Stuart Beam Piston

[JasonBModerator @jasonb]

Andrew, that is not what I have found to be the case.

As I mentioned earlier Model Engineers handbook gives groove sizes for Model use and also shows the industry standard SAE grooves which are for pressures upto 1500psi.

Likewise to use Parker as an example as Ches linked to them if you look at page 3 of this pdf you will again see the grooves are for pressures upto 1500psi or 103.5bar, page 1 shows compression percentages upto 25% which creates a lot more drag than the approx 5% suggested for model use

https://www.parker.com/content/dam/Parker-com/Literature/O-Ring-Division-Literature/O-Ring—Images/Industrial-Reciprocating-O-Ring-Packing-Glands.pdf

If you take the BS 210 ring from that pdf as an example as this is what Ches was supplied with the suggested piston groove dia is 0.758″ which equates to a groove depth of 0.0121″. The ring is 0.139″ cross section so that is 15% compression. Compare that to the figures I gave of a groove depth of 0.132″ and that is only 5.3%

[Chris Taylor 3Participant @christaylor3]

Clive, thanks, that is exactly what I intend to do, everything seems fairly free, and I am intending to use the two rings I bought from Stuart models only after I have made sure everything works properly.

[Nigel Graham 2Participant @nigelgraham2]

Jason and Chas both mention running the engine without rings, though for slightly different reasons.

If the engine is only going to be ambling round at its prototypical 40-ish rpm on a whiff of air, not driving hefty (for it) machinery, the empty groove might function as a very simple labrynth seal.

The principle there is the sudden cross-sectional change for the air oozing past the piston then trying to squeeze into the next few-microns wide leak path, sufficiently inhibits the flow. I think more elaborate, formally-designed versions are used for example as turbine shaft end-seals.

It does demand a properly-fitting piston, though.

 

That curious dental-equipment (as it proved) air-compressor the subject of my thread elsewhere recently, has no grooves, rings, packing or anything like that. The single, double-acting piston is lapped into the bore, but crucially it is proportionately far longer in “thickness” than the disc in most model steam-engines.

[JasonBModerator @jasonb]

The slightly smaller 3/4″ bore 10V and 10H don’t use rings and there must be 1000s of them running with a good few doing real work like generating or in small boats. Their pistons are 1/4″ thick.

It is not much of  astep up to the the Beam and other Stuarts that use the same 1″ x 2″ cylinde, these will also happily run without piston packing or rings as I said in my first reply. These have a 3/8″ thick piston which can be used grooved for rings but may be even better if you just cut a couple of oil grooves.

A few examples of these engines 1″ x 2″ being run ringless. The bores are lapped and the piston “fitted” to the lapped bore – No bore gauges or micrometers were taken out of the draw in their construction!!

 

 

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