Research Papers: Applications

J. Tribol. 2010;133(1):011101-011101-10. doi:10.1115/1.4002729.

A three-dimensional computational thermal contact model is developed. The approach utilizes a combination of the transfer matrix and finite element methods. The frictional heat generated at the contact interface is instantaneously partitioned between the bushing and the shaft. Two methods to couple the heat and temperature at the contact interface are presented. One method automatically accounts for the heat division between contacting bodies by satisfying the heat equilibrium and temperature continuity at interactive surfaces. The other method introduces a fictitious layer between contacting bodies with a specified gap conductance to partition the frictional heat. Application of the model to the heat transfer analysis of journal bearing systems experiencing oscillatory motion is presented. Nonuniformly distributed frictional heat along the axial direction is considered. The model is capable of predicting the transient temperature field for journal bearings. It can also be used to determine the maximum contact temperature, which is difficult to be measured experimentally. Comparison of the simulated resulted along with experimental tests conducted in a laboratory is presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 2010;133(1):011102-011102-12. doi:10.1115/1.4003088.

Multibody dynamics of healthy and faulty rolling element bearings were modeled using vector bond graphs. A 33 degree of freedom (DOF) model was constructed for a bearing with nine balls and two rings (11 elements). The developed model can be extended to a rolling element bearing with n elements and (3×n) DOF in planar and (6×n) DOF in three dimensional motions. The model incorporates the gyroscopic and centrifugal effects, contact elastic deflections and forces, contact slip, contact separations, and localized faults. Dents and pits on inner race and outer race and balls were modeled through surface profile changes. Bearing load zones under various radial loads and clearances were simulated. The effects of type, size, and shape of faults on the vibration response in rolling element bearings and dynamics of contacts in the presence of localized faults were studied. Experiments with healthy and faulty bearings were conducted to validate the model. The proposed model clearly mimics healthy and faulty rolling element bearings.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings & Solid Lubricants

J. Tribol. 2010;133(1):011301-011301-7. doi:10.1115/1.4003066.

ZrB2-containing composite coating was fabricated on pure Ti substrate by laser surface alloying. The microstructure of the composite coating was investigated by means of X-ray diffraction and scanning electron microscopy (SEM). The friction and wear properties of ZrB2-containing composite coating sliding against a GCr15 steel ball at different normal loads and sliding speeds were evaluated. The morphologies of the worn surfaces were analyzed by means of SEM and three dimensional noncontact surface mapping. It is shown that the microhardness and wear resistance of the pure Ti substrate are greatly increased after laser surface alloying, due to the formation of hard ZrB2 phase in the composite coating. Pure Ti substrate sliding against the GCr15 counterpart ball at room temperature is dominated by adhesion wear, abrasive wear, and severe plastic deformation, while ZrB2-containing composite coating involves only mild abrasive and fatigue wear when sliding against the GCr15 counterpart.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2011;133(1):011401-011401-9. doi:10.1115/1.4002953.

Aeolian vibrations represent a threat to the integrity of electrical transmission lines. The fretting fatigue of conductors is thus a major concern. The modelization of the contact conditions at critical points is an important tool in assessing the life of conductors. Treillis points around the last point of contact between the conductor and the pieces of equipment are such critical points. We observe a fully plastic contact condition at these points. Finite element results for the contact between an ellipsoid and a rigid plane and between two wires at different angles are compared with an elastic-plastic microcontact model for elliptical contact areas. These numerical results are then compared with experimental ones for the contact between two wires of a conductor (ACSR Bersfort), showing a very similar relationship between the contact force and the observed contact area. We have a good correlation between the microcontact model and the finite elements ones in the fully plastic contact regime on both the contact area and the contact force for a given interference between bodies. The use of the elastic-plastic microcontact model for elliptical contacts presented in this paper proves to be a strong tool in getting a better understanding of the mechanical behavior at those critical points.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2010;133(1):011501-011501-6. doi:10.1115/1.4002745.

Knowledge on the behavior of lubricant trapped in a surface pocket is important for improving metal forming technology, since the trapped lubricant affects friction and surface finishing. The permeation phenomena at higher reduction in height were quantitatively observed during the upsetting of cylinders with a central conical dent using the new fluorescence direct observation apparatus developed by the authors. Moreover, the permeation phenomena were estimated using a rigid-plastic finite element analysis model incorporating the compressibility of the lubricant. From the experimental results, it was quantitatively observed that over a reduction in height of 37%, the outline of the central conical dent became blurred, and the lubricant trapped within the conical dent permeated into the peripheral real contact area. It was also quantitatively observed that the volume of the lubricant trapped within the conical dent decreased gradually and abruptly with increasing reduction in height up to and above 36%, respectively. From the numerical results, it was estimated that the trapped lubricant permeated when the hydrostatic pressure generated within the lubricant pocket exceeded the die pressure at higher reduction in height.

Commentary by Dr. Valentin Fuster

Research Papers: Friction & Wear

J. Tribol. 2010;133(1):011601-011601-11. doi:10.1115/1.4002543.

This article presents the investigations on modified mechanical and wear characteristics of cement kiln dust (CKD) reinforced homogeneous epoxy composites and its functionally graded materials developed for tribological applications. CKD reinforced homogeneous and functionally graded epoxy composites are developed by simple mechanical stirring and vertical centrifugal casting technique, respectively. Mechanical properties of these graded composites are evaluated and compared with those of homogenously filled epoxy composites. Sliding wear tests are conducted over a range of sliding velocities (105–314 cm/s), normal loads (20–40 N), filler contents (020wt%), and sliding distances (0.5–2 km). For this, a pin-on-disk machine and the design of experiments approach using Taguchi’s orthogonal arrays are used. A theoretical model is proposed for estimating the sliding wear rates for homogeneous, as well as graded composites. The results found from the theoretical model so proposed are found to be in good agreement with the experimental values under similar test conditions. This study reveals that the presence of cement kiln dust particles enhances the sliding wear resistance of epoxy resin and the homogeneous composites suffer greater wear loss than the graded composites. scanning electron microscopy micrograph confirms the graded dispersion of CKD particles in the matrix.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2010;133(1):011701-011701-10. doi:10.1115/1.4002730.

Aerostatic porous bearings have been successfully applied to various precision devices such as machine tools and measuring equipment to achieve a higher accuracy of motion. However, aerostatic porous bearings have a disadvantage in that they are prone to cause pneumatic hammer instability. Therefore, to avoid this instability, a surface-restricted layer that has permeability smaller than the bulk of the porous material is usually formed on the bearing surface. In this paper, the dynamic characteristics of aerostatic porous journal bearings that have a surface-restricted layer are investigated numerically and experimentally. The effects of permeability in bulk porous materials and of a surface-restricted layer on the bearing characteristics are discussed using two kinds of porous material: graphite and metal. It was confirmed that aerostatic porous metal bearings with relatively large permeability could achieve large values of dynamic stiffness and damping coefficients using a low permeability, surface-restricted layer.

Commentary by Dr. Valentin Fuster
J. Tribol. 2010;133(1):011702-011702-8. doi:10.1115/1.4002070.

Considering the freedom of pad tilting and pad translation along preload orientation, an analytical complete model, as well as mathematical method, which contains 2n+2degrees of freedom, is presented for calculating the dynamical characteristics of tilting-pad journal bearing. Based on the motion relationship of shaft and pad, the local coordinate system, the generalized displacement, and the generalized force vector are chosen. The concise transformation of generalized displacement, generalized force, and its Jacobian matrix between the local and global coordinate systems are built up in matrix form. A fast algorithm using the Newton–Raphson method for calculating the equilibrium position of journal and pads is proposed. The eight reduced stiffness and damping coefficients can be obtained assuming that the journal and all pads are subject to harmonic vibration. Numerical results show that the reduced damping coefficients and the threshold speed can be effectively enhanced by giving suitable pad pivot stiffness and damping simultaneously, and this analytical method can be applied to analyze dynamical behavior of the tilting-pad journal bearing rotor system.

Commentary by Dr. Valentin Fuster
J. Tribol. 2010;133(1):011703-011703-12. doi:10.1115/1.4003019.

A model is developed to investigate the effect of radial grooves and waffle-shape grooves on the performance of a wet clutch. Three-dimensional formulation of the governing equations, boundary conditions, and numerical solution scheme are presented for modeling the thermal aspects of the engagement process in a wet clutch. The thermal model includes full consideration of the viscous heat dissipation in the fluid as well as heat transfer into the separator, the friction material, and the core disk. The convective terms in the energy equations for the oil as well as the heat conduction equations in the bounding solids are properly formulated to determine the temperature fields corresponding to the domains between grooves. Roughness, centrifugal force, deformability, and permeability of the friction material with grooves are taken into account. The effects of groove geometry such as groove depth, grooved area, and number of grooves on the engagement characteristic of a wet clutch are investigated. It is also shown that the thermal effects in a wet clutch influence the engagement time and the torque response and should be included in the analytical studies.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(1):011704-011704-9. doi:10.1115/1.4003148.

To improve the performance of a reversible rotation herringbone journal bearing (Rev-HGJB), this study uses reversible elliptical grooves on a journal bearing (Rev-EGJB) and numerically analyzes its characteristics, utilizing the spectral element method. Load capacity, pressure distribution, power loss, and dimensionless radial stiffness of the Rev-EGJB are compared with those of the Rev-HGJB. This comparison shows that the introduced Rev-EGJB exhibits a higher load capacity and a lower power loss than the Rev-HGJB. The pressure region in the Rev-EGJB is higher than that in the Rev-HGJB, which is achieved not only in the pressure-generated region, but also in the pressure-restored region. The load distributions of the Rev-HGJB and Rev-EGJB are also compared in order to determine how the elliptical grooves enhance the load characteristics. The optimum groove parameters of the Rev-EGJB at an eccentricity of 0.1 are investigated by studying the groove parametric matrix, which is given by taking several values in the effective range of each groove parameter. Ultimately, the radial stiffness of the Rev-EGJB with grooved bearing was also shown to be greater compared with that of a Rev-HGJB with optimum geometry; thus, the Rev-EGJB is more stable than the Rev-HGJB when the bearing is grooved.

Commentary by Dr. Valentin Fuster

Research Papers: Mixed and Boundary Lubrication

J. Tribol. 2010;133(1):012101-012101-9. doi:10.1115/1.4003114.

Although some authors work at times with large flat dies and evaluate friction under more realistic conditions than usual, pressure is not totally controlled. In any case, cohesive friction does not appear to have been well studied in literature, but pressure and sliding velocity may provide useful information about preventing the cohesive phenomenon in sheet stamping processes. In this work, the coefficient of friction (COF) for DC-05 electrogalvanized steel is experimentally evaluated under lubrication regime by flat face dies. These tests are also considered to reproduce friction conditions in the die-sheet-blankholder system at some stages of the deep drawing process. High pressure condition in a flat friction system can also be considered for studying the friction behavior in the die radius. This work investigates the influence of contact pressure and sliding velocity of the sheet on the COF value. Adhesion tendency during sliding is also evaluated. Sheets were lubricated with a prelube type mineral oil and different lubricant film thicknesses are present on the sheet as a result of the draining off time effect, an aspect that will be evaluated later. Although sliding velocity has almost no influence on the COF value, pressure has an influence that may be expressed by a potential mathematical function. The COF value tends to be constant for high enough pressure values. This behavior may be explained, in part, from the viewpoint of zinc acting as a typical soft metallic lubricant. Sliding velocity is the most important variable from the adhesion phenomenon point of view, which appears more frequently for low velocity values. The draining off time, which some research works consider fixes the initial lubrication conditions in friction tests, has no significant effect when a mineral oil, typically used as a prelube, is selected as a lubricant. The authors found that pressure is the most important variable for the COF value. Velocity is the determining factor for the adhesion phenomenon in friction processes under mixed lubrication.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2010;133(1):012201-012201-8. doi:10.1115/1.4002929.

The sealing of valve stems is ensured by the traditional systems of packed stuffing boxes. The performance of this type of sealing system, which is also used in rotating equipment, is dependent on the radial contact pressures generated by the packing axial compression. The mechanical behavior of a packing seal is characterized by the transmission ratio of the radial stress over the axial stress known as the lateral pressure coefficient, which is one of the required parameters used to select packing seals. However, the modeling of the packed stuffing box requires the knowledge of other packing seal mechanical characteristics such as compression modulus and Poisson’s ratio. In this paper, the mechanical characteristics of packing seals are obtained using a hybrid experimental-numerical procedure. The experimental study is carried out on an instrumented stuffing box packing test bench. The tests results of the experiments are coupled to the ones obtained by a finite element simulation of the test rig to determine the packing seal mechanical characteristics. Two packing types are used: one based on Teflon and the other one based on flexible graphite. In addition, leak rates are measured for different axial compressive stresses and gas pressures in order to estimate the tightness performance of such seals.

Commentary by Dr. Valentin Fuster
J. Tribol. 2010;133(1):012202-012202-9. doi:10.1115/1.4002218.

Previously, researchers investigated the mechanism of surface defect evolution in rolling. It was highlighted how the lubricant plays an essential role for the final strip surface quality. In some cases the lubricant can be entrapped in pits or in other defects where hydrostatic pressure tends to prevent its elimination; however, when some favorable conditions are satisfied, the lubricant can be drawn out by hydrodynamic actions and defects can be recovered. This mechanism has been described as microplastohydrodynamic lubrication (MPHL) and recent studies report a suitable parameter (the ratio of the oil drawn out from the pit to the initial pit volume) as MPHL characterization coefficient. The present paper deals with the recovery of isolated surface defects in the Sendzimir rolling process of AISI 304 stainless steel; the analyses have been conducted on two rolling conditions, which although quite similar, regularly showed opposite capability of defect recovery, moreover, with a trend that is in contrast with the predictions made by standard MPHL. Two effects, which are usually ignored in literature modeling, have been considered in this work: The former is the back-tension, which has relevant outcome on the contact pressure and the latter is the position of the neutral point, which cannot be assumed to lie at the end of the roll bite. The analytical treatment was supported by FEM simulations, which permitted to put realistic data into the MPHL equations, thus, to explain the experimental behavior. The analysis was then validated with two further rolling schedules that seem to confirm the proposed approach.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Tribol. 2010;133(1):014501-014501-8. doi:10.1115/1.4002604.

In the present investigation, unidirectional grinding marks were created on a set of steel plates. Sliding experiments were then conducted with the prepared steel plates using Al–Mg alloy pins and an inclined pin-on-plate sliding tester. The goals of the experiments were to ascertain the influence of inclination angle and grinding mark direction on friction and transfer layer formation during sliding contact. The inclination angle of the plate was held at 0.2 deg, 0.6 deg, 1 deg, 1.4 deg, 1.8 deg, 2.2 deg, and 2.6 deg in the tests. The pins were slid both perpendicular and parallel to the grinding marks direction. The experiments were conducted under both dry and lubricated conditions on each plate in an ambient environment. Results showed that the coefficient of friction and the formation of transfer layer depend on the grinding marks direction and inclination angle of the hard surfaces. For a given inclination angle, under both dry and lubricated conditions, the coefficient of friction and transfer layer formation were found to be greater when the pins slid perpendicular to the unidirectional grinding marks than when the pins slid parallel to the grinding marks. In addition, a stick-slip phenomenon was observed under lubricated conditions at the highest inclination angle for sliding perpendicular to the grinding marks direction. This phenomenon could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding.

Commentary by Dr. Valentin Fuster
J. Tribol. 2010;133(1):014502-014502-7. doi:10.1115/1.4002605.

The effect of particles size and shape on erosion rates and erosion mechanisms of 5117 steels are investigated using slurry whirling-arm ring. Six different sized silica sand particles are used as erodent. These particles are characterized in terms of their average diameter, aspect ratio, and circularity factor. The measured average diameter varies from 112.7μm to 516.4μm. The wear tests are carried out at impact velocity of 15 m/s and 30 deg and 90 deg impact angles using a sand-water mixture of 1wt% concentration. Analysis of erosion rates shows that there exists threshold energy of impacting particles at which a transition in erosion rate is noticed for sizes of 200μm. It is also observed that the erosion rate increases with the increase in shape factors (aspect ratio and circularity factor). The surface morphology of the eroded surface at impact of 30 deg shows that below 200μm, the erosion mechanism is indentation and material extrusion and above 200μm, the erosion mechanism is ploughing.

Commentary by Dr. Valentin Fuster

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