Many mobile equipment operators in the construction, forestry, mining and transportation industries across the northern U.S. and Canada continue to change out hydraulic fluids for summer and winter use. However, recent evaluations have shown that switching to a single multigrade fluid for year-round operations can improve fuel efficiency for greater cost savings while maintaining - and even improving - lubrication protection.

High and low temperature impact on viscosity

When it comes to hydraulic fluids, overall pump efficiency relies on obtaining the ideal balance between hydro-mechanical efficiency and volumetric efficiency. More simply stated: the viscosity of the hydraulic fluid must be thin enough for the hydraulic pump to start easily, especially at low temperatures, and to prevent it from working too hard (mechanical efficiency), but thick enough to prevent internal pump leakage, loss of pressure, and effective lubrication (volumetric efficiency).

Excessive viscosity at low temperatures can result in reduced mechanical efficiency of the hydraulic system and, in more extreme situations, to lubricant starvation and cavitation. During lubrication starvation, the loss of the thin lubricant film protecting parts creates high contact temperature, excessive wear, and ultimately results in pump seizure. Cavitation is created due to excessive pressure drop at the pump inlet. This eventually leads to metal fatigue and spalling, generating abrasive metal particles in the fluid and reducing pump life.

If viscosity is too low when oil temperatures increase, volumetric efficiency drops due to increased internal leakage. Also, if the fluid is too thin, moving parts are not properly protected - resulting in metal-to-metal contact, overheating, parts wear, and eventual pump seizure.

Thus, if viscosity is too high or too low, the resulting wear in parts leads to an additional decline in volumetric efficiency. As a result, the hydraulic pump must work harder to produce the required flow to hydraulic actuators. The engine must, therefore, burn more fuel to produce the desired amount of hydraulic work. Higher fuel consumption equals higher costs. It also equals greater carbon dioxide (CO2) emissions, which are damaging to the environment. An ideal hydraulic fluid self-adjusts to the widest possible changes in temperature, ensuring a smooth startup as well as efficient peak-temperature operation.

Multigrade hydraulic fluids are formulated with high viscosity index base oils and additive packages to self-adjust to a wider temperature range than monograde hydraulic fluids. The viscosity of a multigrade is lower below 104° F than a monograde oil having the same ISO viscosity grade. This means the hydraulic fluid will flow faster in cold temperatures, contributing to improved mechanical efficiency. In increasing temperatures, the viscosity of a multigrade fluid decreases less than that of a monograde fluid, providing the necessary volumetric efficiency at higher temperatures. Hydraulic fluids with very high viscosity index (VHVI) base oils provide the best performance.