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. 

In the most basic of terms, an electric motor is a simple device that converts electrical energy into rotary motion that, when transmitted to a driven load, will perform mechanical work. If applied correctly, the motor will run effortlessly. If set up correctly, the motor will perform reliably at minimal cost.

The motor will have a long service life if maintained regularly and correctly. This is so much so that, for most motors, only 2% of their total lifetime cost will be attributed to the original purchase price, with the remainder to energy costs.

Although the world is witnessing a renaissance of the electric motor with the advent and continued proliferation of e–motor vehicles, their fundamental design and purpose remain the same. Similarly, application, setup, and maintenance will continue to influence the performance, reliability, and life cycle of the e-motor.

Application, setup, and regular maintenance are all cornerstone elements of precision maintenance, providing a simple, effective, and straightforward approach to ensuring your motors work efficiently for a long time.

Application

A new machine pretty much guarantees the motor has been sized correctly to its intended load. Application problems arise when the maintenance department changes a defective or failed motor with one that’s not “like for like.” Motor application mismatching can also occur when the production team modifies operating conditions by:

• Changing the line or machine speed beyond its original design range.
• Changing the raw material that’s being worked.
• The machine was re-purposed for a different use than the one for which it was initially designed.

An oversized motor for the load condition can provide adequate performance. Still, light loading will often be inefficient and consume excess energy under regular operation, increasing the unit’s energy footprint.

Conversely, an undersized or too-light motor will, at best, stall or trip under load, may fail to operate, and could cause motor damage. In a worst-case scenario, the motor continues to run and overheats, causing a fire.

In particular, in applications when a heavy startup load is expected to reduce significantly once the equipment is running at operational speed, i.e., in a loaded conveyor drive system, there are two options: We can use reduced-horsepower motors that 1) are governed mechanically by a fluid coupling that allows the drive to come up to speed slowly without tripping the motor; or 2) are governed electrically through use of a Variable Frequency Drive (VFD).

If a motor size is suspect, ensuring the unit is sized correctly requires reviewing the original manufacturer’s literature (design specification, Bill of Material, spare parts list, etc.). If no literature is available and you are unsure if the replacement motor is the same size as the original unit or if you wish to change to a high-efficiency model, confer with your local motor supplier’s engineering department for assistance.

Setup: Balancing

Purchasing a new, reputable, name-brand electric motor should provide some assurance that the unit’s main shaft is balanced. When it comes to new, inexpensive, offshore “no-name” motors, operations should err on caution and have the balance checked and certified by a reputable motor shop. Rebuilt motors should have a balance certificate if purchased from a reputable rebuilder.

Unbalanced motor shafts are noisier, vibrate more than usual, require significantly more energy (and money) to operate over time, and often fail prematurely.

Always check a new or rebuilt motor’s balance using your vibration-analyzing equipment, or have it certified independently by a reputable local motor shop, and do so before you place the unit in operational use.

Setup: Alignment

The correct alignment between the drive and driven shaft is arguably the most essential part of any motor setup.

Misalignment comes in two forms: 1) Angular, in which the shafts line up center-to-center but not in a straight line, and 2) Offset, in which the shafts do not line up center-to-center. Both conditions can occur simultaneously. Both will place tremendous stress on the driver and driven bearings and couplings, assuring rapid wear, premature failure, and a considerable increase in motor energy consumption.

Remember: Poor alignment will rapidly wear out shaft bearings, sprockets and chains, belts, and sheave pulleys.

Setup: Soft-Foot Check

A soft-foot condition check should always be included in the alignment process. Aligning with a laser-type alignment system is easy, as virtually all laser alignment systems feature a soft-foot check feature.

Soft foot occurs when one or more motor base feet are not as flat as the others (think of a table with a short leg). Soft foot can also occur when the motor base grout is not flat or square. Both situations are easily remedied with precision shims and correct tie-down-bolt torquing.

If the soft foot persists, excess vibration will eventually loosen the bolts and cause the motor to vibrate more, which will transfer across the drive train and its components. Again, the result will be premature wear and failure, combined with excessive energy consumption.

Regular Maintenance: Lubrication

The reality is that most electric motors are grossly overlubricated. Incorrectly lubricated motors prematurely fail due to the simple act of neglecting to undo the grease drain plug.

This forces any excess grease to build up pressure that eventually ruptures the shaft seal. In that event, grease is free to purge into the motor winding, causing massive overheating, premature failure, and, once again, excessive energy consumption.

Some sub-fractional motors come equipped with grease nipples, even though they contain lifetime lubricated bearings and no drain port to allow excess grease entry to escape.

All motors should be assessed to understand their lubrication requirements and placed on an engineered lubrication program.

Regular Maintenance: Cleanliness

The simple act of keeping a motor dirt—and oil-free will combat the buildup of a debris/dirt-based thermal blanket, allowing the unit to cool as designed and, just as important, use no more energy than designed.

Regular Maintenance: Maintaining the Driven System

Simple maintenance of the driven systems can significantly reduce motor loads and increase energy use efficiency. Among other things:

• Ensure drive belts and chains are tensioned regularly and correctly.
• Use matched drive belts on multiple belt systems.
• Always replace sprockets while the worn chains are being replaced.
• Lubricate drive chains regularly.
• If the motor is coupled to a gearbox, ensure the lubricant has the correct viscosity.
• Ensure the driven component is balanced and lubricated regularly.
• When aligning with direct coupling, use the least-expensive non-flex style that’s required and enforce accurate alignment techniques.
• Check driven-system air filters or fluid-system filters regularly.

What’s the payback from following the steps described here? Remember: With a precision-maintenance approach, an electric motor will virtually always outlast its driven system.

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.

Condition-based greasing is an old and desirable idea. It is the finesse approach to fulfilling grease-based bearing care when done with precision. Efficient. Effective. Reliability-Enhancing. The most desirable objective for grease-lubricated bearings.

It can be systematically and utterly destructive when done improperly (without precision).

Whether this mode of grease replacement inherently enhances or kills your machine’s longevity is related to the practices developed and followed at your site.

In this article we’ll explore the following:

• A review of condition-based greasing
• Challenges with modern data collection prescriptions
  • Data collection and quality issues
  • Cost and efficiency challenges related to determining true condition-based intervals
• A recommended technique for fulfilling the promise of condition-based greasing
• The benefits of condition-based greasing

What is Condition-based Greasing?

Condition-based grease application is a grease replenishment practice triggered by increased bearing decibel values, the first hint of dry bearing surfaces.

Graphic 1 represents a snapshot of the interaction between a race and an element. The surfaces are separated by a thin film of oil, which could come from grease or oil in a reservoir. For grease-based applications, we expect the oil to either bleed from or volatilize from the thickener with time and shear stress.

As this occurs, the available oil reservoir dissipates, the oil film gets thinner, and eventually, the high spots on the races and elements begin to bump and rub. When this occurs, these contact points create sound waves that are well beyond the sensitivity of human ears. We refer to this commercially as ‘ultrasound.’

Sharp points inside the red circle in Graphic 1 are called asperities. Machining and finishing form asperities on machine surfaces, including gear faces, cylinder faces, element bearings, and other parts. These asperities serve the purpose of preventing oil from being easily displaced from the load-contact point. However, it can cause problems if the oil film dissipates.

Ultrasound Detection for Condition-based Greasing

Ultrasound is created whenever asperities at machine surfaces collide. The sound waves emanate from the collisions in all directions.

The effect is similar to if you took a tuning fork designed to produce 35 kilohertz sound waves and pounded the tabletop vigorously. You would only hear the sound if you had a device that would convert the high-frequency sound waves into a frequency range acceptable for human perception.

To describe the wave created by contact energy, please consider Graphic 2, a steel ball dropped onto a steel plate. The moment of contact creates a compression wave that transmits at an exceptionally high speed through the solid plate.

This wave is measured and represented in decibels, a standard unit for measuring sound, as a measure of intensity. The sound waves are very short and occur at, and above 30-kilohertz frequencies, so we need help ‘hearing’ the contact event.

Graphic 3 represents what follows the initial contact. The weight of the ball causes displacement of the plate. The plate flexes up and down following displacement. The upward and downward movement of the plate can be depicted in a waveform that could be measured and reported in various engineering units—measurement of displacement energy central to what occurs during vibration analysis.

The upward and downward energy waves occur at a much lower speed than the waves passing through a solid steel surface. Each form wave energy can be accurately measured with the proper tools.

Compression wave energy transmits very well through solid steel surfaces, so if we could find a way to accurately register that the moment of contact has occurred, then we could use the feedback to alter the risk of any further contact occurring by replacing the oil film intended to keep surfaces apart.

ALL lubrication practices aim to create conditions where that oil film is perpetually healthy, and the components can permanently float on the oil film.

If our underlying decisions are accurate, then, regardless of speed, load, and bearing surface area, we can make the machine parts’ float’ on a film of oil that is thinner than the width of a red blood cell.

Suppose we accurately calculate the oil thickness required for a machine surface to ‘float’ the load-bearing components operating at a given temperature, speed, and load. In that case, those asperities never or rarely have a chance to bump into one another.

The objective of the calculations is to identify precisely which lubricant is needed for the given surface area, surface speed, unit load, and operating temperature relative to the dimensions (height) of the asperities, how often it should be provided, and how much is required to keep surfaces apart.

If we have an accurate plan, we achieve a ‘Lambda’ value (aka specific film thickness) of one (1) or greater. Graphic 4 represents the factors for asperity dimensions (r) and oil film thickness (h).

When the oil film thickness (h) in microns is twice the asperity height (r), then we achieve a Lambda value of one (1), or one times the required oil film thickness. Graphic 1 (above) shows the result of having a Lambda of 1 or greater. Graphic 5 shows the result of a Lambda value of less than one.

Any influence on the ‘steady state’ supply of the right lubricant, in the right volume and time interval, can cause the film to dissipate and produce the effect shown in Graphic 5.

Recognizing this undesirable state, we set about to replenish the lubricant through a ‘just right’ daily care and feeding plan to ensure that the correct volume is always present at the element, race, gear face, or any other interacting machine parts.

We love the notion of condition-based greasing because it promises us the means to hear when the surfaces are bumping and rubbing and provides just enough to stop this failure root cause before damage to the bearing can occur.

That is a very appealing prospect.

Conducting the calculations to make proper decisions, including the timing of grease replacement, is relatively easy, but someone must take the time to fulfill this need.

With this information as a backdrop, let’s consider what it takes to accurately measure this faint energy level when it begins to occur.

Data Collection and Data Quality Practical Challenges

As with oil analysis condition-based machine health measurement, sample collection quality is at the top of the list of Key Success Factors. There is potential for substantial signal intensity (attenuation) loss for various reasons. This condition monitoring technique will only fulfill expectations if the data collection method produces highly repeatable, high-quality signal recognition.

The two predominant challenges with compression wave collection are:

1. Single attenuation through reflectance and refraction
2. Quality of the sensor configuration in use

Both represent a potentially severe dissipation of signal strength during measurement. Let’s take a shallow look at each of these.

Signal Attenuation

Attenuation means the loss of signal strength as measured in decibels (dB). Low signal strength during readings can occur for a variety of common reasons, including:

• capability (sensitivity) of the sensor and instrument
• repeatability in the data collection process
• differences between the type of metal at the data collection point versus the signal generation point (aluminum zerk fittings against steel pipe nipple)
• very low shaft speed (<30 rpm)
• large bearing housing mass
• sensor positioning that causes reflection and refraction

While all of these are common, the last can produce a substantial damping impact on dB readings for a few good reasons.

With each mechanical interface, the sound waves reflect and refract, causing substantial losses to occur to signal strength.

If the sensor is placed around the top of the housing, such as on the zerk fitting at the top of the housing (the 12:00 position location for Graphic 4), as is a common recommendation for instrument providers) then the signal must pass across multiple interfaces, including those of dissimilar metals.

With each new interface (inner race to shaft at the 6:00 position, inner race to shaft at the 12:00 position, outer race to housing, housing to pipe nipple, pipe nipple to zerk, etc.), a part of the signal bounces back (reflection) and a part bounces off in another direction (refraction).

Aside from all the signal bending and bouncing, most asperity contact occurs at the center of the load zone where the oil is under maximum displacement pressure. For optimum signal detection, the sensor should be placed at the center of the load zone either axially (at the 5:00 to 7:00 positions under the shaft) or radially (perpendicular to the shaft on a plane equal to the lowest element position in rotation), or as close to the center as possible.

Poor signal placement forces the technician to make full-confidence judgment calls without clearly understanding surface conditions. In practical terms, the technician doesn’t hear asperity contacts because the signal is being ‘lost’ due to data collection methods.

Consequently, the technician only supplies grease once bearing distress is excessive. The technician inadequately lubricates the bearing by putting an insufficient amount of grease into the housing, appearing adequate due to the diminished signal.

Quality of the Sensor Configuration

There are two points of sensor configuration to consider.

The sensor’s sensitivity (can it detect a 35 kHz signal) and the mode of sensor placement wherever it may be placed.

Companies promoting compression wave detection are addressing the sensor frequency quality issues well enough to be satisfied that if all other conditions are met, the technology delivers on the promise of advanced oil film health measurement.

The second aspect of Sensor Configuration involves the precise method of attaching the sensor to the machine. As demonstrated by Graphic 7, the nature of the contact mode is essential for maximum dB detection at high-frequency ranges. Options one, two, and three are popular modes for placing sensors on surfaces because they are straightforward, but they are not very helpful.

Options four, five, and six require management and technicians to put substantial effort into sensor placement to avoid any energy leakage.

The most promoted approach employs stinger probes or 2-pole and flat magnets due to their relative ease of use and low setup cost (no gluing or drill/tap/thread work). However, coupled with the Signal Attenuation issues already discussed, a poor sample collection technique can compromise the detection of whatever signal might be present.

Proper technique is most crucial at frequencies above 10 kilohertz. As noted previously, compression waves travel at 30 kilohertz.

Per Wilcoxon Corporation, the ‘best practice’ for high-frequency energy capture is to use adhesive with mounting pads (lower cost approach), adhesive with permanently mounted sensors, or a stud-mounted sensor.

If options such as 4, 5, or 6 are not possible, management should abandon the notion of strictly condition-based greasing. If the asset carries a top criticality level, then this upgrade is well worth the relatively low cost of the effort.

Challenges Associated with ‘True’ Condition-Based Greasing Practices: Timing of the Visits

There are three choices for setting the visit schedule to measure for a dB change that signals a need to replenish. To be clear, success with this technique means catching the ‘dry bearing’ conditions at the earliest possible stage and supplying just the quantity needed to re-float the surfaces.

The options are as follows:

1. True Condition-Based Approach – Measure every bearing daily until you have an interval.
2. False Condition-Based Approach – Pick an interval and incrementally adjust from there.
3. Calculate the ‘best fit’ interval and refine it to a condition-based approach by making adjustments until the best fit is determined.

Let’s look at the cost and duration to perform each of these as applied to electric motor lubrication practices. Instrument providers often prioritize this as a sales focus due to the challenging nature of proper motor lubrication.

For the sake of an apples-to-apples comparison between these three modes, let’s assume that the bearing manufacturer has told us that each motor requires replenishment at 244 days based on the conditions noted in Graphic 8.

At first glance, this is counter-intuitive because we are accustomed to lubricating motors on 6- and 12-month cycles. After all, that’s just what we’ve always done!

Assume the following:

1. There are 100 critical 75 HP motors, and you have configured measurement for the most technically viable data collection options.
2. We believe (from the calculation) that the best frequency is 128 days.
3. Each ‘visit’ to the motor bear will take 5 minutes.
4. The labor rate is $60.00 per hour, making each visit worth $5.00 in cost.

Scenario 1: Measure every day (the ‘True’ approach).

Cost:

• 244 * 5 minutes * $1.00 per minute = $1,200 per motor.
• $1,200 per motor * 100 motors = $120,000

Work hours required:

• 244 * 5 minutes each = 1,220 minutes per bearing.
• 1,220 minutes / 60 minutes per hour = 20.33 hours per motor.
• 33 hours * 100 motors = 2,033 expended work hours for the 100 motors

Scenario 1 requires a full work year from the technical trades to fulfill the promise of condition-based lubrication for only 100 hours.

Scenario 2: Use a repeating pattern to reduce cost and shorten the determining cycle

Measure bearings starting from some repeating pattern, and make adjustments until the ‘best fit’ is determined.

A repeating pattern could be anything you like. Since we have a ‘village lore’ based history of lubricating motors on month-increments (6 months, 12 months, etc..), we could start with what is already in place.

We will lubricate and adjust (extend) the interval with each interval until we identify the dry bearing condition. Knowing we’ve gone too far, we’ll take a fraction of the last adjustment and shorten the interval. For instance:

Lubricate the bearing, and then check six months later (condition-based lube check – visit 1). We find no grease is needed in our apples-to-apples comparison of the 244-day expectation. Lubricate again, and check in 12 months.

Visit 2: At 12 months (now 18 months into the investigation), we see that the interval is too long, so we lubricate again, deduct one-half of the LAST interval adjustment (6 months is shortened to a three-month adjustment to (12 – 3 = ) a new interval of 9 months.

Visit 3: We return at nine months and see that the bearing is dry again. (now 27 months into the investigation). We relubricate, cut the last ‘adjustment’ by half, and reset the new interval to (9 months less 1.5 months = ) 7.5 months, or 225 days.

Visit 4: We return at 225 days (now 25.5 months in) and see that the bearing does not yet need lubrication. We EXTEND the interval by ½ of the last adjustment (7.5 months plus 3/4 of a month = ) 8.25 months, and we find that the dB value has risen slightly, and it is time to lubricate based on condition.

In sum, using this method, it has taken 33.75 months to ‘find’ the suitable interval based on condition, and we have made four visits and expended ($5.00 per visit * 4 visits = ) $20.00 in labor for the motor. For 100 motors, we have spent (5 minutes per minute at $1.00 per minute * 100 motors = ) $2000.00 and taken three years (rounded) to make decisions about the 100 critical motors. Certainly a better outcome, but we didn’t take the purist approach with a strictly condition-based estimate.

Scenario 3: Calculate a ‘near ideal’ interval, and shorten the correction cycle.

This approach, like scenario 2, is NOT a true condition-based plan. Like scenario 2, we will pick a starting point and adjust as needed by small increments to extend or shorten the cycle to find the ‘best fit’ interval.

Unlike scenario 2, this approach will begin with standardized engineering practices. The LubeCoach software in Graphic 8, is based on DIN Standard 51825 (with slight modification for practitioner use) to calculate the interval best fit based on the bearing in its actual operating conditions.

The DIN standard was pioneered through the effort of a working committee of bearing manufacturers in the mid-1980s and is considered the best option available to make this type of determination.

The original DIN standard incorporates measures for the C/P Load factor (static to dynamic loading ratio – something we are not privy to with a machine that has been in operation for years) and ALSO a grease life factor (F10, hours, a value known by lubricant manufacturers, but is NOT published) as well. If you’d like to learn more about the variations, see these articles (PDF) on Optimizing Lubrication Effectiveness Part 1 and Part 2. 

Scenario 3 requires that the reliability engineer goes to the motor, records the shaft speed, bearing numbers, shaft orientation (horizontal or vertical), temperature, moisture load, particulate load, and vibration level, and plugs values to the appropriate locations in the calculator. The output, as shown in Graphic 8, provides a specific interval for service, as follows:

Visit 1: @ 244 days, measure dB value for replenishment requirement. If the bearing is dry, replenish and reduce the interval by 10%. If the dB values are still low (oil film is good), extend the interval by 10%.

Visit 2: At either 220 or 268 days, measure the dB value for the replenishment requirement and make another incremental adjustment based on 50% of the last adjustment value.

Visit 3: (now between 464 and 512 days in) Repeat step 2 and adjust similarly as needed. Experience suggests that two adjustments will enable the technician to achieve the desired ‘condition based’ regrease interval.

With this method, the site is in search mode for between 48 and 42 months and has an expenditure for field checks at $15.00 per bearing and $1,500 for the proposed lot of 100 motors.

Extrapolating Condition-Based Regrease Practices to the Entire Bearing Population

Even a small production site (150 to 200 assets) will have a large population of oil and grease-lubricated bearings operating at different speeds, loads, temperatures, and operating environments. The oil-lubricated bearing maintenance is simple: make sure the oil in the machine is the right type and at the right level.

The grease-lubricated bearings will create a morass of complicated scheduling and schedule monitoring based on the intervals that could be scattered between weekly to multiples of years based on the sizes, speeds, and other operating conditions.

It is possible to produce, execute, adjust, and eventually finalize a proper condition-based interval.

However, that doesn’t mean it is cost-effective or operationally efficient to do so. If/when the exercise from start to finish is measured in years and requires years of technicians’ time, and costs well above six figures even for a small production site, then true condition-based intervals (scenario 1) selection is not feasible.

Scenarios 2 and 3, which eventually identify the condition-based interval, are much more cost and time efficient, with scenario 3 producing the most efficiency.

Benefits of Condition-Based Greasing Practices

The core benefit of condition-based machine care of any sort is the opportunity to keep the asset operating in a productive state. Bearing replacements are disruptive and costly before you factor in the cost of lost production time. Lost production time triggers the worst negative consequence of parts replacement – the risk of losing customers due to missed shipments.

Condition-based, or modified condition based, regrease activities should be directed toward the most critical assets. Assuring that the proper lubricant is applied in the appropriate volume and at the right time is the first line of defense against machine repairs.

Refining condition-based greasing with a well-defined starting point enable the technicians to determine the optimum state without wasting time and money.