ISO VG selection is not complicated, but it does require understanding three things: what viscosity does inside a bearing, how speed and size interact, and how temperature changes the viscosity you actually get.
Get those three things right and you’ll make better viscosity decisions than most facilities in North American industry. This article walks through the complete selection process — from the ISO VG classification system through the kappa factor calculation — with worked examples and reference tables you can apply directly in the field.
01 — What ISO VG Actually Measures
The catalog number ≠ operating viscosity
The ISO Viscosity Grade system (ISO 3448) classifies industrial lubricants by kinematic viscosity at 40°C. The classification contains 18 grades, from ISO VG 2 through ISO VG 1500. Each grade is defined by its midpoint viscosity in centistokes (cSt), with ±10% tolerance. Each successive grade is approximately 50% higher than the previous one.
ISO VG 68 = 68 cSt at 40°C. At 80°C operating temperature, it may deliver only 22–28 cSt. That operating viscosity — not the catalog grade — determines film adequacy.
Your bearing doesn’t operate at 40°C. It operates at whatever temperature results from the combination of ambient conditions, load, speed, housing design, and lubrication method. The selection process must connect the ISO VG catalog number to actual operating viscosity at actual operating temperature — which requires knowing the Viscosity Index (VI) of the lubricant and the expected operating temperature.
02 — Why Viscosity Matters: The EHD Film
The film that prevents metal contact
A rolling element bearing lubricates through elastohydrodynamic (EHD) film formation — a thin film of pressurized lubricant that forms in the contact zone between rolling elements and raceways as the bearing rotates. When that film is adequate, rolling elements and raceways are fully separated and the bearing runs on fluid friction, not solid contact. When it’s inadequate, metal surfaces make contact, generating heat, accelerating wear, and consuming bearing life far faster than the L10 calculation predicts.
The goal: the minimum viscosity that produces an adequate film at operating conditions — not the highest viscosity available. More is not better above κ = 4.
The Kappa Factor (κ) — ISO 281:2007
| κ | Viscosity ratio — the key indicator of lubrication film adequacy |
| v | Actual kinematic viscosity of the lubricant at operating temperature (cSt) |
| v1 | Minimum reference viscosity required for the bearing at its operating speed (cSt) |
03 — Calculating the Minimum Required Viscosity (v1)
Bearing size + speed → minimum film requirement
| v1 | Minimum required kinematic viscosity at operating temperature (cSt) |
| k | ~45,000 when (n × Dm) > 1,000 | ~75,000 when (n × Dm) < 1,000 |
| n | Rotational speed (rpm) |
| Dm | Mean bearing diameter = (bore diameter + OD) / 2 (mm) |
| Bearing 6313 | bore 65 mm, OD 140 mm, speed 1,500 rpm |
| Dm | (65 + 140) / 2 = 102.5 mm |
| n × Dm | 1,500 × 102.5 = 153,750 → k = 45,000 |
| v1 | 45,000 / √153,750 = 45,000 / 392 ≈ 11.5 cSt |
| κ = 2 target | v = 11.5 × 2 = 23 cSt at operating temp |
| κ = 4 target | v = 11.5 × 4 = 46 cSt at operating temp |
| ISO VG 68 @ 70°C | ≈ 22–28 cSt → κ ≈ 2.0–2.4 Correct range |
| ISO VG 32 @ 70°C | ≈ 10–12 cSt → κ ≈ 0.9–1.0 No margin |
04 — From v1 to ISO VG: Making the Selection
Three steps to the right grade
Step 1 — Estimate Operating Temperature
Measured: Use an infrared thermometer or thermocouple at the bearing housing. Housing temp typically runs 10–20°C above ambient; contact zone is 5–15°C above housing. Housing temperature is a reasonable proxy for selection.
Estimated: 60–80°C is a reasonable baseline for general industrial rotating equipment. When in doubt, use the higher estimate — the penalty for over-selecting viscosity is modest; the penalty for under-selecting is early failure.
Step 2 — Use Viscosity Index to Find Required ISO VG Grade
Viscosity Index (VI) describes how much a lubricant’s viscosity changes with temperature — higher VI means less change per degree. It is not included in the ISO VG number. Mineral oils typically have VI 90–110. Group III and PAO synthetics: VI 120–160+.
Key implication: a synthetic ISO VG 32 (VI 150) delivers significantly more viscosity at 80°C than a mineral ISO VG 32 (VI 95). They are not equivalent at operating temperature, even though they’re identical at 40°C.
Step 3 — Quick Reference Table
| ISO VG | At 40°C | Speed / Load Profile | Typical Applications | Approx. nDm |
|---|---|---|---|---|
| VG 22 | 22 cSt | Very high speed, light load | High-speed spindles, centrifugal compressors | > 500,000 |
| VG 32 | 32 cSt | High speed, light–moderate load | High-speed motors, centrifugal pumps, blowers | 300k–500k |
| VG 46 | 46 cSt | High speed, moderate load | Pumps, fans, some gearboxes | 200k–400k |
| VG 68 | 68 cSt | Moderate speed, moderate–heavy | General industrial motors, most process pumps | 100k–300k |
| VG 100 | 100 cSt | Moderate speed, heavy load | Large motors, heavily loaded pump bearings | 75k–200k |
| VG 150 | 150 cSt | Low–moderate speed, heavy load | Slow gearboxes, large bearings, conveyor drives | 50k–150k |
| VG 220 | 220 cSt | Low speed, heavy–very heavy | Rolling mill bearings, large slow equipment | 30k–100k |
| VG 320–460 | 320–460 cSt | Very low speed, severe load | Crushers, vibrating screens, very slow conveyors | < 75,000 |
nDm ranges are approximate. Verify against the v1 calculation for each specific bearing. Operating temperature significantly affects actual viscosity delivered.
05 — The Kappa Factor in Practice
What your operating condition actually is
Once you’ve selected an ISO VG grade and know the operating temperature, calculate the actual kappa for your specific bearing application. The practical message: more viscosity is not always better. Above kappa 4, excess viscosity generates churning heat, accelerating oxidation, shortening grease life, and potentially reducing bearing life.
| κ Value | Regime | What It Means | Common Cause |
|---|---|---|---|
| < 0.4 | Severe film failure | Full asperity contact, very high wear. EP additives required immediately. | Wrong viscosity grade, extreme temp, or severely undersized bearing |
| 0.4–1.0 | Inadequate film | Metal-to-metal contact on asperities. Use AW additives (0.8–1.0) or EP (< 0.8). | Viscosity too low for speed/size; running hotter than expected |
| 1.0–2.0 | Marginal film | Minimum film present but no safety margin. Acceptable with AW additives and clean lubricant. | Often acceptable in practice with additive-enriched grease |
| 2.0–4.0 | Optimal range | Full EHD film. Rolling elements fully separated from raceways. Maximize bearing life. | Target zone for all critical rotating equipment |
| 4.0–10.0 | Excess viscosity | Film more than adequate, but increasing viscous drag and heat. No life benefit above κ 4. | Viscosity grade too high for speed/temperature |
| > 10.0 | Excessive | High churning losses — elevated temperature accelerates oxidation. Can reduce bearing life through heat. | Significantly wrong viscosity — common in high-speed equipment |
06 — Grease vs. Oil: How Viscosity Selection Differs
The base oil viscosity is what matters — not the NLGI grade
For grease-lubricated bearings, the ISO VG grade refers to the base oil viscosity contained within the thickener system — not to grease consistency (which is the NLGI grade). This is where many viscosity selection errors originate.
NLGI 2 describes stiffness, NOT viscosity. Find base oil viscosity on the technical data sheet — listed as ‘base oil viscosity at 40°C’ or ‘ISO VG equivalent.’
The Synthetic Advantage
Where a mineral ISO VG 68 (VI 95) might deliver 22 cSt at 80°C, a PAO ISO VG 68 (VI 150+) may deliver 28–32 cSt. That difference can move a bearing from κ 1.8 to κ 2.5 — with no change in ISO VG grade, only by specifying a higher viscosity index product.
07 — 5 Common Viscosity Selection Mistakes
And how to avoid them
08 — Step-by-Step Selection Procedure
Apply this to every bearing position
Bore diameter (d) and outside diameter (D) from maintenance records or bearing catalog.
Dm = (d + D) / 2
Use IR thermometer or thermocouple at the bearing housing under normal operating conditions.
Using v1 = k / (n × Dm)0.5 or a bearing manufacturer nomograph.
v1 × 2 (minimum) to v1 × 4 (optimal for critical equipment).
Find the ISO VG grade that delivers the target viscosity at operating temperature.
Specify for the high-temperature condition; check pour point for cold starts.
ISO VG grade, target kappa range, operating temperature basis — review if operating conditions change.
Viscosity selection is the most consequential lubrication decision for rolling element bearings — and the one most often made by default. ISO VG grade tells you a lubricant’s viscosity at 40°C. It doesn’t tell you whether that lubricant will form an adequate film in your bearing at your operating temperature.
The kappa factor is the tool that connects those variables. Calculate v1, target κ between 2 and 4, use viscosity index to connect catalog grade to operating viscosity at temperature, and document the specification. 15 minutes per bearing position. Lubrication decisions grounded in engineering, not habit.
