[PStechPaul]

I thought I would see what might be involved in making a planetary reduction drive that might be applicable to wheel motors or as an adapter for a motor so that speed reduction and increased torque might be obtained. Here is what I came up with:



This is what I want to make as a working model and then see what it would take to make it for something like a 10HP wheel motor. This model uses 48 pitch nylon gears which are 1/8" thick. See:
http://www.mcmaster.com/#plastic-gear-racks/=ipn9go

The center (sun) gear is 24 teeth, 57655K16, $3.07
The planets are 80 teeth, 57655K29, $4.34
I was going to make the ring from 12.56" of 57655K61, #4.24/ft

I can use a piece of 4" PVC DWV pipe for the outer ring, or maybe a toilet flange which will have holes that might be used to mount the outer ring to a wheel. I'll use an 8" lawn tractor tire. This planetary drive should have a 8.5/1 reduction, which would cause a rotation of 423 RPM with a 3600 RPM motor. That's a ground speed of about 423 ft/min or 5 MPH. Just about right for a fast lawn tractor. If I use a 1/4 HP motor I should get about 3.1lb thrust per wheel. So I need more reduction or a more powerful or slower motor. This is just a conceptual test anyway. Not bad for less than $20 worth of parts.

[PStechPaul]

I made another design, which fits pretty nicely inside a 4" PVC pipe coupling. The ID is 4.50" and the OD of the ring gear is about 4.625. I think I'll be able to machine that into the PVC. Plastic can be a challenge to machine, so I'll have to see how it goes. Here is this design:



This uses a 20 tooth sun gear, 96 tooth planets, and 214 tooth ring. I will need to see how well everything fits. These gears are only 1/8" wide, so they won't transmit much power. This should have a reduction of about 1+214/20=11.7:1, for 3.66 MPH and 4.15 lb thrust per wheel.

But the same gears are also available in steel, although I'll probably use 16 pitch, which are 1/2" wide. For that, the sun gear will be 12 tooth ($16), the planets will be 30 tooth ($31 each), and the rack will be about $30. So this reduction drive would be about $135.

I'm also thinking about machining my own gears, although mostly for the experience. But gear cutters are highly specialized so you need to know exactly what you want before selecting the cutters. And they're about $30-$60 each.

I want to play around with the planetary drive to see how it might be used as a multi-speed transmission or being capable of shifting in or out as needed, using electromagnetic clutches or other means.

[PStechPaul]

Here'a pretty cool site showing a triple planetary gear system made of wood.
http://woodgears.ca/reader/walters/planetary.html

Here's an explanation of the operation of a planetary gear system:


Or the military version as of 1953:

[McRat]

Take apart an electric screwdriver and look inside.

[PStechPaul]

Originally Posted by McRat

Take apart an electric screwdriver and look inside.

Yes, I have one that I can use. Thanks for the idea. I also have some small motor reduction drive heads with planetary drives. And I think many electric drills also have similar mechanisms. For larger examples, I could probably get some motorcycle or tractor transmissions from a junkyard. But I am also lured by the challenge of trying to build it myself, from scratch.

[Woodsmith]

Have you thought about looking at a Laycock type overdrive unit?





It would provide a sizable single planetary gear cluster. If you run it back to front you get a gear reduction.

Don't know if the ratio is any good for you though, depends on your application.

[PStechPaul]

That looks a little bit like the Sturmey-Archer 3 speed hub, without the third reduction mode. I think that would be fairly easy to add, and it would be perfect for a simple transmission. But usually for electric motors an overall reduction drive is needed, so the overdrive would require an additional reduction.

Also the roller clutch adds a degree of complexity and inefficiency to the design, and might not work well for reversing. But it gives me some ideas.

Thanks!

[few2many]

There's also the Gear Vendors unit.