"We will soon need to choose a new oil for one of our hydraulic systems but are concerned about the mixing of hydraulic oils. Do we have reason for concern?"

There are dozens of additives available for use when blending hydraulic oils. Some anti-foam additives, for example, may prevent the buildup of foam on the oil's surface, but at the same time may actually retard the release of air trapped in the bulk oil. As a result, the mixing of hydraulic oils with different anti‑foam agents may actually increase the foaming of the oil.

Some hydraulic oils may be blended to allow water to mix or emulsify with the oil. On the other hand, some oils are formulated to ensure that the water separates from the oil. Therefore, the mixing of hydraulic oils with differing water separation characteristics may cause a reduction in emulsification characteristics or eliminate these characteristics altogether, thereby causing undesirable operating conditions.

Hydraulic oils should be chosen based upon the equipment manufacturer's specifications, keeping in mind the temperature range in which the equipment is to be operated. Occasionally, the manufacturer's specifications may recommend an oil that may not provide the necessary protection due to unique or unusual operation conditions. If these situations occur, detailed consultations with the equipment manufacturer and oil supplier, in cooperation with an independent oil analysis laboratory, are recommended.

Generally speaking, however, it is suggested that top-quality hydraulic lubricants should meet the following requirements.

  • Oxidation and thermal stability
  • Hydrolytic stability (which is the ability to resist chemical reactions with any water present)
  • Anti-rust capability
  • Demulsibility (the ability to separate water so that excess water can be drained off)
  • Anti-wear capability (which is critical in today's high-pressure systems)
  • Corrosion control
  • Filterability (should be at least 5 microns to control contamination, particularly for high-pressure systems)
  • Anti-foam and air-release capability
  • The oil must be shear stable and must be compatible with seal and hose materials

In addition, hydraulic fluids must be maintained in order to ensure their long life and reliability. Some recommendations include:

  • Keep hydraulic fluids cool. The bulk oil temperature at the exterior of the reservoir should never exceed 60 degrees C (140 degrees F).
  • Keep hydraulic fluid clean. There is general agreement among hydraulic experts that 75 to 80 percent of hydraulic system failures are caused by fluid contaminated with dirt, wear particles and other foreign material. In today's high-pressure systems, clearances between wear surfaces are very small, making contamination control critical.
  • Keep hydraulic fluid dry. Water and condensation content should never exceed a maximum of 1,000 ppm, depending on the systems design.
  • Immediately repair fluid leaks. If oil can escape, dirt and dust can re-enter the system. Also, a fluid leak of one drop per second is equal to 400 gallons in a 12-month period.