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What is a Cam and Follower System 

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 What is a cam and follower system?

A cam and follower system is system/mechanism that uses a cam and follower to create a specific motion.  The cam is in most cases merely a flat piece of metal that has had an unusual shape or profile machined onto it.  This cam is attached to a shaft which enable it to be turned by applying a turning action to the shaft.  As the cam rotates it is the profile or shape of the cam that causes the follower to move in a particular way.  The movement of the follower is then transmitted to another mechanism or another part of the mechanism.

If we examine the image below we can see how the profile of the cam imparts a particular motion on the follower.  In this case the motion is an up and down motion. 

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Examining the diagram shown above we can see that as some external turning force is applied to the shaft  (for example: by   motor or by hand) the cam rotates with it. The follower is free to move in the Y plane but is unable to move in the other two so as the lobe of the cam passes the edge of the follower it causes the follower to move up.  Then some enternal downward force (usually a spring and gravity)pushes the follower down making it keep contact with the cam.  This external force is needed to keep the follower in contact with the cam profile. This is probably best displayed in the animation. 

With an external downward force  Vs Without an external downward force

  

With an external downward force

Without an external downward force

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Displacement Diagrams 

    Displacement diagrams are merely a plot of two different displacements (distances).  These two dispalcements are:

  1. the distance travelled up or down by the follower and
  2. the angular displacement (distance) rotated by the cam

 

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In the diagram shown opposite we can see the two different displacements represented by the two different arrows.  The green arrow representing the displacement of the follower i.e. the distance travelled up or down by the follower.  The mustard arrow (curved arrow) shows the angular displacement travelled by the cam. 

  Note: Angular displacement is the angle through which the cam has rotated.

If we examine the diagram shown below we can see the relationship between a displacement diagram and the actual profile of the cam.  Note only half of the displacement diagram is drawn because the second half of the diagram is the same as the first.  The diagram is correct from a theoretical point of view but would have to changed slightly if the cam was to be actually made and used.  We will consider this a little more in the the following section - Uniform Velocity.
 

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Uniform Velocity 

 First we will consider what uniform velocity means without any referance to cam and follower systems.  Uniform Velocity means travelling at a constant speed in a fixed direction and as long as the speed or direction don't change then its uniform velocity.

In relation to cam and follower systems, uniform velocity refers to the motion of the follower.

Now let's consider a typical displacement diagram.  As we have already discussed, a displacement diagram is merely a plot of two different displacements (distances).  These two displacements are:

  1. the distance travelled up or down by the follower and
  2. the angular displacement (distance) of the cam

Considering the case of a cam imparting a uniform velocity on a follower over a displacement of 30mm for the first half of its cycle. 

If we consider this and take the cycle in steps.  Firstly if the cam has to impart a displacement of 30mm on follower over half its cycle then if must impart a displacement of 30mm¸180║ for every 1║ turned by the cam i.e. it must move the follower  0.167mm per degree turn.   This distance is to much to small to draw on a displacement diagram so we will consider the displacement of the follower at the start, at the end of the half cycle, the end of the full cycle and at certain other intervals (these intervals or the lenght of these intervals will be decided on later).
 

 

Angle the cam has rotated through 

Distance moved by the follower 

Start of the Cycle 

0║ 

0mm 

End of first Half of the Cycle 

180║ 

30mm 

End of the Full Cycle 

360║ 

0mm 

If we consider this in terms of a displacement diagram:

Firstly we will plot the graph. Before doing this we must first consider the increments that we will use. We will use millimeters for the follower displacement increments and because 1║ is too small we will use increments of 30║ for the angular displacement.

Once this is done then we can draw the displacement diagram as shown below.  Note a straight line from the displacement of the follower at the start of the motion to the displacement of the follower at the end of the motion represents uniform velocity.

Displacement Diagram for Uniform Velocity

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Simple Harmonic Motion

 For this type of motion the follower displacement does not change at a constant rate.  In other words the follower doesn't travel at constant speed.  The best way to understand this non-uniform motion is to imagine a pendulum swinging.

If you examine the pendulum as it swings you can see that as it swings towards A it slows down until it finally stops at A.  Then it starts to swing back in the other direction.  As it does so it starts to gain speed until it reaches it max speed at O.   Once the pendulum passes O it starts to slow down on its approach to B.  At B the pendulum stops and begins to swing back.  Again it speed up as it approaches O, reaches its maximum speed at O, then slows down on its approach to A, stops at A and then swings back in the other direction.  This cycle then keeps repeating itself.  If you watch the swinging pendulum shown below you should be able to notice the non-unifromity of its motion.

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Displacement diagram for SHM

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Uniform Acceleration and Retardation

This motion is used where the follower is required to rise or fall with uniform acceleration, that is its velocity is changing at a constant rate.


Displacement diagram for Uniform Acceleration and Retardation

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To conclude this section:

A cam can impart three types of motion on its follower:

Uniform velocity Simple harmonic motion Uniform acceleration and retardation

Each of these motion cam be represented by a displacement diagram.


   

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