Piston rings

[Peter Simpson 3Participant @petersimpson3]

Looking at plans for a stream triple expansion engine, the piston rings are machined elliptically. Unlike IC engines. anybody know why ?

[SteviegtrParticipant @steviegtr]

have watched quite a lot of youtube vide’s of people making CI rings. For both Steam & IC. I made mine round for the IC engine. But I did watch a lengthy video of Mr Crispin making some for a Steam engine & I am sure they were perfectly round.

Steve.

[Mike HurleyParticipant @mikehurley60381]

Complicated. But in essence I think it’s to do with more efficient sealing at different stages of the temperatures in the cylinders. IC engines have used this concept, mainly in racing / high performance engines.

Quite a ‘deep’ subject when you get into it.

Regards Mike

[duncan webster 1Participant @duncanwebster1]

There is a very good bit about piston rings in one of Tubal Cain books (the UK one, not the American usurper) it was in ME many years ago, but I think that had misprints in some of the formulae

 

[JasonBModerator @jasonb]

Does it actually describe them as being made elliptical or does the drawing simply show a round machined ring in it’s “sprung” shape? As they may not be round when the gap is opened up and heat treated. What is the engine?

[bernard towersParticipant @bernardtowers37738]

How is an elliptical ring going to fit a round bore?

[Peter Simpson 3Participant @petersimpson3]

The piston ring dimensions. There is also a jig shown on how to achieve these dimensions

[JasonBModerator @jasonb]

That looks mor eeccentric than elliptical.

[Andy_GParticipant @andy_g]

I believe the design is an attempt to create a uniform wall pressure from a split ring – I think it is Frederick Lanchester’s invention. I’ve no idea if it’s relevant for model sized cylinders.

 

[duncan webster 1Participant @duncanwebster1]

 

[duncan webster 1Participant @duncanwebster1]

Dunno what went wrong there!

[mike robinson 2Participant @mikerobinson2]

That is the Bolton Triple, something I am still making – 3 years so far – the part count is 1,248 including studs and nuts. The piston rings are not fun to make especially the for the HP cylinder which, per the drawing was far too strong. You machine a (brass) concentric ring with the outside diameter larger than the bore size, cut the ring slit at 45 degrees (holding is not easy, I slit mine on the lathe). That slit or ring gap eventually creates the radial ring load. Then starting with the LP ring, turn a finely made mandrel and a 5/16” thick clamp washer you clamp the washer up to hold the ring firmly but not tight then attempt to wrap wire around it to compress until the slit fully closes up (I used stainless wire locking wire). Then you set the ring to give you the required eccentricity as measured with a DTI (where the wire gets in the way) then clamp the washer up fully, check eccentricity again and then machine the outside diameter to match the cylinder bore size (noting the mandrel O.D. has to be say .010” smaller than each cylinder size, so start with the LP ring and work your way down. The eccentricity is now on the inside with regard to the bore but there is clearance in the ring gap of the piston halves.

When I can work out how to upload photos I will do…..

[mike robinson 2Participant @mikerobinson2]

Found the way….

[John OlsenParticipant @johnolsen79199]

There are a lot of different opinions on how piston rings should be made. I’ve only tried the Chaddock method, which works fine for me. This involves turning a ring which fits the bore, then breaking it at one point. The ring is then fitted to a jig which holds it with a gap, and heat treated. For the heat treatment, the rings needs to be protected from air and not overheated, I believe just under red heat is actually ideal.

The engine in Dancer , my 30 foot steam launch, has rings made by this method and so far they are giving excellent service. They range in size from one and 3/4 inches up to five inches. There are four sets of rings since both valves are piston valves.

Rings don’t need a lot of pressure against the cylinder wall, since the gas pressure(steam or IC) gets behind them and presses them out to the wall. For this reason they don’t need too much radial width or wall pressure since they must be flexible enough for the gas pressure to push them out. They also need some axial clearance in the ring groove for the gas to get behind them. But in high speed engines they must not have too much axial clearance, since they swap sides in the groove at each dead centre and if the groove is too wide the shock will bust them up.

Rings don’t need the ends to be angled like in the above pictures, nor are clupet style rings really needed. Note that modern car engines, at least the good brands, can do hundreds of thousands of miles without needing any attention to the rings other than regular oil changes.

Incidentally the rings on the piston valves in my launch engine are not pinned in place. Pinning is necessary with large ports like on two stroke motorcycle engines. My valves have many small ports all the way around the valve, so the ports are not wider than about 10% of the bore. With small ports there is no problem with the rings catching in them, at least they have not done so yet after about four years of operation. It is better for rings not to be pinned where possible, since by turning in the groove they help keep the grooves clean.

if your rings are too stiff to be sprung over the piston crown to fit them, they are too thick radially.

John

 

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