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What is ISO VG and how is it selected?
ISO VG classifies industrial oils by their kinematic viscosity at 40 °C. The number on the label is the mid-point viscosity in centistokes (mm²/s) at that temperature: ISO VG 46 means roughly 46 mm²/s at 40 °C.
What the numbers mean
The grades run 2, 3, 5, 7, 10, 15, 22, 32, 46, 68, 100, 150, 220, 320, 460 and 680; each step is about 1.5 times the one before it. The tolerance is ±10%, so an ISO VG 46 product may measure anywhere between 41.4 and 50.6.
That spacing is deliberate: it makes the difference between two neighbouring grades meaningful in service, because moving one step up noticeably increases film thickness.
Choosing the grade for a machine
The starting point is always the window given by the equipment builder. Where none exists, speed, bearing type, operating temperature and load are weighed together — lower grades for high-speed bearings, higher grades for slow, heavily loaded gearing.
Ambient temperature matters just as much: for cold starts the pumpability limit governs, while in hot service the operating viscosity has to stay high enough to maintain the minimum film.
A common mistake
Assuming that thicker is always better is the usual error. Excess viscosity increases churning losses and oil temperature, and on a cold start it can starve the pump. The sound approach is to stay inside the builder's window and choose against actual operating temperature.
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