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Engine balancing (technical question)
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Author:  Warren [ Mon Aug 30, 2004 12:10 pm ]
Post subject:  Engine balancing (technical question)
I have a question about engine balance, in specfic, cross counter-balancing. Am I correct in my understanding of cross-counterbalacing that the egine is dynamically balanced instead of statically balanced in order to reduce to amount of dynamic augment at a specific rotative speed by off setting the balance weights,either in plus or minus degrees figuring the engine's both sides to counter balance to reciprocating mass? Is there a factor included for angular forces generated by the effort of the mainrod when on a power stroke (vertically up or down)? Is there more to it than that meets the eye? Is the balance figuered on 60% of the reciprocating mass as it is in internal combustion engines or is it figured on a combination of reciprocating and rotating mass (i.e. main rods and side rods)?
Author:  Bobk [ Mon Aug 30, 2004 2:07 pm ]
Post subject:  dynamic balancing
A good reference on this topic is Ralph Johnson's book The Steam Locomotive, but I'll try to comment from what I remember. First, the main balance mass for each side of the engine is determined by a number of factors including both reciprocating and rotating masses. As in internal combustion practice, a fraction (the amount of which I don't recall, but is on the order of 50%) of the main rod mass is allocated for reciprocating mass and the rest for rotating. The other connecting rods count as rotating masses. The cross-counterbalance mass is then determined to offset the imbalance brought about by the typical 90 degree offset between the wheels on each side of the locomotive.

To my knowledge, no steam locomotive was actually dynamically balanced i.e. the parts being rotated and imbalances measured and compensated for as is done for automobile wheels and tires. However, the cross-counterbalancing did serve to reduce the dynamic augment. The amounts and locations of both counterbalance and cross-counterbalance weights were determined by formulas essentially based on geometry and empirically derived constants. Note that Johnson's book only gives the formulas for normal balancing as I recall. Cross-counterbalancing formulas can best be found in old AAR documents or could be derived using texts on balancing.

The difficulty in balancing is that one is never able to completely compensate for both reciprocating and rotating masses. This is also complicated by the fact that driver sizes limit the amount of weight that can be added to compensate. Lighter weight rods are a big help in this regard. The Norfolk & Western probably was the most advanced in this area of US roads as their small drivered 4-8-4s with rollerbearing type light weight tandem rods could achieve 100 MPH without damaging track or structures.

Reply by Wolf Fengler of the AT&SF 3751
Author:  johnacraft [ Mon Aug 30, 2004 2:11 pm ]
Post subject:  Re: Engine balancing (technical question)
There really isn't a single answer to your question. Over time AAR developed and issued guidelines and recommendations based on experience, but judgements were made in each new locomotive design. A low-speed 2-8-0 designed in 1900 is a very different beast than, say, N&W's J-class 4-8-4, which probably has the most sophisticated balancing of any American locomotive.

In brief, there are three components that require balancing.

1. The rotating mass. Side rods, and a portion of the main rod and valve gear (use the center-of-percussion method to divide the rotating and reciprocating portions of these masses).

2. The reciprocating mass. The rest of the main rod and valve gear not accounted for in (1), piston, crosshead.

3. The piston force. This force varies.

One can counteract some of the reciprocating mass and piston force in the lead and trailing trucks, by making the centering mechanisms "stiff."
But on a two-cylinder quartered steam locomotive, one cannot completely balance the running gear dynamically (static balance is moot, as the locomotive moves at a variable speed). Where you draw the line is a judgement.

There simply is no single answer to your question.

JAC
Author:  whodom [ Mon Aug 30, 2004 4:23 pm ]
Post subject:  Re: Engine balancing (technical question)
Warren wrote:
I have a question about engine balance, in specfic, cross counter-balancing. Am I correct in my understanding of cross-counterbalacing that the egine is dynamically balanced instead of statically balanced in order to reduce to amount of dynamic augment at a specific rotative speed by off setting the balance weights,either in plus or minus degrees figuring the engine's both sides to counter balance to reciprocating mass? Is there a factor included for angular forces generated by the effort of the mainrod when on a power stroke (vertically up or down)? Is there more to it than that meets the eye? Is the balance figuered on 60% of the reciprocating mass as it is in internal combustion engines or is it figured on a combination of reciprocating and rotating mass (i.e. main rods and side rods)?


Warren,

As some of the other posters have intimated, this is a very complicated subject, but I think I can clarify a couple of issues.

Cross-counterbalancing refers to adding weight to the drivers on one side of an engine to help balance the rotating weights on the other side of the engine. This is necessary because the counterbalance weights in the driver are not (and can't be) in the same vertical plane as the rotating parts. If you look at a set of drivers from the front, you can see that much of the counterweight is within the driver (directly over the rail) and some hangs outside of the driver. The rotating weights you are trying to balance mostly hang far outside of the driver (the crank pin, eccentric crank, rods, etc.). A weight has to be added to the driver on the opposite side of the engine to compensate for this fact. Johnson's book has some good simple illustrations to clarify this.

The percentage of reciprocating weight to be balanced was usually done by rule-of-thumb, and some engines really had problems as a result (the Atlantic Coast Line R-1 4-8-4's for example). IIRC, they were balanced at the current AAR recommendation of 40% of the reciprocating weight. After massive problems ensued (drivers bouncing off the rails), the percentage was reduced to somewhere around 25% and this was found satisfactory. The AAR later revised their recommendations based on the problems several roads experienced. I believe the N&W ignored the AAR rules altogether and came up with their own formulas for balancing which were obviously successful. Later ideas suggested limiting the reciprocating balance to some fractional figure based on the weight on the drivers. You correctly surmise that the angular forces of the main rods possibly played a factor in the "bouncing driver" problem that some RR's experienced; they simply didn't have the necessary tools to analyze the problem at the time. (Sounds like a good project for some industrious ME student...)

The bottom line is, no matter what figure was chosen, the balance on a 2-cylinder locomotive HAD to be a compromise.

Some British engines (all 3-cylinders I believe) dispensed with reciprocating balance altogether and it has been suggested that this may have worked for 2-cylinder engines as well (with appropriate leading and trailing truck design).
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