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104
D) Larger cams mean more revolving weight and in high-speed
machines this leads to increased vibrations in the machine.
E) The inertia of a large cam may interfere with quick starting and
stopping.
The maximum pressure angle α
m
should, in general, be kept at or
below 30 degrees for translating-type followers and at or below 45
degrees for swinging-type followers. These values are on the
conservative side and in many cases may be increased considerably, but
beyond these limits trouble could develop and an analysis is necessary.
In the following, graphical methods are described by which a cam
mechanism can be designed with translating or swinging roller followers
having specified maximum pressure angles for rise and return. These
methods are applicable to any kind of time-displacement diagram.
Determination of Cam Size for a Radial or an Offset Translating
Follower.—
Fig. 9 shows a time-displacement diagram. The maximum
displacement is preferably made to scale, but the length of the abscissa,
L, can be chosen arbitrarily. The distance L from 0 to 360 degrees is
measured and is set equal to 2πk from which
π
=
2
L
k
k is calculated and laid out as length E to M in Fig. 10. In Fig. 9 the two
points P
1
and P
2
having the maximum angles of slope, τ
1
, and τ
2
, are
located by inspection. In this example y1 and y
2
are of equal length.
Angles τ
1
and τ
2
are laid out as shown in Fig. 10, and the points of
intersection with a perpendicular to EM at M determine Q
1
and Q
2
. The
measured distances
11
tan
τ
= kMQ and
22
tan
τ
=
kMQ
are laid out in Fig. 11, which is constructed as follows:
Draw a vertical line R
u
R
o
of length h equal to the stroke of the
roller follower, R
u
being the lowest position and R
o
the highest position
Fig. 9. Displacement Diagram
Fig. 10. Construction to Find
ktanτ
1
and ktanτ
2
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