[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.
[Illustration: FIG. 2.]
To set this arrangement around at right angles, or where the shafts
will bring the wheels together, as for bevel gears, a bent lever arm
would need to be used, as shown in Fig. 2, but the bend in the
connecting arms brings in another feature that must be provided, as it
allows the wheels to turn either with or against each other, and
leaves two places where the bent arms will come to a dead center. What
is needed here is another element that will take all the twisting
strain on the rod and keep the pitch of both arms alike in every
portion of a revolution. To do this the ball and socket joint will
need to be replaced by a gambrel joint like a ship's compass, and
arranging the bent driving arms as shown in Fig. 3; then the driving
end of the connecting frame will move about in a true circle,
producing as great a tendency to turn the driving wheel in one
position as another. In this arrangement there must be at least six
nicely fitted journals and their bearings, four of which will be
required to take care of the forked connecting rod that joins the
wheels together. Besides all this the bearings must all line up with
the same center that the shafts are centered from or there will be a
"pinch" somewhere in the system. It may seem at first that there must
be more or less end-on movement provided for, and that the bearings
should be spherical; but that it is not the case will be noticed when
all the points are understood to be working from one center similar to
that provided for in bevel gears.--_Boston Journal of Commerce._
[Illustration: FIG. 3.]
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