Linkages: Reverse, Parallel, Bell Crank & Crank and Slider

This lesson covers linkage mechanisms — assemblies of rigid bars connected by pivots that transmit and transform motion. Linkages are a key topic in AQA GCSE Design and Technology (8552), Section 3.1.5.

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What Is a Linkage?

A linkage is a mechanical system made of rigid bars (links) connected by pivots (joints). Linkages are used to:

• Change the direction of motion
• Change the type of motion (e.g. rotary to linear)
• Amplify or reduce the distance moved
• Transmit force from one point to another

Linkages are found in everyday products such as scissors, windscreen wipers, folding chairs, desk lamps and car steering systems.

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Fixed and Movable Pivots

Pivot Type	Description	Diagram Symbol
Fixed pivot	Attached to a frame or chassis — the link can rotate about this point but the pivot does not move	Triangle/ground symbol
Movable pivot	Connects two links — both the pivot and the links can move	Small circle at the joint

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Reverse Motion Linkage

A reverse motion linkage makes the output move in the opposite direction to the input. When the input bar moves up, the output bar moves down (and vice versa).

How It Works

• A central bar is attached to a fixed pivot in the middle.
• The input link is connected to one end of the central bar.
• The output link is connected to the other end.
• Pushing the input up causes the central bar to rotate, pulling the output down.

Real-World Examples

Example	How the Linkage Is Used
Seesaw	One end goes up while the other goes down
Windscreen wipers (some types)	Input motion from the motor is reversed to drive wipers in opposite arcs
Nut crackers (some designs)	Squeezing the handles causes the jaws to close
Pedal bin	Pressing the pedal down causes the lid to lift up

Properties

• Output motion is in the opposite direction to the input.
• The speed and distance of output can be varied by changing the position of the fixed pivot along the central bar.
• If the pivot is central, input distance = output distance. If off-centre, one side moves more than the other.

AQA Exam Tip: To identify a reverse motion linkage in a diagram, look for a central bar with a fixed pivot in the middle and links on either side that move in opposite directions.

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Parallel Motion Linkage

A parallel motion linkage makes the output move in the same direction and by the same distance as the input, while remaining parallel.

How It Works

• Two equal-length bars are connected to two fixed pivots on a frame.
• A connecting bar joins the free ends of the two bars.
• When the input bar is pushed, the connecting bar moves in the same direction, remaining parallel to the frame.

Real-World Examples

Example	How the Linkage Is Used
Car windscreen wipers	Both wipers sweep across the screen in the same direction simultaneously
Toolbox drawers	Drawers slide out while remaining level and parallel
Vehicle suspension	Ensures the wheel remains vertical as the suspension moves up and down
Folding clothes airer	Arms fold out in parallel to create a uniform drying space
Lazy tongs / pantograph	Extending mechanism that maintains parallel motion

Properties

• Output moves in the same direction as the input.
• The connecting bar remains parallel to the frame at all times.
• Useful for keeping components level or aligned during movement.

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Bell Crank Linkage

A bell crank linkage changes the direction of motion by 90° (a right angle). It is essentially an L-shaped lever with a fixed pivot at the corner.

How It Works

• An L-shaped crank is attached to a fixed pivot at the bend.
• The input force is applied to one arm of the L.
• The output motion appears at the other arm, at 90° to the input direction.

Real-World Examples

Example	How the Linkage Is Used
Bicycle brake	Squeezing the horizontal brake lever applies a vertical force to the brake pads via a bell crank
Door bell (old-fashioned)	Pulling a wire horizontally causes a striker to swing vertically — hence the name "bell crank"
Aircraft control surfaces	Control rods in the fuselage change direction to operate rudders and ailerons
Machine tool feed	Converts horizontal handle movement to vertical tool movement

Properties

• Changes direction of force/motion by 90°.
• The mechanical advantage depends on the ratio of arm lengths of the L-shape.
• If the input arm is longer than the output arm, force is multiplied (but distance is reduced).

AQA Exam Tip: The bell crank is one of the most commonly tested linkages. Remember: it changes direction by 90 degrees. Sketch the L-shape with a fixed pivot at the corner and show the input and output directions clearly.

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Crank and Slider Mechanism

A crank and slider mechanism converts rotary motion into reciprocating (linear) motion, or vice versa.

How It Works

1. A crank (a disc or arm) rotates around a fixed axis.
2. A connecting rod links the crank to a slider.
3. As the crank rotates, the connecting rod pushes and pulls the slider back and forth in a straight line.

Real-World Examples

Example	Input Motion	Output Motion
Car engine (piston)	Reciprocating (piston) → Rotary (crankshaft)	Note: in an engine, the input is reciprocating
Sewing machine needle	Rotary (motor) → Reciprocating (needle up/down)
Jigsaw (power tool)	Rotary (motor) → Reciprocating (blade up/down)
Train wheel linkage	Rotary (wheel) → Reciprocating (piston/valve)	Steam engine connecting rod
Reciprocating saw	Rotary (motor) → Reciprocating (blade back/forth)
Air compressor	Rotary (motor) → Reciprocating (piston compresses air)

Key Relationships

• Stroke length (the distance the slider moves) = 2 × crank radius.
• A larger crank radius gives a longer stroke.
• The speed of the slider varies — it is fastest at the mid-point and momentarily stationary at each end (called the dead points).

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Crank and Connecting Rod

A close relative of the crank and slider, the crank and connecting rod is used when the output is oscillating rather than strictly linear. The connecting rod swings through an arc rather than following a perfectly straight path.

Example: Windscreen Wipers

1. The electric motor provides rotary input.
2. A crank arm converts this to oscillating motion via a connecting rod.
3. The wiper arm oscillates across the windscreen.

This is an example of rotary → oscillating conversion.

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The diagrams below show how a reverse motion linkage and a parallel motion linkage transmit motion differently:

graph TD
    subgraph "**Reverse Motion Linkage**"
        RI["Input bar\nmoves DOWN ⬇"] --> FP1["Fixed Pivot ⚖\n(central bar rotates)"]
        FP1 --> RO["Output bar\nmoves UP ⬆"]
    end
    subgraph "**Parallel Motion Linkage**"
        PI["Input bar\nmoves RIGHT ➡"] --> FP2["Two equal bars\non fixed pivots"]
        FP2 --> PO["Output bar\nmoves RIGHT ➡\n(stays parallel)"]
    end

Choosing the Right Linkage