Friction Force Characteristics Research Articles

Study on the Effect of Starved Lubrication on the Dynamic Characteristics of Locally Failed Roller Bearings

When the lubricant is not enough to maintain the ideal lubrication state of the bearing, the deep groove ball bearing will enter the starved lubrication state, which leads to the change of the vibration response of the deep groove ball bearing with localized defects. Previous bearing dynamics models mainly consider the lubrication state as an ideal lubrication condition, which may not reveal the effect of starved lubrication on the vibration characteristics of defective bearings. In this paper, the internal interaction of the system under starved condition is taken into account, and a dynamic model of defective deep groove ball bearings with insufficient lubricant is established to investigate the effects of different starved lubrication degrees on the contact force, friction force, cage speed, and system vibration characteristics. The results show that the lubricant insufficiency significantly changes the contact force inside the bearing, and the increase in friction caused by the increase in the degree of starved lubrication leads to the suppression of the degree of bearing slipping. The root mean square (RMS) value of the time-domain signal and the outer ring fault frequency both increase with the increase of the starved degree. The results of the study are helpful for fault diagnosis and condition monitoring of related equipment.

Mechanical modeling of friction phenomena in social systems based on friction force

Friction is not an inherent attribute in natural science. Indeed, there are various friction phenomena present in social systems. In this study, the researchers developed the general concept of social friction and distinguished the two-dimensional structure of explicit-implicit social friction phenomena. Specifically, explicit friction includes institutional friction, economic friction, behavioral friction, and migration friction. Implicit friction includes cultural friction, cognitive friction, interpersonal friction, technological friction, and information friction. Physical theories such as friction force, classical mechanics, and functional principles have been introduced to construct a social friction force model. This paper describes the emergence and evolution of social friction phenomena from the mechanics perspective and clarifies the various characteristics of social friction force in different areas. This study also provides a new theoretical perspective for examining social friction phenomena and adds innovative content to intersection studies in physics and the social sciences.

Experimental and simulation study of the dynamic characteristics of friction force under third-body intrusion behaviour

Although the noise and wear of frictional pairs in third-body behaviour have been widely researched, the dynamic characteristics of the frictional force under third-body intrusion behaviour are seldom studied. In this study, a spring–block structure driven by a conveyor belt with third-body particles is established. The results indicate that the interference of third-body particles changes the positive correlation between the frictional force and velocity to a negative correlation. A multi-contact frictional model is established using a cellular automata modelling approach to estimate the distribution of particles. The results reveal that this model can effectively predict the negative correlation under particle intrusion. This correlation may be attributed to the decrease in the third-body particle number with velocity. SiO2 third-body intrusion complicates the frequency characteristics; however, the intrusion of PVC particles lead to a smaller amplitude of the frictional force. Altering the size of the interface has a certain effect on the fluctuation mode of stick–slip under an extremely small surfaces separation.

Numerical investigation on hydrodynamic performance of a novel shaftless rim-driven counter-rotating thruster considering gap fluid

Shaftless rim-driven thruster (RDT) has recently become the research focus for marine propulsion, primarily due to low vibration, low noise, and energy saving as its advantage. This study is based on CFD theory and used the Ansys-Fluent software to examine the hydrodynamic performance of a novel rim-driven counter-rotating thruster (RDCRT). It takes a No.19A+Ka4-70 duct propeller and a 20 kW RDT as examples, as it verifies the feasibility of the simulation method. It establishes three geometric models for RDCRT's hydrodynamic performance to determine whether it is necessary to consider the motor stator/rotor gap. It examines the flow distribution characteristics of the gap fluid friction force and flow channel and investigates the gap's influence on the hydrodynamic performance. Relevant case studies indicate that, when considering the gap, the calculation outcomes of the simulation model are between the stationary model and the rotational model of the rotor inner wall when ignoring the gap. In the Forward and Aft regions, the total frictional power of the gap channel correspondingly accounts for 1.7% and 1.35% of the rated power. Additionally, compared to situations with a gap, the pressure coefficient of the inner surface of the Forward and Aft rim without a gap is more significant. Thus, the hydrodynamic simulation model should not ignore the gap. For the RDCRT, the thrust coefficient, the torque coefficient, and the maximum efficiency value are more significant than those of the single-propeller RDT, hence validating its advantages.

Investigating the frictional force distributions and inter-stage imbalance of a two-stage brush seal

Although a two-stage brush seal (TSBS) plays an important role in gas turbine machinery, few studies have investigated the complex flow and tip frictional force distributions of the TSBS. This article contributes to the ongoing investigation of the TSBS by proposing a 3D slice model employing computational fluid dynamics to analyze the flow and tip frictional force characteristics of a contacting TSBS using the torque balance principle and linear superposition methods. Our calculations reveal that as the inter-stage imbalance of the pressure decreases, so do the aerodynamic and tip frictional forces between the upstream and downstream bristle packs. In addition, the effects of the number of bristle axial rows and protection clearance height on the inter-stage imbalance are analyzed. The results indicate that the TSBS exhibited significantly less leakage and lower aerodynamic and tip frictional forces than a single-stage brush seal. However, the inter-stage pressure differential results in an inter-stage imbalance in the aerodynamic and tip frictional forces that can be aggravated under high pressure conditions. This study determined that reasonable adjustments can be made to the number of bristle axial rows and protection clearance heights of the upstream and downstream stages to alleviate this inter-stage imbalance. The findings of this study contribute to the design and implementation of more efficient TSBSs.Although a two-stage brush seal (TSBS) plays an important role in gas turbine machinery, few studies have investigated the complex flow and tip frictional force distributions of the TSBS. This article contributes to the ongoing investigation of the TSBS by proposing a 3D slice model employing computational fluid dynamics to analyze the flow and tip frictional force characteristics of a contacting TSBS using the torque balance principle and linear superposition methods. Our calculations reveal that as the inter-stage imbalance of the pressure decreases, so do the aerodynamic and tip frictional forces between the upstream and downstream bristle packs. In addition, the effects of the number of bristle axial rows and protection clearance height on the inter-stage imbalance are analyzed. The results indicate that the TSBS exhibited significantly less leakage and lower aerodynamic and tip frictional forces than a single-stage brush seal. However, the inter-stage pressure differential results in an inter-stage i...

Effects of friction models on simulation of pneumatic cylinder

Abstract. This study examines effects of three friction models: a steady-state friction model (SS model), the LuGre model (LG model), and the revised LuGre model (RLG model) on the motion simulation accuracy of a pneumatic cylinder. An experimental set-up of an electro-pneumatic servo system is built, and characteristics of the piston position, the pressures in the two-cylinder chambers and the friction force are measured and calculated under different control inputs to the proportional flow control valves. Mathematical model of the electro-pneumatic servo system is derived, and simulations are carried out under the same conditions as the experiments. Comparisons between measured characteristics and simulated ones show that the RLG model can give the best agreement among the three friction models while the LG model can only simulate partly the stick-slip motion of the piston at low velocities. The comparison results also show that the SS model used in this study is unable to simulate the stick-slip motion as well as creates much oscillations in the friction force characteristics at low velocities.

Wear Characteristic Evaluation of Electrical Discharge Machined Ti6Al4V Surfaces at Dry Sliding Conditions

The low tribo-characteristics of Ti6Al4V (Ti64) have delimited its range of applicability. To elucidate the underlying mechanism, the friction and wear behavior of Ti64 at elevated pin speeds (200 rpm, 300 rpm and 400 rpm), constant load (50 N) and temperature (ambient) conditions were initially evaluated. The wear, coefficient of friction and frictional force characteristics of the material were identified to follow incessant unsteady transitions, during test run. The pin speed of 300 rpm marked the highest wear, governed by the frictional heating, asperity flattening, centrifugal forced clearance (of the wear track) and third-body abrasion-assisted delamination. The reduced wear trend at 200 rpm and 400 rpm could be attributed to the protective coatings formed by tribo-oxidation and mechanically mixed layer, respectively. Further, the electrical discharge machined Ti64 samples were developed and subjected to friction and wear analyses at varying loads of 50 N and 100 N. In contrast, at similar testing conditions (load 50 N, speed 200 rpm, temperature ambient, sliding distance 1000 m), the electrical discharge machined Ti64 (with a peak wear value of 220 μm) showed enhanced tribo-characteristics compared to that of bare Ti64 (with a peak wear value of 370 μm). The improved tribo-behavior of the former was due to the presence of hard, fine-grained and non-etchable recast layer, which got spalled off at an increased load of 100 N.

Performance characteristics of compressed air-driven free-piston linear generator (FPLG) system – A simulation study

The integrated model of the two-stroke free-piston engine system driven by compressed air is established using MATLAB/Simulink software and the relationship and the influence of the reciprocating motion, friction force, cylinder process and electromagnetic characteristics of permanent magnet linear generator are fully investigated. The piston displacement, velocity, acceleration, thrust, air pressure in cylinder and generator voltage and their relationship are simulated and analyzed, and the motion performance, the characteristics and interaction relationship of the system are obtained. The experimental platform of air driven free-piston linear expander system is tested. Simulation results are compared with experimental data and the whole model is validated. The developed model is then used to study the influence of piston assembly mass, the timing of air intake and exhaust on piston motion characteristics. The peak voltage output, voltage harmonic distortion and efficiency of permanent magnet linear generator under different air pressure and the output power and efficiency with pressure of FPLG under different loads are obtained. The results show change rules of piston displacement, velocity and frequency changes with air pressure, piston assembly mass. Finally from the simulation result, the air pressure and air intake position of highest efficiency of the linear generator are found.

Microstructural and Wear Characteristics of Friction Stir Processed Al-7075/SiC Reinforced Aluminium Composite

Abstract Friction stir processing (FSP), a solid-state surface modification technique, derived from friction stir welding (FSW) process has been evolved as an efficient energy processing technique to fabricate aluminium matrix composites (AMCs). In the present investigation, FSP was used to fabricate 7075-T651 aluminium alloy surface composites using micro sized silicon carbide (SiC) particles as reinforcement. Since Aluminium 7075 alloy has a vast applications in the automotive and aerospace and having the highest strength among all commercial Al-alloys, has been selected for the matrix phase. AMCs were fabricated at different combinations of rotational speed and welding speed. The different combinations were tested for single and triple pass. The fabricated surface composites were examined for microstructure using SEM equipped with EDXS. The micro-hardness was measured and sliding wear behaviour was also evaluated using a pin on disc apparatus. It was observed that the average hardness of the FSP zone composite was 1.65-2.15 times higher than that of parent metal (78VHN). Wear rate along with the frictional force characteristics were found to be promising than the parent metal.

The Statistical Characteristics of Static Friction

Friction is one of the fundamental issues in physics, mechanics and material science with many practical applications. However, the understanding of macroscopic friction phenomena from microscopic aspect is still on the way. In this paper, molecular dynamics simulations are performed to investigate the static friction between two planar crystal surfaces. The friction force experienced by each atom is tracked and the statistical characteristics of atomic friction force are illuminated. More importantly, the influences of normal load and temperature on the statistical features are generalized. This study provides a new insight on the micro-states of friction.

Experimental Estimation of Friction and Friction Coefficient of a Lightweight Hydraulic Cylinder Intended for Robotics Applications

Recently, hydraulic actuator has been used in several engineering applications such as: aeronautics, construction and robotics. This is due to the need of high torque and power density in such engineering applications. Despite these advantages, hydraulic actuators are fabricated from metallic materials, which provoke their heavy weight, which necessitate the development of a lightweight hydraulic actuator, fabricated of composite materials. Using composite materials in hydraulic cylinders, it is important to study the friction force characteristics and to estimate the friction coefficient between composites and O-rings, which is presented in this paper. This paper deals with the estimation of Coulomb friction and friction coefficient in the lightweight hydraulic cylinder fabricated mainly of composite materials. The actuator is presented by its dynamic equation of motion, where each term is discussed including the stiffness coefficient, the viscous damping coefficient, the kinematics and the pressure parameters. Meanwhile, these coefficients and parameters are obtained according to data recorded from conducted experiments. As a result, the new methodology which uses the experimental measurements combined the dynamic model has succeeded to evaluate the friction inside the hydraulic cylinder which has been estimated and found to be around 166[Formula: see text]N, while the corresponding coefficient of friction is computed (about 0.61 as average value). These results will be important for further optimization of the material choice and actuator design, which will help in the amelioration of the hydraulic cylinder.

Investigation of nano-scale scratch and stick-slip behaviors of polycarbonate using atomic force microscopy

Abstract To investigate the nano-scale scratch and friction behaviors of polycarbonate (PC), atomic force microscopy (AFM) was utilized to conduct the nano-scratch tests under different normal loads with a probe of 20 nm tip radius. Then a probe of 1 nm tip radius was employed for the scanning of scratched surface morphology. The stick-slip phenomena are found for all studied normal load levels. The nano-scratch deformation at different normal loads can be categorized into three modes: sliding on asperities, elastic deformed groove and permanent scratch groove. After the analysis of amplitude-frequency characteristics of friction force, a good correlation is found among scratch deformation, friction force and stick-slip. Those findings can be a good guidance to understand and improve the nano-scratch performance of polymers.

Effect of surface conditions of one-side patterned electrodes applied to electrorheological gel

The use of a patterned electrode on one side only is expected to be a suitable solution for omitting the wiring to the movable electrode using electrorheological gel. Electrorheological gel is a functional material that changes the force absorbance and frictional force characteristics of its surface in response to the magnitude of an applied electric field. One problem in both the design and the application of electrorheological gels is the wiring of the electrodes that must sandwich the electrorheological gel. Thus, a one-side patterned electrode can be a desirable solution. We investigated the shear stress produced by electrorheological gel applied to such a patterned electrode during rotational and translational motion. Variation of the material of the opposing part reveals that the mechanical and electric characteristics of the electrode meaningfully affect the experienced shear stress.

Influences of thermal stability, and lubrication performance of biodegradable oil as an engine oil for improving the efficiency of heavy duty diesel engine

Nowadays, the development of biodegradable products is important in improving energy efficiency and green environment, particularly in energy conservation and rotation of machinery systems. In the case of sliding components, lubrication system and lubricant quality plays an important role for energy efficiency as it is directly involved with frictional force and components wear characteristics. The conventional mineral oil-based lubricant is used for machinery lubrications; however, it is nonbiodegradable and is an environmental pollutant. This investigation attempts to develop biolubricant acquired from various vegetable oils to replace mineral oil-based lubricants. This study evaluates the physicochemical properties, thermal stability, and lubricating and tribological characteristics of olive oil and its comparative analysis with commercial lube oil. A four-ball tribotester was used to measure the friction and wear properties of the sample according to the ASTM 4172 method. Olive oil has an excellent oxidation stability due to the presence of high percentages of oleic acid in fatty acid composition. Olive oil showed higher viscosity index and kinematic viscosity than other vegetable oils; hence, it is better for boundary lubrication. Thermogravimetric analysis showed that olive oil persists thermally steady up to 390°C. Olive oil showed a lower coefficient of friction, wear scar diameter, and worn scar surface area than commercial lube oil. Therefore, due to the better lubricating performance, olive oil has high potential for use as an engine lubricating oil for improving efficiency of heavy-duty engines in the automotive applications.

Numerical simulation of contact force on bi-directional pig in gas pipeline: At the early stage of pigging

Periodic pigging of oil and gas pipelines for dewatering, cleaning and inspecting is a standard industry procedure. Predicting the motion of a pig to estimate its velocity, position and required driving pressure is essential before pigging. To enhance these predictions, the frictional force between pig and pipeline is a key determinant and insufficient understanding of the frictional force characteristics leads to significant inaccuracies. As the frictional force acting parallel to the pipeline wall plane is a function of contact force in the radial direction, a 2D axisymmetric nonlinear finite element model was proposed to predict the contact force experienced by bi-directional pig by using ANSYS. The impacts of four parameters (including interference, thickness, chamfer dimension and clamping rate of the sealing discs) on the contact force and the deflection angle of the sealing disc are studied at different differential pressures over the pig. Results obtained in this study shows that the thickness of the sealing disc has the greatest impact on contact force of the bi-directional pig, followed by the chamfer dimension, then the clamping rate and finally the interference. A linear relationship between the contact force and the differential pressure can also be observed from the results. Research findings on the characteristic of contact force and deflection angle of sealing disc may provide useful information for the formulation of theoretical or mathematical models to predict the frictional force on the pig, and pigging risks may be efficiently reduced by establish mitigating strategies on the basis of prediction.

Experimental investigation of friction behavior in pre-sliding regime for pneumatic cylinder

Friction always presents in pneumatic cylinders and causes difficulties in controlling position and velocity of pneumatic systems. In order to improve the control performance of the pneumatic systems, it is necessary to fully understand behavior of friction in the pneumatic cylinders. So far, dynamic friction behavior of pneumatic cylinders has been investigated but mainly focused on the friction behavior in sliding regime. In pre-sliding regime, friction behavior has not been investigated. In this paper, experimental investigations of friction behavior of a pneumatic cylinder in pre-sliding regime are made. The friction force is calculated from the equation of motion of the piston using the measured values of pressures in the two cylinder chambers and the piston displacement. The pressures are controlled by using two proportional pressure control valve. The friction force versus piston displacement characteristics are measured and analysed under various operating conditions of the applied force and the pressures. Experimental results show that: i) the piston motion in pre-sliding regime exhibits a nonlinear spring behavior; ii) hysteretic behavior with nonlocal memory is verified; iii) the pressures have influence only on the size of the hysteretic loop. These experimental results can be applied to develop a friction model for pneumatic cylinders.

On the Stability and Agility of Aggressive Vehicle Maneuvers: A Pendulum-Turn Maneuver Example

We present a dynamic stability and agility study of a pendulum-turn vehicle maneuver. Instead of optimizing the controlled inputs to mimic expert human driver performance, we focus on understanding the stability and agility performance of the vehicle motion using professional racing car driver testing data. We propose to use the rear side slip angle, rather than the vehicle mass center side slip angle, as one state variable to obtain the precise stable region. A hybrid physical/dynamic tire/road friction model is used to capture the dynamic friction force characteristics. We also introduce the use of vehicle lateral jerk and acceleration information as the agility metrics to compare maneuvering performance under the racing car driver and a typical human driver. The analysis and testing results show that during the pendulum-turn maneuvers, the professional driver operates the vehicle outside the stable regions of the vehicle dynamics to achieve superior agility performance than that under typical human driver control. Comparison results also show that the racing car driver outperforms in both the traveling time and the agility metrics. It is ongoing work to design a control strategy for autonomous aggressive maneuvers by using the new stability and agility results presented in this paper.

Time-delayed feedback control of friction-induced instability

Time-delayed feedback control of different types of friction-induced instabilities is discussed. Mainly, three well-known mechanisms of friction-induced instabilities are considered: (1) Stribeck instability governed by velocity-weakening characteristics of friction force, (2) mode-coupling instability, and (3) sprag-slip instability. Control force is constructed utilizing the delayed difference of a single measurable state (displacement here) as proposed by Pyragas. Regions of local stability of the equilibrium are constructed in the plane of control parameters (gain and delay). Global stability regions are also constructed for the Stribeck type instability. A comprehensive methodology for selecting the control parameters is discussed. Analytical results are substantiated by direct numerical simulation.

Friction and wear of resin-based dental materials

Commercially available resin-based dental composites were studied. Their friction and wear kinetics were determined, together with the microhardness of materials. Tribological examinations evidenced that friction force characteristics are different for various composites – equilibrating slower or faster. However, in some cases, friction may also present a constant value during the whole experiment. The friction force characteristics have been found being related to abrasion kinetics of dental composites. Microindentation experiments provided interesting information on the hardness of dental composites. It has been found, that in the most cases, composites studied exhibit the surface gradient of hardness. Generally, the harder and the stiffer the material the higher its abrasion, what follows a micromechanical model of friction.

Dynamic Behavior of Feed Drive Systems around Quadrant Changes in Circular Motion

This paper describes the dynamic behavior of feed drive systems at quadrant changes in circular motion, and then discuss about the generation model of quadrant glitches, which are often seen on the circular trajectories of NC machine tools. In this paper, the velocity and acceleration of the axis under circular interpolation motion were precisely measured by means of linear scales and servo-accelerometers, respectively. The velocity and acceleration at quadrant changes are simulated by a dynamic model which has two degrees of freedom. In addition, a friction model is newly proposed by considering the non-linear characteristics of friction force. From the experimental and simulation results, it is found that the proposed model well expresses the dynamic behavior of the axes, and that any axis does not stay at zero velocity when the motion direction changes. The Influence of the inertia of the feed drive mechanisms on quadrant glitches is also discussed through simulation results.