One of the several videos available that shows engine 268 in her final operating year(s?) has a quick scene where she is taking off from a stop with a decent train. When she starts, because of the geometry of the short main rod, it throws the engine left and right quite violently. A substantial amount of torque must have been lost at these moments as the engine is now being lifted as well as being pushed.
A steam engines thrust is when the main rod is nearing and passing the lowest and highest point on the drivers. The longer the rod, the more of that thrust that is transferred to linear motion and torque. Same can be said about a automobile engine, this is why some racers will put longer rods with shorter pistons. Part of this is the reason the C-17's and C-18's could really handle more of a train than a C-19. I believe the down fall comes with the longer rod adds more reciprocating weight, and can cause more wear and balance issues. A guess would be that this torque issue was discovered as engines grew in weight and power, and after companies had made several locos with the shorter rods. And, so after creating the new design, still offered the older design to those who desired them. The best combination is not so much which driver the main rod connects to, but at what angle the rod is "used" at.
Another bit of trivia: The Cylinders on some (most?) rod engines were actually offset above the centerline of the drivers. This helped to compensate the torque between the two sides of the piston. Because the rod was on the back side of the piston, the forward stroke had less cubic inches of steam driving it. Raising the cylinders put the main rod at more of a straight line to allow more torque transfer to the drivers. The rearward stroke would have more steam, but because of the greater angle, would loose a bit of torque...
Casey
Edited 1 time(s). Last edit at 11/19/2021 08:08PM by Casey Akin.
