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In order to study the influence of tooth root cracks on the fatigue life of case-hardened gears, taking an involute case-hardened gear as the research object, the mechanism of the tooth root fatigue crack growth is analyzed based on the fracture mechanics method and fatigue crack growth theory. The residual life analysis model of the tooth root crack propagation for case-hardened gears is established, considering the influence of load, initial crack size and initial crack location, etc. The evolution of stress intensity factor and crack propagation mechanism at different root crack propagation stages is studied. According to the bending fatigue test data of an involute case-hardened gear pair, the calculation model is analyzed and verified, and the accuracy of the model is proved. The results indicate that, compared with type Ⅱ and type Ⅲ, type Ⅰ stress intensity factor is the highest, which gradually decreases from the tooth surface to the direction of crack depth. With the increase of factors such as load, crack length, crack width, and initial crack distance from the center position of the tooth width, the residual life of crack propagation decreases accordingly.

Compared with ISO 6336:1996 edition，there are great changes in the latest edition of ISO/FDIS 6336-1 to 3：2019, in particular, the calculation of the helix angle factor for helical gear has a subversive change. In order to facilitate the reader to understand the changes of the new ISO 6336, the main changes in the content of different editions of ISO 6336 are introduced, the influences of the helix angle factors and single pair tooth contact factor on safety factor before and after modification are analyzed, and the influence of helix angle on the calculation results of contact strength and bending strength are discussed here with examples.

The rack test device for the Three Gorges ship lift is mainly composed of four parts: driving gearbox,driving gear-rack,loading gear-rack and loading gearbox. In order to better calculate and analyze the transmission efficiency of the rack test device,through the establishing gearbox transmission efficiency calculation model,transmission efficiencies of the four parts and the transmission efficiency and the power consumption of the total whole unit are calculated respectively,the calculation results are contrastive analyzed by the input speed and torque,output speed and torque of the test data,the accuracy of calculation model and test data is verified,the research basis of transmission efficiency of gearbox and gear rack for low speed and load duty is established.

The gearbox of rack test for the Three Gorges ship lift is low speed heavy duty four stages transmission,and need frequent fast reversing operation in the process of test,due to the influence of the gear meshing backlash,elastic torsional deformation of shaft and elastic deformation of gear tooth and other factors,the operation performance of gearbox will be changed during reversing process,especially the output speed and torque. In order to analyze the influence on the transmission performance of gearbox,the calculation models of the influence of the gear meshing backlash and the elastic torsional deformation of the shaft on the rotational speed are setup respectively,the output speed of the gearbox in the reversing process is calculated and compared with the output speed of gearbox measured by experiment,which verifies the accuracy of the calculation model and the test data.

Through the analysis of the influence factors on transmission efficiency of gearbox,a theoretical analysis model of gearbox transmission efficiency is set up from the power consumption,and the influences of gear meshing loss,bearing friction loss,oil loss,oil seal friction loss and windage loss on gearbox transmission efficiency are considered. The analysis model is validated according to the test data of a low speed and heavy duty gearbox,and the influences of speed and torque on the transmission efficiency of gearbox are discussed,the changing curves of gear efficiency with the rotational speed and torque are obtained,the foundation for further research on the efficiency of the gearbox is laid.