CAMS AND ECCENTRICS
More or less confusion arises from the terms _cams_ and _eccentrics_. A
cam is a wheel which may be either regular in shape, like a
_heart-wheel_, or irregular, like a _wiper-wheel_.
The object in all forms of cams is to change motion from a regular into
an irregular, or reversely, and the motion may be accelerated or
retarded at certain points, or inverted into an intermittent or
reciprocating movement, dependent on the shape of the cam.
A cam may be in the shape of a slotted or grooved plate, like the needle
bar of a sewing machine, where a crank pin works in the slot, and this
transmits an irregular vertical movement to the needle.
A cam may have its edge provided with teeth, which engage with the teeth
of the engaging wheel, and thus impart, not only an irregular motion but
also a turning movement, such forms being largely used to give a quickly
rising or falling motion.
What are called _wiper-wheels_ are designed to give an abrupt motion and
such types are used in trip hammers, and to operate stamp mills. In
harvesters, printing presses, sewing machines, and mechanism of that
type, the cam is used in a variety of forms, some of them very ingenious
and complicated.
[Illustration: _Fig. 88. Heart-shaped._]
[Illustration: _Fig. 89. Elliptic._]
[Illustration: Fig. 90. Double Elliptic.]
Cams are also used for cutting machines, or in tracing apparatus where
it would be impossible to use ordinary mechanism. All such forms are
special, requiring care and study to make their movements co-relate with
the other parts of the mechanism that they are connected up with.
Simple Cams.--Fig. 88 shows a form of the most simple character, used,
with some modifications, to a larger extent than any other. It is called
the _heart-shaped_ cam, and is the regular type.
Fig. 89 is an elliptical cam, which is also regular. What is meant by
_regular_ is a form that is the same in each half portion of its
rotation.
Fig. 90 is a double elliptic, which gives a regular movement double the
number of times of that produced by the preceding figure, and the
differences between the measurements across the major and minor axes may
vary, relatively, to any extent.
[Illustration: _Fig. 91. Single Wiper._]
[Illustration: _Fig. 92. Double Wiper._]
[Illustration: _Fig. 93. Tilting Cam._]
Wiper Wheels.--Wiper wheels are cams which give a quick motion to
mechanism, the most common form being the single wiper, as shown in Fig.
91.
The double wiper cam, Fig. 92, has, in some mechanism, a pronounced
difference between the lengths of the two fingers which form the wipers.
The form of cam shown in Fig. 93 is one much used in iron works for
setting in motion the tilt hammer. Only three fingers are shown, and by
enlarging the cam at least a dozen of these projecting points may be
employed.
Cam Sectors.--Fig. 94 shows a type of cam which is designed for rock
shafts. The object of this form of cam is to impart a gradually
increasing motion to a shaft. Assuming that A is the driving shaft, and
B the driven shaft, the cam C, with its short end D, in contact with the
long end E of the sector F, causes the shaft B to travel at a more
accelerated speed as the other edges G, H, approach each other.
[Illustration: _Fig. 94. Cam Sector._]
[Illustration: _Fig. 95. Grooved Cam._]
[Illustration: _Fig. 96. Reciprocating Motion._]
Cylinder Cam.--Fig. 95 shows one form of cylinder A with a groove B in
it, which serves as a means for moving a bar C back and forth. The bar
has a projecting pin D, which travels in the groove.
This form of movement may be modified in many ways, as for instance in
Fig. 96, where the drum E has a sinuous groove F to reciprocate a bar G
to and fro, the groove being either regular, so as to give a continuous
back and forth movement of the bar; or adapted to give an irregular
motion to the bar.
[Illustration: _Fig. 97. Pivoted Follower for Cam._]
Double Cam Motion.--Cams may also be so arranged that a single one will
produce motions in different directions successively, as illustrated in
Fig. 97. The horizontal bar A, hinged at B to the upper end of a link C,
has its free end resting on the cam D.
The arm A has also a right-angled arm E extending downwardly, and is
kept in contact with the cam by means of a spring F. Connecting rods G,
H, may be hinged to the arm E and bar A, respectively, so as to give
motion to them in opposite directions as the cam revolves.
Eccentrics.--An eccentric is one in which the cam or wheel itself is
circular in form, but is mounted on a shaft out of its true center. An
eccentric may be a cam, but a cam is not always eccentric in its shape.
The term is one in direct contrast with the word _eccentric_.
[Illustration: _Fig. 98. Eccentric._]
[Illustration: _Fig. 99. Eccentric Cam._]
Fig. 98 shows the wheel, or the cam, which is regular in outline, that
is circular in form, but is mounted on the shaft out of its true center.
In this case it is properly called an eccentric cam but in enginery
parlance it is known as the eccentric, as represented in Fig. 99.
Triangularly-Formed Eccentric.--Fig. 100 illustrates a form of cam which
has been used on engines. The yoke A being integral with the bar B,
gives a reciprocating motion to the latter, and the triangular form of
the cam C, which is mounted on the shaft D, makes a stop motion at each
half-revolution, then produces a quick motion, and a slight stop only,
at the half turn, and the return is then as sudden as the motion in the
other direction.
[Illustration: Fig. 100. Triangularly-formed Eccentric.]