How to Choose Correct Hydraulic Viscosity (ISO VG)

One of the most important criteria in the selection of a hydraulic fluid is viscosity. A low viscosity fluids can lead to increased friction and pump wear. A fluid that is too high in viscosity, on the other hand, will cause poor mechanical efficiency, startup issues and wear due to cavitation. Therefore, selecting the proper fluid involves optimizing oil viscosity in terms of volumetric and mechanical pump efficiency. To accomplish this, the requirements of the hydraulic components, as well as the operating temperature range, must be taken into consideration.

To select a hydraulic fluid using the Temperature Operating Window criteria, determine the lowest ambient temperature at startup and the highest fluid temperature in use. For instance, consider a machine shop with low temperature startups of 45ºF and a top system temperature of 150ºF. Compare this temperature range to the TOW chart. ISO VG 46 and ISO VG 68 fluids cover the upper and lower bounds of the 45ºF to 150ºF range.