This paper describes some experiments on the coefficient of kinetic friction between two cylindical surfaces considered to be approximately equal to the contact of gear teeth surfaces. The experiments were made for finding out the coefficient of kinetic friction between the contact surfaces of plastic gear teeth. The followings are found from the experiments. The coefficient of kinetic friction was influenced by the lubricating conditions and sliding velocity, and slightly depended on the load. It was hardly influenced by the dimensions of both cylinders so far as present experiments are concerned. As the summary of these experiments, the coefficient of kinetic friction jet concerning the present experiments can be expressed as the following :
μ=C0Pn-0.1·V0.23
where, C0; coefficient to be determined by the combination of materials and lubricating conditions, Pn; load (N), V; sliding velocity (cm/s).