Hydraulic fluid - getting the viscosity right

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Most hydraulic systems will operate satisfactorily using a variety of fluids. These include multi-grade engine oil and automatic transmission fluid (ATF), in addition to the more conventional anti-wear (AW) hydraulic fluid - provided the viscosity is correct.

Viscosity is the single most important factor when selecting a hydraulic fluid. It doesn't matter how good the anti-wear, anti-oxidization or anti-corrosion properties of the oil are, if the viscosity grade is not correctly matched to the operating temperature range of the hydraulic system, maximum component life will not be achieved.

Defining the correct fluid viscosity grade for a particular hydraulic system involves consideration of several interdependent variables. These are:

  • starting viscosity at minimum ambient temperature;
  • maximum expected operating temperature, which is influenced by maximum ambient temperature; and
  • permissible and optimum viscosity range for the system's components.

Once these parameters are known, the correct viscosity grade can be determined using the viscosity/temperature curve of a suitable type of fluid - commonly AW hydraulic fluid defined according to ISO viscosity grade (VG) numbers.

Automatic transmission fluid, multi-grade engine oil and anti-wear, high VI (AWH) hydraulic fluid are commonly used in hydraulic systems that experience a wide operating temperature range. These fluids have a higher Viscosity Index (VI) than AW hydraulic fluids due to the addition of VI improvers. The higher the VI a fluid has, the smaller the variation in viscosity as temperature changes.

In simple terms, this means that if you are running ATF(46) in your skid-steer loader, you can operate the hydraulics with a higher fluid temperature before viscosity falls below optimum, than you could if you were running ISO VG46 AW hydraulic fluid.

When selecting a high VI fluid, the component manufacturer's minimum permissible viscosity value should be increased by 30% to compensate for possible loss of viscosity as a result of VI improver sheardown.

And VI improvers can have a negative effect on the demulsification and air separation properties of the oil and for this reason some hydraulic component manufacturers recommend that these types of fluids only be used when operating conditions demand.

As far as oil recommendations go, for commercial reasons relating to warranty etc, I always advise following the machine manufacturer's recommendation. But for equipment that has a history of satisfactory performance and component life, there is usually no compelling reason to change the type of fluid being used.

Related articles:

Which hydraulic oil should I use?
How to consolidate your hydraulic oil inventory
How to make your hydraulic oil last longer

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