Limit switches prevent over-winding and over-lowering of the hook block. The upper limit prevents the hook block coming Into contact with the rope drum and sheaves.
The lower limit will keep two full turns of rope on the drum when the hook Is In Its lowest working position, and may prevent the hook making contact with the ground. Limit switches must be tested by the
driver before the beginning of each shift.
Most cranes have two upper Ilmit switches to prevent overwinding:
Some older cranes are fitted only with a whole current self-resetting upper limit switch.
The shunt/working upper limit switch Is the first to come Into operation and will stop the hook block before It reaches the whole current non-self-resetting limit switch.
The whole current non-self-resetting upper limit switch Is set above the shunt/working limit switch and only operates If the shunt/working limit switch falls. It will stop all hoisting motion until the fault Is fixed.
A qualified electrician must be called to reset a non-self-resetting upper limit.
The whole current self-resetting upper limit switch Is used where there Is no other upper limit switch. It will self-reset once the lower control Is activated.
Most cranes have a shunt/working limit as a lower limit. If the crane operates above a single level floor this limit prevents the hook coming Into contact with the floor, causing a slack rope.
Crane drivers must be careful about using the limit switches as a method of stopping the load at the maximum height. The contacts can burn out If this Is done to excess.