Selection of Hydraulic Fluid for Optimal Performance and Enhanced Durability
I. Definition of Hydraulic Fluid:
A hydraulic fluid is the medium for power transmission in hydraulic systems.
II. Functions of Hydraulic Fluid:
- Optimal lubrication performance;
- Thermo-oxidative stability;
- Anti-corrosion performance;
- Rapid air release and water separation (demulsibility) capability.
III. Formulation:
Hydraulic fluids are formulated from base oils, selected based on:
- Application and equipment characteristics;
- Additive packages for anti-wear, oxidation resistance, anti-corrosion, anti-foaming, etc.
IV. Classification of Hydraulic Fluids:
- By kinematic viscosity at the reference temperature of 40°C – according to ISO 3448. Based on specific applications and system characteristics, hydraulic fluid viscosity at 40°C can range from ISO VG 10 to ISO VG 150.
- Based on performance level – OEM standards and specifications:
- Recognized international performance standards – ISO, DIN, ASLE, AFNOR, SEB, AIST.
- OEM specifications – Parker (Denison), Bosch Rexroth, Vickers, and Cincinnati Milacron, etc.
Ensuring optimal performance and extended service life can be achieved through correct hydraulic fluid selection.
Key Factors in the Selection Process:
I. Type of Equipment/Hydraulic System
The primary selection criterion for a hydraulic fluid relates to the type of hydraulic pump.
- For systems with vane pumps, it is recommended to choose an anti-wear hydraulic fluid, as high wear rates may occur between vanes and the cam ring.
- Piston pumps (the most durable) can produce higher operating pressures. For these, an anti-wear / EP (Extreme Pressure) hydraulic fluid is recommended.
- Gear pump hydraulic systems (they are the least efficient) can only provide relatively low pressures, are easier to lubricate, and can handle a relatively higher level of contamination.
II. Severity of Operating Cycle and Range of Operating & Ambient Temperatures
- Hydraulic equipment used indoors under normal operating conditions:
Anti-wear hydraulic fluids (generally conventional additive package) in ISO VG 32, 46, and 68 viscosity grades are recommended. The viscosity of lubricants for these hydraulic systems provides lubrication and protection for the hydraulic system components within the normal operating temperature range.
Kinematic viscosity should be optimal – low enough to ensure an adequate fluid flow rate and system energy efficiency, yet high enough to protect components from wear and to ensure correct servo valve operation.
Verify the equipment manufacturer's viscosity requirement and recommended performance level.
- Hydraulic equipment and systems operating outdoors in extreme temperatures:
These systems operate under harsher cycles, at higher pressures, in extreme temperatures, and are also exposed to contamination by moisture, dust environment, etc. The hydraulic fluid should allow for an easy start-up at low temperatures (winter) and maintain sufficient viscosity to protect the system components from wear at high temperatures (summer). Ideal hydraulic fluids for extreme ambient temperatures should have a high viscosity index.
III. Application for Use
- Most hydraulic fluids are formulated from mineral base oils and contain anti-wear, antioxidant, anti-corrosion, anti-foam additives, and, if necessary, viscosity index improvers. These formulations are recommended for typical equipment and hydraulic systems operating indoors or outdoors.
- Fluids can also be formulated with synthetic base fluids, providing better oxidation stability, which translates into extended durability compared to mineral-based fluids.
- For special applications, specially formulated hydraulic fluids are available.
- In environmentally sensitive applications (e.g., agriculture, horticulture, forestry), some hydraulic equipment requires biodegradable hydraulic fluids (e.g., vegetable oil-based for low pressures and temperatures, or synthetic esters for higher pressures and temperatures).
- For hydraulic systems with a potential risk of fire or explosion (e.g., steelmaking, mining, glass manufacturing), fire-resistant hydraulic fluids are formulated:
- Water-based fluids (typically water-glycol type) for low pressures and temperatures,
- Phosphate ester-based fluids for higher pressures and temperatures.
Conclusions and Recommendations
For a correct selection a hydraulic fluid, please consult with your lubricant supplier to verify the characteristics of your hydraulic system/equipment and if the fluid is approved, recommended, or meets the equipment manufacturer’s performance level requirements (international or national standards).
The hydraulic fluid should be supplied with an appropriate cleanliness level to ensure optimal performance.
Monitoring the fluid according to the established maintenance interval ensures extended durability of both fluid and hydraulic system components.
Our team of experts is ready to offer specialized consultancy and support to help you identify the most effective solutions for your needs.