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Miniature epicyclic friction drive

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BillClark

Mechanical
Jul 25, 2009
32
US
I appreciate you taking the time to read this and will attempt to keep it short. Engine Tips is my go-to when I'm stumped and I'm about stumped
I am designing a miniature planetary reduction unit based off of the friction drive concept. The unit will be driven by a <35w electric motor via the pinion. Maximum torque the unit will be required to handle is 4oz/in. The pinion will turn at 30k rpm with a final (ring) ratio of around 5:1. I have machined a test fixture that allows me to test a single friction element and so far I have achieved the torque goal with the single element. The design will have 3 elements total (3 planets) so I am in good shape so far. To paint a clearer picture the pinion is either an o-ring or a machined aluminum flat wheel with a textured finish (to aid in friction) with a diameter of approx. 5mm. the planets are approx. 9.5mm and the ring about 24mm. the planets use 1/16" wide, shore a 70 nitrile O-rings and the ring is aluminum with a rough surface finish (bead blasted). I was very impressed at how well an off-the-shelf o-ring did even with .002-.003" compression. once loaded it seemed to have increased friction probably due to heat at the point of contact but at no time was I able to detect that heat by touch after a lengthy test. The pinion is the area of concern though because it has the least amount of load carrying area compared to the ring/planet interface
Using either of the 2 pinion configurations stated above. Failure was achieved by overloading the element. The o-ring pinion would rip apart and the aluminum pinion would melt the planet O-ring. I would like to raise the point of failure and I believe this can be achieved by using a pinion material with high COF properties and better durability so I started looking at Hydrogenated Nitrile O-rings (shore a 90) which is going in the right direction but still may lack durability or possibly a non metallic material like a phenolic composite (aggressive asperities?) but am not sure what the appropriate grade would be. Something machineable, obviously. The only real constraints are that it has to be something I can purchase relatively inexpensively and is readily available in small quantities. Any ideas on a suitable material. Thanks
 
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From further research it seems a rubber/phenolic hybrid might posses the properties I am seeking but finding such a material is proving to be a bit of a challenge. I recently tested a phenolic pinion which I believe to be paper or cloth impregnated. Although heat and durability were not an issue the COF was low even compared to the aluminum pinion.
 
Staying with the o-ring, FKM (Fluorocarbon, ie Viton) has very good high temperature and wear properties, particularly in the higher durometers. Just make sure it comes from a reputable source, there is a lot of junk "Viton" out there and there are many different grades of the good stuff. With the phenolic, G10 (glass fiber/epoxy resin used for circuit boards & insulators) might work with the right surface texture. I'm not aware of rubber/phenolic hybrids but they might exist.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
 
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