Fatigue Test And Failure Analysis Of Ball Screw Pair

Fatigue life is an important reliability index of ball screw pair. At present, the material selection and heat treatment process of ball screw pair have been improved through plenty of researches, while there have been few studies on the fatigue life of ball screw pair. As a result, the reliability performance, especially precision and fatigue life, of domestic ball screw pair is far behind that of international high-end products. Therefore, it’s of great significance to carry out the fatigue life test based on the material selection and process optimization, which can improve the quality of domestic ball screw pair.Fatigue test and failure analysis were performed on domestic GCr15 steel ball screw at first, to explore the defects of traditional material and process. Based on this, new material was selected and heat treatment process was optimized for the Φ80 mm domestic ball screws of large specification. Then fatigue test was carried out on the improved Best 55CrMo steel, Taiwan 50CrMo4 steel and Japan 50CrMo4 steel ball screw pairs of the same specification. Failure analysis was followed by means of field emission scanning electron microscope, chemical composition, metallographic observation and hardness gradient distribution detection.The results show that the main failure cause of traditional GCr15 steel ball screw is the unreasonable hardness distribution of hardened layer at the race bottom which is in close relationship with unreasonable material selection and surface induction quenching process.55CrMo steel is selected as the new material for the ball screws of Φ80 mm and above. Quenching and tempering is used as pre-heat treatment process. Similarly, optimized surface induction quenching process with four-turn coil is chosen. Therefore, the 55CrMo steel ball screw treated by the optimal heat treatment process has a reasonable surface hardened layer and matrix microstructures and better performance, which ensure the quality of the ball screw of large specification.The fatigue test results show that under the same condition, the 55CrMo steel ball screw has the longest life which is followed by the Japan 50CrMo4 steel and the Taiwan 50CrMo4 steel ball screw. So there is a conclusion that the performance of the 55CrMo steel ball screw meets or exceeds that of the similar products from Taiwan and Japan.The failure analysis results show that the main failure reason for the 55CrMo steel ball screw pair is the damaged GCr15 steel balls of the B nut which cause scratches on the surface of the nut raceway and are related to gathered carbides. The failure of the Taiwan 50CrMo4 steel ball screw pair is mainly due to serious peeling of the A nut raceway, which is connected with the coarse acicular martensite in the surface layer microstructure of the A nut. The major failure cause of the Japan 50CrMo4 ball screw pair is the obvious indentation and pitting on the A and the B nut raceways, which is associated with the band structure in the nuts. The 55CrMo steel ball screw is superior to the similar products from Taiwan and Japan in the depth and the hardness gradient distribution of the hardened layer.From the results of fatigue test and failure analysis, it can be seen that the 55CrMo steel ball screw adopting the surface induction quenching process with the four-turn coil after quenching and tempering can satisfy the quality requirements of the ball screw of Φ80 mm and above. These results have presented the potential of application of the 55CrMo steel ball screw in future.