Bearing Fundamentals: Types, Nomenclature, and Standards

What Bearings Do

A bearing is a machine element that supports a rotating or sliding shaft, controls the direction of motion, and transfers load to the surrounding structure. Without correctly selected and maintained bearings, rotating equipment fails rapidly. In industrial maintenance — whether you are working on a conveyor drive, a marine propeller shaft, or a pump — understanding bearing fundamentals directly affects equipment uptime and worker safety.

Before handling any bearing or bearing housing, verify that the machine is de-energized and locked out following your facility's lockout/tagout (LOTO) procedure per OSHA 29 CFR 1910.147. Wear cut-resistant gloves when handling bearings; sharp cage edges and press-fit races can cause lacerations. Eye protection is required when using compressed air to clean bearing housings.

Two Major Families

Rolling-element bearings use balls or rollers to keep the inner and outer rings separated. Friction is low, they are suitable for moderate to high speeds, and they are found everywhere from electric motors to gearboxes.

Plain bearings (sleeve or journal bearings) have no rolling elements — the shaft rides directly on a bearing surface, usually bronze, babbitt metal, or a polymer composite with a lubricating film. They are compact, quiet, and common on marine stern tubes, large pumps, and heavily loaded slow-speed shafts.

Rolling-Element Bearing Types

Deep-groove ball bearing — The most common industrial bearing. Balls ride in deep, uninterrupted grooves. Handles radial load well and moderate thrust load in either direction. Suitable for high speeds.

Angular contact ball bearing — The inner and outer ring raceways are offset so the load line passes through the contact points at an angle (commonly 15°, 25°, or 40°). Designed for combined radial and thrust loads; usually mounted in pairs to handle thrust from both directions.

Self-aligning ball bearing — Has a spherical outer raceway so the bearing can tolerate a few degrees of shaft misalignment. Used where housing bores are hard to align precisely.

Cylindrical roller bearing — Uses cylinders instead of balls, giving a larger contact area and higher radial load capacity than a ball bearing of the same size. Carries radial load primarily; thrust capacity is limited or absent depending on the ring flange arrangement (designations NU, N, NJ, NF, NUP indicate flange configuration).

Tapered roller bearing — Tapered rollers and raceways allow the bearing to handle heavy combined radial and thrust loads. The cone (inner ring) and cup (outer ring) are sold separately. Axial clearance must be set during installation — this is a critical maintenance step.

Spherical roller bearing — Two rows of barrel-shaped rollers in a spherical outer raceway, giving significant self-aligning capability. Used for heavy radial loads with some thrust, and common on industrial fans, conveyors, and marine line shaft bearings.

Needle roller bearing — Very thin rollers of high length-to-diameter ratio. Excellent radial load capacity in a small radial envelope. Common in rocker arms, universal joints, and compact gearbox locations.

Thrust ball bearing — Flat-washer style; handles axial (thrust) load only, not radial. Common on vertical pumps and machine tool spindles.

Thrust roller bearing — Similar to thrust ball but uses rollers for higher load capacity. Tapered-roller thrust and spherical roller thrust variants handle thrust with some moment load.

Plain Bearing Types

Sleeve (journal) bearing — A split or one-piece cylindrical liner. The shaft is supported by a hydrodynamic oil film at operating speed. Requires a continuous, reliable lubricant supply. Common on large motors, marine main propulsion line shafts, and turbines.

Flanged sleeve bearing — A sleeve bearing with an integral flange to locate the shaft axially as well as radially.

Thrust washer / thrust collar — A flat plain bearing surface that reacts axial loads on a plain-bearing shaft.

Load Direction

Understanding load direction is required to select and classify bearings correctly:

• Radial load acts perpendicular to the shaft centerline (the weight of a pulley, for example).
• Thrust (axial) load acts along the shaft centerline (the force from a helical gear or a marine propeller).
• Combined load has both radial and thrust components simultaneously.

Always determine which loads are present before selecting a bearing type.

Nomenclature and Standards

The American Bearing Manufacturers Association (ABMA) and ISO publish interchangeable dimensional standards so that a bearing made by any manufacturer to the same designation number is dimensionally identical. The ABMA designation system for ball bearings assigns a two-digit code (e.g., ABMA 02 series) based on bore and outside diameter relationship. The bore code (last two digits of the base number, multiplied by 5 for codes 04 and above) gives the bore in millimeters. For example, bore code 08 = 8 × 5 = 40 mm bore.

A typical bearing number such as 6308 breaks down as:

• 6 — deep-groove ball bearing type
• 3 — dimensional series (width and OD relationship)
• 08 — bore code (08 × 5 = 40 mm bore)

Suffix letters add detail: C3 means extra internal clearance; ZZ means double metal shield; 2RS means double rubber seal.

Why This Matters for Certification

Both NCCER Industrial Maintenance and MSSC CPT assessments include questions on mechanical components and preventive maintenance. Being able to look at a bearing, state its type, name its parts (inner ring, outer ring, rolling element, cage, seal or shield), and identify the load direction it handles is a foundational competency you will use every day on the job and need to demonstrate during your credential exams.