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[kno3]

Hello!

Does anybody have experience with using Teflon as a replacement for the rubber o-rings that seal the piston in steam cylinders?
I read on Regner's website that they recommend Teflon tape, twisted to a round thread. I tried this with the piston of a flame eater and it seems to work, it seals well and the friction is low.

So I was thinking of replacing the rubber o-rings of my steam engines with Teflon, to reduce friction. With o-rings the friction is considerable.

Did anybody try this?

[Archibald H.]

Hi kno3,
Many steamers on the dutch 'Modelbouwforum' use the Regner-teflon-technique. On the forum there is someone that prefers hempfibres in stead of teflon, apparently the hempfibres absorb moisture and through swelling close the gaps between piston and cylinder.
Hope this helps.
A!H.

[kno3]

Yes, thanks. I was mainly thinking about reducing the friction, which in case of rubber o-rings is considerable. This would, in my view, improve the ability of the engine to run slowly without stalling.

[SteamboatPhil]

The trouble with O rings is you have to fit them properly, and they do have close tolerances to get them to work efficiently. I have used grafite yarn in the past, the trouble is if you leave them unattended / no oil (heavens forbid) for a long period,the yarn goes hard and you end up having to knock the bl**dy piston out (guess how I found out).
I have used teflon tape as you suggested Kno3 not only for the pistons but in the gauge glass  and pump ramps. In my humble opinion much much better, and easier to work with. To date no problems (one season of running every weekend) Pumps have never worked so well, and the engine has got it's "grunt" back (and its 30 years old--I was just too lazy to make a new piston)
Hope this helps some.

[Bernhard]

Hi.....Phil...is it the same tape,,used for pluming,,and can you show how you did it
regards Bernhard

[kno3]

Phil: Thanks, I will try it on my steam engines too. Did it only to the flame eater so far.

Bernhard: Yes it's the Teflon tape you can buy for plumbing. You just roll it to a thread and wind it a few times in the piston's groove.

[sheerline]

If it's of any use, I have a small steam loco which has preformed piston seals apparently made out of Teflon. The seals had to be fitted to the pistons and inserted into the cylinders.  It was recommended the whole lot was then boiled for five minutes whilst assembled and after cooling, found to be perfectly sealed. The strange thing is that having been pre-assembled by someone else who had failed to carry out this procedure, the pistons were found to leak- by quite considerably, despite endless steaming to bed them down. It would appear Teflon seems ok for moderate steam use but I am not sure if it will withstand exceedingly high temp superheated steam, however it certainly gives a good frictionless seal under normal conditions.

[kno3]

By the way, how much should the Teflon packing protrude from the piston groove?

[sheerline]

I also used the plumbers white ptfe tape on a ' V' twin I built and wound it around the piston until I coud just get it in the cylinder. My pistons had a groove in them and were a good snug fit but after a few high speed runs the seal began to leak. I tried adding more tape but it was never truly effective. In the end, I resorted to carefully machining the piston groove to accomodate soft silicone rubber 'o' rings and once I got it right, the engine runs quite happily with those in place. I have given it a few hours running and a damned good thrash just to see if they wear out but it still seems to be ok.

[Proteus]

if you are going to try other materials get some new O rings so you have a yardstick don't forget the O rings don't like WD40 in case you use it to assemble as I did once.also a picture of the type of water gauge you need and a picture of the water pump that is an extra.

also the safety valve is a  5/16  x   26

Proteus

[Bee]

Don't forget that o-rings come in different paterials. there are some plain plastic ones for plumbing that won't take too well to being hot. there is often bursts of mail about ptfe o-rings in Model Engineer magazine. Possibly just a scare story or a bad batch but some were reported to breakdown under superheated steam and release acid resulting in corosion. When fitting I recall it is important to make the groove wide enough to allow the ring to roll as the piston direction is changed.

[derekwarner]

Teflon and Viton have totally differing levels of hardness...so much so that teflon hardness measurement scale as Shore D [rigid material], where as Viton measurement as Shore A [flexible material] - Teflon is approx 60D, Viton is approx 83A...or Teflon is greater than twice as hard as Viton

Teflon is rated to 260 degrees C, Viton to 200 degrees C in application - nitrile [NBR] is only rated to 100 degrees C so is unacceptable for steam applications

 
Raw Teflon will not pressure energise at 2 Bar nor 20 Bar...if Regner have Teflon piston seals I suspect they have used a common industrial hydraulic seal being a composite two piece Step Seal......comprising a Viton O-ring with a Teflon outer ring - the fluid under pressure energises & flattens the O-ring forcing the Teflon ring to the cylinder wall & creates the dynamic seal

One of the difficulties in using step seals is the Teflon element must be boiled in water and expanded in diameter to slip over the piston major diameter.........& then resized    by compression 
 
Even though Teflon has a low coefficient of friction, the only way to create a seal would be by interference fit between the cylinder wall & the piston seal cavity ID, however this would create high stiction
 
If conventional PTFE tape is being twisted & wound into piston seal cavities, I suspect it is only loosely wound so to provide some 'give' to enable the compression & hence to energise & form a seal......Derek

[derekwarner]

Hi ............& Bee posts.... Possibly just a scare story or a bad batch but some were reported to breakdown under superheated steam and release acid resulting in corosion

Not sure if I am on the correct subject here

ViITON elastomer if exposed to & burnt produces 'hydrofluoric' acid......this is well documented in OH&S issues as the same acid if contacted onto human skin can result in the need for amputation of the exposed joint ....just GOOGLE hydrofluoric acid   

Having said this ....treat burnt VITON elastomer in accordance with the nominated MSDS & all is OK........Derek ...an extract follows.....

Hydrofluoric acid is corrosive and a contact poison. It should be handled with extreme care, beyond that accorded to other mineral acids, in part because of its low dissociation constant, which allows HF to penetrate tissue more quickly. Symptoms of exposure to hydrofluoric acid may not be immediately evident. HF interferes with nerve function and burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury.[4] HF is known to etch bone, and since it penetrates the skin it can weaken bones without destroying the skin.[5] More seriously, it can absorb into blood through skin and react with blood calcium, causing cardiac arrest.

In the body, hydrofluoric acid reacts with the ubiquitous biologically important ions Ca2+ and Mg2+. In some cases, exposures can lead to hypocalcemia. Thus, hydrofluoric acid exposure is often treated with calcium gluconate, a source of Ca2+ that sequesters the fluoride ions. HF chemical burns can be treated with a water wash and 2.5% calcium gluconate gel[6][7][8] or special rinsing solutions.[9][10] However, because it is absorbed, medical treatment is necessary — rinsing off is not enough. In some cases, amputation may be required.

[BarryM]

If memory serves right - isn't it hydrofluoric acid that is supplied with DIY windscreen etching kits?

Barry M

[sheerline]

You could be right Barry as this acid does dissolve glass. As for skin contact, the aforementioned text on this subject is absolutely correct and it is something to be treated with great respect. I am not on possesion of any figures on temperatures but the chemical breakdown of some of these materials sufficient to produce such acids takes place at higher temperatures than simple 'wet steam'. Model locos run exceedingly high temperatures and pressures and it would come as no surprise to learn that this applied use could result in degradation.
If in doubt, play safe and try some traditional methods... like graphited yarn for instance. It works for the locos and they are much harder pressed than a marine steam plant.