| name | ball-bearing-selection |
| description | Ball bearing selection — deep groove, angular contact, dynamic/static load capacity, L10 life (ISO 281), speed limits, preload, fits, lubrication. |
| metadata | {"priority":8,"promptSignals":{"phrases":["ball bearing","bearing selection","bearing life","L10 life","deep groove ball bearing","angular contact bearing","bearing load capacity"],"minScore":3}} |
Ball Bearing Selection — Complete Skill
Bearing Types Overview
| Type | Radial | Thrust | Speed | Notes |
|---|
| Deep groove (DGBB) | High | Moderate | High | Most versatile; single row |
| Angular contact (ACB) | High | High (1-dir) | High | Must mount in pairs for thrust both ways |
| 4-point contact | High | Both dir | Moderate | Compact; higher friction |
| Self-aligning | Moderate | Low | Moderate | Tolerates misalignment |
Load Capacity (ISO 281)
Dynamic load rating C [kN]: from catalog; basis for life calculation
Static load rating C_0 [kN]: maximum load before plastic deformation (rolling contact stress < 4200 MPa)
L10 Life Calculation (ISO 281)
Basic rating life:
L_10 = (C/P)^p [millions of revolutions]
p = 3 (ball bearings); p = 10/3 (roller bearings)
Equivalent dynamic load:
P = X F_r + Y F_a [N]
X, Y = radial/axial factors from catalog (depend on F_a/F_r and F_a/C_0)
For DGBB: if F_a/F_r ≤ e, P = F_r; if F_a/F_r > e, P = 0.56F_r + Y F_a
In hours:
L_10h = L_10 × 10⁶ / (60 × n) where n = speed [rpm]
Modified Life (ISO 281:2007, a_ISO)
L_nm = a_1 × a_ISO × L_10
a_1 = reliability factor: L_10: a_1=1.0; L_5: a_1=0.62; L_1: a_1=0.21
a_ISO: function of κ (viscosity ratio) and fatigue load limit C_u/P
κ = ν_actual / ν_1 (ν_1 = reference kinematic viscosity from chart)
High κ (good lubrication) → high a_ISO → better life
Speed Limits
Reference speed n_ref [rpm]: from catalog (thermal reference speed)
Limiting speed n_lim [rpm]: max mechanical speed (cage/lubricant constraint)
Reduce n_ref by: cage type, lubrication type, temperature factors
ndm parameter: n × d_m [mm·rpm] where d_m = (d+D)/2
DGBB: n d_m ≤ 600,000–1,200,000 (grease); ≤ 1,500,000–2,000,000 (oil)
Preload (Angular Contact Bearings)
Preload eliminates internal clearance → removes play, increases stiffness, improves accuracy
Fixed preload: springs or shims apply fixed axial force F_pre
Position preload: spacers fix axial position (preload varies with temperature)
Recommended preload: F_pre = 0.005 C to 0.02 C (light to medium)
Effect on life: preload adds to load P_total → shorter life; trade-off vs. stiffness
Bearing Fits
Inner ring (rotating inner ring is standard):
Shaft fit: interference (k5, m5, n5 for normal-heavy loads)
Inner ring rotates → must be interference fit to prevent creep
Outer ring:
Housing fit: sliding (H7, G7 for normal; P7 for heavy shock)
Fixed outer ring → transition fit acceptable; floating end → sliding fit
Clearance vs. preload:
Standard: CN (C0) clearance → slight internal play before mounting
Light interference mounting → C2 (less than CN) or CN preloaded to zero play
Lubrication
Grease: 70–80% of all bearing applications
Fill factor: 30–50% of bearing free space (over-packing causes overheating)
Re-lubrication interval: L_10 / 5 to L_10 / 3 typical
Oil (circulating or bath): high speed or high temperature
Oil viscosity at operating temperature: ν_min from ISO chart for bearing size and speed
SKF/FAG Catalog Usage
- Determine bore d, OD D, width B (series 6000, 6200, 6300, 7000, 7200)
- Read C and C_0 from catalog
- Compute P = XF_r + YF_a
- Calculate L_10h — compare to design life requirement
- Check speed vs. n_ref
- Specify fit and lubrication
Common Series
| Series | Bore range | Notes |
|---|
| 6000 | 10–150 mm | Extra light; low noise |
| 6200 | 10–150 mm | Light series; most common |
| 6300 | 10–150 mm | Medium; higher C |
| 7200 | 10–150 mm | Angular contact 15° |
| 7300 | 10–150 mm | Angular contact 25° |
Output
Provide: bearing designation (e.g., SKF 6210), C [kN], C_0 [kN], P [N], L_10h [hours] vs. design target, speed [rpm] vs. n_ref, fit specification (shaft k5, housing H7), lubrication type and interval.
