Viscosity is a primary hydraulic fluid characteristic influencing system efficiency, control response, wear protection, and operating stability. Correct viscosity selection is defined by system design parameters and operating conditions rather than by fluid type alone.
This page explains how viscosity requirements are determined for hydraulic systems and how viscosity grades are interpreted within technical documentation.
Viscosity and hydraulic system behaviour
Viscosity describes a fluid’s resistance to flow and directly affects leakage, internal friction, and energy losses within hydraulic components. If viscosity is too low, internal leakage and wear may increase. If viscosity is too high, flow resistance and energy consumption may rise.
Hydraulic systems are designed to operate within a specific viscosity range at normal operating temperature.
Temperature influence
Viscosity changes with temperature. At low temperatures, viscosity increases and can restrict flow, delay response, and increase pump load. At high temperatures, viscosity decreases and may reduce film thickness and wear protection.
Viscosity selection must account for expected ambient temperature, cold start conditions, and normal operating temperature rather than relying on a single reference value.
Load, speed, and component design
System load and operating speed influence the required viscosity range. High load conditions may require sufficient viscosity to maintain protective film strength, while high speed operation may favour lower viscosity to reduce friction and heat generation.
Component design, including pump type, clearances, and control valve tolerances, also affects viscosity sensitivity.
ISO viscosity grade selection
ISO viscosity grades classify hydraulic oils by kinematic viscosity at 40 °C. Equipment manufacturers typically specify an ISO VG grade or acceptable range based on system design and operating conditions.
ISO VG classification provides a standardised reference point but does not define the complete performance requirements of the hydraulic fluid.
Interpreting viscosity requirements
Viscosity requirements should be interpreted alongside other system considerations, including filtration level, contamination sensitivity, operating pressure, and duty cycle.
Always refer to original equipment manufacturer documentation when determining acceptable viscosity grades or operating ranges.