Wheelsure - Clamp force: still at the nub of a deadly problem

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Clamp force: still at the nub of a deadly problem
Article courtesy of Transport Engineer magazine

Most truck and bus operators in the UK now recognise the serious risks associated with lost or loose wheels and employ various ways of minimising it, acknowledges Wheelsure managing director Gerhard Dodl. But one crucial engineering fundamental continues to be overlooked.

I read this month about yet another tragic road death caused by a wheel-loss incident. This time it was a 39-year-old woman from Stafford whose car was hit by a wheel from a skip lorry. In November it was a 56-year-old man from Irvine who died when his car was struck by another vehicle which had moments before been hit by a loose wheel. In October it was a 44-year-old mother of five in Cornwall who died when she was hit by a wheel as she walked home from taking her children to school. Then just last month, Irish president Mary McAleese’s car was hit by a wheel that had become detached from a commercial vehicle. Luckily this time there were no injuries. I can’t help asking myself: how many more people have to die needlessly?

My experience over the past 15 months or so since starting the Wheelsure business is that the majority of operators take this issue very seriously and employ various methods by which to try and ensure the safety of their fleets and other road users. The fundamental point still being overlooked, however, is that even the most rigorously maintained fleets will suffer the risk of wheel loss. Let me briefly explain why I arrive at that conclusion.

Wheel loss is caused simply by a loss of clamp force. From the very first wheel-change onwards, our research shows that clamp force can diminish well below the recommended levels, even when the correct torquing regime is being followed. The reason for this is that of all the torque energy applied to a wheel-nut, 90 per cent is used up in overcoming the friction in the threads and bearing surfaces, leaving only 10 per cent to generate the clamp force necessary to keep the wheel in place.

Any one of several factors such as contamination from grease, rust or paint, incorrect fitting, consistent over-tightening, excessive heat or human error can then give rise to a further disastrous loss of clamp force. This invisible menace leaves the fleet operator with a truly difficult task as regular retorquing will not improve the situation (see illustration below).

In this example, all the wheel-nuts have been torqued to 670Nm. But tests revealed a clamp-force range of between 56kN and 114kN, well short of the Society of Automotive Engineers’ recommended levels of between 180 and 280kN. In fact, the readings found were low enough for a wheel loss to occur within a few miles if factors beyond the control of the operator, such as road vibration, acted as a catalyst. It is often assumed that as long as the proper level of torque is applied to wheel-nuts, using a calibrated torque wrench, the required clamp force will be achieved. This assumption could have devastating consequences.

Wheel loss and wheel-nut loosening cost UK operators tens of thousands of pounds daily in prohibition costs; vehicle recovery and repair costs; vehicle down-time/lost loads; increased insurance premiums and legal costs, to say nothing of the cost of a lost life. How much is your professional reputation worth to you? I know of an operator who recently paid £15,000 to recover, repair and get back on the road a vehicle that suffered an accident as a result of a wheel loss. Even routine vehicle maintenance to prevent wheel-nuts from loosening (torquing wheel-nuts, replacing damaged nuts and studs) costs an estimated £2,500 per vehicle over a five-year period.

So where do we go from here? What we need is an engineering solution to an engineering problem. As professional engineers we need to view the bigger picture rather than looking at wheel-loss in isolation. By this, I mean taking a long-term approach and working with OEMs (original equipment manufacturers) to bring about product improvements while taking short-term steps that eliminate risk in a practical and cost-effective manner. ~

Let us follow the example of the safety-critical aviation industry where they employ both rigorous and disciplined maintenance as well as failsafe fastening systems. We can do the same today and do so economically. What are we waiting for?

Gerhard Dodl is managing director of Wheelsure, the Ampthill, Bedfordshire-based company that markets the Wheelsure wheelnut-locking device.

This article was originally published in Transport Engineer, July 2005

Sales: 01525 840557 - Fax: 01525 403918 - E-mail: sales@wheelsure.co.uk
WS Group Limited - 8 Woburn Street, Ampthill, Bedfordshire, MK45 2HP, United Kingdom