Mechanical systems – what you need to know Analyse and design simple mechanical systems of the type used for pop-up cards and interactive pages in educational and story books Levers and linkages – select appropriate levers, linkages and rotating discs to design mechanisms that bring about required changes of movement. Select components for joining and acting as pivots, paper fasteners, eyelets, plastic rivets Pop up systems – based on V-fold mechanisms, multiple layers (parallelogram) mechanisms

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V-fold mechanism

Multiple layers mechanism

Rotary mechanism

Other simple pop up - mouth

Parallel slide mechanism

Types of motion There are four basic types of motion in mechanical systems: Rotary motion is turning round in a circle, such as a wheel turning. Linear motion is moving in a straight line, such as on a paper trimmer. Reciprocating motion is moving backwards and forwards in a straight line, as in cutting with a saw. Oscillating motion is swinging from side to side, like a pendulum in a clock. Many mechanisms take one type of input motion, and output it as a different type of motion.

Cams Cams are used to convert rotary motion into reciprocating motion. The motion created can be simple and regular or complex and irregular. As the cam turns, driven by the circular motion, the cam follower traces the surface of the cam transmitting its motion to the required mechanism.

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.

Cam follower design is important in the way the profile of the cam is followed. A fine pointed follower will more accurately trace the outline of the cam. This more accurate movement is at the expense of the strength of the cam follower. As the cam turns it has a tendency to push the cam follower to one side. To overcome this a separate cam follower and push rod can be used as in this mechanism. Here the cam follower drags over the cam surface, accurately tracing the surface of the cam. Any movement of the cam follower is transferred directly to the push rod

Levers A lever is a length of rigid material, such as card, with a pivot somewhere along it. The lever rotates about the pivot. By putting the pivot in different positions you can increase or decrease the amount of movement that takes place at the end of the lever.

Linkages Linkages are made by joining two or more levers together with pivots. There are 2 types of pivot that are used. A fixed pivot attaches the linkage to the background while a movable pivot joins two parts of the linkage together. Reverse-motion linkage A reverse-motion linkage changes the direction of motion. In the diagram below, note how the linkage looks a little like a "Z". See how the central rod moves around a central fixed pivot. By pulling (or pushing) the linkage in one direction, it creates an exact opposite motion in the other direction. If the fixed pivot was not central, it would create a larger or smaller motion in the opposite direction.

Parallel-motion linkage A parallel-motion linkage creates an identical parallel motion. In the diagram below, note how the linkage looks a little like an "n". This time, it is the two side rods that move around two central fixed pivots, while the top of the "n" moves freely. By pulling (or pushing) the linkage in one direction, it creates an identical parallel motion at the other end of the linkage.

Bell-crank linkage A bell-crank linkage changes the direction of movement through 90°. A bell-crank linkage tends to look a little like an "L" or, as shown in the diagram below, a mirror image of an "L". By pulling (or pushing) the linkage in one direction, it creates a similar motion at the other end of the linkage. For example, a bell-crank linkage could be used to turn a vertical movement into horizontal movement, as in a bicycle braking system.

Pivots When making mechanisms from card or thin plastic you can use paper fasteners, eyelets and click rivets as pivots. The position of the pivot is very important in all linkage mechanisms. It can affect how much the mechanism moves and which direction it moves in.