Interference connection is to achieve the purpose of fastening connection by the interference value produced by the fit of the hole and the shaft. After the interference fitting is assembled, the diameter of the shaft is compressed and the diameter of the hole is expanded. Due to the elastic deformation of the material, the pressure is generated between the fitting surface of the shaft and the hole. When working, the friction force generated by the pressure is used to transmit torque, axial force or composite load. The mating surfaces of interference fit connection are mainly cylindrical and conical. This paper mainly introduces several assembly methods of interference fit of cylindrical surface.
Technical requirements of interference fit
① Appropriate interference. Too small interference can not meet the requirements of torque transmission, and too large interference will increase the difficulty of assembly. ② It has high matching surface precision. The fitting surface should have high shape, position accuracy and fine surface roughness. During assembly, pay attention to maintain the coaxial requirements of the shaft hole. To ensure a high degree of neutralization. ③ Have proper chamfering. In order to facilitate assembly, the hole end and shaft end shall be chamfered.
Assembly method of interference fit connection
The assembly methods of cylindrical interference fit mainly include hammering method, pressing method and temperature difference method. When the fit size is small and the interference is small, hammering method and pressing method can be used.
2.1 hammering method
Assembly points: ① before assembly, the hole end and shaft end should be chamfered. ② Remove the burr on the surface of parts and wipe them clean. The mating surface should be coated with lubricating oil. ③ When hammering, a soft metal pad should be placed on the hammering part of the workpiece. ④ Insert part 1 into hole 2 of Part 1 and put it in the right position. Place the base plate on the end face of Part 1 and level it. Knock the base plate with a hammer. When knocking, the striking force should not be skewed and strike symmetrically and evenly.
Features: this method is simple, but the guidance is not good, prone to skew. It is suitable for single piece, small batch production and connection with low matching requirements and short matching length.
2.2 press in method
Press in method can be pressed by press or auxiliary tooling. Tighten the nut with a wrench so that the sleeve is pressed into the assembly position. This method is suitable for small interference and small batch production. The press can be selected according to the pressure range and working table, and its guiding property is better than that of percussion pressing. However, due to the influence of factors such as the size of pressed parts and mass production, it has not been widely used in the assembly process of fully mechanized coal mining equipment.
Assembly points: ① before assembly, the hole end and shaft end should be chamfered. ② Remove the burr on the surface of parts and wipe them clean. The mating surface should be coated with lubricating oil. ③ The pressing process must be continuous and the speed should not be too fast. ④ When pressing in, the axis of the hole and the shaft must be consistent, and inclination is not allowed.
2.3 temperature difference method
Temperature difference method can be divided into hot charging method and cold charging method.
① Hot charging. It is assembled by using the physical characteristics of metal material thermal expansion and cold contraction. The method is that in the two parts with interference fit, the containing part is heated to make it swell, and then the contained part is installed into the matching position.
The heating method of hot charging depends on the size of the package. Gas furnace, electric furnace, oil bath heating, acetylene flame heating, induction heater heating and ceramic heater heating can be selected. The heating temperature is generally 80 ℃ – 120 ℃. Induction heater and ceramic heater are widely used because of their safety, reliability, clean operation environment and convenient heating. As shown in Fig. 3 induction heater: temperature controllable, easy to operate, safe, fast heating speed. It is mainly used for hot charging bearings, gears, crusher cutter teeth seat, etc.
Ceramic flexible heater: it can conduct overall or local heat treatment on metal parts, and control the temperature and time of heating, cooling and constant temperature by special temperature control box. It is especially suitable for small holes or super large parts that can not be heated by induction heater. It can heat multiple parts at a time. In the process of scraper conveyor assembly, it is mostly used to assemble half coupling.
② Cold charging method. The method of cooling the contained part during assembly to reduce its size, and then install the containing part to make it reach the matching position.
Small fitting parts and thin-walled sleeves with small interference can be cold shrinked with dry ice to – 78 ℃. For the fitting parts with large interference, liquid nitrogen can be used to cool down to – 195 ℃. For small and medium-sized parts, low-temperature freezer can also be used for cooling. The temperature of low temperature freezer is adjustable, which can be selected according to the production needs. In the component assembly of scraper, it is mostly used for cold mounting bearing, pin shaft in reducer planetary carrier, etc. the temperature control range is different according to different compressors.
Assembly points of temperature difference method: ① heating or cooling parts, to make it produce enough shrinkage. ② Parts should be assembled quickly and accurately, otherwise it will be stuck in the middle of the assembly, resulting in waste products.
Before assembly, it is necessary to determine the assembly method and the selected equipment according to the size of the parts, assembly relationship, interference and other factors. The application of induction heater, ceramic heater and low temperature freezer improves the assembly quality and efficiency of interference joint products, which is worthy of promotion.