QuisizyxOK. I got HSR but crapped out on HAL.
It's simple: 'heavy axle load'. There have been some pretty good conferences on the issue (at least some of which were partially sponsored by Trains Magazlne) and that would be a very good place to start looking at what improvements do and don't help with the peculiar stresses generated by big steam power.
I hadn't considered the weight distribution. I can see how they can actually be lighter on the rail than current DEs. Plus the larger wheels mean a larger contact area.
Normally there will be increased loading on the drivers simply because they're the only wheels that do any work. Around the turn of the 20th Century it was considered good design practice to 'taper the loading' (which meant to use less weight on the 'engaging' and 'leaving' driver pairs than the ones in the middle); this would have more effect on relatively light and flexible track, but Kiefer's testing established (at least to my satisfaction) that stiff and well-graded track is the only sensible option for high speed with reciprocating power.
I'm starting to feel like I'm among some University student engineering graduates. Voyce Glaze?
He designed the balancing of the N&W J-class 4-8-4, notable for being able to run freely in excess of 110mph with 70" drivers, and do that effectively with spoked cast centers with minimal reported spoke or rim breakage. The specific details of this are preserved in the NWHS document collection and are a highly useful reference for would-be designers...
There are "lateral" issues?
Tremendously significant ones, even before we get into issues where swing (call it 'yaw' of the engine going into and out of curves couples with the suspension characteristics.
The issue of controlling shock and motion at the first driver pair (however many pairs there are) must not be underestimated. It is interesting to see the methods Stroudley used (admittedly on fairly light locomotives) in getting an 0-4-4 express locomotive to track properly -- I recommend you look them up to see how he did it. They could be made to work effectively on power with a good lead truck, too -- although defined lateral-motion devices, which you should also look up, do a much better job on larger and heavier modern locomotives, which depend on defined tread profile and limited flange contact to work best.
I get the impression that backing the centipede tenders could be an exercise for derailment.
The actual surprise is that there is a problem with that long "rigid" wheelbase so seldom. What is notable is that the 'breakover' is remarkably sudden; even a slight increase in curve radius over the critical amount produces derailing (and then there is great difficulty in getting the last pair to go back onto curved rails...
Augment? Yet another term I'm not familiar with. At least not in this context.
The actual term is 'dynamic augment' and strictly speaking its most important application is to resonant amplification of the forces involved, with the inertial forces developed by the mass of the reciprocating parts often being of more importance than the vertical motion allowed by the suspension. If you can find a copy of Johnson's book on the steam locomotive (I believe I found and referenced an online-readable version in a different thread a few months ago) read his section on balancing, which is a reasonable primer on the important aspects. Some of the more sophisticated developments in balancing reflect things that conventional balancing discussion doesn't quite address fully.
I have heard of steam engines "hammering" the rails due to the imperfect attempt to counterbalance the flailing rods.
The problem is actually more complicated than that. The problem with a typical double-acting two-cylinder engine is that the mass of the rods doesn't act in just one plane, but effectively in two: the "up and down" direction that produces wheel bounce, and the "fore and aft" direction that produces surge and nosing. Add to this that the peak effect on lifting the driver on one side comes at the time there is zero effect on the driver on the other side, meaning the axle is being effectively accelerated upward at one end (followed by its being 'dropped') -- this is the source of what the English liked to call 'hammer blow' (the outer driver being the 'head' of the hammer and the inner one being the fulcrum around which it is lifted). Dynamic augment is usually calculated as the downward force on the railhead, the typical metric of greatest concern in large reciprocating-locomotive design, and it is easily possible to produce a two-cylinder engine that has essentially 'zero overbalance' (several Australian designs were actually constructed that way in the '50s). These ride remarkably smoothly, but have vicious surge and hunting characteristics, in turn requiring very stiff lateral control at the leading truck, and poor riding at the rear (especially in the absence of a formal trailing truck) unless very careful arrangements are used to have the tender 'steer' the rear of the locomotive chassis actively).
Even the J, therefore, retains a certain measure of overbalance; it just puts a large percentage of this in driver pairs other than the main. A relatively late and unsung development, the Langer balancer (of 1947, patent assigned to Westinghouse) actually doesn't do anything for 'balance' augment; it is on the locomotive centerline and can very accurately reduce the surge moment of, say, a passenger locomotive (allowing much less formal overbalance, hence less high-speed augment, hence much higher practical top speed without excessively stiff compliance in lead and trailing truck that has the effect of increasing the effective rigid wheelbase.
Never heard of augment. By all means, please educate me.
Start by reading up on the theory known as 'angle' balancing, a variant of cross-balancing, as practiced for example on the later ATSF 4-8-4s. That will bring you up to speed with most of the practical considerations involved in reducing dynamic augment. Then look at the precise details of the setup on the British 'Evening Star' 2-10-0s, which were capable of remarkably high speed with only a couple of details different from 'typical' balance practice.
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