J balancing considerations

[NW Mailing List]

As part of an ongoing discussion, and at the suggestion of Dave Stephenson:

What is the rotating component of mass for the J main rods?  (I 
presume this was done via center-of-percussion measurement, but while 
it would be nice to know exactly how it was calculated, I suspect 
just the figure would be appropriate for immediate purposes).  (I am 
presuming this includes the bearing and race mass as installed - tell 
me if that is wrong, and provide what the effective bearing mass, 
fully lubricated, would have been...)

Likewise, what was the effective rotating component of mass for the 
eccentric rod in the Baker gear?  (This might be extremely small, but 
I suspect N&W measured it and used good math to make balance calcs 
from the measurements).

Finally:  Were precise calculations of the mass distribution of the 
eccentric crank made, both in effective radius and in the horizontal 
plane?  (These would probably be present in the calculations as a 
mass expressed as a 'virtual' radius and acting at a specific lateral 
displacement from locomotive centerline)?

Thesubject under discussion is the degree of running imbalance that 
would be caused by taking down the valve gear and main rod on a J, to 
move the engine 'only on one side'.  We have a Trains Magazine report 
of an incident in the mid-Fifties, between Roanoke and Bristol, where 
the Bristol shop forces took down one side's valve gear and main rod, 
and remarked on how smoothly the result seemed to run -- the 
possibly-apocryphal follow-on being that crews ran the locomotive 'on 
one side' up to some speed approximating 80 mph with a train.  It is 
specifically the high-speed 'augment' and imbalance that would 
characterize that operation that I am trying to develop a model to determine.

It was my opinion, before hearing about this, that if bobweights were 
not available it would have been sensible practice to replace at 
least the eccentric crank on the end of the main pin after the main 
rod would have been taken down.  "Apparently" the Bristol shop people 
did not think this was necessary, which seems a little peculiar as it 
wouldn't have required much time or effort to reinstall and clamp the 
eccentric crank once the main and bearing parts had been removed.

There is no particular time pressure to find or transmit this 
information, and of course it isn't a particularly critical 
request.  Thanks in advance for any numbers you can find.


Robert M Ellsworth