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There are two distinct sources of variation when making viscosity measurements of lubricating oils at low temperature and low shear stresses -- the variations attributable to the instrument and the variations from the materials being measured. Oil variation arises primarily from the ‘crystal forming’ characteristics of the fluids tested at low shear stress. In this study, the viscosities of several fluids were measured at low temperature and low shear stress using the Mini-Rotary Viscometer (MRV), which contains nine separate viscometer cells that are cooled and controlled in unison. The testing procedure was to measure the viscosities of nine samples of each oil in a single test. Data were analyzed to compare the variances and standard deviations of the normalized viscosities. The results showed that the precision of the test on PAO-type oils was better than that for fully-formulated commercial oils.

Current specifications for automatic-transmission fluids and gear oils have viscosity limits which are determined by ASTM D 2983. However, that test is plagued by poor precision. This paper describes the development of a method using the Mini-Rotary Viscometer to make the determination of apparent viscosity at the same nominal shear stress as ASTM D 2983. In this test procedure, samples are cooled in a manner similar to that described in ASTM D 2983. Experimental data were obtained on a mixture of 17 automatic-transmission and gear-oil fluids that included a number of different formulation strategies and commercial products. The results of this method yield a nearly one to one correlation with the results determined by ASTM D 2983.

A 2.3 Liter, overhead cam engine was motored in a sub-zero temperature room to measure the flow characteristics of several SAE 5W-30 commercial lubricants. The lubricant pressure and rate of pressure rise were measured at 8 different points in the engine. The engine was cooled either by an extended overnight cooling program or a constant cooling rate program. These cooling programs were similar to those used in ASTM D4684 and D5133 respectively. During each test the power to motor the engine, as well as the lubricant pressurization time was monitored. A video record was made of each of the tests for later review. In these tests, a correlation was found between lubricant pressurization time and pumping viscosity as measured by the bench test methods, ASTM D4684 and D5133. Significant differences were found between the engine's response to an oil and its ranking by the pumping viscosity bench tests.

The viscosity of engine lubricants was measured at multiple shear rates by a unique Scanning Brookfield apparatus. These oils included both Pumpability Reference Oils (PRO) and a selection of today's commercial multigrade oils. It was found that the viscosity of “flow-limited” oils remained constant when the shear rate decreased. Oils with “air-binding” properties exhibited an increase in viscosity with decreasing shear rate. The magnitude of a change in “slope of the temperature/viscosity profile was found to suggest the degree of air-binding character of an oil. The flow characteristics of PRO 1, 3, 9, 11, and 16 were measured at several shear stresses and temperatures in the Mini-Rotary Viscometer. The Scanning Brookfield technique and the Mini-Rotary Viscometer were found to yield similar results when the shear stress of the Mini-Rotary Viscometer was reduced from 525 to 35 Pascals.