Engineering Mechanisms: Cams
What Is A Cam:
Cams are mechanical devices used to convert the rotation of a shaft into simple or complex reciprocating linear motion. They can be sliding or rotational pieces in a mechanical linkage.
What Is A Cam Made Of:
The cam itself is the small mechanical part that looks like a guitar pick or a tear drop. Devices that utilize cams to transform one type of motion into another are typically composed of three main parts: the cam, the shaft (or rotating wheel), and the lever (also called a cam follower).
- Cam: The cam is the teardrop shaped device. The cam is defined by three radii (the base radius, the follower radius, and the prime radius). The base radius is the distance from the center of the cam to the circumferance of the smallest circle that is traced by the motion of the cam. The follower radius is the distance from the outside of the base radius to the circumferance of the largest circle that can be drawn from the motion of the cam. The prime radius is equal to the base radius plus the follower radius (it is the distance from the center of the cam to the circumferance of the largest circle that is traced by the motion of the cam).
- Shaft (Rotating Wheel): The cam is typically mounted in a fixed position on a shaft or rotating wheel. As the shaft rotates the pointy end of the cam comes into contact with the cam follower once every 360 degrees of angular motion.
- Cam Follower (Lever): The cam follower (or lever) is a long, linear part that comes into contact with and is linearly displaced by the pointy end of the cam once every 360 degrees of cam motion.
How Do Cams Work:
Mechanisms that utilize cams are typically designed to transform rotational motion into consistent reciprocating linear motion. Perhaps the most common example of a cam is an internal combustion engine. Car motors operate through a system of cams mounted on a cam shaft that open and close valves to regulate inputs (air and fuel) and outputs (exhaust) within the piston-cylinder assembly.
How Does Cam Design Affect The Characteristics of The Device:
The characteristics of the linear motion are dictated by the shape of the cam and the cam follower. The profile of the cam can be altered to achieve different characteristics in the overall mechanism. For example:
- Frequency: Cams are usually fairly small which helps to accomplish more rapid reciprocating motion. The larger the circumferance the longer it will take to rotate a full 360 degrees and the less frequent will be the resultant linear motion in the cam follower.
- Distance: The linear distance of travel in the cam follower is equivalent to the follower radius. Stated differently, the linear motion in the cam follower is represented by the difference between the highest point in the cam (the largest or prime radius) and the lowest point in the cam (the smallest or base radius). If you need more or less reciprocal motion simply increase or decrease the follower radius.
- Quickness of Motion: Using a sharp drop off from the largest (prime) radius to the smallest (base) radius of the cam will cause a quick dropping or punching type action of the cam follower. This type of profile can be used to achieve a quick release or quick return feature. Additionally, designing the cam with multiple changes in radius in order to achieve rapid linear displacements of the follower.
How Does Cam Follower Design Affect The Characteristics of The Device:
The geometry of the cam follower is important as well.
- Margin of Error: The smaller the point of the cam follower is, the more accurately the movement of the follower will resemble the design of the cam. That is, the less play or error margin there will be in the overall motion.
- Durability: A tradeoff with margin of error is the durability of the cam. Smaller cams are not as durable as larger cams and can be more prone to breaking. Thicker points offer more durability, but will not respond as well to smaller variations in the cam.
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