Design of special hob for machining star wheel of

2022-06-22
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Design of special hob for star wheel machining of single screw compressor

1 introduction

it is an inevitable trend for the development of power air compressor to replace piston compressor with screw compressor. Screw compressors are divided into twin-screw and single screw. A pair of rotors of a twin-screw compressor is essentially a pair of helical gears, which belong to plane meshing. At present, its tooth shape mainly depends on imports, while the CNC grinding machine used for rotor finishing completely depends on imports. At present, there is a certain gap between the performance of domestic twin-screw air compressors and that of foreign countries. Therefore, imported twin-screw compressors occupy a dominant position in the domestic market. They are not only expensive, but also the maintenance and parts supply are not timely. This situation will have an adverse impact on the safety of the national economy

screw compressors are widely used in national defense forces, air cooling and other fields. For example, the navy has strict requirements on the technical performance of the air compressor: submarines can only surface after the water is discharged with high-pressure air, while submarines with high noise are easy to be captured by enemy sonar. Therefore, the compressor is required to have the advantages of low vibration, low noise, good reliability and energy saving. After a strict demonstration of the existing air compressors, the US Navy believes that the single screw compressor can fully meet the requirements of submarines for high-performance compressors because of the above advantages. However, the rotor of single screw compressor is a spatial meshing pair, and its meshing theory research and processing technology development are more difficult than that of twin screw compressor. At present, only a few countries such as Japan and the United States can produce it after introducing French patents, which are mainly used for air compressor, air conditioning compressor and process compressor. Single screw compressor is the only high-grade compressor that has not been introduced in China so far. China began to tackle key problems in the research and development of single screw compressors in the 1970s. The research and development work has been supported by the former National Science and Technology Commission, the Commission of science, technology and industry for national defense, and relevant ministries and provinces. Some progress has been made, but the key technologies such as high-precision special machine tools and star wheel processing have not been solved. The developed single screw compressors still need to be further improved in terms of service life and energy consumption

The meshing pair composed of a screw (worm) with 6 slots and a star gear (worm) with 11 teeth in a single screw compressor is essentially a pair of special high-precision hourglass worm pairs. Its special feature is that the shape of the screw is not toroidal but cylindrical (or the shape of the screw is the same as that of the ordinary cylindrical worm, and its tooth shape is the same as that of the toroidal worm)

1) traditional tooth profile

since there are two contact lines on one tooth of the worm gear (one on the mother surface or bus, and the other on the secondary envelope surface) when the toroidal worm pair works, researchers generally believe that the meshing pair of the single screw compressor is also in double line contact

2) new tooth profile

20 years ago (about 10 years earlier than abroad), the author found that there were three contact lines on one tooth of the star gear at the same time, and the tooth surface of the star gear was composed of three parts: mother surface (or bus) 3, secondary envelope surface 4 and special surface 5 (see Figure 1). The characteristic surface is also called the third working area. The appearance of the third working area enlarges the effective lower working tooth surface area of the star gear by 5 times, and the service life of the star gear is also extended several times. This newly discovered tooth profile is called "new tooth profile" of star gear

w1 -- angular velocity of star wheel; W2 - screw angular velocity

1. Starwheel 2 Screw 3 Busbar 4 Secondary envelope 5 The second working area

figure 1

design of 3 star wheel hob

this new tooth profile of star wheel has obvious advantages in improving the sealing performance and working life of the meshing pair by taking advantage of the policy of made in China 2025, but its processing technology must be solved. When machining this new tooth profile with a conventional hob, the following problems will occur: ① the reasonable rake angle of the hob blade should vary from end to end by nearly 30 °. However, it is difficult to grind such a rake face in a deep and narrow groove; ② The tooth profile and tooth width of the machined star gear will not be allowed to change after the hob is reground; ③ It is difficult to guarantee the accuracy of the shovel grinding process. In order to solve the machining problem of the new tooth profile of the star wheel of the single screw compressor, a special hob is developed

1) short hob structure

when the imaginary hob is used to roll the star wheel and calculate the cutting allowance of each edge, it is found that the cutting allowance of the left edge of the hob is only a few microns. If the left end of the hob is removed, the resulting tooth profile error will be very small. Therefore, the hob is shortened in the design. This short hob structure not only reduces the manufacturing difficulty of the hob, but also reduces the cutting force

2) design of hob blade

the truncated hob is divided into four sections. Its main advantages are as follows: ① the blade that is difficult to grind in the narrow gap is changed into the blade on the open end face, which greatly facilitates the grinding and detection of the blade; ② The feature of 6-head hob can be easily used to shorten the blade to 1/6 of the original length, which not only greatly reduces the cutting force and ensures the stability of the cutting process, but also reduces the change of the original rake angle of nearly 30 ° to 5 ° ~ 6 °. Therefore, the constant rake angle can be reasonably selected on the premise of ensuring the machining effect, significantly reducing the manufacturing difficulty of the hob; ③ After regrinding of the blade, the correct position of the blade in the tool body can be maintained by grinding the end face or adding shims between the end faces, so multiple regrinding of the hob can be realized

4 practical effect

the designed short hob is used for real cutting test. The results show that the hobbing process is stable and smooth, the meshing clearance of the machined star gear after installation meets the design requirements, the contact spots on the tooth surface are evenly distributed, and the contact area is greater than 90%. The measured displacement is more than 1% higher than the international advanced level. On this basis, the tooth width of the starwheel was changed from the original left and right half teeth to unequal tooth width, and the "energy-saving and long-life single screw compressor" was invented. The energy-saving and long-life single screw compressor uses this special hob to process the star wheel with unequal teeth. Compared with the international advanced level, its performance reduces energy consumption by 0.5kw/m3 · min-1, the volumetric efficiency increases by more than 5%, the pressure tapping point is unfair, and the star wheel life increases by more than 10%. Its industrialization has been supported by the science and technology innovation fund for small and medium-sized enterprises of the Ministry of science and technology, The performance and technical level of the product have been highly praised by many academicians of the Ministry of science and technology and welcomed by users. At present, the product has been serialized and put into mass production. The product can save hundreds of millions of kilowatt hours of electricity every year

5 epilogue

compared with the overseas star wheel which uses this kind of cooking grade oil to improve the traction performance by contributing to the heat dissipation of the tread rubber, the star wheel hobbing process has the advantages of high efficiency and low cost. At present, as the only enterprise in China that has mastered the design and manufacturing technology of this special hob, Zhengli precision machine is making new efforts to further expand China's advantages in basic research, process technology and process equipment of single screw compressor. (end)

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