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The weight of planetary gear transmission device is proportional to the weight of gear in general. The weight of reducer motor and gear has a great relationship with its material and heat treatment hardness. For example, under the same power, the weight of carburized quenched gear will be about one third of the weight of quenched and tempered gear. Therefore, according to the structural characteristics of planetary gear reducer and the load characteristics of gears, hard gear should be widely used. There are many heat treatment methods to obtain hard surface gear, such as surface quenching, whole quenching, carburizing quenching, nitriding and so on, which should be selected according to the characteristics of planetary gear reducer.
1. Surface hardening
The common surface quenching methods are high frequency quenching (for small size gears) and flame quenching (for large size gears). When the hardened layer of surface hardening includes the bottom of the tooth root, the effect is the best. The common material for surface hardening is steel with carbon content of about 0.35%~0.5%. The hardness of tooth surface can reach 45~55HRC.
2. Carburizing and Quenching
Carburized and quenched gears have relatively maximum load-bearing capacity, but finishing process (grinding) must be used to eliminate heat treatment deformation to ensure accuracy.
Alloy steel with 0.2%~0.3% carbon content before carburizing is commonly used for carburizing quenching gears, and its hardness is usually in the range of 58~62HRC. If the strength of tooth surface is lower than 57 HRC, the brittleness increases when the strength is higher than 62 HRC. The core hardness of gear teeth is generally 310 ~ 330 HBW. The hardness of carburized and quenched gears should be gradually reduced from the surface to the deep layer, while the effective carburizing depth should be gradually reduced from the surface to the deep layer, while the effective carburizing depth should be defined as the depth from the surface to the hardness 52.5 HRC.
The effect of carburizing and quenching on the bending fatigue strength of gear teeth not only improves the hardness of the center, but also relies on the residual compressive stress on the surface, which can reduce the stress in the maximum tensile stress zone of gear teeth. Therefore, when grinding teeth, the root part of the teeth cannot be grinded. When hobbing teeth, the retained abrasion hob should be used.
The hardness and wear resistance of the tooth surface can be guaranteed by nitriding under the condition of minimal deformation. After heat treatment, the final finishing can not be carried out and the bearing capacity can be improved. This is of special significance for internal gears which are not easy to grind teeth.
4. Hardness Combination of Gears for Meshing
When both big and small gears have soft tooth surfaces, the hardness of small gears should be higher than that of big gears. When the hardness of both wheels is higher, the hardness of the two wheels is the same.
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