A spate of serious incidents has prompted a warning for crane owners to ensure hoist limits always comply with the manufacturer's instructions.
In Queensland, there have been three incidents in three months where hook blocks have fallen from crane booms—two on tower cranes and one on a mobile crane. All three were avoidable and occurred because the hoist limit didn’t stop the hook block before it ‘two blocked’.
Hoist limits are designed to stop the crane’s hook block hitting the crane boom—known as ‘two blocking’. Winch motors on cranes are powerful and can often cause the winch rope to break or other parts of the crane to fail if the upwards motion of the hook is not stopped by the hoist limit switch.
All cranes (except some pick-and-carry mobile cranes) designed to two block, are fitted with hoist limits. On most mobile cranes, the hoist limit includes a small weight hanging from a chain hanging from the boom tip. As soon as the upward motion of the hook block hits this weight, a switch is activated, and various crane functions are disabled. On most tower cranes, the limit switch is a geared mechanism attached to the hoist rope drum, which includes a deceleration limit and a final stop limit.
In the two tower crane incidents, the hook block weighing around 400kg fell 50m and 100m respectively causing significant damage below. In one, the hook block nearly punched through a slab then bounced and destroyed an air conditioning duct. In the other, the hook block hit a perimeter screen then landed in the garden of an apartment block next door to the site. In the mobile crane incident, the hook block was pulled over the boom tip and hit the crane deck. All these incidents had the potential to be fatal.
It’s important hoist limits are set up following the crane manufacturer’s instructions, with the hook block stopping well before it can physically hit the boom tip of the crane. However, this may not guarantee that two blocking will not occur—if there is any doubt about the limit switches being able to stop two blocking, seek guidance from the crane manufacturer. The hoist limit must allow for variation in subsequent applications of the hoist limit and the added momentum when the hoist winch is in high speed. The hoist limit needs to be adjusted for the number of rope falls (lines of rope). For example, for a single fall the hook block will approach the boom much faster than a crane with four falls of rope based on the same winch speed.
When packing up a luffing crane, the operator should hoist up before raising the boom, so that the hoist rope between the boom tip and the hook block can be seen.
During the daily inspection, the operator should always check the hoist limit switch is functioning correctly before lifting.
The only legitimate reason for temporarily overriding the hoist limit switch is for rigging purposes, including when the crane is commissioned, dismantled or the rigging set up on the hook block is being changed.
When selecting the tower crane for a site, it is vital the hook height is high enough to carry out lifting activities without need to override the crane’s safety features.
Read the Mobile and tower crane hoist limits.