PURPOSE: To simplify the structure and facilitate assembly by furnishing a sun roller consisting of two rollers and a planet roller having two conical surfaces, and by putting conical surfaces of the sun roller in slide contact with the conical surfaces of planet roller.
CONSTITUTION: A planet roller 23 is put between a sun roller 21 and a ring roller 22 in such a manner as in friction contact therewith. Conical surfaces formed on the sun roller 211, 212 have higher level where a ball cam 26 is installed, and are sunk gradually to the left and right. The shape of the conical surfaces of the planet roller 23 is concave in the center and high at the ends so as to well mate with the cones of the sun roller 21. This constitution is free from provision of any thrust bearing, that should simplify the structure, facilitate assembly, and suppress the cost.