Gear is the most widely used mechanical transmission part in modern machinery. Gear transmission transmits the motion and power between any two shafts in space through the meshing of gear teeth, and can change the form and speed of motion. Gear transmission has a wide range of use, constant transmission ratio, high efficiency and long service life. In the design and manufacturing process of mechanical parts products, not only must the properties of the materials be able to adapt to the working conditions of the parts and make the parts durable, but also the materials are required to have better processing performance and economy in order to increase the productivity of the parts and reduce Cost and reduce consumption. If the gear material is not selected properly, premature damage or even failure of the parts will occur. Therefore, how to select and use metal materials reasonably is a very important task.

 

　　Meet the mechanical properties of the material

 

　　The mechanical properties of the material include strength, hardness, plasticity and toughness, etc., reflecting the characteristics of the material during use. When the gear is meshing, there is contact stress at the tooth surface contact, and the tooth root has the maximum bending stress, which may cause the tooth surface or tooth body strength failure. Each point of the tooth surface has relative sliding, which will cause wear. The main failure modes of gears include tooth surface pitting, tooth surface gluing, tooth surface plastic deformation and gear tooth fracture. Therefore, the gear material is required to have high bending fatigue strength and contact fatigue strength, the tooth surface must have sufficient hardness and wear resistance, and the core must have a certain strength and toughness.

 

　　For example, when determining the hardness of the large and small gears, attention should be paid to make the tooth surface hardness of the small gear 30-50HBS higher than the tooth surface hardness of the large gear. This is because the small gear receives more loads than the large gear, and the pinion tooth root is larger. It is thin and has lower strength than large gears. In order to make the teeth of the two gears close to the same strength, the tooth surface of the pinion gear is harder than that of the large gear.

 

　　On the other hand, after the material grade is determined according to the use performance of the material. It is necessary to clarify the mechanical properties or material hardness of the material, and then we can achieve the required hardness range through different heat treatment processes, thereby giving the material different mechanical properties. For example, a gear made of 40Cr alloy steel, when oil quenched at 840-860℃ and tempered at 540-620℃, the quenched and tempered hardness can reach 28-32HRC, which can improve the structure and comprehensive mechanical properties; when oil quenched at 860-880℃ , When tempered at 240-280℃, the hardness can reach 46-51HRC, the surface wear resistance of steel is good, the core toughness is good, the deformation is small; when 500-560℃ nitriding, the nitrided layer is 0.15-0.6mm , The hardness can reach 52-54HRC, the steel has high surface hardness, high wear resistance, high fatigue strength, high corrosion resistance and anti-adhesive performance, and minimal deformation; when it is electroplated or surface alloyed After lining, it can improve the friction 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. The manufacture of gears must go through several processes such as forging, cutting and heat treatment. Therefore, attention should be paid to the technological properties of the materials when selecting materials. Generally speaking, carbon steel has better forging and cutting process performance, and its mechanical properties can meet the requirements of general working conditions. But the strength is not high enough and the hardenability is poor. The alloy steel has good hardenability and high strength, but its forging and cutting performance is poor. We can improve the process performance of materials by changing the process regulations and heat treatment methods.

 

　　"For example, 20CrMnTi steel is selected for gears in automobile gearboxes. This steel has high mechanical properties. After carburizing, quenching and low temperature tempering, the surface hardness is 58-62HRC and the core hardness is 30-45HRC. 20CrMnTi has better process performance, and normalizing is used to improve its machinability after forging. In addition, 20 CrMnTi also has good hardenability. Due to the influence of the alloy element titanium, it is not sensitive to overheating, so it can be directly cooled and quenched after carburizing. And 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 bear high-speed, medium-load, impact and friction. Therefore, it is more appropriate to select 20CrMnTi steel according to the working conditions of the gear.

 

　　Material economy requirements

 

　　The so-called economy refers to the minimum cost and maximum economic benefit. Under the premise of satisfying the performance, the gear material should also be selected to minimize the total cost of the parts. We can consider from the following aspects:

 

　　Consider from the price of the material itself. The price of carbon steel and cast iron is relatively low. Therefore, selecting carbon steel and cast iron under the premise of satisfying the mechanical properties of the parts not only has better processing performance, but also reduces costs. From the perspective of metal resources and supply, the import volume of materials and the use of expensive materials should be reduced as much as possible.

 

　　Consider from the cost of the gear production process. First of all, the relative processing costs of using different heat treatment methods are different. For example, the cost of carburizing and surface quenching of 12CrNi3A steel is much less than the cost 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 under medium impact conditions, the original medium alloy high-grade carburizing steel 18cr2Ni4WA material is selected, which has been carburized at 910-940℃, quenched at 850℃, and tempered at 180-200℃. The tensile strength ≥1177Mpa, the yield strength ≥834Mpa, the elongation ≥10%, the reduction of area ≥45%, the impact toughness ≥980kJ/m2, and the hardness is 58-62HRC. Although it can meet the performance and process performance of gears, the price of parts is high. Now choose the relatively cheap low-carbon medium alloy, medium hardenability carburizing steel 20CrMnTi. After carburizing at 910-940℃, quenching at 870℃, and tempering at 180-200℃, the 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 few and concentrated as possible to facilitate procurement and management. With the development of gear shapes, sizes and materials in the direction of multi-variety, multi-series and individualization, especially with many models and small output, in the production technology of gear forging, machining and heat treatment, there is a large amount of design and production. Long cycle, low efficiency, high cost, high energy consumption, difficult management, and difficult quality assurance. Therefore, selecting, optimizing and compressing material grades and specifications when selecting gear materials will help improve the generalization, serialization and standardization of materials. , Improve the utilization rate of materials, improve the planning of material procurement, to reduce inventory backlog, accelerate capital flow, facilitate storage and storage, and reduce material cost consumption. Finally, we can also improve economic efficiency by improving the process. For example, the die forging process for the production of die forgings has broken through the requirements of the traditional process. When providing forming blanks, the cutting process can be used. The die forging and mechanical finishing can be combined to partially or completely replace the cutting process to directly produce parts, or in production The use of group technologies and processes can also improve product quality, production efficiency and reduce costs.

 

　　Conclusion

 

　　In summary, when selecting gear materials. It is necessary to understand the form of my country’s industrial development, combine my country’s resources and production conditions, proceed from reality, and comprehensively consider issues such as mechanical performance, process performance, and economy. Only reasonable selection of materials can ensure gear quality, reduce product costs, and improve market competitiveness. .