Most common system being used in the US is the "rope gripper" manufactured by Holister-Whitney. The device mounts on the machine bedplate or machine beams under the machine area and surrounds the hoistropes so the ropes pass through the jaws of the device. When a over speed condition is determined the jaws set tight against the ropes "gripping" them (think of holding a straw betwen your fingers and try to pull it in either direction, it is easy to hold the straw tight with little force) and stopping movement.

Hi,the double brake system is to hold the motor sheave when there is no physical movement in the motor. Usually the brake force is measured in Newton which is the force applied on the motor sheave to block the movement.

As per the EN 81 the double brake is given in the motor for the safety reason. so the coil of the brake units are to be given in parallel connection. in case if one brake coil fails, the other will try to hold the sheave but there will be a lot of slip depends on the load condition.

The rope grippers are provided for additional safety which are working in parallel to the brake on the motor sheave.

Hope my answer is apt to your question.