Steam Locomotive Question

 believe it has a lot to do with which motion the eccentrics are and the power/performance they want out of it.  It also has to do with the rest of the locomotive.  Did you look to see what else would have to me moved to put it in the same place on each locomotive?

The placement of the "eccentric crank" isn't the key, but the placement of the main crank pin on the corresponding main drive wheel set. The placement of the main crank pin, which the "big end" of the main rod is connected to, is generally determined by the drive wheel diameters, and the piston stroke. The eccentric crank only drives and times the valve on its respective side.

Weights and balancing the reciprocating motion plays a big role too. Not many engines with drivers over 70" had a monstrous main rod connecting at the 3rd driver...

That is correct, and how this eccentric crank pin is located will vary based on which valve gear is used, a Baker or a Walscheart valve gear is very different in design.

Here is a great link for good information...

http://www.jghtech.com/html/ecc-crank-1.html

Thank you for the informative responses!

A steam locomotive is a huge balancing act. Everything - driver diameter, boiler and firebox size, cylinders - is designed as part of a greater whole. Unlike mass-produced diesels and automobiles, steam locomotives were designed by individual railroads for best performance under local conditions. Even USRA designs were modified after World War I.

Go to UP 844 vs N&W J611 and read that vigorous debate for all kinds of takes on this topic.

Exceptions abound. Atlantics (4-4-2's) usually had main crankpins on the second (rear) set pf drivers. But Milwaukee HIAWATHA 4-4-2's and a few Santa Fe 4-4-2's had main crankpins on the front drivers.

Let's keep this simple for a bit and realize that 8-drivered locomotives - a 4-8-4 or 4-8-2 will have the main rod to the #2 driver whereas a 2-8-4 will have the main rod to the #3 driver. In almost every case this is true to limit the length and weight of the main rod on a locomotive with a 4-wheel "engine" truck.

Look how far back from the rear cylinder head is the first driver on a 4-8-4. The design choice to attach the main rod to the #2 driver derives from the problem of balancing a much longer ( 73"or 78" longer ?) main rod if it were connected to the #3 driver - not a good choice. You have a problem of main rod "whip" when it is that long at the higher rotational speeds of a passenger engine (4-8-4 or 4-8-2). PLUS the added weight of those 73 to 78 inches of longer main rod could not be counterweighted.

Excellent explanation. Thank you.