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Preventive maintenance (PM) has the following meanings:
- The care and servicing by personnel for the purpose of maintaining equipment and facilities in satisfactory operating condition by providing for systematic inspection, detection, and correction of incipient failures either before they occur or before they develop into major defects.
- Maintenance, including tests, measurements, adjustments, and parts replacement, performed specifically to prevent faults from occurring.
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:
- Preventive maintenance is conducted to keep equipment working and/or extend the life of the equipment.
- Corrective maintenance, sometimes called "repair," is conducted to get equipment working again.
Difference Between Preventive and Predictive Maintenance 
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.
- An individual bought a incandescent light bulb. The manufacturing company mentioned that the life span of the bulb is 3 years. Just before the 3 years, the individual decided to replace the bulb with a new one. This is called preventive maintenance.
- On the other hand, the individual has the opportunity to observe the bulb operation daily. After two years, the bulb starts flickering. The individual predicts at that time that the bulb is going to fail very soon and decides to change it for a new one. This is called predictive maintenance.
- The individual ignores the flickering bulb and only goes out to buy another replacement light bulb when the current one fails. This is called corrective maintenance.
See also 
- Reliability centered maintenance
- Corrective maintenance
- Operational maintenance
- Department of Defense Dictionary of Military and Associated Terms
- Predictive maintenance
- Value driven maintenance
- Michael Quinion, PREVENTATIVE OR PREVENTIVE, World Wide Words.
- What term is more correct preventive or preventative?, Answers.com.
- OIT Style Guide: How should I write that word? An A to Z, Office of Information Technology.
- Bobby Joseph, Letter to the Editor: What's the good word—preventive or preventative?, International Journal of Epidemiology, Vol. 30, N. 6, p. 1498.]
Further reading 
- Wu, S. and Zuo, M.J. (2010). Linear and nonlinear preventive maintenance, IEEE Transactions on Reiability, Volume 59, No. 1, 2010, 242-249.
- Maintenance of Hydraulic Systems
- Maj. Ricky Smith, US Army. "Walter Reed Building 18 - It Could Happen Anywhere - So Don't Let It Happen To You".
- International Facility Management Association
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
- The Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland, College Park has a PHM group dedicated to providing a research and knowledge base to support the advancement of preventive and predictive maintenance with a focus on electronics.