Improvement of ball screw of CNC machine tool

With the extensive promotion of electromechanical integration technology, the machining accuracy of machine tools is getting higher and higher. Precise ball screw pair is widely used in numerical control equipment and other advanced equipment due to its superior performance. The ball screw is mainly developed for the purpose of driving device of machine tool. The purpose of its development is to realize the high speed, high precision and smoothness of the machine tool.

Working principle and characteristics of the ball screw pair: the ball screw nut mechanism puts an appropriate amount of ball between the lead screw and nut to make the lead screw drive between the lead screw and nut change from sliding friction to rolling friction. Ball screw pair plays a role in mechanical transmission, which can change rotary motion into linear motion, and can also change linear motion into rotary motion.

Compared with the traditional lead screw pair, the ball screw pair uses rolling friction instead of sliding friction, so it has the following characteristics: small friction loss, high transmission efficiency. Small wear and long service life. High axial stiffness. Small friction damping and smooth movement. It cannot be self-locking and has the reversibility of transmission. With the development of high-speed and high-precision machine tools, the technical requirements for high-speed ball screw pairs are increasing. High-speed ball screw pair is the combination of precision ball screw pair and large-lead ball screw pair.

Therefore, there are two ways to improve the driving speed: one is to increase the speed of the lead screw, and the other is to use large lead. Increasing the speed n is limited by the dn value (d is the nominal diameter of the ball screw). When n increases, d decreases. According to the data, the dn value of the ball screw pair produced by foreign companies has reached 200000. However, excessive increase in speed will cause adverse effects such as lead screw heating and resonance. Moreover, too small d will also lead to poor rigidity and easy deformation of the system, thus affecting the machining accuracy. In order to improve dn value, the structure of ball screw pair must be improved.

When the lead P is too large, it not only increases the manufacturing difficulty of the ball screw pair, but also increases the starting torque of the servo motor. Therefore, in order to achieve both high speed and high precision, the balance between speed and lead must be achieved, and the two aspects should be considered comprehensively. Therefore, it is more important to improve the high-speed rotation ability of the ball screw. The circulation mode of high-speed ball screw pair is that the lead of high-speed ball screw pair is large, so the structure form of external circulation and intubation is adopted. This structure is widely used in various machine tools because of its simple manufacturing, smooth turning and easy ball return. It is often made into single-row or double-row structure. It is necessary to adjust the axial clearance and pre-tighten the ball screw pair, eliminate the clearance and pre-tighten the ball screw nut, and realize high-speed and high-precision transmission.

Advantages and disadvantages of several commonly used clearance adjustment and preloading methods:
(1) The advantages of the nut gasket type are that only the gasket is added, the structure is simple and reliable, the rigidity is good, and the loading and unloading is convenient. However, because the adjustment requires grinding the gasket, it cannot be adjusted at any time during the work. Therefore, this method is only applicable to mechanisms with general accuracy.

(2) The double-nut threaded type is characterized by simple structure, adjustable at any time and convenient. However, the preload must be adjusted by the user, which is difficult to control, poor in accuracy and reliability, easy to loosen, long in axial dimension and inaccurate in axial displacement adjustment.

(3) The double nut tooth difference type is characterized by accurate and reliable adjustment and high positioning accuracy, but its structure is complex.

(4) The features of single nut variable pitch self-tightening type are simple structure, compact size and low price. But it is inconvenient to adjust. There is a disadvantage in the widely used preload forms, that is, the preload cannot be measured directly in the work. It is determined indirectly by measuring the dynamic preload tension moment, so it can not accurately compensate the increased clearance due to wear at any time, and can not guarantee stable accuracy. Therefore, a new way of gap adjustment and preloading is needed.

The continuous adjustable preload device and the transmission clearance of the ball screw pair mainly refer to the axial clearance, that is, when there is no relative rotation between the lead screw and the nut, the maximum axial displacement between the lead screw and the nut, in addition to the clearance of its own structure, also includes the displacement caused by elastic deformation after the axial load is applied. When the ball screw rotates in the opposite direction, it will produce a return error, which will affect the transmission accuracy and reduce the stiffness of the ball screw pair. Therefore, in the precision ball screw pair with strict requirements for axial clearance and axial stiffness, measures must be taken to eliminate the axial clearance and improve the axial stiffness.

In order to eliminate the axial clearance, the elastic displacement curve of the ball screw nut with preload and without preload is compared. It can be seen that when the applied load is exactly three times of the preload, the rigidity of the ball screw with preload is twice of that of the ball screw without preload, and the elastic displacement is half of that of the ball screw without preload.

Controlling the elastic deformation after preloading within a certain range can not only eliminate the gap. It can reduce errors and improve feed accuracy and positioning accuracy. However, excessive preload will increase friction resistance, reduce transmission efficiency and greatly shorten service life. How to easily control and accurately adjust the axial displacement is the problem we need to solve.

It is composed of continuously adjustable preloading device, which is the combination of piezoelectric ceramic sensor and piezoelectric ceramic driver. The continuously adjustable preloading device is mainly based on piezoelectric ceramics. The shape variation of piezoelectric ceramic under the action of electric field is very small, which is not more than one tenth of its size at most. But don't look down on this small change. The precise control mechanism based on this principle - piezoelectric ceramic actuator.

Due to the high electric field strength and small limit strain required by piezoelectric ceramic strain, piezoelectric ceramic actuators often use multilayer piezoelectric ceramic chips bonded by conductive polymer to overlap. These stacked blocks can also be used side by side in order to expand the cross-sectional area. The superposition method not only makes the piezoelectric ceramic chips very thin, thus reducing the voltage required by the actuator, but also increases the displacement of the actuator. A plurality of laminated piezoelectric ceramic sheets are uniformly installed in the packaging box with elastic expansion. The piezoelectric actuator can adjust the axial clearance at any time, and has the advantages of high rigidity, small size and long adjustment distance.

The continuous adjustable device is used to pre-tighten the double nut, which can be used to measure the combined force of pre-tightening and the axial force in the double nut, so as to control the pre-tightening force. In the absence of comfortable signal current, first give the whole device a basic preload, which is about one-third of the maximum working load. The system can also move forward normally under the condition that the preload ensures that the driver is switched in order.

The basic pre-tightening force is taken as the starting point. According to the processing status and operation status, the pre-tightening force required for pre-tightening is controlled by the extension and contraction of the driver by using the signal current in the computer program. The preload is under control, so that it is easy to control and accurately adjust the axial displacement.

The controllability of preload is the basis for the ball screw to work with an ideal preload. The sharp reduction of friction energy can increase the fatigue life of the system with less heat and friction. Moreover, by increasing the preload to a certain extent beyond the basic preload, the required torsion and wear can cause changes in the preload, and then the preload, and then the dynamic preload torque. This effect can be compensated by controlling the preload, thus improving the positioning accuracy of CNC machine tools.

The continuous adjustable preloading device has a wide application prospect in ball screw. Although its application research in ball screw is still at the initial stage, there are still some problems to be solved, such as small strain, brittleness and high supply voltage of piezoelectric ceramics, we firmly believe that. Through continuous theoretical research and application practice, we will continue to solve problems, and finally open a new way for the wide application of continuous adjustable preloading device in ball screw.

Source: Shanghai Huiteng