Property of a fluid to act against resistance to the opposite laminar movements of two neighbouring layers (DIN 1342 and IDN 51 550).
A distinction is made between dynamic viscosity and kinematic viscosity, which is preferred for use in hydraulic systems as it is easy to measure (DIN 51 562).
The viscosity is a very important property of hydraulic fluids, which plays a role in its selection and the usage limits of several devices (Figure V 27).
In order to compare the viscosity of different fluids, they must relate to a uniform temperature based on their viscosity/temperature behaviour. This is done using viscosity classes.
Figure V 27: Viscosity of different hydraulic fluids