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TECHNICAL PAPERS

J. Tribol. 2003;125(3):469-479. doi:10.1115/1.1582880.

A dynamic model of a contacting mechanical seal for down-hole tools, operating in the mixed lubrication regime, has been constructed. Two dynamic cases are examined: the behavior of the seal after it is subjected to a nonequilibrium initial condition and the behavior of the seal as it tracks axial runout of the rotating face. In both cases the seal experiences much larger leakage and inflow (incursion) rates than under steady-state conditions, although in the former case the duration of the increased rates is very short.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):480-486. doi:10.1115/1.1537264.

A new method of reducing the noise of the linear guideway type recirculating linear ball bearing (linear bearing) was studied. In the experiments, the overall sound pressure levels of linear bearings with steel balls or ceramic (Si3N4) balls were measured, and sound frequency analyses were carried out. Moreover, based on the assumption that the main cause of the noise may be the collision between the ball and the carriage, the overall sound pressure levels of the linear bearings were analyzed combining the Hertzian theory and the results of previous studies on collision sound. From the results of experiments and analyses, the conclusions were obtained as follows: (1) The overall sound pressure level of the linear bearing with steel balls can be reduced by about 4.5 dB by using Si3N4 balls having the same diameter; (2) The overall sound pressure levels of linear bearings with steel balls or ceramic (Si3N4) balls were both increased by about 9.8 dB as the linear velocity increased an octave; and (3) The analytical results, based on the assumption that the main cause of the noise emitted from the linear bearings is the collision between the ball and the carriage, match the experimental results well.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):487-498. doi:10.1115/1.1538618.

This research presents an analytical method to calculate the characteristics of the ball bearing under the effect of the waviness in its rolling elements and the centrifugal force and gyroscopic moment of ball. The waviness of rolling elements is modeled by using sinusoidal function, and the centrifugal force and gyroscopic. moment of ball are included in the kinematic constraints and force equilibrium equations to produce the nonlinear governing equations. To improve the convergence of the numerical solution of the nonlinear governing equations, it includes the derivatives of the gyroscopic moment and load-deflection constant of each race in the Newton-Raphson formulation. The accuracy of this research is validated by comparing with the prior research, i.e., (i) the contact force, contact angle in case of considering only the centrifugal force and gyroscopic moment of ball, and (ii) the contact force and vibration frequencies in case of considering only the waviness, respectively. It investigates the stiffness, contact force, displacement and vibration frequencies of the ball bearing, considering not only the centrifugal force and gyroscopic moment of ball but also the waviness of the rolling elements.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):499-506. doi:10.1115/1.1538190.

A finite element analysis, for an elastic perfectly plastic sphere normally loaded by a rigid flat, is combined with an approximate analytical solution to evaluate the maximum tangential load (static friction) that can be supported by the spherical contact at the inception of sliding. Sliding inception is treated as a failure mechanism based on plastic yield rather than a Coulomb friction law with a certain friction coefficient. Two different failure modes are identified, either on the contact area or below it, depending on the elastic-plastic status of the normal preloading. A limiting normal preload is found above which the contact cannot support any additional tangential load. Simple analytical expressions for an “internal static friction coefficient” are presented for both the elastic and the elastic-plastic regimes.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):507-512. doi:10.1115/1.1538192.

The paper focuses on the effect of spin motion upon elastohydrodynamic (EHD) traction in a CVT traction drive. The effect of the spin is determined upon the traction curves, traction coefficient and the slope of linear section of the traction curves. A kinematic study of a standard EHD contact leads to the general formula that governs the spin motion within the area of contact. Subsequently, this equation is applied to a traction drive with conical discs, balls and support ring. Furthermore it is determined that the spin motion at the contacts between balls and supporting ring creates additional loss. A new, original type of supporting ring that may ameliorate the spin loss is presented. This new support ring changes the type of contact between balls and supporting ring by replacing one heavily loaded contact between a ball and the ring with two spin-optimized contacts.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):513-522. doi:10.1115/1.1538193.

A plane isothermal elastohydrodynamic problem for a lubricated line contact is studied. The lubricant represented by a base stock with some polymer additive undergoes stress-induced degradation due to scission of polymer additive molecules. The polymer molecules have linear structure. The degradation process of a polymer additive dissolved in a lubricant while the lubricant passes through the contact is described by a kinetic equation. The kinetic equation is solved along the lubricant flow streamlines. The solution of the kinetic equation predicts the density of the probabilistic distribution of the polymer molecular weight versus polymer molecule chain length. The changes in the distribution of polymer molecules affect local lubricant properties. In particular, the lubricant viscosity changes as polymer molecules undergo scission. These irreversible changes in the lubricant viscosity alter virtually all parameters of the lubricated contact such as film thickness, frictional stresses and pressure. As a result of the polymer additive degradation the lubricant experiences a significant viscosity loss. The viscosity loss (up to 60 percent), in turn, leads to a noticeable reduction in the lubrication film thickness (up to 12 percent) and frictional stresses applied to contact surfaces in comparison with the case of a nondegrading lubricant. Moreover, the pressure distribution in degrading lubricants exhibits extremely sharp spikes of about 2.15 to 2.82 (depending on the slide-to-roll ratio) times greater than the maximum Hertzian pressure. That may lead to noticeable variations in fatigue life of the contact surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):523-532. doi:10.1115/1.1537750.

A wall-slip model including limiting shear stress and the occurrence of slip at the interfaces between the lubricant and the adjacent surfaces is presented. The lubricant model is applied to EHL line contacts using smooth surfaces and isothermal conditions. The main part of the model concerns the lubricant velocities at the surfaces that are decoupled from the corresponding surface velocity giving two new variables in the EHL equations. The lubricant velocities at the surfaces are related to the corresponding shear stresses. As long as the value of the shear stress is below the limiting shear stress, the lubricant velocity is equal to the surface velocity. However, when the shear stress reaches the limiting shear stress, interfacial slip appears and the lubricant velocity differs from the surface velocity. Some initial results are presented and compared to a Newtonian analysis.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):533-542. doi:10.1115/1.1538617.

The Spacer Layer Imaging method has been used to investigate the influence of three-dimensional roughness features on the thickness and shape of elastohydrodynamic (EHL) films. An array of near-hemispherical bumps was employed to represent asperities. A micro-EHL film developed at the bumps whose orientation depended on that of the inlet boundary at the location at which the bump had entered the contact. Rolling-sliding conditions induced a micro-EHL film with a classical horseshoe shape at the bumps. The flow of lubricant around the bumps appeared to differ between thin and thick films.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):543-551. doi:10.1115/1.1539058.

A multigrid numerical solution algorithm has been developed for the laminar (Stokes) flow of a compressible medium in a thin film. The solver has been applied to two model problems each representative of lubrication problems in a specific way. For both problems the solutions of the Stokes equations are compared with the solutions of the Reynolds equation. The configurations of both model problems were chosen such that based on the ratio film thickness to contact length (H/L) the difference between the Reynolds and the Stokes solutions will be very small, so the geometry of the gap itself does not lead to a significant cross film dependence of the pressure. It is shown that in this situation the compressibility can still lead to a cross-film pressure dependence which is predicted by the Stokes solution and not by the Reynolds solution. The results demonstrate that limitations exist to the validity of the Reynolds equation related to the compressibility of the medium.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):552-557. doi:10.1115/1.1537752.

In the past there has been considerable interest in the possibility of using liquid lubricants containing dispersed, solid particles in the 1–50 micron size range to reduce friction and wear. These particles are used in greases and some industrial oils. Researchers are now directing their attention to the behavior of much smaller colloidal particles in the range of 5 nm to 200 nm diameter. Such systems are formally known as “colloidal sols” and have been claimed to influence friction and wear. Further reasons for studying such colloidal particles is that they are present in soot-contaminated engine lubricating oils, as wear debris and as partially-soluble additives. Thus, the objective of the work derived in this paper was to investigate the mechanism of action of colloidal solid particles in the range of 5 to 200 nm diameter in lubricating oils. Of particular interest was the effect of slide-roll ratio on particle entrainment and the influence of the ratio of particle diameter to elastohydrodynamic lubricant film thickness on particles’ behavior. This study has shown that in thin film contacts, colloid nanoparticles penetrate EHD contacts mainly by a mechanism of mechanical entrapment. It is found also that in rolling contacts at slow speeds, colloids formed a boundary film of at least 1 or 2 times the particle size. This film influence friction and wear. However, this film is lost at high speed and the film thickness reverts to the colloid-free fluid. The results of this study have enabled a mechanism of lubricating action by colloid sols to be derived.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):558-561. doi:10.1115/1.1538620.

In the original derivations of the first order and the second order slip models of the generalized Reynolds equation in the literature [3,4], a length scale equal to the mean free path of the gas molecules was used in a Taylor series expansion of the mean velocity field. The coefficients of the correction terms in the derived lubrication equation depend on that length scale. This choice of the length scale is arbitrary to some extent. In this paper, new first order and the second order slip models are derived using a somewhat more physical approach, in which the requirement that the expansion length scale be the mean free path is relaxed. In this approach the momentum transfer rate across each surface element is obtained by summing up the contributions from each group of molecules impinging on the surface at an angle θ to the surface normal within a solid angle dω. The new second order slip lubrication equation appears to be preferable to the original one when the inverse Knudsen number is small, and it is free of any contact pressure singularity, whereas the new first order slip model continues to contain the unacceptable pressure singularity in the limit as the spacing approaches zero, as does the original first order model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):562-570. doi:10.1115/1.1537269.

We numerically investigated contact characteristics of a contact pad with a rough disk surface and the possibility of contact/near-contact sliders, using a single-degree-of-freedom (1-DOF) slider and a random wavy surface model with random roughness. Contact characteristics of a contact pad are numerically calculated based on a modified Greenwood-Williamson model, considering the bulk deformation of the surface due to all other asperity contact forces. It was found that contact stiffness and other characteristics are mainly determined by asperity contact, to the extent that the contact pad penetrates into the upper standard deviation of asperity peak height. However, the contact stiffness tends to approach a constant value as the pad penetrates into the average asperity height because the bulk deformation becomes predominant. From the numerical simulations of a 1-DOF air bearing slider model in contact and near-contact regimes over a random wavy surface with random roughness, a typical example of design condition of disk surface waviness in terms of the tracking ability and wear durability are shown, and the possibility and difficulty of a contact/near-contact slider is discussed. Finally, we analyzed meniscus effects on the contact characteristics and found a hysteresis process of the touch down and take off of a slider due to the meniscus force.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):571-575. doi:10.1115/1.1540124.

At a flying height of 10 nanometers, contacts between slider and disk are likely to occur, and control of contact-induced slider vibrations is an important design consideration. In this study, slider vibrations during contact are investigated using a digital laser Doppler vibrometer (LDV). The noise level of the digital interferometer is compared with that of a conventional analog LDV. In addition, acoustic emission (AE) sensors are used to evaluate the contact behavior of the slider. A comparison of AE and LDV data is performed. The results show that the noise level of the digital LDV is lower than that of the analog LDV, and that suspension sway mode vibrations and torsion mode vibrations are excited during contact as a function of the skew angle.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):576-581. doi:10.1115/1.1537265.

Thermosonic ball bonding is the method of choice for many interconnections to integrated circuits. This study investigated the effects of bonding parameters on bonding strength using a thermosonic bonding machine and a shear tester. Theoretical analyses were conducted to relate bonding strength to interfacial contact phenomena. Our results show that bonding strength of thermosonic wire bonds can be explained based on frictional energy intensity and real contact area. When the ultrasonic power is small, the bonding strength increases with increasing real contact area mainly caused by the bonding force. Increasing the bonding force can hardly increase the frictional energy intensity, but it can increase the real contact area, thus increasing the shear force. For larger ultrasonic power, the ultrasonic power plays an important role in increasing the bonding strength at the interface between the wire and the pad. Increasing the ultrasonic power increases both the frictional energy intensity and the real contact area, thus increasing the shear force until before the frictional energy intensity reaches a critical point. Moreover, increasing the welding time increases both the frictional energy intensity and the real contact area, thus increasing the shear force before the critical frictional energy intensity is attained; this is far smaller than the critical frictional energy intensity when the ultrasonic power is varied.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):582-586. doi:10.1115/1.1538632.

Prior work has shown that there exist a sub-ambient fluid pressure at the interface between a rigid flat and the polishing pad during chemical mechanical polishing (CMP). This sub-ambient fluid pressure can have a significant impact on the polishing process since its magnitude may be similar to the applied load, depending on conditions. Further results have shown that there is a relationship between pad soaking time and the magnitude of this sub-ambient fluid pressure. This paper addresses measurements of the pad soaking time versus the magnitude of the sub-ambient interfacial fluid pressure. Experiments utilized a Rodel IC1000 polishing pad made of foamed polyurethane with average void size of 30 to 50 microns. Pad soaking tests indicated that the weight of the pad increased with soaking time due to water absorption. There is a high rate of water absorption initially before the pad becomes saturated and the mass of the pad stabilizes. It is also observed that the pad material is impermeable to water and most of the water penetrated only the topmost layer of voids in the material. These experiments suggest that the water progressively “softens” the top layers of the pad during the soaking and causes the sub-ambient fluid pressure to increase in magnitude. A model of the sub-ambient fluid pressure increasing as the elastic modulus of the pad decreases is also suggested.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):587-591. doi:10.1115/1.1538194.

The lubrication by thin film has become a very important role in micro machine, magnetic storage device and so on. As the thickness of lubricant film becomes thinner to several nanometers, the conventional law of lubrication becomes unable to use. Nonequilibrium molecular dynamics simulation (NEMD) was carried out to investigate the dynamic behavior of thin lubricant film confined between walls. The model used in these simulations is composed of two solid walls and fluorocarbon polymer lubricant. One of the walls is supporting a load and at the same time moving at constant velocity. Results indicate that the frictional behavior of confined lubricant varied with load; velocity field in the film retain liquid like structure under low load conditions, on the other hand, under high load conditions lubricant film becomes solidified and periodical stick and slip motion is observed at the layer near the wall. At the same time periodically vibrating friction force is observed. In this case, radius of gyration of lubricant molecules also changes periodically. It is concluded that the periodical vibration of friction force is caused by stick-slip with molecular deformation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):592-599. doi:10.1115/1.1538621.

Honeycomb damper seals with convergent-tapered clearance have been proposed to improve static and dynamic characteristics of liquid annular seals employed in pumps. Their characteristics are experimentally investigated and compared to those for a conventional straight (no taper) annular seal with smooth surface and a straight damper seal with identical honeycomb pattern in seal stator. Three convergent-tapered honeycomb damper seals are used in the test, and have different inlet clearance (maximum clearance) and almost the same outlet clearance (minimum clearance). Their outlet clearance is almost the same as the clearance of the straight smooth seal and is slightly smaller than the clearance of the straight damper seal. Experimental results show that the convergent-tapered damper seals as well as the straight damper seal have lower leakage flow rate and cross-coupled stiffness coefficients, and larger main damping coefficients than the straight smooth seal, resulting in larger effective damping coefficients. These results are mainly due to surface roughness in the seal stator such as a honeycomb pattern used in the present analysis. The convergent-tapered damper seals also have larger main stiffness coefficients than the straight smooth and damper seals, which is mainly due to the convergent-tapered clearance and yields larger radial reaction force for a small concentric whirling motion. Consequently, the convergent-tapered damper seals have better seal characteristics than the conventional straight smooth seal and the straight damper seal with the same roughness pattern from the viewpoints of decreasing the leakage and improving the rotor stability capacity.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):600-607. doi:10.1115/1.1472456.

The development of thermal bows in rotors is frequently reported. The cause of the bows is an asymmetric temperature distribution as a result of uneven heating. The heat source can be a friction contact between rotor and seal. Both partial and full annular rub can give asymmetric heating. A general model for these two cases is a rotor with an arbitrary elliptic orbit and an eccentric position in an elliptic seal. Depending of these kinematics, different regions of the rotor and seal are heated and the heat flow resistance effected. Closed-form solutions of the heat conduction in three spatial dimensions are well known for the point source and after integration this is suited for friction heat analysis. When the temperature distribution is expressed the thermal bow follows as a volume integral. This paper focuses on the relation between rotor vibration (via friction heating in the seals) and the thermal bow. The coupling from rotor bow back to vibrations is treated in a later work. The main contribution of this paper is a semi-analytical solution of the thermal bow dynamic function.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):608-616. doi:10.1115/1.1510885.

After a short presentation of state-of-the-art experimental studies on the thermal behavior of non-contacting face seals, the literature about numerical models for thermal effects is investigated. Next, the geometry, kinematics and dynamics of a steady state three-dimensional model are developed. Simplified Navier-Stokes equations, a generalized Reynolds equation and an energy equation with proper boundary conditions are established for flow regimes, varying from laminar to turbulent. The numerical computer code for solving the governing equations is presented and representative results are shown. It is demonstrated that face distortions strongly modify the seals’ thermal behavior. An original test rig has been developed in order to ensure full fluid film conditions. This apparatus and the experimental procedure are described. The ability of the numerical model to simulate real configurations is also illustrated. Theoretical and experimental results are in good agreement. Yet an improved model of heat transfer on boundaries is still needed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):617-627. doi:10.1115/1.1510886.

In this paper, the influence of the design and operating parameters on the TEHD behavior of Mechanical Face Seals (MFS), in steady dynamic tracking mode, is analyzed for two different types of applications. First, an extensive parametric analysis of a typical MFS with very low leakage is presented. Then, the influence of rotational speed, sealed fluid temperature and pressure, rings materials, shape, waviness and misalignment of the rotor, are successively examined. The use of an original dimensionless parametric analysis leads to a very simple and overall description of the results. It is shown that ignoring the thermoelastic distortions of the rings could be misleading as far as the evaluation of MFS performance is concerned. In the final part, a hydrostatic MFS with a very large gap and flow rate is studied. The increase of the rotational speed induces a progressively turbulent radial flow. In this case, it is shown that neither thermal effects nor fluid flow regime significantly affect seal behavior.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):628-637. doi:10.1115/1.1538189.

A new method has been developed to directly measure valve train friction as a function of crank angle using specially designed timing belt pulley torque transducers fitted to the inlet and exhaust camshafts of a single-cylinder gasoline engine. Simultaneous and instantaneous friction torque of both the inlet and exhaust camshafts at any engine speed can be measured, with no apparent detrimental effect of timing belt loading on the output reading. Experiments are reported for valve train friction at a range of motored engine operating conditions with different lubricant formulations, with and without a friction modifier. These are compared with the predictions of an existing valve train friction model based upon elastohydrodynamic lubrication theory. Measured friction decreased with increasing engine speed but increased with increasing oil temperature and the fuel economy benefit of friction modifiers was observed. The model yielded similar magnitudes of friction at medium engine speeds and above but predicted much lower friction with high oil temperatures at low speed. Comparison of theory and experiments also suggests that some oil may leak from hydraulic lash adjusters during the cam event with a consequent reduction in geometric torque.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):638-642. doi:10.1115/1.1537751.

In the wear testing of prosthetic joints, the optimal lubricant protein concentration is disputed. The effect of protein concentration of calf serum based lubricant on the wear of ultra-high molecular weight polyethylene against CoCr was studied with a 12-station, circularly translating pin-on-disk device. The wear factor first steeply increased with increasing concentration, reached a peak at 10–20 mg/ml, and then slowly decreased. Below 20 mg/ml, the wear mechanisms were not entirely representative of clinical wear. Above this value, the morphology of the UHMWPE wear surface resembled that of retrieved cups. The results indicated that the concentration should not be below 20 mg/ml. The scope of this recommendation is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):643-648. doi:10.1115/1.1537745.

Two new technologies have recently been developed that can help to solve some of the wheel rail contact problems. The first is a method of top of rail lubrication (TOR) or friction modification (FM). The second is a technique of laser glazing of steel rails. Both technologies help in reducing the friction, wear, and energy consumption in the wheel rail contact. This paper introduces the two technologies and presents some specific aspects of both methods. A 1:12 scale wheel/rail simulator (LA 4000) was used to study the potential of these two new technologies on energy savings. In order to develop an efficient top of rail lubrication system, all parameters affecting FM consumption rates have been studied. These parameters include speed, angle of attack, load and lubricant quantity. LA 4000 friction/wear studies were conducted to evaluate the effect of laser glazing and TOR lubricant on the lateral slip forces between a simulated wheel/rail. Three conditions under dry and lubricated environments were studied: unglazed wheel and an unglazed rail, an unglazed wheel against a glazed rail, and a glazed wheel against a glazed rail. The results of the tests indicate that the use of TOR and laser glazing does indeed reduce the lateral forces, which are an indirect measure of the damage caused to the wheel, rail and track.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):649-660. doi:10.1115/1.1537747.

A high-speed photographic study has been made of the chip-tool interface and its evolution when cutting pure metals with optically transparent sapphire tools. The use of a high speed camera in conjunction with an optical microscope has enabled details of the interface, including the velocity field along the interface, to be resolved at high spatial and temporal resolution while cutting at speeds between 1 mm/sec and 2000 mm/sec. The results show the chip-tool contact along this interface to be composed of four distinct regions: a region of stagnation at the cutting edge, a region of retardation adjoining the stagnation region, a region of sliding beyond the retardation region, followed by a region of metal transfer or “sticking” that is located furthest away from the cutting edge alongside the boundary of the contact. The chip and tool appear to be in intimate contact over the stagnation, retardation, and sliding regions, with sliding occurring at the interface over much of this zone of intimate contact. These observations have provided direct experimental evidence for a model of the contact conditions proposed by Enahoro and Oxley based on analytical considerations. Cutting experiments with non-oxide tools such as aluminum and high speed steel suggest that this description conditions is equally applicable to tool materials other than sapphire.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):661-669. doi:10.1115/1.1540123.

Tribological properties of several kinds of polyoxymethylene (POM) composites were evaluated for the purpose of developing a polymeric tribomaterial especially suited for mating with aluminum parts having low surface hardness. POM composites containing small amounts of silicon carbide (SiC), POM/SiC; those containing a small amount of calcium octacosanonoate besides SiC, POM/SiC/Ca-OCA; and the one blended with 24 wt % of polytetrafluoroethylene, POM/PTFE(24); were injection-molded into pin specimens and their tribological properties were tested by means of a pin-on-disk type wear apparatus using an aluminum (A5056) mating disk in comparison with a 303 stainless steel (SUS303) disk. Evaluation was focused on observation of the sliding surfaces of the pin specimens and the mating disks by a scanning electron microscope and an optical microscope together with the measurement of surface roughness. In the case of mating against a SUS303 disk having high surface hardness, all pin specimens did not roughen the disk surfaces even after long time of rubbing. Only POM/PTFE(24) composite obviously made a transfer film on the disk surface, while the other composites made an extremely thin one on it. POM/SiC(0.1)/Ca-OCA(1) composite, containing SiC 0.1 wt. % and Ca-OCA 1 wt. %, was found to show the lowest coefficient of friction and the lowest wear rate forming extremely thin transfer film on the mating disk. On the other hand, against an A5056 disk which has lower surface hardness than that of SUS303 disk, unfilled POM and POM composites except POM/SiC(0.1)/Ca-OCA(1) composite roughened the disk surfaces. However, the sliding surface of the A5056 disk rubbed with POM/SiC(0.1)/Ca-OCA(1) composite was significantly smoother and that of the pin specimen was also quite smooth in comparison with other pin specimens. Further, when each POM composite was rubbed against the A5056 disk, formation of transfer film was not obvious on the disk surfaces. For POM/SiC(0.1)/Ca-OCA(1) composite, the wear rate was the lowest of all POM composites, and the coefficient of friction was as low level as 60 percent of that of unfilled POM, but slightly higher than that of POM/PTFE(24) composite. For POM/SiC(0.1)/Ca-OCA(1) composite, the nucleating effect of SiC and Ca-OCA, which accelerated the crystallization of POM during its injection molding to form a matrix containing fine spherulites, must have resulted in increasing the toughness of the matrix and lowering the wear rate. Also, the lubricant effect of Ca-OCA should have lowered the coefficient of friction of the same matrix for rubbing against aluminum mating disk. POM/SiC(0.1)/Ca-OCA(1) composite was concluded as an excellent tribomaterial for mating with aluminum parts.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(3):670-677. doi:10.1115/1.1538191.

A two-dimensional friction model has been developed for cold metal rolling in the “mixed” lubrication regime. Roughness is modelled using superimposed short and long wavelength asperities with a lay orientated along the rolling direction. The hydrodynamic pressure in the lubricant is solved using Reynolds’ equation, coupled with the crushing process of the two-wavelength roughness. This allows for the solution of film thickness and contact area ratio and hence friction coefficient through the roll-bite. The model extends the authors’ earlier model [15] by allowing for a variation in hydrodynamic pressure across the width of the contact. Predictions for both the surface roughness and the friction coefficient are in reasonable agreement with published measurements.

Commentary by Dr. Valentin Fuster

TECHNICAL NOTE

J. Tribol. 2003;125(3):678-681. doi:10.1115/1.1537271.

A nearly ideal two-dimensional scotch yoke mechanism was constructed to test a model of wear depth as a function cycle number. Model variables include the reciprocating mass, a two dimensional wear-rate, crank radius, and angular velocity. The model originally developed by T. A. Blanchet (1997), was nondimensionalized and simplified under conditions of large numbers of cycles, to predict the importance of including coupling based solely on a ratio of maximum allowable wear depth to the crank radius. Experiments show a linear progression of wear over two distinct regions, suggesting a sudden transition in wear modes just after 1.5 million cycles. The need for cycle or time dependent wear rates in analysis, which is a potentially far more significant source of error, is clearly illustrated by the experiment and discussed.

Topics: Wear , Cycles , Mechanisms , Errors
Commentary by Dr. Valentin Fuster

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