Hydrostatic Pocket Research Articles

COMPENSATIONOF WORKTABLEMOTIONERROR BYANACTIVELY CONTROLLEDHYDROSTATIC GUIDEWAY

The accuracy of machine tools is limited by many factors,such as geometric and thermal errors. Current error reduction approaches include e.g. volumetric compensation. However, if a machine tool has only linear axes, angular errors cannot be compensated since there is no controllable axis that could change the tool or workpiece inclination. Therefore, an actively controlled hydrostatic guideway was developed to compensate for angular and other geometric errors. This paper aims to experimentally verify the hybrid use of constant resistance and the pilot stage of an industrial proportional pressure relief valve for regulation of oil flow through hydrostatic pockets and control of these pockets as well as active compensation of machine tool geometric errors. The error reduction of the tested linear axis is presented.

HYDROSTATIC THRUST BEARING WITH REDUCED POWER LOSSES

In many cases in rotating machinery, axial load is carried by tilting pad thrust bearings which have been developed since the beginning of 20th century. For high reliability and simplicity, most commonly the bearings are bath lubricated. In the times of sustainable development, however, minimization of friction losses becomes an important criterion for machinery assessment, and a strategic goal of their development. Performed calculations, based on elementary rules of fluid dynamics, showed that shearing losses in specially designed hydrostatic bearings can be considerably smaller than the losses in tilting pad hydrodynamic bearings. The aim of the research described in this paper was to check if the preliminary results presented earlier and conclusions of benefits of the further increase of the size of the hydrostatic pocket can be confirmed with the use of CFD calculations.

Analytical Solution of Hydrostatic Pocket Tilting

Tilting (parallelism error) of guiding surfaces may cause reduction of load capacity of hydrostatic (HS) guideways and bearings in machine tools (MT). Using coupled finite element (FE) computational models of MT structures, it is nowadays possible to determine the extent of guiding surfaces deformation caused by thermal effects, gravitational force, cutting forces and inertia effects. Assessment of maximum allowable tilt has so far been based merely on experience. The paper presents a detailed model developed for description of the effect of HS bearing tilt on the load capacity characteristics of HS guideways. The model allows an evaluation of the tilt influence on the change of the characteristics as well as determination of the limit values of allowable tilt in interaction with compliant machine tool structure. The proposed model is based on the model of flow over the land of the HS pocket under extended Navier-Stokes equation. The model is verified using an experimental test rig.

Effect of presence of lifting pocket on the THD performance of a large tilting-pad thrust bearing

Abstract Hydrostatic assistance is a commonly used method to improve limited load carrying ability of tiltingpad thrust bearings during transient states of operation of vertical shaft hydro-generators. Despite of special hydraulic equipment (as pumps, valves, etc.), it also requires manufacturing of special recesses/pockets at pad sliding surfaces, into which oil is injected under high pressure. It allows to lift the rotor before start-up of the machine and form a hydrostatic film between pads and collar. There is a quite wide variety of geometry of recesses (shape, depth, and size) met in practical large bearing applications. The presence of a hydrostatic pocket (usually located in the sliding surface above the pivot area, where thin film, high oil pressure and temperature are observed) affects bearing performance under hydrodynamic operation. In theoretical researches, there is an almost common practice not to include hydrostatic recess in thermohydrodynamic (THD) or thermoelastohydrodynamic (TEHD) analysis. This is probably due to the problems with obtaining solution for oil film geometry with pocket, the order of magnitude of the pocket depths being larger than gap thickness. In this paper, an attempt was taken to study the effect of lifting pocket on THD performance of a large tilting-pad thrust bearing of Itaipu power plant. Bearing performance was evaluated including recess shape for several cases of its depth. The results show that hydrostatic recess changes calculated bearing properties quite significantly, especially in vicinity of the pocket.

Using Hydro-Pneumatic Accumulators to Increase the Operation Safety of Heavy Duty Vertical Lathes

This paper presents the principle diagrams that can be used to manufacture hydraulic systems that can prevent the seizing of the circular hydrostatic guideways of the table in the case of heavy-duty vertical lathes, with over 4000 mm diameter. The proposed systems will provide the lubrication of the guideways of the table as long as this one rotates up to a possible stop caused by an accident. Lubrication is achieved even in the absence of the machine supply voltage because it is provided by systems that do not need electrical power to operate. The absence of hydrostatic pressure, even in only one of the hydrostatic pockets, caused by the breaking of a hose, can be detected and will trigger the safety system activation.

Experimental Research Regarding the Influence of Pocket Pressure Values on the Stiffness of Hydrostatic Guideways

A very important parameter which must be considered when designing a hydrostatic guideway of high stiffness is the pressure from the hydrostatic pockets. Modifications of this parameter during functioning are caused by variations of flow and supply pressure and also by the defective geometry of the hydrostatic pockets. The pocket pressure has a major importance for achieving the hydraulic sustentation, because it determines the load capacity, which thenceforth determines the film thickness and the flow. The latter is used for determining the hydraulic resistance of lands and also for deciding the geometry of the hydrostatic guideway as a whole. Based on these considerations, the analysis of the pocket pressure variations represents an important criterion which should be taken into account when designing a hydrostatic guideway with a very high stiffness. The present paper contains a new method for analysing the hydrostatic guideway behaviour with regard to its stiffness, for different pocket pressure values. The experimental research was carried out on a closed hydrostatic guideway, with the moving element having the dimensions of 500x260 [m and the pocket dimensions of 150x88 [m.

Device for Supplying the Hydrostatic Pockets with Compensation of the Fluid Temperature

The supply devices of the hydrostatic guideway pockets are elements with a major influence on the rigidity of the machine tool guideways. The value of the flow that circulates through these devices depends on the fluid viscosity as well. The researches in this field show that an important increase of the fluid temperature is produced when it passes through various orifices, slits and interstices that belong to the hydrostatic systems. The increase of the working temperature leads to decreasing the kinematic viscosity, with undesirable effects on the functioning of the hydrostatic guideways. In order to minimize this disadvantage, this work is presenting a constructive solution of a fixed supply device (flow control valve) with fluid temperature compensation. This type of device avoids the negative effects of increased fluid viscosity caused by the temperature increase as well, by automatically modifying the flowing cross section that is performed through a compensation bushing. The constructive solution of the new supply device is simple and involves relative low workmanship and costs. This supply device has been experimented on a rotary table model MRD 1500x2000, on the linear axis of 2200 mm travel. Through the comparative analyses of the new solution versus the standard one that was using as supply device a capillary type resistor, an increase of the guideway rigidity by 13% has resulted.

Comparative Experimental and Numerical Analysis of Flow and Pressure Fields Inside Deep and Shallow Pockets for a Hydrostatic Bearing

This article presents the experimental and numerical study of the development of flow patterns and pressure profiles inside a rectangular pocket of a hydrostatic journal bearing where its depth is the main varying parameter. Other parameters are the pocket's aspect ratio, restrictor inlet conditions, and shaft velocity. The flow visualization procedure used a full-flow field tracking method, a Lagrangian method used to follow microsphere particles injected in the flow stream and, through them, visually reconstruct the flow pattern in the pocket. A high-intensity pulsed YAG laser combined with an optical train of lenses was used to create a thin sheet of light that illuminates the particles contained in its plane. A long-distance microscope (LDM) coupled with a video camera digitally recorded the particles’ positions, thus creating instantaneous representative images of the flow structure. The experimental results presented herein are accompanied by a numerical simulation that uses CFD-ACE+, a commercial software package (ESI Group, Hunstville, Ala, 2006). The computational engine employed the full three-dimensional Navier-Stokes equations to simulate the steady-state flow in a hydrostatic pocket when an incompressible Newtonian fluid with constant properties was used. The numerical results display velocity vectors and streamline maps as well as the pressure patterns in both the shallow- and deep-type pockets. These results are then qualitatively compared with the experimental flow patterns acquired for the same type of pockets.

Three-Dimensional Flow and Pressure Patterns in a Hydrostatic Journal Bearing Pocket

The laminar flow in a hydrostatic pocket is described by a mathematical model that uses the three-dimensional Navier-Stokes equations written in terms of the primary variables, u, v, w, and p. Using a conservative formulation, a finite volume multiblock method is applied through a collocated, body fitted grid. The flow is simulated in a shallow pocket with a depth/length ratio of 0.02. The flow structures obtained and described by the authors in their previous two dimensional models are made visible in their three dimensional aspect for both the Couette, and the jet dominated flows. It has been found that both flow regimes formed central and secondary vortical cells with three dimensional corkscrew-like structures that lead the fluid on an outward bound path in the axial direction of the pocket. In the Couette dominated flow the position of the central vortical cell center is at the exit region of the capillary restrictor feedline, while in the jet dominated flow a flattened central vortical cell is formed in the downstream part of the pocket. It has also been determined that a fluid turn around zone occupies all the upstream space between the floor of the pocket and the runner, thus preventing any flow exit through the upstream exit of the pocket. The corresponding pressure distribution under the shaft for both flow regimes is presented as well. It was clearly established that both for the Couette, and the jet dominated cases the pressure varies significantly in the pocket in the circumferential direction, while its variation is less pronounced axially.

Pressure and flow characteristics in a shallow hydrostatic pocket with rounded pocket/land joints

The paper studies the development of the flow and pressure maps in a shallow hydrostatic bearing pocket and, on a comparative basis, discusses the effects of the pocket-to-land exit geometry when it takes different shapes (sharp 90° angle, and rounded with different radii of curvature). The numerical simulation uses a dimensionless formulation of the Navier-Stokes equations written in primitive variables for a body fitted coordinate system, and applied through a collocated grid. The model includes on one hand the coupling between the pocket flow and a finite length feedline flow, and the pocket and the adjacent lands on the other hand. Geometrically, all pockets have the same footprint, same land length, and same capillary feedline. The numerical simulation uses the Reynolds nuber ( Re) based on the runner velocity, and the inlet jet strength ( F) as dynamic similarity parameters, while the radius of curvature of the pocket/land joint is used as a geometric parameter. The study treats the laminar ranges of the Re number.

Closure to “Discussion of ‘Effects of the Feedline and the Hydrostatic Pocket Depth on the Flow Pattern and Pressure Distribution’” (1995, ASME J. Tribol., 117, p. 232)

Closure to “Discussion of ‘Effects of the Feedline and the Hydrostatic Pocket Depth on the Flow Pattern and Pressure Distribution’” (1995, ASME J. Tribol., 117, p. 232)

Effects of the Feedline and the Hydrostatic Pocket Depth on the Flow Pattern and Pressure Distribution

The paper treats on a comparative basis the development of the flow and pressure maps in deep, and respectively, shallow hydrostatic bearing pockets. The numerical simulation uses a dimensionless formulation of the Navier-Stokes equations written for a body fitted coordinates system, and applied through a collocated grid. The present work is a continuation of the simulations performed by Braun et al. (1993, 1994a, 1994b) to cases where the length and diameter of the restrictor feedline is of consequence to the flow in both the shallow and the deep pocket. The model includes the coupling between the pocket flow and a finite length feedline flow, on one hand, and the pocket and the adjacent lands on the other hand. Geometrically, all pockets have the same projected footprint, lands length, and capillary feedline. This numerical study uses the Reynolds number Re based on the runner velocity (laminar range only), and the inlet jet strength F as the dynamic similarity parameters, while the dimensionless clearance C is used as a geometric parameter. The flow structures, and the physical reasons underlying the causes of the pressure variation inside the deep and shallow pockets [either longitudinally (radially) or transversally (circum-ferentially)], are discussed quantitatively and comparatively. It is further shown that the transversal pressure distributions under the runner are highly dependent on whether the flow is dominated by the rotation of the runner (hydrodynamic effects), or by the strength of the hydrostatic jet (F). Finally the longitudinal pressure curves in the depth of the pocket, and restrictor are presented and the nature of their variation discussed. The back step and Rayleigh step effects are also discussed on a comparative basis for the two types of pockets, and in conjunction with the Couette, and the jet dominated flow. The modification of the central vortical zones of the deep pockets into oblong vortical zones for the shallow pockets is presented, and its consequences are discussed.

Flow Patterns and Dynamic Characteristics of a Lightly Loaded Hydrostatic Pocket of Variable Aspect Ratio and Supply Jet Strength

The flow in a hydrostatic pocket is numerically simulated using a dimensionless formulation of the Navier-Stokes equations. In essence, this paper continues the work of Braun et al. (1), (2) by extending it to the study of the effects of certain geometric (α and λ) and flow (R and F) parameters on stiffness and damping in the hydrostatic pocket. The numerical simulations evaluate parametrically the effects of the aspect ratio, the Reynolds number and the jet strength on the flow pattern, pressure magnitudes, runner equilibrium position, stiffness and damping. This synthesis analysis has direct pertinence to hydrostatic journal bearing parametric design, especially so for configurations when α << 1.

Effects of Hydrostatic Pocket Shape on the FlowPattern and Pressure Distribution

The flow in a hydrostatic pocket is numerically simulated using a dimensionless formulation of the 2‐D Navier‐Stokes equations written in primitive variables, for a body fitted coordinates system, and applied through a collocated grid. In essence, we continue the work of Braun et al. 1993a, 1993b] and extend it to the study of the effects of the pocket geometric format on the flow pattern and pressure distribution. The model includes the coupling between the pocket flow and a finite length feedline flow, on one hand, and the pocket and its adjacent lands on the other hand. In this context we shall present, on a comparative basis, the flow and the pressure patterns at the runner surface for square, ramped‐Rayleigh step, and arc of circle pockets. Geometrically all pockets have the same footprint, same lands length, and same capillary feedline. The numerical simulation uses the Reynolds number based on the lid(runner) velocity and the inlet jet strength F as the dynamic similarity parameters. The study aims at establishing criteria for the optimization of the pocket geometry in the larger context of the performance of a hydrostatic bearing.

Transient Flow Patterns and Pressures Characteristics in a Hydrostatic Pocket

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.

The Effects of a Hydrostatic Pocket Aspect Ratio, Supply Orifice Position, and Attack Angle on Steady-State Flow Patterns, Pressure, and Shear Characteristics

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&gt;1), and shallow (α≪1) hydrostatic pocket. The effects of the variation of the location and angle of the hydrostatic jet are also investigated.

フルイドダンパを付加したポケット型静圧気体ジャーナル軸受の軸の応答特性

Pneumatic hammer instability has been the greatest problem in realizing a high stiffness gas bearing with a large pocket in the bearing surface. The authors have already succeeded in realizing a high stiffness and heavy damping hydrostatic thrust gas bearing with a large pocket by the use of fluid damper. In this paper, the shaft response characteristics and static performances in a hydrostatic pocket journal gas bearing are investigated both experimentally and theoretically. The following conclusions are obtained. (1) Fluid damper has a good influence on the improvement of the trangent motion and the prevention of pneumatic hammer instability. (2) When an impact force acts on the shaft, it is possible to make the amplitude of rigid body as small as that of elastic body. (3) A high stiffness journal gas bearing with a large pocket can easily be realized by the use of fluid damper. (4) As the settling time is greatly affected by the pocket size, the pocket depth must be made as small as the bearing clearance in order to realize the critical damping condition.

Optimization of Conical Hydrostatic Bearing for Minimum Friction

Equations for the flow rate, load capacity, and friction torque for a conical hydrostatic bearing were developed. These equations were solved by a digital computer program to determine bearing configurations for minimum friction torque. Design curves are presented that show optimal bearing dimensions for minimum friction torque as a function of dimensionless flow rate for a range of dimensionless load capacity. Results are shown for both laminar and turbulent flow conditions. The results indicate the hydrostatic pocket friction is a significant portion of the total friction torque. However, the bearing dimensions for a minimum friction design are affected very little by inclusion of pocket friction in the analysis. For laminar flow the values of the outerland radius ratio X3 and outer bearing radius ratio X4 did not change significantly with increasing friction factor. For turbulent flow, the outer bearing radius ratio X4 did not change with increasing friction factor; therefore, the value determined for X4 in the laminar flow case is valid for all turbulent flows.