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Bleeding Air from a Hydraulic System

Although manufacturers may state that bleeding air from a hydraulic system is not necessary, many engineers prefer to perform this maintenance action.

Broadly speaking, over time air will usually find its own way out of a hydraulic system. Even air that has become trapped in oil will usually get transported back to the reservoir at some point.

However, bleeding air from a hydraulic system can speed up this process. It’s common knowledge that air trapped in a system can cause mechanical damage and lead to erratic operation.

For example, air contamination damage may include:

  • Erosion caused by air bubbles in oil collapsing under pressure
  • Decrease and sometimes loss of lubricity that goes on to cause scoring, abrasion and other friction damage between surfaces that should be lubricated
  • Compression leading to ignition of the mixture of air and oil in the system, burning seals and damaging the cylinder.

Why would manufacturers tell any of us that bleeding or purging of oil is not necessary? Our thoughts are that if there is damage caused, then they can sell you replacement parts as they won’t be under warranty. However, there is also a chance that you could get injured as you bleed the air from the system if they condone it as a general practice and for this you could sue them for hundreds of thousands of pounds. In short, they are protecting themselves. Basically, it brings about a liability issue. They have a point: It’s not a beginner’s job to bleed the air from a hydraulic system and you truly do need to know what you’re doing to take this on.

A less dangerous but somewhat effective approach is to minimise any air on start-up. You can do this by filling any voids as you start up. For example, if you were to replace the cylinder, then you would fill it with clean hydraulic oil before you connect it. The same goes with other components such as the pump, intake lines etc.

Although it’s not possible to completely fill every component, it does offer a safe way to help to expunge any air in the system and that is helpful. 


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