How to Use Pulleys
Pulleys can do more than redirect the pulling tension of a rope. They can be used to gain mechanical advantage, reducing the force needed to move or lift a load, much as a lever does. For example, some nautical block-and-tackle systems have mechanical advantages as high as 32:1.
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Things You'll Need
- winch
- pulleys
- tackle (chain, rope)
- block and tackle hoist
- tripod or other hanging support
Instructions
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1
Determine the direction in which the load will be moved.
For example, if the load is to be raised by a pulling effort on the same level, then a fixed pulley is needed to change the direction of the tackle from some support above.
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2
Determine the mechanical advantage needed.
The mechanical advantage is the ratio of the force output by the pulley system to the force input. For example, if a puller applying a tension of 10 Newtons can lift a 100 Newton load, the mechanical advantage is 10:1.
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3
Determine the number of pulleys needed to meet the mechanical advantage requirement.
By doubling up the tackle, multiple pulleys create mechanical advantage.
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4
Attach one block of pulleys to a support, and the other block to the load.
A tripod can be used as a pulley support if the load is to be lifted.
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5
Use a winch to apply the effort if non-mechanical effort is insufficient.
In this way, the power of a motor can be introduced into the pulley system.
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Tips & Warnings
The doubling up of tackle to create mechanical advantage can be understood as follows. To lift a load, one set of pulleys must rise as much as the load, requiring doubled-over stretches of tackle to shorten the same length. Four doubling overs mean the puller must pull four times as much rope to lift the load the same distance. This is analogous to a lever, in which the load end moves slower than the work input end of the lever moves, but less force is required to move the load than if the lever was not used.
Orient the tackle so as to reduce oblique application of force. The tension of the tackle should be in the direction in which the load is to be moved. This is why pulleys do not tend to be separated, but instead combined into blocks, as in a block-and-tackle hoist. If lifting pulleys are laterally spaced, a lateral force component is created on the pulleys, and in the rope. Instead of all the tension going to lift the rope, some portion of the tension is wasted on pulleys pulling laterally on each other. The tension must then be greater than the weight of the load to lift it. Overlapping pulleys would not need lead to these lateral forces.
References
Resources
- Photo Credit U. of Ark. Agricultural Science Dept.
