bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effects of a Hydrostatic Pocket Aspect Ratio, Supply Orifice Position, and Attack Angle on Steady-State Flow Patterns, Pressure, and Shear Characteristics
Authors: Braun M. J. , Choy F. K. , Zhou Y. M.
Published: June 2008

The flow in a hydrostatic pocket is described by a mathematical model that uses the Navier-Stokes equations written in terms of the primary variables, u, v, and p. Using the conservative formulation, a finite difference method is applied through a staggered grid. The power law scheme is applied in the treatment of the convective terms for this highly recirculating flow. The discussion pertaining to the convergence of the numerical scheme and the computational error, shows that the strict convergence criteria applied to both velocities and pressure were successfully statisfied. The numerical model is applied in a parametric mode to the study of the velocities, the pressure patterns, and shear forces that characterize the flow in a square (α = 1), deep (α>1), and shallow (α≪1) hydrostatic pocket. The effects of the variation of the location and angle of the hydrostatic jet are also investigated.
Application of a Mixed Lubrication Model for Hydrostatic Thrust Bearings of Hydraulic Equipment
Authors: Kazama Toshiharu , Yamaguchi Atsushi
Published: June 2008

On mixed and fluid film lubrication the characteristics of hydrostatic bearings for hydraulic equipment are studied numerically. By applying a mixed lubrication model derived in a previous paper to the bearings, we clarify the effects of the surface roughness, eccentric or moment loads, supply pressure and speed of rotation on the friction, flow rate, and power losses. Introducing the concept of a ratio of hydrostatic balance, we show that the minimum power loss is given as the ratio becomes close to unity.
Observations on the Nonlinear Fluid Forces in Short Cylindrical Squeeze Film Dampers
Authors: Zhang J. , Ellis J. , Roberts J. B.
Published: June 2008

Recent theoretical work by Tichy and Bou-Said (1991) and El-Shafei and Crandall (1991) has resulted in new theoretical expressions for the nonlinear inertia forces for both short and long cylindrical squeeze film dampers (SFDs). This paper provides alternative derivations for the short cylindrical SFD using as a starting point a simplified two-dimensional Navier-Stokes equation. The resulting expressions for the fluid inertia forces are similar to the Tichy and Bou-Said/El-Shafei and Crandall expressions except for differences in certain numerical constants which can be explained by the different averaging methods used within the squeeze-film thickness. The analyses give additional insight into the temporal and convective origins of the various coefficients. The theoretical results are compared with published theoretical and experimental work involving nonlinear cylindrical SFD behavior. The paper highlights the importance of convective inertia terms when cylindrical SFDs operate at large values of eccentricity ratio.
Clearance Effects on Corresponding Annular and Labyrinth Seal Flow Leakage Characteristics
Authors: Rhode D. L. , Hibbs R. I.
Published: June 2008

A previous Navier-Stokes finite difference computer code is extended in order to compute seal leakage directly from given upstream and downstream reservoir pressures. The numerical results are in excellent agreement with previous measurements, the discrepancy being less than eight percent. Annular seals are found to leak approximately twenty percent more than corresponding labyrinths over the entire range of realistic clearance. A rather unexpected finding is that a dramatic increase of swirl velocity occurs near the discharge of small-clearance annular seals, which does not arise in corresponding labyrinth seals. The results, which are used to explain this finding, show that a large density drop occurs near the small-clearance annular seal exit, which provides the swirl velocity increase in accordance with angular momentum conservation.
Mechanical Analysis of an Annular Plate Transversely Loaded at an Arbitrary Point by a Concentrated Force
Authors: Ciavatti V. , Dragoni E. , Strozzi A.
Published: June 2008

An annular plate clamped at the inner rim, free at the outer edge and loaded at an arbitrary point by a concentrated force is mechanically analyzed via a purely flexural model. A series solution is achieved, whose coefficients are analytically evaluated with the aid of an algebraic manipulator. A series acceleration technique is also developed. Experimental data are obtained for a particular plate geometry, and the results retrieved in terms of deflections and stresses are compared to the analytical forecasts. A technically relevant diagram detailing the theoretical deflection of the plate at the loading point for a variety of plate proportions and load locations is also presented.
Response of Composite Beams to an Internal Actuator Force
Authors: Chaudhry Z. , Rogers C. A.
Published: June 2008

Shape memory alloy hybrid composite materials have demonstrated numerous control capabilities. One such capability is the controlled bending of structures. In this paper the response of a cantilevered beam to an internal actuator is examined. The modeling of the compressive force exerted by the induced strain of the actuator on the beam is discussed. The results obtained from treating the force as an external follower force are presented. The response to an internal force such as exerted by an internal shape memory alloy actuator is quite different from that produced by loads due to sources external to the beam. Contrary to normal expectations such an internal force although compressive does not produce any buckling tendencies or any other instabilities in the beam. This principle which is already in use in the design of civil engineering structures is discussed in detail. If the actuators are embedded off of the neutral axis, then due to the eccentricity the beam bends, but again without any buckling tendency. The experimental results obtained for this configuration are also presented.
Instantaneous Kinematics of Parallel-Chain Robotic Mechanisms
Authors: Kumar V.
Published: June 2008

This paper addresses the instantaneous kinematics of robotic manipulators which have an in-parallel scheme of actuation. Hybrid geometries which require both serial and parallel actuation are also considered. Multifingered grippers, walking vehicles, and multiarm manipulation systems in addition to robot arms with a parallel structure can be included in this broad category. The direct and inverse kinematics (and statics) of these devices is discussed with particular attention to applications in control. An analytical method based on screw system theory for obtaining transformation equations between joint and end-effector coordinates is described. Special configurations in which the end-effector gains or loses a degree of freedom, which are also known as geometric singularities, are an important consideration in this study. This is because the number of special configurations or singularities in the workspace is far more for in-parallel manipulators than that for serial manipulators. The special configurations for a planar dual-arm manipulation system, which can be kinematically modeled as a 5-R linkage, are discussed in some detail as an example.
Automatic Selection of Mechanism Designs from a Three-Dimensional Design Map
Authors: Kota S.
Published: June 2008

Often a simple (four-bar linkage) mechanism to trace a desired path is sought. Dimensions of linkages to trace a specified coupler curve exactly are difficult to determine. Precision point techniques do not guarantee that the desired path will be generated. The methodology presented in this paper is based on indirect synthesis approach to select four-bar linkages that trace a given desired path. A practical, computer-aided three-dimensional design chart for the selection of four-bar linkage candidate designs that trace symmetrical paths is presented. It is based on hypothesis that any arbitrary path can be approximated by a symmetrical path at least for the purpose of choosing an initial design. Four-bar linkages that generate symmetrical paths are easier to design and exhibit a definite pattern of changes in their shape as the linkage parameters are varied. General design categories and the corresponding solution subspaces in the 3-D design map are identified to aid in the selection of candidate designs. The automation of the design-selection process requires a satisfactory means to quantitatively compare the “desired” and the “known” coupler curves. This paper addresses the issues involved in quantitative comparison of two arbitrary curves using parameterization and shape evaluation. A design example of automated selection of mechanism designs is also presented.
Characterization of Workspaces of Parallel Manipulators
Authors: Kumar V.
Published: June 2008

The workspaces and kinematic characterization of serial chain manipulator geometries and the geometric optimization have been studied extensively. Much less is known about workspaces for manipulation systems which possess several serial chains arranged in parallel. In this paper, two well known workspaces, the reachable workspace and the dexterous workspace, are investigated for parallel manipulators. A general method for obtaining these workspaces is presented. The existence of numerous special configurations in the workspace present problems in manipulator control. Therefore the controllably dexterous workspace is proposed as a useful measure of kinematic performance. The methodology of delineating the workspaces and its limitations are illustrated with examples.
Determining General and Overclosing Constraints in Mechanism Mobility Using Structural Finite Element Joint Freedoms
Authors: Bagci C.
Published: June 2008

Article presents the most general form of the mobility equation, and primarily describes general and three types of overclosing constraints. Then, it offers a method of determining numbers of the general and overclosing constraints in single and multiloop, planar and spatial mechanisms. It makes use of the number of external joint freedoms and the number of internal element end deformations in finite line element models of the mechanisms for force and torque analysis. The number of degrees of statical indeterminacy determines the number of constraints in the mobility equation. Redundant and passive freedoms, general, and overclosing constraints are determined for several single and multiloop mechanisms, including the Sarrutt’s, Bennett’s, and Bricard’s single loop mechanisms.
Effect of Finite Rotation on the Propagation of Elastic Waves in Constrained Mechanical Systems
Authors: Gau Wei-Hsin , Shabana A. A.
Published: June 2008

In structural systems, impact-induced longitudinal elastic waves travel with finite speeds that depend on the material properties. Using Fourier method of analysis, the exact wave motion can be described as the sum of infinite number of harmonic waves which have the same phase velocity. In this case the medium is said to be nondispersive, since the phase velocities of the harmonic waves are equal and equal to the group velocity of the resulting wave motion. In mechanism systems with intermittent motion, on the other hand, elastic members undergo finite rotations. In this investigation, the effect of the finite rotation, coefficient of restitution, and impact conditions on the propagation of the impact-induced waves in costrained elastic systems is examined. The system equations of motion are developed using the principle of virtual work in dynamics. The jump discontinuities in the system variables as the result of impact are predicted using the generalized impulse momentum equations that involve the coefficient of restitution. It is shown that the phase velocities of different harmonic waves are no longer equal, that is, dispersion occurs in perfectly elastic mechanism members as the result of the finite rotation. The analysis presented in this paper shows that the finite rotation has more significant effect on the phase velocity of the low frequency harmonics as compared to the high frequency harmonics. A rotation-wave number that depends on the material properties and the wave length is defined for each harmonic wave. It is shown that if the angular velocity of the elastic member becomes large such that the rotation-wave number of a mode exceeds one, the associated modal displacement is no longer oscillatory.
The Constrained Least-Square Optimization of Spherical Four-Bar Path Generators
Authors: Angeles J. , Liu Z.
Published: June 2008

In this paper, the optimization of the spherical RRRR four-bar linkage for the problem of path generation is addressed. The problem is formulated as a two-loop minimization of the error between the path-generating point in the coupler curve and the prescribed position, while decoupling the linkage parameters from the configuration variables, namely, the input angles. The synthesis problem consists of evaluating a set of input angles {ψk}1m defining m linkage configurations and the linkage parameters independently. This leads to a constrained overdetermined system of nonlinear equations. The orthogonal decomposition algorithm, introduced elsewhere, is employed to solve the problem. Continuation and damping techniques are used in the numerical procedure to ensure convergence and speed up its rate. The optimization scheme is developed on a general basis and can handle the problem for any number of given path points. Three numerical examples are included.
Position Kinematics of the Generalized Lobster Arm and Its Series-Parallel Dual
Authors: Murthy V. , Waldron K. J.
Published: June 2008

The generalized lobster arm is a six revolute open kinematic chain with 3 consecutive intersecting pairs of axes. A new solution of the inverse position kinematics problem of this arm which takes advantage of its specific geometry is presented. A comparison is made with the direct position kinematics problem of the series-parallel dual mechanism. The equations governing the two problems show strong similarity and can each be reduced to a sixteenth degree univariate polynomial equation. The dual series-parallel mechanism is the one that exhibits, with the lobster arm, the symmetry that exists between the wrench and the velocity motor. Although the results presented here have intrinsic interest, a more generally important feature is the relationship between the solutions to the inverse kinematics of the serial mechanism and the direct kinematics of the parallel mechanism. Although the series-parallel duality has not been shown to hold in the position domain, except in terms of very general characteristics, it is shown here that the two solutions are of the same degree and have other features in common.
Geometric Work of Manipulators and Path Planning Based on Minimum Energy Consumption
Authors: Chou Li-Shan , Song Shin-Min
Published: June 2008

The energy efficiency of the robots of today’s generation is in general very poor due to the existence of “geometric work.” The geometric work is geometry dependent and can be eliminate by adopting a special geometry which decouples the gravitational motion from the horizontal motions. Instead of adopting a special geometry, this paper studies the geometric work of a regular open-chained manipulator and applies it to the path planning for minimum energy consumption. For a given manipulator geometry and end-effector position, the zones of velocity with zero geometric work are determined analytically. A map which describes these zones of zero geometric work at various positions in workspace is then constructed for path planning with zero geometric work. The path planning for minimum energy consumption is generated by the dynamic programming method and the results are compared with the map of zero geometric work. It is found that the end-effector tends to move within the zones of zero geometric work as much as possible. If the end-effector has to cross the boundary of a zone at some point, it again moves within the zones after crossing the boundary. The presented method can also be used to arrange the pick and place positions for minimum travel energy consumption. That is, the two positions should be selected so that a continuous path which connects them with zero geometric work and with monotonously ascending or descending features is available.
Application of New Homotopy Continuation Techniques to Variable Geometry Trusses
Authors: Arun V. , Reinholtz C. F. , Watson L. T.
Published: June 2008

A VGT, or Variable Geometry Truss, can be thought of as a statically determinate truss that has been modified to contain some number of variable length members. These extensible members allow the truss to vary its configuration in a controlled manner. Some of the typical applications envisioned for VGTs are as booms to position equipment in space, as supports for space antennae, and as berthing devices. Recently, they have also been proposed as parallel-actuated, long chain, high dexterity manipulators. This paper will demonstrate the use of homotopy continuation in solving the kinematics of relatively complex variable geometry trusses (VGTs) including the octahedron and the decahedron. The procedural aspects are described in detail with the help of examples.
Kinematic Synthesis and Analysis of the Rack and Pinion Multi-Purpose Mechanism
Authors: Ananthasuresh G. K. , Kramer S. N.
Published: June 2008

The general procedure for synthesizing the rack and pinion mechanism up to seven precision conditions is developed. To illustrate the method, the mechanism has been synthesized in closed form for three precision conditions of path generation, two positions of function generation, and a velocity condition at one of the precision points. This mechanism has a number of advantages over conventional four bar mechanisms. First, since the rack is always tangent to the pinion, the transmission angle is always 90 deg minus the pressure angle of the rack. Second, with both translation and rotation of the rock occurring, multiple outputs are available. Other advantages include the generation of monotonic functions for a wide variety of motion and nonmonotonic functions for a full range of motion as well as nonlinear amplified motions. In this work the mechanism is made to satisfy a number of amplified motions. In this work the mechanism is made to satisfy a number of practical design requirements such as completely rotatable input crank and others. By including the velocity specification, the designer has considerably more control of the output motion. The method of solution developed in this work uses the complex number method of mechanism synthesis. A numerical example is included.
The Inverse Kinematic Solution of the Tetrahedron Based Variable-Geometry Truss Manipulator
Authors: Subramaniam M. , Kramer S. N.
Published: June 2008

The Tetrahedron Based Variable Geometry Truss Manipulator is comprised of a series of tetrahedrons stacked upon one another in a spiraling manner. When one link of each “cell” is made variable in length, the system can be effectively used as a remote manipulator. The manipulator is kinematically equivalent to a serial link manipulator consisting only of revolute joints where the angle between successive axes is 120°. In this paper, a closed-form analytic solution for the six-celled (6R series) variable geometry truss manipulator is obtained. It is shown that a sixteenth order polynomial in one of the joint angles is found which leads to at most sixteen solutions.
Closed-Form Inverse Kinematic Analysis of Variable-Geometry Truss Manipulators
Authors: Padmanabhan B. , Arun V. , Reinholtz C. F.
Published: June 2008

A variety of applications for variable-geometry truss manipulators (VGTMs) have been demonstrated or proposed in the literature. Most of these applications require solution to the inverse kinematic problem, yet only a few isolated examples of closed-form solution methods have been presented to date. This paper provides an overview to the general problem of inverse kinematic analysis of variable-geometry truss manipulators and presents new closed-form solution techniques for problems of practical importance.
Forward Displacement Analyses of the 4-4 Stewart Platforms
Authors: Lin W. , Griffis M. , Duffy J.
Published: June 2008

A forward displacement analysis in closed-form is performed for each case of a class of Stewart Platform mechanisms. This class of mechanisms, which are classified into three cases, are called the “4-4 Stewart Platforms,” where each of the mechanisms has the distinguishing feature of six legs meeting either singly or pair-wise at four points in the top and base platforms. (This paper only addresses those 4-4 Platforms where both the top and base platforms are planar.) For each case, a polynomial is derived in the square of a tan-half-angle that measures the angle between two planar faces of a polyhedron embedded within the mechanism. The degrees of the polynomials for the first, second, and third cases are, respectively, eight, four, and twelve. All the solutions obtained from the forward displacement analyses for the three cases are verified numerically using a reverse displacement analysis.
Tailoring Composite Materials Through Optimal Selection of Their Constituents
Authors: Karandikar H. M. , Mistree F.
Published: June 2008

The use of composite materials has provided designers with increased opportunities for tailoring structures and materials to meet load requirements and changing and demanding environments. This has led to their increased use in structural applications. As with traditional materials the selection of an appropriate material for a design is important. In case of design using composite materials the selection of a material consists of selecting a fiber-resin combination which meets all design requirements. This involves choosing the fiber, the resin, and the proportion of these two constituents in the composite material. The phrase “material selection” refers to the problem of laminate selection. This corresponds to the task of choosing a fiber and resin combination based on technical and economic factors. Materials tailoring, on the other hand, involves manipulating the composition of the composite material to achieve desired properties and it is the selection of a fiber and resin simultaneously but separately. In this paper we present, through an example, a multiobjective optimization-based method for assisting a designer in tailoring composite materials for specific technical and economic objectives.
A Convergent Convex Decomposition of Polyhedral Objects
Authors: Kim Yong Se , Wilde D. J.
Published: June 2008

A convex decomposition method, called Alternating Sum of Volumes (ASV), uses convex hulls and set difference operations. ASV decomposition, however, may not converge, which severely limits the domain of geometric objects that the current method can handle. We investigate the cause of non-convergence and present a remedy; we propose a new convex decomposition called Alternating Sum of Volumes with Partitioning (ASVP) and prove its convergence. ASVP decomposition is a hierarchical volumetric representation which is obtained from the boundary information of the given object based on convexity. As an application, from feature recognition by ASVP decomposition if briefly discussed.
Maximum Intersection of Spherical Polygons and Workpiece Orientation for 4- and 5-Axis Machining
Authors: Tang K. , Woo T. , Gan J.
Published: June 2008

Orienting the workpiece in such a way as to minimize the number of setups in a 4-axis or a 5-axis Numerical Control (NC) machine is formulated as follows: Given a set of spherical polygons (that are representations of curved surfaces visible to a 3-axis NC machine), find a great circle (the 4th axis) or a band (the 4th and the 5th axes) containing a great circle that intersects the polygons maximally. While there are potentially infinitely many solutions to this problem, a sphere is partitioned into O(N2) regions based on the N polygons. Within each of these regions, it is shown that it requires O(NlogN) time to determine maximum intersections and all the solutions are congruent. Central projection mapping is employed so as to present the algorithms in the plane.
Modal Analysis of Multistage Gear Systems Coupled With Gearbox Vibrations
Authors: Choy F. K. , Ruan Y. F. , Tu R. K. , Zakrajsek J. J. , Townsend D. P.
Published: June 2008

This paper presents an analytical procedure to simulate vibrations in gear transmission systems. This procedure couples the dynamics of the rotor-bearing gear system with the vibration in the gearbox structure. The modal synthesis method is used in solving the overall dynamics of the system, and a variable time-stepping integration scheme is used in evaluating the global transient vibration of the system. Locally each gear stage is modelled as a multimass rotor-bearing system using a discrete model. The modal characteristics are calculated using the matrix-transfer technique. The gearbox structure is represented by a finite element model, and modal parameters are solved by using NASTRAN. The rotor-gear stages are coupled through nonlinear compliance in the gear mesh while the gearbox structure is coupled through the bearing supports of the rotor system. Transient and steady state vibrations of the coupled system are examined in both time and frequency domains. A typical three-geared system is used as an example for demonstration of the developed procedure.
Kinematical Optimization of Spiral Bevel Gears
Authors: Fong Zhang-Hua , Tsay Chung-Biau
Published: June 2008

Kinematical optimization and sensitivity analysis of circular-cut spiral bevel gears are investigated in this paper. Based on the Gleason spiral bevel gear generator and EPG test machine, a mathematical model is proposed to simulate the tooth contact conditions of the spiral bevel gear set. All the machine settings and assembly data are simulated by simplified parameters. The tooth contact patterns and kinematic errors are obtained by the proposed mathematical model and the tooth contact analysis techniques. Loaded tooth contact patterns are obtained by the differential geometry and the Hertz contact formulas. Tooth surface sensitivity due to the variation of machine settings is studied. The corrective machine settings can be calculated by the sensitive matrix and the linear regression method. An optimization algorithm is also developed to minimize the kinematic errors and the discontinuity of tooth meshing. According to the proposed studies, an improved procedure for development of spiral bevel gears is suggested. The results of this paper can be applied to determine the sensitivity and precision requirements in manufacturing, and improve the running quality of the spiral bevel gears. Two examples are presented to demonstrate the applications of the optimization model.
Dynamic Analysis of Geared Rotors by Finite Elements
Authors: Kahraman A. , Ozguven H. Nevzat , Houser D. R. , Zakrajsek J. J.
Published: June 2008

A finite element model of a geared rotor system on flexible bearings has been developed. The model includes the rotary inertia of shaft elements, the axial loading on shafts, flexibility and damping of bearings, material damping of shafts and the stiffness and the damping of gear mesh. The coupling between the torsional and transverse vibrations of gears were considered in the model. A constant mesh stiffness was assumed. The analysis procedure can be used for forced vibration analysis of geared rotors by calculating the critical speeds and determining the response of any point on the shafts to mass unbalances, geometric eccentricities of gears, and displacement transmission error excitation at the mesh point. The dynamic mesh forces due to these excitations can also be calculated. The model has been applied to several systems for the demonstration of its accuracy and for studying the effect of bearing compliances on system dynamics.
Working With Multiple Representations in a Concurrent Design System
Authors: Cutkosky M. R. , Tenenbaum J. M. , Brown D. R.
Published: June 2008

The essence of concurrent engineering is to address multiple concerns (e.g., function, geometry, manufacturing) as a design evolves rather than waiting until the geometry of the design is complete. To support concurrent engineering a computational system must therefore be able to work with multiple representations and to keep them consistent. We have built environments for the design of assemblies and machined components in a prototype concurrent design system called Next-Cut and we use these environments to illustrate how such capabilities can be achieved. We discuss the strengths and limitations of the approach taken in Next-Cut and conclude with a discussion of directions for future work.
Refined Models for Hydrodynamic Lubrication in Axisymmetric Stretch Forming
Authors: Hsu Tze-Chi , Wilson William R. D.
Published: June 2008

Two mathematical models for axisymmetric stretch forming with a spherical punch are developed. The models combine a finite element representation of the sheet deformation with a hydrodynamic lubrication model. In one model the influence of sheet bending stiffness is taken into account while in the other only the membrane stiffness is considered. Comparison of the predictions of the models with the film thickness measurements of Hector and Wilson indicates that the inclusion of elastic effects is important in predicting lubricant film thickness. The results of the bending model are in particularly good agreement with the experimental data. A useful analytical method for predicting the film thickness at the center of the conjunction at the onset of yield is also developed.
Perturbation Approach to Hydrodynamic Lubrication Theory
Authors: Myllerup C. M. , Hamrock B. J.
Published: June 2008

Three perturbation approaches that apply for regular hydrodynamic lubrication problems are discussed: a cross-film coordinate perturbation, an iterative scheme, and a regular perturbation in terms of the film aspect ratio. The methods are used to derive higher order terms for a driven corner flow with a Newtonian lubricant of constant properties. Reasons for preferring the regular perturbation scheme are presented, and this method is used to obtain the correct curvature correction in an infinitely long journal bearing. Criteria for identifying singular geometries in hydrodynamic lubrication are set fourth.
The Low-Pressure Rheology of Ultra-Thin Lubricant Films and Its Influence on Sliding Contact
Authors: Streator J. L. , Gerhardstein J. P. , McCollum C. B.
Published: June 2008

Lubricant rheology governs the friction in any lubricated contact. In hydrodynamic lubrication, the genesis of friction is well-understood. However, when asperity contacts occur, the situation becomes more complex. In this “mixed” lubrication regime, it is known that lubricants can deviate dramatically from Newtonian behavior, but the source of this effect has not been identified. In particular, the question arises as to whether the non-Newtonian behavior of the lubricant is due to the extreme thinness of the film or to the very large shear rates to which it is subjected. In the current work, we analyze friction force measurements in the magnetic slider/disk interface to help resolve this question. Because of its precision geometry, the slider/disk interface is ideal for such an investigation. Results of the analysis indicate (1) the lubricant retains its bulk viscosity in films as thin as 11–12 molecular diameters; (2) the rheological state of the lubricant is determined by a parameter we introduce here as the “Newtonian” shear stress, and (3) the rheology of the lubricant at high Newtonian shear stress may indicate a newly discovered property of liquids.
On Parallel Hybrid Guide Bearings Under Combined Sliding and Small Amplitude Vibration
Authors: Chen S. H. , Mote C. D.
Published: June 2008

An original hybrid bearing model, operating under constant volumetric incompressible lubricant supply rate Q*, is proposed for 2-D parallel hybrid guide bearings subject to simultaneous translation and small amplitude transverse vibration. The model may describe the typical fluid film constrained between a translating/rotating saw blade and a saw guide, where lubricant is fed directly into the oscillating film. The inner boundary, or recess, pressure is time varying and coupled to the external lubricant supply. Unsteady fluid inertia resulting from vibration is measured by the squeeze Reynolds’ number Res and modeled. A methodology for analytical solution is developed to predict the amplitude and phase of the dynamic bearing load Wt*. A sample hybrid bearing, used to demonstrate the film pressure generating mechanisms in hybrid squeeze film, generates a 1.1 to 7.1 percent larger Wt* with a 5 to 21 deg phase lag for 1 ≤ Q* ≤ 4 when compared to Wt* produced in a hydrodynamic squeeze film bearing at the same Res. This phase shift can be significant when bearings are used for vibration control purposes.
A Study of Fluid Velocities in Tribological Fluid Film
Authors: Tieu A. K. , Kosasih P. B. , Mackenzie M. R.
Published: June 2008

Recently a model of Reynolds stress in turbulent lubrication theory was proposed by Tieu and Kosasih (1992) based on a modified Van Driest mixing length formula. It was developed from a study of Poiseuille flow velocities in narrow gaps. As a continuation of that study, this paper describes an investigation into fluid velocities in fluid film bearings. Experimental velocity profiles of planar flows in various film geometries are compared with the profiles calculated using the mixing length model in the transition-turbulent regime. Excellent agreements have been attained, confirming the validity of the formula in the superlaminar theory. The effects of Reynolds number and pressure gradient on nonplanar velocity profiles are also presented.
Transient Flow Patterns and Pressures Characteristics in a Hydrostatic Pocket
Authors: Braun M. J. , Zhou Y. M. , Choy F. K.
Published: June 2008

The flow in an open lid, shear flow driven cavity with a jet penetrating at its bottom is described by a mathematical model that uses the Navier-Stokes equations written in terms of the primary variables, u, v, and p. Using a time dependent conservative formulation, a finite difference method is applied through a staggered grid. The power law scheme is used in the treatment of the convective terms for this highly recirculating flow. The time dependent numerical experiments use both geometric (α = d/l, λ = c/l and γ = b/l), and dynamic similarity parameters [Reynolds number (R) and jet strength (F)] to study the development of the flow patterns, velocities, pressures and shear forces when the top plate impulsively entrains the cavity driving shear layer. The experiments are performed in two-dimensional cavities of square geometry (α = 1). The discussion pertaining to the convergence of the numerical scheme and the computational error shows that the strict convergence criteria applied to both velocities and pressures were successfully satisfied.
Experimental Test Results for Four High-Speed, High-Pressure, Orifice-Compensated Hybrid Bearings
Authors: Franchek N. M. , Childs D. W.
Published: June 2008

In this study, four hybrid bearings having different geometric configurations were experimentally tested for their static and dynamic characteristics, including flowrate, load capacity, rotordynamic coefficients, and whirl frequency ratio. The four bearings included a square-recess, smooth-land, radial-orifice bearing (baseline), a circular-recess bearing, a triangular-recess bearing, and an angled-orifice bearing. Each bearing had the same orifice diameter rather than the same pressure ratio. Unique to these test results is the measurement of the added mass terms, which became significant in the present tests because of high operating Reynolds numbers. Comparisons of the results were made between bearings to determine which bearing had the best performance. Based on the parameters of interest, the angled-orifice bearing has the most favorable overall performance.
Step Response of Hydrostatic Thrust Bearings With a Self-Controlled Restrictor Employing a Floating Disk
Authors: Yoshimoto S. , Anno Y. , Kanemoto T.
Published: June 2008

This paper describes the step response of a hydrostatic thrust bearing with a self-controlled restrictor employing a floating disk. This type of bearing can achieve a very high static bearing stiffness by controlling the mass flow rate of the fluid entering the bearing clearance, using the floating disk. Many parameters affect the step-response characteristics of this type of bearings. Influences of each parameter on the step response are theoretically investigated, and theoretical and experimental results are compared in order to verify the theoretical predictions. It was consequently found that the step response of this hydrostatic thrust bearing with the self-controlled restrictor was stable over a wide range of values of design parameters.
Viscoelastic Squeeze Film Characteristics With Inertia Effects Between Two Parallel Circular Plates Under Sinusoidal Motion
Authors: Hashimoto H.
Published: June 2008

In this paper, viscoelastic squeeze film characteristics subjected to fluid inertia effects are investigated theoretically in the case of parallel circular type squeeze films. In the development of modified lubrication equations, the nonlinear Maxwell model combining the Rabinowitsch model and Maxwell model is used as a constitutive equation for the viscoelastic fluids, and the inertia term in the momentum equation is approximated by the mean value averaged over the film thickness. Applying the modified lubrication equation to parallel circular type squeeze films under sinusoidal motion, the variation of the pressure distribution with time is calculated numerically for various types of fluids such as Newtonian, pseudo-plastic, linear Maxwell and nonlinear Maxwell fluids. Some numerical results are presented in graphic form, and the effects of inertia forces on the viscoelastic squeeze film characteristics are discussed.
Maximum and Average Flash Temperatures in Sliding Contacts
Authors: Tian Xuefeng , Kennedy Francis E.
Published: June 2008

The surface temperature rise for a semi-infinite body due to different moving heat sources is analyzed for the entire range of Peclet number using a Green’s function method. Analytical and approximate solutions of maximum and average surface temperatures are obtained for the cases of square uniform, circular uniform, and parabolic heat sources. Considering the heat partition between the two contacting bodies, solutions of interface flash temperature are presented for the general sliding contact case as well as for the case of sliding contact between two moving asperities.
Nanoindentation Hardness Tests Using a Point Contact Microscope
Authors: Lu C. J. , Bogy D. , Kaneko R.
Published: June 2008

The Point Contact Microscope (PCM), developed in NTT’s Kaneko Research Laboratory, is used to conduct hardness tests on polycarbonate and gold at indentation depths in the range of about 5-100 nm. Different diamond indenters, which are attached to single leaf springs of various stiffnesses, are used to study the effects of tip radius and apex angle on the measured hardness values. The indentation depth versus force and hardness versus force relations for various tip conditions are examined. It is found that the hardness value obtained increases for small values of indentation load and approaches a tip-independent value for larger loads. The hardness is sensitive to the indentation force and tip radius in the limit of small indentation depths. In this case, a standard indenter and a fixed load should be used to compare the hardnesses of different materials. The effect of the apex angle on the hardness is relatively insignificant.
Lubricant Film Flow in Pin-Disk Testing
Authors: Berg B. O. , Ettles C. M. , Hagerup H. J. , Calabrese S. J.
Published: June 2008

The spread of lubricant across the wear track of a pin-disk machine may be adequately modeled assuming axisymmetric, viscous flow with zero shear stress at the lubricant-air interface. Waves can develop according to the shape of the initial profile.
A Line-Contact Micro-EHL Model With Three-Dimensional Surface Topography
Authors: Chang L. , Webster M. N. , Jackson A.
Published: June 2008

A mathematical model is presented in this paper that can be used to analyze the effect of 3-D surface topography on the thermal, transient micro-elastohydrodynamic lubrication (EHL). The model efficiently incorporates the surface deformation due to the 3D pressure rippling and the lubricant side flow around the asperities. The resulting computer implementation requires little additional storage space and does not reduce computational efficiency from its 2-D counterpart. The model is shown to sensibly describe the physical problems. The results presented in this paper and in a separate paper (Chang et al., 1993c) show that the lubricant local side flow significantly affects the contact conditions of the EHL of rough surfaces, especially under high sliding. The work reported thus far represents the authors’ continuing effort to develop an analytical/computational model for tribo-systems operating in the micro-EHL/mixed-lubrication regime. Work in the future will model and integrate the asperity contact mechanics and lubricant-surface tribo-chemistry in the micro-EHL environment.
EHL of Coated Surfaces: Part I—Newtonian Results
Authors: Elsharkawy A. A. , Hamrock B. J.
Published: June 2008

A complete solution for the Newtonian elastohydrodynamic lubrication of coated surfaces in line contact is introduced in this paper. The formulation is based mainly on solving the Reynolds equation coupled with the elasticity equations of the multilayered elastic half space. The effects of pressure inside the lubricated conjunction on the lubricant viscosity and density are considered. The mathematical model for this problem is a nonlinear one, and an iterative Newton-Raphson scheme is used for numerical solutions. The effects of coating material and coating thickness on the pressure profiles and the film shapes are presented. Two different coating materials (molybdenum disulfide as a soft coating and titanium nitride as a hard coating) are considered. The results show that the pressure spike is higher for hard coatings than for uncoated surfaces but that the pressure spike disappears for soft coatings. Furthermore, the coating thickness has a significant influence on the minimum film thickness and the maximum contact pressure for both soft and hard coatings.
Micromechanics Modeling of Crack Initiation Under Contact Fatigue
Authors: Cheng W. , Cheng H. S. , Mura T. , Keer L. M.
Published: June 2008

Using dislocation pileup theory, a model is given for the prediction of crack initiation life under contact fatigue. Near surface crack initiation is investigated by introducing the sliding contact boundary condition. Crack initiation originated at the surface and substrate are treated as extreme cases. The new model physically explains how a surface crack can be initiated and shows that the surface crack initiation life should be shorter than the subsurface crack initiation life under the same stress amplitude conditions. A discussion is given about the influence of residual stress, hardness, temperature, irreversibility of the plastic deformation, as well as other parameters that affect the crack initiation life. Preliminary comparisons show that the new model agrees well with the experimental observations of surface and near surface crack initiation.
Thermal Elastohydrodynamic Analysis Using a Generalized Non-Newtonian Formulation With Application to Bair-Winer Constitutive Equation
Authors: Khonsari M. M. , Hua D. Y.
Published: June 2008

The governing equations together with a solution methodology are given which enables one to effectively handle an EHL line contact problem with simple non-Newtonian fluids including thermal effects. A computational algorithm is proposed that determines the equivalent viscosity as a function of shear strain rate for a specified constitutive equation. It is shown that the method effectively handles Bair-Winer’s rheological equation in its original form and without the need for an approximate perturbation analysis. Among the performance parameters presented are the local behavior of the shear stress as predicted by the Bair-Winer’s model and its comparison to that of the Ree-Eyrings constitutive equation. It is shown that these rheological equations predict a qualitatively similar trend for the traction coefficient. Nevertheless, depending on the operating conditions, the local shear stress as predicted by the Ree-Eyring equation may exceed the material limiting shear stress. A comparison study of the traction coefficient as predicted by the Bair-Winer’s fluid model and actual experimental measurements is also presented. The results are found to be in good quantitative agreement.
Elastohydrodynamic Analysis of Reverse Pumping in Rotary Lip Seals With Microundulations
Authors: Salant R. F. , Flaherty A. L.
Published: June 2008

An elastohydrodynamic analysis of a rotary lip seal containing microundulations, incorporating both the fluid mechanics of the lubricating film and the elastic behavior of the lip, has been performed numerically. The results indicate that, under dynamic conditions, the undulation pattern deforms such that it produces reverse pumping. The reverse pumping rate is substantial, and overwhelms the natural leakage induced by the sealed pressure, thereby preventing leakage through the seal. The results also show that the undulations hydrodynamically generate sufficiently high pressures, within the film, to provide load support and maintain the integrity of the film.
Parametric Study of the Dynamic Performance of Gas Lubricated Ringless Pistons
Authors: Gommed K. , Etsion I.
Published: June 2008

A parametric study is performed for a relatively large size helium cryocooler which utilizes a ringless piston. The results are presented in a dimensionless form and may be useful for other ringless-piston systems as well. Altogether twelve different parameters were studied and their effect on piston stability and sealing performance was analyzed and discussed. Although the cylindrical piston shape is commonly used for ringless piston applications, it seems that this is not the best choice for an optimum design.
Dynamic Structural Properties of Compliant Foil Thrust Bearings—Comparison Between Experimental and Theoretical Results
Authors: Ku C.-P. Roger
Published: June 2008

This paper describes an experimental investigation into the dynamic characteristics of corrugated foil (bump foil) strips used in compliant surface foil thrust bearings. This study provided the first opportunity to quantify the dynamic structural stiffness and equivalent damping coefficients of bump foil strips for a wide range of operating conditions. The experimental data were compared to results obtained by a theoretical model developed earlier. The effects of bearing design parameters, such as static loads, dynamic displacement amplitudes, bump configurations, pivot locations, surface coatings, and lubricant were also evaluated. An understanding of the dynamic characteristics of bump foil strips resulting from this work offers designers a means for enhancing the design of high-performance compliant foil bearings.
Structural Damping of Self-Acting Compliant Foil Journal Bearings
Authors: Heshmat Hooshang , Ku C.-P. Roger
Published: June 2008

This paper describes an experimental investigation into the dynamic characteristics of corrugated foil (bump foil) strips used in compliant surface foil journal bearings and dampers. In the experimental method described herein, a test facility with a journal supported by a compliant foil journal bearing was built. The nonrotating journal was driven by two shakers which were used to simulate dynamic forces acting on bump foil strips. The dynamic structural stiffness and equivalent viscous damping coefficients are calculated based on the experimental measurements for a wide range of operating conditions. The results are compared to the analytical predictions obtained by a theoretical model developed earlier.