Hydraulic Oil Viscosity (ISO VG)
It is the measure of the resistance the oil shows to flow. In the ISO VG classification the number gives the oil's kinematic viscosity at 40 C in centistokes: ISO VG 46 oil has a viscosity of about 46 cSt at 40 C. The most common classes in mobile hydraulics are VG 32, VG 46, and VG 68.
Viscosity selection balances two risks. Oil that is too thick leads to cavitation at the suction, high pressure drop, and hard starting in the cold. Oil that is too thin cannot carry the oil film, increases internal leakage, and lowers volumetric efficiency.
- A lower class is preferred in cold climates or winter use, and a higher class in hot climates and under heavy load.
- The pump manufacturer usually gives an operating viscosity range; the aim is to keep system temperature within that range.
- Mixing different classes gives an intermediate viscosity, but the additive packages may be incompatible; be careful when changing brand and type.
From a proportional control standpoint viscosity is felt directly: in cold oil the deadband appears to widen, motion starts late, and response gets sluggish. For this reason calibrations such as minimum current must be done after the oil has reached operating temperature; a setting made cold causes the machine to jump on the first command once the system warms up.
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