6.4.11 Hoisting and Lowering Limits

For safety reasons, to prevent the bottom hook ‘over travelling’ and causing damage to the hoist, a hoisting or upper limit is used. Most power operated hoists are also fitted with a bottom limit to prevent the bottom hook from lowering beyond the designed extended dimension, but it should be noted that this is not always the case, particularly with older hoists. The type of limit used will depend on the hoist design and may be a mechanical device, e.g. a slipping clutch, or an electro mechanical device which uses a mechanical method of actuating a limit switch. Whichever type is employed, hoist limits are not intended for regular use, they must be considered as emergency safety devices. Where positional limits are required, a secondary system should be used, and most manufacturers are able to supply these.

A slipping clutch is generally only used on power operated chain hoists. This is placed between the motor and the final drive. It employs spring loaded discs which will slip when the bottom hook or a stop pushes against the underside of the hoist body framework when hoisting. Similarly, a stop may be fitted to the slack end of the chain to activate the clutch when lowering. It should be noted that the motor keeps running when the clutch is operated and that the operative must release the motion control button to stop the motor and thereby apply the brake.

There are several electro-mechanical methods of actuating a limit which all utilize the movement of the mechanism to disconnect the power to the motor and thereby apply the brake.

In most cases, hoisting and lowering limits are easily reset by reversing the direction of the hoist however in some cases manual resetting may be necessary.