A Cam is a mechanical element used to drive another element, a follower, through a specified motion by direct contact. A cam converts an input motion (usually rotary motion) into either reciprocating or oscillatory output action. A cam system consists of the cam itself and the follower, which provides the output motion.
Cam devices are versatile, and almost any arbitrarily-specified motion can be obtained. In some instances, they offer the simplest and most compact way to transform motions. The problem in cam design is how to determine a cam contour that will ultimately deliver a specified motion with acceptable velocity, acceleration and jerk. In all cam systems it is important that the follower is always in contact and following the motion of the cam. This is achieved a number of ways including gravity, using a mechanical constraint system (i.e groove), using a spring force and using a pneumatic or hydraulic force.
Classification of Cam Mechanisms:
Cam mechanisms can be classified by:
a. Modes of input/output motion.
b. The type and the arrangement of the follower.
c. The shape of the cam.
a. Modes of input/output motion:
The most common modes of input/output motion of cam follower system are:
• Rotating cam translating follower.
• Rotating follower.
• Translating Cam-translating follower .
• Stationary Cam-rotating follower.
b. The type and the arrangement of the follower:
• Knife-edge • Flat-face
• Roller • Spherical-face
The follower can be arranged in two different configurations; in-line follower and offset follower. In in-line follower, the centerline of the follower passes through the centerline of the camshaft. On the other hand, in Offset followers, the centerline of the follower does not pass through the centerline of the camshaft. These two configurations are shown in the figures below.
c. The shape of the cam:
A few commonly used cam shapes are illustrated in the following figures.
Cam Terminology :
Trace point: A theoretical point on the follower corresponding to the point of a fictitious knife-edge follower. It is used to generate the pitch curve. In the case of a roller follower the trace point is at the center of the roller.
Pitch curve: The path generated by the trace point at the follower is rotated about a stationary cam.
Working curve: The working surface of a cam in contact with the follower. For the knife edge follower of the plate cam, the pitch curve and the working curves coincide. In a close or grooved cam there is an inner profile and an outer working curve.
Pitch circle: A circle from the cam center through the pitch point. The pitch circle radius is used to calculate a cam of minimum size for a given pressure angle.
Prime circle (reference circle): The smallest circle from the cam center through the pitch curve.
Base circle: The smallest circle from the cam center through the cam profile curve.
Stroke or throw: The greatest distance or angle through which the follower moves or rotates.
Follower displacement: The position of the follower from a specific zero or rest position (usually it's the position when the follower contacts with the base circle of the cam) in relation to time or the rotary angle of the cam.
Pressure angle: The angle at any point between the normal to the pitch curve and the instantaneous direction of the follower motion. This angle is important in cam design because it represents the steepness of the cam profile.
Next Time we will illustrate the Design of Cam. so keep up with us.
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