O rings for pistons

[duncan webster 1Participant @duncanwebster1]

Before I make an expensive leap in the dark I hope I can get some advice. I'm about to bore my gunmetal cylinders. Diameter is 28mm. I can get Viton rings in 1.5, 2, 2.5 and 3mm sections. Which ones do I want? Viton seems to be OK at steam temperatures.

**LINK**

Eventually I want to try Peek rings. Anyone done this? If so what grade and section (width/depth) did you use, and did you put an O ring underneath to energise the ring?

[duncan webster 1Participant @duncanwebster1]

[Paul KempParticipant @paulkemp46892]

Duncan,

My 5" gauge 'Speedy' was fitted with O rings by the original builder, I have replaced them once due to one side wearing excessively, I didn't make any measurements, just replaced like for like (the originals were red in colour but no idea of the grade). The wear was apparently caused by all the oil from the lubricator going to one cylinder as the good one was 'wet' and the badly worn one 'dry' so at the same time I made a new twin element lubricator to feed each cylinder individually although the loco had run for about 30 years on the originals.

I made PTFE rings for my father's 2" Fowler ploughing engine when he broke one of the original cast iron rings. No science applied to that either! Made the rings about 5 thou narrower than the existing groove and about 50 thou larger on the bore than the piston register if I remember correctly (it was a few years ago now!). Then cut a strip of hard brass shim a free fit in the groove and crimped it to form a wavy spring. Popped the rings on over the top with about a 60 degree chamfer cut to get them over. Spring pushed them out on the bore. He ran it at rallies for three or four years and I ran it for another couple after and did a one mile 'road run' with it on a few occasions at one event. Ran fine. Not used it lately as it needs a trousers down boiler inspection and I can't get the enthusiasm to get the cladding off but assume it will still run as it did. There were no indications of any problem last time it was run.

Paul.

[Fowlers FuryParticipant @fowlersfury]

I think, from my research, you're raising one of the most controversial topics in m.e. !
Likewise having GM cylinder castings (1.5" D) I began looking for suitable metal rings – far too expensive to buy ready-made (e.g. in stainless or PB).
Ordinary O rings were regularly reported to be intrinsically unsuitable for piston rings – according to many experts online & in mags.
So then I purchased the expensive Fluorosint (PTFE+mica) and made rings with scarf joints. More dire warnings surfaced about its considerable expansion at steam temps and consequential need to reduce their diameter at room temp to allow for this.
But of course it was then impossible to run the engine on air…….bugger.
Next, I saw an article on David Carpenter's website about the use of PEEK for rings. "Much preferable to PTFE blah, blah…." The article contained compicated equations for allowing an overlapping joint to close up at steam temps whilst still allowing running on air. Eventually I contacted the author who explained the maths and acknowledged some errors. But ~ the job required several jigs to be made. I decided to buy PEEK-CA30*.
I made the jigs, rings and fitted them. (Finding old articles on http://modelengineeringwebsite.com is difficult so if you're interested, I can provide a copy as well as some of the many pitfalls not covered by the author).
Satisfied after all this effort, I put steam at 40 psi into the GM cylinders only to find they were extremely porous castings and were fit only for scrap……….bugger again, even more so.
Now in the process of maching cast iron cylinders & will joyfully use CI rings.

I appreciate there are innumerable reports of successful use of O rings over many years and PTFE rings similarly. But searching the properties of PTFE and various additives (graphite, mica, glass fibre etc) makes the cautious-minded hesitate if the prospect of running on air is a literal & metaphorical non-starter.

*
PEEK CA30 has a linear thermal expansion coefficient of half that of Fluorosint Mica and a melt point of 340⁰C. In addition the manufacturers state:-

“PEEK CA30 offers higher stiffness and strength than both PEEK and PEEK HPV. In addition it has superior long term creep properties at elevated temperatures of up to 250°C. These features make PEEK CA30 ideal for applications involving high static loads.
• Wide operating range (-20°C to +250°C)
• Excellent hydrolysis resistance
• Excellent mechanical strength / stiffness
• Outstanding dimensional stability
• Exceptional wear resistance”

[Fowlers FuryParticipant @fowlersfury]

Duncan….
Herewith url for the article on PTFE-FILLED rings.
My apologies, I thought it concerned PEEK but I adapted his methodology for PEEK CA30.
Offer to provide additional information stands.

**LINK**

[duncan webster 1Participant @duncanwebster1]

Thanks for the link, I'll read it with interest, but I'm going to use O rings to start with to get it going. I'll fit Peek later on, but at least I now know how. I'll pm you if (when) I get stuck! Just from a quick scan if I use the methodology in the link but halve the expansion coeff I should be OK. Looks like the section is 0.1" square, but not at all clear.

Edited By duncan webster on 07/08/2018 17:19:19

[Another JohnSParticipant @anotherjohns]

Hi Duncan;

A note of support:

My little Tich has had Viton O rings in it, and on the valve spindles, for about 2 decades. They used to have graphite packing, but that "disappeared" when inspected for lack of power. Pistons re-fitted with O rings.

They have not been out since installation, so I can't remember the thickness of the rings, but I don't think it matters much.

As you'll know – a tiny bit of compression, and some space for "roll" and Bobs' your Uncle.

Have not had the Tich out since our track got the "frost heaves" a couple of years ago (REALLY steep grades between frost-heaved posts and those which did not heave) but I'd like to get it out again later this summer, as the club elevated track is getting well-deserved remedial work.

I think the Viton rings (brass pistons, GM cylinders) are 100% successful.

[John PurdyParticipant @johnpurdy78347]

Duncan

When I built my 5" 0-4-0, which has gunmetal cylinders and pistons, I used standard Buna-N "O" rings (I couldn't find Viton at the time) on the pistons (#212) and all the glands. It has run since being completed in the the spring of 1991 with them with no problems. About 5 years ago I found a source of "Viton" rings so decided to change them all over. The original piston "O" rings when removed showed only minimal wear.    The cylinders when made were not lapped in any way just bored to the finest finish I could get (between centres boring bar). So that was 20 + years with minimum wear for the Buna-N rings. So from my own experience, "O" rings for piston seals in gunmetal cyliders are perfectly satisfactory. I found the "O" rings also have an added benefit in that they start to squeak when the lubricator runs out of oil !!

John

Edited By John Purdy on 07/08/2018 18:38:47

Edited By John Purdy on 07/08/2018 18:40:12

Edited By John Purdy on 07/08/2018 18:41:16

Edited By John Purdy on 07/08/2018 18:42:00

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