Pulley & Rope Systems

Written by ned millis
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Pulley & Rope Systems
A pulley with two grooved wheels facilitates increased lifting power and pulling power. (Hemera Technologies/PhotoObjects.net/Getty Images)

Pulley and rope systems aren't very complicated. By changing the direction of an applied force, pulleys create a mechanical advantage that magnifies the pulling or lifting power. Pulley systems make it possible to pull or lift very heavy loads. When you use two or more pulleys in combination, the device is referred to as a block and tackle.

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Description

Pulley wheels are grooved so rope or cable will not ride off of the wheel and jam the assembly. Pulley wheels are mounted in a housing with a fixed hook or hanger loop at one end of the housing. The pulley housing may contain a single wheel or several wheels for increasing the lifting or pulling power. Early pulleys and housings were carved from wood. Modern pulley assemblies utilise metals and polycarbonates for greater strength and durability. Early ropes of sisal or cotton are still in use but metal or plastic rope is used in many instances.

Fixed Pulleys

A fixed pulley is attached to an overhead beam, a ceiling or a wall. It takes more effort to lift objects with a fixed pulley but the task seems easier since the object is raised by pulling downward on the rope instead of lifting. The pulling force applied to the rope must be equal to the weight being lifted.

Movable Pulleys

Movable pulleys require less effort to lift or pull the load because the moving pulley applies greater leverage to the load. The pulley moves upward as a lifting force is applied to one end of the rope. Because rope on two sides of the pulley supports the weight, it requires half the lifting force to raise the weight.

Combination Pulleys

Adding a second pulley to a lifting assembly makes it a block and tackle. By using a block and tackle, the effort to lift a load is reduced to half the load to be lifted, or less.

Rope and Pulley Uses

Visualise the massive sails of an ancient sailing ship and you will see a classic example of ropes and pulleys. It took a lot of power to lift heavy canvas and wooden spars into place, and that power was supplied by extensive use of pulley systems. Watch a flag being raised on a tall flagpole, and you will see a single pulley at the top of the pole. Visit a high-rise under construction and you will see giant cranes with huge cable and pulley systems lifting heavy steel beams high into the air for assembly.

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