Mechanical engineering

What interviewers look for

• You prevent breakdowns, not only repair them: a maintenance plan by run hours or calendar, based on the manufacturer's manual.
• A method for fault finding: you ask, look, listen and measure, and you can explain why a part failed before you replace it.
• Records: job cards, a maintenance log with run hours and parts used, and a spare parts list with a minimum stock for critical items.
• Solid basics: how pumps, diesel engines, air conditioners and bearings work, explained simply and correctly.
• Safe habits: lock-out before working on machines, guards on rotating parts, safe lifting, and care with hot, pressurised and fuel systems.

Questions they ask

1. 1“What is the difference between preventive and corrective maintenance? Give an example from your work.”

   Why they ask: Whether you plan maintenance or only react to breakdowns. The panel wants a real example with the interval and the result.

   How to answer

   • Preventive maintenance is planned before failure: oil and filter changes, belt checks, cleaning, at intervals from the manual (run hours or calendar).
   • Corrective maintenance is the repair after a fault is found or a machine breaks down.
   • Mention condition-based checks too: listening, temperature, vibration and oil condition.
   • Give one example where a planned check found a problem early, and what it saved.

   Preventive maintenance is work we plan before anything fails, based on the manual and the run hours. Corrective maintenance is the repair after a fault. At a hotel in Hargeisa I kept a service sheet for the two standby generators: daily checks by the operator, and oil, filters and a belt inspection at the hours in the manual. During one routine check I found a cracked fan belt and replaced it that day. If it had broken during a power cut, the engine would have overheated and the hotel would have had no power.

2. 2“A diesel generator will not start. Walk us through how you find the cause.”

   Why they ask: Whether you work in a logical order, from the simplest and most likely cause, and stay safe while you do it.

   How to answer

   • Start with what you can see: fuel level, fuel valve open, emergency stop reset, battery and terminals, and any alarm on the controller.
   • If it does not crank: battery voltage, the starter motor, the connections, and the start signal from the controller.
   • If it cranks but does not fire: fuel supply, blocked filters, air in the fuel line (bleed it), the air filter, and the preheat system if the engine has one.
   • Fix the cause, test on load, and write the fault and the fix in the log.

   I start with the quick checks: fuel in the tank, the fuel valve open, the emergency stop reset, and any fault code on the controller. Then I listen. If the starter does not turn the engine, I measure the battery voltage and check the terminals and starter connections. If it cranks but does not fire, I look at the fuel: blocked filters, or air in the line after someone let the tank run empty, which is common. I bleed the system, start it, run it on load, and record the fault and the fix in the log.

3. 3“A borehole pump is running but delivering very little water. What do you check?”

   Why they ask: Whether you understand how pumps and pipelines work together: water level, head, flow, power supply and wear.

   How to answer

   • Check whether the water level in the borehole has dropped, so the pump must lift further or is sucking air.
   • Check the supply: voltage, and the direction of rotation on a three-phase pump; on a solar pump, the sunlight, the panels and the controller.
   • Check the pipeline: leaks, blockages, a partly closed valve, and read the pressure gauge.
   • If all of that is normal, the pump may be worn by sand; compare its output with the pump curve and plan to pull it out.

4. 4“An air conditioner is running but not cooling. What would you check?”

   Why they ask: Whether you know the refrigeration cycle well enough to find common faults, and whether you handle refrigerant safely.

   How to answer

   • Start simple: the settings, dirty filters, a dirty or blocked outdoor coil, and whether the outdoor fan runs.
   • Check that the compressor runs: capacitor, contactor, supply voltage and current.
   • Then the refrigerant: pressures with gauges, and oily marks that show a leak. Find and fix the leak before adding gas.
   • Say that you recover refrigerant with the right equipment instead of releasing it into the air.

   At an office in Mogadishu, a split unit was running but blowing warm air. The filters were clean, so I checked the outdoor unit and found the fan not turning; the compressor was overheating and cutting out. I isolated the unit and tested the fan capacitor with my meter, and it was weak. After replacing it, I checked the pressures and the current, and measured the air temperature at the indoor unit to confirm it was cooling. I then added a monthly filter and coil check for that office.

5. 5“How do you manage spare parts so a machine is not stopped for weeks?”

   Why they ask: Whether you plan for long delivery times, which are common when parts come from outside the country, and whether you control stock.

   How to answer

   • Keep a list of critical spares for each machine: filters, belts, seals, bearings, fuses, a starter battery.
   • Set a minimum stock level for each and reorder when you reach it, allowing for the delivery time.
   • Record every part in and out against a job card, so the stock and the costs are true.
   • Buy genuine or approved parts from trusted suppliers; a cheap fake filter can destroy an engine.

   At a water utility I kept an Excel stock list for the pumps and the three standby generators. For each critical part I set a minimum level, based on how many we used in a year and how long an order took to arrive from outside the country. When a filter or seal reached the minimum, I raised a purchase request the same day. Every part used was written on the job card with the machine number. When one pump's mechanical seal failed, we had a spare on the shelf, and the pump was back in service the same day.

6. 6“Why do bearings fail early, and how do you prevent it?”

   Why they ask: Whether you know the common causes (lubrication, alignment, contamination) and treat the cause, not only the part.

   How to answer

   • Causes: too little or too much grease, the wrong grease, dirt or water getting in, misalignment, belts that are too tight, and poor fitting.
   • Signs: noise, heat, vibration, and grease that leaks or changes colour.
   • Prevention: grease type and amount from the manual, alignment after every installation, correct belt tension, and clean fitting with the right tools, never by hammering the bearing.
   • After a failure, look at the old bearing to find the cause before you fit a new one.

7. 7“Which design software do you use, and what have you made with it?”

   Why they ask: Whether your AutoCAD or SolidWorks claim is real. Panels often follow up with a short test at the computer.

   How to answer

   • Name each program and how often you use it: every week, on one project, or only in class.
   • Describe one real drawing: a part, a bracket, a pump base or a duct layout, with dimensions and tolerances.
   • Say what you have not used yet and what you are learning now.

   I use AutoCAD most weeks for 2D drawings: workshop drawings of brackets, pump bases and pipe layouts with dimensions and tolerances. In my final-year project I modelled a small pump stand in SolidWorks, with a part file for each piece, an assembly and a drawing sheet, but I have not used SolidWorks at work yet. I have never used simulation tools professionally. If you give me a sketch today, I can produce a dimensioned AutoCAD drawing of it.

Example answers are in English, the language most panels use. Say it in your own words.

Topics to revise

• Preventive, corrective and predictive maintenancePreventive is planned by run hours or calendar; corrective is the repair after a fault; predictive (condition-based) uses checks such as vibration, temperature and oil condition to act before failure. They may ask which suits a hospital generator.
• Centrifugal pumps: head, flow and pump curveHead is the height a pump can lift water; flow is how much it delivers per hour. The pump curve shows that as head rises, flow falls. Total head = static lift + friction losses in the pipes + pressure needed at the outlet.
• Cavitation and NPSHIf the pressure at the pump inlet is too low, vapour bubbles form and collapse, making a gravel-like noise and eating the impeller. Prevent it with a short suction lift and a large, clear suction pipe, so the NPSH available stays above what the pump needs.
• Pressure and head (1 bar ≈ 10 m)A column of water about 10 m high gives a pressure of about 1 bar. Useful for gauges, tanks and pumps: a tank 15 m above a tap gives about 1.5 bar at the tap when no water flows, and less when it does.
• Refrigeration cycleThe compressor pumps refrigerant gas to the condenser (outdoor coil), where it gives off heat and turns to liquid; the expansion device drops its pressure; in the evaporator (indoor coil) it takes heat from the room and boils back to gas.
• BTU, tons and kWAir conditioners are rated in BTU per hour or tons of cooling: 1 ton = 12,000 BTU/h, about 3.5 kW. A “1.5 ton” split unit is 18,000 BTU/h. They may ask you to convert between them.
• Four-stroke diesel engineIntake, compression, power, exhaust. Diesel fuel ignites from the heat of compression, so clean fuel, clean air and correct injection matter most. Know the fuel, lubrication, cooling and air systems, and the daily checks for each.
• Diesel smoke coloursBlack smoke often means too much fuel for the air: overload, a blocked air filter or injector faults. White smoke can be unburnt fuel when cold, or water or coolant in the cylinders. Blue smoke means the engine is burning oil.
• Bearings, alignment and beltsBall bearings suit lighter, faster loads; roller bearings carry heavier loads. Too little grease wears them and too much makes them run hot. Misaligned couplings and wrong belt tension cause vibration and early bearing and seal failure.
• Materials: mild steel, stainless, cast iron, aluminiumMild steel is strong, cheap and easy to weld but rusts; stainless steel resists corrosion; cast iron is strong in compression but brittle; aluminium is light and resists corrosion. Know yield strength and hardness, and how paint or galvanising protects steel.
• AutoCAD and SolidWorks (tolerances and fits)AutoCAD for 2D workshop drawings; SolidWorks for 3D parts, assemblies and the drawings made from them. Know tolerances (the allowed variation in a size) and fits: clearance for parts that move, interference for parts pressed together, such as a bearing on a shaft.
• Job cards, maintenance log and min/max stockA job card records each job: machine, fault, work done, parts, hours and who did it. The log gives each machine's history. Min/max levels tell you when to reorder spares. Excel or a CMMS both work if they are kept up to date.

Practical tasks you may get

1. 1Hands-on fault finding in a workshop: a generator, pump or engine with a planted fault. Talk through each check aloud, lock off before you touch anything, and name the tools you need.
2. 2Written test (about 60 minutes): pump head and pressure, BTU to kW, simple power or torque sums, and questions on maintenance intervals. Show the formula and units in every answer.
3. 3Maintenance plan case study, for example three generators and two pumps: build a table in Excel with tasks, intervals by run hours, the person responsible and a spare parts list. Practise by building one from a real manual.
4. 4Drawing test: turn a hand sketch of a part into a dimensioned AutoCAD or SolidWorks drawing in 30–45 minutes.
5. 5Parts and nameplates: identify bearings, seals, filters and gauges, and read a motor or pump nameplate aloud, explaining each number.

Mistakes to avoid

• Talking only about repairs, with no word about prevention, intervals or records.
• Changing parts one after another until the fault goes away, instead of finding the cause.
• Unsafe answers: opening a hot radiator cap, working under a vehicle held only by a jack, or working on a machine that is not locked off.
• Topping up refrigerant without finding the leak, or releasing refrigerant into the air.
• Claiming SolidWorks or a certificate you do not have; panels test software at the computer.
• Ignoring the manufacturer's manual for oil grade, torque values and service intervals.

Quick check

5 questions. Answer each one to see the explanation.