How do linkages change the direction or type of motion, and where are they used in everyday products?
Describe common linkages, including reverse-motion, push-pull, bell-crank and parallel-motion linkages, and explain how each changes the direction or nature of motion
A focused answer to the O-Level Design and Technology outcome on linkages. Reverse-motion, push-pull, bell-crank and parallel linkages, how they change motion direction, and example uses.
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What this dot point is asking
SEAB wants you to describe common linkages, reverse-motion, push-pull, bell-crank and parallel-motion, and explain how each changes the direction or nature of motion. Linkages are mechanisms made of rigid links and pivots that route and redirect movement. You should recognise each type, know what it does to a motion, and give example uses.
The answer
What a linkage is and does
A linkage is a mechanism made of rigid bars (links) joined at pivots. Its purpose is to transfer and change motion: a linkage can change the direction of a motion, move a motion from one place to another, or alter the size of a movement. By arranging links and pivots, a designer routes movement through a product, for example turning a push at one point into a pull or a turn elsewhere.
Fixed and moving pivots
Linkages use two kinds of pivot. A fixed pivot is anchored to the frame and does not move; the link rotates about it. A moving pivot joins two links so they can move relative to each other but is not anchored. The placement of fixed pivots determines how the linkage behaves, so identifying them is key to explaining any linkage.
Reverse-motion linkage
A reverse-motion linkage makes the output move in the opposite direction to the input. A rigid link has a fixed pivot at its centre; the input connects to one end and the output to the other. Pushing the input end up rotates the link about the central pivot, so the other end moves down. Input up gives output down. It is used where two parts must move oppositely at once.
Push-pull (parallel) linkage
A push-pull linkage transfers motion in the same direction from input to output, often to move a control at a distance. Links connected in line pass a push or pull along, so pushing the input pushes the output the same way. It changes the position of a motion rather than its direction. Used for remote controls and connecting rods.
Bell-crank linkage
A bell-crank linkage changes the direction of motion through about a right angle. A bent (L-shaped) link is pivoted at the corner, so a horizontal input produces a vertical output, or vice versa. Bicycle brakes use a bell crank to turn the pull of a cable into a sideways squeeze on the wheel; the old bell-pull systems that rang servants' bells are the origin of the name.
Parallel-motion linkage
A parallel-motion (parallelogram) linkage keeps a part moving parallel to its original position as it moves. Two equal links pivoted in a parallelogram arrangement let a platform rise and fall while staying level. Used in angle-poise lamps, toolboxes that open with trays staying level, and parallel rules. It changes position while keeping orientation.
Examples in context
Example 1. Bicycle brake bell crank. Pulling the brake cable applies a horizontal pull, but the brake pads must move sideways onto the wheel rim. A bell-crank arrangement pivoted at the corner converts the cable's pull into the sideways squeezing motion of the pads. The linkage changes the direction of the motion through roughly a right angle so a single cable pull can clamp the wheel.
Example 2. A folding toolbox with a parallel linkage. When the toolbox lid is lifted, parallel-motion linkages swing the inner trays outward and upward while keeping them level, so tools do not tip out. Because the equal links form parallelograms, the trays stay horizontal throughout the movement. The linkage changes the trays' position for access while preserving their level orientation.
Try this
Cue. Name the linkage that makes an output move in the opposite direction to the input, and state where its fixed pivot goes. Answer: a reverse-motion linkage; its fixed pivot is at the centre of the link, between the input and output points.
Cue. State what a bell-crank linkage does to a motion. Answer: it changes the direction of motion through about a right angle (for example horizontal input to vertical output).
Cue. Give one use of a parallel-motion linkage and say what it keeps constant. Answer: e.g. an angle-poise lamp or a level-rising toolbox tray; it keeps the moving part parallel to its original orientation (level) as it moves.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksA designer needs a mechanism in a toy so that pushing one rod up makes another rod move down at the same time. (a) Name a suitable linkage. (b) Explain how it produces this opposite motion, and state where a fixed pivot is needed.Show worked answer →
(a) A reverse-motion linkage is suitable.
(b) A reverse-motion linkage uses a rigid link with a fixed pivot at its centre. The input rod is connected to one end and the output rod to the other end. When the input end is pushed up, the link rotates about the central fixed pivot, so the other end moves down: the output moves in the opposite direction to the input. The fixed pivot must be at the centre of the link (between the two connection points) for the motions to be equal and opposite.
What markers reward: naming the reverse-motion linkage, explaining that a central fixed pivot makes the two ends move in opposite directions as the link rotates, and correctly placing the fixed pivot between the input and output points.
Original4 marksExplain the purpose of linkages in mechanisms, and describe what a bell-crank linkage does.Show worked answer →
The purpose of a linkage is to transfer and change motion: it can change the direction of a motion, change a motion from one place to another, or alter the size of a movement. Linkages are made of rigid links joined at pivots, and they let a designer route and redirect movement through a mechanism.
A bell-crank linkage changes the direction of motion through (typically) a right angle. It uses a bent link pivoted at the corner, so an input pushing horizontally produces an output moving vertically (or vice versa). It is used, for example, in bicycle brakes to turn the pull of a cable into a sideways squeeze, and historically in bell-pull systems.
What markers reward: linkages defined as transferring and changing motion (direction, position, size) using rigid links and pivots, and a bell crank correctly described as changing the direction of motion through about a right angle with an example.
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