Because Reactive Maintenance is even more costly, and using time to schedule maintenance is easy.
Not doing PM’s leads to a world of reaction, where crisis drives plant activity instead of planned effort. Costs and resources spike up and down based the “fire du jour”, and the facility has no opportunity to strategically assign their resources to an unplanned effort.
Time-based PM’s can get you out of the reactive zone (most of the time), the problem is that time is not really the trigger that leads to a fluid related event. There are so many variables that impact fluid condition over time, that time alone does not offer a very cost effective management tool. In fact its extremely wasteful. For PM’s to take you out of the reactive zone, they have to be conservative by nature. They have to be done at a frequency driven by the lowest common denominator and when you have variability in your process, that lowest common denominator can be anything from chip loading, to tramp oil build-up, to biological activity etc. Multiply that lowest common denominator across all the “similar” machines in the facility and you are multiplying the waste, while still leaving your process at risk to an event since time is not the real trigger to a fluid related event.
Companies are forced to use time based systems when the effort has not been made to truly understand the triggers that drive performance. Once cause and effect is understood and a condition based program is established that tests for these triggers and provides timely corrective action as appropriate, its possible to significantly extend the period between PM’s.
Time based PM’s are the opposite of process control since the variable (time) used to trigger the control action (PM) is not related to the event it is trying to prevent.
Its true that aggressive PM schedules help reduce unplanned events from occurring, however in a world where we need to look to minimize waste and wasteful activities, time based PM’s are a luxury industry can no longer afford.