Put your rear in worm gear.

"Berang" (berang) 07/21/2020 at 22:14 • Filed to: final drive

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Worm gear from a Ford Model TT (Ton Truck).

The owner’s manual does not say why the truck uses a worm gear final drive. I have to assume given the Model T’s wimpy transmission band brake, the worm drive would prevent a run away if the brakes failed or the truck stalled going up a steep hill.

The final drive reduction was 7.25:1 making for a top speed of just about 25mph, vs. the Model T’s 40 or so.

wafflesnfalafel > Berang 07/21/2020 at 22:22

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my favorite worm drive, (and probably has just about as much power as that TT)

old-busted-hotness-still-cant-comment > Berang 07/21/2020 at 22:24

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eeny oony wanah! Worship the salt.

Distraxi's idea of perfection is a Jagroen > Berang 07/21/2020 at 23:47

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7. 25:1 is more than you can easily get out of a single stage bevel gearset. Maybe they figured a  worm drive was cheaper than a 2 stage final drive.

HammerheadFistpunch > Berang 07/22/2020 at 00:14

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What Distraxi said, you can get a higher numerical final from a worm gear setup. Plus then its really easy to to a high pinion design.

Crazy though.

My GX has worm gears in it though, hanging out in the center diff.

The Planetary gears in the T-3 (Type-C) are worm gears running on a worm wheel sun gear, or the other way around I forget. Anyway, it’s interesting where you find some old technology doing cool new tricks. I wish all AWD cars had torsen centers, they are FREAKING AMAZING. Compared to the open over viscous in the Land Cruiser, the Torsen in the GX is so much faster and with so much better bias. The only reason to lock the center is for low range and to turn off VCS.

Distraxi's idea of perfection is a Jagroen > HammerheadFistpunch 07/22/2020 at 01:04

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I don’t think that technically yours (or mine - the 86 has a Torsen rear end) are worm gears, they’re just really wide helical gears with a high helix angle. To be worm gears they have to run at 90 degrees to the mating gear (and its been a long time since I was at engineering school but I’m pretty sure that implies a different tooth profile - you can’t just rotate a worm 90 degrees and run it as a planetary, whatever you do to the profile for the teeth on the sun wheel). Some of the earlier Torsens used actual worm gears, arranged in a spider with their axes tangential to the output gear, but the later ones have the planetaries all parallel.

The practical effect’s the same however - lots of helix angle means lots of surface slip means lots of friction and if you select the helix angle *just so* you can use that friction for a limited slip effect which allows a controllable amount of traction slip and ignores cornering wheelspeed differential (unlike some other LSD technologies). It’s a seriously cool technology.

The tricks which can be played with force and friction balances in a gearbox of any type are incredible. Which is why most self-shifting transmissions will do your head in if you try to understand them at a detail level when rebuilding. Just follow the darn manual and accept that transmission engineers know shit you never will.

Berang > Distraxi's idea of perfection is a Jagroen 07/22/2020 at 01:13

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Makes sense. They had a 5.17 rear end available I think (the high speed version!) - I’m wondering if perhaps the worm gear offered more contact area than the plain bevel gears then in use, and was simply a good way to prevent cracking teeth under high loads. As well, in the Model T the entire drive train, engine, trans, and axle share oil. I think the cars used a 3.6-something to 1 final.

Michael > wafflesnfalafel 07/22/2020 at 10:02

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I have the table saw version of that :D