Therefore, if no preventive maintenance or replacement action is performed on aworkstation, 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 inperiod 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).