I have discovered a few failures of hooks mounted on industrial cranes. These failures are not due to overloading of the hook, but are due to long-term fatigue. In these failures, a fatigue crack propagates in the upper part of the hook neck that is usually hidden inside the pulley block. In typical cases these failures cannot be predicted under routine maintenance. Consequently, the crane user does not get any prior warning of the failure.

The cranes that are most at risk appear to be cranes under heavy and frequent usage. Hook failures are more likely when the crane has intense operation with high loads and regular crane/trolley travel motion. Several rope falls, particularly six or more, tend to multiply the problem.

The fatigue cracks occur because there is a bending moment created in the hook neck. This bending moment arises due to friction, either in the crosshead hinge or in the sheaves. In both cases the friction prevents the hook from aligning fully with a load swinging horizontally. Even though this load swing is small and anticipated in the overall crane design, it is enough to cause fatigue in the hook neck due to a high stress concentration.

The pre-existing hook standards do not consider properly the moment at the hook neck and the high stress concentration as it relates to fatigue. High-tension steels are particularly susceptible.

The traditional crane standards specify the hoist machinery – and therefore the hook – by a classification based on runtime of the machinery, without any precise connection to number of lifting cycles. Fatigue of most elements in the hoisting system, hook included, is however dependent on the number of lifting cycles.

As the shortcomings of these two standards are combined in the final selection of a hook, the result may be inadequate, even though the crane design and the use comply formally with the standards and specified classification.

The standardisation bodies should take these matters into consideration when developing new crane standards.

Cranes, hoists and their components always have a limited useful lifetime specified in the crane or component classification. The actual life of the crane is determined by how well the classification and the real duty match to each other. Until the classification correctly matches the real duty, cranes will continue to fail.

Follow-up of the actual crane duty and comparison to the original classification is an obligation of the crane owner. Crane owners are advised to carry out a special assessment in line with ISO 12482-1, Cranes – Condition monitoring – Part 1: General. In the case of hooks, the assessment must be based on the number of lifts and the amount of hook swing, instead of the machine’s design class.

Hook safety is best assured by a special assessment referred above, by which hooks operating well above their classified duty can be found. Physical crack inspection is a useful tool also, but it only helps for a short period, as it does not take long for a fatigue crack to grow from a detectable size to a critical size. Certified crane inspection organisations and manufacturers may be able to give assistance in hook duty surveys and inspections.

Walter Heinrich,
DNV Inspection senior crane expert, Sweden