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Cam Mechanism

The transformation of one of the simple motions, such as rotation, into any other motions is often conveniently accomplished by means of a cam mechanism. A cam mechanism usually consists of two moving elements, the cam and the follower, mounted on a fixed frame. 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.

A cam may be defined as a machine element having a curved outline or a curved groove, which, by its oscillation or rotation motion, gives a predetermined specified motion to another element called the follower. The cam has a very important function in the operation of many classes of machines, especially those of the automatic type, such as printing presses, shoe machinery, textile machinery, gear-cutting machines, and screw machines. In any class of machinery in which automatic control and accurate timing are paramount, the cam is an indispensable part of mechanism.

A cam is a specially shaped component that serves to guide the motion of a component called a follower. The cam has a rotary or linear motion. The most important advantage of cam principle is that it is quite conveniently possible to introduce pauses of any desired length into the motion. This advantage is widely used in machinery of all kinds, such as packaging machines and many others. Using cams it is possible to perform simple sliding movements or oscillatory. It can also precisely controlled movements of elaborate shape.

For e.g, Guiding a milling cutter along a curved outline of any desired shape. A cam mechanism (Figs.1.2 & 3) basically consist three parts: the frame or base (a), on which the cam (b) is mounted, and the follower (c) whose motion is controlled by the cam, which is given a linear (Fig.1) or a rotary (Figs.2 and 3) motion. A follower is a component which is specially shaped component of cam used to guide its motion.

A suitable example of a cam mechanism is the valve gear of an internal-combustion engine (Fig.4). The rotating cam has an approximately pear-shaped profile comprising two circular curves joined by two straight lines which are tangential to those curves. The follower consists of a roller tappet which is moved up and down by the cam and imparts this motion to the rod that controls the opening and closing of the valve. The center of the roller traces a curve of similar shape to the cam profile.

In Fig.5 the stroke, the speed and the acceleration of the roller tappet have been plotted against the angle of rotation the acceleration undergoes a sudden change in value, which imparts a jerk to the tappet. This occurs every time the radius of curvature of the cam profile changes abruptly e.g., transition from circular curve to tangent and vice versa. Conversely, it is possible to start from a certain acceleration curve that comprises no abrupt changes (red curve in Fig.5) and design a cam so shaped as to gentle acceleration, free of jerks.

Fig.6 shows a cam mechanism whose two paths can so move a milling cutter in two mutually perpendicular directions that a cut conforming to a specific shape (in this case the letters HB) can be produced. The two cam paths are determined as follows: The trace is subdivided into a number of approximately equal portions (22 in the present example).

Then the distances that the cutter has to move in the horizontal and vertical directions, respectively, to reach this point from the initial position 0, are plotted in two diagrams for all the points (0 to 22). Then the circumference of the cam disc is also divided into 22 equal parts. From 19 to 22 the cutter must moreover be lifted off the work, since this constitutes the return motion to the starting point.