The oil pump draws in so much air that it can no longer be released. Reasons for this can be: worn seals, leaks at the hydraulic pump or the pipelines, too low or too high filling level in the tank, changed flow conditions in the tank or in front of the suction nozzle of the pump.
These are often dust, sealing compound, condensate, water (especially with grease solvent), lubricating grease or assembly paste, residues of metalworking fluids or antifreeze (e.g. from a refill canister).
Incompatibility with other oils
Oils are not always "compatible" with each other. Especially when a synthetic ester-based bio-oil is mixed with a mineral oil, the surface tension of the fluids changes. The antifoaming agent does not suffice anymore to cause the air bubbles of the foam to burst. A complete oil change or the addition of a defoamer will remedy the situation.
Failure of the defoamer
Oil filtration can filter out the antifoam additives and therefore, they must be added again.
If oil containers are stored for more than 5 years, the additive may partially separate from the oil. A slight movement of the container causes the defoamer to immediately go back into solution
The OELCHECK tip on the subject of "foaming":
If foam occurs in any of your plants, indicate "foaming" on the Sample Information Form. OELCHECK finds the cause!