Hydraulic oils must be able to withstand particular conditions and still perform their primary function of transferring power from one point to another. As such, they have characteristics that make them unique from regular oils.
Viscosity of Hydraulic Oil
The viscosity of an oil is one of the most essential characteristics, especially for hydraulic oils, as they must transfer power. As such, the viscosity-temperature characteristic of hydraulic oils is also critical. As the temperature of the oil increases, its viscosity decreases (or becomes thinner). Similarly, if the temperature of the oil decreases, its viscosity will increase (or become thicker).
For hydraulic oils, some manufacturers plot their viscosity against temperature to help customers determine the ideal viscosity for their system based on the system’s operational temperatures. Figure 8 shows a chart for Shell Tellus S2MX, illustrating the varying viscosities as the temperature changes.
Choosing the wrong viscosity can cripple power transmission before the system even starts.
For instance, if the system is running at 60°C and the oil needs to have a viscosity of 30cSt, then an ISO VG 68 would be the most ideal oil. However, if the system is running at 40°C, and the viscosity needs to be 30cSt, then an ISO 46 oil would be more appropriate.
The relationship between viscosity and temperature is known as the viscosity index. For hydraulic oils, the higher the viscosity index, the less susceptible the viscosity is to changes in temperature. As a reference, mineral base oils have a natural viscosity index (VI) of 95-100, while synthetic ester-based base oils have a VI of 140-180, and polyglycols have a natural VI of 180-200.
Oxidation and Thermal Stability
The TOST (Turbine Oil Oxidation Stability Test) is usually used to determine the oxidation stability of an oil. Although the test name mentions “turbines”, it can also be applied to hydraulic oils.
Another test that can be used to evaluate whether oxidation has taken place or not would be the RULER® test, which quantifies the remaining antioxidants in the oil. Overall, determining the oxidation and thermal stability of the oil provides the user with an average estimate of the oil’s life expectancy when subjected to environmental extremes.
Foam and Air Release Properties
Due to the operating environment of hydraulic oils, air tends to become trapped in them. This can become a problem as it can easily lead to cavitation inside pumps (the most prevalent form of wear for these systems). Therefore, hydraulic oils must have good air release and anti-foaming properties.
Good air release allows the dissolved or trapped air to coagulate and rise to the surface, where it can then be dissipated. This is where the issue of foam “arises” as it further impedes the oil’s ability to form a full wedge between the two surfaces in any system.
Demulsibility and Water Content
Demulsibility refers to the ability of oil to repel water. Typically, hydraulic oils are designed to operate in environments with some water or high humidity, where water can easily enter the oil.
For hydraulic oils containing detergents or dispersants (DD), fluids (such as water) or other fine contaminants are usually held in suspension. Therefore, the demulsibility test, in which water and oil are mixed and then allowed to separate, will not be effective in determining the water separation characteristic of these hydraulic oils. Filtration should be used for these DD oils, which become contaminated with water.
Water in hydraulic oil isn’t harmless — it’s a silent trigger for failure.
On the other hand, for those oils that do not contain detergents or dispersants, the demulsibility test (ASTM D1401) can be performed. For this test, equal parts of oil and water are mixed at a specific temperature to create an emulsion and then allowed to separate. The amount of oil, water, and emulsion is recorded at 5-minute intervals.
If the viscosity of the oil is less than 90 cSt and there is 3 mL of emulsion or less after 30 minutes, the oil is acceptable. If the oil has a viscosity greater than 90cSt, the result is taken at the end of 60 minutes (if the value of the emulsion is less than 3 mL, it is acceptable). The results are usually recorded in the format: mL oil / mL water / mL emulsion (time recorded in minutes).
Corrosion Protection
Many hydraulic systems contain copper metals, brass, or bronze, especially in cooling systems, pumps, bearing elements, or guides. Therefore, hydraulic oils must be resistant to copper corrosion, as this could compromise the entire system. One such test, which can be used to identify the corrosivity of these oils, is the Copper Strip Corrosion test.
In this test, the copper strip is placed in hydraulic oil for a typical duration of 3 hours at 100°C. The results can be quantified based on the level of discoloration, which correlates with the degree of corrosion.
A similar type of test can also be done with steel / ferrous corrosion. In this case, the oil is mixed with distilled water or artificial seawater and stirred constantly for 24 hours at 60°C while the steel rod is submerged in the mixture. Afterwards, the steel rod is examined for corrosion and allocated ratings accordingly.
