Coefficient Of Friction Parameters Research Articles

Energetic coefficient of friction applied to cylinder liners lab tests

PurposeThis paper aims to present a proposal for evaluating the coefficient of friction (COF), under a reciprocating test that considers the energy dissipated by the friction force. In addition, this new parameter is compared to average COF, which is often used to evaluate COF in reciprocating tests.Design/methodology/approachSamples of compacted graphite iron were extracted directly from an internal combustion engine block. The piston ring used was a nitrided martensitic stainless steel with an asymmetrical profile, and the lubricant oil was the SAE 30 CF, controlled at 40°C. Different testing conditions were carried out in a CETR-UMT-Bruker tribometer, varying loads between 25-125 N, frequencies between 1-12.5 Hz and track length between 3-10 mm. Three maps comparing the average COF and the energetic definition were built, allowing to discuss their similarities.FindingsIn general, both parameters had similarities especially for low frequencies and small tracks. However, for test conditions that imposed higher accelerations (i.e. longer track lengths and higher frequencies), the energetic COF (COFe) was lower than the average COF (COFa) and presented better agreement in Stribeck-like curves – independent on the experienced lubrication regime along the stroke. As the COFe can be interpreted as a weighted average of instantaneous COF in relation to in-track displacements, an immediate consequence is that instantaneous COF closest to mid-stroke is considered more significant. Furthermore, perturbations associated with the intrinsic accelerations of the movement test are minimized in the COFe formulation.Originality/valueThe energetic COF parameter (COFe) is presented and compared to the average COF. The new parameter presented less data dispersion and is attractive to evaluate the COF behavior in reciprocating tests, as its formulation minimizes perturbations associated with the intrinsic accelerations of the movement (mainly in the initial and final part of the track where the acceleration has its greatest magnitude).Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2019-0324

Energetically consistent model of slipping and sticking frictional impacts in multibody systems

A unifying slipping and sticking frictional impact model for multibody systems in contact with a frictional surface is presented. It is shown that the model can lead to energetic consistency in both slip and stick states upon imposing specific constraints on the coefficient of friction (CoF) and the coefficient of restitution (CoR). A discriminator in the form of a quadratic function of the pre-impact velocity is introduced based on an isotropic Coulomb constraint such that its sign determines whether the impact occurs in the sticking mode or in the slipping mode just prior to the contact. Solving for the zero-crossings of such a function in terms of the CoF and the CoR variables leads to another discriminator called Critical CoF, which is the lowest static CoF required to prevent the subsequent impulse vector violating the isotropic friction cone constraint. Investigating conditions for the energetically consistent impact model reveals that the maximum values of either CoR or CoF should be limited depending on the stick or slip state. Furthermore, it is shown that these upper-bound limits in conjunction with the introduced Critical CoF variable can be used to specify the admissible set of CoR and CoF parameters, which can be represented by two distinct regions in the plane of CoF versus CoR. Finally, a case study using the Kane’s example for impact in an MBS involving frictional impacts occurring in both slip and stick states are presented to support the analytical results.