Mechanical Maintenance
What the exam tests
Classification of maintenance strategies, condition-monitoring tools, and simple reliability arithmetic.
Core rules
- Breakdown (run-to-failure) suits cheap, non-critical items; preventive maintenance follows a calendar or running-hour schedule; predictive or condition-based maintenance acts on measured data; proactive maintenance removes the root cause.
- Condition monitoring tools: vibration analysis for unbalance and bearing frequencies, oil analysis for wear debris and water, thermography for electrical joints and hot bearings, ultrasound for leaks, and performance trending.
- Lubrication: mineral or synthetic oil and grease. High speed favours low viscosity, heavy load favours high viscosity; grease suits sealed and slow parts.
- Shaft alignment uses a dial indicator or laser; correct soft foot first, then align, and check alignment hot.
- Bearings: a press fit on the shaft and a push fit in the housing for a locating bearing; self-aligning bearings tolerate small misalignment.
- MTBF = total running time ÷ number of failures; MTTR = total repair time ÷ number of repairs; availability = MTBF/(MTBF + MTTR).
- A machine history card and maintenance record are the basis of any planned maintenance system.
Worked example
A pump ran 9000 hours in a year with 3 failures and 30 hours of total downtime. MTBF = 9000/3 = 3000 h, MTTR = 30/3 = 10 h, availability = 3000/3010 = 99.7 %. This tells the planner whether to invest in condition monitoring (raise MTBF) or in spares and repair technique (cut MTTR).
Traps
- Calling a daily oil-level check predictive; it is routine preventive.
- Quoting MTBF as the answer when availability is asked.
- Aligning a machine before correcting soft foot.