What piston in a bronze cylinder?

[firebird]

Hi

I have started a portable engine build over in the "a work in progress" section. I'm thinking a little way ahead here. I have a bronze cylinder casting that with a little modification will be useable. I'm thinking of using a stainless steel piston. I will be steaming this model, hopefully coal fired, by the way. It won't be doing any real work just fired occasionally.

Can I run a stainless piston directly in the bronze cylinder?

Iv'e heard mention of making a PTF piston ring which has I believe low friction?

As always any advice will be most welcome.

Cheers

Rich

 

[Jasonb]

You will want piston rings or packing rather than just running metal on metal. With a bronze cyl iron rings may be a bit much better to have a couple of grooves to take either graphite yarn or the modern PTFE inpregnated equivalent. Silicon "O" rings are another option if you have a nice smooth lapped bore, the minnie in my avitar uses them.

I don't really see the need for stainless unless you have a bit hanging around, with rings bronze or brass will do and even CI though slightly more prone to rust. Alloy is another option with rings, the lighter weight giving a smoother running engine, thats what I have gone for in my 2" Fowler.

Whats the bore going to be BTW?

J

 

[Ramon]

Hi Rich, whilst I can't comment on using a stainless piston in a bronze bore I have gone the reverse as such and used bronze pistons in cast iron bores running both on steam (basically dried, not S/H) and air. These pistons, approx .002" down on the bore size, have all been fitted with a PTFE seal which has lasted a very long time without loss of seal. The seals were made from PTFE impregnated yarn which is obtainable from Reeves. It comes in three sizes and is square in section. I made the grooves .005" wider than the nominal section and .005" less in depth. Fitting it to the bore can prove a bit testing but by slightly tapering the very end of the bore for about an 1/8 really eases matters without affecting performance.

I should add that these engines never 'worked' just ticked over on low pressure. Lubrication was never more than just a good squirt of steam oil injected into the inlet before coupling up and doing the same at the end of the day and running it through for a few minutes when cleaning down to put it away. Never ever experienced any corrosion problems doing this.

Hope this helps some

Regards - Ramon

 

[firebird]

Hi Jason, Ramon

I don't run my models very often which is why I like to use non corroding materials ie brass/bronze/stainless/copper etc.

I have just measured the bore of the casting its .850 so I guess the finished bore would be 1 inch.

cheers

Rich

 

[portlandron]

A chart I copied of the internet has SS listed as poor when used with Bronze.

The following is the listing for bronze(E=Excelient, G=Good, P=Poor & B=Bad)

E for Cast Iron, Nickel Iron, Steel SAE 4140 H.T. & Tool Steel
G for Nylon Steel SAE 1020 & 1040 & Teflon
P for Oilite & Stainless Steel
B for Bronze & Cast Brass

 

[Springbok]

Piston in this instance should be cast iron and you can decide if you wish rings you wish to use..

Bob

 

[firebird]

Hi

Thanks for those answers. It looks like cast iron is the one to use. Are there any tips to prevent the iron from rusting?

Cheers

Rich

 

[Jasonb]

At the end of the day turn the lubricator over by hand a few times to pump some steam oil in and then turn the engihe over by hand a few times, if you are only having a displacement lubricator then just manually get some oil into the cyl.

J

 

[Till]

Don't use PTFE rings as they condukt heat poorly, heat up and thus emittoxicfumes at steamer or IC temperatures.
More info at http://en.wikipedia.org/wiki/Polymer_fume_fever

 

[Jasonb]

Well I doubt Rich will be running steam at anywhere near the pressure (800psi - 55bar!!!) and therefore temps needed for that problem to arrise so carry on with PTFE packing or solid PTFE rings ;D

J

 

[Ramon]

Hi Rich, I've just realised I made a mistake in my in post above oh:

The .005" wider and shallower groove for the PTFE packing should have read .002" - I guess I was getting my .05mm's mixed up with thous :-[ . (Don't usually work nor think in thous these days.)

Regarding lubrication - Just to elaborate a little I used to run, before I sold it, a Twin Victoria both on steam and latterly on air about once a year at a local model event. After those runs it sat in the workshop covered with a cloth without attention until the next year when the process was repeated. This went on over about fifteen years. About five years after the event began I built a Lang Bridge double diagonal engine which has run (on air ony) in similar vein ever since. Both engines had cast iron liners and bronze pistons fitted with the PTFE packing. Neither engine had a lubrication system fitted and though it was intended to do so on the Twin Vic it never did get done.

Before each running session I would inject with a syringe a healthy dose of proper steam oil into the inlet then couple up the steam or airline and let it carry it through and emanate out of the exhaust into a small tray (the ubiquitous tobacco tin, something else that did not get improved On steam I would do this perhaps three/ four times throughout the days running but only once/ twice at most on air. At home I would clean the engines down ready for storage - inject a good dose of the same oil and run it slowly through on air, turn off the valve and put it away until the next year.

Now I'm not suggesting here that you don't fit a lube system but doing this I never once experienced any form of corrosion in the bores or valve chests over the best part of twenty years so I can state that it worked - for me. Maybe this would not be so good if true rings were fitted but the PTFE never showed any effects and the seals remained/remain as good as when fitted. Should point out I guess that the workshop is warm, with no humidity problems which probably is a big factor too.

Despite good intentions at the end of the build to fit a decent lubricator to the Vic other matters simply lead to total prevarication and this just sufficed. :



Good luck with your engine

Ramon

 

[firebird]

Hi

Thankyou gentlemen for all the valuable info. A piece of 25mm dia cast iron arrived today so thats the piston sorted and I'll go with the ptfe rings I think. Its not the same ptfe that we use for sealing the threads on pipes is it? if so I have several rolls of the stuff.

Cheers

Rich

 

[Jasonb]

No its a cord that is impregnated with PTFE

https://vault1.secured-url.com/reeves2000/shop_item_details.asp?id=43/226

Though on small engines several lengths of plumbers PTFE can be twisted to make a rope. and used for gland packing, for pistons use the proper stuff

better description below
http://www.heritagesteamsupplies.co.uk/Z-Heritage-Steam-Supplies/Gland-Packing/p-297-154

Some people actuallt turn a solid ring from PTFE bar but I've not tried it myself

 

[Ramon]

That's the stuff Jason.

Rich - here is what has gone into my Waller table engine. Don't be put off by the dirty look -it looks much worse than it actually is. It started out looking just as in the link but will readlly pick up the slightest grubiness off your hands.

I have never tried turning solid PTFE either but thinking about it's application here as against this packing it would have to be turned to a very specific size to fit the bore without being too tight. The packing on the other hand has a high degree of compressablity and will 'adjust itself' providing the groove is right as above.

Personally what I like most about it, apart from it's ease of fit to a bore, is the fact that it exerts very little drag on the bore for a high degree of seal which is an ideal attribute for an engine that is just going to sit on the bench and tick over gently for display.

Regards - Ramon

 

[firebird]

Hi

Thanks Jason, very useful links.

Ramon, can you give me some dimensions of the piston you have shown.

Cheers

Rich

 

[Ramon]

Hi Rich,

The piston is 27mm dia by 13 thick.

The groove is a nominal 3/16 toleranced as previously stated for '3/16' packing.

The piston is fitted on a taper turned on the end of the rod then finished to final diameter to ensure concentricity
(That's not intending to teach granny BTW - just how it was done )

Ramon

 

[compspecial]

Hi Rich, I was just thinking that 150thou seems like a very large machining allowance for a casting, is it possible the finished bore should be 875 (7/8)? wouldn't want to leave the wall too thin.
Stew

 

[Jasonb]

I'd say 1" sounds right. a 0.875" finished bore would only give you 0.012" (875-850/2) to clean up which is very little to play with.

J

 

[firebird]

Hi

Thanks Ramon, its sometimes hard to judge sizes from a photo. I had planned on a piston of 3/8 (.375) thick with a 1/8 (.125) groove.

Stew, Jason. The piston will probably end up smaller than 1 inch. I'll machine as little as possible, the smaller the piston the less steam it will require. The casting walls are roughly .410 thick so theres plenty to go at.

Cheers

Rich

 

[firebird]

Hi

Slight change of plan here. I think I will be using a cast iron cylinder now so I'm assuming I should do the reverse and use a bronze piston ??? ???

Cheers

Rich