Air and Aeration in Hydraulic Oil

What does aeration in hydraulic oil mean?

Hydraulic oil always holds dissolved air - about 9 percent by volume at atmospheric pressure. As long as the air stays dissolved it barely matters. When pressure drops below the saturation point, air comes out as fine bubbles and forms undissolved air. At the surface it collects as foam. These undissolved bubbles make the oil compressible and cause the typical problems.

What damage does air cause in the system?

Undissolved air makes the otherwise near-incompressible oil soft: the cylinder feels spongy and control becomes imprecise and sluggish. When bubbles are compressed suddenly in the high-pressure zone, extreme temperature spikes appear - the so-called diesel effect burns the oil locally and cokes seals. Cavitation-like implosions erode material at the pump and valves.

• Spongy, jerky motion of the actuators.
• Loud, humming or knocking pump noise.
• Accelerated oil oxidation from local heat and oxygen intake.
• Coked seals and dark lacquer deposits (varnish).
• Increased wear at the pump, control valves and bearings.

Where does the air enter the oil?

Air usually enters on the suction side or through leaks under atmospheric pressure. Clogged suction filters, undersized lines or a low oil level create a vacuum that releases dissolved air. A poorly routed return line that ends above the oil level also whips air into the fluid.

Frequently asked questions