Effect of rheological behaviour of lithium greases on the friction process

Abstract

PurposeThe aim of this work is to investigate the relationship among rheology, tribology and traditional standardized technological parameters of lithium lubricating greases.Design/methodology/approachLubricating greases having the same composition but differing in processing protocols have been manufactured and characterized in order to isolate the rheological behaviour from the formulation.FindingsSome successful empirical correlations between rheological (viscous and viscoelastic) and technological standardized parameters, with the friction factor obtained from a ball‐disc tribometer, have been established in order to elucidate the role of the rheological behaviour of lubricating greases on the friction process. In addition to this, an energetic evaluation of the structural degradation of greases during the friction process has been carried out by performing stress‐growth experiments. Thus, the storage energy density, which is related to the grease capacity to accumulate energy in the elastic deformation, and the limiting energy density, which represents the dissipation of energy in the flow process, have been satisfactorily correlated with the friction factor.Research limitations/implicationsThe complex rheological behaviour of lubricating greases, the extreme deformations and the high‐shear stresses resulting in a tribological contact imply that it is difficult to develop a model to describe their behaviour in an elastohydrodynamic lubricating contact.Originality/valueThis paper provides a resource of practical data to be applied in tribological systems.

Influence of soap/polymer concentration ratio on the rheological properties of lithium lubricating greases modified with virgin LDPE

“…The rheological modification induced by polymeric additives affects the viscoelasticity of lubricating greases, which is important for preventing loss of lubricant or reinforcing sealing properties, but also the flow properties under working conditions. Thus, some correlations between rheological parameters and tribological data have been previously reported [11,12]. As suggested by Guo et al [13], polymers may reinforce the role of lubricants by modifying the film thickness in a lubricated contact.…”

Tribological Properties of Greases Based on Biogenic Base Oils and Traditional Thickeners in Sapphire-Steel Contact

“…To some extent, explanations for these effects can be found in the microstructure of the thickening agents. Metallic soaps usually form threedimensional networks [13] of polymorphic soap fibers that confine the oil with secondary valence bonds. It is well known that lithium-greases generate longer and more densely arranged and entangled fibers than calcium-greases [14][15][16][17].…”

Tribological and Rheological Characterization of New Completely Biogenic Lubricating Greases: A Comparative Experimental Investigation

“…The rheological energy density can be calculated by the integration of the second part of the τ vs. t curve at a constant shear rate ( . γ) as follows [17]: where t max is the time necessary to reach the maximum shear stress (τ max ) and t is the shearing time. Moreover, the following empirical model was introduced for the shear stress as a function of time [18]:…”