Processing technology of shaft forgings

2024-08-14 15:40:46

Shaft parts are a common type of component in machines. It mainly plays the role of supporting transmission components and transmitting torque. The shaft is a rotating body part, mainly composed of inner and outer cylindrical surfaces, inner and outer conical surfaces, threads, splines, and transverse holes. Shaft parts can be divided into optical axis, hollow shaft, half shaft, stepped shaft, flower chain shaft, cross shaft, Eccentric shaft, crankshaft, and camshaft according to their different structures.

The main technical requirements for shaft forgings.

Dimensional accuracy and geometric accuracy, the journal of the shaft is an important surface of shaft components, and its quality directly affects the rotational accuracy of the shaft during operation. The diameter accuracy of the journal is usually 1T6 according to usage requirements, sometimes up to 1T5. The geometric shape accuracy (roundness, cylindricity) of the journal should be limited within the diameter tolerance. Axes with high precision requirements should be specifically marked with shape tolerances on the drawing.

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Position accuracy, the coaxiality of the mating journal (the journal for assembling transmission components) relative to the supporting journal (the journal for assembling bearings), and the perpendicularity between the journal and the supporting end face are usually required to be high. The radial runout of the mating journal relative to the supporting journal for ordinary taper shafts is generally 0.01-0.03mm, and for high-precision shafts it is 0.001-0.005mm. The end face circular runout is 0.005-0.01mm.

Surface roughness is required for all machined surfaces of shaft components. Generally speaking, the minimum required surface roughness value for the supporting journal is R α 0.63-0.16, followed by the surface roughness value for the mating journal, which is R α 2.5-0.63. The picture is a schematic diagram of a lathe spindle, indicating the main technical requirements.

Materials, blanks, and heat treatment of shaft components.

The material for shaft components is commonly 45 steel. For shafts with medium tip and high rotational speed, alloy structural steel such as 40Cr can be used; For high-precision shafts, bearing steel GCrl5 and spring steel 65Mn can be selected; For shafts with complex shapes, ductile iron can be used; For shafts working under high-speed and load bearing conditions, low-carbon alloy steels such as 20CrMnTi, 20Mn2B, 20Cr, or 38CrMoAl nitride steel should be selected.

The blanks for shaft parts, the most commonly used blanks for shaft parts are round bar materials and forgings: some large shafts or shafts with complex structures use castings. After heating and forging the blank, the internal fiber structure of the metal can be uniformly distributed along the surface. Thus obtaining higher tensile, bending, and torsional strength. Therefore, it is generally preferred to use Forged shafts instead of the required ones. According to the size of the production batch, the forging method of the blank is divided into two types: free forging and die forging.

The heat treatment of shaft components is not only related to the type of steel selected, but also to the heat treatment used. Before processing, forging blanks need to be subjected to normalizing or annealing treatment (including carbon steel and alloy steel with a carbon content greater than Wc=0.7%) to refine the internal grain size of the steel and eliminate forging stress. Reduce material hardness and improve cutting performance.

forging

In order to obtain better comprehensive mechanical properties, shaft forgings often require quenching and tempering treatment. When the blank allowance is large, quenching and tempering should be arranged after rough turning and before semi rough turning to eliminate the residual stress generated during rough turning; When the blank margin is small, quenching and tempering can be arranged before rough machining. Surface quenching is generally arranged before precision machining, which can correct the high deformation caused by quenching. For shafts with high precision requirements, low-temperature aging treatment (long-term low-temperature aging in 160 ℃ oil) is also required after local quenching or rough grinding to ensure dimensional stability.

For nitride steel (such as 38GrMoAl), quenching and low-temperature aging treatment should be carried out before nitriding. The quality requirements for quenching and tempering are also very strict. It is not only required that the martensite structure be uniformly refined after quenching and tempering, but also that the mass fraction of ferrite carbon in the layer 8-10mm away from the surface should not exceed Wc=5%, otherwise it will cause nitride brittleness and affect its quality.