Gear material and forming

1. Practice has proved that the general machine tool gears are made of medium carbon steel and treated by induction heating surface quenching. The hardness, wear resistance, strength and toughness can meet the requirements, and the induction heating surface quenched parts have the advantages of not easy oxidation, small decarburization and deformation, high productivity and so on. Induction heating surface quenching of gears can be divided into two types: one is large module gears (M > 4), which mostly adopts tooth by tooth heating quenching method; The other is small module gear (m < 3.5), which generally adopts integral heating and quenching method.

2. The gears of automobiles and tractors are mainly installed in the gearbox and differential. In the gearbox, it is used to change the speed ratio of engine, crankshaft and main shaft gear; In the differential, it is used to increase the torsional torque and adjust the speed of the left and right wheels. The power of the engine is transmitted to the driving wheel through the gear to drive the car and tractor. The working conditions of automobile and tractor gears are much heavier than those of machine tools. They often work under high load and wear much. The brittle fracture or bending fatigue failure of gear teeth will be caused due to the sudden change of impact load and periodic change of bending load; Because the working face of gear teeth bears large compressive stress and friction, it will cause pitting contact fatigue damage and deep peeling; Due to frequent gear shifting, the end of the tooth is often impacted, which will also cause damage to the end of the tooth. Therefore, the requirements for wear resistance, fatigue strength, core strength and impact toughness are higher than those of machine tool gears, and high-frequency quenching is not competent in some cases. Practice has proved that it is more suitable for automobile and tractor gears to be made of carburized steel and used after carburizing heat treatment.

JB-150 truck gearbox gear is shown in the figure

3. The driving gear of the starter is the key part of the starter. When starting the engine, the power is transmitted through the contact of the tooth surface, and its failure form is generally tooth surface wear, block falling or tooth breaking. It is required that the gear has good wear resistance, strength, toughness and fatigue resistance, and has a working life and reliability without failure after 10000 starts. It is better to use carbonitriding process instead of carburizing process.

4. Domestic heavy truck gear steel basically follows the brand of the former Soviet Union. For a long time in the past, 20CrMnTi steel has dominated the world. Not only the variety is single, but also the steel composition fluctuates widely, the hardenability bandwidth and many inclusions, resulting in large gear heat treatment deformation and low service life. With the introduction of foreign advanced models and the localization of various gear steels, the level of gear steel in China has reached a new level. At present, Cr Mn series steel in Germany, Cr Mo series steel in Japan and sae68 series steel in the United States have been localized, which basically meets the domestic demand for medium and small modulus automotive gear steel.

According to the analysis of the current situation of domestic heavy vehicles, the two problems of overload and poor road conditions are more serious and can not be overcome in the short term, which makes the gears often bear large overload impact load. Overload impact load is between fatigue strength and fracture stress. It has a great impact on the service life of gears and often leads to early failure of gears. From this point, Cr Ni or Cr Ni Mo series steel should be selected for large modulus and heavy load automobile gears. Referring to the analysis of foreign brands, we believe that 17 C NR mio6 steel in Germany can be used as the preferred steel.

5. Nearly 50% of the gears in construction machinery are of boss structure. The traditional die forging or direct cutting leads to the decline of gear production efficiency, resulting in material waste and difficult to form large-scale production. With the improvement of forging technology in China, the development of forging technology is transitioning to the direction of silence, dust-free and vibration free, and has been widely used in production practice. Precision forging machine and roll forging machine replace forging hammer, and pressing and rolling process replace die forging and free forging, which not only improves the quality of forgings, but also reduces the amount of machining. The forging stock allowance of shaft and disc parts is generally 1.2-2.5mm. The rolling process is implemented to form the forging stock, which greatly saves the steel consumption and has achieved good economic benefits in production.

Many manufacturers at home and abroad use precision forging technology to change the processing technology of short teeth and straight teeth into precision forging tooth profile and then extrude the tooth profile, which changes the traditional gear hobbing technology, improves the gear accuracy and durability, and greatly improves the production efficiency. The tooth blank adopts precision forging process and has the following characteristics:

1) Improve material utilization and save raw materials;

2) Reduce the metal cutting allowance, improve productivity and reduce labor intensity;

3) improve the quality of gear blank and provide convenience for subsequent processing;

4) Improving the internal structure of gear blank creates favorable conditions for improving the strength of gear.

6. The gear adopts welded structure, which has the characteristics of light structure, shortening production cycle, saving raw materials and reducing manufacturing cost. Therefore, it is widely used in industrial developed countries. Some famous gear manufacturing companies, such as Japan’s Kawasaki, Mitsubishi and Germany’s lunk company, produce large section gears with welded structure, accounting for more than 40%. China has introduced the manufacturing technology of welded gears since the early 1980s. After more than 10 years of research and development and technical exploration, the manufacturing technology of carburized quenched welded gears can fully meet the design requirements by using the optimized process technology, which has explored a new way for the manufacturing technology of large and heavy-duty gears in China.

Large welded gear quenching in Luoyang mining machinery plant and 20CrMnMo in Shanghai heat treatment plant. The research and development of heat treatment of large steel welded gears summarizes their successful experience and technical progress in the processing and heat treatment technology of large heavy-duty welded gears.

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