There are two effective ways to maintain the system to perform a high or required level of reliability and availability: one is spare units for key units; the other is several maintainers for the system. To solve reliability indexes of complex mechanical system under these assumptions, a system model was established, which consists of two dissimilar subsystems in series and each subsystem is warm standby system, which consists two same units. We divided maintain time into replace time and

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... time. In this paper, we arrange two maintainers for the system, and take 'the system in normal states' as maintenance strategy. Taking high efficiency of cooperative maintainers into account, the formula of system availability, system reliability, rate of occurrence of failure (ROCOF) and mean time to first failure (MTTFF) can be derived by operating the state transition probability matrix. This research provides evidence for further studies of complex mechanical system reliability. two warm standby units. The failure time and the repair time of the online unit have general distributions while in standby situation the failure rate is a constant. A twounit cold standby repairable system with one maintainer and use priority is often used. Zhang et al. [17] studied a cold standby repairable system consisting of two dissimilar components and one repairman. And the repair time distribu tions of the two components are both exponential and component 1 is given priority in use. The explicit expression for the long-run average cost per unit time of the system is evaluated. Rakesh [18] studied that whenever an operative unit fails, a delay occurs in locating the repairman and getting him to the system location due to certain administrative actions. Yang et al. [19] established a model for a discrete-time oneunit repairable system with delay repair. It is supposed that the system has three states: normal, repair and waiting repair, and that the system life, repair time and waiting repair time all have general distribution. Yang et al. [20] also analyzed the continuous-time model and discrete-time instantaneous availability model of the delay repairable system, established the instantaneous availability models in continuous and discrete time are proposed for the one-unit-delay repairable systems with exponential distribution, and get the numerical solutions for the continuoustime model and the discrete-time model. Zhang et al. [21] considered a replacement policy N based on the number of failures of the system. They determined the optimal policy N such that the long-run expected profit per unit time is maximized and showed that the model for the multistate system forms a general monotone process model which includes the geometric process model as a special case. Lam [22] reported a maintenance model for two-unit redundant system with one repairman. Under this model, he studied two kinds of replacement policy, respectively based on the number of failure and the work age for two units. The long-run average cost per unit time for each kind of replacement policy is derived. Sridharan [23] , Mokaddis[24] and Parashar[25] investigated the probabilistic analysis of a two-unit standby system with two types of repairmen and patience time. The first repairman, usually called regular repairman, always remained with the repair facility. An expert repairman was called for the system if and only if the regular repairman is unable to do the job, within some fixed time or on system failure. Most mechanical systems can be equivalent to series systems. Maintaining the system to perform a high or required level of reliability and availability is often an essential requisite. In practice, the key units are often equipped with spare units. The system can be boiled down to several standby systems in series. During the standby time of the units, because of the aging effect and the environment influences, many units (especially some precise instruments) will break down. So the subsystems are warm standby systems. In this paper, maintenance time divides into two parts. One part is replace time and the other part is maintenance time. In the series system which has two parts, each part is equipped with an identical spare unit as replace unit in fault state. The standby units have risk of failure. In this system model, there are two maintainers, who have the same work efficiency, also can repair one failure unit at the same time. To make sense, cooperative efficiency must be more than one maintainer"s efficiency, and less than sum of two single maintainers". Different maintenance strategy makes the result of reliability and availability difference. We take "the system in normal states" as the first priority. For example: if one unit of the system is failure, and the spare unit is in normal state at the moment, the two maintainers stop the current work, instead, they cooperate to replace the failure unit by installing the spare part. Another time, if there is only one failure unit, two maintainers can cooperate; If there are two failure u nits, two maintainers work on separate units. Furthermore, we assume that the work lives