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Imaginary Locomotives


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On 20/10/2021 at 14:22, 313201 said:

Very feasible,  the Brittania locomotives were 2 cylinder pacifics

I realised that just after I logged off!

 

Why were two cylinders not used before the standards though?  Was it only feasible because of the higher boiler pressures?

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Pacifics in the UK were regarded as express passenger engines in the inter-war period, and it had been established by then that express passenger locos needed 3 or 4 cylinders in order to keep the outside cylinders within the loading gauge, reduce loading on the individual big end conrods and associated cranks, and provide a smoother ride for the passengers with less 

‘surge’. 
 

By the 1940s, ‘light pacifics’ were beginning to appear with driving wheels of a size more in line with mixed traffic work, though the reduction in driving wheel diameter was a trend anyway.  These were 3 cylinder engines, but the post-war labour shortages generated a need for locos that could be more quickly and easily prepped, so it was considered desirable not to have to go between the frames to oil around.  Swindon, bless ’em, carried on building 4-cylinder express locos regardless…
 

For this reason, the spec for Riddles’ Standards was 2 cylinders, and all of them were to be capable of mixed traffic work with the exception of the 9F, which acquitted itself pretty well on passenger duties,  Britannia and Clan pacifics were thus 2-cylinder mixed traffic ‘light pacifics’ of a general type that had been common on heavy outer suburban work for over 20 years in the US, but previously unseen here. 
 

71000 was to use standard components as far as possible, including the 6’2” driving wheels, but was conceived as an 8P express passenger loco.  In that sense, it is perhaps best considered as a one-off, by definition not a ‘Standard’ and never intended to be the prototype of a class.  

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Which are pretty reasonable objections.   Even over here, width was a consideration.    Most of the American builders were in the North-East.   I forget which locomotive class it was, but at least one of the superheavy-types built for the Western states had to be delivered with its cylinders off, for clearance east of the Mississippi.   

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The prime operating advantage of a pacific, derived from atlantics, is that a large boiler can be steamed from a wide firebox, able to burn lower grades of coal that may be more easily and cheaply available.  This is seen in larger 4-8-4 types used abroad as well, and the wide firebox advantage is carried through to oil burning.  The use of high grade Welsh steam coal is often correctly cited as one of the reasons the GW produced 4-6-0s instead of pacifics.  A large free-steaming boiler is a very useful thing if you want to run heavy passenger trains at high speeds over long distances, as it is able to produce steam to satisfy sustained high demand from the cylinders. 

 

Driving such a loco is a balance between using the steam and emptying the boiler of it, and sustained high output on a loco with large cylinders and an insufficiently large or inefficient boiler will lead to shortages of steam and loss of pressure, which can only be rectified to a limited extent by the fireman or mechanical stoker.  Of course, if high output is required for a short period, typically climbing banks, the boiler can be mortgaged and the mortgage paid off on the downhill run the other side of the summit.  OTOH, the combination of a large boiler with cylinders too small to handle it's output will lead to pointlessly wasted steam, coal, and water, and will be regarded as not successful, especially by the accountants and the firemen.

 

Express steam locomotives are particularly interesting because they are inevitably compromises between the demand for higher continuous power on one hand and the physical limitations of axle load, loading gauge, human firemen, or mechanical stoking on the other.  Getting this balance right, particularly with the UK demand for high power within route availability and loading gauge restrictions, is a fine line, as the operating costs of the loco have to be taken (literally) into account as well.  Express passenger work in the days befor motorways and high percentages of car ownership was a sound earner for the railways, but the locos still had to earn their living; the purpose of a railway is to provide profit for it's shareholders and carrying freight or passengers is a secondary consideration and only a means to that end.

 

Similar interesting compromises can be found in Blue Riband Transatlantic Ocean Liner design, where the space allocated to engine rooms and machinery has to compete with space for service areas, passenger cabins and large luxury areas for fisrt class in a hull shape suitable for 30knot+ all weather operation, and in post WW1 battleships, where an elusive optimum balance of speed, armour, and firepower had to be found within the treaty tonnage limits, though not all countries played the game strictly to the rules. 

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3 hours ago, The Johnster said:

Pacifics in the UK were regarded as express passenger engines in the inter-war period, and it had been established by then that express passenger locos needed 3 or 4 cylinders in order to keep the outside cylinders within the loading gauge, reduce loading on the individual big end conrods and associated cranks, and provide a smoother ride for the passengers with less 

‘surge’. 

 

I once read the first few pages of Chapelon's book (in English translation) and I recall that he was unconvinced that more than three cylinders were needed in British conditions.

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Gresley and Bullied clearly agreed with him, but the GW maintained the de Glehn 4-cylinder divided drive setup until the final delivery of Castles in 1950, and Ivatt was still building pacifics to this layout in 1947.  The LMS seemed to specialise in 3-cylinder 4-6-0s during the inter-war years, but never build a 3-pot pacific.

 

I suspect the cause of 3-potters on the GW was not helped by being taken up by Prof. Tuplin, who nobody takes seriously...   An issue with 3-cylinder engines is arranging the inside cyldinder valve gear to be driven by the outside valve gear motion, which is why BritCaprotti was used on 71000.  With a 4-potter, it is easy to arrange Walchearts or Stephenson valve gear in pairs, but 3 pots mean that care must be taken in the valve settings in order to provide a reasonably even exhaust beat and smooth power delivery.  The rather groovy noises made by high mileage Gresley locomotives illustrate this very well, conjugated apparently meaning sychopated, it don' mean a thing if it ain' got that swing.

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Here's a thought (probably repeated somewhere in the last 350 pages.)

 

Would there have been potential in a steam-motor setup?  Could a bank of multiple cylinders, with an overall greater volume than would be possible with two-, three-, or four-cylinder conventional drives, attached to a jackshaft, provide higher potential output inside a given loading gauge?

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24 minutes ago, AlfaZagato said:

Here's a thought (probably repeated somewhere in the last 350 pages.)

 

Would there have been potential in a steam-motor setup?  Could a bank of multiple cylinders, with an overall greater volume than would be possible with two-, three-, or four-cylinder conventional drives, attached to a jackshaft, provide higher potential output inside a given loading gauge?

Some of the later Sentinel steam railmotors had up to 12 cylinders IIRC.

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When you consider the extreme sophistication of the complex shaped GWR rockers in order to make for as accurate as possible valve events it seems unlikely they would have been happy with the compromises forced by 3 cylinder implementations. 

 

Cook says in his book "GW locomotives had extreme regularity in their exhaust beats and we certainly could not permit of anything like two beats and a wooffle which was noticeable on some rival lines"

Edited by JimC
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21 hours ago, The Johnster said:

I’m a bit worried about the rear driving axle, which goes through the ashpan or the lower part of the firebox.  The coupled wheels need moving rearwards so that this axle is behind the firebox, which aligns the front driven axle with that of the original single driver’s, and reduces the rear overhang.   Might mean the motor intruding into the cab, though.  

True, but I can assure you about where the motor goes. It fits like a glove and it perfectly fits in fine. Would it help if I shared with you photographs of the chassis on its own?

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2 hours ago, Miss Prism said:

Not imaginary, but some very stupid and ugly GWR specimens are explored in the GWS Going Loco blog. authored by our @didcot. Always worth a read.

 

Some good stuff in the first blog (haven't had time for more yet), but the largest driving wheels on a British steam loco were not the 10 footers on 'Hurricane' but the 10'6" monsters of the LNWR's Cornwall, which still exist on the loco.  In the loco's original form the boiler was carried beneath the driving axle to enable the loco to feature such large drivers.  The idea was that larger wheels would improve the ride and enable slower piston and bearing speeds at higher running speeds, a major concern in those early days being the effectiveness of the lubricants.

 

Hurricane might have worked a bit better, and it could hardly have been worse, had the water been carried above the 'engine' section of the cavalcade, and there was no reason for it not to have been..  Same goes for Thunderer, but there would have been less room for it with all that clockwork in the way.

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9 hours ago, Miss Prism said:

Not imaginary, but some very stupid and ugly GWR specimens are explored in the GWS Going Loco blog. authored by our @didcot. Always worth a read.

 

Very good, but I can't help think it's a bit unkind to the Krugers. They were, as stated, experimental, and not anywhere as unsuccessful as the other two. And the Aberdares were directly descended from them, and were very successful engines, lasting for not far off half a century. Not great lookers, I suppose, but there's a certain chunky, no nonsense air about them.

Looking at Hurricane & Thunderer though, they do look a bit like an early attempt at a Garrett. If only IKB had hit on the idea of having drivers at both ends, and the weight of the boiler carried on them........

Edited by rodent279
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It is a bit misleading to blame IKB for Thunderer and Hurricane, which he signed off on but were actually designed by a bloke called Harrison.  He was at least attempting, albeit hoplessly unsuccessfully, to solve a major problem of the time, and criticism should bear this in mind.  Even then, though, it should have been pretty obvious that these locos had no tractive weight and were unlikely to work; I can well believe that they were barely capable of moving their own weight on anything but the driest rail, and the jointed steam pipes must have been a 'mare to deal with.  It is actually quite difficult to make a steam locomotive that doesn't work at all, but these must be close contenders for the title.  And I still think water tanks and ballast on top of the engine part of Hurricane would have made her a potential success, though the jointed steam pipes would have still been a problem.  We'll never know now; she's an unlikely candidate for a replica unless you power the carrying wheels beneath the boiler by diesel or electrickery...

 

100mph?  Highly unlikely, but something fairly fast by the standards of the time might have been possible in favourable conditions, by which I mean experienced driver (rare as rhs in those days, but apparently he was a Stockton & Darlington man), dry rail, light or no load, strong  following wind.  He must have had nerves of steel; a loco that difficult to get moving is next to impossible to stop for the same reasons.  Speed assessment was pretty subjective back then, few people had accurate watches that could withstand the shaking and rattling, and the firm baulk road would have created a fairly strong impression at 70+.

 

Wind forward to the Gauge Commission, a very revealing read concerning actual operation and the attitudes of the big name engineers giving evidence, and we have McConnel from the LNW's Southern Section, he of 'Bloomer' fame, stating that he could easily provide a locomotive that could reach 100mph if it was required, and that speeds close to that had already been reached on his railway.  He was, of course, giving evidence to show that the broad gauge had no speed advantage in particular over the standard, but one assumes that he was being truthful to the best of his knowledge.  100mph on the rougher permanent way of the day, with unbraked vehicles and on cast iron leaf springs, would have been highly dangerous and the ride would have dissuaded most drivers from trying it, notwithstanding that they were a pretty tough and fearless breed!

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I'd like to see some replicas of some of the earlier locos, not that early, but maybe from the 1860-80 period. A working replica of a Dean single, an LNWR Jumbo etc. I guess mainline work would be very limited, due to the loads and speeds they could reach.

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You might be surprised at some of the speeds.  The GW were running services timed for 70mph top speeds with the Iron Duke class as far back as 1850, and 60mph average point to point timings were common by your period.  Loads were light, and passenger locos were free-running singles with small (by our standards) boilers, low centres of gravity that rode well enugh to encourage fast running.  Much of what is claimed for the era is subjective and often exaggerated, whether consciously or not, but the quaint looking little engines do not necessarily indicate the bucolic pace of life we associate with their later use on branch line ambles.  

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6 hours ago, The Johnster said:

It is a bit misleading to blame IKB for Thunderer and Hurricane, which he signed off on but were actually designed by a bloke called Harrison.  He was at least attempting, albeit hoplessly unsuccessfully, to solve a major problem of the time, and criticism should bear this in mind. 

A good point. There's nothing particularly unusual in a client putting forward a specification that looks impossible in the hopes that the supplier will know more than they do. I recall in my brief industrial chemistry days my employers were asked for a chemical stripper that would remove nylon coating from aluminium zip fasteners so that those that failed QA could be recoated. I wasn't allowed to send out my proposal to dissolve out the aluminium and recover it from solution... Mind you who knows, 40 years later maybe that's possible.

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10 hours ago, rodent279 said:

I'd like to see some replicas of some of the earlier locos, not that early, but maybe from the 1860-80 period. A working replica of a Dean single, an LNWR Jumbo etc. I guess mainline work would be very limited, due to the loads and speeds they could reach.

 

No need for a replica Jumbo:

 

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Neither the Improved Precedent nor the Dean Single is from the period you mention, both being products of the 1890s. 

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Without wishing to reject anyone's enthusiasms for some really good historic locos, shouldn't this start with a specification of what the present need is for new steam engines? At present I'd say most heritage railways need to haul up to 8 (30-ton?) coaches at no more than 25 mph on branchlines that vary in ruling gradient from 1:100 to 1:50 (yes, the West Highland Extension has a short stretch that's even more severe). That's about a 300 ton load (including the loco), requiring 450 hp for 1:100 and 900 hp for 1:50 just to overcome gravity. Stephenson's approximation was that 75% of power was used by gravity for a 1:100 rising grade, with the rest on rolling resistance and acceleration, so the gravity-only isn't too bad a starting point for loco power.

 

I fear the older locomotives just don't have the power, beautiful and intriguing as they are.

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10 minutes ago, DenysW said:

Without wishing to reject anyone's enthusiasms for some really good historic locos, shouldn't this start with a specification of what the present need is for new steam engines? At present I'd say most heritage railways need to haul up to 8 (30-ton?) coaches at no more than 25 mph on branchlines that vary in ruling gradient from 1:100 to 1:50 (yes, the West Highland Extension has a short stretch that's even more severe). That's about a 300 ton load (including the loco), requiring 450 hp for 1:100 and 900 hp for 1:50 just to overcome gravity. Stephenson's approximation was that 75% of power was used by gravity for a 1:100 rising grade, with the rest on rolling resistance and acceleration, so the gravity-only isn't too bad a starting point for loco power.

 

I fear the older locomotives just don't have the power, beautiful and intriguing as they are.

 

Well, that's true for a certain idea of what a heritage railway should be. For myself, a far more interesting day out than a large locomotive on eight Mk 1s or Mk 2s would be a ride in some reproduction 42 ft radials behind a reproduction Precedent (or equivalent for some other pre-grouping company) - at least trying to give the experience of 19th century travel. I guess that's why I prefer narrow gauge lines such as the Lynton & Barnstaple or Talyllyn to many of the standard gauge lines.

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