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97
The arrangements in Figs. 1a, 1b, and 1c show open-track cams. In
Figs. 2a and 2b the roller is forced to move in a closed track. Open-track
cams build smaller than closed-track cams but, in general, springs are
necessary to keep the roller in contact with the cam at all times. Closed-
track cams do not require a spring and have the advantage of positive
drive throughout the rise and return cycle. The positive drive is
sometimes required as in the case where a broken spring would cause
serious damage to a machine.
Displacement Diagrams.—Design of a cam begins with the
displacement diagram. A simple displacement diagram is shown in Fig.
3. One cycle means one whole revolution of the cam; i.e., one cycle
represents 360°. The horizontal distances T
1
, T
2
, T
3
, T
4
are expressed in
units of time (seconds); or radians or degrees. The vertical distance, h,
represents the maximum “rise” or stroke of the follower.
The displacement diagram of Fig. 3 is not a very favorable one
Fig. 2a. Closed-Track Cam
Fig. 2b. Closed-Track Cam with
Two Rollers
Rise Interval Dwell Interval Dwell Interval
Return
Il
h=
Follower
Stroke
One cycle = 360°
Fig. 3. A Simple Displacement Diagram
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