Large mechanical parts cold assembly of industrial refrigerator

In the assembly process of mechanical parts, the interference fit between the hole and the shaft usually adopts pressing method or temperature difference method. For small parts with low assembly requirements and small interference, pressing method is often used, that is, hammering or machine pressing method. The defect of this method is that it can’t realize the assembly with large interference and high holding force, which is easy to cause the scratch and erosion of the mating surface, resulting in the low actual interference and matching accuracy of the assembly body, and the small holding force making the interference fit invalid. The temperature difference method can only be used for large parts with large interference.

Large interference fit of large parts refers to the large size of two or one of the assembled parts, and the size of the contained surface of the matched surface is larger than that of the contained surface. The fit tolerance zone is usually h7u6 or u7h6, with high fitting accuracy and large interference, which is a fixed connection that cannot be disassembled. Generally, the size of parts with holes is small, the thermal expansion rate of materials is large and the toughness is large, so the hot assembly process can be used; the size of parts with shafts is small, the cold shrinkage rate of materials is large and the toughness is good, so the cold assembly process can be used. In this paper, the main frame of mp800 is a part with a hole, which is very large in size, heavy in weight, and made of cast iron, so it is not suitable to bear impact and tension, and it is easy to cause deformation and crack after uneven heating and cooling; the frame is to complete the assembly of the main shaft in the process of processing, and then complete the processing of the other dimension chains with the installed main shaft as the new benchmark; The main shaft is relatively small in size, small in weight, good in toughness and made of chrome steel. The fitting accuracy of the hole and the shaft is high, and the interference is large. After assembly, the elastic deformation or expansion of the material results in a large radial pressure between the mating surfaces. When working, the friction between the mating surfaces is used to overcome the torque and the tangential force. It is a large part with large interference and non detachable fixed connection, which is suitable for cold assembly process.

The cold assembly process of industrial refrigerator is to use industrial refrigerator to cool the contained parts to a certain temperature, so that the outer diameter of the parts can be slightly reduced, and there is a certain gap between the parts and the parts, and then the parts can be assembled on the positioning device. Because the cold assembly method only freezes the relatively small shaft to reduce its size, and does not heat or freeze the hole, there is no stress in the hole before assembly; after assembly, due to the expansion of the shaft, the hole is subject to radial compressive stress and uniform stress, so it is not easy to produce cracks. When working, the torque and load are transmitted by the friction force generated by the radial pressure. The maximum friction force or torque is usually referred to as the “holding force”. The material, interference, structural form, surface roughness assembly process and working conditions of the parts can have a great impact on the holding force. Therefore, the control of the conditions and parameters in the assembly process is very important for the holding force to meet the use requirements. The processing of mp800 main frame is different from that of the past, which is to complete the assembly of the main shaft in the process of processing, and to complete the processing of the remaining dimension chain with the installed main shaft as the new benchmark. Many dimensional accuracy requirements of mp800 mainframe are about 0.1mm, and local dimensional tolerance is even as high as 0.05mm. Therefore, in order to ensure that the assembly accuracy of the spindle can meet the interference required by the customer and ensure the processing quality, it is necessary to strictly control the assembly method and process of the spindle.

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