I have noticed that on steam engines with more than 6 drive wheels ( 2-8-2, 4-8-2, etc. ) have the main rod attached on different drive axles. For instance, on the 2-8-2 it's couple to the 3rd set of drivers but on a 4-8-2, it's on the second. What's the purpose of these different locoations?

CofG
"The Right Way"

Size of drive wheels and length of main rod.

A lot also rides on how the engine is sprung. That allows the wheels to "flex" up and down with the rail. If the main drive was one long rod the axle could not benefit from the springs.

All of the above, plus the space taken up by the second axle of a 4 wheel pilot truck. This additional wheelset moves the drivers farther back from the cylinders.

If in this case the main rod was mounted on the third driver instead of the second, it would need to be much longer and heavier. That in turn, that creates more problems in counter-balancing for the main rod.

Ed Bommer

R.R.

Driver springing has nothing to do with main rod placement on real steam locomotives. It is all about driver diameter, main rod length/weight, crank pin placement, boiler pressure, and piston speeds.

This message has been edited. Last edited by: Hot Water,

Thanks for the replies. I just thought it was odd ( atleast all the pictures I've ever seen ) that all freight locos had the main rod on the 3rd set and all passenger locos on the 2nd set.

CofG
"The Right Way"

I would think the balance of all the turning parts of the drive assembly has to balance out. The passenger engines will turn allot faster and would be harder to balance if the entire drive train were too heavy.
I know from the literature that I have on the CP selkirk it was limited to about 55 mph on the calgary west to field as anything faster was not that well balanced and would tear the track up too much.
GN's big 2-8-8-2 was limited to 35 mph with it 63 inch drivers as with over a 1,000,000 lbs of engine it was way too hard on the track. The thrust of the drive rod ="dynamic augment" would pound the living daylights out of the rail and ties.