Machine tools cannot produce accurate parts if performance degradation due to wear in their subsystems (e.g., spindle units) is not identified and controlled. Appropriate maintenance actions delay possible deterioration and minimize machining system stoppage time that leads to lower productivity and higher production cost. Measuring and monitoring machine tool condition has become increasingly important because of the introduction of agile production and increased requirements for product accuracy. Condition Based Maintenance (CBM) techniques, such as vibration monitoring, are becoming a very attractive method for companies operating high-value machines and components. One of the most common problems of rotating equipment, such as machine tool spindle units, is the condition of bearings. Vibration analysis can diagnose bearing damage by measuring the overall vibration of a spindle or, more precisely, by high-frequency techniques such as enveloping. This paper focuses on the use of vibration analysis to monitor and analyse the condition of machine tool spindle units. The aim of the paper is to present the important factors in a vibration analysis of machine tool spindle units. The method is a case study at an automotive manufacturing company in Sweden. CBM, using vibration monitoring, is implemented on different types of machine tools, including turning machines, machining centres, milling machines and grinding machines. The results of the implementation, as well as a vibration analysis of a spindle unit and its cost effectiveness, are presented in the paper. The results indicate that detecting faults or damages by vibration monitoring of complex structures, such as spindle units, is challenging because there are different sources of frequencies from spindle bearings, gearboxes, gear meshes, etc. However, with help of advanced vibration analysis, such as high-frequency measuring methods, it is possible to detect bearing damage in spindle units from a very early stage of the damage.
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