Viscosity Index
This is the dimensionless number showing how little an oil's viscosity changes with temperature. The higher the index, the less the oil thickens when cold and the less it thins when hot. While standard mineral oils are typically around 90-100, additized multigrade hydraulic oils can go above 150.
Viscosity index is not a fixed viscosity value but a measure of slope. Two oils in the same ISO VG grade can have the same viscosity at 40 °C, yet one stays far more fluid in winter.
- Low index oil: motions are sluggish on the first start of the morning and you have to wait for the system to warm up; in the afternoon the oil thins and internal leakage increases.
- High index oil: the same machine behaves similarly at −20 °C and at +60 °C. The advantage is clear on mobile equipment working across a wide temperature range.
- A high index is generally achieved with additives; these additives can degrade over time and under high shear, so the oil change interval must be observed.
If you do not want your proportional control calibration to drift from season to season, a high index oil is a sensible choice. AXI's retrofit kits are specified to operate between −20 °C and +85 °C; at the lower end of this range, what really limits the lever's response is not the electronics but the oil itself.
Where this term is used: What Is a Hydraulic Lever Retrofit Kit? Definition, Components, Operating Principle