Overload protection devices instantly cut off input and output when an overload occurs, protecting machinery. Overloads are detected accurately and the drive key removed from the groove to cut off input and output. Once the overload is resolved, operation can continue automatically at the same torque from the same indexing position. Torque is set using a built-in torque spring, so there is no troublesome fine-tuning. Torque limiters also serve as flexible couplings, and can be used either by butting two shafts together or by wrapping. Since power is transmitted by a drive key, they can be used safely even in oil.