PRECESSIONAL SLIP AND MICRO-INCLUSION EFFECT ON FATIGUE LIFE OF BEARING ROLLING ELEMENT: AN INTEGRATED LIFE ESTIMATION MODEL THROUGH EXPERIMENTAL AND ANALYTICAL INVESTIGATION

This is an Integrated Mathematical Model to predict the fatigue life of the bearing rolling element under the influence of both the Micro-Inclusion Effect and the Precessional Slip. The relationship between Micro-Inclusion Effect and Precessional Slip is investigated for the bearing ball undergoing Hertzian contact fatigue by performing the failure analysis. The predicted ball's life and Experimental cycles to failure are compared to validate the life model. The model is designed in such a way that the failure on the ball is singled out without damaging the cage and raceway. The presence of Larger Inclusions is measured using X-ray Spectroscopy and destructive testing. The presence of larger defects results in low fatigue life and vice versa. Chemical Composition of the Inclusion is one of the main key factors that affect the damage. For the rolling element, a Statistical approach is necessary since micro inclusions are not always subjected to maximum stress, due to the presence of Slip and Precession. To identify the statistical distribution of the inclusions, a preliminary statistical analysis of the extreme values (sev) was performed. Experimental data was collected from the raw material. To study the influence of the micro inclusion on the material, the material specimen was subjected to 4 points rotating bending test. After failure analysis, optical inspection of the fractured surface along with scanning electron microscope (SEM) analysis is performed, to obtain the dimensions and chemical dimensions of the micro-inclusion. The bearing ball is subjected to a fatigue test using a test rig. The bearing was modified by removing some of the balls to increase the contact pressure on the ball and single out the failure only on the balls, without damaging the inner ring, the outer ring, and the cage. The stereo-microscope inspection was performed on each failed ball, to evaluate if the failure started from an inclusion. If so, the chemical composition and the dimension of the inclusion were measured using a Scanning Electron Microscope (SEM) and the inclusion depth was measured using a gauge meter. Eshelby-Mura model was used to simulate stress distribution of micro inclusion. Geometry, location, and chemical composition were experimentally found and the calculated stress field was used to obtain equivalent Tresca Stresses. This estimation of the fatigue life on the ball involves a complex approach since spin and precession are present. To statistically determine how often the inclusion is loaded, Ball Spin Frequency was estimated. Studying the ball motions in-depth in terms of spin, precession, and transverse slip at the ball-race contact provides the estimate of the frequency at which the inclusion contacts the race during one complete shaft cycle. Now we can easily calculate the life of the ball using the Zaretsky model. Life factor that accounts for race and conformity defects is introduced as a correction. Zaretsky's model is further modified to include the estimates of the slip, precession, and Mirco-Inclusion effects. Finally, life is estimated with and without inclusion effect. These life predictions compared with the experimental cycle of failures validate our mathematical model.