What causes the Grinding Cracks in high-precision Ball Screws?
Large ball screws usually adopt the medium-frequency induction hardening process. However, during production, after quenching (tempering) treatment, the ball screw kits often exhibit axial or radial cracks at the arc of the thread raceway, which can occur after thread grinding and magnetic particle inspection. Even some ball screw assemblies have cracks that can be directly seen with the naked eye during thread grinding, which directly leads to the scrapping of the linear ball screws.
Analysis of the causes of grinding cracks in medium-frequency quenched ball screws
1. Poor raw material quality
There are overproof structures in CCr15 material, such as flaky pearlite and carbide network overproof structures. Based on the analysis of carbide inhomogeneity and microstructure of cracked ball screws, approximately 40% of all unqualified ball screws are attributed to excessive network carbides or issues with the spheroidizing annealing structure.During induction hardening, due to the uneven distribution of carbides, the hardness and internal stress distribution on the surface of the linear motion ball screw are uneven, resulting in stress concentration inside the carbides. In the process of grinding the ball screw, if the internal stress at the grinding part exceeds the yield strength of the material, grinding cracks will occur. In addition, the presence of flaky pearlite will lead to coarse grains on the surface of the ball screw after induction hardening, which reduces the yield strength of the steel. When the internal stress generated during the grinding of the linear motion ball screw is greater than the yield strength of the material, it will also cause grinding cracks.
2. Defects in the heat treatment process of medium-frequency screw quenching
Excessively high quenching temperature or insufficient tempering are the main problems. According to analysis and statistics, the grinding cracks caused by this reason account for about 20%-30% of the total.
When large ball screws are subjected to medium-frequency quenching, if the medium-frequency output power is too high or the cooling rate of the workpiece during quenching is too slow, the quenching temperature of the ball screw sets will be too high, which will cause the martensite structure grade of the ball screws after quenching to reach grade 5 or even exceed the highest grade. For CNC ball screws with a coarser martensite structure, the toughness of the steel will decrease. During the grinding process, when the generated internal stress is greater than the yield stress of the steel, grinding cracks will appear.
In addition, during the quenching process, large-sized ball screws will generate large internal stresses, including thermal stress and structural transformation stress. If tempering is insufficient, such as too low tempering temperature or too short a time, the internal stress generated in the ball screw pair during quenching cannot be eliminated. After quenching and tempering, the residual internal stress in the ball screw and the stress generated by grinding are superimposed. When the superimposed stress exceeds the yield strength of the steel, cracks will be generated on the surface of the high-precision ball screw.