Abstract: The tooth profile contours of a rigid internally toothed gear and a flexible externally toothed gear in this strain wave gearing are stipulated by: meshing portions which each mesh with the opposing gear; tooth-crest-side convex surface portions which, respectively, are smoothly connected to the addendum-side ends of the meshing portions and extend from said ends to the apices of the tooth crests; and tooth-bottom-side concave surface portions which, respectively, are smoothly connected to the dedendum-side ends of the meshing portions and extend from said ends to the deepest parts of the tooth bottoms. The meshing portions and tooth-crest-side convex surface portions are machined portions that are simultaneously machined by topping gear cutting, and there are no edges on the teeth of either gear on the tooth-crest side.

Abstract: A strain wave gearing has a cup-shaped externally toothed gear that is provided with a body formed with external teeth, a diaphragm extending from an end of the body, and a rigid boss integrally formed in the diaphragm. The ratio B/A is set equal to or less than 0.59 where A is the reference pitch circle diameter of the external teeth and B is the outer diameter of the boss. The radial length from the inner peripheral edge of the diaphragm to the outer peripheral edge thereof in the present externally toothed gear is large. This enables to reduce bending stress generated in each part of the externally toothed gear due to coning, and a meshing state of the external teeth with the internal teeth in the tooth trace direction can also be improved, whereby further flattening of the externally toothed gear can be realized.

Abstract: A mounting groove of a seal structure using an O-ring is defined by: a groove bottom surface; an acute-angle-side groove side surface inclined at an acute angle contiguously to the radially inner edge of the groove bottom surface; and an obtuse-angle-side groove side surface inclined at an obtuse angle contiguously to the radially outer edge of the groove bottom surface. The O-ring is mounted in a state of being pushed and expanded to the acute-angle-side groove side surface. The O-ring is constrained, in the mounting groove, between the groove bottom surface and the acute-angle-side groove side surface. The O-ring is prevented from falling off from the mounting groove. A groove width is constant or increases from the groove bottom surface toward a groove opening. Work for mounting the O-ring on the mounting groove is easy, and processing for the mounting groove is also easy.