Bulleid wheelslip question

The unusual bit is the distance covered by the slide- I suppose I'm thinking that this particular slip looks less 'natural' due to the fact that the locomotive is in reverse, and that I more readily associate slipping with one that is moving forward. On balance, its probably just me.

 

Is it my imagination, or every time it starts to slip the outside crank pin is at 8pm?

 

Assuming we are talking about the sequence where she's backing in to the platform, the loco is not slipping. Brakes have been applied, wheels have locked and the loco is sliding along the track. Just like a car skidding when braking as opposed to spinning its wheels when accelerating.

 

Yes, we are talking about the same section. As such, there is a need to differentiate between what constitutes a slip (ie. movement under power) or a slide (ie. continued movement when power has been disengaged and brakes applied), as I thought both were interchangeable. Regardless, I still find it an interesting clip.

 

Slightly confused by this. On one hand you say there is a need to differentiate between a slip and a slide but on the other you say they are interchangeable.

 

an interesting discussion, but im afraid a bit off the mark!

when starting, the Bulleid Pacifics had a problem with axlebox lubrication and the BFB wheels would have pockets of oil that would then find their way onto the treads. this was peculiar to the BFB wheels. it was also the main cause of cladding fires when the oil in these pockets was thrown upwards.

slipping at speed on the unrebuilt locos was examined in depth at Rugby and the test report is well worth studying.

cheers,
julian

 

Would this be the tests carried out on 35022 during 1952 to 1954?

 

Where can we find the report or please could you summarise the findings for us

 

hi johnofwessex,
if you pm me with your email address i will send you a pdf copy of the rugby test report.
cheers,
julian

 

Nothing confusing from where I'm sitting, but I'll put it another way that might make it clearer. I thought the terms 'slip' and 'slide' were interchangeable, but it seems from what you suggest that they are not and there is a need to differentiate between them.

 

So are you saying slip is both different to and the same as a slide? If so it doesn't make sense.

 

No I'm not. I'm saying I thought both were the same, but they are clearly not from what you have said. Makes perfect sense to me.

 

Right. We've got there in the end.

 

Phew...

 

In relation to a railway locomotive, I would understand the term "slip" to mean the wheel spinning, generally when under power, and "slide" to mean the wheel stationary (or near stationary), generally under braking. One significant difference being that during a "slip", the wheel is turning faster than would be suggested for the linear speed of the vehicle along the ground, and during a "slide" the wheel is turning more slowly than would be suggested for the speed of the vehicle.

In extreme cases, the two modes would risk different damage: in particular a slide may lead to flat spots on the rim, i.e. the wheel becomes non round; a slip will cause even wear to the wheel rim but may cause damage to the rail or, in extreme cases, damage to the motion, especially if the wheel regains grip and the slip stops very suddenly.

The root cause is the same either way: the force on the wheel rim causing it to turn (or stop turning) exceeding the frictional force between the wheel and rail.

Tom

 

Totally off topic, but there is rather strange phenomenon involving The L&YR 'High Flyer' atlantics, referred to in several works. This was 'slipping with steam off'. Various loco inspectors wouldn't believe it, but became convinced on experiencing it themselves. I don't think a cause was ever convincingly identified. Eric Mason in 'The Lancashire & Yorkshire Railway' (Ian Allan, London (1954) says:

Throughout almost their entire existence, then, the Atlantics were hard-worked, serviceable, reliable machines. They had their faults, of course. They tended to slip badly at starting. Skilful drivers could reduce this to a minimum by judicious application of the steam sanding apparatus. One thing they could not do, however, was to prevent certain engines of the class from that alarming habit known as "slipping with steam off". This phenomenon has always puzzled locomotive men, and several theories have been advanced to explain it. The question as to whether the engine is "slipping" or "skidding" has been virtually settled in favour of the former, because the tremendous vibration which the machine sets up could not be produced by the mere locking of the coupled wheels, whereas anyone with footplate experience knows that when an engine "picks up its wheels", as we say, the normal footplate vibration is considerably reduced. The inference is, therefore, that at certain times, just after the regulator is closed, the coupled wheels race away much faster than they would if the engine was running normally. A likely explanation is that the coupled wheels are excessively balanced; that is, too great a proportion of the reciprocating masses is balanced, and the balance weights on the coupled wheels produce an unbalanced vertical force which tends to ease the weight on the rails when the balance weights are moving through the upper semi-circle. Also, when steam is shut off and the momentum of the engine and train is driving the engine, the crank is driving the piston, and the pressure of the crosshead is on the bottom slide bar, instead of the top. The effect of this on the connecting rod is to force the crank upwards, and this assists to reduce the load on the driving wheels. This reduction in load, due to both causes, may be such that the driving wheels are almost lifted off the rails and revolve for some time almost free from frictional resistance, due to their own momentum, until speed is reduced or the engine reaches a spot where the rails are less greasy. Slipping without steam always occurs at high speeds in a tunnel or cutting where the rails are greasy. As cases of this nature are seldom if ever heard of on later six-wheel coupled engines, or modern types, it seems evident that, not only are the weights on the coupled wheels greater, but also that balancing receives more careful attention nowadays.

Strange but true!

 

'slipping with steam off' is indeed very mysterious. Apart from anything else, just before steam is shut off the wheels are rotating at the normal rate for the speed at which the loco is travelling. Regardless of what up-and-down forces may exist, how on earth could the rate of rotation increase?

Apropos "slipping" and "sliding"; these terms do indeed have overlapping meanings in general, but not in the context of railway locomotives. Might "sliding" be more often referred to as "wheel lock"? It certainly continues to happen on rolling stock of all sorts, and the resultant wheel flats are often evident when one rides in passenger stock.

 

That's an interesting question. To increase the rotational speed of the wheel, there has to be an input of energy from somewhere. The only realistic possibility is residual steam in the steam chest (i.e. a hysteresis between shutting the regulator and steam actually stopping reaching the pistons), together with some kind of unloading of weight on the driving wheels as @LMS2968 decsribes so that a small amount of additional energy can give a large increase in rotational speed. Clearly track-sensitive too: did the L&YR have a particular reputation for the state of its permanent way (either good or bad?)

All very odd!

Tom

 

Cook mentions the phenomenon in 'Swindon Steam' : "Occasionally a phenomenon is experienced on locomotives of slipping at high speed with steam shut off. It is a very peculiar occurrence and I do not know of anyone who knows the reason for it but it does occasionally happen. It cropped up at very odd times with us."

 

This is surely either diabolical driving on the lines of the 60532 incident, or a stuck regulator. I imagine the loco must have come very close to catastrophic damage, and I would love to see the state of the railheads afterwards!