HISTORY OF BEARINGS

For centuries, man had to rely on his own power to push or pull large objects over the earth. The concept of a bearing – to lessen friction between an object and the surface over which it is moved – is nearly as old as man himself. The first solution to relieving some of this sliding friction was recorded as early as 3,500 B.C. It was then that Mesopotamians were using one of the first bearings known to man, an invention called the wheel. Where the wheel and axle touched, they put a bearing made of leather or wood and lubricated it with animal fat. Ancient drawings from 1,100 B.C. show the Assyrians and Babylonians moving huge rocks for their monuments and palaces with rollers, illustrating the basic bearing principle – to lessen friction. But this was sliding – not rolling – friction.

The roller and ball bearings of today may bear little resemblance to their predecessors but the concept has remained the same: to lessen friction. Today, bearings are used in almost every imaginable application, such as roller skates and bicycles, where two surfaces are turning or moving against each other. They are used in thousands of ways, from the minute internal workings of a clock to large turbine engines in a ship.

The bearings with which we are concerned fit two basic categories – ball and roller. We will discuss both categories, and cover bearing types, installation, operating conditions, maintenance and troubleshooting. In addition, we will feature one particular type – the tapered roller bearing – which has numerous fleet applications.

The parts of a bearing

A bearing’s smooth performance is assured by a combination of four basic working parts

• Outer race (also called outer ring or cup)

• Inner race (also called inner ring or cone)

• Rolling elements (either balls or rollers)

• Separator (also called cage or retainer)

The outer race, or cup, is the bearing’s exterior ring. Since it protects the bearing’s internal parts, it must be machined smoothly and accurately. The inner race, or cone, is the part of the bearing that sits directly on the shaft. The word “radial” means in the direction of a radius: moving from the circumference inward, or the center outward. In this case it moves from the outside in. A radial load pushes down, from the outer race inward to the balls, cage and inner race at the center of the bearing. The load is at right angles (90º) to the shaft on which it is being supported.  “Thrust” means a pressure or pushing force exerted by one part against a touching part. Pressure is exerted sideways, pushing the shaft either right or left. This shaft movement then pushes the inner race of the bearing in the same sideways direction. The line of pressure, that is, the load, runs

parallel to the shaft  An “angular” load is actually a combination of radial and thrust loads. As the load moves at an angle toward the shaft, it pushes against the corner of the inner race. Pressure is transmitted diagonally, through the corner of the race, cage and rolling elements, to the opposite corner of the outer race

Guide moving parts

The third function, to guide moving parts, is a result of the other two functions. By supporting a load while reducing friction, a bearing guides shaft operation. It assists the movement of crucial shafts, wheels and pivots. Without a bearing, the rotating part could not continue operating on a smooth, constant basis