Ball Screw Principle Analysis
Ball screw is the most commonly used transmission element in tool machinery and precision machinery. Rotational motion can be converted into linear motion, and can also be converted into linear motion. So what is the principle of the ball screw? What is the selection method of the ball screw? Let's take a look at the detailed introduction.
What is a ball screw?
The ball screw consists of a screw rod, a nut, a steel ball, a pre-compression sheet, a reverse device and a dust filter. Its function is to convert rotary motion into linear motion, which is a further extension and development of Acme screw. The significance of this development is to change the bearing from sliding to rolling.
The principle of ball screw.
1. According to the national standard GB/T17587.3-1998 and its application examples, the ball screw (which has basically replaced the trapezoidal screw, commonly known as the screw) is used to convert rotary motion into linear motion; or convert linear motion into rotary motion The actuator has the advantages of high transmission efficiency and accurate positioning.
2. When the ball screw is used as the active body, the nut will be converted into linear motion according to the rotation angle of the corresponding specification, and the passive workpiece can be connected with the nut through the nut seat, so as to realize the corresponding linear motion.
Application of ball screw.
In order to adapt to various uses, ball screw bearings provide a variety of standardized products. It is widely used in machine tools, and the circulation methods of balls include circulation conduit type, circulator type, and end cap type. Preloading methods include positioning preloading (double nut type, positioning preloading) and constant pressure preloading. The appropriate type can be selected according to the application. The screw has a precision ball screw with high-precision grinding (accuracy is divided into 6 grades of CO-C7) and a cold-rolled ball screw bearing formed by high-precision cold rolling (accuracy is divided into 3 grades of C7-C10). In addition, in order to cope with the urgent need for delivery by users, as well as the finished products that have been processed to the shaft ends, semi-finished products and cold-rolled ball screw bearings can be freely processed to the shaft ends. As the peripheral parts of the bearing, necessary screw support units, nut holders, lock nuts, etc. are used. It has also been standardized for users to choose from.
Application of ball screw
Ultra-high DN value ball screw high-speed machine tool, high-speed comprehensive machining center machine.
End cap ball screw: quick handling system, general industrial machinery, automated machinery.
High-speed ball screw: CNC machinery, precision tool machines, industrial machinery, electronic machinery, high-speed machinery.
Precision grinding ball screw: CNC machinery, precision tool machines, industrial machinery, electronic machinery, conveying machinery, aerospace industry, other antenna actuators, valve switch devices, etc.
Nut rotation (R1) series ball screws: semiconductor machinery, industrial robots, woodworking machinery, lightning processing machinery, handling devices, etc.
Rolled grade ball screw: low friction, smooth operation, fast supply and low price.
Heavy-duty ball screws: all-electric injection molding machines, punching machines, semiconductor manufacturing equipment, heavy-duty brakes, industrial machinery, forging machinery.
Ball screw type.
There are two common circulation methods: outer circulation and inner circulation. The ball is sometimes out of contact with the lead screw during the cycle, which is called the outer loop; it is always in contact with the lead screw, which is called the inner loop. Each closed loop of the sphere is called a column, and the lead contained in the closed loop of the sphere is called the number of turns. There are 2, 3, 4, and 5 rows for each nut of the internal circulation ball screw pair, and each row has only one circle; the external circulation has 1.5 circles, 2.5 circles and 3.5 circles.
1) External circulation: The external circulation refers to the circulation in which the balls return to the nut through the spiral groove or cannula on the outer surface of the nut after the circulation process is completed. As shown in Figure 2-3, the return methods of the external circulating ball screw nut when the ball is circulated are mainly end cap type, intubation type and spiral groove type. Commonly used external circulation methods are end cap type; intubation type; spiral groove type. For the end cap type, longitudinal holes are machined on the nut as the return passage for the balls. There are return ports for the balls on the cover plates at both ends of the nut, and the balls enter the return tube to form a circulation. Intubation type, using the elbow as the return pipe, this structure has good manufacturability, but because the pipe protrudes outside the nut, the radial dimension is large. The spiral groove type is to mill a spiral groove on the outer circle of the nut, and drill through holes at both ends of the groove, which are tangent to the screw raceway to form a return channel. Such structures are smaller in radial dimension than cannulated structures, but are complex to manufacture. The outer circulation ball screw has a simple structure and manufacturing process and is widely used. The downside is that ball seams are hard to get smooth.
2) Inner circulation: The inner circulation adopts the reverser to realize the ball circulation, and there are two types of reversers. The cylindrical convex key reverser, the cylindrical part of which is embedded in the nut, and the end is provided with a reverse groove. 2. The reverse groove is positioned by the cylindrical outer surface and the round key 1 at the upper end to ensure alignment with the thread raceway direction. The oblate insert reverser is an ordinary round head flat key insert, the insert is embedded in the groove of the nut, and the end is provided with a reverse groove 3. Position with the outer contour of the insert. Compared with the two reversers, the latter is smaller in size, thereby reducing the radial dimension of the nut and shortening the axial dimension. However, the dimensional accuracy of the outer contour of the reverser and the groove on the nut is higher.
The characteristics of the ball screw.
Small friction loss and high transmission efficiency;
Since there are many balls rolling between the screw shaft of the ball screw pair and the screw nut, higher motion efficiency can be obtained. Compared with the previous sliding screw pair, the driving torque is less than 1/3, that is, the power required to achieve the same motion result is 1/3 of the sliding screw pair. This is very helpful for power saving.
The second is high precision.
The ball screw pair is generally continuously produced by the world's highest level of mechanical equipment, especially in the process of grinding, assembly and inspection in the factory environment, and the temperature and humidity are strictly controlled. Due to the perfect quality management system, the precision is fully guaranteed.
Possibility of high-speed feed and micro-feed.
The ball screw pair adopts ball motion, the starting torque is extremely small, and it will not crawl like sliding motion, which can ensure precise micro-feeding.
High axial stiffness;
The ball screw pair can be preloaded, because the preload can make the axial clearance reach a negative value, so as to obtain higher rigidity (the inside of the ball screw applies pressure to the balls, when it is actually used in mechanical devices, etc., due to the ball The repulsive force can enhance the stiffness of the silk nut).