Edward Thompson: Wartime C.M.E. Discussion

How fast and for how long was P2 timed on the level?
If the Big Wheels means higher Speed is true, the P2 would be able to run 138 kmh from P10 measurement.
A gentleman here stated that S15 were impossible to fire at/over 50mph and a mrBradley had a school comrade that timed S15 going to school during a year and they often ran 70.

 

Surely the relevant numbers will concern the next series of measurements?

There's something I'd have never expected to find myself saying, not so long ago!

 

And Gresley himself was not averse to timing trains. He took his chronometer on board the press run of 2509 and informed the crew they’d broken 112 twice. No speedo on 2509 at the time.

 

The work for which the P2s were built did not require high speed running. The original remit was to replace double heading on the heaviest trains at the time.

 

Big wheels are heavy,cost money and destroy tracks.
Are You suggesting that Gresley used shareholders' money unwise?

 

No. I'm suggesting that you are talking wee bit of nonsense.

 

Arguing about steam locomotive design is childish anyway,but interesting.

 

A high speed passenger locomotive with tendencies to hotboxing,crankbreaking and track destruction was not an interwar deal breaker?

 

In the good old steam locomotive days rails were laid further apart in curves to ease pasage of rigid wheelbases of 17 19 feet.
Is that still so today where all vehicles are on bogies?

 

Reply to Posting above No. 4312 by jnc.

I'm not sure that anything could be done easily with the P2s as regarding the crank axle failures as originally built. Bearing in mind that they were built to the limits of the load gauge (they were 1-3/8" wider over the cylinders than an A3) , this gave no scope to make the rigid coupled wheelbase more flexible. If Gresley had opted to go for (say) 19-1/2" diameter cylinders and 250 psi (not brought in until a couple of years later with the A4s) the outside piston rod centres could have been increased by 1-1/2" to allow the coupled wheels to have some side play. As I understand it, the P2 Group have retained the design piston rod centres, reduced the cylinder diameters and upped the boiler pressure to 250 psi to compensate, this I understand, to reduce the overall width of the cylinders by 1-1/2" for route availability reasons. From the original Doncaster G.A drawing for 2001 there appears to be no thinning of wheel flanges to the 2nd or 3rd coupled wheel sets to help matters either.

Having looked at their website (www.p2steam.com/design-study/) regarding the crank axle, it appears that there were anything up to five failures of them that has been attributed to the high torque that could be applied to the crank axle by the cylinders. The question I ask myself, and am uneasy about, is why did all these fractures occur at the 9-5/8" diameter axle behind the driving wheel rather than at the 8-1/4" diameter section adjacent to the inside cylinder crank web which I presume also has a key and keyway?

My feeling is that with the rigid wheelbase like the P2s had, on going round curves a sideways pressure was exerted to the driving wheel rims and via the spokes to the hub, thus putting a rotating bending force on the axle just behind the wheel hub. To demonstrate this take a length of flexible hose and with both hands hold the ends and pull the hose into a curve. Now rotate it and it can be seen that any given point around the circumference is being alternately stretched and compressed.

As the P2 Group website only refers to torque action it appears to me that they have not taken this rotating bending force into account at all, and that their 'stress relief groove' could indeed turn out to be a stress inducing one instead. In short, I don't think that they have fully investigated the problem which as currently designed and manufactured, could yet cause another failure.

As an example of this effect that I have personally witnessed was on a Gardner 6LXB Diesel engine in a Foden lorry. Possibly though over tightening the timing chain, thus putting a sideways load on the camshaft sprocket, the 1" diameter steel camshaft sheared immediately behind the chainwheel. It is difficult to imagine this happening, but it did.

 

Given the problems which the long fixed wheelbase of the P2 appeared to cause on the twisty Aberdeen route, did 9Fs ever appear on it after nationalisation?

 

Surely the 9F wth 5ft drivers and a middle roller wheel was a very different beast to a P2 with 6ft 2ins drivers.

Peter

 

Thanks for that considered reply. It's a little scary; the thought that the 'new' P2 could also break axles is fairly boggling. I had assumed that with the wealth of advanced simulation tools at their disposal, the Steam Locomotive Trust could have put their finger on the exact cause of the original failures (something which might not have happened back in the day, from my admittedly poor knowledge of them), and tweaked the design to rectify it. It will certainly be extremely eye-opening if the new P2 breaks an axle (if so, that will certainly cast an interesting light on the potential wisdom of the decision to rebuild them). Although it may take a while (especially if your suggested cause is accurate), as it's apparently a long-term fatigue thing (given the infrequency of the original failures). Well, I hope for the best, but whatever happens, I'm sure it will cast some illumination.

Noel

PS: By "decision to rebuild them", I mean the one back in the 40's.

 

That's some impressive analysis. Scarey, but impressive and sent me straight to the lined page on the P2 site. Like everyone else, I've been unquestioning of the computer modelling done, forgetting that computers will only provide answers to questions posed in the first place. The linked page mentions an increase to 10" diameter, but given the nature of @Eightpot's case, that leaves an unanswered question.

Another point elsewhere, just yesterday, got me mulling the 'duplex' layout, as per the Pennsylvania T1, where, if I'm understanding what's on their website (big "if", I'll grant you!), this very point underpinned the original rationale of that design i.e. two independent 4-coupled units on one rigid frame. That, of course, is a very different kettle of fish, design wise and not vaguely applicable to the P2, but indicative of an all too real problem, as described by Eightpot.

I'll watch this one with some interest and not a little trepidation.

 

Maybe not 9Fs but surely the Grangemouth-based ex WD 2-10-0s 90750-774 with their 4' 8.5" driving wheels and their 34215 lb tractive effort would have been a regular sight.

 

We're back to the claim that the P2s' long rigid wheelbase caused problems on the Aberdeen road, but was that really so, or did the idea arise from a combination of hearsay and misunderstandings? If there really was a problem, was it due to the long rigid wheelbase alone or to that in combination with inadequate guidance from the pony truck (which is one of the design flaws that have been addressed in the new build)?

Can Simon please remind us whether the P2s' very poor availability (which was the reason for Thompson absolutely needing to do something with them) was mostly due to the crank axle failures (which is the other major design flaw addressed in the new build) or by other faults besides?

 

It seems to me that speculation that the A1 trust have forgotten to consider a particular design flaw in the original is at the far reaches of internet guesswork. I doubt the trust or the regulators have missed the potential problem.
But in any case we now have effective ND testing and the locomotive will be running low mileages by historical standards so there's plenty of potential for an inspection routine to catch anything early.