Bulleid wheelslip question

It's not just Bulleids that can slip quite easily. These two videos below (not my videos) of 6024 show that even the mighty GWR Kings could get into difficulty sometimes. Some brilliant driving here as 6024 hauls an early morning Railtour over the WSR captured at two locations. She almost comes to grief but the Driver manages to encourage her along.

 

No surprise that loco was slipping so much. Those massive numbers on the front must have totally unbalanced it.

 

No, they're there to counterbalance all the brass and copper that festoons the loco

That was a really good trip, despite the very cold, very early start. I seem to recall we were over an hour late out on to the mainline at Taunton, but after a storming run including a brilliant run through the centre road at Reading and plenty of main line running, we were early into Paddington.

 

Don't think there's any doubt that every locomotive can and will slip given the wrong conditions. One that couldn't would be absurdly underpowered!

The original question was whether there was anything in the design of the Bulleids (and others in the reverse direction) that led to a tendency to slip more than other reasonably comparable classes. Regulator design and characteristics of 3 cylinder locomotives are themes that have come up more than once.

 

Any where you have a metal to metal contact the friction between them is going to be dependant on the absence of any thing that will weaken that friction now take one locomotive add oil, grease, general muck and what is a very narrow area of contact between the rail head and the tyre on the driving wheel, and you make a situation where adhesion can be difficault to attain, add on to this the delay between closing the regulator and the steam shutting off at the pistons and valves, and yes in the right conditions any loco can slip, I have known rebuilt Bullied’s slip on bad rail head conditions, so i don’t think you can blame it completely on the chain driven valve gear and oil bath, but saying that, if its dropping oil everywhere and that’s finding its way onto the motion ,then onto the wheels and track, there’s every chance you will pick your feet up.

 

Wheel tyre condition can certainly have an effect on adhesion. I was very surprised at the transformation of 30926 on the NYMR once the tyres had been skimmed to restore the profile.

 

Porta described adhesion as the kingdom of railway stupidity or similar. Things like drains and oil being allowed to get onto the driving wheels/railhead

 

In the very early days of steam locomotives, some designers used rack-and-pinion on the assumption that a smooth metal wheel on a smooth metal rail wouldn't work. Then it turned out to work OK, and it still does, most of the time. But slipping remains a live issue, with some modern diesel locos having fancy control systems to allow very slight slipping (creep?) but to ease off on the power to the affected axle if there's any more than that. On a steam loco, controlling a slip is down to the skill of the driver, complicated by such things as a jammed regulator, the amount of steam already on its way to the cylinders, or a reverser that moves itself to full gear when given the chance. And sometimes there just isn't enough adhesion to pull a train up a bank, no matter what the driver does, as has been discussed recently for the line to Blaenau.

 

Out of interest how much does a driving wheelset from a Bulleid pacific weight compared to a similar sized conventional spoked wheelset? If they lighter then I presume this would have an impact on the likelihood of a slip occurring?

Chris

 

No, it's the weight of the wheel plus the proportion of the loco's weight it is supporting, the latter being by far the higher amount. The individual wheel's or axle's contribution to adhesive weight is comparatively minimal. A lighter wheel/axle assembly does though reduce the unsprung mass, which does give advantages elsewhere, specifically as to ride quality, mechanical and track damage.

 

Some interesting comments, some way of the mark.
I am lucky enough to drive on the main line nowadays.
I have driven a large variety of large loco's (class 7 & 8).
One of the main things to consider is regulator design, touched on earlier.
A Bulleid pacific you can very quickly get full steam pressure in the cylinders if you do not watch what you are doing. Net result very good chance of wheel slip, even on a dry rail.
On virtually all GWR loco's the only way you can do that is by driving like a numpty!
You can not fully open the regulator on a GWR loco when in full froward.
To get into 'second regulator' you would need to virtually need to be in mid gear to be able to get the regulator fully open, THEN wind it into full gear. Not the smartest thing to do!
So comparison with Bulleid's not really relevant.
One of the places in the sixties to see Bulleid's at work was Waterloo.
Uphill and on a bend, with lots of points thrown in.
Is it any wonder they slipped out of there, particularly with all the oil on the rails from loco's standing for periods of time. Reputation born.
In my experience a Bulleid is a very sure footed machine. I would rather have one than any of the other large loco's available if I have to do any 'hill climbing'.
How often does Clan line slip with the pullmans on the Surrey hills circuit when climbing out of Shalford with, sometimes, over 600 tons on it's tail?
To give a comparison, if in your car you put it in first gear, then put your foot hard down on the accelerator, drop the clutch, would you get wheel spin? That's what happens if you do not watch what you are doing driving a Bulleid.
It takes just seconds with the regulator just cracked open to suddenly find yourself with 250 psi in the steam chest.
Because we tend to drive heavy trains nowadays, 12/13 coaches, loco's are driven very close to their adhesion limit when starting away. But you can easily get caught out if you are distracted momentarily.
Once you get them past 10 mph they are masters of adhesion.
So when you hear someone saying the driver is 'pumping' the regulator, what they are doing is opening it to let say 100 psi into the cylinders, then shutting it, reopening it as the pressure falls away, reopening etc etc.
The pressure will fall away very quickly so the impression is the regulator being pumped. As speed builds up greater steam chest pressure can be given and, at the same time, cut-off shortened to lessen the need to keep closing the regulator.
Hope this helps explain a few things, carry on debating chaps!

 

What a great post, thank you! I admit that it's not really on topic, but I was particularly interested in what you say about GWR regulators and was wondering, is there an interlock that prevents the regulator from being pushed all the way to fully open when the reverser is in full gear? I would be interested to learn more.

Kind regards,
Chris

 

Thanks for your reply, that all makes sense.

I don't think I was very clear on what I was getting at in my initial post, I was trying to establish/ask whether the weight of the wheelset has an effect in itself, not in terms of adhesive weight but in terms of the wheelsets in effect becoming a large flywheel.

So a lighter wheelset would require less energy accelerate the point of breaking traction, and similarly less to stop it, whereas a heaver wheelset would be initially more difficult to 'spin' however would be more difficult to stop once started? Similarly, a crank axle (and counter weighted driving wheels) presumably 'store' more energy being heavier than a a gear on a straight axle?

The reason it has struck me is a recent dilemma with rebuilding a friends landrover with a new engine and having to try and work out which flywheel was most appropriate for the new engine we fitted- surely the principal is the same?

Thanks again

Chris

 

I see what you mean, although I'm not sure I know the answer. You can't really compare an internal combustion engine with a locomotive: all the reprocating masses of the former have an equal and opposite force from another cylinder (single and some twin cylinder engines excepted), whereas this doesn't necesarily apply with a loco, and certainly not with a two-cylinder one. Three- and four-cylinder locos should be self balancing, but it would depend also on the relative angles of the cylinders and connecting rod lengths - they aren't normally the same. But the reciprocating forces should damp themselves out anyway as they reverse direction each half revolution, so do they contribute to the flywheel effect or not? I don't know, and it's been too long a day to think it through!

 

No loco is perfectly balanced so there is an out of balance force to consider. At low speed this is probably insignificant but at high speed it can have a significant effect. This was the main reason for the clamp down on the speed exploits of the 9F's; it was calculated that the out of balance forces were sufficient to momentarily lift the wheels off the rails at high speeds. Of course, this upward force at one point of revolution becomes a downward force at the opposite point of revolution, creating significant hammer blow.

 

Mainline steam was around on the Southern until I was 15 so I saw them slip and equally get away surefooted. (School boy spotting at the London end of Winchester City fairly regular) Remember one instance going north at Basingstoke on a train with several false start attempts, slip, set back, try again, slip, repeat until we got going but have no log of the date/engine. Worst example of slipping I've personally seen, and with my camera too, was 4472 trying to get a Scarborough Spa Express out of York. Finally again with 4472 I remember being a passenger (in probably coach 2) of an eastbound SSE in the early 1980s and from the sounds heard she appeared to slip momentarily at speed several times on the straighter, faster, bits of track.

Incidentally excellent and informative thread this.

EDIT - PS Two other bad instances but both due to rail conditions and with non-pacific locos. Ivatt Atlantic on the KWVLR (Oiled rails - proven vandalism) and 4767 in a tunnel restart at Sunderland when double heading with 2005. Again oily rails from DMUs etc. Have photo of the Ivatt.

 

Its all madness really, imagine being told to pull a heavy cart up long slight ramp of shiny steel with smooth shoes on...

 

.............covered in oil & diesel!!

 

But as Hedley proved circa 1814 it could be done by steam worked adhesion locomotives and offered alternatives to muscle power. The world could then be tamed.


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Blenkinsop had the right idea. No problem with slipping with his locos!