ivanj
Involved Member
Posts: 64
|
Post by ivanj on Nov 28, 2006 16:24:18 GMT
Back to my King!! This engine has piston valves which are stainless steel running in bronze, no rings. Should I make PTFE piston rings to effect a better seal or is the existing set up OK?
|
|
|
|
Post by baggo on Nov 28, 2006 17:45:12 GMT
Hi Ivan,
The combination of stainless valves in bronze liners should be ok, in fact it's standard practice. I've never seen reference to fitting rings in bronze liners, they are normally only used for cast iron. That doesn't mean it's never been done though!
The secret is getting the right fit and surface finish and that's where most people struggle I think. A plain turned finish on the valve is not usually good enough as even the finest of cuts still leaves minute ridges on the surface. These then wear off after a short while leaving the valve undersize and it leaks.
I always refer to Don Young's method of fitting the valves. He recommended turning the valves to a TIGHT fit in the bores such that they had to be knocked into the bores with a hammer and suitable drift. Before doing this the valves are coated with Molybdenum Disulphide grease. They are then tapped backwards and forwards in the bore until they become free enough to push through by hand. Every now and then remove the valve, clean it off and apply fresh grease. This process leaves the valve and the bore with a mirror finish and Don reckoned that the valves should then remain steam tight for the life of the engine. I've seen reference to situations where someone has removed a piston valve fitted in this way and then couldn't get it back in because the heat of their hand had expanded it slightly! That's the sort of fit you should be aiming for.
John
|
|
|
|
Post by Shawki Shlemon on Nov 29, 2006 10:04:34 GMT
Hi
I made mine from PTFE and am very happy with it .See my site .
|
|
|
|
Post by Laurie_B on Dec 1, 2006 21:23:46 GMT
A while ago there were a few articles in Model Engineer about the use of PTFE for rings for piston valves.
Mention was made of 'filled' PTFE,this supposedly being more suitable for this sort of application than normal PTFE.
I think it is PTFE with a graphite mixed in,though I might be wrong here.
Does anyone have any knowledge or experience of 'filled' PTFE?None of the usual model engineering suppliers appear to stock 'filled' PTFE as far as I can see.
|
|
|
|
Post by baggo on Dec 1, 2006 21:41:48 GMT
Hi Laurence,
I'm experimenting with a material known as PEEK HPV bearing grade which is a plastic type material loaded with10% carbon fibre, graphite, and PTFE. I'm using it for bearings in the loco I am building at the moment, also the piston valves (see my posting on fabricated cylinders). Unfortunately I won't have any definite results regarding it's suitability for piston valves until the loco is steamed in the New Year so I can't recommend it's use yet. Watch this space!
Jim Ewins used a material for piston valves called Fluorosint which is PTFE filled with mica. It's much harder than plain PTFE and has an expansion rate close to that of aluminium and only 1/5th that of plain PTFE. If you do a Google for Fluorosint you'll come up with suppliers and data sheets.
John
|
|
