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turcite slide valve

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duncan webster05/11/2021 19:12:47
3928 forum posts
61 photos

A question for the tribologists

according to my sums, the piston force on my latest loco is ~80 lbs, but the force pushing the slide valve onto its face is 82 lbs. I can't find values for friction of gunmetal on gunmetal, all the references quote bronze on steel. Values vary from ~0.08 dynamic to 0.16 static, both lubricated.

Many moons ago I seem to remember people using stainless steel valves, or at least a stainless facing 1/8" thick like a picture frame silver soldered onto a bronze backing. I'm reluctant to use a solid carbon filled ptfe valve as the centre where the valve rod slot crosses the nut slot is only 50 thou thick and has 80 psi on one side.

I have considered a turcite picture frame glued to a bronze backing, but the recommended glue is 2 pack epoxy, and I doubt that would stand the 160 C or more temperature.

I know tradition has gunmetal valves on gunmetal cylinders, but the questions are

  1. would stainless facing be better
  2. anyone know of a glue for turcite which would stand steam temperature

I'm about to email Trelleborg, but considering I'd only need a few sqins I don't suppose they will be all that interested

Jeff Dayman05/11/2021 19:48:47
2223 forum posts
47 photos

Any way you could trap the turcite plate in a milled recess at the steam chest to cylinder block joint? If you could, the high temp epoxy would just need to seal the turcite to the cylinder face rather than securing it and sealing it. Just a thought.

I know epoxy and JB Weld etc are supposedly rated to 500 deg F / 260 deg C but in my experience they do not perform well at high temps especially if steam is present.

Pete Rimmer05/11/2021 19:56:59
1219 forum posts
63 photos

I use Araldite 2013 but it's only good for 70DegC

Loctite 9492/9497 would seem to fit your needs good for 180degC

HOWARDT05/11/2021 20:28:23
901 forum posts
39 photos

Turcite is good for 80deg C and Moglice not much better. Facing with Peek would work.

duncan webster06/11/2021 14:36:34
3928 forum posts
61 photos

Thanks for the info, looks like Turcite is a non starter. I've looked into Peek, and it might be possible to make it out of solid, it's a lot stronger, but this peek says the heat deflection temp is 150-170C. Steam at 80 psi is ~160C

I though of the recess idea, but I need to keep it flat as well, so it needs to be glued. However, a pusher each end is probably a good feature to take some of the shearing load if I do go for Peek facing. Stainless is looking good at present to avoid similar materials.

I've made cylinders and steam chest, so I'm not looking for major rethinks.

old mart07/11/2021 14:00:44
3721 forum posts
233 photos

What about some shim brass or bronze? That would glue and comes in plenty of different thicknesses.

Edited By old mart on 07/11/2021 14:01:00

David George 107/11/2021 17:24:25
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1808 forum posts
503 photos

Have you thought of using PTFE sheet it has a temperature range up to 260 deg C.

David

Michael Gilligan07/11/2021 17:54:16
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20096 forum posts
1042 photos
Posted by duncan webster on 06/11/2021 14:36:34:

Thanks for the info, looks like Turcite is a non starter. I've looked into Peek, and it might be possible to make it out of solid, it's a lot stronger, but this peek says the heat deflection temp is 150-170C. Steam at 80 psi is ~160C

[...]

.

I don't know what material they use, but IKEA does a range of silly-cheap kitchen utensils which appear to be heat resistant to such temperatures. The black ones appear to be particularly good.

MichaelG.

.

Ref. https://www.ikea.com/gb/en/p/gnarp-3-piece-kitchen-utensil-set-black-30335841/

Edited By Michael Gilligan on 07/11/2021 17:57:53

Michael Gilligan08/11/2021 19:29:04
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20096 forum posts
1042 photos

Presumably of no interest to Duncan, but may be useful to someone:

The black plastic is rated 220°C [428°F], and is identified as >PA66(GF15)<
… whatever that might be.

It saws and files nicely, but I haven’t tried machining it

MichaelG.

duncan webster08/11/2021 21:17:47
3928 forum posts
61 photos

According to the interweb PA66 is glass loaded nylon and is rated for 110C long term PA66

Following advice on another forum I'm going for Phos Bronze, sufficiently different in hardness to avoid any problems, although with decent lubrication no-one seems to have a problem with gunmetal on gunmetal

Michael Gilligan08/11/2021 22:18:10
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20096 forum posts
1042 photos
Posted by duncan webster on 08/11/2021 21:17:47:

According to the interweb PA66 is glass loaded nylon and is rated for 110C long term PA66

.

For info. … It’s the GF15 bit that denotes the addition of Glass Fibres, PA66 being the matrix.

MichaelG.

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