Air from oil

Environmental pollution and climate change continue steadily. In response to these dramatic developments, laws have been passed to reduce pollution, such as the Euro standards for exhaust emission. Complying with these pollution limits calls for additional measures, such as installing extra exhaust treatment systems next to the combustion engine. This takes up a lot of space inside the machine, however. This and the high cost pressure required to remain competitive on a global basis finally result in the hydraulic tanks being given increasingly complex and smaller shapes. These designs significantly limit the amount of air separation that can take place inside the tank, so more free air circulates inside the hydraulic system.

Free air in hydraulic systems still represents a major technical challenge today. As long as the air is dissolved in the oil, it does not change its technical characteristics.

Undissolved air, however, i.e. air bubbles, causes the following:


  • Signs of corrosion on pumps and controls,
  • It reduces the efficiency of pumps and hydraulic motors
  • It causes increased compressibility and thus possible stuttering movements of the output element,
  • Accelerated oil aging, varnish
  • Noise
  • Component damage (e.g. cavitation)
  • Escaping oil, foam development in the tank
  • Temperature increase
  • Dynamic operation problem
  • Micro-diesel effect
  • Reduced filter capacity
  • Changes to the viscosity

The air enters the circuit during assembly work, due to leaky areas in the underpressure range and when the oil flows back into the tank. Depending on the separation capacity of the filter tank system, the air in the tank rises only slowly and is drawn in again by the pump.

Air is dissolved in the oil proportional to the pressure (up to about 300 bar). If any air is still dissolved at high pressure, it is separated at low pressure (e.g. downstream of needle valves). As the separation speed is higher than the dissolution speed, air bubbles still remain in the oil for a while after a new pressure increase.

Large bubbles naturally rise to the surface faster and the hydraulic system can degas. If there are many small air bubbles, they are moved on due to their small size in the worst case, and conveyed by the hydraulic system. If air is now fed to the system, so many small air bubbles accumulate in the oil in the hydraulic system that it gets cloudy. At exposed points of cylinders or on brake pistons and after an appropriate settling time, air may accumulate in the oil. Small bubbles merge to form large bubbles that e.g. allow a boom or a hydraulically operated brake to become soft hydraulically. One should bear in mind that transmission drives and brakes of self-propelled work machines are safety-relevant components. Therefore, especially for such safety-relevant systems, degassing of the system is essential