Cams

A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel (e.g. an eccentric wheel) or shaft (e.g. a cylinder with an irregular shape) that strikes a lever at one or more points on its circular path. The cam can be a simple tooth, as is used to deliver pulses of power to a steam hammer, for example, or an eccentric disc or other shape that produces a smooth reciprocating (back and forth) motion in the follower, which is a lever making contact with the cam.

• Overview

The cam can be seen as a device that rotates from circular to reciprocating (or sometimes oscillating) motion. A common example is the cam shaft of an automobile, which takes the rotary motion of the engine and translates it into the reciprocating motion necessary to operate the intake and exhaust valves of the cylinders.

Cams can also be viewed as information-storing and -transmitting devices. Examples are the cam-drums that direct the notes of a musical box or the movements of a screw machine's various tools and chucks. The information stored and transmitted by the cam is the answer to the question, "What actions should happen, and when?" (Even an automotive camshaft essentially answers that question, although the music box cam is a still-better example in illustrating this concept.)

• Displacement Diagram

Certain cams can be characterized by their displacement diagrams, which reflect the changing position a roller follower (a shaft with a rotating wheel at the end) would make as the cam rotates about an axis. These diagrams relate angular position, usually in degrees, to the radial displacement experienced at that position. Displacement diagrams are traditionally presented as graphs with non-negative values. A simple displacement diagram illustrates the follower motion at a constant velocity rise followed by a similar return with a dwell in between as depicted in figure 2.^[4] The rise is the motion of the follower away from the cam center, dwell is the motion where the follower is at rest, and return is the motion of the follower toward the cam center.

• Plate Cam

The most commonly used cam is the plate cam which is cut out of a piece of flat metal or plate. Here, the follower moves in a plane perpendicular to the axis of rotation of the camshaft. Several key terms are relevant in such a construction of plate cams: base circle, prime circle (with radius equal to the sum of the follower radius and the base circle radius), pitch curve which is the radial curve traced out by applying the radial displacements away from the prime circle across all angles, and the lobe separation angle (LSA - the angle between two adjacent intake and exhaust cam lobes).

The base circle is the smallest circle that can be drawn to the cam profile.

Problems For Practice

1. Design an edge cam to impart a rise of 35 mm at uniform velocity motion, during 180⁰ of rotation; rest during next 90⁰, and a uniform fall of 35 mm during the remaining 90⁰ of revolution. Assume base circle radius to be 35 mm and angular rotation of cam in clockwise direction?
2. The knife edge following for a plate cam is to have the following motion, 90⁰ lift of 30 mm with simple harmonic motion then 90⁰ dwell, 180⁰ falls of 30 mm with uniform velocity. The cam is to rotate clockwise and the least cam radius is to be 50 mm. Construct the cam profile full size?
3. A radial cam, rotating clockwise, operates an offset roller follower and gives it the following motion; 120⁰ lift of 36 mm with S.H.M. 60⁰ dwell and 180⁰ falls of 36 mm with uniform acceleration and retardation motion. Construct the cam profile, full size, if the follower center line is offset 25 mm to the cam center of rotation. The minimum radial cam thickness is to be 30 mm and the roller is 10 mm?
4. Construct the profile of line knife edge follower for a disc cam with following motion: 180⁰ lift through 40 mm with S.H.M., 60⁰ dwell and 120⁰ fall of 40 mm with uniform acceleration and relation motion. The least radius of the cam is 20 mm and it is to rotate in an anticlockwise?
5. Determine the profile of a cam capable of giving the roller ended follower a rise of 60 mm and a fall of 60 mm with uniform velocity motion for the one revolution of the cam. Roller diameter 25 mm, spindle diameter 50 mm, least distance from the spindle to the cam profile 20 mm. Assume the line of stroke to pass through the axis of the cam?

Question For Self Examination

1. A cam rotating anti clock wise at uniform speed is required to give knife edge following to satisfy the following motion: - (a)    Follower to move outward through 20 mm during 120⁰ of cam rotation with acceleration & retardation motion. (b)   Follower to dwell for next 60⁰. (c)    Follower to fall the rest of the cam rotation with acceleration & retardation motion. Draw the profile or the cam displacement diagram, when the kine of the stroke of the follower passing through the center of the center of the camshaft. The minimum radius of cam = 20 cm. Use scale 1:4 or 1:5.
2. Draw profile of  a cam  with following data :- (a)    Least radius – 30 mm. (b)   Lift of following – 30 mm.
3. The follower is lifted with S.H.M. during 120⁰ of cam rotation, and then remains at rest for next 60⁰ and returns to the starting point during next 180⁰ of cam rotation with uniform motion. The follower is knife edged.
4. Draw profile of cam with following data :- (a)    Least radius = 40 mm. (b)   Lift of follower = 25 mm. The follower is lifted with S.H.M. during 90⁰ of cam rotation, then remains at rest for next  90⁰ and return to the starting point during next 180⁰ of cam rotation with uniformly accelerated and retardation motion. The follower is knife edged.
5. Design an edged cam to impart a rise of 35 mm at uniform velocity motion during 180⁰ of revolution rest during next 90⁰ and a fall of 35 mm during the remaining 90⁰ of revolution with uniform velocity motion. Assume base circle radius to be 35 mm and angular rotation of the cam in an anticlockwise direction?
6.  Draw the profile of an edge cam to give uniform up word motion of 45 mm during the first half of the revolution and again uniform return motion through the  remaining half of the revolution. Maximum distances from cam center to the edge of the follower is 50 mm. Diameter of the shaft is 36 mm. The cam makes the wedge ended follower to reciprocate with uniform velocity motion?
7. Draw the profile of a cam which allows the follower to oscillate with uniform angular velocity about a fixed canter. Every revolution of the cam completes one oscillation of the follower . Distance between the center of cam and roller is 35 mm. Lift = 75 mm, Lever diameter = 12 mm and length of lever = 140 mm?