grease during start-up when seeking an alternative.
Exploring the melt and solidification of grease Greases used in a vehicle’s engine will undoubtedly experience a range of temperatures.
During oscillation temperature ramps conducted on the four greases spanning from -5°C to 150°C, all samples exhibit evident softening upon heating. Subsequently, each grease sample undergoes hardening upon cooling; however, this process does not directly mirror the heating phase, a clear hysteresis occurs.
Automotive manufacturers are required to carefully assess the performance of lubricating greases under varying temperature conditions, encompassing both heating and cooling phases. This scrutiny is essential in identifying alternative lubricants that exhibit consistent and desired behaviour across all operational conditions within the engine.
By fitting an exponential model to the recovery curves, it is possible to determine an exponential time constant, which quantifies the rate at which the greases recover towards their viscosity or modulus prior to shearing. A larger value for the time constant signifies a slower recovery process.
Figure 7: Time constant values for recovery time.
With a lower value for time constant, lithium grease is the quickest to reach a plateau, while conversely, calcium sulphonate is the slowest. In assessing the recovery attributes of an alternative grease, moly grease demonstrates a recovery pattern most closely resembling that of lithium grease.
Figure 5: Complex modulus curves across temperature ramps.
Three-step-thixotropy – complex modulus Thixotropy quantifies the recovery of viscosity or structure of a material after it has been exposed to high stress conditions. Each grease sample in our study shows a delayed recovery following the cessation of the high shear breakdown step.
Conclusion With lithium sources expected to deplete over the coming years, the commonly used lithium grease may need to be substituted with greases comprised of different thickeners. Through rheological analysis of four greases, Lithium, Calcium Sulphonate, Copper, and Molybdenum Disulphide, the Centre for Industrial Rheology has produced useful rheological insights that highlight the variation in behaviour of the different greases.
Whilst there is not an immediately apparent alternative to lithium grease, automotive engine designers and manufacturers can use rheological analysis to better understand how alternative greases may behave in their engines under different conditions and can choose an alternative depending on their exact requirements.
www.rheologylab.com Figure 6: Thixotropic recovery curves for grease samples.
LUBE MAGAZINE NO.180 APRIL 2024
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