Millwright Certification Bearing Installation, Maintenance, and Failure Analysis 2 — Questions and Answers

Question 1: What is the primary purpose of using an induction heater when installing a bearing on a shaft?

To harden the bearing races
To expand the inner ring for an interference fit (Correct answer)
To remove contamination from the bearing
To anneal the bearing steel

Correct answer: To expand the inner ring for an interference fit

An induction heater uniformly expands the bearing inner ring so it can slide over the shaft without force, preserving the interference fit without damaging the rolling elements.

Bearings with interference fits require the inner ring to be heated (typically 80-120 degrees C above ambient) so it expands enough to slip onto the shaft. Induction heaters provide uniform, controlled heating without open flames. Once the bearing cools, it contracts and grips the shaft tightly. Using a press or hammer on larger bearings risks brinelling the raceways.

Question 2: Which bearing failure mode is characterized by small pits forming on the raceway surface?

Spalling (Correct answer)
Fretting corrosion
Brinelling
Smearing

Correct answer: Spalling

Spalling occurs when subsurface fatigue cracks propagate to the surface, causing small pieces of material to flake away and leave pits on the raceway.

Spalling is the most common end-of-life failure mode for rolling element bearings. It begins with microscopic cracks below the raceway surface caused by cyclic contact stresses. Over millions of revolutions, these cracks grow and eventually reach the surface, causing material to break away in flakes or chips. This is different from brinelling (static overload dents), fretting (micro-motion wear), and smearing (sliding contact damage).

Question 3: When checking bearing clearance with a feeler gauge, what does excessive internal clearance typically indicate?

The bearing is properly seated
The housing bore may be oversized or the bearing is worn (Correct answer)
The shaft is misaligned
Normal break-in has occurred

Correct answer: The housing bore may be oversized or the bearing is worn

Excessive internal clearance suggests the housing bore is too large, the bearing outer ring is worn, or the fit has been lost, all of which reduce load-carrying capacity.

Bearing internal clearance must fall within manufacturer specifications. Excessive clearance can result from an oversized housing bore (machining error or wear), outer ring wear, or improper bearing selection. Too much clearance allows the rolling elements to skid rather than roll, generating heat and accelerating wear. Conversely, too little clearance from a tight housing fit can preload the bearing and cause premature failure.

Question 4: What is the recommended method for removing a press-fit bearing from a shaft without damaging it?

Strike the outer ring with a brass drift
Apply force to the inner ring using a bearing puller (Correct answer)
Heat the outer ring with a torch
Pry behind the bearing with a flat bar

Correct answer: Apply force to the inner ring using a bearing puller

A bearing puller applies controlled axial force to the inner ring (the press-fit ring), preventing damage to the rolling elements and raceways that would occur from hammering or prying.

When removing a bearing, force must be applied to the ring with the interference fit, typically the inner ring on shaft-mounted bearings. A mechanical or hydraulic bearing puller grips behind the inner ring and applies even axial force. Striking the outer ring transmits shock loads through the rolling elements, causing brinelling. Heating the outer ring would loosen the housing fit, not the shaft fit. Prying risks damaging the shaft shoulder and bearing.

Question 5: Which condition would most likely cause a bearing to run hotter than normal during operation?

Using the correct grade of grease
Maintaining proper internal clearance
Over-greasing the bearing housing (Correct answer)
Operating below rated speed

Correct answer: Over-greasing the bearing housing

Over-greasing causes excessive churning of lubricant, which generates heat from internal friction and can lead to seal damage and premature bearing failure.

Over-greasing is one of the most common causes of bearing overheating. When a housing is packed too full, the rolling elements must constantly push through excess grease, generating friction heat. This can raise temperatures enough to break down the grease, damage seals, and ultimately cause bearing failure. Best practice is to fill bearing housings only 30-50% full and use calculated relubrication intervals based on bearing size, speed, and temperature.

Question 6: What does false brinelling in a bearing indicate?

The bearing was overloaded during installation
Vibration caused wear marks while the bearing was stationary (Correct answer)
The raceway was damaged by electrical arcing
Corrosion formed due to moisture ingress

Correct answer: Vibration caused wear marks while the bearing was stationary

False brinelling occurs when external vibration causes micro-movement of rolling elements against raceways while the bearing is not rotating, wearing shallow depressions that resemble true brinelling.

False brinelling (also called fretting wear) affects bearings in stationary equipment subjected to vibration from adjacent machinery. The tiny oscillations between rolling elements and raceways wear away the lubricant film and create shallow, polished depressions. Unlike true brinelling from static overload, these marks show wear patterns rather than plastic deformation. Prevention includes isolating equipment from vibration, periodic shaft rotation, and using vibration-dampening mounts.

Millwright Certification Bearing Installation, Maintenance, and Failure Analysis 2

What is the primary purpose of using an induction heater when installing a bearing on a shaft?