The pulley block is composed of several crown blocks and moving pulleys, which can save labor and change the direction of force. In use, how much labor is saved and how the rope is wound depends on the use effect of the pulley block. The moving pulley is borne by two ropes, that is, the force borne by each rope is a fraction of the total weight of the object and the moving pulley. Number, the principle is: when n is odd, the rope is driven by pulley. When using a moving pulley, you have to bear three sections of rope, and then you add two sections of rope for each additional moving pulley. If n=5, two movable pulleys (3+2) are needed. When n is an even number, the wire rope starts from the crown block. At this time, all the moving pulleys are only borne by two ropes. If n=4, two movable pulleys (2+2) are needed. Secondly, the number of crown blocks is determined according to the needs. The principle is: generally, two ropes are equipped with a moving pulley, and generally, a moving pulley is equipped with a crown block. When there is no need to change the direction of force, even-numbered ropes can reduce one crown block; To change the direction of force, you need to add a crown block. To sum up, the design principles of pulley blocks can be summarized as follows: (1) odd motion and even determination; Move a certain number, even number MINUS a certain number, change direction plus a certain number. A simple machine that can rotate around the central axis consists of a grooved disc that can rotate around the central axis and a flexible cable (rope, adhesive tape, steel cable, chain, etc.). ) across the disk. Pulley is the deformation of lever, which belongs to lever simple machinery. In China, as early as the Warring States Period, there was a record about pulleys in the book Mo Jing. The pulley with the fixed central shaft is called the crown block, which is a deformed equal arm lever, and can change the direction of force without any effort. The pulley whose central axis moves with the weight is called the movable pulley, which is a deformed unequal arm lever, and can save half the force without changing the direction of the force. In practice, a certain number of moving pulleys and crown block are often combined into various pulley blocks. The pulley block can save labor and change the direction of force. The differential pulley (commonly known as chain block) commonly used in factories is also a kind of pulley block. Pulley blocks are widely used in cranes, winches, elevators and other machinery. There are two kinds of pulleys: crown block and moving pulley, which are combined to form a pulley block. (1) The crown block is essentially an equal arm lever, which is labor-saving and effortless, but it can change the direction of force. The characteristic of the crown block is that it is not labor-saving to pull the hook code through the crown block. Whether there is a crown block or not, the reading of the spring scale is the same. It can be seen that the direction of force can be changed easily by using the crown block. Many times, changing the direction of force will bring convenience to work. Principle of crown block The crown block is essentially an equal-arm lever, with power L 1 and resistance L2 arm equal to the pulley radius. According to the lever balance condition, it can also be concluded that the crown block is not labor-saving. (2) Moving pulley The moving pulley is essentially a lever whose power arm is twice as large as the resistance arm, which saves the force of 1/2 and takes the distance of 1 times more. The characteristics of moving pulley The use of moving pulley can save half the power and spend more distance. This is because when using the moving pulley, the hook code is hung by two ropes, and each rope only bears half the weight of the hook code. Although the use of moving pulley saves power, the distance of power moving is greater than the distance of hook code rising, that is, the distance is wasted. The principle of moving pulley is essentially a lever, and its power arm (L 1) is twice as big as the resistance arm (L2). (3) Pulley block: Pulley block consisting of crown block and moving pulley can save energy and change the direction of force. The pulley block uses several lengths of rope to lift the object, and the force used to lift the object is a small part of the total weight. The free end of the rope bypasses the movable pulley, but it does not count. Although the pulley block is labor-saving, it takes a long distance, and the distance of power movement is greater than that of heavy objects. Pulley. When using the pulley block, the labor is saved. The pulley block hangs the object with several ropes, and the force used to lift the object is a fraction of the weight of the object. The characteristics of pulley block are used in experiments. It is easy to see that although the pulley block is used, it saves labor, but it consumes distance-the distance of power moving is greater than the distance of goods rising.

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