Maintenance, Diagnostics and Repair
Core concepts
Know maintenance strategies: reactive (breakdown), preventive (time-based), predictive (condition-based) and reliability-centred maintenance. Understand condition monitoring: vibration analysis, thermography, oil analysis, ultrasound and motor current signature analysis. Cover structured fault finding: observe, gather information, form hypotheses, test, confirm, document. Understand mean time between failures (MTBF), mean time to repair (MTTR) and availability = MTBF / (MTBF + MTTR).
Traps in questions
- Confusing preventive with predictive maintenance.
- Replacing parts without diagnosing the root cause.
- Ignoring safety isolation and lockout/tagout before repair.
- Forgetting to document the repair and update maintenance plans.
- Misinterpreting vibration frequency: bearing defects appear at characteristic frequencies, imbalance at 1× running speed.
Worked example
A machine runs 2000 h per year with MTBF = 500 h and MTTR = 10 h. Availability = 500/(500 + 10) ≈ 98 %. Expected failures per year = 2000/500 = 4. Downtime = 4 × 10 = 40 h. Rule of thumb: improving MTTR is often cheaper than improving MTBF, because spare parts and procedures are directly controllable.
Practical competence
Be able to write a short fault-finding report: symptom, measurement, cause, corrective action, prevention. Understand spare-parts strategy and the role of manufacturer documentation. Always apply lockout/tagout and verify absence of voltage before working on electrical equipment.