Continued from page 13 Figure 2: Viscosity of grease samples at higher shear. When applying shear at 0.01s-1 the lithium grease,
calcium sulphonate grease, and the copper grease have similar viscosities whilst the moly grease is more viscous. Once the shear is increased to 100s-1
, it’s the
calcium sulphonate grease that demonstrates a higher viscosity whilst the moly grease behaves more like the commonly used lithium grease.
As with all liquids and semi-solid materials, viscosity alone is not enough to characterise and compare the four greases. In fact, grease displays a commonly observed set of rheological attributes that collectively describe structured liquids. Structured liquids can be thought of as “solids that can be made to flow”; they exhibit liquid-like flow behaviour under sufficient applied stress but revert to soft-solid, elastic behaviour when under gentle stresses.
Figure 3: Complex modulus curves for each grease.
Oscillatory rheology measurements In contrast to high shear viscosity testing, small amplitude oscillatory shear measurements enable the probing of a material’s microstructure through its responses to the application of dynamic sinusoidal stresses. The applied stresses gently “wobble” the sample and the resulting strain, or deformation is monitored. From the ratio of stress to strain, a measurement known as complex modulus is obtained. Complex modulus can be defined as the resistance to deformation. By altering the conditions of the test, such as the applied stress or the measurement temperature, the change in a sample’s behaviour can be observed.
Understanding a grease’s behaviour at start up Performing an oscillation stress sweep, the laboratory observed variations in the yield stresses of the grease samples. Yield stress is defined as the stress required to initiate significant flow. The calcium sulphonate sample has the highest yield stress and the moly sample has the lowest, more similar to the lithium grease sample.
Figure 4: Yield stress values for grease samples. Lithium Grease on a Rheometer for testing.
Yield stress values may impact the start-up energy. An engine using calcium sulphonate grease will need to apply more stress to its grease to make it flow, compared to an engine using moly grease. Automotive manufacturers must assess the performance of each
14 LUBE MAGAZINE NO.180 APRIL 2024
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