In July 2014, a mobile crane at a mine site was being used to unload a truck. After a daily pre-start inspection, the crane took the weight and lifted the load. As the crane started to slew, the nylon head sheave (rooster sheave) weighing about two kilograms, catastrophically failed. Part of the sheave fell around 10 metres to the ground, narrowly missing a rigger working in the exclusion zone below.
When the sheave was inspected later, it appeared to have been damaged prior to the lift. The weight of the load was within the crane's safe limit. There was no side loading to the boom head sheave during the lift. The sheave was an original equipment manufacturer (OEM) supplied part.
Direct causes
The damaged head sheave failed and fell from height during a lift.
Contributory causes
Damage to the head sheave was not identified during the daily pre-start inspection of the crane by workers.