Case study on analysis of horizontal machining center field failure data

Based on the field failure data of 14 HMCs collected from a large automobile engine plant over ten months, the nonparametric estimation values of reliability level of HMCs were calculated by means of Kaplan-Meier method and Weibull transformation, and the Weibull Plotting Paper (WPP) plots were obtained. According to the curve shape of these WPP plots, the two-fold Weibull mixture model was applied to fit the field data set. To avoid solving the multi-dimensional non-linear likelihood equations directly, the original problem was transformed to a constrained nonlinear optimization problem which was solved by the interior point method and the BFGS formula. The confidence interval of parameter was obtained under the confidence level of 95%. The test of goodness of fit for this model was carried out by K-S test model. Result showed that such a life distribution fitting model was accepted under a significance level of 5%. Finally, the failure rate function of Horizontal Machining Center (HMC) was obtained. It was a bathtub curve with a decreasing failure rate, which meant that the HMC was running at the early stage of failure. The Mean Time Between Failures (MTBF) value of HMC was calculated to be 367 hours approximately.