A block and tackle is a mechanism used for moving heavy loads easily. Consisting of pulleys in a housing and a length or rope, the device distributes the force required to move the load. This changes the physics of moving the load from more force with less work to less force with more work, making it easier for one person to move very heavy objects.
Design
The block and tackle system consists of an attachment bracket of some kind fixed to an elevated stationary surface. The bracket holds a housing, called the block, which contains one or more large pulleys seated on an axle. This block also has an anchor point, to which a rope or cable attaches. Another block with pulleys forms the moving portion of the system. It also has a bracket that attaches by a hook to a heavy load. A rope or cable threads from the anchor point through the pulleys in both blocks in a continuous loop and down to a final length, to which a pulling force is applied to lift the load.
Physics
A lever and fulcrum can distribute force in a way that makes it easier to lift a heavy load. In a block and tackle system, the axles act as the fulcrum and the pulleys themselves act as levers. The rope then moves across the pulleys, which distributes the force better to allow it to more easily lift the load—the more pulleys in the block and tackle, the better the force distribution and the less force required.
Advantages
Because the block and tackle system distributes the load, it requires a relatively low amount of force to move a very heavy object. A single person can lift loads far heavier with a block and tackle than without one. Block and tackle systems can also lift or lower loads to greater distances, such as a crane lifting an iron beam many stories into the air or a person lowering a small boat from a ship down many decks into the water.
Disadvantages
Because the block and tackle distributes the force across a long rope or cable, a greater amount of actual work is required. This means to move a heavy load, a lot of rope or cable must pull through a block and tackle system—much more than if the load were lifted straight up on a rope without the block and tackle. This requires more time to execute the task and better rope or cable control than might otherwise be necessary.
Uses
Industrial and construction cranes typically use block and tackle systems. Sailing vessels, from small sailboats to large ocean-going ships, also use block and tackles for tasks such as hauling sails, lowering lifeboats or loading cargo. This device also helps lift heavy objects in farming, manufacturing and other commercial environments.
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