Pulley Systems: Single, Compound, Block & Tackle, Belt Drives

This lesson covers pulley systems and belt drives — mechanisms that transmit force and motion using wheels and flexible connectors (ropes, cables or belts). Pulleys are part of AQA GCSE Design and Technology (8552), Section 3.1.5.

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What Is a Pulley?

A pulley is a wheel with a grooved rim through which a rope, cable or belt runs. Pulleys are used to:

• Change the direction of a force
• Multiply force (reduce the effort needed to lift a load)
• Transmit rotary motion between shafts (belt drives)

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Single Fixed Pulley

A single fixed pulley is attached to a fixed point (e.g. a ceiling beam). The rope runs over the pulley.

Properties

Property	Detail
Mechanical advantage	MA = 1 (no force multiplication)
Direction change	Yes — pulling down on the rope lifts the load up
Effort required	Equal to the load (ignoring friction)
Distance moved	Effort distance = Load distance

Why Use a Single Fixed Pulley?

Although it does not reduce the effort needed, it allows you to pull downwards (using your body weight) to lift an object upwards. This is much more convenient than trying to push a load up directly.

Example: A flagpole — you pull the rope down at ground level, and the flag rises to the top.

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Single Movable Pulley

A single movable pulley is attached to the load itself. One end of the rope is fixed to a support point.

Properties

Property	Detail
Mechanical advantage	MA = 2 (effort is halved)
Direction change	No — you pull up to lift the load up
Effort required	Half the load (ignoring friction)
Distance moved	You must pull twice the distance the load moves

Why Use a Single Movable Pulley?

The load is shared between two rope sections, so the effort is halved. The trade-off is that you must pull the rope twice as far as the load travels.

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Compound Pulley (Block and Tackle)

A block and tackle system combines multiple fixed and movable pulleys to achieve greater mechanical advantage.

The diagram below shows a 4-rope-section block-and-tackle: the upper (fixed) block is anchored to the ceiling, the lower (movable) block carries the load, and four rope segments span between them. The effort is applied to the free end.

graph TD
    Ceiling["===== Fixed Anchor / Ceiling ====="]
    Ceiling --> Upper["Fixed Block (Upper Pulleys)"]
    Upper -->|"rope 1 (supports load)"| Lower["Movable Block (Lower Pulleys)"]
    Upper -->|"rope 2 (supports load)"| Lower
    Upper -->|"rope 3 (supports load)"| Lower
    Upper -->|"rope 4 (supports load)"| Lower
    Upper -->|"effort end (does NOT count)"| Effort["Effort applied here<br/>by operator"]
    Lower --> Load["Load (e.g. 400 N)"]
    Load -.->|"MA = 4 rope sections<br/>Effort = 400 / 4 = 100 N"| Result["Pull 4x distance<br/>load travels"]

How It Works

Number of Rope Sections Supporting the Load	Mechanical Advantage	Effort Required
2	2	Load ÷ 2
3	3	Load ÷ 3
4	4	Load ÷ 4
6	6	Load ÷ 6

General Rule:

$MA = \text{Number of rope sections supporting the load}$MA=Number of rope sections supporting the load

$\text{Effort} = \frac{\text{Load}}{MA}$Effort=MALoad​

Worked Example

A block and tackle system has 4 rope sections supporting the load. The load is 400 N.

$MA = 4$MA=4

$\text{Effort} = \frac{400}{4} = 100 \text{ N}$Effort=4400​=100 N

However, you must pull the rope 4 times further than the distance the load moves.

If the load needs to be raised by 2 m:

$\text{Rope pulled} = 4 \times 2 = 8 \text{ m}$Rope pulled=4×2=8 m

Applications of Block and Tackle

Application	Why a Block and Tackle Is Used
Sailing	Adjusting sails requires multiplying the sailor's effort
Construction cranes	Lifting heavy building materials
Theatre rigging	Raising and lowering scenery, curtains and lighting bars
Rescue operations	Lifting casualties from ravines, buildings or water
Garage engine hoist	Lifting heavy car engines for maintenance

AQA Exam Tip: To find the MA of a block and tackle, count the number of rope sections that directly support the load (go to and from the movable pulley block). Do NOT count the rope going to the person pulling.

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Belt Drives

A belt drive uses a flexible belt connecting two pulleys (wheels) to transmit rotary motion between shafts that may be some distance apart.

Types of Belt Drive

Type	Description	Direction of Rotation
Flat belt	Simple, flat belt wrapped around smooth pulleys	Both pulleys rotate in the same direction
V-belt	Wedge-shaped belt sits in a V-groove on the pulley	Same direction; better grip than flat belt
Toothed (timing) belt	Belt with teeth that mesh with toothed pulleys	Same direction; no slipping
Crossed belt	Belt is crossed between pulleys	Pulleys rotate in opposite directions

Speed and Torque in Belt Drives

Belt drives follow the same principles as gears — the ratio of pulley diameters determines the speed change:

$\text{Speed ratio} = \frac{\text{Diameter of driver pulley}}{\text{Diameter of driven pulley}}$Speed ratio=Diameter of driven pulleyDiameter of driver pulley​

$\text{Output speed} = \text{Input speed} \times \frac{\text{Driver diameter}}{\text{Driven diameter}}$Output speed=Input speed×Driven diameterDriver diameter​

Worked Example

A motor drives a pulley of diameter 50 mm at 1500 RPM. The belt connects to a driven pulley of diameter 150 mm.

$\text{Speed ratio} = \frac{50}{150} = \frac{1}{3}$Speed ratio=15050​=31​

$\text{Output speed} = 1500 \times \frac{50}{150} = 1500 \times \frac{1}{3} = 500 \text{ RPM}$Output speed=1500×15050​=1500×31​=500 RPM

The output speed is 500 RPM — slower, with increased torque (same trade-off as gears).

Applications of Belt Drives

Application	How the Belt Drive Is Used
Washing machine	Motor drives the drum via a belt and pulley system
Drill press	Speed is changed by moving the belt to different-sized pulleys on a stepped cone
Car engine	Timing belt drives the camshaft; alternator and water pump driven by belt
Sewing machine	Motor drives the needle mechanism via a belt
Bicycle (in some designs)	Belt drive replaces the chain for quieter, cleaner operation

Belt Drive vs Gear Drive