A Study on the Effects of Ball Defects on the Fatigue Life in Hybrid Bearings Ching-Yao (Tony) Tang*, Chad E. Foerster*, Michael J. O’Brien*, Brian S. Hardy*, Vinay K. Goyal*,  Benjamin A. Nelson*, Ernest Y. Robinson**, Peter C. Ward* and Michael R. Hilton* Abstract Hybrid ball bearings using silicon nitride ceramic balls with steel rings are increasingly being used in  space mechanism applications due to their high wear resistance and long rolling contact fatigue life. However, qualitative and quantitative reports of the effects of ball defects that cause early fatigue failure  are rare. We report on our approach to study these effects. Our strategy includes characterization of  defects encountered in use, generation of similar defects in a laboratory setting, execution of full-scale  bearing tests to obtain lifetimes, post-test characterization, and related finite-element modeling to  understand the stress concentration of these defects. We have confirmed that at least one type of defect  of appropriate size can significantly reduce fatigue life. Our method can be used to evaluate other defects as they occur or are encountered. Introduction Recently, silicon nitride (Si 3N4) ceramic balls have become an important component of advanced  bearings used in a wide range of applications. The greatest commercial success for Si 3N4balls have  been in hybrid bearings, which combine the ceramic balls with steel races. Compared to the steel ballsthat they replace, the ceramic balls are harder and less dense and offer many other advantages, such as  higher compressive strength, better corrosion resistance, and reduced susceptibility to lack of lubricant or  lubricant breakdown. These benefits make the hybrid bearings ideal for severe high-speed applications. However, during the development of hybrid bearings for space mechanism applications, we became  aware of ball flaws, particularly scars, scratches, C-cracks, and traction cracks that could induce early  fatigue failure [1]. Implementation of visual screen ing methods subsequently eliminated the most severe of these incidents. However, we realized that available data on flaw definition, size, stress concentrations,  and life-limiting effects of ball defects in general were limited or included specific varieties of hybrid materials used under specific conditions [2-6].   In response, The Aerospace Corporation embarked on a long-term Independent Research and  Development program to develop appropriate laboratory and analytical tools to improve the understanding and criteria for screening defects that induce infant mortality fatigue failure. To that end, full-scale bearing rolling contact fatigue testers (Figure 1) have been used to study the fatigue life of newer classes of properly-inspected, defect-free hybrid bearings. In this paper, we present our approach to study the effects of ball defects on fatigue life. Our strategy  included characterization of defects encountered in use, generation of similar defects in a laboratory setting, execution of full-scale bearing tests to obtain lifetimes, post-test characterization, and related  finite-element modeling to understand the stress concentration of these defects. The rolling contact  fatigue life was measured for as-manufactured bearings and for bearings with artificially-induced defects  on the Si 3N4balls. * Engineering and Technology Group, The Aerospace Corporation, El Segundo, CA ** Civil and Commercial Operations, The Aerospace Corporation, El Segundo, CA /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 129