Homework 7 Linkages. A linkage is a system of connected rods and levers that are used to transmit motion and force.

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1 Homework 7 Linkages Linkages A linkage is a system of connected rods and levers that are used to transmit motion and force. Linkages may be used to: 1. Change the direction of motion and force 2. Change the size of the output motion compared to the size of the input motion 3. Create parallel motion 4. Change one type of motion into another type of motion. Reverse Motion Linkage A linkage which makes the output move in the same direction as the input is called a PUSH-PULL LINKAGE. Linkages can be used to make things move in opposite directions. The movement is reversed by using a lever to form the linkage. If the pivot point (fulcrum) is at the centre of the connecting lever then the output movement will be the same as the input movement but it will act in the opposite direction. If the pivot point is not in the centre of the connecting lever then the movement of the output will not be equal to the movement of the input. By careful positioning of the fulcrum, it is possible to design a linkage system which moves an exact distance. It is also possible to design the linkage to produce mechanical advantage. The linkage above illustrates how the direction of motion and force may be changed by moving the pivot/fulcrum. The diagram illustrates how a linkage may be used to: 1. Change the direction of motion and force 2. Change the size of the output motion compared to the size of the input motion by moving the pivot/fulcrum. 3. Change the size of the output force compared to the size of the input force by moving the pivot/fulcrum.

2 Parallel Linkage Linkages are also used to make objects move parallel to each other. These are called PARALLEL MOTION LINKAGES and are used in the manufacture of pantographs for drawing instruments, lazy tongs, parallel hinges on a tool box and are used by electric trains to pick up power from overhead cables. The diagram illustrates how a linkage may be used to create a parallel motion Parallel Motion - Pantograph Linkage The diagram illustrates a pantograph linkage. A pantograph is used to copy and rescale shapes and drawings. Parallel Motion - Lazy Tong Linkage The lazy tong linkage is used for reaching and lifting loads..

3 Bell Crank Sometimes you want to change the direction of a movement or force by 90. This type of linkage is called a BELL CRANK and this idea is used in the design of brakes for bicycles. If you want to change the direction of movement or force through 90 you can use a linkage device called a bell crank (so called because it was used in Victorian times in linkages used to operate doorbells and servants bells). A common device which uses this mechanism is the brakes of a bicycle. Here the force from the handlebar lever is turned through 90 to squeeze the brake block against the wheel rim. If the fulcrum is at an equal distance from the input and output then the movement of the output will be equal to the movement of the input. Otherwise the movement will be different and the system will have some degree of mechanical advantage. Crank & Slider The diagram illustrates how a linkage is used in a crank and slider mechanism and how rotary motion is converted into reciprocating motion.

4 Question The Pedal Bin Problem Learning Objective To develop students' problem solving capability by guiding them through a mechanical systems problem. Situation Pedal bins are a convenient place to store rubbish until the rubbish is finally removed for recycling or disposal. One advantage of pedal bins is that the lid is lifted, i.e. swung open, simply by stepping on a pedal. As designs for pedal bins have improved, pedal bins have become an essential part of many people's fashion items, in homes, offices and school etc. The Pedal Bin System The known parts of the pedal bin system are: the input is Human energy pressing on the pedal the output is a lid that swings open the lid swings and closes when the input is removed. Problem solving 1. Devise a mechanism that will lift the pedal bin lid when the pedal is pressed down and lower it again when the pedal is released. What you must do 1. Analyse the problem so that you understand it fully. 2. Make a list of mechanisms that would swing the pedal bin lid open when the pedal is pressed, using notes and sketches to record your ideas. 3. Choose the mechanism that solves the problem the most effectively. 4. Use notes and sketches to illustrate your best design, showing modifications that you have made to the structure of the bin and the bin lid. 5. Evaluate your design and modify it if necessary. You may use research methods to find information about suitable mechanisms, e.g. product analysis library search / computer software search internet search experiments - model your ideas, e.g. in card interview - ask an expert. Present Your ideas on the drawing overleaf.

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