We could look up the definition in Wikipedia, but I prefer to summarize it in one sentence:
Preventive maintenance is the set of actions necessary to keep the machines running, reducing breakdowns and unexpected stops
Differences between corrective maintenance, preventive maintenance and predictive maintenance
When we talk about corrective maintenance, we refer to repair the faults once they have appeared. The main drawback is that the breakdown can mean stopping a machine, and it is necessary to plan the intervention, allocate the necessary human resources, stock up on spare parts, prepare tools, develop security procedures and intervention that were not foreseen. Corrective maintenance is inevitable, because it is impossible to predict and avoid all breakdowns. However, it is preferable to reduce this system as much as possible and combine it with other maitenance management systems if possible.
In preventive maintenance, we seek to avoid breakdowns by acting before they arise. It is usually done by replacing wearing parts before the end of their useful life. It can also be cleaning or lubrication actions. The clearest example is the maintenance of vehicles, in which oil, belts, filters and other elements are replaced in a programmed manner, before their wear causes breakdowns. This system allows planning the intervention, since the machine or installation works correctly. By knowing in advance the necessary resources, you can plan a preventive stop that affects production as little as possible.
A clear drawback is the difficulty of predicting when preventive action should be carried out, since:
Shortening times means increasing resources. If an oil has a shelf life of one year and I replace it every ten months, instead of making ten changes in ten years, I will have to do twelve, increasing the necessary materials and human resources.
Lengthening the times supposes more faults. Following the previous example, if I change the oil every fourteen months, I run the risk of increasing the wear and tear and causing a breakdown, which could be very expensive.
Predictive maintenance is a variant in preventive maintenance, which consists of analyzing and measuring the wear of the elements to replace them as soon as they show symptoms that predict the failure, before the failure materializes. They usually use work order software and analytical techniques such as thermography, diagnosis by vibrations, electrical measurements (such as the increase in consumption of an engine). This system is the optimum in terms of reliability, because it allows to know with certainty that an element must be replaced. It also has drawbacks, the main one is that measuring all the elements that can fail is usually very laborious. In machines with critical elements, automatic measuring equipment is usually installed, which activates an alarm indicating that it is necessary to intervene. The conditions of work of a machine can be different according to the environmental conditions or the over-efforts to which it may be subjected. That is why it is better to measure than to predict, to adjust the works to the real circumstances.
Which system is better
In practice, companies do not focus on just one of these systems, but combine the different types depending on the case. Ideally, predictive maintenance should be applied as a default option, if it is not feasible to discard it by the preventive one, and if it is not, continue with the corrective.
It is necessary to value the costs of each option. A very extended solution is to carry out preventive (time-based) inspections that include some predictive procedures (based on measurements).