Bulleid wheelslip question

Can the engineers here explain to mortals like me why Bulleid pacifics even the Merchant Navies - were so prone to wheelslip especially when starting? If it was just the light pacifics that had a problem I guess the answer might be power to weight but it wasn't.
Just to kick off the MNs had quite a high axle load so it can't simply be a power to weight issue. Secondly the axles and frames are rigid so it shouldn't be anything that distinguishes a Bulleid from comparable engines, unless there is a "flex" factor which also makes me wonder if it's a characteristic of the box-pok wheel over a spoked wheel, or a problem with unblanced counterweights, and uneven power surge from the piston (also unlikely) or sudden cylinder steam pressure surge leading to uneven force applied to the driving wheels?
Is there a concept in steam locos of sprung and unsprung mass just as there is in diesel and electric locos?
I'm sure this has been covered on threads passim so this is for newbies like me. ta.

 

GWR engineers seemed to credit the relative immunity of GWR locomotives to slip as being down to superior regulator design rather than anything intrinsic in, for example 4-6-0s against 4-6-2s.
I have trouble stomaching the weight transfer argument - although I haven't done a proper mathematical analysis I just can't see enough weight transfer happening when you consider we might be talking two or three tons of drawbar load three feet above the rails and a locomotive weight of 70 tons or more with a CofG in excess of 15 feet in front of the rearmost wheel.I've seen a cogent argument for driving wheels being unloaded on uneven track/pointwork leaving terminuses though.

 

Part of the problem with the Bulleid pacifics is that there is a very large volume of steam between the regulator valve and the cylinders so any tendency to slip is not so easy to control and keep in check. To successfully start without slipping requires a technique of pumping the regulator handle, that is repeatedly opening and rapidly closing the regulator until the loco starts to move. This is quite alien to GWR and LMS men whose engines can be driven hard from a standing start !

David

 

I'd better dig out some of my sound recordings of GWR and LMS locos slipping then. Was it not a Duchess that went into an uncontrollable slip and nearly cut through the rails?

 

Just about anything will slip if conditions are conducive to low adhesion, although some types are more prone than others. I suspect that with all urban myths, the tale improves with the telling.

 

It was definitely a characteristic Martin. When I were a lad our school train was steam hauled from Christchurch to Brockenhurst and the locos rostered for the 8 coach train varied from a light pacific to standard 5s 4s and 4MTs. There were probably hundreds of train spotters on board in those days with their Ian Allan books so we all know the tractive efforts, driving wheel diameters and so on, and we knew that (in theory) the WC or BB should outperform a standard 5, but a standard 5 could power up Hinton Bank without slipping whereas the WC was always very ginger on starting. A WC didn't get much above 40mph before slowing for New Milton. In terms of getting us home on time, the WC was always the "prestige" engine but even a standard 4MT tank could match it due to better acceleration from standing! Anyway now I'm daydreaming - sorry!

 

I hadn't heard the Merchant Navies were particularly bad for slipping. Can anyone comment?
The light pacifics on the other hand were notorious back in the day. I suppose Bulleid would argue that there was demonstrably just enough adhesion for the power to be available when needed.
Durrant comments in 'Swindon Apprentice' "Bulleid Pacific... performance was diametrically opposite to that of a GWR Castle. The latter would make a brave and impressive start, stamping away with confident surefooted adhesion, but once out of earshot of the station, notched up and eased off considerably, continued at plodding gait to the next stop. By contrast, the Bulleid locomotive started off with several violent slips, and after suitable delay got into its stride with offbeat., sneezing exhaust, but once under way sped uphill and down dale with exhilarating vigour. There was little to choose between overall times..."

 

The other factor with all the Bulleid pacifics before rebuilding was the uncertainties in valve timings due to the unique valve-gear on these engines. To get the valve-gear into an oil bath it was made fairly compact and the motion amplified by a rocking shaft. But any minor mis-adjustment on the valve-gear was emphasised by the mechanical amplification. The oil baths were also inclined to leak, and if oil got onto the wheel(s) or rail that would worsen the situation! The valve-gear was adjusted by a steam-powered system rather than direct gearing, and I understand that could cause problems as well.

 

Because of the relatively short Piston stroke (24 inches versus 30 on the GWR) . the power is being delivered on a shorter crank ? this would have a gearing effect on the power transmited to the wheel rim?

 

As mentioned earlier: 4-6-2 wheel arrangements have a bit of weight transfer onto the trailing truck that 4-6-0s don't.

And Bulleid light Pacifics are very powerful for the size and weight of the loco, with a very long steam system after the regulator.

 

In 'Mendips Engineman' Peter Smith describes a Hall taking a heavily loaded train away from a signal stop at Branksome without any problems but a Light Pacific on the following train failed to restart and a loco had to come out from the depot to give it a push.

Also there was the occasion when Swanage went into an uncontrollable wheel spin in Combe Down Tunnel

 

several relevant issues contribute to the slipping tendency:

• regulator with limited travel to full open & little "feel" compared to others
• large superheat volume
• 4-6-2s bridge uneven track compared to 4-6-0s
• uneven valve events
• steam reverser difficult to position accurately

 

Having three cylinders probably has something to do with it. They are always harder to start than two (and four) cylinder locos. Because they have a smaller cylinder, they have less starting torque and you then tend to give it more steam with a consequent slip when the other cylinders get steam.

 

On the other hand, a 3-cylinder design ought to give smoother torque, and so be less prone to slipping - see the recent discussion on the relative merits of GWR 28xx vs. NER 3-cyl 0-8-0. And the small cylinder argument should make an equivalent 4-cyl design even worse than a 3-cyl, shouldn't it?

 

Some of it relates to torque. A three cylinder engine has a factor of adhesion of 33% compered with 25% for a two cylinder. This is because you have six pulses of power per revaluation rather than four so there is a more even torque. By this you can have a larger wheel for speed yet still have the same number of cylinder power cycles per distance traveled. Secondly the stroke to wheel diameter is poor. The ideal is to have the longest stoke possible for the piston so the pulse of power is smoothed out. The drawback is high piston speeds, which can be off set by using streamlined steam cycles.

If you then relate this to diesels where you have thousands of power pulses per revaluation you can see why they have a higher torque to weight ration.

A lot of the other points by others are valid but these here are poorly understood.

 

Once on the move, a three cylinder loco does have a smoother torque but, at the very point of starting it suffers from the fact that, at certain points of rotation, only one cylinder is effective. One is open to steam at approximately mid stroke, one is open to steam but at the start of a stroke so providing little effective torque whilst the third is open to exhaust. With a two cylinder loco, with cranks at 90 degrees the starting torque curve is much better. Because there are three cylinders, they are of smaller diameter, compared with a two cylinder equivalent and individually, the cylinders produce less torque. A four cylinder loco (Lord Nelsons excepted) still has the cranks at 90 degrees so it is essentially identical to a two cylinder loco, except for the angularity effect. Draw out a torque/rotation curve for two and three cylinders at the point of starting and you will see the difference.

 

Apart from the LN class, 4-cylinder locos work with their cylinders in pairs.

Edit : Should have read Steve's post first.

 

I think 30865 had the standard 4 beats per revolution set up.

http://www.semgonline.com/steam/lnclass_1.html

 

I don't think you're correct with regard to adhesion factors for three cylinder locos. I don't know of any which reach the dizzy heights of 33%. For example, a Schools, has a factor of 0.268 and a Light Pacific a lowly 22.1%. Even an A4 only comes in at 24%
Here's a table of Adhesion ratios for NYMR related locos I produced some while ago.