**9. Maintenance strategies**

The oldest and most common maintenance and repair strategy is "fix it when it breaks." The appeal of this approach is that no analysis or planning is required. The problems with this approach include the occurrence of unscheduled downtime at times that may be inconvenient, perhaps preventing accomplishment of committed production schedules. These problems provide motivation to perform maintenance and repair before the problem arises. The simplest approach is to perform maintenance and repair at preestablished intervals, defined in terms of elapsed or

operating hours. This strategy can provide relatively high equipment reliability, but it tends to do so at excessive cost (higher scheduled downtimes) [24]. A further problem with time-based approaches is that failures are assumed to occur at specific intervals. The only way to minimize both maintenance and repair costs and probability of failure is to perform ongoing assessment of machine health and ongoing prediction of future failures based on current health and operating and maintenance history [25–27].

This is the motivation for prognostics: minimize repair and maintenance costs and associated operational disruptions, while also minimizing risk of unscheduled downtime. Preventive maintenance is the strategy organized to perform maintenance at predetermined intervals to reduce the probability of failure or performance degradation. It can be classified into constant interval, age-based or imperfect maintenance:

