Comment
Due to complexity of contact between the vibrating rod and a cylinder, for explanation of the principle of motion of the standing wave ultrasonic motor a more simple geometry shown here was considered (see paper 22). The bottom sphere is moving under some angle to the normal of the upper sphere in an intermitting contact with it. While the rotation of the bottom sphere is prevented, the upper one can rotate. For explanation of the reason of rotation, in the paper 22 a more simple problem was investigated: tangential stresses between two nonrotating spheres were calculated. The stress resultant taken over the contact period is always positive.