Oil chiller　Technical Information　3-2.About oil viscosity

Viscosity plays an important role in the function of oil.
Viscosity is the most important property that governs the function of lubricating oil, and is a numerical expression of "stickiness" or "smoothness", which is the ease of fluid flows. Adjust the viscosity to suit your purpose.

The standards of viscosity

Oils used for industrial purposes are generally divided into "mineral oils (petroleum-based hydrocarbons = natural ingredients)"and "synthetic oils (artificially processed oils that do not exist in nature)".

Industrial lubricant viscosity classification by ISO (JIS K 2001)
=18 grades is defined on viscosity grade between ISO VG2 to ISO VG1500
The viscosity grade value is the center value of kinematic viscosity at 40°C. However, low viscosity grades (ISO VG2, 3, 5, and 7) are 2.2, 3.2, 4.6, and 6.8, respectively, which are the central values of the kinematic viscosity at 40°C summarized as integers. The kinematic viscosity range allowed for each viscosity grade is ±10% of its central value.

Absolute viscocity
Absolute viscosity is the resistance force acting on a fluid.
Kinematic viscosity (viscosity)
The absolute viscosity divided by the fluid's density. The unit is usually centitokes cts, but the SI unit is mm²/s.

The role of viscosity

As shown in Figure (a), when the lower surface moves at a speed U, the fluid is dragged into the gap between the ends due to its viscosity, and the fluid molecules push against each other, generating pressure.
This is called pressure generation due to the "wedge film effect." In addition, even in a gap where both sides are parallel, as shown in (b), if the gap decreases at a speed of V, the fluid resists being pushed out of the gap due to its viscosity, and pressure is also generated. This is called pressure generation due to the "squeeze film effect" or "squeeze effect."
Fluid lubrication is a lubrication method that supports loads using such generated pressure.
In both cases, pressure generation increases as the viscosity of the fluid increases.

Temperature change in viscosity

The viscosity of lubricating oil (hydrocarbon oil) changes significantly when the temperature changes, so when writing the viscosity, it is necessary to write the temperature at which the viscosity is expressed at the same time. To consider the kinematic viscosity and temperature of lubricating oil, you need the kinematic viscosity and temperature chart below.
The vertical axis represents kinematic viscosity and the horizontal axis represents temperature. The scale on the vertical axis is the logarithm of the kinematic viscosity, and the horizontal axis is the logarithm of the temperature. By drawing a straight line between them, you can estimate the kinematic viscosity of the oil at other temperatures.
Viscosity Index VI is a numerical value that represents the degree of change in kinematic viscosity due to temperature of lubricating oil.
This corresponds to the slope of the straight line in the figure below, and the larger the value, the smaller the change in kinematic viscosity due to temperature.
VI is based on empirical values, and was measured using base oil of Pennsylvania crude oil with excellent viscosity/temperature characteristics as VI=100 and base oil of Gulf Coast crude oil with poor viscosity/temperature characteristics as VI=0. An index was established to express the position of oil between these two types using a fixed number.

The following Walther's empirical formula is widely used to calculate the relationship between the viscosity and temperature of hydrocarbon oil.

log log(v+k)=n-mlog T
v:Kinematic viscosity [cSt]
T:Absolute temperature [k]
k・m・n: Constant determined by oil
This calculation formula can also be used to estimate kinematic viscosity at any temperature.

If you look at the straight line between VI100 and VI200, two sample oils with the same viscosity at 100℃, you will see that the viscosity changes significantly with temperature (particularly for VI100).
In particular, it can be seen that the kinematic viscosity increases rapidly in the low temperature region.
However, in the case of hydrocarbon oil, the measured viscosity at low temperatures moves above of this straight line. This means that the viscosity of hydrocarbon oil at low temperatures is even greater than the value estimated from the diagram.

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