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Post by Roger on Feb 13, 2018 20:32:53 GMT
Hi Roger You wrote "This is my solution for using PTFE for piston rings. Originally I was going to peg them like I did for the piston valves, but thought they probably wouldn't rotate in the bores so I didn't bother." Yes you are right as long as the rings do not rotate and the gaps meet and create a leak path. In the back of my mind is something from the past in that it is not good practice to have two o rings on the inside of the piston head as you can get pressure build up caused by the pumping action of the piston and subsequent damage to the static seals. Maybe not a concern at our relatively low pressures? Hello Wilf. It is almost certainly due to the fact that the wall of the piston ring groove and its clearance to the piston ring is an integral part of the seal system. Hence if you try to put two rings in one groove this would not be the case. I stand to be corrected by those who know more about this than me but isn't it standard practice with ic engines to only ever have one groove with one piston ring? Cheers Malcolm Hi Malcolm, If the rings do rotate, I'll peg them. My gut feeling is that they will stay put though. I'm not sure if there's any pumping action with my ring arrangement, there's bound to be some leakage. If the rings can move at all sideways, the pressure ought to be similar to what's on the side where there's a small gap. Time will tell, it's an interesting experiment though. |
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Post by Roger on Feb 13, 2018 20:36:12 GMT
So guys, why are you going to all this effort and expense? What do you gain that a simple rubber ring doesn't provide? Pete. Hi Pete, That's a very good question, and if something works well enough for you, I certainly wouldn't change it. Some people have experienced short life with 'O' rings though, and the higher the temperature, the shorter the life. I'm hoping to achieve the best possible seal with the smallest drag, at all temperatures, and with the longest life. |
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jma1009
Elder Statesman
Posts: 5,922
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Post by jma1009 on Feb 13, 2018 21:03:18 GMT
So guys, why are you going to all this effort and expense? What do you gain that a simple rubber ring doesn't provide? Pete. Hi Pete, Because those of us who make our own locos from scratch want to get the very best design detail and in my case the best performance and longevity operating our creations for the public and passengers. I can tell you without any shadow of a doubt that the prospect of dismantling a miniature loco with inside cylinders because the pistons blow is not a prospect I look forward to. I want to get it right first time. I don't want to be sat at a station with a train load of paying punters and open the regulator with the wheels not moving and just a great 'WHOOSH' going up the chimney! Even on an outside cylinder loco getting the piston rod free of the crosshead can be quite a pain. Taking the piston out plus rod will also probably require removing the front buffer beam in lots of designs. Cheers, Julian |
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Post by joanlluch on Feb 14, 2018 7:17:36 GMT
Hi Roger and Malcom. To answer your question about long term success of these seals. Short answer is Yes. In Catalonia and Spain model engineers adopt two kind of approaches, both of them based on off-the-shelf items. - First approach is using metal rings -possibly cast iron- that are sold as spare parts for small IC engines such as the ones in chainsaws or in electric power generators. - Second approach is using hydraulic seals as the one I discussed above. Choice depends on personal preference and whether you want to have a split piston. There's at least one known locomotive that operates every Sunday pulling a long string of passenger cars for several hours in a club near Valencia (that's Spain) that uses these kind of hydraulic seals. However, that's a non-superheated locomotive. I have never heard on the social networks and local FB groups about these seals being ever replaced, but I am going to ask to the owner of the locomotive to be absolutely sure. Joan Hi Joan, That's interesting, it will be good to get some feedback and also to find out if anyone uses them with Superheated steam. In reality, it didn't take long to turn up a couple of cylinders and slit them, so I would probably stick with what I've designed anyway. I prefer the idea of accommodating the expansion at the gaps rather than in stresses and movement in the material itself. To me is seems to be more technically sound. I also like experimenting! I got a reply from the owner of the locomotive using the commercial PTFE based rings with o-ring underneath. This is his literal answer (translated): “On the ‘Olga’ they have been running for 4 years, or about 250 hours so far” “And on the ‘Maria’ they are still the originals from Marcel, the locomotive is 25 years old, I think”. Olga and Maria are of course the names he gives to his locomotives, Marcel is the former owner of one of them. Olga is a freelance locomotive with ‘decapod’ wheel arrangement (2-10-0 in British notation). Maria is a small locomotive with two axles. None of them are superheated though. Joan. |
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Post by Roger on Feb 14, 2018 8:14:34 GMT
I got a reply from the owner of the locomotive using the commercial PTFE based rings with o-ring underneath. This is his literal answer (translated): “On the ‘Olga’ they have been running for 4 years, or about 250 hours so far” “And on the ‘Maria’ they are still the originals from Marcel, the locomotive is 25 years old, I think”. Olga and Maria are of course the names he gives to his locomotives, Marcel is the former owner of one of them. Olga is a freelance locomotive with ‘decapod’ wheel arrangement (2-10-0 in British notation). Maria is a small locomotive with two axles. None of them are superheated though. Joan. Hi Joan, That's promising though. It will be interesting to see how they perform with Superheaters. |
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smallbrother
Elder Statesman
Errors aplenty, progress slow, but progress nonetheless!
Posts: 2,269
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Post by smallbrother on Feb 14, 2018 9:12:15 GMT
So guys, why are you going to all this effort and expense? What do you gain that a simple rubber ring doesn't provide? Pete. Hi Pete, Because those of us who make our own locos from scratch want to get the very best design detail and in my case the best performance and longevity operating our creations for the public and passengers. I can tell you without any shadow of a doubt that the prospect of dismantling a miniature loco with inside cylinders because the pistons blow is not a prospect I look forward to. I want to get it right first time. I don't want to be sat at a station with a train load of paying punters and open the regulator with the wheels not moving and just a great 'WHOOSH' going up the chimney! Even on an outside cylinder loco getting the piston rod free of the crosshead can be quite a pain. Taking the piston out plus rod will also probably require removing the front buffer beam in lots of designs. Cheers, Julian Hi Julian, Well, never mind from scratch, I designed and built many things from a blank sheet of paper in my time but was never reluctant to go with the simplest solution. However, I take your point about inside cylinders. The O rings I changed must have been there 20+ years. I would hope that anyone intending hauling passengers would examine a loco more than once in over 2 decades. Just my 1/2p worth! I was just curious why an initial question about a simple solution on a straightforward loco (Simplex) gets so involved. Pete. |
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Post by rwilliams on Feb 14, 2018 9:25:59 GMT
It is almost certainly due to the fact that the wall of the piston ring groove and its clearance to the piston ring is an integral part of the seal system. Hence if you try to put two rings in one groove this would not be the case. I stand to be corrected by those who know more about this than me but isn't it standard practice with ic engines to only ever have one groove with one piston ring? Cheers Malcolm Hi Malcolm,
That's the case, but in most car based engines there are three separate rings, and three grooves as well, so you still have multiple rings for the sealing effect. Even then you can still get blowpast all three rings. (Which is why diesel engines turn that lovely honey coloured oil into black sludge quite quickly). I believe some high speed/high compression engines run 4, but conversely some racing cars trade the reduction in friction against power and only run 1, however they do tend to have shorter working lives and short service intervals. With cars the main trade off in design is the friction of the rings in the cylinder against the sealing ability.
Given the high temperature/high pressure environment of an IC engine there may be some inspiration for the model engineer, but if you did go that route then you'd probably need a long hard look at what the lubrication side of things was like - down to what kind of oil you use. And that, as they say, is a whole different story!
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Post by joanlluch on Feb 14, 2018 14:57:48 GMT
Hi Joan, That's promising though. It will be interesting to see how they perform with Superheaters. That should not be any different than other PTFE based designs. On all PTFE based piston seals: 1 - Viton o-rings (if used) are there waiting to be chemically damaged by hot steam. 2 - Filled (or virgin) PTFE is there waiting for mechanical failure due to reduced strength at high temperature. So who knows (?). PTFE based piston seals have been used for a while by model engineers, but after almost my 3 years in this forums, nobody has ever answered that simple question in an experience based way. (It's hard to believe that PTFE has never ever tested on a superheated loco, but yet there are no answers for that) Joan. |
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Post by Roger on Feb 14, 2018 15:44:33 GMT
Hi Joan, That's promising though. It will be interesting to see how they perform with Superheaters. That should not be any different than other PTFE based designs. On all PTFE based piston seals: 1 - Viton o-rings (if used) are there waiting to be chemically damaged by hot steam. 2 - Filled (or virgin) PTFE is there waiting for mechanical failure due to reduced strength at high temperature. So who knows (?). PTFE based piston seals have been used for a while by model engineers, but after almost my 3 years in this forums, nobody has ever answered that simple question in an experience based way. (It's hard to believe that PTFE has never ever tested on a superheated loco, but yet there are no answers for that) Joan. Hi Joan, The only question I have is whether the extra elongation through thermal expansion is going to cause permanent deformation of the ring so that it is then loose in the bore when it cools down. The Viton 'O' rings might lose some of their performance is the temperature is very high, so I suppose they might not provide as much force. My design only relies on the 'O' rings to gently hold the rings outwards to form an initial seal. The main force comes from steam that gets under the rings. I suspect the the 'O' rings in the solid commercial Hydraulic seals play a bigger part by providing enough force to deform the solid ring. I don't think we'll get a high enough temperature to cause the PTFE to fail. That was the limiting factor in choosing how many Superheaters I used. Do we know what material the solid ring is made from in the commercial hydraulic seals? If it's Fluorosint then I can see how this may well work due to the low thermal expansion compared to PTFE |
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Post by joanlluch on Feb 14, 2018 18:40:08 GMT
Hi Roger, I understand your points, and I can see why you think that a thinner PTFE ring with cuts and slight o-ring springing can work better than the commercial one. But we are all talking based on speculation, not actual data or tests, so we can't really know. I also understand your concern about they working both cold and hot. All I can say is that the commercial product works similarly to yours, in both cases it's the fluid pressure what guarantees the seal, and I do not think that there can be permanent deformation at the locomotive working pressures. The o-ring in the commercial seal actually feels tight under the PTFE ring, as you suggest, but it's not enough to actually deform it. It's rather the opposite. At low or no pressure and ambient temperature it is the elasticity of the PTFE -which gets compressed radially in the cylinder bore- what seems to push it inwards. It almost feels as if the o-ring is there only to absorb any inward expansion of the PTFE ring, and to provide effective seal in that area, not to produce any effective push to the outside, which is definitely provided by the fluid pressure. Also the trapezoidal shape prevents seizing thanks to slight titling of the seal under pressure according to the manufacturer. I think you may over-estimate both the expansion and the strength of the PTFE ring. I mean, it's a thermoplastic, and it becomes softer as it gets more temperature / thermal expansion. Its elongation at yield strength (i.e while still in the elastic area) is enormous compared with any metal, so I don't expect any permanent deformation given that the outside diameter is perfectly controlled and it's free to expand inwards. In case you have not found it this is the spec according to the manufacturer: www.seals-shop.com/content/medias/technical_datasheets/Piston_Turcon%20Glyd%20Ring%20T.pdfAbout materials you can chose Bronce filled PTFE, and their proprietary "Turcon M12" which they do not disclose much but they say "mineral filler" which can well be mica, so I suppose it may behave as Fluorosint. If you fancy watching at a very catching commercial then click on this youtube video: youtu.be/Bkm1mAkakQo They rate the seals to max 200 ºC, but it's the Viton o-ring what puts the limit. In fact, I would be more concerned about the Viton o-ring than the PTFE ring. Joan. |
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robmort
Hi-poster
3.5" Duchess, finishing 2.5" gauge A3 and building 3.5" King
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Post by robmort on Feb 14, 2018 20:07:10 GMT
.......... The O rings I changed must have been there 20+ years. I would hope that anyone intending hauling passengers would examine a loco more than once in over 2 decades. Just my 1/2p worth! I was just curious why an initial question about a simple solution on a straightforward loco (Simplex) gets so involved. O-rings work in this case with low superheat, assuming you have a single superheater, but would they work for higher superheat as is common in many other boilers? |
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Post by Roger on Feb 14, 2018 20:44:38 GMT
Hi Roger, I understand your points, and I can see why you think that a thinner PTFE ring with cuts and slight o-ring springing can work better than the commercial one. But we are all talking based on speculation, not actual data or tests, so we can't really know. I also understand your concern about they working both cold and hot. All I can say is that the commercial product works similarly to yours, in both cases it's the fluid pressure what guarantees the seal, and I do not think that there can be permanent deformation at the locomotive working pressures. The o-ring in the commercial seal actually feels tight under the PTFE ring, as you suggest, but it's not enough to actually deform it. It's rather the opposite. At low or no pressure and ambient temperature it is the elasticity of the PTFE -which gets compressed radially in the cylinder bore- what seems to push it inwards. It almost feels as if the o-ring is there only to absorb any inward expansion of the PTFE ring, and to provide effective seal in that area, not to produce any effective push to the outside, which is definitely provided by the fluid pressure. Also the trapezoidal shape prevents seizing thanks to slight titling of the seal under pressure according to the manufacturer. I think you may over-estimate both the expansion and the strength of the PTFE ring. I mean, it's a thermoplastic, and it becomes softer as it gets more temperature / thermal expansion. Its elongation at yield strength (i.e while still in the elastic area) is enormous compared with any metal, so I don't expect any permanent deformation given that the outside diameter is perfectly controlled and it's free to expand inwards. In case you have not found it this is the spec according to the manufacturer: www.seals-shop.com/content/medias/technical_datasheets/Piston_Turcon%20Glyd%20Ring%20T.pdfAbout materials you can chose Bronce filled PTFE, and their proprietary "Turcon M12" which they do not disclose much but they say "mineral filler" which can well be mica, so I suppose it may behave as Fluorosint. If you fancy watching at a very catching commercial then click on this youtube video: youtu.be/Bkm1mAkakQo They rate the seals to max 200 ºC, but it's the Viton o-ring what puts the limit. In fact, I would be more concerned about the Viton o-ring than the PTFE ring. Joan. Hi Joan, Agreed, my analysis largely supposition rather than being based on hard facts. Turcon is states as being a PTFE based material, but so is Fluorosint. It's quite possible that the thermal expansion is much less than virgin PTFE. In fact, it's hard to see how it can work unless it has a lower rate, since we already know that a solid ring of PTFE runs into trouble unless it leaks when cold. I guess that the only way to prove the point is to try it. I guess you could use a Silicone 'O' ring to extend the tempterature range, but it may well not be necessary. As discussed before, the maximum working temperature of 'O' ring materials only mark the point at which there is no degradation due to heat. I doubt if the piston reaches anywhere near the temperature of the inlet steam anyway. If this does work, then there's no reason why you can't make exactly the same thing from Fluorosint since it's just a ring. |
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Post by joanlluch on Feb 14, 2018 22:56:00 GMT
Hi Roger, were your rings made out of Fluorosint, or another grade of filled PTFE? Actually, I think that if the rings are to be split then making them of PEEK-HPV would be possibly better. It's a lot more consistent and dimensionally stable at high temperature.
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smallbrother
Elder Statesman
Errors aplenty, progress slow, but progress nonetheless!
Posts: 2,269
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Post by smallbrother on Feb 14, 2018 23:00:06 GMT
O-rings work in this case with low superheat, assuming you have a single superheater, but would they work for higher superheat as is common in many other boilers? [/quote] Hi Rob, I don't know. I was just curious how what appeared to be a simple proposal for a fairly simple loco got into such a deep discussion. I don't believe the loco in question was doing any particularly onerous work so i would have thought a simple O ring replacement would meet the operating requirements. After all, it was twine for 20 years the first time round. Pete. |
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Post by Roger on Feb 14, 2018 23:25:24 GMT
Hi Roger, were your rings made out of Fluorosint, or another grade of filled PTFE? Actually, I think that if the rings are to be split then making them of PEEK-HPV would be possibly better. It's a lot more consistent and dimensionally stable at high temperature. Hi Joan, Mine are actually natural PTFE because it doesn't matter how much they expand, the gap in them won't close because it's large enough to accommodate the expansion. You may be right about other materials being better, time will tell if PTFE is good enough. PTFE rings are extremely flexible because they are quite thin. I'm not sure if it's beneficial to make them more rigid or not. If they're flexible, I imagine they will conform to the bore more closely than if they were more rigid. Of course the bores ought to be perfectly parallel but in reality they will be slightly inaccurate. I do have some Fluorosint which I might use for the piston valves. Ideally the axial expansion should be as small as possible and the rings rigid enough to stop them from trying to migrate into the port openings. I've got the fixtures that make machining them easy, so I'll probably make a set to try instead. |
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Post by suctionhose on Feb 15, 2018 0:06:41 GMT
Exploring the possibilities of different materials is an important aspect of the hobby and justification alone to do something different.
I posted on another thread last year my findings on orings. Rubber to metal has high static friction. Fine with hydraulic oil. Fine with wet steam (probably). Fine under small pressures - the majority of models in the ilk of Speedy spin their wheels with 30psi steam chest.
Therein lies the limits of orings for piston seals. I use them for piston rod glands but only with external lubrication.
The time I left it to the cylinder lubricator to keep the orings oiled I had short life. Wear on the oring was consistent with being dragged by the shaft.
I have since tried ptfe rod seals. Like an oring, these have nip on ID and OD. clearance in groove axially and a scarfed joint (by razor blade) to accommodate expansion. Seems ok.
I'd probably do the same in a gunmetal cylinder. Still like iron rings in iron cylinders myself though there is long experience here using ptfe on piston valves. I reckon squelching in and out of the ports deals with expansion to a fair extent.
I don't see why a solid ptfe ring (or gapped with razor losing no material) energized by an oring underneath wouldn't be perfect?
Not sure why Roger has two layers - because of gap? Answer: have no gap!
oring and space under ring will accommodate increased ptfe volume due to expansion?
Keep piston clearance small to prevent extrusion. Here a decision is needed on whether the piston is guided by crosshead and gland, not touching the bore in theory or whether gland floats on rod with piston bearing against bore.
To have three points of support is unnecessary and dubious design ignoring practical alignment issues, working clearances, wear and flexing.
For PTFE piston rings, I would make the piston fairly neat to prevent extrusion of the ptfe and have a floating gland like a metallic packing ring. All has neen dpne before with success.
However, figuring all this out for yourself is where the fun is. For a builder, it's more about the journey than the destination!
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Post by Roger on Feb 15, 2018 8:33:34 GMT
Not sure why Roger has two layers - because of gap? Answer: have no gap! oring and space under ring will accommodate increased ptfe volume due to expansion? However, figuring all this out for yourself is where the fun is. For a builder, it's more about the journey than the destination! Hi Ross, I agree entirely with your sentiments about the fun of experimentation. To answer your question, the four rings are indeed to accommodate a large amount of expansion, the ring is expected to grow by about 1mm in length. I imagine you can pretty much ignore this with CI rings, but you can't with PTFE. If you're going to have gapped rings, you have to do something about leaks at the gaps, both at the ends and underneath. As far as I know, it's the only design that provides a true seal while allowing the material to expand naturally. (unless you can make PTFE Clupet rings) It's still experimental, so until it's had a proper extended steam test, it's only a curiosity. Others have tried thick natural PTFE rings, and those can't accommodate the changes in temperature, leaking when cold. Split thin rings do indeed seem to work, but I find that technically unsatisfying. I can only assume that the split allows the material to move sideways, or one end under the other? Who knows. Either way, I don't like it. Joan's solid rings may prove satisfactory , and that may work if the filled PTFE has a low enough to restrict the expansion at the highest temperature. Again, I don't find that technically satisfying, it seems to me that there's an awful lot of stress in the material that has to be absorbed internally. |
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Post by joanlluch on Feb 15, 2018 10:00:29 GMT
Hi Roger,
We all have different views on why things work or not. Of course physics is physics and things work or not based on nature rules that are generally a lot more complex than anybody thinks. While I fully understand why your rings arrangement may (or will) work, I still think that you have several misconceptions on the way solid PTFE rings are supposed to work. In all your explanations about the subject I can't help to get the impression that you think on materials as if they all had 'metal' properties. Wording such as "awful lot of stress" or "low enough expansion at high temperature" makes me think on that. The reality is that you can only hold that rings between your fingers, with only the minimal finger pressure to prevent dropping them, by implying noticeable but non permanent deformation to them. Such an amount of elastic deformation that would not be possible on a metal ring, or that would require actual force to either cause it to break or produce permanent deformation. Maybe you need to think on these rings in terms more similar to rubber properties (like o-rings) than metals ones, to get a more approximate appreciation on why they work.
About your comments on thick natural PTFE rings seizing when hot or leaking when cold, you obviously refer to an article that appeared long ago in ME magazine and that has been reproduced several times on the internet. Yes, that's true. It was found that it was really tricky to get the right diameter tolerance for it to work in hot steam. And once the right diameter was found, they leaked in cold air. However, that design was essentially flawed because they had to allow some gap below the ring in order to account for expansion. That gap was filled when hot and the ring sealed in such condition, but it caused leaks when cold. Well, the way to allow for ring expansion while preventing leaks under the seal is placing a slightly sprung o-ring there. The o-ring removes the need for ultimate accuracy in the seal diameter, as you can just make the PTFE ring with an interference fit on the cylinder bore. Such interference fit is what makes it supposed to work at all temperatures. You also prevent leaks under it thanks to the o-ring. That's a marked design difference that can't be compared with the "thick natural PTFE" rings that you referred. It's not only a case of choice of material, as I believe that virgin PTFE would still work for a while with this design if we forget its reduced wear resistance compared with filled grades.
Joan
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Post by Roger on Feb 15, 2018 12:20:07 GMT
Hi Roger, We all have different views on why things work or not. Of course physics is physics and things work or not based on nature rules that are generally a lot more complex than anybody thinks. While I fully understand why your rings arrangement may (or will) work, I still think that you have several misconceptions on the way solid PTFE rings are supposed to work. In all your explanations about the subject I can't help to get the impression that you think on materials as if they all had 'metal' properties. Wording such as "awful lot of stress" or "low enough expansion at high temperature" makes me think on that. The reality is that you can only hold that rings between your fingers, with only the minimal finger pressure to prevent dropping them, by implying noticeable but non permanent deformation to them. Such an amount of elastic deformation that would not be possible on a metal ring, or that would require actual force to either cause it to break or produce permanent deformation. Maybe you need to think on these rings in terms more similar to rubber properties (like o-rings) than metals ones, to get a more approximate appreciation on why they work. About your comments on thick natural PTFE rings seizing when hot or leaking when cold, you obviously refer to an article that appeared long ago in ME magazine and that has been reproduced several times on the internet. Yes, that's true. It was found that it was really tricky to get the right diameter tolerance for it to work in hot steam. And once the right diameter was found, they leaked in cold air. However, that design was essentially flawed because they had to allow some gap below the ring in order to account for expansion. That gap was filled when hot and the ring sealed in such condition, but it caused leaks when cold. Well, the way to allow for ring expansion while preventing leaks under the seal is placing a slightly sprung o-ring there. The o-ring removes the need for ultimate accuracy in the seal diameter, as you can just make the PTFE ring with an interference fit on the cylinder bore. Such interference fit is what makes it supposed to work at all temperatures. You also prevent leaks under it thanks to the o-ring. That's a marked design difference that can't be compared with the "thick natural PTFE" rings that you referred. It's not only a case of choice of material, as I believe that virgin PTFE would still work for a while with this design if we forget its reduced wear resistance compared with filled grades. Joan Hi Joan, I don't think PTFE behaves like metal. I'm just aware that all materials expand, and that material has to go somewhere when that happens. If you restrict where it can go, ie the diameter can't increase, then it must go somewhere else. The stresses in the material will therefore be such that they exhibit enough force to move the material into those other places. If that results in permanent deformation, then it won't completely return to its former shape when the material cools down. It may be that the hydraulic seals retain their seal when cold because the 'O' rings exert enough force to make the hot rings stay against the walls as they heat up and cool down. PTFE does spring back to some extent, but nothing like the amount that rubber does. It tends to take on a permanently deformed state when it's deformed. You can see that when it's used as a gasket. In the article you referred to, I don't think the gap underneath was filled when the ring was hot. It shouldn't matter if there's a gap under the ring, because when steam is applied to one side, the ring will seal against the end of the groove, thus sealing the underside anyway. This is no different to the situation with Cast Iron rings. |
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Post by joanlluch on Feb 15, 2018 13:18:58 GMT
Roger, neither you or me are the inventors of the commercial rings, so we can discuss forever about the way they are supposed to work and yet get to the wrong conclusions. I think they work essentially elasticaly and they never get to the point of permanent deformation, conceptually working like continuous weld rail. They even tilt elasticaly against the pressurised face (by design) on every single stroke, which pushes the pressurised edge against the bore. It seems that we are already running around in circles, so I guess I’d stop it now.
Joan
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