Effect of linear speed of rolling

I think that it is easy to just think of speed of rotation as the dominant quantity in the selection of a ball bearing. Yet it is the speed of rolling that matters. It matters for two reasons, the first is that this is the speed at which the cyclic deformation of the race and the balls takes place and secondly the speed of rolling affects the lubrication. On both counts lower speeds are to be preferred.

 

Ball races are used at speeds at least up to 30,000 rpm but they run mainly at much lower speeds. For instance the wheels of a small car at 60 mph will rotate at about 1,000 rpm. The ubiquitous single-phase motor runs at 3,000 rpm. The ball races associated with these two applications will have typical diameters for the path of the balls. For wheel bearings this will be about 40 mm and electric motors about 25 mm or smaller. In these applications the linear speed of rolling is quite low.

 

However, one must not overlook the fact that the linear speed of rolling in a ball race is less than half that of the linear speed of the moving track where the track is the line of potential contact between the ball and the race. This is evident from figure 16-27 where the centres of the balls will move at half the speed of the upper block.

 

It is easy to make a plot of the rolling speeds in terms of the rotational speed and the diameter of the path of the balls allowing for the balls rolling at about a half the rotational speed. I have done this in graph 16-13. It is the graph of rolling speed  where  is the diameter of the inner track and  is the speed of rotation of the inner race. Note the rotational speeds are in geometric progression.  There is a dotted line for a rolling speed of 3 metres/sec. This is about twice unhurried walking speed and a speed that I can visualise and think of as low. Many of the races in common use have much lower rolling speeds.