Originally Posted by RalliSpec
What is the exact cause of failure in these gearboxes? We've done some testing in this area. While many people have promoted case distortion as the cause of the failure (we were probably guilty of promoting this theory very early on)....strain guage testing of the case showed only a small level of distortion even at very high torque levels. While even a small amount of distortion is bad because it misaligns bearings and gears increasing friction and heat generation, it is not the main cause of failure. The main cause of failure is excessive gear tooth separation due to shaft flex. We found that at only 275lb-ft of input torque the gear tooth separation exceeded the recommended engineering limit for a helical gear profile. This separation occurrs because the mechanical leverage provided by the gears also creates a reaction force that wants to push the gears away from each other. The more torque you apply to the gears the more force there is trying to separate them. The input shaft is particularly thin so it can flex quite a bit. When the teeth separate the bending loads on the root of the tooth go up quite a bit. Ultimately the weakest tooth fails and creates a domino effect by wedging between the next set of teeth tearing them free and so on. It is very common to see every tooth on the gear torn clean off.
What about dog gears, aren't they a lot stronger? Most dog gears use a straight cut (spur) gear design instead of the helical gear profile used by Subaru. Spur gears are actually weaker than helical gears for the same width and tooth pitch. This is because there are fewer teeth meshed at the same time and the curve of the helical profile creates more surface area. However, replacing the synchronizers with dogs allows the manufacturer to significantly widen the gear widths so this loss in strength is reclaimed and then some. We've seen that some of the cheap aftermarket gearsets (dog or synchro) can be actually more prone to failure than an RA set. This is usually a result of insuficient heat treatment or excessively hardening the gears (creating a brittle gear not capable to absorbing high shockloads).
Quick question regarding these two paragraphs.
I broke an aftermarket gearset. Since it's no longer manufactured, I had to have a custom replacement made. The original was helical, but I asked for a straight cut second on the new shaft. I was under the impression that spur gears are stronger, ince they don't produce the axial loads that a helical gear design does. Is a spur gear design less likely to produce shaft flex?
If the spur design is not going to be as strong...I may have to call and change my order.