Maintenance timing is the central problem of the discipline. Too early and the team is consuming life, labor, and spares on equipment that did not need attention. Too late and the equipment fails during production, consuming ten times the cost in downtime and emergency response. A CMMS is the instrument that lets maintenance leaders move from calendar-based habit to evidence-based timing, one asset class at a time.
Evocon’s “World-Class OEE” study analyzed more than 3,500 machines across 50-plus countries and found discrete manufacturing averages 66.8 percent OEE, versus the Nakajima world-class threshold of 85 percent. Much of the gap is availability loss driven by untimed maintenance, either reactive failures or over-aggressive interventions that consume the production window unnecessarily. The Society for Maintenance and Reliability Professionals’ Body of Knowledge defines the supporting metrics (MTBF, MTTR, PM compliance, schedule compliance) that let a reliability engineer actually move timing toward the optimum.
The Four Timing Regimes
Maintenance timing comes in four distinct forms. Most plants run all four, and the CMMS has to support all four cleanly.
- Reactive. Fix it when it fails. Acceptable for non-critical, low-cost, easily replaceable assets. A problem when it is the default for critical equipment.
- Calendar-based PM. Inspect or service every X weeks. Simple, cheap, effective on stable-duty assets.
- Runtime-based PM. Service every X hours, cycles, or production units. Better for assets with variable duty.
- Condition-based. Service when sensor data (vibration, temperature, oil analysis, current) indicates developing failure.
The right mix depends on asset criticality, failure-mode profile, and sensor availability. A CMMS that forces every asset into one regime is the wrong CMMS.
Why Calendar-Based Alone Is Insufficient
Calendar-based PM is the easiest to administer and the hardest to defend on assets with variable duty. A pump that runs 24 hours a day gets the same PM cadence as one that runs four, and the first one fails early while the second is over-serviced. Plant Engineering’s reporting on rising mean-time-to-repair (from roughly 49 to 81 minutes in recent years, per the ExxonMobil-sponsored Annual Maintenance Study) reflects, in part, plants getting the timing wrong on assets whose duty profile has changed.
Preventive maintenance workflows that allow a mix of calendar, runtime, and condition triggers let the reliability team match the timing regime to the asset.
What Moving to Runtime or Condition-Based Timing Looks Like
A pragmatic sequence:
- Rank assets by criticality. A class first. C class usually stays on calendar.
- Review historical failures from the CMMS on each A-class asset to identify which failure modes the current PMs actually prevent.
- Select the timing regime that matches the dominant failure mode.
- Retire PMs that do not prevent real failures. Paper-pushing PMs consume labor without benefit.
- Pilot condition-based triggers on two or three assets where sensor data is already available.
Typical outcomes after a timing review
- 15 to 30 percent reduction in PM labor without increase in failures (from retiring ineffective PMs)
- 20 to 40 percent reduction in unplanned downtime on A-class assets
- 10 to 25 percent extension of useful life on well-instrumented rotating equipment
- 5 to 15 percent OEE lift on production lines where critical PMs are correctly timed
- 30 to 50 percent reduction in “PM-caused” failures (the post-PM start-up problem)
The Role of Trusted Data
Timing decisions live on work-order history. If the CMMS data is incomplete or retrospective, the reliability engineer is guessing. Two disciplines make the data trustworthy:
- Mobile work-order closure at the point of work, not at end of shift
- Failure codes with enough granularity to distinguish wear from misoperation from lubrication failure
Without those, analytics and reporting dashboards produce the right charts with the wrong story.
Industry Application: Food and Beverage
CIP pumps, homogenizers, and fillers run on tight sanitation schedules. PM timing has to fit the production window without creating sanitation-related risk. Runtime-based PMs on motors and pumps paired with calendar-based sanitation PMs give the reliability team both views.
Industry Application: Energy and Utilities
Transformers and switchgear run for decades between major maintenance events. Calendar-based PM misses condition-based signals that predict failure years in advance. Oil analysis and thermographic inspection feed directly into asset management records that drive condition-based work orders.
Industry Application: Facility Management
HVAC RTUs, chillers, and boilers benefit from a combination of seasonal calendar PMs and runtime-based PMs on compressors. Property and facility teams that shift from “spring and fall service visits” to a hybrid schedule typically cut emergency repairs 20 to 30 percent in the first year.
Where Teams Get Stuck
The most common mistake is treating timing as a one-time project. Timing needs to be reviewed annually, at minimum, because asset duty profiles change, production shifts, new equipment is added, and failure patterns evolve. Reliability teams that run an annual PM effectiveness review keep the program from calcifying.
Frequently Asked Questions
What is the target PM compliance rate? 90 to 95 percent is credible on a well-designed program. Compliance above 98 percent often indicates PMs too light to be meaningful.
How do we know if a PM is working? Look at the failure rate of the modes the PM is designed to prevent. If failures are stable after PM removal, the PM was not adding value.
When is condition-based monitoring worth the investment? On A-class assets where unplanned failure carries high downtime cost, and where failure modes have detectable precursors (vibration, temperature, lubricant chemistry).
What if our assets do not have sensors? Operator-rounds readings (on mobile) are a credible condition-based input for many assets. Temperature checks, visual inspections, and sound assessments can drive work orders without IoT sensors.
How do we adjust PM timing without risking failure? Adjust one asset class at a time, measure for 60 to 90 days, then expand. Do not change timing globally in one release.
Does AI help with timing decisions? Pattern detection on work-order history can surface overdue assets and underperforming PMs for human review. The final timing call is still a reliability-engineer decision.
Getting maintenance timing right is where operational excellence programs earn their name. Book a Task360 demo to see the discipline applied to your equipment base.
