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

I went for a mainline trip once from London to Stratford upon Avon behind Tangmere. On the return trip leaving Stratford, the driver seemed to have a massive struggle getting the train to get started, took about 4 attempts, with wheels slipping each time, with him even reversing a short way to try again. Once it did get underway then it was fine and accelerated gradually. I was watching out of the window of the first coach and interesting to see. Is it uphill leaving this station ?

I did read once about the Schools Class at the NYMR and it said they had a lot of trouble with that slipping all the time, but then at some point the driving wheels were removed and the tyres turned again back to the correct profile and this then cured the issue. So perhaps if the wheel profile is worn this causes an issue on other locos too. ?

 

It’s uphill all the way from Stratford to Wilmcote Summit. The reason for reversing up has been explained above, I’ve seen that happen with LNER three cylinder Pacifics on the curve at the old Peterborough Station, driver opens regulator, nothing happens, reverse a couple of feet and have another go.

 

I've had exactly the same experience behind a Duchess struggling to start from Stratford.

 

The Ajax doors are I believe an American design and were used widely on latter day hand fired locos in many countries. There is a technique to their correct use but it is a technique that is no harder to master than the art of shovelling itself. As Tom says they are often air operated if the loco has an air supply, just as power reversers are often air rather than steam overseas. I remember riding on a Garratt in Zimbabwe a few years ago. The fireman was on assessment by a firing instructor and diligently used the door in the correct manner - stepping on the treadle as he swung his shovel, then releasing his foot pressure momenterally before taping the treadle again before the door banged shut, thereby cushoning the blow. He did this religiously the entire time the instructor was with us but as soon as he left it was back to the lazy way of doing things and the doors just chashed shut. The fitters would soon sort them out when they got too worn through misuse - that what fitters were there for.

When 34081 visited the NYMR a couple of years ago I fancied a go with these doors but although the assembly was complete the steam supply had not been connected yet so I could not use it. I understood that the intention was to have it working though.

I wonder if fitting them to Bulleid Pacifics has anything to do with that other rare feature (for this country) that they possessed, namely thermic syphons and steel fireboxes.

Peter

 

When 34092 came to the NYMR I suggested to my fireman that he might like to use the doors as designed. He turned the steam on and put his foot on the pedal. The doors shot open very rapidly and loudly and shut similarly when he took his foot off. He immediately said he wouldn’t use them. I then screwed the steam valve almost closed and the door movement became much more controlled and gentle and the fireman soon gained the necessary confidence and skill to use them. Is that the root cause of the perceived problem?

 

Unfortunately we still haven't got round to getting them operating again but it is still our intention to do so.

 

Had a ride on the footplate of a QJ in China. Air operated Ajax style doors and the fireman was well versed in the coordination of hand and foot throughout the journey. The sound recording I mate is punctuated by the rhythmic hissing of the doors in action.

 

A couple of observations spring to mind having fired and driven a few Bulleid locos if all 4 existing flavours.

I think the Ajax doors were fitted to control the secondary air better. The firehole on the pacific boiler is big and with the doors open you get a nice big cone of cold air hitting the syphons with all the resultant effects, withthem shut the air flow is turbulent as it goes through the baffled holes.
Steam and air are quite different in the way they operate a mechansim and the design and construction has to take this into account. You can see this if you use the 'wrong' type for testing, eg valve setting on an Ashford Product. It would be interesting to see Ajax doors operating on air.
As for regulators, pumping the regulator is a reliable method of starting off, it limits the amount of steam in the chest and if it does slip, you already have the regulator shut thus limiting the slip. The other method is to use the steam chest pressure gauge. Open reg a bit, watch gauge rise slowly until it stops. If not moving repeat. When you are moving you then have a temporary limit to work to. We can see why pumping the reg is the prefered method. In both cases patience and a light hand on the throttle is the key to a good start.
Swindon stuff always feels solid and rugged to me as a driver, the pilot valves seem well proportioned, you can open it to the stop with confidence and as soon as you are able, get the engine well into second valve.
I think one reason steam reversers were fitted was to encourage notching up. With the later slide valve engine that had reached the size limit, pole reversers can be very heavy to use. The temptation is to get the engine in a reasonable running cut off and leavit there. That's how I drove the E4. We should have got one with a screw reverse.
The point about the wheels holding the oil only to distribute it later is an interesting one that I hadn't thought of.
Lastly, in the 90's the 500cc Grand Prix motorbikes were getting almost unrideable because of their power. Yamaha made thier bikes so called 'Big Bang'. That is the firing pulses were close together, the idea being that if the wheel slipped the gap before the next power impulse would enable the tyre to regain grip, whereas even power impulses keep the wheel slipping between 'bangs'. The crashes caused were known as a 'high side'. The parallels with multi cylinderd engine are obvious.

 

I was thinking long and hard before posting my comment concerning large express locos (with more than 2 cylinders). I've read much on the matter and heard plenty directly from those who've driven 'em. I'm certainly not about to discount your viewpoint, as you've infinitely more footplate experience than me, but reviewing your post, I see nothing to suggest both seemingly disparate cases are mutually exclusive.

The more constant torque one might expect from either Nellies or OVSB's beasties (sorry, but I'm close to clueless about LNER & LMSR locos) certainly would make for a smoother operation ..... however .... the sheer constant power being put down on the rails by such machines surely needs to be accounted for too. No expert, me, but I'd imagine keeping all that oomph successfully harnessed depends a great deal on a drivers' skill with the regulator i.e. if a slip isn't detected and corrected early enough, that constant torque is actually better at producing runaway slipping.

Where two cylindered machines are concerned, I'd expect some effect I can only describe as a sort of inverse of cadence braking (i.e. intermittent forces applied to the rail head), which although it won't completely prevent slipping (I've witnessed enough dodgy starts with 2cyl machines to be sure of at least that statement!), effects more of a pulsing delivery of power than you'd obtain from 3 and 4 cyl locos. Just how, or if, locos with divided drive perform in comparison to those where all cylinders drive one axle is another matter again!

 

Slipping certainly seems to be endemic to Pacifics and certainly the Bulleid ones have a particular reputation.

There is no doubt that oil getting out and on the rollers caused slipping on the test plant at Rugby. Where there were delays to the tests while they were waiting for replacements to arrive after rods had bent.

I suspect that the amount of steam in between the regulator and the pistons may have been large - the smoke box was long, the steam pipes to match and the Merchant Navies
were designed form scratch with full knowledge of internal streamlining - you would not have got laminar flow in undersized pipes. The Bullied Pacifics never had smokebox
regulators which would have helped but some of the LNER ones did. Usually the regulator is the quickest way to control a slip and the nearer it is to the valve chest the better
- though if you had a really quick and reliable power reverser with a really convenient control - fitted with a foot pedal - it could whip the valves into midgear and the whole
question of the steam in the valve chests, superheater elements, the superheater headers and the the pipes to and from them inside the boiler and smokebox would be
negated. To my knowlege whatever happened with Porta in South America and Wardale in South Africa this has never been tried here, did it ever occur to anyone amid the
modifications to the Bullied reversers that that a really triumphant redesign might have helped the slipping as well?

The full original pressure of 280 psi cannot have helped before it was reduced to 250 - there would have been 12% more volume steam pipes and all etc , nearly an eighth extra,
and of course as much more tractive effort to start the slip.

(Beyer Garratts with the combination of usually low axle load, long pipes to the cylinders at both far ends which were distinctly different lengths front and back and completely separate adhesion at each end which varied inescapably as fuel and water were used up differentially: hence a basic problem with one chassis slipping while the other gripped.When this happened they would have been effectively reduced to under half power before you shut off. They had one Hadfield power reverser but there was not a regulator to each end.
Fairlies have shorter steam pipes, identical lengths to both sets of cylinders and two regulators - and advantage is taken of these to the point that the sanding was decidedly underdeveloped, becoming barely possible on the move with modern Health and Safety.)

 

And on a bit of a curve, which will add to the rolling resistance of the stock.

 

Somewhere, I have a video shot of 6024 really struggling to restart a train from Stratford upon Avon, illustrating what an awkward place it is to get away from. Even 4-6-0s have trouble. It was a windy and damp day.

I was lucky enough to have a footplate ride on a Chinese QJ on the JiTong line one evening. The mechanical stoker wasn't in use but the powered Ajax doors were. 2 firemen were taking it in turns to shovel more-or-less powdered coal into the firebox. The "resting" one would operate the firebox door pedal while the other swung the shovel. Their timing was pretty good but once in a while the doors wouldn't open quite quickly enough, and one time the guy's foot slipped on the pedal at the crucial moment. The worst I saw happen was the doors didn't open on cue, the shovel hit them (hard!) and coal dust went everywhere! When they did open correctly, the shoveller didn't need to put any effort into his swing as the draft sucked the powder off the shovel. All he did was tilt the shovel a bit, to try to direct it to the right part of the fire - though I'm not sure how much control he really had!

 

To this casual observer I’d suggest the biggest contributing factors to the suggested propensity to slip lay in the Bore to Stroke/wheel ratios and the BFB wheels, both of which lead to a reduced rotating mass but also a more “rev-happy” machine.

Slightly counter intuitively I find it easier to visualise in IC format. Bore to stroke ratios approaching 1 make for very freely revving engines, go above 1 and you get in to super-bike/V12/10 F1 style engines. These engines are noted for their ability to produce high power but lower torque figures.

A Bullied is running a B/S of 0.75 against an A4’s 0.71 or a Stanier’s 0.59. Incidentally a King is 0.58...

Couple this with the regulator position and it’s easy to see how slips can become easier to instigate. Once the power is on, that whole rotating assembly is naturally inclined to spin away more freely than the classes mentioned above. Then there is all the pent up steam forward of the regulator to feed the slip even when the reg is closed.

Extending the analysis to stroke/wheel diameter, or where the power is applied to the wheel relative to the wheel diameter (an effective leverage ratio of you like):

Bullied 0.324
A4 0.325
Stainer 0.34
King 0.35

The Bullied has the lowest value of its peers.
The application of the steam pressure/force to the wheel is that much more difficult the lower this value becomes until you hit the lower bound of zero where no rotational force is possible. That would be pushing directly on the axle centreline.(1 being rods pinned to tyres a la GWR 28XX/BR 9f) So when a slip does occur there is proportionally more energy in the system to be released.

It’s a small differential between the peer group Iv picked, but as we all know steam locos are multi-variable
beasts. When those variables add up or multiply we get distinct characteristics.

As has already been pointed out the advantages of the bullied design are a loco with low hammer blow and a great tendency to pick up the train and accelerate well once moving, and be smooth at speed when running freely. The bullied must be one of if not the smoothest of the big Pacific’s at speed which is remarkable when you consider it has the smallest wheel diameter of the group.

In an ideal world you would have varying crank angle positions relative to wheel position and relative speed. Ideally you would “draw the rods in to the wheel centres at speed” and “push them out” when starting off for effective leverage to pick up the momentum from static to moving. This is precisely what a reverser is trying to replicate in valve timing events, effective leverage at low speed. At high speed of course it’s function becomes more about efficiently.

To go full loop here this is what VVTi, Vanos,Vtec are all trying to compensate for. The fact that it’s incredibly difficult to have variable stroke/crank angle position relative to speed of the machine and still deliver a reliable and powerful output.
It’s 10 time’s easier to fiddle with valve timing events than crank angle timing events.

I’m going to stop here before I get in to the thorny subject of infinitely variable length runners relative to valve events and crank position.

Suffice to say, mathematically there is a perfect crank angle for wheel position for every discrete wheel speed.

Creating this in practice is extremely complicated and difficult. Whatever values are presented above are compromises on this basic mathematical function.

What’s easier to do from an adhesive perspective is break the link between crank angle position and torque application altogether, which is how we got to diesel electric/hydraulic.

Oh dear, I’ve taken quite the tangent here. Apologies.

 

That's an interesting analysis. Does anyone know how surefooted the turbomotive was?

 

No, but it was always fitted with a speedo, even after these had been removed from everything else during the war, as it was impossible otherwise to tell from the footplate that a slip was happening.

 

Also worth mentioning (in the same context) that Bulleids have bigger valves than most of the comparable classes, especially when compared to the bore sizes. 11" piston valves for both Pacific classes, 18" bores on the MN and 16.375" on the lights, compared to 9" valves on an A4 with 18.5" bores.

 

I'm not sure how relevant that is. According to my (limited) understanding of high performance IC engine designthe main factors driving towards short stroke engines for high performance are the greater surface area available for locating valves in the cylinder head, hence larger (especially inlet) valve area in proportion to displacement, and lower piston speed making higher rpm possible. The first definitely doesn't apply to steam engines, and the second only to a limited extent.

 

The move to multi valve IC heads was partly due to the desire to have lighter valves for higher speeds.

 

On a railtour in 1987 behind 5051 Drysllywn Castle, we climbed out of Stratford with 13 on, in the worst conditions you can imagine, which was steady snow fall all day. BR provided a a 47 to help push us up the bank which was not needed, not a single trace of slip, and also out of Tysley Carriage sidings later that day, no banking engine required. Video of our departure from Didcot, and it was the same at Stratford and at Tysley