Bulleid Power Classification Question

The power classification of a Bulleid Light Pacific is 7P 5FA (though the rebuilds were briely classed as 7P 6F, I believe?)..............but my question is: What does the "A" mean? Is is the old Southern power class "A" (in which case why wasn't an MN 8PA?)
Or is it route availabilty?
Anyone know the answer?

 

The 'A' (or a 'B') denotes brake power of the locomotive. This was one area where BR(S) differed from the rest of BR.

Regards

 

Thanks! But why no braking power for an MN?

 

On the subject of BR power classifications, how did BR decide to grade them? I was told once that tractive effort was measured at 25mph for freight locos and 50mph for passenger engines and these measurements would determine the classification. Is this true, or have I been spun a big fib?

 

Yes, something like that.....@ 85% of boiler pressure too, which does not take into account a free-steaming Bulleid boiler either, eh?

 

Are you sure? At 85% boiler pressure, that is.

 

Oh yes, max tractive effort always quoted at 85% of boiler pressure!

 

But we aren't talkiing about max tractive effort, we are talking about a tractive effort at certain speeds, which is certainly not the maximum. And, in any case, not everyone quotes a theoretical tractive effort at 85% of BP.

 

Hmmmm, yes maybe I'm a bit muddled here, but in all the books, say "the Observers Book Of Steam Locomotives" etc... (showing my age here!), the max tractive effort was always shown as "at 85% of boliler pressure", and I've always assumed that the power classification was based on the max tractive effort, and followed that same rule....?
Any further answers to this?!

 

I heard a chap from the Bluebelle railway, when quizzed about what the 5FA stood for, replied, "Simple mate F****** Awfull"

 

So (got a good one here!), were Bulleid's Pacifics classified before or after the max boiler pressure was reduced from 280psi to 250psi?

 

They went through several classifications throughout their working lives under BR. In 1949, the unrebuilts were classified (for some reason) 6MT; from December 1953 7P5F; from November 1957 7P6F. The rebuilts (I'm not getting into a terminology argument before some bright spark starts, I realise some people like to say that they were merely 'modified') started at 7P5F, becoming 7P6F from November 1961. So ironically, the power classification went up after the boiler was reduced in pressure! Source is from the SEmG. What I'm not sure about here is why this wasn't reflected by a change in cabside classification following the upgrading to 7P6F...

 

I've read that the rebuilds were 7P 6F...............but as far as I know, this classification was never shown on any cabside photo I've ever seen?
And to answer twr12's question, I'd say after bolier pressure reduction, almost certainly?

 

Well, despite the unrebuilts having pressure reduced between 1954 and 1956, they retained their 1953 7P5F classification until 1957. They were also upgraded to 7P6F anyway, which infers that BR(S) was constantly rethinking its classification system for steam locomotives. So to answer the original question, they were classified 7P 5FA before.

 

South African Railways certainly quoted TE at 75% of boiler pressure - partly to allow for the high ash content coal that was used at some depots. Whether this was an original policy or something imported from UK / German or US locobuilder I don't know....

Cheers, Neil

 

The Midland Railway always used 80% of boiler pressure in establishing the nominal tractive effort of their locos. The Hunslet Engine Co. used 75% in its calculations, leading to the anomaly that the Hunslet Austerity 0-6-0ST had a different quoted TE to the J94 although they were identical locos! These are just some examples and there are others. At the end of the day, the traditional formula only gave a comparative figure and not the true value. The actual tractive effort produced by a loco varied throughout a wheel revolution and was dependent on so many variables that it was a very complex calculation.
Some info on the derivation of the BR power classification is given here:
http://www.rmweb.co.uk/community/in...r-ratings/page__pid__28333__st__0&#entry28333
but I can't vouch for its accuracy.