Breakaway Friction Research Articles

Influence of the breakaway friction on the seismic response of buildings isolated with curved surface sliders: Parametric study and design recommendations

The breakaway friction coefficient of curved surface sliders (CSSs) governs the transition between the sticking and the sliding behavior of the isolators, and hence affects the response of an isolated building during an earthquake. When the inertia forces induced by low-to-moderate intensity excitations are not able to overcome the breakaway frictional resistance of the CSS isolation system, the structure behaves as a fixed-base building, thus experiencing higher accelerations, interstorey drifts and internal forces than the isolated building. The majority of structural analysis programs disregard the static coefficient of friction, and implement the dynamic friction coefficient only throughout the response history analysis, which may imply an increased prediction of displacement demand for the isolation system but may lead to an unsafe design for the superstructure. In this paper, the frictional resistance to sliding before the breakaway is simulated through a bidirectional plasticity domain, which has been coded in a finite element of the isolator formulated in OpenSees to incorporate the transition between the breakaway friction in the sticking phase and the velocity-dependent friction model in the subsequent sliding phase. Based on this formulation, the influence of the breakaway friction on the response of buildings isolated with CSSs is investigated numerically through an extensive parametric study comprising more than 9000 bidirectional nonlinear time history analyses (NLTHAs). The parameters cover a range of friction coefficients, superstructure properties and a large group of natural spectrum-compatible bidirectional ground motions having different intensity levels and frequency contents. The results from NLTHAs are processed statistically to elaborate regression formulae and design recommendations that can be useful to predict the trigger acceleration at which sliding motion starts, as well as to suggest how to achieve more accurate estimates of the seismic response when the breakaway friction is ignored in the structural analysis model.

Comparison of Four Friction Models: Feature Prediction

In this paper, we provide not only key knowledge for friction model selection among candidate models but also experimental friction features compared with numerical predictions reproduced by the candidate models. A motor-driven one-dimensional sliding block has been designed and fabricated in our lab to carry out a wide range of control tasks for the friction feature demonstrations and the parameter identifications of the candidate models. Besides the well-known static features such as break-away force and viscous friction, our setup experimentally demonstrates subtle dynamic features that characterize the physical behavior. The candidate models coupled with correct parameters experimentally obtained from our setup are taken to simulate the features of interest. The first part of this work briefly introduces the candidate friction models, the friction features of interest, and our experimental approach. The second part of this work is dedicated to the comparisons between the experimental features and the numerical model predictions. The discrepancies between the experimental features and the numerical model predictions help researchers to judge the accuracy of the models. The relation between the candidate model structures and their numerical friction feature predictions is investigated and discussed. A table that summarizes how to select the most optimal friction model among a variety of engineering applications is presented at the end of this paper. Such comprehensive comparisons have not been reported in previous literature.

Efficient Break-Away Friction Ratio and Slip Prediction Based on Haptic Surface Exploration

The break-away friction ratio (BF-ratio), which is the ratio between friction force and the normal force at slip occurrence, is important for the prediction of incipient slip and the determination of optimal grasping forces. Conventionally, this ratio is assumed constant and approximated as the static friction coefficient. However, this ratio varies with acceleration rates and force rates applied to the grasped object and the object material, which lead to difficulties in determining optimal grasping forces that avoid slip. In this paper, we propose a novel approach based on the interactive forces to allow a robotic hand to predict object slip before its occurrence. The approach only requires the robotic hand to have a short haptic surface exploration over the object surface before manipulating it. Then, the frictional properties of the finger-object contact can be efficiently identified, and the BF-ratio can be real-time predicted to predict slip occurrence under dynamic grasping conditions. Using the predicted BF-ratio as a slip, threshold is demonstrated to be more accurate than using the static/Coulomb friction coefficient. The presented approach has been experimentally evaluated on different object surfaces, showing good performance in terms of prediction accuracy, robustness, and computational efficiency.

Effect of shaft roughness and pressure on friction of polymer bearings in water

In this study, the frictional behavior of selected commercially available unfilled polymers, namely, polyether ether ketone, polytetrafluoroethylene, polyethylene terephthalate, and ultra-high molecular weight polyethylene against an Inconel shaft was investigated using a journal bearing test configuration in water-lubricated sliding contact. Dynamic friction curves were obtained for various shaft roughness values and polymer combinations. The results showed a significant influence of shaft surface roughness on running-in and steady state friction in water-lubricated conditions. Contact angle measurements revealed a significant increase in wettability of Inconel counterfaces. The X-ray photoelectron spectroscopy (XPS) analysis of the surfaces suggests formation of a reaction layer on worn Inconel surfaces when sliding against the polymers. The influences of counter surface roughness and load on frictional response of polymers were studied through intermittent tests by obtaining dynamic and breakaway friction maps for different polymer materials, shaft roughness values, and pressure combinations. In general, a trend of decreasing friction was obtained with increasing contact pressure; however, the materials’ frictional responses to variations in counter surface roughness were different. These results indicate that although a reduced counter surface roughness may be beneficial for dynamic friction of polymers in all lubrication regimes, it can adversely affect the materials’ breakaway friction response.

Static/dynamic friction and wear of some selected polymeric materials for conformal tribo-pairs under boundary lubrication conditions

Abstract This work is aimed at investigating the friction and wear performance of different polymeric materials having potential for hydraulic system components under lubricated sliding conditions against a steel counter face. A pin-on-disc test configuration was used for the experimental study. The different polymeric materials selected for these studies were commercial polyimides (PI), polyether ether ketone (PEEK), and flouropolymers. Some of these materials were bulk materials whereas others were used as coatings applied on to the cast iron substrate. The tribological characteristics of the polymers were compared with a reference grey cast iron. The frictional characteristics were evaluated in both static and dynamic conditions. The results have shown that by using polymeric materials it is possible to reduce breakaway friction by an order of magnitude compared to grey cast iron. However, the breakaway friction increased significantly after the wear tests. The polymeric materials having lowest breakaway friction have shown the highest wear with the exception of the PEEK-PTFE coating which showed low wear. PI with graphite fillers also showed low wear but it resulted in relatively high friction. The carbon fibre reinforced materials resulted in unstable friction as well as higher wear compared to the PI materials with graphite fillers.

Break-away friction of PTFE materials in lubricated conditions

This study investigates the tribological characteristics at initiation of sliding (break-away friction) of several polytetrafluoroethylene based materials. Four PTFE composites, pure PTFE, and white metal were tested in a reciprocating tribo-meter with the block on plate configuration against a steel counter-surface. Apparent contact pressure and oil temperature were varied from 1 to 8MPa and 25 to 85°C respectively. SEM investigations revealed wear patterns of the PTFE materials and the abrasive nature of hard fillers.Bronze filled, carbon filled, and pure PTFE were found to provide lower break-away friction and less variation over the course of testing and generally superior properties.

Performance Simulation and Design of New Series Hydraulic Hybrid Bus

Due to large mass and frequent start-stop, the advanced hybrid propulsion technologies enabling regenerating and reusing of significant amounts of braking energy are critical to achieving future fuel economy goals for city buses. The initial breakaway friction and inertia of the hydraulic hybrid bus can be overcome by energy that would ordinarily already have been wasted, and when running the engine, can operate in more efficient modes. Therefore energy-saving and environmental protection of hydraulic hybrid buses has important significance in application. The simulation model of series hydraulic hybrid bus is created in AMESim. The simulation results of the SHHB model show that it is very prominent in saving energy and reducing harmful emissions.

Improving backdrivability in geared rehabilitation robots

Many rehabilitation robots use electric motors with gears. The backdrivability of geared drives is poor due to friction. While it is common practice to use velocity measurements to compensate for kinetic friction, breakaway friction usually cannot be compensated for without the use of an additional force sensor that directly measures the interaction force between the human and the robot. Therefore, in robots without force sensors, subjects must overcome a large breakaway torque to initiate user-driven movements, which are important for motor learning. In this technical note, a new methodology to compensate for both kinetic and breakaway friction is presented. The basic strategy is to take advantage of the fact that, for rehabilitation exercises, the direction of the desired motion is often known. By applying the new method to three implementation examples, including drives with gear reduction ratios 100-435, the peak breakaway torque could be reduced by 60-80%.

Dual Speed Control Scheme of Servo Drive System for a Nonlinear Friction Compensation

Servo motor drive systems with ball-screw and timing-belt are widely used in a numerical control, robot, factory automation and industrial applications. Most of the servo motor and drive system transfer the motor torque to loads via mechanical connections. However, the nonlinear friction of coupled mechanical devices reduces the performance of a servo drive. Especially, in a low speed range, tracking errors are serious due to the break-away friction and Stribeck effects. In this paper, a new dual speed controller is proposed for the compensation of nonlinear friction torque. The proposed dual speed controller has an outer speed controller and an inner friction torque compensator. The friction torque compensator adds additional torque corresponding to a nonlinear friction of mechanical devices, so the actual speed quickly tracks the reference value. Since the proposed nonlinear friction torque compensator uses the actual speed information without any motor parameters and mathematical model, the proposed compensator has a very simple structure and high stability. The proposed control scheme is verified by computer simulation and experimental results.

An inclined plane test for the breakaway coefficient of rolling friction of rolling element bearings

An ISO/ASTM standard test has been used for over 20 years for detecting the presence of lubricating films on plastics (usually in web form). The test uses a wire paperclip (on end) as the rider on an inclined plane. A need for a test to measure breakaway friction of rolling element bearings for instrument pivots prompted a study to determine if the “paperclip” test could be adapted to screen bearings for low rolling friction. This paper describes the test modification and the results obtained in intralaboratory tests aimed at making the test an ASTM standard. The test was modified by replacing the paperclip rider with a rolling element bearing with the inner race fixed to an axle on a balanced rider. The inclined plane is raised to imitate rolling of the outer race on the inclined plane. The tangent of this angle is used as the test metric. Seven random ball bearings, some new, some used, were tested by six laboratories in the USA to determine if the test is repeatable enough to be an ASTM standard. The coefficient of variation was quite good within lab, and reasonable between labs. The test seems to be a fast, simple friction test for rolling element and other bearings that should be standardized.

New UHMWPE/Rubber Bearing Alloy

A new polymer bearing alloy has been developed that overcomes the high wear nonhydrodynamic deficiencies of UHMWPE when water-lubricated and the shaft wear and high breakaway friction of rubber bearings (Orndorff, Jr., R. L., 1988, “Thermoplastic-Rubber Polymer Alloys and Method for Producing the Same,” United States Patents, 4,725,151 and 4,735,982). Testing covered clean-water high-load and light-load highly abrasive wear testing as well as bearing friction testing. A number of competing materials were also wear tested with the new alloy comparing favorably. Shaft (drum) wear was determined to be zero on the abrasive test. The wear of the polymer alloy decreased by 70% when the high-load sample axial length was reduced by 50% to increase the contact pressure. Rubber bearing wear increased by almost ten times under the same condition. It is hypothesized that the higher-pressure clean-water test sample of the new alloy adjusted to run in the hydrodynamic regime through elastic deflection, creep and a minimal amount of wear. The breakaway coefficient of friction decreased by 67% compared with rubber bearings and the kinetic fiction decreased with increasing load as did the wear. The new alloy will allow water-lubricated rubber bearing length-to-diameter ratios to be reduced from the present value of four to one or less. Projected pressures can be increased five to ten times with the new alloy. [S0742-4787(00)04901-8]

Inertia effects on static friction measurements of laser textured media [magnetoresistive recording heads

Mechanical and magnetic requirements for Magneto-Resistive (MR) recording heads have caused media data zones to become increasingly smoother. Tribology conditions of the landing zone still require some roughness, however. Because of this, dual zone textured media has become a common solution for MR applications. Although some disks have been dual zone textured mechanically, laser textured landing zones have become an industry standard. Laser textured media has unique static and dynamic friction properties, and if careful attention is not paid to measurement set-up, measured static friction values could be extremely inaccurate (up to 100%). This is likely due to system inertia differences and a difference in /spl zeta//sub k/ (The ratio of Dynamic to True Static Friction). While mechanically textured media has typical /spl zeta//sub k/ values ranging from 0.4 to 0.6, laser textured media has typical values close to 1.0. A theoretical model of breakaway friction measurements by Li et al. predicts the above mentioned trend. The model shows that /spl zeta//sub p/ (The sensed to true static friction ratio) varies as a function of /spl zeta//sub i/ (The inertia force to true static friction ratio). This function is different for different values of /spl zeta//sub k/. The model predicts that laser textured media will be much more dependent on inertia force than mechanically textured media. The trend predicted by this model is now demonstrated experimentally. By using a small DC motor to drive a spindle, it is possible to reduce the instantaneous acceleration to an extremely low value. When measuring stiction at these extremely low acceleration rates, it is demonstrated that the measured values are much lower than they are when measured at higher acceleration rates. Even though laser textured media with a carbon coated MR head exhibits this phenomenon, a mechanically textured disk with a non-carbon coated inductive head does not (in the range of accelerations tested).

Disturbance Observer-Based Nonlinear Friction Compensation and Application to Table Drive System

Nonlinear friction in machine elements is one of the inherent nonlinearities, which causes stick-slip motion in very low velocity control range and tracking errors in trajectory control due to the break-away friction and Stribeck effect. Since those greatly affect the control accuracy of servo drive systems, the appropriate modeling and/or compensation for friction is indispensable to achieve the higher performance motion.The authors have already proposed the compensation scheme for nonlinear friction by using a remarkable disturbance observer which is designed on the basis of Coulomb friction characteristic. In this paper, the dynamic characteristic of nonlinear friction is examined by means of a friction model with elastic bristles between sliding surfaces. The analysis reveals that the nonlinear friction behaves in Coulomb friction under the dynamic motion at velocity reversal and, as a result, it is concluded clearly that the proposed disturbance observer is quite useful to estimate the instantaneous friction. Experimental results using a table drive system show the significant performance improvement of the proposed scheme in precise trajectory control.

Disturbance Observer-Based Nonlinear Friction Compensation in Servo Drive System.

Solid friction in various machines is one of dominant nonlinearities limiting the control performance of servo drive systems. Nonlinear friction gives tracking errors and undesired stick-slip motion due to stiction (breakaway friction) and/or Stribeck effect. A wide variety of studies have been reported, which formulated nonlinear friction models, identified its parameters, and compensated for the friction. Those compensation schemes can significantly improve the conventional control performance, however, deterioration of the control accuracy is still the problems due to the inaccurate friction model and/or the restricted bandwidth of the compensation control loop.This paper presents the disturbance observer-based compensation for nonlinear friction in servo drive systems to improve the control performance. The proposed scheme uses a new disturbance observer which is designed on the basis of Coulomb friction characteristics. This scheme can realize the precise speed/position control without response delay which is caused by the restricted bandwidth in the conventional compensation scheme. Experimental results and analytical studies show the distinct performance improvement of the proposed scheme, comparing to the conventional compensation schemes.

A Theoretical Analysis of Breakaway Friction Measurement

The transient response of an I-beam friction measurement system to a constant disk acceleration is theoretically analyzed. The friction measurement system is modeled as a single-degree-of-freedom system. It is found that the true breakaway friction is different from the measured breakaway friction, and that the true breakaway time is also different from the measured breakaway time. The system measurement error is evaluated as a function of the acceleration, the effective mass, the spring stiffness, as well as the static and kinetic coefficients of friction. A closed-form dimensionless solution is derived for the system measurement error. If the dynamic effect of the measurement system can be neglected by selecting an appropriate effective mass and acceleration, the maximum possible discretization error can be estimated as a dimensionless function of the discretization frequency, the breakaway friction, the spring stiffness, and the acceleration.

Computer analysis of multi-port operation using analogue variables

The full capabilities of electrical circuit analysis programs can be utilized in evaluating the performance of multi-port systems by defining analogous conjugate port variables. These variables represent intensive (across) and extensive (through) variables such that their product represents a universal system quantity such as energy or power. A three-port electro-mechanical-thermal system is used as an example. This system consists of a full-wave rectifier circuit driving a permanent-magnet DC motor. Mechanically, the motor is represented by inertia, friction, stiction (breakaway friction), step-up speed gear and a nonlinear fan load. The thermal equivalent circuit tracks the thermal behavior in time and includes thermal resistance, thermal capacitance and thermal power sources.

Further Results on Frictional Properties of Teflon Bearings

Further results on the frictional properties of Teflon-steel interfaces in sliding bearings are reported. Values of the breakaway (static) and sliding coefficients of friction of woven Teflon in contact with polished stainless steel for sliding velocities of up to 10 in./sec (254 mm/s) and bearing pressure of 6,500 and 10,000 psi (44.9 and 69 MPa) are reported. These values are sufficiently low to be of interest in applications of seismic base isolation. Furthermore, the effect of dwell of load on the breakaway coefficient of friction of sheet type Teflon-steel interfaces is studied in a single test in which the load is maintained on the specimen for 20 months prior to testing. The test demonstrated no difference in the breakaway friction in comparison to a test with only one-half hour of load dwell.

Teflon Bearings in Base Isolation II: Modeling

A mathematical model of the factional behavior of Teflon sliding bearings for conditions of interest in base isolation is developed. The calibration of the model is based on extensive experimental data that were presented in an accompanying paper. The model is capable of accounting for: (1) Unidirectional and multidirectional motion at the Teflon‐steel interface; (2) velocity and pressure dependence of the coefficient of sliding friction; and (3) breakaway (or static) friction effects. The model is characterized by four parameters. These are the minimum and maximum values of the sliding coefficient of friction, the ratio of breakaway to sliding coefficient of friction at initiation of sliding and a parameter that describes the variation of the sliding coefficient of friction with velocity. Values of these parameters are presented for sixteen combinations of type of Teflon, bearing pressure and condition (surface roughness) of mating steel surface. Applications of the model in the analysis/design of a sliding isolation system are presented and the effects of bearing pressure and breakaway friction are evaluated. Furthermore, an assessment of the implications of using Coulomb's constant friction model rather than the developed model is presented.

Development of an elastomer with a high static or breakaway friction coefficient

AbstractThis paper reviews the literature concerning the measurement of friction coefficients (μ) for elastomers and summarizes our data measured under static or breakaway conditions against epoxy painted steel using ∼ 176 psi contact pressure. Although largely related to missile launch system applications, the μ values presented are specific for interface pairs, normal loading force, and speed. Therefore, this article should serve as a starting point for those requiring specific friction data for selected elastomers.

Test on a brake lining damper for structures

Tests were carried out on a brake lining damper intended to dissipate energy between parts of a structure having relative motion during earthquake attack. The device operates by the sliding of a steel plate clamped against a brake lining. Hysteresis loops for the damper showed peaks for the breakaway friction force, when the value of the force was up to twice that obtained for continuous movement. Also values of the coefficient of friction derived from the test were lower than those quoted elsewhere. Further testing is required prior to the use of the method in structures.