What is preventive maintenance (PM)?
Preventive maintenance (or preventative maintenance) is maintenance that is regularly performed on a piece of equipment to lessen the likelihood of it failing. Preventive maintenance is performed while the equipment is still working, so that it does not break down unexpectedly.
Preventive maintenance is planned so that any required resources are available.
The maintenance is scheduled based on a time or usage trigger. A typical example of an asset with a time-based preventive maintenance schedule is an air-conditioner which is serviced every year, before summer. A typical example of an asset with a usage-based preventive maintenance schedule is a motor-vehicle which might be scheduled for service every 10,000km.
Preventive maintenance is more complex to coordinate than run-to-failure maintenance because the maintenance schedule must be planned. Preventive maintenance is less complex to coordinate than predictive maintenance because monitoring strategies do not have to be planned nor the results interpreted.
Assets suitable for preventive maintenance include those that:
- have a critical operational function
- have failure modes that can be prevented (and not increased) with regular maintenance
- have a likelihood of failure that increases with time or use
Unsuitable applications for preventive maintenance include those that:
- have random failures that are unrelated to maintenance (such as circuit boards)
- do not serve a critical function
Advantages of preventive maintenance
Advantages compared with less complex strategies
Planning is the biggest advantage of preventive maintenance over less complex strategies. Unplanned, reactive maintenance has many overhead costs that can be avoided during the planning process. The cost of unplanned maintenance includes lost production, higher costs for parts and shipping, as well as time lost responding to emergencies and diagnosing faults while equipment is not working. Unplanned maintenance typically costs three to nine times more than planned maintenance. When maintenance is planned, each of these costs can be reduced. Equipment can be shut down to coincide with production downtime. Prior to the shutdown, any required parts, supplies and personnel can be gathered to minimize the time taken for a repair. These measures decrease the total cost of the maintenance. Safety is also improved because equipment breaks down less often than in less complex strategies.
Advantages compared with more complex strategies
Preventive maintenance does not require condition-based monitoring. This eliminates the need (and cost) to conduct and interpret condition monitoring data and act on the results of that interpretation. It also eliminates the need to own and use condition monitoring equipment.
Disadvantages of preventive maintenance
Disadvantages compared with less complex strategies
Unlike reactive maintenance, preventive maintenance requires maintenance planning. This requires an investment in time and resources that is not required with less complex maintenance strategies.
Maintenance may occur too often with a preventive maintenance strategy. Unless, and until the maintenance frequencies are optimized for minimum maintenance, too much or too little preventive maintenance will occur.
Disadvantages compared with more complex strategies
The frequency of preventative maintenance is most likely to be too high. This frequency can be lowered, without sacrificing reliability when condition monitoring and analysis is used. The decrease in maintenance frequency is offset by the additional costs associated with conducting the condition monitoring.