What Is a Block & Tackle?
A system of multiple pulleys connected to a load by a rope and hook, the block and tackle is a fundamental tool for efficient lifting of heavy objects. This simple rope and pulley device trades distance traveled for force applied. Does this Spark an idea?
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Parts
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The shackle anchors the device to a stable leverage point and the hook attaches the system to the load. Either pulley assembly is called a block, while tackle refers to the other gear.
Adding Distance
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The block and tackle increase the apparent distance the load travels. Lifting is easier, but the total effort is essentially the same because the actual distance is unchanged.
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Splitting Force
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A typical block and tackle include four times as much rope in the lift as the distance the load is actually moved. The force needed to put the load into motion is one-quarter the force needed for direct lifting.
Simple Hoists
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Simple hoists have four to six pulleys split between two blocks. The friction losses of multiple pulleys reduce the overall efficiency of the block and tackle.
Hybrid Hoists
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Common modern versions of the old block and tackle incorporate a ratcheting winch and levering handle. Built with metal parts and cable, the modern come-along lifts loads a simple hoist cannot.
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Resources
- Photo Credit Photo and drawing courtesy of Wikipedia Commons at http://en.wikipedia.org/wiki/File:Mercator06.jpg
