Author Topic: Cylinder & Piston Materials  (Read 781 times)

Offline Reggleston

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Cylinder & Piston Materials
« on: April 26, 2018, 02:31:32 PM »
My recent post showing my 2x Rudy K's vertical engine utilized Delrin material for the Cylinder, piston, steam chest, eccentric and rotary valve. Compression is good and runs well after I got the valve sealing problem solved with the thrust bearing modification. Valve plate still leaks a little but not enough to prevent continuous running with my small compressor air supply. Bored the cylinder for a nice sliding fit with the Delrin piston. Just wondering if anyone else ever used Delrin(Acetal) for cylinder and piston construction? Not sure how it would hold up to steam, will have to wait till I get around to building a boiler. Delrin machines nicely and is a much better material for this application than Nylon in my opinion.

Offline b.lindsey

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Re: Cylinder & Piston Materials
« Reply #1 on: April 26, 2018, 02:47:32 PM »
While Delrin doesn't absorb water as much as Nylon, the expansion rate for both materials is highter than metals so I wouldn't try it with steam. Delrin is also a lower temp thermoplastic than Nylon. It might be ok for piston rings even under steam but have never tried it. If you must use plastic, Teflon would be the preferred choice.

Bill

Offline kvom

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Re: Cylinder & Piston Materials
« Reply #2 on: April 26, 2018, 04:31:26 PM »
Based on listed coefficient of thermal expansion of Delrin, I can confidently predict that application of steam will cause the cylinder-piston to jam tightly.

Offline bent

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Re: Cylinder & Piston Materials
« Reply #3 on: April 26, 2018, 08:26:54 PM »
Delrin breaks down by hydrolysis in hot water, so it's a no-go with steam.  Go with PEEK, polyimide (kapton), polysulfone PSU or polyphenyl sulfone PPSU or polyphenylene oxide (PPO or Noryl) for steam.  Teflon works to, but it's soft.  And all plastics will have expansion coefficients about 20-50 times larger than typical metals.

But for anything up to 180 F, Delrin (acetal) is lovely stuff, it machines so much better than other plastics (PEEK does too, but it's so dang expensive!).

Offline steamer

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Re: Cylinder & Piston Materials
« Reply #4 on: April 26, 2018, 10:27:58 PM »
A split Teflon piston ring works well.   It needs to be lap joint type, but they're not that hard to make.


I use two on my steam boat and they've been awesome at 160 psi saturated steam.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!