In order to share maintenance set-up costs and reduce system breakdown, the group maintenance
policies are widely used for complex multi-unit systems. In this paper, an optimal age-based group
maintenance policy is proposed for a multi-unit series system whose components are subject to different
gradual degradation phenomena. When the degradation level of a component reaches a given critical
size, it is replaced by a new one and the other components undergo a preventive maintenance (PM)
action; otherwise, a planned group PM is performed for the whole system at operational age T40. The
problem is to determine an optimal group maintenance time Tn such that the system’s average long-run
maintenance cost per unit time is minimized. The explicit expression of the objective function is derived
and sufficient conditions for existence and uniqueness of the optimal solution are obtained. Finally, the
proposed maintenance policy is applied to a group of wind turbine bearings and the results are
compared with the case without planned maintenance (i.e., reactive response) and with an individual
age-based maintenance policy.