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

Well the theory that is mine is the short piston stroke is a factor, giving a much less even torque than for example a Hall with 30" stroke.

Dave

 

Do I not recall all steam locos 'sit back' on starting under load, meaning all locos with a trailing truck are going to be somewhat less sure-footed than, say, a 4-6-0? I wish we still has a 'pacific' tank (an LNER A6 would do very nicely, IMHO), even if only to discover whether they shared the predisposition to slip under similar circumstances. C'mon folks, there have been much dodgier rationalisations for newbuild proposals!

Presumably, certain locations were more likely than others to induce slipping. I've noticed that even the FfR's all-adhesion double Fairlies (along with pretty much everything else!) rgeularly lose their footing departing Tan-y-Bwlch in the up direction.

On the Southern, heavy boat trains departing London Victoria were immediately faced with the adverse gradient to cross Grosvenor Bridge. Exeter Central, Ilfracombe and Folkestone Harbour were likewise places where trains faced notoriously steep exits .... all locations where misbehaving locos could embarrass themselves in the full glare of public view.

One of the most notorious slipping incidents in the preservation era didn't even involve a Bulleid loco, as he'd long departed the LNER by the time Peopercorn's A2 appeared. You'll likely recall the serious incident which did so much damage to 60532 Blue Peter

When Bulleid's locos behave themselves, they're often truly impressive. I well recall 34092's sparkling performances on the mainline (was it really over 30 years ago?). On one Scarborough Spa Express working, the phrase 'accelerating like an electric' was used, with BR traction inspectors most complimentary.

I suspect that the power put down by all multi cylindered express locos leaves most susceptible to slipping under the right (wrong?) circumstances. If the tendency is more associated with OVSB's beasties than most, that's possibly because (a) when first introduced, Southern's drivers had seen nothing remotely close to locos that powerful and (b) a fair number have survived and steamed, quite a few doing so on the big railway.

 

In principle, a three-cylinder loco (or a four cylinder one with a Lord Nelson arrangement of cranks) should be less prone to slipping by virtue of a more even torque distribution over a wheel revolution. However, that only applies once the loco has done a complete wheel revolution and there is steam right through the system. On starting, there are certain dispositions of cylinders where you get less starting force because only one cylinder actually is actually receiving steam. That might lead to an attempt to compensate with higher steam chest pressure, which then suddenly causes problems as a second cylinder receives steam and the available torque suddenly jumps, possibly beyond the limit of adhesion since it will likely occur at a point of poor rail condition.

As a complete diversion, the loco I know with the lowest factor of adhesion (i.e. lowest ratio of adhesive weight to TE) is the Adams T3 - worse than a Schools and much worse than a Bulleid Pacific, which are generally reputed to be slippery. It will be interesting to see how it handles when it is restored. My hunch is that, being a saturated loco, there will be much less lag between regulator opening and anything happening, which will be helpful - but it will still be an interesting thing, I think.

Tom

 

I'm not convinced there's anything in that according to the reasons you cite; the short stroke on the Bulleids gives them quite a high ratio of connecting rod length to stroke. The larger that ratio the more even the fore and aft piston strokes become.

I know the GWR types have cited their regulator design as an advantage, but their small superheaters probably were too. Lag is the enemy of control system design, hence multiple valve regulators in the dry header, which did not save Blue Peter from a high speed slipping incident, but that's a different problem. I think @Jamessquared has previously shared guidance on starting from an experienced driver of Bulleids, which centred around managing the steamchest pressure. A less careful driver, thinking "It's not moved yet, I'll give it some more," is going to have a much more dramatic slip when it does do something, with all the volume of the superheater and large steam pipes on a Bulleid.

 

The “short” piston stroke of MN & WC locos is a legacy of the design of the loco for running among EMUS and on railways way out west.
Most class 7 & 8 locos have 6”7” diameter driving wheels, Bulleids have 6”2” diameter wheels for better acceleration among electric trains and better hill climbing in Devon & Cornwall, than if they had 6”7” wheels.

To keep the mean piston speed within acceptable limits at the design speed of 80mph, the piston stroke has to be shorter.
I think the mean piston speeds of an A4 and a MN at the same road speed are almost identical. Of course the Bulleid wheels, axles & motion are moving faster at any given road speed.

 

I have read several statements from locomen that said they did tend to have a tendency to slip.

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I have watched drivers who drove these engines on BR pull away on greasy rails without a single slip, and I have seen others who slip and slide on the same metals, so it has to be down to driver knowledge, the experienced ones, always pump the regulator, on starting, and only open her out once they are moving, i have been on the footplate with two very experienced ex BR drivers, on a wet October gala day on an west country, and we slipped and slided up medstead bank, but kept going, with 8 on, another was on 30506 on the freight , again we picked up , several times, but with skilful driving we plodded up into the station, handled properly an Bulleid is no worse than any other loco.

 

At various times over the past forty years I have driven a variety of pacifics (A1, A2, A3, A4, Princess, Britannia, and Bulleids (both Rebuilt and Unrebuilt). I have to say that of these, the Bulleids required the most care when starting from rest, but are fine once under way. I tend to agree with Jamessquared about the issues with starting three-cylinder locomotives, but cannot explain why other three-cylinder examples such as A4s are so much more sure-footed when getting a train away from rest.

Andy

 

Indeed on my last visit to Nine Elms the track was so bad that this happened!

 

I assume that Singles would have had the lowest factors of adhesion and hence be most prone to slipping, at least with anything more than a very light load.

The preserved Great Northern Stirling Single appears to have worked on the Great Central during the 1980s. Does anyone know how well it performed?

 

It usually hauled three Mk1's and a 6 wheel brake. It did spin the drivers on starting from Quorn then settled quickly. On the ride that I had, it went quite "briskly" with its NRM driver at the regulator........

 

Didn't the Bulleid pacifics originally have a steam operated reverser?

I seem to recall these could not always be used with the same finesse as a manually operated one and the indication could also be wrong, allowing a driver to start with the reverser in a different setting to that intended.

 

Yes they did and by all accounts they were unreliable. I believe they could fly into full gear when the driver tried to change the setting. For that reason a lot of drivers would set the cut off after starting and just leave it, driving on the regulator.
On another point, the originals had a lethal steam operated fire hole doors which could quite easily trap the fireman’s hand. Does anyone know if this feature has been retained on any of the preserved engines?

 

34092 definitely still has steam-operated firehole doors.

On the reverser: it is somewhat maligned, but the story is rather more complicated than is generally assumed. Suffice to say, the location of the reverser on the first ten Merchant Navies was very awkward, trapped in a narrow space under the boiler, and that probably led to a lack of maintenance - and for a steam reverser, maintenance is key, particularly ensuring the hydraulic cylinder is kept topped up with oil. On the later locos, the position moved making maintenance easier. There were other modifications in service, notably adding a pilot valve to the steam supply, which allowed more finesse in operation. The low mass of the moving parts of the valve gear probably didn't help either - had the valve gear been heavier, it would have moved slower and therefore more controllably. Some of the early reports of locos moving into back gear at speed were probably when drivers tried to notch up and the reverser moved too quickly; that problem was resolved by adding the pilot valve. The end result was that the problems of the reverser were largely ironed out, but the reputation largely gleaned from the first ten Merchant Navies has stuck.

It's probably fair to say that a steam reverser isn't the most forgiving of beasts for fine control; you can't easily make the same subtle changes of cut-off that you can with a screw reverser. On the other hand, they save labour. As I understand, Bulleid tried to get a screw reverser into the loco, but couldn't make it fit with the valve gear design. There is also another advantage that I haven't seen ever really elaborated, which is that I suspect a steam reverser is possibly an easier and cheaper device to manufacture during wartime than a screw reverser - important at a time when manufacturing capacity was at a premium.

When all was said and done, if the hydraulic cylinder and hydraulic control valve was properly maintained, they worked reliably enough, and the locos were very forgiving, seemingly giving good performance more or less however they were driven - wide open regulator pulled up tight, or left at a longer cut-off and part-open regulator.

Tom

 

Once you get used to them, the steam reverser is fairly easy to operate, and by applying a bit of finesse small cut-off changes can be made. As a driver, I have always preferred the un-rebuilt pacifics. They are a lot easier and quicker to oil-up than their later counterparts.

I seem to remember that in the preservation era 34092 had steam-operated firehole doors at one time, and I believe that those on 34081 were scheduled to be reinstated after the last overhaul. They are only dangerous if you put your arms inside the firebox whilst firing and then take your foot off the treadle. Most firemen choose not to do this.

Andy

 

I was going by something Bert Hooker said in a talk he gave, he witnessed a fireman being injured and after that would not let any of his fireman use it. The rebuilds also have a trap for the unwary but not when in steam. If you’ve been working in the firebox it’s very easy to grab the ratchet bar to pull yourself out and have the doors close on your torso. I’ve never done it but I’m sure someone has.

 

I am not a big fan of the firehole door design used on Bulleid pacifics. In comparison with those used on other locos they require greater physical effort to operate. This is probably why steam operation of the doors was first used. There is a lot to be said for the LNER 'letterbox' or the GWR 'flap' when looking at ease of use.

Andy

 

However I'm led to believe that with low reciprocating weight and so on, slipping was a much less dramatic and damaging affair. Presumably, too, few were ever superheated.

I was just looking at the headline figures, admittedly never a very safe guide, but the light pacifics don't appear especially outrageous in power against adhesive weight. The GW manors, for instance, have very nearly the same tractive effort, and appreciably less weight on the driving wheels.

 

When 34092 visited the Bluebell a few years ago, I saw the owner adjusting the doors in the morning, and they seemed to work very smoothly; however, I didn't get a line trip to see them in action. I agree with @Andy Williams that the unpowered doors on a Bulleid are a lot of work.

I did get a cab ride on a NZR Ja 4-8-2 that had identical Ajax doors to a Bulleid pacific, but operated with compressed air. The fireman seemed to have complete faith in their operation, stepping on the treadle and flicking the coal in one easy movement and confident the doors would open on cue. I do wonder whether there was any difference in reliability between steam and compressed air operation.

Tom

 

I don’t think ergonomics was a known science when most steam locomotives were designed and they all seem to have faults. The LMS one lever operation of the firehole doors seems a lot easier than the awkward ratchet arrangement on a Bulleid but why did they locate the blower valve above the fire hole door? In the event of a blowback you would have to put your hands into the flames to operate it. I’m sure that, in the incident with Can Pac a few years ago, the crew appreciated the Bulleid arrangement, easily accessible by both driver and fireman.