The preload of a ball screw bearing refers to the intentional application of a small axial force to eliminate any clearance between the rolling elements (balls) and the raceways of the bearing. This preload helps to minimize backlash and improve the positioning accuracy and stiffness of the ball screw system. The preload force is typically applied by using spring elements or by adjusting the bearing arrangement. The specific preload value depends on the application requirements, such as the desired rigidity, operating conditions, and load characteristics. Preload is usually specified as a percentage of the dynamic load rating (e.g., 2% of the dynamic load rating). However, it's important to note that the actual preload force might vary depending on factors like the manufacturer's recommendations, size and type of ball screw bearing, and the specific application requirements. It is recommended to consult the manufacturer's documentation or contact their technical support to obtain the specific preload values and guidelines for a particular ball screw bearing.