Be aware of what viscosity is needed in your system. Viscosity is about how ‘thick’ a fluid is, and it is measured on the ISO VG scale (International Standards Organization Viscosity Grade). Hydraulic fluids often have ISO VG 32, ISO VG 46 or ISO VG 68, but there are many more viscosity grades to choose from. The higher the figure, the thicker and more viscous the fluid – and of course vice versa, so the lower the figure, the thinner and more easily flowing.
Viscosity is very much affected by temperature. Generally speaking a lower viscosity is used in low temperatures and a higher one in higher temperatures, since oil naturally becomes thinner when hot and thicker when cold. A hydraulic system that struggles in the cold probably doesn’t have the optimum fluid viscosity for the operating temperature. To clarify how the viscosity of a hydraulic fluid changes with temperature, a Viscosity Index or VI is used. The VI defines the oil’s ability to resist changes in viscosity when the temperature changes: the higher the VI, the better the oil’s viscosity stability as the temperature fluctuates. So a product with a high VI will retain its viscosity and flow better also when the temperature falls, while a low VI product will become more viscous in lower temperatures.
Generally speaking, hydraulic fluids for outdoor use should have a VI of 150 or higher. This means the product will work within a broad temperature range, which safeguards operation whether it’s warm or cold. Modern hydraulic systems work under high pressures and heavy flows, which places high demands on the hydraulic fluid’s ability to maintain the right thickness and to separate air and water. Having said that, a high VI is less important in industrial hydraulic systems, for example, which tend to operate in consistent temperature ranges and generally require different key properties.
