Preventive maintenance is the practice of performing maintenance on equipment at planned intervals — by time, usage, or condition — to reduce the probability of failure before it occurs.
Expanded Definition
Unlike reactive maintenance (repairing things after they fail), preventive maintenance is proactive: work is performed on a schedule, independent of whether the equipment has shown symptoms. Common PM tasks include lubrication, filter replacement, belt tension checks, fastener torque verification, cleaning, and calibration.
The core assumption behind PM is that most failures are preceded by a period of deterioration — and that intervening during that deterioration period costs significantly less than addressing a full failure.
Time-Based vs. Condition-Based PM
Time-based PM (calendar or interval PM) is the most common approach. Maintenance is performed every X days, weeks, months, or years regardless of actual equipment condition. A monthly filter change or an annual boiler service is time-based PM. It’s straightforward to schedule and easy to track, but it can result in over-maintaining equipment that isn’t degrading as expected, or under-maintaining equipment with unusually heavy use.
Condition-based PM (meter-based or usage-based PM) ties maintenance to a measurable threshold rather than a calendar. Service every 500 operating hours, or every 10,000 cycles, or when vibration readings exceed a threshold. Condition-based PM is better matched to actual equipment wear, especially for assets with variable utilization.
In practice, many PM schedules combine both: perform maintenance at whichever trigger comes first (every 90 days or 500 hours, whichever is earlier).
Setting PM Frequency
PM intervals should be based on:
- Manufacturer recommendations — the standard starting point, though manufacturers tend toward conservative intervals
- Historical failure data — if you know this asset has typically failed at around 1,200 hours, a PM interval at roughly half that provides margin
- Consequence of failure — higher-consequence assets justify tighter intervals
- Operating environment — equipment in dusty, wet, or high-temperature environments typically requires more frequent service
Intervals should be reviewed periodically as failure data accumulates. If PMs are consistently completed with nothing to address, the interval may be extended. If failures are occurring between PMs, the interval needs to come in.
PM vs. Predictive Maintenance
Predictive maintenance goes a step further by using real-time sensor data — vibration analysis, thermography, oil sampling — to detect developing failures and schedule maintenance only when data indicates it’s necessary. Predictive maintenance minimizes unnecessary maintenance events and can catch failure modes that interval-based PM misses.
Predictive maintenance requires instrumentation investment and technical expertise to interpret the data. It’s most cost-effective on critical, high-value assets. Most organizations begin with time-based PM and add predictive techniques selectively where the economics support it.
Why It Matters in Maintenance
A robust PM program is the primary lever for improving equipment reliability. PM compliance rate — what percentage of scheduled PMs are completed on time — is one of the most direct indicators of a maintenance program’s health. Organizations with mature PM programs consistently achieve higher equipment availability and lower reactive maintenance costs than those operating primarily reactively.
Related Terms
See also: Preventive Maintenance Scheduling | Preventive vs. Reactive Maintenance