Classification, function and surface characteristics of screw rod

The screw is a rotary motion into a linear motion of the transmission vice parts, is an important original machine tool to complete the feed movement. The screw of the machine tool can not only transmit the movement correctly, but also need to be able to transmit a certain amount of power. Therefore, there are certain requirements in terms of accuracy, strength and wear resistance. Then, let's understand the classification, function and surface characteristics of the screw together!

The screw rod is a kind of high-precision parts, it can accurately locate the coordinate position of the worktable, convert the rotary motion into linear motion, transfer surface and certain power, so it has high requirements in precision, strength and wear resistance. Therefore, the processing of the screw rod must carefully consider all the processes from the blank to the finished product to improve its processing accuracy.

1. Classification of screw rod

The machine tool screw can be divided into three types according to its friction characteristics: sliding screw, rolling screw and hydrostatic screw. The screw is simple in structure and easy to manufacture, and is widely used in machine tools.

The tooth profile of the sliding screw is mostly trapezoidal. The tooth profile has the advantages of good compensation effect, good transmission performance, high precision and convenient processing.

The meaning of rolling thread is divided into two kinds of ball screw and roller screw. Compared with the roller screw, the ball screw has low friction, high transmission efficiency and high precision, so it is often used, but its manufacturing technology is complicated.

Hydrostatic screws have many advantages and are often used in the feeding mechanism of precision machine tools and CNC machine tools. The shape of the thread is the same as that of a standard trapezoidal thread. However, the tooth profile exceeds 1.5~2 times of the standard thread of this specification, in order to obtain a good oil seal and improve the loading capacity. However, the adjustment is troublesome, and the hydraulic system is required, the technology is complicated, and the cost is high.

2. Structural characteristics of screw rod

1) Process characteristics of screw structure

The screw is a slender flexible shaft, the ratio of length l to diameter d is large, generally 20~50, and the rigidity is poor. The structure is complex, the thread surface requirements are high, and there are steps, grooves, etc., which are easy to deform during processing.

2) Accuracy class

In the national standard GB785-65, the accuracy of regular trapezoidal thread is divided by pitch diameter tolerance. There are five basic parameters: outer diameter d, inner diameter d1, middle diameter d2, spacing t and tooth profile half angle/2. Since the screw must transmit the correct movement, according to the provisions of the JB2886-81, the accuracy of the screw and the pitch is divided into six grades: 4, 5, 6, 7, 8 and 9 according to the requirements of the screw.

The application range of precision screw at all levels is as follows. Level 4 is currently the higher level and is rarely used in general. Level 5 is used for precision instruments and confidential machine tools such as coordinate boring machines and thread grinding machines. Level 6 is used for precision instruments, precision machine tools and numerical control machine tools. Level 7 is used for precision thread lathes, gear processing machines and numerical control machine tools; Level 8 is used for common machine tools such as horizontal lathes and milling machines. Level 9 is used for feeding mechanisms of planers, drilling machines and common machine tools.

Generally speaking, precision screws refer to grade 5, 6 and 7 screws. Precision screws are quenched screws and non-quenched screws. The former has good wear resistance and can maintain the processing accuracy for a long time, but the processing technology is complex and requires high-precision thread grinding machines and special heat treatment equipment, while the latter only requires precision thread lathes. The accuracy grade of ball screw pair and ball screw is also divided into 6 grades.

3) Technical requirements

The technical requirements for the screw rod are divided into the following items.

Accuracy class;

Surface roughness;

Allowable range of individual spacing and cumulative allowable range of fixed length

Allowable Range of Pitch Diameter Roundness

Allowable range of equal outside diameter

Allowable range of outside diameter runout

Allowable range of tooth shape half angle

Allowable tolerances for the middle, outer and inner diameters.

The above introduction is the classification and function of the screw, surface characteristics, if you need to know more, you can contact us at any time!