How many rings per piston?

[Arthur Jones 2Participant @arthurjones2]

Hi all,

I’m building a 3 1/2″ gauge “William” 2-6-2T (Martin Evans) but rather than use gunmetal cast cylinders I’ve machined the cylinders from Meehanite blocks – not least because I’m still learning and did not want to waste money if I scrapped one or more!  I realise that 3 1/2″ gauge is at the bottom end of usual size for CI cylinders and that I’ll need to be very diligent about draining and oiling after runs.  I’ve nearly finished the cylinders now, just the mounting holes to drill and tap and the valve faces to finish-machine.  Steam chests, cylinder end covers etc. are similarly just about made, just the chest covers, valves and pistons still to make.

I am planning to use piston rings (rather than O-rings, packing etc.) and wonder whether one or two are required per cylinder.  The cylinders are 1 1/8″ bore and I have managed to get both of them more or less dead size (something like +- 0.0005″).  They are now honed.  I am proposing to buy the rings ready made from Reeves (yes, I know that making them isn’t terribly difficult but I am balancing time spent vs. cost, and time spent may be more valuable at the moment as I’m extremely busy!).

My question is: will one piston ring per piston be adequate or are two needed?  I’m tempted to go with one but realise that more are typically used on IC engines and full size locos.

Any advice would be appreciated!

Kind regards

Arthur

[Brian Baker 2Participant @brianbaker2]

Greetings Arthur, you seem to be making good progress, well done.

I think cylinders machined from Meehanite is the way to go, and intend doing that on my next build.

As you are using plain rings, I would be inclined to use two per piston, they could go in the same slot, but better if you can to separate them and make sure the ring gaps are 180 degrees apart, ie opposite.

Regards

Brian B

[duncan webster 1Participant @duncanwebster1]

Commercial piston rings tend to be a lot wider than need be, and hence cause more friction. Reeves won’t let me into their site unless I accept cookies, so I won’t be going there. I made the rings for on of my locos, 0.045″ square and 1.5″ diameter. I had access to a heat treatment furnace. As Brian says, 2 grooves better, one of the leak paths is underneath the ring, it’s a lot more tortuous with 2 rings in 2 grooves

[Clive Brown 1Participant @clivebrown1]

My Simplex has 2 x 1/8″ wide rings and seems to work well. The rings are from Reeves. The shorter piston of “William” with two similar rings might make the lands between the grooves rather narrow. I suspect one ring would be OK. It’s more important to get a good fit between the piston and bore.

After running and blow-down I always inject a goodly squirt of oil into the cylinders through the four draincocks, using a hypodermic syringe.

[SillyOldDufferModerator @sillyoldduffer]

Two on a steam piston.   In full-size often pinned so the gaps can’t rotate into alignment.

Trivia corner, they’re called Ramsbottom Rings after their classy Victorian inventor .

Dave

[Tim StevensParticipant @timstevens64731]

It might help you to know how piston rings actually work. This can help at the machining stage so you can concentrate on getting the sealing surfaces smooth – as long as you know which sides of what does the sealing.

What follows is based in ICE practice, but there are really only two differences between petrol and steam, in piston ring terms: The ICE piston works with pressure always on the same side as the sparking plug. Steam is more normally used on both sides alternately (and without any ‘gaps’ in the flow of power). What happens is this: The pressurised gas goes down between the cylinder head and the piston until it gets to the ring, which it pushes against the bottom of the groove. The pressure then can get behind the ring and acts to force the ring outwards against the cylinder wall. So, the rings rely for sealing on good continuous smooth surfaces on the lower face of the ring and the lowest face of the ring groove. On the way back up in a steam engine, the same process happens, relying on the fit of the opposite face of the ring and the opposite face of the ring groove. And it doesn’t seem to improve matters if you use a complex joint where the two ends of the ring meet up – but it does help to reduce the ring gap in the fitted condition as much as you can, on first build.

Petrol and diesel engines often have some identification etched or marked on their rings, and if you follow the pressure faces argument as described above, you will see why these marks are always on the top surface of the ring (as they are marked on a non-sealing surface). Both sides are critical for double acting (steam) engines, so both sides of the ring and its groove must be as near dead flat and smooth as you can get them.

The proper name of piston rings – just in case anyone else is interested in such details – is Ramsbottom’s Metallic Packing. John Ramsbottom was Chief Engineer of the Manchester & Birmingham Railway, later forming part of the LNWR, and he also invented tamper-proof safety valves and the most wonderful invention for small boys, the water trough between the rails so the speeding train could force water into the boiler at speed, and drench anyone who happened to be looking out of the windows. How do I know?

Tim Stevens

[Nigel Graham 2Participant @nigelgraham2]

Tim –

….. the water trough between the rails so the speeding train could force water into the boiler at speed, …

Not the boiler. The water scooped from the troughs went into the tender. The injectors fed the boiler, from the tender.

[noel shelleyParticipant @noelshelley55608]

Thank you Nigel, you beat me to it ! Noel.

[Nigel Graham 2Participant @nigelgraham2]

I had problems logging-in then accidentally logged out so I apologise if this appears twice.

 

Tim –

… the water trough between the rails so the speeding train could force water into the boiler at speed,…

Errr, I think you mean the scooped-up water feeds the tender. The boiler is fed by the injectors, from the tender.

[Arthur Jones 2Participant @arthurjones2]

Hi all,

Thanks for your replies – very much appreciated.  The Reeves rings I was thinking of are 1 1/8″ x 1/16″ so just a bit wider than the 0.045″ rings suggested by Duncan, but certainly narrow enough to fit two of them into a 1/2″ wide piston (three lands each 1/8″ wide with 1/16″ grooves in between).  The put-off might be the cost of four of them at £11 each plus postage, plus perhaps a spare or two given that I’ve never fitted rings before (sheltered youth, I guess).

I might have a go at making my own – I was hoping to avoid that.

Point taken about fit of piston in bore – but I also thought that some play was needed to avoid seizing during warm-up.  The school I was very fortunate to attend had a loco built by the pupils (unthinkable these days) which is still going strong 50+ years later (in the hands of another ex-pupil!) after much remedial maintenance.  The story goes that the piston ended up having to be machined down by quite a lot as it kept seizing when hot.  That piston is aluminium and the rings are/were motorbike rings.  I was going to use stainless steel for the piston (probably 316, not sure of exact grade as it’s unmarked) just to try to cut down on risk of rust buildup.  Its expansion is a bit more than for CI but not as much as aluminium.  I could make it of CI as I have some (limited) stock also. Any thoughts?

Kind regards to all and thanks again

Arthur

[Charles LamontParticipant @charleslamont71117]

A cast iron piston will be fine, with a normal running fit. And much easier to machine than stainless.

[bernard towersParticipant @bernardtowers37738]

If you make your own you can make them with a thinner wall therfore lowering the wall pressure and friction. Plus it adds to your skills which is what this is all about isnt it?

[Clive Brown 1Participant @clivebrown1]

I’d also stick with cast iron for pistons. Rings undoubtedly have advantages, especially for a loco that is intended for a lot of use but there are thousands of successful models made with just soft packing. Piston / cylinder sealing is far more important for an ic engine, which won’t run at all unless it has good compression. Model steam engines are rather more tolerant.

[Charles Lamont]

On Charles Lamont Said:

A cast iron piston will be fine, with a normal running fit. And much easier to machine than stainless.

Agree, and I suggest making a piston from an unknown Stainless Steel is positively asking for trouble.   There any many different stainless steels, many of them tricky to machine.  Worse, stainless steels are prone to gall, making them a particularly poor choice as a bearing material.    Stainless resists corrosion, but apart from that, it’s risky material in this application.

Dave

[ChrisLHParticipant @chrislh]

Arthur. If you decide to make your own rings, I would recommend perusing an article by the once well known professor Denis Chaddock. It actually refers to construction of a 5cc IC engine but covers design and manufacture of the required piston rings. It’s all very practical and does not require a degree in anything at all to understand it. Please PM me if you would like a copy.

[Arthur Jones 2Participant @arthurjones2]

Thanks again everyone for the advice.  Charles, Bernard, Clive and Dave: making the pistons out of SS is clearly not such a good idea as I’d imagined!  I’ll make them from CI once I have the piston rings sorted out.

Knowing what to make and what to buy in is always a bit of a balancing act – for example I’ve bought in the axle pump because it was not much more expensive than the castings!

On reflection I’ll have a go at the rings – as Bernard says, it’s a chance to learn.  Thanks ChrisLH for the advice on Prof Chaddock’s article.  I think this must be 21-Apr 1967 vol. 133 issue 3,319 p. 396.  I don’t have a copy of this and I’m not going to the club this week (clash with something else) so if it’s OK Chris I’ll PM you regarding that.  I do remember another article in the mid 70s that gave detailed instructions (but I don’t think involved lapping or finishing the OD) but am struggling to identify it.

Kind regards and thanks everyone again

Arthur

[duncan webster 1Participant @duncanwebster1]

Also have a look at Tubal Cains articles (the English one, not that American imposter)). Chaddock was doing rings for IC engines, TC allows for steam. When I get home I’ll try to dig out the reference

[Howard LewisParticipant @howardlewis46836]

It is important that the surface finish of the side faces  of the rings and of the ring grooves, is good.

A poor finish, and excessive clearance, will act as a leak path.

I C engines often use gas backed rings where cylinder pressure increases the wall pressure.

This improves sealing, and decreases friction when the piston is off load. Which is important in a four stroke engine, but less so in a steam engine, where the piston is likely to be loaded for both directions of travel for a double acting engine, such as a loco.

The ring gap should be as small as is safe. (If the ring ends butt, breakage  and bore damage is likely).

In an IC engine, minimal ring gaps (cold which will close up at running temperatures) reduce blowby, which is a loss of gas pressure that could be put to better use above the piston. Since pressures are lower in a steam engine, reduction of leakage becomes more important.

Howard

[John PurdyParticipant @johnpurdy78347]

Arthur

Besides Chaddock’s original article there some others that I have noted over the years that detail making piston rings.

ME Apr. ’67 Chaddock’s article

ME 5 OCT. ’73 Manson

ME 2 Jul. ’76 Chaddoch’s reply to Manson

EIM Oct ’79 John M. Tulloch

ME 17 Aug. ’84 G. Trimble

ME 21 Jan. ’94 Tubal Cain (a whole series)

ME 23 Jul. ’04 snippet of an article not sure of the author

ME 24 Apr ’09 Alan Beasley

ME 22 Feb ’13 John Dickinson

ME 27 Nov. ’15 Ben Crowdy

ME 20 Oct. ’23 Martin Gearing

These are what I have in my file and I have used the info to make rings with complete success. Hope this is some help.

John

 

 

[duncan webster 1Participant @duncanwebster1]

TCs information is also in his Model Enginners Handbook

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