GEAR DETAIL INFO


 

 

Why gear detail?

 

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.

20x magnification     120x magnification  

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. What amazes me is that most rebuild shops leave this sort of flaw.

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 yet if you ever get a chance to check out the gears in any high-end transmissions you will find that all the gears are detailed and chamfered. 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 and it's on its 4th season of use without failure.

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.

  2010 TRE