Measurement Of Friction Coefficient Research Articles

Exploiting Turmeric’s Coloring Capability to Develop a Functional Pigment for Wood Paints: Sustainable Coloring Process of Polyamide 11 Powders and Their Strengthening Performance

Currently, the wood coatings industry is focusing on creating unique, vibrant finishes using new functional pigments. Simultaneously, there is a growing adoption of eco-friendly bio-based materials, reflecting trends in other sectors and supporting the circular economy. Thus, the aim of this study is to unveil a straightforward, cost-effective, and notably sustainable process for exploiting the coloring potential of turmeric powder and coloring polyamide 11-based fillers, employed as multifunctional pigments for wood coatings. Through the incorporation of this additive into a wood paint, the study demonstrates its dual effect of enhancing the aesthetics of the final composite layer while leveraging the beneficial protective properties inherent to polyamide 11. The impact of these additives on sample aesthetics is assessed through optical observations, as well as measurements of color, gloss, and surface roughness. The strengthening contribution of the functional pigment is evaluated using the Taber abrasion resistance test, static friction coefficient measurements, and Buchholz surface hardness test. Finally, the aesthetic consistency of the bio-based filler and the coloring efficiency of the sustainable process are tested by subjecting the samples to aggressive conditions, including the UV-B chamber exposure test, cold liquids resistance tests, and water uptake test. Ultimately, the study illustrates how this functional bio-based pigment not only provides sufficient protection but also meets current eco-requirements, thereby contributing to the sustainability of the wood coatings industry.

Operational risk management – Measurement and evaluation of friction coefficients for geotextile sandbags for flood protection

AbstractThe present work focuses on the friction coefficient for various geotextile sandbag structures especially dikes under different conditions. Based on the review of existing investigations there are several gaps to close. Former investigations do not include the whole scope of needed surfaces and conditions. Especially the friction coefficients for polypropylene and even for jute as material mainly used for small geotextile sandbags are not fully available. Additionally, the influence of the surface moisture is not consequently investigated, particularly for jute. Due to this, eleven test series (three with sandbag on sandbag configuration and eight with sandbag on ground surface) with 780 single readings are finalized. In comparison to former studies there are results for surface combinations of good harmonization and those with differences. A comparison of the earlier studies already shows different results Summarized it can be stated that the presented results are representative friction coefficients for calculation of realistic sandbag structures e. g., sandbag dikes or spring cascades.

Effect of orientation and hardness of rubber tread block on the friction coefficient under glycerol lubrication and its underlying mechanisms

This study examines the influence of orientation and hardness of rubber tread blocks on friction under glycerol lubrication, focusing on negative fluid pressure. Measurements of fluid pressure, film thickness, and friction coefficient were conducted at the contact interface. Results showed that when tread blocks aligned parallel to the sliding direction, the friction coefficient increased by 57%–191%, attributed to either reduced film thickness or enlarged real contact area from greater negative fluid pressure. Conversely, with a perpendicular orientation, decreased hardness led to increased friction, due to thinner fluid films from enhanced negative pressure.

Safeguarding public facilities: a study on enhancing safety in mosque amenities through slip-resistance and surface innovations

PurposeThis study critically examines the safety aspects of ablution spaces in United Arab Emirates (UAE) mosques, emphasising the evaluation of smooth ceramic tile floorings prone to water accumulation. As an integral practice in Islamic worship, ablution spaces cater to a diverse demographic, making their safety a paramount concern. This study aims to meticulously assess slip-resistance properties and surface characteristics to provide practical recommendations for enhanced safety measures.Design/methodology/approachThe investigation involves in situ measurements of traction properties and an extensive analysis of surface features in thirty mosques across the UAE. This comprehensive approach allows for identifying the unique challenges posed by ceramic tile floorings in ablution spaces. The study uses a portable tribometer for dynamic friction coefficient measurements, ensuring accurate slip-resistance evaluations. Surface texture analysis uses a portable profilometer to quantify roughness parameters.FindingsThe study’s findings significantly advance the comprehension of safety considerations in ablution spaces. Through empirical evidence and evidence-based insights, the research identifies immediate safety concerns and provides practical recommendations to create secure and inclusive environments within mosques. The emphasis lies on safeguarding the well-being of worshippers and contributing to the broader goal of safety in religious spaces.Originality/valueThe original contribution of this study lies in its dual focus on immediate safety concerns and the deeper understanding of unique safety challenges within ablution spaces. By offering evidence-based insights and practical recommendations, the research strives to make meaningful contributions to creating secure and inclusive environments within mosques.

An analytical friction model for point contacts subject to boundary and mixed elastohydrodynamic lubrication

An analytical EHL model suitable for highly loaded point contacts operating in mixed and boundary regimes of lubrication is presented. The results are compared and validated with the experimental measurements of friction coefficient from an MTM tribometer. Measurements are carried out upon a PAO40 oil at medium to high temperatures. In general, a good agreement in trend and magnitude over a wide range of operating conditions is observed. Both the experimental findings and the model predictions show an inverse relationship between friction and contact load in mixed and boundary regimes of lubrication, an effect not hitherto reported in literature.

Wall shear stress measurement using a zero-displacement floating-element balance

The floating-element (FE) principle, introduced nearly a century ago, remains one of the most versatile direct wall shear stress measurement methods. Yet, its intrinsic sources of systematic error, associated with the flow-exposed gap, off-axis load sensitivity, and calibration, have thus far limited its widespread application. In combination with the lack of standard designs and testing procedures, measurement reliability still hinges heavily on individual judgement and expertise. This paper presents a framework to curb these limitations, whereby the design and operation of a FE balance are leveraged by an analytical model that attempts to capture the behaviour and predict the relative contribution of the systematic sources of error. The design is based on a parallel-shift linkage and a zero-displacement force-feedback system. The FE has a surface area of 200×200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$200\ imes 200\\,$$\\end{document} mm, and measurement sensitivity is adjustable depending on the surface condition and the Reynolds number. It is thus suitable for application in a wide range of low-speed, boundary-layer wind tunnels, small or large scale. Measurements of the skin friction coefficient over a smooth wall show a remarkable agreement with oil-film interferometry, especially for Reθ>1.3×104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\rm Re}_{\ heta } > 1.3 \ imes 10^4$$\\end{document}. The discrepancy relative to the empirical Coles–Fernholz relation (κ=0.39\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\kappa = 0.39$$\\end{document} and C=4.352\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C = 4.352$$\\end{document}) is within 0.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.5\\,$$\\end{document}%, and the level of uncertainty is below 1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1\\,$$\\end{document}% for a confidence interval of 95\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$95\\,$$\\end{document}%.

Low cycle fatigue of doped tetrahedral amorphous carbon coatings by repetitive micro scratch tests

In contrast to the conventional progressive scratch testing where load is increased up to a critical damage level, the repetitive scratch testing is an assessment of coating behavior under repeatedly applied subcritical load. It often represents a better approximation to actual application conditions of well-designed coating systems where overcritical conditions typically do not occur. In this work, the cyclic micro scratch behavior of differently doped amorphous carbon coatings with similar coating thicknesses were studied using repetitive scratch tests at loads that correspond to 40 %, 60 % and 80 % of the critical loads from progressive scratch testing. Depending on the doping element, the structural and mechanical properties changed, in some cases considerably, which also had a clear effect on the test results. The damage progression was examined using optical microscopy, friction coefficient, and residual depth measurements. The progression of failure phenomena was categorized into different stages. Furthermore, the critical number of cycles for coating failure was recorded at the various load levels. For the identification of the critical cycles, the use of the deviation of the friction coefficient from its mean value has proven to be a valuable parameter. The approach of testing at different load levels allows the creation of low cycle fatigue curves to evaluate the coating behavior. It was found that the hardness and residual stresses of the coatings had different effects on the lifetime of the coatings at the various load levels, i.e. higher hardness and residual compressive stresses were only beneficial for moderate stress levels with low plastic substrate deformation. The results demonstrate that the cyclic scratch test is a powerful tool for comparative damage assessment at different load scenarios.

Tribological and Structural Effects of Titanium Carbide and Hexagonal Boron Nitride Reinforcement on Aluminum Matrix Hybrid Composites

AbstractThis research involves the synthesis of a hybrid composite by adding titanium carbide (TiC) and hexagonal boron nitride (hBN) powders in certain weight ratios (2.5–5%) to pure aluminum (Al) powder. When previous studies were examined, it was seen that TiC and hBN powders were added separately to Al matrix powders; however, a hybrid composite was not produced as in this study. The obtained hybrid composites were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD) analysis. Microstructure, hardness and wear tests were carried out under 3 different loads (10 N, 20 N and 30 N) and dry conditions. Weight loss and coefficient of friction measurements were obtained for each hybrid composite during the wear tests. The TiC–hBN-reinforced specimen exhibited a significantly higher hardness value of 37.08% compared to the pure Al composite. It was also found that the synthesized Al–TiC–hBN hybrid composite exhibited a 59% reduction in the wear loss value for 10 N load, 30% for 20 N load and 60% for 30 N load compared to the pure Al sample. It is believed that the hybrid composites produced in this study have the ability to compete with Al matrix materials and exhibit the potential for longer durability and cost reduction in industries that use the production of aluminum parts.

Fatigue life and transmission efficiency prediction of marine timing gears under 3D mixed lubrication state

The timing gears are the core transmission components of marine diesel engine, whose working conditions are complex and harsh, transient and coupled with the microscopic lubrication state, meshing in the mixed lubrication environment with lubrication-contact coexistence, local asperity contact brings stress concentration, further affects the fatigue life and friction loss. In this study, based on a three-dimensional (3D) line-contact mixed lubrication model, dynamic subsurface stress calculation, fatigue life prediction, and friction loss power study were carried out on timing gears, taking into account transient operating conditions and actual surface roughness. For the established simulation model, an equivalent simulation test of line-contact friction of timing gears was carried out, which can realize the measurement of friction coefficient under the condition of different slip-roll ratios, and make up for the deficiency that the traditional test equipment can only carry out the test under the fixed slip-roll ratio. Although the gear meshing process can only be simulated by the disc specimen due to the limitation of the specimen form, this limitation is similar to the simplified form of the simulation model, which can meet the needs of the verification test. The influence of structure and material on the lubrication state, fatigue life and friction power consumption are studied, which provide theoretical guidance for the tribological optimization design and reliability analysis for the marine timing gears.

AFM probe with the U-shaped cross-sectional cantilever for measuring the ultra-low coefficient of friction of 10−6

AbstractAccurately measuring the coefficient of friction (COF) is the fundamental prerequisite of superlubricity research. This study aimed to reduce the COF measurement resolution Δμ of atomic force microscopy (AFM). Based on the theoretical model, a distinctive strategy was adopted to reduce Δμ by optimizing the cantilever’s cross-section of the AFM probe, inspired by civil engineering. Δμ can be reduced by decreasing the width of the horizontal side wR and the wall thickness t and increasing the width of the vertical side wH. Moreover, the I-shape demonstrates the highest reduction in Δμ, followed by the U-shape. Considering the processability, the AFM probe with the U-shaped cross-sectional cantilever was investigated further, and the dimensions are 35 µm wR, 3.5 µm wH, 0.5 µm t, 50 µm l (cantilever length), and 23 µm htip (tip height). The finite element analysis results confirm its reliability. After being fabricated and calibrated, the AFM probe achieves the minimal Δμ of 1.9×10−6 under the maximum normal force so far. Additionally, the friction detection capability of the fabricated AFM probe improves by 78 times compared to the commercial tipless-force modulation mode (TL-FM) AFM probe with the conventional solid rectangular cross-sectional cantilever. This study provides a powerful tool for measuring 10−6 COF.

The effect of C2H2 addition to (cyclopentadienyl) molybdenum (dicarbonyl) (nitrosyl) complex gas system for the tribological property of MoCx/a-C:H films on bearing materials

The (cyclopentadienyl)molybdenum(dicarbonyl)(nitrosyl) complex as a metal-organic precursor and acetylene gas were used to produce the MoCx/a-C:H film by the PECVD. When the acetylene gas is mixed with a vaporized metal-organic precursor, the friction coefficient and hardness would be superior for the MoCx/a-C:H film, whereas the specific resistivity slightly high. The friction coefficient measurements by the ball-on-disk wear tester dramatically reduced from 0.25 to 0.06 as the acetylene flow rate increased. The electrical specific resistivity slightly high from 5.74 mΩ-cm to 203.32 mΩ-cm following the increasing acetylene partial pressure. XPS analysis indicated that the Mo-C bond in the Mo3d peaks diminished, and the sp3/sp2 ratio in the C1s peaks reached 0.37 in the condition of acetylene gas addition.

Nanotribological Characteristics of the Al Content of AlxGa1−xN Epitaxial Films

The nanotribological properties of aluminum gallium nitride (AlxGa1−xN) epitaxial films grown on low-temperature-grown GaN/AlN/Si substrates were investigated using a nanoscratch system. It was confirmed that the Al compositions played an important role, which was directly influencing the strength of the bonding forces and the shear resistance. It was verified that the measured friction coefficient (μ) values of the AlxGa1−xN films from the Al compositions (where x = 0.065, 0.085, and 0.137) were in the range of 0.8, 0.5, and 0.3, respectively, for Fn = 2000 μN and 0.12, 0.9, and 0.7, respectively, for Fn = 4000 μN. The values of μ were found to decrease with the increases in the Al compositions. We concluded that the Al composition played an important role in the reconstruction of the crystallites, which induced the transition phenomenon of brittleness to ductility in the AlxGa1−xN system.

Slip Risk Prediction Using Intelligent Insoles and a Slip Simulator

Slip and fall accidents are the leading cause of injuries for all ages, and for fatal injuries in adults over 65 years. Various factors, such as floor surface conditions and contaminants, shoe tread patterns, and gait behavior, affect the slip risk. Moreover, the friction between the shoe outsoles and the floor continuously changes as their surfaces undergo normal wear over time. However, continuous assessment of slip resistance is very challenging with conventional measurement techniques. This study addresses this challenge by introducing a novel approach that combines sensor fusion technology and machine learning techniques to create intelligent insoles designed for fall risk prediction. In addition, a state-of-the-art slip simulator, capable of mimicking the foot’s motion during a slip, was developed and utilized for the assessment of slipperiness between various shoes and floor surfaces. Data acquisition involved the collection of pressure data and three-axial accelerations using instrumented shoe insoles, complemented by friction coefficient measurements via the slip simulator. The collected dataset includes four types of shoes, three floor surfaces, and four surface conditions, including dry, wet, soapy, and oily. After preprocessing of the collected dataset, the simulator was used to train five different machine learning algorithms for slip risk classification. The trained algorithms provided promising results for slip risk prediction for different conditions, offering the potential to be employed in real-time slip risk prediction and safety enhancement.

Tribological Characterisation and Modelling for the Fused Deposition Modelling of Polymeric Structures under Lubrication Conditions.

In recent years, additive manufacturing technology, particularly in plastic component fabrication, has gained prominence. However, fundamental modelling of the influence of materials like ABS, PC, and PLA on tribological properties in fused deposition modeling (FDM) remains scarce, particularly in non-lubricated, oil-lubricated, and grease-lubricated modes. This experimental study systematically investigates the effects of material type, lubrication method, layer thickness, and infill density on FDM component tribology. A tribology analysis is conducted using a TRB3 tribometer. The results indicate a coefficient of friction (CoF) range between 0.04 and 0.2, generally increasing and decreasing with layer thickness and filler density. The lubrication impact hinges on the material surface texture. The study models the intricate relationships between these variables via full-factor analysis, showing a strong alignment between the modelled and measured friction coefficients (an average error of 3.83%). Validation tests on different materials affirm the model's reliability and applicability.

Microstructure mechanical properties and tribological properties of high entropy M2AlC-MC (M = Ti, V, Zr, Hf, Ta) composites prepared via spark plasma sintering

M2AlC-MC (M = Ti, V, Zr, Hf, Ta) HECs were successfully synthesized via SPS. The microstructure were characterized by XRD, SEM, TEM, EDS, and Maud software was used for Rietveld refinement analysis of phase composition content. The results showed that Ti, V, Zr, Hf, and Ta elements were successfully dissolved in both M2AlC and MC. On this basis, the density, hardness and mechanical properties were tested. The results showed that hardness and mechanical properties of M2AlC-MC HECs were better than those of traditional M2AlC and their composites except for a slight decrease in fracture toughness. For the first time, the tribological properties of M2AlC-MC HECs were studied, and the measured friction coefficient was less than 0.66, and the wear rate was 1.76 × 10−4 mm3 N−1m−1. This work can provide important reference for the subsequent preparation of M2AlC-MC HECs and further expand the development field of friction resistant materials.

The possibilities of utilising the skiddometer T2GO for forensic engineering

The aim of the paper is to verify the applicability of the T2GO skiddometer for measuring the coefficient of friction for forensic practice in accordance with the legislative framework of the Czech Republic. In the introduction, the article discusses the problem of friction coefficient measurement. The results of the comparison with dynamic measuring devices used in the Czech Republic and the determination of the mutual compliance rate with these devices, based on data obtained within the National Comparative Measurement Action, are presented. The article concludes with an interpretation and discussion of the results. In cases where no sufficient mutual compliance was found, an analysis and reasoning of these results is provided. Last but not least, based on the results, a procedure is proposed for future use of the T2GO skiddometer in practice within the legislative conditions of the Czech Republic.

Measurement of road friction coefficient using strain on tire sidewall

If road friction coefficients can be measured in real time, the performance of advanced driver-assistance systems (ADASs), such as anti-lock braking systems (ABS) and automatic braking systems, can be improved. Generally, ADASs use information obtained from wheel speed sensors and acceleration sensors. However, it is difficult to accurately measure the road friction coefficients using these sensors. Therefore, many studies have been conducted to measure road friction coefficients using strains at the inside surface of a tire tread, which are the only places that ground road surfaces. However, sensors installed on the inside surface of a tire tread, where significant deformation occurs locally because of protrusions on road surfaces, can easily be peeled off or damaged. Therefore, this study investigated a method for measuring road friction coefficients using strains on the sidewalls of the tires. If the sidewalls of tires are used instead, stable measurements can be expected because the sidewalls of tires are harder to deform locally compared with the inside surface of a tire tread. First, using an experimental device that simulates the complete slip state of a tire, we measured the relationship between the triaxial direction loads acting on the ground surface of the tire and the strain induced on the sidewall of the tire. Subsequently, we established experimental formulas that express the relationship and devised methods for measuring the road friction coefficient. It was confirmed that the friction coefficients of multiple friction surfaces, even under triaxial direction loads, could be accurately measured using the proposed method. In conclusion, the proposed method was confirmed to be able to measure the load acting on a tire and friction coefficient of the tire grounding surface at low speeds and under full-slip conditions.

Influence of the steel plate on the friction behavior of automotive wet disk clutches

AbstractFriction behavior is the key parameter for the design of automotive wet disk clutches. On the one hand, the Coefficient of Friction (CoF) level should be high to transmit torque efficiently. On the other hand, the clutch requires a positive slope of the CoF over sliding speed for good controllability, high comfort, and a low tendency to shudder. Clutches used in automatic or dual-clutch transmissions mostly use organic friction lining. These friction systems tend to have low CoF at low sliding speeds due to their high requirements regarding shifting comfort. Nevertheless, they show high values of CoF at high sliding speeds.This study investigates the influence of different steel plate finishes on friction behavior in different application-relevant operation modes such as brake shift, unsteady slip, and micro slip. Each of these operation modes requires an accurate CoF measurement at different sliding speed ranges. Therefore, we use different test rig setups. We characterize the steel plates by their areal surface topography measured with an optical system using focus variation. We discuss differences in the friction behavior of the corresponding tribological systems at different operating conditions.Results show an influence of the steel plate surface finish on the CoF level. Therefore, the surface finish of the steel plate influences the functional behavior of wet disk clutches and engineers should consider the surface finish in the early design phase. We discuss the correlation between commonly used surface parameters and friction parameters.

Establishment and Calibration of Discrete Element Model for Buckwheat Seed Based on Static and Dynamic Verification Test

Aiming at the lack of accurate and reliable discrete element simulation parameters for the design of buckwheat metering devices and seeders based on the discrete element simulation method, an accurate buckwheat seed model was obtained by calibrating the discrete element simulation parameters. In this study, buckwheat seed particle models were established based on the manual and automatic filling methods. In order to improve the accuracy of the models, discrete element simulation parameters were calibrated, and the static cylinder-lifting test and dynamic seed-metering test were used to verify the simulation results. A 3D model of buckwheat seed was obtained using the CT scanning method, and a manual filling 7-sphere particle model and an automatic filling multi-sphere particle model were established. The physical parameters and contact parameters were measured using the uniaxial compression test, the drop test, and the friction coefficient measurement test. The Plackett–Burman test and steepest ascent path were used to obtain the optimal parameter combination based on the static cylinder-lifting test. We conducted dynamic seed-metering tests using the two particle models under the optimal parameter combination. The results show that, compared with the measured values of stacking angle, the relative errors of the simulation values of the manual filling 7-sphere and automatic filling 36-sphere particle models are 1.04% and 0.50%, respectively. When the rotation speed range of the seeding wheel is 20~60 r/min, the average relative errors between the simulated value and the measured value are 15.85% and 4.69%, respectively. When the effective working length range of the seeding wheel is 20~40 mm, the average relative errors between the simulated value and the measured value are 22.18% and 9.07%, respectively. Regardless of whether the manual filling 7-sphere or the automatic filling 36-sphere particle model of buckwheat seed was used for static motion parameter simulation or dynamic motion simulation, the automatic filling 36-sphere particle model has a higher accuracy. The buckwheat seed particle model established in this study will provide support for the design of buckwheat special seed-metering devices and improve the quality of buckwheat mechanized sowing operation.

Effect of surface texture positioning in grease lubricated contacts

It is generally established that use of well-designed artificially enhanced surfaces in certain lubricated conjunctions can potentially improve the tribological performance under specific operating conditions. However, there is a considerable lack of understanding of the effect of texture features, which are not exposed to conditions in the contact footprint, to the overall tribological performance. Three discs have been manufactured, one without texture features and two with texture features but in different locations with respect to contact footprint. The discs were tested in a ball on disc tribometer under representative contact conditions from practical application cases as such contacts can experience a multitude of regimes of lubrication during the operation. The contacts were lubricated by grease. Texture features placed directly beneath the contact were shown to provide a negative effect by increasing the measured coefficient of friction relative to the untextured disc. Textures placed outside, but at very close proximity of the contact footprint track on the disc, were shown to reduce the measured friction coefficient and wear simultaneously.