Wheels coming off minibuses, buses and coaches, while not a common event, still happens all too often. But, says the DVSA, which has issued new updated guidance, there is no mystery as to why it happens and you can prevent it.
Some operators blame the ending of left-handed threads, others the ‘mystery’ of wheel loss. But, as the Driver and Vehicle Standards Agency (DVSA) points out in its newly-updated guidance to operators, wheel loss is no mystery.
Its free booklet Careless Torque Costs Lives sets out what operators can do to stop wheel loss and the best practice that should be adopted.
The buck stops here
â€œToo often wheels become detached from commercial vehicles during use,â€ says the DVSA. â€œThese wheels can cause serious injury or death. Many people refer to loss of wheels from commercial vehicles as a mystery.
â€œResearch shows that there is no mystery. Careless torque was found to be one of the reasons for wheel loss.
â€œSomeone is responsible for wheels becoming detached from commercial vehicles.
â€œFollowing the guidelines given here may help to ensure that it is not you.â€
As an operator you are responsible for having correct procedures in place. Your drivers are the first line of defence as a proper daily walk-round check means that they should be able to spot the tell-tale signs.
However, after a wheel is replaced, your fitters also have a responsibility.
In the worst-case scenario, if you don’t have proper systems in place, or don’t stick to them, and your negligence can be proved as a factor, then you and your company could both face a corporate manslaughter charge in the event of a fatality.
Care of wheels
The DVSA recommends making wheel-fixing maintenance an important part of your maintenance schedule. In particular:
- When refitting wheels lightly oil all wheel fixing threads and lubricate the nut to captive washer interface
- Always use a calibrated torque wrench to tighten wheel fixings; do not use power tools or long bars for final tightening
- Tighten to the vehicle manufacturer’s recommended torques or procedures and in the proper sequence; if a sequence is not quoted use the principle of gradually tightening approximately diametrically opposed nuts in turn
- Check for cracks in wheels especially around the fixing holes, and in studs, nuts and washers; if in doubt, renew
- Check for wear and distortion of wheel and nut seats and seating areas; over tightening on cone or spherical sets may have raised a lip around the edge which will affect seating of twin wheels; if in doubt, renew
- Check all mounting interfaces â€“ hub-mounting face, wheel faces and nut/washer faces must be free from corrosion, damage and dirt
- Pay special attention to paint â€“ it may improve the appearance but could be fatal; any paint on the interfaces should be microscopically thin; paint softens under heat generated from braking and will cause looseness
- Ensure that wheel fixings are checked regularly, preferably at the start of each shift; wheel fixings can be checked for any looseness by using a calibrated torque wrench, a socket and short bar or maybe by striking the nuts with a small hammer and listening to the noise generated; if the driver is made responsible for this check, make sure they are properly trained
- Do not simply retighten very loose wheel fixing or wheels which repeatedly become loose; find out why they are loose and whether any damage has been caused
- Use trained personnel and keep records of all wheel and fixings work, including which parts were renewed and when.
There has been a lot of research into the problem of wheel loss; major findings were:
- The use of low-quality replacements parts. Replacement parts from vehicle manufacturers were â€œgenerally satisfactoryâ€ but parts from other sources were found to have poor quality machine-cut threads and excessive tolerances including thread-to-face or seat squareness
- There is a British Standard specification for road wheel nuts, studs and bolts for commercial vehicles, BS AU 50, Part 2, Section 3, 1994. Included in this standard is a recommendation that threads and serrations should be formed by rolling rather than being machine cut; the standard is not mandatory but DVSA advises the use of parts which conform to the standard
- Early relaxation of tension in the wheel fixing after initial tightening following wheel refitting. The wheel stud or bolt is like a very stiff spring that stretches when the fixing is tightened; settlement, even with the vehicle stationary and not subject to vibration, reduces the stretch and the wheel is not clamped sufficiently tightly
- To correct this relaxation, the wheel fixing should be retightened to the recommended torque after 30 minutes if the vehicle is stationary or within 25-50 miles (40-80km) if the vehicle is used.
This relaxation will take place regardless of whether the thread of the fixing is right hand or left hand. Regular checking for looseness will still be necessary.
Types of wheel fixing
Most heavy commercial vehicles now use spigot-mounted wheels where a central hole locates the wheel on the hub. The nuts have captive, rotating, flat-faced washers.
Other vehicles may have wheels with straight-sided cone seat type fixings (BS Conical) or spherical seat type fixings (European, Din Standard) â€“see diagrams (below).
These different types are not interchangeable: make sure you use the correct type.
Composite or dual-purpose wheels are available which have, either, cone or spherical seating plus the spigot location. The DVSA says: â€œTheir use is not recommended.â€
By all means investigate the use of the various alternative locking or safety devices, says the DVSA, but consider:
- Whether they address the symptoms or the cause of the problems. Remember that the research showed that early relaxation occurred without the vehicles moving; therefore, without vibration or rotating
- Preventing nuts from rotating and thus completely unscrewing from the stud may avoid some wheel loss incidents, but loose wheel nuts still cause wear and eventually failure
- Make sure that if you use alternative fixing devices, you do not reduce your maintenance or driver daily checks.
There is the danger that serious injury, or death, can occur from a wheel loss incident, and that is aside from the bad publicity and unwelcome media exposure â€“ and the inevitable appearance at a court, then a Public Inquiry.
And an increase in your insurance premium. And, if the worst happens, the potential for a corporate manslaughter prosecution.
It’s all about setting standards, maintaining them and enforcing them. This new guidance is a helpful addition to the weapons in your arsenal to guard against wheel loss.
Many of the recommendations, plus further details, are given in a British Standard Code of Practice for the selection and care of tyres and wheels for commercial vehicles, BS AU 50 Part 2 Section 7a 1995.
British Standards publications are available to buy at www.bsigroup.com.
Adds the DVSA: â€œMainten-ance may be onerous, it may be a costly nuisance but we are not yet in the age of the maintenance-free commercial vehicle.
â€œLook after your wheels and there is a greater chance of them staying where they should be: On your vehicle.â€