redmog
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Post by redmog on Feb 15, 2008 14:06:52 GMT
I'm building Bantam Cock, a 3 1/2" 2-6-2 tender loco. What shall I use in my cylinders made from Reeves cast gunmetal? - rings - graphite yarn - or 'O' rings, and what should the pistons be made from. Advice welcome.
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simonwass
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Cecil Pagets 2-6-2 of 1908. Engine number 2299. Would make a fascinating model....
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Post by simonwass on Feb 15, 2008 16:50:40 GMT
I'm building Bantam Cock, a 3 1/2" 2-6-2 tender loco. What shall I use in my cylinders made from Reeves cast gunmetal? - rings - graphite yarn - or 'O' rings, and what should the pistons be made from. Advice welcome. Well I'd go for polished bores (garnet papered then a bit of brasso on a rag), turn pistons to 40 thou undersize and fit o rings to correctly sized piston groove to allow ring to roll. A piston with properly fitted o rings is low friction with correct lubrication. The undersize piston means there is no chance of the polished finish ever being damaged if the lubricator packs up, but even slightly wet steam is get home lub for a properly fitted o ring. Pistons can be ali if you wish, all it's there for is to hold the o ring.
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redmog
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Post by redmog on Feb 15, 2008 17:40:13 GMT
Thanks simonwass
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abby
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Post by abby on Feb 15, 2008 18:09:35 GMT
Where does the idea that all the piston does is hold the rings come from ? I have always believed that the rings were there to seal the piston ! Does the cart push the horse ? 40 thou clearance is in my opinion far too much and 1 thou per inch of bore is the accepted clearance , but its your engine and you can do what ever pleases you.
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Post by mutley on Feb 15, 2008 19:44:55 GMT
If you want to use O rings then you need to hone your bores and the small honing kits available are unlikey to be capable of producing the goods. If your cylinders are gunmetal then fit bronze rings or use graphite packing. Piston clearance only needs to be a few thou and should ideally be of the same material as the cylinders to allow for expansion. Diffrent materials and you need to think about different expansion rates.
Andy
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simonwass
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Cecil Pagets 2-6-2 of 1908. Engine number 2299. Would make a fascinating model....
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Post by simonwass on Feb 16, 2008 16:02:32 GMT
Where does the idea that all the piston does is hold the rings come from ? I have always believed that the rings were there to seal the piston ! Does the cart push the horse ? 40 thou clearance is in my opinion far too much and 1 thou per inch of bore is the accepted clearance , but its your engine and you can do what ever pleases you. No matter what shape the end of your piston, it will have the same effective area, a 20 thou gap between the edge of the piston and the wall of the cylinder is no problem. OK you could go down to a thou but there is a risk that it can droop and rub the polished bore, I also have huge clearance on my piston rod glands so have no mechanical support, it simply isn't needed. It has served my engines fine, the Clayton has done mile after mile (8:1 gear ratio to boot!) and the bores are still mirror finish. The Tich did enough miles to completely wear out the valvegear, again the piston bores are untouched. So long as the groove comes to more than the halfway point of the rings cord dia (at the sizes I use 0.1"dia) then a groove depth of 0.09" is perfectly fine to support the ring. The ring does seal the piston but having a 1 thou gap doesn't seal any more effectively than a 20thou gap! It would if the ring was square but they are round (well they do squash a bit!). The technology difference between O rings and mechanical packing is larger that people believe. There certainly isn't any need to use bronze rings in a gunmetal bore, O rings are so easy to use and work if used correctly. I was just passing on my findings, if it doesn't work for you, then thats fine. I'm sticking to what works with no risk of bore wear! Graphited string is difficult to get just right, either you put in too little and have a low friction leaky piston, or you put in too much and get a steam tight and mechanically tight piston. If I were using iron cylinders then I would use a properly fitting piston with tight clearances and have properly designed and made piston rings. We've had the same discussion at the club, some just don't like O rings.
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abby
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Post by abby on Feb 16, 2008 17:58:19 GMT
I have no problem with the use of O'rings , but your suggestion that the piston is merely support for the rings is incorrect, all of the power generated by an engine is transmitted by the piston , not by the rings. A piston with excessive clearance will rock along its axis, placing pressure and later wear on the piston and rod . This may be unimportant in a model sized engine but I prefer to stick to full sized practice where I can. I am also not convinced that O'rings "roll" , given that the ID must be smaller than the OD the force required to compress the OD and stretch the ID during a rolling motion is quite a lot and the friction of the ring to bore contact must therefore be sufficient to apply this amount of force. I have stripped many pneumatic cylinders that use O'rings and from the wear on the rings it was obvious that no "rolling" had occured. I am by no means an expert on this subject and would welcome the views of anyone with experience to enlighten me. I have made PTFE rings of a "top hat" section and these worked very well , but at the end of the day almost anything will work at a model level.
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simonwass
Part of the e-furniture
Cecil Pagets 2-6-2 of 1908. Engine number 2299. Would make a fascinating model....
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Post by simonwass on Feb 16, 2008 23:06:17 GMT
A piston with excessive clearance will rock along its axis I would like to see how this could happen, pressure acts equally over the surface. The O ring also gives equal pressure around the periphery of the piston so it keeps it aligned radially. How could it 'rock' if fitted tight to the rod? I am also not convinced that O'rings "roll" Sorry but you are definitely wrong here, O rings DO roll, the groove allows for this, the rolling action is a feature of O rings used for axial movement. Basically it doubles the life/wear on the ring. Obviously if someone in error machined the groove to nip the ring axially then it wouldn't roll but for instance a ring of cord dia 0.139" should be fitted to a groove 0.170" MINIMUM! Surely someone else has input here and can back this up? I have stripped many pneumatic cylinders that use O'rings and from the wear on the rings it was obvious that no "rolling" had occurred. A pneumatic cylinder may be fitted differently to a steam cylinder, I don't know though as I have no experience with pneumatics. My 20 year experience with O rings will only go on, and on, and on due to absolutely no failure or problems. This speaks for itself I believe, if I had fitted them incorrectly surely I'd have experienced problem after problem? I use then on pistons, piston/valve rods, stop valve spindles, water pump rams, gauge glasses, flanged pipe fittings, clacks (with winged valves), they are brilliant.
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Post by baggo on Feb 17, 2008 1:09:35 GMT
The best source of information on O rings I have found so far is the Parker O ring Handbook: www.nedc.com/literature/nedc-parker-o-ring-catalog.pdfThis seems to go into a bit more detail than the info from James Walker etc. To quote a couple of sections from the handbook: 1) "O-ring seals must be radially compressed between the bottom of the seal groove and the cylinder wall for proper sealing action. This compression may cause the seal to roll slightly in its groove under certain conditions of piston motion, but the rolling action is not necessary for normal operation of the seals." 2) "In a reciprocating seal, there is relative reciprocating motion (along the shaft axis) between the inner and outer elements. This motion tends to slide or roll the O-ring, or sealing surface at the O-ring, back and forth with the reciprocal motion. Examples of a reciprocating seal would be a piston in a cylinder, a plunger entering a chamber, and a hydraulic actuator with the piston rod anchored." End of quotes. My personal thoughts are that in our situation where we work with good lubrication and relatively low pressures there will be insufficient friction between the ring and the cylinder wall to cause the ring to roll completely and the movement will be mainly a sliding one. (Try rolling an O ring inside out between your fingers - it takes quite a bit of effort) There may be a slight rolling backwards and forwards at the end of each stroke as the piston changes direction and the ring moves from one side of the groove to the other. It's my understanding that the extra groove width is to allow the ring to spread sideways as it is compressed in the bore, otherwise the friction between the ring and the bore will be excessive. James Walker recommend a maximum clearance between the piston and the bore of about 5 thou (varies slightly with the ring section) to prevent the ring being extruded through the gap if the piston is pushed to one side. However we're talking pressures up to 1500 psi here so we could get away with larger gaps at 100psi say. With a large gap you could assume that the ring itself would keep the undersize piston central in the bore so long as the crosshead was a good fit in the slide bars and took all the sideways thrust from the connecting rod and the piston rod has a slight clearance in the cylinder cover boss. If the crosshead is a sloppy fit in the slide bars and the piston rod a running fit in the cylinder cover boss then up and down movement in the crosshead will cause the piston to be forced to one side of the bore causing rapid wear of the bore and the piston rod. Mind you, this will happen even if the piston is a close fit in the bore. Like Simon, I use O rings for virtually everything and so far have had no problems apart from when using them for sealing steam pipe connections with high superheat - the silicon ones go gooey! I took the piston out of one of Helen Longish's cylinders recently after about a 100 hours of running and the O ring looks no different to the day I first fitted it. No sign of any wear at all. The secret with O rings is in the finish of the bore. If you can't get a really good finish forget them as they won't last five minutes. Stick to graphite packing, or, as Abby suggests, use PTFE rings instead. John
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steam4ian
Elder Statesman
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Post by steam4ian on Feb 17, 2008 4:17:45 GMT
G'day all.
I have no direct, as yet, experience with the use of O rings for piston rings but I do know the following. If they were only and truly a rolling seal then they would have to move along the piston the same distance they moved along the bore. You can test this with a round pencil and a scale; roll the pencil between the scale and a piece of paper and mark the distance the piece of paper. The stroke of the piston will be twice the distance the ring moves which means the piston will have to be much longer than half the stroke so that the ring does not drop off the end. Putting a groove in the piston limits the ring rolling to a comparatively small amount. If the ring does roll in the groove it simply presents a new wear face on the bore.
If you don't believe try the experiment yourself, use different diameter pencils (rollers) and the outcome is the same.
regards, Ian
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Post by freddo on Feb 17, 2008 9:49:37 GMT
Ian says it all - if the ring doesn't roll for half the piston movement, then it has to skid and consequently wear out.
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waggy
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Post by waggy on Feb 17, 2008 9:51:36 GMT
Morning all, I've used O rings in my engines and had no trouble with wearing or tearing. The pistons are all aluminium with side clearance of two or three thou. The depth of the groove for the ring is machined so the ring sits proud of the piston by seven to eight thou, so giving a five thou compression of the ring when it's in the bore. The groove width is fifty thou wider than ring diameter so it can roll slightly in the groove. The idea of this is so the ring picks up fresh oil every time it rolls due to the piston changing direction. I once tried making the groove nearly as wide as the piston, leaving a thin retaining edge on the piston skirt. This worked but I found the ring could roll too much and got twisted, no good. Fifty though works. Now, here's the dodge! Don't make your pistons with parallel skirts. Set the compound slide to approx five degrees and turn the piston skirt to a taper, working away from the groove towards the outer edges of the piston faces. This allows the piston to collect oil from the cylinder and forces it to go where it needs to be, on the ring. Parallel pistons will push the oil along the bore before it can get to the ring which will increase wear of the ring. I've used this method in cast iron and gunmetal cylinders, bored on the lathe with a between centres bar and polished with the finest wet and dry you can find, oiled and worked by hand along the bore. Waggy.
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Post by freddo on Feb 17, 2008 10:39:47 GMT
Mind you, Waggy, in full size, there's little use of gunmetal cylinders and O rings - I have to say I favour Prof Chaddock's ideas for serious use.
So reklawwc, why not poke a cast iron sleeve in the cylinders and do it properly?
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waggy
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Post by waggy on Feb 17, 2008 13:23:24 GMT
The opening thread refers to a 3.5" loco, hardly full size!
Reklawwc asked for advice which was freely given. The final decision is his regarding the way he builds his engine, however, he will find that some of us can "ease the way" , as it were!
Waggy.
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Post by alanstepney on Feb 17, 2008 15:27:03 GMT
I have always favoured the "traditional" graphite yarn in bronze cylinders.
IMHO, one of the troubles with O rings is not the rings themselves, but that many people make them far too tight.
On the only engine in which I used them, I did some basic experiments. The "best" fit was such that the O ring on is own would stay in the bore when the latter was vertical. However, the O ring fitted to the picton would only just stay put in the same circumstances, and a quick shake would cause the piston to fall down.
We only use low pressures compared to, for example, hydraulics, which is one of the common places where O rings are used.
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simonwass
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Cecil Pagets 2-6-2 of 1908. Engine number 2299. Would make a fascinating model....
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Post by simonwass on Feb 17, 2008 15:49:29 GMT
I have always favoured the "traditional" graphite yarn in bronze cylinders. IMHO, one of the troubles with O rings is not the rings themselves, but that many people make them far too tight. On the only engine in which I used them, I did some basic experiments. The "best" fit was such that the O ring on is own would stay in the bore when the latter was vertical. However, the O ring fitted to the picton would only just stay put in the same circumstances, and a quick shake would cause the piston to fall down. We only use low pressures compared to, for example, hydraulics, which is one of the common places where O rings are used. Anything is only as good as the person building/operating it! The test idea is a good one, when I made my Scotsman cylinders, with normal machine oil the pistons & rods can be 'shook' to move within the bore. Also to show how free of friction a correctly fitted O ring can be, my dads 'Doris' ticks over in its stand on 3psi of air, I tested this at work in an instrument calibration workshop. 5psi to start then air could be backed off to 3psi without the wheels stopping. The mention of hydraulics and our pressures is useful, even I would admit my usual 20thou clearance may be enough for the ring to be 'extruded' through the gap at 1500psi, 80psi though and it seems (is) fine.
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