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Post by joanlluch on Apr 21, 2015 6:03:53 GMT
personally in gunmetal/bronze cylinders i would fit good old fashioned square braided section graphite asbestos packing in the pistons. if properly fitted it is quite long lasting and none of the problems discussed above. cheers, julian Hi Julian, That's a fair comment, based on the concept that if something works why to change it. I just want to add to it that 'Asbestos' is classified as a carcinogen substance. I have been working long years in the chemical industry attempting to reduce the 'Asbestos' content in a particular grade of synthetic fillers, and have worked to keep it below certain levels for applications that would potentially get humans in contact with the product.
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Post by joanlluch on Apr 21, 2015 6:20:45 GMT
The main problem with the above method is the same problem I face, I seem to have two modes of operation 1: running on my own without a heavy load (currently works very well indeed) and 2: under a heavy load carrying 5-8 passengers this does not work as it induces higher pressures in the cylinders and therefore higher temps it would be good to get a solution that works well for both types of running. That's an interesting observation and exactly the same concern I have. I really hope that Roger's solution proves to be one that works, because I am a fan of plastics being used in technical applications and replacing metal wherever possible. Said that, I learnt that most locomotives built in my country use standard motorcycle pistons and rings. People just buy a motorcycle piston head complete with hard metal rings, and adapt them by removing most of its bottom half, to be used in their locomotives. I understand that this is not something that would be acceptable by "purist" (<- please replace this by a more suitable word if needed) model engineers, but also to my understanding this is something that should work well at all temperatures and loads.
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Post by RGR 60130 on Apr 21, 2015 6:36:01 GMT
personally in gunmetal/bronze cylinders i would fit good old fashioned square braided section graphite asbestos packing in the pistons. if properly fitted it is quite long lasting and none of the problems discussed above. cheers, julian Hi Julian, That's a fair comment, based on the concept that if something works why to change it. I just want to add to it that 'Asbestos' is classified as a carcinogen substance. I have been working long years in the chemical industry attempting to reduce the 'Asbestos' content in a particular grade of synthetic fillers, and have worked to keep it below certain levels for applications that would potentially get humans in contact with the product. Graphite impregnated braided packing, or greasy packing as it was traditionally known at sea, hasn't actually contained any asbestos for decades. Even when the use of asbestos was common place it was only used for high temperature applications. The packing I fitted to my tank engine 30 years ago is still in there and I too would endorse its use for non-ferrous cylinders. Reg
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Post by Roger on Apr 21, 2015 7:27:54 GMT
Hi fowlersfury, Hopefully by the time you get to finishing your engine, we'll have a lot more data and experiments behind us and life will be a little simpler. It's interesting that the writer of that article says about the datasheets telling of little or no thermal expansion, I don't think they could have said that because it's simply untrue. If you design assuming that's the case, you'll always be in trouble because the exact opposite is the case. I'm convinced that there is a workable solution amongst the designs we're exploring but it's too early to know as yet. Watch this space!
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Post by Roger on Apr 21, 2015 7:32:39 GMT
Hi Julian, That's a fair comment, based on the concept that if something works why to change it. I just want to add to it that 'Asbestos' is classified as a carcinogen substance. I have been working long years in the chemical industry attempting to reduce the 'Asbestos' content in a particular grade of synthetic fillers, and have worked to keep it below certain levels for applications that would potentially get humans in contact with the product. Graphite impregnated braided packing, or greasy packing as it was traditionally known at sea, hasn't actually contained any asbestos for decades. Even when the use of asbestos was common place it was only used for high temperature applications. The packing I fitted to my tank engine 30 years ago is still in there and I too would endorse its use for non-ferrous cylinders. Reg That's interesting, I wonder what that backing material is then? "The packing I fitted to my tank engine 30 years ago is still in there..." but how much does it leak? There's simply no way of knowing unless you do a bench test. What you do know is that it doesn't leak excessively, so it's certainly good enough.
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Post by RGR 60130 on Apr 21, 2015 8:44:14 GMT
Packings are like motor vehicles, there are many different types for many different applications. The backing material may be yarn, Kevlar, PTFE, carbon fibre based or many others. Some of them are 100%PTFE. When using packing it is essential that they are well fitted and supported i.e. in a piston application for example they will fill the groove 100% and the piston will be a close fit in the bore. For a bench test the actual sealing mechanism isn't too dissimilar from a valve gland and this use is proven over many thousands of instances. They are also widely used in the glands of reciprocating steam pumps without problem. Chesterton produce quite a few variations www.chesterton.com/ENU/Products/Pages/ProductLineDetails.aspx?ProductLine=MPDJames Walker also do a range www.jameswalker.biz/nb/product_types/29-compression-packingsAs a little bit of a thought provoker, the Chesterton catalogue used to have an opening gambit that stated "Packings are not to prevent leakage but to control it". In a valve application a touch of leakage isn't really necessary but in a rotary or reciprocating application a touch of leakage provides a bit of lubrication. This leakage may only be in the region of a couple of drops per minute but it is enough to prevent the packing burning (literally burning if it is pulled down too hard on a rotary pump). Reg
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Apr 21, 2015 8:48:14 GMT
hi roger,
there is a well known fullsize test that can be applied in miniature to see whether any pistons leak. we have mentioned it before and is in the BR 'Handbook for Railway Steam Locomotive Enginemen' p.95.
briefly - put loco in position where side to be tested has crank at 90 degrees to FDC or BDC, put handbrake/steambrake on, cylinder cocks closed. with loco in mid gear open regulator a small amount put into full gear. if blow up chimney piston packing/rings defective. if put into forward and back gear and continuous blow then valves leaking.
cheers, julian
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Post by Deleted on Apr 21, 2015 9:01:22 GMT
Personally in gunmetal/bronze cylinders i would fit good old fashioned square braided section graphite asbestos packing in the pistons. If properly fitted it is quite long lasting and none of the problems discussed above.
J
Graphite impregnated braided packing, or greasy packing as it was traditionally known at sea, hasn't actually contained any asbestos for decades. Even when the use of asbestos was common place it was only used for high temperature applications.
The packing I fitted to my tank engine 30 years ago is still in there and I too would endorse its use for non-ferrous cylinders.
Reg
Well why not - as Joan says if it works why change it .
Interesting though that something that comes from the absolute dawn of steam technology still works well in some modern applications . Earliest engines used water seals and 'junk' packing . Don't know why it was called 'junk' but it was just hemp rope saturated with tallow .
MichaelW
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Post by Roger on Apr 21, 2015 9:19:12 GMT
Three great answers that are very useful. I'm guessing that the voids in a graphite style packing allow the material to compress when it expands due to heat. That's a handy way of testing for possible leaks too. I guess the answer to the question 'why change it if it works?' is that it's interesting to experiment, and graphite square packing seems a bit crude. One has to ask the question why people use cast iron rings in cast iron cylinders and not graphite packing if it's really that good? Anyway, it's all good discussion and long may it continue.
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Post by Deleted on Apr 21, 2015 9:22:29 GMT
dug007red :
I think we've identified most of problems with using ptfe in steam cylinders but not yet come up with any complete solutions .
For the remaining part of this running season I strongly suggest that you do a quick fix using soft packing or O rings . My preference would actually be soft packing - at least it is a known quantity .
There are now more uncertainties over use of ptfe than ever .
Perhaps we should all be looking in another direction . I'll have to have a think about that .
MichaelW
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Post by Deleted on Apr 21, 2015 9:32:23 GMT
fowlersfury ,
Very interesting posting .
You have actually had all the experiences that have been predicted so that's useful information in itself .
Problem is that very few model engineers that have tried ptfe in more exacting applications ever tell us what their actual experiences were after five minutes of running . Your posting telling of actual experience and of some failures was very refreshing .
Personally I always take the view that any solution to a problem that takes endless development and fiddling every time it is used is no solution at all .
MichaelW
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Apr 21, 2015 9:43:11 GMT
all very very interesting!
it is always great to have the likes of Reg and Michael comment on these sorts of topics.
so far as Doug is concerned, just a few points....
it isnt the pressure of the steam with a load, but the increased temperature of the steam due to the superheaters and fire being much hotter as the loco is worked hard.
Doug's original non viton 'O' rings hardened up ( Doug - i know you said they were viton rings fitted by your grandfather but they didnt look like viton to me!).
Doug then fitted graphite string instead of proper square braided soft packing. this quickly disintegrated!
the square braided stuff now sold by the ME suppliers is IMHO useless being (so far as i can tell) nothing more than plastic rope covered in graphite that falls off when the rope is bent to fit the pistons. the stuff i bought was consigned to the bin!
there is a knack to fitting square braided packing. i was taught how to do this many years ago. i do not recall a really good description of fitting soft packing in ME or EIM. i have even watched an ex-BR fitter from steam days fit piston packing to a club loco and get it all wrong!
cheers, julian
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Post by Deleted on Apr 21, 2015 10:02:36 GMT
Hi fowlersfury, Hopefully by the time you get to finishing your engine, we'll have a lot more data and experiments behind us and life will be a little simpler. It's interesting that the writer of that article says about the datasheets telling of little or no thermal expansion, I don't think they could have said that because it's simply untrue. If you design assuming that's the case, you'll always be in trouble because the exact opposite is the case. I'm convinced that there is a workable solution amongst the designs we're exploring but it's too early to know as yet. Watch this space! I'm more convinced than ever now that figures for 'thermal expansion' are very misleading . PTFE certainly expands a lot when heated but how much depends on the way that it is heated and how much liquid is present . Also PTFE expansion at higher temperatures probably has a ratcheting characteristic - a hysterisis like property where it expands when heated but doesn't go all the way back to original size when cooled . This would particularly affect thin sections such as rings . Looking at my frying pan this morning - PTFE bonded to Aluminium . Good mechanical strength and good abrasion resistance ??? MichaelW
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Post by Roger on Apr 21, 2015 11:09:26 GMT
Hi fowlersfury, Hopefully by the time you get to finishing your engine, we'll have a lot more data and experiments behind us and life will be a little simpler. It's interesting that the writer of that article says about the datasheets telling of little or no thermal expansion, I don't think they could have said that because it's simply untrue. If you design assuming that's the case, you'll always be in trouble because the exact opposite is the case. I'm convinced that there is a workable solution amongst the designs we're exploring but it's too early to know as yet. Watch this space! I'm more convinced than ever now that figures for 'thermal expansion' are very misleading . PTFE certainly expands a lot when heated but how much depends on the way that it is heated and how much liquid is present . Also PTFE expansion at higher temperatures probably has a ratcheting characteristic - a hysterisis like property where it expands when heated but doesn't go all the way back to original size when cooled . This would particularly affect thin sections such as rings . Looking at my frying pan this morning - PTFE bonded to Aluminium . Good mechanical strength and good abrasion resistance MichaelW That's certainly confirmed by the figures I posted earlier, something very different seems to be going on between 100 and 200 degrees where the expansion becomes much greater. This is why I like the idea of either thin walled seals or those where rings can freely expand. Hopefully we can test these ideas out and come to some useful conclusions. How on earth they manage to bond PTFE to anything is a mystery to me...
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Post by joanlluch on Apr 21, 2015 13:07:22 GMT
Problem is that very few model engineers that have tried ptfe in more exacting applications ever tell us what their actual experiences were after five minutes of running . Your posting telling of actual experience and of some failures was very refreshing .Personally I always take the view that any solution to a problem that takes endless development and fiddling every time it is used is no solution at all . MichaelW That's indeed very true. But I have never seen a report of a setup consisting on two split PTFE rings -next to each other- with orings underneath. Basically, a simplification of Roger approach by eliminating the inner concentric ring. With this setup the o-rings help to push the ring against the cylinder wall, the steam pressure does it as well. PTFE rings are split with wide enough cuts to cope with large expansion. Sealing is achieved because no gaps are present in the radial direction, and the cuts on the two contiguous rings are at different angular positions. I think the above should work provided that PTFE stands its shape at any given temperature, I mean it does not distort when returning to room temperature and back to working temperature. Virgin PTFE may not meet this condition but maybe some filled grade does. The question is whether someone has actually tried this simple approach already.
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Post by Roger on Apr 21, 2015 13:16:26 GMT
Problem is that very few model engineers that have tried ptfe in more exacting applications ever tell us what their actual experiences were after five minutes of running . Your posting telling of actual experience and of some failures was very refreshing .Personally I always take the view that any solution to a problem that takes endless development and fiddling every time it is used is no solution at all . MichaelW That's indeed very true. But I have never seen a report of a setup consisting on two split PTFE rings -next to each other- with orings underneath. Basically, a simplification of Roger approach by eliminating the inner concentric ring. With this setup the o-rings help to push the ring against the cylinder wall, the steam pressure does it as well. PTFE rings are split with wide enough cuts to cope with large expansion. Sealing is achieved because no gaps are present in the radial direction, and the cuts on the two contiguous rings are at different angular positions. I think the above should work provided that PTFE stands its shape at any given temperature, I mean it does not distort when returning to room temperature and back to working temperature. Virgin PTFE may not meet this condition but maybe some filled grade does. The question is whether someone has actually tried this simple approach already. This is where I started with my design but rejected it because it leaks so badly when cold. You need a really large ring gap and I didn't think it was acceptable for it to leak that much until it warmed up. If that doesn't bother you, then it's worth pursuing.
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Post by joanlluch on Apr 21, 2015 13:29:16 GMT
Just to mention yet another alternative, if we want to stick with polymeric materials, we may consider to use PEEK (Ketron). Some grades are reinforced with carbon fibers and contain both PTFE and graphite (Ketron Peek HPV) to keep good dimensional stability at high temperatures while maintaining low friction coefficient and wear according to the manufacturer. Possibly very expensive though.
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Post by joanlluch on Apr 21, 2015 13:34:54 GMT
This is where I started with my design but rejected it because it leaks so badly when cold. You need a really large ring gap and I didn't think it was acceptable for it to leak that much until it warmed up. If that doesn't bother you, then it's worth pursuing. Roger, it bothers me. And we are all looking for the best -yet simpler- possible approach. You keep the lead by actually having tested things, so: Why do you think that this leaks when cold?. If there is no radial gap -the oring sealing that- and the two contiguous PTFE rings are set at different angles, where is the passage that causes it to leak?. I can not see it. I am referring to main piston, not the valve. Thanks.
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Post by fowlersfury on Apr 21, 2015 14:06:30 GMT
Hi Foulersfury. I have already referred to that article in this same thread. See this. modeleng.proboards.com/post/122177/thread Roger suggested that one possible issue was an unappropriated shape of the valve ports, and/or no rings internal clearance. The author also seems to assume that ring expansion would small and thus no provision for it is made in the design. As already posted before the original link for this article is found here: www.modelengineeringwebsite.com/PTFE_valve_rings.html Thanks to all who read through my long-winded post; a few additional points.... (1) I was the author of the Model Engineering Website article, so no question of plagiarising "Crankpin"! David Carpenter doesn't have a 'Letters' page of course but he agreed to put the article in should anyone have any input but none was forthcoming. The glass-filled Fluorosint piston valve 'rings' are shown on there. I modified the design of Geo Thomas for the valve itself as his idea of easy axial adjustment along the valve spindle seemed very good. (2) On this question of thermal expansion of virgin PTFE and glass or mica filled PTFE, I hadn't meant to imply the data sheets indicated little or no expansion. The data sheets showed that under 150 oC the expansion was significantly less than above that temperature. Accordingly, under conditions of superheated steam at 150psi in the compound, the Fluorosint expansion was likely to problematic to say the least. I found the most instructive information on the filled PTFE polymers came from Quadrant Plastics. linkThe comparative data on thermal expansion is:- Fluorosint 207: Coefficient of linear thermal expansion: – average value between 23 and 100 °C m/(m.K) 85 x 10-6 – average value between 23 and 150 °C m/(m.K) 90 x 10-6 – average value between 150 and 250 °C m/(m.K) 155 x 10-6 (3) I did think seriously about using graphited "yarn" since like many of the posters in this thread, I believe it has many desirable properties. In fact on an earlier engine build, I had used 1/4" square graphited "yarn" in GM cylinders. I did find though, maybe because I'd packed it too hard, the engine ran very tight when cold. Once warmed through, it ran freely enough. Whatever, I didn't have enough remaining in stock for this build and at the time, thought it was no longer available. (4) My reference to Dupont's Kalrez O rings. On the basis of their data sheet, Kalrez would fulfill all the demands of HP superheated steam, GM cylinder bores, presence of oil and longevity. linkBut being a tight-fisted model engineer, the very high prices couldn't be justified. Furthermore, whenever I searched on Fleabay for cheaper Kalrez O rings, they were never available in the sizes I needed and couldn't be sure they were genuine DuPont and not grey imports. Yet....I may live to rue my decision because the propsect of a future, total strip down to replace the Clupet piston rings and Fluorosint valve 'rings' fills me with dread, never mind the cost already incurred for the 6 Clupet rings and 12" of Fluorosint .
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Post by Doug on Apr 21, 2015 14:23:36 GMT
all very very interesting! it is always great to have the likes of Reg and Michael comment on these sorts of topics. so far as Doug is concerned, just a few points.... it isnt the pressure of the steam with a load, but the increased temperature of the steam due to the superheaters and fire being much hotter as the loco is worked hard. Doug's original non viton 'O' rings hardened up ( Doug - i know you said they were viton rings fitted by your grandfather but they didnt look like viton to me!). Doug then fitted graphite string instead of proper square braided soft packing. this quickly disintegrated! the square braided stuff now sold by the ME suppliers is IMHO useless being (so far as i can tell) nothing more than plastic rope covered in graphite that falls off when the rope is bent to fit the pistons. the stuff i bought was consigned to the bin! there is a knack to fitting square braided packing. i was taught how to do this many years ago. i do not recall a really good description of fitting soft packing in ME or EIM. i have even watched an ex-BR fitter from steam days fit piston packing to a club loco and get it all wrong! cheers, julian Hi Julian the orings that started it all off were unknown type (fitted by my greatgrandad in 1962 ish) no idea if they worked very well as the loco seemed un-tested and all bent out of shape when i got it (miss alignment issues on the frame and axleboxes) I fitted viton O rings and they expanded (i now know this is because of tight/poor fit) and siezed up, this was before my first boiler test. i then fitted yarn from Square packing and tried again with great results running two open days without issues, then my gland packing poped out so i stripped down and found the yarn had been stripped of all the graphite so i was left with just wet string AWSOME!!!! anyway i then tried silicon orings that behaved the quite well till i tried to run with a load they then also expanded and locked up (poor fit again?) then we get to the PTFE which exhibits the same as the Silicon O rings except the lockup is not total desipite a heavyer load. So I am thinking to design a ring with lateral float so when it expands it will expand into this space giving me a good seal hopefully. I am being told 20tho would be a good starting figure.
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