Mechanical Maintenance Interview Questions Part I: Bearings, Pumps, Gearboxes & Reliability
Calculate Smarter. Work Faster.
40 practical questions (Q1–Q40) on mechanical fundamentals, bearings, alignment, pumps, compressors, gearboxes, boilers, condition monitoring and reliability — each with a definition, formula, key points and a model answer.
How to Use This Question Bank
These 40 questions cover topics that interviewers commonly explore for mechanical maintenance roles in process plants, utilities and manufacturing: rotating-equipment fundamentals, bearings and lubrication, alignment and couplings, pumps, compressors and fans, gearboxes, boilers, condition monitoring and reliability. Each answer gives a short definition, the formula where one applies, the key points an interviewer listens for, and a first-person model answer to adapt with your own plant experience.
Answers are reviewed for technical consistency with common industry practice and written to be region-neutral. Treat numbers such as tolerances, lubrication intervals and vibration limits as typical; the equipment manufacturer's data and the standards that apply at your site always take priority. Safety steps such as isolation and lockout/tagout follow your site procedures and local regulations.
Abbreviations used: LOTO — lockout/tagout; RCA — root cause analysis; MTBF — mean time between failures; MTTR — mean time to repair; NPSHA / NPSHR — net positive suction head available / required; BEP — best efficiency point; FFT — fast Fourier transform; GMF — gear mesh frequency.
The framework behind most troubleshooting answers. Interviewers want to hear a safe, evidence-based order rather than a parts-replacement list:
- Safety & LOTO
- Understand the symptom
- Collect data
- Inspect & measure
- Failure mode
- Root cause
- Correct
- Test
- Monitor
- Standardise
Calculators and guides that match each module are listed at the end of it, a quick formula sheet follows the last module, and the electrical interview series is linked at the bottom of this page.
No question matches that filter. Try a shorter term, such as “bearing”, “pump”, “alignment” or “gearbox”.
Mechanical Fundamentals
Force, torque, power, speed, stress, strain and friction — the basics behind every rotating-equipment answer.
Q1What are force, torque, work, power and energy?
Definition
Force is a push or pull on a body; torque is the turning effect of a force about an axis; work is force × displacement in the direction of the force; power is the rate of doing work; energy is the capacity to do work.
Formula
F = m × a
T = F × r
Work = F × d
P = Work / time
Rotating: P = T × ω, ω = 2πN / 60
P (kW) = 2π × N × T / 60 000
T (N·m) = 9550 × P (kW) / N (rpm)Example: 30 kW at 1500 rpm → T = 9550 × 30 / 1500 ≈ 191 N·m; at 750 rpm the same 30 kW means ≈ 382 N·m. (9550 is 60 000 / 2π, rounded.)
Model interview answer
“Force is a linear push or pull, while torque is its rotational effect. In rotating equipment power, torque and speed are linked by P = Tω. I use this for motors, gearboxes, conveyors and pumps: for the same power, lower speed means higher torque, which is exactly why a gearbox reduces speed to increase output torque.”
Q2Explain the relationship between speed, torque and power.
Formula
P = T × ω
T (N·m) = 9550 × P (kW) / N (rpm)
Constant power: T ∝ 1 / NExample: 20 kW at 1500 rpm ≈ 127 N·m. Through an ideal reducer to 100 rpm ≈ 1910 N·m; the real figure is lower because of gearbox efficiency.
Key points
- Speed ↑ → torque ↓ (at constant power)
- Speed ↓ → torque ↑ (at constant power)
- Real drives lose a few percent per gear stage, so output torque = ideal torque × efficiency
Model interview answer
“Power is torque multiplied by angular speed, so at constant power torque is inversely proportional to speed. That is the basic principle of a gearbox — speed is reduced and torque increased, less the losses in the gear stages.”
Q3What are stress, strain, Young's modulus and factor of safety?
Formula
Stress σ = F / A
Strain ε = ΔL / L
Young's modulus E = σ / ε (elastic region)
FOS = relevant failure strength / design (allowable) stressThe basis depends on the design case and code: for ductile steels the failure strength is usually the yield strength, for brittle materials the ultimate strength, and fatigue loading needs its own endurance-limit check.
Logic
- Stress: internal resisting force per unit area
- Strain: deformation per unit original length (dimensionless)
- E: material stiffness, valid only in the elastic range
- FOS: margin between failure strength and working stress
Model interview answer
“Stress is the internal resisting force per unit area and strain is deformation per unit length. Young's modulus is the material's stiffness in the elastic range. Factor of safety is the margin between the failure strength and the allowable working stress. I use these when checking shafts, couplings, structures, lifting equipment and pressure parts.”
Q4What is friction? Explain static, sliding and rolling friction.
Definition
Friction is the resistance to relative motion between two surfaces in contact.
Formula
Limiting static friction: F_s,max = μ_s × N
Sliding (kinetic) friction: F_k = μ_k × N (μ_k < μ_s usually)
Rolling resistance: F_r ≈ C_rr × N (C_rr much smaller than μ)These are the basic dry-contact (Coulomb) models; N is the normal force. Rolling resistance comes mainly from deformation at the contact, so it depends on geometry, material and load. Real friction also varies with surface condition, lubrication regime, speed and temperature.
Types and examples
- Static friction must be overcome to start motion and is usually higher than sliding friction
- Sliding (kinetic) friction acts once surfaces move — e.g. plain/sleeve bearings
- Rolling friction is much lower — e.g. ball and roller bearings
- Gear teeth see a mix of rolling and sliding contact
Model interview answer
“Friction is essential in brakes, belts and clutches, but unwanted friction causes heat, wear and energy loss. In rotating machinery, lubrication separates the surfaces with a film, reducing metal-to-metal contact, friction and wear.”
Calculators & guides for this module
Bearings & Lubrication
Bearing types and selection, failure causes, L10 life, grease vs oil and relubrication errors.
Q5Explain ball and roller bearings. How do you select them?
Selection factors
- Radial and axial load
- Speed
- Temperature
- Shock load
- Shaft size and fits
- Lubrication method
- Misalignment tolerance
- Required life
Ball bearing
Uses balls with point contact. Suits higher speeds with moderate radial and axial load.
Roller bearing
Uses rollers with line contact and carries higher radial load and shock. Cylindrical: high radial, little or no axial. Spherical: high radial, moderate axial, self-aligning. Tapered: combined radial and axial, usually used in pairs. Needle: high radial load in a small radial space.
Model interview answer
“I select a bearing from the radial and axial load, speed, temperature, shock, lubrication and required life. Ball bearings generally suit higher speeds, while roller bearings are chosen where higher load capacity or shock resistance is needed.”
Q6What are the common causes of bearing failure?
Sequence
Major causes
- Wrong or degraded lubricant
- Over-greasing or under-greasing
- Contamination (dust, water)
- Misalignment
- Excessive load
- Incorrect installation (hammering, heating errors)
- Wrong shaft/housing fit
- Excessive vibration
- Electrical fluting in VFD-fed motor bearings
Model interview answer
“For repeated bearing failure I don't assume bearing quality is the problem. I check lubricant type and quantity, alignment, shaft and housing condition, fits, load, vibration and installation practice, and read the damage pattern on the failed bearing to identify the root cause.”
Q7What is L10 bearing life?
Definition
L10 is the basic rating life that 90% of a large group of identical bearings are expected to reach or exceed under the same conditions (ISO 281).
Formula
Ball bearings: L10 = (C / P)³ [million revolutions]
Roller bearings: L10 = (C / P)^(10/3)
Hours: L10h = 10⁶ × L10 / (60 × n)C = basic dynamic load rating, P = equivalent dynamic bearing load, n = speed in rpm. Doubling the load cuts ball-bearing life to 1/8 and roller-bearing life to about 1/10. This is the basic rating life; the ISO 281 modified rating life adds factors for reliability, lubrication and contamination.
Model interview answer
“L10 life is the basic rating life at 90% reliability. For ball bearings life is proportional to (C/P)³, so even a moderate load increase — from misalignment, belt over-tension or imbalance — can reduce bearing life dramatically.”
Q8Grease vs oil lubrication — which will you use?
Selection depends on
- Speed
- Load
- Temperature
- Cooling requirement
- Contamination
- Sealing arrangement
Grease
Simple to apply, helps sealing, stays in place — common for electric motor and many rolling bearings.
Oil
Removes heat better, suits high speed and heavy load, can be filtered and circulated — common for gearboxes and large machines.
Model interview answer
“I select grease or oil from speed, temperature, load, cooling needs and equipment design. Grease is common for motor bearings, while circulating oil is preferred for gearboxes and high-speed machinery where cooling and contamination control matter.”
Q9What happens with over-greasing and under-greasing?
Sequence
Over-greasing
Churning, temperature rise, seal damage, leakage and higher drag.
Under-greasing
Metal-to-metal contact, friction, wear, noise, vibration and overheating.
Model interview answer
“More grease is not better lubrication. Over-greasing causes churning and heat, under-greasing causes wear. I follow the OEM grease type, quantity and interval. If temperature rises right after greasing, I check the grease, quantity and relief path, then monitor until it settles.”
Calculators & guides for this module
Shaft, Coupling & Alignment
Misalignment types, dial-gauge and laser alignment, soft foot, coupling types and coupling failures.
Q10What is shaft alignment? Explain angular and parallel misalignment.
Definition
Alignment means positioning two coupled shafts so their rotational axes are collinear within the specified tolerance at operating conditions.
Effects
- Bearing failure
- Coupling damage
- Seal failure
- High vibration (often 1× and 2× running speed, with high axial)
- Shaft fatigue
- Higher power consumption
Parallel (offset)
Shaft centrelines are parallel but displaced.
Angular
Shaft centrelines meet at an angle.
Model interview answer
“Both offset and angular misalignment add bearing and coupling load and vibration. I correct soft foot first and then do precision alignment with a dial indicator or laser system, to the OEM or alignment-standard tolerance.”
Q11Explain shaft alignment using a dial gauge or laser system.
Sequence
Key points
- Laser systems measure relative shaft positions and calculate vertical and horizontal corrections
- Rim-and-face or reverse-dial methods are used with dial gauges
- Allow for thermal growth where the OEM specifies cold offsets
- Check bar sag when using dial-gauge brackets
Model interview answer
“I start with safe isolation and soft-foot correction, then rough and precision alignment with a dial gauge or laser. I correct vertical offset with shims and horizontal offset by moving the machine, tighten to the specified torque, recheck, and confirm vibration on the trial run.”
Q12What is soft foot?
Definition
Soft foot is when one or more machine feet do not sit flat on the base, so tightening the hold-down bolts distorts the casing.
Effects
- Changes alignment
- Adds bearing load
- Raises vibration
- Stresses the coupling
- Causes repeated alignment problems
Detection
Loosen one foot at a time and measure the movement with a dial indicator, or check gaps with feeler gauges; laser systems have a soft-foot routine.
Correction
Clean surfaces, remove burrs, use correct shims (few, clean stainless shims) and repair a damaged base or foot.
Model interview answer
“Soft foot must be corrected before final alignment, because tightening a soft foot distorts the machine and changes the alignment you just set.”
Q13What is a coupling? Explain the main types.
Definition
A coupling connects two shafts to transmit torque.
Types
- Rigid coupling
- Flexible types: jaw/spider, tyre, pin-bush
- Gear coupling
- Grid coupling
- Disc (membrane) coupling
- Fluid coupling
Model interview answer
“A coupling transmits torque between shafts. Flexible couplings accommodate limited angular, parallel and axial misalignment and absorb some shock, but they must not be used to hide poor alignment — the misalignment still loads the bearings.”
Q14What causes coupling failure?
Sequence
Common causes
- Misalignment
- Incorrect coupling gap (DBSE)
- Worn element
- Excessive vibration
- Overload
- Wrong or missing lubrication (gear/grid couplings)
- Loose foundation
- Thermal movement
Model interview answer
“The usual causes are misalignment, incorrect installation, overload, wear and foundation problems. If a coupling fails repeatedly, I investigate alignment and the real mechanical load rather than just replacing the element.”
Calculators & guides for this module
Pumps
Centrifugal pump principle, head and pressure, NPSH and cavitation, trip troubleshooting and sealing.
Q15Explain the working principle of a centrifugal pump.
Definition
A centrifugal pump converts shaft power into hydraulic energy: the impeller accelerates the liquid, and the volute or diffuser converts much of that velocity into pressure.
Formula
Hydraulic power: P_h = ρ × g × Q × H
Pump shaft power: P_shaft = ρ × g × Q × H / η_pump
Motor input ≈ P_shaft / η_motorSI units: ρ in kg/m³, g = 9.81 m/s², Q in m³/s, H in m → P in watts.
Sequence
Model interview answer
“The impeller raises the liquid's velocity and the casing converts that velocity into pressure. Hydraulic power is proportional to density, flow and head, and I divide by pump efficiency for shaft power — and by motor efficiency again if I need electrical input.”
Q16What is pump head? How is pressure related to head?
Definition
Head is the energy given to the liquid per unit weight, expressed as a height of liquid column.
Formula
H = ΔP / (ρ × g)
Water: 1 bar differential ≈ 10.2 m
H (m) ≈ 10.2 × ΔP (bar) / SGΔP is the pressure rise across the pump (discharge minus suction, corrected for gauge elevation and velocity where significant). Example: 5 bar rise on water ≈ 51 m; with SG 0.8, the same 5 bar ≈ 64 m of that liquid.
Model interview answer
“Head is energy per unit weight of liquid. Pressure and head are linked through density and gravity; for water about 1 bar equals 10.2 m. That is why pump curves are given in head — the same pump develops the same head on different liquids, but a different pressure.”
Q17What is NPSH and how does cavitation occur?
Definition
NPSH (net positive suction head) is the margin of suction pressure above the liquid's vapour pressure. The system provides NPSHA (available); the pump requires NPSHR (required), which varies with flow and speed and comes from the manufacturer's test data.
Formula
Condition: NPSHA > NPSHR, with a safety marginNPSHR is usually defined at a 3% head drop, so some cavitation already exists at NPSHA = NPSHR. Guidance such as ANSI/HI 9.6.1 recommends a margin depending on the application.
Cavitation symptoms
- Crackling, gravel-like noise
- Vibration
- Flow and pressure fluctuation
- Reduced head
- Impeller pitting at the vane inlet
- Bearing and seal damage
Remedies
Raise suction level or pressure, reduce suction losses, clean the strainer, open the suction valve fully, lower liquid temperature, and check the pump is not running far right of its best efficiency point.
Model interview answer
“Cavitation occurs when local pressure at the impeller eye falls below the liquid's vapour pressure, so vapour bubbles form and collapse violently. I compare NPSH available with NPSH required and check suction restrictions, liquid temperature, suction piping and the operating point.”
Q18A centrifugal pump repeatedly trips. How will you troubleshoot?
Electrical
Motor current, voltage, phase balance and the actual trip indication.
Mechanical
Bearing temperature, coupling, alignment, shaft and impeller rub.
Hydraulic
Suction and discharge pressure, flow, cavitation, valve position and strainer blockage.
Process
Excess demand, wrong operating point, dry running or a closed-valve condition.
Model interview answer
“I first find out whether the trip is electrical, mechanical, hydraulic or process-related. I read the trip indication and current, then check bearing temperature, alignment, coupling, suction and discharge pressure, flow and cavitation. I don't keep resetting without finding the cause.”
Q19Mechanical seal vs gland packing.
Seal failure causes
- Dry running
- Misalignment
- Shaft runout or deflection
- Excessive vibration
- Wrong face or elastomer material
- High temperature
- Incorrect installation or setting
Mechanical seal
Very low leakage and better sealing, but needs precise installation and correct operating conditions.
Gland packing
Simpler and cheaper, but needs a controlled leakage to cool and lubricate the packing.
Model interview answer
“Mechanical seals give better leakage control but depend on correct installation and operating conditions. Packing is simpler and suits duties where controlled leakage is acceptable. Repeated seal failure means checking runout, alignment, dry running, vibration and process conditions.”
Calculators & guides for this module
Compressor, Fan & Blower
Pumps vs compressors, air compressor systems, high discharge temperature and boiler draft fans.
Q20What is the fundamental difference between a pump and a compressor?
Definition
A pump moves liquid, which is nearly incompressible; a compressor handles gas and raises its pressure, which also changes its density and temperature.
Commonly quoted classification (guide only)
- Fan: small pressure rise (often quoted as a pressure ratio up to about 1.11)
- Blower: moderate rise (often quoted as about 1.11–1.20)
- Compressor: substantial compression (above about 1.20)
- Boundaries vary between standards and manufacturers — treat them as a guide
Model interview answer
“A pump transfers liquid and develops flow and head, while a compressor raises gas pressure with significant changes in density and temperature. That's why compressor systems need attention to pressure ratio, discharge temperature, cooling and moisture removal.”
Q21Explain the basic working and components of an air compressor system.
Sequence
Major components
- Motor and drive
- Compressor element (screw, reciprocating, centrifugal)
- Oil system and separator (oil-injected)
- Coolers
- Air receiver
- Dryer
- Filters
- Safety valve
- Pressure and temperature sensors
Model interview answer
“The compressor converts mechanical energy into compressed-air energy. After compression the air is separated from oil where applicable, cooled, stored in the receiver, and dried and filtered before it reaches the plant.”
Q22Compressor discharge temperature is high. What will you check?
Sequence
Possible causes
- High ambient or poor ventilation
- Dirty or blocked cooler
- Low oil level or wrong oil
- Thermostatic valve fault (oil-injected)
- High pressure ratio / high discharge pressure
- Valve problem (reciprocating)
- Blocked filter
- Excessive loading
Model interview answer
“I first confirm the temperature reading and operating condition. Then I check cooler cleanliness, oil level and quality, discharge pressure, ambient temperature, ventilation and loading. If all are normal, I investigate the compressor element or valves.”
Q23What are the functions of ID, FD and PA fans in a boiler?
If the ID fan fails
- Furnace draft changes
- Flue gas removal is affected
- Boiler interlocks trip the firing
- Unsafe combustion if not tripped
- Boiler shutdown
FD fan
Forced draft — supplies combustion air to the furnace.
ID fan
Induced draft — extracts flue gas and maintains furnace draft.
PA fan
Primary air — carries or dries fuel where the firing system needs it (e.g. pulverised coal).
Model interview answer
“The FD fan supplies combustion air, the ID fan removes flue gas and holds furnace draft, and the PA fan provides primary air for fuel transport and combustion. They are interlocked with the boiler because wrong air or draft conditions create serious safety risks.”
Calculators & guides for this module
Gearbox & Transmission
Gear ratio, speed and torque, overheating, noise and vibration diagnosis and gear tooth failure modes.
Q24How does a gearbox change speed and torque?
Formula
GR = N_input / N_output
Ideal: T_out = T_in × GR
Actual: T_out = T_in × GR × ηExample: 1500 rpm in, 50 rpm out → GR = 30 : 1.
Model interview answer
“A gearbox reduces speed and increases torque by the gear ratio, less the efficiency losses. When selecting one I consider speed, required output torque, service factor, duty cycle and load type.”
Q25What causes gearbox overheating?
Sequence
Possible causes
- Low oil level
- Wrong oil viscosity
- Overfilling (churning)
- Overload
- Bearing failure
- Gear damage
- Misalignment
- Cooling problem (fan, cooler, fins)
Model interview answer
“I first verify the actual temperature and load, then oil level and viscosity, cooling, alignment, bearing condition, vibration and gear condition. If the gearbox is overloaded, changing the oil will not fix the root cause.”
Q26How do you diagnose abnormal gearbox noise and vibration?
Formula
Gear mesh frequency: GMF (Hz) = Z × N / 60Z = number of teeth on the gear, N = its shaft speed in rpm. Sidebands around GMF spaced at shaft speed often indicate a damaged or eccentric gear.
Checks
- Oil level and contamination
- Gear tooth condition
- Bearing condition
- Backlash
- Alignment
- Shaft runout
- Foundation
- Load
- Vibration spectrum
Model interview answer
“I relate the noise to speed and load, inspect the oil and gears, check bearings and alignment, and use vibration spectrum analysis where available to separate gear-mesh, bearing, alignment and looseness sources.”
Q27What are the common gear tooth failure modes?
Pitting
Surface fatigue producing small pits on the tooth flank.
Scoring / scuffing
Lubricant film breakdown under heavy sliding, leaving welded and torn tracks.
Wear
Progressive material removal, often from contamination or poor lubrication.
Tooth breakage
From overload or bending fatigue; fatigue cracks usually start at the tooth root.
Model interview answer
“Gears fail by pitting, wear, scoring or scuffing, and tooth breakage. I look at the contact pattern, lubrication, load, alignment and vibration to identify which mechanism is at work.”
Calculators & guides for this module
Boiler & Pressure Equipment
Boiler working principle, safety devices and interlocks, and drum-level control.
Q28Explain the basic working principle of an industrial boiler.
Sequence
Major components
- Furnace
- Steam drum (and water drum where fitted)
- Economiser
- Superheater
- Air preheater
- Feedwater system
- Blowdown system
Model interview answer
“A boiler converts the chemical energy of fuel into heat by combustion and transfers it to water to generate steam for process heating, power generation or other plant needs.”
Q29What are the important boiler safety devices and interlocks?
Key points
- Safety valves
- Low drum-level trip
- High steam-pressure trip
- Flame failure protection
- Furnace pressure protection
- Feedwater protection
- Fan interlocks
- Burner and purge interlocks
- Emergency trip
Model interview answer
“Interlocks prevent operation under unsafe conditions. I never bypass a critical boiler interlock to keep production running; any temporary bypass must follow the site procedure, a risk assessment and formal authorisation.”
Q30Explain single-element and three-element drum-level control.
Why three-element
- Swell: on a sudden steam-demand increase, drum pressure drops, bubbles expand and level briefly rises
- Shrink: on a demand decrease the level briefly falls
- Level-only control reacts the wrong way during these transients
Single-element
Controls feedwater from drum level only.
Three-element
Uses drum level, steam flow and feedwater flow together.
Model interview answer
“Three-element control uses drum level, steam flow and feedwater flow. Steam flow acts as a feed-forward, so feedwater follows the actual demand instead of being misled by shrink and swell. It is most valuable where rapid load changes make level-only control inadequate.”
Calculators & guides for this module
Maintenance & Condition Monitoring
PM vs PdM vs breakdown, vibration analysis, sudden vibration rise and thermography vs vibration.
Q31Preventive vs predictive vs breakdown maintenance.
Preventive
Time- or usage-based, e.g. a monthly gearbox inspection.
Predictive
Condition-based: vibration, thermography, oil analysis, ultrasound.
Breakdown
Repair after failure — acceptable only for non-critical, low-consequence items.
Model interview answer
“Preventive maintenance is planned by time or usage, predictive maintenance by measured condition, and breakdown maintenance happens after failure. For critical rotating equipment I combine PM with condition monitoring to reduce unplanned downtime.”
Q32What is vibration analysis and why is it important?
Parameters
- Displacement, velocity, acceleration
- Frequency and FFT spectrum
- Overall levels judged against the applicable part of ISO 20816 (formerly ISO 10816) or OEM limits
What the spectrum suggests (clues, not a diagnosis)
Dominant 1× running speed radial: imbalance. High 2× and high axial 1×: misalignment. Non-synchronous peaks at bearing defect frequencies: bearing damage. Gear mesh frequency with sidebands: gear problems. Many harmonics: looseness.
Model interview answer
“Vibration analysis is one of the most useful predictive techniques for rotating equipment. It identifies developing imbalance, misalignment, bearing, gear and looseness problems before they become failures.”
Q33Machine vibration suddenly increases. What will you do?
Sequence
Model interview answer
“First I decide whether the level is unsafe and needs a shutdown, and whether the increase is real. Then I compare with the trend, check load, bearings, alignment, coupling, foundation and lubrication, and use spectrum analysis. I don't replace parts based only on an overall vibration number.”
Q34Thermography vs vibration monitoring.
Thermography detects
Hot bearings, friction, lubrication problems, thermal overload and hot spots.
Vibration detects
Imbalance, misalignment, bearing defects, gear defects, looseness and resonance.
Model interview answer
“Different failure modes show different physical symptoms. I use thermography for abnormal temperature patterns and vibration analysis for dynamic mechanical problems. For critical assets, combining techniques gives better diagnostic confidence.”
Reliability & Maintenance Management
MTBF, MTTR and availability, RCA with a bearing example, and a maintenance strategy for critical equipment.
Q35Explain MTBF, MTTR and availability.
Formula
MTBF = operating time / number of failures
MTTR = total repair time / number of repairs
Inherent availability (simplified) = MTBF / (MTBF + MTTR) × 100 %Example: MTBF 1000 h, MTTR 10 h → 1000 / 1010 × 100 ≈ 99.01 %. Operational availability is lower once waiting for spares, logistics and planned maintenance downtime are included.
Model interview answer
“MTBF is the average operating time between failures, MTTR the average repair time, and availability the share of time the equipment can run. I use them to see whether the main problem is frequent failure or slow restoration.”
Q36Explain root cause analysis using a bearing failure example.
Sequence
RCA tools
- 5-Why
- Fishbone (Ishikawa)
- Fault tree
- Failure history
- Inspection evidence (damage pattern, grease sample)
Corrective actions
Correct the grease specification, update the lubrication chart, train technicians, improve stores control, and monitor temperature and vibration to verify.
Caution
Each 'why' is a hypothesis until evidence confirms it.
Model interview answer
“RCA identifies why a failure happened, not just which part failed. I use evidence, failure history and tools such as 5-Why or fishbone, implement the corrective action, and then monitor to confirm the failure doesn't repeat.”
Q37How will you develop a maintenance strategy for critical rotating equipment?
Sequence
What each step covers
- Criticality: production, safety, environment, repair time, spares availability
- Strategy: PM, PdM, inspection, lubrication, alignment, condition monitoring
- Spares: bearings, seals, couplings, gear parts, lubricants
- KPIs: MTBF, MTTR, availability, repeat failures
Model interview answer
“I classify equipment by production and safety criticality, study its failure history and define the right PM and predictive strategy. Critical spares, lubrication, alignment, condition monitoring and RCA are built into the plan and tracked with KPIs.”
Senior Practical Troubleshooting
Repeated pump bearing failure, a stopped critical conveyor, and a reliability plan for the whole plant.
Q38A pump bearing repeatedly fails. What will you do?
Bearing
Correct bearing number, fit, installation method and damage pattern.
Lubrication
Correct lubricant, quantity, contamination and interval.
Alignment
Motor–pump alignment, soft foot and coupling.
Shaft and housing
Runout, journal condition, bore condition and fits.
Process
Pump load, cavitation and operating point versus BEP.
Condition monitoring
Temperature, vibration and spectrum.
Model interview answer
“If a pump bearing repeatedly fails I don't treat replacement as the solution. I investigate lubrication, fits, shaft and housing condition, alignment, soft foot, coupling, hydraulic load and vibration, identify the actual failure mechanism, correct it, and follow up with condition monitoring.”
Q39A production-critical conveyor suddenly stops. How will you troubleshoot?
Sequence
What to check
- Drive pulley and gearbox
- Coupling
- Bearings
- Belt or chain, tension and tracking
- Idlers and rollers
- Alignment
- Jammed material or blockage
- Electrical: motor trip, overload, VFD fault, contactor, supply
Model interview answer
“I make the conveyor safe with isolation and LOTO, then find whether the stop came from the drive, gearbox, coupling, belt, bearing, a blockage or electrical protection. After repair I do a controlled trial and monitor it, and for a critical or repeated failure I complete RCA rather than only restoring production.”
Q40As a senior mechanical maintenance engineer, how will you improve plant reliability?
Focus areas
- Criticality: critical, bottleneck and safety-critical assets
- Failure history: repeats, frequency, modes, downtime
- PM optimisation: avoid over- and under-maintenance
- Predictive: vibration, thermography, oil analysis, ultrasound
- RCA for recurring failures
- Lubrication excellence: right lubricant, quantity, interval, clean storage
- Precision alignment: soft foot, coupling, thermal growth
- Critical spares
- KPIs: MTBF ↑, MTTR ↓, availability ↑
- Safety: LOTO, PTW, hot work, lifting, confined space, guarding
Model interview answer
“My focus is equipment reliability, not only attending breakdowns. I start with criticality and failure history, optimise PM and predictive maintenance, carry out RCA on repeated failures, and build lubrication, alignment and critical-spares discipline, tracked through MTBF and MTTR. The goal is higher availability, less unplanned downtime, controlled cost and safe operation.”
Calculators & guides for this module
Quick Formula Sheet
The core relationships used in the answers above, in one place.
| Quantity | Formula |
|---|---|
| Mechanical power | P (kW) = 2π × N × T / 60 000 |
| Torque | T (N·m) = 9550 × P (kW) / N (rpm) |
| Stress | σ = F / A |
| Strain | ε = ΔL / L |
| Young's modulus | E = σ / ε |
| Friction (dry) | F_s,max = μ_s N; F_k = μ_k N |
| Ball bearing L10 | L10 = (C / P)³ [10⁶ rev] |
| Roller bearing L10 | L10 = (C / P)^(10/3) |
| L10 in hours | L10h = 10⁶ × L10 / (60 × n) |
| Pump hydraulic power | P_h = ρ × g × Q × H |
| Pump shaft power | P = ρ × g × Q × H / η |
| Head | H = ΔP / (ρ × g) |
| Water pressure ↔ head | 1 bar differential ≈ 10.2 m |
| NPSH condition | NPSHA > NPSHR + margin |
| Gear ratio | GR = N_in / N_out |
| Gearbox output torque | T_out = T_in × GR × η |
| Gear mesh frequency | GMF = Z × N / 60 |
| MTBF | Operating time / failures |
| MTTR | Repair time / repairs |
| Inherent availability | MTBF / (MTBF + MTTR) × 100 % |
These answers summarise common industrial practice for interview preparation. Tolerances, clearances, lubrication intervals, vibration limits and statutory requirements vary by equipment, manufacturer and country — always follow the OEM documentation, applicable standards and your site's safety procedures.
Frequently Asked Questions
How should I prepare for a mechanical maintenance engineer interview? +
Revise the core relationships — torque, power and speed, stress and strain, pump head and NPSH, bearing L10 life, gear ratios and MTBF/MTTR — then practise explaining real failures in a clear order: safety and isolation, symptom, data, inspection, failure mode, root cause, correction, testing and monitoring. Interviewers usually value a structured, evidence-based method and genuine plant examples more than memorised definitions.
What is the difference between NPSHA and NPSHR? +
NPSHA (available) is the suction head the system provides above the liquid's vapour pressure at the pump inlet; it depends on suction level or pressure, friction losses and liquid temperature. NPSHR (required) comes from the manufacturer's test data for the pump at a given flow and speed, usually defined as the point where head drops by 3% because of cavitation. NPSHA must exceed NPSHR with a safety margin, otherwise the pump cavitates.
Why does bearing life drop so much when the load increases? +
Basic rating life follows L10 = (C/P)³ for ball bearings and (C/P)^(10/3) for roller bearings, so life falls with the cube (or 3.33 power) of the load. Doubling the equivalent load cuts ball-bearing life to one-eighth and roller-bearing life to about one-tenth. That is why misalignment, belt over-tension and imbalance, which all add bearing load, shorten bearing life so sharply.
Why must soft foot be corrected before shaft alignment? +
If a machine foot does not sit flat, tightening its hold-down bolt distorts the casing. That shifts the shaft position and can change the alignment after it has been set, and it also loads the bearings and raises vibration. Checking each foot and correcting soft foot with clean, correct shims first makes the final alignment repeatable.
Which vibration frequencies point to imbalance, misalignment and bearing damage? +
As a general guide, imbalance shows as a dominant 1× running-speed peak in the radial direction; misalignment often shows high 2× and high axial 1× vibration; rolling-bearing defects appear at non-synchronous bearing defect frequencies and their harmonics; gear problems show at gear mesh frequency with sidebands; and looseness produces many running-speed harmonics. Diagnosis should always be confirmed with trends, phase readings and inspection.
Electrical Interview Questions
Preparing for a combined E&M or utility role? The 120-question electrical maintenance series covers the electrical side of the same plant.