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Preventive maintenance (PM) has the following meanings:
The primary goal of maintenance is to avoid or mitigate the consequences of failure of equipment. This may be by preventing the failure before it actually occurs which Planned Maintenance and Condition Based Maintenance help to achieve. It is designed to preserve and restore equipment reliability by replacing worn components before they actually fail. Preventive maintenance activities include partial or complete overhauls at specified periods, oil changes, lubrication and so on. In addition, workers can record equipment deterioration so they know to replace or repair worn parts before they cause system failure. The ideal preventive maintenance program would prevent all equipment failure before it occurs.
Preventive maintenance can be described as maintenance of equipment or systems before fault occurs. It can be divided into two subgroups:
The main difference of subgroups is determination of maintenance time, or determination of moment when maintenance should be performed.
While preventive maintenance is generally considered to be worthwhile, there are risks such as equipment failure or human error involved when performing preventive maintenance, just as in any maintenance operation. Preventive maintenance as scheduled overhaul or scheduled replacement provides two of the three proactive failure management policies available to the maintenance engineer. Common methods of determining what Preventive (or other) failure management policies should be applied are; OEM recommendations, requirements of codes and legislation within a jurisdiction, what an "expert" thinks ought to be done, or the maintenance that's already done to similar equipment, and most important measured values and performance indications.
In a nutshell:
Predictive maintenance tends to include direct measurement of the item. Example, an infrared picture of a circuit board to determine hot spots while Preventive Maintenance includes the evaluation of particles in suspension in a lubricant, sound and vibration analysis of a machine.