Strain wave gearing

1. A strain wave gearing comprising:

a flexible internally toothed gear capable of flexing in a radial direction;

a support mechanism provided with a supporting inner peripheral surface for supporting the internally toothed gear from an outer peripheral side thereof;

a flexible externally toothed gear disposed concentrically inside the internally toothed gear; and

a wave generator fitted inside the externally toothed gear so as to be capable of rotating relative to the externally toothed gear, the wave generator causing the externally toothed gear to flex in a non-circular shape and pushing the externally toothed gear toward the internally toothed gear supported by the supporting inner peripheral surface, and causing the externally toothed gear to mesh with the internally toothed gear at a plurality of locations that are set apart in a circumferential direction,

a meshing state of external teeth of the externally toothed gear relative to internal teeth of the internally toothed gear being an overlapping meshing state in which pitch circles of the internal teeth and the external teeth overlap each other.

2. The strain wave gearing according to claim 1 ,

wherein the internally toothed gear is a cup-shaped or top-hat-shaped internally toothed gear provided with an internally-toothed-gear cylinder part capable of flexing in the radial direction, an internally-toothed-gear diaphragm that extends radially outward or radially inward from one end of the internally-toothed-gear cylinder part, and internal teeth formed on an inner peripheral surface portion on the other-end side of the internally-toothed-gear cylinder part;

the support mechanism has a floating ring provided with the supporting inner peripheral surface that supports an outer peripheral surface of an internal tooth formation portion in which the internal teeth in the internally-toothed-gear cylinder part are formed, the supporting ring being in a floating state that allows displacement following a state of radial flexing in each position of the internal teeth along a tooth trace direction; and

an inner diameter of the floating ring is set based on an outer diameter of the internal tooth formation portion that is caused to flex in the non-circular shape by the wave generator via the externally toothed gear.

3. The strain wave gearing according to claim 2 ,

wherein the externally toothed gear is a cup-shaped or top-hat-shaped externally toothed gear provided with an externally-toothed-gear cylinder part capable of flexing in a radial direction, an externally-toothed-gear diaphragm that extends radially outward or radially inward from one end of the externally-toothed-gear cylinder part, and external teeth formed on an outer peripheral surface portion on the other-end side of the externally-toothed-gear cylinder part; and

the wave generator is fitted inside an external tooth formation portion in which the external teeth in the externally-toothed-gear cylinder part are formed, the wave generator being fitted inside the external tooth formation portion so as to be capable of rotating relative to the external tooth formation portion, and the wave generator causing the external tooth formation portion to flex in a non-circular shape and pushing the external tooth formation portion toward the internal tooth formation portion supported by the circular inner peripheral surface of the floating ring.

4. The strain wave gearing according to claim 3 ,

wherein the wave generator causes the external tooth formation portion to flex in an ellipsoidal shape and causes the external teeth to mesh with the internal teeth at positions on both major-axis-direction ends of the ellipsoidal shape; and

an inner diameter of the supporting inner peripheral surface of the floating ring is set based on a major diameter of the outer peripheral surface of the internal tooth formation portion caused to flex in the ellipsoidal shape by the wave generator via the external tooth formation portion.

5. The strain wave gearing according to claim 2 ,

wherein the floating ring is a ring having a prescribed width in an axial direction of the internally toothed gear;

the support mechanism has a ring support part for supporting the floating ring; and

the ring support part is configured to support the floating ring in a state that enables both axial-direction side portions of the floating ring to be radially displaced about positions midway along the axial direction in an outer peripheral surface of the floating ring.

6. The strain wave gearing according to claim 5 ,

wherein the support mechanism is a cylinder member provided with an inner-side cylinder and an outer-side cylinder that are disposed concentrically, and an annular web that links the inner-side cylinder and the outer-side cylinder, the cylinder member having a cross-section in the shape of a letter H;

the inner-side cylinder in the cylinder member is the floating ring; and

the annular web and the outer-side cylinder in the cylinder member are the ring support part.

7. The strain wave gearing according to claim 5 ,

wherein the support mechanism has a ball bearing;

an inner race in the ball bearing is the floating ring; and

balls and an outer race in the ball bearing are the ring support part.

8. The strain wave gearing according to claim 2 , further comprising:

an elastic body to restrict movement of the floating ring in an axial direction of the internally toothed gear.