Experimental investigation of the relation between the surface integrity and bending fatigue strength of carburized gears

Bending fatigue is an essential parameter that needs to be considered in the improvement process of the power density and reliability of gear drives. Quantitative relations among the manufacturing parameters, surface integrities, and fatigue performance are not clear, which seriously limits the effectiveness of an anti-fatigue design. For this work, tooth-bending fatigue tests of carburized gears with different surface integrities were performed using a pulsator. The effects of the manufacturing parameters and surface integrities on the gear fatigue, such as surface hardness and residual stress, were investigated. The experimental results revealed that due to the improvement of surface integrities after shot peening, the nominal bending stress number(fatigue limit) increased by 6.3%–31.1%, with an amplitude range of 39–143 MPa. A supervised learning algorithm of a random forest was implemented to determine the contribution of the surface hardness and surface residual stress to the nominal stress number. An empirical formula was proposed to predict the nominal stress number considering the surface integrities. The prediction error was less than 7.53%, as verified by several gear-bending fatigue tests. This provided theoretical support for the modern, anti-fatigue design of the gears.

Evaluation of contact fatigue risk of a carburized gear considering gradients of mechanical properties

Carburized gears are widely used in geared machines such as wind turbines.Contact fatigue problems occur in engineering practice,reducing reliabilities of machines.Contact fatigue failures are related to many factors,such as gradients of mechanical properties of the hardening layer.In this work,an elastic-plastic contact model of a carburized gear is developed based on the finite element method to evaluate contact fatigue failure risk,considering variations in hardness and strength.The Dang Van multiaxial equivalent stress is calculated via Python coding within the Abaqus framework.The gradient of yield strength along the depth from case to core is considered.The concept of local material fatigue failure risk is defined to evaluate the probability of pitting failure.The effects of design factors,such as the case hardening depth(CHD),surface hardness,and contact pressure on fatigue failure risk,are studied.As the CHD increases or the surface hardness decreases,the risk of deep spalling failure reduces.The increase in surface hardness leads to a decreased risk of pitting failure,while the variation in CHD hardly affects the pitting failure risk.

Study on contact fatigue of a wind turbine gear pair considering surface roughness

Contact fatigue issues become more and more crucial in gear industry as they significantly affect the reliability and service life of associated mechanical systems such as wind turbine gearboxes.The contact fatigue behavior is mostly determined by the mechanical properties of materials and stress fields near the contact area,which is further influenced by the lubrication and surface roughness due to pressure fluctuations.In this study,a numerical model incorporating the lubrication state,tooth surface roughness,residual stress,and mechanical properties of the material is developed to determine the contact fatigue behavior of a megawatt level wind turbine carburized gear.The variations of the hardness and residual stress along the depth were characterized by the Vickers hardness measurement and X-ray diffraction test,respectively.The elastohydrodynamic lubrication theory was applied to predict the contact pressure distribution,highlighting the influence of the surface roughness that stemed from the original measurement through an optical profiler.The stress histories of the studied material points during a complete contact loading cycle were fast calculated using the discreteconcrete fast Fourier transformation(DC-FFT)method.Modified Dang Van diagrams under different working conditions were determined to estimate the contact fatigue failure risk.The effect of the root mean square(RMS)value of the surface roughness on the failure risk at critical material points were discussed in detail.Results revealed that the surface roughness significantly increases the contact fatigue failure risk within a shallow area,and the maximum risk appears near the surface.