Oil grade (ISO VG) selector for bearings
Enter bearing size, speed and temperature. The tool finds the ISO VG grade that keeps a healthy oil film at running temperature, shows kappa for the grades either side, and checks your lubrication method.
Worked example
Inputs: 60 mm bore, 130 mm outside diameter, 1,500 rpm, 65 °C bearing temperature, oil bath, normal load.
| Grade | κ | Zone |
|---|---|---|
| ISO VG 22 | 0.80 | Mixed |
| ISO VG 32 (pick) | 1.17 | Full film |
| ISO VG 46 | 1.68 | Full film |
- Method. At standstill, fill to the middle of the lowest ball or roller. Check it on a sight glass.
- Oil change. For a bath, about every 6 months, or sooner if oil analysis says so.
This is the live calculator output for the default values above.
What kappa (κ) tells you
Kappa is the ratio of the oil's actual viscosity at running temperature to the viscosity the bearing needs. Below 0.4 the surfaces touch (boundary). From 0.4 to 1 the film carries part of the load (mixed). From 1 to 4 a full oil film separates the surfaces, which is the target. Above 4 the film is more than enough and extra thickness only adds drag and heat.
How the grade is found
- Required viscosity at running temperature = 4472 ÷ √(n × dm), where n is rpm and dm is the mean diameter in mm.
- The oil gets thinner as it warms, so the selector divides by the fraction of its 40 °C viscosity that remains at your temperature (a standard viscosity-temperature curve, VI about 95).
- It picks the first standard ISO VG grade at or above that figure, then shows kappa for the grade below and above.
Method limits
Oil bath is normally used up to about n·dm 500,000, circulating oil to about 1,000,000, and oil mist to about 1,200,000. Jet and oil-air go higher. The selector flags a mismatch.
Related tools and guides
Questions
What ISO VG oil should I use for a bearing?
It depends on speed, size and running temperature. The selector computes the required viscosity at running temperature and returns the ISO VG grade that gives a kappa between 1 and 4.
What does ISO VG 68 mean?
ISO VG 68 is an oil with a kinematic viscosity of about 68 mm²/s at 40 °C. The number is the viscosity grade.
Why does the running temperature matter so much?
Oil thins quickly as it warms. At 80 °C a typical mineral oil keeps only about a quarter of its 40 °C viscosity, so a grade that looks thick at 40 °C may be barely enough when hot.
Results are starting points from standard engineering practice. Confirm critical selections with the bearing or lubricant maker.