| name | bearing-preload |
| description | Bearing preload — angular contact/tapered roller, axial preload types (rigid/spring), contact angle change, stiffness vs. preload, heat generation, DB/DF/DT arrangements, FAG/SKF preload tables. |
| metadata | {"priority":7,"promptSignals":{"phrases":["bearing preload","angular contact preload","axial preload bearing","bearing stiffness preload","duplex bearing","face-to-face back-to-back"],"minScore":3}} |
Bearing Preload — Complete Skill
Purpose of Preload
Preload: internal axial force applied to bearing before external loading
Benefits:
- Increases stiffness (critical for precision spindles)
- Eliminates clearance → no dead-band in response
- Improves running accuracy (reduces runout)
- Reduces vibration susceptibility
Drawbacks:
- Generates heat (friction loss)
- Reduces fatigue life if excessive
- Temperature sensitive (rigid preload)
Duplex Bearing Arrangements
DB (Back-to-Back / O arrangement):
Contact angle lines diverge outward
Better moment capacity (wide reaction span)
Best for: combined radial + overturning moment loads
Thermal expansion changes preload slightly
DF (Face-to-Face / X arrangement):
Contact angle lines converge inward
More sensitive to misalignment (shorter reaction span)
Less rigid than DB; better for short overhanging shafts
Thermal expansion partially relieves preload (more stable)
DT (Tandem arrangement):
Both bearings carry load in same direction
Higher axial capacity (both share thrust load)
Cannot handle bidirectional thrust; use with opposing pair
Notation: DB/DF/DT for paired bearings; universal match (angular contact) grindable for any arrangement
Preload Quantification
Rigid preload (spacer-controlled):
Inner and outer spacers of different length → creates exactly δ_preload of axial displacement
F_preload = k_pair × δ_preload [N; k_pair = combined axial stiffness of pair [N/m]]
Spring preload (constant force):
Spring stack gives constant F_preload regardless of temperature
Better for temperature-cycling environments
Preload classes (angular contact 7xxx series):
| Class | Symbol | Preload [N] (typical 30 mm bore) |
|---|
| Light | L or LP | 10–50 |
| Medium | M or MP | 50–200 |
| Heavy | H or HP | 200–800 |
Exact values: from manufacturer tables for each designation (SKF, FAG, NSK preload tables)
Stiffness vs. Preload
Hertz contact (ball-raceway):
δ = K_Hz × F^(2/3) [nonlinear spring; stiffness increases with load]
k = dF/dδ = (3/2) × F^(1/3) / K_Hz [N/m; stiffness higher at higher load (preload)]
Axial stiffness of preloaded pair:
At zero external load (only preload F_p):
k_axial = (3/2) × F_p^(1/3) × K_stiffness [N/μm; K from bearing geometry]
Effect of preload on stiffness:
Double the preload → stiffness increases by 2^(1/3) ≈ 1.26× (nonlinear)
Example (7206 bearing, F_p = 100 N):
k_axial ≈ 35 N/μm (light preload)
F_p = 400 N: k_axial ≈ 55 N/μm (heavy preload)
Contact Angle Change with Preload
Free contact angle α₀ (from catalog: typically 15°, 25°, or 40° for angular contact)
Under axial preload F_a:
cos α = cos α₀ × (1 + (F_a / K_const)^(2/3))^(-1/2) [contact angle increases with axial load]
Effect on load capacity:
Higher contact angle → higher axial capacity; lower radial capacity
Typical change: 15° → 20–25° under operating preload
Heat Generation from Preload
Bearing friction power (SKF model):
P_friction = M_total × ω [W; ω in rad/s]
M_total = M_rolling + M_sliding + M_seal + M_drag
Preload contribution to M_rolling:
M_rolling ∝ F_p^(1/3) × ν_oil^(0.6) × n^(0.6) × d_mean [proportional to preload; increases with speed]
Temperature rise (equilibrium):
ΔT = P_friction / (k_thermal × A_housing + Q_oil) [°C; k_thermal = housing conductance; A = surface area]
Rigid preload: ΔT increases preload → more friction → higher ΔT → risk of thermal runaway (rare but possible)
Thermal runaway check:
k_bearing × α_shaft × L × ΔT > F_preload_limit → unsafe
Use spring preload or calculate maximum allowable ΔT for rigid preload
Selecting Preload Level
Machine tool spindle (high precision):
Medium to heavy preload; k_axial target = 100–500 N/μm
Heavy preload at low speed; light preload at high speed (some spindles use adjustable preload)
High-speed spindle (> 20,000 RPM):
Speed parameter dn = n × d_bore [mm·RPM]
For dn > 10⁶: use light or super-light preload to limit heat
For dn = 0.5–1.0×10⁶: medium preload OK
General industrial:
Light to medium preload; stiffness less critical than life
Testing and Verification
Measuring preload (after assembly):
Running torque method: measure starting torque at low RPM; compare to torque vs. preload curve from manufacturer
Vibration/noise test: run-up; check 1× runout and spindle frequency response
Runout specification (ISO 1132):
Single bearing runout; axial + radial; affected by preload uniformity
Machine tool spindle: axial + radial runout ≤ 1–5 μm (depending on class)
Standards and References
| Standard | Scope |
|---|
| ISO 15241 | Angular contact ball bearing dimensions |
| ISO 5593 | Rolling bearing terminology |
| FAG / SKF Bearing Catalogues | Preload tables by designation |
| JIS B 1521 | Ball bearing preload (Japanese) |
Output
Provide: bearing arrangement (DB/DF/DT), preload class (L/M/H), preload force F_p [N], axial stiffness k_axial [N/μm] at preload, contact angle change α₀ → α [°], friction power P_friction [W], temperature rise estimate ΔT [°C] at operating speed, thermal runaway check, speed parameter dn [mm·RPM] vs. limit, spring vs. rigid preload recommendation, running torque verification value [N·mm], and applicable standard (ISO 15241, SKF/FAG preload tables).
