Figure 2. ISO VG 46 operates in a wide range of extreme temperatures. Courtesy Lubrizol Corp.The challenge is different with mobile equipment — used for offroad construction, timber harvesting, municipal wast e and utilities, for example. Mobile equipment can see high pressures, severe duty cycles, and wide temperature extremes during year-round use in areas with seasonal climates. Their fluid needs to work well at low temperatures for winter time start up, while maintaining enough viscosity at high temperatures to protect during severe service or hot weather.
The ideal hydraulic fluid for mobile applications is one that flows like a lighter oil at low temperatures but protects like a heavier oil at high temperatures: a multigrade hydraulic oil. This is similar to using SAE 5W-30 or 10W-30 multigrade engine oil in our cars, which can be used year-round without problems. In Figure 3, note that the multigrade fluid has the low-temperature fluidity of the lighter Fluid A, while maintaining the hightemperature viscosity of the heavier Fluid B.
Multigrade and conventional fluids
What differentiates multigrade hydraulic fluids from conventional fluids is that they contain polymer additives, called viscosity modifiers (VM) — also known as viscosity index improvers — in addition to the hydraulic performance package. These VMs keep the fluid viscosity more consistent over the temperature range encountered during use.