Instantaneous effective crank


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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' stroke and that the difference is larger than accounted for by the 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.)