Bearings — Engineering Reference
See also (Tier 3 family index): Bearings Taxonomy
1. At a glance
A bearing is the engineering element that localises one moving part against another while minimising the resistance to relative motion. Every shaft that spins, every linear slide that translates, every joint that articulates inside a gearbox, motor, pump, vehicle, robot, hard drive, wind turbine, or aircraft engine rides on one or more bearings. The global bearing market is roughly USD 130 billion (2024) and the four largest producers — SKF (Sweden), Schaeffler (FAG + INA, Germany), NSK (Japan), and Timken (US) — together ship something north of 10 billion bearings per year.
Bearings split into two thermodynamically distinct families that the designer must distinguish before any other selection step:
- Rolling-element bearings. A set of rolling elements (balls, cylindrical / needle / tapered / spherical rollers) is trapped between an inner race on the shaft and an outer race in the housing, held in angular position by a cage (also called retainer or separator), and protected by seals or shields. The rolling elements convert what would be sliding friction at the shaft surface into rolling contact with a coefficient of friction roughly μ ≈ 0.001 – 0.003 — one to two orders of magnitude lower than dry sliding steel-on-steel. Friction torque is small, breakaway is small, life is statistically defined.
- Plain bearings (sleeve bearings, bushings, journal bearings). A cylindrical bushing supports the shaft directly. Contact is sliding, not rolling. Friction is controlled either by a fluid film between shaft and bushing (hydrodynamic / hydrostatic / aerostatic / magnetic) — in which case μ can be even lower than rolling, ~10⁻⁴ — or by a low-friction solid material (bronze with graphite plugs, PTFE-lined, polymer) for the boundary-lubrication regime where speeds are low or starts/stops are frequent.
The five selection drivers — in roughly the order they constrain the design:
- Load type, magnitude and direction. Radial only, axial (thrust) only, combined; static, rotating, oscillating, shock.
- Speed regime. Surface speed at the bore, expressed as d_m × n (mean-bore-diameter mm × rpm) or DN value. Low: < 100 000. High: > 500 000. Ultra-high (hybrid ceramic, machine-tool spindle): > 1 500 000.
- Required service life. L_10h hours of 90 %-survival rating. Industrial pump: ~25 000 h. Machine-tool spindle: ~40 000 h. Wind turbine main: 175 000 h+. Auto wheel hub: ~3 000 h equivalent at design loads.
- Environment. Temperature (cryogenic, ambient, > 200 °C), contamination (dust, water, chips, swarf), lubrication (grease, oil bath, oil mist, dry), chemistry (acid, alkali, sour gas), vibration.
- Precision / runout / preload / cost. Spindle bearings need micron-class runout; conveyor pillow blocks tolerate 100 μm. Precision costs money; tolerance grade ABEC-7 / P4 is roughly 3–10× the cost of ABEC-1 / P0.
Where it sits in the design stack: bearings are the interface between shaft / housing tolerancing (fits, surface finish, alignment), lubrication engineering (viscosity selection, EHL film thickness), statics & dynamics (load path, gyroscopic loads, unbalance), fatigue (sub-surface rolling-contact fatigue is the designed end-of-life mode), and condition monitoring (vibration analysis is dominated by bearing-defect frequencies). Most rotating-machinery failures trace to a bearing — and most premature bearing failures trace to lubrication or contamination, not to the bearing itself.
2. First principles
2.1 Rolling-element contact — Hertzian theory
Two curved elastic bodies pressed together do not contact over a single point or line; they elastically deform into a small finite contact patch with a non-uniform pressure distribution. Hertz (1882) solved this for general curvatures:
- Ball-on-race (two surfaces with different principal radii of curvature in two planes): the contact patch is an ellipse, the pressure distribution semi-ellipsoidal, the peak pressure p_0 = (3/2) · p_mean.
- Roller-on-race (line contact, ignoring end effects): the contact patch is a thin rectangle, the pressure distribution semi-cylindrical, peak p_0 = (4/π) · p_mean.
Typical operating peak contact pressures in service bearings sit in the range p_0 ≈ 1.5 – 3.5 GPa (yes, gigapascals — well into the material’s compressive yield range, sustained only because the loaded volume is small and triaxially constrained). 52100 bearing steel survives this because it is through-hardened to HRC 60–64, with retained austenite and a martensitic matrix dispersing fine carbides.
The Hertzian stress state below the contact patch peaks not at the surface but at a depth of roughly z ≈ 0.78 · a (a = half-width of contact for line contact; 0.48 · a for point contact). The maximum orthogonal shear stress sits at that subsurface depth and is the driver of the canonical bearing failure mode: subsurface-initiated rolling-contact fatigue spalling. The bearing does not fail at the surface in clean conditions — it fails from below, after the shear-stress cycle count reaches Weibull-distributed fatigue.
2.2 The Stribeck curve — lubrication regimes
The friction coefficient between sliding or rolling lubricated surfaces is not a constant of the material pair; it is a strong function of the lubricant film thickness relative to the surface roughness. Stribeck (1902) plotted μ versus the Hersey number N = (η · ω) / P, where η = dynamic viscosity, ω = sliding speed, P = unit load. Four regions:
| Regime | λ = h_min / σ_composite | μ typical | Wear mode |
|---|---|---|---|
| Boundary | < 1 | 0.08 – 0.20 | adhesive + abrasive; high |
| Mixed | 1 – 3 | 0.02 – 0.08 | partial film; moderate |
| Elastohydrodynamic (EHL) | 3 – 10 | 0.001 – 0.01 | low |
| Hydrodynamic / full-film | > 10 | 0.001 – 0.005 | negligible (fluid only) |
Here λ (Lambda ratio) = minimum elastohydrodynamic film thickness h_min divided by composite surface roughness σ_composite = √(σ₁² + σ₂²). Design target λ ≥ 3 for adequate fatigue life. Below λ = 1, surface asperities are punching through and the bearing is wearing rather than rolling.
2.3 EHL — elastohydrodynamic lubrication
In a rolling-element contact, the lubricant is dragged into the converging gap and pressurises to GPa levels. At those pressures the lubricant piezo-viscously stiffens (η rises exponentially with pressure, η = η₀ · exp(α · p), α ≈ 1–3 × 10⁻⁸ Pa⁻¹ for mineral oils) and the surfaces simultaneously elastically deform — creating a remarkably uniform 0.1–1 μm thick oil film that physically separates the rolling element from the race. The Hamrock-Dowson equation gives the central film thickness:
h_c / R = 2.69 · U^0.67 · G^0.53 · W^(-0.067) · (1 - 0.61 · e^(-0.73·k))
with dimensionless parameters U (speed), G (material), W (load), k (ellipticity). The practical engineering form used by SKF and Schaeffler is:
κ = ν / ν₁
where ν = actual kinematic viscosity at operating temperature and ν₁ = required viscosity from the catalogue chart (function of d_m and rpm). κ ≥ 4 indicates full-film (long life); κ = 1 indicates boundary-mixed (life heavily reduced); κ < 0.4 is starvation territory.
2.4 Hydrodynamic plain-bearing film — Reynolds equation
A loaded shaft in a slightly oversized cylindrical bushing, rotating, drags lubricant into a converging wedge by viscous shear and generates a pressure field that lifts the shaft off the bore. The pressure distribution is governed by the Reynolds equation (a reduced Navier–Stokes for thin films):
∂/∂x ( h³/(12η) · ∂p/∂x ) + ∂/∂z ( h³/(12η) · ∂p/∂z ) = (U/2) · ∂h/∂x + ∂h/∂t
Solutions for the finite-width journal bearing give the Sommerfeld number S = (η·N / P) · (r/c)² (η = viscosity, N = rps, P = projected load, r = journal radius, c = radial clearance) which parametrises eccentricity, minimum film thickness, friction, attitude angle, and flow.
Practical design rule for hydrodynamic journal bearings: c/r ≈ 0.001 (1 mil per inch of radius), L/D ≈ 0.5 – 1.0, h_min ≥ 5 × composite roughness for full-film operation. Below the lift-off speed the bearing operates in mixed / boundary mode (start-stop wear); above it, μ is dominated by fluid shear and is independent of load.
2.5 Internal clearance and preload
Rolling-element bearings ship with radial internal clearance — a small play between rolling elements and races at zero load (codified in ISO 5753: C2 < CN < C3 < C4 < C5). Once mounted with an interference fit on the shaft and warmed to operating temperature, the operating internal clearance shrinks. Designers select clearance so that:
- Operating clearance ≈ 0 to slightly negative for noise / runout — light preload.
- Operating clearance > 0 to accommodate thermal differential expansion — standard fit.
- Heavy preload (negative clearance forced by spacer / spring) for spindle stiffness — but raises temperature and shortens life.
Angular-contact and tapered-roller bearings are designed to be preloaded as opposed pairs. The preload sets stiffness, runout, and contact angle uniformity at the cost of friction and heat.
3. Practical math / design equations
3.1 Equivalent dynamic load
Combined radial F_r and axial F_a loads are reduced to an equivalent radial load P that produces the same fatigue damage:
P = X · F_r + Y · F_a
X and Y are factors from the bearing catalogue, dependent on bearing geometry and the ratio F_a / F_r relative to a threshold e (also catalogue-tabulated). For a deep-groove ball bearing (typical):
| F_a / (Y₀ · C_0) | e | X (F_a/F_r > e) | Y (F_a/F_r > e) |
|---|---|---|---|
| 0.025 | 0.22 | 0.56 | 2.0 |
| 0.07 | 0.27 | 0.56 | 1.6 |
| 0.13 | 0.31 | 0.56 | 1.4 |
| 0.25 | 0.37 | 0.56 | 1.2 |
| 0.50 | 0.44 | 0.56 | 1.0 |
When F_a / F_r ≤ e: X = 1, Y = 0 (use pure radial). For angular-contact and tapered roller bearings the X / Y / e tables are different (Y is single-valued at the bearing’s nominal contact angle).
3.2 L10 fatigue life — ISO 281 basic rating
The bearing basic dynamic load rating C is the constant radial load (or axial, for thrust bearings) that 90 % of a population of identical bearings will survive for one million revolutions of rolling-contact fatigue. Life at any other load follows the Lundberg–Palmgren / Weibull law:
L_10 = (C / P)^p × 10⁶ revolutions
with p = 3 for ball bearings, p = 10/3 ≈ 3.33 for roller bearings. Doubling the load drops the life by 8× (ball) or 10× (roller); halving it multiplies the life by 8× (ball) or 10× (roller).
Converted to operating hours at speed n (rpm):
L_10h = L_10 / (60 · n) [hours]
3.3 Modified life — ISO 281 (2007) L_nm
Modern ISO 281 augments the bare Lundberg–Palmgren result with reliability and lubrication-contamination corrections:
L_nm = a_1 · a_ISO · L_10
| Reliability | a_1 |
|---|---|
| 90 % (L_10) | 1.00 |
| 95 % (L_5) | 0.64 |
| 96 % | 0.55 |
| 97 % | 0.47 |
| 98 % | 0.37 |
| 99 % (L_1) | 0.25 |
a_ISO is a multi-variable function of contamination level e_C, viscosity ratio κ = ν / ν₁, and the load ratio C_u / P (where C_u is the fatigue load limit — the load below which no fatigue at all is theoretically expected, ~C_0 / 8 for steel bearings). a_ISO ranges from 0.1 (heavy contamination + boundary lube) through 1.0 (clean + full film) to >50 (very clean + κ > 4 + low load) — yes, modern clean conditions can extend life 50× beyond the catalogue L_10.
3.4 Static load capacity
For bearings that rotate < 10 rpm, oscillate, or carry shock loads while stationary, the static load rating C_0 governs — not the dynamic. C_0 is defined (ISO 76) as the load producing 0.0001 · D_ball permanent indentation at the most heavily loaded contact. Design rule:
P_0 ≤ C_0 / s_0
with safety factor s_0 = 1.0 – 1.5 for normal operation, 2.0 – 3.0 for shock or oscillating duty. Exceeding C_0 produces Brinelling — permanent indents at each rolling-element position — and the bearing will run rough and noisy thereafter.
3.5 Limiting speed
Catalogue values give two limiting speeds per bearing — grease-lubricated (lower, heat-dissipation limited) and oil-lubricated (higher). These are not absolute limits; they are the speed at which catalogue-default lubrication is adequate. Above them: oil-mist or oil-jet cooling, special low-friction cages, special lubricants. The product d_m × n (mm × rpm) is a useful normalised metric:
| d_m × n | Regime |
|---|---|
| < 100 000 | low speed, grease, standard everything |
| 100 000 – 500 000 | normal industrial; standard grease or oil bath |
| 500 000 – 1 000 000 | high speed; oil bath / oil-mist; polymer cage |
| 1 000 000 – 1 500 000 | machine-tool spindle; oil-air or oil-jet, polymer cage |
| > 1 500 000 | hybrid ceramic bearings, very clean lube |
3.6 Worked example 1 — 6206 deep-groove ball at moderate combined load
Given: 6206 deep-groove ball bearing (30 mm bore × 62 mm OD × 16 mm width), C = 19.5 kN, C_0 = 11.2 kN, limiting speed 13 000 rpm (grease). Loads F_r = 3 kN radial, F_a = 1 kN axial, speed n = 1 500 rpm.
Step 1 — F_a / C_0 ratio for X, Y table:
F_a / C_0 = 1.0 / 11.2 = 0.089
Interpolating the catalogue table at 0.089 gives e ≈ 0.29, Y ≈ 1.5.
Step 2 — check whether axial component matters:
F_a / F_r = 1.0 / 3.0 = 0.333 > e = 0.29 → use X = 0.56, Y = 1.5
Step 3 — equivalent dynamic load P:
P = X · F_r + Y · F_a
= 0.56 × 3 000 N + 1.5 × 1 000 N
= 1 680 + 1 500
= 3 180 N ≈ 3.18 kN
Step 4 — L_10 in revolutions (p = 3 for ball):
L_10 = (C / P)^3 × 10⁶
= (19 500 / 3 180)^3 × 10⁶
= (6.132)^3 × 10⁶
= 230.6 × 10⁶ rev
Step 5 — convert to hours:
L_10h = L_10 / (60 · n)
= 230.6 × 10⁶ / (60 × 1 500)
= 230.6 × 10⁶ / 90 000
= 2 562 h
2 562 h L_10h versus a typical industrial 25 000 h design target — the 6206 is undersized at this load.
Options to recover margin:
- Step up to 6306 (medium series, same 30 mm bore, C = 28.1 kN). New P unchanged at 3.18 kN. L_10 = (28 100 / 3 180)³ × 10⁶ = 690 × 10⁶ rev. L_10h = 7 670 h. Still under.
- Step up to 6406 (heavy series, same bore, C = 43.6 kN). L_10h = (43 600 / 3 180)³ × 10⁶ / (60 × 1500) = 28 700 h. ✓ Meets target.
- Or accept L_10h = 2 562 h and rely on a_ISO. Clean installation + κ ≥ 4 + light load multiplies effective life by ~5–10×, bringing the 6206 to roughly 13 000 – 25 000 h. Marginal — better to upsize.
3.7 Worked example 2 — paired tapered roller bearings, cantilever shaft
Given: Motor pinion shaft on two tapered-roller bearings 200 mm apart, with the pinion 100 mm beyond the inboard bearing. Pinion load 8 kN radial + 4 kN axial. Bearings: 30206 at the pinion end (C = 47 kN, e = 0.37, Y = 1.6), 30210 at the motor end (C = 76 kN, e = 0.43, Y = 1.4). Speed n = 1 750 rpm.
Reactions (sum moments about each bearing):
F_r,A (pinion end, 30206) = 8 kN × (200 + 100) / 200 = 12 kN
F_r,B (motor end, 30210) = 8 kN × 100 / 200 = 4 kN
Induced axial from each tapered bearing (F_a,ind = 0.5 · F_r / Y):
F_a,A,ind = 0.5 · 12 / 1.6 = 3.75 kN
F_a,B,ind = 0.5 · 4 / 1.4 = 1.43 kN
Thrust distribution. External thrust 4 kN drives bearing A in the loaded direction, so A takes its induced plus the external; B takes only its own induced:
F_a,A = 3.75 + 4.0 = 7.75 kN
F_a,B = 1.43 kN
Equivalent loads. A: F_a/F_r = 0.646 > e = 0.37 → X = 0.4, Y = 1.6 → P_A = 0.4·12 + 1.6·7.75 = 17.2 kN. B: F_a/F_r = 0.358 < e = 0.43 → P_B = F_r = 4.0 kN.
L_10 (p = 10/3 for roller):
L_10,A = (47 / 17.2)^3.333 × 10⁶ = 29.8 × 10⁶ rev → L_10h,A = 284 h
L_10,B = (76 / 4.0)^3.333 × 10⁶ = 13 970 × 10⁶ rev (dominates by 470×)
System life ≈ L_10,A = 284 h — catastrophically inadequate. Engineering action: upsize the pinion-end bearing. Re-running with a 30307 (C = 92 kN) gives L_10h ≈ 2 990 h — still under typical 25 000 h industrial target. The real fix is design-side: shorten the overhang. This is the kind of analysis that flags geometry trouble before steel is cut.
3.8 Worked example 3 — lubrication κ check
Given: 6206 bearing (d_m = (30 + 62)/2 = 46 mm), at n = 1 500 rpm, operating temperature 80 °C. Lubricant: ISO VG 32 mineral oil. Compute the κ ratio.
Step 1 — required viscosity ν₁ from SKF / Schaeffler chart, as a function of d_m and n:
For d_m = 46 mm, n = 1 500 rpm → ν₁ ≈ 21 mm²/s (cSt)
(Chart is a log-log nomogram; the relationship is approximately ν₁ ≈ 4.5 × 10⁴ / (n · d_m^0.5) at moderate speeds.)
Step 2 — actual viscosity at 80 °C. ISO VG 32 means kinematic viscosity = 32 mm²/s at 40 °C. Viscosity falls steeply with temperature; for a typical mineral oil with viscosity index VI ≈ 100, the Walther equation (ASTM D341):
log₁₀(log₁₀(ν + 0.7)) = A − B · log₁₀(T_K)
gives, for ISO VG 32, ν(80 °C) ≈ 9.5 mm²/s.
Step 3 — κ ratio:
κ = ν / ν₁ = 9.5 / 21 = 0.45
κ = 0.45 is poor. Boundary / mixed regime; a_ISO will be heavily penalised, expect a_ISO ≈ 0.3 – 0.5 even with clean contamination. The L_10 of 2 562 h from worked example 1 becomes L_nm ≈ 0.4 × 2 562 ≈ 1 000 h.
Remedial action: step up to ISO VG 68 (ν(80 °C) ≈ 18 mm²/s, κ = 0.86 — still poor) or VG 100 (ν(80 °C) ≈ 26 mm²/s, κ = 1.24 — adequate, a_ISO ≈ 0.7) or VG 150 (ν(80 °C) ≈ 38 mm²/s, κ = 1.81 — good). Alternatively, lower bearing operating temperature (cooler housing, more efficient oil cooler) to push ν up at the bearing.
This is why viscosity selection and operating-temperature management dominate real-world bearing life far more than the catalogue C / P ratio.
4. Reference data
4.1 Common deep-groove ball series (DIN 625 / ISO 15)
Single-row deep-groove ball, basic dimensions and ratings (open variant; sealed (-2RS) similar dimensions, slightly lower limiting speed):
| Designation | Bore × OD × W (mm) | C (kN) | C_0 (kN) | n_lim grease (rpm) | n_lim oil (rpm) |
|---|---|---|---|---|---|
| 6004 (light 60-) | 20 × 42 × 12 | 9.36 | 5.0 | 24 000 | 28 000 |
| 6204 (med 62-) | 20 × 47 × 14 | 13.5 | 6.55 | 20 000 | 24 000 |
| 6304 (heavy 63-) | 20 × 52 × 15 | 16.8 | 7.8 | 19 000 | 22 000 |
| 6904 (extra-light 69-) | 20 × 37 × 9 | 6.37 | 3.65 | 28 000 | 32 000 |
| 6006 | 30 × 55 × 13 | 13.8 | 8.3 | 17 000 | 20 000 |
| 6206 | 30 × 62 × 16 | 19.5 | 11.2 | 13 000 | 17 000 |
| 6306 | 30 × 72 × 19 | 28.1 | 16.0 | 12 000 | 15 000 |
| 6906 | 30 × 47 × 9 | 7.28 | 4.55 | 19 000 | 22 000 |
| 6208 | 40 × 80 × 18 | 30.7 | 19.0 | 11 000 | 13 000 |
| 6308 | 40 × 90 × 23 | 41.0 | 24.0 | 9 500 | 11 000 |
| 6210 | 50 × 90 × 20 | 35.1 | 23.2 | 9 000 | 11 000 |
| 6310 | 50 × 110 × 27 | 61.8 | 38.0 | 7 500 | 9 000 |
Series suffixes: 6900 extra-light (thin-section), 6000 light, 6200 medium (the common workhorse), 6300 heavy. Same bore → progressively higher C as you walk the series; the cost is greater OD and width.
4.2 Bearing types and primary use cases
| Type | Radial load | Axial load | Misalignment | Speed | Typical use |
|---|---|---|---|---|---|
| Deep-groove ball | ★★ | ★ | very low (~ 2′) | very high | universal default |
| Angular-contact ball | ★★ | ★★★ (one direction) | none | very high | spindles, paired sets |
| Self-aligning ball (1xxx, 2xxx) | ★ | ★ | high (~ 2.5°) | high | textile shafts, conveyors |
| Cylindrical roller (NU, NJ, N) | ★★★★ | none (free) | low | high | high-radial machinery |
| Needle roller | ★★★ | none | low | medium | gearboxes, compact spaces |
| Spherical roller (22xxx, 23xxx) | ★★★★★ | ★★ | high (1.5–2°) | medium | mining, paper mills, wind |
| Tapered roller (3xxxx) | ★★★ | ★★★ (one direction) | none | medium | wheel hubs, gearboxes, paired |
| Crossed roller | ★★ | ★★ | none | medium | robot joints, rotary tables |
| Thrust ball (51xxx) | none | ★★ | none | low | low-axial slow rotation |
| Cylindrical thrust roller (81xxx) | none | ★★★★ | none | low | screw jacks, lifting |
| Spherical thrust roller (29xxx) | ★ | ★★★★★ | high | low | heavy axial + misalignment |
4.3 Plain bearing variants
| Type | Friction μ | Speed limit | Load | Lubrication |
|---|---|---|---|---|
| Oil-impregnated sintered bronze (SAE 841) | 0.05 – 0.20 | low (PV < 50 000) | low–medium | self-lubricated |
| Solid bronze (SAE 660 / C932) | 0.05 – 0.15 | medium | medium–high | external grease / oil |
| PTFE-lined (DU, DX bushings — GGB, Garlock) | 0.02 – 0.20 | medium | medium–high | dry / boundary lube |
| Sintered iron (SAE 840) | 0.10 – 0.25 | low | medium | oil-impregnated |
| PEEK / PI polymer (Vespel, Igus iglide) | 0.10 – 0.30 | low | low | dry; chemical resistance |
| Ceramic (Al₂O₃, SiC) | 0.10 – 0.20 | high | medium | dry / process fluid |
| Hydrodynamic journal | 0.001 – 0.005 (full film) | high | high | flooded oil, dedicated supply |
| Hydrostatic | 10⁻⁴ – 10⁻³ | very high | very high | external high-pressure pump |
| Aerostatic (air bearing) | 10⁻⁵ – 10⁻⁴ | extremely high | low | external air supply |
| Magnetic | ~ 0 (contactless) | extremely high | medium | active control system |
4.4 Radial internal clearance (ISO 5753-1)
For a 30 mm bore deep-groove ball (6206) in μm of pre-mount radial play:
| Code | Range (μm) | When to use |
|---|---|---|
| C2 | 1–11 | quiet running, light interference fit |
| CN / N (normal) | 6–20 | general; default |
| C3 | 13–28 | hot operation, interference fit on shaft |
| C4 | 25–41 | very hot, high interference |
| C5 | 37–58 | extreme thermal / heavy press fit |
4.5 Tolerance grades
| ISO 492 / DIN 620 | ABMA / ABEC | Use |
|---|---|---|
| Normal (P0) | ABEC 1 | general industrial |
| P6 | ABEC 3 | small electric motors |
| P5 | ABEC 5 | medium machine tools |
| P4 | ABEC 7 | spindles, high-precision |
| P2 | ABEC 9 | metrology, scanners |
4.6 Grease NLGI grades
| NLGI | Worked penetration (0.1 mm) | Consistency | Use |
|---|---|---|---|
| 000 | 445–475 | fluid | central lube systems |
| 00 | 400–430 | semifluid | gearboxes |
| 0 | 355–385 | very soft | low-temp, central lube |
| 1 | 310–340 | soft | low-temp, centralised |
| 2 | 265–295 | medium (default) | general bearings |
| 3 | 220–250 | firm | vertical shafts, high temp |
| 4 | 175–205 | very firm | special |
| 5–6 | 85–175 | block-like | sealed industrial |
Base-oil viscosity matters more than NLGI grade. A grease specced “NLGI 2” with ISO VG 32 base oil and one specced “NLGI 2” with ISO VG 460 base oil behave completely differently in service — the first runs cooler at high speed, the second carries far higher load.
5c. Variants & topologies
5c.1 Rolling-element family
- Deep-groove ball (6xxx) — universal default; radial + modest axial both directions; non-separable; mild misalignment tolerance (≤ ~ 2 arc-min); easy to seal (Z, 2Z, RS, 2RS). The first bearing to consider; excluded only by extreme load, severe misalignment, or pure thrust.
- Angular-contact ball (7xxx) — 15° / 25° / 40° contact angle; combined radial + heavy axial one direction; used in face-to-face (DF), back-to-back (DB), or tandem (DT) preloaded pairs (DUL/DUM/DUH = light/medium/heavy preload). The machine-tool spindle bearing.
- Self-aligning ball (1xxx, 2xxx) — two rows of balls in a spherical outer race; up to 2.5° static misalignment. Lower load rating; long-shaft / textile / conveyor duty.
- Cylindrical roller (NU, NJ, N, NUP) — line contact gives 2–4× the radial capacity of a same-envelope ball. NU = ribs on inner (outer floats), NJ = single rib on outer (one-direction thrust), NUP = full ribs (axial both). The floating bearing in locate-and-float arrangements.
- Needle roller — rollers with L/D ≥ 3:1, 3–5 mm thick. Maximises radial in minimum envelope. Drawn-cup (HK, BK) and heavy-duty (NA, NK) variants; needle-and-cage assemblies run directly on hardened shaft + housing. Universal in automotive gearboxes (planet pinions, conrod little-end) and helicopter swashplates.
- Spherical roller (22xxx, 23xxx) — barrel rollers in a spherical outer race; enormous combined load + 1.5–2° misalignment. Paper-mill rolls, mine crushers, wind-turbine main shafts.
- Tapered roller (3xxxx) — frustum-conical rollers with apex on the bearing axis; combined radial + thrust one direction; used in opposed pairs to take thrust both ways and set preload; cone + roller assembly separable from cup. The automotive wheel-hub bearing; Timken’s home court (inventor, 1899).
- Thrust bearings — thrust ball (51xxx, low load), cylindrical thrust roller (81xxx, higher load lower speed), tapered & spherical thrust roller (29xxx series) for heavy axial + misalignment (mill-roll necks).
- Four-point-contact ball (QJ) — split Gothic-arch race takes axial in both directions in a single row; replaces a paired angular-contact set where envelope is tight.
- Crossed-roller (CRBA / CRBH / RB) — adjacent rollers rotated 90°; single bearing handles radial, bidirectional thrust, and tilt moment. The robot-joint and rotary-table bearing (Harmonic Drive, RV reducer outputs).
- Slewing rings (200 mm – 6 m) — single- or double-row ball or three-row roller, often with integral gear teeth. Cranes, excavators, wind-turbine yaw / pitch, satellite antennas.
5c.2 Plain bearing family
- Sintered bronze (SAE 841 “Oilite”) — ~20 % porosity filled with SAE 30 oil; lifelong self-lubrication for low PV duty. Cheap; the appliance / hobby workhorse.
- PTFE-lined laminated — GGB DU (steel / sintered bronze / PTFE+Pb), DX (lead-free), Garlock DU. Dry-running; standard for sealed actuators, robot joints, suspension bushings.
- PEEK / polymer (Igus iglide, Vespel SP-21) — chemical resistance, dry, low cost; PV limited.
- Solid bronze (SAE 660, 64, 65) — high load, external lube; worm gears, low-cost engine big-ends.
- Babbitt-lined hydrodynamic — tin- or lead-based white metal cast on a steel shell; the classic plain bearing of large engines, turbines, motors. Conforms to shaft, embeds debris.
- Tilting-pad journal & thrust — segmented self-aligning pads (Kingsbury, Michell, Waukesha). Steam turbines, large compressors, generators.
- Hydrostatic, aerostatic, magnetic, foil — external-pressure or contact-less film bearings for very high speed, very high precision, or oil-free duty (machine-tool slides; telescope drives; turbomolecular pumps; air-cycle machines).
5c.3 Specialty rolling-element variants
- Hybrid ceramic — Si₃N₄ (silicon nitride) balls in steel races. Ball density 40 % of steel → 40 % lower centrifugal load on the outer race at high speed → ~25 % higher d_m·n capability. Also harder, lower friction, electrically insulating (kills VFD shaft currents). Used in machine-tool spindles, electric-vehicle motors, dental drills, F1 wheel hubs. Cost: 3–10× steel.
- Full-ceramic (Si₃N₄ or ZrO₂ races + balls) — corrosion-immune, electrically insulating, non-magnetic; used in semiconductor process equipment, medical, vacuum.
- Polymer cage (TN9 / TVH suffix — polyamide PA66 + glass fibre; TPI = PEEK) — lower mass than brass / steel, smoother at high speed, quieter. Most modern deep-groove balls > 20 mm bore ship with polymer cage unless otherwise specified.
- Full-complement (no cage — designation suffix V) — packs more rolling elements into the same envelope at the cost of speed (rollers rub against each other). Used for slow heavy duty (gantry-crane wheel bearings, sheave bearings).
- Insulated bearings (INSOCOAT / INS suffix) — aluminum-oxide ceramic coating on the outer race OD (or inner race bore) provides ≥ 100 MΩ electrical isolation. Mitigates VFD-driven motor shaft currents.
6c. Selection criteria
Decision sequence, in order of constraint hierarchy:
- Load type & magnitude. Pure radial → deep-groove ball, cylindrical roller, needle. Pure axial → thrust ball or thrust roller. Combined with F_a < 0.3·F_r → deep-groove ball. Combined with F_a > 0.3·F_r → angular contact or tapered roller (paired). Heavy combined + misalignment → spherical roller. Moment / tilt load → crossed-roller, four-point-contact, or paired tapered.
- Speed (d_m × n). High d_m·n favours ball over roller (lower friction); hybrid ceramic above ~ 1 000 000. Low speed / oscillating: rolling-element bearings false-brinell under vibration with no rotation — consider plain bushings.
- Misalignment. < 5 arc-min → any rigid bearing. 0.5–2.5° → self-aligning ball or spherical roller. Heavy long-shaft deflection → spherical roller or split-housing spherical plain.
- Environment. Wet → contact seals (2RS) and consider 440C stainless. High temperature (> 150 °C) → polyurea or PFPE grease, brass cage, C3 / C4 clearance. Cryogenic → 440C or hybrid ceramic. Vacuum / cleanroom → solid-lubricant bearings, low-outgassing seals. VFD electric motor → insulated bearing and/or shaft grounding ring.
- Required life. L_10h per ISO 281 × a_ISO; aim ≥ 2× the design service life because real-world a_ISO is usually < 1.
- Precision. P0 / ABEC-1 default; P5 / ABEC-5 for machine-tool feeds and robot joints; P4 / ABEC-7 or better for spindles, scanners, metrology.
- Cost / availability. Deep-groove ball is the cheapest per kN. Hybrid ceramic, precision grades, specialty stainless: 3–20×. Non-stock = weeks of lead time — check SKF / Schaeffler / NSK / NTN stock status before designing in an oddity.
7c. Datasheet decoding
A standard rolling-bearing data line in any major catalogue:
Designation 6206-2RS1/C3
Bore d 30 mm
OD D 62 mm
Width B 16 mm
Basic dynamic C 19.5 kN
Basic static C_0 11.2 kN
Fatigue limit C_u 0.475 kN
Reference speed 17 000 rpm
Limiting speed 13 000 rpm
Mass 0.20 kg
What each item really means:
- C (basic dynamic load rating, kN) — load for 10⁶-rev L_10 fatigue. Increases with size and is the headline number for selection. Note that some pre-2008 catalogues used a different formulation than ISO 281:2007; modern bearings have been re-rated and C values may be ~10 % higher than older same-part data.
- C_0 (basic static load rating, kN) — load producing 0.0001·D ball-indent. Governs for slow / oscillating duty.
- C_u (fatigue load limit, kN) — Lundberg–Palmgren-style theoretical fatigue threshold; feeds into a_ISO calculation. Roughly C_0 / 8 for steel bearings.
- Reference speed — thermal-equilibrium speed at which catalogue conditions produce 70 °C operating temperature with oil-bath lube; used as input to a_ISO.
- Limiting speed — mechanical/kinematic ceiling beyond which cage stress, lube starvation, or other issues become design concerns. Grease and oil values differ. Not absolute — special arrangements can exceed it.
Suffix codes (SKF / industry-common):
| Suffix | Meaning |
|---|---|
| Z, 2Z | one / two metal shields (non-contacting) |
| RS, 2RS, RZ, 2RZ | one / two rubber-lip contact seals; RZ = low-friction lip |
| NR | snap-ring groove on outer race for axial location |
| C2 / CN / C3 / C4 / C5 | radial internal clearance class (tight → loose) |
| P0 / P6 / P5 / P4 / P2 | tolerance grade (default → precision) |
| J | pressed-steel cage |
| M | machined brass cage |
| TN9 / TVH | polyamide (PA66+GF) cage |
| TPI | PEEK cage |
| V | full-complement (no cage) |
| 2RS1 | seal-type rubber NBR (versus 2RS2 fluoroelastomer for hi-temp) |
| /VA228 | OEM-special grease / clearance pre-set (varies by maker) |
| /HC5 | hybrid ceramic balls, ANSI/ABMA-5 tolerance |
| /W64 | solid-oil-filled (SKF Solid Oil) for low-maintenance |
| /VL024 | INSOCOAT outer-race coating (insulated bearing) |
Tapered-roller designations (e.g. 30206) decode differently: first digit = 3 (tapered family), next two digits = series, last two × 5 = bore in mm. So 30206 → bore 30 mm, light-medium series.
Spherical roller designations (e.g. 22206 EK): 22 = double-row barrel, last two × 5 = bore (30 mm). E = energy-efficient enhanced design (1990s+ standard).
8c. Mounting, shafts, sealing
Bearings only deliver catalogue life when shaft / housing / seal / lubrication / installation are all right. Most “premature bearing failures” are actually mounting failures.
8c.1 Shaft & housing fits
Selection is governed by which race rotates and how heavily the bearing is loaded. ISO 286 fits, applied to bearing seats:
| Condition | Shaft fit (rotating inner) | Housing fit (stationary outer) |
|---|---|---|
| Light load, deep-groove ball | j5, j6 | H7, J7 |
| Normal load, ball bearing | k5, k6 | J7, K7 |
| Heavy load / shock, roller bearing | m5, m6, n6 | K7, M7 |
| Very heavy / shock | p6, r6 | N7, P7 |
| Rotating outer (e.g. gantry wheel) | g6, h6 (loose on shaft) | M7, N7 (tight in housing) |
Rule of thumb: the race that rotates relative to the load gets the interference fit (so it doesn’t creep / wear in its seat); the race stationary relative to the load gets the clearance fit (so thermal expansion can accommodate without preloading the bearing).
8c.2 Preload
Two methods to preload paired bearings:
- Solid preload — ground spacer between paired bearings sets the inner / outer spacing precisely. Stable with temperature only if shaft and housing CTEs match; otherwise preload drifts with heating. Standard on spindle bearings.
- Constant-pressure preload — a wave spring or Belleville stack pushes one bearing toward the other with a metered force. Preload is constant regardless of thermal growth; stiffness is determined by the spring rate. Standard on smaller machine spindles and electric motors.
For tapered roller, preload is set by adjusting nut (KM-series locknut) and verified by starting torque measurement or bearing setting clearance with feeler gauge / dial indicator.
Set screws on bearings — avoid. They cause fretting at the shaft contact, mark the shaft, can come loose, and produce uneven preload. Better: tapered adapter sleeve (H-series), thrust collar with hydraulic nut, or shoulder + locknut.
8c.3 Sealing
Three categories of seals:
- Integral bearing seals — Z (metal shield, non-contact), 2Z (two shields), RS (rubber contact lip, one side), 2RS (both sides), RZ (low-friction non-rubbing rubber). The 2RS variant adds friction and limits speed but is the practical default for any non-clean environment.
- External shaft seals — radial lip seal (the classic spring-energised rubber V-lip, ISO 6194), V-ring (Forsheda V-seal), labyrinth seal (no contact, gap labyrinth + sling), magnetic / face seal (high-pressure or zero-leak duty). Lip seals run on a ground shaft surface (Ra ≤ 0.8 μm, no spiral lay).
- Bearing isolators — labyrinth-type magnetic-or-mechanical sealed cartridges installed outboard of the bearing. Garlock Klozure, Inpro/Seal, Chesterton DSR. Standard practice for ANSI process pumps and electric motors above 50 kW where lip-seal life is poor.
8c.4 Lubrication management
- Grease relubrication interval — depends on temperature, speed, bearing size. SKF / Schaeffler chart: at 70 °C and d_m·n ≈ 200 000, ~3 000–10 000 h between grease additions for an open bearing; 5–10× shorter at 100 °C. Halve the interval for every 15 °C above 70 °C.
- Grease quantity — for a normally-rotating bearing, fill bearing 100 %, housing free space 30–50 %. Over-fill causes churning, heat, and accelerated grease breakdown.
- Oil bath — fill to centre of lowest rolling element at rest. Higher = churning losses. Used in gearboxes; oil splashed by gear lubricates the bearing.
- Oil mist / oil-air — atomised oil fed by compressed air; spindle and high-speed bearings; minimal lubricant, no churning.
- Centralised lube — pump + distribution block + measured doses every 15 min – 8 h. Standard on machine tools, paper mills, mining equipment.
8c.5 Installation
- Cold press — press only on the race that has the interference fit. Never push axial load through the rolling elements (brinells the race).
- Heating — induction heater (SKF TIH-series) or oil bath to 80–120 °C above ambient produces enough thermal expansion to slide the bearing onto a shaft interference-fit without force. Standard practice above ~50 mm bore. Never exceed 125 °C on sealed bearings (degrades grease/seal).
- Hydraulic / oil injection — for large bearings on tapered shafts, oil pumped through a shaft channel under the bearing race lifts it off the seat. Standard for large rolling-mill bearings.
- Adapter sleeves (H/H2/H3/H4) — split tapered sleeve with locking nut; mounts bearing on a parallel shaft without machining a precision seat. Common on textile / fan / conveyor shafts.
8c.6 Removal
- Bearing puller — two-jaw or three-jaw, mechanical or hydraulic (SKF TMMA, Enerpac). Pull on the race with the interference fit only — never the loose race.
- Induction heater — for assembly with shrink fit; also useful to aid removal.
- Oil injection — for shrink-fitted large bearings, oil under 1500 bar between shaft and bore physically lifts the race off the shaft.
- Cut-and-replace — for shaft-side races that have run a damaging interference and won’t pull. Grinder + cold chisel + new bearing.
9c. Real parts & sourcing
The bearing industry is more concentrated than nearly any other mechanical-component sector. Five companies hold > 50 % of the global market (SKF, Schaeffler, NSK, NTN, Timken). Counterfeit bearings are an industry-wide problem — particularly for high-value designations on grey-market online channels.
| Maker | HQ | Strength |
|---|---|---|
| SKF | Sweden | Market leader (~USD 11 B revenue 2024); deep-groove ball + spherical roller + condition monitoring; de-facto reference catalogue |
| Schaeffler (FAG + INA) | DE | Cylindrical & needle roller (INA), automotive (FAG); largest private bearing maker |
| NSK | JP | Machine-tool spindles, automotive |
| NTN | JP | Automotive + industrial broad range |
| Timken | US | Inventor of the tapered roller (1899); heavy industrial + rail |
| JTEKT (Koyo) | JP | Automotive + steering systems |
| NACHI-Fujikoshi | JP | Bearings + machine tools |
| MinebeaMitsumi | JP | Miniature precision bearings (< 10 mm bore); HDD, instruments |
| RBC Bearings, Barden, Kaydon | US/UK | Aerospace, spindle, large slewing |
| Boca Bearings | US | Hobby / small / ceramic specialty |
| Liebherr / Rothe Erde | DE | Large slewing rings (cranes, wind, mining) |
| GGB / Garlock, Igus | US/DE | Plain bushings (PTFE-lined, polymer iglide) |
| Kingsbury, Waukesha | US | Hydrodynamic tilting-pad journal & thrust |
Counterfeit risk is significant. Real-world counterfeit rate on common designations (6205, 6206, 6306) from non-authorised channels has been audited at 10–25 %. Telltales: poor packaging, missing or misregistered laser-etched part numbers, wrong cage colour, inconsistent dimensions. Buy through authorised distributors only: Motion Industries, Applied Industrial Technologies, BDI, MSC, Bearing Service, Kaman Industrial.
Standard bore series: miniature (< 10 mm, 1 mm increments — 683, 684, 685, … in the 9-radix; 624, 625 in the 6-radix), small (10–50 mm, 5 mm increments), medium (50–200 mm), large (> 200 mm, per-application), slewing (> 500 mm up to 6 m+, bespoke).
Pricing rough order of magnitude (2024 commodity-channel, single unit):
| Bearing | Price |
|---|---|
| 608-2RS (skateboard) | USD 0.50 (commodity) – 5 (premium) |
| 6205-2RS | USD 3 – 15 |
| 6206-2RS | USD 5 – 25 |
| 6308-2Z | USD 15 – 50 |
| 22216 EK spherical | USD 200 – 600 |
| 30206 tapered | USD 10 – 40 |
| Wind-turbine main spherical (~2 m) | USD 30 000 – 200 000 |
Ceramic / hybrid premiums: 3–10×. Aerospace / P4 / specialty: 5–50×.
10c. Failure modes & derating
ISO 15243:2017 classifies bearing damage in six primary modes with twelve sub-modes — this is the language used in failure-analysis reports across the industry. Every operator of rotating machinery should be able to read a damaged bearing and assign it to one of these.
10c.1 Subsurface fatigue (rolling-contact fatigue, RCF)
The designed end-of-life mode. A subsurface crack initiates at the depth of maximum orthogonal shear stress (≈ 0.5 mm below the race surface for a typical 6206), propagates parallel to the race for some millions of cycles, then turns upward and spalls a flake of material out of the race surface. The bearing then runs progressively rougher until vibration trips a protection threshold (or it eats itself). L_10 is the statistical 10 %-failure point of this mode under perfect lubrication and zero contamination. In clean modern installations with κ > 4 and good filtration, subsurface RCF is rare — the bearing tends to outlive other components.
10c.2 Surface-initiated fatigue (micropitting, peeling)
Common in boundary / mixed lubrication (κ < 1). Surface asperities are loaded directly; small flakes (10–100 μm) peel away. Macroscopically the race appears matte / grey. Causes premature life shortfall but doesn’t immediately fail the bearing — it accelerates wear and eventually triggers macro-spalling. Mitigation: raise κ via viscosity or temperature management; install finer filtration.
10c.3 Wear
- Abrasive wear — hard contamination particles ground between rolling element and race; the race takes on a uniform dull-grey appearance, dimensional loss is measurable. Source: ambient dust, broken-off paint flakes, weld spatter, casting sand. Mitigation: better seals, oil filtration (10 μm or finer absolute), keep-clean during install.
- Adhesive wear — under boundary lubrication; asperities cold-weld and tear, transferring material between surfaces. Often associated with starvation or extreme load.
10c.4 Corrosion
- Moisture corrosion — red rust, pitting, sometimes extensive after only days of water ingress. Mitigation: contact seals (2RS, lip seal); store bearings sealed in original packaging until install; use corrosion-resistant grease (lithium-calcium with rust inhibitor).
- Fretting corrosion — between bearing inner race and shaft (or outer and housing) with insufficient interference and small vibratory motion. Red-brown oxide debris (Fe₂O₃). Damages the shaft / housing surfaces, not the bearing per se, but the resulting wear creates running clearance loss. Mitigation: correct fit (don’t make the fit too loose), avoid set screws, hydraulic-fit assembly.
- False brinelling — closely related to fretting; vibration of a stationary bearing (e.g. shipping vibration of a stored motor, or stop-go duty of a paused conveyor) erodes the lubricant film at each ball/race contact and produces shallow polished spots at ball-pitch spacing. Looks like brinell indents but is wear, not plastic deformation. Mitigation: rotate stored equipment monthly; transport on isolators; use grease with anti-fretting additive (MoS₂, Zn-dithiophosphate).
- Chemical corrosion — acid / alkali / hydrogen sulphide attack. Mitigation: stainless or hybrid ceramic.
10c.5 Electrical erosion (the modern problem)
Variable-frequency drive (VFD) common-mode voltages induce shaft voltages of 5–40 V peak in AC motors. When this voltage exceeds the breakdown strength of the lubricant film (typically when speed is low and film is thin), it discharges through the bearing — punching microscopic pits in the race surface. Two visible signatures:
- Fluting — visible parallel washboard pattern around the race circumference, axial-direction grooves. Pathognomonic of bearing currents.
- Frosting / grey track — dense field of microscopic pits without macroscopic grooves; early-stage damage.
Mitigation:
- Insulated bearings (INSOCOAT / INS) on at least one motor bearing (typically the non-drive end).
- Hybrid ceramic balls (electrically insulating, $$$ but excellent — preferred for high-power EV motors).
- Shaft grounding rings (AEGIS SGR by Electro Static Technology, or Inpro/Seal MGS) — a carbon-fibre microbrush ring that gives shaft currents a non-bearing path to ground.
- Common-mode chokes at the VFD output — reduce CMV magnitude at source.
Industry standard for VFD-driven motors ≥ 22 kW: insulated NDE bearing and shaft grounding ring (NEMA MG 1 Part 31 / IEC 60034-25).
10c.6 Plastic deformation (overload)
Brinelling — static load above C_0 produces permanent indents at each rolling-element position; sources include shock loads on a stationary bearing, dropping the assembly, hammering on the wrong race during install. Smearing / scuffing — rolling element slides instead of rolling under lube starvation or sudden acceleration; localised welding and tearing. Skidding — at very light load or high acceleration the cage cannot drag rolling elements at full kinematic speed.
10c.7 Fracture & cage failure
Race fracture from gross overload, fatigue propagation, or thermal shock. Cage failure is more common: cage-pocket fatigue at speeds above the limiting value, or cage collapse from lube starvation that locks the rolling elements and transmits torque the cage was not designed for. Once the cage breaks, rolling elements bunch and the bearing seizes within seconds.
10c.8 Seizure
Terminal loss of bearing function from total lubricant loss, gross contamination, severe overload, or internal-clearance lockup from thermal differential. Usually the terminal event of a cascade that started weeks earlier — a healthy bearing does not seize.
10c.9 Lubricant degradation
Oxidation (base oil + heat + oxygen → polymerised residue; accelerates above 80 °C), contamination (water emulsifies grease, particulate causes abrasive wear, incompatible-thickener mixing turns grease to liquid), churning (over-filled bearing whips lubricant, oxidises it), and drying out (base oil migrates out of grease over years; thickener-only residue cannot lubricate — the end-of-life signature for sealed-for-life bearings).
10c.10 Derating
Catalogue L_10 is for ideal conditions. Real-world derating via a_ISO:
| Condition | a_ISO range |
|---|---|
| Heavily contaminated, marginal lube (κ < 0.5) | 0.1 – 0.3 |
| Typical industrial, moderate filtration (κ ≈ 1) | 0.5 – 1.0 |
| Clean process, good lube (κ ≈ 2, fine filtration) | 1 – 5 |
| Clean (filter ≤ 5 μm), excellent lube (κ ≥ 4), low load (P < C_u) | 5 – 50+ |
So an industrial bearing with catalogue L_10 = 30 000 h might actually deliver anywhere from 3 000 h (dirty + boundary) to 300 000 h (clean + full-film, very lightly loaded). Lubrication and cleanliness routinely dominate bearing life by 100× — this is what condition-based maintenance focuses on.
10c.11 Condition monitoring — vibration analysis
Bearing defects produce characteristic vibration tones at predictable frequencies, computable from geometry:
BPFI = (n/2) · (1 + (d_b/d_m) · cos α) · z (ball-pass frequency, inner race)
BPFO = (n/2) · (1 − (d_b/d_m) · cos α) · z (ball-pass frequency, outer race)
BSF = (n/2) · (d_m/d_b) · [1 − (d_b/d_m · cos α)²] (ball spin)
FTF = (n/2) · (1 − (d_b/d_m) · cos α) (fundamental train / cage)
with n = shaft rpm, d_b = ball diameter, d_m = pitch diameter, α = contact angle, z = number of rolling elements. A spectrum showing rising amplitude at BPFI / BPFO / BSF / FTF (and their harmonics, plus sidebands at shaft speed) is the diagnostic signature of bearing damage.
Tools: accelerometer + FFT analyser (Brüel & Kjær, SKF Microlog, Emerson CSI 2140, Fluke 805 / 810), envelope-detection demodulation (extracts impacts buried in broadband noise), shock-pulse method (SPM Instrument Sweden), acoustic emission, ultrasound (CTRL, UE Systems, SDT).
Practitioner certification: ISO 18436-2 vibration analyst categories I (data collector), II (basic analysis), III (full diagnosis + condition assessment), IV (root-cause + corrective recommendations). Major vendors (SKF, Schaeffler, Emerson, Brüel & Kjær) operate condition-monitoring services around their analyser hardware.
10c.12 Engineering judgement on derating
- Always design for a_ISO < 1. Assume your installation will not be cleaner than typical industrial. Catalogue C/P should yield ≥ 2× the required life before a_ISO is applied — that gives margin for the inevitable real-world degradation.
- For VFD motors, always specify insulated bearing + shaft grounding ring on motors ≥ 22 kW. This is no longer optional; bearing-current failure has overtaken classical fatigue as the dominant cause of premature motor-bearing replacement in modern industrial fleets.
- For oscillating duty (servomotors holding position with small dither, wave-energy converters, oscillating linkages), use plain bushings or four-point-contact bearings designed for oscillation. Rolling-element bearings false-brinell.
- For high-shock applications, double the static-load margin (s_0 ≥ 2). Brinelling is irreversible.
- For high-temperature service (> 150 °C operating), check three things: lubricant temperature limit (most greases drop out above 150 °C; PFPE-based are good to 250 °C), seal compatibility (NBR fails at 120 °C; FKM to 180 °C; PTFE to 250 °C), and operating clearance (use C3 or C4 to absorb thermal differential expansion).
- For long-storage duty, rotate the assembly periodically (monthly is the standard interval per OEM service docs) and ship on vibration isolators. False brinelling has destroyed motors and gearboxes during shipping.
11. Cross-references
- mechanics-of-materials — Hertzian contact stresses and the subsurface shear-stress field that drives RCF
- materials-steel — bearing steels: AISI 52100 / 100Cr6 (through-hardened), AISI 8620 / 4320 (case-hardened), AISI 440C and BG42 (corrosion-resistant)
- materials-ceramics — Si₃N₄ (silicon nitride) hybrid bearings; Al₂O₃ and SiC plain bearings
- electric-motors — every motor has bearings; VFD-induced shaft currents and the insulated-bearing / grounding-ring mitigation
- vibration-dynamics — bearing defect frequencies BPFI / BPFO / BSF / FTF used in condition monitoring
- gears-power-transmission — bearings + gears together inside every gearbox; preload, alignment, lubrication interactions
- bearings — Stribeck curve, EHL theory, viscosity selection
- machining — shaft and housing fits, surface finish for bearing seats, runout requirements
- fasteners-bolts — KM-series locknuts with MB tab washers used to retain tapered-roller and spherical-roller bearings on shaft
- seals-taxonomy — radial lip seals, bearing isolators, labyrinth seals
- manipulator-design — crossed-roller and four-point-contact bearings in robot joints; harmonic drive output bearings
- motors-electric — servo-motor bearing failure modes; current-induced damage in robot drive trains
12. Citations
- Harris, T. A.; Kotzalas, M. N. “Rolling Bearing Analysis,” 5th ed. (2 volumes), CRC Press, 2007. The canonical text — Hertzian contact, Lundberg–Palmgren life theory, kinematics, and thermal analysis from first principles.
- SKF General Catalogue, SKF Group, current edition (2024). Industry-standard practical reference — dimensions, ratings, suffix codes, lubrication, installation. Free PDF from skf.com.
- Schaeffler Technical Pocket Guide (TPI 144), Schaeffler Technologies AG, 2018+. Compact reference covering FAG and INA brands.
- Timken Engineering Manual (Bearing Selection, Mounting, Maintenance), Timken Co., current. Authoritative on tapered roller bearings.
- Hamrock, B. J.; Schmid, S. R.; Jacobson, B. O. “Fundamentals of Fluid Film Lubrication,” 2nd ed., CRC Press, 2004. Reynolds equation, hydrodynamic and EHL theory; Hamrock–Dowson film-thickness equation.
- Stachowiak, G. W.; Batchelor, A. W. “Engineering Tribology,” 4th ed., Butterworth-Heinemann, 2013. Standard graduate tribology text.
- ISO 281:2007 “Rolling bearings — Dynamic load ratings and rating life.” Defines C, P, L_10, L_nm, a_ISO, κ. Amended A1:2010, A2:2020.
- ISO 76:2006 “Rolling bearings — Static load ratings.” Defines C_0.
- ISO 5753-1:2009 “Rolling bearings — Internal clearance — Part 1: Radial internal clearance for radial bearings.” Codifies C2 / CN / C3 / C4 / C5.
- ISO 15243:2017 “Rolling bearings — Damage and failures — Terms, characteristics and causes.” Standardised vocabulary for failure-mode analysis.
- ISO 492:2014 “Rolling bearings — Radial bearings — GPS and tolerance values.” Defines P0 / P6 / P5 / P4 / P2.
- ABMA Std 9-2015 “Load Ratings and Fatigue Life for Ball Bearings”; ABMA Std 11-2014 for roller bearings — US equivalents of ISO 281 (closely harmonised).
- DIN 625-1:2024 “Rolling bearings — Radial deep groove ball bearings — Single row.” European dimension series for the 6xxx family.
- IEC 60034-25:2014 “Rotating electrical machines — Part 25: AC machines used in power drive systems”; NEMA MG 1 Part 31 — bearing-current mitigation requirements for VFD-driven motors.
- ISO 18436-2:2014 “Condition monitoring and diagnostics of machines.” Defines vibration-analyst certification categories I–IV.
- Lundberg, G.; Palmgren, A. “Dynamic Capacity of Rolling Bearings,” Acta Polytechnica Mech. Eng. Series Vol 1 No 3 (1947) & Vol 2 No 4 (1952). The foundational (C/P)^p life papers.
- Ioannides, E.; Harris, T. A. “A New Fatigue Life Model for Rolling Bearings,” ASME J. Tribology Vol 107 (1985), 367–378. The Ioannides–Harris extension that introduced C_u and the modified-life methodology in ISO 281:2007.