lubricant-selection

name	lubricant-selection
description	Lubricant selection — viscosity grading (ISO VG, SAE), viscosity index, mineral vs. synthetic (PAO, ester, polyglycol), additive packages (AW, EP, DI), application matching (gears/bearings/engines/compressors), operating temperature range, compatibility, oxidation stability, ASTM D445/D2272, lubrication regime (boundary/mixed/full film).
metadata	{"priority":7,"promptSignals":{"phrases":["lubricant selection","lubricant grade","oil viscosity selection","gear oil selection","lubricant specification","ISO VG grade"],"minScore":3}}

Lubricant Selection — Complete Skill

Viscosity Fundamentals

Viscosity Parameters

Dynamic (absolute) viscosity: τ = μ × (du/dy) [Pa·s; τ = shear stress; du/dy = velocity gradient; μ = dynamic viscosity]

Kinematic viscosity: ν = μ / ρ [m²/s; or cSt = mm²/s; 1 cSt = 10⁻⁶ m²/s]

Viscosity at 40°C (ν₄₀) and 100°C (ν₁₀₀): standard measurement points per ISO standards

ISO VG (Viscosity Grade) — ISO 3448

ISO VG grade defined at 40°C:

ISO VG	ν₄₀ [cSt]	Range ν₄₀ [cSt]
22	22	19.8–24.2
32	32	28.8–35.2
46	46	41.4–50.6
68	68	61.2–74.8
100	100	90.0–110
150	150	135–165
220	220	198–242
320	320	288–352
460	460	414–506

Viscosity Index (VI)

VI: describes how viscosity changes with temperature; higher VI = less viscosity change VI < 0: poor (steep viscosity-temperature relationship) VI = 0–100: conventional mineral oils (VI = 95–100 typical) VI = 100–120: high VI paraffinic mineral or VHVI Group II VI = 150–200+: synthetic (PAO) or multigrade with VI improver

ASTM D2270 VI calculation: VI = (L - U) / (L - H) × 100 [L, H = reference oil kinematic viscosities at 40°C; U = oil being tested at 40°C; from ASTM D2270 tables]

Pourpoint: lowest temperature at which oil flows; ASTM D97; typically -15°C to -50°C for mineral; -60°C for PAO

Viscosity at temperature T (Walther equation): log(log(ν + 0.7)) = A - B × log(T) [A, B = constants from two-point viscosity data; T in Rankine or Kelvin]

Base Oil Types

Group Classification (API)

API Group	Characteristics	VI	Sulphur	Applications
Group I	Solvent refined mineral; conventional	80–120	> 0.03%	General industrial; lower cost
Group II	Hydroprocessed mineral	80–120	≤ 0.03%	Premium industrial/automotive
Group III (VHVI)	Hydrocracked mineral (essentially synthetic)	> 120	≤ 0.03%	Automotive engine oils
Group IV	PAO (polyalphaolefin) synthetic	120–150+	0	Premium gear/bearing/engine
Group V	Other (esters, polyglycols, silicones)	varies	varies	Specialty applications

PAO (Polyalphaolefin — Group IV)

Advantages over Group I/II mineral:

• Higher VI (130–150): less viscosity change with temperature
• Lower pour point (-60°C to -70°C): excellent cold starts
• Better thermal/oxidation stability: longer service life
• Clean (no wax, sulfur, aromatics): compatible with seals
• Lower evaporation (Noack volatility ASTM D5800): reduced oil consumption

Limitations:

• Poor solvency for additives (requires ester co-solvent, typically 5–15% ester)
• 3–5× cost vs. mineral; 2× cost vs. Group III
• Dissolves some elastomers (nitrile) → verify seal compatibility

PAO viscosity grades: PAO 2, 4, 6, 8, 10, 40, 100 (numbers = ν₁₀₀ [cSt]) PAO 4 → ISO VG 32 blended oil; PAO 6 → ISO VG 46; PAO 100 → ISO VG 460

Ester Oils (Group V)

Diester: low viscosity (VG 22–46); aviation turbine oils (MIL-PRF-23699); excellent lubricity Polyol ester (POE): refrigeration compressor lubricants (miscible with HFC refrigerants R134a, R410A); MIL-PRF-32073 Phosphate ester: fire-resistant hydraulic fluid (ISO 12922 HFDR); aircraft hydraulic systems (Skydrol) Trimellitate: very high-temperature stability (> 200°C); industrial gear/bearing

Caution: esters hydrolize in presence of water → acid + alcohol → corrosion; must keep dry

Polyalkylene Glycol (PAG — Group V)

Water-soluble or water-insoluble types: Water-soluble PAG: used as machining fluids, quench media (polymer quench ASTM D6351) Water-insoluble PAG (PG): gear oil base for worm gears and hypoid (lower friction than mineral)

PAG properties: VI = 150–200 (excellent) Completely miscible with water (some grades): caution — absorbs moisture from air → water contamination Excellent high-temperature stability; low carbon deposit formation NOT compatible with mineral oil (phase separation); requires flush when changing

Compatibility: PAG NOT compatible with Group I–IV; must use pure PAG fill or complete flush

Additive Packages

AW (Anti-Wear) Additives

ZDDP (zinc dialkyldithiophosphate): most common AW additive; forms protective film by mild decomposition on metal surfaces (tribofilm) Mode: thermal/tribochemical reaction at asperities → glassy phosphate film < 20 nm thick → low wear Temperature activation: > 80°C for efficient film formation Conflicts: high performance dispersants can compete with ZDDP film formation; pH sensitive

Alternative AW (low SAPS — sulfated ash, phosphorus, sulfur): MoS₂ (molybdenum disulfide): solid lubricant in grease or oil; low shear strength interlayer MoDTC (molybdenum dithiocarbamate): liquid-soluble Mo compound → fuel economy improvement in engine oils Organic phosphate esters: effective AW without Zn, P, or S (good for sensitive DPF/catalyst systems)

EP (Extreme Pressure) Additives

Application: gears (high contact stress), hypoid axles, cutting fluids Mechanism: chlorine, sulfur, or phosphorus compounds react with steel at high flash temperature → iron halide/sulfide/phosphate film; softer than steel → sacrificial; low friction → prevents scuffing

FZG test (DIN 51354, ISO 14635): 12-stage gear oil test; increase load per stage; record load stage at which scuffing occurs Stage 11+: conventional gear oil (GL-4); Stage 12+: GL-5 or premium EP; Stage 14: exceptional EP

API GL ratings: GL-4: bevel gears, synchromesh transmissions; moderate EP; lower sulfur/phosphorus GL-5: hypoid gears, rear axles; severe EP; sulfur-phosphorus additives; ASTM D7038 corrosion concern with yellow metals GL-6: high-torque high-sliding speed hypoid (mostly obsolete) GL-5 not backward-compatible with GL-4 in some synchromesh gearboxes (aggressive toward Cu-based synchronizer rings)

Detergent-Inhibitor (DI) Package — Engine Oils

Components: Detergent (Ca/Mg sulfonates, phenates): keeps combustion deposits in suspension; neutralizes acids Dispersant (succinimide PIBSI): holds soot particles dispersed; prevents sludge Antioxidant: ZDDP + phenolic/amine antioxidants; extend oil life Pour point depressant (PPD): polymethacrylate; controls wax crystal growth; improves cold pumpability Viscosity index improver (VII): polymer (PAMA, OCP); multigrade viscosity control

ILSAC GF-6 (2020): current US passenger car motor oil standard; API SP; improved fuel economy; LSPI protection (low-speed pre-ignition); better sludge control

Application Matching

Rolling Bearings

Viscosity selection: Operating viscosity ν₁ from bearing speed and bore diameter: ν₁ = K × (n × d)^(-0.5) [K factor from SKF bearing catalog] Ratio κ = ν_actual / ν₁_required: κ = 1: full film (EHD); κ < 1: mixed/boundary → need EP or AW additive Target: κ = 1–4 at operating temperature (not too viscous → drag losses; not too thin → wear)

Example: Ball bearing: d = 50 mm; n = 3,000 rpm; n × d = 150,000 mm·rpm; ν₁ = 12 cSt at operating T At 60°C operating: ISO VG 46 mineral → ν₄₀ = 46; ν₆₀ ≈ 25 cSt → κ = 25/12 = 2.1 → good (full film)

Grease vs. oil: Grease: simpler sealing; lower speed (nDm < 300,000 mm·rpm for NLGI #2) Oil mist/jet: high speed (nDm > 500,000 mm·rpm); better cooling; more complex system

Gears (Industrial)

Viscosity selection from pitch line velocity: v_pitch < 5 m/s: ISO VG 460 or higher (heavy; sufficient film at slow contact speed) v_pitch 5–15 m/s: ISO VG 220–320 v_pitch > 20 m/s: ISO VG 100–150 (too viscous → churning losses; heating) Worm gears: PAG preferred (lower friction; worm has high sliding)

AGMA 9005 gear oil viscosity recommendations: Enclosed gears at 50°C sump: AGMA Grade 4 (ISO VG 150), Grade 6 (320), Grade 7 (460) by pitch line speed and load

Engine Oils (Automotive)

SAE viscosity grades (SAE J300): Monograde: SAE 30, 40, 50 (summer use); rarely used in modern engines Multigrade: SAE 0W-20, 5W-30, 10W-40, 15W-40 (W = winter) Prefix 0W, 5W, 10W: low-temperature (cranking) viscosity grade (−30, −25, −20°C start) Suffix 20, 30, 40, 50, 60: high-temperature viscosity grade at 100°C (ν₁₀₀ range)

Current trends: 0W-16, 0W-20 for fuel economy (lower viscosity → less drag → +0.5–1.5% fuel economy)

Hydraulic Systems

ISO 6743-4 hydraulic oil categories: HM (anti-wear mineral): most common for industrial; ISO VG 32, 46, 68 HV (high VI anti-wear): for mobile hydraulics (wide temperature range); VI > 120 HVLP (HV + friction modifier): premium mobile HFAS, HFAE, HFBS: water-based fire-resistant; for underground mining HFDR (phosphate ester), HFDU (vegetable ester): synthetic fire-resistant; aircraft

Hydraulic viscosity range: Min viscosity at max temperature: 10–15 cSt (pump minimum) Max viscosity at min temperature: 1,000 cSt (pump starts; cavitation risk) Operating range: ISO VG 32–68 for most industrial; VG 15–22 for aircraft

Compatibility and Approval

API Service Category: SN, SP (gasoline engines); CI-4+, CK-4 (diesel) OEM Specifications: BMW Longlife 01/04, VW 507.00, Mercedes-Benz 229.31 (additive treatment and base oil requirements beyond API) Seal compatibility: NBR (nitrile): Group I–III mineral, some PAO; FKM (Viton): all groups including esters; EPDM: avoid ester

Standards

Standard	Scope
ASTM D445	Kinematic viscosity measurement
ASTM D2270	Viscosity index calculation
ISO 3448	Industrial liquid lubricants — ISO viscosity classification
ASTM D2272	Rotating pressure vessel oxidation test (RPVOT) — oil life
DIN 51354 / ISO 14635	FZG gear test
API GL-4/GL-5	Gear lubricant performance requirements
SAE J300	Engine oil viscosity classification
ISO 6743-4	Hydraulic oil classification

Output

Provide: application type (rolling bearing/gear/engine/compressor/hydraulic), operating speed [rpm] or pitch line velocity [m/s], operating temperature range [°C], load type (normal/high/EP required), viscosity required at operating temperature ν_op [cSt] with selection method (κ = ν/ν₁ ≥ 1 for bearings; AGMA table for gears), ISO VG grade selected, base oil type (Group II mineral/PAO/ester/PAG) with justification, VI [−] and pour point [°C], additive package required (AW/EP/DI; specific: ZDDP, GL-5, ILSAC GF-6), API or AGMA specification, OEM approval if applicable, seal material compatibility (NBR/FKM/EPDM), service interval recommendation (oxidation life from ASTM D2272 RPVOT > 1,000 min target), and applicable standard (ISO 3448, ASTM D445, SAE J300).