Cage motions in ball bearings are investigated using a dynamic analysis program. Increases in the cage friction coefficient induce unstable motions of the cage. The instability is more likely to occur under high load and low speed conditions due to less ball-race sliding. A simple theory of cage instability is developed, and a critical cage friction coefficient formula is proposed, which is a function of the cage mass, ball-race traction, ball-cage contact stiffness, cage rotational speed and number of balls. The prediction of this formula agrees with the results of the dynamic analysis. With a nonuniform separation between the balls, a high speed whirl is superimposed on the normal whirl with the ball group speed. The direction of the high speed whirl is the same as the cage rotational direction in inner race rotation, but they are opposite in outer race rotation. These results agree with some experimental results in the literature and validate the dynamic analysis.