GCSE D&T — Mechanical Devices, Gears and Cams Explained
GCSE D&T — Mechanical Devices, Gears and Cams Explained
Mechanical devices are one of the most frequently examined topics in GCSE Design and Technology -- and one of the most rewarding to revise, because the questions follow consistent patterns and the calculations are straightforward once you know the formulae. On AQA 8552, mechanical content contributes to the 15% minimum mathematical requirement, so you can expect at least one calculation question on gear ratios, velocity ratios, or mechanical advantage every year.
This guide covers every mechanical device on the specification, explains the four types of movement, and works through calculation examples in the same format you will see on the exam paper.
Types of Movement
Every mechanical device converts one type of movement into another, or changes the direction, speed, or force of movement. You need to know four types:
| Type of Movement | Description | Example |
|---|---|---|
| Linear | Movement in a straight line in one direction | A drawer sliding open, a train on a straight track |
| Rotary | Movement in a circle around a central point | A wheel turning, a clock hand, a drill bit spinning |
| Reciprocating | Repeated back-and-forth movement in a straight line | A sewing machine needle, a piston in an engine |
| Oscillating | Repeated swinging movement along an arc (like a pendulum) | A playground swing, a metronome, a clock pendulum |
A key skill is identifying how a mechanism changes one type of movement into another. For example, a cam and follower converts rotary motion into reciprocating motion. A crank and slider converts rotary motion into reciprocating motion (or vice versa).
Levers
A lever is a rigid bar that pivots around a fixed point called the fulcrum (or pivot). Levers are used to move a load by applying an effort. The mechanical advantage depends on the relative positions of the fulcrum, load, and effort.
First Class Lever
The fulcrum is positioned between the effort and the load.
Layout: Effort --- Fulcrum --- Load
Examples: Scissors, seesaw, crowbar, pliers, claw hammer
The mechanical advantage can be greater than, equal to, or less than 1, depending on where the fulcrum is positioned. Moving the fulcrum closer to the load increases the mechanical advantage.
Second Class Lever
The load is positioned between the fulcrum and the effort.
Layout: Fulcrum --- Load --- Effort
Examples: Wheelbarrow, nutcracker, bottle opener, stapler
Second class levers always have a mechanical advantage greater than 1, meaning the effort required is less than the load. They amplify force but reduce the distance the load moves.
Third Class Lever
The effort is positioned between the fulcrum and the load.
Layout: Fulcrum --- Effort --- Load
Examples: Tweezers, fishing rod, human forearm, broom
Third class levers always have a mechanical advantage less than 1 -- you apply more effort than the load. However, they amplify distance and speed, allowing you to move the load over a greater distance or at a greater speed than the effort.
Linkages
Linkages are systems of rigid bars connected by pivots. They transmit motion and force from one point to another. You need to know four types:
Reverse motion linkage: A fixed pivot in the centre causes the input and output to move in opposite directions. Push one end down, the other end goes up. Used in: windscreen wipers, balance scales.
Parallel motion linkage: Two or more bars are connected so that the output moves in the same direction as the input while maintaining the same orientation. Used in: tool boxes with rising trays, desk lamps (such as the Anglepoise), extensible mirrors.
Bell crank linkage: Changes the direction of movement through 90 degrees. An input force in one direction produces an output force at right angles. Used in: bicycle brake mechanisms (pulling the brake lever horizontally causes the brake pads to move vertically onto the rim).
Crank and slider: Converts rotary motion into reciprocating motion, or reciprocating motion into rotary motion. A rotating crank arm is connected to a slider that moves back and forth in a straight line. Used in: internal combustion engines (piston and crankshaft), sewing machines, jigsaws.
Gears
Gears are toothed wheels that mesh together to transmit rotary motion. They can change the speed, direction, or turning force (torque) of rotation. This is one of the most calculation-heavy areas of the specification.
Simple Gear Train
Two gears meshed together form a simple gear train. The gear connected to the input (motor or handle) is the driver gear. The gear it drives is the driven gear (also called the follower).
Key rules:
- When two gears mesh, they rotate in opposite directions
- A larger driven gear rotates more slowly than the driver but with more torque
- A smaller driven gear rotates faster than the driver but with less torque
Gear Ratio Formula
Gear ratio = number of teeth on driven gear / number of teeth on driver gear
Worked Example 1: A driver gear has 20 teeth. The driven gear has 60 teeth.
Gear ratio = 60 / 20 = 3:1
This means the driver gear turns 3 times for every 1 turn of the driven gear. The output speed is reduced by a factor of 3, but the output torque is multiplied by 3.
Worked Example 2: A driver gear has 40 teeth. The driven gear has 10 teeth.
Gear ratio = 10 / 40 = 1:4 (or 0.25:1)
This means for every 1 turn of the driver, the driven gear turns 4 times. The output speed is multiplied by 4, but the output torque is reduced by a factor of 4.
Calculating Output Speed
Output RPM = Input RPM / Gear ratio
Worked Example 3: A motor drives a gear with 15 teeth at 600 RPM. It meshes with a driven gear that has 45 teeth.
Step 1: Gear ratio = 45 / 15 = 3
Step 2: Output RPM = 600 / 3 = 200 RPM
The driven gear rotates at 200 RPM.
Worked Example 4: A driver gear has 50 teeth and rotates at 120 RPM. The driven gear has 25 teeth. What is the output speed?
Step 1: Gear ratio = 25 / 50 = 0.5
Step 2: Output RPM = 120 / 0.5 = 240 RPM
The driven gear rotates at 240 RPM -- faster than the input because the driven gear is smaller.
Compound Gear Train
A compound gear train uses an idler gear (or intermediate gear) between the driver and driven gears. The idler gear reverses the direction of rotation, so the output gear rotates in the same direction as the driver.
If the idler gear is the same size as the other gears, it does not change the overall gear ratio -- it only changes the direction. However, if you have a compound system where a small gear and a large gear are fixed on the same shaft, the overall gear ratio is found by multiplying the individual ratios:
Overall gear ratio = gear ratio of stage 1 x gear ratio of stage 2
Worked Example 5: Stage 1: Driver gear A has 20 teeth, driven gear B has 40 teeth. Stage 2: Gear C (on the same shaft as B) has 10 teeth, driven gear D has 50 teeth.
Gear ratio stage 1 = 40 / 20 = 2 Gear ratio stage 2 = 50 / 10 = 5
Overall gear ratio = 2 x 5 = 10:1
If gear A rotates at 1000 RPM, gear D rotates at 1000 / 10 = 100 RPM.
Other Gear Types
Worm and wheel: A worm (a screw-like gear) meshes with a worm wheel. It creates a very high gear ratio in a compact space. The worm drives the wheel, but the wheel cannot drive the worm -- this self-locking property is useful in lifting mechanisms (winches, guitar tuning pegs). One turn of the worm advances the wheel by one tooth.
Rack and pinion: Converts rotary motion into linear motion (or vice versa). A circular gear (pinion) meshes with a flat toothed bar (rack). Used in: steering mechanisms, pillar drills, sliding gates.
Bevel gears: Two cone-shaped gears that mesh at an angle (typically 90 degrees). They change the axis of rotation. Used in: hand drills (where the handle turns at right angles to the drill bit), differential gears in vehicles.
Cams
A cam is a shaped piece fixed to a rotating shaft. A follower rests against the cam surface and moves up and down as the cam rotates. Cams convert rotary motion into reciprocating motion.
Cam Types
| Cam Shape | Motion Produced | Characteristics |
|---|---|---|
| Pear cam (drop cam) | Gradual rise followed by a sudden drop | The follower rises slowly as the cam rotates through the long curved section, then drops rapidly. Used when a sudden fall is needed, such as in a stamping mechanism. |
| Circular (eccentric) cam | Smooth, continuous rise and fall (sinusoidal) | The cam is a circle mounted off-centre. The follower moves up and down smoothly with no sudden changes. Used in mechanisms requiring gentle, even reciprocating motion. |
| Heart-shaped cam | Uniform rise and fall at a constant velocity | The follower rises and falls at a steady rate. The motion is even and predictable. Used in sewing machines and textile machinery. |
| Snail cam (drop cam) | Very gradual rise followed by an instantaneous drop | Similar to the pear cam but with a more pronounced sudden drop. The follower rises gradually over almost the full rotation, then drops vertically. Used in feed mechanisms. |
Key terms:
- Dwell: A period where the follower remains stationary even though the cam continues to rotate. A flat section on the cam profile produces a dwell.
- Rise: The follower moves upward as the cam radius increases.
- Fall: The follower moves downward as the cam radius decreases.
Exam questions often present a cam profile and ask you to sketch or describe the motion of the follower over one complete rotation. Practise reading cam profiles and plotting the follower displacement against the angle of rotation.
Pulley Systems
Pulleys use a wheel and rope (or belt) to transmit force or change the direction of a force.
Simple Pulley
A single fixed pulley changes the direction of the effort but provides no mechanical advantage. Pulling down on the rope lifts the load upward. The effort required equals the load.
Compound Pulley
A compound (or block and tackle) pulley system uses multiple pulleys to multiply the force. The mechanical advantage equals the number of rope sections supporting the load.
Worked Example 6: A block and tackle system has 4 rope sections supporting the load. The load weighs 200N.
Mechanical advantage = number of supporting rope sections = 4
Effort required = Load / MA = 200 / 4 = 50N
However, the trade-off is that the effort must be applied over a greater distance. To lift the load by 1 metre, you need to pull 4 metres of rope.
Belt and Pulley Systems
Two pulleys connected by a belt work similarly to gears but transmit motion over a greater distance and slip under excessive load (which can be a safety feature).
Velocity ratio = diameter of driven pulley / diameter of driver pulley
Worked Example 7: A driver pulley has a diameter of 50 mm. The driven pulley has a diameter of 150 mm.
Velocity ratio = 150 / 50 = 3:1
The driven pulley rotates 3 times more slowly than the driver, but with 3 times the torque. If the driver rotates at 900 RPM, the driven pulley rotates at 900 / 3 = 300 RPM.
Mechanical Advantage
Mechanical advantage (MA) tells you how much a mechanism multiplies the input force.
MA = Load / Effort
Worked Example 8: A lever is used to lift a 450N rock. The effort applied is 150N.
MA = 450 / 150 = 3
This means the lever multiplies the input force by 3.
Worked Example 9: A pulley system has a mechanical advantage of 5. What effort is needed to lift a 600N load?
Effort = Load / MA = 600 / 5 = 120N
Worked Example 10: A gear system has a driver gear with 10 teeth and a driven gear with 80 teeth. A motor provides an input torque of 2 Nm. What is the output torque?
Gear ratio = 80 / 10 = 8
Output torque = Input torque x Gear ratio = 2 x 8 = 16 Nm
The output speed will be reduced by a factor of 8, but the torque is multiplied by 8.
Common Exam Calculation Questions
Here are the question types you are most likely to encounter, with a strategy for each:
"Calculate the gear ratio." Identify the driver and driven gears. Apply the formula: driven teeth / driver teeth. Express your answer as a ratio (e.g., 3:1).
"Calculate the output speed." Find the gear ratio first, then divide the input speed by the gear ratio. Include the unit (RPM) in your answer.
"How many times does the driver gear turn for one complete rotation of the driven gear?" This is simply the gear ratio. If the ratio is 4:1, the driver turns 4 times for every 1 turn of the driven gear.
"Calculate the effort required to lift a load." Use Effort = Load / MA. If you are given a pulley system, count the rope sections supporting the load to find the MA.
"Describe the movement produced by this cam." Read the cam profile. Note where the radius increases (follower rises), where it decreases (follower falls), and where it stays constant (follower dwells). Describe each phase in order around one full rotation.
"Explain why a worm and wheel is used in this product." Key points: high gear ratio in a compact space, self-locking (load cannot back-drive the mechanism), precise control of position.
Revision Approach for Mechanical Devices
This topic rewards structured practice. Start by learning the definitions and types of movement, then work through each mechanism category (levers, linkages, gears, cams, pulleys) in turn. For each mechanism, make sure you can:
- Name and describe it
- Sketch a simple diagram
- Explain what type of motion conversion it performs
- Give a real-world example
- Perform any associated calculations
Once you are confident with the individual mechanisms, practise past paper questions under timed conditions. Most mechanical device questions on AQA papers follow the patterns described above, so familiarity with the question formats is just as important as knowledge of the content itself.
LearningBro courses for GCSE Design and Technology include dedicated modules on mechanical devices with step-by-step worked examples and practice calculations. Working through these modules systematically is an efficient way to build both your understanding and your exam technique for what is one of the most predictable and scoreable areas of the paper.