Flat strain wave gearing

A flat strain wave gearing comprising:

• a first rigid internal gear;
• a second rigid internal gear;
• a cylindrical flexible external gear capable of meshing with the first and second rigid internal gears;
• a wave generator that flexes the flexible external gear into a non-circular shape to partially mesh with the first and second rigid internal gears and that moves meshing positions therebetween in a circumferential direction;
• and
• a restraining mechanism that limits movement of the flexible external gear in an axial direction,
• wherein the restraining mechanism has:
• annular first and second restraining members for limiting the movement of the flexible external gear in the axial direction;
• an annular first recess where the first restraining member is accommodated in a floating state in which it is movable in the axial direction and in a radial direction by a predetermined amount and is movable in a circumferential direction;
• and
• an annular second recess where the second restraining member is accommodated in the floating state;
• and
• wherein the first restraining member faces a first external-gear end face on one side of the flexible external gear in the axial direction, and the second restraining member faces a second external-gear end face on the other side of the flexible external gear;
• and
• further comprising:
• a first housing to which the first rigid internal gear is attached;
• and
• a second housing to which the second rigid internal gear is attached,
• wherein the first recess is open inward in the radial direction and is formed between the first housing and a first internal-gear end face of the first rigid internal gear on a side of the first external-gear end face;
• the second recess is open inward in the radial direction and is formed between the second housing and a second internal-gear end face of the second rigid internal gear on a side of the second external-gear end face;
• and
• the first and second restraining members have a surface hardness having a value within a range from 90% to 99% of the surface hardness of the first and second external-gear end faces of the flexible external gear.