Accepted Manuscripts

Avinash A. Thakre and Arun Kumar Singh
J. Tribol   doi: 10.1115/1.4039029
Present study includes the investigation on the frictional dynamics of hard and soft solid interface using low velocity linear tribometer. Effects of gelatin gel concentration, nanoparticles concentration, normal stress and sliding velocity on the static and dynamic frictional shear stresses acting on the sliding gel block are studied using response surface methodology. The experiments are conducted in steady sliding regime well above the critical velocity. L31 orthogonal array consisting of five levels for each factor is selected for the experimentation and second order quadratic model has been generated for both the responses. The mathematic models are validated with the available trends mentioned in the literature.
TOPICS: Response surface methodology, Tribometers, Shear stress, Dynamics (Mechanics), Gelatin, Stress, Nanoparticles
John F. Booker and Stephen Boedo
J. Tribol   doi: 10.1115/1.4038985
An 'inverse' formulation is described for general problems of unsteady elastohydrodynamic lubrication (EHL). Spatial discretization gives an explicit initial-value ODE problem with (interior) nodal film thicknesses as state variables. Numerical results are compared with published experimental results for normal approach of a spherical surface to an elastic foundation.
TOPICS: Elastohydrodynamic lubrication, Film thickness
Huaidong Yang and Itzhak Green
J. Tribol   doi: 10.1115/1.4038984
This work presents a finite element study of a 2D plane strain fretting model of a half cylinder in contact with a flat block under oscillatory tangential loading. The two bodies are deformable and are set to the same material properties (specifically steel), however, because the results are normalized, they can characterize a range of contact scales (micro to macro), and are applicable for ductile material pairs that behave in an elastic-perfectly plastic manner. Different coefficients of friction (COFs) are used in the interface. This work finds that the edges of the contacting areas experience large von-Mises stresses along with significant residual plastic strains, while pileup could also appear there when the COFs are sufficiently large. In addition, junction growth is investigated, showing a magnitude that increases with the COF, while the rate of growth stabilization decreases with the COF. The fretting loop (caused by the tangential force during the fretting motion) for the initial few cycles of loading is generated, and it compares well with reported experimental results. The effects of boundary conditions are also discussed where a pre-stressed compressed block is found to improve (i.e., reduce) the magnitude of the plastic strain compared to an unstressed block.
TOPICS: Finite element analysis, Displacement, Plane strain, Tangential loading, Junctions, Boundary-value problems, Cycles, Cylinders, Friction, Steel, Stress, Materials properties
Technical Brief  
Yankui Liu
J. Tribol   doi: 10.1115/1.4038952
The elastic approach of a hollow roller compressed by two flat plates is a basic building block in roller bearing design. According to the theory of contact mechanics, a finite element model was established in this paper to study the contact problem of a hollow roller. Research results show that deformation of the hollow roller due to contact has a strong relationship with roller's hollowness ratio. A new equation for calculating the contact deformation of a hollow roller is proposed. In addition, it is found that the accuracy of existing calculation method for bending deformation is also worth studying, and a new equation for calculating bending deformation of a hollow roller is established by data fitting. Experimental results are also presented to support the results of the present work.
TOPICS: Rollers, Deformation, Contact mechanics, Design, Finite element model, Fittings, Flat plates, Blocks (Building materials), Roller bearings, Computational methods
Patrick Smyth and Itzhak Green
J. Tribol   doi: 10.1115/1.4038958
Biotribology and biomechanics are evolving fields that draw from many disciplines. A natural relationship particularly exists between tribology and biology because many biological systems rely on tribophysics for adhesion, lubrication, and locomotion. This leads to many biomimetic inspirations and applications. The current study looks to mimic the function of articular cartilage in purely mechanical systems. To accomplish this goal, a novel coupling of phenomena is utilized. A flexible, porous, viscoelastic, material is paired with a hydrodynamic load to access the feasibility and benefit of a biomimetic thrust bearing. This study presents the dynamic properties of the coupled system, as determined from transient to steady operating states. The results indicate that bio-inspired bearings have application in certain tribological systems, including biomechanical joint replacements, dampers, flexible rotordynamic bearings and seals.
TOPICS: Biomimetics, Storage, Tribology, Biomechanics, Bearings, Dampers, Engineering disciplines, Transients (Dynamics), Lubrication, Adhesion, Viscoelastic materials, Stress, Thrust bearings, Cartilage, Arthroplasty, Biotribology, Biology
Harpreet Singh, ParamPreet Singh and Hiralal Bhowmick
J. Tribol   doi: 10.1115/1.4038957
The present study is focused on the performance evaluation of MoS2, H3BO3 and MWCNT used as the potential oil additives in base oil for aluminium metal matrix composites (AMMC)-steel (EN31) tribo contact. Al-B4C composite is used for this purpose based on a set of preliminary investigation under unlubricated and fresh oil lubrication using three different types of AMMCs (Al-SiC, Al-B4C and Al-SiC-B4C) were used. A pin-on-disc tribometer is used for all the friction and wear tests under operating condition of load 9.8 N and sliding velocity of 0.5 m/s. From the particle based wet tribology, it is clear that both the additives H3BO3 and MWCNT improve the friction as well as wear behavior for selected composite contacts. MWCNT emerged out as superior amongst all the additives, whereas MoS2 additives shows marginal enhancement in frictional performance under given operating conditions. Fractography and morphological study of pin specimens are carried out to identify the underlying friction and wear mechanisms.
TOPICS: Steel, Tribology, Multi-walled carbon nanotubes, Friction, Wear, Composite materials, Tribometers, Lubrication, Aluminum, Particulate matter, Metal matrix composites, Stress, Disks, Fractography, Performance evaluation, Wear testing
Jie Zhang and Jingxuan Xie
J. Tribol   doi: 10.1115/1.4038959
Rubber O-rings are widely applied in the static and dynamic seal of machinery, energy, chemical, aviation and other fields. Mechanical behavior and sealing performance of the O-ring were investigated in this paper. Effects of pre-compression amount, fluid pressure, friction coefficient on the static and dynamic sealing performances of the O-ring were studied. The results shows that the maximum stress appears on the inside but not surface of the O-ring. The static sealing performance increases with the increasing of fluid pressure and compression amount. Reciprocating dynamic seal performance of the rubber O-ring is different with its static seal, the stress distribution and deformation are changing in reciprocating motion. Sealing performance in outward stroke is better than it in inward stroke. Overturn of the O-ring occurs when the friction torque is greater than the torque caused by fluid pressure in inward stroke. Distortion, bitten and fatigue failure are the main failure modes of the O-ring in dynamic seal. Those results can be used in design, installation and operation of rubber O-rings in static and dynamic seals.
TOPICS: Seals, Rubber, Sealing (Process), Fluid pressure, Friction, Compression, Torque, Aviation, Reciprocating motion, Fatigue failure, Machinery, Deformation, Stress, Stress concentration, Design, Failure mechanisms, Mechanical behavior
Jun Sun, Xiang Huang, Guangsheng Liu, Xiaoyong Zhao, Enming Miao, Guixiang Zhu and Yunqiang Li
J. Tribol   doi: 10.1115/1.4038960
The status of the lubricating oil transport in the piston skirt-cylinder liner has important influence on the lubrication of piston assembly frictional pair, the consumption of lubricating oil, the emission and the performance degradation of lubrication oil. In this paper, based on the model of piston secondary motion, fluid lubrication and lubricating oil flow, the status of the lubricating oil transport between the piston skirt and the cylinder liner on different engine operating condition is calculated, and the quantity of lubricating oil retained on the surface of cylinder liner is mainly analyzed when the piston skirt moves from the top dead center to the bottom dead center. The results show that the variation of the quantity of retained lubricating oil is almost same in the corresponding stroke on different engine operating condition, the quantity of retained lubricating oil is dissimilar at different moment and is equal in principle at the piston top and bottom dead center. The quantity of lubricating oil retention is dissimilar at different moment in the intake stroke or expansion stroke, but the quantity of lubricating oil retention is equal in principle at the top and bottom dead center on different engine operating condition. When the engine is on the same load condition, as the engine rotational speed increasing, the quantity of retained lubricating oil is decreased in the whole intake stroke and the middle and later part of expansion stroke, but the quantity of retained lubricating oil is increased in the front part of expansion stroke. When the engine is on the same rotational speed condition, the quantity of retained lubricating oil increases with the increasing engine load in the front part of expansion stroke, it does not vary with the engine load in principle in the middle and later part of expansion stroke, the variation that the quantity of retained lubricating oil varies with the engine load is dissimilar in the intake stroke on different engine rotational speed condition.
TOPICS: Engines, Cylinders, Lubricating oils, Pistons, Stress, Lubrication, Fluids, Manufacturing, Emissions, Flow (Dynamics)
Xiujiang Shi, Liqin Wang, Qinghua Zhou and Qian Wang
J. Tribol   doi: 10.1115/1.4038953
This paper reports a new three-dimensional model for heat conduction in a half space containing inhomogeneities, applicable to frictional heat transfer, together with a novel combined algorithm of the equivalent inclusion method (EIM) and the imaging inclusion approach for building this model. The influence coefficients (ICs) for temperature and heat flux are obtained via converting the frequency response function (FRF) and integrating Green's function. The model solution is based on the discrete convolution and fast Fourier transform (DC-FFT) algorithm using the ICs, convenient for solving problems involving multiple elliptical inhomogeneities with arbitrary orientations. A group of parametric studies are conducted for understanding the thermal fields in the inhomogeneous half space due to surface frictional heating, influenced by the properties of the inhomogeneity, its depth and orientation.
TOPICS: Heat conduction, Heating, Algorithms, Enterprise information management, Fast Fourier transforms, Frequency response, Imaging, Three-dimensional models, Heat flux, Temperature, Heat transfer
Xiuying Wang, Liping Shi, Wei Huang and Xiaolei Wang
J. Tribol   doi: 10.1115/1.4038864
Spiral groove is one of the most common types of structures on gas mechanical seals. Numerical research demonstrated that the grooves designed for improving load carrying capacity or film stiffness often lead to the leakage increase. Hence, a multi-objective optimization approach specially for conflicting objectives is utilized to optimize the spiral grooves for a specific sample in this study. Firstly, the objectives and independent variables in multi-objective optimization are determined by single objective analysis. Then, a set of optimal parameters i.e., Pareto-optimal set is obtained. Each solution in this set can get the highest load carrying capacity under a specific requirement of the leakage rate. Finally, the collinearity diagnostics are performed to evaluate the importance of different independent variables in the optimization.
TOPICS: Pareto optimization, Load bearing capacity, Leakage, Optimization, Stiffness
Dara Childs
J. Tribol   doi: 10.1115/1.4038867
New models are developed for flexibly-mounted stator (FMS) and flexibly-mounted rotor (FMR) mechanical seals that incorporate the radial reaction force components produced by supporting O-rings due to relative squeezing motion across the O-rings. Supporting data come from tests done in relation to O-ring supports for ball bearing races. The reaction-force model is linear but a nonlinear function of excitation frequency. The model accounts for the axial displacement doz of the O-ring from the mass center of the seal stator (FMS configuration) or seal rotor (FMR configuration), which couples the radial and pitch-yaw motion of the model's stiffness and damping matrices. Greens' coned-face-seal model is used to define the reaction moment arising across the seal faces via stiffness and damping matrices. The damping matrix does not not coincide with Green's. His is constant; the matrix developed here contains terms that are harmonic at twice the precession-frequency. When averaged over one precession cycle, the new average damping matrix coincides with Green's result. When the averaged damping matrix is used, the resultant model is linear. However, because of the viscoelastic reaction-force and reaction-moment models used for the O-ring coefficients, most of the stiffness and damping matrices are strong functions of the assumed precession frequency. The new FMR model contains a skew-symmetric stiffness matrix due to the O-ring damping terms. In rotordynamics, skew symmetric stiffness matrices due to internal damping in the rotor can lead to rotordynamic instabilitities.
TOPICS: Seals, Structural dynamics, Pumps, Vibration analysis, Damping, Stiffness, Rotors, Flexible manufacturing systems, Stators, Excitation, Yaw, Rotordynamics, Ball bearings, Cycles, Displacement
Andreas Kaufmann, Tino Lindner-Silwester and Thomas Antretter
J. Tribol   doi: 10.1115/1.4038863
The wear of dynamic sealing elements, i.e. elements that seal against a moving counter-surface, is highly complex. In dry-running reciprocating compressors, these sealing elements, commonly referred to as packing rings, have to seal the compressed gas against the environment along the reciprocating rod. Since the packing rings' seal effect arises from the differential pressure to be sealed, it is of paramount importance to take into account the gas pressure drop across the dynamic sealing surface. This paper presents a numerical model that allows us to calculate how the wear of such a packing ring evolves with time. An analytical solution is used to verify the numerical model.
TOPICS: Wear, Compressors, Sealing (Process), Modeling, Pistons, Pressure drop, Packings (Cushioning), Packing (Shipments), Computer simulation, Pressure
Gengxiang Wang and Hongzhao Liu
J. Tribol   doi: 10.1115/1.4038806
Three-dimensional (3D) wear of the clearance spherical joint in 4-DOF parallel mechanism is predicted based on Archard's wear model. The flexible moving platform is treated as thin plate element based on Absolute Nodal Coordinate Formulation. The tangent frame is introduced to formulate the constraint equation of universal joint. One of the spherical joints is treated as clearance joint. The normal and tangential contact forces are calculated based on Flores contact force model and modified Coulomb friction model. In order to predict 3D wear, the normal contact force, tangential contact velocity and eccentricity vector are decomposed in the global coordinate system. Simulations show that 3D wear occurred in three directions are not uniform each other.
TOPICS: Wear, Clearances (Engineering), Parallel mechanisms, Friction, Engineering simulation, Coulombs, Simulation, Universal joints
Noel Brunetiere, Antoinette Blouin and Guytri Kastane
J. Tribol   doi: 10.1115/1.4038805
An experimental test rig has been used to analyze the lift-off condition of a squeeze film thrust bearing. It is composed of a vibrating flat plate linked to a piezo-actuator, a cylindrical mass, and two displacement sensors. The frequency and magnitude of oscillation are varied as well as the mass of the solid, to identify the lift-off conditions. The experimental results are compared to numerical simulations. The model solves the transient compressible Reynolds equation coupled with Newton's law for the levitated mass. The model is then used to extend the experimental results to other operating conditions. A dimensionless analysis of the results is performed to study the lift-off conditions and the average film thickness during levitation.
TOPICS: Levitation, Film thickness, Flat plates, Piezoelectric actuators, Thrust bearings, Transients (Dynamics), Displacement, Oscillations, Sensors, Computer simulation
Mamoun Fellah and Linda Aissani
J. Tribol   doi: 10.1115/1.4038103
The aim of the present research was focused on the study of the effect of replacing vanadium by niobium and iron on the tribological behavior of Hot Isostatic Pressed (HIPed) titanium alloy (Ti-6Al-4V) biomaterial, using a ball-on-disk type Oscillating tribometer, under wet conditions using physiological solution conditions (Ringer solution) in accordance with the ASTMG 99, ISO 7148-1:2012, and ASTM G 133–95 standards. The tests were carried out under a normal load of 6 N, with an AISI 52100 grade steel ball as a counter face. The morphological changes and structural evolution of the nanoparticle powders using different milling times (2, 6, 12 and 18 h) were studied. The morphological characterization indicated that the particle and crystallite size continuously decreases with increasing milling time to reach the lowest value of 4 nm at 18 hours milling. The friction coefficient and wear rate were lower in the samples milled at 18 h (0.226, 0.297 and 0.423) and (0.66 x10-2, 0.87x10-2 and 1.51x10-2 µm3.N-1.µm-1) for Ti-6Al-4Fe, Ti-6Al-4Nb and Ti-6Al-4V, respectively. This improvement in friction and wear resistance is attributed to the grain refinement at 18 hour milling. The Ti-6Al-4Fe samples showed good tribological performance for all milling times
TOPICS: Wear, Titanium alloys, Milling, Sliding friction, Biomedicine, Tribology, Friction, ASTM International, Tribometers, Wear resistance, Physiology, Stress, Nanoparticles, Disks, Iron, Steel, Particulate matter, Biomaterials
Joseph Polly, David Talbot, Ahmet Kahraman, Avinash Singh and Hai Xu
J. Tribol   doi: 10.1115/1.4038412
In this study, load-independent (spin) power losses of a gearbox operating under dip-lubrication conditions are investigated experimentally using a final-drive helical gear pair from an automotive transmission as the example system. A dedicated gearbox is developed to operate a single gear or a gear pair under given speed and temperature conditions. A test matrix that consists of sets of tests with (i) a single spur, helical gears, or disks with no teeth, and (ii) helical gear pairs is executed at various temperatures, immersion depths and pinion positions relative to its mating gear. Power losses from single gear and gear pair at identical operating conditions are compared to quantify the components of the total spin loss in the form of losses due to gear drag, gear mesh pocketing, and bearings and seals.
TOPICS: Mechanical drives, Gears, Helical gears, Temperature, Spin (Aerodynamics), Particle spin, Rotation, Lubrication, Drag (Fluid dynamics), Stress, Bearings, Disks, Automotive transmissions
Janith Samarasinghe, Wyatt Culler, Bryan D. Quay, Domenic Santavicca and Jacqueline O'Connor
J. Tribol   doi: 10.1115/1.4037461
Fuel staging is a commonly used strategy in the operation of gas turbine engines. In multi-nozzle combustor configurations, this is achieved by varying fuel flow rate to different nozzles. The effect of fuel staging on flame structure and self-excited instabilities is investigated in a research can combustor employing five swirl-stabilized, lean-premixed nozzles. At an operating condition where all nozzles are fueled equally and the combustor undergoes a self-excited instability, fuel staging successfully suppresses the instability: both when overall equivalence ratio is increased by staging as well as when overall equivalence ratio is kept constant while staging. Increased fuel staging changes the distribution of time-averaged heat release rate in the regions where adjacent flames interact and reduces the amplitudes of heat release rate fluctuations in those regions. Increased fuel staging also causes a breakup in the monotonic phase behavior that is characteristic of convective disturbances that travel along a flame. In particular, heat release rate fluctuations in the middle flame and flame-flame interaction region are out-of-phase with those in the outer flames, resulting in a cancellation of the global heat release rate oscillations. The Rayleigh integral distribution within the combustor shows that during a self-excited instability, the regions of highest heat release rate fluctuation are in phase-with the combustor pressure fluctuation. When staging fuel is introduced, these regions fluctuate out-of-phase with the pressure fluctuation, further illustrating that fuel staging suppresses instabilities through a phase cancellation mechanism.
TOPICS: Fuels, Combustion chambers, Nozzles, Flames, Heat, Pressure, Fluctuations (Physics), Gas turbines, Flow (Dynamics), Oscillations

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