Hoist Mechanism

The hoist mechanism is the winch that raises and lowers the load. It sits on the counter-jib, behind the slewing centre, so its own weight works as part of the counterweight.

Construction

Grooved hoist drum, planetary or helical reduction gearbox, service and emergency brakes, and a wound-rotor or frequency-controlled motor. The drum carries the hoist rope through a slack-rope limiter and an over-hoist limiter before the rope runs out to the jib.

Role on the machine

It sets the hoisting speed and, more importantly for precast work, the inching speed — the fraction of a metre per second at which the load can be brought to the fixing point. Frequency control also gives smooth acceleration, which keeps the rope from swinging when a heavy panel starts to move.

Inspection and maintenance

• Check drum rope spooling and the rope for broken wires or kinks; replace on the discard criteria, not on appearance.
• Test the service brake, the emergency brake and both limiters before each shift.
• Check gearbox oil level and look for leaks at the drum bearings.