PTFE Bobbin material

[johnd]

Need a bit of advice as to what is the best material for a set of piston valve bobbins.
Looking to make new valve PTFE bobbin assembly’s for the Hielan lassie after reading excellent artice on John Baguley's web description.
Have a choice of 25% Carbon filled, 25% Glass filled or virgin PTFE for the bobbin sleeve, but not sure which one would be the best option.
Any advice welcome.

Daggers

[Roger]

If you can afford it, Fluorosint has about a third of the thermal expansion and has the same melting point as virgin PTFE. That helps overcome the biggest issue. ie it growing in size when it gets hot.

[andyhigham]

It appears to me that piston valves are a big problem in bronze cylinders. Cast iron cylinders are easy, the valve can have a cast iron piston with cast iron rings.
I know it sounds a bit like "I wouldn't start from here" but If I was building a piston valve loco I would go down the cast iron route.
Another possibility following IC engine techniques would be cast iron rings in a "Nikasil" plated bore

[Deleted]

+1 for fluorosint, it's expansion rate is similar to bronze. I wouldn't use virgin PTFE although this was probably what most used when firsr going the PTFE route. The other popular type among model engineers is PEEK which may be slightly cheaper than fluorosint.

Pete

[johnd]

Just to clarify the cylinders and valve body’s I’m restoring are bronze and completely machined it just the fit of the bobbins that concerns me.
Therefore i am remaking the bobbins using ptfe sleeves. I have ptfe in stock in the three varieties mentioned above just not too sure which of the three would be best.

[Roger]

Personally I'd use the Graphite loaded one.

[John Baguley]

I've always had very good success with the virgin PTFE for both piston valves and piston rings but that doesn't mean that it's the best choice

I keep meaning to try some of the graphite filled stuff for piston rings as I think it is harder and springier which will help to keep the ring against the cylinder bore. For larger cylinders using virgin PTFE I put an O ring under the ring to keep the ring against the cylinder wall. I find that's not needed for smaller cylinders. I rather baulk at the very high price of Fluorosint so haven't tried that!. Possibly the bearing grade PEEK (containing PTFE, Graphite and Carbon Fibre) that I tried originally to make the solid piston valves might be good for valve heads and piston rings?

I think the secret with plain PTFE valve bobbins is to keep head as thin as possible so that expansion becomes less of an issue. The heads on my Helen Long are only 0.063" thick and are as steam tight now as when I fitted them many years ago. I'm surprised that they haven't failed due to the very high steam temperatures that the superheaters produce but they are still going strong.

I'd agree with Roger though - give the graphite filled PTFE a try.

John

[johnd]

John/Roger

Thanks for you interest.
After reading your comments i will give the carbon loaded material a try. The bobbins ar 1/2” diameter and i intend to make the wall thickness 0.063” as mentioned.
What i may do is make some test sleeves, boil them and measure the difference in diameter between virgin and the graphite loaded. Not very scientific but may be interesting.
Daggers

[oldnorton]

I do agree with all of John B's comment, a lot of sense. The thin bobbin head idea perhaps originated in Australia. The 25% graphite filled PTFE does feel a bit firmer to machine so it is a better bet for that reason. I have never seen any reported data on the lower thermal expansion rates for graphite filled vs. virgin. Since it is 25% filled is it just 25% less expansion? It is hard to see how it could be better.

What most people don't appreciate is the significant expansion of PTFE around room temperature, or between 16 -26 degC. This is just the range over which it will warm when on the lathe, which means that parts that fitted when cold suddenly will not fit when they come off a warm mandrel. The graph below is one that I copied from the web somewhere. I have seen similar from other sources.

Norm

[chris vine]

I think the filling material, even if only 25%, could have a larger effect that one might think.
The carbon filling should have quite a high stiffness, so it could constrain the expansion of the raw ptfe.

Chris.

[oldnorton]

Chris

I appreciate your thoughts on this, because I have wondered about it. If the carbon/graphite is put in the PTFE as a powder when the polymer is made, is the graphite not like a lot of unattached currants in a bun? Or does the carbon chemically bond? There is also glass filled PTFE which I understand is sold as being dimensionally stiff.

Norm

[Jim]

This is the process I followed in making the Teflon rings for Boadicea's piston valves which interestingly have the lands for the rings machined .003" undersize to allow for the rings expansion. Sadly I can't say if they are carbon filled, glass filled or virgin PTFE all I can say is that the material feels 'soapy' to touch.

modeleng.proboards.com/thread/6349/70036-boadicea?page=6

Not much help I'm afraid.

Jim

[Gary L]

Mar 6, 2021 17:29:03 GMT oldnorton said:

Chris

I appreciate your thoughts on this, because I have wondered about it. If the carbon/graphite is put in the PTFE as a powder when the polymer is made, is the graphite not like a lot of unattached currants in a bun? Or does the carbon chemically bond? There is also glass filled PTFE which I understand is sold as being dimensionally stiff.

Norm

I used plain PTFE for valve rings in a Speedy, after reading John Baguley’s description, and like him I found them long lasting and completely satisfactory. They do expand quite a lot when hot, thus there is blow-by at normal temperatures, but that was never a problem; they warm up in seconds. That isn’t to say that a filled PTFE wouldn’t be even better, I wouldn’t know. But I don’t like the sound of glass reinforcement; isn’t that going to cause wear and be counterproductive in this application?

Gary

[John Baguley]

I'm sure I read some years ago that the glass filled PTFE is quite abrasive so probably not suitable for this use. I'm not sure what it's intended use is. Maybe for some static aplication?

John

[oldnorton]

Agreed, I wasn't suggesting the glass filled would be our choice for sliding components like rings. I think it is used for more rigid seals and low speed rotative bearings as it overcomes the PTFE plastic creep. I was referring to its similarly lower thermal expansion, like graphite filled.

Norm

[Deleted]

I'm not sure what the make up of Fluorosint is but having machined it I'm pretty sure that there's nothing abrasive about it. It doesn't form chips when cutting, it's just long continous strings of soft plastic, very strange.
If it wasn't for the spec sheets I wouldn't think it suitable for our purpose.
If anyone has specific knowledge of the Fluorosint material I'd be very interested to hear their view.

Regards

Pete

[John Baguley]

I've always been under the impression that Fluorosint is bronze filled but looking it up online suggests that it's filled with synthetic mica? No mention of bronze.

John

[Deleted]

Mar 7, 2021 12:47:31 GMT John Baguley said:

I've always been under the impression that Fluorosint is bronze filled but looking it up online suggests that it's filled with synthetic mica? No mention of bronze.

John

I recall reading something like that too, one spec that gained my interest was it's expansion rate being very similar to bronze. Having bronze cylinders myself it made good sense to use it. When the stock arrived it looked/felt as I had expected, that is, very rigid and hard. Once machined though it becomes much softer, perhaps I should take another look at the bobbins I made to see if they have hardened while stored?

Pete

[oldnorton]

OK managed to post the graph that shows the non-linear expansion of PTFE - see above original text.

[John Baguley]

Mar 7, 2021 13:02:12 GMT @greenglade said:

I recall reading something like that too, one spec that gained my interest was it's expansion rate being very similar to bronze.

Ah, that's where I got the bronze bit from !

John