triple expansion engine piston rings

On stripping the Bolton/Bertinat triple expansion engine that I had machined and assembled in the 1990s (never run because I hadn't by then built a boiler of sufficient size), I find that there are no piston rings in the IP and LP cylinders.

The design calls for hard brass rings of varied cross section and since the LP is 1.5" diameter I now remember that the material was not to hand - still isn't.

Smaller engines seem to run OK when fitted with graphite string packing in leu of rings but I'm concerned about its fixing in the larger grooves (already machined)

Any suggestions of suitable materials to go in 1/4 x 1/8" deep gunmetal piston grooves which will run on steam?

Tony

Cast iron rings made by the Chaddock method are very good, but that section is too thick and you would never be able to spring them on over the pistons, especially if the smaller pistons have the same groove size. If it was mine I would be making new pistons with grooves according the proportions suggested by Chaddock.

John

I'm wondering if the design calls for hard brass rings because the cylinders are cast-iron? If so Bronze might be suitable instead of Hard Brass. How about 1½" diameter PB1 from Noggin End, yours for only £10 per inch.

Who told me this was a cheap hobby?

Dave

Would it not be OK to use 2 no. x 1/8 wide rings, to overcome the ring rigidity problem mentioned?

Solid rings necessitate multi piece, built up pistons, and will be less flexible than a ring with a gap. Alignment of the components of a built up piston, at least three with two rings, plus piston rod? may be less accurate..

If the rings have a gap, it should be possible to expand them to pass over the piston. After all, that is the method used in I C engines which are subject to much higher pressures and temperatures than a steam engine, even with superheated steam. The CLUPET ring design, although probably not easily produced in such a small size allows ring expansion whilst reducing leakage to a minimum.

The fitted gap should be 0.001" per Inch of bore diameter, to miminise leakage through the gap, and may probably accommodate any slight out of roundness that may occur in running. Since the metal sections around the cylinder are unlikely to be completely uniform, as temperatures move away from the ambient when machining took place, the bore is unlikely to remain absolutely "as machined"., and cylinder temperatures will change along the cylinder, and during running, so dimensional changes are quite possible.

Howard

From the photos here **LINK** it appears the engine has gunmetal cylinders, so I don't reckon cast iron rings are going to suit. I'd make some new pistons and use o-rings with only about 4-5% nip. Still needs new pistons as a 1/4" wide groove is far too much.

As the LP is 1.5" diameter, the HP will be quite a bit smaller, so even bras rings 1/8" thick would be a challenge to get on without distorting

Edited By duncan webster on 16/09/2020 20:11:05

The Bolton triple uses two piece pistons so no problem getting rings into place, rings shown with a diagonal cut.

If you don't want to go with teh brass then a reducer could be fitted then you can use O ring, graphite yarn or whatever you fancy.

Is it not 1.75" LP?

Edited By JasonB on 16/09/2020 20:35:30

I hadn't realised that the Bolton has gunmetal cylinders. Would brass rings run OK on that or are the materials too similar? Cast iron will run happily on cast iron, but that is about the only thing that will run well on itself.

On the split piston thing, if a ring is stiff enough to need a split piston to be installed, then it is too thick to do any good anyway. They need to be flexible enough for the gas pressure behind them to give the sealing force.

O rings are fine for this sort of engine, especially since they don't usually do a lot of work, much less than a typical; model loco for instance. You do want a good smooth finish on the bore.

John

The engine has indeed got gunmetal cylinders and two part gunmetal pistons. The hard brass rings of the design have a scarfed joint (gap) and an excentric bore giving a 3/32 thickness opposite the gap which is at a 1/16 thickness.

I have made one for the HP cylinder which will have steam at about 80psi inlet. Since i'm going to the effort of a complete rebuild I think I will get some material and follow the design for the IP and use graphite in the LP which exhausts into the condensor. It is not an easy engine to work on once assembled and has required more modified spanners and nut runners than any other project I have undertaken.

Thanks for the suggestions

Tony