a helicopter rescue hoist - traction sheave assembly including a hoist cable engaging sheave and first housing , a drive motor having a drive shaft mounting the sheave and a second housing mounting the motor and positioned in adjustable relation to the sheave housing , a rocker arm fixed to the motor - mounted housing , an alarm switch located near the rocker arm , and a torque compensating spring attached to , and normally retaining the rocker arm in a neutral position out of engagement with the switch when the drive motor is applying a normal amount of traction to the cable through the sheave and automatically moving the rocker arm to an engaged position with , and closing the switch in the event of a malfunction resulting in a substantial reduction in the motor - applied traction .