• Driver counterweights

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

Moderators: slide rules, Typewriters

  by BobLI
How did they determine the weight of the driver counterweights and did each wheel have a different weight on it?

How did they prevent track damage? I'm wondering why a steam engine would damage track without the right weights on the drivers.
  by mp15ac
Actually, there were many instances where steam locomotives were built with incorrect counterbalancing, with the result that track was damaged. Most 2-10-0 and 2-10-2 locomotives were restricted to a maximum speed of 35 mph because of the limits of counterbalancing the heavy main rods. The Atlantic Coast Line had 4-8-4's, and the New Haven 4-6-4's which had counterbalancing problems because Baldwin miscalculated the weights involved.

  by Pat Fahey
The weight of the counterweights on the drivers , is also determanted by the weight of the side & main rods . Pat.
  by Eliphaz
the problem with a wheel out of balance is that the force pressing the wheel to the rail is not constant, but varies as a sine wave. As the heavy part of the wheel rises up the back side and over the top, centrifugal force will tend to lift the wheel off the rail; then as the heavy part heads down the front of the wheel centrifugal force will combine with gravity to produce a rapid increase in downward force. At high rotational speed this can produce a loss of adhesion in the first case and a "hammer blow" on the rail in the second. Damaging to the rails certainly and not good for the engine axles, pins, and bearings either!
  by Engineer Spike
In the case of the New Haven, its mechanical department was able to calculate the correct counterweight, and they were modified. In the end, the I5 class went on to run successfully until replaced by the Alco PA diesels.

Baldwin got the contract to build them because they were the low bidder. New Haven was bankrupt, so its hands were tied. If they had the choice, I'm sure they would have remained loyal to Alco.

Sorry to the Baldwin fans, but I've got to be partial to Alco, since I married into a family of former Alco employees.
  by Allen Hazen
Re: Baldwin vs Alco. I've read a few things that make me think that Alco was a bit ahead of Baldwin when it came to calculating driver balancing. The balancing was a very complicated engineering problem, because you had TWO things you wanted to balance: rotating mass (so as not to get the hammer-blow effect Eliphaz describes), but also reciprocating mass: the piston, piston rod, and driving rod are going back and forth, and their momentum would tend to make the locomotive yaw back and forth. Trouble is, you can't balance both: the balancing weights you need to do one tend to make the other worse.

Now, again this is my impression from what I've read: I am not a trained mechanical engineer. But one of the advances in locomotive design around 1930 was the introduction of one-piece, cast, locomotive beds in place of the earlier built-up frames. What this meant was that the problem with reciprocating masses became less important: the locomotive structure was more rigid, so could resist the forces better, and also more sturdy, so less likely to get bent out of shape or loosened by the effect. So Alco's tendency was, I think, not to try as hard to balance the reciprocating masses: late Alco designs for the New York Central still had some counterweighting positioned to counteract the effects of the reciprocating parts, but they didn't try to compensate for as large a proportion of this effect as earlier designs had. So the weighting could be placed to do a better job of compensating for the rotating masses, and so do a better job of eliminating hammer-blow. New York Central Hudsons (an Alco design) and Niagaras (another Alco design) were smooth-running locomotives. ACL 4-8-4 and New Haven 4-6-4 (Baldwin designs) had problems when first delivered. (I guess that the fixes involved moving counterweights-- often these involved, I think, lead weights put in "pockets" in the steel driving wheel-- so as to approximate better to the sort of balancing that Alco got right the first time!)

As I say, this is the impression I have gotten from reading about a number of locomotive classes. It could be all wrong… or it could be that the "drive train dynamicists" (I'm sure that wasn't an official job description, but I think it describes the relevant part of the engineering task) at Schenectady in the late 1930s were better at it than their counterparts at Eddystone.


Leading to a nice bit of counterfactual history. Suppose Baldwin and Alco's bean-counting had come out a bit differently, so that Alco got the New Haven order. It's time to work out the details. Alco's engineers tell the New Haven's "I know you haven't used boosters in the past, but they would let you get a high tractive force when starting up with smaller, lighter, pistons in the main cylinders, and I don't think I have to tell you what that would mean for operations and maintenance. And if you agree to that, we can shave the price a bit, because we can use some available casting designs and not have to come up with something new…" And the New Haven people see the light and agree. Result: an Alco I-5 with different enough streamlining sheet metal to disguise it, but mechanically a copy of the New York Central's J-3.
  by mp15ac
Two other factors also helped reduce reciprocating weights were:

1. Light weight alloy rods, both main and side. The light weight side rods reduced the rolling weights to be balanced, and the light weight main rods reduced some of the rolling weight and some of the reciprocating weight.

2. Higher boiler pressure meant that smaller diameter cylinders could be used while maintaining the same amount of tractive effort. Since the pistons were smaller in diameter that meant that they weighed less, which reduced the amount of reciprocating weight.

On the N&W Class J 4-8-4 the rolling weight was balanced so that the engine would have to be traveling 140 mph before a driver would lift off the rail (using a 70" diameter driver no less!). The reciprocating weights were managed by the use of cast bed machinery frames, and having the pilot and trailing truck lateral springing being made very stiff. This meant that it required more force to move the pilot and trailing trucks out of center. It also meant that the J's had to be kept on high quality track. When N&W started using them as freight engines they had trouble on some of the freight sidings in which they would derail because of the truck springing.

  by Allen Hazen
Thanks, Stuart! The information about the N&W J -- both the technical details about stiff centring of the trucks and the operational details of what it led to -- is very helpful! Exactly what is needed to get a "feel" for the design issues.
  by Pneudyne
I had seen previously comment about the lateral stiffness of the N&W J but it is not an aspect that has gained much attention. Rather the design has been much lauded for its ability to run very fast even though it had 70 inch drivers as compared with the 80 inch size, give-or-take, that was then the norm for fast passenger 4-8-4s. But of course, that capability did not come for free, and the tradeoff in this case was unusually high lateral stiffness. No doubt all of that was carefully considered by the N&W whose excellent track and roadbed could accommodate this kind of machine, and where running at the very highest speeds was not a major proportion of the J’s operations. For extended running at very high speeds, larger diameter drivers might have been beneficial in that rotational and piston speeds would be lower, possibly with lower wear rates. The J was also at the low end of the range for factor of adhesion (FA), at around 3.9. Again, the N&W seemed to be able to handle numbers like that. On other roads in other environments, such as winter on the Wasatch, one suspects that the J might have been somewhat on the slippery side.

I suppose the antithesis of the N&W J amongst the “big” 4-8-4s was the UP FEF-2/3, which was apparently quite flexible in a lateral sense, having for example lateral motion devices on three of its driving axles, as compared with the more usual two, and an FA above 4.2. The slightly smaller D&H K-62 was similarly endowed.

  by rlsteam
Photos of the Milwaukee Road's F-6 4-8-4s show holes in the driver counterweights. Here's an example: http://www.railarchive.net/randomsteam/milw128.htm I presume this was an attempt to adjust the rotating balancing. Note these were Baldwin locomotives, like the ACL 4-8-4s and NH 4-6-4s.

By the way, what is the function of the vertical pipe running down the left side of the smokebox on No. 128 and 135 in these photos provided from Carl Weber's collection? (Possibly a booster exhaust, but if so not very efficiently contoured.)