Skip to main content
Back to module
Mechanical Systems & Maintenance · Maintenance Practice

Preventive maintenance and CMMS

Reading40mLesson 2 of 4 in this module

Compare maintenance strategies and write a preventive maintenance work order the way a CMMS stores it.

Why it matters

A plant that only fixes things when they break spends nights and weekends on emergency repairs, buys parts at rush prices and loses production. Planned maintenance flips that around. Mechanics do not just do the work: they write and improve the PM tasks, record what they find and close work orders in the computerized maintenance management system (CMMS). Good records are how a team learns.

Maintenance strategies

  • Reactive (run to failure): fix it when it breaks. Fine for cheap, non-critical items such as a light bulb in an office. Expensive for anything that stops production.
  • Preventive (PM): tasks on a fixed schedule, by calendar (every month) or by usage (every 500 operating hours). Examples: change filters, grease bearings, inspect belts.
  • Predictive / condition-based (PdM): measure the machine's condition (vibration, temperature, oil analysis, ultrasound) and act when the data shows a problem developing. You will see more of this in the PLC course's reliability lesson.
  • Reliability-centered maintenance (RCM): a structured method that asks how each asset can fail and what the consequences are, then picks the right strategy for each failure mode.

Most plants mix all of these. The skill is matching the strategy to the asset's criticality.

Key metrics

  • MTBF (mean time between failures) = operating time / number of failures. Higher is better.
  • MTTR (mean time to repair) = total repair time / number of repairs. Lower is better.
  • PM compliance = PMs completed on time / PMs scheduled. Many teams target 90 percent or more.
  • Planned vs reactive ratio: the share of maintenance hours that were planned.

Worked example. A conveyor ran 2,000 hours last quarter and failed 4 times, with repairs of 3, 5, 2 and 6 hours.

MTBF = 2,000 / 4 = 500 hours
MTTR = (3 + 5 + 2 + 6) / 4 = 4 hours

What a CMMS does

A CMMS is a database of the plant's maintenance. Typical records:

  • Assets in a hierarchy (site, area, line, machine, component), each with an ID, make, model, serial number, criticality and manuals.
  • Work orders: requests, corrective repairs and PMs, with status (requested, approved, scheduled, in progress, complete, closed).
  • PM schedules that generate work orders automatically.
  • Parts inventory with stock levels, bin locations and reorder points.
  • Labor and history: who did what, how long it took, what was found.

Common systems include IBM Maximo, SAP PM, Fiix, Limble and UpKeep, and many small shops use spreadsheets. The concepts are the same.

Anatomy of a good PM work order

WO number:     PM-0412          Asset: CNV-03 Conveyor 3 drive
Type:          Preventive       Frequency: every 4 weeks / 500 h
Priority:      3 (routine)      Estimated time: 45 min, 1 mechanic
Safety:        LOTO per procedure LOTO-CNV-03; gloves, safety glasses
Parts/tools:   Belt tension gauge, grease gun (NLGI 2), straightedge
Tasks:
  1. Lock out and verify zero energy at the drive disconnect.
  2. Inspect V-belts for cracks, glazing, wear. Record condition.
  3. Measure belt deflection force. Spec: 4-6 lbf at 0.5 in. Record.
  4. Check sheave alignment with straightedge. Record OK / adjusted.
  5. Grease both pillow block bearings: 6 strokes each. Record.
  6. Refit guard, remove locks per procedure, test run, listen.
Readings:      Tension ____ lbf   Bearing temp after run ____ °C
Findings:      _______________________________________________
Follow-up WO:  _______________________________________________
Completed by / date / actual time: ___________________________

Notice what makes it good:

  • Specific and measurable: "measure deflection force, spec 4 to 6 lbf", not "check belts".
  • Safety first: the lockout procedure is referenced by number.
  • Parts and tools listed so the mechanic does not walk back to the crib.
  • Readings recorded, so trends show up over months.
  • A place for findings and follow-up work, so a worn belt becomes a planned corrective order, not a breakdown next week.

Closing the loop

The work order is not done when the wrench goes down. Record actual time, parts used, readings and what you found, then close it. Failure codes (problem, cause, remedy) on corrective orders let the team spot repeat failures. If a PM never finds anything, it may be too frequent; if failures happen between PMs, it may be too infrequent or checking the wrong things.

Common mistakes

  • Vague tasks such as "inspect pump" that two mechanics would do differently.
  • Signing off a PM without recording readings.
  • Pencil-whipping: marking tasks done that were not done. It hides real problems and can be a safety issue.
  • Never reviewing PMs after they are written.

Beyond the platform

Industrial machinery mechanic roles usually require an apprenticeship or an associate degree in industrial maintenance with supervised hours, plus site lockout and qualified-person training. Optional credentials include NIMS (https://www.nims-skills.org/) and SMRP's CMRT (https://smrp.org/). Registered apprenticeships are listed at https://www.apprenticeship.gov/ (Checked on 2026-10-23.)

Recap

  • Match strategy to criticality: reactive, preventive, predictive, RCM.
  • MTBF = operating time / failures; MTTR = repair time / repairs.
  • A good PM work order is specific, measurable, safety-first and records readings.

Try it

Open the Do tab to write your own PM work order.

Also in this lesson: