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I have a question regarding steam locomotive side rod configurations.  In the case of a 2-8-4 or 4-8-4 wheel arrangement, why is the main rod connection sometimes on the second axle back, as in a locomotive usually used in passenger service, and sometimes on the third axle back, as in a locomotive usually used for freight service?  Does one location provide higher net speed (more RPM) and the other more torque for starting heavier trains?




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@MELGAR posted:

There was a recent Forum discussion of this question. I'm not sure we ever got to the definitive reason because the design depends on multiple factors as well as the judgment of the designer. Here is the link:


I remember that discussion. While I did not follow it, I did read some of the comments by those that know way more about steam than I know about what my grass looks like in the morning when I step outside. Needless to say that it is a great read, and we'll worth it in my opinion.

On a side note, I had to ask Rich a question recently which I thought was a load of bull. Rich confirmed that it was indeed bull, but went out of his way to explain how steam was used in a few different engines, even though I didn't ask. I am grateful he did because I am sure that eventually I would have a need to know whether just curious. Always brings a smile.

The placement of the crankpin and main rod on a steam locomotive was not dictated by the kind of service (freight or passenger) the engine was typically assigned to do. That choice was the result of a number of different design considerations.

The running gear of a steam locomotive is, by design, inherently out of balance. Trying to balance the rotating mass of the wheels with the reciprocating mass of the rods and pistons was a very difficult engineering task. The engineers also had to look at weight distribution among all the axles, to properly equalize the axle loading on each axle.

The out-of-balance forces generated by the running gear was referred to as “Dynamic Augment.” Minimizing Dynamic Augment was the goal of the design engineers when balancing the running gear. The placement of the main crankpin had a lot to do with the control of Dynamic Augment.

On a steam locomotive with a 2-wheel lead truck, like the 765, the proper place for the main rod was on the third axle. Putting it there achieved the right balance between main rod length, main rod big end Dynamic Augment, axle loading and good operating angles between the main rod and the crosshead.

On a steam locomotive with a 4-wheel pony truck, the cylinders were moved forward a bit, to place a bit more weight on the two axles of the lead truck. Placing the crankpin on the third driver axle would have resulted in a very long main rod. It would have too much reciprocating mass on the big end to be able to balance it with the main driver counterweight. By moving the crankpin and main rod to the second axle, better overall balance was achieved and Dynamic Augment was minimized. The diminished Dynamic Augment allowed for higher speeds, which was a good thing for an engine destined to run mostly in passenger service.


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Slightly off beat trivia, comparative overview of connecting rod length factors.:

In motor sports we were made aware of the  effect of rod length ratio to bore diameter and it's effect on performance.  Early on in the development of Chevrolet V-8 truck engines.  The truck blocks had a raised deck to accommodate a longer rod to increase torque for hauling loads.  Truck blocks at that time frequently had smaller bores to bump the torque and a lower R.P.M. range.

In contrast light load high speed engines took an opposite approach, comparatively larger bore short stroke and higher R.P.M. range.

In boat racing we destroked engines for less displacement to make a particular class but found our power range was higher.

Where this get interesting in relationship to steam engines is we ran a longer rod with the piston pin boss set higher in custom pistons to reduce rod angle & maintain the torque needed due to the high drag friction of boat racing.

Some pins were so high they got into the oil scrapper groove.

Connecting rods are  unique  because they experience both the rotating motion of the crankshaft or loco driving wheel and the reciprocating motion of the piston.

Delete error.

Longer rods also made balancing difficult, frequently there was a need to either use a harmonic balancer with steam locomotive type off set cast in weights and or heavy metal machined into the crank throws.

Rich,  Thanks for realigning my head. 

Last edited by Tom Tee
@Tom Tee posted:
...One of the trade offs in rod length is the way a short rod promotes piston skirt wear/cylinder scuffing and a long rod reduces piston skirt wear/cylinder wall scuffing...would you have cylinder maintenance comments concerning comparative rod lengths?

Making this comparison is a bit of apples and oranges.

In a gasoline or diesel engine, the connecting rods are directly attached to the piston via their connection at the wrist pin. Therefore, any angularity in the connecting rod is transmitted directly to the piston and to the cylinder walls. The resulting scuffing with short connecting rods is a direct result of this angular force imparted to the piston.

In a steam locomotive, the crosshead assembly sits between the main rod and the piston/cylinder. All of the angular forces from the main rod are absorbed by the crosshead. The piston rod always moves in a straight line with the piston and the cylinder, thus there is no additional wear in the cylinder when a short main rod is used.

I can see where there may be some additional stresses and wear in the crosshead with a short main rod, but I don't have any specific data to confirm that.

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