Lubricated Friction and the Hersey Number

Abstract

Most frictional contacts are lubricated in some way, but is has proven difficult to measure and predict lubrication layer thicknesses and assess how they influence friction at the same time. Here we study the problem of rigid-isoviscous lubrication between a plate and a sphere, both experimentally and theoretically. The liquid layer thickness is measured by a novel method using inductive sensing, while the friction is measured simultaneously. The measured values of the layer thickness and friction on the disk are well described by the hydrodynamic description of liquid flowing through a contact area. This allows us to propose a modified version of the Hersey number that compares viscous to normal forces and allows us to rescale data for different geometries and systems. The modification overcomes the shortcomings of the commonly used Hersey number, adds the effects of the geometry of the configuration on the friction, and successfully predicts the lubrication layer thickness.

High velocity impact on a thin (non-Newtonian) fluid layer

“…2004; Bruus 2008; Veltkamp et al. 2021; Veltkamp, Velikov & Bonn 2022). In this case, a hydrodynamically steady state is present and the friction is a result of the shearing of the fluid layer.…”

“…In previous work (Veltkamp et al. 2021, 2022) we have analysed this type of flow in detail. It is concluded that, for the relatively gentle flows probed in these studies, the viscosity of the liquids makes the dominant contribution to the generated lift.…”

“…An important distinction should be made between two different manners in which a contact can be lubricated. On the one hand, a lubricating layer can establish itself between two surfaces due the lift that is generated hydrodynamically as a consequence of parallel motion (Hamrock et al 2004;Bruus 2008;Veltkamp et al 2021;Veltkamp, Velikov & Bonn 2022). In this case, a hydrodynamically steady state is present and the friction is a result of the shearing of the fluid layer.…”

“…In this case, a hydrodynamically steady state is present and the friction is a result of the shearing of the fluid layer. In previous work (Veltkamp et al 2021(Veltkamp et al , 2022 we have analysed this type of flow in detail. It is concluded that, for the relatively gentle flows probed in these studies, the viscosity of the liquids makes the dominant contribution to the generated lift.…”

Experimental characterization and modeling of the inter-ply sliding behavior of unidirectional prepreg in the preforming process

“…The Stribeck curve is widely used to investigate the inter-ply slip behavior of prepreg [14,15]. In this curve, a new variable the Hersey number is defined by viscosity, velocity and pressure [16,17]. In the hydrodynamic lubrication region of the Stribeck curve, the Hersey number is linearly related to the friction coefficient.…”