Cylindrical gear processing technology and common process equipment

Gear drives are used in a wide range of modern machines and instruments. Their function is to transmit motion and power in a prescribed speed ratio.

The gear structure has various shapes due to different requirements, but from the perspective of the process, the gear can be regarded as consisting of two parts, a ring gear and a wheel body. According to the distribution form of the teeth on the ring gear, it can be divided into straight teeth, helical teeth, herringbone teeth, etc.; according to the structural characteristics of the wheel body, gears are roughly divided into disc gears, sleeve gears, shaft gears, sector gears and teeth. Bars and so on, as shown in Figure 9-1.

Of the above-mentioned various gears, the most widely used is a disk gear. The inner diameter of the disk gear is mostly cylindrical holes and spline holes with high precision. Its rim has one or more ring gears. The single-girdle ring gear has the best structure technology, and can process gear teeth in any one of the tooth-shape processing methods; multiple-tooth ring gears such as double or triple gears (Fig. 9-1b, c). When the axial distance between the rims is small, the choice of machining method of the small-tooth ring tooth profile is limited, and usually the pinion can only be selected. If the precision of the small ring gear requires high precision and needs fine-rolling or grinding-tooth machining, and the axial distance is not allowed to increase in design, this multi-ring gear can be made into a single-tooth ring gear combination structure to improve machining. The craftsmanship.

Second, the technical requirements of gears

The manufacturing precision of the gear itself has a great influence on the working performance, carrying capacity and service life of the entire machine. According to its use conditions, gear transmission should meet the following requirements.

(I) Accuracy of transmission movement

It is required that the gears transmit motion more accurately and the transmission ratio is constant. That is, it is required that the rotation angle error of the gear in one rotation does not exceed a certain range.

(B) the smoothness of the transmission movement

It is required that the gear transmission be smooth in order to reduce impact, vibration and noise. That is, it is required to limit the change of the instantaneous speed ratio when the gear rotates.

(c) Load distribution uniformity

When the gear is required to work, the tooth surface contact should be uniform so that the gear will not cause the contact stress to be too large due to the uneven load distribution when the power is transmitted, and cause the tooth surface to wear prematurely. The contact accuracy includes the contact area and the contact position in addition to the tooth surface contact uniformity.

(4) The rationality of the backlash of the transmission

When the gear is required to work, there is a certain gap between the non-working tooth surfaces to store the lubricating oil, to compensate for dimensional changes due to temperature and elastic deformation and some errors in machining and assembly.

The manufacturing accuracy of the gears and the backlash of the gears are mainly based on the use and working conditions of the gears. For gears used for indexing, the main requirements are high accuracy of the gears; for high-speed power transmission gears, in order to reduce impact and noise, there is a higher requirement for the accuracy of the work stability; for gears for heavy-duty low-speed transmission, The tooth surface is required to have a high contact accuracy to ensure that the gear will not wear out prematurely; for the gears used in the reversing drive and reading mechanism, the backlash should be strictly controlled and, if necessary, the clearance must be eliminated.

In B10095?88, 12 precision grades are specified for gears and gear pairs, which are sequentially reduced from 1 to 12. Among them, grades 1 to 2 are grades of precision to be developed, grades 3 to 5 are grades of high precision, grades 6 to 8 are grades of moderate accuracy, grades 9 and below are grades of low accuracy. Each tolerance class has three tolerance groups that specify the various tolerances and deviations, as shown in Table 9-1.

Table 9-1 Gear Tolerance Group

Tolerance group

Tolerances and deviations

Impact on transmission performance

I

Transfer movement accuracy

II

Transmission stability, noise, vibration

III

△Fβ, △Fpx

Load uniformity

Third, gear materials, heat treatment and blanks

(I) Gear Material and Heat Treatment

1. Material selection

The gears should be selected according to the working conditions in use. The suitability of the gear material has a direct influence on the processing performance and the service life of the gear.

In general, for a low-speed heavy-duty transmission gear, the tooth surface is subjected to pressure to cause plastic deformation and wear, and the gear teeth are easily broken. Should use mechanical strength, hardness and other comprehensive mechanical properties of better materials, such as 18CrMnTi; high linear speed transmission gear, tooth surface is easy to produce fatigue pitting, so the tooth surface should have a higher hardness, available 38CrMoAlA nitrided steel; For the transmission gear subjected to impact load, good toughness materials should be used, such as low-carbon alloy steel 18CrMnTi; non-force transmission gears can be selected from non-hardened steel, cast iron, cloth bakelite, nylon and other non-metallic materials. General-purpose gears are made of medium carbon structural steel such as 45 steel and low carbon structural steels such as 20Cr, 40Cr, 20CrMnTi, etc.

2. Heat treatment of gears

Two types of heat treatment processes are arranged according to different purposes in gear machining.

(1) Heat treatment of blanks Prepare heat treatment before and after processing of blanks—normalizing or tempering. Its main purpose is to eliminate the residual stress caused by forging and rough machining, improve the cutting performance of the material and improve the comprehensive mechanical properties.

(2) tooth surface heat treatment After the tooth shape processing, in order to improve the tooth surface hardness and wear resistance, often carburizing quenching, high-frequency quenching, carbonitriding and nitriding treatment and other heat treatment processes.

(b) Gear blanks

Gear blanks mainly consist of bar stock, forgings and castings. Bars are used for gears that are small in size, simple in construction, and require less strength. When the gear strength is required to be high and wear resistance and impact resistance are required, forging blanks are often used. When the diameter of the gear is larger than Φ400~Φ600, the cast tooth blank is commonly used. In order to reduce the amount of machining, gears can be cast directly for large-size, low-precision gears; for small-size, complex-shaped gears, precision casting, pressure casting, precision forging, powder metallurgy, hot rolling, and cold extrusion can be used. New processes, such as the production of gear teeth with billets, in order to increase labor productivity and save raw materials.

Fourth, tooth billet processing

The gear machining before gear machining is called gear blank machining. The inner hole (or journal), end face or outer circle of the gear blank is often the reference for gear machining, measurement and assembly. The accuracy of the gear blank is important for the machining accuracy of the gear. Impact. Therefore, gear blank processing occupies an important position in the entire gear processing.

(I) Precision of tooth blank processing

In the machining of the tooth blank, it is mainly required to ensure the dimensional accuracy and shape accuracy of the reference hole (or journal) and the positional accuracy of the reference end face relative to the reference hole (or journal). The tolerances and surface roughness requirements for different precision holes (or journals) are listed in Table 9-2, Table 9-3, and Table 9-4, respectively.

Table 9-2 Tooth Blank Tolerance

Gear accuracy level 1

5

6

7

8

9

Hole size tolerance

Shape tolerance

IT5

IT6

IT7

IT8

Shaft dimension tolerance

Shape tolerance

IT5

IT6

IT7

Top circle diameter 2

IT7

IT8

IT8

1 When the tolerance levels of the three tolerance groups are different, the tolerance value is determined according to the highest accuracy level.

2 When the top circle is not used to measure the tooth thickness reference, the dimensional tolerance is given by IT11, but should be less than 0.1mm.

Table 9-3 Radial and Face Runout Tolerances ( μm) of Gear Reference Plane

Dividing circle diameter (mm)

Accuracy level

more than the

To

1 and 2

3 and 4

5 and 6

7 and 8

9 and 12

0

125

2.8

7

11

18

28

125

400

3.6

9

14

twenty two

36

400

800

5.0

12

20

32

50

Table 9-4 Surface Roughness Parameters Ra( μm)

Accuracy level

3

4

5

6

7

8

9

10

hole

Neck end

Face

Top circle

≤ 0.2

≤0.1

0.2~0.1

≤ 0.2

0.2~0.1

0.4~0.2

0.4~0.2

≤ 0.2

0.6~0.4

≤0.8

≤ 0.4

0.6~0.3

1.6~0.8

≤0.8

1.6~0.8

≤1.6

≤1.6

3.2~1.6

≤3.2

≤1.6

≤3.2

≤3.2

≤1.6

≤3.2

The selection of the gear blank processing scheme is mainly related to the wheel body structure, technical requirements, and production volume of the gear. For gear blanks of shaft and sleeve gears, the machining process is similar to that of general shaft and sleeve parts. The following is a brief introduction to the gear blank machining plan for the disk gear.

1. Medium and small batch production of gear blanks

Medium and small batch production as far as possible the use of general-purpose machine processing. For the cylindrical hole tooth blank, the rough car-finishing car processing plan can be used:

(1) The parts of the roughing gear on the horizontal lathe;

(2) The inner hole and the reference end face of the finishing car in a single installation to ensure that the reference end faces the beating requirement of the inner hole;

(3) The inner hole is positioned on the mandrel, the outer circle, end face and other parts of the finishing car.

For spline hole blanks, the roughing-lathing-finishing car processing scheme is used.

2. Mass production of gear blanks

In high-volume production, regardless of spline holes or cylindrical holes, high-productivity machine tools (such as broaching machines, multi-axis automatic or multi-tool semi-automatic lathes, etc.) are used. The processing scheme is as follows:

(1) The outer circle is positioned to machine the face and holes (remaining broaching allowance);

(2) support the pull hole with the end face;

(3) positioning the hole on the mandrel, roughing the outer circle, end face, and notch on a multi-knife semi-automatic lathe;

(4) Without removing the mandrel, continue to finish the car's outer circle, face, notch, and chamfer on another lathe, as shown in Figure 9-2.