Static Friction Coefficient Research Articles

INFLUENCE OF CONTACT TIME WITH LUBRICANTON POLYMER-POLYMER SLIDING PAIR INTERACTION

This paper presents tribological investigations of pairs of same-type construction polymers: PET, PA6, andPOM, which for different durations were in contact with a lubricant. Hydraulic oil HLP 68, used for heavilyloaded drive systems, was the lubricant. Test samples were subjected to storage in this lubricating mediumfor different lengths of time: ranging from no storage to 2 weeks of storage. The static friction coefficient,which has a significant bearing on the startup of systems, was studied. Furthermore, investigations aimed atidentifying the phenomena involved in the interaction of the polymer-polymer sliding pair were carried out.The duration of the storage of the polymeric materials in the lubricating medium has been found to have avisible effect on static friction coefficient values and surface hardness and condition.

Effect of load and pulling speed on the static coefficient of friction of cross-country skis on snow

Effect of load and pulling speed on the static coefficient of friction of cross-country skis on snow

Experimental study on the maximum static friction coefficient of fiber winding based on semi‐geodesic method

Experimental study on the maximum static friction coefficient of fiber winding based on semi‐geodesic method

On the dynamics of transporting rolling cylinders

AbstractA common, yet hazardous, method of transporting cylindrical tanks used to carry compressed gas involves rolling both tanks at opposite angles of inclination to the vertical. By propelling one of the tanks while maintaining point contact between the tanks, both tanks can be moved such that their centers of mass move in a straight line. The purpose of this paper is to explore this locomotion mechanism. First, the problem of supporting an inclined cylinder in point contact with a rough surface is examined. The analysis shows that dependent on the geometry of the cylinder and the coefficient of static friction, a wide range of angles of inclination are feasible. The presence of non-integrable constraints on the motion of the rolling cylinder is explored using the concept of a holonomy. The problem of transporting two cylinders using the aforementioned mechanism is then analyzed with the help of Frobenius’ integrability criterion for constraints and numerical simulations. The results show the mechanical advantage of transporting a pair of cylinders, the range of possible angles of inclination, and the forces needed to sustain the motion.

Evaluation of static and rolling friction coefficients of granulated sugar using the discrete element method in a rolling mode rotating drum

PurposeThis research investigates the behavior of granulated sugar particles of different sizes in a rotating drum at varying speeds, using the discrete element method (DEM) as a mathematical modeling approach.Design/methodology/approachThe study conducted a data scan to determine both static and rolling friction coefficients. Based on benchmark studies, the Hertz–Mindlin contact model with rolling history elastic-plastic spring-dashpot (EPSD) and CDT (directional constant torque) models were employed to simulate the behavior of granulated sugar particles in a rotating drum under varying speeds.FindingsIn this research, the static and rolling friction coefficients presented the best values for granulated sugar near 0.60 and 1.5, respectively, applying the CDT model. The method demonstrated great accuracy in replicating experimental data.Originality/valueThis study enables comprehension of the behavior of the particles and particle system in a rotating drum at different speeds. The method may develop models that characterize and predict the main effects of particle systems to reduce project time and expense, especially in the food industry.

Transferability of Model-Based Static Coefficient of Friction

The accurate determination of static coefficients of friction (COFs) is crucial in engineering design, yet standard reference values often show considerable variability. As a result, engineers frequently need to perform experimental COF measurements to ensure the reliable transferability of model-based COFs to real-world components. However, the effectiveness of cost-efficient laboratory tests, typically conducted on standardized samples, in reflecting actual component performance is often questioned as it is not trivial to transfer and scale the tribosystem. This study addresses this issue by conducting friction coefficient experiments on interference fits and flange connections, comparing the results with laboratory-based COF tests. The findings reveal a strong correlation when the tribological conditions of the real assembly are replicated in the lab. This research offers a method to enhance the accuracy and transferability of COF values from lab tests to practical applications, providing engineers with a more reliable approach to friction testing.

Interaction properties of wine grapes: DEM analysis and experimental investigation

The processing of wine grapes is both labor-intensive and time-consuming, necessitating equipment improvements to enhance productivity. Virtualizing machining processes is crucial for designing and researching equipment aimed at improving food quality and safety standards. Accurate modeling requires detailed knowledge of material and interaction properties. Therefore, the contact properties of wine grapes were obtained using the progressive optimization method. Experimental and simulated results of particle collision and rolling during processing were compared to select accurate input parameters. The experiment concluded that the coefficient of restitution (CoRg-g), coefficient of rolling friction (CoRg-g), and static friction coefficient (CoSg-g) between grape particles were 0.178, 0.400, and 0.459, respectively. Similarly, the corresponding values for interactions between grapes and stainless steel were 0.411, 0.027, and 0.643, respectively. Notably, these values have been independently verified through simulations.

Test and sensitivity analysis of base‐isolated steel frame with low‐friction spherical sliding bearings

AbstractIn Japan, spherical sliding bearings with low friction coefficients are gaining popularity for base‐isolating low‐rise frames that are relatively lightweight. However, the complete dataset of isolators and a base‐isolated frame for evaluating the model sensitivity and response uncertainty are limited. This study first presents the design process, isolation unit‐ and frame‐level testing, and a blind prediction contest conducted on the occasion of full‐scale shaking table testing of a three‐story base‐isolated hospital specimen. The design process utilizes a numerical model that accounts for the velocity and contact pressure dependencies and requires soft‐ and hard‐case simulations with nominal friction coefficients plus and minus standard deviation to consider the uncertainties associated with the bearing behavior. The pre‐shipment isolation unit and frame shake table testing yielded an invaluable dataset for bearings under normal and low contact pressures, low and high velocities, and constant and varying axial loads. The accompanying blind prediction contest provided a valuable dataset for rethinking the impact of modeling uncertainty. In‐depth data analysis and sensitivity analysis were conducted. The sliding coefficient increased under low‐contact pressure and low‐velocity conditions. The static friction coefficient was 1.9 to 4.5 times higher than the dynamic coefficient, but this had little impact on the residual displacement, cumulative travel, and maximum story shear force. The axial force fluctuation, vertical motion, and two‐directional input did not significantly affect the bearing behavior in the test. The test and the following simulations confirmed that the low friction coefficient helped the building contents, that is, medical equipment in this study, remain in order under near‐fault and long‐period ground motions.

Investigation the yarn to yarn static and kinetic friction based on the response surface method

One of the most significant physical characteristics of textiles is friction, which has a direct bearing on how they behave mechanically. This characteristic has been researched in a variety of methods thus far, demonstrating its significance. The static and kinetic coefficients of friction in yarn to yarn interaction have been examined in this paper by considering a number of variables, including yarn count (33, 50, 57, and 65 Tex), yarn movement speed (30, 45, and 60 mm/min), angle of yarn placement (0, 45, and 90 degrees), and surface reaction force (5, 10 and 15 g). To this aim, 32 different experiments were designed using the central composite design approach in order find the static and kinetic coefficients of friction. The outcomes were then statistically analyzed. The relationship between the friction coefficients and the aforementioned parameters was investigated as well. Next, a model with 93% and 85% accuracy for the static and kinetic coefficients of friction, respectively, was obtained using the response surface method. Additionally, the relationship between the friction coefficients and each of the aforementioned parameters was assessed.

The effectiveness of chalk as a friction modifier for finger pad contact with rocks of varying roughness

The application of chalk (magnesium carbonate) in rock climbing is common practice as climbers attempt to improve their grip by removing moisture from their hands with the aim of increasing friction at the finger pad-rock interface. This novel work investigated the effectiveness of chalk as a friction modifier on four different rocks (sandstone, granite, dark limestone and light limestone) typically found in areas of the UK where the sport of climbing is undertaken, with varying surface roughness. The static coefficient of friction was measured for dry and wet fingertip conditions with and without chalk, under normal (‘grip’) forces of 5, 10 and 15 N. Results showed that the effectiveness of chalk as a friction modifier is dependent on a number of factors such as moisture level and the gradient of the asperity at the rock surface, however, in general chalk applied to dry fingertips had a more positive effect on the static coefficient of friction than in simulated sweaty conditions. During lab tests, chalk was also seen to be beneficial by making the static coefficient of friction more consistent across most test conditions. The results of this study, and the explanation of friction mechanisms involved, provides guidance for the use of chalk with consideration of the type of rock which is being climbed.

Sticky feet: a tribological study of climbing shoe rubber

This study examines the tribological properties of climbing shoe rubbers, challenging the common belief in the climbing community that softer rubbers are inherently grippier. This study investigates the mechanical and wear characteristics of climbing shoe rubbers by employing a high-precision modular mechanical testing environment (Bruker UMT TriboLab) and representative granite counter-surfaces. Key parameters, including surface roughness, Shore A hardness, interfacial adhesion, static and dynamic friction coefficients, and material wear patterns, were analyzed. The mechanical properties of each rubber compound were characterized through Shore A hardness testing and ball indentation–retraction tests, measuring indentation force, energy, and adhesive properties. Sliding friction tests, simulating real climbing conditions, were conducted to understand the tribological behavior of each rubber compound under different loads, further analyzing static and dynamic friction coefficients and wear characteristics. The findings of this study indicate that rubber performance is a convolution of several factors, including material hardness, surface roughness, and interfacial adhesion. Contrary to popular belief, softer rubbers did not consistently exhibit superior tribological characteristics. The findings of this study suggest that climbing shoe selection and design should consider a broader range of material characteristics beyond hardness, emphasizing the role of surface roughness and adhesion in determining overall frictional performance. This research offers valuable insights for the climbing community, providing methodologies to benchmark climbing rubber material characteristics.

基于 EDEM 离散元方法的盐碱土壤参数标定与测试

In this study, saline soil parameters were calibrated based on EDEM discrete element method, soil density, soil elastic modulus, soil shear modulus, soil particle size distribution and Poisson's ratio were determined. The Hertz-Mindlin with Johnson-Kendall-Roberts (JKR) model was used to simulate the characteristics of soil stress and strain of particles under external forces. The JKR contact parameter between saline soil particles was used as a test factor, and the soil accumulation angle was used as a test index to carry out the saline soil contact parameter calibration test. It was finally determined that an elastic recovery coefficient of 0.262, a static friction coefficient of 0.263, a dynamic friction coefficient of 0.234, and a JRK surface energy of 10.084 were the optimal combinations of contact parameters for saline soils.

Frictional Properties of Biomimetic Micro-Hexagonal-Textured Surfaces Interacting with Soft Counterfaces under Dry and Wet Conditions.

Biomimetic micro-hexagonal-textured surfaces have sparked interest for their application in fields that demand high friction and adhesion, such as micro-robotics and biomedicine. Despite extensive research conducted on this specific microstructure, its friction behavior against soft counterfaces remains a topic that has not been fully investigated yet. This study examines how micro-hexagon textures behave when they come into contact with engineered and biological materials like gelatin and chicken skin in dry and wet conditions. The results show clearly that under dry contact conditions, flat surfaces generate higher friction compared to hexagon micropattern surfaces. Under wet conditions, hexagon micropattern surfaces generate higher friction compared to flat surfaces. In wet conditions specifically, the static coefficient of friction is up to 13 times greater than that of a flat specimen against glass, up to 11 times greater against gelatin, and up to 6 times greater against chicken skin. For the dynamic coefficient of friction, the patterned surface demonstrates a maximum increase by a factor of 28 against glass, 11 against gelatin, and 5 against chicken skin. These results further develop our knowledge of these hexagon micropattern surfaces and pave the way for their utilization in future technological advancements in which soft and wet counterfaces are to be considered, such as in biomedical applications that can benefit from increased friction in wet conditions for better control and stability.

An elementary approach based on variational inequalities for modeling a friction-based locomotion problem

We propose an elementary proof based on a penalization technique to show the existence and uniqueness of the solution to a system of variational inequalities modeling the friction-based motion of a two-body crawling system. Here for each body, the static and dynamic friction coefficients are equal.

Characterization of friction between metal-polymer material pairs with different speeds and surface pressures

Abstract Engineering design requires accurate knowledge of the friction conditions of material pairs. The friction coefficient (static or dynamic) depends on many parameters in addition to the pairs of materials used (e.g. surface roughness, temperature, humidity, surface pressure, etc.). In this study, based on our own measurements, we present the friction conditions of an aluminum (Al 6082-T6) – polypropylene homopolymer surface pair under different surface loads (5-15 MPa) and different relative speed differences (4-8–48-96 mm/min). The results of the experiments show that the static and dynamic friction coefficients decrease with the increase of surface load. As relative speed increases, the dynamic coefficient of friction increases, while the static coefficient of friction can be considered constant.

Assessment of Physical Properties of Black Gram and Green Gram Seeds in Relation to the Design of Seed Metering Mechanisms

In 2023, a comprehensive study was carried out at the Department of Agricultural Engineering, IARI, New Delhi, India, focusing on the investigation of vital physical properties encompassing size, sphericity, test weight, bulk density, true density, coefficient of static friction, and angle of repose for black gram and green gram seeds. Black gram seeds were characterized by mean dimensions of 4.90 mm (length), 3.74 mm (breadth), 3.57 mm (thickness), and a geometric mean diameter of 4.02 mm. In contrast, green gram seeds exhibited corresponding values of 5.08 mm, 4.06 mm, 3.99 mm, and 4.45 mm. Sphericity percentages were quantified as 82% for black gram and 86% for green gram. Mean bulk density measurements of 0.86 g/cc (black gram) and 0.87 g/cc (green gram) were accompanied by true density values of 1.36 g/cc and 1.38 g/cc, respectively. The mean angle of repose measurements were recorded as 24.06° (black gram) and 23.94° (green gram). The mean coefficient of static friction exhibited variability across different materials: 0.50 (aluminum), 0.45 (mild steel), 0.47 (galvanized iron), and 0.60 (plywood) for black gram seeds; while for green gram seeds, corresponding values were 0.48, 0.43, 0.45, and 0.57. Additionally, the mean thousand-grain weight was found to be 41.82 g for black gram and 46.18 g for green gram.

Influence of Fault Dip Angle and Strength on Fault Slip Characteristics of Gas Storage

The alternating stress caused by periodic high-pressure injection and extraction in gas storage can potentially induce fault slippage, compromising the sealing integrity of faults within these storages sites. Understanding the mechanical behavior of faults under alternating stress is crucial for ensuring the long-term stability and safety of gas storage operations. To explore the impact of fault dip angle and fault gouge strength on fault slip characteristics, fault samples were prepared with uniaxial compressive strengths of 20.1, 30.2, 42.4, and 51.4 MPa at two distinct dip angles. Triaxial compression experiments were conducted under alternating stress conditions corresponding to operational pressures at a specific gas storage site in China. The results indicate that faults with dip angles of 30° and 45° tend to fail at their weakest points. The increasing strength of fault gouges shifts failure mechanisms from interfacial failure between gouges and the surrounding rock towards internal gouge failure, often accompanied by shear failure across sections, resulting in characteristic “X”-shaped conjugate shear failures. The decrease in the ratio of bedrock strength to fault gouge strength elucidates the observed phenomena of an initial reduction followed by increased fault deformation. Transition points for faults with 30° and 45° dips occur around the strength ratios of 1.7/1 and 1.2/1, respectively. Fault damage exhibits a negative correlation with fault gouge strength and a positive correlation with fault dip angle. Samples with a higher-strength fault gouge at a 30° dip angle generally incur less damage compared to those with a lower-strength fault gouge at a 45° dip angle. Moreover, higher maximum static friction coefficients denote greater fault resistance to slipping, with 30° faults consistently demonstrating superior resistance compared to 45° faults. Additionally, a higher-strength fault gouge consistently enhances slip resistance under identical dip angles.

Modeling static friction behavior of elastic–plastic spherical adhesive microcontact in full-stick condition

The static friction behavior of an elastic–plastic spherical adhesive microcontact between a rigid flat and a deformable sphere under combined normal and tangential loading is studied by the finite element method (FEM). The contact between the sphere and the rigid flat is assumed to be full-stick, and the sliding inception is related to a loss of tangential stiffness. The intermolecular force between the rigid flat and the sphere is assessed by the Lennard–Jones (LJ) potential, which is applied to the sphere and the rigid flat by a user subroutine. The evolution of the adhesive force with tangential displacement in the full-stick condition is revealed. The results indicate that the increasing effect of adhesive energy on the static friction coefficient gradually diminishes with an increase in the adhesive energy and the external normal load. Finally, based on an extensive parametric study, an empirical dimensionless expression is obtained to predict the static friction coefficient of the spherical adhesive microcontact considering the intermolecular force.

A systematic evaluation of DEM input parameters measurement and calibration

Rotary drums are widely used in industry, however, determining granular dynamics within them is still a complex task, thus remaining a focal point of research over the years. In this context, experimental techniques require numerical support to obtain data that is not easily acquired experimentally. One well-established numerical technique for studying particulate systems is the Lagrangian approach embed with Discrete Element Method (DEM). Although, DEM has limitations related to the definition of input parameters. It is crucial that DEM input parameters be measured and/or calibrated to reliably represent the granular behavior. These parameters are related to particle properties such as density, Young’s modulus, Poisson’s ratio, among others, and particle interactions, such as coefficients of restitution, static friction, and rolling friction. This study explores the experimental determination of interaction parameters preceding the acquisition of DEM input parameters in a rotary drum. Statistical analysis highlights the importance of each parameter and their combined interactions with the static angle of repose. The granular materials used were: soybeans, glass beads, polyacetal, 6 mm steel, and 4 mm steel. The coefficient of restitution was determined via free-fall methodology using equipment that verifies particle trajectory, static friction coefficient through equilibrium between weight and resistance forces, rolling friction coefficient using a launching device on surfaces of different roughness, and static angle of repose in a rectangular box with sections where particles rested. After removing the partition, angles were measured at both top and bottom sections. Regression techniques using a neural network created in Python were employed for both measurements. Experimental investigations and numerical simulations using DEM indicated that the coefficient of restitution depends on the thickness and type of material of the flat surfaces, showing an asymptotic behavior with increasing surface thickness. The static friction coefficient increased on rougher surfaces and in particles with greater imperfections. Rolling friction coefficient also increased on rough surfaces. Analysis of the angle of repose revealed that granular properties influence the response, such as static and rolling friction coefficients, whereas the coefficient of restitution had no significant influence. Neural network correlation coefficients exceeded 0.95, indicating a strong association between the variables. Our findings revealed that mathematical models can be used in conjunction with static repose angle experiments to calibrate DEM parameters for more robust and complex simulations. This research also quantifies and provides insights into the interactions between DEM parameters in direct measurement experiments, promoting more efficient calibration for future research.

Static and dynamic friction and wear of cobalt-based coatings at elevated temperatures

Advanced ultra-supercritical power plants, in which steam temperatures exceed 700 °C, demand the development of protective coatings, particularly for valves governing the steam flow where sealing surfaces are subjected to high static tribological solicitations. In this context, we systematically investigated the friction (static and dynamic) and wear of two cobalt-based coatings and one nickel-based substrate. Results indicate that oxidation kinetics exhibits a double-edged sword effect in terms of tribological properties at elevated temperatures. Fast oxidation kinetics promotes the sintering of oxides, generating a smoother oxide tribofilm and lowering the dynamic coefficient of friction (COF). However, this can also significantly increase the static COF over loading time due to welding/sintering of oxidized asperities, leading to increased torque requirements for valve operation.