Which axle?

A question came to me today and I really wonder why I never thought of it before. What engineering factor(s) determine the point of
connection of a steam engine's main rods to the drive unit? Example (based on four hours of consulting my limited literature): Consolidations, Mikados, Berkshires, Mountains and Northerns all have eight drivers, yet the first three classes seem to have the main rod connected to the third set of drivers while the second set of drivers is utilized for the last two groups. It doesn't seem to be a simple function of driver diameter or era. Can anyone provide an explanation?
Seeing I'm inquiring about rules, permit me to also ask about exceptions. I know that the Santa Fe had a class of Atlantics in which the first rather than the second set of drivers was employed. IIRC Moguls have used both the second and third set of drivers. Does anyone know if exceptions exist for the examples given in the first paragraph?
Thank you in advance for your aid.
Jerry
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"Angularity of the main rod" is the reason to connect the main rod to a driver far back from the cylinder. Weight and mass of the rod would be a reason to connect it to a driver further forward. There's a compromise there. Mountains and Northerns have a four wheel lead truck, so the drivers are further back from the cylinders, so it makes sense to connect the rod to the second drivers. The other classes have a two wheel lead truck, so the third drivers are in a good connecting place.
Angularity of the main rod referrs to the fact that when the crank pin is 90 degrees off of dead center, the piston is not in the center of the stroke. This affects valve timing and events. The greater this angularity, the more bad effect there is on the valve events.

I don't think here are real rules, just best compromises. You want a long main rod, but not one so long that it will be impossible to counterbalance with wheel weights.
--
Bill Kaiser
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A really good answer.
Thanks.
-Pete
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" snipped-for-privacy@hotmail.com" wrote:

Those rods are very heavy! The coupling rods (between wheels) can be fully balanced by the moon shaped balance weights on each wheel, but the connecting rods are describing a circle at one end and a back and forth movement at the other so the average movement is an elipse. If you fully balance the circular motion then the back and forth motion tries to throw the loco ends from side to side. If you try to balance the back and forth motion then the up/down imbalance becomes greater and the drivers alternately lift off the rails and hammer down onto the rails at speed. One can reduce the effect by shortening the connecting rod to the second driver, but then the angle that the power from the cylinder is greater, representing wasted power and mechanical haemoraging. So, connecting to the third axle is more likely on an eight coupled intended for pulling power. Connecting to the second axle is more likely on a loco intended for higher speeds.

There are always exceptions! ;-)

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Jerry, It is too hard to explain here. I would suggest that you get a copy of J. Parker Lamb's "Perfecting the American Steam Locomotive" published by Indiana University Press http://iupress.indiana.edu . An excellent book that answers your question and many others. Paul

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