In the process of gear transmission design, especially closed transmission, if the hardness of the gear is not fully considered in the design, it will have a great impact on the performance of the transmission system. Such as common gear failure: tooth surface pitting, tooth root fatigue fracture, etc., are related to gear hardness and gear quality. The gear of gear reducer is divided into soft tooth surface and hard tooth surface according to tooth surface meshing, which is reflected in the following application: 1： Soft tooth surface gear transmission (tooth surface hardness ≤ 350 HBS), at least one of a pair of gears is soft tooth surface, which is often used in general medium and low speed gear transmission with low precision requirements. 2： Hard tooth surface gear transmission (tooth surface hardness > 350 HBS), both gears are hard tooth surfaces. It is often used in gear transmission with strong bearing capacity and small volume. In specific equipment, such as winch, soft tooth surface is generally used, while hoist now mostly uses hard tooth surface. The following three points are usually considered in the design: 1： In the closed soft tooth surface gear transmission under general working conditions. Soft tooth surface (≤ 350 HBS) steel gear transmission: mainly based on tooth surface contact strength, designed according to tooth surface contact fatigue strength; Then check according to the bending fatigue strength of the tooth root. 2： For gear drives with closed hard tooth surfaces. Hard tooth surface (> 350 HBS) steel gear and cast iron gear transmission: mainly based on the bending strength of the tooth root, and designed according to the bending fatigue strength of the tooth root; Then check according to the contact fatigue strength of tooth surface. 3： For open gear drives. The main failure forms are tooth surface wear and tooth fracture caused by wear. The design criterion is: design and calculate only according to the bending fatigue strength of the tooth root, determine the modulus, and increase the designed modulus by 10% - 20% to consider the influence of wear
