Bulleid wheelslip question

A few thoughts to add. Could the flexibility of the springing on locos can affect the adhesion? I have direct experience on 5" gauge models. A friend built a MSWJR 2.6.o which was nearly impossible to drive as it slipped as soon as the regulator was opened. He changed the springs on the driving axles so that they were more flexible, which cured the problem. I believe the dynamic effects as locos move over small track irregularities require the wheels to be able to move to to maintain contact; this is especially relevant when near the limit of adhesion. Could this be part of the issue with the Bulleid pacifics?

Interesting to note that the Bulleid Q1 class were very sure footed. My Father told me that they would easily start the loaded coal trains from the up siding at Shepherdswell whereas an N class would struggle with the same load despite having a slightly higher adhesive weight. Perhaps the Q1s had negative lead in full gear due to the Stephenson valve gear.

Wheel lift at high speed was not only a problem for the 9Fs. The 4-8-2s on the Romney, Hythe and Dymchurch suffered the same problem. However this was not found by calculation but by putting someone on a flat truck in front of the loco and running it at speed. I am sure this was true as I was told by Arthur Binfield - he was the poor beggar on the truck. I guess young apprentices were expendable in the 1920s.

 

Spring rates are as much an art as they are a science. Too firm and you lose grip and to some extent suspension travel, particularly with close coupled axles on rough surfaces or where there is low adhesion. Too soft and the wheel will bounce particularly if you have an un-damped suspension system and unbalanced rotating masses. Luckily with a steam loco you have more axle control via the horns than you do with other systems where the leaf springs also often control some of the axles fore, aft and twisting motion as well as controlling the ride. One day someone will attempt a new build steam loco with rail friendly air suspension which will give the driver more control options.

 

At the risk of following up on the slightly off the Bullied topic re springing. The earliest (or if not about the earliest) loco springs were in fact the steam springs on George Stephenson's early creations so a form of air suspension is nothing new as far as ideas go. I forget exactly why they failed long term, probably lack of effective gland packing options, but they did improve performance. IIRC due to allowing valve timing to be more accurate but that is from memory as I can't access the correct books at present to check.


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I'm surprised to learn that as I thought that mechanically the Bear was a Star with an extra 6 tons on the coupled wheels.

Was it perhaps in comparison with 4700 during the period it was used on fast freight?

 

Well clearly not just a Star with more adhesion as it had an extra pair of wheels too! Which, given that this thread has demonstrated that the causes of slipping can be quite a subtle combination of effects, must surely be relevant?

The reference to the loco being a bad slipper, incidentally, came from Winkworth's book on Bulleid Pacifics. I don't suppose there is a lot of surviving film of "The Great Bear"! I do still wonder whether at least some of the latter-day reputation that Bulleids have for slipping is simply the result that there were still very large numbers of them working at a time when cine cameras had got to a cost level where they could be relatively widespread. Add in their use from termini such as Waterloo that had a hard start and it is perhaps not surprising that film of them slipping is more common than for many other comparable classes of loco.

Tom

 

I've not seen any reference to the Bear being slip prone in any of the books I have. I wonder what his source was. Mind you there are references to alterations in the weight distribution and my main reference to the Bear is not in my opinion the most objective of sources! By contrast I've seen references to the *light* Pacifics being prone to slipping in contemporary references. I can't lay my hand on it right now but I recall one in which a Southern engineman described them as "would slip on sandpaper". I don't recall seeing as much about the Merchant Navies though. Wayne's comments certainly tell us that its much easier to slip on starting with a Bulleid than some other designs due to the regulator design. No doubt this design has advantages as well as disadvantages.

This thread tells us a lot about human nature and bias in general and enthusiasts in particular though: I was very amused by all the posts saying ah yes, but XXX will slip too, and enthusiastically providing examples: posts which are, essentially of minimal relevance. Every steam locomotive can and will slip given (un)suitable conditions and handling.

 

Ah yes, but on NatPres, the plural of anecdote is clearly data, as witness the Royal Duchy thread amongst others. Which amongst other reasons is why I am interested to know if there are any objective data on the relatively propensity of different classes of locos to slip. Or, more realistically, simply to point out that a lot of observations of Light Pacifics slipping can't be entirely disentangled from the fact that they existed in large numbers and were in operation right to the end of steam, and moreover there may well have been an element of unofficial record breaking in the summer of '67 leading drivers to try for very smart starts out of stations.

In summary: I'm sure that they were somewhat more prone to slipping, in an absolute sense, than other comparable classes, but possibly not as much more prone as their reputation suggests.

Tom

 

There is of course so many variables, anything will slip if the railhead is bad even a 'ten-coupled' such as a 9F.
Also, as touched previously, worn tyres etc. Wheel diameter also effects matters, the smaller the wheel the less chance of slipping there should be.

 

Also emptie sand boxes ,I knew some passed cleaners who went on loan to Nine Elms from the Midland to cover shortage's in the mid sixties, who weren't keen on emptying smoke boxes let alone carrying buckets of sand to fill the the boxes.