Silicon seals a success story

[jma1009]

hi doug,

if the loco ran well without a heavy load there is no logical reason why the 'O' rings on your pistons would cause a sudden problem when starting with a heavy load. the temperature difference in the cylinders would be very small if nothing at all, unless for the first time ever you had a raging fire and radiant superheaters fitted and too mean with the delivery of steam oil from the pump.

for many years Viton 'O' rings have been sold and recommended as the only suitable grade for steam locomotive cylinders. this is due to the temperature of the cylinders/pistons, and the effect of steam oil on the 'O' rings. steam oil is usually a compound of mineral and vegetable oils specially blended and can do strange things to 'O' rings.

please note that if Viton 'O' rings were fitted be very careful when inspecting them as in certain circumstances after use with steam oil they give off a burning acid. not nice on the fingers!

ordinary silicon 'O' rings wont last 5 minutes on miniature steam loco pistons.

as previously stated unless the bores are dead smooth 'O' rings on pistons are useless and wear quickly, even if you get the fit and clearances right.

cheers,
julian

[pault]

Hi Doug
Earlier in the thread you mentioned that the fit of the pistons in the bore was so good that they hardly needed sealing. This might be your problem. When the loco is working hard the pressures are higher and so is the temperature of the steam. The mass of the piston is very low relative to the mass of the cylinder block, so it follows that the piston may well reach a higher temperature than the block under certain conditions. This is especially true under transient conditions as things are heating up from relative cool. Should that be the case the minimal clearance between the piston and bore may well disappear and become it becomes an interference fit. Once it cools down normal service is resumed.
I would suggest skimming a couple of thou (about 51 microns) off the dia of the piston. This will not have any detrimental effects on the O ring seals but will give clearance to allow for differing expansion. It will also give clearance to allow oil to reach the rings which will improve their life.
Good luck sorting it out
Regards
Paul

[Doug]

Aug 18, 2014 9:59:30 GMT jma1009 said:

hi doug,

if the loco ran well without a heavy load there is no logical reason why the 'O' rings on your pistons would cause a sudden problem when starting with a heavy load. the temperature difference in the cylinders would be very small if nothing at all, unless for the first time ever you had a raging fire and radiant superheaters fitted and too mean with the delivery of steam oil from the pump.

for many years Viton 'O' rings have been sold and recommended as the only suitable grade for steam locomotive cylinders. this is due to the temperature of the cylinders/pistons, and the effect of steam oil on the 'O' rings. steam oil is usually a compound of mineral and vegetable oils specially blended and can do strange things to 'O' rings.

please note that if Viton 'O' rings were fitted be very careful when inspecting them as in certain circumstances after use with steam oil they give off a burning acid. not nice on the fingers!

ordinary silicon 'O' rings wont last 5 minutes on miniature steam loco pistons.

as previously stated unless the bores are dead smooth 'O' rings on pistons are useless and wear quickly, even if you get the fit and clearances right.

cheers,
julian

Hi Julian
not wanting to mislead anyone the loco was a little tight before but not solid as it became with a heavy load, the piston bores are super smooth so i am not too worried this combined with my three to four outings a year then they should last, i did try Viton orings first but had a simular experiance in that the whole lot locked up solid right from the off thats when i went for graphite yarn which worked really well for about 5-6 outings although i didnt have much power (think it was bypassing quite a bit)the tightness i have now i am sure is mostly from the piston and valve rod seals as these are squashed up (a big no no aparently) so i will free them off and give them the clearance they should have and i will report back on what happens next. The extra power it has since the silicon orings have been fitted is very significant it trots round the track notched back so far its almost in mid gear and can handle the gradent without moving the reverser just opening the regulator a bit so something must be right. i just need to get this issue sorted and i am a happy boy.
all the best Doug

[Doug]

Aug 18, 2014 10:12:24 GMT pault said:

Hi Doug
Earlier in the thread you mentioned that the fit of the pistons in the bore was so good that they hardly needed sealing. This might be your problem. When the loco is working hard the pressures are higher and so is the temperature of the steam. The mass of the piston is very low relative to the mass of the cylinder block, so it follows that the piston may well reach a higher temperature than the block under certain conditions. This is especially true under transient conditions as things are heating up from relative cool. Should that be the case the minimal clearance between the piston and bore may well disappear and become it becomes an interference fit. Once it cools down normal service is resumed.
I would suggest skimming a couple of thou (about 51 microns) off the dia of the piston. This will not have any detrimental effects on the O ring seals but will give clearance to allow for differing expansion. It will also give clearance to allow oil to reach the rings which will improve their life.
Good luck sorting it out
Regards
Paul

thanks paul could be onto something there

[Doug]

Aug 18, 2014 9:37:22 GMT Roger said:

Hi Doug,
I'd revisit the amount of compression the rings are supposed to have, you may had them too tight. Can you get Silicone 'X' rings in the size you need? I can't help but think those ought to be a better bet, ie more compliant and better sealing edges.

Hi Roger the piston orings are spot on both latterally and raidialy the rod seals however,,,,,,
bit of an issue there i am afraid the design was for graphite packing so the glands natrally squash up the selaing media and in the casse of orings obviously not good (only just found this out doing reaserch into the seals for speedy) so i need to modify things a bit to stop them squashing and actually give them clearance to roll in the groove (about half the section dia of the ring) i could always just knock up some solid PTFE seals but i am enjoying the learning curve so no rush dump the rings. once i have a working solution i can apply it to Speedy happy that it will work first time.
i will drop the cyllinder end caps off and have a look at the bores if Paul is correct (i think he prob. is) then there will be some scoring on the bores either way i still need to get the orings right or they will wear very quickly indeed.

[pondok]

Hi Doug, I see you mention PTFE already, it's the perfect solution here. I do tend to bash on about it but not because I have shares in PTFE, I don't, but because its always so great to see someone else grinning from ear to ear with a loco that is transformed by its unique sealing and smooth running properties. Its also so easy to machine up - no doubt you've got more than one o-ring groove in the piston, so turn down the metal between grooves so you have one large groove/gap to be filled with a PTFE ring. I did this the other day and just used some alu tube to protect the piston rod from the chuck jaws. Turn a ring to be an easy push fit in the bore. I don't even bother with an o-ring or 2 underneath - no need - but I leave a mm or so clearance between the groove bottom and the ring, so it can expand inwards if need be. After parting off so it's an easy push fit in the groove (using the piston to check this while the ring's being turned), you usually need to clean up the inside edge burrs a bit if you like. I then cut the ring with a thin craft knife, a stepped cut, not that it makes any difference, and slip it over the piston, rolling it on the table to centralise the ring, and back they go. You might find some leakage with compressed air, but under steam you'll be very pleased. I have yet to hear of anyone not totally delighted with it for piston and valve rings, it's used so much now. It also doesn't care if lubrication fails. I've always enjoyed how you can crack open the regulator and just wait, the seals are so good pressure slowly rises until it starts to move smoothly, instead of the usual heavy regulator that's needed with pistons and valves that leak....

[Doug]

Aug 18, 2014 11:15:55 GMT pondok said:

Hi Doug, I see you mention PTFE already, it's the perfect solution here. I do tend to bash on about it but not because I have shares in PTFE, I don't, but because its always so great to see someone else grinning from ear to ear with a loco that is transformed by its unique sealing and smooth running properties. Its also so easy to machine up - no doubt you've got more than one o-ring groove in the piston, so turn down the metal between grooves so you have one large groove/gap to be filled with a PTFE ring. I did this the other day and just used some alu tube to protect the piston rod from the chuck jaws. Turn a ring to be an easy push fit in the bore. I don't even bother with an o-ring or 2 underneath - no need - but I leave a mm or so clearance between the groove bottom and the ring, so it can expand inwards if need be. After parting off so it's an easy push fit in the groove (using the piston to check this while the ring's being turned), you usually need to clean up the inside edge burrs a bit if you like. I then cut the ring with a thin craft knife, a stepped cut, not that it makes any difference, and slip it over the piston, rolling it on the table to centralise the ring, and back they go. You might find some leakage with compressed air, but under steam you'll be very pleased. I have yet to hear of anyone not totally delighted with it for piston and valve rings, it's used so much now. It also doesn't care if lubrication fails. I've always enjoyed how you can crack open the regulator and just wait, the seals are so good pressure slowly rises until it starts to move smoothly, instead of the usual heavy regulator that's needed with pistons and valves that leak....

thanks it does seem to be the right material to use, Baggo said the same thing to me yesterday he had silicon orings in his Helen Longish, had some issues and swapped to PTFE.
i will have to find some larger dia stock as i only have 16mm currently (i have some 2" white 'something' but not sure if its nylon). i am using PTFE for my valve bobbins on Speedy as i know they are good from all the good reports i have had about them quite easy to fit on the bobbins too not so good on the piston though but your diagonal cut should work ok. i am going to remove all the shaft seals and the piston from one side then make my decission on whether to remake PTFE seals of stay with it, albeit set up correctly. although i am leaning towards the PTFE route a bit now as i keep hearing the same thing, "tried silicon it didnt work, i use PTFE now" mmmm tricky.

[Deleted]

Looking at the specs, I don't think any of the 'normal' O ring materials are particularly suitable for steam use as the temperature is getting towards the upper limits of their comfort zone. Most people seem to use Viton though which probably is the most suitable out of the bunch. Silicon is not recommended as it's very soft and easily damaged and will wear rapidly unless the bore is perfect. I believe it absorbs water as well so it will swell when exposed to steam. PTFE O rings may be ok but apparently are not very flexible and may need a 2 part piston to get them on. The more exotic materials such as Kalrez cost an arm and a leg!

Andy, interesting that you don't use a backing O ring as your pistons must be quite big diameter. I haven't used them either yet but the pistons I've used PTFE on have had a maximum diameter of 7/8".

Next time I would like to try the carbon filled PTFE as an experimant. I've read that it needs to be finish machined pretty much like a cast iron ring as it's a lot harder and springier than the virgin stuff.

Doug - did you make your O ring grooves in the pistons to the dimensions given in the data books? If so, they may well be much too tight to begin with. Those dimensions are to withstand pressures far in excess of those we will ever see in cylinders.

John

[Doug]

Aug 18, 2014 13:41:39 GMT John Baguley said:

Looking at the specs, I don't think any of the 'normal' O ring materials are particularly suitable for steam use as the temperature is getting towards the upper limits of their comfort zone. Most people seem to use Viton though which probably is the most suitable out of the bunch. Silicon is not recommended as it's very soft and easily damaged and will wear rapidly unless the bore is perfect. I believe it absorbs water as well so it will swell when exposed to steam. PTFE O rings may be ok but apparently are not very flexible and may need a 2 part piston to get them on. The more exotic materials such as Kalrez cost an arm and a leg!

Andy, interesting that you don't use a backing O ring as your pistons must be quite big diameter. I haven't used them either yet but the pistons I've used PTFE on have had a maximum diameter of 7/8".

Next time I would like to try the carbon filled PTFE as an experimant. I've read that it needs to be finish machined pretty much like a cast iron ring as it's a lot harder and springier than the virgin stuff.

Doug - did you make your O ring grooves in the pistons to the dimensions given in the data books? If so, they may well be much too tight to begin with. Those dimensions are to withstand pressures far in excess of those we will ever see in cylinders.

John

thats not good!

the grooves on Butch were cut by my Great grandfather and may be a little tight (the oring is slightly loose) it is no where near as loose as the Speedy ones i just did as per manufaturers instructions (which seem sloppy)
i am not yet conviced that the piston rings are the problem i still "feel" like its the rod seals as i know these are way too tight now i know how much space they should have so i am keen to rectify that first and move on from there the recifying may be to make the seals from PTFE as i have the stock for that and can knock them out in a few mins. then try a steam up again.
as i will ahve to take the pistons out i will check the clearances as well while i am at it and see if there is any damage to the orings i could make some piston rings from 1mm sheet i have but  i will see how i get on first.

[pondok]

Yes John my pistons are 2 3/4", pretty big (too big for the average club load, I'll sleeve it down one day to 2 1/4" bore). I never did try the inner 0-rings back in the day with my first loco, I used just the PTFE rings from the start and never had an issue. The expansion seems to settle after a while, but even at first steaming with the new rings, and a firm push fit when cold, the loco is not especially hard to push by hand, when its all hot, so I assume its expanded inwards. From memory the rings are about 3/8" deep. No worries at all about siezing.

[Roger]

I've just found this about Silicone rubber. Scroll down to Moisture and Steam resistance. If I interpret that correctly, it's possibly ok if the temperature and pressure isn't too high. It's tempting to use a Silicone 'O' ring to provide the backing  pressure for a PTFE ring but the piston would need to be made in two parts to get it on.

[jma1009]

i dont know much about 'O' rings as it all gets rather complicated and there seem to be far more types available now than a few years ago, but it is my understanding that ordinary silicon 'O' rings are totally unsuitable in miniature steam locomotive pistons or for that matter in piston rod glands, and only Viton 'O' rings are remotely suitable due to the temperature, effect of steam and condensation, and effects of compound steam oil.

personally i wont have 'O' rings as packing on piston rod glands and valve rod glands or pistons in my locos from bitter experience of the sort of problems on other locos that doug has encountered on his BUTCH.

doug, you havent said what grade of 'O' ring you used. this is very important!

if they are Viton 'O' rings then be very careful when inspecting them as the acid i mentioned earlier is hydrofluoric acid which is very corrosive and occurs due to the chlorine in this type of 'O' ring when exposed to 'high' temperatures and certain types of oil.

there was an awful lot of research undertaken on so called 'condensation' in steam cylinders. i wont bore you all with the details, which in any event are still today hotly debated, but as saturated steam expands before the exhaust point it must lose some of it's temperature which in turn stops the cylinders overheating and in theory causes a cyclical temperature change inside the cylinder and on the piston with each stroke. superheating is designed to (amongst other things) prevent condensation occuring, but the same temperature change takes place albeit at a higher temperature range. if this arguement is accepted then there will be a maximum 'working' temperature of the cylinders and pistons, depending on the initial temperature of the steam when it enters the cylinders and the amount of expansion and exhaust events. (and yes, i am well aware of compression too!)

i would never fit Viton 'O' rings to a miniature loco fitted with efficient radiant superheaters!

cheers,
julian

[suctionhose]

Silicon can take the temperature. I doubt it can take the abrasion as a dynamic seal.
As static seal, eg a gauge glass, perfect.
I keep three engines operational an run them all frequently. Pulling big loads is my thing, so glands are put through their paces.
Vitons in glands have given me 20 years without trouble BUT they must be lubricated and the rod should not touch metal and score. (See somewhere discussion on neck ring clearance)
I concede a graphite packing is self lubricating, however I find providing an oil ring in the gland and an oil cup is also perfect.
Without sufficient oil the oring grabs the shaft and rolls with it -you can see the worn cross section by cutting the ring and veiwing with a glass.
Normally, like everyone else,y piston rods are stainless however on my traction engine, where the revs are high hence surface speed is high, I used Chrome Bar.
Lastly, I did some investigation of friction caused by orings under pressure (balanced slide valves) and discovered the frictional loads were quite high, especially static friction even dripping with oil.
Metal to metal seals had very much lower friction under pressure. I therefore concluded that vitons were fine in small sizes (glands) but be wary in large sizes (pistons) due rubber to metal friction.
Lastly, alignment is everything and no packing will work well if the rod is constantly pushing it away.

[Deleted]

Julian,

You're looking at temperatures of over 400°C for decomposition of the O rings so unlikely to happen in cylinders but great care should be taken if they have been burnt. This problem was brought to attention I think when people were salvaging parts from cars that had had a fire where any O rings would have been subjected to suitably high temperatures. As you say, they break down to produce hydrogen fluoride which forms hydrofluoric acid on contact with any moisture - very nasty stuff indeed. It can take up to 24 hours for the effects to appear.

John

[Doug]

Aug 18, 2014 21:42:19 GMT jma1009 said:

doug, you havent said what grade of 'O' ring you used. this is very important!



cheers,
julian

Hi Julian the oring type is mentioned in the thread title and in my first post in it they are Silicon orings not Viton I mentioned that I had already tried them(Viton) and they didn't work at all.

as an addon I had a quick look last night and the loco is now VERY free running so what ever seals that were rubbing are now gone so I will have to replace them so I am going to go with PTFE rod seals and I have a bit of an idea for piston seals but I will look into that once I get the pistons out and check they are not rubbing. 

[suctionhose]

Re reading this thread it seems its title ain't quite the truth are it?!
I'm assuming the cylinders are gunmetal which I have no experience of using. Iron cylinders with iron rings of own manufacture has been my only experience other than slide valve balancing which I mentioned above and drew conclusions.
For a gunmetal cylinder I reckon PTFE. Consideration though to what guides what.
Of the 3 potential points of contact, you really only want 2 and relieve the third. Ie piston and crosshead but not gland or gland and crosshead not piston.
I would be going with a wide PTFE ring, gapped for expansion and oring under to give intial wall tension (like Roger found).
Make the piston 30 thou smaller the the bore so it never touches and guide it with crosshead and long gland.My guess it would be smooth as silk!
Ross

[jma1009]

where gunmetal cylinders and bronze pistons are concerned in the 3.5"g and 5"g locos i play with i am a complete luddite and use square section braided asbestos graphite packing. once you have got the knack of how to fit it properly i find they give long service. it still comes up on ebay from time to time.

otherwise proper cast iron rings on cast iron cylinders!

piston rod packing and valve rod packing is also graphite asbestos packing in both cases.

doug, apologies for missing the obvious re the thread title, and if you used ordinary silicon 'O' rings im not surprised they are now causing problems.

i set my valve gear using lung pressure or lung suction via the cylinder drain cock tapped holes to give accurate events. i would be somewhat concerned if using PTFE rings on pistons using this accurate method for setting valves if the pistons leak when 'cold' and you cant get the sensitivity needed for setting the valves.

one of the advantages of using graphite asbestos packing on piston rod glands is that the crosshead doesnt need removing to add new packing. i dont think one should disturb the cotter or taper pins in the crosshead in small locos once originally fitted unless absolutely necessary.

i dont see any reason to depart from the piston fits specified by LBSC.

all fun and games!

cheers,
julian

[Doug]

Oh dear and it started so well

my first few runs were indeed a success however i may have been on a bit of a hair trigger regarding the results
never mind its all good stuff i can store up for later.

just to clarify where i am regarding seals,,

Butch has a boatload of silicon orings 6 in total and is the one that has siezed up under load. this needs checking to see if its the piston or the seals. however i have decided that whatever happens i am replacing 4 off oring rod seals with PTFE. leaving only the piston ring seals with a question mark. the choice on that depends on what i find once its apart.

Speedy has just had one of the pistons machined and a silicon oring sized groove cut in it (this could be used for PTFE though)

at this point i cant add anything useful till i get it apart and understand what has happened.
i would like to belive that Silicon Orings will work as they are sold for this very purpose (these were all bought from Blackgates) and its only my poor instalation that has caused the problem. We shall see.....

[Roger]

Just to muddy the water even more, I wonder how piston rings made of Vesconite would perform. I think the material is rigid enough to make them in a similar fashion to cast iron ones, ie on a compressed after slitting an holding them on a mandrel for finish turning. Again, they could have a Silicon 'O' ring as a backing spring. SPEEDY has pretty long pistons so I guess you could even fit to piston rings in the space so the gaps are not opposite each other.

[Doug]

Aug 19, 2014 10:28:47 GMT Roger said:

Just to muddy the water even more, I wonder how piston rings made of Vesconite would perform. I think the material is rigid enough to make them in a similar fashion to cast iron ones, ie on a compressed after slitting an holding them on a mandrel for finish turning. Again, they could have a Silicon 'O' ring as a backing spring. SPEEDY has pretty long pistons so I guess you could even fit to piston rings in the space so the gaps are not opposite each other.

i think it would probably work very well, i had come up with the idea of using sheet PTFE, punching the inner dia (not critical size as a gap is needed for expansion) then mounting on a mandrel to finish turn to outside dia then do a diagonal cut. two sheets could be used together so as to give a laberinth seal for any small leak through the cut

quite a few of the pistons in hydralic rams have simualar material to Vesconite backed with an oring and they work very well (albeit without the heat).