Millwright Certification Bearing Installation, Maintenance, and Failure Analysis 1

Question 1

When repacking a bearing housing with grease, what is the generally recommended fill level to prevent overheating from churning?

Accepted Answer

One-third to one-half of the free space in the housing

Answer

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:

Accepted Answer

Fretting corrosion from micro-movement between mating surfaces

Answer

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?

Accepted Answer

Only the inner ring, which is the ring receiving the interference fit

Answer

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?

Accepted Answer

Electrical current passing through the bearing from a VFD or stray ground

Answer

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:

Accepted Answer

Keeping them in original manufacturer packaging, stored horizontally in a clean, dry area

Answer

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.

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Millwright Certification Bearing Installation, Maintenance, and Failure Analysis 1