A pulley is a small wheel with grooves around it that can rotate around an axis. A simple machine that can rotate around a central axis, consisting of a grooved disc that can rotate around a central axis and a flexible cable (rope, tape, steel cable, chain, etc.) that spans the disc, is called a pulley. The pulley is a deformation of the lever and belongs to the simple lever type machine.
The pulley block is a simple machine assembled from multiple movable pulleys and fixed pulleys. It can save labor and change the direction of force. The labor saving of the pulley block is determined by the number of rope strands, and its mechanical efficiency is determined by the gravity of the object being pulled, the gravity of the moving pulley, and friction.
The function of pulleys and pulley blocks
The pulley supports and guides the movement of the rope and plays two roles of guidance and balance. The pulley block is composed of a certain number of movable pulleys and fixed pulleys. It has the characteristics of both movable and fixed pulleys. The pulleys are divided into fixed pulleys and movable pulleys. Fixed pulleys only change the direction of the force.
Moving pulleys save effort. Therefore, the pulley block can not only change the direction of force but also save effort. And it can be used as a deceleration or speed-increasing device. When the lifting weight is large, a pulley block with a larger magnification should be used to reduce the tension of the wire rope; when the lifting weight is light, a pulley block with a smaller magnification should be used to reduce the number of threading and winding of the wire rope and improve work efficiency.
The pulley block composed of wire rope, fixed pulley and movable pulley is an important part of the crane lifting mechanism. According to the function of the pulley, it is divided into two types: labor-saving pulley block and speed-increasing pulley block. The pulley block is divided into two types: single pulley block and double pulley block.
Pulley scrap standards
The pulley should be scrapped under one of the following conditions:
① Cracks (repair welding is not allowed);
② The uneven wear of the wheel groove reaches 3mm;
③ The wear of the wheel groove wall thickness reaches 20% of the original size;
④ The diameter of the bottom of the wheel groove is reduced by 50% of the diameter of the wire rope due to wear;
⑤ Others that meet the conditions for scrapping.
Pulley block overhaul
(1) The single-girder bridge crane is disassembled, cleaned and inspected for cracks;
(2) Maintenance of pulley groove;
(3) Inspection of shaft hole;
(4) assembly;
(5) The pulley shaft must not have cracks, the journal must not wear 30% of the original diameter, and the taper is not greater than 5%. If it exceeds this value, it should be replaced;
(6) Use a sample to check the groove shape of the pulley. The radial wear should not exceed 30% of the wall thickness, otherwise it should be scrapped. Those that do not exceed the standard can be repaired. After overhaul, use a sample to check. The bottom and lateral gaps should not be greater than 0.5mm. The deviation between the center line of the wheel groove and the center line of the pulley should not be greater than 0.2mm. The deviation between the center of the rope groove and the end face of the profile should not be greater than 1mm;
(7) After the overhaul of single-girder bridge crane, the shaft hole is allowed to have defects not exceeding 0.25CM2, and the depth should not exceed 4mm;
(8) After assembly, it should be able to rotate flexibly by hand, and the lateral swing should not exceed D/1000. D - Nominal diameter of pulley.
(Source: Lifting Business Communication)