While ideal rotational surfaces of the pivot and the race of ball bearings can touch the spherical ball surfaces only at mathematical points, these surfaces actually “flatten” at the contact, due to elastic compression so that contact occurs over a finite area. It is shown in this paper that as a result of this flattening, owing to unavoidable small relative slipping over some of the contact areas, even the most precise and flawless ball bearings possess definite friction. This source of unavoidable friction is analyzed, and the frictional torque due to it is computed for an individual ball, for the whole bearing, and for both bearings holding a rotor or gimbal.

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