Millwright Certification Bearing Installation, Maintenance, and Failure Analysis 1 — Questions and Answers
Question 1: When repacking a bearing housing with grease, what is the generally recommended fill level to prevent overheating from churning?
- Fill the housing cavity completely to ensure full lubrication
- One-third to one-half of the free space in the housing (Correct answer)
- Just enough grease to coat the rolling elements and cage
- Two-thirds to three-quarters of the housing cavity
Correct answer: One-third to one-half of the free space in the housing
Overfilling a bearing housing with grease causes the rolling elements to churn through excess lubricant, generating heat and ultimately degrading the grease. One-third to one-half fill allows adequate lubrication while leaving room for grease expansion and circulation.
Question 2: A technician finds a reddish-brown powdery debris between a bearing outer ring and its housing bore after disassembly. This condition is most accurately identified as:
- Spalling caused by subsurface fatigue
- Fretting corrosion from micro-movement between mating surfaces (Correct answer)
- False brinelling from vibration during standstill
- Galvanic corrosion from dissimilar metals
Correct answer: Fretting corrosion from micro-movement between mating surfaces
Fretting corrosion occurs when two clamped surfaces experience very slight oscillatory relative motion, causing micro-welding and tearing that produces the characteristic reddish-brown iron oxide powder. It is common at bearing-to-housing fits and shaft-to-ring fits.
Question 3: When pressing a bearing onto a shaft using a mechanical press, to which surface should the pressing force be applied?
- The outer ring, so force is transmitted through the rolling elements to seat the inner ring
- The cage, to distribute the load evenly across all rolling elements
- Only the inner ring, which is the ring receiving the interference fit (Correct answer)
- Alternately to both rings in small increments to equalize stress
Correct answer: Only the inner ring, which is the ring receiving the interference fit
Press force must be applied only to the ring receiving the interference fit — in this case the inner ring for a shaft press-fit. Transmitting force through the rolling elements causes brinelling (indentations) on the raceways, permanently damaging the bearing.
Question 4: During inspection, a millwright observes a regular corrugated or washboard pattern of shallow grooves running around the circumference of a bearing raceway. What is the most likely cause?
- Misalignment between the shaft and housing during installation
- Electrical current passing through the bearing from a VFD or stray ground (Correct answer)
- Excessive axial thrust load in a radial-only bearing
- Contamination by abrasive particles in the lubricant
Correct answer: Electrical current passing through the bearing from a VFD or stray ground
The corrugated or fluted pattern is the signature damage of electrical current passing through a bearing. Electric discharge machining (EDM) erodes the raceway in a repeating pattern. This is increasingly common with variable frequency drives (VFDs) that produce stray currents through the motor shaft.
Question 5: New bearings that will not be installed for several weeks should be stored by:
- Removing original packaging and applying a fresh coat of light machine oil for inspection
- Keeping them in original manufacturer packaging, stored horizontally in a clean, dry area (Correct answer)
- Standing them vertically on their outer rings to prevent brinelling from static loads
- Removing factory grease and repacking with fresh lubricant graded for the anticipated service
Correct answer: Keeping them in original manufacturer packaging, stored horizontally in a clean, dry area
Bearings should remain in their original sealed packaging until installation. The packaging protects against contamination and moisture. A clean, dry, vibration-free storage environment on flat shelving preserves the factory-applied corrosion protection and lubricant.
Question 6: In the context of angular contact or tapered roller bearings, what does 'preload' mean?
- The maximum dynamic load rating stamped on the bearing by the manufacturer
- A deliberate elimination of internal clearance that places the rolling elements under a compressive load (Correct answer)
- The initial radial force applied with a press to seat the bearing fully on the shaft
- The amount of grease pre-filled into a sealed bearing at the factory
Correct answer: A deliberate elimination of internal clearance that places the rolling elements under a compressive load
Preload removes all internal clearance and introduces a controlled compressive stress on the rolling elements and raceways. This increases bearing rigidity, reduces vibration and noise, and improves accuracy of shaft position — critical in machine tool spindles and precision gearboxes.
Millwright Certification Bearing Installation, Maintenance, and Failure Analysis 1
When repacking a bearing housing with grease, what is the generally recommended fill level to prevent overheating from churning?