Most readers of this column are well aware that the viscosity of a hydrocarbon-based hydraulic fluid is inversely proportional to temperature. As temperature increases, fluid viscosity decreases and vice versa. This is not an ideal situation for several reasons. In fact, the ideal hydraulic fluid would have a viscosity index (the change in a fluid’s viscosity relative to temperature) represented by a horizontal line intercepting the Y axis at 25 centiStokes.
This temperature-viscosity shows that an ideal hydraulic fluid would exhibit no change in viscosity regardless of temperature.
Unfortunately, no such fluid exists for hydraulic machine efficiency and longevity. And it’s unlikely such a fluid will be developed in my lifetime. But if such a fluid was developed and patented, its creator would hold the key to a gold mine. For now, we have multi-grade hydraulic oil. These fluids have a high viscosity index, so their viscosity is less sensitive to changes in temperature than a monograde oil.
Fluid viscosity is one of the factors that determines whether full-film lubrication is achieved and maintained. If load and surface speed remain constant, but elevated operating temperature causes viscosity to fall below that required to maintain a hydrodynamic film, boundary lubrication occurs; this creates creating the possibility of friction and adhesive wear.
On the other hand, there is a viscosity range where fluid friction, mechanical friction, and volumetric losses are optimal for hydraulic system performance. This is the viscosity range where the hydraulic system will operate most efficiently: the highest ratio of output power to input power.