Hydraulic Oil: Viscosity

Hydraulic Oil



    The animating element of a hydraulic system is the hydraulic oil.  For something so important you would think it would get more attention.  We buy it in bulk so we can get the best price but we give little attention to how we store it and transfer it into our equipment.  We allow it to overheat and get dirty and yet we expect it to perform everyday and keep our machines running.  


Fortunately, hydraulic oil is quite forgiving and even under abuse and neglect, it continues to perform.  But if we treat it properly, keep it clean and moderately cool, it will take care of our equipment and take care of our customers.  This is the first part of a four part blog series we will discuss the concept of viscosity.  



     Most of us understand what viscosity is, but have a hard time defining it with words.  Visualizing a fluid that is thick and pours slowly is one that has higher viscosity. But in a molecular sense, it is the resistance to deformation or change.  Viscosity in hydraulic oil is a function of Temperature. As the temperature of the oil increases the viscosity decreases and at some point the viscosity decreases to a value that is detrimental to the hydraulic system.  Selecting the correct viscosity is a function of operating parameters and climate. 


Given the typical climate across the United States, ISO46 is the standard hydraulic oil used in our equipment.  The viscosity level of this oil is prime for keeping hydraulic systems working in North America.  In sub zero temperature regions lower IOS weight hydraulic oils with lower viscosity are a little more appropriate.  Hydraulic oil fills the gap between moving parts; lubricating them and allowing pressure to build in the system.  When the viscosity of the oil doesn’t match the system requirement the lubrication of the internal parts may suffer or the oil may not have sufficient thickness to fill the spaces between moving parts allowing pressure to build in the component.  Having the correct oil viscosity is essential to maintaining lubrication and the pressure capability of the system.


Written by Pat Kinnison and Chrystal Bates

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