Selecting a suitable grease

Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing specificationBearing executionSealing, mounting and dismounting

Selecting a suitable SKF grease

The assortment of SKF greases for rolling bearings provides adequate choice for most application requirements. These greases have been developed based on the latest knowledge of rolling bearing lubrication and their quality is continuously monitored.
Using LubeSelect and selection rules
SKF LubeSelect is an online tool that lists SKF greases that fulfil the demands of your specified operating conditions. The analysis performed by the tool is based on generalized selection rules that have been carefully developed by SKF lubrication experts.

The same selection rules are used in the SKF bearing grease selection chart [PDF], where the speed, temperature and load range are used as the primary operating parameters for selecting a suitable grease.

Temperature, speed, and load ranges for grease selection

The terms used to specify the ranges of temperature, speed and load, for grease lubricated bearings, are defined in table 1, table 2 and table 3.

Consistency, NLGI

Consistency is a measure of the stiffness of the grease. Classification of greases by consistency is in accordance with the National Lubricating Grease Institute (NLGI), ISO 2137. Greases with a metallic soap thickener and a consistency grade of 1, 2 or 3 (soft to stiff) on the NLGI scale are typically used for rolling bearings. The most commonly used greases have a consistency of grade 2.

Mechanical stability

During rotation of a bearing, the grease is mechanically worked and a change in consistency may result. This property is known as the mechanical stability of the grease and is measured in standardized tests, ASTM D217 and/or ASTM D1831. Greases that soften may leak from the bearing cavity. Those that stiffen may restrict bearing rotation or limit oil bleeding. The mechanical stability should not change drastically if operation is within the specified temperature range of the grease. 

Corrosion protection

In applications where water or condensation is present, the corrosion inhibiting properties of the grease are very important. The corrosion inhibiting ability is determined by the properties of the rust inhibitor additive and/or the thickener type. The performance is measured using the EMCOR test, ISO 11007. For applications where water or condensation is present, the rating should be 0-0.

The most important technical specifications for SKF greases are provided in Technical specifications for SKF greases [PDF].

The SKF traffic light concept for grease temperature performance
The temperature range over which a grease can be used depends mainly on the type of base oil, thickener and additives. The relevant temperature limits are schematically illustrated in diagram 1 in the form of a double traffic light, with additional details provided in diagram 2.
  • The low temperature limit (LTL) is determined by the low temperature frictional torque test according to ASTM D1478 or IP 186. The LTL is determined by the temperature at which the starting torque is equal to 1 000 Nmm and the running torque is 100 Nmm.

  • The high temperature limit (HTL) is the temperature at which a grease loses its consistency and becomes a fluid. It is determined using the dropping point (ISO 2176).
The low and high temperature limits for reliable operation, indicated by the green zone in diagram 1, are:
  • low temperature performance limit (LTPL), defined as the temperature at which grease no longer shows sufficient oil bleed as measured in DIN 51817. Diagram 2 provides the LTPL values for roller bearings. The LTPL values for ball bearings are approximately 20 °C (35 °F) lower.

  • high temperature performance limit (HTPL), determined by the SKF R0F grease life test
Within these two limits, the grease fulfils its function reliably and the relubrication interval or grease life is predictable. Because the definition of the temperature performance limits is not standardized internationally, care must be taken when interpreting data from grease suppliers other than SKF.

At temperatures above the high temperature performance limit (HTPL), grease degrades with increasing rapidity. Therefore, temperatures in the amber zone, between the high temperature performance limit (HTPL) and the high temperature limit (HTL), should only be allowed to occur for very short periods.

An amber zone also exists for low temperatures, between the low temperature limit (LTL) and the low temperature performance limit (LTPL). In this zone, the temperatures are too low to provide sufficient oil bleeding. The width of the amber zone depends on the grease type and bearing type. Serious damage can result when the bearings are operated continuously below the LTPL. Short periods in this zone, such as during a cold start, are generally not harmful because the heat caused by friction brings the bearing temperature into the green zone.

Additional factors and considerations when selecting a grease

Verify the lubrication condition, consider EP/AW additives

The lubrication condition κ, is evaluated by using the base oil viscosity as described in Lubrication condition – the viscosity ratio, κ. In the lubrication condition domain defined by κ below 1, EP/AW additives are recommended.

EP/AW additives of the sulphur-phosphorus type, which are the most commonly used today, may also have a negative influence on the fatigue life of the bearings. This is because in the presence of humidity, which can never be completely avoided, sulphur and phosphorus acids are produced which induce a more aggressive chemical process at the rolling contact. This effect increases with temperature and, for temperatures above 80 °C (175 °F), a lubricant with EP/AW additives should only be used after careful testing. SKF greases have been tested and can be used above 80 °C (175 °F) until the HTPL is reached.

Low speeds

Bearings that operate at very low to low speeds (table 2) under heavy loads should be lubricated with a grease that has a high viscosity base oil and contains EP additives. The thickener should contribute to the surface separation. Sufficient oil bleeding should assure oil replenishment during operation.
Solid additives, such as graphite or molybdenum disulfide (MoS2), should be considered for a speed factor ndm < 20 000 mm/min. SKF LGEV2 is successfully used up to ndm = 80 000.

Heavy and very heavy bearing loads

For bearings subjected to a load ratio C/P < 4, the calculated relubrication interval may be so short that it dictates the use of continuous relubrication or oil lubrication.

Miscibility with other greases

If it becomes necessary to change from one grease type to another, consider the miscibility of the greases and their ability to be mixed without adverse effects (table 4 and table 5). If incompatible greases are mixed, the consistency of the grease mix can change dramatically such that bearing damage because of severe leakage could result. Note that PTFE-thickened greases are not compatible with other grease types.

Miscibility with preservation oils

The preservative oils with which SKF bearings are treated are compatible with the majority of lubricating greases, with the exception of synthetic fluorinated oil based greases using a PTFE thickener, for example, SKF LGET 2 grease. For PTFE-thickened greases, the bearing preservatives must be removed before applying the grease. White spirit is recommended as a solvent. Make sure all remnants of solvent have evaporated and then immediately apply the grease.

Assessing the suitability of non-SKF greases

Greases from suppliers other than SKF must be approved by the supplier. Use diagram 3 to evaluate the temperature performance and grease life prediction. Where relevant, take into account the considerations specified for SKF greases.

Lubrication systems

Continuous lubrication can be achieved via single­point or multi­point automatic lubricators, e.g. SKF’s SYSTEM 24 or SYSTEM MultiPoint.
Centralized lubrication systems, such as SKF MonoFlex, SKF ProFlex, SKF DuoFlex, SKF MultiFlex (table 6) and Lincoln Centro Matic, Quicklub and Dual Line can reliably deliver grease in a wide range of quantities.
For additional information about SKF lubrication systems → Lubrication solutions.
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