Hydraulic Oil Classification and Selection: Performance and Viscosity
Hydraulic fluid selection affects system efficiency, wear control, and reliability. Understanding fluid types and viscosity grades, while considering temperature, operating conditions, and equipment specifications, helps ensure suitable performance in different hydraulic systems.
Hydraulic systems transmit power through pressurized fluid. In addition to power transmission, hydraulic oil provides lubrication, heat transfer, corrosion protection, and contaminant transport. Since systems differ in pressure, speed, temperature, cleanliness, and fire-risk requirements, fluid selection should follow equipment specifications rather than product names or price alone.
Common Types of Hydraulic Fluids
Conventional and anti-wear hydraulic oils: Conventional mineral oils vary in performance by formulation. Anti-wear oils contain additives designed to protect components and are widely used in industrial hydraulic systems. The required performance category must match the equipment specification.
Ashless hydraulic oils: These use additive systems without metal-containing components. “Ashless” does not automatically mean superior performance; anti-wear protection, oxidation stability, filterability, and material compatibility still need evaluation.
High-pressure and low-temperature fluids: High-pressure systems require suitable wear protection and viscosity. Cold-weather applications need appropriate low-temperature flow, start-up performance, and viscosity-temperature behavior.
Semi-synthetic and synthetic hydraulic oils: Base-oil composition can influence oxidation stability, temperature capability, and service life. Actual performance depends on the formulation and operating conditions.
High-cleanliness hydraulic oils: Used where particulate contamination must be tightly controlled. Confirm cleanliness requirements, filtration compatibility, and maintenance procedures.
Fire-resistant hydraulic fluids: Used where fire risk or equipment specifications require them. Fluid types differ in composition and properties, so system compatibility must be verified.
Hydraulic transmission fluids: Designed for specific fluid-transmission systems and should not be substituted for ordinary hydraulic oil unless the equipment manufacturer permits it.
How to Select the Right Fluid
First, check the equipment manufacturer’s required fluid type and performance specification. ISO 11158:2023 covers mineral-oil hydraulic fluids in categories HH, HL, HM, HV, and HG. These categories are not interchangeable with descriptions such as synthetic, ashless, or fire-resistant.
Next, select the correct viscosity grade. Common ISO viscosity grades include ISO VG 10, 22, 32, 46, 68, and 100. The grade represents a kinematic-viscosity range at a specified reference temperature. Excessively low viscosity may reduce lubrication and sealing, while excessive viscosity can increase flow resistance and start-up load.
Finally, consider ambient temperature, operating pressure, pump design, contamination control, and seal compatibility. Do not mix different fluids without confirming compatibility. When changing fluids, follow the equipment manufacturer’s maintenance procedures.
