During World War II, ball bearing factories were important bombing targets. Military leaders knew that, without bearings, the machinery of war would literally grind to a halt. Today, ball bearings are no less important for keeping production machinery running smoothly.

A radial ball bearing consists of four main components: an inner ring, an outer ring, balls and a separator. In addition, a shield or seal can be installed on one or both sides of the bearing to protect the bearings from contamination.

The inner ring is mounted on the shaft of the machine and is usually the rotating part. The bore can be cylindrical or tapered. The outer ring is mounted to the housing of the machine. In most cases, it does not rotate. The outside surface of the outer ring can be smooth, or a groove or flange can be machined into it to help secure the bearing in position.

The balls travel in grooves, or races, in the inside surface of the outer ring and the outside surface of the inner ring. The radius of the race is slightly larger than the radius of the ball. There's usually a small amount of play between the inner and outer rings. This enables the bearing to spin freely and helps accommodate slight angular misalignment between the shaft and the housing.

The separator keeps the balls evenly spaced and prevents contact between them during operation. The separator can be a flat ring with open or closed pockets to hold the balls, or it can be a loop of formed wire. The separator doesn't even have to be one part. Rather, several parts, such as toroids, cylinders or helical springs, can separate the balls.

Balls and rings are usually made of high-carbon steels, such as AISI 52100, or stainless steels, such as 440C. But, other materials are available for specialized applications. For example, plastic bearings filled with plastic or glass balls are used in magnetic resonance imaging equipment, because steel bearings could distort the images. Plastic bearings are also used in pool cleaning equipment, because they can run underwater without lubrication.

Balls can also be made of silicon nitride. Balls made of this ceramic weigh less than comparably sized steel balls. This decreases the centrifugal force imparted to the rings, which enables the bearing to handle high speeds. Because the modulus of elasticity of silicon nitride is 50 percent higher than steel, bearings with ceramic balls are stiffer than bearings with steel balls. And, because silicon nitride has a low coefficient of thermal expansion, bearings with ceramic balls are less apt to experience changes in contact angle during operation, which can cause variations in preload.

When specifying radial ball bearings, three dimensions are important: the outside diameter, the inside diameter and the width of the rings. It's also important to specify the axial, radial and moment loads the bearing will handle; the operating speed; and how many hours the bearing is expected to last.

The life of a ball bearing is inversely proportional to the cube of the load. Thus, if the load doubles, bearing life will decrease by a factor of eight. Other factors that affect bearing life include speed, operating temperature, lubrication, contamination and alignment.

Bearing life is the number of revolutions the bearing can turn, or the number of hours it can run at a constant speed, before the balls or the rings show signs of fatigue. In catalogs, the L-10 life of a bearing refers to the number of revolutions that 90 percent of a set of identical bearings, operating under identical conditions, will attain or surpass before the bearing material fails from fatigue. It's usually expressed in millions of revolutions.

The basic load rating of a bearing is the maximum, constant, radial load that the bearing can theoretically endure for 1 million revolutions.