Re: Broken parts...

From the pictures of the side rod in question I
felt it was a stress failure. If you remember the pistures, the rod broke almost exactly straight across and the broken ends were as smooth as if they had been machined.
This type of failure is caused by repeated stress
of the type that would occur if there was a small error in tramming the locomotive frame or in the
quartering of the wheels.
A good example of the importance of accuracy in this work is the results the C&NW got with their transit layout of locomotive frames, shoes and wedges. The use of modern materials and transit layout resulted in locomotives that ran 400,000miles between class 3 repairs and that required no class 5 repairs between the class 3s.
In a rod driven locomotive any inaccuracy in the
wheel quartering, rod length, crankpin location,
or shoes and wedges can result in these parts "fighting" each other and build up tremendous stresses. Stresses which can reverse
at each end of the piston stroke, as in side rods, and eventually cause a stress fracture.
The fact that locomotives ran as long as they did
is a testimony to the efforts the machinists took
to do accurate work with the relatively crude measuring equipment they had to work with.
Jim