Gallery, Projects and General > How do I??
Stainless Oscillating Pistons?
Stilldrillin:
--- Quote from: Darren on April 21, 2009, 05:33:30 AM ---Did the PTFE piston expand with steam or were you talking about the brass one?
So you pinned the piston then, that's good info to know :thumbup:
--- End quote ---
Darren,
This plastic piston lark isn`t quite straightforward........ ::)
The engine ran perfectly at room temp, on air.
Steam heat expanded the piston, locking it solid, as far as the engine was concerned. :scratch:
Piston into lathe, take off "half a thou" per side. Back onto engine....... Several times!
The lathe chuck is "true"....... But not that good, so the piston is not truly round now.
Tried a little `speriment this morning. Dunked a couple of cylinders & machined ptfe into a mug of boiling water.
Got it to slide in one cyl. But not in the other (which has a thinner cyl wall).
Measured o/d when cooled....... It`s only .001" or so below std (.3125")!!! :bugeye:
I get different readings from mic & vernier. Also depends weather the sun`s shining or not....... :bang:
I think I`ll have to steam individual piston/ cyl sets before mounting to the 3cyl engine...... :scratch:
The piston s are tapped 1/8 Whit & screwed onto the rod, with a spot of loctite.
David D
sbwhart:
David
One Little trick we use to do with nylon parts was to boil them, just like an egg, many plastics take up moisture and grow, I know that with nylon it can take up to about 1% of its own weight, don't know what it is for PTFE but you could try it before you machine to size.
Cheers
Stew
Stilldrillin:
--- Quote from: sbwhart on April 21, 2009, 08:13:19 AM ---David
One Little trick we use to do with nylon parts was to boil them, just like an egg, many plastics take up moisture and grow, I know that with nylon it can take up to about 1% of its own weight, don't know what it is for PTFE but you could try it before you machine to size.
Cheers
Stew
--- End quote ---
Sounds good to me Stew! :clap:
I`ll try anything....... :thumbup:
The used plastic piston measures around .309".
The boiling water plug measures around .312".
I guess steam is hotter than boiled water...... And it shows in the differing diameters!
David D
Darren:
I did mention plastics absorb water, I didn't think it was that much though !
bogstandard:
So now you lads understand when I go on about the correct materials for making cylinders and pistons out of.
That is why I always ask if it is going to be run on just air or steam. It makes a very big difference.
Darren,
If you are going to make the twin engine for running on steam, I would stay with the recommended metals. David's engine is a lot different in design, and yours will have a lot more heat, friction and stresses involved. You could modify my design to use PTFE piston rings, but I don't think there would be any advantage over the standard o-rings.
John
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