An important requirement of a design is to make sure the cam and cam-follower bearing are always in contact.
There are two designs that make sure there is contact between the cam and the cam-follower: Force-Closed and Form Closed.
Note: This topic shows cam types with Cam-Roller Bearings, only.
An EXTERNAL FORCE makes sure the cam-roller remains in contact with the cam profile. External Forces include: Gravity, Springs, Air, and Hydraulic-Cylinders.
The External-Force that cam-types illustrate below is a Spring.
RECIPROCATING CAM-FOLLOWERS (also called Translating Cam-Follower)
Oscillating Cam Follower (also called Swinging-Arm Cam-Follower.
Force-Closed Cams: Advantages and Disadvantages
•Only one cam to manufacture
•Less manufacturing precision to give an operating cam
•Less expensive to manufacture
•Zero backlash (play) - when the follower does not lift off the cam!
•Reduced Scuffing: The surface-speed of the cam and the cam-follower bearing will be identical at low speeds. This reduces or eliminates scuffing wear on the cam-profile. At high speeds, or where the radius of the cam varies rapidly, there may be some scuffing (mismatching velocities) as the roller cannot accelerate or decelerate enough.
In the case of a machine jam, you can:
•Configure the mechanism so that the radius of the cam decreases as the mechanism moves towards where it is most likely to jam. Say, as the tool moves into the assembly-area, the cam-follower will lift off the cam profile.
•Alternatively, it is possible to use an air-cylinder to actively lift the cam-follower off the cam-profile in response to a sensor detecting a jam in the machine.
•Cam-Lift: You must calculate the spring (or cylinder) parameters (Free-Length and Spring-Rate) to make sure the cam-follower does not lift off the cam-profile at all operating speeds.
•Cam-Force: The contact-force on the cam-profile is more with a spring than with a Form Closed Cam. This may reduce the operating life of the cam.
•Machine Stiffness: The increase to contact-force will also increase other forces on the machine frame, especially when the pressure angle is high. You may need a stronger and more rigid machine frame.
•Residual Energy after Stop/E-stop: At stand-still, the torque on the cam from the spring will tend to turn the cam.
•Cam-Shaft Speed Fluctuation The increase to contact-force will also increase the torque fluctuation on the cam-shaft. At slow speed, the torque fluctuation due to a spring may be very high, again, if the pressure angle is high.
The speed of the cam-shaft may fluctuate because the spring resists the rotation during the 'rise', and assists the rotation during the 'return' segment (or vise versa, if arranged in the opposite sense). If the speed fluctuation is excessive (>10%) - the machine may damage itself - see Torque, Overrun, and Power.
Form Closed Cams are also called Body Closed Cams, or sometime, Positive Drive Cams. An Internal Force makes sure the cam-follower remains in contact with the cam.
There are three different design types.
* Cam-Bearings are nearly always circular - and given the name Cam-Roller - and they usually roll. However, other shapes are possible. In this topic we use the term cam-bearing for the element that should remain in continuous contact with the cam.
Cam-Bearings can have a number of different shapes, each with advantages and disadvantages.
The Cam-Bearing is a child to the Cam-Follower. The Cam-Follower may be a swinging-arm-follower or a translating-arm-follower.