The basic principle of the cam is to turn a
circular motion into a linear one. This is
referred to as reciprocating movement.
In it's simplest form you turn a handle to
make something move up and down.
The cam-follower is connected to,
and part off, a shaft known as the Push-Rod
The push-rod controls the direction of motion and
transfers the cam's movement. The cam-follower
should be designed with a smooth end that can
easily follow the cam's contours and movement.
This is very important as the cam and follower
will jam if not properly designed.
They are found in many machines and toys.
An concentric cam
is a disc with its centre of rotation positioned 'off centre'. This means as the cam rotates the flat follower rises and falls at a constant rate. This type of cam is the easiest to make and yet it is one of the most useful.
As it rotates it pushes the flat follower upwards and then allows it to drop downwards. The movement is smooth and at a constant speed.
A mechanical toy based on a series of concentric cams is seen below. As the handle is turned the shaft and the cams fixed to it rotate. Placed above the cams are a number of segments representing a 'snake'. As the cams rotate some of the flat followers are pushed upwards while others drop down. This gives the impression that the snake is moving.
Below is a mechanical toy based on a CAM mechanism. As the handle on the concentric cam is turned the top part of the egg shell lifts to reveal a face. The basic construction of the toy is also shown below. The 'flat' follower moves upwards and downwards as the cam rotates. Although the design is simple it must be made accurately or the mechanism will stick.
In order to design a cam you need to know
what you want it to do. It may have just one or
several movements per revolution.
Cams turn on a shaft and so need to be offset
to create movement. If you have a circle with
the shaft running through the center then
nothing happens. However, if you offset it you
can create a mechanism that can lift.
The cam-follower has
lifted by this amount.
So the more you offset
the cam, the greater
the amount of lift you
It is very easy to calculate the amount of lift
by simply taking the measurement from the
center of the drive shaft to the lowest point of
the cam and subtracting this from the
measurement to the highest point. This
calculation will give the amount of lift the cam
The concentric cam, is a circle with an offset
center. By offsetting the center you
produce the lift. The further you move away
from the center point the greater the amount
of lift you will produce. Don't overdo it. It
is better to make a larger cam that rises
gently than a small one that rises rapidly.
They will both do the same job but the smaller
cam is more likely to jam.
If you need to produce lift to a specific
height, the following formula is simple
and shows you how to
work out the fixing point for the drive shaft:
Every millimeter that you move away
from the cam's center point, you must
double, in order to calculate the amount of
lift generated by the cam.
To eliminate the turning affect you can either
build stops to prevent turning, (this can affect
the overall look of your automata) or put
guides either side of the cam.
A thin, card cam when used
with a wooden dowel camfollower
may jam. To avoid this
a circular cam-follower known
as a Plate
should be used.
Because of it's large, flat contact
area, it is less likely to jam. This
type of follower works best with
concentric and some lobed
cams. It will not work on cams
with complicated shapes.
This cam produces a smooth
uplift which suddenly drops
down. It is often referred to
as a snail cam
because of its
shape or contour. This cam
can only work in one direction.
If you turn it the other way the cam-follower
would jam. You need to bear this in mind
when you are designing cams.
To ensure the rotation is smooth, the vertical centre line of the snail/drop cam is positioned slightly to the left of the slide.
This cam produces several
short up and down
movements from one
This cam produces
three very distinct
movements from one revolution.
You can combine as many
movements as your cam will allow.
Remember that the cam-follower
has to work smoothly. If you try
to make it do too much or
make the contours too
steep, such as this
one on the right, it will
jam. The cam-followers
can only move on gentle
curves, make them too tight
and you will have problems!
Lobed and Droped
From the basic round cam you can increase
the diameter across one axis, to produce an
egg-shaped, or Lobed
you can create a recessed area that drops
below the circumference of the circle,
producing a Drop
cam. You can combine
these two elements in a single cam.
If you raise part of the circumference, you
produce a lobe, hence the name lobed cam.
This will lift the cam follower by the maximum
height from the tip of the lobe to the
circumference of the circle. When the camfollower
returns to the circle it will pause and
this is referred to as the dwell angle. You can
produce a pause or dwell angle on top of the
lobe if you design it properly.
If you dip below the circumference of the circle
then the cam follower drops, hence the term
You can calculate the drop of the cam by
measuring from the lowest point of the drop to
A very popular form of drop cam is called the
snail cam. This has a sudden drop that slowly
rises to the next drop point. This cam is used a
lot in automata and is a blend of both drop and
An offset cam not only moves
things up and down but also in a
circular motion. You must make
sure that the cam contacts the
cam-follower plate either side
of the cam shaft. If it contacts
directly underneath then it
will only lift. Offsetting two
cams either side produces
movement in opposite
directions, giving you both
up and down as well as a
side to side movement.
Note that the closer
the cam is to the center of
the followerf the
faster and further it will
rotate, moving away from the
center has the opposite effect.
The skew cam has a thin plate which is
attached to the drive shaft at an angle. As it
turns, it contacts a forked lever which it turns
from side to side. This twists a vertical rod and
so transfers the movement.
The skew cam is like a wobbly plate and turns
a circular motion into a side to side one.
This can be adapted to form the axle of a pull
The Pull-along Toy