Abstract
The reliability, efficiency and accuracy of CNC machines as work cells of intelligent manufacturing systems (IMS) are criteria to measure the processing level of the latter. In order to improve the reliability of the IMS and reduce the maintenance cost, very sound preventive maintenance and management strategies concerning the CNC machines should be defined. We realized a parameter estimation of our reliability model for CNC machine units in an IMS environment, carried out a linear correlation test and a distribution fitting test for the model and obtained the failure distribution function and failure rate function. We then built a post-failure maintainability model and realized a maintainability evaluation. Following the above analyses, we built a cost-based preventive maintenance cycle model and obtained its optimal value by using the particle swarm optimization (PSO) algorithm. This research and its result can on the one hand guide the setting-up of preventive maintenance planning and management schemes and on the other hand reduce the production cost and enhance enterprise efficiency.
Highlights
In order to build the preventive maintenance cycle model based on failure rates, we prescribed the following assumptions: 1. After each preventive maintenance, the failure rate of the machine tool returns to original level
The formula of the preventive maintenance cycle model which is based on failure rates is:
Assuming on the one hand that N failures occurred to a machine tool, each of which demanded the repair time d f (i) and the repair cost cf (i), in which i=1, 2, ...N, and on the other hand M preventive maintenances were carried out on the machine tool, each of which demanded the repair time d p ( j) and repair cost cp ( j), in which j=1, 2, ...N, we obtained the following formula:
Summary
According to previous reliability studies on machining centers, the cumulative failure distribution function abides by the Weibull distribution [15]. The two-parameter Weibull distribution function is :. According to the two-parameter Weibull distribution, formula (1) can be linearly transformed as following:. The value F(t) needs to be estimated before calculation. The median rank is used to estimate F(t),that is :. According to the least square method, the parameter estimation is drawn as below:.
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