Originally Posted by
Kontact
If you watch the balls for awhile it will be clear that their outer circumferences are not able to roll on the static outer race or the moving inner race at the same rate. You can see that the red dot on the ball is not moving as quickly as the red dot on the inner race. If all three had teeth, the structure would not turn at all.
If they were rolling on both races at the same rate the bearing would need to have infinite diameter i.e. be a straight line. Because they roll at different rates on each race the balls also circle the races, at a rate determined by the difference in the two race diameters.