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Gears fabricated using powder metallurgy technique (PMT) have the potential to replace traditional wrought gears due to their very high forming efficiency and relatively low cost. However, their practical applications are restricted due to the lack of understanding of their fatigue properties and failure mechanisms. This work investigates the contact fatigue performance and corresponding failure mechanism of a typical Fe-based small-module gears fabricated by PMT. The fatigue tests were conducted under grease lubrication condition, using a power-opened small-module gear testing rig. The experimental results show that the current PMT fabricated small-module gears exhibit contact fatigue properties comparable to those of traditional wrought steel gears. It has been found that the subsurface cracks induced from pores are the dominant cause of spalling of PMT fabricated small-module gears during rolling contact. The cracks tend to propagate between adjacent pores, playing a vital role in the overall fatigue life. Thus, this study suggests that PMT can be a successful method for fabricating Fe-based small-module gears with excellent contact fatigue property if formation of pores during fabrication is well inhibited.

Based on the indentation fracture mechanics, a thermal-assisted grinding model is established to investigate the effect of heat source on subsurface crack propagation. Combining the complex function method with the continuous distribution technique of dislocations, the stress intensity factors (SIFs) near the crack tip under various thermal and mechanical loads are calculated numerically. Results show that whether the median crack propagation is inhibited or promoted by the heat source mainly depends on the relative positions between thermal loading and mechanical loading. The stronger heat source in front of the abrasive grain can inhibit the growth of subsurface crack. When the heat source is located behind the abrasive grain, a minimum value of SIF can be obtained if the grinding parameters are well controlled. Especially, with the increase in heat source intensity, the maximum and minimum SIFs occur when the heat source is located at 1.5 times the distance of half crack length behind and in front of the abrasive grain, respectively. [ABSTRACT FROM PUBLISHER]

Abstract: In this paper, the development of a gear spalling depth prediction model is reported. The model is based on the ligament collapse spalling mechanism in gears, which was proposed by Ding et al. [Y. Ding, R. Jones, B.T. Kuhnell, Numerical analysis of subsurface crack failure beneath the pitch line of a gear tooth during engagement, Wear 185 (1995) 141–149; Y. Ding, R. Jones, B.T. Kuhnell, Elastic–plastic finite element analysis of spall formation in gears, Wear 197 (1996) 197–205] and subsequently validated by a thorough experimental investigation [Y. Ding, Spalling mechanism for spur gears, Ph.D. Dissertation, Department of Mechanical Engineering, Monash University, Australia, 1998; Y. Ding, N.F. Rieger, Spalling formation mechanism for gears, Wear 254 (2003) 1307–1317]. The mathematical model is established by utilising the numerical data obtained from the previous Finite Element Analysis [Y. Ding, R. Jones, B.T. Kuhnell, Numerical analysis of subsurface crack failure beneath the pitch line of a gear tooth during engagement, Wear 185 (1995) 141–149; Y. Ding, R. Jones, B.T. Kuhnell, Elastic–plastic finite element analysis of spall formation in gears, Wear 197 (1996) 197–205]. The spalling depths, calculated by the model due to various contact loads and crack sizes, are compared with the experimentally measured spalling depths obtained under corresponding loading conditions. A good agreement between the predicted spalling depth and experimental results is found. It is concluded that the model provides a reliable prediction for the depth of spalling on gear tooth contact surfaces, given the parameters of geometry and material of a gear set, as well as input loading conditions. [Copyright &y& Elsevier]

Abstract: Surface ring crack defects found on the surface of ceramic balls can reduce the rolling contact fatigue life (RCF) performance. In this study, various crack sizes were selected to identify the failure mechanisms in various lubricants. A modified four-ball machine was used to perform rolling contact fatigue testing. The crack within the contact path was carefully oriented and positioned the same way for all tests. Experimental results showed that the RCF life was influenced most by load, lubricating fluid and crack size. Examination of spalled and unspalled balls showed that the original ring crack propagated during fatigue testing and that crack branching occurred. Also, secondary surface cracks were observed. The consistency of results showed that crack growth was an integral part of the failure mechanism. [Copyright &y& Elsevier]

Abstract: Surface ring crack defects are normally found on the surfaces of ceramic balls. These ring cracks are circular extending for approximately one quarter of circumference of a circle and these cracks can significantly reduce rolling contact fatigue life. In this study, a boundary element analysis was carried out to identify the main factors governing the propagation of sub-surface cracks. The calculations of the main stress intensity factors (SIF), K I and K II, were made to determine which factors were important for crack propagation. Various geometries of the ring crack were selected to investigate their effect on the subsurface propagation of the ring crack and calculated results showed the most important factor of crack geometry was shown to be the subsurface crack length with crack radius. Crack angle and arc length was shown to have relatively little effect. The K II values were mainly affected by load, crack length and crack face friction. Increasing the load or crack length resulted in an increase in absolute K II values, but high crack face friction led to lower K II. Lubricants with less ability to penetrate a crack and thereby increase crack face friction should therefore give a better rolling contact fatigue performance. The numerical results were verified by a comprehensive experimental study , which show that present predictions of subsurface crack growth are consistent with the experimental observations. [Copyright &y& Elsevier]