As we have said before, the main purpose of a cam is to move a follower in a specific way. A good cam design can be made to create almost any desired follower movement. This is done by generating a follower displacement diagram. The x-axis represents one full revolution of a cam while the y axis represents the follower's displacement. Rise is the beginning of the stroke, when the cam begins to push on the follower, dwell is the period in which there is no change in displacement, and return is the period where the cam returns to the starting position, usually using it's own momentum, gravity, or a spring attached to the follower.
Base Circle (Sometimes Base Radius): The circle that the design begins with. It is centered on the cam's axis and changes to it determine the change in displacement of the follower.
Cam Profile: The part of the cam that comes in contact with the follower.
Pitch Curve: The path followed by the trace point. This is determined first and the cam profile is derived from it.
Pitch Point:This is where the pressure angle is greatest
Pressure Angle: This is the angle between the normal to the pitch curve and the instantaneous direction of the follower. This will determine the cam profile's steepness. If the cam is too steep, the follower may fail to keep contact with the cam profile.
Prime Circle: The prime curve the trace point would follow if the cam profile was the base circle.
Trace Point:The point where a follower makes contact with the cam profile. On a roller follower, this is the center of the wheel.
The design process begins by generating a follower
displacement diagram. The x-axis, or abscissa, of the diagram is split into equal parts, either based on time elapsed or angle of the base circle.
In the figure below, the displacement diagram and the cam were split into 12 equal parts. The slope of the displacement diagram determines the shape of the cam. This is a very simple and "ball-park" process. If the diagram is split into more pieces, then the cam will be more efficient, but the mathematical complexity of determining the shape of the cam will increase. For this reason, and the design limitations mentioned below, most cam design is done on computers with dedicated programs.
Some other considerations to keep in mind when designing a curve are keeping pressure angle to a minimum by having a gentle slope, with care being taken to not make the cam too large and heavy. Other design limitations include friction, material strength of the components, desired velocity of the follower, and the tolerances in the variation of the parts due to the machining process. All of these considerations are vital to the proper function of the cam and follower.