Bearings Taxonomy — Family Index
A consolidated reference covering the major bearing families — rolling-element, plain/journal, fluid-film, magnetic, and specialty — with ISO designation conventions, suffix codes, materials, lubrication, and selection heuristics. SI units throughout. Designations cite real product lines from SKF, Schaeffler (FAG/INA), Timken, NSK, NTN, Koyo/JTEKT, NACHI, THK, HIWIN, igus, Kaydon, Kingsbury, Waukesha, Garrett, Capstone, and S2M.
1. At a glance
Bearings constrain relative motion between machine elements while reducing friction. They split into five fundamentally different load-transfer mechanisms:
- Rolling element (RE) — balls or rollers between concentric races. Low starting torque, predictable L10 fatigue life, ubiquitous. ~80 % of all motor and gearbox bearings.
- Plain / journal — sliding contact in a bushing, usually with boundary or mixed-film lubrication. Quiet, high load density, tolerant of contamination if sealed.
- Fluid-film — full hydrodynamic or hydrostatic separation of journal and bush by a pressurised oil/air film. Effectively zero wear at speed; required for turbomachinery >10 MW.
- Magnetic — active or passive magnetic suspension, no mechanical contact. Used where lubrication is impossible (turbomolecular pumps, oil-free chillers).
- Jewel — sapphire/ruby pivot in a polished cup. Used in watches, instrument gimbals, galvanometers.
Standards governing the field:
- ISO 15 — rolling bearing boundary dimensions for radial bearings (bore × OD × width series).
- ISO 281 — dynamic load rating and rating life L10 calculation.
- ISO 76 — static load rating C0.
- ISO 5753 — radial internal clearance.
- ISO 492 — tolerances for radial rolling bearings (P0/P6/P5/P4/P2 classes).
- ABMA Standard 9 / 11 — American Bearing Manufacturers Association equivalents (Std 9 ball, Std 11 roller) for load ratings; cross-referenced to ISO 281 since 1990.
- DIN 620 — German tolerance class definitions (legacy, now harmonised with ISO 492).
Mean diameter × speed parameter n_dm = (D + d)/2 × n_rpm is the universal speed metric — grease-lubed RE bearings typically operate at n_dm ≤ 500 000 mm·rpm; oil-jet ceramic-hybrid can reach 3 × 10⁶ mm·rpm.
2. Rolling-element types
2.1 Ball bearings
- Deep-groove ball (DGB), 6xxx series — workhorse radial bearing. Conrad-assembly. Modest combined axial load. SKF 6204, FAG 6206-2RS1, NSK 6308DDU. Open / shielded (Z/2Z) / sealed (RS/2RS) variants.
- Angular-contact ball, 7xxx series — contact angle 15° / 25° / 30° / 40° (suffix B/AC/A — varies by maker). Single-direction axial load, paired for both directions. SKF 7008CD/P4ADGA (matched pair), NSK 7212BWG. Used in machine-tool spindles, pump duplex sets, automotive wheel hubs (older designs).
- Self-aligning ball, 1xxx / 2xxx series — two rows on a single spherical outer raceway. Tolerates ±2–4° misalignment. Light loads only. SKF 1207 ETN9, FAG 2308-K-TVH-C3. Common in agricultural shafting and long-shaft conveyors.
- Four-point contact (QJ series) — single-row, split inner ring, takes axial in both directions. Used in gearbox layshafts. SKF QJ305 MA.
- Thrust ball, 51xxx (single) / 52xxx (double) — pure axial only, low speed. SKF 51208. Watch-crown / lazy-Susan duties.
2.2 Cylindrical roller bearings
- NU — flangeless inner, two-flanged outer; allows axial float (locating-free).
- N — flangeless outer, two-flanged inner.
- NJ — one inner flange; locates shaft in one axial direction.
- NUP — NJ with loose flange ring; locates in both directions, used as a fixed end.
- NF / NCF / NJ-HJ — variants with retained flange ring or auxiliary HJ angle ring.
- Full-complement (NCF, SL) — no cage, more rollers, ~40 % higher C, but speed-limited. SKF NCF2934.
- Multi-row cylindrical (FC / FCD / FCDP) — rolling-mill backup rolls; >1 m bore.
2.3 Tapered roller bearings (TRB)
- Single-row Timken-numbering: 4-digit cone (e.g. 30206, HM218248) + cup. ISO standard inch series LM/HM/JLM/JM.
- Double-row TDO / TDI — cup outer or cone inner shared; preloaded face-to-face / back-to-back. Wheel hubs (TDO), pinion supports (TDI).
- Four-row — rolling-mill chocks, pass-line rolls.
- Contact angles 10°–30°; classic Timken 06200 series ≈ 14°.
2.4 Spherical roller bearings (SRB), 2xxxx / 222xx / 223xx / 240xx / 241xx
Two rows of barrel rollers on a single concave outer raceway → self-aligning ±0.5–2°. Highest load capacity per envelope; tolerates shaft deflection. SKF Explorer 22220 EK + H320 adapter sleeve. Used in mill stands, paper machines, vibrating screens (CC suffix for vibration duty).
- CARB toroidal, C-series (SKF only, 1995) — long roller, self-aligning and axial float in one component. C2208, C3032.
2.5 Spherical roller thrust, 294xx / 293xx / 292xx
Single row of asymmetric rollers at 45°, axial + low radial. 294xx = high-load series. SKF 29440 E. Wind-turbine yaw / pitch, hydro turbine thrust.
2.6 Needle roller bearings
- Drawn-cup HK / BK — thin-wall, low cost; HK is open-end, BK closed-end. INA HK1210.
- Machined-ring NA / NKI — heavy duty with inner race.
- Full-complement RNA / NCF — no cage, max capacity, low speed.
- Yoke / stud cam followers — NUKR / KR / KRV — track-roller for cams and linear runways. INA NUKR47.
- Needle thrust AXK / AS — axial low-profile, planetary gearsets.
2.7 Crossed roller bearings
Single-row of alternately crossed cylindrical rollers at 90° — one bearing carries radial + axial + moment. THK RB / RU / RA (RB = separable inner, RU = integrated mounting holes, RA = thin section). IKO CRB / CRBC. Schaeffler XSU / XSI. Robot wrists, optical mounts, indexing tables.
2.8 Thin-section (Kaydon Reali-Slim, INA / Schaeffler XU)
Constant cross-section regardless of bore (1/4” × 1/4” up to 1” × 1”). Robotics, gimbals, medical scanners. Kaydon KA / KB / KC / KD / KF / KG / KH / KAA / KBB.
3. Boundary dimensions (ISO 15)
Radial RE bearings carry a five-digit (or four-digit) basic number where the last two digits encode the bore:
| Digits | Bore (mm) |
|---|---|
| 00 | 10 |
| 01 | 12 |
| 02 | 15 |
| 03 | 17 |
| 04 and up | × 5 (so 20 → 100, 21 → 105 …) |
Bore numbers ≥ 96 use a slash, e.g. 6228/500 = bore 500 mm. The first digit (or two) is the type code (6 = DGB, 7 = ACBB, 2 = SRB, 3 = TRB, etc.). The next digits are dimension series = width series + diameter series.
Worked example: 6204
- 6 — deep-groove ball
- 2 — diameter series 02 (light)
- 04 — bore 20 mm
→ OD 47 mm, width 14 mm.
Worked example: 22220 EK
- 22 — spherical roller, double-row, dimension series 02
- 20 — bore 100 mm
- EK — Explorer geometry, tapered bore K (1:12)
4. Designation suffixes
Suffix order varies by maker; the table below covers SKF / FAG / NSK conventions.
4.1 Closures
- Z / 2Z — one / two metal shields (non-contact, retain grease).
- RS / 2RS / 2RS1 / DDU / LLU — one / two contact lip seals (NBR or HNBR).
- RZ / 2RZ / LB — low-friction non-contact seal.
4.2 Internal clearance (ISO 5753)
| Code | Meaning | Notes |
|---|---|---|
| C2 | Tighter than normal | Low-noise, light interference fits |
| CN / (blank) | Normal | Default |
| C3 | Looser than normal | Most common for electric motors and where some interference or thermal expansion is expected |
| C4 | Looser than C3 | Hot-running gearboxes |
| C5 | Looser than C4 | Heavy heat / interference fits |
4.3 Tolerance class (ISO 492)
| Code | Class | Use |
|---|---|---|
| P0 / (blank) | Normal | General industrial |
| P6 | Higher accuracy | |
| P5 | Precision | Pumps, fans |
| P4 | Super-precision | Machine-tool spindles |
| P2 | Ultra-precision | Grinding spindles |
4.4 Cage codes
- J — pressed-steel cage, ball-guided.
- M / MA / MB / MP — machined brass cage; MA = outer-ring guided, MB = inner-ring guided, MP = roller-guided.
- TN / TN9 / TVH / TVPB — glass-fibre-reinforced polyamide 6,6 (PA66) — temp limit ~120 °C.
- CA / CAC / EC — pressed-steel window cage for spherical / cylindrical rollers.
- PEEK (TB) — high-temp polymer cage, ≥200 °C continuous.
4.5 Special features
- VL0241 / VL2071 (SKF) — outer or both rings ceramic-coated insulating (≥1000 VDC) for VFD-driven motor shafts — prevents EDM fluting.
- HC5 — hybrid ceramic balls (Si₃N₄) with steel races.
- E / EM / ECP — reinforced internal geometry (more or larger rollers, higher C).
- K — tapered bore 1:12 (or 1:30 for big SRBs), mounted via adapter sleeve H / H3 or withdrawal sleeve AH.
- W33 — annular groove + three lubrication holes in outer ring (SRBs).
- DGA / DGB / DGC — universal pre-grade (matched any-pair) ACBBs.
5. Plain bearings (bushings)
5.1 Boundary-lubricated metallic
- SAE 841 Oilite (sintered bronze, oil-impregnated) — porous Cu-Sn-graphite, 18 % oil by volume. Self-fed, PV ≤ 50 000 (psi · ft/min). Beemer Precision / Bunting / GGB. Low cost, throw-away.
- PTFE-lined steel-backed (DU / DX, Glycodur, GGB DU) — steel backing → sintered bronze → PTFE+Pb or PTFE+MoS₂ overlay. Dry-running, maintenance-free, P ≤ 250 MPa static. GGB DU (PTFE-Pb), DP4 (lead-free PTFE/PEEK), DX (greased). Norglide, KS Permaglide P10/P11.
- Aluminum tin (AlSn40) — engine crank/conrod main bearings.
- Babbitt (white metal SnSb8Cu4, PbSn10) — turbomachinery tilting-pad surface, embeds contamination.
5.2 Plastic plain (self-lubricating polymer)
igus iglidur lineup — injection-moulded tribo-polymers with embedded solid lubricant:
- G — general purpose, PV ≤ 0.41 MPa·m/s, –40 to 130 °C.
- J — low coefficient, mating shafts < 50 HRC.
- X — high-temp 250 °C continuous.
- Z — heavy duty / high load, 150 MPa static.
- W300 — long-life rotating.
- A180 / A350 — FDA-compliant for food contact.
Other polymer-plain materials: POM (Delrin / acetal), UHMW-PE, PEEK + PTFE/CF/graphite, PI (Vespel SP-3 / SP-21), PAI (Torlon 4301 / 4501).
5.3 Hydrodynamic plain journal (Sommerfeld regime)
- Plain cylindrical journal — simplest; subject to oil-whip instability above ~2 × first critical.
- Lemon (elliptical) bore — bore machined elliptical → vertical preload → stabilises.
- Multilobe (3-lobe, 4-lobe, offset-halves) — preloaded segments improve stiffness asymmetry, suppress whirl.
- Tilting-pad journal (Mitchell / Kingsbury 1905) — 4 / 5 / 6 pivoted pads; each pad self-aligns to form converging wedge. Industry standard for >MW turbines, large compressors. Waukesha, Kingsbury, Miba, Pioneer Motor Bearing.
5.4 Hydrodynamic thrust
- Plain washer (taper-land or step-pad) — small machines.
- Kingsbury / Mitchell tilting-pad thrust (1905) — 6 / 8 / 12 pads, leading-edge groove; modern designs add directed lube, copper-backed Babbitt, or PTFE-faced pads for shaft-startup robustness. Standard on steam / gas / hydro turbines and large vertical pumps.
6. Fluid-film, advanced
- Hydrostatic — externally pressurised oil supply (~3–10 MPa) lifts the journal before rotation. Used on machine-tool grinding spindles (Hardinge, Mori-Seiki super-precision), large telescope mounts (Mauna Kea Keck 380-ton azimuth ride), radio-telescope bearings, and rolling-mill work rolls.
- Tilting-pad journal — see §5.3. Mitchell (Australia) 1905 patent; Kingsbury independently in US, 1898–1910. Industry standard for turbomachinery; API 670 / 684 design rules.
- Foil air bearing (compliant) — bump-foil + top-foil supports a journal on a self-generated air film at >30 000 rpm. Used in microturbines (Capstone C30/C65/C200, Garrett auxiliary power units), air-cycle machines on aircraft, oil-free turbocompressors (Mohawk Innovative, R&D Dynamics). Lift-off speed ~5 000–10 000 rpm; intermittent solid contact at start/stop wears a sacrificial PTFE-doped polymer top coat.
- Gas-magnetic hybrid — gas film augmented by AMB during startup; experimental for high-temp helium turbines (HTGRs).
- Water-lubricated — rubber-lined (Cutless / Thordon SXL) or nitrile journal bushings for pumps, ship sterntubes, hydro-turbine guide bearings; abundant lubricant, no oil contamination risk.
7. Magnetic bearings
7.1 Active magnetic (AMB)
Electromagnets actively control rotor position via inductive / eddy-current displacement sensors and a digital controller (PID or μ-synthesis). Zero contact, oil-free, vibration-monitoring built in.
- SKF / S2M (France, acquired 2007) — turbomolecular pumps, oil-free compressors, energy-storage flywheels. e-Mag and AMB Generation X.
- Waukesha Magnetic Bearings (Dover) — oil-free industrial compressors.
- Mecos (CH) / Calnetix / Synchrony — chillers (Danfoss Turbocor TT/TG), turboexpanders.
- MAN ESS, Siemens DATUM-C — pipeline compressors.
Limitations: bandwidth-limited (~kHz), require touchdown / catcher bearings (a duplex ACBB pair with 0.1 mm radial clearance) for de-energised landings; failure of power = rotor drops onto catchers.
7.2 Passive magnetic (PMB)
Permanent magnets only, no controller. Earnshaw’s theorem forbids full 3-DOF stable levitation with only PMs, so PMBs partially levitate and combine with a mechanical or AMB axis. Halbach-array PMBs in flywheel energy storage (Beacon Power, Active Power); cryogenic / vacuum applications.
7.3 Hybrid AMB-PMB
Permanent magnets supply bias flux (reducing electromagnet current and copper loss); the AMB modulates around the bias. Used in next-gen flywheels (Boeing Phantom Works concept), in long-life satellite reaction-wheel bearings, and in some heart-assist pumps (HeartWare HVAD, HeartMate III centrifugal).
8. Specialty
- Linear-motion ball (LM guides) — recirculating ball blocks on hardened-steel rails. THK SR / SHS / HSR, NSK NH / NS, Bosch Rexroth R1623, Hiwin HG / EG. Accuracy classes Normal / H / P / SP / UP; preload classes Z0 / ZA / ZB / ZC.
- Linear plain — igus drylin T / W / Q / N, Schaeffler PAR / PFI with FRP rails; food / clean-room / wet.
- Ball screws — precision-rolled (P5–P7 lead) economy class; precision-ground (P1–P3) for machine tools and aerospace actuators. NSK, THK, Bosch Rexroth, Hiwin, Steinmeyer, Korta. Single-nut, double-nut preload, ZF preload, oversize ball preload. Variants: ground-end roller-screw (Rollvis / SKF / GSA) for higher load / longer life.
- Cam followers — NUKR / KR / KRV / KRE stud-type, NUTR yoke-type, eccentric-collar versions. McGill (acquired by Emerson, 1995), INA, IKO.
- Spherical plain (rod-end) — INA GE, SKF GE…ES-2RS, GEH heavy; chrome-on-bronze, PTFE-lined (E-suffix) or steel-on-steel. Aircraft control linkages, hydraulic-cylinder ends.
- Jewel bearings — synthetic sapphire (Al₂O₃) or ruby cups holding a hardened-steel pivot. Wristwatches (Rolex / Patek 17–32 jewels), galvanometers, mechanical compasses, inertial-platform gimbals.
- Slewing rings — large-diameter (0.3–10 m) single-row 4-point, double-row, three-row roller, or crossed-roller assemblies. Rothe Erde, Kaydon, IMO, ThyssenKrupp. Excavators, wind-turbine pitch / yaw, crane base rotation.
9. Comparison table (~30 rows)
Load codes: R = radial, A = axial, M = moment; ★ = strong, ◐ = moderate, ✕ = none.
| # | Type | R | A | M | Speed (n_dm, mm·rpm) | Accuracy | Start friction | Lube | Typical use |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Deep-groove ball 6xxx | ★ | ◐ | ✕ | 500 k | P0–P5 | Very low | Grease/oil | Motors, pumps, fans |
| 2 | Angular-contact 7xxx (single) | ★ | ★ (one way) | ✕ | 1.0 M | P4–P2 | Very low | Oil mist | Spindles |
| 3 | ACBB duplex DB/DF | ★ | ★★ | ◐ | 0.8 M | P4 | Low | Oil mist | Spindle pairs |
| 4 | 4-point QJ | ★ | ★★ | ✕ | 0.6 M | P0–P5 | Low | Grease | Gearbox layshafts |
| 5 | Self-aligning 1/2xxx | ◐ | ✕ | ✕ | 0.4 M | P0 | Very low | Grease | Long shafts |
| 6 | Cylindrical NU / N | ★★ | ✕ | ✕ | 0.6 M | P0–P5 | Low | Oil | Heavy radial + axial float |
| 7 | Cylindrical NJ / NUP | ★★ | ◐ | ✕ | 0.6 M | P0–P5 | Low | Oil | Locating ends |
| 8 | Full-complement NCF | ★★★ | ✕ | ✕ | 0.2 M | P0 | Moderate | Oil | Mill, vibrator |
| 9 | Tapered roller (single) | ★★ | ★ | ✕ | 0.4 M | P0–P5 | Moderate | Oil | Wheel hubs, pinion |
| 10 | TRB double-row TDO | ★★ | ★★ | ◐ | 0.3 M | P0 | Moderate | Oil | Auto hubs |
| 11 | Spherical roller 22/23xxx | ★★★ | ◐ | ✕ | 0.5 M | P0 | Moderate | Oil/grease | Mills, screens |
| 12 | Spherical roller thrust 29xxx | ◐ | ★★★ | ✕ | 0.3 M | P0 | Moderate | Oil | Wind yaw, hydro |
| 13 | CARB toroidal (SKF) | ★★ | ✕ (float) | ✕ | 0.4 M | P0 | Low | Oil | Paper machine |
| 14 | Needle drawn HK | ★ | ✕ | ✕ | 0.2 M | P0 | Moderate | Grease | Compact radial |
| 15 | Needle full-complement | ★★ | ✕ | ✕ | 0.1 M | P0 | High | Oil | Oscillating duty |
| 16 | Thrust ball 51xxx | ✕ | ★ | ✕ | 0.2 M | P0 | Low | Grease | Lazy-Susan |
| 17 | Crossed roller (THK RB) | ★ | ★ | ★ | 0.3 M | P4–P2 | Low | Grease | Robot wrists |
| 18 | Thin-section Kaydon | ◐ | ◐ | ◐ | 0.4 M | P5 | Low | Grease | Robotics |
| 19 | Plain bushing (Oilite) | ★ | ◐ | ✕ | n.a. | — | High | Self-fed oil | Throw-away |
| 20 | PTFE-lined DU/DX | ★ | ◐ | ✕ | n.a. | — | Low–moderate | Dry / grease | Hinges, dampers |
| 21 | iglidur G | ◐ | ◐ | ✕ | n.a. | — | Moderate | Dry | Food, clean |
| 22 | Hydrodynamic plain journal | ★★★ | ✕ | ✕ | unlimited | — | Very high (boundary) → 0 (film) | Oil flood | Big motors |
| 23 | Tilting-pad journal | ★★★ | ✕ | ✕ | unlimited | — | High start, 0 at speed | Oil flood | Turbomachinery |
| 24 | Tilting-pad thrust (Kingsbury) | ✕ | ★★★ | ✕ | unlimited | — | High → 0 | Oil flood | Turbines, hydro |
| 25 | Hydrostatic | ★★★ | ★★★ | ◐ | unlimited | μm-level | 0 (pre-loaded) | Pressurised oil | Grinders, telescopes |
| 26 | Foil air | ★ | ◐ | ✕ | >2 M | — | Solid until lift | Air | Microturbines |
| 27 | Active magnetic AMB | ★★ | ★★ | ◐ | >2 M | μm | 0 | None (gas only) | Turbo pumps, chillers |
| 28 | Passive magnetic (Halbach) | ◐ | ◐ | — | high | — | 0 | None | Flywheels |
| 29 | LM ball block (THK SHS) | linear | — | ★ | linear speed 5 m/s | UP / SP | Low | Grease/oil | CNC axes |
| 30 | Ball screw (NSK precision) | linear thrust | — | — | DN ~140 k | C0–C5 | Low | Grease/oil | Servo positioning |
| 31 | Slewing ring (Rothe Erde) | ★ | ★ | ★★ | n_lim low | — | Moderate | Grease | Cranes, wind pitch |
10. Materials
10.1 Rolling-element / race steels
- AISI 52100 / DIN 100Cr6 / JIS SUJ2 — through-hardened chromium steel, ~58–64 HRC, the universal standard. Carbide-stable to ~150 °C. Cleanliness grades vary: VAR (vacuum arc remelt) > VIM-VAR > ESR > air-melted; SKF Explorer / Timken Premium / NSK EM-LH grades guarantee VIM-VAR + tight oxide-inclusion limits → 2–3× rating life.
- Case-carburised 8620 / SAE 4320 / 17NiCrMo6 — tough core, hard case (60 HRC); used for tapered rollers and large SRB rings where shock toughness matters.
- M50 / AMS 6491 (4Cr4Mo4V) — through-hardened, 0.8 % C, heat-stable to ~315 °C. Standard aerospace mainshaft (jet-engine) bearing.
- M50NiL (case-carburised M50 derivative) — tougher core for hybrid loadings; PW / RR mainshafts.
- 440C / X65Cr13 / SUS440C — martensitic stainless, ~58 HRC, mild corrosion resistance. Medical, food, marine. ~25 % lower C₀ than 52100.
- BG42 / Cronidur 30 (X30CrMoN15-1) — high-N stainless, corrosion + ~58 HRC, used in high-end aerospace and weapons.
10.2 Hybrid and full-ceramic
- Hybrid Si₃N₄ ball + steel race — 60 % lower density → lower centrifugal hoop stress on outer race, 40 % higher dN, electrically insulating, near-immune to micro-pitting under EHD breakdown. Standard in machine-tool spindles and high-speed VFD motors. SKF HC, FAG HCB, NSK HSP.
- Full-ceramic (Si₃N₄ or ZrO₂ races and rollers) — vacuum, semiconductor wet etch, dry-running. Low C; brittle. Cerobear, GMN, Saint-Gobain.
10.3 Cage / retainer
Pressed steel (low C, high T), machined brass (CuZn40Mn — quiet, T ≤ 200 °C), PA66-GF25 polyamide (T ≤ 120 °C), PEEK (T ≥ 200 °C), phenolic-cotton laminate (high speed quiet spindles).
10.4 Seals / closures
NBR (nitrile, std), HNBR (hydrogenated nitrile, –40 to 150 °C), FKM / Viton (–20 to 200 °C, chemicals), PTFE-lipped, all-PTFE labyrinth (chemical pumps), magnetic face seals (Inpro / Garlock — bearing isolators).
11. Sizing — rating life basics
11.1 Dynamic equivalent radial load
For combined R + A loading:
P = X·Fr + Y·Fa
where X, Y come from manufacturer tables that depend on (e·Fa/Fr) crossover (e.g., e ≈ 0.19–0.44 for DGBs, e ≈ 1.5·tan α for ACBB / TRB).
11.2 Rating life L10
ISO 281:
L10 = (C / P)^p × 10^6 [revolutions]
where p = 3 for ball bearings, p = 10/3 for roller bearings, C is the basic dynamic load rating (catalogue), and P is the dynamic equivalent load. L10 in hours:
L10h = L10 / (60 · n_rpm)
Modified life accounting for cleanliness, lubrication regime, and reliability:
Lnm = a1 · aISO · L10
- a1 — reliability factor (a1 = 1 for L10/90 %; a1 = 0.21 for L1/99 %).
- aISO = f(eC · Cu / P, κ) — combines contamination factor eC, fatigue load limit Cu, and viscosity ratio κ = ν/ν1. Capped at 50.
11.3 Static safety
S0 = C0 / P0_max
Recommended S0 ≥ 1 for normal duty, ≥ 4 for shock load / oscillation.
11.4 Speed rating
- n_lim — thermal / cage-stress speed limit, listed in catalogue per lubricant.
- n_dm — practical parameter, see §1; ceramic hybrid + jet oil pushes well past catalogue n_lim.
12. Lubrication
12.1 Grease (Bearings <70 % of grease consumption worldwide)
- NLGI 2 — most common general-purpose consistency (Pen 265–295).
- NLGI 3 — high-temp / vertical-shaft / vibration; less migration.
- NLGI 1 — central-lube pumpable.
- NLGI 0 / 00 / 000 — low-speed open gears, gearcase substitute.
Base oils:
- Mineral (MO) — default, –30 to 120 °C.
- PAO (polyalphaolefin synthetic) — –50 to 150 °C; wide-temp gearbox, automotive sealed-for-life.
- Ester (POE, diester) — high-thermal-stability, miscible additives; aerospace.
- PFPE (perfluoropolyether, e.g. Krytox) — –70 to 260 °C, vacuum-stable, oxygen-compatible, semiconductor and rocket valves. Chemours Krytox GPL, Solvay Fomblin, Dupont Demnum.
- Silicone — wide T range but poor for steel-on-steel EHD; ok in low-load oscillating duty.
Thickeners:
- Li-soap (most common), Li-complex (≥160 °C drop point), Ca-sulphonate (water tolerant), Polyurea (sealed motor bearings, long life, low noise), PTFE (PFPE only), Bentonite (high-T, no drop point).
12.2 Oil
- Bath / sump — large gearboxes, slow journals.
- Splash / ring-oiled — vertical pumps, motors.
- Pressure-fed circulating — turbomachinery.
- Jet oil — aero-engine mainshafts: nozzle aimed at cage / inner-race shoulder.
- Oil-mist / oil-air — high-speed spindles; tiny droplets entrained in air, minimum quantity lubrication.
Lambda ratio λ = h_min / σ (composite roughness) determines regime: λ > 3 full EHD, 1 < λ < 3 mixed, λ < 1 boundary.
12.3 Sealed-for-life
Pre-greased + 2RS / 2DDU sealed. Standard for small DGBs in fractional-HP motors, automotive alternators, idlers. Life calculated from grease re-greasing interval per manufacturer chart (function of n_dm and T).
13. Vendors
13.1 Global top tier
- SKF (Sweden, 1907) — broadest line; pioneers of self-aligning ball (1907, founder Sven Wingquist), CARB toroidal (1995), Explorer steel, S2M magnetic, Lincoln lubrication.
- Schaeffler Group (Germany) — FAG (1883) full-range; INA (1949) needle / cam follower / linear; LuK clutches. Schaeffler engineering portal (medias) is a reference for designation parsing.
- NSK (Japan, 1916) — full line, leader in machine-tool spindle and ball-screw.
- NTN (Japan, 1918) — automotive bias, large SRB/spherical thrust.
- Timken (US, 1899) — invented tapered roller; acquired Torrington (2003), MPB Bearings, Philadelphia Carpenter & Paterson. Strong in industrial drives, rail, off-highway.
- JTEKT / Koyo (Japan, merged 2006) — Toyota-affiliated, automotive + industrial.
- NACHI-Fujikoshi (Japan) — automotive bias.
13.2 Specialty
- RBC / Roller Bearing Company (US) — aerospace, defense.
- Kaydon (US, now SKF Aeronautical) — thin-section, slewing rings.
- McGill (US, now Emerson) — cam followers (CAMROL).
- MRC, Fafnir, New Departure, Hyatt — legacy US brands rolled into SKF / Timken portfolios.
- igus (Germany) — polymer plain, drylin linear, energy chains.
- GGB / Glacier Garlock — DU / DP4 / DX PTFE-lined.
- THK (Japan, 1971) — invented LM Guide; ball screws, crossed roller.
- HIWIN (Taiwan, 1989) — linear, ball screws.
- Bosch Rexroth (Germany) — linear, ball screws.
- IKO Nippon Thompson (Japan) — needle, crossed-roller, linear.
- Kingsbury (US, 1912) — tilting-pad journal & thrust.
- Waukesha Bearings / Dover (US) — fluid-film, magnetic.
- Miba, John Crane, Pioneer Motor Bearing — Babbitt / tilting-pad.
- S2M (Skf Magnetic Mechatronics) — AMB.
- Synchrony / Calnetix — AMB for turbomachinery.
- Cerobear, GMN, Saint-Gobain — ceramic.
- Rothe Erde / IMO / ThyssenKrupp — slewing rings.
14. Selection heuristics
| Need | First choice | Why |
|---|---|---|
| Light radial, low cost, sealed | Deep-groove ball 6xxx 2RS | Lowest cost, low friction, available everywhere |
| Combined radial + axial, precision | Angular-contact 7xxx duplex DB | Axial both ways with preload |
| Heavy radial + shock + misalignment | Spherical roller 22/23xxx | Highest C per envelope, self-aligns |
| Misalignment, light load | Self-aligning ball 1/2xxx | Lowest friction self-aligner |
| High-speed turbomachinery | ACBB hybrid ceramic 7xxx HC + jet oil | Low DN losses, electrical isolation |
| Massive thrust (turbines) | Kingsbury / Mitchell tilting-pad | Zero-wear film at speed |
| Cleanroom / vacuum / no lubrication | Ceramic full + Krytox / dry-film | No outgassing, no metal wear |
| Food, pharma, wash-down | SS 440C + H1 food-grade grease, or igus iglidur | Corrosion + FDA |
| Oscillating, no rotation | Needle full-complement or PTFE-lined plain | No discrete contact cycles |
| Heavy moment + radial + axial in one part | Crossed roller / slewing ring | Single bearing carries M |
| Robotics wrist / RV reducer | Crossed-roller THK RB / Kaydon thin-section | High M-stiffness, low envelope |
| VFD-driven motor NDE | Insulated bearing (VL0241) or shaft grounding | EDM fluting prevention |
| Microturbine / oil-free | Foil air bearing | No oil system, lift at speed |
| Energy-storage flywheel | Hybrid AMB+PMB | No drag, no maintenance |
| Linear CNC axis | Recirculating ball LM guide + ball screw | Stiffness + repeatability |
15. Failure modes
| Mode | Signature | Root cause |
|---|---|---|
| Subsurface fatigue spalling | Pitting at race-ball/roller contact band; classical L10 endpoint | Cyclic Hertzian stress past material endurance limit |
| Surface-initiated fatigue (micropitting / peeling) | Frosted matte band on raceway | Thin EHD film (λ<1), surface roughness asperity contact |
| Lubrication failure | Discoloration, smearing, melted cage | Grease run-out, oil starvation, wrong viscosity |
| Contamination | Indentations + initiated spalls from particles | Poor sealing, dirty assembly |
| Brinelling (true) | Plastic-deformed indentations at ball positions | Static overload, hammering, vibration during shipment |
| False brinelling | Polished depressions at ball positions, no plastic flow | Micro-vibration with no rotation (transport, idle equipment) |
| Smearing | Material transfer streaks along race | Roller skidding on entry to load zone, insufficient load |
| Fretting corrosion | Reddish-brown oxide between inner ring and shaft | Loose interference fit + vibration |
| Electrical fluting (EDM) | Washboard pattern across race, frosted | High-frequency shaft currents from VFD common-mode → use ceramic balls / VL0241 / shaft brushes |
| Cage failure | Broken cage pockets, debris | High vibration, misalignment, excessive speed |
| Corrosion / wash-out | Rust pits | Water ingress, condensation, wash-down |
| Overload fracture | Cracked rings | Press-fit error, impact, locked-axial expansion |
References: ISO 15243:2017 Rolling bearings — Damage and failures, terms, characteristics and causes.
16. Cross-references
[[Engineering/bearings]]— Tier 2 single-page reference.[[Engineering/Tier3/gears-taxonomy]]— counterpart gear types (most appear together in gearboxes).[[Engineering/Tier3/seals-taxonomy]]— bearing closures, lip seals, labyrinths, magnetic face seals.[[Engineering/Tier3/couplings-taxonomy]]— adjacent power-transmission element; misalignment couplings reduce moment on bearings.[[Engineering/Tier3/electric-motor-taxonomy]]— most bearing failures in industry are motor bearings; VFD-bearing currents tie back here.[[Engineering/bearings]]— deeper EHD / λ-ratio / grease selection.[[Engineering/bearings]]— wear modes, contact mechanics.[[Engineering/vibration-dynamics]]— fluid-film bearing stiffness/damping, oil-whirl criterion.
17. Citations
- Harris, T. A. & Kotzalas, M. N. Rolling Bearing Analysis, 5th ed., CRC Press, 2007 (two-volume: Essential Concepts + Advanced Concepts). The reference text for Hertzian contact, cage dynamics, and EHD in RE bearings.
- Hamrock, B. J., Schmid, S. R., Jacobson, B. O. Fundamentals of Fluid Film Lubrication, 2nd ed., CRC Press, 2004.
- Khonsari, M. M. & Booser, E. R. Applied Tribology — Bearing Design and Lubrication, 3rd ed., Wiley, 2017.
- SKF General Catalogue (latest digital edition at skf.com); SKF Bearing Calculator.
- Schaeffler medias online catalogue and Technical Pocket Guide (TPI 113).
- Timken Engineering Manual (2024 revision) and Timken Tapered Roller Bearing Catalog.
- NSK Rolling Bearings Catalog CAT. No. E1102.
- NTN Ball and Roller Bearings CAT. No. 2202.
- ISO 281:2007 Rolling bearings — Dynamic load ratings and rating life.
- ISO 76:2006 Rolling bearings — Static load ratings.
- ISO 15:2017 Rolling bearings — Radial bearings — Boundary dimensions, general plan.
- ISO 5753-1/2 Rolling bearings — Internal clearance.
- ISO 492:2014 Rolling bearings — Radial bearings — Geometrical product specifications.
- ISO 15243:2017 Rolling bearings — Damage and failures.
- ABMA Standard 9 Load Ratings and Fatigue Life for Ball Bearings; ABMA Standard 11 Load Ratings and Fatigue Life for Roller Bearings.
- API 670 Machinery Protection Systems and API 684 Rotordynamic Tutorial (Kingsbury / tilting-pad bearings for turbomachinery).
- Schweitzer, G. & Maslen, E. H. Magnetic Bearings — Theory, Design, and Application to Rotating Machinery, Springer, 2009.
- DellaCorte, C. & Bruckner, R. J., NASA TM publications on foil air bearings (Capstone-class turbines).
- igus iglidur Engineering Manual (current edition at igus.com).
- GGB Engineered Bearing Solutions manual (DU / DP4 / DX series).