Potty Simpson and Shipton Short Stroke Engine

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sbwhart:
Thanks for you're comments Guys

Bill:- yes one or two tight spots the bearing centres are not quite correct had to slacken thing off to stop things tightening up needs a little shimming

Your correct about the action, the origonal had spring loaded sides, it a succesfull design only two made, only one of these ran. To start with I had 0.25mm end float from the piston this was far too much lapped the cylinder width down to reduce this to 0.05mm. It is getting better the more i run it, I've also spotted areas for improvement, but happy with it, I leave it aloan for a bit and keep running it I want to make a ball valve for it and a base so i'll get on with that in the mean time.

Stew

NickG:
Very nice again Stew, well done :beer:

sbwhart:
Cheers Nick :beer:

Still a bit along the learning curve to go with this engine.

To days lesson:- I made the piston from PTFE it seemed a good idea at the time, I know PTFE has some strange properties and that it has a high figure of coefficient of linear expansion, but I though that shouldn't pose any problem, I measures the width of the piston and the cylinder up and made them so that I had about 0.2mm clearance fit, I reduced this down to 0.05 to try and get a better run, but as I was doing this I was having problems getting a repeat size for the piston, this morning I stripped the engine down and measured things up the clearance was size for size, great i thought, tried it for fit and it all rolled over smoothly I fiddled about a bit, tried it for fit and things had tightend up, measured up again and I had an interferance fit :scratch:

Time for my morning coffee :coffee: so stuck the piston in the fridge for 1/2 hr measured up again and it had shrunk by 0.15mm.

Just done a google search on the properties of PTFE and as far as I can tell it has coefficient some 5 times greater than brass.

Time for a change of material i have some nice shash weight cast iron that John gave me I'll give that a try.

Stew

lordedmond:
Stew

Try some PEEK that what I use on piston valves , just polo mint looking bits with a stainless bobbin

but the big but and its a big un the price it OTT, or try Polly Models they do have some PPS Polyphenylene Sulphide again sit down when asking the price

Stuart

NickG:
Stew,

If the engine isn't going to be run on steam have you considered graphite for the piston? It worked perfectly in the two flame gulpers I made which require a very close fit and low friction. The way the graphite machines and its self lubricating properties make it an excellent choice for this. I don't know what it's coefficient of expansion is, but it must be very close to cast iron as those engines get very hot when they're running. If there was much difference it would either stop running or bind up ... it did with an aluminium piston.

Just a thought.

Cheers,

Nick

:offtopic:ps. are you going to Harrogate this year? Can't remember what day I said I'd go, will have to check the roster as will be helping on the club stand at some point!

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