Piston material choice.

[Johan Maritz]

Hi to you all. I'm in the very final stages of my Elmer Verburg no 29 engine. I need to machine the piston. I made the cylinder out of brass. I was thinking in the direction of making the piston out of cast iron. Not sure if this is a good combination, i must add that i doubt i will ever run it on steam. It will most probably only run on compressed air. Then i started thinking in the direction of teflon ( vesconite) for piston material. Would that be a good combination with the brass cylinder? Any other suggestions. By the way, my hat off for people who has build this engine. This was seriously challenging in terms of very small fiddly parts.
Regards
Johan

 

[minh-thanh]

Hi Johan !
See post # 59

https://www.homemodelenginemachinist.com/threads/trouble-shooting-a-new-stirling.31105/page-3

 

[Johan Maritz]

Thanks, that is a nice table to work from.

 

[Johan Maritz]

Thanks, that is a nice table to work from.

 

[deverett]

Although Teflon is a good material to work against brass, you may find it tricky to machine.
The table given in the above reply refers to steam engines (see the end note). For compressed air, I don't think you would have any problem with aluminium. Mild steel would also work, but make sure you have cylinder lubrication for running.

Dave
The Emerald Isle

 

[abby]

I use brass pistons with brass cylinders in my G1 steam locomotives , and Viton "O"rings.
I have also experimented with back to back cast iron rings , these work very well and overcome the "stick/slip" problem encountered with the "O"rings.
However machining and fitting such small (1/2 " bore) rings is time consuming , they have to be very thin to go over the pistons and break very easily
The PTFE plastics are good for steam engine piston rings , especially the "filled" variety. However they do expand quite considerably with the heat so some experimenting with clearances is necessary.
Dan

 

[Jasonb]

I use aluminium quite a lot as there is less moving mass it can help balance an engine that otherwise may develop some vibration. Though I usually like to run mine nice and slow its always interesting to see what the will do when the regulator is open.

If you are not going to be loading the engine and just running on air for the fun of it you don't even need to fit the rings, leave them out and get a smooth runner. On that small a bore I would not even think of rings, just an oil groove or two will be fine.

I've just finished a small 12mm x 12mm and that ticks over at just above 100rpm despite a small flywheel and ran to 2300rpm

 

[Steamchick]

Among the various engines I have, there are brass cylinders with brass pistons (ex boat use) that have done a lot of use on steam. Well worn, they all still run with only a couple of grooves as labyrinth seals. But they do leak past the pistons so need far more air than originally built. One with completely worn out main and big-enforcement bearings runs with a couple of thou clearance and it looks like the pistons and bores have worn about the same. I plan to make new crank and pistons and re-bore the cylinders to round, but will not bother with rings as the labyrinth seals are adequate for the slow running on air at shows. I suggest you make brass pistons, close clearance with 2, 3 or 4 labyrinth seal grooves and don't bother with rings. If not happy, you can usually re-machine pistons for o-rings or piston rings.
Enjoy!
K2

 

[adolfgalland]

I have made several air powered models and have went to the high performance plastics for the pistons. You might think Teflon would be a good choice because it has the least amount of friction but has the greatest amount of thermal expansion with temperature change (stuck piston). I have had great luck with PET-P (Ertalyte). Very low friction and expansion and self lubricating like most high performance plastics (never need oiled) and very easy to machine. There are other good ones too. Gary

 

[Johan Maritz]

Thanks everybody for your advice and experience shared here. I have decided to make the piston out of good old cast iron. I have a nice piece of fine grain cast iron round bar that i will use. At a bore of 12.7 mm i will not attempt proper rings or even a o'ring. I will just machine a few oil grooves as per Elmer's drawing to work as labyrinth seals. The engine will not be loaded when running, thus a good fit should suffice. I'm glad that you guys warned me about teflon, i was on my way to make the piston out of that, did not know that it has such a big thermal expansion, not that i think the engine will get that hot running on compressed air.

 

[plipoma]

For running on low pressure air (20 psi or less), I've very sucessfully used graphite for pistons. It's very light, slippery and easy to machine. I do not use rings or a snug fit. At this low pressure and 50 to 100 RPM, there is very little blow-by, particularly if shallow grooves are cut into the piston. A fit to prevent blow-by adds too much friction, less efficiency. I sandwich the graphite piston between two aluminum discs of a smaller diameter. Works great!

 

[Johan Maritz]

Now the graphite piston sounds clever to me. Definitely something that i would like to try, do you buy this as stock from a supplier, or can you salvage it from something redundant.

 

[Steamchick]

I have made several air powered models and have went to the high performance plastics for the pistons. You might think Teflon would be a good choice because it has the least amount of friction but has the greatest amount of thermal expansion with temperature change (stuck piston). I have had great luck with PET-P (Ertalyte). Very low friction and expansion and self lubricating like most high performance plastics (never need oiled) and very easy to machine. There are other good ones too. Gary

Does the PET-P (Ertalyte) work on steam? - or just air?
Cheers,
K2

 

[ChazzC]

From Aetna Plastics, the Coefficient of Thermal Expansion of PET-P Ertalyte is 3.3 - 8 10(-5) in./(in. °F.) [33 - 80 10(-6) in./(in.°F.)], which is 3 - 8 times that of brass (see attachments). In theory, a 1/2" Ø piston would grow 0.000015" - 0.00004" more than the brass cylinder; not a lot, but enough to cause problems if there isn't much clearance to begin with. Moisture uptake may be more of a concern.

 

[adolfgalland]

I agree with ChazzC that the high moisture in steam would not be a good mix with any of the plastics in a steam powered engine. None of my air engines are for steam and all but one have ran fine and thats's with machining the piston to where it will not drop through the cylinder bore with your finger held over one end. I learned about not using Teflon many years ago when I machined 2 pistons for a V-twin in a 60 degree shop and let them sit in the bores overnight upstairs in the 75 degree house. The next morning they were stuck big time and would not come out. I was dumbfounded till I researched Teflon's specs. No more Teflon pistons. Gary

 

[plipoma]

I bought round bar graphite years ago from an outfit in Dallas that sold EDM supplies. I think they also have flat stock, saw some 1/2" thick on an EDM machine at a show.
Check EDM suppliers!

 

[GregNixon]

How about Delrin. This could be a trade name for more a more common plastic. It does machine easily though.

 

[Steamchick]

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

 

[GregNixon]

Delrin is advertised as a material to the hobby community. I've used it, but I've never really bothered to look closely at its properties. A quick google around tells me that it is Polyoxymethylene (POM) also called polyformaldehyde or acetal. Low moisture absorption is also listed as one of its properties. Here's a fact sheet which summarizes its properties and uses. Acetal Delrin: The Complete Guide - Is It The Right Material For You? | Jiga (jiga3d.com).
Try it and see. Its not expensive and I feel that it could be used to replace metal in a lot of applications.

Cheers
Greg

 

[ajoeiam]

Delrin is advertised as a material to the hobby community. I've used it, but I've never really bothered to look closely at its properties. A quick google around tells me that it is Polyoxymethylene (POM) also called polyformaldehyde or acetal. Low moisture absorption is also listed as one of its properties. Here's a fact sheet which summarizes its properties and uses. Acetal Delrin: The Complete Guide - Is It The Right Material For You? | Jiga (jiga3d.com).
Try it and see. Its not expensive and I feel that it could be used to replace metal in a lot of applications.

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.