All hydraulic oils contain air in solution, which usually has no affect on the operation of the system as long as it remains dissolved. However, if it comes out of solution and becomes entrained in the hydraulic fluid as bubbles, it can cause numerous problems. Entrained air is the most problematic form of air in a hydraulic fluid. Common causes are normally a faulty system or a result of poor system design. Normal system faults causing air intrusion are leaks in the pump suction, large height level differences between hydraulic components compared to the reservoir tank, or when the hydraulic return line discharges oil above the surface level of the oil in the reservoir. Pre-filling components and proper bleeding of the hydraulic system during start-up will usually help to reduce the concentration of free air. Small amounts of foam are merely cosmetic and will generally not be a problem. However, if large enough volumes of foam are present causing a reservoir overflow, this can be a symptom of a more serious air contamination and/or fluid degradation problem. Negative effects from entrained air can be an increase in fluid temperature, increased oxidation and thermal degradation, cavitation and erosion, a noise increase and decreased pump efficiency.  

Top left: Initial air bubble.

Top right: Initiation of bubble collapse

Bottom left: Forming of liquid jet

Bottom right: Impact and metal extrusion  

Cavitation occurs when the pressure acting on a fluid is below the saturation pressure of the dissolved gas within the fluid. When air bubbles, following the fluid stream, passes through a region of higher pressure, the bubbles will collapse as illustrated in the figure above. The process may be violent, depending upon the load pressure of the hydraulic pump. This can cause broad, high-frequency vibrations, noise, material damage and thermal degradation of the oil.

An important property of a hydraulic oil is its ability to release entrained air. The air release properties vary between different oils based on viscosity, additive composition, age and wear. FRAS Technology has newly acquired an apparatus that enables us to measure the air release properties of hydraulic oils according to the international standards (ASTM D3427, DIN 51381, IP 313, NFT 60149). By measuring the density of the oil before and after air instrusion, and measuring the time it takes for an oil to achieve 99,8% of original density after air intrusion, a quantitative measure can be obtained. In our experience, many oil producers do not focus on this property, as the minimum requirements are very liberal. If you experience troublesome operation due to air problems, a survey of the air release property of your oil may be a valid tool in resolving such issues. 

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