The pulley group is made up of several fixed pulleys and movable pulley, which can achieve the purpose of saving effort and changing the direction of force action. In use, how much effort and rope around the method, decided to use the effect of the pulley block.
The pulley is assumed by two ropes, i.e. each rope bearing object and movable pulley
Force is a fraction of the total weight of the object and the movable pulley. Number, the principle is: N is odd, the rope from the moving pulley as the beginning. With a movable pulley, there are three pieces of rope to bear, after which each additional movable pulley adds two sections of rope. For example: n=5, two movable pulley (3+2) are required. When n is even, the rope starts from the fixed pulley, when all movable pulley are held with only two pieces of rope.
such as: n=4, you need two movable pulley (three). Secondly, according to the requirements of the number of fixed pulley, the principle is: general: two strands of rope with a movable pulley, a movable pulley generally with a fixed pulley.
When the direction of force is not required to change, even section of the rope can reduce a fixed pulley; To change the direction of forces, a fixed pulley should be added. In summary, the design principle of pulley block can be summed up as: odd motion and fixed; a moving match must, even minus must, change to add certain.