Power Classifications, how and why?

How exactly were power classifications decided?

For example the SDJR 7F's had a higher tractive effort than the Stanier 8F, while the difference in Tractive effort between a class 4 & 5 4-6-0 was minimal

 

You're talking about the Nominal Tractive Effort, which is virtually meaningless in practice. The Applied Tractive Effort varies with speed - usually downwards - and since the loco moves the train at a speed, the TE at these higher speeds is what counts. The power classifications also took boiler output into account.

Many shunting locos had a high TE, but not above 20 mph. They might move the same train as a Black Five, but they wouldn't work an express!

 

They might havr been on use on the Midland but some of the Western Region classifications were clearly nonsense, the 4MT prairies ranged from the miniature 45xx with 21250 lb TE and 16.6 sq. ft grates to the high pressure rebuilds of Churchward's largest prairies, the 3100 with 31170 tractive effort, despite larger wheels, and 20.6 sq. ft grates.

I presume the WR just stuck ABCDE to decide which locos to use!

 

Don't forget if all other railways went one way then the GW went another way.
Just had to be different.

 

I suspect that one of the operational benefits of assigning power classifications is to help staff (especially shed foremen) assess the capabilities of unusual locomotives. A duty may have been performed effectively by a King Arthur year after year, but the shed staff would need to know whether a new BR 73xxx would be likely to be able to perform the same duty or not - hence the need for some kind of standardised classification. The usefulness of such a classification depends on whether it answers real-world operational problems.

When the Southern Railway formed, there was a requirement to make such comparisons between the a multitude of locomotives from three different pre-grouping companies - a stock of 2,254 locomotives as of 1 July 1923, ignoring those on the Isle of Wight. A scheme was devised to classify all those engines and compiled together so that senior officers and shed staff would, at a glance, be able to assess the capabilities of "foreign" engines.

The scheme was as follows:

Type letter

• Passenger engines with 6ft or larger wheels able to run fast trains - P
• Passenger engines with wheels smaller than 6ft, primarily tank engines or tender engines suitable for mixed traffic - M
• Goods engines - G
• Shunting engines - S

Grade numeral
A classification based on estimated hauling power, measured by finding the heaviest train that the loco could start on a 1 in 100 grade, thus taking into account the useful haulage at the drawbar.

Power numeral
A classification based on nominal power (which thus allows for the fact that TE and power are not necessarily related: a BR Class 08 shunter has a similar Tractive Effort but much lower power than an A4 pacific, so should be able to start a similar load on a gradient, but couldn't work a lighter train at 100mph!)

Range numeral
A classification that related water capacity to steam consumption, and thus gave an estimated range.

In each of the last three classifications, the higher the number, the stronger / more powerful / greater the range.

Data on all the locos was then compiled into a book. For reference, in the summary pages, SE&CR locos of each classification were recorded in a final column, on the grounds that many ex-SE&CR officers had positions of responsibility in the locomotive and running departments, so it gave them an easy reference point back to what they were familiar with!

Shunting engines were not given power and range classifications as it was considered unimportant.

The classification was thus Type / grade / power / range

Some sample data for locomotives currently preserved, and other notable types, include:

P (Passenger) engines, of which there were 247 from the SECR, 130 from the LBSCR and 363 from the LSWR

P 5 / 8 / 3 - LSWR N15 (later, with modifications, the King Arthurs)
P 5 / 8 / 1½ - LBSCR L class 4-6-4T
P 5 / 7½ / 1¼ - SECR River Class 2-6-4T
P 4 / 7 / 1½ - LBSCR J class 4-6-2T
P 3 / 8/ 2 - LBSCR H2 class 4-4-2 (i.e. Beachy Head)
P 3 / 6½ / 2¼ - LSWR T9
P 3 / 5½ / 2¼ - SECR D
P 2½ / 5½ / 2¼ LBSC B1 (Gladstone class)

M (Mixed traffic) engines, of which there were 227 from the SECR, 228 from the LBSCR and 360 from the LSWR

M 4½ / 5½ / 1¾ - SECR J class 0-6-4T
M 3½ / 5½ / ¾ - LSWR M7 class 0-4-4T
M 3½ / 4½ / 1 - SECR H class 0-4-4T
M 3 / 5 / ¾ - LSWR 0415 class 4-4-2T (Adams Radial Tank)
M 3 / 4 / ¾ - LSWR o2 class 0-4-4T (i.e. Isle of Wight Calbourne)
M 2½ / 4 / ½ - LSWR 0298 class 2-4-0T (i.e. Beattie Well Tank)
M 1½ / 2 / ¾ - LBSCR A1x class 0-6-0T (Terrier)
M 1½ / 2 / ¾ - SECR P class 0-6-0T

G (Goods) engines, of which there were 250 from the SECR, 250 from the LBSCR and only 96 from the LSWR

G 5 / 8/ 3 - LSWR S15 class 4-6-0
G 5 / 7 / 2¾ - LBSCR K class 2-6-0
G 5 / 7½ / 2¼ - SECR N and N1 class 2-6-0
G 4 / 4½ / 2¾ - SECR C class 0-6-0
G 4 / 4½ / 2 - LBSCR E4 class 0-6-2T (i.e. Birch Grove)
G 4 / 4 / ¾ - LBSCR E1 class 0-6-0T (i.e. as preserved on the Isle of Wight)
G 3½ / 4½ / 1¾ - SECR O1 class 0-6-0T

S (Shunting) engines, of which there were 6 from the SECR, 10 from the LBSCR and 87 from the LSWR

S 8 - LSWR G16 4-8-0T
S 3½ - LSWR B4 0-4-0T (i.e. Normandy)

There are some interesting comparisons to be drawn in amongst there!

Tom

 

Thinking about it, how did the 2MT do at the recent WSR Gala? I find it hard to believe that they cant do as well as a 45XX which is a class 4

 

I've always wondered what power classification you'd get by double heading 2 0P/F?

 

What a question! Tom ( Jamessquared ) is on the mark when he writes that the power classifications were a tool that allowed staff to judge the capabilities of engines with which they were familiar against those that they were not. Very useful in the nationalised era. Not without flaws though. Take the S & D 7f. It has a higher T.E. than the Stanier 8f so if you wanted to start/move a heavy train you would pick Clayton's finest. It also has phenomenally good brakes for a steam locomotive. Sadly it lacks steaming capacity and the convoluted steam passages don't help it either. But if heavy, unfitted freight is to be dealt with over heavy gradients then the 7f is what you would choose. The 8f has the better steaming capacity and superior steam passages. So it will fulfil Churchwards requirement for a heavy freight locomotive - that it could work an express passenger train if the need arose, far better than the S & D machine. Yes those 28XXs can shift if required, I would dearly like to see one let loose on the mainline.
The A4 and K4 have similar tractive effort figures. One is an 8P the other a 6MT. The A4 will produce double the horsepower of the K4. As the technology moves on a K4 equivalent produced today would have a much improved speed profile with associated increase in horsepower.
The power classification was useful but did not tell the full story. T.E. does not give much of a lead when it comes to how much horsepower a design might develop. A useful tool? Yes. An absolute guide? Sadly, no.

 

There's good information on the LMS Locomotive Power Classification and its BR derivative in Vol.5 of the RCTS History of BR Standard Steam Locomotives. This includes the formulae used, which includes cylinder T.E. and boiler T.E. Boiler T.E. utilises a firing rate of 130lb/sq ft of grate with an upper capacity of firemen of 4480 lb/hr. There are differences for wide and narrow firebox boilers and superheated and saturated boilers. In establishing cylinder T.E. there are graphs which give mean effective pressure as a function of engine speed in revs/sec and for long lap superheated locos, short lap superheated locos and saturated locos. It isn't a simple answer and, if you want to know the full detail I suggest that you get hold of this book.
I understand that the Western region didn't like the results obtained in this way and upped the classification of some of their locos!

 

In an idle (bored) moment yesterday evening, I decided to do a calculation on a GWR loco using the info in the RCTS book to see what the results were. I chose a 56XX because I have a lot of experience with 6619 and, although they are rated at Cl.5, I always considered them to be significantly inferior to a Black 5.
Locos classed as passenger/mixed traffic were assessed according to the formula: Free gas area (ft) x grate area (ft) x T.E. (lbf) / 10,000 This gave a 2-3 figure no. and then there is a table relating this no. to the power class. Freight locos were based simply on the lesser of T.E. and adhesive weight/4.5. Taking it a mixed traffic loco and using that criteria, the derived figure for a 56XX is 194 and that puts it in the Cl.4 range (181-300) and much nearer Cl.3 than Cl.5. Applying the freight criteria puts it firmly in the Cl.5 category.

Edit: changed 'adhesion ratio' to 'adhesive weight' in the above to correctly state it

 

just to point out that the GWR was there first by a considerable margin so it was in fact the other railways that went another way. I hate the "bash the GWR at every opportunity" mentality

 

Jubilees and B17/6s seem to have very similar leading dimensions but the B17/6s are somewhat "marked down" in regard to power classification. With all the debate on the Galatea thread about the Jubilees' temperamental steaming was there something about the LNER locos that somehow just made them less effective?

 

Maybe so but there are Swindon acolytes who are just as bad and dismiss anything that isn't Churchward or Churchward inspired.

 

and that justifies it then?

 

On the GWR at least there was a chart which showed what power class locomotives were permitted to haul what freight wagon loads on what gradients - there's one in GWR Goods Wagons by Atkins Beard and Tourret. Presumably there were also such charts for other types of traffic and quite possibly individual lines, all of which had been worked out in the light of experience. So my guess is that changing to the rather different BR power classes on the WR would have meant a considerable body of work in completely reworking all the material relating to weights and loads, and for no actual operational benefit. So its unsurprising there was little appetite for taking on a considerable body of administrative work in doing all that.

So there was rather more to changing systems than just painting 5F instead of the letter D on the cabside. It didn't help that in GWR practice just about all locomotives (except the Kings) were effectively mixed traffic, so (for example) 28s needed to be classified to take passenger trains when the need arose, and Castles might be used on fast mixed freight turns alongside the 47s. I wonder too if the different design philosophies affected things. The small prairies have been mentioned, which under the BR formula were rated 4MT like the large prairies, but in GWR terms were class C, not class D like the large prairies. In the other example mentioned above both large Prairie and 5600 were class D on the GWR.

 

That is a very very strange formula. Free gas area needs to be roughly proportional to grate area, and there's a somewhat rougher proportionality between grate area and TE. So the formula would make the power classification roughly proportional to the cube of the grate area.

 

Probably not but GW aficionados nearly always take the bait when it is dangled.

 

something wrong with posters who set out to bait people.....

 

But you could say that for some a bit of fishing does not always require water.

With regard to comparing the Jubilee class with the LNER B17 the North British designed machine was built down to a weight limit but could produce good work in spite of the problems created by this. The boiler design remains well thought of and steaming problems were seldom an issue. As far as sustainable power output is concerned I can quote 1360 edbhp at 42.5 mph working at 45% cut off and full regulator for a period of 20 minutes under steady conditions. This was during a test in 1936. A new build, when it arrives, could answer a few questions.

 

I had wondered if the smaller grate area of the B17 might have been part of the reason for a lower classification? That test result seems to belie this though, given that the performance was sustained for 20 minutes. I wonder if there any comparable data for Jubilees?

Dave