Skid steer loaders, compact track loaders, and mini-excavators typically use petroleum-based hydraulic fluids. How much do you know about the hydraulic fluids you work with every day? Let's see if we can share something you don't know yet!
Here are a few other Shop Talk Blog posts you might find of interest:
- Terminology for Final Drive Motors, Part 1
- Maintaining Skid Steer Final Drives for Optimal Performance and Reliability
- Hydraulic Hose Fittings: A Beginner’s Guide
Mobile hydraulic equipment like skid steer loaders, compact track loaders, and mini-excavators typically use petroleum-based hydraulic fluids. In this Shop Talk Blog post, we’ll talk about the basics of this type of hydraulic fluid.
Key Characteristics of Petroleum-Based Hydraulic Fluids
Synapticrelay, CC BY-SA 4.0, via Wikimedia Commons
Viscosity is probaby the most critical factor in hydraulic fluids. It directly relates to the thickness of a fluid, with higher viscosity fluids moving more slowly. However, if it’s too high, cavitation, problems at equipment startup, and reduced efficiency result; if the viscosity is too low, however, greater friction levels and accelerated wear result.
ISO VG is the Viscosity Grade, a standardized reference for the viscosity of lubricants, including hydraulic fluid. The values provided in ISO VG tables represent the kinematic viscosity at 104ºF measured in cSt (centistokes). The temperature of 104ºF is intended to represent the operating temperature within the equipment. For petroleum-based hydraulic fluids, the viscosity range is typically between 10 to 100 cSt and is measured using viscometers to determine the fluid’s resistance to flow. Kinematic viscosity tests are often conducted at standardized temperatures (e.g., 40°C and 100°C) following methods such as ASTM D445.
The VI, or Viscosity Index, is used to express how much the viscosity of a fluid varies with temperature. This can be extremely important in applications with significant temperature variations. Note that viscosity index improvers are a common additive for hydraulics fluids. Standardized procedures, such as those outlined in ASTM D2270, provide a numerical value indicating how much the fluid’s viscosity changes with temperature.
The oxidation stability of a petroleum-based hydraulic fluid is also essential and relates to how resistant the fluid is to natural aging and degradation. Aging compromises the performance of the fluid and leads to problematic byproducts such as sludge and varnish. In fact, oxidation reduces the service life of lubricants by half, for every 18॰F increase in fluid temperature above 140॰F. Oxidation stability is usually measured using accelerated oxidation tests.
Lubricity is usually evaluated with wear tests. Methods such as the four-ball wear test measure the wear scar on a metal surface after prolonged exposure to the fluid. These tests help quantify the fluid’s ability to form a protective film that minimizes friction and wear. The video below illustrates such a test but for grease instead of hydraulic fluid.
Thermal stability is related to the fluid’s ability to dissipate heat, and most fluids do a good job of dissipating heat under normal operating conditions -- but they have limits. Heat will degrade hydraulic fluids. Thermal stability is measured by exposing the fluid to elevated temperatures and monitoring any changes in its properties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) indicate how the fluid behaves as it degrades, while ASTM methods such as ASTM D5800 evaluate oxidation stability and determine how the fluid’s viscosity and performance are affected by heat over time.
Seal compatibility is key because not all hydraulic fluids are compatible with all seal materials. When incompatibility exists, the seal will begin to degrade over time, leading to contamination and leaking seals. Elastomer seals are exposed to the hydraulic fluid under controlled conditions, after which changes in tensile strength, elongation, hardness, and swelling are measured, usually following ASTM D471.
Hydraulic Fluid Additives
Additives are used to enhance certain aspects of hydraulic fluid performance and usually serve one of three purposes related to the base oil: enhance existing properties, suppress undesirable properties, or impart new properties. These additives usually comprise 2 - 10% of the total fluid volume in petroleum-based hydraulic fluids. Note that the type and volume of additives used are what differentiate gear oil, engine oil, and hydraulic fluid.
R&O (or RO) is a rust and oxidation inhibitor. Oxidation inhibitors extend the life of hydraulic fluid by reducing the oxidation aspect of aging, as discussed earlier. Rust and corrosion inhibitors form a chemical barrier to protect the metal surfaces from moisture.
AW stands for anti-wear and its job is to reduce wear, prevent metal loss, and reduce friction during surface-to-surface metal contact. They are key to preventing issues such as scoring and seizing, as well. When heat is generated, they form a protective film.
EP stands for Extreme Pressure and achieves similar results as AW but does so by chemical reaction. In addition, EP is more aggressive than AW in preventing wear and metal loss. EP additives begin protection under high loads when high contact temperatures are triggered. EP additives are usually responsible for the sulfurous smell of some oils and fluids.
Viscosity index improvers, abbreviated VI, prevent hydraulic fluid from becoming too thin when temperatures increase and also aid in better oil flow at low temperatures. Note that HVI stands for high-viscosity index and can include VI-improved R&O and VI-improved AW.
The pour point of a fluid is the lowest temperature at which an oil will still be fluid. This can be a major issue with petroleum-based fluids because of wax crystals that form when temperatures drop too low. Pour point depressants allow fluids to be used at lower temperatures.
Defoamants are added to hydraulic fluids to prevent the formation of foam by weakening bubbles that form. They also contribute indirectly to preventing oxidation and the accelerated aging associated with it.
In addition to these additives, colored dyes are often added to make it easier to identify the presence of leaks.
Conclusion
Understanding petroleum-based hydraulic fluids is essential for keeping your final drive motors, pumps, and hydraulic systems running at peak performance. From viscosity and oxidation stability to seal compatibility and additives, each aspect of hydraulic fluid plays a crucial role in preventing wear, reducing heat damage, and ensuring smooth operation. Choosing the right hydraulic fluid helps extend the lifespan of your equipment and reduce costly downtime.
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