Viton or silicone to make O rings

[mbrown]

Do any forum members know of a source of 1/16" dia Viton or silicon string for making up one's own O rings? I have a hank of Nitrile, acquired from a friend years ago, and used it to make a large diameter O ring seal for my smokebox door. It seals perfectly when cold but I am worried that Nitrile won't stand the heat when running and I'd like to replace it with a Viton or silicon ring. The O ring is about 3" dia - so a ready made-up O ring of over 3.5" could be cut and spliced to suit.- but I don't know where to source one of those in Viton or silicon either....

Any suggestions, guys?

Malcolm B

[Roger]

I you type 'O ring cord' into eBay you'll find loads of it.
You can also find it here, including the glue.

[uuu]

Simply Bearings list 1.5mm section viton rings in 1mm diameter increments up to 103mm

simplybearings.co.uk

Wilf


Edit - how about that - same supplier as Roger. I've bought from them several times.

[Roger]

Jan 24, 2015 14:31:34 GMT uuu said:

Simply Bearings list 1.5mm section viton rings in 1mm diameter increments up to 103mm

simplybearings.co.uk

Wilf


Edit - how about that - same supplier as Roger. I've bought from them several times.

Yep, they seem as good as anyone, I use them a lot.

[Deleted]

I use this company for my O rings:

www.polymax.co.uk/

They list cords in 1.5 and 1.6mm. There is a minimum order of £10.00 and usually a minimum order of 20 for O rings but postage is free between £10 and £15.

John

[steamcoal]

Viton is the only O-ring material to use due heat. That is what i was advised to use.

[mbrown]

Thanks all - I'll follow these up.

Best wishes

Malcolm

[andyhigham]

I use a viton O ring as the head gasket on my Greeves. Another happy customer of Simply Bearings, they are in the next town to me

[SteveW]

Guys, do we all know the bit about getting Viton too hot and causing it to decompose into components including hydrofluoric acid. That's the stuff that causes very severe burns in flesh requiring either an expensive and short shelf life antidote or (I've heard) surgery.

See Viton's nasty side..

Agreed, the temperatures quoted are a lot higher that we expect but if you discover a manky Viton 'O' ring it's worth knowing what that mank could be. I believe the other high temperature fluoro plastics like PTFE can be dangerous.

I was told of a machinist who was smoking while also machining PTFE. minute flakes of swarf got on to the hot bit of his cigarette and nearly killed him. Then there was the 6 months or so off work and poor health thereafter.

Folk need to be aware of this stuff.

[Roger]

Many thanks for sharing that, I had no idea these things could be potentially dangerous.

[andyhigham]

315 deg C is in the realms of firebox temperatures. If the steam circuit got that hot the paint would be burning off the cylinders, the lubrication oil would self ignite

[SteveW]

Guys, just so you know:

A data sheet.

If you read down there's mention of smoking in an area where Viton dust is present.

Clearly, Viton 'O' rings provide a very good and safe solution to some/many applications but it has to be regarded as a bit exotic and in this case not thrown on the fire when it's knackered.

[jma1009]

if i might be allowed to digress for a moment,

it isnt actually the smokebox vacuum that matters per se. it is the volume of gas that is ejected through the chimney. the draughting needs to be arranged on a coal burning loco to draw sufficient air through the fire through the resistance of the ashpan grate and coal fire bed and draw the gases of combustion through the resistance of the tubes. there are many more factors than just pure vacuum.

a small leak on the smokebox door will have little effect. the machining of the smokebox door and front of the smokebox and hinges and darts etc would have to be pretty bad to cause a draughting problem.

therefore i see no need whatsoever to fit 'O' rings or 'O' ring material to the smokebox door of a miniature steam loco. of far more importance is a properly designed and concentric draughting arrangement, a free steaming boiler, a grate with thin bars and ample air space (gaps), coal of the right size, and sufficient ashpan volume and openings. plus a bit of top air via the firehole door (which has pretty much the same effect as leaving the smokebox door partly open so far as vacuum is concerned through the firebed and grate and ashpan!)

smokebox vacuum in miniature is far lower than in fullsize and the amount of air drawn into the smokebox via a leaky door will be extremely small

cheers,
julian

[mbrown]

Thanks Julian - I quite agree. It's just that I like experimenting... Plenty of full sized locos had a packing ring around the joint between the door and the smokebox (from memory, I think it was a normal Darlington practice)usually using asbestos rope. When I was machining my smokebox door ring I trepanned a 1/16" groove in it for that purpose and, having got the groove, I wanted to see how good a seal I could get - hence the query! If the loco doesn't steam without it, I agree that something must be seriously wrong elsewhere. But I am an inveterate meddler, I am afraid.

Malcolm

[pault]

Andy, 315 degrees C is actually below smokebox gas temperatures. I have measured almost 500 deg C at the smokebox end of tubes and almost 1000 deg C gas temp in the fire box. That said whilst the dangers of PTFE and Viton being heated are possible this makes interesting reading.

www.hse.gov.uk/mvr/topics/fluoroelastomers.htm

[Jim Scott]

Jan 25, 2015 13:31:44 GMT pault said:

Andy, 315 degrees C is actually below smokebox gas temperatures. I have measured almost 500 deg C at the smokebox end of tubes and almost 1000 deg C gas temp in the fire box. That said whilst the dangers of PTFE and Viton being heated are possible this makes interesting reading.

www.hse.gov.uk/mvr/topics/fluoroelastomers.htm

Pault.

Thanks for posting that link to the HSE website. It seems to have been a bit of a show stopper as no one has commented since.

I hope everyone can and has read it, but in a nutshell HSE are likening this to an urban myth.

A couple of quotes from the Guidance Sheet:

"The truth

As with many urban or industry myths, there may be a tiny element of truth that makes the story more believable.........."

"Fact

There is no verifiable incident related to HF from burnt out vehicles anywhere in the world........"


I would love to hear the full story of the 'element of truth' incident. A 'test' of a Viton o-ring at 400C and 600psi in a sealed container? Dismantling under pressure? HF acid squirting out? More info needed; half a working life in engineering R&D has made me a great sceptic....

Several years ago I committed to using Viton o-rings as piston seals running in polished bores in my 5"G Terrier Earlswood. I have no qualms about this, even though I have since measured steam chest inlet temperatures between 300 and 350 C (on my Maid of Kent fitted with radiant superheaters). I am expecting that in operation the cylinder wall/piston temperature will be around half this value (much as Jim Ewins data suggests) and therefore below the manufacturers temperature limit for onset of degradation. The proof of the pudding etc. and this this will be a bit in the future but I will be far more interested to see how Viton o-rings fare regarding wear.

This is not to say I think this is the best way of making piston rings, who's to say I wouldn't use PTFE if I were to do them again today.

As has been said earlier, there really is no need to use a sealing ring on a smokebox door door given reasonable machining of same.

Jim S

[andyhigham]

Temperatures in excess of 315 Deg C may be attained within the smokebox but the smokebox shell and door will remain relatively cool due to conduction of heat to the air. Touching a piece of metal at 315 C would remove skin instantly

[jma1009]

in the summer of 1983 i received a nasty burn on my arm near the elbow when it came into contact with the chimney of the club's stationary boiler. skin removed, nasty blister etc. i still have the scar to prove it!

the boiler only worked at 50psi to supply steam for a large table full of stationary engines, but on that particular summer's day that chimney must have been pretty hot!

cheers,
julian

[SteveW]

Guys, the 'Urban Myth' is associated with burning cars. The alleged threat being from viton seals buried deep in the engine getting cooked and posing a danger to firefighters. I expect we've all seen the boiling water in a paper cup demonstration so will understand that the engine seals shouldn't pose a risk. Similarly, modern cars are full of plastics so burning cars do provide a real risk to health but from a different vector.

A decaying viton 'O' ring on a fire door exposed to hot gases or of unknown history can pose an unknown hazard. It's not quite in the realms of a radio active turd but simply treat the manky ones with caution.

In fact, if the makers include details of a hazard, whether its arse covering or not, it could be real. Buttered bread will drop butter down.

[Roger]

All duly noted though, and 'O' rings on smokebox doors are clearly not needed as long as a reasonable job is made of the two faces.