06-05-2014, 04:22 PM
CAMS
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Introduction
A cam is a mechanical member used to impart desired motion to a follower by direct contact.
The cam may be rotating or reciprocating whereas the follower may be rotating, reciprocating or oscillating.
Complicated output motions which are otherwise difficult to achieve can easily be produced with the help of cams.
Cams are widely used in automatic machines, internal combustion engines, machine tools, printing control mechanisms, and so on.
They are manufactured usually by die-casting, milling or by punch-presses.
A cam and the follower combination belong to the category of higher pairs.
Necessary elements of a cam mechanism are
A driver member known as the cam
A driven member called the follower
A frame which supports the cam and guides the follower
According to Shape
1) Wedge and Flat Cams
A wedge cam has a wedge W which, in general, has a translational motion.
The follower F can either translate [Fig.(a)] or oscillate [Fig.(b)].
A spring is, usually, used to maintain the contact between the cam and the follower.
In Fig.©, the cam is stationary and the follower constraint or guide G causes the relative motion of the cam and the follower.
Spiral Cams
A spiral cam is a face cam in which a groove is cut in the form of a spiral as shown in Fig.
The spiral groove consists of teeth which mesh with a pin gear follower.
The velocity of the follower is proportional to the radial distance of the groove from the axis of the cam.
The use of such a cam is limited as the cam has to reverse the direction to reset the position of the follower. It finds its use in computers.
Conjugate Cams
A conjugate cam is a double-disc cam, the two discs being keyed together and are in constant touch with the two rollers of a follower (shown in Fig.).
Thus, the follower has a positive constraint.
Such a type of cam is preferred when the requirements are low wear, low noise, better control of the follower, high speed, high dynamic loads, etc.
Globoidal Cams
A globoidal cam can have two types of surfaces, convex or concave.
A circumferential contour is cut on the surface of rotation of the cam to impart motion to the follower which has an oscillatory motion (Fig.).
The application of such cams is limited to moderate speeds and where the angle of oscillation of the follower is large.
Flat faced or mushroom follower.
When the contacting end of the follower is a perfectly flat face, it is called a flat-faced follower, as shown in Fig. 20.1 ©.
It may be noted that the side thrust between the follower and the guide is much reduced in case of flat faced followers.
The only side thrust is due to friction between the contact surfaces of the follower and the cam.
Motion of the Follower
The follower, during its travel, may have one of the following motions.
Uniform velocity
Simple harmonic motion
Uniform acceleration and retardation
Cycloidal motion