Millwright Certification Predictive and Preventive Maintenance 1 â Questions and Answers
Question 1: Which condition monitoring technique uses ultrasonic sound emissions to detect early-stage bearing defects before they appear in vibration spectra?
- Infrared thermography
- Oil debris analysis
- Airborne/structure-borne ultrasound (Correct answer)
- Motor current signature analysis
Correct answer: Airborne/structure-borne ultrasound
Ultrasound detection picks up the high-frequency stress waves (typically 20â100 kHz) produced by microscopic spalling and asperity contact in bearings long before the defect energy is strong enough to register as a classic bearing fault frequency in a vibration spectrum, giving earlier warning than vibration analysis alone.
Question 2: A millwright reviews a vibration spectrum and notices dominant peaks at the ball-pass frequency of the outer race (BPFO) with sidebands at running speed. What does this pattern most likely indicate?
- Misalignment between shaft and coupling
- A localized defect on the bearing outer race (Correct answer)
- Mechanical looseness at the bearing housing
- Mass unbalance in the rotating assembly
Correct answer: A localized defect on the bearing outer race
BPFO harmonics with running-speed sidebands are the classic spectral signature of a localized spall or pit on the outer race. The sidebands appear because the defect impact is modulated by shaft rotation, distinguishing this fault from looseness (which produces sub- and super-harmonics of running speed) or unbalance (pure 1x).
Question 3: What is the purpose of establishing a baseline vibration reading on newly installed or recently overhauled equipment?
- To confirm the equipment meets design speed before startup
- To provide a reference point for trending and detecting future changes in machine condition (Correct answer)
- To satisfy insurance requirements for new machinery
- To calibrate the vibration analyzer to the specific machine
Correct answer: To provide a reference point for trending and detecting future changes in machine condition
A baseline (also called a 'fingerprint') reading captures normal operating vibration when the machine is known to be in good condition. All future readings are compared against this baseline so that gradual changes â which may still fall within absolute alarm limits â are caught through trend analysis before they become failures.
Question 4: During a preventive maintenance inspection, a millwright finds that a v-belt drive has a static deflection greater than specified when tested at mid-span. What corrective action is required?
- Replace the belts immediately with a wider cross-section
- Increase belt tension by adjusting center distance until deflection meets specification (Correct answer)
- Reduce sheave diameter to lower the load on the belts
- Apply belt dressing to restore friction and correct the slack
Correct answer: Increase belt tension by adjusting center distance until deflection meets specification
Static deflection (typically 1/64 in per inch of span) is the standard field method for setting belt tension. Excessive deflection means the belt is under-tensioned, causing slip, heat, and premature wear. The correct fix is to increase center distance (or use the tensioning adjustment on the motor base) until the specified deflection force produces the correct deflection depth.
Question 5: Which lubrication practice is most critical to preventing premature rolling-element bearing failure during a preventive maintenance re-greasing task?
- Using the highest-viscosity grease available to maximize film thickness
- Purging all old grease before refilling to the housing capacity
- Adding grease slowly while the machine runs to distribute it evenly and avoid over-packing (Correct answer)
- Mixing compatible greases from different manufacturers to improve performance
Correct answer: Adding grease slowly while the machine runs to distribute it evenly and avoid over-packing
Over-greasing is one of the leading causes of bearing failure. Running the machine while adding grease slowly allows excess grease to purge through the relief fitting and ensures the grease distributes evenly around the rolling elements. Filling to full housing capacity or mixing incompatible greases can cause churning, heat build-up, and seal damage.
Question 6: A predictive maintenance program uses motor current signature analysis (MCSA). What type of fault is this technique specifically well-suited to detect?
- Outer-race spalling in rolling-element bearings
- Broken or cracked rotor bars in induction motors (Correct answer)
- Misalignment between a pump and its driver
- Cavitation in a centrifugal pump impeller
Correct answer: Broken or cracked rotor bars in induction motors
MCSA analyzes the frequency spectrum of the AC supply current drawn by an induction motor. Broken rotor bars cause characteristic sideband frequencies at (1 Âą 2s)fâ around the supply frequency (where s is slip and fâ is line frequency), making MCSA the standard non-invasive method to detect rotor bar faults without disassembling the motor.
Millwright Certification Predictive and Preventive Maintenance 1
Which condition monitoring technique uses ultrasonic sound emissions to detect early-stage bearing defects before they appear in vibration spectra?