3.2 Introduction to Bearings

Why Bearing Fundamentals Matter on the Exam

Module 15209, Introduction to Bearings, is a Millwright Level 2 module (15 curriculum hours) that lays the vocabulary foundation for everything the exam asks about bearings — and bearings show up constantly across the test, not just in the 8.0%-weighted Bearings and Fasteners domain. Bearing terminology bleeds into pump questions (bearing housings, thrust loads from impellers), alignment questions (bearing clearance and shaft centerlines), and troubleshooting questions (bearing failure signatures in vibration analysis). Get the basic bearing taxonomy wrong here and it costs points across the whole exam, not just one domain.

What a Bearing Actually Does

A bearing supports a rotating or sliding shaft, reduces friction between moving surfaces, and locates the shaft precisely relative to a housing or frame. Every bearing does some combination of three jobs: carry radial load (perpendicular to the shaft), carry axial/thrust load (along the shaft's centerline), and/or allow rotation with minimal friction and heat.

Two Fundamentally Different Friction Mechanisms

Plain (sleeve/journal) bearings have no rolling parts at all. The shaft rides on a thin hydrodynamic film of oil that develops between the shaft and a smooth, often babbitt-lined sleeve once the shaft is turning fast enough to "float" on the film. There is no metal-to-metal contact in normal operation — friction is fluid friction, not sliding friction. Plain bearings are common on large, slow-to-moderate-speed equipment like some pump and turbine main bearings.

Anti-friction (rolling-element) bearings use hardened balls or rollers riding between an inner race (against the shaft) and an outer race (against the housing). Contact is rolling, not sliding, which is why these bearings tolerate a much wider speed range and need far less lubricant volume than a plain bearing — but they concentrate load onto small contact points or lines, which is exactly why fatigue failures like spalling occur over time.

The Rolling-Element Bearing Family

A guide bearing is a special case worth knowing by name: it centers and positions a shaft radially with very light load, while a separate dedicated thrust bearing (often mounted elsewhere on the same shaft train) carries the actual axial load. This split arrangement shows up on vertical shafts, such as vertical pumps, where gravity and process thrust must be handled by one bearing while a lightly loaded guide bearing simply keeps the shaft centered.

Mounted Bearing Units

Millwrights rarely install a bare bearing race by itself in the field — they typically work with a pre-packaged mounted bearing unit that combines a bearing (often a self-aligning ball or spherical roller insert) with a housing:

• Pillow block bearing — mounted to a flat surface with bolt holes on either side of the bearing centerline; the most common mounted bearing style on conveyors and general drive shafts.
• Flanged (flange) bearing — mounted through a flat face (commonly 2-bolt or 4-bolt patterns) to a vertical or angled surface, such as the side of a frame.
• Take-up bearing — mounted in a slide rail or take-up frame that allows the whole bearing (and the shaft it carries) to be repositioned, most often used to tension a conveyor belt or chain as it stretches over time.

Bearing Designation (Numbering) Systems

The exam expects familiarity with the ABMA (American Bearing Manufacturers Association) numbering convention used to size and specify bearings. The key rule tested most often is the bore code:

• For bore codes 04 and above, multiply the last two digits of the number by 5 to get the bore diameter in millimeters (example: a bearing number ending in "08" has a 40 mm bore, since 8 x 5 = 40).
• Codes 00, 01, 02, 03 are special cases that do not follow the x5 rule: 00 = 10 mm, 01 = 12 mm, 02 = 15 mm, 03 = 17 mm.
• Additional digits and letters ahead of the bore code identify the bearing series (light, medium, or heavy cross-section, which drives load capacity) and type (deep-groove ball, angular contact, spherical roller, and so on).

Fit Selection: Interference vs. Clearance

Bearing fits are chosen based on which race rotates relative to the load: the race that rotates relative to the applied load (usually the inner race on a shaft) gets an interference (press) fit so it cannot creep or spin loose against the shaft. The race that stays stationary relative to the load (usually the outer race in the housing) gets a clearance (slip) fit so it can be installed and removed without excessive force and can accommodate slight thermal growth. Getting this backward — a loose inner race, or an over-tight outer race that can't float with thermal expansion — is a common root cause of premature bearing failure that the exam tests through failure-diagnosis scenarios.

Exam Traps to Watch

• Confusing ball bearings (point contact, higher speed tolerance) with roller bearings (line contact, higher radial load capacity) — the exam tests which geometry suits which load/speed combination.
• Forgetting that spherical roller bearings are specifically valued for self-aligning tolerance to shaft misalignment, unlike cylindrical or tapered roller types.
• Misreading the ABMA bore code special cases (00-03) as if they followed the same x5 rule as 04 and above.