Careless torque 02 June 2015

Almost two decades have passed since the DfT published its ‘Careless torque costs lives’ advice on how to crack wheel security issues – yet the problem persists. Brian Tinham examines current thinking

It seems little short of incredible that, in 2015, DVSA inspectors are still seeing loose wheel nuts and stretched studs at roadside inspections – findings that lead to embarrassing maintenance examinations and appearances before the traffic commissioners. Worse, the emergency services are still being called to accidents caused by wheel detachments. Most only result in damage, but some lead to injuries and a few to fatalities. The numbers may be small (about half of one per cent of accidents involving HGVs), but the outcome for all concerned is devastating – and that may well include the truck driver, the operator, its management team and its workshop people.

Why does it happen? The trite response might be that if we knew that, it wouldn't. But we do know. As TRL's 2006 report for the DfT (Department for Transport) 'Heavy vehicle wheel detachment: frequency of occurrence, current best practice and potential solutions' concluded: "The causes [of wheel nut loosening] are now well understood". Paraphrasing, the study states that the clamping load/force (of the wheel to the hub) is critical to security, and this must be sufficiently high "to withstand all applied forces, despite any effects of joint relaxation, variation in the torque-to-clamp ratio, or relaxation due to temperature". It also made the obvious point that this clamping force must not be so great as to exceed the studs' yield point.

Crucially, though, the report went on to assert that current wheel fixing designs (which, to all intents and purposes, have remained unchanged) are capable of achieving those requirements, but only if "joint relaxation is accounted for, with re-torquing, and all components are in very good condition". Just as important: "The factor of safety [provided] means that considerable maintenance of the joint is required to maintain its effectiveness over several years' service."

So there you have it: regular inspection and proper maintenance procedures that absolutely include re-torquing, but also surface cleaning and, by inference, appropriate stud lubrication, are key to ensuring wheel security. It sounds simple enough, but the plain fact that wheel detachments and near misses still occur (thousands of fixing defects are still found at roadside inspections annually – including many with movement indicators in place) says something is going wrong.

How can that be? For the most part, yet again depressingly little seems to have changed since TRL's 2006 report. It made the point that there are several good practice guides for fitting and maintaining wheels (including the IRTE's own publication – see panel), but that, while there is plenty of commonality between them, there is also divergence. That manifests itself particularly in terms of "torque levels and the issue of lubrication, where there is no standard approach". And none has been forthcoming, largely because of variations in detail design, but also differences in individuals' interpretation of the engineering behind achieving and maintaining that clamping load.

No surprise then that, although the vast majority of operators and their workshops (owned or contracted third party) now have formal policies and procedures for wheel fixings and their maintenance, they don't always precisely match the truck, axle, hub or wheel manufacturer's recommendations. No data is available, but the anecdotal evidence is clear – and this problem is amplified as trucks age and non-OEM replacement parts are acquired, especially at breakdowns when the pressure is on to get vehicles back on the road.

Equally, though, just as in 2006, too many drivers do not seem to understand what good practice looks like and fail even to complete their mandatory daily walk-around checks, which should include a visual inspection of the wheels. Sadly, even fewer understand why it matters – which is worrying, given TRL's additional finding that "those drivers or operators who better understood and adhered to [these] requirements reported a lower incidence of wheel fixing problems".

Talking of 'understanding', there is today another factor blamed for some wheel failures: that of incorrect hub and wheel combinations (resulting from the now readily available variety) causing early metal fatigue. John Ellis, managing director of Motor Wheel Service Distribution (MWSD), insists that poor awareness even of the existence of non-circumferential hubs (star and spider – some fitted as OEM) is a serious and largely hidden safety issue.

The European Parliament and European Council agree, and hence their classification of commercial vehicle wheels as 'safety critical', which was enshrined in European Law in the EU Roadworthiness Package in May 2014. That package comprises three European directives, the most relevant of which concern periodic roadworthiness testing and roadside inspection of commercial vehicles. The upshot is that wheels and hubs will have to be routinely assessed for fitness for purpose as part of both those encounters when the UK amends national legislation – which must happen by 20 June 2018. That should smarten up procurement, fitting and maintenance acts.

As yet, however, there is no date, and the DfT's provisional report 'Heavy Vehicle Wheel Fatigue Study', earlier this year, appears to cast doubt on the scale of this problem. Its survey, sent to 11,000 members of the IRTE, FTA (Freight Transport Association) and RHA (Road Haulage Association) suggests that wheel defects are not catastrophic and typically identified during routine maintenance. In particular, it finds no proof that fatigue life is shortened by the use of variable shaped hubs.

Ellis counters that less than 1% (107) of operators responded to the survey and that limiting it to members of organisations that promulgate best practice "skewed the results". He also argues that the report demonstrates a lack of awareness, even among the great and the good of hub variants – which invites the question, how would anyone be aware of the issue of early fatigue caused by incorrect wheels on non-EUWA (Association of European Wheel Manufacturers) approved hubs.

Further, he points to wheels giant Maxion's literature, which warns technicians to heed ISO 4107, DIN74361-3 and SAE 1694, all of which assume hub shapes with a continuous diameter for European standard wheel types. Maxion explains that EUWA members have tested wheels with star shaped hubs and, although they passed relevant tests, reduced fatigue life was experienced.

Either way, MWSD is calling for the DfT to adopt the EU Roadworthiness Package without delay to raise the standards of CV wheel testing. In the meantime, ATS Euromaster, Bus Eireann, Pirelli, Stagecoach and TruckForce are among big names working with the company to improve wheel safety.

IRTE wheel security guide

The IRTE, ATS, FTA and SITA UK best practice guide on wheel security, launched late in 2009, is as relevant today as it was then. And with an introduction that reads: "When wheels become detached from a moving vehicle, they can accelerate up to around 150km/h ... reaching a height of 50m before colliding with other vehicles or road users – at an equivalent force of 10 tonnes", fleet managers are left in no doubt as to the importance of getting this right.

The guide draws on TRL's evidence and cites the same issues around maintaining the clamping force (compression of the wheel, hub and drum together). It agrees that failed or worn studs are the primary causes of incidents (45% and 23% respectively). Key reasons: settlement; insufficient torquing; over-torquing; and incorrect lubrication of threads and interfaces, leading to friction losses.

As for the mechanisms of detachment, it suggests the wheel starts moving relative to the hub, which results in side loadings and loosening of any nuts present. That leads to elongated stud holes, fatigue failure of the studs, fretting fatigue cracks and ultimately catastrophic failure.

The IRTE guide advises that, although nut movement ought to be easy to identify, settlement is more difficult to detect. It also states that 19% of wheel-fixing problems reported by DVSA from roadside checks involve trucks fitted with nut movement indicators or similar devices. Be warned.

Brian Tinham

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ATS Euromaster Ltd
Department for Transport
M Wheels Ltd
Maxion Wheels
Pirelli Tyres Ltd
Society of Operations Engineers
Stagecoach London

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