CAMS and FOLLOWERS

Have you ever looked closely at a simple mechanical toy ? If you have the opportunity to study one closely you will see that it is made up of mechanisms, usually including CAMS. Can you name any mechanical devices that use cams as part of its movement?

A CAM changes the input motion, which is usually rotary motion (a rotating motion), to a reciprocating motion of the follower. They are found in many machines and toys

A CAM has two parts, the FOLLOWER and the CAM PROFILE. Diagrams one to six show a rotating cam pushing a follower up and then allowing it to slowly fall back down

Dia 1 Dia 2 Dia 3 Dia 4 Dia 5 Dia 6

In diagram 1 the cam is in a vertical position and it slow rotates in an anticlockwise direction. As it rotates the follower drops down. In diagram 4 the follower is in its lowest position. The cam continues to rotate and the follower begins to rise in diagrams 5 and 6 until it returns to its original position.

KEY PHRASES ONE CYCLE One rotation/revolution of the cam.

DWELLWhen the cam rotates but the follower does not rise or fall.

THE RISEThat part of the cam that causes the follower to rise.

What do you think is the 'FALL' ?

TYPES OF FOLLOWER

FLAT POINT/KNIFE ROLLER OFFSET

The cam seen below is a 'pear shaped cam'. As the handle is turned the cam profile rotates. The model moves forwards and backwards as a result. The movement is gentle although the faster the handle is turned the faster the movement of the model becomes.

CAM PROFILESEXAMPLE USE OF A CAM

Below is a typical use of a cam. A simple toy based on a cartoon character (Ed the Handyman) is controlled by a cam and follower. As the cam rotates in an anticlockwise direction, the follower falls (diagram 2) and so does the arm of the toy. As the cam continues to rotate the arm begins to rise as seen in diagram 4. The cam shown is a pear shaped cam (named after its shape).

Cams can be shaped in any number of ways and this is determined by the way the follower is to move. The shape of the cam is called the PROFILE. Examples of various cam profiles can be seen below.

PEAR CIRCULAR HEART DROPPear shaped cams are used on the shafts of cars. The follower remains motionless for about half of the cycle of the cam and during the second half it rises and falls.

Circular cams or eccentric cams produce a smooth motion. These cams are used in steam engines.

Heart shaped cams allow the follower to rise and fall with ‘uniform’ velocity.

What type of movement do you think this cam profile will give ?

PRACTICAL EXAMPLE - ECCENTRIC CAM

An eccentric 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.The diagrams (1 to 7) seen below show the cam rotating in an anticlockwise direction. 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 eccentric 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 whilst others drop down. This gives the impression that the snake is moving

The eccentric cam below pushes the 'fish' upwards and allows it to drop downwards at a uniform speed. The overall speed is controlled by the speed the handle is turned.

QUESTION:

1. Design a toy or practical item based on a series of eccentric cams, working in a similar way as the ones seen above.

THE SNAIL / DROP CAM

Eccentric, heart shaped and pair shaped cams generally allow for a slow rise and fall of the follower. However, a snail drop cam is used where the drop or fall of the follower must be sudden.

The example snail/drop cam shown opposite rotates in an anticlockwise direction. Rotating in a clockwise direction would probably lead to the entire mechanism jamming. This highlights one possible disadvantage of using this type of cam profile. Also, to ensure the rotation is smooth, the vertical centre line of the snail/drop cam is positioned slightly to the left of the slide (see diagram).

The diagrams below shows the rotation of the snail/drop cam. When rotating for one complete revolution the follower stays level for approximately the first 120 degrees (diagrams 1 to 4). The follower then rises slowly (diagrams 5 to 6) and then suddenly drops when it reaches and passes the peak (diagram 7).

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The mechanical toy seen below has a snail/drop cam as its main part. The follower is connected to the characters arm by a wire link. As the cam rotates, the follower rises and the wire link lifts the characters arm. This gives the appearance of the character lifting a fork full of food towards his mouth.

As the cam continues to rotate the follower suddenly falls and also the characters arm and fork

Another example of a drop cam is seen below. As the cam profile rotates the foot of the model rises slowly and then suddenly drops.

ANOTHER MECHANICAL TOYAs the mechanical duck is pulled forward the wings flap upwards and downwards. This is achieved through the use of a eccentric cam fixed to each axle. The diagrams below show how the mechanism works

As the wheels and axle rotate the eccentric cams also rotate. As the eccentric cams move upwards they also push the wings upwards. As the eccentric cam rotate downwards the wings drop down. The faster the toy is pulled forward the faster the wings flap.