There are several reasons to lubricate plain bearings with grease:
As a result of the lower end leakage, the amount of lubricant required for the bearing is lower.
When a grease-lubricated bearing is stopped for any period of time – with the flow of lubricant shut off – the high apparent viscosity of the static grease reduces end leakage sufficiently so the grease usually does not completely drain or squeeze out.
Some grease remains on the bearing surfaces so that a fluid film can be established almost immediately upon startup. As a result, torque and wear during the starting phase may be greatly reduced.
During shutdown periods, retained grease also acts as a seal to prevent the entry of dirt, dust, water, water vapor and other environmental contaminants and, thus, protects the bearing surfaces against rust and corrosion.