Hydraulic fluids are designed to transmit power through hydraulic systems, such as excavators, forklifts, presses and injection molding machines, while also protecting components, managing heat and maintaining cleanliness.
These fluids typically use high-purity base oils, with high oxidation resistance, water-separability and air-release properties.
To perform reliably across a wide range of temperatures and operating pressures, they are often formulated as high‑viscosity‑index “multigrade” fluids, thanks to polymer-based viscosity modifiers that support efficient flow and stable lubrication under both hot and cold conditions.
These additives help address advanced challenges such as varnish formation, hydrolytic stability, filterability and shear degradation under increasingly demanding system pressures, temperatures (up to 500 bar and 100 degrees Celsius) and environmental conditions. As a result, today’s hydraulic fluids must exceed rigorous OEM and industry specifications, like Denison, Bosch Rexroth and Eaton, while delivering improved equipment life, energy efficiency and reduced downtime.
Composition & Formulation
Base stocks (~90–99%) comprise most of the volume of a hydraulic fluid. Typically, they are API Group II mineral oils, Group III, PAOs, esters or polyalkylene glycols. Ester-based fluids can be saturated branched (SBE) or saturated linear (SLE), offering high oxidative stability and biodegradability. SBE often performs best for seal compatibility and wear resistance.
Additives: Types & Roles
Additives (0.5–2%) include anti-wear or extreme pressure, such as ZDDP or ashless EP packages for protection under high pressure, oxidation and thermal stabilizers, corrosion inhibitors, air-release, demulsifiers, foam suppressors, filterability enhancers and seal swellers are common.
- Antiwear/EP: ZDDP and ashless types protect metal surfaces during pressurized operation
- Oxidation inhibitors: Phenolics, metal deactivators extend fluid life
- Corrosion inhibitors & demulsifiers protect against moisture and acid, maintain cleanliness
- Defoamers and air-release agents reduce bubbles and ensure stability
- VI improvers: Polyacrylates or PMA-based polymers adjust fluid thickness at different temperatures, such as polymer-based viscosity index improvers are used to maintain fluid properties across temperature ranges.
Applications
Key required properties include non-compressibility, heat transfer, seal lubrication, foam control, filterability, demulsibility, air release, and environmental performance (biodegradability, low toxicity). Hydraulic fluids are used in:
- Mobile machinery: excavators, backhoes, dump trucks
- Industrial systems: presses, presses, lifts, turbines
- Transportation & vehicles: brakes, power steering, automatic transmissions
- Safety-critical systems: aircraft controls, fire-resistant systems