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!’
Double-pulley in a Popular Culture
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!