In this column, we will discuss the viscosity and viscosity index.
Viscosity and the viscosity index are terms that are applied not only to quenchants, but also to many other areas in machining, lubrication, and tribology. Viscosity is a measure of a fluid’s resistance to flow at a given temperature. If the temperature is measured in English units, then the units for viscosity are Saybolt Universal Seconds (SUS). If the viscosity is measured using the metric system, then viscosity is reported in centistokes, or cSt. Typically, 100°F and 210°F are used for kinematic viscosity measurements in English units, and 40°C and 100°C are used for viscosity measurements using the metric system.
There are two types of viscosity — dynamic viscosity and kinematic viscosity. Dynamic viscosity, or absolute viscosity, is a measure of the internal resistance of the fluid to motion. The (metric) units are g/(cm-s) or centipoise. Kinematic viscosity is the ratio of dynamic viscosity to density:
where μ is the dynamic viscosity, ρ is the density of the fluid, and ν is the kinematic viscosity.
Viscosity is measured by ASTM D 445 [1]. In the viscosity test, the fluid is placed in a specially designed glassware tube (Figure 1) and allowed to equilibrate at the desired temperature. After the sample is at temperature, it is pulled by suction to the measurement section. The time that it takes to drain through a specific volume is the viscosity. This time is dependent on the tube used (capillary size) and the temperature. Temperature during measurement must be controlled very accurately, generally to less than ± 0.1°C. Even minor differences in temperature, such as the difference between 100°F (37.8°C) and 40°C (104°F) results in typically a 10 percent difference in viscosity.
In oil quenchants, changes in the viscosity can indicate oxidation, or the presence of contamination with a lighter or heavier fluid. Confirmation tests with flashpoint or other tests can confirm oxidation or contamination. Viscosity is used to determine concentration of polymer quenchants. Viscosity is also important in sizing pumps or heat exchangers. For instance, the viscosity of the fluid is important in sizing pumps, to know how hard the pump will have to work to move the fluid. It is much easier to move a thin fluid (low viscosity) than a thick fluid (high viscosity).
Viscosity Index
The viscosity of a fluid changes with temperature. As temperature increases, the viscosity decreases, and the fluid becomes thinner. Viscosity determines film thickness and film strength in machinery and infers the lubrication capability. It also determines how well a quenchant will wet a part during quenching.
In the case of lubrication, too high a viscosity could result in inadequate oil flow. In the case of a car engine, this could mean oil starvation and dry start-ups. Close tolerance engines (aircraft and high-performance cars) require that the oil is circulating before the engine is started to prevent excessive wear. If the viscosity is too low, then an inadequate film thickness would be present, resulting in greater mechanical wear and friction [2].
The change in viscosity as a function of temperature is determined by the viscosity index (VI). The methodology is described by ASTM D2770 [3]. In this method, the viscosity of a fluid is measured at 40°C and 100°C. This is compared to two reference oils — Pennsylvania crude (paraffinic) was established as the reference for low changes with temperature. It was assigned a viscosity index of 100. Texas Gulf crudes, which are naphthenic and show a large influence of temperature on viscosity, was assigned a viscosity index of 0.
The viscosity index is calculated from the equation [4]:
where L is the viscosity at 40°C of an oil with VI = 0; H is the viscosity at 40°C with a VI = 100; and U is the viscosity of the tested oil at 40°C. The values of L and H for a specific viscosity of oil is available in ASTM D2270 [3]. These values of L and H are only valid for an oil with a viscosity of less than 70 cSt at 40°C. For oils with a viscosity greater than 70 cSt, L and H are calculated from [3]:
where Y is the viscosity of the fluid at 100°C.
For example, using the viscosity of a typical quench oil (Figure 1) and the tables of ASTM D2270, the viscosity index can be calculated.
Once the viscosity index (VI) is obtained, the viscosity of the fluid can be determined over a wide range of temperatures. This is accomplished using ASTM D341 [5]. Using special charts, the viscosity of the fluid is plotted at 40°C and 100°C. A straight-line is drawn between the two points. The viscosity at the desired temperature can then be directly read. While this is very simple, the charts are somewhat difficult to read. However, there are many on-line calculators available that can do the job much quicker and more accurately [6]. The results showing the calculated viscosities at different temperatures for the oil in Table 1 are shown in Figure 2.
This is very useful. For instance, if I have a thick martempering oil, and it is delivered in the middle of winter, I want to make sure my pump can pump the product. By calculating the viscosity at the desired temperature it will enable me to determine if my pump can handle the increased viscosity.
Using the example of quench oil again, viscosity is used for determination of the heat transfer coefficients used for sizing a heat exchanger. Using these charts, the viscosity can be readily determined, and the proper sized heat exchanger can be selected.
Conclusions
In this short column, we briefly described viscosity and its measurement. We illustrated the viscosity index (VI) and how it is calculated. Lastly, we showed how the viscosity index and charts can be used to calculate the viscosity of a product as a function of temperature. Several application examples were provided.
Should you have any questions regarding this column, or suggestions for future columns, please contact the editor or myself.
References
- ASTM, “Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity),” ASTM, 2018.
- J. Fitch, “Don’t Ignore Viscosity Index When Selecting a Lubricant,” Machinery Lubrication, no. June, 2012.
- ASTM International, “Standard Practice for Calculating viscosity index from Kinematic Viscosity at 40 °C and 100 °C,” ASTM International, Conshohocken, PA, 2016.
- J. C. Cragg and E. A. Evans, “Viscosity Measurement and Viscosity Index,” J. Inst. Petroleum, vol. 29, no. April, pp. 99-109, 1943.
- ASTM International, “ASTM D341-03 Standard Test Method for Viscosity-Temperature Charts for Liquid Petroleum Products,” ASTM International, Conshohocken, PA, 2003.
