Thursday, July 3, 2014

CL450 Racer - Engineering Failure Analysis and Improvement

With all the news lately about the GM ignition switch recall, I wanted to illustrate an example of how a component failure should be addressed and improved upon by a responsible company.

I have worked as a test and r&d engineer for 4 years, and have developed an eye for spotting the warning signs and causes of part failure. Some are blatantly obvious, while others are only noticeable after they have already broken. In this case, I found a component in my Honda CL450 racer that was under-designed and made from a sub-par metal, but which had not yet failed. 


The part above is the brake actuation cam for the rear wheel drum brake. While disassembling the wheel to replace the brake pads, I found that the rod which spins the cam had twisted from the torque of applying the brakes. This was clearly indicated by the twisted splines at the end of the rod. The splines are keyed to the brake actuation lever to prevent it from slipping.

The part on the left is the original part, and it appears to be made from either a casting or a forging. This can be determined by the grainy surface of the metal and the lack of machine tool marks on many of the features. The cam on the right is from a later model, and demonstrates how Honda recognized the failure and improved the design to compensate. Based on the surface finish, this later part was made from a higher grade of steel. It lacks the grainy surface of the earlier part (large grain size is an indication of a weak material). It also has a noticeably darker color, which may be indicative of higher carbon content or a hardened outer surface. Finally, there are numerous machine tool marks all over its surface, which means this part was machined from a blank piece of material.

I have since replaced the original with the sturdier part. This cam is an excellent example of a part revision that has eliminated a potentially dangerous flaw.

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