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"We are looking to change our hydraulic oil. Will switching to a multigrade hydraulic oil with a high viscosity index (VI) improve hydraulic efficiency? Are there any documented cases to prove this?"
Hydraulic efficiency in terms of hydraulic fluid depends on a few factors, all of which relate to the viscosity of the hydraulic fluid. Low viscosity, either from poor fluid selection or high temperatures, results in a loss of volumetric efficiency, overheating and wear. High viscosity, either from poor fluid selection or low temperatures, results in poor mechanical efficiency, difficulty starting up and wear.
Hydraulic pump and motor manufacturers have often based hydraulic fluid recommendations on viscosity requirements at startup, optimum running states and extreme running states. With this being the case, temperature control and/or controlling the viscosity fluctuations will be critical in managing the two main drivers of hydraulic pumping efficiency.
Choosing a multigrade hydraulic fluid with a high viscosity index will correlate to improvement in overall hydraulic pump efficiency, including hydromechanical and volumetric efficiency. Typically, the desired operating hydraulic fluid viscosity is around 25 to 36 centistokes in order to maximize the overall hydraulic efficiency. When a hydraulic system is required to perform at temperature extremes, it is almost certain that a multigrade hydraulic fluid will be necessary to provide the highest efficiency.
However, there are some aspects of multigrade hydraulic fluids that can pose significant risks. One of these relates to the higher concentration of viscosity index improvers. Air separation becomes hindered with these high concentrations, and for the application of a hydraulic fluid, this is of great concern. Any system with poor deaeration will have difficulty with fluids that have high concentrations of viscosity index improvers.
One commonly known solution to this problem is to increase the minimum permissible viscosity limit by approximately 30 percent. By doing this, the maximum allowable operating temperature is reduced, which in turn provides a safety factor for viscosity loss in the event of a sheardown of the viscosity index improvers.
Therefore, on one hand, hydraulic efficiency is somewhat compromised by using a multigrade hydraulic fluid at an average temperature. On the other hand, if the hydraulic equipment is required to operate at temperature extremes, the efficiency is significantly improved by using a multigrade hydraulic fluid. So the decision to use a multigrade hydraulic fluid should ultimately be determined by the fluid’s required temperature range.