Careless Torque Costs Lives
Too often wheels become detached from commercial vehicles during use. These wheels can cause serious injury or death.
Many people refer to loss of wheels from commercial vehicles as a mystery. Research shows that there is no mystery.
Careless torque was found to be one of the reasons for wheel loss.
Someone is responsible for wheels becoming detached from commercial vehicles. Following the guidelines given here may help to ensure that it is not you.
Driving a vehicle in a dangerous condition carries a severe penalty for drivers and is an endorseable offence.
Care of wheels
Make 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 manufacturers 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 – hub mounting face, wheel faces and nut/washer faces must be free from corrosion, damage and dirt
- pay special attention to paint - 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 loss of clamping of the wheel and 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, which is why it is no mystery.
Major findings were:
- the use of low quality replacements parts; replacement parts from the vehicle manufacturer were generally satisfactory 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 we advise the use of parts which conform to the standard, particularly if they are from sources other than the vehicle manufactures
- 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 which 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 40 kilometres (25 miles) to 80 kilometres (50 miles) 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 of looseness will still be necessary
Maintenance may be onerous, it may be a costly nuisance but we are not yet in the age of the maintenance free commercial vehicle.
Look after your wheels and there is a greater chance of them staying where they should be: On your vehicle.
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).
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. Their use is not recommended.
Alternative fixing or locking devices
By all means investigate the use of the various alternative locking or safety devices which are on the market but please 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 and equally without the wheel nuts or bolts 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 of the wheel and fixings and eventually failure
- Make sure that even if you do fit alternative fixings devices that you do not reduce your maintenance or driver daily checks