It has been long understood by engineers that removing
stress risers will increase fatigue strength of a gear. Stress risers can range from
poor design geometry, too tight of a corner radius or
root fillet, defects in the surface finish, improper
manufacturing process and general rough handling of the
part. This page is to provide further understanding of our
services and what TRE can do to improve the durability of
a transmission be it a new transmission, an exchange or a
transmission that is here for a rebuild.
In the photos below is a gear that has a small chip
on the side edge of the root fillet, down between the gear
teeth. This type of defect is due to a combination of the
stress from the hobbing and shaving process and the
embrittlement that occurs during the carburization of the
gear. Edges can become extremely brittle and as the
gear is heat treated the localized stress field exceeded
the cohesive strength of the steel and a chunk of material
about .010" broke off leaving a stress riser which can be the location of the failure when the gear
is heavily loaded with torque. The sides of the gears have
been ground to add contrast, making it easier to view.
You can click on any of these photos to get the larger
photos.
We commonly find defects like the ones shown on this page inside of nearly every
transmission that is in a production vehicle. Long ago, in
the pursuit of building the better and stronger
transmission, we started offering gear detail services
which remove these defects and create a larger corner
radius which in turn makes the gear stronger.
Shown below are some notches that are left from an
improperly adjusted/maintained gear chamfering machine
back at a gear manufacturing plant and is quite common
inside of the Mitsubishi transmissions and the cause of
most gear related failures.
In the photo below is the same gear after it has been
detailed. Notice that the edge and root are now chamfered
and that the stress riser is gone, leaving a stronger gear
tooth. Blending out defects is something that
professionals do and it has proven itself to be worthwhile
by increasing the strength of the gear but it's something
that few shops can offer being that it takes special
equipment. Instead they'll claim it doesn't do anything.
Gear detail is a time consuming process and not every
defect can be corrected; some times a gear will have to be
replaced if it is not acceptable. This gear was able
to be saved and went into an EVO that is making over
500ft.lbs of torque without failure.
After the gears have been detailed we polish them up in
the Almco vibratory finishers. We then improve the
fatigue strength even further by shot peening the
gearset
to maximize the strength for high torque applications.
To understand shot peening please click
here.
