Part 2: Contamination in Hydraulic Systems

Ideally, hydraulic oil should create a lubricating film to keep precision parts separated. To reduce wear, this film should be thick enough to completely fill the clearance between moving parts. The actual thickness of a lubricating film depends on fluid viscosity, applied load, and the relative speed of the two surfaces.


If the contamination particle is larger than the clearance between the surfaces, the result is interference. Friction is increased and damage results. This damage introduces more contaminant particles. Therefore, contaminant size should be kept smaller than the clearance and the number of these particles reduced as much as possible. 


(Ref: Parker filtration)

Types of contamination

Contamination in hydraulic systems can be classified into particle contaminants (Metal particles from wear, dirt ingression) or chemical contaminants (water, air, heat, etc). Examples of damage from contamination are: accelerated component wear, orifice blockage, formation of rust or other oxidation, depletion of additives, formation of other chemicals, oil degradation.


(Ref: Parker filtration)

Particle contaminants

Particle sizes are generally measured in micrometers or microns. Some examples of microns: Grain of salt 100 microns, human hair 70 microns, lower limit of visibility 40 micron, milled flour 25 micron, average bacteria 2 micron. Note that most damage-causing particles in hydraulic or lubrication systems are smaller than 14µm micrometres, so they cannot be seen.

Chemical contaminants


The most common chemical contaminant in hydraulic systems is water. The presence of water in hydraulic oil can have wide-ranging effects on system components because of its effect on the physical and chemical properties of hydraulic oil. Rust in tanks, reduced lubrication characteristics resulting in accelerated metal surface wear are some of the most obvious physical results of excessive water, however the effects could be as diverse as the jamming of components due to ice crystals at low temperatures. Chemical effects include additive depletion or deposition, oxidation, unwanted reactions which can result in the formation of acids, alcohols or sludges. Oil becomes cloudy when it’s contaminated with water above its saturation level. The saturation level is the amount of water that can dissolve in the oil’s molecular chemistry and is typically 200 to 300 ppm at 20°C for mineral hydraulic oil. SKF state that hydraulic oil containing just 0.1% water by volume cuts bearing life in half, while 1% reduces bearing life by 75%.


Air in a hydraulic system can exist in either a dissolved or entrained (undissolved, or free) state. Dissolved air may not pose a problem, providing it stays in solution. When a liquid contains undissolved air, problems can occur as it passes through system components. There can be pressure changes that compress the air and produce a large amount of heat in small air bubbles. This compressibility of air means that control of the system is lost. Air bubbles and frothing in the oil reservoir can cause major damage to pumps or it can also cause oil to “boil” out of the tank.


Excessive heat in hydraulic systems can also result in additive depletion or chemical changes to the oil.


Common sources of contamination in hydraulic systems

In general there are four main sources of contamination in hydraulic oil.

  1. “Native” contamination. These are contaminant particles that were left in the system or any of its components during manufacture or repair and include contaminants such as welding slag, machining swarf, pieces of Teflon tape or excessive sealant.
  2. Contaminated new oil. New oil may not be clean enough for the system (and is generally not clean enough for a modern, high pressure hydraulic system). The manufacturing process and the subsequent handling and storage introduce contaminants.
  3. Ingressed contamination. This contaminate may enter with air flowing into the reservoir through the breather cap. Water will build up in the oil of a system operating in humid conditions if there is no protection against this built in to the reservoir. Another common source is dirt particles riding in on a cylinder rod. No rod seal can totally prevent the entrance of particles, this can be a major source of contamination on earth moving equipment operating in extreme conditions. Furthermore, whenever the system is opened in any way such as when a hose is disconnected or fluid is topped up, there is the potential for contamination to ingress.
  4. Internally generated contaminate. Particles removed from the interior surface of the components will circulate in the system until they are removed. Each impact of one of these particles with a surface causes more damage. This phenomenon is known as the wear regeneration cycle. Included in this is contamination from the catastrophic failure of components within the system.

The need for filtration

It is clear that even sealed system can have contaminate build up, therefore any hydraulic system needs to have a means of controlling contamination.

Reference on our Website

A reminder that this information as well as the other parts of this series will be available on our website for  quick and easy reference.

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