How to Keep Air Out of Your Hydraulic Oil for Smooth Operation

How do I prevent air from getting into my hydraulic oil?


Does it sound like there are marbles in your hydraulic pump? Does your cylinder keep creeping even after you have closed a valve to stop it?

All hydraulic fluid contains air. It is part of hydraulic oil’s makeup to contain up to 12% of dissolved air. Dissolved air in hydraulic oil is not problematic but “Free Air” or “Entrained Air” can cause serious issues. Outside of foaming of oil on the surface air can be present in three forms in hydraulic oil:

  • Dissolved Air – 6% to 12% of hydraulic oil is dissolved air by volume naturally.
  • Entrained Air – these are air bubbles usually 1mm or less spread throughout the oil
  • Free Air – is often a “pocket” of air trapped in part of the system such as a cylinder, hydraulic conductor or in a hydraulic pump.

Free air can usually be minimized by pre-filling and bleeding a hydraulic system prior to start-up. Entrained air occurs most often as a result of air making its way into a hydraulic system via the pump inlet. Leaks in suction lines or low reservoir oil level will allow free air to enter into the inlet of the pump. The free air will become entrained air as it exits the pump and the oil is compressed. Peak temperatures will develop and oxidation of the oil will occur. When these air bubbles pop, cavitation occurs and erosive wear increases significantly causing component damage.

Entrained air can also form from dissolved air when gaseous cavitation occurs. Common causes of this phenomenon are:

  • Clogged suction strainers or inlet filters
  • Turbulence on inlet side of pump
  • Too much lift required between pump inlet and reservoir oil level
  • Suction lines having too much restriction or being too small
  • Intake line being collapsed

Proper design, start-up and maintenance guidelines must be followed to prevent air contamination in a hydraulic system.

Bottom line is your hydraulic oil should not look like a milkshake or a well poured Guinness waiting to settle. Hydraulic oil should be relatively clear.