A “Toolbox” for Forensic Engineers

Component Failure in Road Traffic Accidents 331
Figure 10.9 Frictional heating close to bearing.
rotation as the friction increases acts to unscrew the nut, which tends to relax
the frictional force compared with positive tightening on the right axle. The
driver feels no immediate effect as the wheel detaches but first becomes aware
of the problem when a wheel overtakes the side of the car! A loose wheel
rolling into the path of another vehicle traveling at high speed often has fatal
results, certainly in countries that drive on the left side of the road. The clues
as to the cause of the axle shearing are readily apparent in the state of the
outer bearing, the nut and cotter and the signs of frictional heating in the
vicinity of the bearing, as illustrated in Figure 10.9.
Wheel detachment on both right and left sides of the vehicle may also
occur if worn or faulty bearings fail and pieces of a broken roller or parts of
the cage jam and cause seizure. This can usually be traced by careful examination
of the debris to a worn out or damaged bearing. The detachment
illustrated in Figures 10.8 and 10.9 occurred on a small car that had undergone
a 20,000 mile service only the day before, and this had required removal
of the rear brake drums to inspect the linings. The young mechanic who had
carried out the work was not supervised so full liability for the accident was
accepted by the garage’s insurers.
10.3 Brake Pipe Failures
By far the two most frequent causes of accidents resulting from failure of the
braking system are loss of hydraulic fluid due to either corrosion or fatigue
of the pipework. In both cases the loss begins as a slight ooze at the site of
a perforation or a fatigue crack penetrating the wall and, if this passes