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Piston and valve rings.

Discussion in 'Steam Traction' started by Eightpot, Sep 18, 2022.

  1. 242A1

    242A1 Well-Known Member

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    More research may well be required but lessons which have been learnt are certainly not being applied and there appears to be an unwillingness to apply them.
     
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  2. 242A1

    242A1 Well-Known Member

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    I appreciate that but this subject has been addressed before but perhaps not so directly. We have also mentioned K. J. Cook's paper The Steam Locomotive: A Machine of Precision published in The Engineer in September 1955.
     
  3. Jamessquared

    Jamessquared Nat Pres stalwart

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    Not my field I’m afraid. Posing the question, I wondered about the lithium greases (which are a form of soap suspended in oil) but apart from the issue of how you would apply them, they don’t appear to be very thermally sensitive - and, moreover, the decomposition is catalysed by transition metal impurities, which isn’t a great starting point in an iron cylinder.

    I wonder to what extent drawing ash into the cylinders when coasting accelerated wear? Maunsell’s engines had prominent smokebox-mounted anti-vacuum valves when built, but BR removed them, I wonder if that was a false economy? Would trying to coast with a breath of steam be beneficial - assuming of course the regulator allows sufficient fine control?

    Anecdotally, my feeling is that the issue is worse on superheated locos, which makes sense if the thermal stability of oil is part of the problem.

    Tom
     
  4. David Withers

    David Withers New Member

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    A noteworthy exception to Steve's excellent post: Danish 0-6-0T No. 656 at Nene Valley Railway has slide valves carrying a pressure relief apparatus that is restrained by the valve chest cover. This would prevent the valve lifting off its seat, at least no more than a few thou, to release cylinder water. Instead, the cylinders are fitted with automatic vent valves.
     
  5. Steve

    Steve Resident of Nat Pres Friend

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    I thought it was the done thing to coast with a breath of steam on?
     
  6. Steve

    Steve Resident of Nat Pres Friend

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    That sounds like a balanced valve. I did ponder qualifying my statement but didn’t think there were any locos operating with such valves. Or nearly operating. :)
     
  7. David Withers

    David Withers New Member

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    Very nearly operating! Hydraulic exam passed and formal steam exam to come within the next few days, precisely 20 years since we started converting 656 from 'basket case' into, expectedly, 'top-class condition'.

    Incidentally, a good many locos operating with balanced slide valves can be seen if we care to look beyond our little island. :)
     
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  8. Big Al

    Big Al Nat Pres stalwart Staff Member Moderator

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    I agree. My understanding is that you close the regulator when you are bringing a locomotive to a halt but if you are coasting - and speed limits on the main line make that far more regular than used to be the case - you would keep steam in the steam chest.

    That, by the way, is why setting a maximum for steam - 75, 60 etc - can only ever be seen as indicative. Penalising a crew for letting speed drift over the limit if coasting downhill, for example, is actually against good enginemanship in my view.

    It's never happened but Network Rail would have problems if they started getting overly picky about the actual speed of steam around the limit.
     
  9. twr12

    twr12 Well-Known Member

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    The only way to let drivers use their experience to drive steam hauled trains around the speed limit, rather than up to the speed limit; would be to block GPS signals to the train in question!
     
  10. Steve

    Steve Resident of Nat Pres Friend

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    It's only the merest breath of steam that's required so should not present a problem in controlling speed, even at 25 mph on a heritage line. I shut the regulator then crack it until I see the needle just lift on the steam chest pressure gauge. If no gauge then a wisp of steam at the chimney; much harder on a hot summer's day, though.
     
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  11. 1472

    1472 Well-Known Member

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    Possibly but then most of us see our role as preservation (as was as far as is sensibly possible) and not ongoing development.
     
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  12. 8126

    8126 Member

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    I've been meaning to comment on this thread since it popped up, but well, things got in the way. If we just take the original question regarding Britannias and 9Fs needing ring replacement at 5000 miles, the simple answer is no, they shouldn't have been that bad. Quoted intervals for piston and valve exams on Bulleid Pacifics were around 36000 miles for the notably low maintenance originals and 44-48000 for the rebuilds (Rogers, Bulleid Pacifics at Work - don't shoot me if the numbers aren't quite right). So the Britannias and 9Fs were, if they needed ring replacement at 5000 miles, notably bad by the standards of reasonably contemporary machines. However, as you say, even the Bulleid numbers aren't great by the standards of heavy duty IC engines, although maybe not so awful for a contemporary car.

    The two best reads on piston ring matters in the literature that I'm aware of are Wardale (The Red Devil and other tales from the age of steam) and Holcroft (Locomotive Adventure, can't remember which one), for completely different reasons.

    Wardale is good because he tells you how piston/valve rings should have been done (hint: like in diesels). So material selection matters, detail design matters, correct lubrication matters. Earth shattering stuff, I know. The materials aren't even that exotic: grey cast iron is fantastic stuff for this kind of application, but you do need to pick the correct grades for the rubbing components. Maybe you want a bronze rubbing surface on the valve/piston, maybe you just want one or two bronze rings in your total complement. Lubrication should be directly to the rubbing surfaces if you have hot steam, atomising it in the steam just makes it break down more readily, but if you apply it directly you need to make sure all the relevant surfaces are covered and the liners don't get above the oil breakdown temperature (harder for valves than pistons). Oh, and the ring needs to be flexible and that means a slim section and lots of rings, when compared to standard piston valve practice. He was aiming for a heavy general interval without work on the pistons and valves. The pistons failed, perhaps partly due to implementation errors, but I believe the valves lasted at least for the duration of his association with 3450. It would be fascinating to know how long they lasted in the end.

    Holcroft is interesting because he tells you why it went wrong. There's the tale of Ashford's superb, durable implementation of the Schmidt ring, which would last an overhaul interval without excessive leakage, but was lost when the Southern Railway closed the Ashford foundry and suddenly realised Eastleigh couldn't replicate the material. So they gave up and fitted everything with the sort of narrow rings generally adopted henceforth.

    But, circling back to Wardale, there was no standardisation of materials worth speaking of before the Second World War, at least in the UK. The EN steel numbers that still crop up today are the Emergency Numbers created then to try and provide some measure of standardisation of steels. The likes of Rolls Royce had their own foundries and their own detailed material grades, particularly for aluminium alloys. Wardale quotes a lot of AISI/SAE standards. Now this may be partly because he got a lot of information from Porta, who would probably have been more in the US sphere of influence on these matters. But the SAE published their first standard in 1912 and started to get involved with material standards in the '30s. With appropriate standard grades, you can quote a well matched wearing couple with two grades of cast iron and know that if the material is made to spec it will behave consistently. I'm sure Rolls Royce were very picky about their piston ring and liner materials (to say nothing of Bristol with their sleeve valve radials), whereas I suspect that designers on the Big Four got what the foundries gave them.
     
    Last edited: Sep 20, 2022
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  13. Nigel Day

    Nigel Day Member

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    See Porta’s papers from ASTT or Camden. Form experience they contain good practice.
     
  14. Eightpot

    Eightpot Resident of Nat Pres Friend

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    The lubricants themselves may be part of the problem. Around 1970 we had cause to take the cylinder covers off on a Sentinel DG waggon only to find that the bores, etc., were bone dry with no trace of oil. The oil was being pumped in but lacked 'cling' and was just being blown through the engine. Changed to Shell Valvata which cured the problem.

    Moving on to the 1990s, I recall the S160 2-8-0 on the Mid-Hants suffer damage to the cylinders and valves due to use of an oil that wasn't up to the job. Rumour at the time had it that there was a legal claim against the oil manufacturers. Anyone know more on this?
     
  15. GWR4707

    GWR4707 Nat Pres stalwart

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    Posting this without comment, it popped up as suggested on You Tube and brought to mind this thread, the 'offending' bit is towards its end.

     
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  16. 30854

    30854 Resident of Nat Pres

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    Ever seen 'Super8' footage from Douglas, back before Lord Ailsa's time? There was a shaggy dog story doing the rounds that legendary General Manager A.M. Sheard instructed drivers to depart the terminus, drain cocks fully open, to thwart photographers. Funny story ..... total cobblers, of course!
     
  17. MellishR

    MellishR Resident of Nat Pres Friend

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    Britannia's drain cocks seem to have been open all the time, not only when starting away with the train but throughout the shunting. The only time when less steam was coming out was when the regulator was more or less closed, and even then there was some. There must have been a reason. Any suggestions what that could have been if it wasn't to annoy the photters?
     
  18. std tank

    std tank Part of the furniture

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    Could be bad seatings in the drain cocks.
     
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  19. Jamessquared

    Jamessquared Nat Pres stalwart

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    I wasn't on the footplate, so don't know. But I really doubt the common accusation of leaving draincocks open to annoy photographers, since it would be a very pyrrhic victory. If you've ever been on the footplate of a loco - particularly a big loco - with the draincocks open you'll know it is bl**dy annoying: it's noisy, and it prevents your seeing ahead , including obstructing signal sighting. So I don't think anyone does it more than is really necessary - or, put another way, if the driver had the cocks open for a long period, it is almost certainly because he felt he needed them open; or they were stuck open, rather than just to annoy the photographers.

    If I had to hazard a guess about the video above (and I'll state again: I wasn't there), then two possibilities come to mind. One option is a fault stopping them closing cleanly (as suggested by @std tank ) which can sometimes be cured by judicious opening and closing, but not necessarily something you'd want to do in the first few wheel revs from a standing start. (Notably, the loco does a couple of small slips there on starting, so the driver is clearly concentrating just on getting away). The other possibility, if the loco had been standing a long time with a big fire, is that the fireman has prevented blowing off by slightly over-filling the boiler, and the driver is being cautious against priming until the water level has dropped a bit. Even with a superheated loco, the exhaust can sometimes be a bit wet.

    Tom
     
  20. Wenlock

    Wenlock Well-Known Member Friend

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    And presumably, if the loco had stood for a while, the cylinders would be cold and the steam could be condensing more in the cylinders?
     

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