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Poor Man's Differential with Positraction #483
03/08/2019 07:33 AM
03/08/2019 07:33 AM
Joined: Jan 2019
Posts: 68
Beryl, UT - Middle Nowhere
Doc Offline OP
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Doc  Offline OP
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Joined: Jan 2019
Posts: 68
Beryl, UT - Middle Nowhere
Okay folks looking for comments on this idea. So check me out on the design validity.

Idea of a differential is that when a vehicle goes around a corner the outside wheel turns faster than the inside wheel does (because circumference of a bigger circle is larger than a smaller circle, duh!) Railroads deal with this by using tapered wheels that are smaller on the outside than they are on the inside. A flange on inside keeps car from falling off track (which is generally a big drag for all involved.) Anyway, as train car goes around corner it shifts toward the outside rail so that inside wheel runs on smaller diameter and outside wheel runs on larger diameter. Sneaky way to get away with solid axles... but limited to broad curves.

Okay, along come cars and trucks. They want to go around all kinds of corners. Some bright boy invents the differential. This is a cage that holds a set of "spider gears", which is a set of beveled gears like so:
[Linked Image]

(Although most automotive differentials I've seen use two idler gears between the outside gear sets rather than one as shown in drawing.)

Outside the carrier is a ring gear driven by the driveshaft.

All well and good... until you get on slippery ground. Then a differential will treat a slipping wheel as if it was just going around a really tight corner and allow all your power to be applied to spinning it in the mud or snow just as fast as you apply power. Thus was spawned limited slip or positraction type differentials. These use some kind of clutch mechanism to limit how much slip it will allow to a spinning wheel before it shifts more torque to non-slipping wheel.

I won't go into how positraction works... as for one I'm a bit hazy on it myself.... and for another it doesn't matter anyway to this discussion.

Okay, on to my idea. (Not mine, I stole it off youtube, but I'm modifying it to work better.) Idea is a mechanism that permits the outside wheel to freewheel. Meaning that either wheel is free to turn faster than the other, but not slower.

Useable for go karts. But probably not for tractors. Reason being that it won't work at all in reverse. It is usually desirable for tractors to be able to back up... so no go.

The guy I stole the idea from on youtube used a ratcheting mechanism. Crude, but useable. Subject to heavy wear and easy failure.

So what is different about my idea is that it uses "sprag" directional bearings. These are ball bearing looking and acting, except they only turn freely in one direction. Keyed on both inner and outer diameters they are used in such applications as rollers that will only turn in one direction, but not the other.

Much more sophisticated than ratchet mechanism... and currently on sale at AcerRacing in LA. 35x72x17mm Sprag Bearing $19.99

Okay looking at the specs on the bearings they are rated to apply 175 N/m of torque (129 ft/pounds), which is not phenominal but certainly LOTS to apply to any one wheel.

So okay, how to implement them... build a carrier, similar to a spider gear carrier in a differential. Either built from 3" OD thick walled pipe or turned from a 3" round. A sprocket or pully is welded to outside of this. Driven from say a torque converter off an engine. Two of the "Sprag" Bearings are mounted inside, keyed to sleeve. A shaft from either side each keys into one of the bearings. (35mm.... hey thats just 0.002" bigger than 1 3/8" shaft. Good enough!) Probably want an outside snap ring on inside ends of output shafts and an 1 3/8" pillow block on either side to support mechanism... maybe two on either side to give solid frame support to center carrier / freewheel mechanism. Outside axles can either directly drive wheels or do so with smaller sprockets allowing even more reduction of final ratio if desired for more torque vs slower speed.

So that's the idea so far. Advantage is it also acts like positraction, not losing power to a slipping wheel. Dissadvantage is it won't work at all in reverse... or transfer any engine braking. Requires additional brakes to be mounted after center carrier, either on wheels or on each output shaft.

So.... what do you guys think?
Doc



Warning: Reading is addictive and may serve as a "gateway" drug to the unregulated accumulation of knowledge!
Re: Poor Man's Differential with Positraction [Re: Doc] #484
03/08/2019 05:19 PM
03/08/2019 05:19 PM
Joined: Jan 2019
Posts: 220
Arrowsmith, Il. USA
sonny Offline

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Posts: 220
Arrowsmith, Il. USA
In your drawing, one wheel will rotate left and the other will rotate right from what I envision! a ring gear would still be needed, I think.

Last edited by sonny; 03/08/2019 05:21 PM.

"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands."
Re: Poor Man's Differential with Positraction [Re: sonny] #485
03/08/2019 05:27 PM
03/08/2019 05:27 PM
Joined: Jan 2019
Posts: 68
Beryl, UT - Middle Nowhere
Doc Offline OP
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Doc  Offline OP
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Joined: Jan 2019
Posts: 68
Beryl, UT - Middle Nowhere
The offset bump top and bottom on drawing. Yes, the ring gear drives the entire carrier.
Doc

Last edited by Doc; 03/08/2019 05:28 PM.

Warning: Reading is addictive and may serve as a "gateway" drug to the unregulated accumulation of knowledge!
Re: Poor Man's Differential with Positraction [Re: Doc] #486
03/08/2019 07:11 PM
03/08/2019 07:11 PM
Joined: Jan 2019
Posts: 220
Arrowsmith, Il. USA
sonny Offline

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Posts: 220
Arrowsmith, Il. USA
Now I see!! Gotta forgive me. some days things look different to me! lol!!


"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands."
Re: Poor Man's Differential with Positraction [Re: Doc] #488
03/09/2019 10:16 AM
03/09/2019 10:16 AM
Joined: Jan 2019
Posts: 26
MA
GLyford Offline
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GLyford  Offline
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Joined: Jan 2019
Posts: 26
MA
Planet Junior 2-wheel walking tractors use a simple pawl clutch on both sides between the center chain driven axle and the wheels, but they don't have reverse (single speed, aka "a little too fast"). The other disadvantage is that when you are running downhill both sides are freewheeling and there is nothing slowing it down except you (or nothing if you are riding a trailer or sulky)...no brakes either.

Re: Poor Man's Differential with Positraction [Re: Doc] #489
03/09/2019 11:24 AM
03/09/2019 11:24 AM
Joined: Jan 2019
Posts: 220
Arrowsmith, Il. USA
sonny Offline

MBN enthusiast
sonny  Offline

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Joined: Jan 2019
Posts: 220
Arrowsmith, Il. USA
David-Bradley 2-wheel garden tractors use that setup in both wheels too. posi trac, 1- speed forward, (too fast), no brakes , and 10-4 on the downhill ride! lol!!


"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands."

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