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Post by Doug on Apr 19, 2015 13:31:50 GMT
Hi I'm back with some results, today I tried a full load test this is the second run with the PTFE rings. The results are that the extra heat caused by running a full load has made the rings expand significantly more than an unloaded run. Julian had previously indicated that this could potentially be an issue the loco acts very differently indeed when loaded up. I managed two very sweet laps before it started to get quite stiff, I retired on the third lap as I ended up in full gear with full regulator. Once cold it's free as a bird again. So I need to think of a way to allow for expansion with my design. I may go for a thinner ring backed by an oring as Baggo suggests. This problem was the main reason I wanted to do a full load test today I suspected that it may become an issue as I have no expansion space with my design so it's back to the drawing board, especially with my Speedy.
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Post by Deleted on Apr 19, 2015 15:32:40 GMT
Hi I'm back with some results, today I tried a full load test this is the second run with the PTFE rings. The results are that the extra heat caused by running a full load has made the rings expand significantly more than an unloaded run. Julian had previously indicated that this could potentially be an issue the loco acts very differently indeed when loaded up. I managed two very sweet laps before it started to get quite stiff, I retired on the third lap as I ended up in full gear with full regulator. Once cold it's free as a bird again. So I need to think of a way to allow for expansion with my design. I may go for a thinner ring backed by an oring as Baggo suggests. This problem was the main reason I wanted to do a full load test today I suspected that it may become an issue as I have no expansion space with my design so it's back to the drawing board, especially with my Speedy. Hi dug , You need to think in 3D to use PTFE effectively . Warm PTFE compresses isostatically - long word but means essentially compresses at nearly constant volume under pressure load . So compress it radially into a slot and it will expand sideways if given freedom to do so . All this amounts to is putting the same PTFE rings as you have into slightly wider grooves (or narrower rings into same grooves) and engine should be much more tolerant of variations of pressure and temperature when working . How much wider the grooves need to be is a matter of trial and error but try 15 to 20 thou for a starting point . Same thing for O rings for a somewhat similar reason - O ring grooves should be significantly wider than the O ring diameter to make them much less sensitive to exact fitting dimensions on diameters and to allow ring to unstick by rolling at each change of direction . MichaelW
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Post by Roger on Apr 19, 2015 23:03:33 GMT
I suppose you could just shave a small amount from the ends of the ring to increase the gap when cold. The figures I was given for PTFE are as follows...
30 -100 deg C Dimensional increase is 0.8% 30 -200 deg C Dimensional increase is 2.4%
Your guess is as good as mine as to what temperature yours will reach! On my SPEEDY with a bore of 38mm and 1%, that's Pi x 38mm x 1/100 = 1.19mm for the initial gap if I've got that right. This is why I'm amazed that anyone have managed to make this work just by slitting rings with a craft knife unless the rings are very sloppy in their grooves and the extra length is accommodated there like Michael suggests. Quite whether the rings recover their original dimensions after cooling down is another thing. If there's no ring gap to start with, there doesn't seem to be any difference between a split and a solid ring in my understanding of this. Of course, if the temperature of the steam gets much higher, the gap will need to be even greater or the clearance on the width greater.
The issue of the piston ring width in the groove on my design is one of the areas with a big question mark at the moment. I think we all have a lot of experimentation to do before we get this working as we would like.
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Post by Doug on Apr 20, 2015 5:32:16 GMT
I suppose you could just shave a small amount from the ends of the ring to increase the gap when cold. The figures I was given for PTFE are as follows... 30 -100 deg C Dimensional increase is 0.8% 30 -200 deg C Dimensional increase is 2.4% Your guess is as good as mine as to what temperature yours will reach! On my SPEEDY with a bore of 38mm and 1%, that's Pi x 38mm x 1/100 = 1.19mm for the initial gap if I've got that right. This is why I'm amazed that anyone have managed to make this work just by slitting rings with a craft knife unless the rings are very sloppy in their grooves and the extra length is accommodated there like Michael suggests. Quite whether the rings recover their original dimensions after cooling down is another thing. If there's no ring gap to start with, there doesn't seem to be any difference between a split and a solid ring in my understanding of this. Of course, if the temperature of the steam gets much higher, the gap will need to be even greater or the clearance on the width greater. The issue of the piston ring width in the groove on my design is one of the areas with a big question mark at the moment. I think we all have a lot of experimentation to do before we get this working as we would like. Those values make a lot of sense under no load (just my not inconsiderate bulk) the likelihood is I was running at low ish temp 120ish deg @ 40-60psi typical but cylinder pressure would be very low 10-20psi as there is very little resistance. Under load running 80psi typical cylinder pressure would be almost as high as resistance is considerable, so quite a bit hotter. The conclusion is that if I make a small gap to allow for expansion then it will work. Although things got really tight it didn't seize up completely so only a small alteration will be required to make it effective. I doubt it would be as effective when cold once it's been hot ran but then loco's don't ever run cold so I am not too worried.
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Post by Roger on Apr 20, 2015 7:36:24 GMT
I suppose you could just shave a small amount from the ends of the ring to increase the gap when cold. The figures I was given for PTFE are as follows... 30 -100 deg C Dimensional increase is 0.8% 30 -200 deg C Dimensional increase is 2.4% Your guess is as good as mine as to what temperature yours will reach! On my SPEEDY with a bore of 38mm and 1%, that's Pi x 38mm x 1/100 = 1.19mm for the initial gap if I've got that right. This is why I'm amazed that anyone have managed to make this work just by slitting rings with a craft knife unless the rings are very sloppy in their grooves and the extra length is accommodated there like Michael suggests. Quite whether the rings recover their original dimensions after cooling down is another thing. If there's no ring gap to start with, there doesn't seem to be any difference between a split and a solid ring in my understanding of this. Of course, if the temperature of the steam gets much higher, the gap will need to be even greater or the clearance on the width greater. The issue of the piston ring width in the groove on my design is one of the areas with a big question mark at the moment. I think we all have a lot of experimentation to do before we get this working as we would like. Those values make a lot of sense under no load (just my not inconsiderate bulk) the likelihood is I was running at low ish temp 120ish deg @ 40-60psi typical but cylinder pressure would be very low 10-20psi as there is very little resistance. Under load running 80psi typical cylinder pressure would be almost as high as resistance is considerable, so quite a bit hotter. The conclusion is that if I make a small gap to allow for expansion then it will work. Although things got really tight it didn't seize up completely so only a small alteration will be required to make it effective. I doubt it would be as effective when cold once it's been hot ran but then loco's don't ever run cold so I am not too worried. If you guess at 120 degrees, I think you need to allow for about 1% for the increase in circumference for any given piston diameter. That's going to give a big gap, say 1mm which ought to completely close when it's hot. I don't know if that's what you mean by a 'small gap' but in piston ring terms it's very big for the sort of diameters we're talking about. If you make it any smaller, it will probably start to tighten up again. I guess we both need to make sure that 1% is the available clearance on the width of the ring too.
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Post by Deleted on Apr 20, 2015 10:55:51 GMT
I've done a calculation of the stress induced in a trapped ptfe ring subject to temperature rise .
It's a simple 1D analysis just to get a feel for what is happening . Rectangular ring fitted in same size groove . 150 deg C reference temperature . I used 5 by 3 mm ring in 50 mm cylinder .
Stress induced in ptfe due to temperature rise comes out at 6.4 MPa or about 930 psi . This would also be the surface pressure seen between ring outer and cylinder bore .
Even taking a coefficient of friction as low as 0.05 a force of 250N or about 56 lbf would be needed just to move the piston - no wonder they jam .
Probably going way too far for model designs but just for interest it would be quite possible to decide on the surface pressure desirable and calculate backwards to decide on a configuration of ring and clearances that gives this pressure .
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Post by Roger on Apr 20, 2015 12:54:14 GMT
I've done a calculation of the stress induced in a trapped ptfe ring subject to temperature rise . It's a simple 1D analysis just to get a feel for what is happening . Rectangular ring fitted in same size groove . 150 deg C reference temperature . I used 5 by 3 mm ring in 50 mm cylinder . Stress induced in ptfe due to temperature rise comes out at 6.4 MPa or about 930 psi . This would also be the surface pressure seen between ring outer and cylinder bore . Even taking a coefficient of friction as low as 0.05 a force of 250N or about 56 lbf would be needed just to move the piston - no wonder they jam . Probably going way too far for model designs but just for interest it would be quite possible to decide on the surface pressure desirable and calculate backwards to decide on a configuration of ring and clearances that gives this pressure . That's really interesting, and as you say, no wonder it seizes up! I suppose you could do as you suggest, but what happens if the desired temperature is exceeded, say when running light with a very hot fire? The figures I have seem a bit vague as to the amount of expansion when the temperature exceeds 100C. It seems to me that having a clearance that is always present is probably a better bet so that the PTFE always has somewhere to go. Out of interest, I've been looking into the idea of using a split piston to allow a special piston ring to be used. This is the sort of thing I have in mind. The idea is that the piston ring has 'O' rings underneath it and the top is slightly narrower than the piston ring groove to let steam get under the lip. The piston ring is thin so it's flexible enough to be compressed as the temperature rises and the steam pressure controls the amount of outward force. Experimental piston ring by rogerfroud, on Flickr
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Post by Doug on Apr 20, 2015 13:43:10 GMT
I've done a calculation of the stress induced in a trapped ptfe ring subject to temperature rise . It's a simple 1D analysis just to get a feel for what is happening . Rectangular ring fitted in same size groove . 150 deg C reference temperature . I used 5 by 3 mm ring in 50 mm cylinder . Stress induced in ptfe due to temperature rise comes out at 6.4 MPa or about 930 psi . This would also be the surface pressure seen between ring outer and cylinder bore . Even taking a coefficient of friction as low as 0.05 a force of 250N or about 56 lbf would be needed just to move the piston - no wonder they jam . Probably going way too far for model designs but just for interest it would be quite possible to decide on the surface pressure desirable and calculate backwards to decide on a configuration of ring and clearances that gives this pressure . That's really interesting, and as you say, no wonder it seizes up! I suppose you could do as you suggest, but what happens if the desired temperature is exceeded, say when running light with a very hot fire? The figures I have seem a bit vague as to the amount of expansion when the temperature exceeds 100C. It seems to me that having a clearance that is always present is probably a better bet so that the PTFE always has somewhere to go. Out of interest, I've been looking into the idea of using a split piston to allow a special piston ring to be used. This is the sort of thing I have in mind. The idea is that the piston ring has 'O' rings underneath it and the top is slightly narrower than the piston ring groove to let steam get under the lip. The piston ring is thin so it's flexible enough to be compressed as the temperature rises and the steam pressure controls the amount of outward force. Experimental piston ring by rogerfroud, on Flickr thats reall odd Roger i was thinking the samething yesterday making a piston that splits, I had only got as far as a u shape profile though.
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Post by Doug on Apr 20, 2015 13:48:58 GMT
I've done a calculation of the stress induced in a trapped ptfe ring subject to temperature rise . It's a simple 1D analysis just to get a feel for what is happening . Rectangular ring fitted in same size groove . 150 deg C reference temperature . I used 5 by 3 mm ring in 50 mm cylinder . Stress induced in ptfe due to temperature rise comes out at 6.4 MPa or about 930 psi . This would also be the surface pressure seen between ring outer and cylinder bore . Even taking a coefficient of friction as low as 0.05 a force of 250N or about 56 lbf would be needed just to move the piston - no wonder they jam . Probably going way too far for model designs but just for interest it would be quite possible to decide on the surface pressure desirable and calculate backwards to decide on a configuration of ring and clearances that gives this pressure . Nice work, thats really useful info i think i will apply it to my piston valves, I am making the PTFE sections of the bobbin very thin to try and reduce this effect but its worth doing the calculation to see what we have got. it also goes to explane somwhat why smaller guages do better with PTFE as the sections are smaller so the expansion less and forces lower, for the same given boiler pressures
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Post by Roger on Apr 20, 2015 14:33:34 GMT
That's really interesting, and as you say, no wonder it seizes up! I suppose you could do as you suggest, but what happens if the desired temperature is exceeded, say when running light with a very hot fire? The figures I have seem a bit vague as to the amount of expansion when the temperature exceeds 100C. It seems to me that having a clearance that is always present is probably a better bet so that the PTFE always has somewhere to go. Out of interest, I've been looking into the idea of using a split piston to allow a special piston ring to be used. This is the sort of thing I have in mind. The idea is that the piston ring has 'O' rings underneath it and the top is slightly narrower than the piston ring groove to let steam get under the lip. The piston ring is thin so it's flexible enough to be compressed as the temperature rises and the steam pressure controls the amount of outward force. thats really odd Roger i was thinking the samething yesterday making a piston that splits, I had only got as far as a u shape profile though. In a way, it's similar to having a thin piston ring, but it allows you to have that and not have it so flimsy and fragile. It might be easier to make as two separate rings though, holding this design to do the opposite side is a bit of a challenge. Basically, it's much the same as an oil seal that you see on a shaft if you do that.
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Post by Doug on Apr 20, 2015 14:39:04 GMT
ok with my silly head on I have slipped in to the murky world of MATHAMATICS unless i am doing something silly i work out that the expansion on the PTFE is; @ 1.6mm thick the coefficent of expansion being 0.000135 ish per deg C so at 160 deg i will get an extra 0.032mm this does not seem very large, am i missing something? or do i need to reduce the diameter by just 0.064mm to allow for expansion? i must be getting somethig wrong somewhere surely.
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Post by Roger on Apr 20, 2015 15:29:03 GMT
It's not the thickness you have to worry about, you've got a split ring, so you have to look at the length of the circumference and apply the coefficient to that. Then you can see the amount of gap you need to leave.
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Post by joanlluch on Apr 20, 2015 17:04:04 GMT
To be more precise: Thermal expansion is like applying a global scale factor, all lineal dimensions such as diameter and width are affected in a free expanding object. However in this scenario we are constrained by the cylinder bore diameter, so the resulting effect is a reduction of the cut gap by the expansion of the circumference length. What Roger suggests is that you must allow more than enough gap to cope with the highest temperature.
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Post by Deleted on Apr 20, 2015 18:24:50 GMT
I can't find any definite numbers but I think it likely that ptfe expands by even more than the simple thermal expansion amount when heated by steam and hot water rather than by dry heat in a lab test .
Apart from just seeing what stresses can be generated when constrained ptfe is heated I have learned something else today and that is that that practical performance of ptfe in simple ring installations can be changed radically by very small changes in ring and groove geometry . Variations of a few thou can take the seal from success to failure or vice versa .
Need to find a way of eliminating the sensitivity and obtaining a satisfactory and predictable result every time .
Sealing rings should be fit and forget - make and install them according to a known spec. and they should just work perfectly and survive for required service life .
Need to recognise that ptfe only has two useful properties - low friction and good flexibility when warm .
Mechanical properties of ptfe are poor in many ways - but most relevant to seals are that it has effectively zero elasticity and very uncertain long term stability .
Lack of any elasticity means that ring cannot be set up with enough spring ring effect to guarantee constant contact with cylinder bore under all conditions . Lack of long term stability means that even if enough spring could be contrived on a new ring this would rapidly weaken in service .
A really effective ptfe piston seal will probably always end up being two part - a seal ring in ptfe and a mechanical backup such as a thin conventional metal ring or O ring .
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Post by Roger on Apr 20, 2015 18:33:37 GMT
I can't find any definite numbers but I think it likely that ptfe expands by even more than the simple thermal expansion amount when heated by steam and hot water rather than by dry heat in a lab test . Apart from just seeing what stresses can be generated when constrained ptfe is heated I have learned something else today and that is that that practical performance of ptfe in simple ring installations can be changed radically by very small changes in ring and groove geometry . Variations of a few thou can take the seal from success to failure or vice versa . Need to find a way of eliminating the sensitivity and obtaining a satisfactory and predictable result every time . Sealing rings should be fit and forget - make and install them according to a known spec. and they should just work perfectly and survive for required service life . Need to recognise that ptfe only has two useful properties - low friction and good flexibility when warm . Mechanical properties of ptfe are poor in many ways - but most relevant to seals are that it has effectively zero elasticity and very uncertain long term stability . Lack of any elasticity means that ring cannot be set up with enough spring ring effect to guarantee constant contact with cylinder bore under all conditions . Lack of long term stability means that even if enough spring could be contrived on a new ring this would rapidly weaken in service . A really effective ptfe piston seal will probably always end up being two part - a seal ring in ptfe and a mechanical backup such as a thin conventional metal ring or O ring . I agree with every part of this, and it's not easy to predict what's going to happen. The two part seal might not be necessary though if the steam is allowed to easily get under the ring. The effect of that is really noticeable on my piston valve setup where there's no end float on the rings. When air is first applied to the bobbin, it's easy to move with just the 'O' rings for the seal. That all changes when air eventually leaks under the rings and applies much greater outward force. The force to move the bobbin is maybe three times greater when that happens. Whether this would happen if the rings weren't already being held against the bores is uncertain. I need to look at how the lack of end float is going to affect my design because I really don't know yet. It might bind up and prevent it working, but then again the expansion might be accommodated elsewhere without too detrimental effect. Until I try it at a higher temperature, I won't know.
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Apr 20, 2015 21:37:07 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
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Post by Roger on Apr 20, 2015 22:16:39 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 I'd love to see how that type of seal performs in a scientific test on the bench because I doubt if it performs as well as you think. I'm sure it works well enough to be perfectly satisfactory, but the idea of these experiments is to find alternative methods that seal better over long service intervals, and don't contain banned substances!
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Post by fowlersfury on Apr 20, 2015 22:52:52 GMT
I have been watching this thread with great interest and the sound advice being offered by knowledgable contributors. May I offer the following over-long narrative for comment? The issue has been a constant source of anxiety during the build and until steam testing is possible (if ever), it will remain so !
Use of PTFE Composite for Piston Valve Rings in Gunmetal Castings This issue arose originally because the purchased job-lot of castings for my next (HP compound) locomotive were all gunmetal and received wisdom was that using cast iron rings for both piston valves and main pistons in gunmetal bores was not a good idea. Therefore for the main piston rings, several different materials were compared against their commercial availability, cost and/or ease of manufacture in a home workshop. One new polymer (Kalrez©) appeared to possess all the necessary properties for O rings in the main HP pistons but the price was prohibitive. Rings in stainless steel or phosphor bronze, made to order, also proved far too costly as well as their suitability with gunmetal being questionable. Yet subsequently, after prolonged research and then discussion with the metallurgist-producer of cast iron “Clupet” rings, it was decided to employ these rings for the main pistons with assurances that providing there was good lubrication, there should be no problem using them in the gunmetal bores. I wish I was wholly convinced!
For the piston valves, several articles in M.E. and EIM over the past 10 years have described using either virgin PTFE or mica or glass filled PTFE for the valve “rings” so it was decided to adopt this approach. However, only a few of the several published articles referred to a problem. This was that machining the rings to be a sliding fit in the valve liners at ambient temperature led to their seizure when hot, despite the data sheets showing there was little or no thermal expansion of the polymer. Indeed, no lesser builder than Doug Hewson had experienced just this paradox. To avoid post-build dismantling, the offered solution was that the entire cylinder block, together with the valve spindle and its PTFE composite rings should be repeatedly heated in an oven to something like operating temperature and the rings’ diameter progressively reduced on the lathe until they were free to slide at this elevated temperature. It was said to be an irksome process. Nevertheless, a short length of the very expensive Fluorosint 207 (PTFE with mica) was purchased for use in the high pressure, inner cylinder as piston valve “rings” for this compound locomotive. After machining, some difficulty was experienced in obtaining a smooth fit at room temperature because the Fluorosint seemed to snag and roughen-up on the steam passageways cut into the gunmetal liners despite their smoothed edges. A particular difficulty was also encountered in the simple task of measuring the diameter of each Fluorosint ring. This should have been straightforward but there was very little repeatability in the measurements using a sensitive micrometer; the measured values seemed to vary by about 0.005”. Perhaps consequently, with this middle HP cylinder, the valve spindle felt rather loose in the valve liner at ambient temperature despite the ID and OD dimensions being apparently the same. At this point, the inner cylinder was gladly put aside until the two outside LP cylinders had been machined. Months later, when the two outside cylinder castings had been machined the price of Fluorosint had risen further (to £13.50 per linear inch for 1” diameter rod) and given the surface scoring observed with the inner cylinder’s valve, the somewhat less expensive and probably better wearing, glass-filled PTFE was purchased. The two outside cylinder valve spindles were made to the same design as the inner cylinder and it became obvious that again the rings had to be turned down in situ, on the spindle, once their central (¼”) diameter hole had been drilled on the lathe. This necessitated using a collet and female centre to ensure repeatable concentricity for the ‘skimming’. The diameters of the rings (1” OD) were reduced until a sliding fit in the valve liners was obtained. One of the published articles recommended making the central hole in the rings larger than the diameter of the spindle “to allow some float”. This was done at first by enlarging the hole to 9/32”. Subsequently it proved to be a mistake as the rings would then not run concentrically when later mounted to reduce their diameters. Therefore new rings were made so that they were a tight fit on the ¼” spindle. The bore may be enlarged later to allow some degree of float. The next stage was to find the extent of binding, if any, when the temperature was raised. The inside cylinder was therefore dusted off and together with the 2 new outside cylinder blocks & piston valves, prepared for heating. Because the valve liners had been fixed in place with Loctite 301 there was concern that too much heat might break the seal, so the domestic oven was set to only 150° C. This was rather less than the steam tables suggested the temperature of steam would be at 150 psi. The polymer rings were coated with a light mineral oil and the complete spindles inserted into the 3 cylinder blocks and allowed to equilibrate in the oven for about 30 minutes. After removal from the oven, still at 150°, the valve spindles were all locked solid in their liners due to the expansion of the PTFE composite. So, after waiting until cool, the spindles were returned to the lathe collet & female centre and 0.001” was removed from the ring diameters. The cylinders and valve spindles were reassembled and returned to the oven for another check. The whole tedious procedure was repeated many times until the spindles could be slid with only slight resistance when at 150°C. Not surprisingly by now, when at ambient temperature, the valve spindles were decidedly loose in the valve liners because the rings had contracted. Clearly, the locomotive could never be run on compressed air to bed in the motion work with valves this loose at room temperature. The two “rings” on each spindle (offside and nearside) had been machined to the same diameter using a collet and female centre each time yet their measured diameters were different. This was attributed to the inability to get a consistent feel with the micrometer. Whether using the ratchet thimble or not appeared to make no difference to the poor repeatability of measurements. Conclusions Compared to using cast iron rings, reinforced PTFE was very problematic and appears to prohibit running-in using compressed air because of its thermal expansion. The whole process of fitting and machining the PTFE rings was tedious and very inexact. The temperature at which the fitting was done (dry at 150° C) gives no confidence that considerable tightening will not occur when the loco is steamed and if so, will demand substantial dismantling to rectify the problem The cost of Fluorosint or glass-filled PTFE is considerable and whilst reports suggest that the service life of such “rings” is lengthy, the disadvantages seem also considerable. The product data sheets for Fluorosint quote figures for linear expansion to be extremely low and that it is unchanged up to 150°C, thereafter the linear expansion rises only very slightly. But this does not accord with findings in this application and of other builders where the radial expansion was appreciable. From a sliding fit at ambient temperature to a sliding fit at 150°C some 7 to 10 thousands of an inch had to be removed from the diameter of each ring and moreover, judging a sliding fit was very subjective and therefore imprecise with these glass-filled PTFE rings. I have read no negative comments about using PTFE piston valve rings; indeed the authors have expressed every satisfaction. Perhaps therefore my experiences so far are unusual and due to my ignorance and poor methodology. Regrettably I have passed the stage in the locomotive’s build where I could contemplate scrapping the gunmetal cylinders and reverting to the time-honoured combination of cast iron rings in cast iron liners. Yet I have little confidence that when the locomotive is finally steamed these glass-filled PTFE valve rings as fitted will prove trouble free.
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Post by Doug on Apr 21, 2015 5:38:51 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.
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Post by joanlluch on Apr 21, 2015 5:53:06 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
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