Abstract: The ball screw assembly is a core component of the transmission system in CNC machine tools. Under external axial loads, the contact load distribution among the internal balls is uneven. Variations in the contact load between the balls and the raceways significantly affect the stiffness and lifespan of the ball screw assembly. Existing methods for analyzing the contact load between the balls and raceways in ball screw assemblies typically assume that all balls have the same diameter and that the load is distributed evenly among them. This assumption does not align with the actual load distribution experienced by the balls. To improve the uniformity of the contact load distribution among the balls within the ball screw assembly, a contact load model has been developed based on the existing ball screw load model, which considers variations in ball diameter. Utilizing this model, how installing balls of different diameters can achieve a uniform load distribution is studied, ultimately aiming to enhance the stiffness and lifespan of the ball screw assembly.