## 1 Introduction  
An automatic production line is an important part of the manufacturing system in the modern industrial field owing to the enormous advantages of high yield, good product quality and labor cost saving [1; 2; 3]. However, due to the variety of machine, complex layout and structure, any machine failure may lead to the whole production line shutdown, resulting in enormous economic losses to the production enterprise. The low reliability and high maintenance costs of the production line have become an urgent problem for production enterprises to solve [4; 5]. It is very important to put forward scientific maintenance strategy to guide the maintenance and management works according to the machines' failure characteristics and operation laws of the production line. Because the working conditions of machines in the production line are different and the machine reliability is also different from each other, if all machines in entire production line are performed on breakdown maintenance or preventive maintenance in the same maintenance period, it is difficult to meet the requirements of high reliability, high operating rate and low maintenance costs [6; 7; 8]. In previous literature, the group maintenance method, which meant that a group machines with similar characteristics are maintained together in order to reduce the maintenance times and maintenance costs, was considered to be suitable for the automatic production line [9; 10]. [PERSON] et al. [11] developed an optimized maintenance cost model to determine an optimal interval of condition monitoring and the degradation level after imperfect preventive repairs, which had initiated a new area for the research on cost effective maintenance strategies. [PERSON] and [PERSON] respectively established the maintenance optimization models for the production line using the different methods [12; 13]. [PERSON] et al. [14] developed an opportunity maintenance model for the production line to conduct preventive maintenance on several machines at the same time when one of the machines in the production line breaks down. However, it is difficult to use the opportunity maintenance strategy for the complex production line consisting of many machines. [PERSON] et al. [15] used a heuristic algorithm to group the machines of the production line, and then performed the maintenance to reduce maintenance costs. [PERSON] et al. [16] used the Analytic Hierarchy Process (AHP) and goal planning methods to develop the maintenance strategies based on considering the machine failure rate and maintenance costs, respectively. [PERSON] et al. [17] put forward a delay-time model to the prevent machines' failure during the preventative maintenance period when the machines had serious deterioration. However, these previous maintenance models and maintenance strategies were often developed with single evaluation criterion such as reliability and maintenance costs, or the same maintenance model and maintenance strategy were used for all machines in the automatic production line without distinguishing the machines' criticality. In the present study, a preventive maintenance strategy for the automatic production line was developed based on a group maintenance method. Using the uniform design method, the machines in the production line were classified according to the influence degree of the machine failure rate on the operating rate of the production line. 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. The preventive maintenance models were established for the most and secondary critical machines, respectively. The preventive maintenance model of the most critical machines was established with the shortest shutdown time as a decision objective on the basis of delay-time theory, and that of a secondary critical machine was established based on consideration of reliability-maintenance cost.