Understanding Hydraulic Fluids: Properties, Testing, and Maintenance Considerations

Hydraulic fluid is essential for a range of applications, including automotive braking systems, industrial machinery like forklifts and construction equipment, and hydraulic lifts. It serves to transmit force and mechanical power throughout machines by responding to applied pressure. Viscosity, which affects flow efficiency, is crucial. Viscosities being too high can lead to frictional losses, while too low may cause leaks. Maintaining incompressibility is vital to prevent issues like sponginess in hydraulic systems, often caused by air bubbles or foam.
 
Moreover, hydraulic fluids provide lubrication and protection against wear for system components like pumps and pistons. They contain anti-wear additives to safeguard surfaces under high pressure. However, fluids must also avoid reacting with system materials and causing corrosion or seal damage. Contaminants such as water and oxidation byproducts pose significant risks, leading to corrosion, sticky varnishes, or abrasive particles that wear down components. Effective filtration is necessary to remove contaminants while preserving beneficial additives.
 

Practical considerations include minimizing costs, safety hazards, and environmental impact. Longer service life and the use of non-toxic, environmentally friendly additives are essential for sustainability and safety. Additionally, fire resistance is crucial in certain applications to prevent accidents. Overall, hydraulic fluids play a critical role in various industries, and their proper selection and maintenance are essential for efficient and safe operation.

Testing Hydraulic Fluids for Flow Properties

Many hydraulic fluid tests examine viscosity and fluid flow. ASTM D445 measures kinematic viscosity. When determined at two temperatures, this test shows how the fluid flows as it warms and cools. As it cools and becomes more resistant to flow, pumpability is a concern. The Scanning Brookfield Test (ASTM D5133) examines whether a change in structure (viscosity and/or gelation tendency) may prevent effective pumping at cold temperatures. Low-temperature pumpability might be especially important for hydraulics in a refrigerated warehouse. Other tests measure viscosity loss under shearing conditions. That loss may last only as long as the fluid is under shear forces or may be permanent. Combining tests for high-temperature, low-shear viscosity (SAVLAB TBR) and high-temperature, high-shear viscosity (ASTM D4683 or D6616) at the same temperature reveals both temporary and permanent viscosity loss from shearing. KRL shear loss (CEC L-45-99) or sonic shear (ASTM D2603) tests also show permanent viscosity loss.

Testing for Incompressibility

The Foaming Characteristics (ASTM D892) and Air Release (ASTM D3427) tests relate to keeping air out of the hydraulic fluid so it remains incompressible. In the foaming characteristics tests, air is introduced into the fluid, creating foam. Fluids with desirable anti-foaming characteristics will create minimal foam volumes. The faster the foam dissipates, the more quickly air problems will resolve. Air Release also times a fluid’s return to an acceptable minimum level after air is added.

Testing for Protection

Many tests measure a fluid’s ability to protect machine components from damage. Rust Prevention Test (ASTM D665) determines protection against rust, Copper Strip Corrosion (ASTM D130) against corrosion of yellow metals, and a higher acid number found using Acid Number (ASTM D664 or D974) generally indicates a more corrosive fluid. Several Elastomer Compatibility Tests (ASTM D471, ASTM D7216, or CEC L-112-16) examine how the fluid impacts the elastomer seals, Differences between these Elastomer Compatibility Tests generally involve the specific reference material tested with the fluid, and modifications may be appropriate for application-specific seal materials. Many tests measure protection against wear. A few of these include the FZG Scuffing Test (ASTM D5182), the Four-Ball Wear Test (ASTM D4172), and the Falex Pin and Vee Test (ASTM D2670). These tests use different mechanisms to create wear and may be listed in different specifications. For instance, ASTM D6158-18 specification requires the scuffing test, while FED-STD-791 requires the four-ball test. Consideration of the type of wear likely in a particular system and discussion with a testing expert at Savant Labs may help select a wear test.

Testing for Resistance to Fluid Degradation

Many different hydraulic fluid tests assess fluid degradation. These tests predict how long a fluid will retain its critical properties or measure the actual degradation of an in-service fluid. Oxidation tests, such as Oxidation Characteristics/TOST (ASTM D943), Oxidation Characteristics/Dry TOST (ASTM D4310), and Oxidation by RPVOT (ASTM D2272), artificially apply conditions with increased oxygen and high temperatures to stress the fluid. The measurements determine how long the fluid resists breakdown from these stressors. While results may not predict a specific number of service hours, a better result in an oxidative test indicates a longer expected service life. The Thermal Stability Test (ASTM D2070) is similar but focuses on elevated temperature, not oxygen exposure. When applied to a fresh and in-service fluid, some oxidation tests, such as ASTM D2272 or Oxidation by FTIR (ASTM D7214), measure whether antioxidant additives are depleted. The Hydrolytic Stability Test (ASTM D2619) also focuses on predicting service life. It tests how well fluid can resist changes in the presence of water. Better results imply a longer service life and better suitability for applications where water contamination is likely. Tests for the presence of water, such as the Water Content by Karl Fischer Test (ASTM D6304), and tests to determine the Particle Count (ISO 4406) reveal contamination of in-service fluids. The Water Separability Test (ASTM D1401) evaluates the ease of separating water from samples when they are contaminated. Filterability tests verify that only contaminants, not fluid components, are removed during filtration. Both tests indicate the possibility of extending service life by removing contaminants. Testing for volatility (ASTM D6417) can indicate whether a fluid could lose some of its critical properties as the lightest components evaporate. When applied to an in-service fluid, changes in volatility can indicate contamination. Changes in fluid composition by elemental analysis (ASTM D5185), density (ASTM D1298), or color (ASTM D1500) also indicate contamination or other fluid degradation.

Savant’s Hydraulic Fluid Testing Capabilities

Savant Labs has extensive experience with all of these hydraulic fluid tests. In addition to testing new fluids for specifications, we offer customized projects to address particular problems. In one case study, the customer was evaluating new fluids for their company-wide hydraulic systems. As part of the project, we tested some of the customer’s in-service fluids. Clues like wear metals revealed during elemental analysis, poor oxidation resistance, and permanent viscosity loss due to shearing helped the client identify unmet lubricant needs to be addressed with their new fluid choice. Our analysts also tested several proposed fluids and recommended which would perform best under the customer’s service conditions. Whether you need to confirm your newly developed fluid is meeting specification limits, verify that a fluid is the right one for your application, or monitor your in-service hydraulic fluids, Savant Labs can meet your needs. Contact Savant Labs for a quote on standard hydraulic fluid testing or to learn more about our custom testing capabilities.