In order to construct a system with multi-degree of freedom by only using the motors with one degree of freedom, the same number of motors as the degree of freedom is required. On the other hand, in a spherical motor with three degrees of freedom, it is possible to realize a system with three degrees of freedom by only one spherical motor, and it is possible to reduce the size of the system. However, in many spherical motors driven by electromagnets, a spherical speed reducer is required as a device to extract and transmit the output torque. The spherical speed reducer considered in this study is composed of an output ball and two small balls transmitting torque. Each of the small balls is supported by two small ball casters. The mechanism of the spherical speed reducer requires a model of rotation transmission to control suitably the rotor for a desirable rotation of the output ball. The purpose of this study is to measure angular velocities of the small balls in the case that the output ball is not mounted as the first step to derive a rotation transmission model. From the measurement results, the arrangement of two small ball casters is focused on, and a valid rotation transmission model is derived.
