Variation Of Normal Force Research Articles

Investigation of tribological behavior of polytetrafluoroethylene/graphene composite

AbstractIn this study, we constructed molecular friction models for polytetrafluoroethylene (PTFE)/graphene composites with various defect ratios to represent their different wear stages and simulated their friction processes under different load conditions. The friction and wear mechanisms were revealed by analyzing the variations in their molecular architectures and energies during friction. The results indicate that the friction coefficient decreased with increasing load. When the load was constant, the friction coefficients of the composites containing defections were resemble, indicating that they remained unchanged in the different wear stages. The composite wear continued to increase during friction. However, the wear increment decreased gradually. In addition, by analyzing the changes in MSD, wear can be divided into initial, stable, and accelerated wear stages.Highlights Construct PTFE/Gr molecular models to represent their different wear stages. Simulate the dynamic changes of PTFE/Gr molecular structure during friction. The influence of PTFE/Gr dynamic structural changes was revealed. Reveal the normal force variation from an energy perspective.

Robotic abrasive belt grinding with consistent quality under normal force variations

Robotic abrasive belt grinding with consistent quality under normal force variations

Nonlinear interaction of parametric excitation and self-excited vibration in a 4 DoF discontinuous system

Interaction between parametric excitation and self-excited vibration has been subjected to numerous investigations in continuous systems. The ability of parametric excitation to quench self-excited vibrations in such systems has also been well documented. But such effects in discontinuous systems do not seem to have received comparable attention. In this article, we investigate the interaction between parametric excitation and self-excited vibration in a four degree of freedom discontinuous mechanical system. Unlike majority of studies in which oscillatory nature of stiffness accounts for parametric excitation, we consider a much more practical case in which parametric excitation is provided by a massless rotor of rectangular cross section with a cylinder-like mass concentrated at the center. The rotor arrangement is placed on a friction-induced self-excited support in the form of a frame placed on a belt moving with constant velocity. This frame is connected to a supplementary mass. A Stribeck friction model is considered for the mass in contact with the belt. The frictional force between the mass and the belt is oscillatory in nature because of the variation of normal force due to parametric excitation from the rotor. Our investigations reveal mutual synchronization of parametric excitation and self-excited vibration in the system for specific parameter values. The existence of a stable limit cycle with constant synchronized fundamental frequency, for a range of parametric excitation frequencies, is established numerically. Investigation based on frequency spectra and Lissajous curves reveals complex synchronization patterns owing to the presence of higher harmonics. The system is also shown to exhibit Neimark–Sacker bifurcations under the variation of belt velocity. Furthermore, variation in belt velocity and coupling stiffness is seen to cause a breakup of quasi-periodic torus with small-amplitude oscillations to form large amplitude chaotic orbits. This points toward the possibility of vibration suppression in the system by tuning the parameters for stabilizing the small-amplitude quasi-periodic response. An example of co-existence of different attractors in the system is also presented.

Comparison of wheel load application methods in single-wheel testbeds

This work reviews wheel load application methods in single-wheel testbeds (SWTBs), used widely to study terramechanics. Normal force oscillations have been reported in SWTBs, and visible in time-series data when provided. This has usually been attributed to grousers, but is also visible for wheels without grousers suggesting other system dynamics (e.g. friction in the vertical axis) are the cause. Furthermore, accelerations related to periodic vertical displacements of the wheel (whether caused by grousers or otherwise) cannot explain more than a small fraction of the amplitude of normal force variations. In this work, a 4-bar mechanism was developed that balances the normal load and ensures that a constant (not oscillating) normal force is achieved throughout terramechanics experiments. System behaviour with and without the 4-bar mechanism was assessed, highlighting the need for a 4-bar mechanism or other method of balancing normal loads if a constant normal force is desired. No significant effects on average experimental results were observed due to normal force oscillation, but artifacts were observed in the time-series data. Comparisons of results with a system where wheel load is applied pneumatically and characterization of friction in each testbed provide further support to the conclusion that normal force oscillations are mainly caused by friction in the vertical axis.

Innovative squeak noise prediction: An approach using the harmonic balance method and a variable normal contact force

A flawless interior acoustics is a customer requirement in automotive industry. Any disturbing noise e.g. squeak or rattle must be avoided to meet the customer’s expectations regarding quality. This work presents an improved approach to predict squeak noise. For this purpose, a test rig generating the underlying stick-slip phenomenon by harmonic excitation is used and a corresponding finite element model is built describing the system’s dynamic behavior. The Harmonic Balance Method is applied for solving the nonlinear equations of motion. The major difference to previous approaches consists in the implementation of a contact algorithm allowing variable normal forces in the contact domain. The calculated nonlinear system response is compared to existing methods using a constant normal force. The proposed method is capable of predicting odd harmonics, which is a significant improvement in assessing squeak noise. Furthermore, good agreement between simulation and experiment is observed which validates the approach.

Assessment of micro-abrasive wear tribological properties of H10 tool-steel under conditions of “constant normal force ⇒ variable pressure” and “constant pressure ⇒ variable normal force”

In industrial applications, tools of H10 tool-steel used in mechanical and metallurgical manufacturing processes, such as rolling, extrusion, forging, sheet metal stamping, milling and turning, frequently present significant levels of micro-abrasive wear, which occurs under specific circumstances of “constant normal force”, resulting in a “variable pressure” physical condition, or “constant pressure”, resulting in a “variable normal force” physical condition. Consequently, results of micro-abrasive wear generated under these tribological conditions can help engineers design materials and tools with a longer working life span when they are working under mechanical and metallurgical manufacturing processes. Observing the relevance of this subject, the purpose of this work is to assess the micro-abrasive wear tribological properties of H10 tool-steel submitted to experimental conditions of “constant normal force ⇒ variable pressure” or “constant pressure ⇒ variable normal force”. Ball-cratering wear tests were conducted on quenched and tempered AISI H10 tool-steel; the counter-body was a quenched and stress-relieved AISI 52100 bearing steel sphere and the abrasive slurries were prepared mixing abrasive particles of silicon carbide with distilled water. During the micro-abrasive wear tests, the abrasive slurry was, continuously, agitated and fed between the sphere and the tested material; values of coefficient of friction were, simultaneously, acquired, as well. After the tests, each wear crater was analyzed by optical microscopy, SEM and CAD software. The results were discussed based on the behavior of the “Steady-State Wear Regime”, “wear volume”, “wear rate”, “micro-abrasive wear modes”, “pitch between grooves”, “coefficient of friction” and, finally, validated by ANOVA. With the attainment of the Steady-State Wear Regime, it was possible to present the following conclusions: (i) wear volume and wear rate increased under conditions of “constant pressure ⇒ variable normal force”, (ii) there was the predominance of “grooving abrasion”, with the action of “micro-rolling abrasion” wear phenomenon, (iii) the pitch between grooves presented, approximately, the same value of the average abrasive particles size and (iv) the coefficient of friction remained in a tribological condition classified, statistically, as “constant”.

Dynamic simulation model for different type of wheeled mobile robotic drives on leveled surface

Dimensions of wheel base, payload weight and geometry, and type of drive configuration are some of the major factors to be consider during prototyping of a mobile robotic drive. For analysing the effect of such factors, a dynamic simulation model valid for any drive configuration using simple, omni or mecanum wheels is discussed in this paper. Sub-systems to take into account the inertia, motor characteristics normal force, friction force and wheel slip are discussed. Preliminary simulation results for path anticipation, for a control algorithm, for a four wheels omni drive robot with uneven mass distribution and variation of normal force due to acceleration of the robot are also shown in this paper. The simulation model is developed in Simulink/MATLAB.

Fretting test rig with variable normal force

Fretting is small amplitude reciprocating sliding between surfaces, and it may quickly causes surface cracks, which can continue growing under cyclic loads, until the structure breaks entirely as a result of the fretting fatigue. Fretting can also produce hardened wear particles as a result of adhesive wear, which then accelerates abrasive wear. In this case, the community uses the term fretting wear. The design of heavily loaded contacts, susceptible to fretting, is a difficult task because there is no generally accepted design guide. More extensive fretting research is needed to create them. This paper introduces detailed design phases for a equipment (rig) for a variable normal force fretting test. Supporting high radial and normal forces such that there is minimal run-out between the specimens was the most significant design challenge. The combination of a hydrostatic radial bearing and elastic torque shaft was selected for the detail design phase based on FE-analyses, calculations, and overall evaluation. The frame of the test rig consists of the main frame, which supports mainly the normal force and two torque frames, which support torque cylinders. Many solutions, which were found to be working in the current "ring-ring" apparatus of Tampere University, could be utilized in the new test rig like the tapered connections of the specimens, the elastic rod of the torque lever, axial displacement plate, and contact pressure adjustment system. The designed test rig enables fretting tests with 0 Hz to 20 Hz cycle frequency so that normal and tangential force or displacement can be controlled independently of each other. The normal force cannot change from compression to tension dynamically, but the adhesive force of the contact can be measured by slowly increasing the tension force. The designed fretting test rig fulfills all essential requirements, which were set.

Safety distance model for longitudinal collision avoidance of logistics vehicles considering slope and road adhesion coefficient

With the acceleration of urbanization process, the country’s strong support for the healthy development of the logistics industry has made urban logistics become a hot topic in recent years. With the increase in the number of logistics vehicles, traffic accidents have become more frequent. Intelligent vehicle collision avoidance system is an important part of advanced safety technology. To increase veracity and practicability of logistics vehicle safety collision avoidance, this paper presents a safety distance model for longitudinal collision avoidance of logistics vehicles considering road slope and road adhesion coefficient. Based on the vehicle kinetic theory, the information of surrounding environment for the vehicle is obtained using environment sensing system adequately, a method is designed to estimate the road slope and road adhesion coefficient. Combined with vehicle dynamics and tire normal force variation, the road slope was estimated. Based on the relationship between slip rate and adhesion coefficient, the Least Square Method is used for multivariate fitting to obtain the relationship between rolling resistance coefficient and road adhesion coefficient, estimate the road adhesion coefficient, and the maximum deceleration of vehicle braking is modified. Therefore, the safety distance model is established. In order to verify the accuracy and effectiveness of the model, three cases are designed for verification: case I is verification of the safety distance model considering the slope factor; case II is verification of the safety distance model considering the road adhesion factor; case III is the safety distance verification considering both the factors of slope and road adhesion coefficient. The result shows that it is necessary to take the factors of road slope and adhesion coefficient into the safety distance model to improve the accuracy of the model.

Design, modeling, and performance of a bidirectional stick-slip piezoelectric actuator with coupled asymmetrical flexure hinge mechanisms

A stick-slip piezoelectric actuator with bidirectional motion is proposed and measured, which uses coupled asymmetrical flexure hinge mechanisms and symmetrical indenter to generate controllable tangential displacement. The operating principle of the proposed stick-slip actuator is illustrated, and the normal force variation between the stator and slider is analyzed. A dynamic model based on the method of dimensionality reduction is established to simulate the displacement and load capacity. In order to obtain improved actuator properties, the design rules of the coupled flexure hinge mechanisms are discussed, and the tangential and normal displacements of the indenter are investigated by the finite element method. A prototype is fabricated, and the experiment investigation of the actuator characteristics is presented. Testing results indicate that the actuator achieves the maximum output velocity of 10.14 mm/s and its maximum load reaches 1.5 N under a voltage of 100 Vp–p and a frequency of 850 Hz in the positive x-direction. The maximum efficiency of the actuator is 0.57% with a load of 90 g, a locking force of 5 N, and the actuated velocity of 5.48 mm/s. In addition, experimental results confirm the feasibility of the presented model by comparing numerical simulation results.

Numerical verification of variable friction cladding connection for multihazard mitigation

The motion of cladding systems can be leveraged to mitigate natural and man-made hazards. The literature counts various examples of connections enhanced with passive energy dissipation capabilities at connections. However, because such devices are passive, their mitigation performance is typically limited to specific excitations. The authors have recently proposed a novel variable friction cladding connection capable of mitigating hazards semi-actively. The variable friction cladding connection is engineered to transfer lateral forces from the cladding element to the structural system. Its variation in friction force is generated by a toggle-actuated variable normal force applied onto sliding friction plates. In this study, a multiobjective motion-based design methodology integrating results from the previous work is proposed to leverage the variable friction cladding connection for nonsimultaneous wind, seismic, and blast hazard mitigation. The procedure starts with the quantification of each hazard and performance objectives. It is followed by the selection of dynamic parameters enabling prescribed performance under wind and seismic loads, after which an impact rubber bumper is designed to satisfy motion requirements under blast. Last, the peak building responses are computed and iterations conducted on the design parameters on the satisfaction of the motion objectives. The motion-based design procedure is verified through numerical simulations on two example buildings subjected to the three nonsimultaneous hazards. The performance of the variable friction cladding connection is also assessed and compared against different control cases. Results show that the motion-based design procedure yields a conservative design approach in meeting all of the motion requirements and that the variable friction cladding connection performs significantly well at mitigating vibrations.

Study on the manoeuvring characteristics of axisymmetric underwater vehicles with different slenderness and control fin aspect ratios

This paper presents the effect on the manoeuvring characteristics of an axisymmetric underwater vehicle due to its body slenderness ratio and aft fins. The vehicle is a slender body of revolution with elliptical nose, tapered tail and cylindrical middle body. Five vehicle lengths with slenderness ratio from 10.8 to 14.4 with two sets of aft cruciform fins with different aspect ratios are employed for this study. The effect of manoeuvrability is evaluated by simulation of standard manoeuvres using 6 degrees-of-freedom (6DOF) equations of motion. The variation of normal force and pitch moment at oblique flows/pitch rates due to slenderness ratio and aft fin are evaluated by experimental and numerical methods. It is seen that the straight line stability in horizontal plane improves with slenderness ratio and aft fins size.

Characteristics of the fluid–structure interaction within Darrieus water turbines with highly flexible blades

Vertical-axis water turbines (VAWT), the target of the present study, provide a higher area-based power density when used in arrays, and are a promising alternative to horizontal-axis hydro-kinetic turbines (HAWT). Nevertheless, they operate under highly dynamic conditions near or even beyond dynamic stall at their best-efficiency-point. The abrupt loss of lift and strong increase of drag associated with hydrofoil stall can produce cyclic loads and possible damage of turbomachines due to material fatigue.The effect of flexible structures in a highly dynamic flow regime including separation and stall is here studied systematically in an experimental setup which permits observations of all regimes ranging from quasi-static state up to the occurrence of deep dynamic stall and beyond. The process is studied using a surrogate model consisting of an oscillating NACA0018 hydrofoil in a closed water channel, following a motion law comparable to the real angle of incidence of a Darrieus turbine blade along its rotation. The investigated parameters are the oscillation frequency and tip speed ratio, for one rigid as well as for three flexible hydrofoils of different stiffnesses. The coupling process is therefore investigated for multiple machine designs and working points. Lift and drag measurements have been carried out in a systematic manner.Results show that at tip speed ratios for which highly dynamic flow regimes occur, flexible blades provide not only higher thrust, but also reduced normal forces and reduced peak-to-peak cyclic normal force variations. This reduction of stress loads would translate into significantly increased turbine lifetime. This supports the need for further investigations in order to identify optimal blade flexibility and check further turbine designs.

Variable friction cladding connection for seismic mitigation

Cladding systems are conventionally designed to serve an architectural purpose and provide environmental protection for building occupants. Recent research has been conducted to enhance structural resiliency by leveraging cladding systems against man-made and natural hazards. The vast majority of the work includes the use of sacrificial cladding panels and energy dissipating connectors. These passive protection systems, though effective, have typically targeted a single hazard one-at-a-time because of their limited frequency bandwidths. A novel semi-active friction connection has been previously proposed by the authors to leverage the cladding motion for mitigating blast and wind hazards. This semi-active friction device, termed variable friction cladding connection (VFCC), is designed to laterally connect cladding elements to the structural system and dissipate energy via friction. Its variable friction force is generated onto the sliding friction plates upon which a variable normal force is applied via actuated toggles. Because of its semi-active capabilities, the VFCC could be used over wide-band excitation frequencies and is thereby, an ideal candidate for multiple hazard mitigation. The VFCC in its passive in situ mode has been previously designed to mitigate air-blast effects towards the structure and its semi-active scheme has been applied to wind hazard mitigation. In this paper, a motion-based design (MBD) procedure is developed to apply the VFCC to seismic hazard mitigation, completing its application against multiple hazards. The MBD procedure begins with the quantification of seismic load and performance objectives, and afterwards, dynamic parameters of the cladding connection are selected based on non-dimensional analytical solutions. Simulations are conducted on two example buildings to verify and demonstrate the motion-based design methodology. Results show the semi-actively controlled VFCC is capable of mitigating the seismic vibrations of structures, demonstrating the promise of the semi-active cladding system for field applications.

Variation in aerodynamic coefficients with altitude

Precise aerodynamics performance prediction plays key role for a flying vehicle to get its mission completed within desired accuracy. Aerodynamic coefficients for same Mach number can be different at different altitude due to difference in Reynolds number. Prediction of these aerodynamics coefficients can be made through experiments, analytical solution or Computational Fluid Dynamics (CFD). Advancements in computational power have generated the concept of using CFD as a virtual Wind Tunnel (WT), hence aerodynamic performance prediction in present study is based upon CFD (numerical test rig). Simulations at different altitudes for a range of Mach numbers with zero angle of attack are performed to predict axial force coefficient behavior with altitude (Reynolds number). Similar simulations for a fixed Mach number ‘3’ and a range of angle of attacks are also carried out to envisage the variation in normal force and pitching moment coefficients with altitude (Reynolds number). Results clearly depict that the axial force coefficient is a function of altitude (Reynolds number) and increase as altitude increases, especially for subsonic region. Variation in axial force coefficient with altitude (Reynolds number) slightly increases for larger values of angle of attacks. Normal force and pitching moment coefficients do not depend on altitude (Reynolds number) at smaller values of angle of attacks but show slight decrease as altitude increases. Present study suggests that variation of normal force and pitching moment coefficients with altitude can be neglected but the variation of axial force coefficient with altitude should be considered for vehicle fly in dense atmosphere. It is recommended to continue this study to more complex configurations for various Mach numbers with side slip and real gas effects.

Study of the control process and fabrication of microstructures using a tip-based force control system

The atomic force microscopy tip-based nanomechanical machining method has already been employed to machine different kinds of nanostructures with the control of the normal force of the tip. The previous studies verified the feasibility of the nanomachining approach with the force control. However, there are still some shortcomings of small normal force, small machining scale, high cost and low machining efficiency. Therefore, in this study, a tip-based micromachining system with normal force closed-loop control is established based on the principle of atomic force microscopy. The control parameters are optimized based on an analysis of the control process to enable the production of a constant normal force during machining when using a tip tool. The maximum machining velocity that can be attained using this system while maintaining a constant normal force is obtained based on an analysis of the normal force variations during machining. By controlling nanoscale accuracy and high-precision stage, more complex microstructures, including microsquares, millimeter-scale microchannels and three-dimensional step microchannels, are successfully fabricated using the proposed force control method. Experimental results show that the tip-based normal force control method is a simple, low-cost and versatile micromachining method with the potential ability to machine more complex structures and is likely to find wider applications in the micromachining field.

Modeling and analysis of stick-slip motion in a linear piezoelectric ultrasonic motor considering ultrasonic oscillation effect

Piezoelectric ultrasonic motor is an electromechanical coupling system, which is driven by the interface friction between the stator and the rotor/slider. A novel friction model is developed to study the stick–slip motion in a linear piezoelectric ultrasonic motor using longitudinal and bending modes. In this model, the influence of the ultrasonic oscillation on the dynamic friction coefficient is taken into account, which is described by a friction law related to the vibration amplitude at the interface. In addition, the friction contact model involves intermittent separation non-linearity induced by the variable normal force, which can result in complex stick–slip–separation motion at the contact interface. Analytical transition criteria between stick and slip are established by using the switch model. Based on the developed model, numerical simulations are performed to analyze the force transmission between the stator and the slider involving stick–slip motion. A transformed phase plane is proposed to study various states of the contact interface (stick, slip and separation), on which slip–separation, stick–slip, stick–slip–separation, and pure stick motions are intuitively characterized. The influence of some input parameters on the stick–slip motion is analyzed. Furthermore, the low-voltage dead-zone behavior is clarified by stick motion.

A Dynamical Model to Analyze the Influence of Sliding Friction on Motion on a Curve—An Analytical Method

To demonstrate the influence of sliding friction of motion on a curve, a circular path is considered for simplicity on which a person slides from the highest point to the lowest point. A slide which represents a quadrant of radius 5 m and a person of mass 60 kg are considered for comparison in this paper. A Differential equation for motion considering the fact that the normal force depends both on the sin component of weight and also on the tangential velocity, is established and is solved using integrating factor method, and the motion is analysed for different surface roughness of the slide and is compared using superimposed graphs, also the limiting value of friction coefficient at which the person just exits the slide is determined. The correction factor for exit velocity with friction as compared with the exit velocity for zero friction is determined. The fraction of energy lost to friction at the exit is evaluated. The Variation of normal force with the position of the person on the slide is plotted for different surface roughness of the slide, and the position on the slide where the normal force or the force experienced by the person is maximum, is determined and hence its maximum value is evaluated for different surface roughness. For simplicity, a point contact between the body and the slide is considered.

Analysis of squeeze flow of fluids between solid and porous surfaces

Squeeze flow of fluids between surfaces is important in rheology, material processing, lubrication and biomedical applications. The surfaces for squeezing can be solid and/or porous. In this work, the squeeze flow between a porous surface and a solid surface is examined with Newtonian model fluids and model fabrics used in composite processing. Given that the fluid is expected to impregnate the fabric, the permeability and the wettability of the fluid–fabric combination are of utmost importance. Constant velocity squeeze flow experiments were carried out with polyester and polyol resins, while glass fabrics with different sizings were used as the porous surfaces. To understand the variation of the normal force during squeeze flow, its scaling with squeeze gap is evaluated. The scaling is observed to vary from −3 to −1 for different squeeze flow experiments. Squeeze flow theories are used to examine the normal force variation for different fluid–fabric combinations. It is shown that a slip based squeeze flow model and a permeability based squeeze flow model can be used to understand the wettability and the permeability of a fabric with a specific fluid.

A Semi‐active piezoelectric friction damper

SummaryThis research investigates the development of a semi‐active piezoelectric friction damper for controlling the seismic response of large‐scale structures. The proposed device is made of Duplex steel and leads to high friction capacity, which can be developed either in passive or semi‐active modes. For the later, piezoelectric actuators react against a stiff clamping system and apply a variable normal force on the multiple contact surfaces. To validate the design, a prototype, which contact surfaces were made of stainless steel and brake pad material, was built and tested in both friction modes. Moreover, an analytical model of the damper was developed to estimate the performance of the piezoelectric actuators within the clamping system. Experimental results showed that the proposed device achieves a force range factor of 1.9. These experimental results also compare well with those obtained from the analytical model of the damper. Copyright © 2014 John Wiley & Sons, Ltd.