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Ok, so after watching various wooden gearbox builds from youtubes, it got me invigorated with the idea of building my own gearbox that can be attached to an engine and use it on real life loads and not just as a toy to play with. So I went googling and found this quora thread: https://www.quora.com/Can-you-build-a-small-transmission-out-of-wood-if-the-engine-is-only-8HP where this guy asked if it was possible to build a gearbox out of wood to support a 8HP engine attached to it, to which one answer was a yes but the gear teeth would have to be bigger to compensate for its low strength on an equivalent steel gear teeth size and that the engine would ideally be outputting low RPM but with high torque for it to work.

What I wanted to know is how big these gears have to be, to be strong enough to handle 8HP engine that would idle at about 1000 RPM and floored to 3600 RPM without shattering and whether or not if it's even practical. The guy who said yes never elaborated more on this aside from giving alternative options or vague information.

Let's say the use case is on ratrod you're trying to build, and yes by the same definition, you would have a tight budget for this build and of course everything DIY or custom made is welcomed, hence the wooden gearbox, unless you can find a working metal one for free that is just as light or and small, which would also work.

This ratrod is basically a minimalistic one seater car with a rollcage and soft suspensions for off roading. It also has a tow bar for towing a custom made trailer for it. Let's say the ratrod is also mostly made out of wood as well with some metal and plastic and other materials here and there that wood cannot be used or used well at all and you're wanting to do cool tricks, or race someone in their go kart, with your ratrod as your performance test.

Anymore details I need to add to help you guys answer my question?

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  • $\begingroup$ Check out windmills. $\endgroup$
    – Solar Mike
    Commented Dec 7, 2021 at 14:51
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    $\begingroup$ Just thinking about a fixed ratio gearbox it would seem too big for a car, let alone a transmission. There is also wear. You want strength in a small space for a gearbox. Wood is not going to beat steel there. Most things don't. Yes for windmills though where size doesn't matter. You could always make just two wood gears of a size that would be reasonable and run them for wear and torque. Make it stir water or something for load. Plywood is real soft though. I think your massive stumbling criteria, even if it were reasonable is "last as long as a metal one". $\endgroup$
    – DKNguyen
    Commented Dec 7, 2021 at 15:04
  • $\begingroup$ "[where] wood cannot be used or used well at all" I think the transmission easily fits that criteria more than anything else on the car, other than the engine. $\endgroup$
    – DKNguyen
    Commented Dec 7, 2021 at 15:23
  • $\begingroup$ Let's have some sense of strength-size effect. The strongest wood in shear is OAK that has a shear capacity (max.) of 2600 psi, while A36 steel has a static allowable shear stress of 14400 psi, see the difference? $\endgroup$
    – r13
    Commented Dec 7, 2021 at 17:05
  • $\begingroup$ @SolarMike So I would need gears the size of the ones used in windmills for them to be strong enough for torque-y and speedy applications such as in a car gearbox? $\endgroup$
    – Newb Coder
    Commented Dec 9, 2021 at 12:50

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No

You can build a wooden gearbox for novelty use, but meshing and connecting parts is a poor use for wood.

Specifically

Practical to build: No. As already discussed you would have to make everything larger than you want just to get it to pass the forces you want. Oak's flex strength is 50 MPa, and its tensile yield stress is 5.5 MPa. Compare to mild steel's yield strength of 250 MPa. So wood isn't nearly as strong, your design needs to account for that. You'll have the same problem with hardness, which you need to prevent deformation at the teeth.

That will last just as long: No. You will never get a good oil film to prevent wear on wood, the bearing surfaces just won't support it. You can't get the surface finish you can on steel either. You won't find mechanical components made of wood for exactly this reason. I think clocks are about the limit of what you can do with it. A journal just might work with wood at the right speed, but it'll always be a problem, and you need several in a gearbox.

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  • $\begingroup$ Yeah okay, so how large do I need them to be to pass the forces I want them to go through? Give me a figure, let me visualise it somehow.... $\endgroup$
    – Newb Coder
    Commented Dec 9, 2021 at 13:03
  • $\begingroup$ I could use solid Patagonian Rosewood for the gears, and thus gear teeth, which is apparently the hardest on the Janka scale according to this site: thespruce.com/janka-wood-flooring-hardness-rating-chart-1821655 $\endgroup$
    – Newb Coder
    Commented Dec 9, 2021 at 13:03
  • $\begingroup$ @NewbCoder, no way, I hated Machine Design 2 and I'm never ever going to design a gear again. $\endgroup$
    – Tiger Guy
    Commented Dec 9, 2021 at 13:32
  • $\begingroup$ What is "Machine Design 2"? Is that a program you use to design meshable spur gears and then print out a template onto wood and then cut the wood out in accordance to the template? $\endgroup$
    – Newb Coder
    Commented Dec 14, 2021 at 5:05

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