Traction sheave warning for helicopter rescue hoist systems

In a hoist employing a winch and cable, a traction sheave mechanism for maintaining tautness in the cable at the winch, particularly in the absence of a cable load, said mechanism comprising:

• a traction sheave housing;
• a traction sheave attached to a shaft rotating in bearings in said housing, said traction sheave frictionally engaging the cable from the winch;
• a motor support structure mounted on said sheave housing by means of a bearing permitting rotation of the support structure about the axis of said shaft;
• a motor mounted on said support structure;
• means including a slipping clutch for coupling the motor to said shaft for continuously applying a torque to said sheave in the direction to tension the cable between the sheave and the winch;
• a spring attached to said sheave housing and to said support structure for exerting a rotational force on the support structure that counterbalances the normal traction applied to the cable by the sheave;
• and means responsive to a rotation of said support structure relative to said sheave housing by a predetermined amount in the direction of said spring force to actuate an alarm signalling a low value of sheave traction on said cable.