Selecting a suitable grease
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 selectionThe 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, NLGIConsistency 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 class 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 class 2.
Mechanical stabilityDuring 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 protectionIn 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 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 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).
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
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.
Verify the lubrication condition, consider EP/AW additivesThe 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.
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.