Friction In Mechanical Systems Research Articles

Diving deep into the superior lubrication efficiency: A dynamic approach incorporating covalent organic polymer as strengthening booster in paraffin oil through a holistic blend of experimental and computational approaches

This research addresses the paramount importance of enhancing surface protection and minimizing friction in mechanical systems through the novel design of covalent organic polymer additive (COPA). COPA-1 and COPA-2 were intricately designed to enhance surface protection and minimize wear. Tribological analysis revealed that the addition of an optimized concentration of COPA-1 (25 ppm) to base oil significantly reduced friction (∼60.49% decrease) and wear (∼79.23% decrease) while increasing load-bearing capacity to 588 N. Rheological analysis confirmed excellent shear stability and desirable viscosity behavior. Furthermore, computational tools like density functional theory and molecular dynamics simulations provided valuable insights into the adsorption mechanisms and tribological performance. This integration of experimental and computational insights highlights the transformative potential of COPA in revolutionizing lubrication technology.

Characterization of Sintered Bronze–MoS2 Composite With Solid Lubrication Effect

Abstract The most efficient method to reduce material loss and frictional energy losses is by using lubrication. An alternative is the use of solid lubrication, specifically by using solid lubricants evenly distributed in a metallic matrix, thus forming self-lubricating composites, which are capable to induce low coefficients of friction in mechanical systems. Molybdenum disulfide (MoS2) is a very versatile solid lubricant, suitable for lubrication in critical circumstances such as vacuum, high temperatures, and pressures. Therefore, the aim of this study is to produce samples of sintered composites consisting of homogeneously distributed MoS2 in a bronze matrix obtained by cold uniaxial pressing and to compare the wear-rates and friction coefficient between the MoS2-free bronze and the self-lubricating composites. Different MoS2 percentages were used to characterize the tribological properties of the composites as a function of the MoS2 content. At the end of the experiments, it was found that samples with 20% MoS2 did not sinter properly due to the large amount of lubricant between the bronze particles. It was also found that the mixture with 5.0 vol% MoS2 had proper sintering, satisfactory hardness, achieved lower friction coefficient, and better material wear performance due to the optimal amount and good distribution of MoS2 when compared with the rest of conditions studied.

Colliding solids interactions of earthquake-induced nonlinear structural pounding under stochastic excitation

A simplified analysis model of structural pounding, used in the study of the effects of earthquakes subjected to Gaussian white noise of relatively high intensity, filtered through a Kanai-Tajimi filter is proposed in this investigation. The required distance to avoid pounding is important in structural engineering. As a consequence of frictional contact phenomena, energy is dissipated and the state of a system can change slowly and rapidly, depending on the nature of the contact, continuous or impact condition. Buildings without enough separation often create structural pounding under seismic events. Other effects associated with friction in mechanical systems are the vibration and noise propagation of the system components, nonlinear systems behavior and wear. In this paper, we prove that because of the inherent stochastic nature of the applied earthquake loads or the uncertainty arising from randomness (non-smooth behavior), stochastic p-bifurcations occur at law noise intensities and disappear when increasing noise intensities. Because of the presence of both impact-friction events, p-bifurcations should be observed at weak noise intensities. P-bifurcation occurs and created instability that will increase pounding effects. Some relationship between impact-friction events appears: small noise intensity occurs when the time of friction (continuous) is greater than the impacting events, hence high probability density function (PDF). But if the noise intensity increases, the impact events are great (small friction) with weak PDF. But successive jump effects can create noisy system and great impact. The models stochastic processes of stationary probability density function (PDF) of the earthquake ground motion are set up. The demonstrative application examples which include friction in systems involving contact-impact events are illustrated in Central Africa, a part of Congo Stable Block.

On the Nonlinear Energy Interactions in Harmonically Excited Post-Buckled Flexible Inverted Pendulum in the Presence of Friction

Abstract Nonlinear energy interaction is a fascinating feature of nonlinear oscillators and has been drawing the attention of researchers since the last few decades. Omnipresent friction in mechanical systems can play a crucial role in modifying these interactions. Using post-buckled flexible inverted pendulum as a candidate system we characterize here, theoretically and experimentally, significant changes in the nonlinear energy transfer in the presence of friction at the input side. Particularly, even with relatively low friction, the energy gets transferred in the higher harmonics of excitation close to a resonant mode as against the transfer to higher modes reported previously. We term this new phenomenon as “excitation harmonic resonance locking.” Theoretical modeling and simulations, considering large deformations, based on assumed modes method, and using a simple friction model reasonably capture the experimental observation. In summary, the paper explicates the role of friction in shifting energy transfer frequencies and can be useful in understanding and designing of oscillators and nonlinear vibrating systems.

Modeling and analysis of friction including rolling effects in multibody dynamics: a review

Friction exists in most mechanical systems, and it can have a major influence on their dynamic performance and operating conditions. As a consequence of frictional contact phenomena, energy is dissipated and the state of a system can change slowly and rapidly, depending on the nature of the contact, continuous or impact condition. Other effects associated with friction in mechanical systems are the vibration and noise propagation of the system components, nonlinear systems’ behavior and wear. Overall, the knowledge of the friction regimen, as well as the frictional forces developed at the interface of mechanical parts in contact with relative motion, is crucial for the dynamic analysis of mechanical systems, and has consequences in the design process. Thus, this work is a review of the modeling and analysis of frictional effects in multibody systems with the purpose of better understanding and obtaining accurate responses. In this process, pure dry sliding friction, stick–slip effect, viscous friction, Stribeck effect, and frictional lag are some of the main phenomena associated with friction, which are addressed in depth. Overall, the friction models can be divided into two main groups, namely the “static friction models” and the “dynamic friction models”. The static models describe the steady-state behavior of the relation friction-force/relative-velocity, while the dynamic models allow for the capturing of more physical responses and properties by using extra state variables. In a simpler manner, the static and dynamic friction models differ mostly in the modeled frictional effects, implementation complexity, and computational efficiency. Hence, this research is aimed at analyzing in detail the role of friction modeling in the dynamic response of multibody system, as well as addressing the importance of friction models selection for accurately describing the friction related phenomena. Demonstrative application examples, which include friction in ideal mechanical joints and systems involving contact–impact events, including an example of rolling contact will be considered and investigated to illustrate the main assumptions and procedures adopted in this work. The results from this study indicate that in most cases, a static friction model, which accounts for static friction and avoids the discontinuity at zero velocity, is a suitable choice. A more advanced dynamic friction model has to be developed to be utilized for systems containing high variations of normal load, namely with impact conditions.

Pinning Loss Power Density in Superconductors

The pinning loss power density is theoretically derived based on the resistive energy dissipation when the flux lines are driven by the Lorentz force in a superconductor. The obtained loss power density does not depend on the viscosity or flow resistivity, but is proportional to the pinning force density only, and it possesses the nature of hysteresis loss, as commonly measured in experiments. These features are predicted by the critical state model, which was recently proved theoretically. The obtained pinning force density is consistent with the prediction of the coherent potential approximation theory, a kind of statistical summation theory, for flux pinning. Thus, the irreversible properties associated with the flux pinning can be comprehensively described by these flux pinning theories. The irreversible flux pinning in the superconductor is compared with similar irreversible phenomena such as the motion of magnetic domain walls in ferromagnetic materials and the friction in mechanical systems. The po...

Dynamical Modeling of Constraints with Friction in Mechanical Systems

Modeling of unilateral constraints in multi-body systems is already a challenging task itself, which becomes even more complex if friction has to be considered caused by an active constraint. In this paper, this task is solved by combining two recently developed methods: constraint modeling using power-based restriction functions within Lagrangian mechanics and dynamic modeling of friction effects for numerically robust simulations. The approach is demonstrated on a simulation experiment of a double inverted pendulum.

Theoretical analysis of the dynamic behavior of presliding rolling friction via skeleton technique

Depending on the application, friction in mechanical systems can be a desirable or an undesirable thing. In order to optimally utilize or compensate for the frictional effect in the system, characterization of the frictional behavior is a crucial task. Unfortunately, friction force exhibits strong (hysteretic) nonlinear relationship with sliding velocity, displacement and time, which makes the characterization to become a difficult task to fulfill. The classical Coulomb friction has been widely used, especially in the field of control engineering to compensate for the static force in a system. However, despite its simplicity, the effectiveness of the Coulomb model in capturing the frictional behavior of friction in the presliding regime is very low.This paper deals with the derivation of equivalent modal parameters, namely the stiffness and damping elements, to represent the hysteresis friction element on mechanical systems. The analysis is carried out using the skeleton technique, which employs the instantaneous amplitude and frequency of the excitation input and the response of the system. Subsequently, the dynamic analysis is performed on the equivalent system and the result is compared to that of the original system. The results show good agreement between the equivalent system and the original system for wide range of excitation.

Bifurcations of equilibrium sets in mechanical systems with dry friction

The presence of dry friction in mechanical systems induces the existence of an equilibrium set, consisting of infinitely many equilibrium points. The local dynamics of the trajectories near an equilibrium set is investigated for systems with one frictional interface. In this case, the equilibrium set will be an interval of a curve in phase space. It is shown in this paper that local bifurcations of equilibrium sets occur near the endpoints of this curve. Based on this result, sufficient conditions for structural stability of equilibrium sets in planar systems are given, and two new bifurcations are identified. The results are illustrated by application to a controlled mechanical system with friction.

Simulations and Tests of Innovative Friction Laws in Brake Systems

AbstractModeling of friction in mechanical systems is classically carried out by an incorporation of suitable algebraic functions. The general layout of these functions depends on the system under consideration [1]. For an analysis of brake systems, whose external parameters generally change rapidly, such steady state descriptions do not suffice. A detailed understanding of these systems and their dynamics requires innovative friction laws, which must be able to also capture transient effects in the contact zone. The tribological interface between brake lining material and brake disc is characterised by a closed‐loop interaction between heat, wear and friction, which causes a continuous growth and destruction of smooth and hard surface structures [2]. This equilibrium of flow can be well reproduced by modern automata methods [3]. It is significantly influenced by external load parameters, such as normal force and sliding velocity, which modulate the life cycle and average size of the contact patches and by that the coefficient of friction over time. Based on this principal mechanism, a new mathematical description of friction has been derived, which uses a set of coupled differential equations. This layout gives insights into the third body dynamics during transient friction events. Therewith, predictions of the surface topography, heat distribution and friction characteristics of different brake lining compositions become possible. The performance of the developed model will be illustrated by a discussion of specific braking events, extracted from industrial testing procedures (AK‐Master test). (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear Identification and Observer Based Compensation of Friction in Mechanical Systems

For better dynamic and static precision of pointing or tracking systems, the friction characteristic should be included in the design of modern position control loops. Therefore the actual friction force must be known. As the friction force is normally not measured, an observer for the estimation of this force is designed. The observer is based on a discrete time model of the process, whose parameters are identified during a pre-identification period using a Least Squares Estimator. Then, a state space representation including the friction as a state is derived. First, a linear observer is designed. The feedback vector is calculated by solving the Matrix-Ricatti difference equation. A poor convergence of the estimated friction to the actual friction is characteristic for the linear observer, especially in conjunction with high dynamic movements. Including the nonlinear discontinuous relay characteristic of the friction in the state space model, the convergence of the friction estimation is improved. The estimated value of the friction transformed with the inverse nonlinearity of the process is then added to the normal control action for compensation purpose. An adaptation of the compensation to changing friction is possible due to the observer. The functionality of the presented observer is finally demonstrated with a simulated example.

Modeling Mechanical Stick-Slip Friction Using Electrical Circuit Analysis

A new technique for modeling stick-slip friction in mechanical systems is proposed. The technique uses an electrical circuit analysis program to analyze the electrical circuit equivalent of the mechanical system. The stick-slip characteristic can be altered to take almost any form. The method is easy to apply to a vast range of systems, and two examples are given to illustrate its validity.

A numerical approach on the combined viscous and Coulomb friction motion

Coulomb friction in mechanical systems introduces strong non-linearities. Classical studies on this phenomenon take into account an equivalent viscous damping, but the method can give satisfactory results only near resonance. The present paper approaches the problem with numerical investigation on a one degree of freedom system. Different motion patterns have been identified with a strict correlation to the system parameters. With a frequency response approach the non-linearties effects have been here represented with the presence of higher order harmonics.

The simulation of Coulomb friction in mechanical systems

Coulomb or dry friction occurs in mechanical systems when nonlubricated surfaces come into sliding contact. The force resulting from Coulomb friction has a magnitude dependent on the contact force and a direc tion opposite to the relative velocity. If the re lative velocity is zero, the Coulomb friction assumes the value, within bounds, to maintain zero velocity. This, of course, presents the opportunity for a variety of equilibrium configurations. This paper presents an overview of the common tech niques for the digital simulation of Coulomb fric tion. All these techniques use the relative velocity to determine the friction force and are therefore flawed since they cannot predict multiple equilibrium positions. By means of an example, this paper shows that modeling Coulomb friction as a function of posi tion rather than of velocity allows multiple equili bria and offers features that are useful in modeling high-friction, low-mass systems.

Long-Term Storage Circuit Using a Sampling Technique

A circuit is presented that exhibits stable long-term memory analogous to that provided by Coulomb friction in mechanical systems. The circuit stabilizes the voltage across a capacitor. The capacitor voltage controls the length of a delay whose beginning is synchronized with a stable oscillator. At the end of the delay, the oscillator output is sampled and fed as a current to the capacitor. Periodic repetition of this procedure produces an average capacitor current that is a periodic function of capacitor voltage. Over the voltage range where the peak value of this current exceeds the intrinsic leakage current, every other one of those voltages at which the net average current is zero is a stable voltage. The principles used in this circuit may be applied to cancel the drift of any viscous-type analog memory element. The stored values are discrete but may be made so numerous as to approximate true analog memory to virtually any desired degree.