Mechanical cams remain the "heart of automation," providing precise timing and motion control in high-speed machinery. Understanding the geometry and dynamics of these systems is essential for modern machine design.
Classes of Cams
Cams may, in general, be divided into two classes: uniform motion cams and accelerated motion cams. The uniform motion cam moves the follower at the same rate of speed from the beginning to the end of the stroke. However, as the movement starts from zero to full speed instantly and stops in the same abrupt way, there is a distinct shock at the beginning and end of the stroke if the movement is at all rapid.
In machinery working at a high rate of speed, therefore, it is important that cams are so constructed that sudden shocks are avoided when starting the motion or when reversing the direction of motion of the follower.
The uniformly accelerated motion cam is suitable for moderate speeds, but it has the disadvantage of sudden changes in acceleration at the beginning, middle, and end of the stroke. A cycloidal motion curve cam produces no abrupt changes in acceleration and is often used in high-speed machinery because it results in low noise, vibration, and wear.
Cam Follower Systems
The three most used cam and follower systems are radial and offset translating roller followers and the swinging roller follower. When the cam rotates, it imparts a translating motion to the roller followers or a swinging motion.
Figure 1: Comparison of radial, offset translating, and swinging roller followers.
The motion of the follower is dependent on the shape of the cam. The displacement diagrams explain how a favorable motion is obtained so that the cam can rotate at high speed without shock.
Figure 2: Analysis of open-track vs. closed-track (positive drive) cam systems.
Open-track cams are more compact but generally require springs to maintain contact. Closed-track cams do not require a spring and have the advantage of positive drive throughout the rise and return cycle, which is essential for safety-critical applications.
Pressure Angle and Radius of Curvature
The pressure angle is the angle between the direction the follower is moving and the direction the cam is pushing it. Maintaining an optimal pressure angle is critical for efficiency and longevity.
The maximum pressure angle (αm) should generally be kept:
- ≤ 30 degrees for translating-type followers.
- ≤ 45 degrees for swinging-type followers.
Radius of Curvature
The minimum radius of curvature should be kept as large as possible to prevent undercutting of the convex portion and too high surface stresses. Undercutting occurs when the cam profile is too sharp for the roller to follow.
Figure 3: Visualization of undercutting where the radius of curvature becomes too small for the roller.
To enable milling or grinding of concave portions, the radius of curvature must satisfy:
Rc = ρmin + rf
Cam Forces and Materials
The fatigue failure of a cam is commonly due to surface stress. When calculating forces, inertia is the primary factor at high speeds. For parabolic motion, dynamic forces are often 2.0x the theoretical, whereas for cycloidal motion, the factor is only 1.05.
Materials should be selected based on the surface endurance limits found in the Machinery's Handbook. Common materials include alloy steels hardened to high Rockwell C values.
Modern Cam Manufacturing: The CNC Revolution
In the past, cams were manufactured by tracing a master template. Today, the process is fully digital, bridging the gap between design and manufacturing (CAD/CAM).
1. From SolidWorks to G-Code
Modern engineers use software like SolidWorks to generate cam geometry. Collision detection and motion analysis allow for the detection of undercutting before any metal is cut.
2. The "Electronic Cam" (Servo Motion)
In modern packaging, physical cams are often replaced by Servo Motors following a software-defined motion profile. This "Electronic Cam" allows for instant timing changes via code without mechanical re-tooling.
3. Maintenance and Inspection
A critical maintenance task is checking the Runout. Using magnetic Dial Indicators, technicians verify that the cam still follows the intended displacement diagram.
Comments
does it really? then why is it called a uniform acceleration cam at all?
and are you sure the accn also changes in the middle of the stroke?
The uniformly accelerated (constant acceleration) motion cam has a curve known as parabolic curve. It has constant positive and negative accelerations. It has an abrupt change of acceleration at the terminals and the transition point, which makes it undesirable except at low speeds.
See Parabolic cam profile (s, v and a)
We can see from the figure that the jerk is infinity at the beginning, middle and end of the stroke, while cycloidal motion cam curve has finite value of jerk.
See Jerk comparison between Parabolic and Cycloid
Hope this answers your query.
Ake.