Wedge action in ball bearings

It is essential to lubricate ball and roller bearings; they will not run dry. No one seems to take much notice of the lubricant and appear to regard it as incidental as one might the lubricant applied to the hinges of a door. Yet the lubricant is trapped by the rolling action to produce a wedge action like that in the plain bearing and in the thrust pad. 

 

In figure 16-26 I said nothing about the profile of the races. They could just be two plain, cylindrical rings. I have drawn such an arrangement in figure 16-30. If these balls run in, say, a liquid lubricant, lubricant in the spaces that I have coloured in red would be pushed out of the way be the rolling action of the ball just as the bows of a ship push the water aside to form the bow wave. Inevitably the pushing process will be resisted by the viscosity and by the inertia of the lubricant. The necessary acceleration will be imposed on the lubricant by a distributed force acting on the lubricant and there will be equal and opposite distributed forces acting on the ball and on the race. The magnitude of these forces will depend on the shape of the space in which the ejection of the lubricant takes place. It is always an action that is of the same character as wedge action in plain bearings and thrust pads. The distributed force creates a pressure that tends to separate the ball and the tracks.

 

In the plain bearing and the thrust pad we found that high pressures were developed in converging surfaces that were close together and only converging slowly. This is clearly not possible for the arrangement in figure 16-30. However we have some control over the shape of the space in which this action takes place. In practical ball bearings grooves are cut in both races. They are most likely to have the profile of an arc of a circle. I have drawn the arrangement in figure 16-31. The arc has a larger radius than the ball and the closer that this radius becomes to that of the ball the less space that there will be to accommodate the lubricant being ejected from the space between the balls and the tracks in the races. This can lead to very high pressures in the lubricant and separate the balls and the tracks to eliminate metal-to-metal contact and perhaps accommodate for variation in the clearance resulting from the random variation in the size of the balls. But it must be remembered that the clearances involved are commensurate with the deformation of the balls and the track when under load and also commensurate with the clearances needed to produce high pressures.

 

Fortuitously the provision of the tracks in the races makes possible the construction of ball bearings as single units if the dimensions are chosen so that, if the inner race is placed inside and touching the outer race, the balls can be fitted into the space between them and then the inner snapped across to centralise it with the balls around it. Then the balls can be spaced when the cage can be fitted. See the photograph in figure 16-32. The cage shown is one of two identical pressings. They are fitted one each side and spot welded together. The two races, the balls and the cages form a robust unit that is just called a ball race.