An approach of the internal friction-dependent temperature changes for conventional and pure biogenic lubricating greases

This work investigated the temperature changes inside the bulk of lubricating greases under controlled high-shear stress conditions (250–500 s^-1). For this purpose, a newly developed temperature-measuring cell called Calidus was successfully tested. The temperature changes (ΔT) have been related to the greases' components (thickener, base oil-type, and composition) and the structural degradation of the lubricating greases. Furthermore, a theoretical approach was proposed for calculating the internal temperature change of lubricating greases during shear stress. All greases showed an internal temperature profile characterised by a sudden rise in ΔT within the first 4 h from starting the test and subsequent ΔT decay until it reaches the steady state value. Furthermore, it was found that greases C1 and C5, formulated with lithium and calcium soap, respectively, with different soap content (16.1 wt% and 9.7 wt%, respectively), but the same base castor oil, showed the highest value of the maximum ΔT, c.a. 3.2 K, and the most drastic drop of ΔT. These greases showed both the highest specific densities and heat capacities. In addition, they showed the lowest ratio of expended energies (R_tee), which means more structural degradation in the stressed grease. On the contrary, the grease C3, with 13 wt% of Li-soap but the lowest base oil's viscosity, showed the lowest maximum ΔT and the temperature profile was characterised by a moderate variation of ΔT along the test. The biogenic grease B3 developed a low-temperature change in the group of pure bio-genic greases close to grease C3.