The viscosity of a Newtonian fluid changes as a factor of the temperature: For hydraulic fluids it decreases rapidly as the temperature rises and at low temperatures it approaches the pour point (Figure V 31). This viscosity change can reach a few orders of magnitude. Its characteristic curve is equivalent to an exponential function, which is shown as straight in the dual logarithmic scale (Figure V 32).
This VT characteristic influences the functioning of hydraulic devices: Very high viscosity impairs the suction behaviour of the pumps (danger of cavitation) and can cause switching delays. Very low viscosity worsens the lubricity of the oil and increases the leakage. The operating range of an oil lies in between the two Figure V 33).
If the prescribed temperature cannot be reached with normal mineral oil, an oil with a viscosity improver
must be provided (HV oils) or it must be pre-heated or cooled.
Figure V 31: VT behaviour of two mineral oils
Figure V 32: VT behaviour of flame-resistant fluids as compared to mineral oil
Figure V 33: VT behaviour and permissible operating temperatures