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Replacing a gear on automotive gearbox mainshaft

Replacing a gear on automotive gearbox mainshaft

Replacing a gear on automotive gearbox mainshaft

I've copied this from my original post in the metallurgy forum, since it overlaaps both, with a little luck someone with some experience will have an idea. I'm not after a 50,000 mile fix, if it coveres 500 in the rest of its lifetime it will be a miracle...
My question relates to removing (through grinding) 1st gear on a mainshaft, removing a 2nd gear with a lower ratio from another gearbox, and welding it onto the original shaft.
I had originally hoped I could do this by tig welding, cooling, repeat. This would have avoided annealing shaft and gear.
However, trying this out has proved this to be easier said than done, and to get a decent fillet means the shaft and gear are softened.
This leaves me with a question, can the shaft, once welded to this new gear be heat treated back to something close to original hardness?

The application is a low mileage dirt track car, normally run in one gear, but it needs a "gate" gear to take it to the first bend, hence the fitting of a lower second gear in place of the original 1st.

RE: Replacing a gear on automotive gearbox mainshaft

The original design has the gear integral with the shaft because that's what is necessary to transfer the torque load to the full width of the gear. Welding it will only ever attach the gear for the thickness of the weld bead on either side. Your method of "removing" the original gear needs to maintain concentricity and accurate fit, too.

If 2nd gear in the other gearbox is the correct ratio that you want, why not just use that gearbox and start off on 2nd??

RE: Replacing a gear on automotive gearbox mainshaft

I think you are right in practice, but a nasty bit of stress analysis suggests that if one tooth area X can take a torque T at radius R, then a fillet weld area Y around the entire shaft *2 at radius r might be in with a chance.

Whether it is worth blowing a race and a gearbox to test this horrible calculation is another matter.

Maybe you could broach a spline into the new gear and machine a spline on the old shaft?


Greg Locock

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RE: Replacing a gear on automotive gearbox mainshaft

Note that when you replace the first gear on the mainshaft with a smaller one, you also should change the corresponding gear on the countershaft for a larger one.

It might be simpler, cheaper, and more reliable to fit a higher numerically final drive gearset,
or fit an underdrive box between the transmission and the final drive,
and run most of the race in second gear.

... or just use smaller tires.

Mike Halloran
Pembroke Pines, FL, USA

RE: Replacing a gear on automotive gearbox mainshaft

Thank you both for answering, to answer both your replies in turn, and explain some of my assumptions:-
The original gears were machined from the shaft for cost reasons, and 1st couldnt be anything else sice it is barely bigger than the shaft at its root.
I don't believe there to be an issue with weld strength, since the length of the welds will total around 6 inches for both sides of this shaft/gear.
Dimensional accuracy shouldn't be a problem, i can clock up the original shaft, machine the gear by drilling and boring to ensure concentricity. Likewise the receiving mainshaft can be clocked and ground. I need a low gear for gating, which will take me to the 1st corner, then change to second for the rest of the race.
Greglocok, i agree with your assumtion on strength. It might be worth blowing a race and the work to test this, if it works.
The cutting of new geras is impractical and cannot work.
The crux of this is how to retreat the shaft once complete, but i know nothing, relatively, about that.
Thanks again
Mikehalloran, missed your answer whilst replying. Most of what you suggest is covered by my answer, but suffice to say i need a gate gear, and a slightly higher race gear. It might be possible to do this by using a close 2nd and 3rd, but a new crownwheel and pinion is quite costly also.
Mike, I notice your address, I'm in Pembrokeshire, west Wales, UK.

RE: Replacing a gear on automotive gearbox mainshaft

If you don't know the steel alloy and HT used for the donor shaft and new gear, there is no way to determine if re-heat treating is even possible. Most MT gear teeth are induction or case hardened. The steel alloys used for these parts are not easy to weld. You would also need to make sure the gear teeth are outside of the weld HAZ. The only HT process you can probably do after welding is a low temp thermal stress relief.

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