BEARING LUBRICATION

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.

ANTIFRICTION BEARINGS

Anti friction bearings consists of ball and roller type, needle bearings, tapped roller bearings, etc.

The main duties of ball and roller bearing lubricants are :

to reduce friction between the rolling elements and the separator or cage, and between the rolling elements and the races at any point where true rolling is absent.

to reduce friction between the ends of rollers and the guiding surfaces which form part of the inner or outer race.

to assist in dissipating heat generated within the bearing.

to protect the highly polished working surfaces of the bearing from corrosion and rusting.

to assist in sealing the bearing against the entry of contaminants such as dust and moisture.

Ball and roller bearings may be lubricated by oil or grease, the choice of lubricant usually being decided by operating conditions and bearing design.

OIL LUBRICATION

Oil is generally considered to be more effective lubricant, and is to be preferred if the sealing arrangements are adequate, especially where the operating temperature or surface speed of the balls or rollers is high. Oil having viscosities ranging from that of the thinnest spindle oil to that of a heavy steam cylinder oil is used. The viscosity required for any given set of conditions depends on size, speed, load, and temperature of the bearing. Mineral oils of high quality should be chosen, because of their resistance to form resinous products or acids which might damage the highly polished working surfaces of the bearing.

The method used to apply the oil depends mainly on running speed of the bearing. Use of proper method is especially important with high speed bearing, which are liable to overheat through churning of the oil if it is not correctly applied. The following means of applications are suitable :

Oil bath and splash system - low and medium speeds.

Circulating systems - medium speeds.

Spray or mist - high speeds.

Over-lubrication should be avoided. As a general guide, with oil baths, it can be taken that oil level should not be above the center of the bottom rolling element when the bearing is not rotating.