Preventive maintenance
What is preventive maintenance: the strategy explained
Preventive maintenance is work carried out on a fixed time or runtime schedule to detect, prevent or slow the degradation of an asset before it fails. The US Department of Energy defines it as action taken on a time-based or machine-run-based schedule aimed at sustaining or extending useful life by keeping degradation to an acceptable level. It is also called time-based maintenance or planned maintenance.
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The defining feature is the trigger. A calendar date or a runtime counter says the work is due, not the measured condition of the machine.
What does preventive maintenance cover?
- Scheduled replacement. Filters, seals, belts and lubricants swapped at set intervals regardless of remaining life.
- Scheduled servicing. Lubrication, cleaning, tightening, alignment checks and calibration on a fixed cycle.
- Scheduled inspection. A route where someone looks, listens and records against a checklist. This is where preventive work starts to shade into condition monitoring.
- Statutory and compliance work. Pressure vessel checks, lifting equipment examinations and electrical testing on legally fixed intervals, where the schedule is not negotiable.
How preventive maintenance compares with reactive and predictive
Three strategies, three triggers. Reactive maintenance waits for the failure. Preventive maintenance acts on a schedule. Predictive maintenance acts on measured condition. Every real maintenance programme is a mix of all three, and the mix is the actual decision.
The DOE Operations and Maintenance Best Practices Guide gives the numbers most often quoted in this comparison:
- Cost per horsepower per year, from the Piotrowski study reproduced in the guide: reactive $18, preventive $13, predictive $9, and reliability centred maintenance $6. The dates are old and the dollars are not current, so read the ratios, not the amounts.
- Savings from preventive over reactive: an estimated 12 to 18 percent, with the guide noting that facilities running purely reactive could save considerably more than 18 percent.
- Savings from predictive over preventive: an estimated 8 to 12 percent further.
- The average facility mix: more than 55 percent reactive, 31 percent preventive, 12 percent predictive, 2 percent other.
- Continually top-performing facilities: under 10 percent reactive, 25 to 35 percent preventive, 45 to 55 percent predictive.
Note what the top-performer figures do not say. Preventive work does not disappear at the top end. It stays a quarter to a third of the programme, because a great many assets will never justify a sensor.
Preventive maintenance explained: a worked example
A plant runs 40 process pumps and replaces mechanical seals quarterly. That is 160 interventions a year at roughly 1.5 hours each, so around 240 labour hours plus parts. A teardown review finds that half the seals removed still had meaningful life left. Roughly 120 hours a year went into work that changed nothing, and every one of those interventions carried its own risk of a misalignment or a contamination fault on reassembly.
How much preventive maintenance is too much
Over-maintenance is real and it is expensive. NIST's report on the costs and benefits of advanced maintenance in manufacturing (Douglas S. Thomas, 2018) cites estimates that roughly one third of maintenance costs are unnecessary or improperly carried out, and that preventive maintenance is applied unnecessarily up to 50 percent of the time in manufacturing.
Two failure modes cause it. Intervals inherited from an OEM manual and never revisited against real failure history. And the intervention itself introducing faults, which is why infant mortality spikes after a rebuild often trace straight back to the last planned job.
When preventive maintenance is the right call
It fits best where the failure mode is genuinely age-related, where a failure is expensive or unsafe, and where instrumenting the asset would cost more than the schedule saves. Lubrication regimes, statutory inspections and consumable replacement are natural preventive work.
It fits worst where failures are random rather than age-related, because a schedule cannot anticipate what does not follow a clock. That is the territory predictive maintenance is built for.
The practical middle ground for most installed equipment is the inspection route: a repeatable visual check that produces a comparable record over time. Done properly it turns preventive inspection into something closer to condition monitoring, which is why guided capture of inspection evidence and consistent equipment verification matter more on this strategy than on either of the other two. If nobody records what the pump looked like last quarter, the route generates activity but no trend.
