The most common application of a lubricant is to bearing which are included in an endless variety of mechanical equipment. Two types are considered : plain bearing and rolling  bearing.

PLAIN AND BUSH BEARING

When a journal rotates in a plain bearing which is receiving oil in adequate quantity, a fluid film of oil is built up by virtue of the viscosity of the oil as a result of rotation of the journal in the bearing. Under suitable conditions this oil film prevents metallic contact between the journal and the bearing, while frictional resistance to the movement of the journal depends mainly on the speed of rotation of the journal and viscosity of the oil under the conditions existing in the film. Friction may be reduced by every case, depending on speed and load, there is a lower limit of viscosity below which a fluid film is not fully maintained and metal-to - metal contact is likely to occur. In practice, therefore, it is usual to employ oils of sufficient viscosity to provide a safety margin. The effect of operating temperature on viscosity must not be overlooked.

To ensure fluid film lubrication, the bearing must at all times be flooded with oil, so the oil must be supplied to the bearing at a rate equal to that at which it escapes. The rate of escape depends on the viscosity of the oil (less viscous oils escape more rapidly) and on such factors as bearing clearances and the mechanical condition of the bearing. Speaking generally, the greater the load, the lower the speed; or the larger the clearances, the more viscous the lubricant must be. Conversely, lighter loads, higher speeds and smaller clearance necessitate oils of relatively low viscosity. Viscosity requirement also varies directly with surface specifications and bearing mean pressures.

Cleanliness in handling and applying the lubricant is most important to reduce wear, and every precaution should be taken to prevent impurities and moisture getting into bearings. Correct alignment is also a matter of obvious importance in preventing overheating and wear.

Variation of factors referred above makes it necessary to provide a considerable range of machine oils of different viscosity, so as to give the most effective lubrication under all possible conditions.

Grease - lubricated plain bearings running at normal temperatures require general purpose cup grease of medium soft consistency. Moisture-resistant grease is generally preferable. Where operating temperatures are high, special high temperature grease is required.