1961 West Side Drive, Rochester, New York 14624.
The term ‘instantaneous effective crank’ is a common one
in engineering, but I find that relatively few steam engine
operators are aware of it.
I have read article after article trying to show that the torque
on the ‘out’ stroke was greater than on the ‘in’
neutralization of a part of the piston area by the presence of the
piston rod itself. The statements were true, but the explanation
left something to be desired.
The torque is greater on the ‘out’ stroke because the
instantaneous effective crank is longer during that part of a
revolution utilizing full steam chest pressure on the piston; i.e.
– not during the expansion and exhaust part of the stroke.
The effective length of the crank charger continually throughout
a revolution and when the valve passes through zero as it does
twice per revolution, the engine has no torque at all and is said
to be on ‘dead center’.
The two figures will illustrate the term ‘instantaneous
effective crank’ and how it varies throughout the cycle. In
both Figures 1 and 2, the angles of divergence of the connecting
rods (center lines are D-E) from the crankshaft-crosshead (center
liner B-C) are constructed equally. With the connecting rods being
‘free pinned’ at both ends, they can exert only a push or a
pull and that only along the paths of their centerlines.
In Figure 1, to find the length of the instantaneous effective
crank, we have to extend the centerline of the crankshaft. The
length of that line A, is the effective crank at that instant. In
Figure 2, to find the length of the instantaneous effective crank,
we drop a line perpendicular to the center line of the rod and
again passing through the center of the crankshaft. The length of
that line A2 is noticeably less than that of Al.
(Claude P. Abbert, Project Engineer is retired from the Taylor
Instrument Co. of Rochester, New York and author of The Baker Fan
(I.M.A. Jan.-Feb. 1975) and numerous other articles.)
