LEEA column: The life of a crane

In-service failure of cranes and other lifting gear has many causes, from criminal negligence downwards. One recurrent theme is the failure of equipment that appears to be operating well within its ‘safe working load’ and ‘design life’. Such incidents are seen both in ageing and in relatively new equipment and suggest that those responsible for safe maintenance and operation do not always fully understand the ‘life cycle’ of the crane and its implications for safety. To concentrate directors’ minds it is worth noting that cranerelated accidents can have severe consequences not just for those involved, but also those responsible.

Age-related failures are entirely avoidable provided everyone in the chain of responsibility performs their role, and understands that the modern, finely engineered crane does not necessarily last for ever. They need to be looked after.

Part of the problem may be that the whole approach to the design of cranes has changed in relatively recent times and the simple concepts of ‘design life’ and ‘safe working load’ are not really valid in modern practice.

Greater understanding, combined with the ability of computer aided engineering to enable a less conservative evaluation of stress and strain calculation, allows engineers to design close to the ‘limit state’, not just in terms of static loads but in terms of cyclical or fatigue loading, and in the effects of wear and tear. That has yielded great benefits in terms of initial cost, ease of transport and assembly, and indeed the giant tower cranes seen on construction sites today would not have been possible under the old design approach.

However, this also means that cranes are designed, and classified, for particular patterns of duty. Structures are designed for a lifetime measured not in calendar years but in working cycles, and mechanisms similarly for a life in running hours. Working cycles are related to the load spectrum – the average load handled by comparison with the nominal rated load.

So, for example, a crane rated at 10t and intended to perform occasional maintenance tasks will be designed differently, and have different vulnerabilities, from a 10t crane intended for continual use on a production line. If the inspection, maintenance and use of the crane takes due account of what is now a quite complicated specification – not a ‘design life’ but a ‘design working period’ (DWP) – all should be well, but there is little margin for error.

This does not just apply to structures – there are similar considerations around motors, brakes, wire ropes and other elements. Various components and assemblies may have DWPs, which are not the same as that of the crane as a whole.

It is easy to see how things can go wrong. The intended usage may have been inadequately defined when it was bought or hired; usage may change, perhaps because production increases; it may be used for purposes it wasn’t intended for. It becomes very important to maintain a history of usage and to relate that to the design parameters: that may not be easy – for example, with a hired crane, or when a new owner takes over a site or factory with cranes already installed. It isn’t enough to depend on the periodic inspections and examinations specified under Loler (Lifting Operations and Lifting Equipment Regulations) – incipient failure, whether in structural members or, for example, in wire rope cores, may not be readily visible.

‘BS ISO 12482 Cranes – monitoring for crane design working period’ describes a method of monitoring the actual duty of bridge and gantry cranes relating it to the original duty envisaged in the classification. That then enables the prediction of when design limits are being approached and, in turn, the timely targeting of special inspections, maintenance and refurbishment. The philosophy of BS ISO 12482 should apply to all cranes.