A Stroke Model for Straightening Partially Heat-Treated Ball Screws with Complex Mechanical Properties

Ball screws are surface heat-treated/hardened to improve their fatigue strength. However, the screws inevitably deform during heating. Thus, an additional straightening process is essential. Pressurization through three-point bending is typically adopted; however, determining the appropriate stroke for certain plastic deformations is challenging, mainly due to the complex mechanical properties of surface heat-treated ball screws. In this study, ball screws composed of S55C with various geometric shapes and heat-treated depths were tested with the aim of constructing a stroke model for precise straightening. Three-point bending experiments showed that the plastic deformation of the ball screws after bending was affected by the ratio of the heat-treated area to the total cross-sectional area, as well as the absolute value of the heat-treated area. Response surfaces were constructed according to the given plastic deformation and geometric shape of the ball screws to predict the desired stroke for straightening. Based on the experimental results, a computational model was built by modeling the screws with two materials. By adjusting the mechanical properties of non-heat-treated and heat-treated areas, the computational model showed similar characteristics to the experimental results. It is expected that our approach will contribute to more precise and effective straightening of partially heat-treated ball screws with complex mechanical properties.