Joint shaft structure and horizontal articulated robot

A joint shaft structure comprising:

• a base member;
• an output shaft member that is supported on one side of the base member so as to be rotatable around a rotational axis;
• and
• a strain wave gear reducer that rotates the output shaft member relative to the base member by transmitting rotation of a motor, disposed in a hollow part of the base member, to the output shaft member while reducing a speed of the rotation, wherein:
• the strain wave gear reducer includes:
• a wave generator fixed to a shaft that is rotated around a longitudinal axis coinciding with the rotational axis by a driving force from the motor;
• a substantially cylindrical flexspline having a first end and a second end, an elastic part being provided at the first end, the second end being fixed to the base member by a flange extending radially outward from the elastic part, the elastic part having a shape of a thin- walled cylinder elastically deformable in a radial direction and including a plurality of external teeth arrayed in a circumferential direction on an outer circumferential surface, and the wave generator being fitted inside the elastic part;
• and
• a ring gear that is disposed on a radially outer side of the flexspline and fixed to the output shaft member, and has internal teeth meshing with the external teeth;
• and
• the output shaft member includes:
• an opening larger than an outside diameter of the wave generator, at a position adjacent to the elastic part;
• a cover that is configured to be mounted over the opening so as to completely close the opening and that is configured to be removed from the opening;
• and
• wherein the cover is mounted over the opening without a bolted connection.