Why is the Front Differential Ring Gear machined like this?

GSX_Dan · Apr 29, 2016

Ring gear is out of an early 92 AWD auto, the left half of each tooth tip is machined different than the right half. Every single one. There's actually a step down about 0.05mm where it transitions the smoother to rougher serration. Any ideas if this is normal or a Mitsu f***up, why I'm asking is because there's marks on the gear it meshes with (front output shaft) on the half with the rougher serrations, but not scratches, like glitter you find in tranny fluid.

I compared it to a 91 AWD auto ring gear and it's not like this, just one serration pattern(smoother like on my right half) the length of the tooth, but I do know Mitsu likes to change up helix angles/tooth patterns/counts and whatnot between years...

Also does anyone know why these serrations exist in the first place? Seems to me like these surfaces should be as smooth and rounded off as possible?

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Front output shaft gear (some call it the pinion to this ring gear)

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luv2rallye · Apr 29, 2016

PM twicks69 (tranny rebuilder) about it.

doubleclutch · Apr 30, 2016

The first picture has serrations on the top of the tooth because that never is in contact. Those are the tooling marks from making the gear blank. I couldn't tell you why the small divot is in every tooth. Never seen that before. It appears to definitely be intentional and was done before the gears were finish ground.

As far as the second picture, the serrations you are seeing on the gear flank itself is actually called ridging and is a type of gear damage or wear. It also looks like you have some pitting on the very top side of the gear teeth. All in all, they aren't too bad, but they definitely have seen some load. If it was me I would do a good flush. Ridging and pitting can be caused by particulate in the fluid.

GSX_Dan · Apr 30, 2016

Indeed, I found a lot of glitter in the front diff area and with it being an auto I'm taking it all out and cleaning all the passeges, cavities, valve body etc.

I also found some nasty looking side and pinion gears inside the front diff...

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doubleclutch · Apr 30, 2016

Those diff gears don't look too bad actually. The arks are no in the direction of mesh sliding which is sorta odd. The most concerning appears to be the chunks missing out of the edge of the teeth. Not sure how that would happen unless big pieces of something hard was getting in there.

96dsmftw · Apr 30, 2016

On the tip of the tooth, those 'serrations' are actually grooves from the radius at the end of a lathe insert. The forgings have to be cut down to size on lathe CNC machines. The different depths are from changes in feed rates. The slower you go the smoother the finish is. The step in the middle of the tooth(from top to bottom) is from the fact that you can only lathe one half of the part because the other half has to be chucked up. There are specs for these transition areas as well. Not really a concern as that part of the gear shouldn't see contact. The deeper groove on the one side is a OD mark to make sure the parts are oriented the right way when they get hobbed (the teeth actaully get cut into them). Also, for finishing processes after heat treat, used for giving a nice smooth tooth profile. The reason you see lathe lines in some and not others is because there are different hob processes for cutting gear teeth. One cuts the faces of the tooth and the root, while using the lathe OD size as the overall diameter of the part. Other hob processes cut the faces, root, and cut the tip of the tooth as well, making the tip look smooth.

GSX_Dan · Apr 30, 2016

96dsmftw said:
On the tip of the tooth, those 'serrations' are actually grooves from the radius at the end of a lathe insert. The forgings have to be cut down to size on lathe CNC machines. The different depths are from changes in feed rates. The slower you go the smoother the finish is. The step in the middle of the tooth(from top to bottom) is from the fact that you can only lathe one half of the part because the other half has to be chucked up. There are specs for these transition areas as well. Not really a concern as that part of the gear shouldn't see contact. The deeper groove on the one side is a OD mark to make sure the parts are oriented the right way when they get hobbed (the teeth actaully get cut into them). Also, for finishing processes after heat treat, used for giving a nice smooth tooth profile. The reason you see lathe lines in some and not others is because there are different hob processes for cutting gear teeth. One cuts the faces of the tooth and the root, while using the lathe OD size as the overall diameter of the part. Other hob processes cut the faces, root, and cut the tip of the tooth as well, making the tip look smooth.

Thanks for taking the time to explain it in such detail, you must have machined a few of these in your time!

twicks69 · May 1, 2016

The parts look fine; you are going to be finding debris everywhere else in side the transmission, especially inside the valve body, and the clutch baskets (and inside the piston area too). It looks like your gears were eating metal debris for some time, but the wear patterns on the front diff pinion gears typically look like that. The transfer gears just show it was eating junk from the trans the whole time. The "serrations" on the gear teeth is from the hobbing process, and the larger groove on the teeth is an identification marking. The transfer gears (all 3) have an orientation groove and an identification marking.

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Why is the Front Differential Ring Gear machined like this?

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doubleclutch

GSX_Dan

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96dsmftw

GSX_Dan

twicks69

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