Pad Load Research Articles

Investigation on Pivot Rolling Motion Effect on the Behavior of Rocker Back Tilting Pad Journal Bearings

The rolling motion of the pads with rocker back (RB) and ball and socket pivots is normally neglected in common software programs for the study of the rotor dynamic behavior of tilting pad journal bearings (TPJB). In other words, the theoretical contact point of the pivot is considered fixed. The aim of this work is to provide a novel way to implement in commercial software the effect of the variation of the circumferential coordinate of the theoretical contact point due to the pad rolling motion in RB TPJB. This is done by introducing an equivalent pivot rotational stiffness evaluated with an analytically derived formula, validated through finite element analysis. Such a stiffness is a function of the pad load and the radii of the contact pair, increasing with the load, the radii, and the degree of conformity of the contact. The static and dynamic characteristics of a five pad RB TPJB are then evaluated with a commercial software with and without the rotational stiffness contribution for two different pivot geometries. Non-negligible differences were found, particularly regarding the cross-coupled dynamic coefficients that show the higher sensitivity to the rotational stiffness. The inclusion of a pivot rotational stiffness among the data of commercial software for simulation of RB TPJB could contribute to fill the gap between numerical and experimental results.

Tribological Properties Study of Solid Lubrication with TiO2 Powder Particles.

Titanium dioxide (TiO2), by its tribological behavior, is known as a solid lubricant. TiO2 as a solid lubricant, together with tungsten disulfide (WS2) and molybdenum disulfide (MoS2) decreases friction and excessive wear. By compacting TiO2 powder, pellets are formed. Studies and research on the solid lubricant coatings were conducted with success on a tribometer with the possibility of making two simultaneous contacts, pellet/disk, and slider pad/disk. On the disk of a tribometer, we studied the lubrication characteristics of the TiO2 powder particles as the third body by intentionally transferring. Results show that the TiO2 pellet behaved like an effective oil-free lubricant by self-repairing and self-replenishing. In experiments, a TiO2 pellet is intentionally sheared against the surface of the disk, while the slider pad slips loaded on the lubricated surface until the deposited powder film is exhausted. A theoretical model control volume fractional coverage (CVFC) was used to estimate both the wear rate for the lubricated pellet/disk sliding contact and the friction coefficient at the pad/disk separation surface. According to materials properties, disk velocity, pellet and slider pad load, the pellet wear rate, and slider pad friction coefficient, using the CVFC model, can establish the pellet wear rate, and slider pad friction coefficient. The fractional coverage represents a parameter of the CVFC model that varies with time, and it is useful for estimating the film amount from the third body that covers the disk asperities. Model results well enough describe the tribological behavior of the sliding contacts in experiments, both qualitatively and quantitatively. In addition, the theoretical results obtained by modeling and the experimental those obtained in the process of friction, are compared.

Finding the pad load to suppress wrinkling in free bending of ultra-high tensile strength steel sheets

The critical pad load to suppress wrinkling in free bending is derived by linking the pad lifting behaviour and the wrinkle disappearing condition that depends on the clearance between the punch and the blank generated by pad lifting at the early stage of bending. The clearance is estimated based on the simulated results that the contacting zone of the blank with the pad at the early stage of bending is confined to the vicinity of the punch shoulder. The usefulness of the critical pad load is confirmed by forming experiments of a T-shaped product from high strength steel sheets.

Effectiveness of neoprene pad vibration isolators at high frequencies

The natural frequency, dynamic stiffness, and insertion loss of commercially available neoprene pad vibration isolators have been measured in a simple, single degree of freedom system over a wide range of pad loadings out to a maximum frequency of 10 kHz. The results reveal that dynamic stiffness can vary significantly with pad loading as well as the durometer of the material. It will also be shown that insertion loss follows the theoretical single degree of freedom curve only out to a frequency that is about 5 to 10 times the natural frequency, depending upon the pad durometer rating. Above that frequency wave resonances in the material cause the insertion loss to deteriorate significantly out to a frequency near 1 kHz, above which the insertion loss maintains a relatively constant value, again depending upon the pad durometer rating. In some instances the insertion loss values can approach 0 dB or even become negative at specific frequencies in the frequency region that is 10 to 20 times the natural frequency of the system.

Wrinkle Generation Process in Press Forming of High-strength Steel Sheet

The wrinkle generation process of high-strength steel sheets in compression is investigated by finite element method (FEM) simulation and experiment. When a high-strength steel sheet is compressed under a pad with a constant load and initial clearance, there exist two modes in the wrinkle generation process. One is the wrinkle growth mode with the progress of compression upon lifting the pad. The other is the wrinkle disappearance mode, where the wrinkle generated at the early stage of the compression process vanishes with the decrease in clearance between the pad and the sheet resulting from the increase in sheet thickness. The wrinkle disappearance mode is applicable when the pad load is higher than the initial peak value of the wrinkle pushing force in compression.

Comparison of lapping performance between fixed agglomerated diamond pad and fixed single crystal diamond pad

The ability for self-conditioning is the most important characteristic of hydrophilic Fixed Abrasive (FA) pads using in lapping. The failure mode of diamond abrasives embedded in a FA pad greatly impacts its self-conditioning process. In this investigation, three types of FA pads were prepared. One was embedded with W50 single crystal diamond (SCD) abrasives, one was embedded with W10 SCD abrasives, and the other was embedded with agglomerated diamond (AD) abrasives. The lapping performance was tested on fused quartz workpieces. The pad wear mechanisms and the wear debris of the three pads was evaluated using Scanning Electron Microscopy (SEM). The FA pad embedded with AD exhibited a higher material removal rate and a lower arithmetic average surface roughness (Ra) of the lapped quartz than those embedded with SCD. Pad loading and flattening of the of abrasive grit are the main factors causing the failure of FA pad with SCD. Compared with FA pads with SCD, the rough surfaces of AD grains improves the holding force of pad matrix to them, which in turn leads to a more stable cutting depth of abrasive grain. Size analysis of the debris suggests that AD grains are inclined to micro-fracture rather than to wear flat or pull out. The micro-fracture phenomenon of AD in FA lapping process improves the material removal rate and lowers the average surface roughness of lapped work-pieces. The adoption of AD grains was concluded to be important in improving the self-conditioning performance of FA pads.

The influence of incontinence pads moisture at the loaded skin interface

AimProlonged mechanical loading on soft tissues adjacent to bony prominences can lead to pressure ulcers. The presence of moisture at the skin interface will lower the tolerance to load. Absorbent pads manage moisture in individuals with incontinence, although their role in maintaining skin health is unknown. The present study investigated the effects of moist incontinence pads on skin physiology after periods of mechanical loading. Material and methodsTwelve healthy participants were recruited to evaluate a single incontinence pad design under three moisture conditions: 0% (dry), 50% and 100% fluid capacity. For each pad condition, pressure (9 kPa) or pressure in combination with shear (3 N) was applied to the sacrum, followed by a period of off-loading. Measures included trans-epidermal water loss (TEWL) and inflammatory biomarkers sampled at the skin interface. ResultsResults revealed no change in TEWL in the loaded dry pad condition. By contrast, when the pads contained moisture, significant increases in TEWL were observed. These increases were reversed during off-loading. Inflammatory biomarkers, specifically IL-1α/total protein ratio, were up-regulated during dry pad loading, which recovered during off-loading. Loaded moist pads caused a significant increase in biomarkers, which remained elevated throughout the test period. ConclusionThe study revealed a marked compromise to stratum corneum integrity when the skin was exposed to moist incontinence pads in combination with mechanical loads. These physiological changes were largely reversed during off-loading. Incontinence pads provided some protection in the dry state, although more research is required to determine optimal clinical guidance for their use.

An investigation into the wear mechanism of zirconia-alumina polishing pad under different environments in shape adaptive grinding of WC-Co coating

Shape adaptive grinding (SAG) is a relatively newer finishing process to achieve nanoscale surface roughness of difficult to machine materials. The wear mechanism of zirconia-alumina polishing pads is investigated after performing SAG and chemical assisted SAG (CA-SAG) for the finishing of WC-Co coating. The numbers of active abrasive particles in SAG increases due to the existence of the elastic medium and that leads to a smaller depth of penetration. This phenomenon along with the presence of tough zirconia phase in abrasive particles limits the possibility of the fracture wear. Smooth and elongated grooves along the finishing direction are observed on the wear flats which indicate that the plastic deformation is the predominant wear mechanism rather than micro-fracture. A phenomenological model for the attritious wear of the polishing pads is developed using the experimental findings. The radial pad wear and pad loading are analysed quantitatively using surface texture parameters. The wear mechanism in CA-SAG is immensely affected by the use of chemical reagent in the process. A substantial pad loading is observed owing to the deposition of the passivation layer in the inter-grit spaces. Thus, shelling of the abrasive particles and pad loading are identified as the dominant wear mechanisms.

Component design and accident analysis of fuel assembly for prototype GEN-IV sodium-cooled fast reactor

Abstract The Korea 150 MWe Prototype GEN-IV Sodium cooled Fast Reactor (PGSFR) project has reached the middle of its preliminary safety analysis report (SAR) design phase. The core design studies are being conducted by KAERI. Some design features for the core have been made to comply with the GEN-IV reactor objectives. The main objective to improve the safety of the core design. A negative sodium void worth was achieved thanks to a new fuel assembly design including U-Zr and U-TRU-Zr axially heterogeneous fuel rods, a large cladding/small spacer wire bundle, a sodium bonding between metal slug and cladding, and upper gas plenum above the active core region, and upper/lower reflectors. Finite element calculations were performed to increase the stiffness of the above core load pad and top load pad in order to analyze their effect on the core mechanical behavior during hypothetical radial core flowering and compaction events. More generally, all design choices for the PGSFR have been made with the permanent objective of minimizing the fuel assembly height to decrease the overall costs of the reactor and the fuel cycle. This paper describes the fuel assembly design for the GEN-IV core at the end of the preliminary SAR design phase and studies performed to optimize the performances. The basic design and fuel assembly analysis results were described in this work. In specially, the design analysis results under the accident condition is focused in this paper. As the results of that, the structural integrity of a fuel assembly was satisfied for the PGSFR fuel assembly mechanical design criteria.

On the Two-Scale Modelling of Elastohydrodynamic Lubrication in Tilted-Pad Bearings

A two-scale method for modelling the Elastohydrodynamic Lubrication (EHL) of tilted-pad bearings is derived and a range of solutions are presented. The method is developed from previous publications and is based on the Heterogeneous Multiscale Methods (HMM). It facilitates, by means of homogenization, incorporating the effects of surface topography in the analysis of tilted-pad bearings. New to this article is the investigation of three-dimensional bearings, including the effects of both ideal and real surface topographies, micro-cavitation, and the metamodeling procedure used in coupling the problem scales. Solutions for smooth bearing surfaces, and under pure hydrodynamic operating conditions, obtained with the present two-scale EHL model, demonstrate equivalence to those obtained from well-established homogenization methods. Solutions obtained for elastohydrodynamic operating conditions, show a dependency of the solution to the pad thickness and load capacity of the bearing. More precisely, the response for the real surface topography was found to be stiffer in comparison to the ideal. Micro-scale results demonstrate periodicity of the flow and surface topography and this is consistent with the requirements of the HMM. The means of selecting micro-scale simulations based on intermediate macro-scale solutions, in the metamodeling approach, was developed for larger dimensionality and subsequent calibration. An analysis of the present metamodeling approach indicates improved performance in comparison to previous studies.

The Effect of the Tractor Tires Load on the Ground Loading Pressure

The contribution, based on the experimental measurement, analyses the issue of a specific pad loading pressure in relation to radial wheel load and tire inflation. Tile inflation in terms of allowed radial load, which we wish to be high as far as possible in terms of the power transmission from the wheels to the ground is important for the field tensile works, on the other hand, the pressure in the tire is closely related to the contact area and ground loading pressure. The obtained results show the following: Michelin Multibib 650/65 R38 tire, inflated to a pressure of 80 kPa has a sufficient reserve in radial load. As the measurement results show, ground loading pressure at the highest load of 36.55 kN, which was used during the measurement, is under the value of 100 kPa, which would mean on loamy soil at a humidity of 17 % that the limit of the ground pressure on the ground has not been exceeded.

Finite element analysis of fretting contact for dissimilar and nonhomogeneous materials

Fretting problem arises in the case of relatively small sliding motion between contacting surfaces leading to reduction in fatigue life of these parts significantly. The purpose of this study is to investigate the effects of fretting on the contact region in a cylindrical on flat contact configuration. In order to identify fretting contact, a finite element (FE) model was constructed by using commercial finite element package ABAQUSTM. Fretting contact is studied under two types of loading: tangential loading of the pad and axial bulk stress loading of the specimen. The numerical calculated stresses are compared with Mindlin analytical solution and Nowell and Hills analytical solution (which includes the effect of bulk stress). In order to get a better estimate of fretting in real type structures such as bolted joints and lug type structures, dissimilar materials in the contact region are then studied. For dissimilar materials, in the light of the conducted numerical analysis, Mindlin solution does not give reliable results in terms of shear traction distribution when compared to FE results. Finally, the effect of non-homogeneity in materials is investigated by addition of circular voids.

Uncertainty Analysis of a Tilting-Pad Journal Bearing Using Fuzzy Logic Techniques

This paper is dedicated to the analysis of uncertainties affecting the load capability of a 4-pad tilting-pad journal bearing in which the load is applied on a given pad load on pad configuration (LOP). A well-known stochastic method has been used extensively to model uncertain parameters by using the so-called Monte Carlo simulation. However, in the present contribution, the inherent uncertainties of the bearing parameters (i.e., the pad radius, the oil viscosity, and the radial clearance; bearing assembly clearance) are modeled by using a fuzzy dynamic analysis. This alternative methodology seems to be more appropriate when the stochastic process that characterizes the uncertainties is unknown. The analysis procedure is confined to the load capability of the bearing, being generated by the envelopes of the pressure fields developed on each pad. The hydrodynamic supporting forces are determined by considering a nonlinear model, which is obtained from the solution of the Reynolds equation. The most significant results are associated to the changes in the steady-state condition of the bearing due to the reaction forces that are modified according to the uncertainties introduced in the system. Finally, it is worth mentioning that the uncertainty analysis in this case provides relevant information both for design and maintenance of tilting-pad hydrodynamic bearings.

Steady state and dynamic characteristics for guide bearings of a hydro-electric unit

Experiments are conducted using a 10-MW Kaplan hydropower machine which is outfitted with an extensive array of sensors to determine oil film thickness, pad load and oil temperature in all three guide bearings as well as motion of the shaft in relation to both the bearing housings and the concrete foundation. Test results for all journal bearings are compared to a commercial rotor dynamics model and results for the central journal bearing are compared to a multi-physics model to provide insight into the machine's steady state and dynamic characteristics and their variations during normal operation.

Development of a machine integrated strain-based contact force sensor for pad foot soil compactors

An investigation was undertaken to explore the use of measurable pad strains on a non-vibratory pad foot roller to provide real time continuous evidence of compaction and contact force. Individual pads were instrumented with strain gages in a pattern chosen based on pad finite element analysis (FEA). Different pad–soil contact stress distributions were modeled to simulate a range of soil conditions. The FEA revealed that the contact stress distribution has a significant influence on the observed pad strain field, suggesting soil specific interpretation of pad strains in order to determine contact force. Results from uniaxial laboratory testing of pad loading on dry sand verified the FEA, i.e., experimental strains generally matched within 15% of FEA strains. The contact stress distribution was measured using tactile pressure sensors and found to be moderately parabolic. A soil specific empirical calibration factor relating vertical sidewall strains to contact force was determined. Field testing was performed on the dry sand with multiple instrumented pads installed on a Caterpillar CP56 roller. Pad strain magnitudes increased up to 250% during compaction from repeated passes of the roller. Using the empirical calibration factor, the estimated contact force was shown to increase with compaction, represented by the independently-measured soil unit weight.

The influence of foot posture, support stiffness, heel pad loading and tissue mechanical properties on biomechanical factors associated with a risk of heel ulceration

Heel ulcers (HU) are the second most common type of pressure ulcers. In this work, we developed the first anatomically-realistic three-dimensional finite element model of the posterior heel for studying the risk for HU in bedridden patients. We specifically simulated a heel that is resting on supports with different stiffnesses at upright and inclined foot postures. Our objective was to examine the effects of foot posture and stiffness of the support on strains and stresses within the fat pad of the resting heel. We found that strains and stresses in the fat pad of the heel are considerably reduced when the foot is positioned so that its lateral aspect is at 90° with respect to the horizon compared to an abducted (60°) foot posture. The study therefore indicates that theoretically, an inclined foot posture puts a bedridden patient at a higher risk for HU with respect to an upright foot posture, which may be explained by the anatomy of the heel that faces a lower curvature and better cushioned region against the support when the foot is upright.

A Study on the Oil Whip of Rotor Supported in Tilting-Pad Journal Bearings (2nd Report: Effects of Various Design Parameters)

This paper studies the oil whip stability characteristics of a Jeffcott rotor supported in tilting-pad journal bearings theoretically. The effects of design parameters such as type of bearing seat, pad inertia, length to diameter ratio (L/D), pad number, load direction and stiffness of the shaft on the stability characteristics of the rotor-bearing system over the wide operating conditions are shown. Types of bearing seat make a great effect on the stability characteristics of the rotor-bearing system. In the case of pivotable bearing seat, the stability threshold shaft speeds increase with the preload factor under wide operating conditions. On the other hand, in the case of non-pivotable bearing seat, the stability characteristics do not change much with the increase in the preload factor. Increase in L/D ratio decreases the stability threshold shaft speed when Bearing Modulus λ is low. Pad inertia destabilizes the rotor at the high shaft speeds only when bearing modulus is low. Pad number and load direction make little effect on the stability.

An asperity-based fractional coverage model for transfer films on a tribological surface

An asperity-based fractional coverage (AFC) model has been developed for studying the process of lubricant film transfer on a real surface. A solid/powder lubricant film is transferred to a test disk surface by intentionally shearing a compacted pellet against the rotating disk surface. A slider pad simultaneously riding on the disk can be lubricated or starved during a competition process where the pellet deposits film as the slider depletes it. The tribological behavior of this film transfer process is modeled at the asperity scale to get an understanding of the macro-scale transient and steady-state friction. The AFC model was developed for this study by extending the range of applicability of its predecessor, the control volume fractional coverage (CVFC) model, to a real tribosurface. Topography data for the tribosurface was obtained from an atomic force microscope (AFM) image of the tungsten carbide (WC) test disk. The model predicts the wear rate of the pellet, pellet/disk and pad/disk friction coefficients, and lubricant thickness as a function of the pellet load, slider pad load, disk speed, and material properties.

Influence of Different Loading Modes on Rock Tensile Strength

A typical mechanical character of rock is that the tensile strength is far less than the compressive strength. Meanwhile, the test data of tensile strength is very dispersive. Because the direct tensile tests always result in failure due to the difficulty in clamping the rock sample, the splitting test is used to determine the tensile strength of rock. There are four kinds of loading modes in the splitting test in actual laboratory test: angle pad splitting, round pad splitting, aclinic loading platen splitting, arc loading platen splitting. In this paper, the direct tensile test, the splitting test and the influence of different loading modes on rock tensile strength were studied. In order to study the stress distribution, the progressive splitting failure process was numerically modeled under the four kinds of loading cases by the Realistic Failure Process Analysis code (RFPA2D). Results show that the stress states under angle pad splitting, round pad splitting are similar to the stress states under diametrical compressive state. Regarding that the round pad splitting test is easy to implement, and its numerical results are also stable relatively, the round pad loading mode was suggested to be adopted.

A novel bumping process for fine pitch Sn–Cu lead-free plating-based flip chip solder bumps

Electroplating is the best process for the manufacture of fine pitch flip chip solder bumps. However, certain unstable electroplating parameters usually cause poorer coplanarity, which affects packaging reliability and yield. This paper attempts to utilize a CMP-like polisher to reduce the nonuniform height deviation after electroplating. The optimization of three major polishing parameters—pad hardness, loading pressure, and polishing speed—enables the polisher to have a higher material removal rate (MRR) and an easier manipulation as compared with chemical mechanical polishing (CMP). After polishing at a pitch size of 100 μm, the overall coplanarity could be decreased sharply from 33±2.5 μm (coplanarity=7.5%) to 28±1 μm (coplanarity=3%) and it even reached 26±0.5 μm (coplanarity=1%) after reflow.