Search

The safe and efficient torque transmission of wet disk clutch systems requires high coefficients of friction. To achieve good controllability and high comfort, a positive slope of the coefficient of friction over sliding velocity is ensured by a reasonable formulation of the lubricant and choice of the friction pairing. This results in low transmittable torque at low sliding velocities. Thus, the occurrence of unwanted micro-slip in dynamic operation modes must be considered for the design of safety-relevant clutch systems. This work presents a methodology for the holistic measurement of the friction behavior of wet disk clutches. It is suitable for numerous applications and supports a sound understanding of frictional properties in the range of sliding velocities occurring in brake shifts through forced slip operation down to static torque transmission. The experimental determination of the holistic friction behavior is crucial for developing optimized design guidelines for modern clutch systems. [ABSTRACT FROM AUTHOR]

In ultra-precision equipment, nanoscale rough surface contact widely exists, and the adhesion between interfaces has become a critical component in researching the friction phenomenon. However, adhesion is not considered in traditional friction models. In this study, the adhesion model between fractal contact surfaces is established by proposing the critical truncated area of adhesion, a c u t ′ , and a new friction model considering adhesion is established by combining with the traditional model. The present model shows that adhesion increases as the fractal dimension (D) increases or the surface roughness coefficient (G) decreases when contact occurs between nanoscale rough surfaces. The friction coefficient calculated based on the new model is larger than that of the traditional model, especially when G is small. In addition, a static friction force measurement system was built by using the reticle and vacuum chuck in the lithography system. The experimental system measured the maximum static friction between the chuck and the reticle under different normal loads, and the experimental results show that the new model proposed in this paper is more accurate in characterizing the static friction of nanoscale rough surfaces. [ABSTRACT FROM AUTHOR]

[Display omitted] Contact angle and sliding angle measurements are widely used to characterize superhydrophobic surfaces because of the simplicity and accessibility of the technique. We hypothesize that dynamic friction measurements, with increasing pre-loads, between a water drop and a superhydrophobic surface is more accurate because this technique is less influenced by local surface inhomogeneities and temporal surface changes. A water drop, held by a ring probe which is connected to a dual-axis force sensor, is sheared against a superhydrophobic surface while maintaining a constant preload. From this force-based technique, static and kinetic friction forces measurements are used to characterize the wetting properties of the superhydrophobic surfaces. Furthermore, by applying increased pre-loads to the water drop while shearing, the critical load at which the drop transitions from the Cassie-Baxter to Wenzel state is also measured. The force-based technique predicts sliding angles with reduced standard deviations (between 56 and 64%) compared to conventional optical-based measurements. Kinetic friction force measurements show a higher accuracy (between 35 and 80%) compared to static friction force measurements in characterizing the wetting properties of superhydrophobic surfaces. The critical loads for the Cassie-Baxter to Wenzel state transition allows for stability characterization between seemingly similar superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

The surface dynamic and static coefficient of friction of yarns is one of the most important means of characterizing the surface structure of yarns and can provide an important basis for yarn performance evaluation and quality monitoring. The lack of highly stable, high precision and low-cost instruments for measuring the dynamic coefficient of friction of yarns is a major constraint to yarn performance evaluation and performance improvement. The entanglement method calculates the coefficient of friction between yarns by measuring the input and output tensions on both sides of the yarn at its own kink point, which is easy to use and highly accurate. Based on this, a yarn dynamic friction coefficient measurement system was developed based on the kink principle and the measurement error was evaluated using 28tex cotton yarn and 420 D polyester yarn in turn. After five repeatability tests using 28tex cotton yarn, 36tex aramid and 420 D polyester, the coefficient of friction between the three yarns themselves fluctuated by less than 0.01 and the instrument showed good stability and repeatability. In six comparative tests with the CTT constant tension transfer system from Lawson, USA, the inter-group CV of the coefficient of friction between the yarns themselves was only 3.66% after six tests. A final comparison of the data from the entanglement and the Rhodes method tests shows that the coefficients of friction obtained show the same trend. The yarn running speed and pre-tension are adjustable and the system has a timed test function, which allows the input and output tensions of the yarn and the dynamic coefficient of friction between the yarn and the yarn to be measured and displayed in real time. This improves the measurement accuracy, reduces the price of the product and helps to evaluate the surface properties of yarns and improve product quality. [ABSTRACT FROM AUTHOR]

Mechanical testers have commonly been used to measure the frictional properties of socks. However, the friction values may be susceptible to the level of stretchiness of tested fabrics or human variability. Thus, the aim of this study was to propose a novel method that enables friction measurement of socks in a sock-wearing condition with less human variability effects. Five socks with different frictional properties were chosen. Three experimental ramp tests were performed with an artificial structure shaped like the foot-ankle complex (last) and a ramp tester to quantify the static coefficient of friction (COF) at the foot against sock, at the sock against an insole, and the foot wearing socks against the insole, respectively. The angle where the last slipped while the ramp surface was gradually inclined was used to compute the static COF values for each sock. The reliability was 0.99, and COF values ranged from 0.271 to 0.861 at the foot-sock interface, 0.342 to 0.639 at the sock-insole interface, and 0.310 to 0.614 in the third test. Socks with different frictional properties were successfully distinguished each other. Thus, the suggested protocol could be a reliable option for measuring the static COF values in the tension similar with it found in a sock-waring condition with reduced effects of human variability. [ABSTRACT FROM AUTHOR]

High static friction between functional surfaces poses a serious challenge for industrial valves, particularly for those used in the advanced ultra-supercritical power generation industry that requires raising the steam temperature to 760 °C. In this context, we established the static friction measurement methodology by evaluating the effects of temperature, loading time, and number of static cycles on the evolution of the static coefficient of friction up to 800 °C. This methodology was applied on a nickel-based solid lubricant coating that appears an attractive candidate to withstand very harsh environments. The tribological behavior is explained in terms of a competition of three mechanisms governing the static friction processes: micro-welding or sintering of asperities, creep of asperities, and formation of lubricious oxides. • Establishment of the static friction measurement methodology to simulate valve applications. • First time to systematically report the evolution of static COF with number of static test cycles, time and temperature. • Three main mechanisms governing the static friction processes. [ABSTRACT FROM AUTHOR]

Atmospheric icing on structures and equipment represents a challenge for operation and safety. Passive ice removal by ice-phobic coatings has received much attention over the last decades. The current state-of-the-art methods for quantifying the ice-release properties of such coatings suffer from a range of drawbacks, including poor reproducibility and high complexity test setups. Here, a facile rotational tribometer approach for measuring the static friction between polymeric coatings and ice is presented. The torque necessary to initiate motion at the coating-ice interphase was used as a measure of ice release. For a polydimethylsiloxane-based coating (Sylgard 184), the effects of ice-temperature, normal force, coating thickness, and dwell time (contact time between coating and ice at rest with fully applied normal force prior to applying torque) were established along with the conditions resulting in least data variation. With these conditions, tribology-based friction measurements were carried out on two additional coatings; a two-component polyurethane, and a commercial foul release coating. The outcome of the method, i.e., grading of the coatings in terms of antiicing effect, matched those obtained with a widely used ice adhesion test method based on ice shear adhesion testing. The same trends are revealed by the two methods. However, the findings from the proposed tribology-based method result in consistently lower variation in outcomes and offer more detail on the ice adhesion and friction mechanisms. [ABSTRACT FROM AUTHOR]

A force sensor design utilizing a photo-interrupter is presented for measuring the static coefficient of friction (COF). The measurement of ice friction on polymer surfaces, a process that requires detecting forces in the sub-Newton range, is chosen for the study. The photo-interrupter is coupled with a specially designed sensitive flexure, with structural parameters validated through finite element methods, to detect the small forces. The static properties of the sensor are characterized by calibration techniques. An accompanying rotary table is constructed to measure the COF of ice on polymethylmethacrylate (PMMA) and polyester specimens under refrigeration conditions. The experimental results indicate that the device can be utilized to predict the COF. The designed portable and miniature friction measurement setup can be a compact and cost-efficient alternative to bulky tribo-rheometer equipment. [ABSTRACT FROM AUTHOR]

This work provides the values of both the static and the kinetic friction coefficients for chestnut (Castanea sativa Mill.) of Spanish origin. Knowledge concerning these coefficients has its main application in the study of agroindustrial structures and machinery with timber members in contact. This determination was developed taking into account the timber anisotropy to establish surfaces and directions of slipping. A modified direct shear test device was used to conduct the tests and reproduce the tribological system. This procedure was functional and reliable and considered suitable for standardizing the friction measurement between timber surfaces, since this device is widely distributed in geotechnical and materials laboratories and the European codes do not specify a procedure or device to carry it out. The average values obtained were 0.46 for the static coefficient and 0.33 for the kinetic one, without considering the surfaces and directions of slipping. These values ranged between 0.36 and 0.55 for the static friction coefficient and between 0.28 and 0.39 for the kinetic friction coefficient depending on the direction considered and also taking into account the anisotropy of the timber. A good correlation was obtained between both coefficients, thus allowing for the estimation of the kinetic coefficient from the static one. [ABSTRACT FROM AUTHOR]

The handling of sticky raw material can cause problems during operation of process equipment in the cement production industry. These handling problems are generally observed when raw material (e.g., wet clay) sticks to machine walls and causes blockage of outlets. This leads to frequent production shutdowns and expensive cleaning operations. In this work, the effects of surface material and process parameters on the friction forces between wet clay and surface were investigated. Various surface materials and clay impact speeds were investigated. The results demonstrate that not only the equipment surface material but also the surface roughness influences the observed frictional behavior. The ranking of the materials in terms of effective static friction coefficients fell in two groups with equal performance within the group: (1) Two Teflon-based coatings (Accofal 2G54 and Accolan LB), polished AISI 304 stainless steel, Matrox lining, and a polyurethane-based coating (best performing surfaces), (2) Mild steel, a silicone-based coating, and AISI 304 stainless steel (worst performing surfaces). However, the friction coefficients of the two groups only varied by a factor of two, suggesting that adhesion of wet clay to surfaces is difficult to avoid by the use of coatings. [ABSTRACT FROM AUTHOR]

This study conducted friction experiments of three elastomer block materials on ice (low pressure injection of polyurethane (PU), injection moulding of a thermoplastic polyurethane (TPU) and vulcanization of rubber (RU)), with a linear tribometer operating under four different ambient air temperatures (−10 to 0 °C) and six different sliding velocities (0.3–5 m/s). At low temperatures (−10 °C and −5 °C), the RU material showed the highest dynamic and static friction. However, at high temperatures (−2 °C and 0 °C) the PU revealed the highest dynamic friction and highest static friction at 0 °C. We discuss the physical phenomena's of elastomer friction on ice and in relation to the application of footwear slipping on ice. • Slipping accidents simulated by biofidelic friction testing of elastomers on ice. • PU outsole material on warm ice provides higher static friction coefficient compared to TPU and RU materials. • RU outsole material revealed higher dynamic friction coefficient on cold (−10 to 5 °C) ice compared to TPU and PU materials. • Hybrid outsole constructions combining materials, composites and design solutions may be able to accommodate slip resistance. [ABSTRACT FROM AUTHOR]

The article presents the problems of measuring the static friction force of granular materials of plant origin. Measurements as such do not have clearly defined procedures. This leads to a situation in which it is very difficult to compare the results obtained by different researchers. The tests were performed on a linear tribometer. The authors demonstrated the effect of sample slip velocity on the maximum value of the measured friction force and observed the effect of inertia force on the friction force oscillations in the initial phase of sample slip. The difference between the maximum force T 1max and the minimum force T 1min , recorded in the first friction ripple cycle can be considered as the basic parameter for evaluating the effect of inertia on the measurement results on a given measuring device. To omit the force of inertia, the authors propose conducting tests under quasi-static conditions, at a slip velocity of 0.5 mm/min. [Display omitted] • The problems of measuring the friction force of granular materials were presented. • A significant effect of sliding speed on the value of T 1max and Δ T were demonstrated. • A significant effect of inertia force on the value of friction force were demonstrated. • An analysis of 3 phase of change in friction force from displacement were presented. [ABSTRACT FROM AUTHOR]

The subject of this paper is the solution to specific problems in the measurement of the coefficient of the sliding friction of material pairs using a variable incidence tribometer. The aim of this work was to solve the questions of the measurement of the static coefficient of friction using a variable incidence tribometer from the perspective of metrology. In particular, we intended to research the expression of the credibility of this measurement and the possibility of increasing the credibility of the measurement. Another problem that needs to be solved is the method of carrying out the measurement, which has an impact on its achieved uncertainty. Sliding friction is a phenomenon that depends on many material properties such as contact area roughness, moisture, lubricants, temperature, and relative motion velocity. If environmental conditions are defined, the main input parameter is the relative motion velocity. For this input quantity, friction force becomes a symmetrical problem, and it is only necessary to explore this phenomenon for positive values of velocities. Symmetry in this area simplifies the research of this science topic. [ABSTRACT FROM AUTHOR]

The aim of this study is to measure the maximum coefficient of static friction with a smartphone. To reach this aim a very simple experiment with few materials was designed. Different from conventional experiments, the smartphone was used like an angle meter to determine the slope angle. The experiment in this study can be more practical than the conventional ones. It also requires easily accessible materials. We think that teachers and students should carry out this experiment as a group or demonstration activity in their classrooms. [ABSTRACT FROM AUTHOR]