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Changes in the thermal status of machine tools spindle-bearing system can have a noticeable effect on the performance of the machine itself, and therefore studying the thermal effect on the performance ball bearing during service is important. For this purpose, a study was carried out where a simple rotating shaft system supported by two angular contact ball bearings was taken into consideration. Heat was generated due to the contact between the balls and rings of the bearing. This thermal effect on the dynamics of the system was studied using a transient thermal model. The system was divided into nodes; each was assumed to be a uniform temperature. Thermal energy balance was used on each node to obtain a set of differential equations. ODE solver in MATLAB was used to solve the resulting system of differential equations. The thermal model considered an initial preload as well as the thermal preload that is caused by the uneven expansion of bearing components. In this research, a 5 DOF nonlinear dynamical system model is integrated with a spindle-bearing thermal model and then utilized to study the impact of preload variations on the spindle-bearing system of a grinding machine. The effect of different system parameters such as speed of rotation, type of bearing, ambient temperature, type of oil, initial preload on temperature output and thermal growth within the system was studied. The study shows that the heat generation rate is directly proportional to the rotational speed of the shaft and higher thermally induced preload is reached at higher speeds. It is also noticed that initial preload has a small effect on the heat generation, thermally induced preload, and temperature of the bearing. Also, the dominant frequency values of the spindle system generally increase with changing thermally induced preload values. This study is useful in predicting the thermal profile as well as preload value resulting in the bearing assembly, which in turn will be used to predict variation in the dynamics of the system.

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An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.