How to install ball screw pairs of machine tools a

2022-09-21
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How to install machine tool ball screw pairs accurately and efficiently

the ball screw pair of large CNC machine tools is a screw transmission element with ball as the rolling element between the screw and nut. It is an advanced transmission element with precision, high efficiency, high stiffness, long life and energy saving. It can convert the rotating motion of the motor into the linear motion of the workbench. Therefore, it is widely used in machine manufacturing, especially in CNC machine tools and machining centers, It provides good conditions for the efficient and high-speed of the host. With the increasing accuracy requirements of CNC machine tools and machining centers, the high-precision of ball screw pair has become an inevitable trend of development, and its installation accuracy on the host has gradually become a prominent problem in assembly. In order to meet the requirements of coordinate position accuracy of the machine tool, reduce the lead screw winding, prevent radial and offset loads, reduce the temperature rise and thermal deformation of each link of the lead screw shaft system, minimize the transmission torque of the servo motor and improve the reliability of continuous operation of the machine tool, it is necessary to improve the installation accuracy of the ball screw pair on the machine tool

common installation methods of machine tool ball screw pair:

common installation methods of ball screw pair usually include the following: double push free mode; Double push support mode; Double push - double push mode

large horizontal machining center is an efficient machining equipment of electromechanical integration with high performance, high rigidity and high precision. It is an ideal processing equipment for processing all kinds of high-precision transmission box parts and other large molds. Its three coordinate directions adopt servo motor to drive the rolling lead screw transmission, and the working stroke of the three coordinate directions, namely x, y and Z, is large. Due to the structural characteristics of the ball screw pair, the installation of the ball screw pair in three directions on the host becomes particularly critical

I. there are defects in installing the ball screw pair with the old method:

1. According to the traditional process method, the method of installing the ball screw pair has been using the mandrel and positioning sleeve to connect the supporting bearing seats at both ends and the intermediate screw mother seat together for correction, using the dial indicator to align and parallel the mandrel axis with the guide rail of the machine tool, and making the mandrel drive freely and quickly. This installation method is convenient for small CNC machine tools and machining centers with small travel in three coordinate directions. Due to the matching clearance between the mandrel and the positioning sleeve, the positioning sleeve and the bearing holes supported at both ends and the middle screw seat hole, the coaxiality error between the installed support bearing hole and the screw seat hole is often large, resulting in a series of serious consequences such as the increase of the lead screw winding, the increase of the radial offset load, the increase of the temperature of each link of the lead screw shaft system, the increase of the thermal deformation and the increase of the transmission torque, resulting in the overload and overheating of the servo motor, The servo system alarms, affecting the normal operation of the machine tool. Li Jianbo believes that in the upcoming "classification of combustion performance of building materials and products", in addition, the actual difference between the bearing holes at both ends and the holes of the intermediate screw mother seat cannot be accurately measured, which affects further accurate adjustment. For CNC machine tools and machining centers with large travel in three coordinate directions, because the required mandrel is more than 1500mm, it is difficult to process and it is not easy to ensure the accuracy, so the alignment method of mandrel and positioning sleeve can not be used for the installation of ball screw pairs

in the production of a horizontal machining center, due to the large three coordinate travel of the machine tool, during the installation process using the traditional process method, the coaxiality of the bearing holes at both ends and the middle thread mother seat hole is out of tolerance, resulting in the increase of the radial and offset load of the ball screw, and the phenomena of overload and overheating of the servo motor and alarm of the servo system often occur, making the machine tool unable to operate continuously, At the same time, the service life and transmission accuracy of the ball screw pair are seriously affected, and the maintenance cycle of the host machine is shortened

2. Using other assembly methods, such as moving the sliding saddle, shortening the distance between the screw nut seat and the bearing seat, and aligning the screw nut seat and the bearing seat at both ends respectively, due to the need for two sections to be aligned respectively, plus the matching gap between the inspection rod and the inspection sleeve, the actual application effect is not ideal, and the above problems also exist

II. New assembly speed up the construction of Yangpu and Haikou Port Railway and Haikou and Sanya highway freight transport key construction process methods:

through the on-site technical research of the product, after repeated exploration and production verification, a relatively reliable assembly process method is summarized

1. First, use an integral special mandrel to correct the nut seat hole, so that its positive and lateral parallelism with the reference guide rail is within 0.01/1000

2. After fixing the screw base, use professional measuring fixture to actually measure the positive and lateral distance between the screw base hole and the reference guide rail; Then, also use the integrated special inspection rod to align the positive and lateral parallelism between the bearing hole and the reference guide rail within 0.01/1000, and use the special measuring fixture to actually measure the positive and lateral distances between the bearing hole and the reference guide rail. It is required that the positive and lateral distances between the screw mother hole and the reference guide rail are consistent, and the tolerance is 0.01; Fix the bearing seat. This method adopts an integral special inspection rod, which is not only short in length, but also combines the mandrel and the positioning sleeve into one, eliminating the fit gap between the mandrel and the positioning sleeve, and reliably ensuring the parallelism of the bearing hole, the thread mother seat hole and the guide rail

3. Through the measurement of the actual distance, the coaxiality of the bearing support holes at both ends and the screw base holes is also reliably guaranteed, which reduces the winding and radial offset load of the ball screw pair and improves the installation accuracy of the screw pair

4. In addition, in the process of installing the ball screw, the axial displacement of the ball screw must be strictly controlled. This technical index will directly affect the transmission position accuracy of the ball screw feed system. According to the field verification, it is shown that: first, the bearing in the bearing seat installed at the servo motor end should be assembled well, which plays a major role in the transmission process of the rolling screw, and the axial displacement of the ball screw should be controlled between 0.015~0.02; Then, assemble the bearing in the bearing seat at the other end to control the axial displacement within 0.01. This can effectively ensure the stiffness and accuracy of the ball screw feed system

5. The accuracy of the ball screw is the highest, and the pre stretching of the ball screw shaft is also very necessary. In order to improve the stiffness and accuracy of the ball screw feed system, it is very effective to pre stretch the screw shaft. However, due to the different sections of the screw shaft, and the temperature rise is not easy to set accurately, the pre tension calculated according to the relevant literature should only be stable and the driving can be used as a reference. In production, the lead screw shaft with the target value in the negative direction is often pre stretched, so that the orientation of the positioning accuracy curve of the machine tool workbench is close to the horizontal

III. conclusion:

in production, the no-load torque of the ball screw in the three coordinate directions of a large machining center assembled by the above new process method is significantly reduced, the no-load current is also significantly reduced, the servo motor and servo system work normally, there is no servo alarm in the three coordinate directions, and the machine tool can run continuously for more than 72h. The above results fully show that the new process method can effectively ensure the installation accuracy of the ball screw pair. In addition, this method is not limited by the travel size of the machine tool. The larger the travel of the machine tool, the more its advantages can be highlighted, which provides an effective and reliable method for the installation of ball screw pairs in large CNC machine tools and machining centers

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