Robot arm

A robot arm comprising:

• an upper arm that is installed so as to be swingable around a horizontal first axis line;
• a forearm that is installed at a distal end of the upper arm so as to be swingable around a second axis line perpendicular to a longitudinal axis of the upper arm;
• a first wrist element that is installed at a distal end of the forearm so as to be rotatable around a third axis line perpendicular to the second axis line, and has a hollow part through which the third axis line penetrates;
• a motor that is installed in the forearm, and rotates the first wrist element around the third axis line;
• and
• a speed reduction mechanism that transmits rotation of the motor to the first wrist element while reducing speed of rotation, wherein
• the motor includes a rotation center axis extending obliquely downward along a third plane on the opposite side of the upper arm across a first plane, the first plane including the third axis line and being perpendicular to the second axis line, the third plane being placed closer to the first wrist element than a second plane is, the second plane including the second axis line and being perpendicular to the third axis line, and
• the speed reduction mechanism includes:
• a hypoid gear set that includes an output hypoid gear, which is formed into a ring coaxial with the third axis line and is fixed to the first wrist element, and an input hypoid gear engaged with the output hypoid gear;
• and
• a gear set that includes a gear wheel fixed to the input hypoid gear and a pinion that is fixed to a shaft of the motor and is engaged with the gear wheel.