Piston material choice.

[adolfgalland]

I have used Delrin on air powered engines with good results. Its coefficient of expansion is about 40 percent more that Ertalyte but for a comparison Teflon is 200 percent more. Many of my engines are small and I use the Delrin mostly on bores of 3/8 inch or less. On the larger (up to one inch) I use Ertalyte. Engines run good with out blowby and again no need to lubricate. None of these plastice are good in wet environments if you have close tolerances. Photo of very small Radial with 1/4 inch bore Delrin pistons. Runs good. Gary

 

[L98fiero]

I know for a fact that its used commonly in wheels on the doors on light rail transport wagons.
Would suggest that delrin is a great product for reasonably slippery and quite non moisture absorbent.

https://www.curbellplastics.com/Research-Solutions/Plastic-Properties
Seems like a reasonably comprehensive table - - - - looks like uhmw might also fit this kind of application.

I machine Delrin, regular Acetal and UHMW regularly. Delrin homopolymer is a little harder, more rigid than the generic acetal copolymer and way nicer to machine than UHMW, either of the acetals break into light, easily broken stringy chips and UHMW will create an ugly, stringy mess in seconds because the chip doesn't break, it's about as bad as nylon for that but it's about 2/3 the price of the acetals.

 

[plipoma]

Attached are photos of graphite pistons, one a small 5/16" diameter & cylinder and the second is a 1 1/4" diameter for a Corliss engine that performs very well.

 

[Steamchick]

Thanks Greg! Good stuff!
K2

 

[GrahamJTaylor49]

I think Delrin is Nylon (Glass filled?). I knew someone who didn't appreciate that Nylon is hygroscopic - it absorbs water. He made replacement bushes for the bearings supporting the rudder on his yacht using some "nylon" he picked-up cheaply... Then after fitting them, when he went sailing, the nylon absorbed water to the point where the rudder seized.... So without steering control, pointing the wrong way, and unable to adjust his direction for "home", the local lifeboat launched to tow him back home.... It took him ages to drill out the nylon sufficiently so he could dismantle the rudder and fit Bronze bearings... But cost him a round of drinks for the lifeboat lads in his local! "Materials Choice" is the start of good design. Too many modellers spoil "Excellent workmanship" with "Poor materials choice", IMHO.... (I have done it to my regret). The cost of materials is always a fraction of the cost and endeavour invested in a good model.
Your advice and expertise here is very welcome, Thanks.
K2

Nylon is a wonderful material. When my company sold air tools we sold Dessoutter air drills. In there wisdom they changed the spec of the internal motor vanes from the Tufnol, cloth reinforced, to Nylon. We started to get tools coming back under warranty with the motors seized. It turned out that the nylon vanes were absorbing the water in the compressed air, due to not having proper dryers, swelling up and seizing. No recompense from Dessoutter, all they said was the client should have a refrigeration dryer so it wasn't their problem. We spent hours replacing the nylon vanes with tufnol ones. Typical response from a large company.

 

[Mad]

I have used Delrin on air powered engines with good results. Its coefficient of expansion is about 40 percent more that Ertalyte but for a comparison Teflon is 200 percent more. Many of my engines are small and I use the Delrin mostly on bores of 3/8 inch or less. On the larger (up to one inch) I use Ertalyte. Engines run good with out blowby and again no need to lubricate. None of these plastice are good in wet environments if you have close tolerances. Photo of very small Radial with 1/4 inch bore Delrin pistons. Runs good. Gary

Off topic. But do you have the Drawings for this beautiful little engine?

 

[adolfgalland]

I am sorry Mad but there never were drawings for this little guy. I belong to an E.A.A. pilots group and several years back we were discussing Radials and some of the guys didn't know how the crank / rods functioned so I built and assembled just the parts shown so that you could rotate the crank and watch the movement and the pistons going up and down in the headless cylinders. Then one of the guys challenged me to make it a runner and having never seen a 9 cylinder air powered model I did some redesigning and the rest is history. Gary

 

[Mad]

I am sorry Mad but there never were drawings for this little guy. I belong to an E.A.A. pilots group and several years back we were discussing Radials and some of the guys didn't know how the crank / rods functioned so I built and assembled just the parts shown so that you could rotate the crank and watch the movement and the pistons going up and down in the headless cylinders. Then one of the guys challenged me to make it a runner and having never seen a 9 cylinder air powered model I did some redesigning and the rest is history. Gary

I thought it was to good to be true.
I'll try to use the photos to dream something up.

Thanks anyway.

Mad (Mike)

 

[ChazzC]

Delrin's thermal expansion [6.8 x 10(-5)] is similar to Ertalyte [3.3 – 8.0 x 10(-5)] (see attached). As L98fiero noted, it machines significantly better than UHMW, but I have never used it above 200°F. Per the protolabs link maximum service temperatures are 300°F. (intermittent) and 185°F. (long term). I couldn't find maximum service temperature data for Ertalyte, but ProtoLabs lists 320°F. and 212°F. – 230°F. for PET (also attached).

 

[Henry K]

About 35 to 40 years ago I was one of the engineers in a 1,000+ employee printing and packaging company that also got involved in other things (we made some of our own patented inks, had an injection molding plant 40 employees, etc. Someone decided we needed a plastic roller on some of our large (8 feetx10 feetx60feet) lithographic printing machines.
The plastic had to have an expansion (when soaked in its operating environment of water) of maybe 0.002" diameter on a roller about 14"? in diameter. I checked nylon suppliers all over the world. I finally found a manufacturer in Germany that could meet our specifications. I think we ordered 2 pieces that weighted about 200 pounds each. The project was a success. I do not know if the factory and its nylon variant are still around but it did exist. If you need something like this, try to find it - it did exist.