Many steam locos in the early 20th century came with extended piston rods. Most railroads seem to have found they weren't needed and cut them off. I always wondered how this affected the counterbalancing of the engines. That's a heavy chunk of metal to remove from the reciprocating motion.

Steve

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I guess that's what I really wonder, did they check the balancing and adjust it after they were removed. Is there any evidence it was done? On small driver engines that couldn't be fully balanced, it probably helped no matter what, but on larger, faster engines I would think re-balancing would be important.

Steve

The point is that balancing a 2-cylinder steam locomotive was a compromise. You could achieve perfect rotating balance or perfect reciprocating balance but never both. Any weight added to improve the reciprocating balance screwed up the rotating balance. The compromise that was used was to balance all rotating weights and balance a portion of the reciprocating weights. Something between 25% and 50% of the reciprocating weights were balanced depending on when the engine was built and who built it.

Since the reciprocating weights were only partially compensated for, reducing the weight of the reciprocating parts by removing the tail rods improved the reciprocating balance (it left less of the reciprocating weight unbalanced) without affecting the rotating balance.

If they'd been so inclined, the railroads could have removed some of the balancing material from the drivers to maintain whatever percentage they followed for balancing the reciprocating weights, which would improve the rotational balance, but they didn't have to do anything. If the engine rode satisfactorily with tail rods in place, it'd ride satisfactorily with the tail rods removed and no adjustment to the balance.

How many pounds is even involved? Especially compared to the weight of the piston, remaining piston rod, crosshead and main rod (or some portion of the latter).

Another thing that mathematically exists but probably doesn't matter is that without the extended rod, the force on the piston moving towards the rear is slightly greater than the force going the other way, because of the larger area for steam to work against.

It had to be pretty heavy, most these engines had a 24-30" stroke and the rod must have been 4" or more diameter.

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Its interesting that the formula for counter balancing wasn't satisfactorily perfected until 1940. The formula became more complicated, but it amounted to balancing 25% or less of the reciprocating weight. Compared to 40 to 50% before.

Tom Hamilton

I don't think the formula was so much "perfected" as it was adjusted as locomotive designs changed and grew.

The formula that worked fine for ~1918 light Pacifics didn't work so well for 1938 4-8-4's. The formulas that were used were largely derived empirically (trial and error) as the analysis of steam locomotive/track dynamics was well beyond the technology of the day.

It'd be very interesting to see what a detailed computer analysis would show on this subject.

I'd like to distinguish 'legacy' tailrods (acasein point being the setup applied to many K4s) from the modern use of tailrods to support multiple diesel-style piston rings and the like.

First, the additional reciprocating mass is a comparatively small concern in a system of low or zero overbalance, where 'other methods' (like Langer's geared setup from 1947) are used for surge compensation, and stiff compliance on the trucks, relatively long wheelbase and high effective polar moment of inertia, and the other Voyce Glaze methods are used for the nosing and so forth. We already note that in a good system of even conventional lightweight rods and good balancing, very long stroke (up to 34"!) appeared to be preferable in many designs to increased piston mass in a larger bore to get given volumetrics. Keeping the rear gland from damage on a longer stroke gets to be a problem that rapidly turns larger if there is any play or misalignment in the crosshead... and higher unbalanced main rod forces on the crosshead and guide with increasing speed may contribute to deflection at the gland and weird piston/ring wear even with zero lost motion in the crosshead 'ways'.

Second, the tailrod only needs to be capable of keeping the piston aligned, so it can be hollow-drilled as modern piston rods are, but to a much more significant extent. (In keeping with the general optimization of bending moment in tubes you'd keep the OD fairly large, but there really is not all that much loading involved to keep the deflection and thermal expansion 'bearable'. In a modern system I doubt if anywhere near 100 lb. per side would be required. (This is also a nifty place to introduce cylinder oil to be pumped to radial ports around the piston circumference, without 'pressurizing' the bore of the piston rod itself ... but I digress.)

As noted, if the tailrod is of equal OD to the piston rod the pressures on either side of the piston can be equalized without the usual subtle adjustment of ports, 'steam edge', duration, etc. to equalize thrust. In my opinion that only matters at high speed, or if the engine has an 'artificially low' FA or permitted axle load giving too much propensity to slip when the higher-area side of the piston sees the pressure corresponding to the torque peak.

If care is not taken with the structure of the tail rod GUIDE I suspect it's easy to introduce more issues than you solve, starting with heavier and more complex cylinder heads with a cantilevered load hanging off them, and eventually getting around to lubricating them, servicing the necessary gland(s) and so forth that a conventional DA locomotive doesn't have, and the added maintenance care needed to remove pistons and heads, tram to set rings, adjust for wear, etc. Looking at pictures of the PRR guides, I wonder if the bores were sealed or open ... with a passel of potential problems either way.

In related news, it appears that the tail rods were added back to this 1963 Mitsubishi-built 2-10-2 that was just restored in Argentina:

viewtopic.php?f=1&t=38740

Note that photos of unrestored engines in the same series posted in the thread show no tailrods.