Viscosity is the name given to the characteristic of a fluid , where a fluid exerts a resistance to the laminar movement of two adjacent fluid layers against each other.
The most important parameter when selecting a hydraulic fluid is the viscosity. It increases as pressure increases and rapidly falls as temperature increases.
If viscosity is too high, this leads to high frictional and flow losses, which is indicated by a pressure drop and rapid heating of the oil.
If the viscosity is too low, this leads to leakages , increased wear and again to rapid heating of the oil. We differentiate between kinetic and dynamic viscosity. Kinetic viscosity is preferred in hydraulics as it is easier to measure. It is defined as the relationship of dynamic viscosity to the density of the fluid.
Cf. viscosity η, dynamic and viscosity ν, kinetic.
viscosity η, dynamic
In a moving fluid, the relationship exists between the shearing stress t and acceleration dv/ds. Coefficient η is known as dynamic viscosity.
viscosity ν, kinetic
This is the usual parameter for viscosity in hydraulics. It describes the relationship between dynamic viscosity η and fluid density ρ
– Viscosity classes
When selecting a viscosity class, the specifications given by the manufacturer must be taken into account. If the maximum value is exceeded this may lead to damage e.g. as a result of lack of lubrication. If the minimum value is not adhered to this may lead to increased wear and leakage .
(X) Pressure p
(Y) Dynamic viscosity η
The viscosity of fluids increases as pressure increases.
This characteristic must be taken into account when planning hydraulic systems which use pressures of more than 200 bar. By approx. 400 bar, the viscosity has already doubled. The viscosity-pressure behaviour changes with respect to temperature.
(X) Temperature in °C
(Y) Viscosity in mm²/s
Viscosity of fluids is greatly reduced with increases in temperature. The viscosity index (VI) is used to indicate temperature dependent viscosity changes. The higher the viscosity index, the lower the temperature dependency of the viscosity.
(Y) Viscosity in mm²/s
-Operating viscosity range
This is the term used for the viscosity range within which the system can operate.
We distinguish here between three main values:
• the max. start viscosity, i.e. the maximum permissible viscosity (lowest temperature) at which the system can still be operated.
• the min. operating viscosity, which is determined by the maximum permissible oil temperature, the point at which oil becomes too thin to lubricate properly or where leakage oil leakage loss becomes too great.
• the max. operating viscosity, which is limited by the maximum permissible viscosity of the individual components of the system and by the ascent time of air bubbles.