Determine whether the parts are suitable for cold assembly.
Whether the physical properties, mechanical properties and metallographic structure of mechanical parts change abnormally at low temperature is the key to choose whether cold assembly can be adopted. The allowable cooling temperature shall be determined according to the change of various properties of the metallographic structure of the part material in the low temperature state to a good direction. If the strength of the parts is reduced, the service life is shortened, and the bearing capacity is reduced after the cold installation, the cold installation cannot be selected.
The impact resistance test of the material at low temperature shows that the suitable temperature of the high carbon steel for cold assembly is higher than that of the low carbon steel, which is due to the fact that the impact toughness of the high carbon steel is easier to reduce at low temperature, that is to say, the cold brittleness that we often call occurs. However, the temperature of color parts is the lowest. Combined with the practical application of the author in the factory, it is recommended to adopt cold assembling for non-ferrous parts, especially for casting thin-walled colored sleeves, such as the widely used cast bronze bearing shells. In general, carbon steel should not be cold installed. If industrial refrigerator must be used for cold installation, the temperature should be controlled above – 75 ℃.
2 Calculation of cold charging temperature
When implementing the cold assembly process, the cold assembly temperature and implementation scheme should be selected according to the fit tolerance, interference, surface roughness between the mating parts and the shape, specification and position relationship of the workpiece. The interference is the key factor in the selection of the cold assembly temperature.
3 Calculation of cooling time
The wall thickness, length, diameter, physical and mechanical properties of the parts all affect the required cooling time, and these factors must be taken into account when implementing the cold assembly process.
4 requirements for cold charging medium and equipment
At present, the commonly used industrial refrigerators have cooling temperatures of – 65 ~ – 10 ℃, – 80 ~ – 10 ℃, – 100 ~ – 30 ℃, – 150 ~ – 20 ℃.
5 operation methods and precautions
In order to ensure the cooling effect, the industrial freezer should be selected according to the size and quantity of pre installed parts. First place the parts to be cooled in the industrial freezer to reach the cooling time, take out the parts and quickly install them in the containing parts.
In the process of operation, attention must be paid to: (1) do a good job in labor protection, wear protective glasses and gloves to avoid frostbite; (2) in order to ensure the quality of cold assembly, consider the quick assembly after cooling before cooling the parts, so do a good job in preparation for lifting and operation in advance.
Problems to be considered when adopting cold assembly process
Cold assembly is to use the heat expansion and cold contraction properties of the contained parts to complete the assembly requirements of interference fit parts. When the contained parts return to normal temperature, the contained parts will generate a pressure P on the joint surface of the contained parts. Due to the existence of P, the contained parts will produce “shrinkage phenomenon”, that is, the size of the inner hole will be reduced. In view of this phenomenon, the method of re machining to the design size after assembling into parts is generally adopted. However, some of them are very large in size (such as connecting rod), or complex in shape (such as box), so they can hardly be processed by machine tools. The use of machine tools for processing will cause a great waste of equipment resources, increase production costs, and the relative position requirements are difficult to guarantee. Fitter repair can correct the roundness error caused by shrinkage, but the shrinkage allowance caused by large interference must be removed mechanically.
We use the method of enlarging the cutting amount of inner hole in production. Taking a typical thin-walled copper sleeve as an example, this paper briefly describes the specific process measures.
When the wall thickness of the copper sleeve is uniform, it can be approximately considered that the tangential pressure P at each point of the cross section is equal and the shrinkage rate at each point is equal, then the shrinkage of the inner hole is equal.
For some large-scale thin-walled casings that must be machined after cold assembling shrinkage, considering the influence of shape error caused by shrinkage force, enough cutting allowance must be reserved during machining, and it is recommended to reserve 3-5mm on one side. The accuracy of machine tools and the technical level of workers should be taken into account.
Conclusion
Combined with practical application, the principle, characteristics, application scope, specific implementation process and precautions of cold charging technology are comprehensively discussed. There are not only theoretical derivation process, but also practical application examples. Empirical formula and recommended data are tested by production practice, and application effect is good.