Bearings

Bearings are the most important part that can be tweaked to get better efficiency from a machine with rotating parts. Three most important types of bearings are discussed here.

1.       Bush Bearing: In mines, pumps are the most common mechanical equipment that can be found and for a multistage turbine pump, bush bearing plays an important part. A bush bearing consists of a cast iron housing which houses a brass or gun metal bush and the rotating shaft, rests inside the bush, which is of plane cylindrical shape. A thin film of oil separates the two surfaces.  A bush bearing is used for small diameter shaft (<75mm) rotating at high speed. The bush is tightly fitted in the block and a set screw is driven partly in the bush to prevent its rotation. Oil lubrication is done through a hole provided in the block extended to bush.

Bush bearing

Plummer Block

2.       Pedestal or Plummer Block: It is a form of bearing where the brass of the bush bearing is separated in two halves which are housed in pedestal body of cast iron.  A cast iron cap is bolted on the pedestal body to cover the top brass. To prevent rotation of the brasses within the block, a lug is provided in each half brasses and it fits in the recess formed in the pedestal body. The face, along which the two brasses rest against each other i.e. the joint, is parallel to its length of the shaft. Coefficient of friction between the shaft and the bearing is of the order 0.01.

Split Bush Bearing

Ball Bearing

3.       Ball and Roller Bearing: These differ from the bush bearing discussed above as the rotating shaft is separated from the stationary portion by means of intermediate balls or rollers which roll freely between the two surfaces. A ball/ roller bearing consists of following parts:

·         An inner race: It is fitted tightly with the shaft and revolves with it.

·         An outer race: It is stationary and is push fitted in the enclosed housing.

·         Row(s) of balls (or rollers) running in the groove formed by the races, the difference in the radius of the two races is 5-10% greater than the radius of the balls) or rollers.

·         Cage/Separator: To separate the balls (or rollers) and maintain them at correct distance apart.

The races and ball are of high carbon chrome steel having hard surfaces. They are machined and ground to within limits of 0.0002am, and highly polished. The cage may be of steel, or brass or a synthetic material.

Grease is used for lubrication and preventing rusting and foreign materials entering into the bearings.

Roller Bearing

The roller bearing can carry twice the load of a corresponding ball bearing, and it is able to move axially within its races to adjust for slight variation in shaft length rather more careful mounting than a ball bearing in order to maintain accurate line of contact. The friction is as low as 0.001 in ball/roller bearings, just a bit higher at the start than when running.

exploded view of a ball bearing