Search

Improving operating conditions and reducing the friction-related power losses of hydrodynamic journal bearings could lower emissions and contribute to efforts for a faster green transition. Experimental studies play an important role in evaluating and improving the performance of plain journal bearings. The concept of the experimental setup for friction measurements of journal bearings is well defined in the relevant standards, but the actual design is always left to the researchers conducting a particular study. Our study reveals that common practical assumptions about the frictional power losses caused by components of the experimental setup can lead to misleading conclusions about the measured coefficient of friction. The research premises were tested on the developed experimental setup using two methods for measuring the coefficient of friction and different load interfaces. In particular, the direct torque measurement using strain gauges on the shaft demonstrated the strongest correlation with theoretical values. However, the result was affected by the correct evaluation of the frictional losses of the support ball bearings. Conversely, the Stribeck curves showed that the combination of the indirect cantilever method and load interface with direct radial force application led to inferior results. The intricate interactions between setup components and unknown frictional conditions made straightforward geometric corrections challenging. The study encourages researchers in the field to critically assess their experimental setups and to rely on their own assessment of frictional losses. [Display omitted] • Measurements with shaft torque transducer are the least affected by the setup configurations. • Setup with cantilever transducer requires an interface load rolling bearing. • Interface bearing should be selected with respect to load and measured friction torque. • Numerical correction cannot compensate for a poor design of the experimental setup. • Assumptions about power losses should be avoided when designing an experimental setup. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This work investigates the mechanisms for friction reduction in textured journal bearings. The measured friction of laser-etched connecting-rod big-end shells was compared with a non-textured reference under engine-like conditions. Experiments covered several working conditions, and the friction and lubricant film temperatures were measured for three texture configurations. The origin of the verified friction reduction was explored using a numerical model that simulates the hydrodynamic lubrication and global thermal effects. The simulation results accurately matched the friction measurements, revealing that the microscopic mechanisms of reduction in lubricant shear stress and effective viscosity due to texture-induced cavitation act simultaneously to reduce the friction coefficient for the textured shells. The findings have practical applications for the optimal design of textured hydrodynamic bearings. • Friction in textured journal bearings was experimental and numerically investigated. • A journal bearing model with thermal effects was validated using experimental data. • Textures reduced the lubricant shear stress and viscosity and lowered friction. • The shear area reduction mechanism for textured hydrodynamic bearings was confirmed. [ABSTRACT FROM AUTHOR]

Friction on snow surface, especially in relation to winter sports, is difficult to measure directly because of its complexity and variability. In this work, a wearable snow friction measurement device for cross-country skiing was designed and constructed to calculate the friction force and monitor movement signals through the logical placement of three-component sensors and connection parts on skis. Both theoretical simulation and experiments have been conducted, which proved that the deformation of cross-country skis outputs few abnormal force signals during applying large load. Through the static validation in the laboratory, the output accuracy and stability of the device were confirmed. To verify the availability of the device, measurements on a skiing treadmill were conducted, and the results showed that the device can work well and distinguish the different friction behaviors of various waxes. More important, through the ski field test, the device was able to monitor the technical actions and evaluate the frictional coefficient of different ski waxes on snow surface comprehensively, which provides a scientific reference for athletes and coaches in routine training. [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]

Real contact area is a challenging phenomenon in tribology, where despite established theoretical models, detailed experimental studies are still lacking. This work is focused on the evolution of the real contact area of multi-asperity steel contacts during sliding, by using sub-micron characterisation of entire multi-asperity contact area and simultaneous measurement of the friction force. Steel-sapphire contact was monitored by using an in-situ optical system in real time during 5 mm of unidirectional sliding in surface roughness range R a = 0.1–1.0 µm and normal loads up to the nominal contact pressure equal to steel yield strength (Y). The results showed that the initial real contact area caused by static loading increased during sliding. At low nominal contact pressure of 0.3 Y , the increase in real contact area was the largest: for R a = 0.1 µm it went from 9.5% of the nominal area under static conditions to 15.0% after sliding, a 56% relative increase, and for R a = 1.0 µm it went from 5.0% to 11.5% after sliding, a 106% increase. The increase in real contact area was smaller at higher nominal contact pressures of 0.6 Y and 1.0 Y. At 1.0 Y for R a = 0.1 µm the real contact area went from 18.0% to 21.8% after sliding and for R a = 1.0 µm it went from 9.4% to 12.9% after sliding. The coefficient of friction increased with roughness at the lowest nominal contact pressure 0.3 Y (for R a = 0.1 µm it was 0.24 and for R a = 1.0 µm it was 0.29) but became independent of roughness at higher contact pressures (on average 0.18). • In-situ measurement of real contact area evolution during 5 mm of sliding. • Whole multi-asperity steel contacts observed using optical method. • Real contact area increased due to sliding from the initial static conditions. • The largest increase from static to sliding conditions was at the lowest load. [ABSTRACT FROM AUTHOR]

The (cyclopentadienyl)molybdenum(dicarbonyl)(nitrosyl) complex as a metal-organic precursor and acetylene gas were used to produce the MoC x /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 MoC x /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. [ABSTRACT FROM AUTHOR]