ISO viscosity grade: the ISO 3448 system for industrial oils, in which the grade number is the midpoint kinematic viscosity at 40 C in centistokes and the oil must fall within plus or minus 10 percent of it. The common steps are 22, 32, 46, 68, 100, 150, 220, 320, 460 and 680, each roughly 50 percent thicker than the one before. Hydraulics mostly run 32 to 68, and enclosed gearboxes 150 to 460.
Spec says ISO VG 220 for the drive gearbox, don't grab the 320 just because it's closer on the shelf.
The grade only fixes viscosity at 40 C. A high-VI 46 and a low-VI 46 can be far apart at an 80 C operating temperature.
Also heard as
- VG
- ISO grade
Related terms
Viscosity
A fluid's resistance to flow, the lubricant property that sets how thick an oil film can build.
Viscosity index
Number showing how little an oil's viscosity changes with temperature; higher means flatter.
Turbine oil
Highly refined, lightly additized circulating oil built to last for years in steam, gas and hydro turbines.
Top off
To add oil to bring a gearbox, sump or reservoir back up to its correct level.