Speed reducer with electric motor

A speed reducer with an electric motor, comprising:

• a hollow shaft that is hollow and extends in an axial direction around a first center axis;
• a tubular casing that is directly or indirectly fixed to the hollow shaft and extends in the axial direction on a radial outside of the hollow shaft;
• a fixed part that expands around the hollow shaft and is relatively stationary with respect to the casing;
• an electric motor that generates a rotation motion with respect to a second center axis parallel to the first center axis on the radial outside of the hollow shaft;
• a speed reduction mechanism that decelerates and transmits the rotation motion obtained from the electric motor;
• an output part that is rotatably connected to the fixed part via a first bearing and rotates with respect to the first center axis at a rotation speed that has been decelerated by the speed reduction mechanism;
• and
• a torque sensor that expands around the hollow shaft and is connected to the casing and the fixed part;
• wherein the torque sensor comprises:
• an elastically deformable strain body that has an annular firsts outer ring and an annular first inner ring;
• and
• a plurality of strain sensors;
• the first outer ring is located at an end portion on a radial outside of the torque sensor and is connected to one of the casing and the fixed part;
• the first inner ring is located at an end portion on the radial inside of the torque sensor and is connected to the other of the casing and the fixed part;
• and
• each of the plurality of strain sensors is at least partially located in a radial direction between the first outer ring and the first inner ring.