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Discussion of steam locomotives from all manufacturers and railroads

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 #854302  by jgallaway81
 
A quick question... does anyone have a picture of the layout of the rods of a 3-cylinder?

I presume all were laid out such that the cranks were 120* from each other, but what was the layout of the all the rods of the internal cylinder-set? (not the valve rods, just the main and connecting rods). Obviously the main rod was set to a crank in the axle, but how were the other axles connected to the main driver? Or did the connecting rods for the two outside engines do all the connecting work?

Also, could an axle-mounted stephenson-style eccentric be coupled to a modern valve gear such as a baker or other similar design?
 #854393  by Eliphaz
 
The three cranks are 120 degrees apart. the outside cylinders work on pins on the outside of the drivers in the usual way.
The center cylinder acts on a crank.
These drawings show the motion of Raven S-3 Class 4-6-0 of 1919 for the Northeastern Railway
note that all three sets of eccentrics and Stephenson links are inside the frames as well as the center cylinder. Very neat appearance , but pity the oiler!
Inside cylinders and gear was common practice in Britain well in to the 20th century. Stephenson gear was most common, but others systems such as Joy and Walschaerts were arranged inside as well.
about the S-3 class http://www.lner.info/locos/B/b16.shtml
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 #854399  by Eliphaz
 
While I am on this topic, here are some details of Raven's T-3 class 0-8-0. note that the cylinders are inclined to clear the lead axle, and all three drive on the second axle.
about Raven's T-3 class : http://www.lner.info/locos/Q/q7.shtml
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 #855334  by timz
 
It seems a safe bet that the pistons in the three cylinders were intended to reach the end of their strokes at equal 120-degree intervals of driver rotation-- which would mean if the outside cylinders are horizontal and the inside cylinder is inclined (e.g. 9.5 degrees on SP/UP 4-10-2s and 4-12-2s) then the inside crank will be correspondingly shifted.

Gresley-Holcroft gear-- the 2-to-1 and 1-to-1 levers to operate the inside valve-- is intended to set valve events 120 degrees apart. Some other lever proportions could accommodate some other spacing, if desired, but no reason ever to do that?
 #855351  by jgallaway81
 
Well, if we were talking an engine that had ample clearances, could all three cylinders be inclined parallel to each other? I'm thinking a high-pressure (~310-315psi) cylinder in the center and two low-pressure cylinders outside.

Also, in this case, could axle-mounted eccentrics be used to provide the motion for something like baker valve gear?
 #855383  by Eliphaz
 
In both the Raven designs all three cylinders are inclined parallel. these are both single expansion engines.
On a three cylinder compound engine, you can have one high pressure cylinder in the center and two low pressure cylinders outside, in which case the three are nearly the same bore; or you can have two small high pressure cylinders outside and one whopping big lp cylinder in the center. both systems were employed successfully.
A shaft mounted eccentric strap like the ones shown above can certainly be used to provide the motion for a Walschaerts or Baker type gear, just a question of proportions. only need one per cylinder of course.
 #873638  by Juniatha
 
Hi, everybody - it's me - I'm back at last :wink:



A note on three cylinder engines


The regular case - three cylinder simple expansion:


As in most cases cylinders were given common bore and stroke for equal power outputs, cranks were set to give six equally spaced beats per revolution, that is: 3 x 120 ° - taking into account angle of incline of inside cylinder or inside minus outside cylinders if both were not horizontally mounted (for example due to clearance restrictions).
(And by the way - yes it was sure possible to mount valve gear such as Walschaert's or Baker or Stephenson's if you preferred between frames and have it actuated by either a small crank or an eccentric, too.)
Clearly, three cylinder engines should thus have regular exhaust beats just as two cylinder engines but with 6 instead of 4 beats / revolution. Smart Alecs discovering inclined position of inside cylinder have deducted that such an engine - allegedly having cranks set strictly at 120 ° - could never have regular beats because of that. Every-day practice of so-so maintained engines running with off-point valve gear events had seemingly 'proven' them right. They were not.
However, even with valve events tuned as best can be and sound mechanical condition a three cylinder engine - depending on design specs - may or may not have accentuated beat rhythm mainly because of the necessarily different exhaust conducts and shorter distance from exhaust at valve to blast pipe tip, overlaid by individual design deficiencies such as unsymmetrical arrangement of steam chest chambers at front / back cylinder ends. The former often lead to a somewhat sharper beat from the middle cylinder, creating a decided 3/4 rhythm sounding (while smoothly working) engine while the latter made for a more or less noticeable difference in accentuation of front / back chamber exhaust of the middle cylinder.
Some engine design were given somewhat shorter piston stroke for inner drive - again some with / some without compensation by larger cylinder bore to provide for / forget about fully balanced power outputs. An example of the latter case was the Union Pacific 4-12-2; slightly shorter inner stroke was opted for in view of shorter con rod which again was a consequence of driving second coupled axle while outer drive was to third. This dual axle drive arrangement by itself was to contain amount of super elevation of inner cylinder in view of limited height for exhaust conduct with desirable low blast nozzle position (distance nozzle tip to stack top was always lacking against ideal proportions of draughting with large engines). As concerns mechanical stresses it was sure less than ideal to drive to second coupled axle with six in a row because of bearing loads and summation of play in side rods and axle bearings from drive to last coupled axle. By default, dual axle drives in three cylinder engines were always submitted to oscillating loads and stress to axles / bearings / axle boxes / frames between inner and outer drive which tended to generate extra wear and consequential play which lead to ‘rattling’ in drives as engines ran – i.e. such engines depended on careful maintenance if they were to show smoother running over two cylinder engines – which theoretically was one big advantage of the 3 x 120 ° setting in combination with – basically - just 67% the piston loads or in fact output per cylinder for same engine output. It was this principal advantage which has largely mislead designers to believe the three cylinder SE engine type would also have reduced frame stresses – which by a closer look it does not do, in a nutshell: it just alters them.
Union Pacific and ALCO both had to learn the hard way about the inner secrets of the three cylinder dual axle drive with cylinder / frames bolted connections working loose. The last few of UP ‘Nines’ built with cast steel engine beds incl cylinders successfully proved to stand up to demands, however, UP had then moved on to the Challengers since doubling of drive sets looked more promising for ever rising performance demands than just adding one cylinder to the standard machine.
(European railways were in no ways suffering less from three cylinder working characteristics, yet – shucks – arranged for according classified repairs leaving things much as they had always been; on German state railways strict standardization anyways had effectively petrified steam loco design to a once established level – which had been tolerably progressive when put up in 1925 but was ageing as time went by).
Thus, in America the three cylinder type seemed dated and done with while its development scope for fast revving express locos in combination with integrally cast steel loco beds was allowed to slip unnoticed.


The special case – three cylinder compound:


Pardon me for skipping the Webb type (two small h.p. outside / one large l.p. inside cylinder).
To start with, equal output level in all cylinders was always desirable, no matter which type of expansion – only, it was much more difficult to reach in designing a compound engine and by default was more elusive in that it wasn’t automatically present and was impossible to realize over the entire working range of the locomotive. Concentration on vital working points was asked for: best leveling of torque output at high t.e. demands versus best expansion rates in the upper speed range. Both was difficult enough to combine and lack of it was one key to failure in a couple of these types of engines.
Design could be approached from three very different angles:

-a- crank settings 90 / 135 ° (+/- angle of cylinder inclination)
This type of engine started on l.p. cylinders only, receiving live steam through a starter valve or starting throttle, with the h.p. cylinder floating in steam with much the same pressures to both sides of the piston when working on cut-off at some 70 – 80 % in both h.p. and l.p. – thus effectively emulating a two cylinder machine. As speed began to rise, starting throttle or valve was being closed and cut-off was being brought in according to increasing speed (like working to valve pilot on the NYC).
Although this meant torque would become increasingly uneven per turn of wheels as the single h.p. cylinder gained power while that of l.p. cylinders settled, this particular characteristic of the three cylinder compound engine had little effect to adhesion as t.e. decreased with rising speed and t.e. peaks never exceeded peaks at low speed high t.e. Favorably, the extra torque provided by the middle cylinder was symmetrically divided to the left and right side of axles.
Still, this was a delicate engine type to design, demanding well balanced proportions, relying on sturdy frames structure to provide for smooth working or else it would produce slippery starts and uncertain acceleration. Undoubtedly, by far the best of all locomotives designed with this type of drive was the Chapelon 4-8-4, 242.A.1 which made test run after test run over principal main lines, passing and passing again with flying colors – only to end as a lone fighter allocated at Le Mans among the smaller 141.P class Mikados while the inferior PLM derived 241.P Mountains roamed the Ligne Imperial until electrification and kept on going to write steam’s last chapter in express service on the SNCF.

-b- crank settings 3 x 120 ° (+/- angle of cylinder inclination)
With these crank settings design aimed at gaining compound efficiency while retaining balanced running of simple expansion. Since nothing is for free in engineering, clearly this left design to choose between cuts at either end or both. While it was not too difficult to arrange for level cylinder outputs in both h.p. and l.p. at starting effort by suitable cylinder volume correlation and valve gear settings, what happened at speed as cut-off was brought in was another story. To attain favorable working at short cut-off in the upper speed range often called for modification of the low speed working. For one example, the l.p. / h.p. volume ratio desirable for balanced working at low speed high t.e. was too small for best expansion with short cut-off at speed – or, if a ratio well suiting expansion was chosen, such as 2.5 – 2.8 depending among other parameters on live steam pressure and temperature, then it only yielded a leveled t.e. in both h.p. and l.p. cylinders at low mean pressure in the l.p. cylinders – i.e. the locomotive lacked t.e. and had to live as a lame starter.
This also affected some less lively classes of de Glehn type four cylinder compound engines with inside l.p. cylinders, although for different reason: providing adequate volume for l.p. cylinders between frames was not a simple task, even with the lighter European axle loads and consequently lower t.e. demands. Design seeked compensation by providing ‘full four cylinder simple working’ for highest t.e. demands, i.e. not only was live steam of up to ¾ b.p. admitted directly to l.p. cylinders, exhaust of h..p. stage was then directed to blast pipe instead of receiver, thus taking away receiver pressure from h.p. cylinders exhaust, increasing their work. Clearly this sort of ‘all out’ working was not possible in a three cylinder compound of any crank setting or it would have upset torque, leading to wheel spin.
However, three cylinder compound types naturally had their l.p. cylinders on the outsides so that ample cylinder volume could be provided for adequate t.e. Yet, with valve gear design as it mostly was, ample cylinder volumes tended to result in excessive throttling of steam flow, thus deflating mean working pressure showing loops in diagrams at upper speed range.

-c- crank setting 105 / 127.5 ° (+/- angle of cylinder inclination)
Strange as it may seem at first glance this is simply a setting midways between the preceding two variants. It was proposed by Chapelon but dropped for the 242.A.1 in view of possible irritation of drivers due to the resultant somewhat ‘hopping’ exhaust rhythm (not quite like a three cylinder simple minus middle exhaust). It aims at a compromise between the advantages and disadvantages of the preceding two. In comparison with -a- it provides for lower torque peaks with short cut-off while compared with -b- it helps to spread l.p. torque at low speed long cut-off working when it tends to surpass h.p. torque thus keeping t.e. more leveled out over a turn of wheels.

Look at it from any angle you like – for me nothing compares to a four cylinder compound engine showing speed potential by compact h.p. cylinders, piston valve gear equipped with ample steam chests, while l.p. cylinders are snugly fit between frames complete with independent poppet valve gear and above that a slender boiler of better than 300 psi live steam pressure.

= J =