Why we use ventilation in the hydraulic system

Bleeding hydraulics: why is it important and what should you watch out for?

We have already presented them to you: the hydraulics and hydraulic systems, with their advantages and disadvantages, their individual components, such as hydraulic cylinders or pumps, and how they work. Anyone who uses hydraulic systems knows that correct maintenance and optimal commissioning of the systems will significantly increase their service life.

Today it is all about good maintenance and repair of hydraulic systems: What role does air play in a hydraulic system? Why is bleeding important and what should you watch out for? Are there any indications and help to identify damage and how can errors be avoided? You can find the answers on this page.

Air in hydraulic systems

As planned, there is only one point of air in the hydraulic system, namely in the hydraulic tank, above the fill level, i.e. above the oil. This air has an oxygen content of approx. 21% and is not a problem. The whole thing only becomes critical when the air mixes with the hydraulic oil.

The hydraulic tank

Unintentional mixing of air and oil can happen if a line or inlets are installed too high up and the hydraulic oil flows down into the container from above like a waterfall. The result is an undesirable air-oil mixturewhich can lead to serious damage to the system, including seals, cylinders, pumps and hoses, and cause leaks.


Leaks, for example in a connection, cylinder or valve, mean that more air can get into the system. This happens when individual components are damaged or on seals that are heavily loaded, such as the sealing lips of hydraulic cylinders.

Why do you have to bleed hydraulics?

You are probably familiar with it from your home heating system, which you have to ventilate at regular intervals. If there is too much air in the radiator, the heating becomes less warm, makes adventurous noises and the heating output drops significantly.

It is roughly the same when there is air in the circuit of a hydraulic system. In addition, however, the air in the hydraulic system can lead to small explosions - and these are anything but good, as we will see below.

When we talk about air within a hydraulic system or hydraulic pump, we don't mean large air bubbles, no, we mean air within a hydraulic system small air bubbles that mix with the hydraulic fluid.

These supposedly tiny bubbles should not be underestimated, because they can have an unsightly effect and cause considerable damage. They can lead to premature wear. You have the strength Damage seals, cylinders and other components.

Consequences of air in the hydraulic system

The consequences of unwanted air bubbles in a hydraulic system can vary in severity and become a long-term problem. On the one hand, the control can be influenced: Air in a hydraulic system can influence the control, which makes it less precise to operate.

On the other hand, and this is a particularly unpleasant consequence, it can lead to wear and tear and internal damage to the system: the seals are particularly often affected by this. At high pressures on the mixture of oil and air bubbles, so-called "Micro explosions“Which can damage the seals.

These seemingly small explosions occur when air bubbles expand and are put under great pressure together with the hydraulic oil. In practice, these explosions are also known as Diesel effect.

The diesel effect

At particularly high pressure, air in the hydraulic oil can lead to the so-called diesel effect.

The term “diesel effect” is derived from the way the diesel engine works, as this is also a chemical-physical process: An explosion occurs in which a mixture of oil and gas (air) is strongly compressed. Within a hydraulic system, these explosions can lead to internal wear and damage, even if they appear inconspicuous and often go unnoticed.

When is ventilation necessary?

One is particularly important Bleeding hydraulic systems when using for the first time. By the way: before you fully use your new hydraulic system and hydraulic cylinder, you should check all functions in addition to the ventilation, starting with the suction line and pump, through the cylinders and filters, to the drawbar and the screw connections. It is best to start the first commissioning with only low pressure.

When changing the oil or when lines and valves have been opened, the system must also be carefully vented.

Directional valve from Busch Hydraulik

Evidence of a diesel effect and internal damage

As already mentioned, “micro-explosions” or a diesel effect often happen so inconspicuously that they are not noticeable. However, there is an indication of such internal damage: the small explosions contaminate the hydraulic oil and turn it dark. If you notice the oil turning black, it could be that there is a diesel effect somewhere in your system.

How does ventilation work?

When bleeding hydraulic systems, all functions are activated one after the other while idling with very low pressure but full cylinder stroke. The ventilation should be carried out at the highest point in the pipeline network. Similar to the venting of a radiator, the screw connection is easily loosened to vent a hydraulic system. This allows the air to escape wonderfully with little oil leakage.

As soon as no more foaming oil escapes, the system is vented and you can tighten the screw connection again. Please then refill the amount of oil that escaped during venting so that there is sufficient oil in your hydraulic system and you can generate optimal pressure again.

Special hydraulic systems with vent screws

Some hydraulic systems have vent screws on the lift cylinder. If this is the case, you can carefully vent your system at this point. It is important that the full cylinder stroke is actuated and moved several times so that you can be sure that there are no more air residues in the system.

For cylinders that are arranged horizontally, the vent screws should always be at the top.

Prevention and contributions to the protection of the plant

What can you do so that there is neither a diesel effect nor damage from unwanted air particles? In addition to regular venting, there are now components that are a successful answer to the air problem and can prevent air fractions from gassing into the oil. These include, for example, degassing modules for tanks and lines or venting valves for hydraulic cylinders.

Bleeding Hydraulics - Summary

It is often impossible to tell with the naked eye whether there is air in a hydraulic system. Even a seal that appears intact and fully functional at first glance can be leaky and possibly lead to additional air in the liquid.

So that you can enjoy your hydraulic system for a long time, we recommend that you bleed your hydraulic system at regular intervals and check that the oil turns black.

We hope to have given you answers to all your questions and we look forward to advising you personally.