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Post by suctionhose on Feb 25, 2015 9:16:06 GMT
Static Friction of Orings
I’ve been mucking about with balanced slide valves for years too. Actually, in a 5” model, they are of doubtful value the reason being scale factor. A valve made at 1/10th scale has 100th the area and hence far less force on it proportionally than its prototype. Still experiments are fun! At one point I was using an Oring as the balancing piston. Nothing else. Just an Oring on a spigot on top of the valve.
In measuring the force to move the valve under pressure I found the force required to break the rubber to metal contact was far greater than any gain had by balancing. Tried various things before reverting to metal to metal seals again. There is detectably less force required to move a balanced valve over an unbalanced one but not so much that you’d say it was necessary. Orings were are step in the wrong direction. Even now I have a metal to metal balancing piston with a sticky oring! PTFE soon for that one.
My latest model just started incorporates a Patent Slide Valve by W C Church. I have found 3 items of evidence pertaining to W C Church between 1870 and 1890. One is magazine article from 1876 – a circular, balanced slide valve with curved ports – the second is an actual drawing – the third is a US Patent in 1890, including 10 drawings, for an inside admission slide valve. Together they describe the life of a man obsessed with changing the status quo but failing to do so. Some so called deficiencies don't warrant improvement. But all that is another avenue of research altogether…!
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Post by Deleted on Feb 25, 2015 9:17:36 GMT
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Post by suctionhose on Feb 25, 2015 9:33:06 GMT
Very good Michael! Washers just like I said...
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Post by Roger on Feb 25, 2015 9:44:40 GMT
This is gold dust Ross, thank you so much for sharing. I did wonder if an 'O' ring would become stuck to a ball. I wonder if there's a way to contain the 'O' ring so it could be used with an eccentric driven pump. One way I can picture that would be to contain the 'O' ring in a face pocket and then have a separate ring to close onto that, thus creating four sided pocket with just the corner missing. Alternatively, it might be possible to use your design but fit a thin sleeve inside the bore after the 'O' ring has been installed.
The gland looks really good on the locomotive, exactly how it should. I'm definitely going to use that for the piston valve.
Even though I have no experience of any of this, I can already appreciate your comments about PTFE. It's a strange material and it will be very interesting to see if it's any good in the split piston ring design I have for the piston valves. The 'squelching' into the ports sounds about right to me, I need to conduct some hot tests to see if the ports end up shaving the rings like a cheese grater! I'm hoping that it's not going to be an issue. I think you're right about Pondok, I think he uses thin walled long PTFE piston valve sleeves that have been slit with a craft knife, so presumably those will want to expand into the ports like mine. I don't think he pegs them, so I'm surprised the edges don't catch. There's still so much to experiment with when it comes to PTFE.
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Post by Roger on Feb 25, 2015 9:50:42 GMT
That's a fascinating design, and I can now see how it changes diameter. I guess that's way too fiddly to make in miniature with the segments that have to mate to each other but I can see why this is necessary as the pressures start to climb. |
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Post by Roger on Feb 25, 2015 9:54:14 GMT
Michael Williams kindly sent me a copy of US2333196 which you can view from this link
It's a weird and wonderful shaped set of three segments that slide over each other, and tricky to make. |
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Post by Roger on Feb 25, 2015 10:05:09 GMT
Static Friction of Orings I’ve been mucking about with balanced slide valves for years too. Actually, in a 5” model, they are of doubtful value the reason being scale factor. A valve made at 1/10th scale has 100th the area and hence far less force on it proportionally than its prototype. Still experiments are fun! At one point I was using an Oring as the balancing piston. Nothing else. Just an Oring on a spigot on top of the valve. In measuring the force to move the valve under pressure I found the force required to break the rubber to metal contact was far greater than any gain had by balancing. Tried various things before reverting to metal to metal seals again. There is detectably less force required to move a balanced valve over an unbalanced one but not so much that you’d say it was necessary. Orings were are step in the wrong direction. Even now I have a metal to metal balancing piston with a sticky oring! PTFE soon for that one. My latest model just started incorporates a Patent Slide Valve by W C Church. I have found 3 items of evidence pertaining to W C Church between 1870 and 1890. One is magazine article from 1876 – a circular, balanced slide valve with curved ports – the second is an actual drawing – the third is a US Patent in 1890, including 10 drawings, for an inside admission slide valve. Together they describe the life of a man obsessed with changing the status quo but failing to do so. Some so called deficiencies don't warrant improvement. But all that is another avenue of research altogether…! That's all very interesting, and the theme of 'O' rings or PTFE taking quite a force to start them moving is a recurring one by the sound of it. It certainly is easy to become obsessed by change when you can see a small deficiency in an existing design, but tricky to come up with something that is a sufficient improvement to be worth the effort and complication. Such is the case with my piston valves! |
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Post by suctionhose on Feb 25, 2015 10:21:04 GMT
Roger,
Not sure what your worry is with piston valves. Rings or glands?
Being inside admission the gland is only subject to exhaust pressure an generally don't require any sealing - just a length of engagement of the rod in a reamed hole.
Rings: Well that's an area of wide choice. I've made them as ground bobbins - no rings at all. I've followed common practice of metal clip lock rings although these have been phased out of automotive now in favour of plastics or something. I've seen them with PTFE rings. All work so it's really a personal choice which road you take...
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Post by Deleted on Feb 25, 2015 10:25:37 GMT
Very good Michael! Washers just like I said... I'm not rising to this, I'll agree to disagree and will halt my search for a decent picture. If anyone actually would like to know what is in there on the BR Stds and Bulleids then feel free to PM me and I will continue this via PM The link provided previously is not exactly what is fitted to Standards or Bulleid Pacifics and neither is what you found Roger. Cheers Ben |
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Post by suctionhose on Feb 25, 2015 10:47:55 GMT
Very good Michael! Washers just like I said... I'm not rising to this, I'll agree to disagree and will halt my search for a decent picture. If anyone actually would like to know what is in there on the BR Stds and Bulleids then feel free to PM me and I will continue this via PM The link provided previously is not exactly what is fitted to Standards or Bulleid Pacifics and neither is what you found Roger. Cheers Ben Oh settle down! Tongue in cheek mate! Sheesh sensitive... Clearly Michaels link is not about wsshers. But when you see a crosshead flopping up and down 1/4" then you take all this precision ground stuff with grain of salt. Marshall used metallic packing on their later machines. The portable I had was 1930 something? The packing was a bunch of iron discs like washers and a set of iron cups. There was a drain at bottom of stuffing box as I recall. Clean faces and a good fit on the rod was all that was required. |
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Post by ejparrott on Feb 25, 2015 11:55:42 GMT
Piston Rod Orings These are fitted according to hydraulic cylinder principles with the housing and rod dimensions being nominal inch sizes. BS orings are oversize in section somas to provide the required nip inside and out the establish the seal. Metric different The groove is always wider than the nominal ring section and is formed in this case by pulling the gland up to a shoulder. It looks like a normal gland but it cannot be taken up. It doesn’t need to be. The pressure deforms the oring to one side of the groove. Hydraulic people would use a hard plastic back up ring to provide a concave seat for the oring so it retains its shape better under the very high pressures. What is not commonly appreciated is that in terms of metal to metal contact, pistons, piston rods & crossheads are the bearings with the highest surface speeds on the engine. I would say the limitation of Viton in pistons & rods is likely the surface speed and the surface finish encountered by the ring while deformed under pressure. On one engine I have used hardchrome rod for piston rods instead of stainless. Still lubricated. I often think a bush of PTFE would do the job without oil. You could hardchrome the bores of a bronze cylinder too!!! I haven’t used PTFE for rings although many people do. The inherent characteristic of PTFE that is both its downfall and its success, is being soft with a high coefficient of expansion. Great for piston valve rings where they can squelch in and out of ports. The wall tension comes from expansion with heat. I think the gentleman from Switzerland (Pondok?) talked about his experience before.   That's great Ross, thanks, almost what I was thinking. I had though of incorporating this into the gland already, but definitely will now. I'll probably just stick with stainless for the rods, maybe ground but maybe just as is, I'll see. |
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Post by Roger on Feb 25, 2015 13:44:28 GMT
Roger, Not sure what your worry is with piston valves. Rings or glands? Being inside admission the gland is only subject to exhaust pressure an generally don't require any sealing - just a length of engagement of the rod in a reamed hole. Rings: Well that's an area of wide choice. I've made them as ground bobbins - no rings at all. I've followed common practice of metal clip lock rings although these have been phased out of automotive now in favour of plastics or something. I've seen them with PTFE rings. All work so it's really a personal choice which road you take... Hi Ross, You're probably right about the need or lack of it on the piston valve rod, there's not one on the plans but I thought since there's going to be a gland showing on the detail, it may as well contain an 'O' ring. Plain bobbins are what's shown on the plans, but I fancied making a proper seal having been told that of course there is only one way that wear will go, and that's to increase the leakage. There's a huge amount of my machinations about this earlier on the thread, but to cut through all that, here's what I've ended up with....  20150215_204418 by rogerfroud, on Flickr  Piston valve assembly by rogerfroud, on Flickr All the designs that involve PTFE that I've seen require there to be solid rings which leak when cold, or split rings which rely on the gap being completely closed when hot. I wanted to see if it was possible to create a seal where the ring gaps were unimportant and that would seal when hot or cold. I think this solution is novel, I don't recall seeing piston rings on top of each other in concentric rings before. The idea is a simple one, and that's to cover the end gap with an adjacent ring, and seal the gap top to bottom with an inner ring. It's not that difficult to do as it happens, although the peg slots take a little more thought. The 'O' rings underneath don't really contribute anything to the seal other than to create a labyrinth under the rings. Their purpose is to provide and initial sealing outward force when there's very little pressure. This is experimental of course and it could all go pear shaped! If this doesn't work, I can always fit either solid PTFE or metallic ones. |
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Post by suctionhose on Feb 25, 2015 21:53:23 GMT
Very thorough Roger! Sure to work!
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Post by suctionhose on Feb 25, 2015 22:01:27 GMT
The thread opened with mention of Kalrez. I've only used Kalrez in conjunction with chemical attack - not steam. We found another, more sensibly priced equivalent available from Consolidated Bearings or one of their affiliate seal partners. Can't think what they were called but you could take up that line of enquiry.
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Post by Roger on Feb 25, 2015 22:16:10 GMT
Thanks Ross, I'm crossing my fingers that I haven't missed something with that design.
I enquired about suitable O-ring materials from Ashton Seals and they replied that the best material for the rod 'O' rings was Steam Resistant FPM. Unfortunately that's a highly specialised material that isn't available off the shelf. They offered to have them made at £20 each if I ordered 10 pieces!
They also confirmed that PTFE 'O' rings wouldn't be any good (I think we knew that anyway) because they aren't elastic enough, and that none of the encapsulated 'O' rings were suitable for dynamic seals.
They didn't mention Kalrez at all.
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Post by Deleted on Feb 25, 2015 22:35:57 GMT
Been pondering about making pistons and piston valves out of solid PTFE .
There must be a shape that could be formed into a solid PTFE piston or piston valve such that they distort locally under pressure and create an adaptive pressure seal .
There may not be any need to actually fit seals to a solid PTFE piston or valve .
I'll ponder more .
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Post by Roger on Feb 26, 2015 7:42:37 GMT
Been pondering about making pistons and piston valves out of solid PTFE . There must be a shape that could be formed into a solid PTFE piston or piston valve such that they distort locally under pressure and create an adaptive pressure seal . There may not be any need to actually fit seals to a solid PTFE piston or valve . I'll ponder more . I'm pretty sure someone has tried a plain solid one from PTFE with predictably poor results, but it might be possible to do what you suggest. It would need to seal on both edges of each ring area which makes life more difficult. My guess is that for the PTFE to be thin enough to do what you suggest, it would need to be very thin and you would have to find the sweet spot where it moves enough to seal while not being extruded into the port. It's a tough one. |
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Post by andyhigham on Feb 26, 2015 7:57:48 GMT
The ideal piston valve may be machined from the same material as the liner. Then coated with a thin layer of ptfe. Like an inside out "glacier" bearing?
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Post by Roger on Feb 26, 2015 8:25:45 GMT
The ideal piston valve may be machined from the same material as the liner. Then coated with a thin layer of ptfe. Like an inside out "glacier" bearing? That's an interesting proposition, and it would certainly work. I'm not sure that I would call it 'ideal' though because it would still have to account for the fact that the piston valve is going to heat up much more rapidly than liner, so there needs to be an initial clearance to allow for that while it's cold else it would seize. |
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Post by suctionhose on Feb 26, 2015 11:56:12 GMT
The "ideal" piston valve is one that performs for hundreds of hours without problems and we already have those!
OK. That was a smart arse comment reflecting why Mr Church got nowhere with his multiple renditions of the "Improved Slide Valve" in the 1880's.
I do think Roger's seal arrangement is likely to be unique. It is certainly thoroughly considered in every respect. So in terms of engineering it is wonderful and I shall be an eager listener to the results in steam.
Having said that not much is new under the Sun and sure enough today, I was at a fellow ME's workshop turning some gear blanks and we got talking about the 5" GWR Panier they built 20 years ago.
Turns out piston seals are PTFE with an "springy" element underneath the ring to create wall tension. The fella couldn't remember exactly what it was ultimately but definitely an O ring was tried along with metal springy things of various designs.
And the practise was gleaned from somebody else who successfully did it before them and wrote a story about it in ME or EIM or some similar publication (mid 1990's).
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