Understanding the safety implications of ageing cranes
Ageing cranes can pose safety risks, with the potential for lifting gear failure that leads to injuries or death.
Some equipment failure occurs despite the fact that it appears to be operating well within its safe working load and design life. This suggests 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 ensure safe practice it is crucial that users are fully aware of issues surrounding design working period and maintenance responsibilities. Age-related failures are entirely avoidable, provided that all individuals in the chain of responsibility perform their role and understand that cranes require care.
Greater understanding, combined with the ability of computer-aided engineering to enable a less conservative evaluation of stress and strain calculation, has enabled crane 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.
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.
A crane rated at 10 tonnes and intended to perform occasional maintenance tasks will be designed differently, and have different vulnerabilities, from a 10 tonne crane intended for continual use on a production line. 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 in ways for which the equipment was not designed. It becomes very important to maintain a history of usage and to relate that to the design parameters, which may not be easy.
BS ISO 12482 describes a method of monitoring the actual duty of bridge and gantry cranes, relating it to the original duty envisaged in the classification. This enables prediction of when design limits are being approached and, in turn, the timely targeting of special inspections, maintenance and refurbishment.
The duty holder
Formally, the duty holder’s responsibilities include:
- Ensuring that cranes brought in (bought or hired) are fit for purpose.
- Creating a risk assessment using manufacturer’s data, environmental details and usage information to identify critical or vulnerable components and determine maintenance and inspection intervals.
- Ensuring that cranes are maintained, inspected and thoroughly examined to ensure they are safe to use.
- Ensuring that they are not unduly susceptible to foreseeable failure modes.
- Keeping records of crane use, maintenance, inspections, repairs, modifications, exceptional events and so on so that the history of the crane and thus its remaining safe life can be determined. The duty holder also has to keep and supply information required so that anyone modifying or upgrading the crane can calculate a revised DWP.
- Ensuring that cranes are overhauled or replaced before they reach their DWP.
Unless the company is actually a crane specialist itself, the duty holder will probably not have the required level of knowledge to carry out all of the above adequately and so will delegate to persons competent for the required task. The duty holder is responsible for ensuring that any remedial actions that are flagged are fully and promptly acted upon.
Examiners, inspectors and maintenance staff
The ‘thorough examination’ by a ‘competent person’ is carried out in accordance with statutory requirements. Routine inspections of critical features may be carried out in-house, perhaps by maintenance staff, to a frequency determined by risk assessment — which they may have been responsible for creating.
In accordance with statutory requirements, a competent person will take into account the age of the crane using actual or estimated information on the DWP expended to decide what actions to take or recommend, from deeper inspections to partial or complete disassembly, or precautionary replacement of parts. Records of inspections, actions and recommendations need to be kept and passed back to the duty holder. Planned and preventive maintenance may reset the DWP clock for particular components or assemblies.
Manufacturers, modifiers and resellers
Suppliers have the responsibility of providing all the information necessary for classification and assessing DWP. Modifiers are responsible for assessing and informing the duty holder of the remaining DWP.
Those buying, hiring or letting a contract for crane services have a responsibility for ensuring that the specifications they issue, and the bids they accept, properly reflect the likely usage of the crane, and that it will be able to perform safely under all foreseeable conditions of use, for the specified classification of crane duty, as determined by risk assessment. They must ensure that equipment meets, and is marked and documented for, all relevant safety requirements and standards.
Further information can be found in ‘Guidance on roles and responsibilities for crane Design Working Periods (Document Reference LEEA 074)’ and ‘Roles and responsibilities for ensuring continued safety of in-service lifting equipment (Document Reference: LEEA 072)’, available to download from www.leeaint.com.
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