Experimental Studies of Lubricant Flow and Friction in Partially Filled Gaps

Abstract

Starved lubrication is an important strategy for minimizing the amount of lubricant needed, and also inevitably occurs during idling and fail-safe lubrication. In this regime, however, the flow of the lubricant and the related friction coefficients are yet to be fully understood. This research aims to make fundamental contributions to the understanding of contact mechanics of partially lubricated contacts. Recent experiments with a pin-on-disk tribometer examined the microscopic behavior of partially filled gaps. Using a new experimental setup on a macroscale, new insights into partially filled gaps with rough surfaces were gained. This work presents the systematic analyses of the lubricant flow, friction coefficients, and other variables over a wide range of friction parameters. Distinct friction behaviors were observed, and similar effects occur on both the micro and macroscale. The experimental results show that a typical Stribeck characteristic is visible regarding not only the relative velocity, but also regarding the lubricant filling level in the gap. The fluid exhibits a variety of flow patterns for various velocities and viscosities. The patterns relate to different friction regimes, such as dry friction and mixed lubrication. It is concluded that the filling level is a valid parameter for regulating the transition from dry friction to hydrodynamic lubrication. These findings are quantified regarding the filling level and it is shown that for the identification of the friction regimes the filling level is an independent parameter in addition to the established parameters like speed, viscosity and pressure.

On the Lubricant Flow and the Friction Coefficient in Partially Filled Gaps

“…Depending on the rotational speed, the fluid viscosity and the surface roughness (and correlating gap height) different parts of the Stribeck curve were visible. [6] In conclusion, the research gives new insights into the friction behaviour of partially filled gaps. All experiments were repeated 5 times and showed very good repeatability.…”

“…For the macroscopic experiments the WDI was used [6]. The test bench has a setup in which two glass disks form the contact zone, thus allowing the observation of the fluid flow in the contact zone through the transparent glass.…”

“…Details on the setup for this measurement can be found in [6]. [6] 2 of 2 Section 5: Nonlinear oscillations Second, for all measurement configurations the fluid volume in the gap was varied from a dry gap to a fully filled gap. First, the experiments were evaluated regarding the COF.…”

“…First, the experiments were evaluated regarding the COF. [6] Third, the fluid viscosity was varied by using Glycerol (roughly η = 1P a · s), M1 (viscosity 1/10 of Glycerol) and M2 (viscosity 1/100). Depending on the rotational speed, the fluid viscosity and the surface roughness (and correlating gap height) different parts of the Stribeck curve were visible.…”

“…Therefore, additional experiments have been carried out on a macroscopic scale with the new test bench Wear Debris Investigator (WDI) [5]. A range of validity considerations were undertaken to ensure that those experimental results are related to a technical hydrodynamic friction behaviour and resulted that a cross-scale analysis of the lubricants flow is promising [6].…”