Examples of Bolt Failures

Wheel Stud Failures

Wheel stud failures on vehicles is a relatively common problem. The problem of wheel fixing failure occurs across a range of vehicle types and manufacturers but is predominantly a problem relating to heavy goods vehicles. Metallurgical analysis of a large number of failed wheeled studs have revealed that all had failed from fatigue rather than overtightening.

Wheel Stud Failures There has been a considerable amount of research into the subject of wheel loss from commercial vehicles. A report from the Institute of Road Transport Engineers in 1986 noted that based upon a large sample of machines from the Institutes membership, a wheel stud failure rate of 3 per cent per year and the average distance between failures is 123000 Km (although the pattern is unpredictable). More than 6 per cent of hubs are affected each year, however.

Overtightened stud An analysis from the Accidents and Defect databases of the UK Vehicle Inspectorate shows that up to August 1994, a total of 323 cases of wheel loss had been identified since 1982. Not all cases resulted in the preparation of a defect report but those which were prepared showed that some 72% of the failures were related to maintenance or abuse. Common problems were failure to tighten wheel nuts to the specified torque, overtightening of wheel nuts causing damage and failure to regularly check tightness of wheel nuts. Since not all wheel losses are reported, for the UK it has been estimated a wheel loss incidence of some 3000 cases per year and there have been figures quoted of 10 fatalities per year.

Some consider the cause of the problem to be due to a relaxation of the tension in the studs due to settling between the number of interfaces involved in the wheel assembly. Use of locking devices will not prevent relaxation and since the wheel integrity is dependent upon the friction grip provided by the stud tension, the relative looseness of the wheels will damage the studs and result in eventual wheel loss due to failure of the fixing. The recommended procedure is that following wheel replacement the wheel studs should be tensioned to the torque prescribed by the vehicle manufacturer and retensioned after a period of 30minutes or 40km to 80km road running.

Wheel Stud Failure In the UK, loss of the wheels from the left hand (nearside) rear drive axle seems to be most common and this is thought to be due to the effects of road camber and the fact that these wheels are subject to both braking and driving torque. Wheel StudsHowever, as almost all wheel studs have right hand threads, which are tightened in clockwise direction, there have been several suggestions that the use of left hand threads for the left hand side wheels would overcome the problem. Again whilst this may prevent full unscrewing and loss of the nut it does not address the main cause of loss of tension in the fixing following initial tightening and the wheel would eventually be lost because of fixing and /or wheel wear and fracture.

The British Standard AU50 part 2 expects a tightening torque of 600Nm to produce between 19 and 24 tonnes clamping force from each bolt. This is needed to make sure a spigot located wheel stays tight under the worst conditions. It reports that to maintain these tension forces from 600Nm implies a coefficient of friction of only 0.08 to 0.105 from an oiled nut.

The British Standard recommends retorqueing after 30 minutes or after 40 to 80 km. When this procedure was tested in 1995 it was found that some nuts slackened a little after 160km. For an operation with hilly routes when the brakes get hot, the slackening might be expected to be more pronounced because it has been reported that studs start to creep when temperatures exceed 200 degrees Celsius. Wheelsure device

The Institute of Road Transport Engineers investigations had found that wheels could come loose even though tight when checked. Stud tension and wheel clamping load were easy to maintain when the assembly was new, but deteriorated, with sometimes disastrous results, when wheels had been taken off many times and nuts continually retorqued. Their conclusion was that nut torques were often insufficient to preserve tightness. Elastic yield of the wheel could cause nut slackness to develop and accelerate stud fatigue.

There are a number of devices on the market whose intention is to assist in preventing wheel loss and nut loosening. One such product is shown here. The Wheelsure product incorporates a left hand nut on top of the standard nut is used to ensure that no backing off is possible. A cap fits over both nuts to lock them together.

Wheel Pointers fitted to a truck wheel - Photo from Bolt Science The assumption is generally made that more care in maintenance would cure the problem, or at least prevent it being a danger. Many consider, including Bolt Science, that the fundamental cause of the problem is design, rather than maintenance, related. Poor maintenance practices obviously have played a part in many failures, but manufacturers insisting upon frequent torque re-checks is an indictition that the joint design on many wheel assemblies is marginal. The inherent variation in stud preload as a result of torque tightening can result in the forces acting on some wheel assemblies exceeding the studs capabilities to resist them.

Photo on the right shows a wheel fitted with plastic pointers which are used to give a visual indication of nut rotation.

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