Technical Information

3-2. Oil viscosity

Viscosity is important for the function of oil.
Viscosity is the most important property that governs the function of lubricants, and is a numerical representation of the "stickiness" or "thickness" of the fluid, which is how easily it flows. Adjust the viscosity to suit your purpose.

Viscosity Standards

Oils used for industrial purposes are generally divided into "mineral oil (petroleum hydrocarbon = natural component)" and "synthetic oil (artificially processed oil that does not exist in nature)".

ISO Industrial Lubricant Viscosity Classification (JIS K 2001)
= Viscosity grade: 18 grades specified between ISO VG2 and ISO VG1500
The numerical value of the viscosity grade is the center value of the kinematic viscosity at 40°C. However, for low viscosity grades (ISO VG2, 3, 5, 7), the center values of the kinematic viscosity at 40°C are 2.2, 3.2, 4.6, and 6.8, respectively, rounded up to integers. The allowable kinematic viscosity range for each viscosity grade is ±10% of the center value.

Absolute viscosity
Absolute viscosity is the resistive force acting on a fluid.
Kinematic viscosity (commonly used viscosity)
The absolute viscosity is divided by the density of the fluid. The unit is usually centitokes (cts), but in SI units it is mm ² /s.

The role of viscosity

As shown in Figure (a), when the lower surface moves at a velocity U, the fluid is pulled into the gap due to its viscosity, causing the fluid molecules to push against each other, generating pressure.
This is called pressure generation due to the "wedge film effect." Also, even in the case of a gap where both surfaces are parallel, as in (b), if the gap decreases at a velocity V, the fluid's viscosity will resist being pushed out of the gap, and pressure will also be generated. This is called pressure generation due to the "squeeze film effect" or "squeeze effect."
The lubrication method that supports the load with this generated pressure is called fluid lubrication.
In both cases, the pressure generated by both effects increases with the viscosity of the fluid.

Viscosity change with temperature

The viscosity of lubricating oil (hydrocarbon oil) changes significantly with temperature, so when viscosity is expressed, the temperature at which the viscosity is expressed must also be expressed.The kinematic viscosity vs. temperature chart below is required to consider the kinematic viscosity vs. temperature of lubricating oil.
The vertical axis represents kinematic viscosity and the horizontal axis represents temperature. By drawing a straight line with the vertical axis representing the logarithm of kinematic viscosity and the horizontal axis representing the logarithm of temperature, the kinematic viscosity of that oil at other temperatures can be estimated.
Viscosity Index VI is a value that indicates the degree of change in kinematic viscosity of a lubricating oil due to temperature.
In the graph below, the larger the slope of the line, the smaller the change in kinematic viscosity due to temperature.
The VI is based on empirical data, with Pennsylvania crude oil base oil, which has excellent viscosity-temperature characteristics, assigned a VI of 100, and Gulf Coast crude oil base oil, which has poor viscosity-temperature characteristics, assigned a VI of 0. An index was established to express, as a fixed number, where the measured oil falls between these two types.

The following empirical formula by Walther is widely used to calculate the relationship between viscosity and temperature of hydrocarbon oils.

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

Looking at the straight lines for two sample oils, VI100 and 200, which have the same viscosity at 100°C, we can see that the viscosity changes greatly with temperature (especially for VI100).
In particular, it can be seen that the kinematic viscosity increases sharply in the low temperature region.
However, the viscosity measured at low temperatures for hydrocarbon oils is shifted above this line, meaning that the viscosity of hydrocarbon oils at low temperatures is even greater than the value estimated from the graph.