Mechanical Devices - Cams Gears Pulleys and Levers
Mechanical Devices — Cams, Gears, Pulleys and Levers
Mechanical devices convert one type of motion into another or transfer force and movement. AQA GCSE D&T requires you to understand how these devices work, calculate mechanical advantage, and apply them to product design.
Types of Motion
Before studying mechanisms, you must understand the four types of motion:
| Motion | Description | Example |
|---|---|---|
| Linear | Movement in a straight line | Drawer sliding open |
| Rotary | Movement in a circle around an axis | Wheel turning, clock hands |
| Reciprocating | Back-and-forth movement in a straight line | Piston in an engine, sewing machine needle |
| Oscillating | Swinging back and forth around a fixed pivot | Pendulum, windscreen wiper |
Levers
A lever is a rigid bar that pivots around a fixed point called a fulcrum. Levers multiply force (giving mechanical advantage) or increase the distance/speed of movement.
Three classes of lever:
| Class | Arrangement | Example | Mechanical Advantage |
|---|---|---|---|
| Class 1 | Fulcrum between effort and load | Scissors, seesaw, crowbar | Can be >1, =1 or <1 |
| Class 2 | Load between fulcrum and effort | Wheelbarrow, nutcracker, bottle opener | Always >1 (force multiplier) |
| Class 3 | Effort between fulcrum and load | Tweezers, fishing rod, tongs | Always <1 (distance multiplier) |
Mechanical Advantage (MA):
MA = Load ÷ Effort
If MA > 1, the lever multiplies force. If MA < 1, it multiplies distance or speed instead.
Linkages
Linkages are systems of levers connected together to transmit or change the direction of motion.
| Linkage | Effect | Example |
|---|---|---|
| Reverse motion (push-pull) | Input and output move in opposite directions | Windscreen wipers |
| Parallel motion (push-pull) | Input and output move in the same direction | Tool box lid mechanism |
| Bell crank | Changes direction of motion by 90° | Bicycle brake mechanism |
| Crank and slider | Converts rotary to reciprocating motion (or vice versa) | Piston engine, jigsaw |
Cams
A cam is a shaped disc mounted on a rotating shaft. It converts rotary motion into reciprocating motion via a follower that rides on the cam's surface.
| Cam Shape | Follower Movement |
|---|---|
| Circular (eccentric) | Smooth, gradual rise and fall — gentle reciprocating motion |
| Pear-shaped | Gradual rise, sudden drop — one smooth lift per rotation |
| Heart-shaped (cardioid) | Uniform, steady rise and fall — even speed throughout |
| Snail (drop) | Very gradual rise then sudden drop — used in door locks, music boxes |
The follower type matters too:
- Flat follower — simple, slides on flat surface
- Roller follower — reduces friction, smoother operation
- Knife-edge follower — follows complex cam profiles accurately but wears quickly
Gears
Gears are toothed wheels that mesh together to transmit rotary motion and force. They can change speed, direction, and torque.
Gear ratio = Number of teeth on driven gear ÷ Number of teeth on driver gear
If the driven gear has more teeth than the driver, it turns slower but with more torque (force). If fewer teeth, it turns faster with less torque.
| Gear Type | Description | Use |
|---|---|---|
| Spur gears | Straight-toothed, parallel axes | Clocks, simple machines |
| Bevel gears | Conical, axes at 90° | Hand drill, differential in cars |
| Worm and worm wheel | Screw meshes with gear, 90° axes, non-reversible | Guitar tuning pegs, lifting jacks |
| Rack and pinion | Circular gear on straight toothed bar, rotary to linear | Steering mechanism, sliding gates |
| Compound gear train | Multiple pairs of gears on shared shafts | Gearboxes, clocks (multiple reductions) |
| Idler gear | Placed between driver and driven, changes direction only | Reversing rotation without changing ratio |
Example calculation: Driver gear has 20 teeth, driven gear has 60 teeth.
Gear ratio = 60 ÷ 20 = 3:1 — the driven gear turns 3× slower with 3× the torque.
Pulleys
Pulleys use wheels and belts or ropes to transmit motion or lift loads.
| System | Description | Mechanical Advantage |
|---|---|---|
| Single fixed pulley | Changes direction of force only | MA = 1 |
| Single movable pulley | Attached to load, moves with it | MA = 2 |
| Block and tackle | Combination of fixed and movable pulleys | MA = number of rope sections supporting load |
Belt and pulley systems transmit rotary motion between shafts:
- Same-size pulleys = same speed
- Larger driven pulley = slower speed, more torque
- Crossed belt = reverses direction of rotation
Exam Tips
- Gear ratio calculations appear frequently — always show your working and state the formula
- Know how to sketch each cam profile and describe the follower movement
- When identifying lever classes, mark the fulcrum (F), effort (E) and load (L) clearly on your diagram
- If asked about a mechanism in a product, state what type of motion conversion occurs (e.g. rotary to linear)
- Worm gears are non-reversible — this is a common exam point (the load cannot drive the worm)
- For pulleys, count the number of rope sections supporting the load to find MA
Key Definitions
- Fulcrum: The fixed pivot point of a lever
- Mechanical advantage: The ratio of load to effort — how much a mechanism multiplies force
- Gear ratio: The relationship between the number of teeth on the driver and driven gears
- Cam: A rotating shaped disc that converts rotary motion to reciprocating motion via a follower
- Torque: Rotational force — the turning effect of a force about an axis