As a linear positioning transmission mechanism, ball screw is used as the core mechanism of servo transmission in various high-end precision machine tools, numerical control equipment, aerospace vehicles and other important fields because of its good characteristics of high efficiency, high precision, high load and high speed. Wear is the inevitable result of friction, because the contact surface of two contacting objects will be damaged in the process of relative movement.
Wear is a complex dynamic process, which can be divided into the following categories according to the wear mechanism:
1) Adhesion wear: When there is sliding between the contact surfaces of two objects, the adhesion point will be sheared due to adhesion, and the sheared objects will either fall off into pieces or transfer from one object to another, thus forming corresponding adhesion wear.
2) Abrasive wear: Between the relatively moving contact objects, the external free hard particles enter between the two friction surfaces, causing surface damage and shedding, thus causing abrasive wear.
3) Fatigue wear: The friction surface of two rolling or rolling-sliding objects is subjected to cyclic stress, which causes fatigue peeling and corresponding pits. This phenomenon is called fatigue wear.
4) Corrosion and wear: In the process of friction movement, corresponding chemical or electrochemical interaction will occur between related objects and surrounding substances, damaging the surface of the objects. This phenomenon is called corrosion and wear.
5) Erosion wear: mechanical wear caused by the action of hard impurity particles contained in flowing gas or liquid.
6) Fretting wear: a kind of composite wear caused by the combined action of adhesive wear, abrasive wear, corrosion wear and fatigue wear. Due to the tiny vibration between the friction surfaces in contact with each other, oxidation wear particles will be formed on the contact surfaces, which are not easy to be removed from the contact surfaces, resulting in fretting wear.
In the working process of ball screw, there is not only one kind of wear, but the interaction of various wear forms. At the beginning of the work, the contact surface between the ball of the ball screw and the raceway is very smooth, only containing adhesive wear; However, after a certain process, metal particles will fall off in the contact area between the ball and the raceway, which will lead to wear; At the same time, after the metal particles fall off, another metal surface layer will be formed again, and corresponding chemical or electrochemical reactions will occur under the action of temperature and lubricant, leading to corrosion and wear; In addition, due to long-term cyclic contact stress, the ball screw work will lead to fine cracks on the raceway surface, which will lead to peeling or pitting of objects in the contact area, resulting in fatigue wear.
To sum up, it can be seen that the wear process of ball screw is complex and dynamic.
In order to deeply study the wear of ball screw pair, it is necessary to pay attention to its wear mechanism and its influencing factors.
There are many factors that affect the wear of ball screw, including speed, preload, working load, contact stress, raceway parameters and friction of ball screw. When the screw speed increases, the tangential relative speed of the contact surface between the ball and the raceway also increases, which leads to the increase of contact surface wear. The size and distribution of the load on the ball screw also have a great influence on the wear. When the working load on the ball screw increases, the corresponding contact area between the ball and the raceway will also increase, and the rolling friction torque caused by elastic hysteresis will also increase, which will inevitably lead to an increase in wear. In addition, if the raceway parameters of ball screw are different, the contact area between ball and raceway is also different, which will inevitably lead to different wear areas of ball and raceway.
Through the analysis of various friction types of ball screw pair, the total friction torque of ball screw pair is calculated, and the influencing factors of various friction types of ball screw pair are discussed.