Mechanical joint, mechanical arm, and self-moving cleaning device

A mechanical arm, comprising a mechanical joint which is configured to connect a first arm to a second arm in a mechanical arm, wherein the mechanical joint comprises:

• a motor, wherein the motor comprises a first output shaft, a motor base plate, a stator and a rotor, the rotor is located outside the stator, the first output shaft penetrates through the motor base plate, and the stator is connected to the motor base plate and located on a circumferential side of the first output shaft;
• and
• a planetary speed-reducing mechanism, which comprises a primary gear set and an output end, wherein the primary gear set is connected to the first output shaft and arranged adjacent to the motor base plate, and the output end is configured to drive the second arm to rotate relative to the first arm.