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TECHNICAL PAPERS

J. Tribol. 2001;124(2):229-238. doi:10.1115/1.1387027.

To study the evolution of asperity contact in the sliding contact, a new in situ observation system of sheet metal surface is built up. The workpiece is stationary so that the continuous deformation of a specific group of asperities can be traced. In the case of no strain rate in the substrate of workpiece, the experiments for various materials, roughness patterns, sliding velocities, and normal loads all show that the fractional contact area will increase from its initial state of pure flattening without shear stress, to a saturated value with sliding distance in different rates. The sliding velocity has minor influence on the variation of contact area for same sliding distance. The maximum increasing rate of the contact area is found in the case of medium load. In most cases, the increase in contact area takes place with simultaneously decreasing shear stress and pressure on the flattened asperity peak. The development of adhesive wear can be clearly observed in the case of high normal load and long sliding distance.

Commentary by Dr. Valentin Fuster
J. Tribol. 1999;124(2):239-244. doi:10.1115/1.1387035.

The friction force as a function of humidity was measured between thin carbon films coated onto mica surfaces. The friction force was found to be proportional to the area of contact. The shear stress at 0 percent, 33 percent, and 100 percent relative humidity was measured to be 26 MPa±5 MPa,12 MPa±2 MPa, and 5 MPa±0.5 MPa, respectively, and was independent of the applied pressure for pressures less than 20 MPa. Water acts as a lubricant decreasing the friction between the carbon surfaces. The shear stress at 0 percent relative humidity corresponds to the shear stress of a solid paraffin film, and suggests that the shear may be dominated by a thin organic film adsorbed from air, at least at the pressures less than 20 MPa and a velocity of 1 μm/s. At 100 percent relative humidity, the shear stress for carbon coated surfaces was about double that for mica surfaces, indicating a stronger influence of the water for the more hydrophilic mica surface than the more hydrophobic carbon surface. The friction between one uncoated mica and one carbon coated mica surface resulted in immediate damage and generation of wear debris.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):245-258. doi:10.1115/1.1396344.

A gear-cam adaptor was used to investigate the effect of an extreme pressure (EP) additive in the base oil and roller’s surface roughness on the pitting life of a disk specimen under oil lubrication with rolling-sliding line contacts. Incorporating the mean friction coefficient, increases in the Vickers microhardnesses within the layer of strain hardening can explain the influence of the EP additive on the material toughness of the ratchetting layer. The pull-off forces obtained from the tip/sample’s force-distance curve of an atomic force microscopy (AFM) can further reveal the ever-growing characteristic of the boundary layer, which is benefit to retard the growth of cracks. Weibull plots for disk’s pitting life are provided for three lubrication cases. The use of the EP additive in the base oil can expand the range of pitting-life distribution; on the other hand, increasing the roller’s surface roughness can also elevate the pitting life of the specimen lubricated by the same oil. From the study of contact subsurface, the distributions of four material responses: boundary layer, ratchetting layer, shakedown layer, and elastic layer, can be found. The EP additive in the base oil affects the behavior of the first two layers and roller’s surface roughness, the dominant factor, even does the third one.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):259-265. doi:10.1115/1.1398547.

The effect of lubricant on flyability and read-write performance in ultra-low flying regime has been studied over the disks with lubricant on one half of disk surface thicker than the other half. The dynamics of a slider was monitored using Acoustic Emission (AE) and Laser Doppler Vibrometer (LDV). An instability characteristic of a slider flying over the thick lubricant region has been observed and this instability intensifies as flying height decreases and the step thickness increases. After the slider flies over the disks, it has been found that lubricant re-distribution occurs as lubricant is “carried” by the flying slider from the thick lubricant region and deposited onto the thin lubricant region. Possible mechanisms were discussed to explain the observations. Finally, recording tests were performed and the magnetic spacing loss due to the lubricant steps was estimated.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):266-273. doi:10.1115/1.1398548.

The various existing forms of Reynolds equation are known to yield pressure fields which are unbounded and nonphysical when contact occurs. With a view to developing a Reynolds equation that is suitable for contact, here we revisit Taylor’s plate scraping problem for incompressible flow. This problem has an infinite pressure where the scraper contacts the plate. By suitably modifying Maxwell’s slip condition, the scraping problem can be reformulated so as to lead to finite pressures. This is shown locally via asymptotics, and globally via convergence checks. The approach looks sufficiently promising to consider the further development needed for the compressible flow present in gas lubricated bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;124(2):274-280. doi:10.1115/1.1398550.

Carbon-carbon composites are used as friction materials because of their exceptional mechanical and thermal properties. In particular, carbon fibers offer high specific stiffness and high thermal diffusivity along the fiber axis. However, it was observed that after being subjected to the braking process, especially after rapid deceleration, the friction surfaces across the rubbing tracks become very wavy. These grooves alter the contact characteristics at the sliding interface and may reduce the friction performance. During the rubbing process, a sliding surface is damaged. In this paper, the effect of thermal property variations on the friction surface is presented. Multiple carbon samples were fabricated with different crystallinity distributions and orientations. All experiments were conducted using a Friction Assessment Screening Tester (FAST) under controlled atmospheric conditions. An experiment using a laser beam to provide the temperature variations across the surface was also performed. The results show that the formation of macro-grooves on the friction surface is a function of thermal or/and mechanical property variations, crystallinity distribution and crystallite orientation with a respect to the rubbing direction. However, if the distribution of crystallites is non-uniform, micro-grooves are always observed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):281-287. doi:10.1115/1.1398290.

We have formed a micro-texture on a pico-slider’s air-bearing surface to reduce the vibration when the slider comes into contact with the disk. The contact between slider and disk was controlled by adjusting the ‘interference height.’ Our measurements show that, at a giving interference height there is a significant less vibration in the textured slider. This lower amplitude of vibration is attributed to the lower friction force, which is in turn due to the smaller area of contact. We have also introduced the concept of interference area and found that it provides a good explanation of measured vibration.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):288-296. doi:10.1115/1.1398293.

Electrostatic charge sensing technology has been used to monitor adhesive wear in oil lubricated contacts. Previous work in this area demonstrated that “precursor” charge events may be detected prior to the onset of scuffing. Possible charging mechanisms associated with the precursor events were identified as tribocharging, surface charge variation, exo-emissions and debris generation. This paper discusses the proposed charging mechanisms and details a series of investigative tests using an adapted pin-on-disc (PoD) rig. The PoD tests focused on surface charge variation effects and were of two types, non-contact, where different materials were inserted in the disc, and controlled scuffing tests.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):297-304. doi:10.1115/1.1401019.

This research investigates the dynamic characteristics of a herringbone grooved journal bearing with plain sleeve (GJPS) and a plain journal bearing with herringbone grooved sleeve (PJGS) under static and dynamic load. FEM is used to solve the Reynolds equation in order to calculate the pressure distribution in a fluid film. Reaction forces and friction torque are obtained by integrating the pressure and shear stress along the fluid film, respectively. Dynamic behaviors of a journal, such as orbit or rotational speed, are determined by solving its nonlinear equations of motion with the Runge-Kutta method. Numerical results are validated by the experimental results of prior researchers. A GJPS produces less friction torque than a PJGS so that the GJPS consumes less input power than the PJGS. Under static load, the PJGS converges to the fixed equilibrium position, but the GJPS has a whirling motion due to the rotating groove even at the steady state, which produces the excitation frequencies corresponding to the integer multiple of the rotor speed multiplied by the number of grooves. The variation of rotational speed of a GJPS is always less than that of a PJGS due to less friction torque. Under the effect of mass unbalance, the excitation frequencies of the reaction force in a GJPS and a PJGS are the rotational frequency due to mass unbalance and its harmonics due to the nonlinear effect of fluid film. However, the GJPS has relatively big amplitude corresponding to the multiples of the number of grooves, in comparison with the amplitudes at the adjacent harmonics.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):305-312. doi:10.1115/1.1402178.

The engagement of a wet clutch with skewed surface roughness was investigated. A Weibull asperity height distribution rather than a Gaussian one was utilized in the asperity contact pressure model. The combined effects of surface roughness and skewness on the friction coefficient were studied for new, run-in and glazed wet friction materials. The results show that the engagement time predicted by the Weibull distribution is greater than that obtained using the Gaussian distribution. A torque spike at the beginning of engagement occurs using the Weibull distribution by taking the skewness into account. A positively sloped curve of friction coefficient versus velocity can reduce the torque increase near the end of the engagement. The strain value at the end of engagement obtained by including the skewness is lower than that predicted by excluding it. The surface topography and the friction characteristics change with the engagement wear and thermal glazing. The torque response and the phase plane are presented for the run-in and the glazed wet friction materials as a function of surface roughness, skewness, and friction characteristic.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):313-319. doi:10.1115/1.1402180.

The present study deals with the experimental determination of the performance of a 100 mm diameter plain journal bearing submitted to a misalignment torque. Hydrodynamic pressure and temperature fields in the mid-plane of the bearing, temperatures in two axial directions, oil flow rate, and minimum film thickness, were all measured for various operating conditions and misalignment torques. Tests were carried out for rotational speeds ranging from 1500 to 4000 rpm with a maximum static load of 9000 N and a misalignment torque varying from 0 to 70 N.m. The bearing performances were greatly affected by the misalignment. The maximum pressure in the mid-plane decreased by 20 percent for the largest misalignment torque while the minimum film thickness was reduced by 80 percent. The misalignment caused more significant changes in bearing performance when the rotational speed or load was low. The hydrodynamic effects were then relatively small and the bearing offered less resistance to the misalignment.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):320-326. doi:10.1115/1.1402130.

An experimentally observed reverse flow phenomenon at the end tip of the cavitation zone of a submerged journal bearing is modeled and theoretically investigated. The shape of the cavity, the nature of the reverse flow and the pressure distribution in the bearing are calculated in an attempt to understand previous experimental observations of pressure build up in the cavitation zone. A comparison with the available experimental results reveals that the cavitation shape, the behavior of the reverse flow and the pressure distribution are fairly well predicted by the present model. The reverse flow mechanism is indeed capable to generate the level of the experimentally measured pressures, particularly towards the end of the cavitation zone.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):327-335. doi:10.1115/1.1402131.

This article is concerned with the simulation of a magnetic storage device consisting of a flying head above a rigid rotating disk. The need to improve the transfer rate has led, at present, to very small surface-to-surface distances (air gaps). In this situation it is compulsory to take into account roughness effects. A popular method consists in averaging film-thickness from Mitsuya et al. (1989), with good reported results compared with experimental data. To overcome some limitations that become apparent at very small air gaps, notably when the roughness is two-dimensional, Jai in 1995 introduced a new technique based on a rigorous homogenization theory. Recent developments from Buscaglia and Jai (2000) have greatly reduced the computational complexity of the homogenization-based method, rendering it suitable for practical applications. We propose in this paper an original approach based on strength and rigorous mathematical model to avoid numerical problems which are usually encountered in classical approaches. The validation of the homogenization-based method is in some sense, a generalization of the film-averaging technique suitable for configurations in which some parameters of the latter (such as the Boltzmann correction factor) are not unambiguously defined.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):336-345. doi:10.1115/1.1402132.

A comprehensive model is developed for analyzing the onset of thermoelastic instability in a wet clutch. For this purpose, appropriate governing equations are derived that take into account the porosity and deformability of the friction material. The effect of the thickness of the separator disk and that of the friction material are also included. The model is general and can be used to describe TEI in a variety of other systems such as in a mechanical seal, as a special case. A series of simulations are presented that predict the thermoelastic behavior of a wet clutch from an instability viewpoint.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):346-350. doi:10.1115/1.1402133.

Squeeze film dampers (SFDs) reduce rotor vibrations and control dynamic instabilities in turbomachinery. Depending on damper geometry and operating conditions, the kinematics of journal motion can induce air ingestion and entrapment, produce lubricant vapor cavitation, or both. Air ingestion is the most common condition found in open ended dampers due to the low levels of external pressurization used in practice. The degrading effect of air entrapment on damper performance not only defies predictive models but also constrains the design of SFDs to a costly trial and error process based on prior experience. The present measurements correlate for the first time dynamic squeeze film pressures and pictures of the flow field with the air volume content in the lubricant mixture of a damper performing circular centered motion. The photographs of the flow field at key instances of journal motion show the development of a non-homogeneous flow with large striated cavities of air that persist even in the regions of positive (above ambient) dynamic pressures.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):351-357. doi:10.1115/1.1403456.

The contact between the male and female teeth of involute spline couplings connecting misaligned transmission shafts has been studied using an elastostatic contact model with stick-slip friction based on the boundary integral element method. The effect on the distribution of pressure and on the slip path during shaft rotation, of a wide range of design parameters and of applied torque and misalignment has been explored. The predicted behavior is classified according to the regime of friction (cyclic stick-slip or gross slip) and to that of the pressure history (uniform, cyclic, discontinuous, or toppled). The magnitude of the maximum tooth load, the axial skewness of the distribution of pressure and a maximum wear depth parameter are presented in terms of dimensionless design and operating parameters. The effect of tooth crowning is briefly examined. The results show a number of previously unreported features including a cyclic tooth load—which declines to zero for certain conditions—and an effective slip amplitude of around half the rigid-body value. This may affect the interpretation of laboratory fretting tests.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):358-366. doi:10.1115/1.1403457.
Abstract
Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):367-376. doi:10.1115/1.1403458.
Abstract
Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):377-385. doi:10.1115/1.1405129.

This paper is a report into an experimental and theoretical investigation of the effect of oil thermal properties on the performance of a tilting-pad thrust bearing. Three oils, namely poly-α-olefin, ester and mineral base, were chosen for this study. These oils all have same viscosity grade (ISO VG46) but differ in their rates of viscosity variation with temperature and in their heat capacity and thermal conductivity values. Mineral base oil of a higher viscosity grade (ISO VG68) was also analyzed for comparison. Experimental data were obtained from an equalizing tilting-pad thrust bearing with an outer diameter of 228.6 mm operating in a flooded lubrication mode. Simultaneous measurements of pad and collar temperatures, friction torque, pressures and oil film thickness were taken. In the tests, oil supply temperature and flow rate were held constant for all load-speed combinations. The theoretical analysis of oil performance was based on a three-dimensional TEHD model. In the analysis, thermal effects were locally taken into account and heat transfer into the pads was considered. The displacements of the active surface of the pads, due to pressure and temperature fields, were determined. The effect of initial pad crowning on the oil film thickness is discussed. Experimental and theoretical results are compared and analyzed in terms of the inlet and outlet oil film thickness, bearing operating temperature and power loss.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):386-391. doi:10.1115/1.1405130.

A novel design of planetary tapered roller traction drive was described. The design eliminated the spin motion induced sliding, offering high mechanical efficiency and improved wear performance. Principles of performance calculation have been outlined. Torque and life calculations on a 186 kw (250 hp) prototype indicated a remarkable torque capacity and satisfactory durability.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):392-397. doi:10.1115/1.1405131.

A tapered roller planetary traction drive based on a so-called zero-spin design was built for performance evaluation. The traction drive demonstrated, in many aspects, superb performance characteristics. The zero-spin design increased the torque capacity and enabled the traction drive to operate under both fully lubricated and starved lubrication conditions. The traction drive improved mechanical efficiency compared with traction drives of conventional designs and was able to achieve high efficiency up to 99 percent. The traction drive had satisfactory temperature performance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):398-405. doi:10.1115/1.1405815.

In this paper, the stability of water-lubricated, hydrostatic, conical bearings with spiral grooves for high-speed spindles is investigated theoretically and experimentally. In these bearing types, pressurized water is first fed to the inside of the rotating shaft and then introduced into spiral grooves through feeding holes located at one end of each spiral groove. Therefore, water pressure is increased due to the effect of the centrifugal force at the outlets of the feeding holes, which results from shaft rotation. In addition, water pressure is also increased by the viscous pumping effect of the spiral grooves. The stability of the proposed bearing is theoretically predicted using the perturbation method, and calculated results are compared with experimental results. It was consequently found that the proposed bearing is very stable at high speeds and theoretical predictions show good agreement with experimental data.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):406-413. doi:10.1115/1.1430673.

The dynamic behavior of a rolling element bearing under auxiliary operation in rotor/magnetic bearing systems is analyzed. When contact with the rotor occurs, the inner race experiences high impact forces and rapid angular acceleration. A finite element model is used to account for flexibility of the inner race in series with non-linear ball stiffnesses arising from the ball-race contact zones. The dynamic conditions during rotor/inner race contact, including ball/race creep, are deduced from a non-linear matrix equation. The influences of bearing parameters are considered together with implications for energy dissipation in the bearing.

Topics: Force , Bearings , Rotors
Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):414-419. doi:10.1115/1.1430674.

This paper is focused on the real area of contact for paper-based wet friction materials during the engagement of wet clutches. The deformation of the wet friction material is identified as elastic during the engagement. A microcontact model is proposed considering both surface roughness and skewness. A Weibull density distribution is employed in the model rather than a Gaussian density distribution. This model is compared with the Greenwood-Williamson (GW) model for the cases of positive skewness, zero skewness and negative skewness. The real areas of contact of new, run-in and glazed wet friction materials were investigated using this microcontact model. Both surface roughness and skewness were found to have a great effect on the real area of contact.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;124(2):420-427. doi:10.1115/1.1430675.

The effects of thermal conductivity and kinematics of contacting surfaces and viscosity-pressure coefficient on the film thickness in an elliptical contact under pure sliding conditions in which lubricant entrainment vector is directed along the major principal axis of the contact ellipse are discussed on the basis of experimental and numerical results. In the system where the conjunction is composed of surfaces having a large difference in their thermal conductivities, the film thickness is influenced markedly by the kinematics, in particular when a lubricating oil with a rather high viscosity-pressure coefficient is used. That is, the film thickness is much smaller when the surface having a low thermal conductivity is moving and that having a high thermal conductivity is stationary than in the opposite case. It is also found that a rather large dimple occurred in former case under rather high sliding condition moves toward the exit.

Commentary by Dr. Valentin Fuster

BOOK REVIEW

J. Tribol. 2002;124(2):428. doi:10.1115/1.1432665.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster

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