Stationary Pad Research Articles

Investigation of Lubrication Characteristics for a Soft Contact Lens

It is popular to wear the contact lens nowadays. Also, the output value of the contact lens is estimated more than 4 billion NT dollar every year. Because the phenomena of the contact lenses are very complicated, the relevant mechanisms are not well understood. Thus, the security of contact lens needs to be further investigated. In the lubricated mechanism of the contact lens, there is a layer of tear between the contact lens and cornea. The contact lens’ behavior is akin to that of a slider bearing. The lens represents the slider, the eye plays the role of the stationary pad, and the tear film is the lubricant. Hence, hydrodynamics and contact mechanisms of a contact lens are quite a fascinating subject that is relevant to the science of tribology. In the paper, the lubrication mechanisms include the partial hydrodynamic lubrication (contact lens roughness), contact mechanics and Newtonian fluid mechanics have been established. The parameters of roughness, flow factor, tear film geometry, and hydrodynamic pressure distribution are discussed. The developed technology increases the safety of contact lens.

Temperature in the railway disc brake at a repetitive short-term mode of braking

The finite element (FE) model to determine the transient temperature field in the ventilated disc brake of the traction diesel multiple unit (DMU) has been proposed. The advantage of the developed numerical model is the representation of mutual motion of the stationary pad and the rotating disc, by a heat source of arbitrary shape moving over the stationary disc. Computations were carried out for the pad and the disc separately introducing the heat partition ratio. Both the single and the multiple modes of braking were examined. The calculated distributions in contact temperature were compared with the corresponding results obtained from analytical solutions of the boundary-value thermal problem of friction, and with experimental data determined by the method of thermocouples. It was demonstrated that the calculated mean temperature on the friction surfaces of the brake components and the bulk temperature of the disc during multiple brake application agree well with the corresponding results, obtained by methods mentioned above.

Automotive brake squeal analysis with rotating finite elements of asymmetric disc in time

The new finite element brake squeal model is proposed where the finite elements of a real brake disc rotate in time. Contact nodal forces between the rotating disc and stationary pads are allocated to the moving contact area at every time step. When the proposed model is applied to an asymmetric automotive brake disc, it becomes the periodic time-varying brake system. The stability boundary of the discrete time-varying system is numerically calculated by the Floquet theory. Also, the quasi-static linearized eigenvalue analysis is conducted to show that the unstable modes repeatedly appear at the short interval of the disc rotation angle. The results are consistent with the angle-dependent local phenomenon of squeal termed squeal periodicity in the squeal experiment. In the nonlinear time-domain analysis, the squeal vibration increases and then decays in time for the rotating mode shape functions. It demonstrates that the rotation of an asymmetric disc can change the nonlinear squeal behavior as well as the linear stability character drastically.

Effects of surface texturing in steady-state and transient flow conditions: Two-dimensional numerical simulation using a mass-conserving cavitation model

This paper presents a numerical investigation on the influence of surface texturing on the hydrodynamic performance of a two-dimensional parallel slider bearing operating in both steady-state and transient lubrication. The tribological behavior of the bearing is evaluated by means of a mass-conserving cavitation model based on a finite element discretization of a modified version of the unstationary Reynolds equation. While considering the impact of the operating parameters, the influence of various geometrical features such as the texture ratio, the texture density, or the dimple depth is determined by means of an extended parametric study. Furthermore, particular attention is given to the lubrication mechanisms describing the potential effects produced by texturing the moving pad of the bearing. Thus, it is shown that a textured moving surface can contribute to the generation of hydrodynamic lift on the basis of a squeeze effect being produced at the leading edge of the textured region. Finally, the study reveals how texturing the stationary pad and/or the moving pad of the bearing affects hydrodynamic performance. The analyses show that, for an applied load, texturing the moving pad of the bearing instead of the stationary one leads to a decrease in film thickness, while texturing both pads produces optimum bearing performance.

회전 디스크 브레이크의 스퀼소음에 대한 선형안정성 연구

The squeal propensity of an automotive disc brake system is studied in the theoretical and computational manner. The rotating disc is in contact with two stationary pads and the nonlinear friction is engaged on the contact surface. The friction-coupled equations of motion are derived in the finite element(FE) of the actual brake disc and pad. From the general definition of friction force, the rotation and in-plane mode effects can be included properly in the brake squeal model. The eigenvalue sensitivity analysis and the mode shape visualization at squeal frequencies are also conducted for the detailed investigation. It is found that the squeal propensity is strongly influenced by rotation effect and the in-plane mode can be involved in squeal generation.

Brake features enhancing the wear debris bimodal distribution

Investigated is wear debris separation from dry friction interface of a vehicle brake system. Owing to structural and operational properties of the interface between the disc hard surface and the pad soft surface, the study focuses on the abrasive wear process of the pad profile. A theoretical dependence is sought between debris size distribution and governing parameters such as surface roughness, unevenness, and levels of normal loading.The analysis describes interacting surface profiles in terms of discrete probabilities of their heights. Enforced by motion constraints, the disc hard profile is monitored while clearing its way through the stationary pad overlapped profile by shearing of likely encountered asperities as estimated by their conditional probability. As a result, a single tangential stroke leads to a pad redistributed profile along with a collection of separated debris. A pre-stroke approach of the pad closer to the disc simulates the effect of a normal loading increment. Accordingly, three types of shear strokes may develop, namely; the superficial, the shallow subsurface, and the deep subsurface.It is found that the range of debris size only extends up to the mean height of surface profile. The debris size distribution is generally bimodal with large-size and small-size peaks, both being distinct during superficial strokes. The position and level of either peak depend on the two interacting profiles just before each individual stroke. For a disc Gaussian profile, sharply wavy profile of the pad surface makes the debris large-size peak to lie evidently near the complementary of the disc mean height. An originally Gaussian or a disc-induced profile of the pad surface enhances the level of the debris small-size peak whose position is constant. In addition, deep subsurface strokes yield an invariable debris distribution with a single small-size peak.

Squeal analysis of gyroscopic disc brake system based on finite element method

In this paper, the dynamic instability of a car brake system with a rotating disc in contact with two stationary pads is studied. For actual geometric approximation, the disc is modeled as a hat-disc shape structure by the finite element method. From a coordinate transformation between the reference and moving coordinate systems, the contact kinematics between the disc and pads is described. The corresponding gyroscopic matrix of the disc is constructed by introducing the uniform planar-mesh method. The dynamic instability of a gyroscopic non-conservative brake system is numerically predicted with respect to system parameters. The results show that the squeal propensity for rotation speed depends on the vibration modes participating in squeal modes. Moreover, it is highlighted that the negative slope of friction coefficient takes an important role in generating squeal in the in-plane torsion mode of the disc.

Heat Partitioning Coefficient Calculations for Sliding Contacts with Friction

In a sliding contact with a high Peclet number, frictional heat is partitioned between the contacting surfaces and depends heavily on the geometry, the thermo-physical properties of the contacting materials, and contact conditions. In this investigation, comparisons are made between contact temperatures and heat partitioning coefficients computed according to the analyses of Challen and Dowson and Harpavat. In the Challen and Dowson analysis, the moving surface is treated as a half-space, with convective heat loss only from the stationary pad to the surroundings. In the Harpavat analysis, the moving surface has finite thickness, and both the moving surface and stationary pad transfer heat via convection. Several cases are examined and a comparison of the results from the two models is made. The analyses are useful for designers, particularly those facing thermal management challenges in sliding contacts, and for tribological testing, where an accurate contact temperature estimate and partitioning coefficient are necessary to interpret the results for temperature-sensitive materials such as polymers and polymer composites.

Thermohydrodynamic analysis of infinitely wide cycloidal profiled pad thrust bearing with rough surface and a comparison to plane profiled pad

AbstractThis work reports the thermal analysis of hydrodynamically lubricated cycloidal profiled pad thrust bearing (CPPTB) using an efficient numerical method based on Lobatto quadrature technique. The performance parameters with smooth and rough surfaces of cycloidal and plane profiled pads are presented for various operating inputs. Significant increase in magnitude of pressure and substantial reduction in coefficient of friction are observed in the presence of relatively large amplitude of roughness over the surface of stationary pads. Comparisons are made for performance parameters achieved with plane profiled pad thrust bearing (PPPTB) and CPPTB. The load carrying capacity of CPPTB is found significantly high than the PPPTB for the provided set of operating parameters. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of surface profile on slider bearing performance

Hydrodynamic pad thrust bearings are commonly used in industries for sustaining the axial loads in machinery. Researchers have given great importance to the investigation of performance parameters of hydrodynamic pad thrust bearings for the wide range of operating conditions by using theoretical and experimental approaches. Recently, research is being focused on surface profiling of bearing surfaces and its role over the performance of fluid lubricated thrust pad bearings. Thus, the objective of this paper is to study theoretically and experimentally the performance parameters of pad thrust bearing by providing micro cycloidal surface profiling over the stationary pad. Moreover, performance parameters of pad thrust bearing achieved with cycloidal surface and conventional plane surface profiles are compared for highlighting the importance of micro-surface profiling on pad. Micro-level cycloidal surface profile is generated over the bearing pad using CNC milling machine. Based on this study, it is observed that the pressure profile generated in cycloidal surface pad thrust bearing is significantly more than conventional plane pad thrust bearing. However, temperature rise in lubricating film with both (cycloidal and plane) pad thrust bearings are almost same.

Nonlinear response of a beam under distributed moving contact load

In this paper, the nonlinear behavior of a one-dimensional model of the disc brake pad is examined. The contact normal force between the disc brake pad lining and rotor is represented by a second order polynomial of the relative displacement between the two elastic bodies. The frictional force due to the sliding motion of the rotor against the stationary pad is modeled as a distributed follower-type axial load with time-dependent terms. By Galerkin discretization, the equation governing the transverse motion of the beam model is reduced to a set of extended Duffing system with quasi-periodically modulated excitations. Retaining the first two vibration modes in the governing equations, frequency response curves are obtained by applying a two-dimensional spectral balance method. For the first time, it is predicted that nonlinearity resulting from the contact mechanics between the disc brake pad lining and rotor can lead to a possible irregular motion (chaotic vibration) of the pad in the neighborhood of simple and parametric resonance. This chaotic behavior is identified and quantitatively measured by examining the Poincaré maps, Fourier spectra, and Lyapunov exponents. It is also found that these chaotic motions emerge as a result of successive Hopf bifurcations characterized by the torus breakdown and torus doubling routes as the excitation frequency varies. Various aspects of the numerical difficulties in the solution of the nonlinear equations are also discussed.

A numerical study of performance of a slider bearing with heat conduction to the pad

A numerical study of thermohydrodynamic performance of a high-speed slider bearing with conduction to a stationary pad has been analyzed. The partial differential equations governing the fluid-film mass, momentum and energy conservations coupled with the heat conduction equation for the solid stationary pad assuming temperature-dependent density and viscosity, are solved using streamline upwind Petrov–Galerkin finite element method to yield various bearing characteristics. The influence of the solid pad under various thermal boundary conditions in conjunction with thermal boosting on the load-carrying capacity and frictional drag force of a slider bearing, at different inlet pressures and for various geometric configurations have been analyzed. Interesting features like hot spots and thermal boundary layer are noticed in the temperature flow field. Study reveals that setting the slider at temperature lower than the incoming lubricant increases the effective lubricant viscosity and thereby the laminarity which is vivid from the flow near outlet.

A methodology for the determination of dynamic instabilities in a car disc brake

The dynamics of a car disc brake system is investigated by a combined analytical and numerical method. The disc is rotated past the stationary pads and calliper in sliding friction at constant speed. The modal data of the disc are obtained by means of modal testing whereby a dynamic model for the disc is derived based on the thin plate theory. Then the pads, calliper and mounting are analysed by means of the finite element method. Finally the equations of motion for the whole disc brake system are established through the interfaces between the pads and the disc. The stability of the vibrating system is studied by the method of state space.

The influence of a joint displacement on torque, angle and angular velocity during isokinetic knee extension/flexion. An experimental investigation

In this article we investigated the influence of a displacement of the knee joint in relation to the axis of the isokinetic device on the measured torque, angle and angular velocity during isokinetic knee extension/flexion. We analysed the motion of the knee joint of four young men with the CCD-camera system KINEMETRIX (ORTHODATA GmbH, Liidenscheid, Germany) at different isokinetic speed (60° /s and 180° Is), using a device with a stationary (CYBEX 6000, LUMEX Inc., Ronkonkoma, NY) and a device with an instationary (LIDO ACTIVE, LOREDAN Biomedical Inc., Davies, California) tibial pad. The results show that the displacement of the knee joint in relation to the axis of the device using the instationary (sliding) input cuff is greater than using the stationary cuff. The torque errors range from 0% to -15% using the stationary pad and from +10% to -20% using the instationary cuff. While it is impossible to correct the torque values using the stationary pad (the displacement cannot be measured), the sliding cuff allows a mathematical correction of the torque values after the exercise. The max. torque error can be reduced below 2%. The largest deviations in angle occur at the higher speed (180° Is). At the moment the knee is in full extension, the max. error amounts approximately 20°. At lower speed (60° Is) the max. error amounts around 10°. The device with the stationary input cuff produces more accurate results. The measurement errors are below 10°. Using the instationary cuff, the selected isokinetic speed of 60° /s and 180° /s had been kept well. Considering the angular velocity of the knee joint during a test session, no real isokinetic state could be found. The device with the stationary input cuff has no real isokinetic state of the knee joint during the ROM too. In addition, the speed control during the motion is insufficient. Analysing the different factors which can influence the test results, it must be said that a comparison of test results, receiving from different isokinetic devices, is not possible resp. very questionable at today's state of the art.