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THE WHEEL 0 RI, •SR:11111114E
Page 46
Page 47
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If you think good maintenance, frequent checks and regular re-torqueing is enough to guarantee against wheel loss. Think again. Andy Salter reports.
. A ccording to the Vehicle and Operator Services Agency, there
have been more than 2,500 incidences of wheel nut/stud
defects in each of the last three years — the majority of which resulted in a prohibition Road transport safety pressure group Brake reckons 10 people per year are killed in the UK by runaway wheels and traffic commissioner Tom Macartney recently said -in the past year, 20 people have been killed by detached wheels"
Tragically, last October, a mother of five was killed in Cornwall as a result of a wheel loss incident, which has prompted discussion and investigation in the Houses of Parliament Into the extent of the problem and the solutions.
Transport solicitor Backhouse Jones daims to deal with "cases involving wheel security issues on a daily basis". However, the problem is not confined to the UK. During 1997 in Ontario, Canada, there were 215 reported wheel loss incidences, which prompted the Provincial government to bring in very stringent penalties against hauliers, both for detached wheels and loose or missing wheel nuts. As a result, trucks and trailers are impounded and a penalty of up to $50,000 can be issued
So why does wheel loss occur? The answer is simple. wheel loss and wheel nut loosening are caused by a loss of "clamp force". The design of the spigot or hub-mounted wheel relies totally on the clamp force on the studs to prevent the loosening of the nuts. So when the clamp force diminishes, a variety of factors may then conspire to force wheel nuts to loosen or wheels to completely detach.
These reasons might include: Joint settling after initial tightening and failure to re-torque • Poor condition of the nuts and damaged threads
4 Wheel bearings over heating
Poor road conditions Mating surfaces contaminated by paint, oil, grease or dirt Inadequate vehicle maintenance Differential heat expansion in the joint components Brakes not properly adjusted High wheel vibration Low bolt hardness (not to specification) Nut or bolt threads out of specification The reason for clamp force diminishing is more technical. According to leading wheel nut expert John Owen. When fasteners are new they have an efficiency of approximately 10%; of the 100% of torque applied to a nut, 90% is used overcoming friction in the stud threads and bearing surfaces, leaving only 10% to develop the clamp force' Tests carried out by SE Systems recorded the clamp force developed by a fixed torque of 650Nm.The average reading on n: nuts was 206kN, but old nuts in poor condition exhibited only 95k clamping force Once wheels are removed, the bolt threads beco damaged and mating surfaces can be contaminated with grease, oil and paint, causing higher frictional resistance, resulting in the c force dropping So with the exception of new studs and bolts, 650 of torquing energy will probably not provide sufficient clamp force adequately secure a wheel Many have blamed poor maintenance as a reason for wheel loYet, while excellent maintenance and torquing regimes are essen help reduce the risk of wheel loss and wheel nut loosening, they eliminate the risk completely. A lack of clamp force cannot be see regular re-torqueing will not improve the situation.
In our scenario, all the nuts have been torqued to 670Nm. but w tested a clamp force range of between 56kN and 114kN was foun This falls well short of the SAE's recommended levels of between 180 to 280kN. In fact, the readings were low enough for a wheel loss to o cur within a few miles, if factors beyond the control of the fleet engine: r, eg of rust, amp s.
ial to ill not and road vibration, acted as a catalyst Many assume that as long as the proper level of torque is applied to the wheel nuts. using a calibrated torque wrench, then the required clamp force will be achieved, but this assumption could have devastating consequences.
The costs of suffering wheel nut loosening and wheel loss can be enormous and include prohibition costs, vehicle recovery and repair costs, downtime costs, increased insurance premiums and legal costs, not to mention potential damage to the firm's reputation To help avoid this scenario several areas should be considered: Ensure all vehicles follow a rigorous maintenance regime, replacing any wheel nuts/studs when damaged with the correct tools and good quality replacements.
Devise wheel maintenance and torquing procedures that are followed every time a wheel is replaced. It should incorporate manufacturer's recommendations and include re-torquing after 30 minutes.
Ensure staff are fully trained in these procedures and that only trained staff carry them out. Implement a daily driver defect reporting system and issue drivers with the necessary tools to check/adjust wheel nuts.
Consider fitting a wheel nut locking device.
There are visual and mechanical aids available to assist in managing this problem. Wheel nut indicators provide drivers with a visual aid. However, as clamp force is not visible, indicators can be used as an aid only to determine if a wheel
"A wheel nut indicator nut has moved, they are not an indicator of clamp force. Even then a wheel nut indicator movement of 15 is unlikely to be picked up by even the most vigilant driver, and this is likely to mask a
significant loss of clamp force.
A second category of products can be described as locking wheel nuts and their objective is to lock the wheel nut in place. Although they cannot develop clamp force that does not exist, their purpose is clearly different to the indicators described above. The latest type of product, such as that marketed by Wheetsure, is a locking device that features both a standard thread and a reverse thread. This device leaves the existing standard nut in place, but adds a patented locking cap that automatically tightens if the nut shows the slightest sign of loosening. Operators can fit the device to vehicles utilising the existing bolt, provided it is in good condition.This helps to keep costs down and enables fitting to take place during scheduled routine maintenance.