Development of the Wear Resistant Ball Nut Assembly in the Rack-Based Electric Power Steering

Electric Power Steering (EPS) is rapidly evolving in terms of performance. However, the abnormal noise from steering, which is experienced by the drivers, reduces the perfection of performance. In this study, the deterioration of Rack-based Electric Power Steering (R-EPS) samples by mileage was analyzed to investigate the cause of steering abnormal noise in the R-EPS applied to a large-sized luxury vehicle that is more sensitive to noise. The results from our analysis showed that the ball nut and screw in the ball nut assembly of R-EPS were in the mode of normal wear. However, the steel ball between the ball nut and screw showed irregular wear mode. It was found that the ball phi 5 has a larger contact area than the ball phi 4, which is advantageous for abrasion and irregular wear because stress is dispersed in the load. Moreover, the ball with phi 5 was superior to the ball with phi 4 in the results of abrasive and adhesive wear tests verified by the ball-on-disk test. Therefore, in the future R-EPS development, the ball with phi 5 compare to the ball with phi 4 should be proposed for the ball nut assembly. This is in order to eliminate the potential cause of steering abnormal noise.