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Cams and Followers

A cam is a mechanical device used to transmit motion to a follower by direct contact. The driver is called the cam and the driven member is called the follower. In a cam follower pair, the cam normally rotates while the follower may translate or oscillate. A familiar example is the camshaft of an automobile engine, where the cams drive the push rods (the followers) to open and close the valves in synchronization with the motion of the pistons.

Types of Cams

i. Disk or plate cam

ii. Cylindrical cam

iii. Translating cam

Types of followers:

Based on surface in contact:

(a) Knife edge follower

(b) Roller follower

(c) Flat faced follower

(d) Spherical follower

Based on type of motion:

(a) Oscillating follower

(b)Translating follower

Based on line of motion:

(a) Radial follower: The lines of movement of in-line cam followers pass through the centres of the camshafts.

(b) Off-set follower: For this type, the lines of movement are offset from the centres the camshafts.

Cam nomenclature

Cam Profile: The contour of the working surface of the cam.

Tracer Point: The point at the knife edge of a follower, or the centre of a roller, or the centre of a spherical face.

Pitch Curve: The path of the tracer point.

Base Circle: The smallest circle drawn, tangential to the cam profile, with its centre on the axis of the camshaft. The size of the base circle determines the size of the cam.

Prime Circle: The smallest circle drawn, tangential to the pitch curve, with its centre on the axis of the camshaft.

Pressure Angle: The angle between the normal to the pitch curve and the direction of motion of the follower at the point of contact

Types of follower motion:

Cam follower systems are designed to achieve a desired oscillatory motion. Appropriate displacement patterns are to be selected for this purpose, before designing the cam surface. The cam is assumed to rotate at a constant speed and the follower raises, dwells, returns to its original position and dwells again through specified angles of rotation of the cam, during each revolution of the cam.

Some of the standard follower motions are as follows:

They are follower motion with

i. Uniform velocity

ii. Modified uniform velocity

iii. Uniform acceleration and deceleration

iv. Simple harmonic motion

v. Cycloidal motion

Displacement diagrams:

In a cam follower system, the motion of the follower is very important. Its displacement can be plotted against the angular displacement θ of the cam and it is called as the displacement diagram. The displacement of the follower is plotted along the y-axis and angular displacement θ of the cam is plotted along x-axis. From the displacement diagram, velocity and acceleration of the follower can also be plotted for different angular displacements θ of the cam. The displacement, velocity and acceleration diagrams are plotted for one cycle of operation i.e., one rotation of the cam. Displacement diagrams are basic requirements for the construction of cam profiles.