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Post by Deleted on Dec 8, 2014 18:04:53 GMT
Time out to do a bit of reading as my copy of Riddles Class 6/7 Standard Pacifics by David Clarke has arrived. At first glance it will be an interesting read.
Cheers
Tom
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Post by Jim on Dec 8, 2014 20:39:55 GMT
You'll find it a very handy reference Tom.
Jim
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Post by Deleted on Dec 9, 2014 2:22:04 GMT
I briefly met Robert Riddles at the SVR when we had finished Britannia's part restoration back in the late 1970's ( I helped with the Cannon Axles)......He was guest of honour at the re-naming ceremony and then took the regulator for a short run within station limits.......He died in 1983 so were very lucky indeed.... ------------------- That's me, crouching down by the front driver with a good friend of mine --Mike Heinzman just in front...He was full-time staff and is now retired, whilst the young girl must be a woman in her late 30's by now....
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Post by Deleted on Dec 9, 2014 12:27:12 GMT
Interesting stuff there and the book is good reading as well.
So, is there a running Britannia at the moment? I believe one resides in the museum in York (not far from a pub) but is there another one? I would put it on my list to visit next year.
Cheers
Tom
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Post by ejparrott on Dec 9, 2014 18:10:50 GMT
Britannia herself is usually on the mainline but is currently out of action, Cromwell I'm not currently sure about
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Post by Deleted on Dec 9, 2014 19:04:09 GMT
not in the Houses of Parliament, then ??
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pondok
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My 5" gauge SAR class 15F
Posts: 359
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Post by pondok on Dec 11, 2014 12:05:34 GMT
Hi Tom, cue my stuck-record advocation of PTFE rings for your pistons (and valves). No need for o-rings underneath, simply machine firm push fit rings in the bore, that fit firmly into the original piston grooves. No need for a split piston, cut the PTFE ring with a thin craft knife, round off any burred edges. No need for any fancy type PTFE, just standard. It's very tolerant of less than perfect bore finish (unlike o-rings), it lasts forever (or in my case, 6 years later still perfect seal), friction free, so tolerant of occasional lube failure, makes a perfect seal because of heat expansion which in 3.5" gauge is really important for a good runner. Especially good in used locos where the bore has been slightly belled out due to wear over the years - the PTFE fills the space to compensate. Only possible down side is running on air afterwards might not seal 100% as it's contracted again with the cold. Have fun! cheers andy
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Post by Deleted on Dec 11, 2014 12:21:13 GMT
Thanks for that information Andy. I won't be near my PTFE supplier until the New Year but I want to pick some up then. I definitely want to do that with the valves and I might have a go at a pair of pistons as well.
When cutting the ring, would cutting on an angle be the idea?
Cheers
Tom
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pondok
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Post by pondok on Dec 11, 2014 13:09:40 GMT
Hi Tom,
I don't think it matters much to be honest. I figured if you cut at an angle, you end up with sharper points that could conceivably catch on something, but then again that's only likely with a valve ring that passes over a port, the piston ring doesn't. I always go with a step cut, to reduce the very theoretical chance of steam somehow passing via the cut. It's really splitting hairs, let alone rings! ;-) Btw, I do make them to have a mm or two clearance under the rings in the groove, so that in theory they can expand first outwards into the bore, then any further expansion of the material will go inwards. I've yet to hear of anyone whose tried it like this and regretted it. Fit and forget cheers
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Post by Deleted on Dec 11, 2014 13:26:23 GMT
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Post by Deleted on Dec 11, 2014 17:26:27 GMT
Thanks Pete and Andy for the replies. I followed and saved all those links and did a bit of reading. I am certainly going to have a go with the valves.
The current piston rods are in tough shape as it appears the were scored with a pain of pliers in trying to tighten them in the piston. I have sort of smoothed them out but they are still pretty manky looking. They are locktited in so I will probably just make a new set and insert the ptfe rings.
Cheers
Tom
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Post by Roger on Dec 11, 2014 20:16:18 GMT
Sorry to butt in here, but Andy, could you give some idea of the thickness of PTFE piston rings I would use on a 38mm piston bore and 16mm valve bore?
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Post by joanlluch on Dec 12, 2014 7:20:48 GMT
Andy, this is going to be a brave question. But do you think that PTFE ringed pistons would work for a significant amount of time with NO oil lubrication?
My experience says that in most cases water is all what is needed to lubricate PTFE sliding against a polished, non oxidising, metallic surface such as stainless steel and probably bronze. So I wonder if cylinder liners on a steam engine would maintain wet enough to support this property of PTFE. I mean, water can be the lubricator of PTFE (instead of oil)
A steam engine seems to be the perfect device to use that PTFE property as water is implicitly there in the cylinders.
If the above was true, it would be a game changer on live steam technology because oil lubricators would be no longer needed.
So my question is: Have you eventually run a loco with PTFE rings with no oil lubrication for a significant amount of time to see what happens?
Thanks Joan
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Post by Roger on Dec 12, 2014 8:08:10 GMT
If you're using superheated steam there won't be liquid water in the valves to lubricate them so the PTFE would have to rely on it's natural lubricating properties. I don't know if that would be enough.
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Post by joanlluch on Dec 12, 2014 8:21:16 GMT
If you're using superheated steam there won't be liquid water in the valves to lubricate them so the PTFE would have to rely on it's natural lubricating properties. I don't know if that would be enough. Hi, Roger. I agree. This is what led me to pose the question. I would be more confident about this working on non-superheated engines because in such case some amount of liquid water would presumably be always present in the cylinders. However, I wonder if in superheated engines steam expands enough to reach the dew point and thus creating some 'moisture'. I am also wondering whether the PTFE itself could retain enough moisture in its voids as to create the lubricating effect by itself. Some of this is of course wishful thinking, but the key is whether somebody has ever tried it. Maybe the possibility of this working is so unbelievable and anti-conventional that nobody ever has tried it. Just wondering.
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Post by Roger on Dec 12, 2014 8:32:31 GMT
It's certainly an interesting question. My instincts say that the bores would remain dry once they've reached their running temperature. You can always try out your piston sleeve dry on the bench with a piston valve and heat it up with a hot air gun to try it and see what it feels like. I imagine that lubrication is usually used because there's probably going to be contact between the main piston and the cylinder wall, not just the piston rings.
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Post by joanlluch on Dec 12, 2014 8:45:01 GMT
I imagine that lubrication is usually used because there's probably going to be contact between the main piston and the cylinder wall, not just the piston rings. Roger, I am thinking on some design that would completely avoid metal to metal contact. We can name the PTFE rings as such, but imagine that they are actually part of the piston design in a way that never any metallic part would reach the cylinder wall. For example, PTFE may have a thick (and wide) annular shape, where any metallic part supporting it is far enough from the cylinder wall, in such a way that only after the PTFE has worn significantly (thus the loco stopped working) would metal touch metal.
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Post by Roger on Dec 12, 2014 8:57:13 GMT
It's an interesting idea, I think only some sort of experiment is going to answer that question.
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pondok
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Post by pondok on Dec 12, 2014 10:33:17 GMT
Hi chaps, Roger I reckon its swings and roundabouts to a large extent, ring section of around 5mm square is just fine, the material doesn't have all that much inherent strength, so when you get down to near 2mm or less, there's a small chance it could catch on a port and peel into it, and fly out the blast pipe in pieces. On the other hand, if it's made very thick, the expansion factor comes proportionally into play, so as thin as possible without being too flimsy works for me. Joan, there was a time when I let the lubricator go dry and didn't notice, and ran like that for maybe 10 minutes, not sure how long exactly, and it wasn't affected at all, no squeaking, no noticeable friction drag. Piston rod packing, piston and valve rings are all PTFE. But I was quick to refill the lubricator when I checked and found it bone dry, all the same, it was a busy passenger running day and didn't want any experimentation! cheers all
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pondok
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Post by pondok on Dec 12, 2014 10:57:10 GMT
BTW, I can confirm that there should be no metal to metal contact, clearance of 0.2mm gives plenty of support for the PTFE rings.
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