Bulleid wheelslip question

One most certainly modified to have cranks at 90º.

 

I sometimes wonder if there is objective statistical proof that Bulleids were actually more prone to slipping than other similar locos, or whether it has become an urban legend. (Genuine question - I don't know one way or the other).

There were 140 Bulleid pacifics, more I think than any other British pacific. They also worked a lot in London and right up to the end of mainline steam, so were closely observed, perhaps more closely observed than any comparable class. So I wonder if the reputation for slipping a lot had something to do with simply being more closely observed than some other classes, so the average spotter was far more likely to see a Bulleid slipping than other classes with fewer members? As an example, the GWR "The Great Bear" was apparently prone to slipping, but it doesn't particularly carry that reputation I suspect simply because comparatively few people ever saw her slip. Whereas there were 140 times as many Bulleids, so proportionately, a spotter would be 140 times more likely to see one slip, and no doubt draw the conclusion that they were prone to the habit ...

DW Winkworth quotes some correspondence in The Railway Magazine of observing Duchesses start from Rugby, on a falling gradient: in two weeks of observation, there was not a single clean start, even on dry rail - which puts the Bulleid performance into some kind of perspective. A pacific with a high power to weight ratio would be prone to slipping, but whether a Bulleid was objectively worse than other British pacifics is more in doubt.

As for "pumping" the regulator: a driver acquaintance of mine with considerable BR-era experience on them recommended that technique and was of the opinion that, driven carefully, it was possible to start quite reliably without slipping. But you did need to start carefully, no doubt in part on account of the large steam chest volume.

Tom

 

There's a loco in the NRM, no stranger in its day to routes later made famous by the Bulleid pacifics, that has a higher ratio than all but one of those (0.256).

Interestingly they didn't have a particular reputation for slipping, either on starting or at speed. I suspect in part they were driven by drivers well aware of their overall capabilities: in particular, on the switchback Salisbury - Exeter line, I believe the general technique at the time was to run at fairly constant steaming rates, which meant going up hill in fairly pedestrian style but then keeping steam on and going hell for leather downhill. No superheaters and correspondingly small steam chest volume, and no trailing axle, no doubt helped on starting too.

Tom

 

Steve's beaten me to it in part, but it seems to me that the most comparable machines to the Bulleids are the various LNER three-cylinder pacifics, with a basically similar arrangement and also tending towards the high superheat with a large volume between regulator and piston. So I knocked up a quick listing of adhesion factor by class. My numbers are marginally different to Steve's, I guess from source material, and I can't find a weight table for a rebuilt West Country.

A3 0.222
A4 0.239
A1 (Pep) 0.253
A2 (Pep) 0.273
WC (280) 0.246
WC (250) 0.220
MN (280) 0.266
MN (rebuilt) 0.230

So, in theory the only LNER pacific that should be less slippery than a Bulleid after about 1952 is an A3, and yet it's the Bulleids with the reputation for slipping while an A2 should be the worst of the lot*. I don't know whether the multiple-valve regulator is in the wet or dry header on 60532 (dry header gives better control), I do know practice varied with the Melesco regulators because they required less maintenance in the wet header.

In various texts regulator modification is mentioned in original specification for the Bulleid rebuilds, because it wasn't considered sensitive enough. I don't know whether this was actually done, or whether it was also applied to boilers on original locos as well. There must be someone out there who's driven an LNER Pacific and a Bulleid and can comment on the difference? The steam reverser shouldn't really have mattered on starting, getting it in full gear was rarely the problem.


*Yes, I do know about the 60532 incident. I don't think it's really relevant in this case.

 

Fairly certain that was no.4470 Great Northern actually, as rebuilt. Pretty early on in her career as the lone A1/1 when she was fitted with the GCR style regulator as opposed the balanced type fitted to the other Pacifics.

 

I've never seen a Bulleid do this!

 

I'm finding this implausible. At starting, the valves will be in full forward gear, which means steam at full steam chest pressure (not necessarily full boiler pressure) acting on each piston for a large fraction of its stroke. At speed, the cutoff will be shorter, so full pressure for less of the stroke. So, for any given loco, the variation of torque during one revolution of the wheels should be somewhat less at starting than at speed.

However a three cylinder loco may have a shorter maximum cutoff than for two cylinders, for the sake of longer valve travel and therefore better port openings when the engine is notched up. That could make the torque more uneven (than with two cylinders) in full gear, and very low at some points in the revolution. That shouldn't cause a slip but could indeed make the loco unable to move the train, hence the technique of setting back a short distance. That may have been what was going on recently with Tangmere at Plymouth, with the vagaries of the Bulleid valve gear making the cutoff shorter still on one or more cylinders.

Apropos the technique of opening the regulator and immediately shutting it: I think I have read of this being common practice with LNER pacifics. With any loco, when it is moving very slowly even a small regulator opening will allow enough steam through to give nearly full boiler pressure in the steam chest, hence possibly more TE than the wheel-rail interface can take.

 

I've ridden on the footplate of several types of loco and I can't recall any of them being given full regulator from the off. A common method has been to open the regulator partially and once steam chest pressure has risen sufficiently, close it again. Repeat until safely under way.

 

You may be right about 4470 but I'm certain I've read about this happening with a Duchess and andydrz's clip shows how a Duchess can slip massively under certain conditions.

 

Film must be a fake as everyone knows only a Bulleid slips on starting.

 

There was an incident with a Duchess that took over an hour to get away from, I believe, Liverpool Lime Street on account of severe slipping. (Referenced in Winkworth). Something that has always intrigued me, slipping aside, is that there was seemingly no pilot on the back able to give a shove. I'm not really familiar with station working at Liverpool, but had it been, for example, Waterloo, then there would have been the pilot engine that bought the carriages into the station that could give assistance until the train had cleared the platforms.

Apparently London Bridge platform 7 was a bad place for starting, being on a curve and rising gradient. The practice there if a Bulleid was having difficulty getting away was to call forward the electric train behind to give a shove! Delays were seldom more than a few minutes.

Tom

 

The incident was mentioned by Ossie Nock. Generally, the stock was brought into Lime Street by the station pilot, or the train engine for an arrival, which then unhooked. When the train got away, the pilot / train engine would follow it AT A DISTANCE OF A FEW YARDS as far as the platform starter, where it would await a clear signal. Generally, trains were not banked out of Lime Street.

Although strange, this was standard practice there, and if a loco was to remain at the stop block, the driver had to inform the bobby; there was no track circuit there and the platform road was effectively shorter. This practice continued long after steam finished and was carried on with diesel and electric traction.

It doesn't explain why the Big 'un had no assistance on that day, though.

 

A valid suggestion. One of the contemporary main line drivers would be a good place to start. May I suggest Ray Churchill, who has a broad route knowledge, plus I'm sure that he could give a balanced account of the various foibles of most of the present main line fleet. Of course, he has a reputation of getting the best out of 34067, but would his "masters" give him a free reign in divulging any inside information? Ray's testimony on particular locomotives would make a fascinating account.

Cheers,

Alan

 

Let's hope he writes his memoirs when he has retired.

 

The slipping argument is an interesting one and it's one I looked into a lot as one of the acrophypal stories about thompsons Pacifics.

It was of course true they had the greater potential for slipping compared to the other Pacific classes with reduced adhesion, but in reality a lot of it is down to driver technique as well. Intriguing (in my view anyway) that a change in regulator type as per 4470 can have as much influence if not more on slipping as the adhesion rate.

Begs the question is slipping just down to the engine or more to the driver not being familiar with his charge?

 

Location can have a lot to do with it too. Northbound starts at York could produce all manner of slipping and I doubt that was due to driver unfamiliarity in steam days. All those flanges binding against the curve added a lot of resistance to a train.

 

Very good point.

 

On the basis of that calc. Table a Clan would be 0.217, pretty grippy for a pacific...