| CNC machine tools are the basis and key equipments of military, aerospace and automotive industries, whose performance reflects the manufacturing level of a country. Low reliability has become the shortcoming of homebred machine tools and limited the development of manufacturing industry in China. Hence, it is imminent to improve the reliability of home-made CNC machine tool and its key functional components. As one of key functional components, high-speed motorized spindle directly drives tool cutting which reliability restricts the reliability of CNC machine centers. Therefore, it’s imminent to improve the reliability of high-speed spindle.Reliability test is the basic work to ensure the reliability of high-speed motorized spindle. And it can offer substantial reliability test data, especially the fault data for reliability modeling, reliability analysis and reliability design. The reliability test of high-speed motorized spindle includes field reliability test of machine centers in factory and reliability test in test bench of functional components in the laboratory. Field reliability test collects the operation data, failure data and maintenance data of machine center in enterprise. Since the MTBF of high-speed motorized spindle generally is more than 5000 h, the test cycle of filed test is long and consumes a large amount of manpower, material and financial resources. What’s more, the filed test condition is uncontrollable.However, through constructing motorized spindle reliability test rig and simulating the cutting load of the actual machining process, the reliability test in laboratory not only is time-saving, labor-saving in data acquisition, but also has controlled test conditions. So the test efficiency of reliability test in laboratory is higher than field reliability test.Therefore, based on the reliability test data in the past, author made FMECA of motorized spindle unit, simulated the real working condition and drew the four-dimensional load spectrum, designed and carried out the motorized spindle test with free-load, static load, and four dimensional load spectrum, made the performance monitoring and fault diagnosis of high-speed motorized spindle. In this paper, the main research results are as follows:(1)The development and research status of the reliability of motorized spindle in CNC machine tool are discussed. The background and significance of reliability test of motorized spindle are emphasized. In this part, the process and research status of failure analysis, reliability test with static load and load spectrum are reviewed. The performance monitoring and fault diagnosis technology of motorized spindle are briefly summarized. Finally technical roadmap of high-speed motorized spindle reliability test was extracted.(2)The subsystem of motorized spindle unit was divided based on the composition and system function. Its FMECA was made based on the fault data of field reliability test. As traditional CA ignored the enterprise cost when motorized spindle broke down, author put forward a new FMECA method counting the maintenance cost which included the factory profit, factory artificial fee, machine fee, maintenance fee and maintenance material cost. The new FMECA solved the deficiency of the traditional FMECA method. The new FMECA was applied to instances and the result showed that the harm degree of spindle and clamping unit were highest.(3)The reliability test with four dimensional load spectrum on motorized spindle rig was designed and carried out. First of all, the load limit of the motorized spindle was calculated. Made force analysis of motorized spindle rig. Based on the bearing life and bearing fatigue load, the maximum axial load and maximum radial load of motorized spindle without change of motorized spindle failure mechanism were obtained. Second, the load spectrum of motorized spindle was drew. The previous load spectrum could only got the cutting force spectrum, torque spectrum and rotating speed spectrum respectively, but ignore the corresponding relationship between the cutting forces, torque, rotational speed which was inconsistent with the actual cutting conditions. To solve this problem, author drew four-dimensional load spectrum of motorized spindle. The cutting forces and torque were calculated based on the reliability test load data from filed reliability test. And they were divided into 5 levels. Also the speed was divided into 3 levels. So far, the four dimensional load spectrum considering the speed, axial force and radial force, torque and relative cycles were divided into 75 levels. Finally, within the load limit of the motorized spindle test rig, the reliability test with free load, static load and four dimensional load spectrum were designed and carried out.(4) The radial run-out and axial movement were hardly be monitored when CNC machine center was operated. Aiming at this problem, this paper proposed a real-time monitoring method of radial run-out and axial movement of motorized spindle by acceleration mixed integrated in time-frequency domain. The acceleration signal after high-pass filtering time domain integral by the low-frequency cutoff frequency domain integral, motorized spindle vibration displacement, namely motorized spindle radial run-out and axial movement were got. In order to verify application value of the time-frequency domain integral method, the reliability test on motorized spindle test rig with free-load was carried out. Test data of acceleration sensor and displacement sensor were synchronous collected based on Lab VIEW. Compared displacement calculated by time-frequency domain integral with actual displacement signal, the integral error examine showed that consistency is good. Also compared with five other methods, namely the simple quadratic time domain integral, simple quadratic integration in frequency domain, frequency domain integral by low frequency cut-off, time domain integral by polynomial fitting and time domain integral by high pass filtering, the test results showed that the integral error of frequency mixed time-frequency domain integral method was smallest. The could be used in monitoring the radial run-out and axial movement of motorized spindle when machine center was working.. The acceleration signal mixed integrated in time-frequency domain can be used to monitor the spindle vibration displacements which are radial run-out and axial movement.(5) An fault diagnosis method based on an improved soft thresholding and S transform was proposed for motorized spindle. The new method added an correction parameter to traditional soft threshold algorithm. The acceleration signal was processed by the improved soft thresholding and S transform to search the fault feature frequency to diagnose the fault pattern. The fault data of shaft friction and misalignment on motorized spindle were analyzed by S transform and S transform with improved soft thresholding. Compared with FFT, STFT and HHT, the new method had best property. |
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