The purpose of this safety alert is to highlight the risk of rope sheave failure on mobile cranes. Crane owners, crane operators and workers involved in rigging cranes should apply the principles in this alert to any crane with rope sheaves.
In May 2019, a plastic (nylon) hoist rope sheave on a 130-tonne mobile crane catastrophically failed and broke into pieces during an attempt to lift an 8 tonne load on the main winch rope (refer Photograph 1). As the sheave failed, the load dropped a short distance and applied shock loading to the hoist rope, damaging it. The main winch was rigged with three parts (falls) of rope.
Fortunately, no-one was injured and damage to the crane was minor. However, the incident had the potential for more serious consequences if the lifted load was heavier. With a heavy load, failure of a rope sheave can cause complete failure of a wire rope.
The sheave that failed was a top sheave on the end of the hydraulic boom. On this crane the hoist rope passes through the top sheave and then down to a sheave on the bottom of the boom tip before passing down to the hook block.
It appears the main cause of the incident was that the hoist rope applied a side loading to the sheave, caused by misalignment of the hoist rope. Instead of the hoist rope passing from a top sheave to the sheave directly below (so that it is close to vertical), it passed from the top sheave to the third sheave along (refer Photograph 2). Sheaves are designed for loads to be applied in the direction of the sheave, so that the sheave can freely rotate with negligible side load.
It is acknowledged that when rigging a mobile crane in multiple falls (parts) of rope, there will be a slight rope angle between sheaves due to the position of consecutive sheaves. As the hook block is hoisted down this angle will reduce, and any side loading applied to the sheaves will also reduce. However, for a misaligned rope on the end of a crane boom, the distance between the top and bottom sheaves stays the same and the associated side loading remains constant.
Photograph 1: Failed sheave segments
Mobile cranes are to be rigged in accordance with the manufacturer's instructions. The rigging set up should ensure when a rope passes from one rope sheave to another, the consecutive sheaves are aligned as closely as possible. If inspection shows that the reeving is incorrect and excessive side load is being applied to the sheaves, the sheaves should be thoroughly inspected and, if necessary, replaced with new sheaves that comply with the crane manufacturer's specifications.
Regular inspection and service of rope sheaves should be carried out in accordance with the crane manufacturer's instructions during the crane's periodic safety inspections. Damaged and worn sheaves are to be replaced. At the major inspection, all rope sheaves should be removed from the crane and closely inspected for both their condition and operation. Sheaves should not be painted.
Photograph 2: Example of correct and incorrect rigging on boom head
See more information here