Large shaft is the shaft used for motors with the specified sub core outer diameter of more than 1000mm. The surface roughness of its bearing position is required to be high, usually RA 0.4 μ M (RA: arithmetic mean of the absolute value of contour offset within the sampling length) to reduce wear. Today, Ms. will participate in a special discussion on large motor shaft and briefly introduce the key points and process difficulties.
Large shafts are made of forgings. The manufacturing process of forgings includes smelting, casting, forging, heat treatment, etc. For the shaft of large steam turbine generator, it shall also include rough turning, rough drilling of central through hole, RE heat treatment, machining of outer ring, fine boring of central through hole, sampling inspection, etc. These works are completed by the forging manufacturer.
The manufacturing process of large forgings is very complex. It needs to cast dozens or even hundreds of tons of steel ingots, thousands or even tens of thousands of tons of giant forging equipment, large annealing furnace and heavy lathe. Therefore, the shaft of some large hydro generators is forged by dividing the shaft body and coupling, and then welded. Some giant turbogenerators have also used combined rotors to avoid making large forgings, but combined rotors are rarely used because of their poor mechanical strength and complex machining.
Many large shafts have central through holes, which are designed to remove the parts in the center that are not easy to forge through, eliminate internal stress and improve the quality of workpieces. After the material in the central part is taken out, it can also be used as a sample for analyzing the material composition. The shaft of large steam turbine generator is long and the speed is high. Therefore, the requirement for the shaft is very high.
The shaft blanks are alloy steel forgings. A series of inspections shall be carried out for the shaft blank, including inspection of the central hole wall, residual stress test, material mechanical property test, internal defect flaw detection, chemical composition analysis, sulfur segregation test and magnetic conductivity test. Machining is allowed only after passing the inspection.
Alloy steel shaft must be quenched and tempered in order to give full play to its superior performance. Quenching and tempering treatment is to heat the shaft to about 850 ℃ for quenching, and then temper it at about 500 ℃ to obtain the tempered sorbite crystal structure and improve the impact toughness of the shaft.
