ball-bearing-selection

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

1. Determine bore d, OD D, width B (series 6000, 6200, 6300, 7000, 7200)
2. Read C and C_0 from catalog
3. Compute P = XF_r + YF_a
4. Calculate L_10h — compare to design life requirement
5. Check speed vs. n_ref
6. 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.