Driving force transmission device and robot hand

A robot hand, comprising:

• a driving force transmission device for transmitting a driving force;
• and
• an arm;
• the driving force transmission device comprising:
• a first rotator configured to perform one of an input operation and an output operation of a driving force, the first rotator including a concave surface;
• a second rotator configured to perform the other of the input operation and the output operation of the driving force, the second rotator including a convex surface fitted into the concave surface;
• and
• a plurality of spheres between the concave surface and the convex surface,
• wherein the concave surface has a plurality of holes in which the respective plurality of spheres are received,
• the convex surface has a groove configured to receive parts of the plurality of spheres protruding from the plurality of holes,
• the groove includes at least one bend having a vertex and two sides, and the two sides extend in a portion of the second rotator located toward a first end of the second rotator in an axial direction of the second rotator,
• in response to the first rotator or the second rotator being driven to rotate under an external driving force, the plurality of spheres received in the plurality of holes revolve about a rotation axis of the first rotator and move along the groove between the concave surface and the convex surface being out of contact with each other,
• the plurality of holes and the groove are located to create a pressurized state throughout a rotation range of each of the first rotator and the second rotator, where the pressurized state is a state in which at least one of the plurality of spheres has a pressure angle of 60° or less, and the pressure angle is an angle between a direction normal to an outer opening edge of the at least one bend and a moving direction of each of the plurality of spheres under a driving force transmitted between the outer opening edge and each of the plurality of spheres, and
• the arm is integral with one of the first rotator and the second rotator, and driven to rotate under a driving force transmitted from the driving force transmission device.