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Principle of pulley
- Nov 20, 2018 -

The pulley is a small wheel with a groove around it that can rotate around the axis. A simple machine consisting of a disk that can be rotated around a central axis and a flexible cable (rope, tape, cable, chain, etc.) that spans the disk can be called a pulley.


The pulley block is made up of several fixed pulleys and movable pulleys, which can achieve the purpose of saving labor and changing the direction of force action. In use, the labor saving and the winding method of the rope are determined by the use effect of the pulley block. The pulley is carried by two ropes, that is, each rope bears the object and the moving pulley force is a fraction of the total weight of the object and the moving pulley.


The principle is that when n is an odd number, the rope driven pulley starts. When using a moving pulley, there are three sections of rope, and each additional one of the moving pulleys adds two pieces of rope. For example: n=5, you need two moving pulleys (3+2). When n is even, the rope starts from the fixed pulley, and all the movable pulleys are only used by two ropes. For example: n=4, you need two moving pulleys (2+2).


Secondly, according to the requirements to determine the number of pulleys, the principle is: the general: two ropes with a moving pulley, a moving pulley is generally equipped with a fixed pulley. When the direction of force action does not require change, the even-numbered section of the rope can be reduced by a fixed pulley; to change the direction of force action, a fixed pulley is required.


In summary, the design principles of the pulley block can be summarized as: odd-moving even; one move is fixed, even-number is reduced, and the change is added.


A simple machine consisting of a grooved disc that can be rotated about a central axis and a flexible cable (rope, tape, cable, chain, etc.) that spans the disk and that can be rotated about a central axis. The pulley is a deformation of the lever and belongs to a simple mechanical type of lever. There is a record of pulleys in the book "Mojing" in China as early as the Warring States Period. The fixed pulley of the central shaft is called a fixed pulley, which is a deformed equal-arm lever, which is not labor-saving but can change the direction of the force. The pulley that moves the center shaft with the heavy object is called a moving pulley. It is a deformed unequal arm lever that can save half of the force but does not change the direction of the force. In practice, a certain number of movable pulleys and fixed pulleys are often combined into various types of pulley blocks. The pulley block is both labor-saving and can change the direction of the force.