The type of ball bearings as used on cycles, that is one in which a row of balls run between a race in the form of a cup and an adjustable cone, is not one often used on any other machinery, unless it be of very light construction. Ball bearings for use on industrial machines are designed so that the balls run between concentric rings for taking radial pressure, or between parallel plates for taking thrusts, see Figs. 1 and 5 respectively. In early types of such bearings it was arranged that each ball made four points of contact, i.e., two on each race, so that practically the balls ran in a groove, making contact on each side of the groove. This was found to give bad results and finally it has become universal to cause the balls to make contact at one point only on each ring or plate ball race. As a result of experiments it has also been found that balls running in grooves can support greater load than if running on ungrooved surfaces, providing that the radius of the groove itself bears a certain ratio (within limits) to the radius of the balls, and that the balls make contact at one point only in the groove, i.e., the groove radius is greater than that of the ball radius. Where it is essential to reduce the bearing friction to the absolute minimum for a bearing running at a high speed, but with a light load then the outer ball race is sometimes of a cylindrical form, but if the load is heavy, then it is usual and desirable that the radius of the outer ball race is some 6 to 8 per cent, greater than that of the ball, and that of the inner race may be up to 4 per cent, greater, it is of necessity a compromise between capacity and friction saving. It should be realised that balls and races under load deform slightly, and so introduce a rubbing friction, due to a slight slip taking place.

Fig. 1.

Ball bearings made by makers with a first-class reputation should always be used, and their advice asked on any special points that may arise, such as extra high speed or load, or high temperature conditions. Messrs. Skefks Ball Bearing Co., Ltd., recommend that ballbearings never be run under a temperature condition exceeding 200° C. (392° F.), and that if the bearings are running continuously a maximum temperature of 300° F. only is allowed. They also do not recommend that graphite be used as a lubricant for ball bearings, and put oil forward as the ideal lubricant. This oil should be pure mineral, and have no tendency to gum or become sticky, and should contain no free mineral acid or free alkali or sulphur. For speeds between 3,000 and 5,000 r.p.m. the viscosity should be 200 Redwood sees. (6-9 Engler deg.) at 100° F. For speeds up to 3,000 r.p.m. grease lubrication is suitable, which grease should be chemically neutral, and contain not more than 1 per cent, of neutral saponifiable oil. The melting point of this grease should be 194° F. (90° C). Where the temperature of the application is likely to exceed this value (90° C), oil should be used.

In any ball-bearing care should be taken to hold both the inner and outer ball races in correct alignment with one another, the inner race is held firmly into the rotating shaft and rotates truly with it, while the outer ball race is held in a fixed non-rotating position truly concentric with the inner ball race. Where two ball-bearings are used on the one shaft, making one complete bearing, it is usual to make use of a sleeve between the two inner ball races, see Fig. 1, this sleeve having clearance on the shaft.

Fig. 2 shows the outer bearings on each end of a shaft, one is fixed to locate the shaft, and the other is free in the housing. The shaft on which the inner ball race is to fit should be machined to the tolerance given by the makers, and the ballbearing pressed into its seat, correctly done, little pressure will be required, and this pressure should only be applied direct into the inner race and never into the outer ball race. When entering the outer race into the housing, only a piston fit is required, and the pressure must not be applied via the inner race.

Fig. 2.

The outer ball race has a slight end play allowed it in the housing. In these fitting operations care should be taken to keep all dust, grit or small pieces of metal out of the bearing, their entry means certain trouble.