The ASTM definition of lubricating grease is:
“A solid or semi-solid lubricant consisting of a thickener agent in a liquid lubricant. Other ingredients imparting special properties may be included”
A commercial lubricating grease usually consists of a thickener ( 5 to 35 wt %), base oil ( 65 to 95 wt %) and various additives (0 to 10 wt %). Lubricating greases are classified based on the thickener that constitutes the grease matrix. The most widely used thickeners are based on lithium and calcium. The thickener forms a solid like network and acts as an oil reservoir (Figure 1).
The operational temperature window and grease selection criterion is usually determined by the properties of the thickeners and base oils. However, the lubricity is mainly governed by the properties of the oil and the additives. The consistency of the grease is mostly governed by the physical interaction of the thickener with the base oil. The characteristic feature of lubricating grease is shear thinning, i.e its ability to undergo a decrease in viscosity with an increase of shear rate (Figure 2).
A lubricating grease can act as an effective sealant that prevents the entry of contaminants into the bearing. The requirements on sealing are less stringent since grease does not easily leak out of the bearing. This makes it possible to use low friction seals or even non-contacting shields. Finally, the friction torque and therefore energy losses in grease lubricated bearings is lower than for oil lubricated bearings. As a result around 80-90% of the rolling bearings are grease lubricated.