Dynamic Friction Model of Lubricated Surfaces for Precise Motion Control

A model is developed to describe dynamic friction effects in lubricated surfaces. The model covers the hydrodynamic, mixed and boundary lubrication regions. The dynamic friction model can predict the friction force for time-varying velocity, and is useful in precise motion control. The model presented is for a short journal bearing, but can be extended to other geometries of sliding surfaces, such as point and line contacts or rolling element bearings. The friction is related to a time variable fluid film thickness, resulting from journal vibrations relative to the sleeve. The proposed model agrees qualitatively with experimental results for lubricated line contact. Both show similar hysteresis-type friction curves under oscillating velocity.     

An Interfacial Friction Law for a Circular EHL Contact Under Free Sliding Oscillating Motion

The friction response of a lubricated interface under free sliding oscillating motion is investigated as a function of the contact pressure and the rheology of the lubricant in terms of viscosity and piezoviscosity. For loaded contacts, both velocity dependent friction, referred to as viscous damping, and friction independent of the instantaneous sliding velocity contribute to the energy dissipation. Viscous damping mainly corresponds to the dissipation in the lubricant meniscus surrounding the contact, while dissipation within the confined lubricated interface is mainly independent of the instantaneous sliding velocity. The friction coefficient independent of the instantaneous sliding velocity falls on a master curve for the wide range of tested operating conditions and lubricant rheological properties. The master curve is a logarithmic function of a dimensionless parameter corresponding to the ratio of the viscosity of the confined lubricant to the product of the pressure and a characteristic time. The physical meaning of this latter and the friction law are discussed considering the confined interface as a viscoelastic fluid or a non-Newtonian Eyring fluid.     

Influence of sliding velocity on friction force in rubber seal rings under reciprocal motion

The influence of sliding velocity on the value of maximum and steady friction force in rubber seal rings under reciprocal motion is studied. It is established that the nature of the change in a static friction force and a steady-state friction force as a function of the sliding velocity is the same all other conditions being equal. The greater the sliding velocity, the faster the decrease and stabilization of a friction force after endurance of rubber seal rings in fixed contact with a cylinder. The maximum of the friction force dependence on the velocity in an investigated pair decreases with increasing temperature of a sealed medium.     

转缸驱动转子滑片压缩机运动分析

介绍了一种新型结构滑片压缩机的结构特点和工作原理，该新结构压缩机与传统滑片压缩机相比。改善了低速性能，具有更低的振动噪声和摩擦损失等优势。建立了机构模型，并分析了该压缩机的运动规律。     

A quasistatic manipulation for multifingered robotic hands

A generalized algorithm for the motion/force planning of the multifingered hand is proposed to generate finite displacements and changes in orientation of objects by considering sliding contacts as well as rolling contacts between the fingertips and the object at the contact point. Specifically, an optimization problem is firstly formulated and solved to find joint velocities and contact forces to impart a desired motion to the object at each time step. Then the relative velocity at the contact point is found by calculating velocity of the fingertip and the velocity of the object at the contact point. Finally, time derivatives of the surface variables and the contact angle of the fingertip and the object at the present time step are computed using the Montana’s contact equation to find the contact parameters of the fingertip and the object at the next time step. To show the validity of the proposed algorithm, a numerical example is illustrated by employing the robotic hand manipulating a circular cylinder with three fingers each of which has four joints.     

Theory of viscoelastic lubrication

We study the lubricated (wet) contact mechanics of a smooth hard cylinder sliding on a randomly rough nominally flat surface of a linear viscoelastic solid. We calculate the rolling and sliding friction, and study the transition from the boundary lubrication to the elasto-hydrodynamic lubrication regime. For the viscoelastic contact the minimum (average) separation does not monotonically increase with the sliding velocity, and the Stribeck curve exhibits new structures not shown for elastic solids.     

Internal synthesis of motion and the dynamic characteristics of external friction

On the example of a simple tribosystem in the form of a body sliding (rolling) over an inclined plane, it is demonstrated that the kinematic concept of external friction based on the Amonton-Coulomb law of modifications is not appropriate as a base for the study of the dynamics of external friction. The obtained dynamic friction characteristics in respect to the velocity and acceleration prove that the velocity of motion varies and that the dynamic equations are nonlinear both in respect to velocity and acceleration. The mechanism of dynamic feedback in tribosystems ensures both subordination to external effects and the capacity for effective synthesis of relevant motion types. The problem is analyzed of selecting adequate design types of motion elements of a tribosystem in relation to the frictional self-excited vibrations and the metrology of measuring the coefficients of sliding and rolling friction. In view of the high complexity of a priori modeling of external friction, it is concluded that methods of experimental tribodynamics should be developed.     

Dynamical Evolution of Wear Particles in Nanocontacts

Nanoscale surface modification, by the interaction of sliding surfaces and mobile nanoparticles, is a critical parameter for controlling friction, wear and failure of surface structures. Here we demonstrate how nanoparticles form and interact in real-time at moving nanocontacts, with reciprocating wear tests imaged in situ at the nanoscale over >300 cycles in a transmission electron microscope. Between sliding surfaces, friction-formed nanoparticles are observed with rolling, sliding and spinning motions, dependant on localised contact conditions and particle geometry. Over periods of many scratch cycles, nanoparticles dynamically agglomerate into elongated clusters, and dissociate into smaller particulates. We also show that the onset of rolling motion of these particles accompanies a reduction in measured friction. Introduction of nanoparticles with optimum shape and property can thus be used to control friction and wear in microdevices.     

Friction-induced vibration of oscillating multi-degree of freedom polymeric sliding systems

The purpose of this study is to investigate friction-induced vibration of oscillating systems. Special attention is focused on modeling polymer-on-metal systems. Past experimental and analytical friction results from non-oscillating or unidirectional translational sliding modes are extended into the oscillating sliding mode. Specifically, this refers to the incorporation of a representative functional shape of the friction–velocity relationships estimated from experimental results. Dependent on the relative sliding velocity between the frictionally coupled subsystems, the modeled system exhibits behavior such as a single stick–slip at the beginning of oscillatory motion, a single stick–slip at each motion reversal, or multiple stick-slip events during each half cycle of motion. Additionally, the boundary of incipient friction-induced vibration was identified via a rigorous definition of friction-induced vibration.     

Modeling of Dynamic Friction in Lubricated Line Contacts for Precise Motion Control

A simple dynamic friction model for an elastohydrodynamic lubrication sliding and rolling line contact has been developed. This model uses the technique introduced earlier by Harnoy and Friedland (1). The model includes low-velocity regions where friction is a combination of contact and elastohydrodynamic friction. The study shows that the lime-variable friction is not only a function of instantaneous sliding velocity, but is also a memory function of the velocity history. Simulation of the model for an oscillating velocity exhibits similar hysteresis effects in friction-velocity curves as observed earlier in several experimental studies. The model can be useful for friction compensation to enhance the precision of motion in control systems.     

The effect of longitudinal tangential vibrations on friction and driving forces in sliding motion

The main purpose of the work has been to qualitatively and quantitatively analyse the influence of longitudinal tangential vibrations on friction and driving forces in a sliding motion. Computational models were developed and implemented in a combined Matlab-Simulink environment. Both the dynamic Dahl's and the Dupont's and classical Coulomb's friction models were used. The influence of vibration velocity amplitude on the friction force in the presence of tangential longitudinal vibrations and on reduction of the driving force in sliding motion was analysed. It revealed that the commonly accepted view that the reduction of the average friction force is a consequence of cyclic changes in the sign of the friction force vector, only when the amplitude of vibration velocity is greater than the sliding motion velocity, is erroneous. The phenomenon was also observed without any changes in the sign of friction force vector. The results of simulations were compared with experimental data obtained with the use of a test rig specifically designed for the work. The Dahl's friction model led to the best correlation.     

Influence of temperature on the friction performance of gear oils in rolling–sliding and pure sliding contacts

The friction behaviour of five different gear oils in rolling–sliding and pure sliding contacts and how temperature influences their friction properties were investigated. It is found that increasing temperature decreases boundary friction with gear oils that contain friction modifiers while not for other gear oils, at all contact pressures investigated. In mixed lubrication region, temperature decreases friction at low contact pressures while increases friction at high contact pressures. The effect of slide–roll ratio on friction is significant in boundary lubrication region especially at higher temperature while less significant in mixed lubrication region at both low and high temperatures. The ranking of gear oils for friction in boundary and mixed lubrication regimes is similar both in rolling–sliding and pure sliding contacts, regardless of temperature. Copyright © 2013 John Wiley & Sons, Ltd.     

Molecular Dynamics Simulation of Rolling Friction Using Nanosize Spheres

The rolling resistance of a sphere on an atomically flat surface was studied by molecular dynamics simulation carried out by rolling a rigid Ni sphere on a copper (100) surface. Spheres of 6 and 12 nm diameters were used for rolling simulations after indentation up to 10 Å in depth and the computations carried out using embedded atom potentials of Ni and Cu, assuming either active molecular interaction at the contacts (normal potentials) or the presence of a passivation layer on the sphere. Results showed that the sphere size, penetration depth, and adhesion at the rolling interface strongly affected the rolling friction. When molecular interactions were allowed at the rolling contacts, the average rolling friction coefficient was higher and severe oscillations in the friction force was observed. On the other hand, a sphere with a passivation layer produced more dislocations in the copper substrate during rolling and the motion of the dislocations affected the coefficient of rolling friction and the size of the friction force oscillations. This work also suggested that rolling friction at the nanoscale level was similar to the macroscopic rolling condition during strain hardening of metals due to severe dislocation multiplication underneath the nanosphere in nanorolling.     

一种滚动密封式爬壁机器人动力学建模与分析

介绍了一种滚动密封式爬壁机器人,分析了其密封与运动机理。提出了一种多履带协调运动的爬壁机器人动力学建模方法,推导出该机器人直线运动及转向动力学方程。建立了直线运动及转向时履带牵引力与机器人姿态角之间的关系,分别计算出采用滑动密封和滚动密封方式的机器人在运动过程中的摩擦阻力并进行了比较分析。仿真和样机实验表明该爬壁机器人可沿壁面行走与越障,具有摩擦阻力小、负载能力大及对壁面适应性强等特点。     

The Combined Role of Soot Aggregation and Surface Effect on the Friction of a Lubricated Contact

One of the considered research paths to reduce friction loss consists in optimizing the interactions between surfaces and lubricants. The latter may significantly change with the lubricant ageing. In this framework, the tribological behaviour of aged formulated lubricant is analysed for various low-speed reciprocating motions and with different nature of surfaces. This paper focuses on soot aggregate formation processes in a lubricated contact and on their correlation to friction. Although no aggregates have been observed in pure rolling conditions, pure sliding conditions may lead to the appearance of aggregates moving through the contact as a function of the nature of the surfaces. The analysis of their displacement within the contact is used to discuss their interactions with the surfaces. Moreover, we show that the velocity and the dwell time of the aggregate depend on the sliding speed. The morphology of these aggregates evolves over time, affecting friction behaviour. An additive law combining a contribution from the shear of the aggregates with another one due to the shear of a thin lubricant film surrounding the aggregate is then proposed to interpret friction origin and friction evolution with time of shear. The aggregate motion also varies with the nature of the surfaces: in particular, DLC–DLC couple reduces aggregation phenomena and maintains a low friction without apparent wear.     

Positioning error improvement based on ultrasonic oscillation for a linear motion rolling bearing during sinusoidal motion

Friction can be a major disturbance to precision positioning. This study presents a method for improving positioning error in a linear motion rolling bearing based on ultrasonic oscillations. Experiments were conducted in which a single-axis linear motion rolling bearing was driven in a sinusoidal motion to simulate circular motion. Two ultrasonic actuators excited both the rail and the carriage of the guide to create relative displacements between raceways and rolling elements. The carriage of the linear motion rolling bearing was driven by a frictionless voice coil motor (VCM). The displacement of the carriage and the friction force were measured by a springless linear encoder and the VCM's current, respectively. The early stages of the experiments focused on several oscillating patterns, and their consequent impacts on positioning error during sinusoidal motion were investigated. Finally, the oscillating pattern that maximally improved the positioning error was proposed and tested. By applying the proposed pattern, the maximum displacement error, exhibited just after velocity reversal, was reduced by approximately 40%, while the average error was reduced by 26%.     

Rolling Friction: Comparison of Analytical Theory with Exact Numerical Results

We study the contact mechanics of a smooth hard cylinder rolling on a flat surface of a linear viscoelastic solid. Using the measured viscoelastic modulus of unfilled and filled (with carbon black) nitrile rubber, we compare numerically exact results for the rolling friction with the prediction of a simple analytical theory. For the unfilled rubber, the two theories agree perfectly while some small difference exists for the filled rubber. The rolling friction coefficient depends nonlinearly on the normal load and the rolling velocity.     

The influence of surface roughness and the contact pressure distribution on friction in rolling/sliding contacts

A numerical contact model is used to study the influence of surface roughness and the pressure distribution on the frictional behaviour in rolling/sliding contacts. Double-crowned roller surfaces are measured and used as input for the contact analysis. The contact pressure distribution is calculated for dry static contacts and the results are compared with friction measurements in a lubricated rolling/sliding contact made with a rough friction test rig. The mean pressure is suggested as a parameter that can be used to predict the influence of surface roughness on the friction coefficient in such contacts. The results show two important properties of the friction coefficient for the friction regime studied in this paper: (1) there is a linear decrease in friction coefficient as a function of the slide-to-roll ratio, and (2) the friction coefficient increases linearly with increasing mean contact pressure up to a maximum limit above which the friction coefficient is constant. The absolute deviation of experimental results from the derived theory is for most cases within 0.005.     

点接触弹流润滑性能及应用分析

牵引式无级变速器的传动零件间处于点接触状态，在某一传动比时，相对自转速度为零。本文研究了该状况下各种工况参数如滑动率、滚动速度和载荷等对点接触的弹流润滑性能的影响。研究表明：随着滑动率的增大，摩擦因数增大，油膜最大温升增大；在相同滑动速度下，随着滚动速度的增大，油膜厚度增大，但摩擦因数减小；随着载荷的增大,油膜厚度减小,摩擦因数增大，油膜最大温升增大。     

Experimental investigation of temperature rise in elliptical EHL contacts

A toroidal traction drive continuously variable transmission (CVT) transmits power by the shearing action of lubricant film under heavy loads at contact points on the CVT rollers. In other words, rolling and sliding motions produce traction force. Furthermore, due to the geometry of the toroidal CVT, spin motion is produced at the contact points. These contact points, which are elliptical in shape, are where shear stress of the lubricant generates frictional heat. Temperature rise at the contact point has never been measured under the conditions of high rolling speed (velocity) and minute amounts of sliding (slippage), nor has the influence of spin motion on temperature rise been examined thus far. The authors et al. measured temperature distribution at contact points under conditions of high rolling speed and minute amounts of sliding, such as what is found in a toroidal CVT, using an FZG twin-disk test machine and thin-film sensors. The influence of spin motion on temperature rise was also experimentally investigated.