Piston Rings?

[stantheman]

Probably been done before, but as a relatively new reader I would not know where.
Question is, "what are the general thoughts today on best material to use for piston rings in gunmetal cylinders with piston valves".

Stainless steel/ cast iron rings/ PTFE / old fashioned graphite yarn or what.

All views welcome.

Stantheman.

[greasemonkey]

Hi Stan
General rule is cast iron rings for cast iron cylinders and bronze rings for gunmetal cylinders, but I stand to be corrected. There is some movment now to using PTFE rings with good results but allowance has to be made for the materials expansion at temperature. i have never had much success with graphite yarn or O rings. If you do want to use O rings a honed bore is essential as is a wider slot on the piston head to allow the ring to roll. Baggo has tried a new material called Peek but I am sure he will join in.

Andy

[baggo]

Hi Stan/Andy,

I'm using the Peek for making the actual piston valve bobbins but it probably would be ideal for the piston rings as well. The problem is the prohibitive cost in the larger sizes e.g. a 300mm length of 30mm diameter is about £95

I think the norm for gunmetal/bronze cylinders is to use either packing or O rings. I've seen very little on using actual rings except for cast iron cylinders although I see no reason why bronze rings could not be used.

As Andy says some people have experimented successfully with PTFE rings but getting the right fit can be a problem. This can be made easier by cutting the ring ends at an angle so that they overlap rather than straight as in cast iron rings. This allows the ring to expand without increasing it's diameter.Some people use an O ring underneath the PTFE ring to press it against the bore and allow for expansion.

The cylinders I am working on at the moment are fitted with O rings but the bores must be honed to as fine a finish as possible to give a satisfactory life for the rings and the grooves in the piston must be correctly sized. There is no need for the O rings to be a tight fit in the bores as the steam pressure will seal them against the bore. I have read that the grooves can be made slightly deeper than that recommended to ease the fit of the ring. The sizes given in the manufacturers data sheets are designed for a working pressure far higher than encountered in our use so the fit does not need to be so tight.

One thing I have noticed with O rings is that they tend to 'stick' if left standing for any length of time. Once they have moved slightly they free up again though.

John

[stantheman]

Thanks Andy.

My problems begun when I brought back a 'very nearly' finished 5" 'Brit' that was being built by an old friend over in Ontario. On dis-assembling the chassis for painting and finishing I have discovered a couple of points that needed finishing. One being the cylinder set.
He has used the original castings, from N.Spink but has confused the issue a little. The cylinder castings are of gunmetal. The piston, on drawing, is shown as from gunmetal but what could be mild steel has been used, at least it is showing signs of being magnetic. The rings, as found in a small box, also seem made from a similar material.

So it looks as though I am perhaps better off working new pistons from gunmetal and using suitable rings.

I shall wait a while to see if any other points are raised from this thread. Although not a complete novice I see what problems we can have taking over something that has been started by A.N.Other!

Thanks again Stan.

[baggo]

Hi again Stan,

The piston material may be stainless as some of it is magnetic to a certain degree depending on the grade.

John

[stantheman]

Thanks John.

Of course you may well be right on this, pity there is no way of knowing for sure as my old mate passed on during 2005.
I should admit that I have used PTFE on my old (nearly 27 years) Pannier. I replaced the graphite packing , must be nearly 10 years ago, with PTFE rings that were sliced with a double edged razor blade. Never had any real problems with those. maybe I will go for something similar.

Stan.

[chris vine]

Hi Stan,

Leave the pistons outside while you do other stuff to the loco. Buff them up a bit in the lathe before you do, so there is no oil. If they are stainless they won't rust!!

I do that with odd springs etc that I find and want to put away for something.

Chris.

[John Lee]

stantheman said:

Thanks John.

Of course you may well be right on this, pity there is no way of knowing for sure as my old mate passed on during 2005.
I should admit that I have used PTFE on my old (nearly 27 years) Pannier. I replaced the graphite packing , must be nearly 10 years ago, with PTFE rings that were sliced with a double edged razor blade. Never had any real problems with those. maybe I will go for something similar.

Stan.

I too have used graphite yarn a lot; it seems to last for ages and is very easy to replace when its no use anymore. The simplest solution and very forgiving I feel even though its not a "proper" solution or following full size practice..as if that matters on a hidden bit, or at all.

Another consideration especially with gunmetal cylinders (as you asked in the OP) is that metal rings, made of whatever, need to exert pressure on the bore to seal at a fairly narrow point. Surely this has got to cause more wear?

Regards

John

[Deleted]

In 1988 I started using a type of 'filled' PTFE sold by Columbia Metals which goes under the brand name 'Flourosint' (I seem to recall that it is 'filled' with Mica) for my piston rings (pistons and valve head rings). The beauty of this stuff is that it has a coefficient of expansion very close to bronze. It also machines really nicely and does not tend to deflect from the cutter like pure PTFE. Off cuts makes great sealing washers for steam fittings etc. I've also used this stuff for plugs in the end of the gudgeon pins of my drag racing bikes for years instead of using circlips.

One thing to remember with 'O' rings, especially if dismantling a 2nd hand loco, is that they may be 'Viton'. We all know (?) that Viton can produce a nasty acid if overheated and that the only way to get it off your skin is to have the area surgically removed! I think it has to be heated to the point of melting, so shouldn't be a problem, but I think I would wear gloves!

As to stainless steel, the more common 304 grade has slight residual magnetism whilst 316L which should not have, can become magnetic during machining. I know because we had to scrap several hundred pounds worth of partly machined components after they became magnetic.

I should have a photo of the components for a German 4 cylinder compound I built using Flourosint rings, will try to dig it out of my carefully filed (not!) archive.

[stantheman]

Firstly to Chris, I should think that over the next few days we should have enough 'precipitation' here in the UK to evaluate the situation, will give that a try, thanks.

Thanks to URSA, luckily there are no 'o' rings fitted anywhere at the moment, your warning about the 'Viton' type is a timely reminder to all of us to beware when handling these things. I will try to contact either your source or another through my old working establishment to see if the material is obtainable at reasonable cost.

Isn't it great fun this 'model' engineering.

Thanks to all, Stan.

[stantheman]

This is addressed to Chris basically, but it is return to his earlier suggestion about leaving the piston heads out in the weather for a few days.

I did, and yes, they have turned rusty very quickly. Looking at the finish on the faces before they were left outside they give the appearance of being from cast iron. The 'speed' at which they have turn rusty would also suggest this.

However I will replace them with bronze/gunmetal versions to save any problems later, also as said earlier will try the PTFE approach for the rings.

Thanks for all the help and advice on this subject.

Stan..

[Shawki Shlemon]

Hi
All of your suggestions can be used ,many people will swear by one method or another.I use cast iron for cast iron and PTFE or "O" rings for bronze . I am happy with all the above . My suggestion is you machine the grooves a standard size to give you the option to change the rings if not satisfactory .

[stantheman]

Thanks again for the comments.

As said earlier I did replace the graphite yarn rings on my 5" Pannier with PTFE. I read lots about sizing and questioned several people on the subject. Then decided to make the rings the same o/d as the bore plus a couple of thou', giving more clearance on the width to allow some movement sideways, then sliced them, angled cut, with a razor blade and slipped them on. The i/d was, if I remember correctly a few though larger than the bottom diam of the groove. With some trepidation I steamed it for the first time and was delighted when it ran so well. To date I have not had them out for inspection. Some lines of thought are that in all probability they have, by virtue of the heat, sealed up the angled split by now.
Stan

[Deleted]

I found the photo of the Bavarian S3/6 compound referred to above, however my memory is obviously on the way out as the low pressure pistons have yarn packing (PTFE impregnated I think, but with my memory who knows!). I am sure the piston valves and high pressure pistons used Flourosint rings; trouble is the negative is so rubbish it's hard to see.



If you haven't already guessed the scale is 1/32; gauge 1.
High pressure valves are inside admission and low pressure are outside admission which gives really short passages between the two. There was an interstage superheater and a second regulator to allow high pressure steam access to the low pressure steam chests,in practice this was rarely used.