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Meet the mechanical properties of the material
The mechanical properties of the material include strength, hardness, ductility and toughness, reflecting the properties exhibited by the material during its use. When the gear is meshed, there is contact stress at the contact of the tooth surface, and the root has maximum bending stress, which may cause tooth surface or tooth strength failure. Each point on the tooth surface has a relative sliding, which causes wear. The main failure modes of gears include tooth surface pitting, tooth surface gluing, tooth surface plastic deformation and gear tooth breaking. Therefore, the gear material is required to have high bending fatigue strength and contact fatigue strength, the tooth surface should have sufficient hardness and wear resistance, and the core should have certain strength and toughness.
For example, when determining the hardness of the large and small gears, care should be taken to make the tooth surface hardness of the pinion gear 30-50HBS higher than the tooth surface hardness of the large gear. This is because the pinion gear is loaded more times than the large gear, and the pinion tooth root is compared. Thin, lower than the large gear. In order to make the gear teeth of the two gears close to the same strength, the tooth surface of the pinion gear is harder than the tooth surface of the large gear.
On the other hand, the material grade is determined based on the performance of the material. To clarify the mechanical properties or material hardness of the material, we can then achieve the required hardness range through different heat treatment processes, thus giving the material different mechanical properties. If the material is 40Cr alloy steel gear, when 840-860 °C oil quenching, 540-620 °C tempering, quenching hardness can reach 28-32HRC, can improve the structure and improve the comprehensive mechanical properties; when 860-880 °C oil quenching When tempered at 240-280 °C, the hardness can reach 46-51HRC, then the surface of steel has good wear resistance, good core toughness and small deformation; when nitriding at 500-560 °C, 0.15-0.6mm nitride layer With a hardness of 52-54HRC, the steel has high surface hardness, high wear resistance, high fatigue strength, high corrosion resistance and anti-gluing properties and minimal deformation; when plating or surface alloying After that, it can improve the frictional performance of the working surface of the gear and improve the corrosion resistance.
Meet the process performance of the material
The process performance of a material refers to the ability of the material itself to adapt to various processing requirements. Gears are manufactured through several processes such as forging, cutting, and heat treatment. Therefore, attention should be paid to the material properties of the materials. In general, carbon steel has good process properties such as forging and cutting, and its mechanical properties can meet the requirements of general working conditions. However, the strength is not high enough and the hardenability is poor. The alloy steel has good hardenability and high strength, but the forging and cutting performance is poor. We can improve the process performance of materials by changing process procedures, heat treatment methods, etc.
For example, the gear in the automobile gearbox selects 20CrMnTi steel, which has high mechanical properties. After carburizing and quenching at low temperature and tempering, the surface hardness is 58-62HRC, and the core hardness is 30-45HRC. 20CrMnTi has good process performance, and it is normalized to improve its machinability after forging. In addition, 20 CrMnTi also has good hardenability, and it is not sensitive to overheating due to the influence of alloying element titanium, so it can be directly cooled and quenched after carburizing. Moreover, the carburizing speed is faster, the transition layer is more uniform, and the deformation after carburizing and quenching is small. It is suitable for manufacturing important parts that can withstand high speed medium load and impact and friction. Therefore, it is more suitable to select 20CrMnTi steel according to the working conditions of the gear.
Economic requirements for materials
The so-called economy refers to the smallest cost to achieve the greatest economic benefits. Under the premise of satisfying the performance, the gear material should be selected to minimize the total cost of the parts. We can consider the following aspects:
Consider the price of the material itself. The price of carbon steel and cast iron is relatively low, so carbon steel and cast iron are selected under the premise of satisfying the mechanical properties of the parts, which not only has better processing performance, but also can reduce the cost. From the perspective of metal resources and supply, the amount of materials imported and the amount of expensive materials should be reduced as much as possible.
Consider the cost of the gear production process. First of all, the different processing costs are different for different heat treatment methods. For example, the cost of carburizing surface quenching of 12CrNi3A steel is much less than that of nitriding treatment, and carbonitriding has the characteristics of short production cycle and low cost. Secondly, the cost can also be reduced by improving the heat treatment process. For example, when a gear is working at high speed, medium load and moderate impact, the medium alloy high-grade carburized steel 18cr2Ni4WA material is used, which is carburized at 910-940 °C, quenched at 850 °C, and mechanical properties after tempering at 180-200 °C. The tensile strength is ≥1177Mpa, the yield strength is ≥834Mpa, the elongation is ≥10%, the area shrinkage is ≥45%, the impact toughness is ≥980kJ/m2, and the hardness is 58-62HRC. Although the gear performance and process performance can be satisfied, the price of the parts is high. Low-carbon medium alloy and medium-hardenable carburizing steel 20CrMnTi, which are relatively inexpensive, are now used. After 910-940 °C carburizing, 870 °C quenching, 180-200 °C tempering mechanical properties of tensile strength ≥ 1100Mpa, yield strength ≥ 850Mpa, elongation ≥ 10%, reduction of area ≥ 45%, impact toughness ≥ 680 The hardness is 58-62HRC. With this improvement alone, the material cost is not only greatly reduced, but also meets its performance and process performance. Third, the selected steel grades should be as small and concentrated as possible for procurement and management. With the development of gear shapes, sizes and materials in the direction of multiple varieties, multiple series and individualization, especially in the case of many models and low production volumes, there are large designs and production in the production process of gear forging, machining and heat treatment. Unfavorable conditions such as long cycle, low efficiency, high cost, high energy consumption, difficult management and difficult quality assurance. Therefore, the selection, optimization and compression of material grades and specifications during gear selection are conducive to improving the generalization, serialization and standardization of materials. Improve the utilization rate of materials and improve the planning of material procurement to reduce inventory backlog, speed up capital flow, facilitate storage and storage, and reduce material cost. Finally, we can improve the economics by improving the process. For example, the die forging process produced by die forgings has broken through the requirements of traditional processes. When forming blanks, it can be used with less cutting process, die forging and mechanical finishing, partially or completely replacing cutting directly to produce parts, or in production. The use of group technology and processes can also improve product quality, production efficiency and cost.
Conclusion
In summary, when selecting gear materials. We must understand the form of industrial development in China, combine the resources and production conditions of our country, and proceed from the reality to comprehensively consider the problems of mechanical properties, process performance and economy. Only reasonable selection of materials can ensure the quality of gears and reduce the cost of products, thus improving market competitiveness. .
Gears are the most widely used mechanical transmission parts in modern machinery. The gear drive meshes with the teeth to transmit motion and power between any two axes of the space and can change the form and speed of the motion. Gear transmission has a wide range of applications, constant transmission ratio, high efficiency and long service life. In the design and manufacture of mechanical parts, not only the material properties can be considered to adapt to the working conditions of the parts, the parts are durable, and the materials are required to have better processing performance and economy, so as to improve the productivity of the parts and reduce Cost, reduce consumption. If the gear material is not properly selected, premature damage or even failure of the part may occur. Therefore, how to reasonably select and use metal materials is a very important task.