# Planetary reduction drive



## PStechPaul (May 1, 2012)

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.


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## PStechPaul (May 1, 2012)

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:


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## McRat (Jul 10, 2012)

Take apart an electric screwdriver and look inside.


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## PStechPaul (May 1, 2012)

McRat said:


> 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.


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## Woodsmith (Jun 5, 2008)

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.


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## PStechPaul (May 1, 2012)

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!


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## few2many (Jun 23, 2009)

There's also the Gear Vendors unit.


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## PStechPaul (May 1, 2012)

I did some more looking into reduction drives and I even ordered some parts to see how it would work and what the cost might be. I started with some 32 pitch 40 tooth 1.25" nylon spur gears at $5.37 each and a 1 ft length of matching nylon rack for $5.33. I figured I could bend the rack to fit in a 3.8" inside diameter cylinder (about the size of 3" PVC pipe fittings), but I will need a smaller sun gear. Also, the teeth close up too much at that radius, so I will need to go to a larger diameter and bigger gears and probably 4" pipe which has 4.5" ID for fittings. That requires 14.13" of rack. Perhaps even go to 6" pipe which uses 20.42" of rack. But then I'd need much larger gears (about 1.75" to 2") and the largest in that pitch are 62 tooth and 1.938" pitch diameter for about $10 each. It would probably be better to go to a larger pitch, but this is only for a conceptual prototype. A planetary drive can be built with just two planets, so a reduction drive could be made for about $50. 

To make the same thing using steel, I would probably go for 16 pitch. A 2 foot section of rack is about $25 and for a 6" diameter assembly I could use one 1" diameter 16 tooth sun gear for $20 and two 2.5" diameter 40 tooth planets for $42 each. I think this will give a ratio of 320:16 or 20:1 if the planets are held stationary. So this assembly could be built for about $130. However, I don't know if the gears will mesh properly with the rack if it is bent to this radius. 

Unfortunately, McMaster-Carr does not have internal tooth ring gears. So I found a company on-line Rush Gears, located in PA, that has a wide range of all sorts of gears, but they are all custom made to order so there is little advantage to choosing the standard sizes listed. No prices were listed, so I asked for a quote for components of a similar size planetary drive. The prices I got were about $2000 each for the ring gear and $1300 each for spur gears, for single piece quantity, and $733 and $194 for quantity 10. 

So, I also thought about making the equivalent of a planetary drive by using all spur gears and no ring gear. Here is my concept:










The small (24 tooth) center gear is for the motor shaft, and it drives the two large outer (96 tooth) "planet" gears. These are attached to small gears which drive another large gear as the output. The total reduction should be 16:1. This assembly ( a 9:1 version) can be made from three 20 pitch 0.6" gears at $12 each and three 1.8" gears at $32 each, or a total of $132. It could also be made with three planets for more torque, for an additional $44. I think I will make a prototype using less expensive nylon gears to get a feel for how it works. More later...


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## shortbus (Sep 27, 2011)

Don't think rolling a gear rack into a ring is going to come out like you think. As it's rolled the space between the teeth is going to get so small the pinions won't fit.

And some where in the mix between the motor and the gear train you'll need a one way or sprag clutch. With out one the motor will over speed when the vehicle is coasting. Due to back drive of the gearing.

A junk automatic transmission will give you all of the needed parts pretty cheaply. to mess with.


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## PStechPaul (May 1, 2012)

I did find that I could not use a single 12" rack with the pinion gears in a ring gear of 3.8" diameter. But it seems that a 4.5" or 6.5" diameter might just work, although I would not use it for anything other than a conceptual prototype. I think what happens is that the pressure angle is reduced when the rack is bent on a radius. I think the change of pressure angle can be determined by the number of teeth in the ring, so 20.42" of rack (for the 6.5" diameter) with 653 teeth would introduce a reduction of 360/653 or 0.55 degrees. For the 3.8" 384 tooth ring it would be just about 1 degree. These gears and racks have a 14.5 degree pressure angle. They are also available with 20 degree pressure angle, but not in nylon.

Some information on gears and pressure angle:
http://www.brighthubengineering.com...e-the-pressure-angle-of-a-physical-spur-gear/
http://www.wisegeek.com/what-is-a-pressure-angle.htm
http://en.wikipedia.org/wiki/Angle_of_pressure
http://en.wikipedia.org/wiki/List_of_gear_nomenclature

I have also thought about molding a ring gear. For that, it might work to bend a rack the other way, so the teeth open up. I could use silica to make a high temperature ceramic mold (2500 F), and then use silicon bronze casting alloy (1780 F) to make the gear. But I doubt that I will actually do that. 

That's a good idea to get a junk automatic transmission. I've seen lots of planetary gear sets for cheap on eBay and most of them are automotive. I'd rather get something smaller, perhaps from a motorcycle or go-cart. I have a junk electric drill that might also suffice. 

[edit] I found an interesting article on the details of an automatic transmission:
http://auto.howstuffworks.com/automatic-transmission.htm


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## david85 (Nov 12, 2007)

Wavecrest Laboratories had a design like this a few years ago. Magna International bought them out so they're in EV haven now, but I digress.

Anyway, they used a fixed ratio planetary to allow a higher RPM motor with a decent overall Hp per wheel. If I remember right, it was small enough to fit inside a 14" rim while producing about 45kw each. Pretty peppy for the subcompact bracket they were aiming at and they claimed much lower cost than PML flightlink (also bought out.....).

I'd second the suggestion of an automatic transmission planetary. You'll never break it, and all the design work for you. Gear pitch, clearance and slant is already done. The helical cut will also keep gear noise down.

Keep in mind if you go with a PVC pipe to reinforce the annulus, any flex in the plastic could allow lateral force of the teeth to pry the entire gearset apart (the stresses will try to expand the annulus into a rounded triangle). 

If you look at any planetary drive, you'll notice the sun and planetaries are usually fairly compact, but the annulus is made very thick for rigidity. This is what keeps the gears meshed while under load. Four, five and six pinion gear sets also spread the load more evenly across the circumference of the annulus for this reason. Most light duty autos simply use a three pinion.

Although with such a small sun gear and large planets, it might not be anything to worry about. Either way it should be fun to see how it turns out.


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## shortbus (Sep 27, 2011)

Any motorcycle I've ever worked on uses a sliding gear arrangement in the transmission. Two or more parallel shafts with gears on them. Never saw one with a planetary.

A front wheel drive transaxle uses smaller gear sets than a rear wheel drive does. But they a still pretty strong too.


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## PStechPaul (May 1, 2012)

You are probably correct about motorcycle transmissions. I only worked on my CB-360 once almost 40 years ago and IIRC it was a parallel shaft type, 5 speed, and probably similar to the transmission in my riding mower EV (although it has a reverse, using a chain drive).










I have an old English bike rear wheel with a Sturmey-Archer hub that is a small clever 3 speed planetary drive. It might be suitable for a small motor:

http://www.sheldonbrown.com/sutherland/CB-IGH-4-aw.pdf

Automotive front wheel drive units may be a good option, as you said, and are only about $40 or so:
http://www.ebay.com/itm/1976-1995-D...7RE-front-planetary-gear-set-GG-/200960248663

There are also some smaller gears for R/C models, but they are probably too small to use on even a small tractor:
http://www.ebay.com/itm/NEW-Great-P...4-28mm-Ammo-Motors-GPMG0520-NIB-/290829191404
http://www.ebay.com/itm/NEW-DuraTra...DX450-Motorcycle-4-DTXC4464-NIB-/330611475024

Just for fun, and to make working models that could be scaled up, I might get an assortment of plastic gears such as these:
http://www.ebay.com/itm/261277771952
http://www.ebay.com/itm/171122637717

I'm rather fascinated by mechanical contraptions with gears, sprockets, belts, and chains, but it's really a diversion from my main efforts to get my tractor projects completed. And I should concentrate on electronic designs.


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## dragonsgate (May 19, 2012)

The blow up looks like a typical Dana mower transmission. Those little spines #8 are a weak point. The edges can get rounded and the spring tension gets soft so they pop out of the groove and the trans slips. After awhile they get so bad that they do not hold at all. Another thing is if you can find new replacements it is something like two hundred bucks for four little pieces of metal. The price of a new 5 or 6 speed box is about $600. I picked up a fairly good used one to repair my twenty year old Wizard yard tractor for $50. I have taken a couple of the sturmey hubs apart to look at and wondered why planetary gears are not used in more applications. I have always liked the concept of planetary gears and if I had the tools and smarts not to mention money I would like to build a transposed transmission with planetary gears for my X19.


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## shortbus (Sep 27, 2011)

You might like this link, to Lenco transmissions. They use planetary gear sets but instead of hydraulic pressure to engage/change gears, they use a shifter linked to a wedge typ ball ramp, to apply the gear clutches. Pretty good exploded view of the internals.
http://www.lencoracing.com/CS1MasterPage.html

The Ford Model T used a similar operation, but instead of levers they used foot pedals.


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