[modeleng] Re: Modern Material

Hi Barry,

I've made solid piston valves from PEEK HPV which is a high performance 
bearing grade plastic filled with PTFE, Carbon, and Graphite with some 
success, apart from the difficulty of getting the correct fit at working 
temperature due to the expansion (not a problem with slide valves!). 
Using this for slide valves should be no problem as it will stand the 
temperatures involved and it's very easy to machine. I've used it for 
all sorts of bearings including coupling rod bushes etc.
I made a set of slide valves for my brother's Rob Roy from solid PTFE 
which seem to be ok so far although it hasn't run a great distance yet. 
I have heard mention of PTFE valves distorting but as mentioned above, 
the ones I made haven't. The carbon filled variety would probably be 
more rigid and perhaps more suitable.

John

Barrie Purslow wrote:
> I am currently refurbishing a 5" gauge Simplex which has non ferrous 
> cylinders. The slide valves are rather badly scored so I have skimmed the top 
> of the cylinder block no problem but I am reluctant to skim the valve itself 
> as it is obviously not made of a material with good rubbing characteristics 
> against the cylinder block.
> I have it in mind to make new valves out of carbon filled ptfe which I have 
> previously used for a regulator valve with good results but this duty is a 
> little more onerous. 
> Any thoughts anybody?
> Barrie
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