Kelly Anderson Wrote:
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>
. . . I agree with Johnson Barr, that it was done
> to compensate for the reduced thrust on the for-
> ward stroke due to the piston area taken up by
> the piston rod. How does raising the cylinder
> help? Here’s my opinion. By making the main
> rod closer to horizontal on the forward stroke,
> the vertical load on the crosshead is reduced
> compared to the back stroke, so the frictional
> drag on the crosshead is reduced compared to
> the back stroke. Less drag leaves more of the
> piston’s thrust available to pull on the crankpin.
IMHO, the change to the angle of the force (simple vector physics) is much more important than the friction.
> An interesting side bar is that modern automobile
> engines sometimes have the cylinders offset in
> relation to the crankshaft in order to reduce the
> angularity of the connecting rods on the power
> stroke. Others have the wrist pin hole in the
> pistons offset to give the same effect.
>
> I have never seen a detailed drawing of the en-
> gine unit for a Shay locomotive, but since they
> virtually all ran in reverse for 50% of their
> lives, I would bet $0.05 that their piston rod
> centers were in line with the crankshaft center.
>
> The fly in the ointment of this theory is that
> SP’s cab-forwards also have their piston rods
> about the usual 2” higher than the axle centers,
> even though they ran in reverse in normal
> operation. But like I say, the reason for this
> detail is missing in the builder’s standard
> practices, so it may have fallen through the
> cranks in the Baldwin design offices (or maybe
> SP just wanted more ground clearance under
> their cylinders).
The cab-forwards do present an interesting special case, Kelly -
Maybe the valve timing and/or the port sizes were adjusted to give equal total force to the push stroke and the pull stroke, or maybe the difference of a few hundred pounds out of a total of thousands of pounds of t.e. didn't matter enough to worry about.
- El Abuelo Histœrico, Greengo y Curmudgeoño de los Locomoturas Viejos y Verdes,
aka Der Grossväterlich DünkelOlivGrünDampfKesselMantelLiebHabender
