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WHEELS LINKED WITH A BELL CRANK.


[Illustration: FIG. 1]

There are four ways in which a connecting rod is made use of in
machine work. The first is in linking two wheels together that stand
in the same position, but a slight distance off centers. The rod in
this case has only to lead the driven wheel around by connecting it
with the driver, and consequently has only to endure a pulling strain
in the direction of its length. The second is when the rod is called
upon to stand a pull and a push at every revolution. The third takes
in the matter of the twisting strain that a rod can manage; but the
fourth brings the hardest usage that a connecting rod can be called
upon to endure, and that is by making a lever of the rod to get a
driving action by prying on a fulcrum in the center. In Fig. 1 is seen
a case of this kind taken from a machine in which a disk engine was
made use of. The rod has a chance to turn about on its center from a
ball and socket joint, and engages with both wheels in nicely fitted
journals, and boxes set in line with the center of the socket joint,
so that when one wheel turns, the rod pries the other around by using
the rod as a lever and the ball joint for a fulcrum, giving a uniform
leverage all the while, with no dead centers.