Aeration
Air from outside mixing into the oil. Because air is compressible the system becomes springy: the lever feels spongy, motion becomes jerky and position cannot be held under load. Foam in the tank and bubbles in the return line are the most typical symptoms.
Air mostly enters the system on the suction side, because that side is below atmospheric pressure and the smallest sealing defect draws air in without oil leaking out.
- Places to look: suction hose clamp, pump inlet gasket, shaft seal, loose fitting, low oil level in the tank.
- A return line that ends above the oil level also mixes in air by pouring oil into the tank; the return pipe must always extend down into the oil.
- After a system repair or hose replacement it is normal for air to remain; run a few full cycles without lifting the full load to drive the air back to the tank.
In proportional control the effect of aeration is particularly annoying: no matter how well the controller is tuned, because air adds a spring like flexibility to the system, oscillation and delay become unavoidable. That is why, on a "the lever is shaking" complaint, air and oil level should be checked before touching the controller gain.
Where this term is used: What Is a Hydraulic Lever Retrofit Kit? Definition, Components, Operating Principle