Origin of polltion in hydraulic circuits

1. The component

Origine de la pollution

Most hydraulic failures are due to pollution (75%).

1. Solid pollution

The origins of solid pollution are:

  • Original pollution (assembly of components).
  • Oil fill contamination.
  • Contamination by hydraulic failure (motors, pumps , etc.)
  • Contamination by internal degradation (cylinders, hoses, etc.)
  • External contamination (contamination by cylinder rods, couplers, breather, etc.)
  • Contamination following a maintenance intervention (flexible changes, etc.)
  • Contamination through component wear.
  • Cavitation contamination (pumps, motors, etc.)

Solid pollution is classified into 3 categories:

  • Coarse pollution :

The particle size corresponds to the functional clearance value of the components.

Coarse pollution causes spool seizing and major scratches. Failures are sudden and blocking.

Coarse pollution clogs the nozzles.

  • Fine pollution:

The particle size is less than the functional clearance value of the components.

Fine pollution behaves like grinding paste. It causes wear which leads to internal leaks. The movements of the receivers are slowed down. Leaks cause the oil temperature to rise.

  • Ultrafine pollution :

The particle size is less than 3 microns. We are talking about micronic mud.

The ultrafine pollution increases the friction of the drawers until they block.

Ultrafine pollution accelerates the oxidation of oils and increases the TAN index (acid index).

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2. Thermal pollution

The origins of thermal pollution are:

  • Cooling circuit fault.
  • Climatic or external condition.
  • Excessive lamination (wrong setting).
  • Internal leaks.
  • Oil level.

Thermal pollution causes spool seizures. The internal lubrication of the components is degraded. Failures are sudden and blocking.

3. Liquid or gaseous pollution

The origins of liquid or gaseous pollution are:

  • Contamination by oil top-ups (uncompatible oil mixture, etc.)
  • Contamination by storage (condensation, sealing, etc.)
  • Defective cooling system (exchanger out of order, etc.)
  • Air intake.
  • High-pressure washes.

Liquid or gas pollution causes fluid degradation. Contamination changes viscosity , increases acid number and accelerates oil oxidation. (Aging)

The presence of water can modify the filterability of the fluid. The oil loses its lubricity. Water or a mixture of 2 incompatible oils causes emulsion and poor deaeration of the oil.

The acceptable water content for hydraulic installations is 0.05%.

That is 10 cl for a 200 liter tank

Cavitation is a destructive phenomenon that occurs mainly on hydraulic pumps and motors. The formation of gas bubbles (cavity) is the result of a depression at the suction of the pumps. When these gas bubbles are recompressed, implosions cause tearing of metal.

4. Closing

The most critical pollution for hydraulic installations is invisible to the naked eye.

From 40 µm the human eye can no longer see.

For information :

Grain of fine table salt: 100 µm .

Human hair: 70µm .

White blood cell: 20 µm .

Talc powder grain: calibrated between 3 & 10 µm .

A good visual appearance of the hydraulic oil cannot therefore guarantee that the oil is of good quality, on the other hand the presence of particles in suspension must be considered abnormal.

The milky appearance is the sign of an emulsion(Presence of water or mixture of 2 incompatible oils)

The “foaming” aspect is the sign of aeration. (Presence of air)

The dark colored appearance with a burnt smell corresponds either to rolling (heating of the oil) or the presence of diesel effect (implosion and combustion of the oil).


Inspection of the filters sometimes reveals abnormal pollution. The causes must be sought.


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