bearing-clearance

name	bearing-clearance
description	Bearing clearance — radial internal clearance (RIC), thermal expansion, press-fit reduction, operational clearance, ISO 5753 clearance groups, preload vs. clearance effects on stiffness and life.
metadata	{"priority":7,"promptSignals":{"phrases":["bearing clearance","radial internal clearance","bearing preload","ISO 5753 clearance","operating clearance bearing","thermal clearance bearing"],"minScore":3}}

Bearing Clearance — Complete Skill

Radial Internal Clearance (RIC) Definition

RIC: total free radial movement of inner ring relative to outer ring (before installation) Measured under light measurement load (no load, room temperature)

ISO 5753 clearance groups (radial ball bearings):

Group	Symbol	Clearance range (typically for 60–80 mm bore)
C2	Smaller than Normal	15–35 μm
CN (Normal)	CN	25–50 μm
C3	Greater than Normal	35–70 μm
C4	Greater than C3	55–100 μm
C5	Greater than C4	90–150 μm

(Actual ranges depend on bearing bore diameter — see ISO 5753 tables)

Clearance Reduction from Mounting

Press-fit on shaft (inner ring expansion): δ_inner = Δr_fit × (d / D_inner)^0.6 [approximate; Δr_fit = interference fit amount [μm]] Δ_inner = clearance reduction from inner ring mounting For solid shaft with interference: Δ_inner ≈ 0.7–0.85 × interference [depends on ring stiffness vs. shaft stiffness]

Press-fit in housing (outer ring contraction): Δ_outer ≈ 0.7–0.85 × interference [similar reduction factor]

Total clearance reduction from mounting: Δ_mount = Δ_inner + Δ_outer [μm] If transition fit (interference): significant reduction; if clearance fit: Δ = 0

Thermal Clearance Reduction

Differential thermal expansion: ΔT = temperature difference between inner ring (on shaft) and outer ring (in housing) Δ_thermal = α_shaft × d × ΔT_inner - α_housing × D_outer × ΔT_outer α_steel = 11.7×10⁻⁶ /°C

Typical ΔT: Well-lubricated bearing: ΔT = 5–15°C (inner ring hotter) Poorly lubricated: ΔT = 20–40°C

Example: Bearing bore d = 60 mm; ΔT = 10°C Δ_thermal = 11.7×10⁻⁶ × 60 × 10 × 1000 = 7 μm inner ring expansion → reduces clearance by ~7 μm

Operational (Running) Clearance

Operating clearance: C_op = C_ISO - Δ_mount - Δ_thermal [μm] C_op should be ≥ 0 (positive clearance) for most applications

Target operating clearance: Typical: C_op = 5–20 μm for most industrial applications C_op = 0 (zero clearance): maximum stiffness but no margin; temperature sensitive C_op < 0 (preload): highest stiffness; generates heat; reduces life if excessive

Selection guide:

• C_op > 20 μm: adequate for light loads, high speeds, high vibration environments
• C_op = 5–20 μm: normal operating condition
• C_op = 0 to -5 μm (slight preload): precision spindles, machine tools

Effect of Clearance on Bearing Life and Load Distribution

Load zone angle ψ: For bearing with clearance δ₀, applied load P: ψ = arc cos(1 - 2ε) [load zone half-angle; ε = (δ₀/2)/(C/2); load zone parameter] ε = 0: half-race loaded; ε = 0.5: exactly half; ε → 1: full race (preload)

Stribeck factor Z_r (radial load distribution): C_r = Z_r × n × (P/C)^1 [simplified; accounts for number of loaded balls] Z_r depends on ψ; fewer balls loaded → lower life

Equivalence for ISO life calculation: Higher positive clearance → fewer balls in load zone → concentrated load → lower life Preload → more balls share load → higher C/P ratio → longer life (up to optimum)

Optimal clearance for maximum life: Slight preload (δ₀ = -2 to -5 μm) gives maximum life (all balls share load) Too much preload → excessive contact stress → heat → reduced life

Preload Types

Spring preload: axial preload from spring stack (constant force preload) Rigid preload: spacer sets exact preload; varies with temperature (sensitive)

Preload quantification: F_preload = k_bearing × δ_preload [N; k_bearing = axial stiffness; δ_preload = preload displacement] Typical angular contact pair: δ_preload = 5–20 μm → F_preload = 50–500 N (depends on bearing size)

Stiffness vs. preload: k_axial = dF/dδ = (3/2) C_e × δ^(1/2) × (k_material × contact) [Hertz nonlinear; stiffness increases with preload] Heavy preload: much stiffer but generates heat

Measurement of RIC

Manual gauge method (ASTM): Feeler gauge: not accurate enough for precision bearings Dial indicator with light load (typically 1–5 N): preferred Rock inner ring radially; measure total displacement with dial gauge

Measurement machine: Automated RIC tester (manufacturer); repeatable to ±2 μm

Standards

Standard	Scope
ISO 5753	Internal clearance; all tables
ISO 15242	Bearing measurement methods
SKF, FAG, NSK technical specs	Specific bearing clearance selection guides
ISO 492	Tolerances for radial bearings

Output

Provide: ISO clearance group (C2/CN/C3/C4/C5), RIC from ISO 5753 table [μm] for bearing bore, clearance reduction from inner ring press-fit Δ_inner [μm], clearance reduction from outer ring press-fit Δ_outer [μm], thermal reduction Δ_thermal [μm] at ΔT [°C], operating clearance C_op [μm], assessment (positive/zero/preload), load zone parameter ε and Z_r factor, effect on L10 life [%], preload force if applicable [N], and applicable standard (ISO 5753, ISO 492).