A finite element model of a hard disk drive (HDD) is developed for simulating shock testing and head slap. Individual components of the disk drive are modeled using Hypermesh, a commercially available mesh generator. The following components are modeled: the base plate, the impact surface, the actuator arm, the suspension arm, the head gimbal assembly (HGA) and the disk. The boundary conditions and contact surfaces are defined in Hypermesh [1] prior to importing them to the solver, LS-Dyna [2], for the transient finite element solution. The solution calculates displacement, velocity, acceleration and stress at every time step.

When the flying height is below 5 nm, some physical phenomena that can be ignored for higher flying sliders need to be included in the modeling equations. Another challenging issue is the unavoidable asperity contact problem. It is known that zero spacing at asperities may cause some of the existing modified Reynolds equations to predict unphysical unbounded contact pressure singularities. In this paper we first review the source of these pressure singularities, showing which models have this problem and which do not. We also review here a new derivation of a compressible lubrication equation by using a different slip velocity boundary condition, in which additional slippage at the gas-solid interface due to pressure gradient is introduced. The new equation is free of any contact singularity [1]. Next we review our recent work on the non-negligible effect of the intermolecular forces on the slider’s flying attitude when the spacing is below 5 nm [2]. The additional force is attractive until the spacing is below about 0.1 nm for example and then it is repulsive. Finally we show the effect of these new force contributions on the flying attitude of a low flying slider.

Engineering surfaces are never perfectly flat. They contain micro and nano-scale features on multiple length scales. Predicting the amount of fluid trapped in these minute surface crevices and its controlled release could benefit a variety of practical applications. In a sliding contact, the released fluid could create an ultra-thin film, reducing the direct contact and consequently the boundary friction. Transdermal patches are the least invasive of available subcutaneous drug delivery techniques. The drug is stored in a micro-reservoir and it is released to the skin either through a permeable membrane or through a series of micro needles. The aim of the current paper represents the first attempt to investigate whether a modeling approach encompassing two complementary simulation techniques in an integrated framework can be used to predict the volume of fluid stored in a nano-scale surface feature. Molecular dynamics (MD) simulation could provide accurate modeling of fluid behavior at nano-scale, and statistical mechanics (SM) could provide a fast prediction.

Insufficient friction at the shoe-floor interface causes a large number of slip and falling accidents each year. Developing solutions for enhancing shoe-floor-contaminant friction requires understanding the mechanisms that contribute to slippery surfaces. Over the past several years, our research group has conducted several experimental and modeling studies to reveal the critical tribological mechanisms contributing to shoe-floor-contaminant friction. This extended abstract will discuss the findings of these studies to: 1) determine the lubrication regime(s) that is/are most relevant to under-shoe conditions during slipping; 2) quantify how under-shoe conditions, shoes and flooring affect the two main contributions to boundary lubrication: adhesion and hysteresis; and 3) describe how this information can be used to design slip-resistant shoes and flooring. To identify the lubrication regime, interfacial pressures at the shoe-floor-contaminant interface were measured and coefficient of friction was monitored. Low viscous fluids and shoes with at least 2mm of tread were found to have negligible interfacial pressures and moderate friction coefficients (0.07–0.40). Untreaded shoes combined with high viscous fluids led to high interfacial pressures that supported up to 40% of the normal load and low friction coefficients (<0.01). These results suggest that mixed/elasto-hydrodynamic lubrication is relevant in some untreaded conditions but that boundary lubrication is relevant for most other conditions. In boundary lubrication, the primary factors contributing to friction are adhesion and hysteresis. Experimental data and finite element models demonstrate that hysteresis friction increases with floor roughness and the ratio of shoe to floor hardness. Adhesion friction is dependent on real area of contact and the shear stress required to break junctions. Experimental data suggests that adhesion is dependent on the fluid lubricant, sliding speed, floor roughness and shoe material. Finite element models confirm that a reduction in the real area of contact occurs with increasing floor roughness and sliding speed, consistent with the experimental adhesion effects. Ensuring that the shoe-floor-fluid interface is operating in the boundary lubrication regime requires establishing minimum tread threshold for fluid lubricants that are likely to be found in a given environment. Designing a high hysteresis shoe-floor combination is preferred because it is relatively unaffected by fluid contaminants or under-shoe conditions (i.e. sliding speed). Therefore, ensuring a minimum tread depth is used along with increasing floor roughness and shoe to floor hardness may be effective in addition to minimum tread thresholds.

In many tribological applications, such as journal bearings and gears, a fluid film is used to accommodate velocity between moving surfaces. To model the behavior of this film and to predict its ability to carry load, the Reynolds equation is predominantly employed. As computational processing power continues to increase, computational fluid dynamics (CFD) is increasingly being employed to predict the fluid behavior in lubrication environments. Using CFD is advantageous in that it can provide a more general approximation to the Navier-Stokes equations than the Reynolds equation. Moreover, using CFD allows for the simulation of multiphase flows as could occur during bearing contamination and bearing exit conditions. Because the bearing surfaces move relative to each other as they obtain equilibrium with the fluid pressure, there is a need to incorporate the moving boundary into the CFD calculation, which is a non-trivial task. In this work, a fluid-structure interaction (FSI) technique is explored as an approach to model the dynamic coupling between the moving bearing surfaces and the lubricant. The benefits of using an FSI approach are discussed and the results of its implementation in a lubricated sliding contact model are presented.

The lubricant applied to the disk in a hard drive is a critical component for head-disk interface reliability. In Heat Assisted Magnetic Recording (HAMR), the heat supplied to the disk by the laser will add new thermal considerations to lubricant performance. Investigations into how the lubricant behaves at the small time and length scales seen in HAMR systems need to be conducted numerically. Published works on HAMR lubricant modeling have considered only the van der Waals contribution to disjoining pressure, commonly called the dispersive component, and do not consider the film thickness dependence of viscosity. However, lubricants with reactive end groups such as Fomblin Zdol are widely used, and such simple disjoining pressure and viscosity models do not capture certain lubricant behavior. We have developed a simulation tool that incorporates film thickness dependencies of viscosity and polar and dispersive disjoining pressure into a continuum lubrication model. We investigate the effect of initial thickness on lubricant flow and evaporation under HAMR write conditions considering both components of disjoining pressure and thin-film viscosity. Simulation results indicate the effect of including polar disjoining pressure depends on the initial lubricant thickness. The inclusion of viscosity thickness dependence does not affect simulation results under scanning laser conditions but will be important in reflow simulations.

In normal IC engine operation 1 st compression ring sustains thermal loads effectively. At large radial clearance in low-speed engine start up it has to act as effective seal against thermal loading. Absence of elastohydrodynamic lubricating (EHL) film and secondary displacements expose the ring to adhesive wear in initial engine start up. Isothermal hydrodynamic and EHL models of parabolic-faced ring are developed numerically after incorporating secondary dynamics of piston assembly at a low engine start up speed. Reynolds equation is solved to generate hydrodynamic pressures and film profiles in 4-stroke cycle. In EHL model inverse solution technique and elastic displacements of ring and liner are employed to generate pressures and film profiles. The simulation results suggest that a large ring-to-bore clearance increases eccentricity and affects EHL of 1 st compression ring when a low-viscosity grade oil is used in initial engine start up.

Foil gas bearings are self-acting, compliant-surface hydrodynamic bearings that usually use air or other process gas as their working fluid or lubricant. Foil gas bearings are made of one or more bump foils, which are compliant surfaces of corrugated metal, and one or more layers of top foil. Because foil gas bearing performance parameters, such as load capacity, are dominated by foil material and foil geometric designs, numerical models have been developed to predict the bearing’s performance based on these characteristics. However, previous models often simplify bump foil as elastic foundation with constant stiffness and neglect top foil altogether. Further, they typically use the Reynolds equation to simplify the fluid solution. In this study, ANSYS software is used to build a 3D, fully-coupled, fluid-structure-interaction model for a foil gas bearing to predict key performance parameters such as load capacity and journal attitude angle. The model’s results show good agreement with previously published test data. This not only demonstrates the feasibility of 3D fully coupled fluid-structure-interaction model for a conventional foil bearing using commercial codes, but also shows modeling capability for future generations of foil gas bearing.

In the normal low-speed engine operation, elastohydrodynamic lubrication (EHL) of piston skirts and lubricant rheology reduce friction and prevent wear. In a few initial start up cycles, a very low engine speed and absence of EHL cause adhesive wear. This study models hydrodynamic and EHL of piston skirts in the initial very low cold engine start up speed by using a high viscosity lubricant. The 2-D Reynolds equation is solved and inverse solution technique is used to calculate the pressures and film thickness profiles in the hydrodynamic and EHL regimes, respectively. The work is extended to investigate the effects of three very low initial engine start up speeds on the transverse eccentricities of piston skirts, film thickness profiles and pressure fields in the hydrodynamic and EHL regimes. Despite using a viscous lubricant, thin EHL film profiles are generated at low start up speeds. This study suggests very low speed optimization in the cold initial engine start up conditions to prevent piston wear under isothermal conditions.

In the paper, numerical TEHD results for one of the biggest water power plant Itaipu were compared to available measurement data. Numerical simulations were carried out with the use of bearing model for which the accuracy was previously confirmed with the measurements available for smaller bearings. Several cases were analyzed. The only differences between the cases are the applied boundary conditions. The influence of heat transfer coefficient assumed at bearing pad side walls and collar deformation on predicted bearing performance was determined. Some conclusions concerning large thrust bearing modeling were drawn on the basis of comparisons of the theoretical results to measurements for analyzed bearing system.

This paper presents a finite element method approach for modeling a thermal-electrical-mechanical coupled-field contact comprised of an elastic hemisphere and an elastic half space. The goal of this model is to develop a fundamental understanding of the behavior of a multi-physics contact with a particular interest in the contact area. The results from the model illustrate a clear difference in contact behavior between force control and displacement control in the presence of an applied electrical potential. It is shown that, while Hertzian contact theory can be used to accurately predict the behavior of the contact under force control, a new relationship must be established to accurately predict the behavior of the contact under displacement control. This relationship can be applied to RF MEMS switch contact bumps as well as macro scale electrical contacts.

In this paper, an analytical model for calculating eddy current loss in magnetic thrust bearings (MTBs) is presented. The surface magnetic field intensity, considering the skin effect of eddy current, is varying with the frequency and can be calculated by effective reluctance method. The magnetic field distribution and eddy current loss can be obtained from it. Finally, the validity of the model is verified by the finite element method (FEM) simulation and experimental results.

Granular flows continue to be a complex problem in nature and industrial sectors where solid particles exhibit solid, liquid, and gaseous behavior in a manner which is often unpredictable locally or globally. In tribology, they have also been proposed as lubricants because of their liquid-like behavior in sliding contacts, and due to their ability to carry loads and accommodate surface velocities. The present work attempts to model a granular Couette flow in an annular shear cell using a lattice-based cellular automata (CA) computational modeling approach. The CA framework has the flexibility to employ rule-based mathematics and/or first-principle physics to rapidly model physical processes. The model developed in this work incorporates dissipative effects due to friction between particles and boundaries, in addition to particle spin. This new model also includes a rigorous treatment of boundary-particle interactions. Additional work has been done to account for the transfer of momentum through particle chains, allowing for the modeling of transitional granular flows, which consist of both kinetic and contact flow regimes.

Pellets were formed by compacting MoS 2 powder. In the setup, the pellet is sheared against a disk surface while the slider pad rides on the MoS 2 partially-covered asperities, while depleting the lubricant. A volumetric fractional coverage (VFC) modeling approach was developed to predict the friction coefficients for the lubricated pellet/disk sliding contact by assuming that the friction coefficients at pellet/disk and slider/disk interfaces are functions of the fraction of lubricant occupying the control volume. This fractional coverage is a theoretical parameter for quantifying the amount of third body film covering the disk asperities as a function of time. The model can be applied to a deterministic surface, thus, the proposed theory can predict the behavior of friction coefficients between the pellet/disk and slider/disk interfaces for most topographies, whether they are deterministic (i.e., textured) or real (i.e., random).

When dealing with mechanical systems including contact interfaces, two different mechanics at two different time and length scales have to be accounted for: the contact mechanics and the structural mechanics. The mechanics of the contact and the mechanics of structure are coupled by friction (forces and moments), local body deformations and wear (geometrical coupling). In the past, dynamic and tribological problems were treated separately and the analysis was focused either on the mechanism scales or on the contact scales, as a function of the problem. The analysis of the dynamic response of the mechanical system accounted for the contact interfaces thought global coefficients (global contact stiffness, damping, energy loss factor, etc.); vice versa, the analysis of the local behaviour at the contact interface accounted for the system dynamics imposing global boundary conditions on the contact model (pressure distribution, relative velocities, etc.). During last decades mechanical system design led to a power increase and to a drastic optimization, increasing as well the number of contact interfaces between deformable bodies. Thus, the need for coupling dynamic and tribological analyses arises for solving and understanding either wear phenomena or dynamic response of a mechanical system. On this sense, experimental tools showed their limits on the estimation and prediction of friction and wear phenomena. This paper proposes a first step toward a more general approach aimed to couple the dynamic analysis of the mechanism with the local tribological analysis at the contact, by the coupling of two different numerical models dealing with the two different scales. Two applications are then mentioned: the analysis of “false Brinelling” wear and the understanding of brake squeal instability.

Multi-impact modeling is a challenge in the simulation of processes such as shot peening, with its variants laser peening and ultrasonic peening, or cold spray. Shot peening is commonly used to induce compressive stresses in machine elements subject to fatigue. The large number of impacts involved in the process implies consequent computation time to obtain representative results. In this paper, we present a new approach to model shot peening by using a semi-analytical method that accounts for plasticity. The numerical efficiency of the code along with the use of a small representative area allows reduction in the computation time. It is also possible to observe, with a small number of impacts, the effect of several coverage rates. Thus the improving efficiency of the process with increasing covering rate can be observed as well as potential saturation effect when the covering rate is too high.

While continuum-based approximations have had success modeling tribosystems with thick film powder flows, they do not predict the behavior of thin powder transfer films which can be worn away from the asperity region during mixed lubrication. This is especially the case for in situ self-replenishing transfer films in the form of compacted solids which are set up to undergo deposition and depletion simultaneously. Although control volume models have been developed to describe this type of tribosystem, very little is known about the wear mechanisms that govern the transfer film in sliding contact. The problem is highlighted through the sudden breakdown of the lubricant film which inevitably leads to interfacial starvation. This study examines evidence that will lead to a fundamental description of the deposition and depletion of powder transfer films by leveraging abrasive wear theory. Justification for this theory is provided by examining the relationship between the coefficient of friction (COF), wear rate, and atomic force microscope (AFM) images of wear interfaces; so that a more accurate model of an in situ self-replenishing solid lubrication process may be constructed.

Nonlinear behavior of force and displacements in rolling contacts with the presence of surface defects are studied. Model-based fault assessments in rolling element bearings and gears require detailed modeling and dynamics of faults. A detailed model of rolling element bearings with direct correspondence between parameters of the model and physical components is developed. The model incorporates dynamics of faults, nonlinear contacts, slips and surface separations. Mechanics of contacts with inner race faults (IRF), ball faults (BF), and outer race faults (ORF) are studied using the developed model. Contacts force, displacement and impulse signals are studied for different size and types of surface defects. It is shown that impulse signals contain useful information about the severity of surface defects in rolling element bearing. Results provide model-based diagnostics a deep knowledge of rolling contact mechanics with surface defects to be used for fault assessments.

The mechanical behavior of eukaryotic cell components is critical to cell mobility, division, signaling, and proliferation. Cell components are in such harmony with each other that often mechanical testing of individual components produces less meaningful results than testing performed within living cells. Moreover, cell components are constantly undergoing growth and remodeling, normally through polymerization and depolymerization. An appropriate experimental and modeling framework is therefore essential for cell mechanics. In this study, a special atomic force microscope (AFM) with controlled adhesion area of the cell to a substrate was used to probe the mechanical response of cells to an external force. This experimental set up enables direct control of the applied force and precise measurement of the cell penetration by the AFM tip. A modeling framework that combines the displacement field with the applied force/stress obtained with the AFM is developed to estimate the mechanical properties of each cell component. The tribological relevance of this research is the viscoelastic deformation of cell components under different contact loads.

The Green Tribology is a new concept that was suggested in 2009. We discuss the scope of the new discipline and the publications on Green Tribology which are to appear in 2010 and 2011. The three areas relevant to Green Tribology are: (i) biomimetic and self-lubricating materials / surfaces (ii) biodegradable and environment-friendly lubricants (iii) tribology of renewable and/or sustainable sources of energy. The hypothesis is that these three areas can benefit from each other and, therefore, it is justified to combine these three areas into the novel field of Green Tribology. We discuss recent theoretical, modeling, and experimental research in these areas of interest and present theoretical, modeling, and experimental results in the area of environment-friendly tribology.