• Eccentic Question - Mikes vs. Mountains

  • Discussion of steam locomotives from all manufacturers and railroads
Discussion of steam locomotives from all manufacturers and railroads

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  by Cactus Jack
This is probably a basic question, but why on a Mountain (4-8-2) is the eccentric on the 2nd coupled driver (at least from all the photos I have noticed) and on a Mikado and Consols on the third coupled driver? Each having 4 drivers.

As a designer, how is the bore and stroke determined ? In other words what calculations go into it based on other variables which might include boiler pressure, heating surfaces, boiler size, driver diameter, and what the locomotive is meant to do functionally for the operating department. NKP Berks appear to have longer strokes than some locomotives which is why ? Less dynamic augment as a factor of a longer drive ride mitigating vertical forces ? More power per stroke ?
  by Allen Hazen
Preliminary guess about the first question (I'll try to get back with more informed responses when I have a chance to check some sources).
Mountains tend to have bigger driving wheels than Mikados. You don't want the main rod (the piece that's connected to the end of the piston rod at one end andtothe eccentric on the main driver at the other) to be too short (it would be at too great an angle from the horizontal when the eccentric is at the top or bottom) or too long (it would be too flexible or too heavy), and getting the optimum length requires going back the diameter of an additional driving wheel with the smaller wheels. (Don't have any documentation on that being the actual reason, but it SOUNDS plausible to my -- amateur's -- ear.)
  by timz
Theoretically, a longer main rod means a more even turning moment. It shouldn't make a huge difference, but maybe that's why freight engines tend to have longer main rods. Presumably that's why some? most? 0-6-0s drive the third axle instead of the second.

95%? of 4-8-2s drove the second drivers, but N&W, C&O, DRGW, DL&W, NH had 4-8-2s that drove the third drivers.

None of us has a clue why some 69-inch-drivered 2-8-4s had 34-inch stroke and some had 32-inch.
Last edited by timz on Thu Feb 28, 2013 1:31 pm, edited 2 times in total.
  by Allen Hazen
Timz got in before me! I was going toad that the firstAmerican 4-8-2 type, the Chesapeake and Ohio's J-1 of 1911, had drive onto the third drivers. I'm tempted to take that as supporting evidence for my "theory" that considerations of the length of the main rod were important here: these C&O locomotives had 62-inch driving wheels, a size more typical of 2-8-2 than of 4-8-2 locomotives.

Berkshires are a problem for me, though. Small-drivered 2-8-4 (the original Lima design from the 1920s, and also the P&LE's engines at the very end of steam, both of which had 63-inch drivers) make sense: until you get back to the firebox they are very much like heavy 2-8-2, and have the drive onto the third drivers, as one would expect. But the high-drivered Berkshires (the Erie's 70-inch drivered machines and the 69-inch drivered Berkshires of the Nickel Plate and many imitators) also have drive onto the third drivers, despite having driving wheels as large as those on many 4-8-2. My thoughts on this: (1) the cylinders are set further forward on a 4-8-2 (cylinders tend to be centred over the middle of the wheelbase of the 4-wheel leading truck), so the main rod of a 4-8-2 would be a bit longer than that on a 2-8-4 with the same driving wheel diameter if they both drove onto the same driving axle, and (2) Berkshires at least sometimes had longer cylinder strokes than Mountains, so (since the eccentric on the main driver has to be as far from the centre of its axle as 1/2 the cylinder stroke), the angle above or below the horizontal that the main rod would assume at mid-stroke would tend to be a bit greater on the Berkshire than on the Mountain, other things (wheel diameter, choice of driving axle to drive on) being equal.

More anon.
  by Cactus Jack
Intersting and thoughtful comments, and many thanks. Keep them coming !

These are just a few things I had really not thought about until recently and then was not sure of the "whys" of what I was seeing.
  by Steffen
Hello Kaktus Hans,

the most serious problem is the balance of the weights. Rotation causes the weight of the crankpins and the rods to lift the wheels from the track on moving upward in speed, or increasing the weight by downward movement.... thus you have counter weights on the wheels, just to equal those forces.
But a long drive rod will have a motion power in weight, thus pushing the wheel into the bearing and creating a wanking motion on the drive wheels... also a long drive rod won't be able to deliver strong forces, because it starts slightly bending on brute forces by heavy loads, also on high speeds the end on the crankpin will cause a wip effect on the down and up zero position, causing cruel forces on the crankpin bearing.
Short drive rods won't create that high forces... But, a long drive rod will deliver better power from the cylinders, even if there is more effect on bending, but with the crankpin closer to the center and smaller wheels the angle of the drive rod is smaller that for short rods with large wheels - thus the pushing and pulling of the piston will be more and effectifly transfered to a crankpin... so small wheels and long drive rods generate huge power transfers, better than short drive rods.

So this is a difficult and very important thing to consider... and thus... it depends on force, on power and speeding ability one want's to have the locomotive had to have.
  by Cactus Jack
I note that EAR&H 59 & 60 class Garretts had the eccentric on the #3 driver too.
  by Pneudyne
To generalize somewhat, I think one could say that in American practice, locomotives with four-wheeled pilot trucks drove on the second axle, whilst those with two-wheeled pilot trucks drove on the third axle. In each case that was likely the best trade-off between conflicting requirements, such as avoiding undue angularity of main rod movement on the one hand, and avoiding excessive mass on the other.

As already noted, there are exceptions. But some of these tend to reinforce rather than contradict generalized practice.

The SP and UP 4-10-2s and the UP 4-12-2s had their outside cylinders driving the third, not second axles. But in each case, the locomotives had extra-long guides and piston rods that put the crossheads in more-or-less the same position as would have been the case had they had two-wheel pilot trucks and conventional length guides. Essentially they were equipped with four-wheel pilot trucks less because they were perceived as necessary for the desired road speeds, than because they were needed to carry the extra weight of the three-cylinder front ends. Still, UP was convinced of the merits of the four-wheel pilot truck for freight locomotives, and to a first approximation, its “small” Challengers were a “bent” version of the 4-12-2. The small 4-6-6-4 shared the same grate area as the 4-12-2, which tended to run against the idea that there was a sharp division between locomotives with two-wheel trailing trucks and those with four-wheel trailing trucks, the latter being a necessary (but not sufficient) condition for application of the “Super Power” moniker.

And the Challenger had third axle drive for both engine units. For the trailing unit, that aligns fully with the generalization, as the cylinder placement relative to the drivers was akin to that of a locomotive with a two-wheel pilot truck. For the leading unit, third axle drive was achieved through the use of extended piston rod and guides located somewhat rearwards of the cylinders.

A wider look at articulateds generally and the Pennsy duplexii will, I think, tend to support the generalization.

Genuine exceptions included the low-drivered 4-8-2s with third axle drive, as already noted, including the relatively late Bangor & Aroostock example.

2-6-2s had either second or third axle drive, the former usually associated with drivers above 69 inches (and reputedly not very stable in a yaw sense) and the latter with 63 inch drivers.

4-4-2s had either front or rear axle drive; in the former case with extended spacing between trailing pilot truck axle and the leading driving axle.

2-6-0s usually had centre axle drive, but with longer driving wheelbases than other 6-coupled types with similar driving wheel diameters. E.g. compare the SP 2-6-0 with the Milwaukee 2-6-2.

Re the 69 and 70 inch drivered Berkshires, third axle drive is consistent with the generalization; for example the rod layout essentially aligns with that of the rear unit of the UP 4-8-8-4.

That Berkshires were a problem might be a wider issue than just the drive axle choice, at least for the 63 inch drivered set. Notwithstanding the “hype” associated with the Lima A-1 and its progeny, it does not escape notice that their fate on several roads that purchased that type is not quite consistent with what might be expected for a “wonder machine”. Thus for example the B&M opted for a big 4-8-2 rather than more 2-8-4s in the post-depression era. The IC also opted for the 4-8-2 type, and in its major post-depression rebuilding program, seemed to be a bit diffident about what to do with its fleet when the 4-6-4 conversion idea did not work out, whereas rebuilding its large 2-8-2 fleet in kind seemed to be a key activity. The Mopac rebuilt some of its 2-8-4s into 4-8-4s. And the Santa Fe seemed to be less sure as to where its 2-8-4 fleet fitted in later times, so it escaped any major modernization.

I suspect that the problem was that the 2-8-4 was more powerful than a 2-8-2 of similar adhesive weight, which meant that it would balance at higher speed with any given train weight, but not start a heavier train than the 2-8-2. But satisfactory running at those higher speeds required larger diameter driving wheels, hence the move up to 69 and 70 inches. However, then realizable speeds were probably beyond what some roads were prepared to accept for locomotives with two-wheeled pilot trucks, hence the preference in some cases for large 4-8-2s. And with the latter wheel arrangement, with its higher safe speeds, 73 and 74 inch drivers were feasible if desired.

In fact one might undertake a paper comparison of the relative utilities of a set of 8-coupled locomotives, all normalized to say 270 000 lb on drivers. A possible set of examples could be:

2-8-2 63 inch: A thoroughly modernized USRA heavy, e.g. Atlanta & West Point.
2-8-4 69/70 inch: The Van Sweringen design, e.g. Wheeling & Lake Erie.
4-8-2 73/74 inch: The B&M design
4-8-4 73/74 inch: The Alco WWII standard, say in Rock Island form.
4-8-4 80 inch: The UP FEF-2

Of that list, the UP FEF-2 was perhaps in a class of its own when it comes to very fast passenger haulage with 100 mile/h+ capability.

At the other end, the 2-8-2 was probably nicely balanced for freight haulage where moderate balancing speeds were acceptable; it would start the same weight of train as the others, and might have been an effective helper for the 73/74 inch 4-8-2 an 4-8-4 on grades where it was desire to double the low speed tractive effort but not the power.

For fast freight in lowish grade territory, say not much above 1%, the 73/74 inch 4-8-4 would be about right. (For steeper grades, a 4-6-6-4 might have been considered, particularly if one did not want to sacrifice too much speed capability on level sections within heavy grade divisions.)

That leaves the 4-8-2 and 2-8-4. An obvious role for both would have been on divisions where bridge loading constraints, etc, would not allow the use of a 4-8-4 because of its total weight. But which one? With the same adhesive weight, they would both have started about the same trailing loads, but with lower balancing speeds than the 4-8-4, and perhaps with the 2-8-4 having a slight speed edge over the 4-8-2 with the heavier trains. But the 4-8-2 would have matched the 4-8-4 for top operating speed, albeit with a lighter load. On the other hand, I suspect that some roads at least would have had an aversion to running locomotives with 2-wheel pilot trucks at much above 50 mile/h, even if the capability to go materially faster than this without overstressing the machinery was there, as it probably was for the Van Sweringen design.

I haven’t taken a more holistic look at the relative numbers of 2-8-4s and 4-8-2s built since 1924, when the A-1 arrived, nor at their dispositions and inferred standing in their respective owner’s eyes, but I suspect that if one did that, then the 4-8-2 might come out as the dark horse that won by virtue of greater general utility. But as type, and particularly in its later (say post-1925) form, it has not been subject to the same coherent treatment in the literature as either the 2-8-4 group in toto or its Van Sweringen subgroup. Heresy perhaps...