PTFE Piston Rings

[baggo]

Hi all,

I'm about to start re-assembling Helen's chassis after painting and I would like to try fitting PTFE rings to the pistons instead of the original O rings, not because the O rings were not successful, more as an experiment to see how they will perform.

I've searched the forum and noticed that several members have used PTFE rings in locos and am wondering how best to make them as there seems little information available.

One method mentioned by Malcolm Stride in ME is to use a single ring with a half lap joint to allow for expansion. The method quoted is to turn the ring about 1/8" oversize in diameter, cut it through vertically with a sharp knife, and then cut away alternate sides of each end halfway through to form the overlapping joint. There is no mention of turning the ring to suit the bore after doing this although it seems to me that the ring will only touch the bore in the middle and at each end due to it originally being turned to a larger diameter. Perhaps the material is flexible enough to shape itself to the bore?

Another method mentioned here (by Ron?) is to turn the ring to size and then cut it through at an angle with a suitable gap for expansion. The problem seems to be getting the gap just right - too small and the ring will seize, too big and it will leak.

An idea which occurs to me is to use two narrow rings side by side. Each ring with a suitable gap for expansion but the gaps at 180° apart so there will be no leakage unless the rings rotate until both gaps align. Rotation could be prevented (if necessary?) by pinning the rings.

Any comments on the above method or suggestions for any other methods of manufacture welcome.

The cylinders are gunmetal by the way.

John

[stantheman]

I have to admit that several years ago I spent time and eyesight seeing what I could find about PTFE rings. At that time there were lots of ideas, many trying to seek advice along the lines of expansion, sizing and even using 'O' rings under the PTFE to act as a 'spring' to push it out into the contact with the bore. I took the chance and made my rings the size of the bore (diam) plus about five thou. They were made to fit the original slot in the piston of my 'Pansy', with some small amount of side clearance. (The original packing was graphite in the form of a .250" square) The finished PTFE ring was cut at an acute angle with a razor blade and fitted to the piston in turn squeezed in to the bore. To date, some ten or so years down the line there has never been a problem with them, no seizure, no leaks in fact they have behaved admirably. The cylinders on my effort are also gunmetal. Seems to me there are some that believe in them (PTFE rings) and some that dont, I took the chance and have enjoyed their use so far.

[havoc]

I use PTFE but not as a ring. After giving a solid ring a few tries I settled for PTFE tape (the stiff gaz approved one). Cut off a lenght, wet your fingers with steam oil and roll it into a thread. Wind it stiff into the grove.

[02jcole]

Hello,

I model engineering friend of mine hasd fitted PTFE rings in serveral engines and they are very successful, but be careful of the source of your PTFE as he got an off cut from a plastic engineering company and made a set of rings for a 4 cylinder freelance 5" gauge engine only to find that this certain grade expanded with heat locking a four cylinders solid!! It took quite a while to realise this as he had never had any trouble before.

I also think, but cannot be sure, that I was told that it can be quite poisonous and when turned gives off a nasty gas, but it may have been tufnol not PTFE. Someone said a model engineer died by getting some of the swarf in in pipe and smoking it...?

James.

[peterseager]

You are talking PTFE. From when I first started work in the electronics industry the firms always stopped people in the workshop smoking if they were working with PTFE.

[simonwass]

Polymer fume fever
> Symptoms: flu-like illness with chest tightness and mild cough;
> Signs: leucocytosis; normal chest x-ray;
> Onset after exposure: 4-8 hours;
> Heavy exposure to: pyrolysis products of polytetrafluoroethylene (PTFE, trade names Fluon, Teflon, Halon);
> Resolution: within 48 hours;
> Comments: Polymer fume fever occurs when PTFE is heated above 300o C. When PTFE is heated above 450o C, the pyrolysis products are different and may cause acute lung injury if controls to enclose the fume have not been installed.

[ianengr]

Hi Baggo,

Might I suggest using the pair of PTFE rings side by side with a normal straight or angled joint and using a thin wall full width metal ring underneath to give a light spring tension, spacing the gaps 120 deg apart. I would make the metal ring a neat fit in the width of the groove. I'm not too keen on using an O ring as a "spring" as the pressure would vary a lot for a small dimensional change in the outer ring thickness, whether this be from initial size, expansion or wear. If you wish you could also use just one locating peg fixed near one end of the metal ring in the centre, that would engage both outer rings. (a semi circular hole on each on the inner faces) this would not compromise the PTFE rings in the same way as a notch in the side of a CI ring would affect its wall tension properties.

A three ring pack using all CI rings is something I'm keen to try when the opportunity arises.

Good luck,

Ian

[modeng2000]

Baggo,

Do I remember you saying that you were going to try a material called I think 'PEKE' for the rings? You said that it was expensive so perhaps that is the reason you might have changed to PTFE.

John

[mackintosh]

Hello John
Don't know if this is any use but PTFE expands when hot by how much I don't know. This can effect the outside diameter of the ring. I have a friend who has made and fitted several sets before getting it right. He reckons his loco will hardly run on air when cold. Also the shape of the cutting tool has to be almost like a parting tool to keep the temperature down.
Good Luck Bob

[Nigel Bennett]

I used PEEK (Poly Ether Ether Ketone) as an experiment in my safety valves for "Edward Thomas". Works fine! The stuff is sold as a bearing grade material, and it is expensive. I imagine it would work well as a piston ring, but it would be VERY expensive - probably cost you a hundred quid in piston size for a suitable length at RS type prices.

[baggo]

Thanks to everybody for your comments

I think I'll try the two split rings side by side for a start but without a metal ring underneath. If the rings are turned slightly (?) oversize they should have sufficient spring to seal them against the bore. As they expand with the heat the sealing pressure should increase. I'll try them without a peg at first. If they do rotate until the gaps align, I figure they'll carry on rotating and seal the gaps again!

I did consider using PEEK rather than PTFE but, as Nigel rightly says, a 300mm length of 30mm dia. PEEK from RS would be £94 PTFE is a fraction of the price

I have used PEEK (Bearing grade) in quite a few applications on Helen with success. The coupling rod bushes and various other bearings are Peek, as are the die blocks in the expansion links, and show no signs of wear after a years running. The seating for the regulator (screw down type) is PEEK and works perfectly, sealing with very light pressure and no seizing when cold! (I'm thinking of using PEEK or PTFE seats for steam valves in the future as it seems virtually impossible to get valves with metal seats to seal properly without tightening them excessively).

I eventually abandoned the PEEK piston valves after they seized up on me blasting up the 1 in 40 gradient before the tunnel at Chesterfield but I now think that was probably due to a lubricator problem. I intend to pursue the PEEK valves at a future date, possibly using thin PEEK valve heads on a solid stainless bobbin. The problem with making the whole bobbin from PEEK is the expansion is still too great. Machining the bobbins with enough clearance at running temperature means they leak like a sieve when cold.

John

[baggo]

Spent an hour or so today turning up 3 sets of PTFE rings from solid bar. Very easy to do with sharp tools. Bored the inside to about 0.015" larger in diameter than the bottom of the piston groove and turned the outside to 0.01" larger than the cylinder bore then parted off to thickness. The rings were then split with a sharp knife and the slit widened to give a gap of 0.01" when in the bore. The width of the rings was gauged to give 0.005" sideways clearance in the piston groove. These clearances are all guesswork so we'll see what happens when the loco is run on steam again



I haven't altered the piston grooves at all so if all else fails I can refit the O rings ;D

[stantheman]

All else being okay, albeit with some guesswork, only thing I can say is that I would have split the rings with an angled cut, using not a knife but a razor blade or very sharp scalpel knife. I think some would call it a 'scarf' cut/joint. This does at least offer the theoretical chance that when in use the ring will not have gaps, both cut faces would ride upon each other and more or less keep the opening closed. My understanding also is that with time/use and heat the ring would amalgamate back together almost.
Anyway, best of luck with the trials.

[baggo]

Hi Stan,

I'm hoping to steam the loco again in a couple of weeks so will know better then. If the above rings don't perform well I will try your method.

The idea behind leaving the gaps is to allow the rings to expand freely and still be able to float in the groove when hot. The ring gaps will be at 180° so there should be no leakage through them (in theory!).

I know from past experience that PTFE will permanently deform under heat so it's possible that a virtually solid ring (one with no gap as such) will compress until it fits itself to the bore and then stay at that diameter.

John

[stantheman]

I look forward to your results. I know when I did my first set I was concerned about all of the chat with the obstacles being put in place for 'us novices'. Luckily my own very basic method has worked out for me at least. Stan

[scoobiesdo]

I made PTFE piston rings for a 5" gauge Speedy recently, I made a scarf joint with a scalpel and allowed only 0.001" on diameter for expansion with a snug fit in the piston groove. It ran great on air but siezed soild on steam. After a bit of research I found that PTFE has a high coefficient of linear expansion of 125 to 152 and I should have allowed around 0.008” clearence on the diameter and width - this depends on the section of the ring. I made new rings to the correct dimension and have had no issues since. The cylinders are cast iron and would definately recommend PTFE.

[baggo]

An update on the rings.

I ran the loco under steam on the 17th at our club 2½" day but had problems with the lubricator not delivering enough oil so I abandoned the run after a couple of laps. I had previously run the chassis on air and it did so quite happily on 10psi.

Had another run yesterday after fixing the lubricator problem and did about 8 laps in the pouring rain (must be mad ). The loco seems very free running with no signs of any leakage past the rings and coasts for a long way with the regulator shut which it didn't do with the original O rings. Time will tell but the conversion to the PTFE rings seems to have been a success

Next project is to go back and have a rethink on the Peek piston valves. The originals were a solid bobbin which had to be a pretty loose fit when cold to stop them seizing at steam temperature. Thoughts now are to use two thin walled 'cylinders' of Peek to act as the valve heads and have these floating on a stainless bobbin with O rings between the valve heads and the bobbin to prevent leakage between the heads and the bobbin. Hopefully it will be possible to make the valve heads a closer fit when cold as the thin heads should (?) compress a bit when they expand rather than seize solid.

John

[freddo]

You know, John (Baggo), I've always been a "dyed-in-the-wool" Prof Chaddock CI piston, Cylinder and rings man, and have never had reason to see past his ideas as they work 100%.

What I'm thinking though, is would it be possible to add a proportion of PTFE lubricant to steam oil to bring it's efficiency up to modern day standards? I dunno as I'm too old and a huge believer in - "if it ain't broke, don't fix it" brigade.

Freddo

[tcase]

Expansion for plain PTFE is about 10 times that of aluminium but reduced by the addition of fillers. To counter this in this application its wear resistance is low so it will quite rapidly wear to the point where it no longer binds and then stop due to its self lubricating properties, PTFE has the second lowest coefficient of friction of any material.

[baggo]

In some of his writings, LBSC suggested mixing colloidal graphite with steam oil to improve it's lubricating properties. I think the problem was that it separated out if the loco was not used for some time and clogged up the oil pump.

Will have to look into the PTFE additive though - sounds like an interesting idea. I know you can get PTFE based lubricants in a spray can - I've seen it mentioned recently but can't remember where (memory's not what it was )