The form of gear production can be divided into hard tooth surface and soft tooth surface according to the hardness of the tooth surface:
1) Hard tooth surface: tooth surface hardness HB>350. This kind of gear production has high bearing capacity, quenching, surface quenching or carburizing quenching after gear finishing, and the hardness of the tooth surface is HRC45-65. However, in the heat treatment, the gear is inevitably deformed during the production, so it is necessary to grind, grind or finish after the heat treatment to eliminate the error caused by the deformation and improve the accuracy of the gear.
If the hardened gear is not accurate enough, its load carrying capacity is often not as good as that of the soft toothed surface. The nitriding surface hardness HV ≥ 600, the anti-adhesive ability is high. Due to the low temperature during nitriding, the deformation of the gear is small and can no longer be machined, but the nitriding layer is thin, suitable for manufacturing small-sized gears, but cannot withstand impact loads or abrasive wear.
2) Soft tooth surface: tooth surface hardness HB ≤ 350, fine tooth cutting after heat treatment or normalizing heat treatment. This kind of gear has low production capacity, but it is easy to manufacture and has good running performance. It is used in general machinery where the size and weight of the transmission are not strictly limited and the production is small. Because the weight of the small wheel in the paired gear is heavier, in order to make the working life of the large and small gears substantially equal, the hardness of the small tooth surface is 20 to 50 higher than that of the large wheel.