Summary
The wheel is a small wheel with a groove around it that can rotate around the shaft.
pulley
Definition of pulley
The pulley is a small wheel with a groove around it that can rotate around the shaft.
[A simple machine consisting of a disk that can rotate around a central axis and a flexible cable (rope, tape, cable, chain, etc.) that spans the disk is called a pulley. 】
The pulley is a deformation lever and belongs to a simple type of leveraged machinery, which has a wide range of uses. In China, as early as the Warring States period, the book "The Book of Memories" contained records about pulleys. The pulley fixed to the center axis is called the fixed pulley, which is a deformed equal-arm lever. It is not labor-saving but can change the direction of the force. The pulley driven pulley with the center shaft moving with the weight is a deformed unequal arm lever, which can save half the force, but does not change the direction of the force. In practice, a certain number of moving pulleys and fixed pulleys are often combined into various types of pulley blocks. Pulleys save effort and change the direction of force.
Differential pulleys commonly used in factories (commonly known as chain hoists) are also a kind of pulley block. Pulleys are widely used in cranes, hoists, lifts, pulleys and other machinery.
Type of pulley
According to the production of materials: wood pulleys, steel pulleys and engineering plastic pulleys;
According to the use of methods: fixed pulleys, moving pulleys, and moving and fixed pulleys.
According to the number of pulleys: single pulley, double pulley, three pulleys, four pulleys, and more than one wheel;
According to the action of the pulley: guide pulley, balance pulley;
According to the connection mode: hook type, chain type, ring type and hanging chain type.
Fixed pulley
Definition: When using the pulley, the fixed position of the shaft is called the fixed pulley.
The fixed pulley is essentially an arm lever, which is not effortless, but it can change the direction of the force.
Characteristics of fixed pulley
Pulling objects through pulleys is not a hassle. With or without pulleys, the spring dynamometer reading is the same. It can be seen that using fixed pulleys does not save energy but can change the direction of force. In many cases, changing the direction of force will bring convenience to the work.
The principle of fixed pulley
The fixed pulley is essentially an equal arm lever, and the power arm (L1) and the resistance arm (L2) are equal to the pulley radius. According to the condition of lever balance, it can also be concluded that the pulley is not labor-saving.
Moving pulley
Definition: The pulley with the position of the shaft moving together with the pulled object is called a moving pulley.
The moving pulley is essentially a lever whose power arm is twice that of the resistance arm and saves 1/2 force more than one time.
The use of a movable pulley can save half the force and cost distance. This is because when the movable pulley is used, the hook code is suspended by two ropes, and each rope only takes half of the hook weight. Although the use of mobile pulleys saves power, the distance the power moves is greater than the distance that the hook code rises, which means it takes a long distance.
Without changing the direction of the force, the moving pulley of the moving pulley is essentially a lever whose power arm (L1) is double the resistance arm (L2). (savings)
Pulley
Pulley group: The pulley group composed of fixed pulleys and follower pulleys, which can save labor and change the direction of force.
The pulley group uses several ropes to lift the object, and the force used to lift the object is a fraction of the total weight. The free end of the rope bypasses the moving pulley and the pulley that bypasses the fixed pulley is not considered.
Although the use of pulleys saves energy, but it takes a distance, the distance of power movement is greater than the distance the weight moves. The distance of the fee is mainly determined by the number of ropes of the pulley.
The use of pulleys:
In order to both save and change the direction of power, fixed pulleys and moving pulleys can be combined into a pulley set.
Labor-saving size
When using the pulley block, the pulley block uses several ropes to suspend the object, and the force used to lift the object is a fraction of the weight.
Pulley Features
Using pulleys as an experiment, it is easy to see that the use of pulleys saves the effort, but it takes a long distance - the distance the power moves is greater than the distance the goods are raised.
Several relationships (when pulleys are placed vertically): (1) s=nh (2) F=G total/n (without friction)
Where s: distance at which the rope end moves h: height at which the object rises
GTotal: The total weight of the object and the moving pulley
F: The force applied by the rope end n: The number of ropes pulling the weight
F=1/n×(G+G)
When connecting the pulley block, it is necessary to connect a moving pulley to a fixed pulley, otherwise the connection will fail.
According to F=(1/n)G, without considering the friction and the weight of the pulley, it is necessary to make the force of 2400N into 400N to be a six-segment rope, and then according to the even-odd singularity principle, there are even-numbered ropes, so the beginning of the rope should be from the fixed pulley. At the beginning, because six ropes are required, three parallel moving pulleys are required. Correspondingly, three parallel pulleys are also required. Winding around the hook at the bottom of the fixed pulley group, the first moving pulley is bypassed sequentially. One fixed pulley, the second... until the last piece of rope around the third fixed pulley, then the direction of the rope is down, and the tension will be 400N (without considering the friction and the weight of the pulley).