What is the difference between time-based and condition-based maintenance?
Time-based maintenance performs a task at a defined calendar or operating-age interval because age or exposure meaningfully changes failure probability or because an approved requirement mandates the interval. Condition-based maintenance performs or triggers work from evidence of actual asset condition. The appropriate strategy depends on failure behavior, detectability, consequence, technical effectiveness, data quality, and site capability rather than a universal preference for one approach.
Time-based tasks make sense when age or exposure is technically relevant.
Scheduled restoration or replacement can be appropriate when a component has a defensible age-related failure pattern, wear limit, life limit, degradation interval, regulatory requirement, OEM requirement, or other controlled basis. The interval should come from applicable authority and evidence, not arbitrary calendar habit.
Time-based inspections can also be useful, but an inspection interval is not the same as automatically replacing the component at that age.
Condition-based tasks need detectable evidence and enough response time to act.
A condition-based strategy requires a credible indicator of degradation, a method capable of detecting it, a decision criterion or qualified interpretation, and enough lead time between detection and functional failure to take useful action.
Collecting vibration, temperature, oil, ultrasound, electrical, process, or visual data does not create a condition-based program by itself. The information must lead to a controlled maintenance decision.
Select the task from the failure mode, not from the technology catalog.
Start with the function and credible failure mode. Ask whether failure has an age relationship, whether a detectable condition develops before functional failure, whether the method is technically capable, whether the site can collect and interpret the evidence reliably, and what consequence exists if the strategy fails.
| Question | Time-based implication | Condition-based implication |
|---|---|---|
| Does failure probability change with age/exposure? | May support scheduled restoration or replacement | Not required for condition monitoring |
| Is degradation detectable before functional failure? | May still use periodic inspection | Required for an on-condition task |
| Is there enough response time? | Less central for scheduled replacement | Essential to turn a finding into action |
| Can the site sustain the method? | Needs interval governance and execution | Needs collection, interpretation, and response capability |
Many assets need a combination of strategies.
One component may have lubrication tasks, periodic condition inspections, statutory tests, failure-finding tasks for protective functions, and run-to-failure decisions for low-consequence items. Build strategy at the failure-mode level rather than forcing an entire asset into one label.
The most mature-looking technology can still be the wrong task. Strategy quality is the fit between failure behavior, consequence, evidence, and action capability.
Use execution history to test whether the selected strategy is working.
Review task findings, failure recurrence, corrective follow-up, missed detections, false alarms, interval changes, labor burden, and actual failure mechanisms. If the evidence no longer supports the strategy, revisit the failure-mode decision instead of simply adding more inspections.
Which strategy should you choose?
Choose the technically effective task that controls the relevant failure mode within site capability and governing authority. If neither time nor condition provides an effective preventive task, the correct decision may be redesign, failure finding, run-to-failure with consequence control, or another approved strategy.
- Start with function, failure mode, and consequence.
- Verify whether age or detectable degradation actually supports the task.
- Confirm the site can interpret and act on condition evidence.
- Do not allow technology preference to replace failure-mode reasoning.
Select maintenance strategy at the failure-mode level.
A defensible program can explain why each task exists, what failure behavior supports it, how the condition is judged, and what action follows from the evidence.
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