Discussion on selection and application of design process for ball screw pair

When a body is moving, it is generally necessary to transmit the generated motion directly or through other mechanisms indirectly to the final movement part. In today's world, devices that can represent mechanical and sports mechanisms can be said to have some form of motion transmission mechanism. The ball screw pair is the most reasonable product to turn the rotary motion into a straight line motion or to convert straight motion into rotary motion. As the ball screw directly affects the precision, parameters and manufacturing cost of CNC machine tools, the ball screw pair must be reasonably selected and a series of calculation check must be carried out in the design of the machine tool.

Selection of 1 ball screw

Ball screw is a kind of precise and efficient transmission element. It is widely used in the feed servo mechanism of modern CNC machine for its advantages of high speed, high rigidity, reversibility, long life, synchronism, high sensitivity, no gap, simple maintenance and so on. At the same time, the use of ball screw is also different from the ordinary machine. One of the fundamental features of the bed.  As the ball screw directly affects the precision, parameters and manufacturing cost of CNC machine tools, the ball screw pair must be reasonably selected and a series of calculation and verification must be used in the design of the machine tool.

The selection of 1.1 guide precision

The accuracy of ball screw directly affects the positioning accuracy of CNC machine tools. In the precision parameters of ball screws, the lead error has the most obvious influence on the positioning accuracy of machine tools. In the initial design, the V300P of any 300mm stroke set by the screw should be less than L / 3 ~ 1/2 of the positioning accuracy value of the target location, and it is determined in the final precision checking calculation. The ball screw is divided into 7 precision grades, that is, 1-7 and 10, and the 1 grade is the highest, decreasing in turn.

Selection of 1.2 nominal models

The nominal type mainly refers to the nominal diameter and lead distance of the screw. The initial stage of the design can be based on the technical requirements of the machine tool (mainly including load, speed, servo motor speed and so on) to select the ball screw model preliminarily. Then the calculation can be checked, and the results can be finalized after the design is in conformity with the design requirements.

The choice of the 1.3 support methods

In the design of CNC machine tools, the support way of ball screws usually chooses "fixed one support" or "fixed fixed". The support mode of "fixed support" is relatively convenient, rigid and high precision. It is mainly used in the environment of medium speed, large stroke and heavy load. The "fixed and fixed" support mode is relatively difficult, good rigidity and high precision, and is often used in the environment of high speed, large stroke and heavy load. .

Calculation and checking of 2 screw

2.1 allowable check for axial load

(1) maximum axial load

The medium is the friction coefficient of the guide surface; m is the total mass of the moving parts; F is the drag of the guide surface (when no load); of which, a=v/t, V is the speed of the machine tool, and t is the reaction time of the system.

The axial load is calculated respectively when the path acceleration is accelerated, the speed of the way, the deceleration, the speed of the return, the speed of the return, the speed of the return, the maximum of the maximum absolute value.

(2) the compression load of the ball screw shaft

The correlation coefficient of the middle type ETA 2 for different installation modes; the groove diameter of the d1- ball screw shaft; and the La- installation spacing.

(3) the allowable compression tensile load of the ball screw shaft

If the calculation result is P1>F1 and P2>Fa, then other projects can be checked. Otherwise, the model of ball screw can be re selected for checking above.

The calculation of the allowable speed of 2.2 ball screw

(1) maximum rotational speed of ball screw

The V is the moving speed of the moving parts, M / min and s for the lead screw.

(2) the critical speed of ball screw

In type: Lb installation space.

(3) the allowable rotation speed is N2=DN / D, and D is the center diameter of the ball. If the calculation result is N1>Nmax and N2> Nmax, the lead screw can be used, otherwise the ball screw model should be re selected for checking above.

Check and life calculation of the selection of 3 nuts

The allowable axial load of 3.1 nut

The basic rated static load of the Coa ball screw is calculated; if the calculation result is Famax>Fa, the nut is available, otherwise the model of the nut should be re selected for the above check.

The calculation of the rated life of the 3.2 nut

(1) the calculation of the average axial load

Type: F1, F2 and F3 are the axial loads (only the same direction load) when the path is accelerated, the path is uniform, and the return stroke is decelerated. L1, L2... L6 is for the trip of nuts when the path is accelerated, the speed of the road is uniform, the speed of the road is slowed down, the speed of return is accelerated, the return speed is uniform, and the trip is slowed down.

(2) life calculation

Formula: LR is nut life (total revolutions) FW is the load factor; Ca is the basic rated dynamic load.