bearing-lubrication

name	bearing-lubrication
description	Bearing lubrication — grease vs oil, viscosity selection (ISO VG grades), re-lubrication intervals, contamination control, temperature rise, starvation and over-lubrication.
metadata	{"priority":7,"promptSignals":{"phrases":["bearing lubrication","bearing grease","bearing oil","viscosity selection","re-lubrication","lubricant selection"],"minScore":3}}

Bearing Lubrication — Complete Skill

Purpose of Lubrication

• Separate rolling elements from raceways (elastohydrodynamic film)
• Reduce friction and wear at contact zones
• Carry heat away from bearing
• Protect against corrosion and contamination

Grease vs. Oil

Grease (80% of applications)

Composition: base oil (70–95%) + thickener (lithium, polyurea, calcium) + additives Advantages: simple, sealed, low maintenance, contamination resistance Limitations: cannot cool; limited speed; life finite

Speed factor limit (grease): n × d_m ≤ grease speed rating (typically 300,000–500,000 for lithium NLGI 2) d_m = (d+D)/2 = pitch diameter [mm]; n = speed [rpm]

NLGI consistency grades:

NLGI	Penetration	Consistency	Use
0	355-385	Very soft	Centralized systems
2	265-295	Standard	Most bearings
3	220-250	Stiff	Vertical shafts, vibration

Oil

Advantages: cooling, easy contamination control, high speed, controlled supply Types: mineral (ISO VG 32–680), synthetic (PAO, ester), gear oils (GL grades)

Oil bath: shaft submerged to 30–50% of lowest rolling element at rest Circulation: mandatory for high power/speed; oil temperature ≤ 60°C target

Viscosity Selection

Reference viscosity ν₁ (ISO chart method — SKF/FAG): ν₁ depends on n × d_m and bearing type (ball vs. roller) Ball bearings typical: ν₁ = 7–50 mm²/s at operating temp Roller bearings: ν₁ = 2× that of ball bearings (higher contact stress)

Viscosity ratio κ = ν_actual / ν₁: κ ≥ 1 → adequate lubrication (full EHD film likely) κ ≥ 4 → excellent; κ = 0.5–1.0 → marginal; κ < 0.5 → boundary lubrication → wear

Viscosity at operating temperature: ν₂ = ν₄₀ × exp(B × (1/T_op - 1/313)) (approximate Walther-ASTM equation) Use ASTM D341 viscosity-temperature chart for accurate interpolation

ISO VG grades (kinematic viscosity at 40°C): VG 32: 32 mm²/s; VG 46: 46; VG 68: 68; VG 100: 100; VG 150: 150; VG 220: 220

Re-lubrication Intervals (SKF Method)

Grease relubrication interval: t_f = (14,000,000/(n √d_m) - 4d_m) × f_t × f_s × f_v × f_h × f_l

Correction factors (from SKF catalog):

• f_t = temp factor: 1.0 at 70°C; 0.5 at 90°C; 0.25 at 110°C
• f_s = contamination: clean=1.0; moderate=0.7; severe=0.4
• f_v = vibration: normal=1.0; heavy=0.67; shock=0.5
• f_h = mounting position: horizontal=1.0; vertical=0.5
• f_l = bearing type: DGBB=1.0; CRB=0.5; DGBB open=1.5

Grease quantity per re-lubrication: G_p = 0.005 × D × B [grams] (D = bearing OD, B = bearing width [mm])

Fill Quantity

Grease fill factor = 30–50% of free bearing space Over-greasing causes overheating (shear heat in excess grease) Under-greasing causes starvation → wear

Check for overheating: surface temperature should be ≤ 60°C (normal); ≤ 80°C (elevated) Temperature rise > 15°C above ambient at steady state → investigate

Contamination Control

Contamination factor ISO 4406: ISO Cleanliness code: 18/16/13 = typical hydraulic; 15/13/11 = precision bearing Solid contamination → surface distress → shortened life by factor 5–50×

Sealing: Contact seals (lip seals): effective barrier; friction heat; wear over time Labyrinth seals: non-contact; lower friction; less effective at contamination exclusion Shielded/sealed bearings: 2RS, 2Z — self-contained; 1–3 year sealed life

Solid Lubricants

Used when: oil/grease inapplicable (vacuum, extreme temp, chemical) Molybdenum disulfide (MoS₂): excellent up to 300°C in air; 1200°C in vacuum PTFE: low friction; to 260°C; poor thermal conductivity Graphite: requires moisture; not suitable in dry or vacuum Tungsten disulfide (WS₂): space applications; vacuum stable

Output

Provide: lubricant type (grease/oil), NLGI grade or ISO VG grade, operating viscosity ν [mm²/s], κ ratio, re-lubrication interval [hours], fill quantity [grams or liters], sealing recommendation, temperature limit.