Although this video was developed to illustrate the issue of "secondary balance" with respect to internal combustion engines, it seems like if you substitute connecting rod for main rod, the same geometry would apply although obviously locomotive drive wheels aren't turning at 6000RPM. Basically as the rod deviates from the position where it is in line with the piston direction, it becomes the hypotenuse of a right triangle and the distance between the piston and the crank shortens.

Is this significant enough in steam locomotives to affect balancing, speed, load limits?

https://www.youtube.com/watch?v=C_YNn3ZkJmU

One of those days when I wished I had stayed in engineering.

I know nothing about this, but I have noticed that some locomotives have the main rod to the drivers farther forward and some have longer rods that go to the rearward drivers. The narrow gauge 2-8-0’s are good examples of this, some crank on the 2nd axel and some on the 3rd. I wonder if this had to do with keeping a more consistent piston speed.

The piston velocity is a sinusoid.

The problem has always been that the motion of rotation does not equally cancel a motion of reciprocation. Engines with four cylinders have an advantage here, as well as Fairbanks Morse opposed piston engines.

.

Here is a graph that shows what Kelly is saying.

Zero is rods up. Ninety is front dead center. Stroke is 2, and main rod is 2 long. There is much more swept cylinder volume on the forward half of the rotation, than in the rear half of the rotation. Measurements are with respect to wheel center.

And, unless you have tail rods, the front of the cylinder has more expanding steam volume than the rear half thanks to the piston rod's occupying space. There's a whole lot happening at the same time. I think the traditional way to set valves is the standard because it is simple and achievable and makes the sound seem right.......

Ports and passages are designed to account for the disparity in swept volume. Whether this remains effective at high cyclic rpm probably can't be determined without good multiphysics modeling, and even then the assumptions used for the model might be incomplete or incorrect. The effect could be overcome at high speed with a little Chapelon-style steam injection to 'trim' the expansiom response.

The big issue for me is that the peak machinery speed occurs in the center of the cylinder, away from lubrication delivered through the valves. If there is even momentary starvation under what is still severe steam heating and emulsification, damage between rings and bore may develop suddenly and progress catastrophically.

A much greater concern, to me, is the concomitant peak speed and tribology of the piston valves. As one example: the failure of the N&W J tested by PRR occurred due to valve overheating resulting in seizure, something probably not helped by excessive superheat at high mass flow.

It is my opinion that any modern reciprocating engine meant to be operated at high speed needs to be built with zero overbalance, and the coupled motion in 'hunting' addressed by some means like balance shafts, just as surge is compensated for by the Langer balancer. In most American locomotives, the polar moment of inertia is high enough, and the effective wheelbase long enough, that stiff or controlled lateral in the lead and trailing trucks can effectively control the chassis moments, and of course the wheels will be in much better rotating balance.

Someone here can confirm my understanding that Voyce Glaze eliminated most, but not all, the overbalance in the main driver (distributing it in the coupled wheels, which might not have been the best thing if Chapelon was right about the behavior of Timken thin-section rods!). What may be instructive is why he left "some" overbalance in the main, and why: it is nominally the vertical component of peak thrust on the main rod, which is related dynamically to the issues being raised in the OP. That may be related to a traditional reciprocating-locomotive drive not being a 'crankshaft' but acting as a third-order lever system with respect to actual effect on the contact patch (and hence the effective instantaneous thrust as opposed to torque moment) and to the fact that the main driver is not fixed in bearings as a crankshaft is, but has to accommodate reasonably free suspension movement (albeit heavily loaded for 'damping' effect).

Kelly, didn't Ralph Johnson have a bit on this, right around the section he's covering how to calculate the center of percussion for mains? I don't have my copy anywhere near to consult, and I don't have time to fiddle with Hathitrust... ;-}

I've read that Lima widened a couple of steam ports on the forward end of the valve liner by 7/16 of and inch. This increased the cutoff on the backward stroke which evened out the torque some, on their early Super Power steam locomotives. The backward stroke had a cutoff of 63% and the forward stroke had a cutoff of 60%. I look at it, as equalizing the cutoffs with respect to the crank pin positions. It was called, limited maximum compensated cutoff.

https://patentimages.storage.googleapis ... 772118.pdf

.

This is correct. Someone might find, and secure the rights to post, the diagram of 'resultant' wheelrim torque, in 15-degree increments, that David Wardale furnished in the Red Devil book. There is also the care in three-cylinder engines to angle and counterbalance the inside crank and main when the center cylinder is elevated for rod clearance or whatever. I think Kelly is right in thinking as even a torque as possible ought to be a design criterion.

Of course there are issues of lead and degree of cutoff, and to an extent things like excessive superheat at high cyclic rpm, that spoil some of the static-geometry optimization of this. ISTR some of the Franklin System propaganda noted how poppet valves were supposed to aid in keeping things regular.

The steam-cooled, articulated, 'diesel-style ring' long-travel valves Wardale designed are valuable to study. Wardale discusses valve tribology at that point in the book, part of the diesel-style rings being the possibility of better oil control. It may be difficult to apply those valves as designed to historic-preservation engines, though.

The solution to long-lead is basically the same as for short fixed cutoff (as on the PRR I1s with 50%) -- slot or as Porta called them Weiss ports or equivalent. (The "equivalent" is the most interesting field for study...) These are very small ports, only a couple of 16ths wide, that are uncovered to steam at a 'longer cutoff equivalent' than the steam edges of the valves on the main admission ports. These are scaled to bleed enough steam to help the engine get off center; they then 'starve' rapidly as the cyclic comes up, so increasingly the mass flow is determined by the valves. The effective 'dead space' this adds may or may not be relevant to design or thermodynamics calculations. Wardale's Herdner valves are another approach to providing 'starting steam' at different effective cutoff for starting and very low speed.

I am more than a bit of a nut on the issue of compression control. Okadees are cute, but they don't have the necessary mass flow or response to act efficiently at high speed/short stroke duration (keeping in mind that across a wide variety of high-speed locomotive types, somewhere in the low-40s percent cutoff is what produces maximum speed). The 'solution' I adopted was to paraphrase Jay Carter's design for steam automobiles, where the compression was monitored, and relieved via proportional valves into a well-insulated (and, potentially, heated or jacketed) reservoir, from which the accumulated pressure and mass could be, equally proportionally, released to the cylinder on the early part of the return stroke to bring cylinder pressure closer to admission pressure across unporting at the steam edge.

The C&NW limited cutoff 2-8-4's were equipped with a 3/4 inch pipe that connected the valve bushing and the cylinder bushing, directly. Instead of a 1/8 inch slot starting port feeding into the steam passage. The piston would block the admission of steam from the pipe as the piston approached top dead center. Avoiding the preadmission of steam too early in the return stroke of the piston.

In regards to excessive lead, both classes of D&RGW 4-8-4's and the first batch of Challengers had a variable lead device that gave zero lead at starting and 3/8 inch lead with the valve gear centered. I suppose it would be an advantage in starting a train on a 3% grade.

Porta, in "The Red Devil," advocated for a long lead at starting and zero lead at a short cutoff. If I understand that correctly.

This is where an indicator diagram would really show the positive or negative effects of these changes. They do this electronically, now in IC engine testing. I believe that it's called cycle analysis.

Tom Hamilton