Ball screw can realize the conversion between rotary motion and linear motion, and it is the key functional component of the feed system of CNC machine tools. On the one hand, the accuracy retention of ball screw determines the working performance and service life, and also directly affects the performance of machine tools using ball screw as motion transmission components. On the other hand, there is an increasing demand for high-precision ball screws in industry, which promotes the development of ball screws. The precision loss of ball screw is mainly related to the wear on the contact surface of ball screw and ball nut, and wear is also the reason for the decrease of preload. Although the preload of the ball screw can be returned to the initial level by adjusting the preload structure, if the precision loss exceeds the limit value, it will be judged as precision failure.
If this kind of ball screw is judged to be ineffective in accuracy, it will lose its reliability and safety. Therefore, it is very necessary to put forward a calculation method that can calculate the accuracy loss of ball screw and predict the accuracy change trend. In order to study the law of precision loss of ball screw, the mechanism of precision loss of ball screw is analyzed first, which provides the basis for the subsequent calculation method of precision loss. Then, based on the analysis of the kinematic characteristics of the ball screw, combined with the sliding motion between the ball and the raceway, elastic deformation, livestock characteristics and the relationship between the pre-tightening force and friction torque, the accuracy loss of the ball screw is calculated according to the mechanical characteristics of the double nut pre-tightening ball screw.
The precision loss of ball screw is mainly caused by wear. According to the different wear mechanisms, wear can be divided into abrasive wear, adhesive wear and corrosion wear. There may be many kinds of wear forms of ball screw during its operation. Initial stage: the contact surface between raceway and roller is smooth, only adhesive wear exists, and the accuracy loss of ball screw is also slow; With the adhesive wear, metal particles on the surface of the ball screw fall off, resulting in abrasive wear; The new metal surface formed after shedding may undergo electrochemical reaction under the condition of lubricating medium and temperature, thus causing corrosion and wear. By this stage, the wear of ball screw has increased dramatically. In addition, under the action of long-term cyclic contact stress of ball screw, fine cracks may breed on the raceway surface, resulting in peeling of contact surface materials and fatigue wear.
The accuracy loss of ball screw is a complicated process under the action of wear, and the accuracy loss process may be affected by various kinds of wear. However, before the ball screw wear increases sharply, the process of precision loss may be affected by various wear forms. However, before the ball screw wear increases sharply, the accuracy loss mainly occurs in the adhesive wear stage of the ball screw, that is, adhesive wear is the fundamental reason for its continuous decline in accuracy.
Adhesive wear refers to a kind of wear in which the adhesive node formed by adhesive effect is sheared and the sheared material falls off into abrasive chips. From the microscopic point of view, adhesive wear can be expressed as follows: there are many contact peaks on the contact surface of the ball screw, and the actual contact area is only 0.01-0.1% of the apparent area. When the ball moves in the raceway, the contact peaks squeeze each other, and the surface pressure at the contact peaks increases rapidly, resulting in an instantaneous temperature above 1000℃; Because the volume of friction is much larger than the contact point, once out of contact, the temperature of the peak point drops rapidly, and the duration of local high temperature is generally only a few milliseconds; When the friction surface is in this state, the lubricating oil film and other surfaces will break, which will make the contact peak point adhere, and then the adhesion node will be destroyed during sliding. That is, adhesive wear is an alternating process of adhesion, destruction and re-adhesion. In this alternating process, the accuracy of the contact surface of the ball screw gradually loses until it exceeds the specified accuracy index range.
Theoretically, the wear is proportional to the sliding distance and the normal load, and the yield limit of soft materials is inversely proportional to the friction, so it is necessary to calculate the temperature and analyze the force of the precision loss machine tool. In addition, when the ball screw is used, in order to eliminate the empty return error caused by axial clearance, the structure of double nut pre-tightening is often adopted. The double nut gasket pre-tightening structure has the advantages of simple structure, convenient assembly and disassembly, reliable work and good rigidity, and is widely used.