The purpose of maintenance is to extend equipment lifetime, or at least the mean time to the next break- down whose repair may require significant expenditure [8]. Clearly, too little maintenance may have very costly consequences. On the other hand, maintenance, too, has a price-tag, and it may not be economical to perform it too frequently. In a cost-effective scheme, the two expenditures (the cost incured for doing no maintenance and that for performing maintenance too often) must be balanced.