Piston Ring Materials

A

ac16

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Hi all,

About to put a new set of rings in a 5 inch gauge steamer, it is running through a cast iorn cylinder with semi superheated steam. the original was a streight o ring which gave out (understandably).

I have been looking at a few replacement material options they are as follow.
I think cast iorn will kill my poor machine so that is out.

http://www.quicksales.com.au/ad/750-chb-50-x-25-x-183-mm-steam-lathe-model-weld-drill-og/332687
http://www.quicksales.com.au/ad/delrin-black-50-dia-x-250-mm-lathe-mill-model-weld-bike-4-x/430385

Open to any other suggestions, I had heard teflon is quite good

Rob
 
you might try a transmission shop in your area. automatic transmissions have teflon split rings of all different sizes.
find one close to your size and trim to length. temp will not harm it.
steve
 
Rob,

Bronze and iron is a good combination for cylinders and rings, but normally it's bronze for the cylinder and iron for the rings. Bronze rings would definitely work, but they might wear prematurely against the iron during break in. I wouldn't discount iron for the rings, as long as it's gray iron (with graphite). Iron rings have been working in iron bores for hundreds of years. If you used iron or bronze, the bores would need to be honed in a cross hatch pattern for break in. If there is a good polish on the bores, Teflon may work, but I doubt Delrin would take the heat, or seal very well (too hard).

Tom
 
If it were me, Grey Cast Iron would be the way to go. It will not kill your engine as you presume. I use iron against iron all the time in my small engines. If you prefer. Teflon is a good choice to replace the O Ring.

"Billy G"
 
Hello Rob,
What steam pressure are you operating at?
Do you have an estimate of the superheat?
I agree with Tom G that cast iron rings on cast iron cylinders is the traditional method.
In today's world, other metals like steel or brass might also be possible.

Teflon might be an option if the pressure and temp are not too high.
My experience with Teflon O rings in hot oil piping flanges is that 500 F is the upper limit. This was with the O rings captive in grooves. In a sliding seal situation, the limit is probably lower. One issue is that Teflon starts to distort around 500F. When you remove the seal after exposure to high temp, you probably will not be able to re-install the seal.

Not sure where one would find the proper size CI rings except to make them.
I am told that back in the 1920's mechanical engineering students were required to take fairly extensive machine shop classes.
At the U of Minnesota, one of the projects they did was to fabricate new piston rings for a Ford Model T engine using cast iron sewer pipe as the source material.
I dunno - could work, but I am not sure how one would go about it.

If you run iron on iron, you will need cylinder lubrication. Maybe you already have this. Cast iron is usually porous enough (due to the carbon flakes) that it holds oil well and does a good job of polishing itself when running against an iron cylinder. For a small model engine, a "pressure pot" feeder on the steam line, holding a few drops of oil, is probably enough. You have to periodically refresh the oil charge because under the heat of the steam, the oil is slowly emulsified and carried away with the exhaust steam.

Another item that you probably know: Traditional steam engine design has a slight relief in the cylinder bore at top and bottom center. The relief only needs to be 0.010" or so on the radius. The idea is that the outer ring slightly overruns the relief with each stroke, and thereby does not generate a ridge on the cylinder. The usual overrun of the ring is about 20% of the ring width. Without the relief, steam engines can really get to pound as wear accumulates in the crank and cross-head bearings. (I guess the Teflon rings would not need a relief). If you decide to install CI rings (or other metal rings), you might want to think about boring in a relief at each end of the cylinder.

Post some pics if you get a chance.
I would be interested to see the engine.
Regards,
Terry S.
 
Hi Guys,

The worry about "the machine" was more along the lines of the old myford belt lathe I am using, as a nasty habbit of grabbing and locking up on harder materials.

As far as I can gather (fairly good mates with the fellow who did the boiler code for AU) the steam heat is between 90-100deg c at the cylinder as the superheating elliment is the old LBSC there and back copper job which was natorioulsy innaffective. B
low off pressure on the old girl is 100psi but she usally sits at the 80psi mark.

I had heard of teflon rings used before with some success in 3.5" gauge and smaller 5" locos. Ideally I would love to do cast Iorn as it seems to have the best track record but again it is a case of I have a suspicion my lathe wont take it, has anyone had much success with steel, if so what grade?

At Terry S - She was originally fitted off with a mechanical lubricator which failed (which i have found mechanicals seem to be prone to), so I am in the process of fitting her up with hydrostatic, going is abit slow as i am working on her when i get a break to see the family as I travel around alot for work.

At Tom - Strangely my old mans steamer (a martin evans simplex) is fitted up with the bronze cylinder CI ring arrangement and goes really well - hauls 6 adults not bad for a loco the size of a shoe box.

as for pic here is one from afew years ago Robert_001.jpg

Cheers
Rob

Robert_001.jpg
 
Thanks for the pic Rob,
Great details.
Amazing that a small model engine can develop that kind of traction.

As far as temps go, using the blow off press of 100 psi gives a saturated steam temp of just under 330 F.
I suspect the superheat is pretty minimal since there is not a lot of surface area available in a scale model.
So - it could be that Teflon may be a good candidate at the temps you are running.
PTFE rings have been used in "oil-less" air compressors for many years (both industrial and consumer sizes).
You might be able to find replacement part rings for a compressor that could be modified to fit.

I'm not sure where one would look for these.
Perhaps someone else has rebuilt an oil-less compressor and has experience with replacement rings.

What are you using in the stuffing box for the piston rod?
Regards,
Terry S.
 
Hi,

The old girl (when the old piston ring where working) could haul around 14 passangers on a 1:50 grade.
Piston rod stuffing box, assuming it is the packing gland (its interesting to find differances in terminology on the railways in other countries- actually i think the aussie terminology is a mungrel of the british and american terms) is filled with graphited yarn.

Cheers
Rob
 
could cast iron pipe fittings be used as a source of cast iron to make rings from?
They come in a range of sizes but are they the wrong version of cast iron.
 
Pipe fittings are made form malleable iron, which is basically white cast iron that has been heat treated to make it softer and more malleable. Being white iron, it lacks the graphite and consequently the lubricating properties of gray iron which is used for things like engine blocks and piston rings, things that require good wear properties.

Tom
 
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