There are several definitions or meanings of preventive maintenance, however all preventive maintenance management programs are time-driven. What this means is, performed maintenance procedures are derived on an elapsed time or hours of operation method. Below depicts the statistical life of a typical machine-train. 
The mean-time-to-failure (MTTF) also commonly referred to as the bathtub curve illustrates that a recently installed machine has a higher likely hood of failure because of installation issues that occur during the first few weeks of commissioning. After this initial period, the possibility of failure is comparatively low for an extended period. After this typical machine life period, the probability of failure increases drastically with elapsed time. When it comes to preventive maintenance management, equipment repairs or rebuilds are determined based on the MTTF statistic.
The actual execution of preventive maintenance varies greatly. There are programs that are extremely restricted and consist of only lubrication and minor adjustments. In depth preventive maintenance programs schedule repairs, lubrication, adjustments, and machine rebuilds for all essential plant machinery. The common denominator for all of these preventive maintenance programs is the scheduling guideline—time.
All preventive maintenance management programs assume that machines will degrade within a time frame typical of their particular classification. For example, a single stage, horizontal split-case centrifugal pump will normally run 18 months before it must be rebuilt. However by using the right form of preventive management techniques, the pump would be scheduled to be removed from service and rebuilt around the 17 month time frame of operation. The difficulty with this approach is that the mode of operation and system or plant-specific variables directly influences the normal operating life of machinery. What do I mean when I say this? Well, you need to compare “apples to apples here. The mean-time-between-failures (MTBF) are not going to be the same for a pump that handles water and one that handles harsh fluids or slurries.
The normal result of using MTBF statistics to schedule maintenance is either unnecessary repairs or catastrophic failure. In the example, the pump may not be requiring a rebuild after 17 months. If it was rebuilt, what would have occurred is a wasted labour and material cost in the budget. The second option using preventive maintenance is even more costly. If the pump fails before 17 months, it must be repaired using run-to-failure techniques. Analysis of maintenance costs has shown that repairs made in a reactive (i.e., after failure) mode are normally three times greater than the same repairs made on a scheduled basis.