Pulley
A simple machine that changes direction or transmits power via a wheel.
A pulley is a wheel mounted on an axle or shaft. It allows a taut rope, cable, belt, or chain running over it to change direction or to transfer power between the wheel and its shaft. Many pulleys have grooves between raised edges to keep the cable or belt in place.
The oldest known pulleys come from Ancient Egypt’s Twelfth Dynasty (around 1991–1802 BC) and from early 2nd millennium BC Mesopotamia. They later appeared in Ancient Egypt and Greece, where the mathematician Archytas of Tarentum wrote about them. In Roman Egypt, Hero of Alexandria (c. 10–70 AD) listed the pulley as one of the six simple machines for lifting loads. Several pulleys can be combined into a block and tackle to multiply force, and they are also used in belt and chain drives to send power from one rotating shaft to another. Plutarch’s *Parallel Lives* describes how Archimedes demonstrated the power of compound pulleys by using a block and tackle to pull a fully loaded ship as if it were gliding through water.
A block is a set of pulleys mounted so each wheel spins independently. Two blocks—one fixed and one attached to the load—with a rope threaded through them form a block and tackle. The ideal mechanical advantage equals the number of rope sections supporting the moving block. For example, a gun tackle has an advantage of 2, a luff tackle 3, a double tackle 4, a gyn tackle 5, and a threefold purchase 6.
In a rope and pulley system, a single continuous rope transmits tension around one or more pulleys to lift or move a load. If no energy is lost to friction or stretching, the mechanical advantage is the number of rope parts acting on the load. For a moving block supported by *p* rope parts, each part carries a tension of *W/p*, so the input force needed is *W/p*. Thus, the block and tackle reduces the required input force by a factor of *p*.
The simplest analysis assumes weightless, frictionless pulleys and non-stretching ropes. In equilibrium, the forces on the moving block sum to zero, and tension is the same in every part of the rope. For a single movable pulley supported by two rope parts, each supports half the load.
There are several types of pulley systems: - **Fixed:** The axle is mounted to a support. It changes the direction of force but does not by itself multiply force. Mechanical advantage comes from combining it with a movable pulley or a fixed pulley of a different
- earliest_evidence
- Ancient Egypt (Twelfth Dynasty, 1991–1802 BC) and Mesopotamia (early 2nd millennium BC)
- ancient_described_by
- Archytas of Tarentum (Greek mathematician)
- identified_as_simple_machine_by
- Hero of Alexandria (c. 10–70 AD, Roman Egypt)
- famous_demonstration_by
- Archimedes (as recounted by Plutarch)
- mechanical_advantage_examples
- Gun tackle: 2; Luff tackle: 3; Double tackle: 4; Gyn tackle: 5; Threefold purchase: 6
- types
- Fixed, movable, compound (block and tackle)
Lore & Background
The pulley's history begins in the Twelfth Dynasty of Ancient Egypt and early 2nd millennium BC Mesopotamia. It was later described by the Greek mathematician Archytas of Tarentum. In Roman Egypt, Hero of Alexandria identified the pulley as one of six simple machines used to lift weights. Plutarch's Parallel Lives recounts a scene where Archimedes proved the effectiveness of compound pulleys and the block-and-tackle system by using one to pull a fully laden ship towards him as if it were gliding through water.
A block is a set of pulleys assembled so each rotates independently. Two blocks with a rope attached to one and threaded through both sets form a block and tackle. The ideal mechanical advantage equals the number of rope sections supporting the moving block. For example, a gun tackle has a mechanical advantage of 2, a luff tackle of 3, a double tackle of 4, a gyn tackle of 5, and a threefold purchase of 6. The mechanical advantage can be increased by roving the tackle 'to advantage,' where the rope is attached to the moving block and pulled in the direction of the load.
Belt-and-pulley systems use two or more pulleys with a common belt to transmit power, torque, and speed across axles. Differing diameters provide mechanical advantage. A crowned pulley (slightly convex) keeps a flat belt centered. Cone pulleys and step pulleys provide multiple drive ratios. Lagging—a coating applied to pulley shells—improves friction and extends life, as with rubber-lagged drive pulleys.
Reader's Guide
The pulley is a fundamental simple machine that has enabled human civilization to lift heavy loads and transmit power for millennia. Its earliest known use in Ancient Egypt and Mesopotamia demonstrates its ancient importance. The block and tackle, a system of multiple pulleys, provides significant mechanical advantage, allowing a single person to move loads far beyond their own strength—as famously demonstrated by Archimedes pulling a fully laden ship. The pulley's role in belt and chain drives is equally critical, forming the basis for power transmission in countless machines, from factory line shafts to modern drill presses and vacuum cleaners. The concept of mechanical advantage, where the number of rope sections supporting a load directly determines the force reduction, is a key principle in physics and engineering. The pulley's versatility—in fixed, movable, and compound configurations—and its adaptation to different drive elements (rope, cable, belt, chain) have made it indispensable. Even today, innovations like lagging address practical challenges of friction and wear, ensuring the pulley remains a vital component in mechanical systems worldwide.
Did You Know?
- The earliest evidence of pulleys dates back to Ancient Egypt in the Twelfth Dynasty (1991–1802 BC) and Mesopotamia in the early 2nd millennium BC.
- Hero of Alexandria (c. 10–70 AD) identified the pulley as one of six simple machines used to lift weights.
- Archimedes proved the effectiveness of compound pulleys by using one to pull a fully laden ship towards him as if it were gliding through water.
- A block and tackle 'rove to advantage' increases mechanical advantage by attaching the rope to the moving block and pulling in the direction of the load.
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