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Final Drive Design

Final Drive Design

Final Drive Design


I have been charged with the design of a new final drive for a gearbox, this is for a FWD car that has a flat final drive and needs to be modified for use in mountains. I have two big questions you guyus can help me with:

1.) I have knowledge of design and manugfacturing of ISO gears, from what I see in the donor car a different type of automotive gear is used, anyone has a link to data on these gear profiles? What books references do you have?

2.) The other question is material, I have thought about using 8620 nitrided steel with cryogenic microstructure stabilization, but this can be quite an expensive solution. Anyone can suggest alternative materials and processes, the application is OEM and the car is a 1600cc engine withj modest hp and torque ratings.

Any help would be greatly appreciated

RE: Final Drive Design

The gears are probably cut on dedicated tooling that doesn't necessarily follow the ISO gear sizing modules or pressure angles. But given that you are also going to have to change the pinion gear, that's not to say that you couldn't substitute ISO gears IF the center-to-center distance works out. Your two new gears only have to mesh with each other, not with what's there.

And, given that this is part of a transversely-mounted transaxle, that pinion gear is probably one-piece with the output shaft of the transmission. Have you thought about this?

I've never heard of anyone aftermarket who actually makes non-stock ratios for transmissions like this. Usually one just finds another transmission of the same series but meant for a different vehicle model, which has different internal ratios (but is of the same general design), and swaps parts.

If you are working for the OEM who builds this gearbox, you ought to have plenty of co-workers who can help! What's your usual manufacturing process? Go out in the plant and have a look!

Can't comment on the material choice.

RE: Final Drive Design

The photo provided obviously shows a RH helical gear. The tooth profile looks like it uses a fairly low pressure angle (<20 deg). And the roots look to have a full radius fillet. Production automotive transmission gears have very highly optimized geometry and metallurgy. So unless you have detailed data for the pinion, it will be difficult to do a good job of designing a mating gear capable of higher performance.

One problem with nitriding gears is the limited case thickness. Doesn't provide much allowance for finish grinding. If your budget permits, you might consider using a vacuum melt quality raw material like 9310, rough the blank, shape the teeth, stress relieve/carburize/quench/temper, and then finish grind.

If you prefer to avoid the cost of finish grinding of your gear required with carbrizing, you can use a vacuum melt quality raw material like Nitralloy N, rough the blank, stress relieve/quench/temper, shape/shave the teeth, nitride, and then finish hone the tooth flanks after nitriding. For nitriding, Nitralloy N is a better choice than 8620.

RE: Final Drive Design

8620 is usually carburized to achieve a case depth of at least .5 mm.

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