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Kinematics of Machines Tutorials : Classification of cams and followers

admin — Sun, 13 Feb 2011 20:37:10 +0000

CLASSIFICATION OF CAMS AND FOLLOWERS,

Cams are classified according to their

Basic shapes
Types of follower movement
Manner of constraints of the follower

Classification according to basic shapes of the cams:

Wedge cam: A wedge cam has a wedge of specified contour. The translation motion of the wedge is imparted to the follower which either reciprocates or oscillates. Generally, a spring is used to maintain contact between the follower and the cam

Figure: Wedge cam

Plate cam: A cam made out of a plate in such a way that follower moves radially from the centre of rotation is known as plate cam. These cams are also known as disc cams or radial cams because the surface of the cam is so shaped that the follower reciprocates or oscillates in a plane at right angle to the axis of the cam. By far. the most common is the plate cam. For this reason, we shall restrict our discussion to plate cams, although the concept presented pertains universality

Figure: Plate cam

Cylindrical cam: In a cylindrical cam, a circumferential contour is cut on the surface of a cylinder which rotates about its axis. The follower may cither oscillate or reciprocate as shown in Figures . These cams are also known as drum or barrel cams.

Figure: Cylindrical cam

Spiral cam: A circular plate in which spiral groove is cut and a pin gear follower meshes with the teeth cut on spiral groove, as shown in Figure below, is called spiral cam. Il is also known as face cam. Tne main limitation of such cam is that it has to reverse the direction to reset the position of the follower.

Figure: Spiral cams

Globoidal cam: A globoidal cam may have either concave or convex surface and a circumferential contour is cut on the surface. The follower in these cams has an oscillatory motion.

Figure: Globoidal Cams (Convex globoidal and Concave Globoidal Cams)

Classification according to movement of the follower:

The motions of the followers are distinguished from each other by the dwells ,rises and returns they have.

Rise of a cam: The motion of the cam which tend to lift the follower is known as the rise motion.

Dwell of a cam: The rotation of the cam for which the follower is stationary at its position is known as dwell of the cam.

Return of a cam: The motion (rotation) of the cam for which the follower tends to move its original position is known as the return motion of the cam.

Cams are classified according to the motions of the followers in the following ways:

1. Rise-Return-Rise (R-R-R): In this, there is alternate rise and return of the follower with no periods of dwells (as shown in figure). It’s use is very limited in the industry. The follower has a linear or an angular displacement.

Figure: Rise-Return-Rise Follower motion

2. Dwell-Rise-Return-Dwell (D-R-R-D): In such a type of cam, there is rise and return of the follower after a dwell (as shown in figure). This type is used more frequently than the R-R-R type of cam.

Figure: Dwell-Rise-Return-Dwell Follower motion

3. Dwell-Rise-Dwell-Return-Dwell (D-R-D-R-D): It is the most widely used type of cam. The dwelling of the cam is followed by rise and dwell and subsequently by return and dwell as shown in Fig.

Figure: Dwell-Rise-Dwell-Return-Dwell Follower motion

Figure: Return of the follower is by a fall, the motion may be
known as Dwell-Rise-Dwell (D-R-D).

Classification according to manner of constraint of the follower:

To reproduce exactly the motion transmitted by the cam to the follower, it is necessary that the cam and follower remains in touch at all speeds and at all times. The cams can be classified according to the manner in which this is achieved.

1. Pre-loaded Spring Cam: A pre-loaded compression spring is used for the purpose of keeping the contact between the cam and the follower.

2. Positive-drive Cam: In this type, constant touch between the cam and the follower is maintained by a roller follower operating in the groove of a cam. The follower cannot go out of this groove under the normal working operations. A constrained or positive drive is also obtained by the use of a conjugate cam.

3. Gravity Cam: If the rise of the cam is achieved by the rising surface of the cam and the return by the force of gravity or due to the weight of the cam, the cam is known as a gravity cam. However, these cams arc not preferred due to their uncertain behavior.

This is all about the classification of the cams. In next tutorial, we will learn about the classification of follower. Hope you enjoyed it, Please comment if you feel any difficulty in understanding the topics covered in this tutorial.

References:

1. Theory of Mechanisms and Machines By C. S. Sharma, Kamlesh Purohit

2. Theory of Machines by SS Rattan

Tutorials of Cams:

Part 1: Kinematics of Machines Tutorials : Introduction of Cams

Kinematics of Machines Tutorials : Introduction of Cams

admin — Fri, 11 Feb 2011 14:23:00 +0000

Introduction of Cams

A cam is a mechanical member used to produce desired motion to a follower by direct contact. The cam may be rotating or reciprocating whereas the follower may be rotating, reciprocating or oscillating.

Image: Rotating CAM with reciprocating follower

(Source: http://www.ul.ie/~kirwanp/whatisacamandfollowersyste.htm)

Image: Reciprocating cam with reciprocating follower

Difference between Oscillating motion and Reciprocating Motion

One of the few doubts which arise in the mind of the student is the difference between oscillating and reciprocating motions. The difference between oscillating motion and reciprocating motion is that the oscillating motion follows the equation of Simple Harmonic Motion. We will discuss the details of simple harmonic motion in some other article but right now our definition is that simple harmonic motion is the motion which follows sinusoidal curve. i.e. which is according to the equation X=ASin(wt). Any to and fro motion is known as reciprocating motion. More precise, the definitions are:

Oscillation:

Oscillation, repeating back-and-forth motion, is very common in nature. Often we think of "Simple Harmonic Oscillation" as a motion which varies sinusoidally with time: Many, many systems oscillate in this way, from a plucked string to a ringing bell, to radio waves (x is an electric field, say), and lots of other stuff. Even things that don’t move back and forth can oscillate.

Reciprocation:

"Reciprocation" is most often used to refer to motion that is repetitive and involves some kind of back-and-forth changing of the position. I wouldn’t demand that the position vary sinusoidally with time to say that it is reciprocating (although that is nearly the case with common objects in reciprocating motion). Examples include pistons in pumps and steam engines on trains as things that reciprocate. Reciprocating saws have a straight saw blade that is driven up and down with a motor.

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 follower pair is always a higher pair. i.e. it must have either line or point contact and not the surface contact. The necessary elements of a cam-follower system are:

A driver member known as the cam
A driven member called the follower
A frame which supports the cam and guides the follower

Design of cams:

Cams are widely used in almost all machinery. They include internal combustion engines, a variety of machine tools, compressors and computers. In general, a cam can be designed in two ways.

The profile of a cam is so designed to give a desired motion to the follower, or
To choose a suitable profile to ensure a satisfactory performance by the follower.

The latter procedure simplifies the manufacturing process, and this is why most of the cams are designed in this manner. However, if a particular motion is desired for the performance of a machine, the first procedure is to be adopted.

The cam design is usually simple for low-speed machinery. Although a cam may be designed to impart a particular motion to the follower, certain motions give rise to large velocities, accelerations and jerks over a cycle which will produce high stresses and undue vibration in the machine assembly. These problems are more pronounced for high-speed cams. In such cases the design of a cam is more complicated.

References:

Theory of Machines by SS Rattan, Tata McGraw Hills
Mechanism and Machine Theory by R V Dukkipati, Bohem Press
http://van.physics.illinois.edu/qa/listing.php?id=299