| 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.
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.
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
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. However, 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.
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.
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.
another example of a bolt failure