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101
Smoothness in velocity and acceleration during the stroke is the
advantage inherent in this curve. However, the instantaneous changes in
acceleration at the beginning and end of the stroke tend to cause
vibration, noise, and wear. As can be seen from Fig. 6, the maximum
acceleration values occur at the ends of the stroke. Thus, if inertia loads
are to be overcome by the follower, the resulting forces cause stresses in
the members. These forces are in many cases much larger than the
externally applied loads.
Cam Profile Determination.—In the cam constructions that follow an
artificial device called an inversion is used. This represents a mental
concept which is very helpful in performing the graphical work. The
construction of a cam profile requires the drawing of many positions of
the cam with the follower in each case in its related location. However,
instead of revolving the cam, it is assumed that the follower rotates
around the fixed cam. It requires the drawing of many follower
positions, but since this is done more or less diagrammatically, it is
relatively simple. As part of the inversion process, the direction of
rotation is important. In order to preserve the correct sequence of
events, the artificial rotation of the follower must be the reverse of the
cam's prescribed rotation. Thus, in Fig. 7 the cam rotation is
counterclockwise, whereas the artificial rotation of the follower is
clockwise.
Radial Translating Roller Follower: The time-displacement diagram for
a cam with a radial translating roller follower is shown in Fig. 7(a). This
diagram is read from left to right as follows: For 100 degrees of cam
shaft rotation the follower rises h inches (AB), dwells in its upper
position for 20 degrees (BC), returns over 180 degrees (CD), and finally
dwells in its lowest position for 60 degrees (DE). Then the entire cycle
is repeated.
Fig. 7(b) shows the cam construction layout with the cam pitch curve as
a dot and dash line. To locate a point on this curve, take a point on the
displacement curve, as 6" at the 60-degree position, and project this
horizontally to point 6"' on the 0-degree position of the cam
construction diagram. Using the center of cam rotation, an arc is struck
from point 6" to intercept the 60-degree position radial line which gives
point 6"' on the cam pitch curve. It will be seen that the smaller circle in
the cam construction layout has a radius R
min
equal to the smallest
distance from the center of cam rotation to the pitch curve and,
similarly, the larger circle has a radius R
max
equal to the largest distance
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