Input, Process & Output in Mechanical Systems

This lesson applies the systems approach (Input → Process → Output) to mechanical systems, showing how force, motion and energy flow through mechanisms. This is a core concept in AQA GCSE Design and Technology (8552), Section 3.1.5.

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The Mechanical Systems Approach

Just like electronic systems (covered in Section 3.1.4), mechanical systems can be described using the Input → Process → Output model:

Stage	Function in a Mechanical System	Example (Bicycle)
Input	The force or motion applied to the system	Rider pushes the pedals (human effort)
Process	The mechanism that changes, transmits or amplifies the force or motion	Chain and sprocket system, gears
Output	The resulting force, motion or action	Rear wheel turns, bicycle moves forward

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Input: Forces and Energy Sources

The input to a mechanical system is the effort — the force or energy that drives the system.

Common Input Sources

Input Source	Description	Example
Human effort	Force applied by a person	Turning a handle, pushing a lever, pedalling
Electric motor	Converts electrical energy to rotary motion	Motor in a washing machine, power drill
Engine	Burns fuel to produce rotary motion	Petrol engine in a car, diesel engine in a truck
Spring	Stores potential energy and releases it	Clockwork mechanism, spring-loaded toy
Gravity	Weight of an object provides a downward force	Pendulum clock, water wheel
Pneumatic/hydraulic	Compressed air or pressurised fluid provides force	Pneumatic drill, hydraulic press

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Process: Mechanisms

The process stage is the mechanism itself. It changes the input in one or more of the following ways:

What the Mechanism Changes	Explanation	Example
Type of motion	Converts one motion type to another	Cam converts rotary to reciprocating
Direction of motion	Changes the direction of movement	Bevel gear changes axis of rotation by 90°
Speed of motion	Increases or decreases the speed	Gear reduction slows the output but increases torque
Magnitude of force	Amplifies or reduces the force	A lever multiplies the input force
Distance / range of motion	Changes how far the output moves	A lever can increase or decrease the distance moved

Key Concept: Trade-Off Between Force and Distance

A fundamental principle of mechanics is that you cannot get something for nothing. When a mechanism multiplies force, it reduces distance (or speed), and vice versa. This is based on the conservation of energy:

$\text{Energy input} = \text{Energy output} + \text{Energy lost to friction}$Energy input=Energy output+Energy lost to friction

$\text{Force} \times \text{Distance (input)} = \text{Force} \times \text{Distance (output)}$Force×Distance (input)=Force×Distance (output)

Example — A Lever: If a lever has a mechanical advantage of 3, the output force is 3 times the input force, BUT the input must move 3 times further than the output.

AQA Exam Tip: Whenever you describe a mechanism multiplying force, always mention that the distance moved (or speed) is reduced as a trade-off. This shows understanding of the underlying physics and targets the higher mark bands.

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Output: Work Done

The output is the useful work performed by the system — the result that the mechanism was designed to achieve.

Common Outputs

Output	Description	Example
Movement	An object is moved or rotated	A gear train turns a wheel
Lifting	An object is raised against gravity	A crane lifts a load
Cutting	A blade or tool removes material	A cam-driven press stamps out shapes
Clamping	An object is held firmly in place	A vice uses a screw thread to clamp a workpiece
Pumping	Fluid is moved from one place to another	A cam-driven pump moves water

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Efficiency

No mechanical system is 100% efficient. Energy is always lost to friction, heat and sound. The efficiency of a system tells you what proportion of the input energy is usefully converted to output.

Efficiency Formula

$\text{Efficiency} = \frac{\text{Useful output energy (or power)}}{\text{Total input energy (or power)}} \times 100\%$Efficiency=Total input energy (or power)Useful output energy (or power)​×100%

Example

A motor provides 500 J of input energy to a gear system. The gear system delivers 400 J of useful output energy. The remaining 100 J is lost to friction and heat.

$\text{Efficiency} = \frac{400}{500} \times 100\% = 80\%$Efficiency=500400​×100%=80%

Ways to Improve Efficiency

Method	Explanation
Lubrication	Oil or grease reduces friction between moving parts
Bearings	Ball bearings or roller bearings reduce rotational friction
Precision manufacturing	Tightly fitting parts reduce energy loss from vibration and slack
Material selection	Low-friction materials (e.g. PTFE, nylon) for bearing surfaces
Regular maintenance	Replacing worn parts prevents increased friction

AQA Exam Tip: Efficiency calculations appear frequently in the exam. Always show your working, include the formula, substitute the values and include the % symbol in your answer. Losing marks for missing units is easily avoidable.

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Mechanical Systems in Real Products

Example 1: Hand-Cranked Pencil Sharpener

Stage	Detail
Input	Human hand turns the handle (rotary motion, low torque)
Process	Gear train inside the sharpener increases speed and changes axis of rotation
Output	Cutting blades rotate at high speed around the pencil tip

Example 2: Bicycle

Stage	Detail
Input	Rider pushes pedals (rotary motion applied to the crank)
Process	Chain and sprocket system transmits motion; gear system allows speed/force trade-off
Output	Rear wheel rotates, propelling the bicycle forward

Example 3: Car Windscreen Wiper

Stage	Detail
Input	Electric motor produces rotary motion
Process	Crank and linkage mechanism converts rotary motion to oscillating motion
Output	Wiper blade oscillates across the windscreen, clearing rain

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System Diagrams

A system block diagram is a simple way to represent a mechanical system visually:

graph LR
  A["INPUT<br/>(Human effort or motor)"] --> B["PROCESS<br/>Mechanism<br/>(gear, lever, cam, etc.)"]
  B --> C["OUTPUT<br/>Useful work<br/>(movement, lifting, etc.)"]

The mermaid diagram below applies the same Input → Process → Output model to three real products, showing how the same system structure underlies very different mechanisms.