Speed reducer

1. A speed reducer comprising:

a case;

an input shaft configured to be rotationally driven by an electric motor and having an eccentric shaft portion that is eccentric to an axis of the input shaft;

an externally toothed gear wheel rotatably supported on the eccentric shaft portion;

an output shaft rotatably supported on the case to be coaxial with the input shaft and having an internally toothed gear wheel, which has teeth more than teeth of the externally toothed gear wheel and is engaged with the externally toothed gear wheel;

a guide plate formed with a guide recess that is recessed parallel to the axis of the input shaft, and disposed between the input shaft and the externally toothed gear wheel;

a guide protrusion provided in the case to protrude parallel to the axis of the input shaft so as to be inserted into the guide recess, and configured to guide sliding of the guide recess in a first radial direction that is centered on the output shaft; and

a guide portion provided between the externally toothed gear wheel and the guide plate to guide movement of the externally toothed gear wheel relative to the guide plate in a second radial direction that crosses with the first radial direction about the externally toothed gear wheel as a center, thereby guiding the externally toothed gear wheel so as to be revolvable but non-rotatable in cooperation with the guide recess and the guide protrusion.

2. The speed reducer according to claim 1 ,

wherein the guide recess surrounds the guide protrusion over an entire periphery of the guide protrusion.

3. The speed reducer according to claim 1 ,

wherein the guide plate includes a pair of plate thickness surfaces extending parallel to the first radial direction, and

the case includes a pair of steps configured to come into contact with the pair of plate thickness surfaces, respectively.

4. The speed reducer according to claim 1 ,

wherein the guide plate includes a plate thickness surface extending in the first radial direction, and

the case includes a step configured to come into contact with the plate thickness surface.

5. The speed reducer according to claim 1 ,

wherein the guide protrusion includes a curved portion configured to come into line contact or point contact with the guide recess so as to guide sliding of the guide recess in the first radial direction.

6. The speed reducer according to claim 1 ,

wherein the guide recess penetrates the guide plate parallel to the axis.

7. The speed reducer according to claim 1 ,

wherein the guide protrusion includes a pair of guiding surfaces formed on an end surface thereof so as to extend parallel to the first radial direction,

the guide recess includes a pair of guided surfaces formed on an inner wall surface thereof so as to extend parallel to the first radial direction, and

the guide plate moves relative to the case in the first radial direction by causing the guided surfaces to come into contact with the guiding surfaces and to be slid.

8. The speed reducer according to claim 1 ,

wherein the guide portion includes:

a guide lug provided on the externally toothed gear wheel so as to protrude toward the guide plate parallel to the axis; and

a guide groove provided in the guide plate by cutting out an end of the guide plate such that the guide lug is inserted into the guide groove.

9. The speed reducer according to claim 1 ,

wherein the guide plate is formed of a resin material.

10. The speed reducer according to claim 1 ,

wherein the reducer includes plural sets of the guide protrusions and the guide recesses, each of the sets including the guide protrusion and the guide recess as a set.