Edward Thompson: Wartime C.M.E. Discussion

Most Pullman types, Gresley and Thompson coaches do too. That's why the LNER Pacifics plus the Bulleids all had drop head buckeye couplings.



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The LNER recorded a number of "mega trains" (over the normal carriage numbers) but by far the most impressive was the V2 taking the twenty something carriage evacuation train out of Kings Cross, unassisted.

 

Maunsell and Bulleid carriages had buckeye couplings.

But I think in focusing on that second point, people are missing the first one: the real issue with Bulleids slipping seems to me about an early propensity to slip at speed. By contrast, most locos asked to start a heavy train will, under certain circumstances, slip on starting, for all sorts of reasons discussed. In that context I think a train of screw-coupled carriages can also be considered rigid; the issue is having to move upto 650 tons (including the locomotive) from scratch all in one go, starting at zero revs. You don't do that on any other vehicle: when you start a car, there will be a period in which the clutch is slipping until speed is sufficient for engine revs to match wheel revs. That puts a lot of demands both on rail-wheel interface and throttle sensitivity, and that is before you get into the relationship between where the piston is in its stroke and therefore how many cylinders are receiving steam, where the crank is in relation to applying maximum torque etc.

Tom

 

A further thought on rigid couplings, was there any form of 'flexible buffing gear' (for want of a better description) built into the L&NER articulated stock?

 

I'm not sure about the relevence of buckeye couplings to this discussion. They may be rigid but also have just as much "slack" in them as do tightly screwed up conventional couplings. If you stop at a station on a slight up grade and try to hold a long train on the loco brake the rear will move back a couple of feet as the train brakes release. This is not a good idea if passengers are boarding at the time.

Peter

 

Yes, probably my fault - I was really trying to distinguish between passenger trains (which are quasi-rigid regardless of couplings) and a loose coupled freight. For a passenger train, even if the couplings are compressed, the loco has to be moving the entire weight of the train within a fraction of wheel revolution - even two feet of slack in the couplings along a train is only about one tenth of a wheel rev for a 6' diameter wheel, after which the entire load has to be moved as one. A loose-coupled freight train is rather better in that regard, but in all cases, starting a steam locomotive is considerably different to almost any other vehicle in which there is a high starting inertia, because of the direct relationship between engine speed (i.e. piston speed) and vehicle speed. Nothing else has that same relationship on starting.

Tom

 

Corridor tender LNER Pacifics only, surely? And certainly not on the Bulleid locomotives, although the coaches did have them.

 

Not something I'd previously considered, but perhaps the coupling arrangements on the FfR explain the perennial slipping issues starting heavy trains in the up direction at T-y-B?

 

The LNER and SR used Pullman gangways and buckeye couplers for corridor stock, the LMS and GWR used British Standard gangways and screw coupling (by and large that is, there were odd exceptions built for inter-regional workings)

 

Beat me to it Buckeyes were fitted because the tenders had Pullman gangways and that is part of the set up.

 

My apologies, you’re quite right, I should have qualified that better.

 

The type of couplings will have much less effect than the water spilled from the water cranes a few spots of oil that have dripped from the loco and the whole train being on a curve. I do not know if the station is on a gradient but that would be an additional factor. I did once see the driver of Blanche put a little sand on the rails from a tin, including between the wheels. Despite the wet day the loco started away cleanly.


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Curves on railways are normally gentle enough that the slight coning of the tyres deals with the different distances on the inner and outer rails.

 

I don't think that is true at rest.

If you imagine a train that has come to rest on a super-elevated curve, it will have fallen (by gravity) to the inside of the curve, i.e. the inside flange will be hard against the gauge face of the inside rail. In that situation, the contact point of the inside wheel is, by the shape of the cone, at its largest diameter; and that on the outside rail is at its minimum diameter - which is precisely the opposite way round to what you want.

The other issue for drag is that for a train on a curve, the forces through the drawbar will tend to draw the train towards the inside, dragging the inside wheel against the rail head and increasing friction. (Imagine two vehicles with a shallow angle between them caused by the curve: the pull along the axis from the first vehicle will resolve into a component along the axis of the second, and a second component at right angles, pulling the vehicle towards the centre of the curve).

Tom

 

Hmm! Maybe, but if the wheels are tending towards the outside when the train is moving, when and how would that change as the train comes to rest?

That I can readily agree with.

(BTW the thread drift (lack of slack in couplings) from thread drift (causes of slipping) might belong better in the "Why are Bulleid Pacifics more prone to slipping than other designs?" thread rather than here.)

I once witnessed and videoed a situation where the screw coupling between the loco and the train was so tight that uncoupling was a very hard struggle. This was on one of the RPSI's weekends when the coaches were approved for carrying passengers in the Republic but not in the North, so the train ran Dublin - Dundalk - Drogheda - Dundalk then empty back to Whitehead. The problem came at Drogheda, when they tried to uncouple the loco to run round. The drive put on steam to try to compress the buffers, but those on the inside were already compressed as tight as they would go, as can be seen in this still from my video. Eventually brute force was needed to start slackening the screw coupling.

 

Imagine a train running round a long continuous super-elevated curve at the precise speed that the super elevation is designed for. The wheels will be centred on the rails. If it speeds up, it will tend towards the outside. If it slows down, it will tend towards the inside - in the limiting case, if it stops it will come to rest towards the inside of the curve.

Tom

 

Collect introduced buckeye couplings in the 1920s on some of his high waisted GWR stock, usually distinguishable by bow ends. The ends of sets coupling to locos had flat ends and screw couplings.

 

Not convinced, but I think this particular thread drift has gone too far already.

 

The issue is that its a 1 in 100 gradient on a reverse curve and the rail head is contaminated. Double engines don't have sanders either which makes things tricky in the wet.

Tim

 

It doesn't sound correct to me either, my understanding is that the coning steers the wheelset to the position where the wheel diameters are correct for the difference in distance along the inner and outer rails. Sorry for perpetuating the thread drift.