Preventive Maintenance (PM)
Maintenance performed on a fixed schedule — by calendar or usage — to reduce the likelihood of failure.
Preventive maintenance is scheduled intervention: inspect, lubricate, adjust, or replace at fixed intervals defined by calendar time (quarterly) or usage (every 500 running hours, every 10,000 cycles). It is the workhorse strategy of organized maintenance — predictable to plan, simple to audit, and effective against failure modes that genuinely correlate with age or use: filter loading, belt wear, lubricant depletion, calibration drift.
Its known weakness is that most failure modes are not age-related. Classic reliability studies found that only a minority of failure modes benefit from time-based replacement; the rest fail randomly, where fixed-interval intervention adds cost and intrusion without adding reliability — and every intrusion carries some risk of inducing the infant-mortality failure it was meant to prevent.
Mature programs therefore treat PM as one tool in a portfolio: time-based tasks where age-related evidence supports them, condition-based coverage for the random-failure modes, and periodic PM optimization reviews that retire tasks with no history of findings. A PM library inherited unreviewed from OEM manuals is almost always oversized.
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