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<p> A general study of theory of reliability has been made. The mechanism of failure of mechanical components, the properties of mechanical components, and the interaction of properties responsible for failure have been investigated. Elements of probability and statistics pertinent to reliability theory have been reviewed in brief. The various testing methods for determining component reliability using exponential, normal and Weibull distributions have been investigated. Acceptance sampling procedures for satisfying the necessary reliability requirements have been presented. Use of statistical methods in predicting fatigue life of mechanical components in general and rolling contact bearings in particular have been discussed. </p> / Thesis / Master of Engineering (MEngr)

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Le roulement est l'un des composants les plus importants des machines tournantes. Néanmoins, dans des conditions normales d'utilisation, il est soumis à de la fatigue de roulement qui peut conduire à un défaut d'écaillage. Dans ce travail, nous présentons un suivi de la fatigue d'un roulement de butée grâce à un banc d'essais dédié. L'analyse vibratoire est une méthode qui permet de caractériser et de localiser les défauts dans les roulements. Des mesures successives de ces niveaux de vibrations donnent des indications quant à l'évolution de la sévérité des défauts. Le suivi de cette évolution est fait grâce à un indicateur statistique, la valeur RMS (Root Mean Square) qui peut être corrélée avec la taille d'un écaillage de roulement. L'approche suit le fonctionnement du roulement de butée jusqu'à la dégradation avec une acquisition on line des états vibratoires sous forme de signaux temporels. A l'aide du traitement de signal, on obtient les valeurs des amplitudes vibratoires qui caractérisent l'état vibratoire du roulement. Par conséquent, ces valeurs nous permettent de tracer les courbes de fatigue. Au cours de notre travail expérimental, cette opération est appliquée à un lot de butées à billes pour lesquelles nous avons obtenu des courbes semblables où la tendance de l'évolution suit un modèle mathématique à partir de la détection de l'apparition de la première écaille. Le résultat de ce travail contribuera à prédire la durée de vie résiduelle avant la panne. / The bearing is one of the most important components of rotating machines. Nevertheless, in normal conditions of use, it is subject to fatigue which creates a defect called a rolling fatigue spalling. In this work, we present a follow-up of the thrust bearing fatigue on a test bench. Vibration analysis is a method used to characterize the defect. In order to obtain the fatigue curve more adjusted, we have studied the vibration level according to statistical indicators: the Root Mean Square value (RMS value), which is one of the best indicators to show the evolution of the bearing degradation. The approach follows the working of the bearing until the degradation with an on line acquisition of vibration statements in form of time signals. With the signal treatment, we obtain the values of the vibration amplitudes which characterize the vibration state of the bearing. Consequently, these values allow us to plot the fatigue curves. During our experimental work, this operation is applied for a batch of thrust bearings for which we have obtained similar fatigue curves where the evolution trend follows a mathematical model from the detection of the onset of the first spall. The result of this work will contribute to predict the working residual time before failure.

Rolling element bearings are the one of the most widely used machine elements in the industry. The most important criterion in bearing selection is the endurance life. The first attempts on the prediction of the endurance life of rolling elements bearings are done by Lundberg and Palmgren in 1950s (Harris, 1999). Their work adopted as an ANSI, ABMA and ISO standard which is widely used in industry today. The basic assumption of Lundberg-Palmgren formulation is that no matter how small the load applied on rolling element bearing, all material in the stressed volume is subject to fatigue failure. In this study, four main life theories / Weibull, Lundberg-Palmgren, Ioannides-Harris, and Zaretsky on rolling element bearings have been investigated. Three-dimensional finite element models of a bearing&rsquo / s inner ring and rolling element have been prepared. The stress fields within the inner ring and the ball with respect to the applied load are obtained numerically. The fatigue life of the inner ring has been predicted by two methods that are widely used for fatigue analysis / Total Life Analysis (S-N method) and Crack Initiation Analysis (&amp / #56256 / &amp / #56624 / -N method). Obtained results are compared with ISO formulation. As a result of the investigation, S-N and &amp / #56256 / &amp / #56624 / -N methods are determined to give more conservative results than ISO method for higher loads that cause stresses above the fatigue limit of the material. The used methods for bearing life prediction recognize the existence of the fatigue limit stress. Hence as the stresses within an operating bearing do not exceed the limit stress, the bearing can achieve infinite life. It is also observed that load variation has a direct influence on the bearing life. When the load significantly changes from the levels which create stress above the fatigue limit to the levels that result stress is below the fatigue limit, the bearing would have higher endurance life than predicted by ISO method.

Bearings are employed in a number of applications under extremely demanding conditions. During long operation times, the material undergoes rolling contact fatigue where microstructural damage manifests as dark-etching regions and white-etching areas, which display different properties from the surrounding region. The aim of this study is to identify the mechanisms for such damage and to suggest models that can explain the influence of the initial microstructure and test conditions. In order to appraise the stress state in rolling contacts, two testing techniques were employed and it was examined if the testing methods could reproduce the same damage as in bearing operation. During ball-on-rod fatigue testing, microcracks were generated adjacent to inclusions and some were decorated with white-etching areas. Repetitive push tests showed a similar extent of subsurface hardening compared to the ball-on-rod tests, and allowed the strain per stress cycle to be measured. The microstructural alterations in a white-etching area were studied both on a macroscale and on an atomic-scale. The degree of stress concentration near a microcrack was calculated employing a nite element method. The microstructure, as well as the segregation behaviour of alloying elements in the white-etching area, were investigated by employing transmission electron microscopy and atom probe tomography. A nanocrystalline structure with scattered carbide particles was observed in the white-etching area. Carbon and silicon segregation was highly pronounced in some boundaries of dislocation cell structures. Models were suggested to account for the microstructural alterations during rolling contact fatigue. Carbide coarsening in dark-etching regions was modelled by considering how carbon di usion is assisted by dislocation glide. The predicted hardness evolution was consistent with experimental observation. The kinetics of carbide dissolution in white-etching areas was calculated by taking two processes into account: deformation accumulation and carbon diffusion. These models suggest that the microstructural changes during bearing operation can be controlled by tailoring the initial microstructure and managing the test conditions.