LMS 6202, the "turbomotive"

While much of what you say is true, the ability to work as well in reverse as it can forward was very desirable in an express engine, even if not so much as in a goods engine. On arrival at certain termini, Euston or Lime Street for instance, it was common practice for the incoming engine to bank the empty stock back up the gradient to the carriage sidings. It was hardly reasonable to expect a Jinty or coal tank to take sixteen or seventeen coaches from a standing start up a 1 in 70 gradient. The train engine banking it allowed one to do this, but when 6202 was on the train, an 8F was commonly sent to Euston to remove the stock. Or so I've heard, anyway!

There is also the problem of a possession en-route, where the train's running line might be under repair and wrong line working implemented. This would involve the train passing a trailing crossover, then the engine propelling it back through the crossover on to the opposite road. This would require the engine being placed in reverse, and could cause the Turbo not a little embarassment if she then couldn't get it to move!

 

The diagrams are very helpful; I have never seen most of them before.

http://www.dself.dsl.pipex.com/museum/locoloco/turbom/turbom.htm

The reverse turbine is 4 row, compared to 18 row for forward turbine, though it goes through a further reduction gear of about 2 : 1 judging from the last diagram. The forward turbine is rated at 2400hp, unfortunately no rating is given for the reverse.

 

Rear sanders

Sorry, but rear sanders were fitted to Princess Coronation and Princess Royal classes. I don't think they come any bigger than those.

 

It is not all past tense - the Princess Royal class boiler 6202 carried (as the turbomotive) in the 1930s survives on 6203.

 

This would be boiler No. 9100. It was a 32 element, domeless boiler (although a dome was later fitted) put on to 6202 in June 1935 when she was built. It was soon realised that higher superheat was desirable for turbine propulsion, so a new boiler with dome and 40 elements was built and fitted, this being No. 9236. She carried this engine for the rest of her working life, even after her rebuild. 9100 was then used on 6203 (12/36 - 11/38); 6205 (6/39 - 11/42); 6212 (6/43 - 12/45); 6207 (5/46 - 3/49); 6211 (5/49 - 11/52); 6206 (2/55 - 1/58); 6203 (6/58- present)

9236 was used on other Lizzies while 6202 was out of service: 6210 (9/43 - 8/44); 6204 (8/50 - 5/52). After repair, it passed to 6212 (1/54). I have no record of it appearing on another loco, nor of this engine receiving a replacement boiler. But as she wasn't withdrawn until 11/61, it is very unlikely that she carried it to withdrawal, as she received a Heavy Intermediate (HI) from 3/10/55 - 18/11/55); another HI 4/3/57 - 6/4/57; and two Heavy Generals: 1/9/58 - 12/11/58 and 12/11/59 - 19/1/60. A 'Heavy' usually involved a boiler lift, and while it wasn't mandatory, it seems improbable that it didn't happen on at least one of the four occassions.

 

I believe there is a active preserved 2-8-0 that is turbine powered in Sweden. Its not as elegant as 6202, but must be interesting to see and hear in action.

 

Wonder if it sounds like a SAR Class 25? Weirdest thing I heard - no blast, just a sort of whining roar, a bit like an aero engine as you might expect.

 

Despite the acknowledged weaknesses of the reverse turbine, there is a published photograph (RCTS Journal?) of 6202 storming up Cambden Bank, in reverse, with a good load on, presumably ecs!

 

If it's the one I'm thinking of, it was taken at Bushey rather than Camden with nine bogies in tow, and I'm not sure that 'storming' is the right term!

It was intended to move the engine to and from the shed, rather than to work a train. It is asumed that this particualr working was simply to see if this were possible

 

Turbomotive references

The two references given earlier in this discussion for the Turbomotive are slightly wrong. LMS Journal is published by White Swan not the LMS Society. White Swan does not have a website.

Another reference is The Journal of The Institute of Locomotive Engineers Paper 458. This is the record of the lecture given to the Institute by Robin Riddles on the first ten years of the engine on 30th January 1946. Unfortunately the paper doesn't record the power output of the reverse turbine, but it does record that when, after recording 9,165 miles in service, the clutch broke at Liverpool on 24 September 1935. The clutch was redesigned and the turbine was modified to give greater power because there had been complaints about this. Bearing in mind that the LMS used, for ECS and banking, Cauliflower 2F locos at Euston until the mid-forties when they were replaced by the Midland 2F, it would be reasonable to assume that the intention was that the reverse turbine would have at least the same power output as a Class 2 engine.

Regards

 

Taking up 2968's point about drivers, I look at the long frame base of 6202 in profile and can't help wondering how it would have performed as a 4-8-0 or even 4-8-2, using 56½ inch drivers instead of 78 inch. These would have been available to Stanier in 1933 from the Fowler 7F 0-8-0 parts stock, and from 1935 they were incorporated into the design still so familiar to us as his 8F. But I wonder if they had the techniques in those days to make a product (wheel-axle unit) that spun almost as fast as an express tender or coach wheel but bore much higher loads, and had to run for xx thousand miles between maintenance. Perhaps someone with more knowledge of engineering history could comment.

It's all might-have-beens, but fascinating nonetheless.

 

Don't forget that 6202 had roller bearings, so high rpm should not have been a problem.

 

The Fowler 7F wheels were to a very different design to the Stanier 8F wheels. All Stanier wheels had a built-up triangular rim which was intended to strengthen the wheel. The tyre fixing was different too.

Part of the idea of having a Pacific was to be able to test it against the other Princess Pacifics, so as to get an understanding of the differences between the two concepts, one of the conventional reciprocating engine, the other a non-condensing turbine.

Regards

 

On second thoughts, Stanier wouldn't have needed Fowler 7F wheels at all. The first of his 7Fs (later redesignated 8F) was rolled out in 1935, when 6202 was already an active project, though not yet built. I take Orion's point on why it was a Pacific, though - take a machine that's already in series production, and change only enough in it to enable the new technology to be fitted. The cheapest option, which in this case so nearly succeeded.

It would be so interesting to read Stanier's own thoughts about it. Perhaps they're in some archive at the Inst Mech Eng, FRS or even the NRM.

 

Stanier gave his thoughts on the Turbomotive in the discussion section of the Riddles paper I mentioned in an earlier post. I have tried to upload the files but the upload stops quoting database problems. He doesn't say very much, in fact his comments are very superficial.

In reply to an earlier post re the size of coupled wheels. In any locomotive with a mechanical drive it's important to have a size of wheel which relates to the top speed speed of the engine, just to reduce the forces which are inherent in the rotating mass of the coupling rods (for the Turbomotive) and also the reciprocating masses (in the conventional loco). If the final drive is electrical then the size of wheel can be reduced to the sizes that can be seen everyday on the railway network.

Regards

 

Only partially true! This discussion has perhaps reached the point where its rightful place is in the Mutual Improvement Classes, and the moderators might consider a move?

To get back, there are two issues: balancing the rotational and reciprocating masses. The rotational ones, which entirely incorporate the coupling rods and crank pins, are easilly dealt with. The formula to calculate an out of balance force is F = MxW^2xr, where F is the out of balance force; M is the out of balance mass; W (omega) is the angular velocity in radians per second; and r is the radius of the out of balance mass from the wheel centre. Note that W is a squared function; doubling M or r doubles the force, but doubling W quadruples it.

Rotational balance is easy to achieve: a proportionate weight is simply attached to the opposite side of the wheel to the out of balance mass,the weight and radius being made proportional. Angular velocity will be constant, since both the balance and imbalance forces are on the same wheel. The only issue is that the structure, wheels, crankpins, etc., must be made strong enough to resist the centripetal forces preventing the coupling rods from breaking free.

The difficult issue is reciprocating imbalance. As the piston, piston rod, crosshead, front one third of the connecting rod and, in the case of the Lizzies, union link and lower half of the combination lever, move backwards and forwards, they attain momentum, which is transferred through the connecting rod to the wheel. This is in turn forced backward and forward in the axlebox, which is likewise forced backward and forward in its horn guides. This can be partially overcome by adding weightss to the wheel rim, so as the reprocating masses force the wheel one way, the balance weight forces it in the opposite direction, so balancing it out.

This though leaves a rotational imbalance: this new weight is balanced in the horizontal plane by the reciprocating masses, but by nothing in the vertical plane. So as it rises each revolution, it tries to lift the wheel, then slams it back down as it comes downwards. This is hammerblow.

It is desirable therefore to provide recipracting locos with large wheels, which give a lower rpm (and angular velocity) for a given speed. But a turbine has no reciprocating masses to be balanced, so rpm need not be restricted and the wheels can be run at a far higher rpm without inflicting mechanical damage, hammerblow, and all the other issues inherent with cylinder propulsion. This is why 6202's axle weight was allowed to exceed 22.5 tons which was the usual LMS limit, and the figure of 24 tons was mentioned at the design stage.

As to keeping the same configuration as the Lizzies, this allows a direct comparison to be made between otherwise identical engines, only the turbine v cylinders need be looked at. But the point I was making was that this rather restricted the turbine's performance, so a fair evaluation of the turbine itself wasn't possible.

 

It was precisely because I didn't want to turn this matter into an MIC that I skated over the details. Much of what you say is true although you do ignore some items.

Because there wheren't any reciprocating masses on the Turbomotive the issue of hammerblow didn't arise which was why the Chief Engineer was willing to accept a greater static axle weight.

I don't accept, and I'm quite sure on reading through Robin Riddles paper again he would accept either, that the turbine didn't perform as expected or that it wasn't given a thorough test. There were issues with lubrication and the use of white-metal bearings in the turbine and gearbox but the concept and its realisation worked just fine.

To return to the issue of the coupled wheels, one of the reasons why a mechanical drive loco needs to have driving wheels that are related to its duties is the matter of the coupling of forces. Although the expression you have offered above is true in theory it doesn't take into account the existance of engineering clearances or wear. The coupling rod behaves in a much more uncertain manner due to these clearances and wear in the bearings, hornblocks and, not least, the variable diameter of the tyres. These force the rods into a motion that will generate in time create a 'couple' which will tend to make the engine roll. That extent of that roll can be controlled to a degree by the choice of wheel diameter, because the wheel diameter has an influence on the amount of wear to be expected over a period of time.

Engineering isn't simply a question of learning mathematical expressions in a classroom. Passing the exams is just the start; experience is all.

 

Now there's a comment with barbs in it! In my experience, which is more than you might think, practice does follow what theory says; if it doesn't, it usually means that there are factors which haven't been considered, or at least, not fully so.

"Passing the exams is just the start; experience is all." No, they go hand in hand. For the record, I have both, having done the classroom bit as a student gaining the Honours Degree in Engineering, and am now standing at the front of the class teaching it. But the classroom work started AFTER twenty five years of practical spanner wielding on all sorts of different machinery, including steam locomotives.

Having got that little rant over, I'll admit that there are issues with high rpm, even today. The small-wheeled Virgin Voyager and Pendo sets suffered from bearing problems on introduction, and that was with roller bearings everywhere. But I still maintain that 6202 did not need wheels 6'6" diameter with turbine drive, other than to maintain parity with the other Lizzies. Reducing to 5'0", as in the later 9Fs, would have allowed the fitting of an extra axle and lower axle load. The 9Fs proved capable of 90 mph running, which was about the maximum expected of 6202, and while they were soon banned from such passenger work, this was due to cancerns about piston speeds rather than coupling rods. While understanding that, as a prototype, 6202 had to conform to her half sisters' specifications, I feel that production locos might have been a little more adventurous in their wheeling.

 

2968, I have a question. If the will to experiment further had been there, would it have been possible to convert 6202 to a 4-8-0 or 4-8-2 arrangement using smaller drivers, or would the LMS have had to build another loco from scratch, by which I mean with different frames?

 

It would be started from scratch, I believe. 6202 was very much a prototype, and I'd suggest that a Mark II would be a very different engine incorporating all the lessons learned - which is what a prototype is for.

Let's be fair; 6202 was a very good engine in it's own right, but there were problems: the reverse turbine's low power output (which isn't the same thing as low speed tractive effort) has been discussed, and there were certain weaknesses in the transmission. But a reason was that she was based on a piston-driven loco, not all parts of which were suitable for the different power source. I suspect that any follow up would start with a blank piece of paper, whereby the turbine's attributes could be more fully explored.