Therefore, if no preventive maintenance or replacement action is performed on a workstation, its unavailability increases over time and subsequently results to manufacturing breakdown due to increase of occurrence of unexpected failures. On the other hand, a workstation may become unavailable due to scheduled preventive maintenance or replacement activity. Therefore, the availability function for a repair- able workstation i in period t should be defined in away to reflect two causes of unavailability. Considering these aspects, the availability of workstation i is formulated as Eq. (14) and the availability function of the entire series system of workstations can be derived accordingly as presented in (15).