TRIBOLOGY
Ultra Shear Viscometer (USV)
Accurate measurement of dynamic viscosity at very high shear rates is of particular concern in the Automotive Industry. Lubricants developed for motor vehicle applications are typically non-Newtonian making it necessary to reproduce the shear rate found in engines when measuring the viscosity.
The USV is a computer controlled instrument capable of fully automatic viscosity measurements up to 107 s-1 reciprocal seconds. The instrument can perform HTHS measurements according to ASTM standards D4683 and D4741 at 1,000,000 reciprocal seconds and 150°C and also D6616 at 1,000,000 reciprocal seconds and 100°C.
The main application for the USV is in the research and development of engine oils and additive packages. Its ability to automatically measure viscosity at different temperatures and shear rates means that researchers can fully characterise a candidate lubricant with little or no user intervention required.
Limitations of Existing HTHS
At present most instruments for measuring HTHS viscosity employ the same technique. The shear stress is generated between a tapered steel plug and copper stator. To minimize the effects of thermal expansion, the test results have to be compared with those for a calibration oil at the same temperature.
Time-consuming calibration procedures have to be carried out each time the instrument is used which slows down the test turnarounds and increases uncertainty in the results.
Secondly, extending the measuring range on existing instrument is not feasible due to the temperature rise caused by shear heating of the lubricant. The rise in temperature causes distortion in film shape and drop in test oil viscosity. Consequently, the maximum shear rate is limited at approximately 3 x 106 s.
The Solution: Ultra-Shear Viscometer
The Ultra Shear Viscometer (USV) can carry fully automated viscosity measurements over a shear rate range from 106 s to 107 s. All test parameters are controlled by built-in microprocessor which leads to rapid turn-around i.e. a complete temperature / shear rate map for a lubricant can be completed under 1 hour.
The USV is fitted with DC servo motor capable of speeds up 20,000 rpm and an electromagnetic clutch. The clutch engages the rotor for only a very short period of time (typically 30 ms). This brief shearing minimizes the shear heating.
Secondly, the rotor and stator are made from same material which is heated uniformly to the test temperature. The uniform expansion means the the gap change with temperature is negligible. Since the shear heating is minimal, the thermal distortion of gap is almost insignificant. As a result, the radial gap is constant and there is no need for calibration procedure and each change of lubricant. This constant gap allows the USV to run multi-temperature tests over a wide range of shear rates.
In addition, the temperature is constantly monitored by two independent platinum RTF probes. The main probe slides into the stator and is in close proximity to the test sample. The second probe cross checks temperature at the bottom of the test chamber for safety purposes.
What are the user benefits?
Better repeatability and reproducibility
Each results is the average of five measurements to ensure consistency. As a result, the USV achieves a repeatability of 2% of the viscosity value. For high shear rates, the repeatability is as good as 0.03 mPas and reproducibility is 0.05 mPas. 2.Reduced testing time
It takes 2 minutes to measure the viscosity at one shear rate. Heating and temperature stabilization of test chamber require - on average - 10 minutes. Therefore, testing a sample at 10 shear rates for 3 different temperature would take 3 x (10 x 2 min + 10 min) = 90 minsReduced calibration time
Calibration is required once at the start of the test and is then valid for all subsequent tests. Calibration is simple and fully automatic with an autosamplerVery small sample volume
