Getting a good steam seal on a small engine.

[Gas_mantle]

Hi all,

I'm a beginner to model engineering and at the moment I'm building a small horizontal steam engine, the piston is brass and 16 mm in dia with the plans recommending a graphite packed sealing ring. The smallest piece of graphite rope I've been able to get is 3mm square but this seems to me to be too big on a 16mm piston.

I've never used graphite before so not really sure how to use it, do others think a 3mm piece will be too large ? Can I unpick it or thin it down with a knife ?

Would making a sealing ring from rolled up PTFE 'rope' be a better solution ?

Any help would be greatly appreciated.

Many thanks
Peter

[Jasonb]

I just unravel a length of the graphite yarn and use what I need.

J

[kev]

You can get 1.6mm diameter "string" ebay sometimes or some of the model shops like maidstone or macc models

[b.lindsey]

Peter, how wide/deep is the ring groove?

Bill

[fumopuc]

Hi Peter, if you are located in the UK or Europe, may be this will help you.
http://www.maidstone-engineering.com/graphite-yarn-1-16-twist-approx-5ft-may-contain-asbestos

[Gas_mantle]

Hi all,

Thanks for the replies, because I've not done this before I think I may have bought the wrong stuff as it looks like it won't unravel. I'll try to take a picture later an show you the piece I have.

Now I know it's available in smaller sizes I think the best thing is to buy some more (the stuff that Maidstone sell looks nothing like the piece I have, so that also leads me to think I have the wrong material).

Bill, the piston groove is 1mm deep and 1 mm wide - the piston itself is only 16 mm dia and 5 mm thick so although there can be a bit of flexibility in how I cut the groove the piece of graphite yarn I have is far too big

Thanks
Peter.

[derekwarner]

Peter.......if you have a machining [lathe] facility you could consider

A standard size BS 2-014 O ring sizing is 12.42 ID x 1.78 section.......so that equates to 15.98 OD
So considering this, the piston seal cavity would need to be increased to say 12.5 cavity diameter x a nominal 2.0 height
This would allow for compression on the OD ...in the cylinder bore of 16mm ...and a little float

Naturally the o-ring material should be Viton or Silicone....both of which are totally suitable for the temperature and pressure of a model steam engine

Derek

[jerry kieffer]

Hi all,

I'm a beginner to model engineering and at the moment I'm building a small horizontal steam engine, the piston is brass and 16 mm in dia with the plans recommending a graphite packed sealing ring. The smallest piece of graphite rope I've been able to get is 3mm square but this seems to me to be too big on a 16mm piston.

I've never used graphite before so not really sure how to use it, do others think a 3mm piece will be too large ? Can I unpick it or thin it down with a knife ?

Would making a sealing ring from rolled up PTFE 'rope' be a better solution ?

Any help would be greatly appreciated.

Many thanks
Peter

Peter
       There are actually two considerations when working with a piston of this size.

Of course the piston must seal, but more critical in this size piston is friction.      Anything other than very low friction will require excessive pressure and engine speed in operation if the engine actually runs.    In general, Graphite and "O"rings will cause excessive friction but are most commonly used to compensate for poor quality construction.

If your cylinder and bore are properly lapped, you will have no need for rings in a steam engine of this size piston.

My personal test for proper fit is as follows.

(1) With a dry and clean cylinder/piston, the piston must freely fall through the cylinder under its own weight.

(2) With one end of the cylinder sealed, the piston is pushed completely into the other end.    When released, it must jump out of and free of the Cylinder.

  This type fit will offer extremely low friction as well as a good seal.

If for what ever reason you want rings, I would suggest unhardened cast iron with a cross section no larger than .37 mm and about 1.5 mm tension.

Jerry Kieffer

[sshire]

None of my air/steam engines have rings. The PMR 1/4 horse did but, after running it on steam and having it lock up tight, they were removed. It ran perfectly.
Can't do any better than listening to Jerry.
After boring the cylinder, I turn the piston and use the cylinder as a gage for the fit. I always test the fit using Jerry's technique.
If your fit is good, try running without the packing or rings.

[fcheslop]

For small steam engines I now prefer to use silicone O rings or make teflon cup seals as per a bicycle pump sort of two inverted cups 
Iv used ptfe tape tightly packed into the piston grove at a pinch
Blackgates have a list of seals and the dimensions for the grove required although been a rough ar*se I tend to guauge the ID using drill bits.
Here is a good bit of info on O ring application with the thanks to Sandy who kindly posted it on another site
My steam engines tend to be wobblers for marine use due to there reliability so it maybe of no help
Also a link to get some ideas about cup seals.
http://www.vapeuretmodelesavapeur.com/piston/index.html
This antique 1880s paddle boat engine is running on ptfe seals due to the cylinders having worn a wee bit in the hundred and odd years its sailed
<a href="https://www.youtube.com/watch?v=ZbahxB87K5I" target="_blank">http://www.youtube.com/watch?v=ZbahxB87K5I</a>


kind regards

[Gas_mantle]

Hi all,

Thanks for the help, I now have a few options open to me.

The plans I'm following suggest a graphite seal and I thought that may also help compensate for any slight inaccuracies in my machining (this is only the 2nd engine I've built so I'm no expert).

I did have another go at trying to unravel the graphite yarn I have and found it does unravel around a central core, do I use the strands or the core ? I did photograph the result but still can't manage to upload an image.

In the meantime I'll experiment with all the other options and as my engine nears completion decide which works best.

Thanks
Peter.