carefully, stored in their original packaging materials, in a vibration-free cabinet (to eliminate false brinelling and flat spotting on running surfaces), and housed in a clean, humidity-controlled environment. (Bearings can rust in humid environments.)

Keep in mind that if personnel leave bearings unwrapped, touch their surfaces with bare skin, spin them by hand or with compressed air, drop them on the floor or bench, or expose them to moisture and poor air quality, these components can deteriorate before first-time use and suffer a premature failure.

Stage 4: Installation, Machine Setup

During bearing installation, reaching and surpassing the expected L10/L10a service life will rely on the precision-maintenance skill and knowledge of the tradesperson to correctly install and align the component in place.

Forcing bearings into place using blunt-strike tools can cause immediate surface brinelling. Bearings that fit too loose or tight can cause surface scoring, rapid wear, and fatigue failure.

When bearings are employed in belt and chain-drive systems, accurate motor, pulley, and gear alignment are essential for their survival. Both angular and offset misalignment will excessively and unevenly load a bearing, causing wide ball-path wear on the inner race and non-parallel wear on the outer race, leading to premature failure.

Stage 5: Service-Life Maintenance

Once a bearing has successfully been placed in service, the operating conditions and quality of the lubrication-management program will influence its lifecycle.

For example, a standard 6206 bearing employed in the white-room-controlled environment of a pharmaceutical manufacturing plant is likely to survive significantly longer than its counterpart used in a foundry that requires high process heat and sand (silica) to turn out products.

The end user’s manufacturing process dictates working conditions. Of course, it’s hoped that the design will have been optimized to ensure long machine and bearing life. Whereas a pharma plant presents ideal conditions for bearing life, a foundry presents less-than-perfect conditions, which must be compensated for.

Based on the Arrhenius Rule, cool-running bearings live longer than hot-running ones. A temperature-change-dependent failure rate for materials and fluids operating above 176 F (80 C) will apply.

The Arrhenius Rule shows that “for every 18 F-deg. (10 C-deg.) increase in temperature, the lubricant lifecycle (and bearing protection) will be reduced by half.” Fortunately, the reverse is true: By reducing the lubricant-operating temperature, we can expect to double the lubricant life and gain a correlative increase in the bearing lifecycle.

Note that when bearings are subjected to localized high heat, they can be cooled inexpensively by placing a heat sink/cover between the bearing and the heat source. (A simple aluminum- pie-plate heat sink can reduce the temperature by 3 to 5 degrees.) Bearing covers can also protect against water, sand, and dirt, all detrimental to bearing life.

Whatever stages of life a site’s bearings are in, the maintenance and planning group will perform a vital role. A thorough understanding of a bearing’s lifecycle can help extend it and, in the process, capture a host of benefits for the operation.

This article was originally published in The Ram Review.

Varnish is characterized as a deposit originating from the degradation of lubricating oils. These degradation byproducts are predominantly polar compounds, exhibiting inherent instability within the non-polar phase of the lubricating oil. Due to this chemical incompatibility, they precipitate out of the oil solution, forming adhesive deposits on machine components.

The presence of varnish poses several reliability challenges in mechanical systems. For instance, these deposits can adhere to the internal surfaces of valves, resulting in increased friction and a propensity for the valves to adhere to their seats.

This sticking phenomenon disrupts the precise control of turbines, potentially leading to erratic operation and increasing the risk of unintended shutdowns, referred to as trip events.

Additionally, the accumulation of varnish on the surfaces of heat exchangers significantly impairs their thermal efficiency. The insulating nature of these deposits hinders optimal heat transfer, thereby reducing the overall efficiency of the heat exchanger system.

Oil degradation products also tend to form on bearings, causing many deleterious operational issues. This varnish is typically hard and tenacious and can harm the bearing’s performance.

It can restrict oil flow, reduce heat dissipation, and increase friction and wear. The high temperatures of journal and thrust bearings not only accelerate oil degradation but also facilitate the hardening of these degradation products on the bearing surfaces, further accelerating deposit formation.

This article focuses on the impact of bearing deposits, the mechanisms that cause them, detection methodologies, and remediation strategies.

How Deposits Impact Bearing Performance

Oil degradation products are also well known to form on bearings, causing many deleterious operational issues. This varnish is typically hard and tenacious and can be detrimental to the bearing’s performance in three main ways:

1. Act as an insulator – one of the main functions of an oil is to keep the system cool. However, with the formation of the deposits, these can act as insulators trapping heat. Therefore, the oil can no longer cool the bearing, leading to increased bearing temperature trends. This can also lead to rapid bearing temperature excursions, causing trip alarms and potentially threatening operations.
2. Act as surface asperities – a bearing typically undergoes different phases of lubrication. Ideally, it should maintain a hydrodynamic film (complete separation of the two surfaces) to ensure its efficient operation. With the presence of these deposits in the high load areas, these surface asperities can interrupt the hydrodynamic film, causing the bearing to experience a mixed lubrication regime. In this condition, the bearing will have metal-to-metal contact between the two surfaces, resulting in wear or scoring, as shown in Figure 1 above.
3. Induce vibrations – The mechanical impact of bearing deposits may be sufficient to cause measurable vibrations in the bearings.

Characterization of Bearing Deposits

Two distinct types of bearing deposits form through different mechanisms, each with potentially different remediation strategies.

1. Cold Varnish Deposits: This category predominantly results from oil oxidation, a process where oil molecules react with oxygen, forming polar byproducts. The oxidative degradation pathway is explained in Figure 2.

   These oxidation byproducts are typically aldehydes and ketones, which are quite soluble in the oil. They may undergo aggregation and agglomeration, evolving into larger molecular structures with reduced solubility in the oil.

   This results in the formation of adhesive deposits. As the oil is squeezed through a bearing, it concentrates the amount of degradation products, leading to molecular cross-linking, further reducing the solubility of the degradation products, resulting in deposits.

Detection methodologies primarily include Membrane Patch Colorimetry (MPC, ASTM D7843) and Ultracentrifuge tests. These diagnostic tools are vital in providing early indicators of oil degradation presence and their potential to form deposits, enabling timely implementation of varnish removal and prevention strategies.

2. Shear Stress Deposits (Hot Varnish): This deposit type is distinguished from cold varnish by its formation mechanism and chemical composition. In this case, oil degradation is driven by mechanical forces, where mechanical energy is converted into thermal energy under high shear conditions. This process results in localized temperature spikes, predominantly observed in turbomachinery operating under high-speed and heavy-load conditions.

   The chemistry of these shear stress deposits differs markedly from those derived from oxidative processes as they have a higher concentration of less soluble fatty acids.

According to Chu Zhang (2017), the molecular friction generated from Shear Stress can result in temperatures reaching several hundred degrees Celsius, leading to instant degradation and the formation of localized deposits. Yulong Jiang (2021) also further explains that these high temperatures are isolated and localized in the minimum oil film thickness zone, which occurs at a molecular level.

The “Morton Effect” is a phenomenon of synchronous rotor instability due to non-uniform heating of journal bearings and is said to be caused by shear stress. The Morton Effect also causes vibration issues in turbomachinery. As per Jongh (Sep 17-20, 2018), shear stress is a dominant factor in generating non-uniform bearing temperatures.

Direct observation and temperature measurement of turbine oil during operational phases is impossible. The oil film, often only a few microns thick, precludes in-situ temperature monitoring at these scales. Nonetheless, analyzing bearing deposits can infer indirect evidence of high-temperature occurrences.

As illustrated in Figure 3 below, a bearing pad coated with varnish is examined. Samples of deposits extracted from various bearing regions underwent rigorous chemical analysis.

The findings revealed that the bulk of these deposits were organic, primarily composed of oxidized oil degradation byproducts. This chemistry contrasts with the darkest deposits, which exhibited an inorganic composition, predominantly consisting of phosphorus-based extreme pressure (EP) additives.

Under normal conditions, these EP additives remain inert, typically requiring activation temperatures around 200°C to react. The presence of these activated EP additives in the deposits strongly indicates the occurrence of localized high-temperature zones within the bearing system.

Such micro-temperature fluctuations are challenging to detect with conventional bearing thermocouple probes, underscoring the complexity of monitoring and diagnosing thermal dynamics in turbine oil films.

Primary Drivers of Lubricant Degradation Under Shear Stress

Shear Stress Degradation is influenced by the load exerted on the bearing and its rotational speed. The critical zone of maximum load within a bearing coincides with the region where the oil film attains its minimum thickness.

As illustrated in Figure 4, empirical observations corroborate that this zone of minimal oil film thickness is the primary site for the formation of deposits.

This correlation underscores the significance of these mechanical parameters in the onset and progression of shear stress-induced lubricant degradation.

It has been noted that Shear Stress deposit events occur with a greater frequency in compressors compared to turbines. This can be because the bearings in a compressor typically experience rotational speeds over 50,000 rpm.

On the other hand, the bearings in turbines usually operate at approximately 3,600rpm in the 60Hz North American market and 3,000rpm in the European and Asia Pacific markets at a frequency of 50Hz.

Detection Methods

Oxidatively derived cold varnish can be measured with the MPC or UC test as the bulk oil has experienced degradation. However, oil analysis tests are less valuable when an oil undergoes shear stress degradation.

Observing bearing temperature and vibration trends can detect shear stress deposits. Visual inspections of the bearings during outages are also valuable.

Typically, temperature spikes resembling a sawtooth-like wave pattern, as seen in Figure 5, are expected when varnish is present. A stable sawtooth waveform can also represent the presence of varnish.

Varnish has high film strength. In some cases, the accumulation of bearing deposits can move the entire shaft. Therefore, with systems monitoring the shaft position, utilizing the gap voltage vertical probe measurement will be worthwhile.

As shown in Figure 6 below, the shaft moves away from the center position as the temperature increases due to deposit accumulation. The shaft’s vertical position shift correlates closely with a bearing temperature spike.

Bearing temperature excursions can cause operational disruptions. Below are some noteworthy bearing temperatures based on API 670:

• 110° C Alarm Temperature
• 120° C Trip Temperature
• 132° C Babbitt Creep
• 232° C Babbitt melting point

How can Bearing Deposits be Mitigated?

Cold Varnish

Oxidatively-derived bearing deposits can be removed by installing a resin-based filtration system, such as Electrophysical Separation Process (ESP) technology.

It can also be prevented or removed using a solubility enhancer, such as DECON. The impact of these technologies results in bearing temperature decreases, low MPC values, elimination of bearing wear, and increased efficiency due to lower friction.

Figure 7 shows an example of a bearing in a steam turbine before and after an ESP system was installed.

Hot Varnish

Shear Stress Deposits have been observed in Group I, II, III, and IV base oil formulations. This suggests that simply changing the oil to another base stock formulation will not alleviate the issue. However, there are potential mechanical and chemical solutions.

Mechanical Solutions

From a mechanical aspect, Shear Stress Deposits can also be controlled by reducing the load on the oil. Some mechanical fixes include;

• Reducing the load (this is operationally feasible but reduces compressor output, making it a costly option)
• Expanding bearing clearances (Jongh, Sept 17-20, 2018)
• Offsetting the bearing pivot towards its lagging side has been proven to improve the angle of attack and allow more oil to flow
• Directional lubricating (Bloch, July 2006)

The Chemical Solution

Adding a solubility enhancer is a potential chemical solution to this mechanical problem. DECON has been shown to have an immediate impact on deposits and can lower the temperatures in bearings. It is an oil-soluble cleaner designed to be added at 3-5%.

It has been designed for long-term use, which can provide a permanent solution to bearing deposits. This product effectively mitigates deposit formation by enhancing the solubility of the in-service oil, enabling the dissolution of degradation byproducts.

Often, varnish products comprise depleted antioxidants, inorganic additives, and degraded hydrocarbon molecules. DECON is designed to soften the carbon-based deposits upon which the inorganic deposits (such as the aldehydes, ketones, and fatty acids found on journal bearings) usually adhere. Thus, when these carbon-based deposits soften, they can be easily removed.

When these deposits are dissolved into the oil, they remain inert and pose no operational concerns until they principate out of the solution. Dissolving these deposits back into the oil does not adversely impact the oil’s condition, nor does it pose a risk of catalyzing further degradation.

Figure 8 shows the temperature graph after adding 3% of DECON to a compressor suffering from shear-stress deposits. The temperature excursions stopped after a few hours.

Not all mechanical issues can be solved through chemistry. However, DECON is cost-effective for bearings experiencing shear stress and oxidatively derived deposits.

Summary of Differences Between Hot Varnish and Cold Varnish on Bearings

The following table summarizes the differences between hot and cold bearing deposits.

Summary

Bearing deposits can have a significant impact on the operation of rotating equipment. They lead to high bearing temperature excursions, wear, and increased friction. Two different types of deposits are found on bearings: oxidatively-derived cold varnish and Shear Stress-derived hot varnish.

The traditional methods of utilizing the MPC and UC (Ultracentrifuge) tests to detect the presence of cold varnish in equipment are ineffective in detecting hot varnish or shear stress deposits. By monitoring bearing temperatures and vibration, operators have an increased opportunity to detect the presence of shear stress deposits.

Bearing deposits experiencing cold varnish may be remedied by installing a resin-based filtration system, such as ESP technology, or adding a solubility enhancer, such as DECON.

The remedy to shear stress deposits is often a mechanical fix that may spread the bearing load over a greater oil surface to reduce load, especially in the minimum oil film thickness zone. Alternatively, DECON has been shown to be an effective chemical solution to shear-stress deposits.

 This is article is based on the white paper ” Understanding Shear Stress and the formation of deposits in heavily loaded, high-speed bearing applications” by Greg Livingstone (Fluitec) and Cody A. Evans (Mobil).

References:

1. Chu Zhang, J.-G. Y.-S. (2017). Influence of Varnish on Bearing Performance and Vibration of Rotating Machinery. International Journal of Rotating Machinery, Article ID 9131275, 10 pages.
2. Jongh, F. d. (Sept 17-20, 2018). The Synchronous Rotor Instability Phenomenon – Morton Effect. 47th Turbomachinery & 34th Pump Symposia, Houston, Texas, 12.
3. Yulong Jiang, B. L. (2021). Prediction on Flow and Thermal Characteristics of Ultrathin Lubricant Film of Hydrodynamic Journal Bearing. Micromachines, 12, 1208.
4. Bloch, H. (July 2006). Tilting Pad Thrust Bearings. Machinery Lubrication. July 2006.