| name | bearing-temperature |
| description | Bearing temperature analysis — friction heat generation, SKF friction model, thermal resistance, maximum operating temperature, oil/grease temperature limits, thermal runaway, ISO 15312. |
| metadata | {"priority":7,"promptSignals":{"phrases":["bearing temperature","bearing heat generation","bearing friction power","bearing thermal","bearing overheating","bearing thermal limit"],"minScore":3}} |
Bearing Temperature Analysis — Complete Skill
Friction Heat Generation
SKF Friction Torque Model (Comprehensive)
Total friction torque:
M_total = M_rr + M_sl + M_seal + M_drag [N·mm]
Rolling friction torque M_rr:
M_rr = φ_ish × φ_rs × μ_rr × G_rr × (ν_oil × n)^0.6 [N·mm]
μ_rr = 0.0004–0.0015 (rolling coefficient; material/geometry dependent)
G_rr = f(bearing geometry, d_m)
ν_oil = oil viscosity at operating T [cSt]; n = speed [RPM]
φ_ish = inlet shear heating reduction factor; φ_rs = kinematic replenishment factor
Sliding friction torque M_sl:
M_sl = G_sl × μ_sl [N·mm]
μ_sl = 0.04–0.15 (sliding coefficient; lubrication dependent)
G_sl = f(bearing load, geometry, contact area)
Seal friction torque M_seal:
M_seal = K_S1 × d_S^β + K_S2 [N·mm; d_S = shaft diameter; K_S1, K_S2 from bearing type]
Shielded bearing: negligible; Rubbing lip seal: significant
Drag/churning friction M_drag:
M_drag = V_m × K_drag × (n × d_m)² × ρ_oil [N·mm; V_m = oil level factor; K_drag = geometry factor]
Significant at high speeds or high oil level
Simplified Friction Power
For initial estimates:
P_friction = M_total × ω = M_total × 2π n / 60 [W; M in N·mm; n in RPM]
Divide by 1000: P_friction [W] = M_total [N·mm] × n [RPM] / 9549
Typical range:
Lightly loaded bearing (P < 0.1C): P_friction ≈ 0.5–5 W
Heavily loaded (P > 0.3C): P_friction ≈ 5–50 W
High speed, large bearing: P_friction ≈ 100–1000 W
Thermal Resistance and Temperature Rise
Thermal network model:
T_bearing = T_ambient + P_friction × R_thermal_total [°C]
Thermal resistance components:
R_inner = resistance from inner ring to shaft → shaft temperature
R_outer = resistance from outer ring to housing → housing temperature
R_radiation+convection = outer housing to air
Approximate thermal resistance (per bearing):
| Housing type | R_thermal [°C/W] |
|---|
| Isolated (no oil flow) | 20–50 |
| Good air circulation | 10–20 |
| Oil-circulating (forced) | 1–5 |
| Water-cooled housing | 0.5–2 |
Equilibrium temperature:
T_bearing = T_ambient + P_friction × R_outer
For R_outer = 30°C/W, P_friction = 10 W: ΔT = 300°C → unacceptable → need cooling
For P_friction = 1 W: ΔT = 30°C → acceptable (40°C ambient → 70°C bearing)
Maximum Bearing Temperature Limits
Standard bearing steel (52100):
T_max = 120°C continuous (above this: ring softening → dimensional change → clearance change)
Short-term T_max = 150°C (for special heat-stabilized bearings, marked S1/S2)
Dimensionally stabilized bearings:
S1 (stabilized to 150°C): suffix S1; T_max continuous = 150°C
S2 (200°C): T_max = 200°C; higher T_max steel or ceramics
S4 (250°C): T_max = 250°C; used in baking ovens, sterilization
Ceramic balls (Si₃N₄):
T_max > 500°C for ball itself; limited by steel ring: 120°C standard
Hybrid bearing: Si₃N₄ balls + steel rings; advantage = lower inertia, not temperature
Grease temperature:
Standard grease: T_max = 120°C (mineral oil base)
Synthetic (PAO or ester): T_max = 150°C
High-temperature grease (silicone or fluorocarbon): T_max = 200–260°C
If T_bearing > T_grease_limit → relubrication interval must be shortened drastically
Oil temperature:
ISO VG 46 mineral oil: T_max operating = 90°C; oxidizes above 90°C → shortened oil life
Synthetic oil: T_max = 120–150°C
Relubrication Interval vs. Temperature
SKF relubrication interval tf:
tf = t_f0 × [2^((T_ref - T_bearing) / 15)] [hr; t_f0 = base interval at 70°C; halves for each 15°C increase]
Example:
t_f0 = 5000 hr at 70°C
At T_bearing = 100°C: tf = 5000 × 2^((70-100)/15) = 5000 × 2^(-2) = 1250 hr
At T_bearing = 120°C: tf = 5000 × 2^(-3.33) = 5000 × 0.1 = 500 hr
Thermal Runaway Detection and Prevention
Thermal runaway mechanism:
Preload → friction → heat → thermal expansion → more preload → more friction (positive feedback loop)
Most common in rigid-preloaded angular contact spindle bearings
Check:
dF_preload/dT > k_thermal_dissipation → runaway possible
Check: (k_bearing × α_shaft × L_shaft) > (k_thermal × A_housing) [conservative criterion]
Prevention:
Use spring preload (constant force) for high-speed spindles
Ensure adequate ventilation or active cooling
Include thermal break in spindle housing design
Monitoring
RTD (Pt100) placement:
Mount in housing near outer ring; distance < 5 mm from bearing (thermal lag reduces accuracy)
Response time: sensor + housing thermal mass → typical lag 1–5 min
IR thermometry:
Non-contact; useful for moving shafts; cannot read through oil film
Thermocouple: faster response than RTD; useful for dynamic T monitoring
Alarm and trip settings (typical):
Alarm: T_bearing > 80–90°C (above baseline by 20°C)
Trip: T_bearing > 100–110°C
Standards
| Standard | Scope |
|---|
| ISO 15312 | Thermal speed limits for bearings |
| ISO 14839 | Rotating machinery monitoring |
| ISO 13373 | Condition monitoring procedures |
| SKF Bearing Technical Handbook | Friction model, thermal limits |
Output
Provide: friction torque components M_rr + M_sl + M_seal + M_drag [N·mm], total friction power P_friction [W], bearing equilibrium temperature ΔT [°C] above ambient, check vs. bearing steel limit (120°C), grease temperature limit vs. operating temperature, relubrication interval tf [hours], thermal resistance R_thermal [°C/W] for housing configuration, thermal runaway risk (yes/no with criterion check), temperature monitoring type and alarm setpoints [°C], and applicable standard (ISO 15312, ISO 13373).
