Dual-type strain wave gearing

A strain wave gearing comprising:

• a rigid first internally toothed gear in which first internal teeth are formed;
• a rigid second internally toothed gear in which second internal teeth are formed, the second internally toothed gear being disposed so as to be coaxially aligned in parallel with the first internally toothed gear;
• a flexible externally toothed gear in which first external teeth capable of meshing with the first internal teeth and second external teeth capable of meshing with the second internal teeth are formed in the outer-peripheral surface of a radially flexible cylindrical body, the second external teeth differing in number from the first external teeth, and the externally toothed gear being disposed coaxially inside the first and second internally toothed gears;
• and
• a wave generator which causes the externally toothed gear to flex in an ellipsoidal shape, causing the first external teeth to partially mesh with the first internal teeth and causing the second external teeth to partially mesh with the second internal teeth;
• wherein radial flexing amounts by which the first and second external teeth are flexed respectively by the wave generator are set to be a same radial flexing amount d;
• and the radial flexing amount d satisfies a relationship
• d=(d1+d2)/ω,
• 1.4≤ω≤2.6,
• where ω is a coefficient;
• where, when m1 is a module of the first external teeth, m2 is a module of the second external teeth,
• n1 and n2 are positive integers, 2n1 is a difference in number of teeth between the first external teeth and the first internal teeth, and 2n2 is a difference in number of teeth between the second external teeth and the second internal teeth, and
• d1 is a theoretical value that is an amount by which the first external teeth are radially flexed at major-axis positions and d2 is a theoretical value that is an amount by which the second external teeth are radially flexed when the external teeth are flexed in an ellipsoidal shape, the theoretical values d1 and d2 being respectively represented by
• d1=m1n1 and,
• d2=m2n2
• wherein a gap is formed between a tooth-trace-direction inner-end surface of the first external teeth and a tooth-trace-direction inner-end surface of the second external teeth, the gap having a prescribed width along a tooth trace direction, and the gap having a deepest part along a tooth depth direction at a tooth-trace-direction central portion;
• wherein a relationship
• 0.1L<L1<0.3L
• is satisfied, where L is a width from a tooth-trace-direction outer end of the first external teeth to a tooth-trace-direction outer end of the second external teeth, and L1 is a maximum width of the gap along a tooth trace direction;
• and
• wherein relationships
• 0.9h1<t1<1.3h1 and
• 0.9h2<t2<1.3h2
• are satisfied, where h1 is a tooth depth of the first external teeth, h2 is a tooth depth of the second external teeth, t1 is a tooth-depth-direction depth from a tooth top land of the first external teeth to the deepest part, and t2 is a tooth-depth-direction depth from a tooth top land of the second external teeth to the deepest part.