ETS Baja parts failure analysis: geared for success

When the ETS Baja team had to withdraw from the Baja Auburn 2012 event due to abnormal gear failure, an extensive analysis was required to ensure that problems of this nature would not surface again. After some serious investigation, the conclusion of almost 30 hours of research was unexpected and proved surprisingly simple.

IN THE WORLD OF SAE OFF-ROAD EVENTS, the Baja team at École de Technologie Supérieure (ETS) is known for its strong performances since 1989, and its ability to push the limits of the competition by continuously bringing cars with mechanical components that are rarely seen in the world of Baja. This year, it was decided that developing an active differential would be an interesting feature to have on the car, which would allow the driver to change the stance of the vehicle from handling to mobility by the push of a button. This feature required a lot of space which was not available in the existing gearbox and resulted in a complete redesign, mainly to be able to downsize the gears and the power transmission line to a size that would accommodate the large hydraulic differential assembly. One of the most efficient ways to reduce the size of a gears assembly while keeping the same ratios is by using a planetary geartrain.

Once the whole assembly was completely designed in SolidWorks, fabrication started in our in-house facilities, thanks to ETS. Parts were then sent to St-Hubert Machine Shop, where they managed to cut the gear teeth with exceptionally short lead time. The gears were then case-hardened, bearing surfaces were machined to final size and finally fitted with various bearings supplied by Motion Industries. Once assembly was completed, the team immediately left for Auburn, Alabama. As a result of using a new system that was not properly tested, the team faced an unexpected failure when all the planet gears shattered after a shock loading during the landing of a jump while competing in the Suspension & Traction event. A quick inspection of the broken parts revealed that failure had occurred in the rim of the gears, instead of the usual gear failure that occurs at the root of the teeth. This unexpected failure necessitated further investigation to completely understand the root cause of the failure and prevent a similar situation from recurring. While a review of the calculations revealed that the rim thickness was under-dimensioned, fracture analysis performed on the gears exposed signs of a fragile rupture.