This page shows examples of mechanical failures, in the
main due to metal fatigue, lack of lubrication or other imposed abnormal
stresses.
(CLICK ON THE IMAGES TO SEE A MORE DETAILED VIEW)
PISTON & VALVE DAMAGE
The following three images show an Aermachi 249cc racing single cylinder
piston, exhaust valve and crankcase.
This is what can happen when the engines designed rev limit is exceeded (for
whatever reason) or the piston to valve clearance is too close and the piston
comes into contact with the valve head. Close examination of the valve
revealed that fatigue cracks are evident in various areas of the valve head.
CRANKCASE WELDED REPAIR FAILURE
The crankcases on the same engine had cracked in the past and had been welded
up as a temporary repair. Examination of the welded area revealed fatigue
cracking emanating from the camshaft bearing housing in the casing.
Immanent failure of the cases required a renewal.
SWING ARM (SUSPENSION) FATIGUE FRACTURE (RESULTING IN TOTAL FAILURE)
The following images are of an Aermacchi 344cc rear swing arm. The awing
arm failed in a road race at Chimay, Belgium.
The resultant crash at around 100 mph, severely damaged the bike but the rider escaped with minor injuries.
Rear swing arm after it was removed from the bike and cleaned for examination.
Close up of the right side arm that parted company with the pivot section.
Close up of the separated arm. The striated fracture patterns on the
surface of the metal surface are typical of a fatigue fracture caused by
repeated cyclic events.
This is a close up of the pivot section of the swing arm. The electrical
weld used to join the arm to the pivot section can be seen. A reinforcing
plate has then been welded to the arm and pivot section. The weld has
caused localised hardening of the joined materials. In the short term this
has not been very important. However, intermittent and hard (racing) use
since the manufacture in 1968 has caused fatigue fractures due to cyclic
stresses (bending and twisting moments) and the eventual total failure of the
component.
Further cracks had started to develop at the wheel spindle ends of the swing
arm. The arms on this machine had been strengthened in the past (as was
common on the Aermacchi) by welding reinforcement brackets around the arm ends,
as the original component suffered failures in this area. However, fatigue
fractures have started to propagate and would eventually has caused the rear
wheel to detach from the machine.
This is the other arm end with similar cracking staring to develop.
None of the cracks (that had not resulted in the total failure) were
particularly easy to see before the swing arm was cleaned. I therefore
suspect that the crack that caused the failure and resultant crash may not have
been easy to see either. Moral - Clean machines and examine them
thoroughly before using them, especially if they are very new or very old.
SWING ARM FATIGUE FRACTURE (FOUND BEFORE TOTAL FAILURE)
The following images are of a swing arm from a Maxton framed Yamaha TZ350.
In contrast to the above swing arm the bike was cleaned and examined at regular
intervals. The cracks were detected during checks of the machine between
races and repaired before total failure could occur.
Maxton rear swing arm. The cracked area has been abraded prior to repair.
Close up of un-repaired cracks in swing arm adjacent to wheel spindle mounting.
SEAT FRAME FAILURE
The following images are of a Honda RS125GP aluminium seat sub frame assembly.
The seat frame was found to be loose during checks, dismantling and close examination
revealed a badly bent frame with fatigue cracks and badly welded and failed
joints.
Alluminium seat frame that has been distorted during crashes, repaired and
suffered fatigue failure due to cyclic stress and poor welding techniques.
The lower right side (45°) support mounting has sheared off completely.
Close up of the broken support mounting. The lack of weld penetration is
evident, this has lead to the failure of the mounting.
Close up of crack around the seat mounting caused by repeated bending of the
tube.
Close up of the left side support mounting. Cracks are starting to develop
due to fatigue cause by repeated bending.
PISTON & VALVE FAILURE
Piston from a 4.2 litre Jaguar XJ6. The valve timing chain drive sprocket micro spline sheared on the exhaust camshaft at high speed, causing the valve
timing to go out of sync. The result was contact between the exhaust valves,
inlet valves and pistons. The head of one exhaust valve (shown on top of
the piston crown) had been broken off and was actually found in the sump of the engine. There was
also major damage to the cylinder head.
BIG END BEARING FAILURE
Connecting rod from a racing Suzuki T20. Note the elongation of the big
end eye and damage caused by the big end bearing failure. The engine was
producing normal power at high speed in a race, then within a second there was a
loss of power. The clutch was disengaged and the engine stopped instantly.
DRIVE SHAFT FATIGUE FRACTURE
Drive shaft from a 'sit on' large garden grass mower. The shaft has suffered a torsional fatigue fracture due to repeated hard forward and reverse movements.
There have also been excessive shear forces applied caused by use of the vehicle
on rough ground (it has no suspension other than that given by the tyres).
PISTON LUBRICATION FAILURE
Piston from a Honda RS125 GP single cylinder two-stroke Grand Prix race bike.
The piston has suffered a severe overheating caused by lubrication breakdown.
It was running on a fuel mix of 25:1 fully synthetic mineral based competition oil, 50%
avgas and super unleaded fuel. The oil was changed to a castor based
(Castrol R40) competition oil and no further problems of this nature occurred again.
PISTON AND CONNECTING ROD LUBRICATION FAILURE
Piston, piston pin & connecting rod from an Aermacchi 344cc GP race bike.
This is what can happen when the oil level is allowed to run low! The rest
of the engine damage included, bent valves, bent push rods, cracked barrel, oil
pump big end and main bearings.
BICYCLE PEDAL CRANK SHAFT FATIGUE FRACTURE
Disassembled pedal crank, shaft and bearing assembly from a child's bicycle.
Note the fractured central shaft.
Close up of the same assembly.
The pedal crank shaft. Zoom in to see the fatigue patterns in the
fracture.
The same shaft viewed end on. Note that the threaded hole has been drilled
and tapped off centre. This has contributed to the cause of the
failure by weakening one side of an already weakened component. The shaft
had been drilled and then hardened to support the bearing surfaces. This
had caused the shaft to be very brittle and unsuitable for its purpose.
The fracture of the pedal crank shaft resulted in an accident involving injury.
MORE TO FOLLOW!
