In the paper, the kinetic forces on the balls of the radial ball bearings of the multi-stage gear reducers, i.e. the multiplier of the revolutions of the main shaft, were determined. The kinetic contact forces of balls and circular guides, stationary and moving radial ball bearing due to the occurrence of: a* centrifugal forces of unbalanced gears fixed on the shafts and b* when eccentricity of the center of mass of a pair of balls with one diameter occurs in a radial ball bearing, due to the difference in their mass density, at equal radii. The number of revolutions of the balls in rolling, without sliding, and the change of contact points in which kinetic contact forces occur, for one revolution of each of the shafts and reduced to the main shaft, were determined. It is determined for the cases of radial ball bearings with four pairs of balls and with six pairs of balls in radial ball bearings.