The strength design of bevel gear is carried out from two aspects: the tooth surface pressure and the tooth root strength. With the development of technology and the application of computer, the development of transmission technology in the world tends to adopt hard tooth surface. According to statistics, the adoption of bevel gear with hard tooth surface greatly promotes the light weight, miniaturization and improvement of quality performance of the machine, so that the working speed of the machine is increased by a level.

Because the depth of nitriding layer is very shallow, it is not suitable for vulgar heavy-duty gear transmission, and the nitriding process itself is expensive, so it is rarely used.   

Surface hardening (such as high frequency, medium frequency or flame quenching) has obvious transition interface between hardened layer and non hardened layer, and the distribution of hardness is too large. At the same time, the hardening quality is not uniform, and the tooth root is difficult to harden, which is easy to form surface cracks. The hardness of tooth surface is relatively low (about HRC55), so the application is gradually reduced.

Deep carburizing and quenching grinding high-precision hardened gear has many advantages, such as high precision, high surface hardness (hrc58 + 4) and even hardened layer of tooth surface, which is especially suitable for low-speed and heavy-duty gear transmission. Its surface hardness is high, the contact strength is doubled than that of quenched and tempered gears, and the bending strength is about 50% higher than that of quenched and tempered gears.

High precision hard tooth surface gear represents the development direction of industrial and marine gear transmission device.