The main damage form of gear in soft tooth surface rolling mill is severe plastic deformation of gear surface, which is caused by frequent impact and overload of rolling mill. This overload is not caused by the uneven load of the gear under the above steady load, but greatly exceeds the yield strength of the tooth surface. For example, the center distance of the herringbone gear of the herringbone gear stand of the medium plate Laut mill in a steel plant is 660mm. The maximum torque generated when biting into the steel is 12000000kgf · cm, while the maximum torque generated when rolling is 620000kgf · cm. The maximum brittle failure stress σ of the tooth surface can be calculated according to the following formula.
Therefore, to increase the service life of rolling mill gear is mainly to increase the yield strength of tooth surface. According to our investigation on the gears of rolling mills in ten iron and steel companies, all the gears were damaged due to severe plastic deformation and wear after 2-3 years of use, as shown in the figure. The upper gear σ≥ 65kgf / mm2 and the lower gear σ≥ 75kgf / mm2 of a 850mm rolling mill imported from a factory require high yield strength. After 9 years of use, there is only a little tooth surface plastic deformation, but it can still be used. In fact, the yield strength of rolling mill gear with 35CrMo steel and hb260 modulated hardness is only 50kgf / mm ~ 2. After 3 years of service, the gear surface has been severely plastic deformed. After 6 years of service, the gear has lost its tooth profile and is scrapped. The metallographic structure of the top of the gear is tempered sorbite + ferrite.
High yield strength of gear surface can be obtained by using medium hardness gear with good hardenability, which can effectively improve the service life of rolling mill gear.
Due to the installation, structure and other reasons of the machine, some reducer gears with excessive extra impact load and frequent use will also appear the similar damage form of spider steel machine gears. In order to ensure the normal operation of the gear and improve its service life, the extra impact overload should be eliminated first.