Cams

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.



Concentric Cams

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.
 
More Examples
      
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.


  

Designing Cams

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.

offset.pngThe cam-follower has lifted by this amount. So the more you offset the cam, the greater the amount of lift you produce.


Calculating Lift

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 will produce.

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.

Cam-follower Plate

offset.pngA 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.

Cam 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 revolution.
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

concentric.png From the basic round cam you can increase the diameter across one axis, to produce an egg-shaped, or Lobed cam. Alternatively, 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.

Lobed 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.

Droped Cam


If you dip below the circumference of the circle then the cam follower drops, hence the term drop cam. You can calculate the drop of the cam by measuring from the lowest point of the drop to the circumference. 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 lobe cam.



Offset Cams


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 along toy.

The Pull-along Toy


Troubleshooting

Cam mechanisms work well if they are made accurately. However, any inaccuracy in making the device can lead to the mechanism 'jamming' during rotation. Also, inaccuracy can lead to the movement of the follower being less than smooth when rotating.


The diagrams show a typical CAM, mechanical toy. The follower has jammed as the profile rotates in a clockwise direction. There are possible reasons/faults that lead to this problem;
  1. The slides are too far apart, allowing the follower to jam as the profile rotates.

  2. The shape of the follower means that the movement of the mechanism is likely to jam or at best, move roughly rather than smoothly.

  3. The Shape of the cam is too extreme



The mistakes can be corrected easily by altering;
  1. The shape of the follower. A 'flat' follower is used. This means that the movement of the profile and follower is more likely to be smooth and efficient. Alternatively, you can provide support for the follower:


  2. The slides have been moved close together so that the follower is forced to move vertically, without 'jamming'.

  3. A larger, more subtle shape creates several events with smoother motion

Aligning the follower with the cam


Materials

When you have designed your cam, you will have to think about what to use to make it. The ideal material should be soft enough to cut easily but strong enough not to break or wear out too quickly. Cardboard, for example, can be a useful material. Several thin sheets can be cut to size and then stuck together (or laminated) using wood glue or PVA. This produces a very strong and durable cam. Alternatively you can use thick corrugated cardboard. The cams don't have to be industrial strength, very often a single 2mm or 3mm thick card one will work adequately.

MDF (medium density fibre board or craftboard) can be bought in various thicknesses. 4-6 mm works best and it is fairly easy to cut and can be shaped with sand paper.

Thin pine wood (again 4-6 mm) is another effective material to work with. It takes a little more time to cut and shape but is very durable, works well and looks good.
This entire automaton is made from paper and card. The lobed cam is constructed from thick corrugated card. The cranks (supporting shafts, cranks and push-rods) can be made out of pencils, straws or wooden dowels. Wood glue or PVA can be used to stick all the parts together. The base can be made out or a cereal or other box.

This illustration shows you how the mechanisms are constructed. The head is hinged at the back and is pushed up by the pushrod. The wings are slotted through the body (which acts as a pivot) and are attached to a rod that is in turn connected to the pushrod. This provides the lift for the wings. Both the head, wings and cam follower are assisted by gravity which provides the downward force.
Source: How to Design and Make Simple Automata by Robert Addams