Hello all, has anyone ever used TEFLON bar to make pistons for small steam model engines? If so,what would be the best machining method,[tool,speed etc]. I would appreciate any advice given,and thank you all in advance, All the best for now, John.
Pure Teflon (PTFE) has a high coeffecient of thermal expansion, about
or 6 times that of brass and alumninum and 10 times that of cast iron. If you fit it at room temperature it will expand and bind at operatin temperature. On a 1 inch bore it would have to be almost 0.007 inc undersize at room temperature to fit properly at operating temperatur and with this much clearance an engine will probably not even start.
There are some mica filled PTFE stocks that approach the same expansio rate as alumninum, but these are not commonly available in bar form.
If you are building a display engine to run only on air it will wor OK.
Use very sharp tools to machine it and fit it to the cylinder as eve micrometer pressure can distort it enough to give an undersize reading
-- GailInN
First R/C 1956
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I've used it for a piston in a very low power (coffee cup) stirling engine.
I turned it with a very sharp HSS tool as might be used for nylon. It cut easily, like cheese :)
However, PTFE had a significantly different coeff. of expansion to the cylinder, so to get a consistent sliding fit I ended up turning a metal piston body, fitting a tight teflon sleeve, and finally turning this down very thin to fit; when it was done, it was lovely, but I wouldn't bother again. Might not be an issue with a more powerful engine!
Teflon's slippery and soft, so easily deformed if you tighten up the chuck in an effort to get a good grip; it deflects away from the tool (e.g reamed holes come out undersize) then suddenly grabs and gets cut too deep, so I found I had to take many tiny finish cuts!
HTH Guy
One of the properties of PTFE, even glass or mica filled PTFE to a lesser extent, is that it "creeps" under load, that is it deforms slowly and permanently, so while it has a good co-efficient of friction and a good thermal stability it is not a good bearing material except under very low loads.
Cliff Coggin Kent UK
What is a good mix for a piston/cylinder combo, under high loads, when no lubrication is available?
I was thinking bronze/steel, or steel/steel with bronze "piston rings", but what do I know?
Perhaps a teflon cyclinder AND piston might make an interesting ultra low power Stirling engine.
BugBear
Hello all, Thank you very much for all your advice, All the best for now, John.
Cast iron would be much better than steel here.
Cheers Tim
I have vague memories of something like that, but it sounds potentially dangerous to me! (maybe that's why you can't get it now).
I've got a can of Rocol dry lubricant, which is some sort of PTFE spray. Whether that's still available I don't know, but I'm sure it wouldn't be a permanent solution.
Cheers Tim
One option is a high temperature plastic called PEEK. Radiospares stock it but it is not cheap. Same expansion issue as PTFE (teflon) so normally used as a sleeve on a metal piston in a metal bore. (Material choice would depend on working fluid and temperature).
Fluid is air, helium and oxygen gas, pressure is 200 bar, temperature might reach 200C worst case.
I don't think PEEK will do, especially not the graphited kind!
This is for a high-pressure compressor which will have to work 24/6 with no lubrication and only occasional maintenance. It is fairly small though, so materials cost is not really an important consideration.
If anyone knows where to get a 15l/sec 0.5 cfm 200 bar air compressor (not for oxygen), please can they let me know.
Thanks
NAUGHTY BOY! peter. John.
Eh? What's naughty about it? Please can you explain.
That sounds pretty demanding to me. What is it, a diving compressor? I'm away from the office (and all my literature) for a week, but I'll have a think.
Ground support equipment for model rocketry. It's much smaller than a diving compressor though, only 15 l/min.
It is used to compress air, nitrogen and scavenged helium for general purposes (purging, dumping, tank pressurisation, a few other bits and bobs), but the majority of the time it is used as a cryo-refrigeration pump to make liquid oxygen.
Incidently, it has to undergo a safety inspection for insurance purposes, and CE compliance / marking if I want to sell them as part of a kit (the long-term plan), and I both know and intend to more than follow the safety and regulatory requirements. It might be "wicked", but it's not "naughty".
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