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Why are Bulleid Pacifics more prone to slipping than other designs?

Discussion in 'Steam Traction' started by RASDV, May 29, 2020.

  1. 242A1

    242A1 Well-Known Member

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    If you are not able to control the flow of steam to the cylinders in a suitable manner you will have problems. The better the steam circuit the more important the quality of the control. Where should the regulator be placed in the circuit? Under the dome? At the superheater at the end of the wet pipe? Or in the superheater controlling the flow from the dry steam header to the main steam pipes? Or would it be best placed at the entry point to the cylinder and steam chest?
    Porta wrote of a driver who opened the regulator at the start of a journey, removed the handle and adjusted everything on the cutoff. His nickname was "the Monk", I might well have used the word Jesuit. For this driver the regulator was of no use in working the train so you could ask where for him was the equivalent?

    The ideal steam pressure drop through the circuit between boiler and steam chest should be zero. And the steam pressure drop in the steam chest should be zero too during the period of admission but indicator diagrams show that this is not the case. So circuits are restricted for various reasons and some are more restricted than others which might be a factor in improved starting but is certainly not one from the point of view of efficiency and power output.

    The Bulleid Pacifics produce the power rather well but they deserve a better regulator system to enable a suitably graduated flow at starting. They are not the only designs which would benefit from improvements to the regulator.

    In France the Chapelon Pacifics had a reputation for not slipping. The electric locomotives were witnessed slipping even with their greater adhesion while the 231Es got away with a clean grip on the rails. The young engineer who observed this phenomenon realised that there had to be an equivalent on the electric locomotives to the coupling rods on the Pacifics and he became involved in the design of these systems. A detail that demanded attention.

    The GW Castle shown is a Class 7 like the West Country Class, it might be able to get away quicker than the WC but once the Pacific is away there is no way that the 4-6-0 can out accelerate it. And as for maximum horsepower, remember 34092? So you could say that both designs are deserving of a little treatment, just in different areas.
     
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  2. Big Al

    Big Al Nat Pres stalwart Staff Member Moderator

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    Speedos are essential. When an engine is just about to slip it often transmits that message first through the speedo a second or two before other signals reach the footplate. Is that not why many drivers who are in situations where their loco might slip keep their hand on the regulator? That second or so response time can sometimes make all of the difference.
     
  3. Cosmo Bonsor

    Cosmo Bonsor Member

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    I disagree about speedo being essential.

    There is more than one kind of slip, by which I mean the way it develops and the driver’s response to it.

    Roughly speaking there is the type where you suddenly and completely lose grip and the only action is to shut off, maybe use sand and open up again. This is type of slip at starting or a sudden stretch of poor rail condition, leaves for example.

    Another type is where the loco slips with a gradual increase in wheel revs, this might happen in a yard or poor rail condition. Then you would react cautiously, if you shut off altogether then you risk stalling. You might ease the regulator or pull it up a bit. It is much, much harder to re-start after a stall. Go and check Youtube ;-)

    In general the more gradual slip is indicated by a ‘shudder’ through the loco and sometimes a very slight change in exhaust note. As you feel it coming you can react the right way. I believe the engine is entering ‘creep’ where perversely there is more grip available or it can be ‘quarter slipping’ without letting go entirely.

    You often need not to ‘lose your bottle’ when controlling a steam engine, especially a powerful one say Class 5 and up.

    Interestingly there is a short part in Sharpthorne Tunnel near the south end where the exhaust changes its tone. Trainee drivers sometimes think a slip is developing ( it is a wet tunnel) but it almost never does. The slip is more likely under the northern vent shaft. It can seem like a very long way then.

    It turns out driving steam engines is somewhat tricky. Who knew?
     
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  4. Steve

    Steve Resident of Nat Pres Friend

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    Whilst I look at the speedo fairly regularly it is to confirm the speed I think I’m doing, not to watch for any sign of slipping. You can generally feel the slip first, Certainly before it shows as noise and heightened activity at the chimney. If the slip is gentle, I’ll try and control it by easing the regulator but if it is more violent there is no option but to slam the regulator shut. Whenever I think there is a possibility of slipping the hand stays on the regulator handle.
     
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  5. Big Al

    Big Al Nat Pres stalwart Staff Member Moderator

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    That makes sense to an amateur. I guess that all I was doing was reflecting on a gradual and slow slip at low speeds as @Cosmo Bonsor describes, that can be controlled by adjustment, with slips at much higher speeds when the speedo might be the early indicator.

    And the other advantage of a speedo is that it tells you if the locomotive has picked up its wheels. I have been told of occasions on the big railway when a train is approaching a stop under normal braking and because the driver was watching the speedo he immediately saw when the speedo dropped to zero because the railhead conditions had changed and the loco had lost grip. Not good for tyres but not something you can anticipate.
     
  6. peckett

    peckett Member

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    When a engine slides everything goes a lot quieter on the footplate plate ,because wheels and side rods are not going round. An experienced driver would notice this straight away.This was a regular occurrence with loose coupled freight trains.
     
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  7. peckett

    peckett Member

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    Lose your bottle is quite right. I always had the opinion engine that a steam engine was like a dog ,it has to be shown who is boss.I once was watching a pathetic piece of driving of a freight train. The man standing next to me said .That loco is top side of him.
     
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  8. Flying Phil

    Flying Phil Part of the furniture

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    One of the many delights on the NP website is the detailed replies given by people who actually "do" the various jobs and explain the background....Many thanks!
    It is easy to forget that steam engine driving is very different to most of our "driving" experience ie in cars. If you imagine driving a car by using a 12 foot steel rod on the accelerator/brake/clutch that is only part of the difference.
     
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  9. Wenlock

    Wenlock Well-Known Member Friend

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    Not just on steam!
    I recall shunting a heavy loose-coupled raft of loaded bogie bolsters of track. Someone had opened the trap point while we were on a break. Shutting off power and applying the air brake on the loco resulted only in that gentle hiss of wheel sliding on rail, too close to release brake and let wheels roll again before reapplying brake. Nothing to do but wait for the rumble, rumble, crunch of hitting the dirt.
     
  10. W.Williams

    W.Williams Well-Known Member

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    I guess one of the other delights is that any poster can assume a basic level of understanding within the forum not to have to caveat every single speculative piece of discussion with “but of course others will be more informed/experienced etc” as that surely goes without saying?

    This is the internet after all, a place where anyone’s insane or ill informed notions can be shot down and education can be imparted. If no one says anything none of us learn from those better informed.

    Sometimes something has to be said to elevate the understanding of others. I suppose main thing to remember is not to be offended if you do find yourself being re/educated.
     
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  11. Jamessquared

    Jamessquared Nat Pres stalwart

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    Been there, shut off unnecessarily, had to open up again, got the t-shirt :oops:

    It's interesting how you focus on different sounds on the footplate. As a fireman, I'd never especially concentrated on that spot, since you are concentrating instead on listening for the injector, which is almost invariably running through the tunnel and which you definitely don't want to knock off un-noticed.

    Tom
     
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  12. Steve

    Steve Resident of Nat Pres Friend

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    Noise is an important aspect of footplate work that is rarely appreciated by those not involved. It can be anything from a new one not heard before to the loss of a regular one. The relative silence that happens when the wheels lock and you get into a slide has already been mentioned. It is often your only indication apart from the fact you aren’t slowing down. The change in exhaust sound as it starts to pick the water is another first telltale sound before it shows at the chimney. Tom’s mentioned listening to the injector and I’ve lost count of the times I’ve said ‘is that injector still on?’ because it didn’t sound right. No need to look. Sound is even more important in darkness.
     
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  13. S.A.C. Martin

    S.A.C. Martin Part of the furniture

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    440BCCC2-A6A2-4671-9DCB-75E4028A66F4.jpeg BBEE0D29-5223-4566-A9F9-E9C5A1C08B3F.jpeg

    Whilst looking for something else I found this table in one of Allen’s books. I hope this is of interest.

    I would suggest that factor of adhesion is only part of the story and that a more important factor in this discussion is the design and use of the regulator on the locomotives concerned, together with cut off.
     
  14. Jamessquared

    Jamessquared Nat Pres stalwart

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    The weight given for a “King” equates to an axle load of 22.5 tons, which was the official value from the Swindon fiction office, but the actual number is closer to 25 tons - that puts the factor of adhesion nearer to 4.15 ...

    Tom
     
    Last edited: Jun 14, 2020
  15. Steve

    Steve Resident of Nat Pres Friend

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    Despite steam locomotives being essentially constant tractive effort machines (unlike diesel locomotives, which are constant horsepower machines), tractive effort falls off with increase in speed. This is because the boiler can't supply the steam and the steam can't get in an out of the cylinders. If your operating department want you to haul a given weight of train up a gradient at (say) 60 mph, you have to provide the tractive effort to do so at that speed. Because T.E falls off with speed, it stands to reason the the tractive effort available at starting and low speed will be much higher. The civil engineers limit the axle load so advantage cannot be taken of this higher tractive effort and the ratio of adhesion is inevitably compromised in such circumstances. Unlike a diesel, with a steam locomotive, the only way of limiting tractive effort at low speed is the drivers skill in controlling the admission of steam. For these reasons, locomotives designed to go fast invariably have a low coefficient of adhesion and are thus more inclined to slip when starting.
     
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  16. S.A.C. Martin

    S.A.C. Martin Part of the furniture

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    Proof (if needed) that we should always question the sources! I thought that adhesion looked a bit low for a 4-6-0.
     
  17. Jamessquared

    Jamessquared Nat Pres stalwart

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    There was a discussion here - on the West Somerset thread, of all places:

    The 22.5tons axle load for a King has always looked a bit odd in my eyes (the neatness of three axles all supporting exactly 22.5 tons - how many other large locos have exactly equivalent weight on all driving axles?) It seems inconceivable to me that the real weight wasn't known about at least within Swindon works when first built, but seemingly a veil was drawn over the actual figure, at least in public - whether the civil engineer knew, I don't know. Perhaps he was content, but the undesirability of making explicit reference to the excess weight meant a convenient fiction that the loco was lighter than stated, which the publicity department would be happy with as creating the myth of a loco of such great power at such a low overall weight. Given that it is routine to reweigh a loco after anything that has upset the suspension - which would be after any heavy overhaul in which it was lifted off its wheels - it seems obvious to me that either the real weight was widely known within the works, or some subterfuge of weighing was underway, perhaps weighing empty rather than with a full boiler.

    Whether those in charge of the Locomotives Exchanges were aware I don't know - one assumes that Cecil J. Allen didn't know, or if he did he was keeping decidedly quiet.

    Bringing it back to Bulleids: when 21C1 was outshopped, it was several tons heavier than planned. Assorted fittings went, such as the cast number plates and "Southern" name on the tender; subsequent locos (including those still in the erecting shop) were modified to remove weight - I think by adding holes in the frames.

    Tom
     
    Last edited: Jun 14, 2020
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  18. 46203

    46203 Member

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    Going back a few years now, but I had the privilege to witness a brilliant piece of driving on Princess Royal 462o3 southbound on the S&C at the beginning of leaf fall season. As soon as we left Birkett Tunnel the engine began to slip and did so all the way up to the summit, five miles distant. Speed fell alarmingly with 470 tons on the drawbar, but the former Kingmoor driver left the regulator in 2nd valve - never touched it once, and he controlled the situation with continual use of the reverser and kept the train going - just; we passed the summit board at Ais Gill at 17mph.
     
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  19. LMS2968

    LMS2968 Part of the furniture

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    A similar situation arose with the first Stanier Pacific, 6200, designed for 104T 10cwt. When complete and put on the scales, all the pointes shot around to be hard against their stops at 24T. By adjusting the springs they determined that the real weight was 109t 18cwt, and a similar lightening process begun. The second engine 6201, wasn't outshopped for four months after the pioneer and the weight reduction process might have been a bit more drastic before all the bits had been put on. I read that, by cutting various holes and thinning some castings, the weight of the production engines, still stated as 104t 10cwt, was down to 106t. But the official figure always stood, and great care was taken to ensure that the true weight never reached the ears of the CCE!
     
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  20. Cosmo Bonsor

    Cosmo Bonsor Member

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    This illustrates the choices the driver has to make. I'm not familiar with those pacifics more's the pity but the design of the controls affects the decision you make.
    For example the regulator on a S15 has a small pilot valve meaning you are in second valve on any gradient at more than 15mph or so. Shutting the regulator, killing the slip then getting it into second valve could easily get you stalled. As it happens the screw reverser is a fast thread and nicely balanced so you can control the slip on that.
    I'd be interested to know how they handle it in Yorkshire. Oh, add gravity sanding to the list.
    A BR Std 5 has the horrible screw reverser which I think I would only use in an emergency eg stuck regulator, but the regulators are well designed with short stiff rods to them. Two similar power engines with very different characteristics.
    So on a Bulleid you have a nice regulator but a potentially uncertain steam reverser as described by Tom, so the choice is obvious.
    An aspect of regulator design is the connection from the handle to the valave. The S15 has a long rod running from the backhead to the valve in the dome. This acts like a torsion spring. The slot in the pilot valve and stiction is why you have to open fully and shut the regulator from second valve. The Bulleid pacifics have a better arrangement. There are lots of different layouts.
    On a Youtube vid of the King Arthur at the GCR you can hear the rod banging on an internal support in the boiler. 847's does that.
    Interesting things, Steam engines.
     
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