| name | bearing-selection |
| description | Rolling element bearing selection — L10 life, dynamic/static load ratings, equivalent load, speed limits, preload, mounting, lubrication. Covers ball, roller, thrust, tapered, spherical bearings per ISO 281. |
| metadata | {"priority":8,"promptSignals":{"phrases":["bearing","bearing selection","L10 life","bearing life","ball bearing","roller bearing","radial load","thrust load"],"minScore":4}} |
Bearing Selection — Complete Skill (ISO 281)
Step 1: Load Analysis
Identify:
- F_r = radial load (N or lbf)
- F_a = axial/thrust load (N or lbf)
- n = operating speed (rpm)
- L_req = required life (hours)
- T = operating temperature
Step 2: Bearing Type Selection
| Type | Radial | Axial | Speed | Misalignment |
|---|
| Deep groove ball (6xxx) | Good | Moderate (both) | High | Low (0.1°) |
| Angular contact ball (7xxx) | Good | High (one dir) | High | Very low |
| Cylindrical roller (N,NU,NJ) | Excellent | None (N,NU) or partial (NJ) | High | Very low |
| Tapered roller (30xxx) | Good | High (one dir) | Medium | Very low |
| Spherical roller (22xxx) | Excellent | Moderate | Medium | High (2-3°) |
| Thrust ball (511xx) | None | Good | Low | None |
| Needle roller (NA,RNA) | Very high | None | Medium | None |
Decision rule:
- F_a/F_r < 0.35: use deep groove ball or cylindrical roller
- F_a/F_r = 0.35-0.7: use angular contact (pair) or tapered roller
- F_a/F_r > 0.7: use angular contact (pair) or thrust bearing
- Misalignment expected: use spherical roller
Step 3: Equivalent Dynamic Load
Deep groove ball bearing:
If F_a/(C_0·e) ≤ e: P = F_r
If F_a/(C_0·e) > e: P = X·F_r + Y·F_a
(X, Y, e from bearing catalog — function of F_a/C_0)
Angular contact bearing (paired):
P = 0.4F_r + 0.4F_a (for X=0.4, Y=0.4 — use catalog values)
General: P = X·V·F_r + Y·F_a (V = 1 for rotating inner ring, 1.2 for rotating outer)
Step 4: Required Dynamic Capacity
L10 life: L_10 = (C/P)^p · (10⁶/60n) hours
→ C = P · (L_req · 60n / 10⁶)^(1/p)
Where p = 3 (ball bearings), p = 10/3 (roller bearings)
With reliability factor a₁:
L_10m = a₁ · a_SKF · (C/P)^p (ISO 281:2007 — modified rating life)
a₁ = 1.0 (90% reliability), 0.62 (95%), 0.33 (99%)
Modified: ISO life adjustment aISO:
Considers lubrication (κ = ν/ν₁) and contamination (e_c)
L_nm = a₁ · a_ISO · L_10
Step 5: Viscosity & Lubrication
Reference viscosity ν₁ (mm²/s):
ν₁ from bearing catalog nomogram (function of bearing mean diameter d_m = (d+D)/2 and speed n)
Actual viscosity κ = ν/ν₁:
κ ≥ 2: excellent (full EHL film)
κ = 1-2: adequate
κ < 1: boundary lubrication (reduce L10 or use additives)
Relubrication interval:
t_f = (14000/√d_m - 4·n) · (1/f_t) hours (f_t = temperature correction)
Step 6: Speed Limits
Grease: n_grease = reference speed from catalog (typically 0.5-0.8 × n_ref)
Oil: n_oil = reference speed from catalog
ndm limit: n·d_m < limit (varies by type: ball=500,000-2,000,000 mm·rpm)
Step 7: Static Load Check
Static safety factor: s₀ = C₀/P₀ ≥ 0.5 (smooth) to 2.0 (shock)
P₀ = X₀·F_r + Y₀·F_a (equivalent static load — catalog values)
Step 8: Fits and Mounting
Rotating inner ring (most common):
Shaft tolerance: k5, m5, n5 (light to heavy interference by bore size and load)
Housing tolerance: H7 (normal clearance fit)
Rotating outer ring:
Housing: M6, N6 (interference)
Shaft: g6 (clearance)
Interference fit stress:
σ_hoop = (δ·E)/(2r_i) · (r_o²-r_i²)/r_o² (thick wall cylinder theory)
Check yield at bore
Step 9: Preload (Angular Contact & Tapered)
- Back-to-back (DB): higher stiffness, handles moment loads
- Face-to-face (DF): self-aligning tendency, handles shaft deflection
- Tandem (DT): both carry axial in same direction
Light preload: P_axial = 0.02C (spindle bearings, low vibration)
Medium preload: P_axial = 0.05C (general purpose)
Heavy preload: P_axial = 0.1C (high stiffness required)
Step 10: Bearing Number Selection
Based on required C and bore d, select from catalog:
- Series 62xx: light (d=10-150mm)
- Series 63xx: medium
- Series 64xx: heavy
Select smallest outside diameter that provides required C.
Output
State: bearing type, designation (e.g., 6208), C required, C selected, L10 actual (hours), κ, fit recommendation.
