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Post by baggo on Feb 11, 2007 1:03:05 GMT
This is prompted by SteamJohn's thread on balanced slide valves really.
Has anyone considered using or actually used PTFE to make slide valves instead of the usual bronze or cast iron? In theory it seems a good way to reduce the friction and wear in unbalanced valves. It would be easy enough to machine the valve from a block of PTFE or a slice cut from a piece of bar. I'm machining a set of cylinders for Rob Roy at the moment and I'm thinking of giving it a try.
John
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Post by Shawki Shlemon on Feb 11, 2007 6:48:57 GMT
Certainly it will do both , I have never thought about it although I used PTFE for piston valves . I just finished the cylinders for dukedog in bronze , everything is installed and completed but I may try it on my next project . I think is a good idea , it can't do any harm trying it .  |
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Post by greasemonkey on Feb 11, 2007 8:50:35 GMT
you'd have to machine them undersize to alow for the expansion at temperature or accept a change in the lap and lead. PTFE expands more than bronze or cast iron
Andy
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Post by baggo on Feb 11, 2007 10:26:54 GMT
Hi Andy,
It shouldn't be too difficult to allow for the expansion. A lot less fiddly than fitting a PTFE piston valve! By my reckoning a 1" length of pure PTFE will expand by 0.016" when it's heated to 160° C so the initial length of the valve and the exhaust cavity can be calculated to suit. I've ordered some blocks of PTFE to play with so we'll give it a go.
John
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Post by ron on Feb 11, 2007 11:05:59 GMT
John
It sounds like a good idea, I'd be interested to know how you get on,
Ron
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Post by baggo on Feb 11, 2007 13:03:56 GMT
I'll keep everybody posted Ron . The piston valves I made from the PEEK material have been run on low pressure steam (at 120° C) and seem ok so far. Hope to test them on full blown boiler pressure soon!
John
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abby
Statesman
Posts: 927
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Post by abby on Feb 11, 2007 17:38:23 GMT
It does machine beatifully , I tried a ptfe piston, well front and rear seals with brass centre , in a small engine years ago with favourable results and was planning to use the same method in DOT.
Abby.
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Post by Deleted on Feb 11, 2007 18:19:10 GMT
If I were to do this I would use Flourosint, as it has a similar coefficient of expansion to bronze and is more stable when machining, far easier to deburr also. Being brown it is a lot easier to see what you are doing too!
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Post by steamjohn248 on Feb 11, 2007 22:39:17 GMT
Has anyone any experience with DELRIN? We've just started running a HERCULES with Delrin Die Blocks so too early to tell yet and of course they wont get as hot as a valve or piston but are getting all the oil and grit that seems to be attracted to the motion. (We use it in the boatyard for underwater rudder bearings and thought we'd give it a try).
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Post by gargamel140 on Feb 12, 2007 22:19:46 GMT
Dear all, This guy (http://www.sci.fi/~animato/3003/060o.html) in Finland uses PTFE a lot and drilled small holes in the valve 'rings' to handle the expansion. I think the pictures says enough. Browsing around this great site you will also the same PTFE rings after one year of usage and no signs of real ware www.sci.fi/~animato/steam/460/460m.htmlmichel |
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Post by Shawki Shlemon on Feb 13, 2007 8:43:37 GMT
Hi Michel
I saw the website ,it is great and particularly the piston rings. What I didn't understand is why he soft soldered the dome covers to the domes . What if he needs to open the domes for any reason ? I have never come across that before . just a question and is not meant to be a criticism .
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Post by alanstepney on Feb 13, 2007 9:10:24 GMT
Jan-Erik does some excellent work and is very knowledgeable.
Exploring his website is worthwhile for the ideas one can pick up.
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Post by gargamel140 on Feb 13, 2007 22:19:06 GMT
Hi Shawki,
No idea why he did this soft soldering. I think this guy has some innovative ideas and the use of PTFE in combination with O-rings for sealing was something I had never seen before. To be honest I use the Internet as my main source of information and without doing a lot machining myself I can level a bit with my fellow steam friends who did/do scratch builds. I'm more the Winson/Modelworks type of guy, but then you definitely need to know more about about pistons and valves:)
regards, Michel
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Post by baggo on Feb 13, 2007 23:03:24 GMT
I found Jan-Erik's site some time ago when I was searching for sites on building steam locos. It's been a great source of inspiration for me and prompted me to do a website of my own. H. Watanabe and Erik-Jan Stroetinga's sites are also well worth a look.
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Post by Shawki Shlemon on Feb 16, 2007 9:27:50 GMT
Hi michel
I have seen this PTFE and O ring combination in aircraft industry . It is certainly effective , but not necessary for the models.Every person has his limits and I respect every opinion.
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