Strain wave gear speed reducer, method for manufacturing strain wave gear speed reducer and actuator for link mechanism for internal combustion engine

A method for manufacturing a strain wave gear speed reducer including

• a rigid internal gear having a plurality of internal teeth each of which has a straight tooth profile,
• a flexible external gear having a plurality of external teeth each of which has a straight tooth profile, the flexible external gear being disposed on an inner side of the rigid internal gear, and
• a wave motion generator configured to cause the flexible external gear to be deflected in a radial direction so as to be partially engaged with the rigid internal gear, the wave motion generator being configured to rotate about an axis of rotation, thus causing an engagement portion to move in a circumferential direction,
• the method comprising:
• a first decision step where a radius re of a reference pitch circle of the flexible external gear, a radius ri of a reference pitch circle of the rigid internal gear, and a speed reduction ratio “i” of the strain wave gear speed reducer are decided;
• a second decision step where a shape of the external tooth and a shape of the internal tooth are decided based on both of the radius re of the reference pitch circle and the radius ri of the reference pitch circle;
• and
• a third decision step where a shape of a tooth top of the internal tooth is decided by a movement envelope of the external tooth.

An actuator for link mechanism for internal combustion engine for rotating a control shaft which changes an attitude of a link mechanism of an internal combustion engine, the actuator comprising:

• an electric motor configured to rotationally drive a motor output shaft;
• a strain wave gear speed reducer configured to reduce a rotational speed of the motor output shaft, and to transmit the rotational speed to the control shaft;
• and
• a housing configured to cover the strain wave gear speed reducer, wherein
• the strain wave gear speed reducer includes
• a flexible external gear having a plurality of external teeth each of which has a straight tooth profile, and transmitting a rotational force to the control shaft,
• a rigid internal gear disposed on an outer periphery of the flexible external gear, and fixed to the housing, the number of internal teeth of the rigid internal gear being greater than the number the external teeth, the internal tooth having a straight tooth profile, and a tooth top of the internal tooth having a shape which matches or overlaps with a movement envelope of the external tooth as viewed from a direction of an axis of rotation of the flexible external gear, and
• a wave motion generator rotationally driven by the motor output shaft, the wave motion generator causing the flexible external gear to be deflected in a radial direction so as to be partially engaged with the rigid internal gear, and rotating about an axis of rotation, thus causing an engagement portion to move in a circumferential direction.