Selecting the Right
Base Oil Viscosity for a Grease
It is critical to consider the base oil viscosity when manufacturing or selecting a grease for a specific application. For example: if a lubricant’s viscosity is too high for a given application and its operating conditions – particularly the temperature – the lubricant might not flow properly to where it is needed and hence its main function would be insufficient. On the other hand, if the lubricant’s viscosity is too low, the lubricant might not adequately protect moving parts, which would lead to increased friction and eventually equipment damage.
The kinematic viscosity and the viscosity index (VI) are therefore two of the most critical parameters which describe the usability of the base oil.
Figure 1. Lubrication
In other words, high-VI base oils exhibit a smaller change in viscosity over a wide temperature range compared with low-VI base oils. Ideally, a thicker lubricating film should be provided at high temperatures so that friction protection and wear suppression are ensured. At lower temperatures, correct lubrication needs to be ensured by sufficient fluidity of the lubricant so there is less resistance to movement of mechanical parts. A wide range of mineral and synthetic oils can be used, but VI determination plays a crucial role in selection of the right base oil.
Innovation in viscosity index determination: SVM™ 4001 The highly precise viscometers of the SVM™ series are based on the Couette measuring principle and have an integrated density measuring cell. The small viscosity measuring cell contains a tube which rotates at a constant speed and is filled with sample fluid. A measuring rotor with a built-in magnet floats freely in the sample. The sample’s shear forces drive the rotor while magnetic effects retard its rotation. Shortly after the measurement starts, the rotor reaches equilibrium speed. This speed is a measure of the fluid‘s viscosity. The kinematic viscosity is automatically calculated from the dynamic viscosity and density of the sample.
What is the meaning of the viscosity index (VI)?
Low VI: The viscosity changes considerably at different temperatures. The oil is highly viscous at low temperatures and relatively liquid at high temperatures.
High VI: The viscosity changes only slightly at different temperatures over a wide temperature range.
Dr. Jelena Fischer, Anton Paar GmbH
Anton Paar was established in 1922 and today develops, produces and distributes highly accurate laboratory instruments and process measuring systems, and provides custom-tailored automation and robotic solutions. SVM™ viscometers of Anton Paar have revolutionized viscosity measurements. With an SVM™, a single measuring cycle on a small sample volume yields kinematic viscosity, density, dynamic viscosity, Viscosity Index and more.
The new SVM™ 4001 contains two connected Stabinger Viscometer™ cells. The innovative double-cell design of SVM™ 4001 allows for simultaneous measurements at 40°C and 100°C from a single syringe. As each of the measuring cells also contains an integrated density oscillator, density measurements according to ASTM D4052 at each temperature are simultaneously performed. The minimum sample volume needed for viscosity index determination and the density measurements is only 2.5 mL. The viscosity index results according to D2270 as well as API calculations are automatically shown on the 10.4” touchscreen.
Moreover, SVM™ 4001 allows for simultaneous determination of viscosity and density at any two temperatures between 15°C and 100°C. The SVM™ provides viscosity-temperature extrapolation according to ASTM D341 and freely selectable API grades.
A viscosity range from 0.2mPa.s to 30 000 mPa.s, density range from 0.6g/cm³ to 3.0 g/cm³ and temperature range from 15°C to 100°C are covered with one integrated measuring cell system that works out-of-the-box.
Figure 2. Stabinger Viscometer™ measuring principle
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LUBE MAGAZINE NO.138 APRIL 2017
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