1) gear materials must meet the requirements of working conditions. For example, in order to meet the requirements of small mass, high transmission power and high reliability, alloy silver with high mechanical properties must be selected. Gear transmission in mining machinery generally has high power, low working speed and high dust content in the surrounding environment, so materials such as cast steel or cast iron are often chosen; The power of household and office machinery is very small, but it requires stable transmission, low or no noise, and can work normally with little or no lubrication, so engineering plastics are often used as gear materials. In a word, the requirement of working conditions is the first factor to consider when selecting gear materials.
2) Gear size, blank forming method, heat treatment and manufacturing process should be considered. Large-size gears are generally made of casting blanks, and gear materials can be cast steel or cast iron. Gears with medium or low size requirements often use forged blanks, and forged steel can be used. When the size is small and the requirements are not high, round steel can be selected as the blank.
The methods of gear surface hardening include carburization, ammoniation and surface quenching. When carburizing is adopted, the gear material should be low carbon steel or low carbon gold-bearing steel; Nitriding process can be used for ammoniated steel and quenched and tempered steel; When surface quenching is used, there is no special requirement for materials.
3) Normalized carbon steel, no matter how the blank is made, can only be used to make gears working under stable load or slight impact, and cannot bear large impact load; Quenched and tempered carbon steel can be used to manufacture gears working under moderate impact load.
4) Alloy steel is often used to manufacture gears with high speed, heavy load and working under impact load.
5) For gear transmission in aircraft, the gear size should be as small as possible, and high-strength alloy steel with surface hardening treatment should be adopted.
6) For the soft tooth surface gear made of metal, the hardness difference between the tooth surfaces of the two paired wheels should be kept at 30 ~ 50 HBS or higher. When there is a big difference in tooth surface hardness between pinion and big gear (such as quenching and grinding of pinion tooth surface, normalizing or quenching and tempering of big gear tooth surface); Moreover, when the rotating speed is high, the hard pinion tooth surface will have a significant cold work hardening effect on the soft gear tooth surface, thus improving the fatigue limit of the gear tooth surface. Therefore, when there is a big hardness difference between the paired gear tooth surfaces, the allowable contact fatigue stress of the big gear can be increased by about 20%, but attention should be paid to the high hardness tooth surfaces and the roughness value should be reduced accordingly.