Piston Fit?

[ron]

I've just finished boring the cylinder for my Stuart Victoria and going to turn the piston next. On the smaller ST engines I've built, without piston rings, I just aimed for as good a piston fit as I could manage, but with the Victoria having an O-ring piston seal I wondered if the piston fit was as important or would it actually be better slightly slack?
Ron

[Tel]

You still want to try for as nice a fit as possible - should be able to suck & blow it back & forward by mouth pressure.

Personally I'd lose the O-ring & put soft packing in there, or even twisted up teflon tape, but maybe thats just me.

[ron]

Hi Tel
Thanks for the reply, I think I might try your idea of the teflon tape, I'm not really sold on the O-ring.
Ron

[Tel]

That would be a sensible move IMHO, stationary engines tend to not get run as often as locomotives, and the O-rings will stick. I've had really good results with the teflon tape, both as piston packing and in stuffing boxes and gauge glass assembly

[ron]

Hi Tel
Never thought of teflon tape, I've used good old fashioned graphite packing in the past and turned PTFE piston rings but they are very fiddly to get right. The Victoria uses a cast iron cylinder and piston, I was a bit surprised I expected the piston to be brass or gunmetal.
Ron

[Tel]

yep, the Beam engine was the same. I think that perhaps the g/m pistons are limited to the smaller kits?

I've only ever built two Staurt kits - the 10V & the Beam. All the others have been fabricated. This is my fabricated mill engine - Victoria inspired with a lot of modifications

[ron]

Very nice Tel, I came across a lovely bit of old oak up the loft which after staining and varnishing made a great base for the Victoria, so I'm going to cut a thin piece with the bandsaw and use it as matching cylinder lagging. I like the ST kits, only complaint is the price, bit expensive for what you get. I'll also need to figure out how I get photos from my computer on to here, I'm OK with old cars and steam engines, not so hot with all this modern technology.
Ron

[chris vine]

Hi Ron,

If you have a CI piston and cylinder, why not go the whole hog and put a (or 2) cast iron rings in. There have been several articles in ME about how to make them, Tubal Cain did a series.

His main point for steam engines is that they need very little wall pressure to seal and reduce friction. I put them in a ST double 10 some years ago because the O rings wore out double quick. (maybe my bores were not smooth enough!!??)

Usually where you see O rings in "real" engineering products, they only slide a bit, or slowly or intermittantly. Also the bore will tend to be ground and or honed/lapped.....

Chris.

[nickguk]

I was also going to suggest using cast iron rings if the cylinder and piston is cast iron.

Why not go for o rings, I am sure they will be very effective if you get the clearances right. I think the mistake people make is getting them too tight a fit, it should be just touching the cylinder bore all the way round and the ring should be able to move in its groove. There is probably a standard for the clearances etc.

Nick

[baggo]

An 'O' ring brochure can be downloaded from here:

www.barnwell.co.uk/brochures.html

This gives the available sizes and also the design data for size of grooves etc. required. As Nick suggests, people tend to make the O rings too tight a fit which results in limited life. The pressure of the steam etc. helps to press the O ring against the bore and make it seal properly.

The width of the groove to house the ring should be about 1.5 times the width of the ring. This allows the ring to 'roll' along the bore rather than slide. Also the cylinder bore should be honed or lapped to as high a finish as possible. Leaving the bore as it is after just boring will rapidly destroy any O ring. I use an expanding aluminium lap and 600 grade Aloxite powder (left over from my telescope making days!) mixed with oil. It's important to make the lap from a material softer than the cylinder material so that the abrasive embeds itself in the lap rather than the bore!

John

[ron]

Hi All
I think I'm going to do a bit of experimenting, time permitting, a friend has given me a short length of very nice looking CI [meehanite?] I normally lap ST bores with a home made aluminium hone and very fine lapping paste anyway, I've initially made the piston with the O ring as per ST but plus a fine groove on each side of it to hold oil, Ill use this extra CI to make another couple of pistons, I'll try one with a PTFE ring [which I've done before but found quite tricky to get the clearance right] and another with a CI ring of approx 1/16" sq section, it was suggested that I make a tapered adapter from anything handy to ease the ring onto the piston, and see if there is any difference between them.
My own opinion, not based on any scientific evidence, is on a bore this small, a good fitting plain piston with three oil grooves, on a nicely lapped bore would probably be as good as any, but the efficiency of this would obviously depend on a very good fit and very parallel bore.

[baggo]

Hi Ron,

I'm experimenting with another plastic material as an alternative to PTFE. Depending on who manufactures it, it's called Peek HPV or TecaPeek PVX. It's a bearing grade thermoplastic (containing 10% carbon, graphite, and PTFE) with a much lower expansion than PTFE (but still about twice that of stainless or bronze). It has a low coefficient of friction even without any lubrication, will withstand superheated steam and has a high mechanical strength. My original idea was to use it for the piston valves in the 2-1/2" gauge loco that I am working on but I've also used it for the coupling rod bushes. One suggested application is for piston rings but unfortunately it's quite expensive in the larger sizes.

John

[lancelot]

Hello Baggo,
very interested in this ''PEEK'' stuff,reason being that I am building a large boiler ''TICH'', and recently acquired a part built tich on ebay,mainly for the boiler, which was brand new,also I intend to scavange as many parts as I can to push the build along a bit. Now the connecting rods dimensions vary from my build by 20 odd thou. and I was toying with the idea of rebushing them with this ''PEEK'' as you have done in your build, I also would like to use your idea of making slide valves out of it.
May I ask where you obtained it, what kind of tooling you used, and cutting speeds e.t.c.
All the best for now,
John.

[baggo]

Hi John,

I initially bought some of the rod from RS Components:

rswww.com/

They list it as Peek Rod - Bearing grade and sell it in 300mm lengths. It's by no means cheap e.g 10mm diameter is £9-76, 20mm dia is £36.86, and 30mm is £79-66 ! All prices plus VAT.
They don't actually sell the size that I really wanted so I did a search and found another supplier - Davis Industrial Plastics.

www.davis-plastics.co.uk/

They list it as Peek HPV rod. They have a much greater range of sizes but would only supply the size I wanted (12mm dia) in a 1 metre length at £36.07 + delivery and VAT. The prices are probably no cheaper than RS actually. I did order a length for making the piston valves as they are 7/16" dia.

I must stress that I am using it as an experiment and there is no guarantee that it will prove to be a satisfactory substitute for the normal materials. So far as I know, it's not been used in our context before so it's all unexplored territory!

The only machining I've done with it so far is making the coupling rod bushes and I just used normal tools and methods. The only recommendation I would make is that you make sure your tools are as sharp as possible as the material is quite soft and blunt tools won't cut it cleanly. I found it to be lovely stuff to work with, probably due to it's self lubricating properties.

I decided to try it for the rod bushes as silver steel crankpins sometimes get quite badly scored and it's been suggested this may be caused by using hard phosphor bronze for the bushes rather than a lack of proper lubrication. Using the Peek material should get around this problem.

I've just finished machining up the fabricated cylinder casting mentioned elsewhere so I will soon be ready to have a go at making the piston valve for it. My theory here is that with the material being self lubricating it should be possible to make the valve quite a tight fit without causing too much friction and straining the valve gear. Unfortunately, it will be some time before the loco is running and proper tests can be carried out. I will post any findings when available though.

With regard to use for slide valves, you may find that ordinary PTFE could be used which would be a lot cheaper I think. The problem of PTFE's high expansion would not be a problem in this case and could be allowed for if necessary.

John

[lancelot]

Hello Baggo,
thank you for the information, I will be contacting these suppliers in the near future to obtain some, will let you know how it proceeds.
All the best for now.
John.