Firstly many thanks to Paul Roddison and Burton of Peaks & Pennines for arranging a very interesting evening looking into the failed Mk4 gearbox.
In addition to opening up a failed gearbox Paul also had a two pairs of failed gears. It was these failed gears which provided a good insight into the cause of the gearbox failures.
One gear in particular was a perfect example of gear tooth bending fatigue failure. The fracture face of two of the missing teeth showed classic high load fatigue crack “beach” marks which had emanated in the root, towards one end of the tooth, and then travelled in an arc into the tooth until final failure. In addition a number of the gear teeth which were still intact had visible fatigue cracks in the roots of the tooth again at one end.
From what I observed the cause of Paul Roddisons Mk4 gearboxes is clearly bending fatigue failure of the gears. This could be due to a design weakness of the gears themselves, incorrect gear material or heat treatment, poor manufacture or uneven tooth loading due to deflection of either the main shaft or layshaft. Which of these is the root cause would take further investigation.
I agree with Paul Roddison that the reduction in the final diff ratio when compared to the Mk3 does put a higher torque loading on the gearbox components. However the designer should have taken this into account. Automotive gearbox design is a “mature” industry with well proven design fomulas and I fail to understand how Mazda can get something like this so badly wrong.
Whilst at the moment in the UK it is mainly the Mk4s that have been raced which are suffering gearbox failures it is the nature of gear tooth bending fatigue that failures will occur in other less highly loaded gearboxes, it will just take longer to reach the point of failure.
Paul Robinson