How a Block & Tackle System Works
A block and tackle system is a simple machine like a lever or wedge. A block and tackle system is an arrangement of ropes and pulleys used to gain a mechanical advantage in lifting. A block consists of a shell to support the pulleys and a means of attaching the block to a support or a load.
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Construction
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A block and tackle system consists of two blocks, with one or more pulleys each, and a rope that goes through the pulleys. One end of the rope is attached to one of the blocks. Manila rope or wire rope may be used depending on the weight to be lifted. The pulleys and shells are designed for the load to be carried.
Mechanical Advantage
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A mechanical advantage means that a greater weight can be lifted with a smaller force. A two-to-one mechanical advantage means that 100 lbs. can be lifted by the application of 50 lbs. to the rope. The trade-off in block and tackle systems is that the rope, in this example, must be moved twice as far as the weight is lifted. The theoretical mechanical advantage in a given system can be determined by counting the number of ropes that are shortened when the weight is lifted.
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Examples
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A block and tackle system with two pulleys in the upper block and one in the lower block, and the rope attached to the lower block, has three ropes that are shortened when the weight is lifted. The mechanical advantage of this system is three, which means that 50 lbs. applied to the loose end of the rope will lift 150 lbs.
A block and tackle system with two pulleys in the upper block and two in the lower block, and the rope attached to the upper block, has four ropes that are shortened when the weight is lifted. The mechanical advantage of this system is four, which means that 50 lbs. applied to the loose end of the rope will lift 200 lbs.
Friction Losses
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The actual mechanical advantage of a block and tackle system will be lower than the theoretical advantage because of friction losses in the system. In the first system, the actual advantage would be about 2.7, or about 90 percent. In the second example, it would be about 3.4, or about 85 percent.
Practical Limitations
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Archimedes (born 287 B.C.) is credited with saying that with enough pulleys he could move the world. However, he would have been a victim of friction long before he had enough pulleys. Practical limits are about four pulleys in a block for Manila rope and six in a block for wire rope.
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References
- Photo Credit ship"s block and tackle image by David Smith from Fotolia.com
