Design criteria for gear drives
The different failure modes of gear transmission are unlikely to occur simultaneously in a pair of gears, but they do affect each other.For example, pitting on the tooth surface will aggravate the wear of the tooth surface, and serious wear will lead to tooth fracture.
Under certain conditions, the main failure modes are tooth fracture and tooth surface pitting.Therefore, when designing gear transmission, the main failure modes should be analyzed according to the actual working conditions, so as to determine the corresponding design criteria.
For closed soft tooth surface (hardness ≤350HBW) gear transmission.Good lubrication conditions, tooth surface pitting will be the main form of failure, in the design usually according to the tooth surface contact fatigue strength design, and then according to the tooth root bending fatigue strength check.
0.15 ~ 0.35Si, 0.85 ~ 1.25Cr, 0.35 ~ 0.45Mo.After negotiation with steel mills, a new type of 22CrMoH steel has been produced. Its hardenability index is J9=36 ~ 42HRC, which well meets the requirements of automobile gears.However, the process performance of the steel is poor. The gear forging body can be machined only after isothermal annealing treatment. The hardness is 156 ~ 207HB, and the metallographic structure is proeutectoid ferrite + pseudoeutectoid pearlite.The steel has higher hardenability, and the ordinary normalizing is easy to produce granular bainite. The appearance of granular bainite is extremely unfavorable to the cutting process, which not only reduces the service life of the tool greatly, but also causes the increase of heat treatment distortion of the gear due to the appearance of abnormal structure, which is always accompanied by the inhomogeneity of metallographic structure.4) American automobile manufacturers strive to reduce production costs, and at the same time, improve the reliability and durability of parts, which requires the product's geometric size and mechanical properties to be highly consistent.
For closed gear transmission with hard tooth surface (hardness >350HBW), the pitting corrosion resistance is strong, and the function of tooth breaking is large, in the design and calculation.Usually according to the root bending fatigue strength design, and then according to the tooth surface contact fatigue strength check.
At the present stage, the gear industry should improve the industry concentration degree through market competition and integration, and form a group of large, medium and small enterprises with several billion yuan, 500 million yuan and 100 million yuan of assets;Through the design and development of products with independent intellectual property rights, the company has formed a group of leading enterprises of vehicle driveline (gearbox, drive axle assembly) to integrate the capabilities and resources of the gear industry with the supporting capabilities of leading enterprises.To achieve specialization, network supporting, forming a large number of characteristic technology, characteristic products and rapid response ability of the famous brand enterprises;Through technical reform, realize the transformation of modern gear manufacturing enterprises.At the end of the 11th Five-Year Plan period, the annual sales of China's gear manufacturing industry could reach 130 billion yuan, with the per capita sales rising to 650,000 yuan per year, ranking second in the world.We will add 100,000 units of new equipment annually, that is, we will invest about 6 billion yuan in new equipment every year, and we will purchase 20,000 units of new machine tools with an average unit price of 300,000 yuan per unit.By 2010, China's gear manufacturing industry should have a total of about 400,000 machine tools.
The main failure mode of open gear transmission is tooth surface wear.But because the wear mechanism is more complex, there is no mature design calculation method, so we can only calculate according to the bending fatigue strength of the tooth root, increase the modulus by 10% ~ 20% to increase the tooth thickness, so that it has a longer service life, so as to consider the wear influence.
However, the quality of steel products with qualified chemical composition and mechanical properties is often affected due to the large fluctuation range of hardenability in automobile production.For example, if the hardenability of 20CrMnTi carburizing steel is too low, then the core hardness of the gear carburizing and quenching is lower than the specified value of the technical conditions, the fatigue test, the fatigue life of the gear is reduced by half;If the hardenability is too high, the shrinkage of the inner hole of the gear after carburizing and quenching is too large and affects the gear assembly.Since the hardenability of steel has an extremely significant impact on the hardness and distortion of the tooth core, the Ministry of Metallurgical Industry promulgated the technical conditions for ensuring hardenability of structural steel in China in 1985 (GB includes the chemical composition and hardenability data of 10 kinds of carburizing steels including 20CxMnTiH and 20MnVBH steel in this technical conditions.According to the standard, the hardenability index of 20CrMnTi steel used in the manufacture of gear is 30-42HRC at the point 9 from the water cooling end.
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