The issue for the project is that some of the rear axles for General Motors
GMT800 and GMT900 are found to be leaking. Most of the leaks are spotted
already at the paint facility Paint Tech International (PTI), but some make
it all the way to the customer before being spotted. The leaks are normally
discovered as a wet spot around the vent tube area on the differential, or
in more severe cases as a puddle of lube around the pinion flange. The leak
has already been isolated 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 offers a systematic approach to find and eliminate causes of
variation in a process. IDDOV stands for Identify, Define, Develop,
Optimize and Verify.
The purposes of this project are to find out why some axles leak and to
suggest a design or process change to reduce the number 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 findin...
