Research And Application Of Fatigue Spalling Strength Of Gear Hardened Layer

With the progress of science and technology, the gear drive is developing gradually in the direction of lightweight and low energy consumption. With the rapid development of industrial technology, it puts forward higher request to the load capacity and economy of the gearing which is the core equipment of mechanical transmission system. At present, the surface-hardening treatment has been widely used because the harde surface gear has the advantages of high strength, small volume, light weight. However, the energy saving in heat treatment process is an important ordeal for the heat treatment of the gear. Energy saving is a very important and difficult task in the development of heat treatment technology, and it has an important significance to popularize the application of energy-saving technology in the gear industry.Using hard surface gear instead of soft surface gear in transmission systems is an important means of the lightweight design of gear. However, tooth surface spalling that is the main failure form of hardened surface gear has seriously hindered the large increase of the load capacity of the gear. The failure of gear spalling is related to the shear stress distribution under tooth surface, case depth, and hardness gradient. The case depth is closely related to the strength and reliability of the gear, and is the key to ensure the maximum load capacity of gear. Increasing the case depth can improve the load capacity of the gear and prevent fatigue spalling failure. However, the Increase in production costs and energy consumption may result from a case depth that is too large, which may bring many problems such as the increase of process difficulty, the growth of the process cycle and the larger deformation. The key point of the design of a rational case depth is not only to ensure that the transition zone has enough strength to prevent tooth surface spalling, but also not to increase safety redundancy. The design of an optimum case depth, whether it is to improve the load capacity of the gear tooth surface and product quality, or energy saving is extremely important. Therefore, further research on the optimum case depth, which can prevent the fatigue spalling of gear and is energy saving, has become a very important work in the design of the gear. But so far, there is not an effective design method of case depth can be used to calculate the spalling strength of gears. In this paper, the fatigue spalling strength of gear is studied and the test results are verified from the point of view of spalling. The main contents of the dissertation are as follows:(1) Based on considering the friction factor or not, the distribution of shear stresses under tooth face are analyzed in terms of elasticity theory, and the variation of shear stress under different friction coefficients is analyzed. In order to understand the distribution of the shear stresses under tooth surface more conveniently and intuitively, the distribution map of shear stresses are obtained by Kriging method. At last, the shear stress is calculated by finite element method, and the calculated result is compared with the result calculated by theoretical method.(2) The mechanical meaning of hardened depth for the gear is discussed, and the relationships between radius of curvature, core hardness, residual stress, processing quality and hardening layer peeling are analyzed. According to the maximum shear stress and its criterion, a design method of case depth for preventing tooth surface spalling is proposed. Besides, based on the relationship of shear stress and shear strength, the lowest hardness distribution curve for carburized gear is obtained. On the basis of this, a design method for quantitative determining the effective case depth of gear is put forward.(3) Based on the design method of determining the effective case depth of gear, the formula calculating the distribution of shear stress under tooth surface and the effective case depth of gear is derived, which provides a simple and feasible method to determine the effective case depth of gear quantitatively. Based on the calculating formula of the effective case depth, the model of effective case depth distribution of gear is established and quantitative analysis of the effective case depth of gear is realized. The influencing factors of the effective case depth of gear are studied, the factors of gear parameters that have the closest relations with the effective case depth are found out.(4) Considering the influence of the case depth on the contact fatigue strength of the gear, the calculation model of safety factor and reliability considering the influence of the case depth is established, and the relationship between the case depth and the safety factor,the reliability is obtained.(5) With the design method of the gear anti-spalling case depth, the fatigue spalling strength of the test gear is calculated. According to the distribution of the stress and strength under the tooth surface, flaking failure of the gear and the initiation location of fatigue crack under the tooth surface are predicted, and the results are verified by the gear contact fatigue test.The reasons of spalling failure are analyzed from the aspects of the chemical composition of the material, the detection of the inclusions, metallographic structure analysis, stress analysis, case depth analysis and partial load, overload analysis.(6) Combined with the heat treatment process test, the hardening layer fatigue spalling strength of the inner gear ring for wind turbine gearbox and the rack for Three Gorges Shiplift are analyzed. Through the comparison and analysis of the hardness distribution of the needed and the actual heat treatment along the depth direction, the case depth currently used in wind turbine gear ring and the Shiplift rack are evaluated and verified.