Properties of the intermittent linear winches
Two movable grippers
The intermittent linear winch consists of two grippers mounted in a frame. One of the grippers is able to move back and forth (extend and retract) through the actions of a set of hydraulic main cylinders. When moving, this gripper pulls a steel wire rope, that is passed through the machine.
Load holding wedges
The grippers can hold the wire since each gripper is fitted with wedges. These wedges will clamp onto the steel wire rope. When the moving gripper is pulling, the wedges in this gripper hold the wire rope, moving the rope while it slides through the wedges of the other gripper. The wedges can be opened and closed by hydraulicly powered wedge cylinders inside the gripper.
When the moving gripper moves back and forth (extends and retracts) the other gripper is stationary in the winch frame. The stationary gripper will hold the steel wire rope when the moving gripper moves back to its starting position, resulting in intermittent motion. The design of an intermittent linear winch allows it to build a compact machine which is suitable to work on either offshore platforms with limited or in area’s with limited space and/or crane capacity.
The wire rope pull-in is usually automatically controlled, and a winch operator can adjust the pull speed and the load. The movement of the gripper can be sequenced to either pull-in or pay-out.
Fail to safe
In both the manual and automatic modes, one of the gripper wedges always maintains contact with the wire rope. Since the load-holding wedge is automatically locked and cannot be opened, control of the load is always maintained. Only when there is no load or when the load is released after a pull operation, both wedges can be opened. This allows for inserting and removing wire. The system is “Fail to Safe”. In other words, when a sudden pressure drop occurs, for example when the emergency engine stop is activated, springs will automatically close the wedges and the wedges will hold on to the steel wire rope.