Leaking rear axles: a design for Six Sigma project at General Motors

The problem for the project is that a few of the rear axles for General Motors GMT800 and GMT900 are found to be leaking. The majority of the leaks are spotted already at the paint facility Paint Tech International (PTI), however some make it all the way up to the consumer before being spotted. The leaks are usually found as a wet spot around the vent tube area on the differential, or in more serious cases as a puddle of lube around the pinion flange. The leak has already been separated to the interface between the vent and a plastic cap covering it during manufacturing and assembling.

The project is a Design for Six Sigma project, and follows the IDDOV cycle. IDDOV provides a systematic procedure for locate and get rid of reasons for variation in a process. IDDOV stands for Identify, Define, Develop, Optimize and Verify.

The purposes of this project are to uncover why some axles leak and to advise a design or process change to lessen the quantity of leaking axles. The area of the leaks was verified and the strategy to be used was decided in the projects phase.

The project was broken down into two categories; process study and design study. The process study made comparisons against axles of other sizes as well as differences between different GM assembly plants. In the design study a capability study was made, which showed some shortcomings in the vent, cap and the amount of lube injected in each axle. A component search without any reliable results was also done here. The most interesting finding was from the process study which showed differences in the manufacturing processes between the 8.6” axle and the 11.5” axle. The 11.5” axle got new fresh caps after the paint process to make sure the axle had been pressure equalized.

To find out if the vent and cap pairs are affected by the heat that are applied to them in the manufacturing process, a design of experiments was made in two steps. The first step was to see if heat had an impact, and the second step was later done, with some additional factors, to more precisely determining the impact. From the study, heat and time in the oven were found to impact the amount of pressure it takes to pop off the plastic cap of from the vent. By that it is known that the heat treatment is weakening the cap.

A process improvement was made early in the project. All 8.6” axles have now their caps pulled for a few seconds at PTI. The cap is then replaced when the pressure inside the axle has been equalized. Since the process change, interaction has been held with GM regularly to make sure no axles leak anymore. GM has also made a test to confirm that the axles do not leak anymore. The total amount of savings from the project is estimated to be about $ 29,000.

The suggestion for process change, based on the findings of the project, is to continue with the ongoing task of removing and replacing the caps at PTI. Preferably a new cap should be used instead of the old one, to separate the equalized axles from the nonequalized ones, and also since the cap does get deformed by the heat earlier in the manufacturing process.

Source: Lulea University of Technology

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