J. Echávarri Otero

Analytical model for predicting the friction coefficient in point contacts with thermal elastohydrodynamic lubrication

This article presents analytical friction prediction models applicable to lubricants in point contacts under thermal elastohydrodynamic lubrication (TEHL). The types of models used consider the heat generated and its penetration into the bulk of the contacting solids. Therefore, the increase in temperature is determined, which causes important variations in the operating conditions. The article sets out the hypotheses assumed, the theoretical calculation procedures and the ensuing equations for determining the friction coefficient under TEHL. An experimental stage is performed on a mini-traction-machine, which allows the measurement of friction coefficient in ball–disc contacts under a wide range of control parameters involved in TEHL, such as lubricant bath temperature, load, average velocity, slide-to-roll ratio, and contacting materials. The experimental results for different lubricants are compared to those given by the models, and show the proposed models to be accurate for predicting the friction coefficient.

Models for Predicting Friction Coefficient and Parameters with Influence in Elastohydrodynamic Lubrication

This article shows different friction prediction models applicable to lubricants in point contacts under an elastohydrodynamic regime. The types of models used are two variations of the Newtonian theory, the Limiting Shear Stress model and the one based on Carreaus equation. The article sets out the theoretical calculation procedures and the ensuing equations for calculating the friction coefficient. The aims of the article are to study the effect of the parameters with influence on friction and to compare the models results with those given by an experimental stage performed on a mini traction machine. This test system allows the measurement of friction coefficient in point contacts (ball–disc) under a wide range of variation of parameters such as temperature, slide-roll ratio, lubricant, material, load, or velocity.

Theoretical and experimental evaluation of the flow behavior of a magnetorheological damper using an extremely bimodal magnetic fluid

The flow behavior of a magnetorheological (MR) fluid, consisting of iron particles dispersed in a ferrofluid carrier (MRFF) in a commercial monotube MR shock absorber is studied. The magnetorheological properties of the MRFF suspensions are compared with those of a conventional oil-based MR fluid (MRF). The mechanical behavior of the MR damper, filled with the MRFF or alternatively with the MRF, is characterized by means of different oscillatory force–displacement and force–velocity tests. The MR shock absorber has an internal electromagnet that generates a controlled magnetic field in the channels through which the MR suspensions flow under operation conditions. The results obtained indicate that the damper filled with MRFF shows a reliable and reproducible behavior. In particular, the response of the shock absorber can be controlled to a large extent by adjusting the electromagnetic current, showing a response that is independent of the mechanical and magnetic history of the MRFF. The non-linear hysteresis model proposed for predicting the damping force provides good agreement with the experimental results when the MRFF is employed. The improved response of the damper loaded with ferrofluid-based MRFF (instead of the conventional MRF) is explained considering the physical properties and the internal structure of the suspension.

Optimising lubricated friction coefficient by surface texturing

Surface texturing has proved to be a very useful tool for expanding the behaviour under hydrodynamic and elastohydrodynamic regimes instead of mixed or boundary lubrication regimes, and therefore for reducing the friction coefficient under high-load low-speed conditions. This article presents the texturing of different copper test-samples using photolithography and chemical etching to measure the friction coefficient using a point contact machine. The effects of texture size, texturing density, the initial roughness of the samples and the operating conditions have all been studied. Some combinations of texturing density and texture size achieve up to 30% reduction in the friction coefficient. Taking into account experimental data, artificial neural networks are used as a tool for both predicting and optimising the friction coefficient on the textured surface for any given operating condition.

Elastohydrodynamic Models for Predicting Friction in Point Contacts Lubricated with Polyalphaolefins

This paper shows the results of a comparison between theoretical models for predicting friction in point contacts under elastohydrodynamic lubrication and the experimental results obtained in a Mini Traction Machine (MTM). The types of models used are two variations of the Newtonian Theory, applicable to polyalphaolefins (PAO): the Limiting Shear Stress Model and the Carreau’s model. The experimental stage includes a wide variation of the operating conditions, by using different control parameters, such as temperature, contact materials, slide-roll ratio, sliding velocity and load.