The ball screw, as a key transmission component that can achieve the conversion between rotational motion and linear motion, plays a decisive role in the feed system of modern CNC machine tools. It not only directly determines the transmission accuracy and operational stability of the machine tool but also greatly influences the overall performance and service life of the equipment. With the continuous development of industrial technology and the increasing demand for high-precision, high-efficiency machining, the accuracy retention and reliability of ball screws have attracted growing attention. Especially in the fields of precision machining, automation equipment, and aerospace technology, the performance indicators of ball screws have become an important measure of equipment advancement and competitiveness.
During the operation of a ball screw, accuracy loss is inevitable, and the root cause lies mainly in the wear occurring at the contact interfaces between the screw and balls as well as between the nut and balls. The accumulation of wear leads to a reduction in preload and an increase in backlash, which gradually lowers the transmission accuracy. Although preload can be adjusted through structural mechanisms to partially restore performance, once the accuracy loss exceeds the allowable limit, the screw is considered to have failed. Continuing to use a failed ball screw not only reduces reliability but may also create significant safety risks. Therefore, studying the mechanisms of accuracy loss, establishing reliable calculation models, and predicting accuracy change trends hold both theoretical and practical importance.
The essence of ball screw accuracy loss is a complex process driven by multiple wear mechanisms working together. Depending on the wear mechanism, it can be classified into adhesive wear, abrasive wear, corrosive wear, and fatigue wear. In practical operation, these mechanisms often occur alternately and influence each other. At the initial stage of use, the contact surfaces between the balls and the raceways are relatively smooth, and the dominant wear mode is adhesive wear. This happens as tiny adhesion nodes form between surfaces under rolling and sliding contact, which are repeatedly stretched, sheared, and broken, generating wear debris and causing material loss. At this stage, wear is relatively mild and the loss of accuracy is slow. As adhesive wear continues, detached metal particles remain in the contact area and act like hard abrasives, leading to abrasive wear. At the same time, inadequate lubrication or increased operating temperature can cause the newly exposed metal surfaces to react chemically with lubricants or the environment, resulting in corrosive wear. During this stage, the rate of wear accelerates, and accuracy loss becomes much faster. In long-term service, cyclic stresses acting on the ball and raceway surfaces may cause micro-cracks, which gradually expand and result in spalling of surface material. This type of damage is known as fatigue wear, and once it appears, the ball screw’s service life is near its end, with accuracy decreasing sharply and the risk of failure greatly increasing.
From this analysis, the accuracy loss process of a ball screw can be divided into a slow degradation phase and a rapid failure phase. Before accuracy loss becomes severe, adhesive wear remains the fundamental cause of the continuous decline in precision, making it a central focus of research and control.
From a microscopic perspective, adhesive wear occurs because the surfaces of the screw and balls contain numerous asperities, with the real contact area being only 0.01% to 0.1% of the apparent area. During motion, these asperities are subjected to extremely high local pressures, which generate transient temperatures that may exceed 1000°C. Although this high temperature exists only for a few milliseconds, it is sufficient to destroy lubricant films and allow direct adhesion at contact points. As rolling continues, the adhesion nodes are sheared apart, carrying away fragments of material and exposing fresh surfaces. This cycle of adhesion, fracture, and re-adhesion results in gradual surface damage and continuous accuracy loss. Theoretically, wear volume is proportional to sliding distance and normal load, and inversely proportional to the yield strength of the softer material. This indicates that careful control of load and temperature is essential to reducing adhesive wear and maintaining ball screw accuracy.
To eliminate axial clearance and backlash errors, ball screws often adopt a double-nut preload structure. Among these, the spacer-type double-nut preload system is widely used because of its simple structure, ease of assembly, good rigidity, and reliable performance. By applying preload, the contact stiffness of the nut is increased, ensuring stable transmission accuracy. However, excessive preload is detrimental, as it leads to higher friction torque, increased temperature rise, and accelerated wear. In general, preload should be controlled within 10% of the rated load to balance accuracy, rigidity, service life, and temperature effects.
Lubrication conditions are another critical factor in determining the service life and accuracy retention of ball screws. If lubrication is insufficient, friction and wear rise significantly, leading to shortened life or premature failure. In most cases, mineral oil commonly used for rolling bearings is suitable. At high rotational speeds, oil lubrication is preferred over grease lubrication, since it dissipates heat more effectively and reduces temperature rise. Grease lubrication, on the other hand, has the advantage of longer maintenance intervals, as grease often only needs to be replaced every six months or annually, provided that old grease is removed beforehand. The amount of grease should be controlled to about half the internal space volume of the nut to avoid excessive resistance.
In addition to lubrication, protective measures are equally important. If dust, chips, or foreign matter enter the ball screw, rapid wear and failure will follow. For this reason, protective devices such as bellows or telescopic covers should be used to shield the screw shaft. Dust seals should also be installed at both ends of the nut to prevent contamination. For vertical transmission systems, special safety measures must be designed to prevent nut disengagement, which could cause accidents. When installing a ball screw, the nut should not be removed from the screw shaft directly. Instead, an auxiliary sleeve slightly smaller than the screw root diameter by 0.2–0.3 mm should be used to safely remove or install the nut without allowing the balls to fall out.
Ball screws are highly efficient, with transmission efficiencies ranging from 85% to 98%. Because the difference between static and dynamic friction coefficients is small, they are highly responsive, reduce stick-slip behavior, and improve positioning accuracy. These advantages make them essential in CNC machine tools, machining centers, and advanced manufacturing systems. With the rapid development of high-tech industries, the performance requirements placed on ball screws, particularly in terms of accuracy, rigidity, and reliability, are becoming increasingly demanding.
In conclusion, the accuracy loss of a ball screw is a complex process dominated by multiple wear mechanisms, with adhesive wear being the fundamental cause of gradual precision decline. By optimizing preload design, applying suitable lubrication strategies, and implementing protective and safety measures, the process of accuracy loss can be effectively delayed. At the same time, it is essential to address the non-self-locking nature of ball screws by adopting reliable anti-reverse devices to ensure safe and stable operation. As modern manufacturing technologies continue to advance, further research into the wear mechanisms and accuracy retention of ball screws will remain an important direction to ensure higher precision, longer life, and better performance in next-generation machine tools and automation systems.