A “Toolbox” for Forensic Engineers

Component Failure in Road Traffic Accidents 329
maneuvering or moving slowly, when the bending loads on the inboard end
of the axle are considerably greater than when it is traveling normally on the
road. A typical fracture surface of a steel stub axle shows two fatigue crack
initiation sites, a larger area radiating from the bottommost fiber and a much
smaller one radiating from the uppermost fiber. The final break is a ductile
fracture occupying a band across the middle.
The reason for this unequal development of the fatigue cracking is that
tensile stresses acting on the bottommost fibers are caused by the weight of
the vehicle tending to bend the axle upward plus cornering forces acting to
push the bottom of the wheel outward. Both are of a cyclic nature when the
vehicle is traveling on the road but the cornering force only becomes significant
when the weight of the vehicle is transferred to the axle as it changes
direction. However, the stresses on the uppermost fibers are always lower,
because there is no external gravitational force bending the axle upward as
there is along the bottom of the axle; rather it is the opposite effect —
gravitational force produces compressive stress in the uppermost fibers and
so tends to counteract tensile forces when the vehicle is cornering. The net
result is that the fatigue initiates first at the bottom of the axle and propagates
upward and it is only when the cross-sectional area has been reduced and
the applied loads generate higher stress levels that the cornering forces are
able to initiate a fatigue crack at the top of the axle. This is also the reason
why the final break almost always takes place when cornering, usually at low
speed, because the upward bending forces are greatest when a heavily laden
vehicle is maneuvering.
Torsional overstress which twists the retaining nut off the end of an axle
is a common cause of wheel detachment when the vehicle is traveling at high
speed and usually occurs shortly after wheel bearings have been adjusted
during servicing. In the majority of motor vehicles wheels are mounted on
their axles by a pair of taper roller bearings that must to be critically adjusted
to allow a small amount of free play without becoming sloppy. Some front
wheel drive vehicles are fitted with a pair of ball bearings separated by a
tubular spacer clamped tightly together and the only provision for adjustment
is to place shims at the ends of the spacer. The retaining nut has to be
tightened to a high torque. Some wheel detachments have occurred simply
because a mechanic has mistakenly tightened taper roller bearings.
Taper bearings are retained on the axle by a washer and castellated nut
which has to be carefully set to allow a small amount of free play and is
prevented from coming unscrewed by a cotter pin or “C” clip passing through
a hole in the screwed end of the axle and lining up with opposite castellations
of the nut. The method of making this adjustment is to fit the roller bearings
and tighten them until they bind, then release the nut by one sixth of a turn
and check that the wheel turns freely with minimum end float. The nut is