Unlike water, which has almost the same flowability between 0°C and 100°C, the viscosity of oil varies hugely according to temperature. In addition, operating pressure or factors, such as oxidation or impurities, also influence viscosity. Unfortunately, it gets even more complicated, because the flowability of an oil does not change uniformly, i.e. linearly, with temperature.
When the temperature drops, an oil will always become thicker, i.e. will have a higher viscosity. When the solidifying point is eventually reached, the oil will become so thick that it can no longer move at all. On the other hand, when the temperature increases, the viscosity drops considerably. The oil can become extremely thin. These temperature-dependent changes must be taken into account when choosing a lubricant. Particular care needs to be taken since viscosity-temperature behaviour depends on the type of oil. Even oils with the same viscosity at, for example, 40°C can behave totally differently at 0°C or 100°C.
A temperature-dependent viscosity change will not be linear but can be calculated “double logarithmically”. Temperature differences of, for example, 10°C do not lead to identical number jumps in viscosity changes. The viscosity index (VI), which is calculated with the aid of kinematic viscosity measured at 40°C and 100°C, is used to describe an oil’s viscosity-temperature behaviour. This parameter allows the viscosity behaviour of various oils to be better compared based on temperature. The calculation method described in ISO 2909 was developed approximately 60 years ago. In terms of the viscosity index, the worst mineral oils known at the time were awarded a VI of 0, and the mineral oils with the best viscosity-temperature behaviour were awarded a VI of 100. At the time, there were no synthetic or multigrade oils. Nowadays, viscosity can be influenced by so-called VI improvers or synthetic oils to such an extent that the viscosity index now extends far beyond 100. The following standard values illustrate how high the viscosity index can be taken by modern oils:
Oil or fluid type
~ 95 - 105
~ 140 - 200
~ 135 - 160
~ 140 - 190
~ 195 - 210
~ 200 - 220
~ 205 - 400
A simple and widely-used method for visualising viscosity-temperature behaviour is the viscosity-temperature diagram (VT diagram) according to Ubbelohde/Walther. Using the mathematical conversion (double logarithmical calculation), VT behaviour can be approximated to such a degree using a straight line through two points (usually at 40°C und 100°C), that viscosity at all other temperatures can be read from the diagram.
Different fields of application can be illustrated using the VT diagram. HLVP oil with a higher viscosity index can, for example, cover a broader temperature range.
Oils also become thicker with increased pressure. Viscosity-pressure behaviour is also a lubricantspecific parameter which can however, for the most part, be neglected since, at pressures below 400 bar, it is virtually insignificant. The change in viscosity due to an increase in pressure of 100 bar is disproportionately less than that due to an increase in temperature of 10°C. Designers of highperformance hydraulic systems and components always consider the infl uence that pressure has on viscosity whilst also allowing for the temperature influence which occurs simultaneously.
Amongst other things, lubricants are tasked with protecting the surfaces of pairs of moving parts against wear by building a resilient lubricating film. There is the positive effect that, with traditional lubricating oils, the viscosity in the lubricating film is increased to such an extent on account of the prevailing pressure on it that the surfaces are kept apart.
In metrological terms, the viscosity of a lubricating oil, which has been changed on account of high pressure, is very difficult to determine. Only a handful of institutes, such as the RWTH in Aachen, can also actually carry out such measurements.
Viscosity changes in oil applications
With regards to oil changes, the most important parameter in used oil analyses is the consideration of viscosity changes. The viscosity of an oil can change for reasons other than just temperature and pressure. If the viscosity of a sample differs from the initial values of the fresh oil or the reference of the previous analysis, the causes may be as follows:
Increase in viscosity
- During operations, the oil has absorbed oxygen on account of the temperature and has therefore been oxidised.
- Oxidation inhibitors, ageing-delaying additives have decomposed.
- Ageing and oxidation products, such as acids and oil-insoluble components have formed.
- Varnish-like deposits, such as resin and sludge, have built up.
- Soot, dust, water or residues from alternative fuels are contaminating the oil.
- The wrong oil was used or refilled.
Decrease in viscosity
- VI improvers, additives for improving the viscosity index, were not shear-stable and have decomposed.
- Unburned fuel (poor combustion) has diluted the oil.
- An oil which was too thin or the wrong type of oil was used or refilled.
- The system was cleaned with a thin flushing oil prior to being filled. Flushing oil residues have got mixed in.