Why are Double-pulleys so Rare on the Water-wells?

Often, when I see a water-well, a question bothers me - ‘Why is a double-pulley so uncommon on the water-wells?’ I am in this search of the double-pulley system for a while now, but I have come across none.

On one hand, the complexity in the design of the multi-pulley system, manufacturing and material-cost could be potential reasons for the double-pulley being rare on the wells.

On the other hand, understanding and use of the multi-pulley system are probably not as intituative as one expects them to be. And, that could be affecting a wide spread use of the multi-pulley arrangement on the wells. I would like to know your views about this question which is a main motivation for the post.

Why do we use pulleys?

Firstly, we exploit a fact that a rope, over a pulley (and under a tension due to a load), can redirect the force. This provides us a flexible choice for applying the pulling force.

See Case-1 above. A person is pulling a load on a cart. Let’s assume that a force F is required to keep the cart under a desired motion. A rope under the tension offers some advantage. For example, the person can bend and achieve an optimal pull.

Mechanical advantage with rope-pulley system

With a rope-pulley system, we make use of fact that a sigle rope under a tension, distributes the tension uniformly over its length (under the tension).

In the Case-2 shown above, a deployment of a double-pulley (popularly known as snatch block) results in a reduced tension of say F/2 in the pulling rope. Hence, the cart can be pulled with half the pulling force now. This is a mechanical advantage.

More mechanical advantage with multiple-pulley arrangement

Let’s consider the Case-3 of three pulleys above. It’s descernible that it is more advantageous, as the pulling force can be reduced by a factor of four, i.e. F/4.

Also, the Case-4 in the picture above shows that a combination of two pulleys can also yield the same mechanical advantage as that in the Case-3, i.e. pulling force of F/4. A take-away is - ‘A smart choice of rope-pulleys arrangement is essential!’

We have records that Archimedes had demonstrated the power of multiple pulleys to a king by employing several pulleys and pulling a ship with manual force. Now, after centuries, I would like to see multiple pulley systems highlighted more in the story telling and the popular culture. For example, film makers have been successful in holding audiance’s breaths with a typical scene wherein a protagonist saves a life of a person on the cliff with the help of a rope.

The film makers got to ensure two things for the scene;

  • The protagonist has to have strong muscles and,
  • The protagonist has to really flex the muscles to pull the rope.

For a change, I would like to see the protagonist with smarter tricks in his hat. And, he would swiftly set sophisticated pulley arrangements and win the audiance with the Mechanical Advantage!

  1. Pulley on Wikipedia
  2. A video by Smarter Everyday

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