How Bearings Work

Have you ever wondered how electric motors and many types of industrial equipment spin so smoothly, quietly, and efficiently? The answer is found in a neat, simple little device called a bearing. Without bearings, maintenance personnel would be constantly replacing parts that wear out because of friction.
By Marshall Brain June 1, 2001

Have you ever wondered how electric motors and many types of industrial equipment spin so smoothly, quietly, and efficiently? The answer is found in a neat, simple little device called a bearing. Without bearings, maintenance personnel would be constantly replacing parts that wear out because of friction.

This column explains bearing basics and looks at several different types.

Bearing basics

The concept behind a bearing is very simple: Items roll better than they slide. The reason for the improved performance is that when items slide, friction between the two causes a force that tends to slow them down. But, if the two surfaces roll over each other, friction is greatly reduced.

Bearings reduce friction by providing smooth, metal balls or rollers, and a smooth inner and outer metal surface for the balls to roll against. These balls or rollers “bear” the load, allowing the device to spin smoothly.

Bearings typically deal with two kinds of loading: radial (illustrated below) and thrust. Radial loading is perpendicular to the bearing’s axis or rotation, while thrust loading is along the axis of rotation. Depending on where the bearing is used, it may see all radial loading, all thrust loading, or a combination of the two.

Types of bearings

There are many types of bearings, each with a different purpose. Common configurations include ball, roller, needle, ball thrust, and roller thrust.

Ball bearings (left) are probably the most common type. They are found in everything from inline skates to hard drives, and handle both radial and thrust loads. They are usually used in applications where the load is relatively small.

Ball bearings transmit the load from the outer race to the ball, and from the ball to the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it spin very smoothly. But this condition also means that there is not much contact area holding the load. Therefore, if the bearing is overloaded, balls can deform or squish, ruining the bearing.

Roller bearings (below) are used in applications such as conveyor belt rollers, where they must hold heavy radial loads. In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This arrangement spreads the load out over a larger area, which allows the bearing to handle much greater loads than a ball bearing. However this type is not designed to handle much thrust loading.

Needle bearings are a variation of the roller type. They use cylinders with a very small diameter, which allows the bearing to fit into tight places.

Ball thrust bearings are used primarily for low speed applications, and cannot handle much radial load.

Roller thrust bearings support large thrust loads. They are often found in gearsets, such as between the housing and rotating shafts.