Why Wheel loss occurs and how it affects the road transport industry
Affecting operators of commercial vehicles across the globe, wheel loss is a serious issue. The wheel nuts that secure the wheel to the hub can loosen for a number of reasons with the risk that, in certain circumstances, the wheel will completely detach with possibly fatal consequences. The costs to the operator of suffering wheel nut loosening and wheel loss can be enormous and can include prohibition costs, vehicle recovery and repair costs, vehicle down-time costs, increased insurance premiums and legal costs, not to mention the potential damage to the company’s reputation.
Although there is a huge amount of anecdotal evidence for incidences of wheel loss and wheel nut loosening, there is little in the way of statistics. The 2007/08 Department for Transport sponsored study entitled ‘Heavy Vehicle Wheel Detachment’ estimated the typical annual frequency of these incidents in the UK to be as follows:
- Wheel fixing problems between 7,500 and 11,000
- Resulting in between 150 and 400 wheel detachments
- Of which 50 to 134 would result in damage-only accidents, 10 to 27 in injury accidents and three to seven in fatal accidents
Although this research is continuing with results published in Autumn 2008, one of its four initial recommendations is:
“Potential mandatory use of wheel nut retention devices or movement indicators, if independently shown to be effective”
Wheelsure’s own independent market research also provides further evidence of how this problem impacts on fleet operators. In a survey conducted amongst 400 fleet operators:
- 83% of respondents had experienced wheel nut loosening at some time in their career
- 62% of respondents had experienced wheel loss in their career
- 32% of operators are concerned that despite their best efforts, wheel loss can still occur in their fleet
Fleet operators deal with the problem through maintenance regimes ranging in the most extreme cases from daily torquing regimes through to more orthodox service-based processes. Wheelsure’s products offer an opportunity to manage fleets more confidently and in a manner that will reduce operational costs. However, in order to explain why this is true, it is necessary to understand the science that lies behind vibration wheel nut loosening.
Bolt science and the true cause of wheel loss
Wheel detachments are often described in the newspapers as “freak accidents”. This reflects an everyday lack of awareness amongst the public that contrasts sharply with the research undertaken by Wheelsure amongst professionals in the industry. Wheel loss can be explained very simply by understanding the science that lies behind bolted joints.
The invisible problem: Clamp force and torque
Almost without exception, the wheel of a commercial vehicle is held in place by 6-10 bolts in an assembly known as the spigot or hub-mounted wheel. These bolts have been designed to deliver the correct amount of clamp force required to more than adequately hold that wheel in place when new. However, as the bolts age and degrade, the clamp force diminishes to a point where it is possibly insufficient to secure the wheel adequately.
When the clamp force diminishes, a variety of factors may then conspire to force wheel nuts to loosen or wheels to completely detach, such as:
- Joint settling after initial tightening and failure to re-torque
- Poor condition of the nuts and damaged threads
- 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
Independent vibration testing carried out by Bolt Science on Wheelsure’s devices using a Junkers test machine, clearly illustrates how rapidly a loose wheel nut can back off completely.
See HERE for a demonstration of this.
How often does a wheel get removed for maintenance? Six times a year or more at least when considering brake services, tyre rotation and punctures. This figure could be even higher in some waste management fleets, local authority and public service fleets, where punctures are an occupational hazard.
Each and every time a wheel is changed there is an opportunity, even in rigorously maintained fleets, for human error to occur. Add into the mix the complexity of remote depots, 24 hour logistics operations and third-party subcontractors and it becomes clearer to understand how mistakes can occur.
It is this combination of a maintenance regime that relies purely on accurate torque – without any knowledge of the clamping force achieved-and the potential for human error, which gives rise to the circumstances where wheel loss can occur in even the best managed fleets. Wheelsure’s innovative products are truly unique and the only fail-safe means of preventing unintentional nut-loosening caused by vibration.