What Is a Bearing? The Complete A-to-Z Guide for Mechanical Engineers

A bearing is a precision machine element that supports loads and reduces friction between moving parts by replacing sliding friction with rolling friction through intermediate rolling elements.

Bearings enable shafts to rotate freely with friction coefficients as low as 0.001–0.005 — ten to fifty times lower than plain bushings (SKF Rolling Bearings Catalogue). Global industry consumes over 15 billion bearings annually, spanning applications from 0.37 kW food-processing motors to 8 MW offshore wind turbines (Schaeffler Annual Report 2023). This article covers the full technical scope of rolling bearings for mechanical engineers and maintenance professionals.

Rolling Bearing vs. Plain Bearing — Terminology

Two fundamentally different machine elements share the word "bearing." A rolling bearing (ổ lăn) uses balls or rollers between hardened raceways. A plain bearing (bạc trượt) relies on a sliding film of lubricant between shaft and bore surface. This article focuses exclusively on rolling bearings, defined by ISO 15190.

In Vietnamese, rolling bearings are called vòng bi (northern dialect) or bạc đạn (southern dialect). Both terms refer to the same component. No technical difference exists between the two names.

English terminology hierarchy: bearing (generic) → rolling bearing (formal ISO term) → ball bearing (uses spherical elements) → roller bearing (uses cylindrical, tapered, spherical, or needle elements).

How a Rolling Bearing Works

A rolling bearing operates on one principle: replacing sliding friction with rolling friction. As the shaft rotates, rolling elements — balls or rollers — roll between two raceways. The inner ring attaches to the shaft. The outer ring sits in the housing.

Rolling friction is far lower than sliding friction because the contact area between rolling elements and raceways is extremely small. Ball bearings create point contact — in reality, a tiny Hertzian ellipse due to elastic deformation. Roller bearings create line contact — larger area but still far smaller than a plain bearing surface.

Friction Coefficients by Bearing Type

Source: SKF Rolling Bearings Catalogue, estimated coefficients under normal lubrication conditions.

Basic Friction Torque Equation

Total friction torque M of a bearing comprises two main components: load-dependent torque and speed-dependent torque.

• M₁ = f₁ × F × d_m — load-dependent component, where f₁ depends on bearing type and load ratio, F is the equivalent load, and d_m is the mean diameter.
• M₀ = f₀ × (ν × n)^(2/3) × d_m³ — speed/viscosity-dependent component, significant only at high speeds.

At a cement plant in Hai Phong, Vietnam, replacing plain bushings with NU 2220 cylindrical roller bearings on an induced-draft fan reduced housing temperature from 85°C to 55°C and motor current by 8%.

Bearing Classification — 8 Main Types

Rolling bearings divide into two major groups: ball bearings using spherical rolling elements, and roller bearings using cylindrical, tapered, barrel, or needle-shaped rolling elements. Each type has distinct load capacity and speed characteristics.

Classification Overview Table

1. Deep Groove Ball Bearing (DGBB)

The most common type, accounting for approximately 30% of global bearing production. Carries combined radial and bidirectional axial loads simultaneously. Highest speed limit among rolling bearing types.

Example: the 6205 bearing has d = 25 mm, D = 52 mm, B = 15 mm, C = 14.8 kN. This is the single most popular bearing designation on 1.5–5.5 kW electric motors in Vietnam.

Read more: Deep Groove Ball Bearings — construction, specs, applications

2. Cylindrical Roller Bearing (CRB)

Radial load capacity 50–100% higher than ball bearings of the same envelope size, thanks to line contact. Suited to heavy radial loads, moderate-to-high speeds, and shock conditions.

Example: NU 2210 has C = 88 kN, while ball bearing 6210 with the same 50 mm bore reaches only C = 35.1 kN.

Read more: Cylindrical Roller Bearings — when to choose CRB over DGBB

3. Tapered Roller Bearing (TRB)

Carries combined radial and single-direction axial loads through its tapered contact geometry. Always mounted in opposing pairs to handle bidirectional axial loads. Dominant in vehicle axles and industrial gearboxes.

Example: 30207 has d = 35 mm, D = 72 mm, B = 17 mm, C = 56 kN.

Read more: Tapered Roller Bearings — design, mounting, clearance adjustment

4. Needle Roller Bearing

Uses rollers with a length-to-diameter ratio ≥ 3:1. Extremely thin radial cross-section — ideal for space-constrained applications. Many variants require no dedicated inner ring, using the shaft surface as the inner raceway.

Read more: Needle Roller Bearings — applications and design

5. Self-Aligning Bearing

Includes self-aligning ball bearings and spherical roller bearings (SRB). The outer ring has a spherical raceway, allowing misalignment compensation up to 2–3°. Essential for applications with long shafts, widely spaced supports, or imperfect alignment.

Example: 22228 EK/C3 has d = 140 mm, D = 250 mm, B = 68 mm, C = 580 kN — widely used in crushers, industrial fans, and vibrating screens.

Read more: Self-Aligning Bearings | Spherical Roller Bearings

6. Angular Contact Ball Bearing (ACBB)

Offset raceways create contact angles of α = 15°, 25°, or 40°. Superior single-direction axial load capacity compared to DGBB. Typically mounted in pairs or triplets for CNC machine-tool spindles.

Read more: Angular Contact Ball Bearings — design and arrangement

7. Thrust Bearing

Carries axial loads exclusively — no radial load capacity. Includes thrust ball bearings (series 511xx/512xx) and thrust roller bearings (series 292xx). Applications: rotary tables, screw jacks, crane slewing rings.

Read more: Thrust Bearings — design and applications

8. Spherical Roller Bearing (SRB)

Uses barrel-shaped rollers on a spherical outer raceway. Combines very heavy radial load capacity with self-aligning capability. The primary bearing choice for crushers, vibrating screens, and mining equipment.

Read more: Spherical Roller Bearings — specifications and applications

Key Technical Specifications

Engineers must master five core parameters when selecting bearings: dynamic load rating C, static load rating C₀, basic rating life L₁₀, radial internal clearance, and tolerance class.

Basic Dynamic Load Rating C

The constant load under which a bearing achieves a basic rating life of one million revolutions (10⁶ rev) with 10% probability of failure, per ISO 281. Higher C means longer life under the same operating load.

Basic Static Load Rating C₀

The radial load producing a total permanent deformation of 0.0001 × rolling element diameter at the most heavily loaded contact point. Critical when bearings are stationary, oscillating, or rotating at extremely low speeds (< 10 rpm) under shock loads.

Basic Rating Life L₁₀

The number of million revolutions (or operating hours) that 90% of bearings in a batch will complete before first signs of fatigue appear.

L₁₀ Formula:

• L₁₀ = (C / P)^p × 10⁶ revolutions
• Where p = 3 for ball bearings, p = 10/3 for roller bearings
• P = equivalent dynamic load

Conversion to hours:

• L₁₀h = L₁₀ / (60 × n) — where n is the rotational speed (rpm)

Calculation example: Bearing 6205 (C = 14.8 kN), load P = 3 kN, speed 1,450 rpm:

• L₁₀ = (14.8 / 3)³ × 10⁶ = 119,095 × 10⁶ rev ≈ 119 million revolutions
• L₁₀h = 119 × 10⁶ / (60 × 1,450) = 1,369 hours (basic rating life)

Actual service life (L₁₀m per ISO 281:2007) is 3–10× longer due to modification factors a₁ (reliability) and a_ISO (lubrication and contamination conditions). Read more: Bearing Life Calculation.

Quick-Reference Specification Table

Radial Internal Clearance

Radial internal clearance is the total radial displacement permitted between inner and outer rings before mounting. ISO 5753-1 defines five clearance groups: C2, CN (Normal), C3, C4, C5.

Electric motors account for 70% of industrial bearing demand in Vietnam, and C3 clearance is the industry standard. The reason: during operation, the inner ring runs 10–20°C hotter than the outer ring. Thermal expansion reduces clearance. C3 ensures positive residual clearance after expansion — preventing preload that causes overheating and premature failure.

Read more: Bearing Clearance — when to use C2, C3, C4

Tolerance Class

ISO 492 defines five tolerance classes: P0 (standard), P6, P5, P4, P2 (highest precision). Higher precision = tighter dimensional tolerances, lower runout, higher cost.

Over 90% of industrial bearings in Vietnam use P0 class. P6 serves motors requiring low vibration. P5 and above are reserved for machine-tool spindles.

Read more: Bearing Tolerance Classes — ISO 492 explained

How to Read Bearing Designation Codes

A bearing designation encodes all technical information into a compact alphanumeric string. Mastering the code structure allows engineers to determine specifications on sight, without consulting a catalog.

Decoding Example: 6308-2RS/C3

Result: 6308-2RS/C3 = deep groove ball bearing, 40 mm bore, two rubber seals, C3 clearance.

Decoding Example: 22228 EK/C3

Bore Code Rules

• Bore code 00 = 10 mm
• Bore code 01 = 12 mm
• Bore code 02 = 15 mm
• Bore code 03 = 17 mm
• Bore code 04 and above: bore code × 5 = bore (mm). Example: 08 × 5 = 40 mm, 28 × 5 = 140 mm.

Common Suffixes

Read more: How to Read Bearing Codes — from 6205 to 22328

Bearing Construction — 5 Principal Components

Every rolling bearing consists of five basic components: inner ring, outer ring, rolling elements, cage (retainer), and seals or shields.

Inner and Outer Rings

Two raceways manufactured from 100Cr6 bearing steel, through-hardened to 58–64 HRC, with superfinished raceway surfaces achieving Ra 0.02–0.1 μm. The inner ring mounts on the shaft; the outer ring seats in the housing. Raceway geometry defines bearing type — circular-arc grooves for ball bearings, flat raceways with ribs for cylindrical roller bearings, tapered raceways for tapered roller bearings.

Rolling Elements

Balls made from 100Cr6 steel or Si₃N₄ ceramic (hybrid bearings). Ceramic balls are 60% lighter, harder, and electrically insulating — ideal for variable-frequency-drive motors to prevent shaft current damage. Cylindrical, tapered, and needle rollers use standard 100Cr6 steel.

Cage (Retainer)

Maintains uniform spacing between rolling elements, preventing mutual contact. Common materials: pressed steel cage (suffix J), machined brass cage (suffix M), and glass-fiber-reinforced polyamide (suffix TN or P). Polyamide cages are lightweight, reduce inertia, and suit high speeds — but are limited to 120°C continuous temperature.

Seals and Shields

• Rubber seals (2RS): Contact the inner ring. Effective against dust and moisture ingress. Lower speed limit due to contact friction.
• Metal shields (2Z): Non-contact design. Block coarse particles only. Higher speed limit than rubber seals.

Common question: What is the difference between 2RS and 2Z? The 2RS seal is water-tight, suited to dirty or humid environments — but generates more friction and heat. The 2Z shield allows airflow, suited to clean environments and higher speeds.

Read more: Bearing Construction — analysis of 5 components

Bearing Materials

Material selection determines fatigue life, temperature capability, and speed limits. Two primary material families: bearing steel and ceramic.

100Cr6 Bearing Steel (AISI 52100)

The standard material for over 95% of rolling bearings worldwide, per ISO 683-17. Chemical composition: C 0.95–1.10%, Cr 1.30–1.60%, Mn 0.25–0.45%, Si 0.15–0.35%.

Heat treatment process: austenitize at 830–850°C → oil quench → temper at 160–180°C → achieve 58–64 HRC. Required microstructure: tempered martensite, uniformly distributed carbides, retained austenite ≤ 12%.

Bearings with the S suffix (stabilized) undergo additional tempering at 200–250°C to reduce retained austenite below 5% — stabilizing dimensions when operating above 120°C, at the cost of 1–2 HRC.

Si₃N₄ Ceramic (Silicon Nitride)

Ceramic balls have a density of 3.2 g/cm³ versus 7.8 g/cm³ for steel — 60% lighter. Harder than steel (HV 1,500 vs. HV 700). Low thermal expansion coefficient. Electrically insulating — breaks the conductive path between shaft and housing.

Hybrid bearing applications (ceramic balls + steel raceways): CNC spindles above 20,000 rpm, VFD motors requiring shaft current protection. Cost is 3–5× higher than all-steel bearings.

Material Comparison Table

Read more: Bearing Materials — 100Cr6 vs Si₃N₄ vs stainless steel

Bearing Lubrication — Grease and Oil

Lubrication accounts for 40–50% of actual bearing service life, per SKF Lubrication Guidelines. Inadequate or incorrect lubrication is the leading cause of premature bearing failure.

Grease Lubrication

Over 90% of industrial bearings use grease. Grease consists of a base oil, thickener, and additives. Three common grease types:

Grease fill volume: 30–50% of bearing cavity for normal speeds, 15–30% for high speeds. Overfilling causes churning, raises temperature, and shortens life. At a paper mill in Binh Duong, Vietnam, a technician filled an exhaust fan bearing cavity to 100% — housing temperature climbed from 65°C to 95°C within two hours, forcing an unplanned shutdown to drain excess grease.

Relubrication Intervals

SKF formula for estimating relubrication interval (hours):

t_f = k × [(14 × 10⁶) / (n × √d_m)] − 4 × d_m

Where: k = bearing type factor (1.0 for DGBB, 0.4 for TRB), n = rpm, d_m = mean diameter (mm).

Example: 1,450 rpm motor, bearing 6308 C3 (d_m = 65 mm), temperature 70°C:

• t_f = 1.0 × [(14 × 10⁶) / (1,450 × √65)] − 4 × 65 ≈ 937 hours (basic estimate)
• In practice, sealed-for-life motors with polyurea grease achieve 8,000–12,000 hours before bearing replacement.

Oil Lubrication

Used when speeds are very high (n × d_m > 500,000), temperature exceeds 150°C, or active heat removal is required. Methods: oil bath, oil mist, oil jet, oil-air.

Common Bearing Failure Modes

Early identification of failure modes enables accurate root-cause analysis and prevents recurrence. According to the NSK Bearing Failure Analysis Guide, six primary failure modes account for over 95% of real-world cases.

Six Primary Failure Modes

When Should You Replace a Bearing?

Three primary warning signals:

1. Abnormal noise: Squealing (grease starvation), clicking (cage damage), humming (uniform wear).
2. Increased vibration: Measure with an accelerometer — if velocity exceeds 4.5 mm/s (ISO 10816-3 Zone C), investigate immediately.
3. Housing temperature rise: An increase exceeding 15°C above normal baseline warrants inspection. Electric motor bearings typically operate at 60–80°C at the housing. If temperature exceeds 95°C, shut down and inspect.

Bearing Brand Landscape

The global industrial bearing market is dominated by six major brands: SKF (Sweden), FAG/Schaeffler (Germany), Timken (USA), NTN (Japan), NSK (Japan), and ZVL (Slovakia). Each has distinct strengths.

Tier 1 Brand Comparison Table

ZVL — European Quality Available in Vietnam

ZVL manufactures at Kysucké Nové Mesto, Slovakia, under full European EN/ISO standards. Technical specifications match other Tier 1 brands: ZVL 6205-2RS carries C = 14.8 kN, equivalent to SKF 6205-2RSH. The key differentiator: ZVL offers significantly more competitive pricing than SKF and FAG, driven by Eastern European manufacturing costs — not by lower quality.

ZVL provides a complete product range: deep groove ball bearings, spherical roller bearings, tapered roller bearings, cylindrical roller bearings, and angular contact ball bearings.

Choosing a Brand

At a steel production facility in Ba Ria–Vung Tau, Vietnam, the maintenance department ran a parallel trial of ZVL 22220 EK/C3 and SKF 22220 E/K/C3 on identical blast-furnace induced-draft fans. After 14 months of monitoring, actual service life and operating temperature showed no significant difference. Procurement cost decreased substantially with ZVL, while operational performance remained equivalent.

Brand selection should be based on: C/C₀ ratings matching actual loads, verifiable genuine origin, and post-sale technical support — not brand name alone. All Tier 1 brands (ZVL, SKF, FAG, Timken, NTN, NSK) meet quality requirements for standard industrial applications.

Read more: SKF vs FAG vs NSK Comparison | Japanese vs European Bearings

Bearing Selection Guide — 7-Step Process

Correct bearing selection follows a strict sequence: operating conditions → bearing type → size → clearance → tolerance class → lubrication → brand. Skipping any step risks a mismatch.

Step 1: Determine Loads

Measure or calculate radial load (Fr) and axial load (Fa) at each bearing position. Load sources: rotor weight, belt tension, gear forces, centrifugal forces.

Step 2: Determine Speed

Rotational speed (rpm) directly influences bearing type selection and lubrication method. Ball bearings suit high speeds. Tapered roller bearings have lower speed limits.

Step 3: Select Bearing Type

Use the decision table:

Step 4: Select Size (Calculate Required C)

From the desired L₁₀h life and equivalent dynamic load P, back-calculate the required C:

C = P × (L₁₀h × 60 × n / 10⁶)^(1/p)

Select a bearing designation with catalog C ≥ required C.

Step 5: Select Clearance

• Press-fit inner ring: clearance reduces → select C3 or C4
• Temperature differential inner–outer > 10°C: select C3
• Standard application: CN
• Electric motors: C3 (industry standard)

Step 6: Select Lubrication

• Normal speed, scheduled maintenance: lithium complex grease
• Sealed-for-life motors: polyurea grease
• High speed, n × d_m > 500,000: oil
• Wet/humid environment: calcium sulfonate grease

Step 7: Select Brand

Prioritize Tier 1 brands (ZVL, SKF, FAG, Timken, NTN, NSK) with ISO 9001 certification and verifiable origin. Compare C/C₀ values across brands — ratings should be equivalent. Select the brand with appropriate technical support and delivery capability in your region.

Read more: How to Select the Right Bearing — A to Z process

Cross-Reference Table Between Brands

Maintenance engineers frequently need equivalent designations when substituting bearings from one brand to another. All Tier 1 brands manufacture to the same ISO standards, so boundary dimensions (d, D, B) are identical for the same base designation — only internal suffixes differ.

Common Cross-Reference Designations

Note: Suffixes differ across brands, but d × D × B dimensions and C load ratings are equivalent among Tier 1 manufacturers.

Read more: Bearing Cross-Reference Table — complete interchange guide

The Vietnamese Bearing Market

The Vietnamese bearing market has distinct characteristics that engineers should understand when selecting products and suppliers.

Demand by Industry

Electric motors represent the largest segment — approximately 70% of industrial bearing consumption. Pumps and fans follow at 15%, gearboxes and power transmission at 10%, and other applications at 5%.

Best-selling bearing designations in Vietnam: 6205, 6206, 6207, 6305, 6306, 6308, 6309, 6310 — all deep groove ball bearings in the 62xx and 63xx series for electric motors.

Counterfeit Product Issues

Counterfeit bearings are a serious problem in the Vietnamese market. Fakes typically carry SKF, FAG, or NSK markings but are manufactured with inferior steel, improper heat treatment, and loose tolerances. The consequence: service life reaching only 10–20% of genuine products, causing unplanned shutdowns.

Identification methods: inspect laser markings (genuine bearings show sharp, uniform engraving), verify packaging (barcodes must scan correctly), and demand certificates of origin. Purchase from authorized distributors with manufacturer appointment letters.

Read more: How to Identify Fake Bearings — 7 telltale signs

Trends in Vietnam

Three trends worth watching over the next 3–5 years:

1. Predictive maintenance: Vibration and temperature sensors monitoring bearings online, detecting degradation 2–6 months before replacement is needed.
2. Hybrid bearings for VFD motors: Shaft currents from variable-frequency drives cause electrical erosion on standard bearings. Ceramic hybrid bearings eliminate this failure mode entirely.
3. Shift from cheapest-price to lifecycle-cost purchasing: Professional facilities are calculating total lifecycle cost instead of purchase price alone. Tier 1 bearings cost more upfront but last 3–5× longer than generic alternatives.

Application Examples by Industry

Electric Motors

Electric motors represent the most common bearing application. Two-pole motors (2,900 rpm) use deep groove ball bearings at both ends. Four-pole motors (1,450 rpm) and six-pole motors (960 rpm) typically use a deep groove ball bearing at the non-drive end and either a DGBB or CRB at the drive end.

Standard bearing assignments for 4-pole motors:

Read more: Motor Bearings — designation selection, clearance, lubrication

Centrifugal Pumps

Pumps commonly use a ball bearing + ball bearing or ball bearing + cylindrical roller bearing pair. The pump (wet) end carries axial thrust from fluid pressure — requiring angular contact ball bearings or deep groove ball bearings with axial load capacity. C3 clearance for pumps operating above 60°C.

Conveyors

Conveyor idler shafts use deep groove ball bearings with 2RS seals due to dusty environments. Series 6204–6208 are most common. Loads are light, but effective sealing is critical — bearings fail from contamination ingress far more often than from fatigue.

Crushers and Vibrating Screens

Spherical roller bearings (SRB) series 222xx or 232xx handle the severe operating conditions. Heavy shock loads and structural misalignment demand bearings that compensate 2–3° misalignment while carrying very heavy radial loads. C3 or C4 clearance.

Example: A ball mill at a cement plant in Ninh Binh, Vietnam uses 22336 EK/C3 on the main trunnion — radial load 180 kN, speed 16 rpm, planned replacement interval 18 months.

Correct Bearing Mounting Practices

Incorrect mounting causes premature bearing failure in many real-world cases across Vietnam. Three errors account for the majority of mounting-related failures:

Error 1: Direct Hammer Blows

Hammering transmits force through the rolling elements, creating permanent indentations (brinelling) on the raceways. Correct method: use a mounting tube positioned against the tight-fit ring, and apply force with a hydraulic press or lock nut.

Error 2: Skipping Preheating for Interference Fits

Bearings with bore > 50 mm and interference fit require heating to 80–110°C (never exceeding 120°C) using an induction heater before mounting. Heating with a torch produces uneven temperatures and risks localized overheating.

Error 3: Incorrect Orientation

Tapered roller bearings and angular contact ball bearings have a fixed mounting direction. Reversing them eliminates axial load capacity. Failure occurs immediately upon operation.

Mounting Checklist

Relevant ISO Standards for Bearings