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Cams

Cam is a common mechanism element that drives a mating componentknown as the follower. The cam accepts an input motion similar to a crankand imparts a resultant motion to a follower.

Cam Mechanisms

The transformation of one of the simple motions, such as rotation, into anyother motions is often conveniently accomplished by means of a cammechanism a cam mechanism usually consists of two moving elements, thecam and the follower, mounted on a fixed frame. Cam devices are versatile,and almost any arbitrarily-specified motion can be obtained. In someinstances, they offer the simplest and most compact way to transformmotions.

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 apredetermined specified motion to another element called the follower. Thecam has a very important function in the operation of many classes ofmachines, especially those of the automatic type, such as printing presses,shoe machinery, textile machinery, gear-cutting machines, and screwmachines. In any class of machinery in which automatic control and accuratetiming are paramount, the cam is an indispensable part of mechanism. Thepossible applications of cams are unlimited, and their shapes occur in greatvariety. Some of the most common forms will be considered in this chapter.

Classification of Cam Mechanisms

Plate or Disk Cams are the simplest and most common type of cam. Thistype of cam is formed on a disk or plate. The follower moves in a planeperpendicular to the axis of rotation of the camshaft.

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Cylindrical or Drum Cams this type of cam is formed on a cylinder. Agroove is cut into the cylinder; w/c varies along the axis of rotation.Attaching a follower that rides in the groove gives the follower motion alongthe axis of rotation.

Wedge Cam this type of cam is formed on a translated block. Gives thefollower motion perpendicular to the plane of translation.

B. Follower Position

In-line follower exhibits straight line motion, such that the line of

translation extends through the center of rotation of the cam.

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Offset follower exhibits straight line motion, such that the line of motion isoffset from the center of rotation of the cam.

C. Follower Shape

Knife-edge follower Consist of a follower that is formed to a point anddrags on the edge of the cam.

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Roller follower Consist of follower that has separate part, the roller, that ispinned to the follower stem.

Flat-faced follower Consist of a follower that is formed with a large, flatsurface available to contact the cam.

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Spherical-face follower Consists of a follower formed with a radius facethat contacts the cam.

Definitions

Rise is the part of the cycle that pushes the follower up.

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Dwell is the part of the cycle that keeps the follower stationary

Return is the part of the cycle that brings the follower back down

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Base Circle is the smallest circle tangent to the cam profile centered aboutthe cams center of rotation

Trace Point is a reference point on the follower for representing its motion.

Pitch Curve path of trace point relative to the cam.

Pressure Angle steepness of the cam profile. it is the angle betweennormal to the pitch curve and the direction of motion of follower.

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Follower Displacement Diagram

A plot of follower displacement versus time, or cam angular displacement.

When the cam is assumed to rotate at a constant velocity, time can berelated to angular displacement and vice versa.

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rotation angle of the cam during rise or fall interval (deg)

speed of the cam (deg/time)

total time period for the rise and fall interval

can also be used to determine the required speed of the cam, by observingthe time consumed during one cycle.

where is the total time for one cycle

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Follower Motion Schemes

Constant velocity characterized with a straight line displacement diagrambecause the velocity is uniform.

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Motion Curves

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Constant A cceleration

The displacement diagram for a rise or fall interval is divided into two halves,one of constant acceleration and the other of constant deceleration. Theshape of each half of the displacement diagram are mirror-image parabolas.

Motion Curves

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Harmonic M otion simple harmonic motion is a type of periodicmotion where the restoring force is directly proportional to thedisplacement.

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Cycloidal Motion it is the motion of a point on a disk rolling on a straightline.

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