## 6 Conclusions  
1. A maintenance strategy for automatic production line was developed on the basis of considering both the reliability, operating rate and maintenance cost. Furthermore, the different maintenance methods were employed according to the criticality of the machines in the production line. The maintenance strategy was practical. If this maintenance strategy was used to guide the maintenance and management work for the production enterprise, the economic efficiency and production efficiency of the automatic production line should increase obviously. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Machine** & \(C_{0}\) **(RMB)** & \(C_{m}\) **(RMB)** & \(C_{fm}\) **(RMB)** \\ \hline M01 & & 275 & 3470 \\ M08 & & 370 & 2405 \\ M12 & & 300 & 3190 \\ M16 & & 330 & 2575 \\ M18 & & 315 & 2640 \\ M19 & & 290 & 3895 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The maintenance cost data of each secondary critical machine. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Machine** & \(T_{se}\)**/day** & \(T_{s}\)**/day** & \(F_{m}\)**(\(T_{s}\))**/\%** \\ \hline M01 & 14.5 & 13 & 0.266 \\ M08 & 14.3 & 13 & 0.269 \\ M12 & 13.2 & 13 & 0.296 \\ M16 & 13.3 & 13 & 0.291 \\ M18 & 13.3 & 13 & 0.292 \\ M19 & 17.4 & 13 & 0.218 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The cumulative failure probability of secondary critical machines. 2. The uniform design method and simulation method were combined to establish the regression model between the failure rate of machine and the operating rate of the production line. The influence extent of the machine failure rate on the operating rate was used as the criterion to evaluate criticality of the machines. Then, the machines were classified into three groups: the most critical machines, the secondary critical machines and the general machines. The general machines were performed on the breakdown maintenance. 3. The preventive maintenance model for the most critical machines was built based on delay-time theory. The relation between the preventive maintenance period for the most critical machines and total shutdown time of the production line was gained. The best preventive maintenance period of the most critical machines was determined by taking the shortest total shutdown time as the decision objective. 4. The preventive maintenance model for the secondary critical machines was built based on the reliability-maintenance costs. The preventive maintenance period was gained according to take the parameters (\(C_{0}\), \(C_{m}\), \(C_{f\mu}\), \(F_{\max}\), \(\alpha_{m}\), \(\beta_{m}\)) into the model which were determined by the statistics and analysis of failure data and maintenance data of the secondary critical machines. 5. A case study on an automotive part automatic production line was carried out. The results showed that the optimal preventative maintenance period for the most critical machines' group comprised of M02, M05, M15, and M20 was seven days while the production line reliability was ensured. In addition, the secondary critical machines' group comprised of M01, M08, M12, M16, M18, and M19 was 13 days while the average maintenance costs were lower. 6. This study assumed that the system would be in a new state after the maintenance, but, in fact, this assumption was difficult to achieve under the limitation of maintenance capability. Therefore, it is necessary to investigate the synchronous imperfect preventive maintenance strategy for the automatic production line in the future. **Author Contributions:** Supervision and Funding Acquisition, G.L. ; Writing--Original Draft Preparation, Y.L. and X.Z. ; Data Curation and Analysis, C.H. and B.X. ; Writing--Review and Editing, J.H. and J.C.  
**Funding:** This research was funded by National Science and Technology Major Project of the Ministry of Science and Technology of China (2017 ZX04002001) and Jilin Scientific and Technological Development Program (20170204007 GX, 2018031204 ZZG).