TECHNICAL FIELD

The present invention relates to a spline structure that is, for example, integrated into a precession speed-reducing or -increasing apparatus, or used as a constant velocity joint.

BACKGROUND ART

A precession speed-reducing apparatus includes a first crown gear, a second crown gear, a cam unit, and a spline structure (refer to Patent Literature 1). The first and second crown gears face each other, and are different in the number of teeth. The cam unit causes the first crown gear to incline with respect to the second crown gear in such a manner that the first crown gear meshes with the second crown gear, and causes the first crown gear to precess in such a manner as to move the meshing position. When the first crown gear precesses, the second crown gear reduces the speed and rotates by the difference in the number of teeth.

CITATION LIST

Patent Literature

Patent Literature 1: WO 2016/140234 A

SUMMARY OF INVENTION

Technical Problem

In the known precession speed-reducing apparatus, a rolling element is disposed between the cam unit and the first crown gear in such a manner that the first crown gear precesses smoothly. A rolling element rolling portion where the rolling element rolls is formed on the cam unit. Therefore, the cam unit is made of steel to increase durability. However, the cam unit rotates at high speed together with an input shaft. There is a problem that if the cam unit is made of steel, especially when the diameter of the cam unit is large, the inertia of the cam unit is increased, and controllability is reduced.

Hence, an object of the present invention is an object of providing a precession speed-reducing or -increasing apparatus that can reduce the inertia of a cam unit.

In order to solve the above problem, one aspect of the present invention is a speed-reducing or -increasing apparatus that includes a first crown gear, a second crown gear facing the first crown gear, and a cam unit configured to cause the first crown gear to incline with respect to the second crown gear in such a manner that the first crown gear meshes with the second crown gear, to cause the first crown gear to precess in such a manner as to move the meshing position, and to be coupled to an input or output shaft. Further, this apparatus according to the present invention includes a rolling element between the cam unit and the first crown gear, and the cam unit includes each of a first member configured to be coupled to the input or output shaft and a second member made of steel including a rolling element rolling portion where the rolling element rolls, where the second member is configured to be incapable of rotating relatively to the first member, and at least part of the first member has a lower specific gravity than the second member.

According to the aspect of the present invention, at least the part of the first member of the cam unit has a lower specific gravity than the second member. Accordingly, the inertia of the cam unit can be reduced. Moreover, the second member of the cam unit having the rolling element rolling portion is made of steel. Accordingly, it is possible to prevent the durability of the cam unit from decreasing.

