There are several basic tests that should be used to determine the condition and health of hydraulic oils. These include:
- Viscosity (ASTM D445) – Generally, if this value falls below or above10-15%, there is cause for concern. Any increase in viscosity (outside of the system limits) can lead to the system experiencing higher pressures. Conversely, any decrease in viscosity outside of the limits will not allow for the full transfer of power through the fluid.
- Water content (ASTM D6304) – Too much water in a system is always a bad thing (except in a swimming pool). In particular, if the water content starts trending upwards of 500 ppm, the source of water ingression should be found and eliminated at once. This can hinder the transmission of power in the system, making it less efficient.
- Presence of wear metals (ASTM D5185-05) – These values will differ depending on the system in which the hydraulic oil is being operated. It would be a good idea to contact the OEM about the limits for the wear metals for your system to ensure that no irregular wear is occurring. The presence of these wear metals may also act as catalysts for other reactions, potentially leading to the degradation of the oil.
- Particle Count (ISO 4406) – This also depends on your system, as varying levels of cleanliness are typically aligned with different systems. However, with hydraulic systems, there is usually some guidance on the tolerance levels. The presence of these particles can hamper the transmission of power in the system or block clearances.
Specialty tests for hydraulic oils also exist. These include tests for monitoring antiwear and extreme pressure, foaming, or oxidation stability characteristics of the oil. For determining the antiwear or extreme pressure properties, tests such as the Vickers Vane pump test, 4 Ball test, and FZG rating test can be used.
