Fluid Dynamic Bearings Research Articles

An experimental study on the applicability of acoustic emission to detection of lift-off state in gas foil bearings

Gas foil bearings (GFBs) are fluid dynamic bearings with applications in high-speed lightweight machinery. One of the important parameters in GFBs is the lift-off speed. It indicates a speed above which there is no occurrence of dry friction and wear between the rotor and bearing, the so-called lift-off state. There are various techniques for determination of the lift-off state. However, they come with major limitations and complexities for use in real-world applications, especially in high-speed machinery. In recent years, monitoring the lift-off condition in journal bearings based on acoustic emissions (AE) has been investigated and shown to be suitable for this type of bearing. Nevertheless, its applicability to GFBs is yet not known, as there has been no practical investigation on AE signals in GFBs. In the present work, the applicability of AE measurements to detection of lift-off state in GFBs is explored experimentally based on previous studies on journal bearings. The results demonstrate that AE measurements are a potential alternative to conventional methods in determining the lift-off state in GFBs. The comparison between features of AE signals, measured at two different locations on the bearing, with the friction torque as the reference exhibits the applicability of AE measurements to detection of lift-off state.

Hematite Suspension based Absorbent Pad Inclined Slider Influenced by Slip and Squeeze Velocity with Altering Film Ratio

The effects of various entities like slip and squeeze velocities, inlet-outlet film ratio, and the material parameter have been fairly explored in a hematite suspension based absorbent (porous) pad inclined slider. Mathematical expressions for pressure, load capacity (lifting force), friction, friction coefficient, and position of centre of pressure (COP) in terms of the above physical parameters have been acquired. Jenkins model has been employed as a mathematical set of governing equations. It has been found that an increase in the squeeze velocity has enhanced the load capacity and diminished the friction coefficient whereas the escalating values of slip velocity and material properties have reversed the trends. Besides, the optimum value of the inlet-outlet film ratio for maximum load capacity has reduced with a rise in the squeeze velocity. Improvement in material parameters shifted the position of COP slightly towards the inlet while an enhancement in the squeeze velocity and film ratio shifted the same slightly towards the outlet. The results acquired in the present paper will be helpful in designing and modifying the various types of fluid dynamic slider bearings.

Characteristics of Modified Spiral Thrust Bearing through Geometries and Dimension Modifications

This research focuses on the optimum design of fluid dynamic bearing (FDB) named modified spiral design. The objective is to improve the pressure and velocity distribution inside the FDB. In this paper, the current spiral design has 12 number of grooves while the modified spiral design has 24 number of grooves. Both design can be classified into two, bearing with seal and without seal. Air was chosen to replace the oil as the lubricant. Results show that the modified spiral bearing design has comparable characteristics compared to conventional spiral design in terms of pressure and velocity distribution. Modified spiral geometries also shows a tendency to replace the function of seals in FDB. This makes it possible to simplify bearing design without using any seal just by modifying its geometries based on novel modified spiral geometries. Experimental verifications also proved that the modified spiral bearing design has better air leakage control compared to the conventional ones. This phenomenon occurs in both design parameters, where when the designs are compared with respect to increase of rotational speed and air film thickness.

Finite-element coupled analyses of the Reynolds and Hagen–Poiseuille equations to calculate pressure and flow of fluid dynamic bearings with a recirculation channel

Abstract A finite-element analysis was proposed to evaluate pressure, flow, friction torque, and load capacity of coupled fluid dynamic thrust and journal bearings (CFDBs) with a recirculation channel (RC). The numerical program was developed to simultaneously solve the discretized equations of Reynolds lubrication for the CFDBs and the Hagen–Poiseuille flow for the RC. The proposed method was validated using a two-step procedure. As a first step, the measured flying height was compared with the simulated height. As a second step, pressure, friction torque, and flow calculated from the proposed method were compared with those calculated using computational fluid dynamics software. These results confirmed that the proposed method can effectively predict performances of the CFDBs with an RC.

Height analysis of the air-oil interfaces in the non-operating and operating FDBs in a tied shaft of HDDs

We proposed a method to predict the heights of upper and lower air-oil interfaces of the non-operating and operating fluid dynamic bearings (FDBs) in the tied shaft of hard disk drives. To predict the height of upper and lower air-oil interfaces, we formulated the linear pressure and volume equations in both cases of non-operating and operating conditions, respectively. The initial heights of upper and lower air-oil interfaces are determined by pressure and volume equations during non-operating condition. Pressure equation consists of capillary pressure, atmospheric pressure, hydrostatic pressure and applied pressure, and volume equation consists of the volumes of the clearance and the injected oil. In case of operating condition, hydrodynamic pressure generated by grooved bearings is additionally considered in the pressure equation, because heights of air-oil interfaces vary due to hydrodynamic pressure during operating condition. The linear pressure and volume equations were solved simultaneously to calculate the heights of upper and lower air-oil interfaces both in non-operating and operating conditions. In case of non-operating conditions, the height of upper air-oil interface increases with the increase of pressure difference because pressure of lower air-oil interface is higher than that of upper air-oil interface. Since the volume of injected oil is constant, height of lower air-oil interface decreases due to pressure difference. In case of operating conditions, the height of upper air-oil interface decreases according to rotating speed because more oil flows inside the FDBs and recirculation channel than upper seal. On the other hand, the height of the lower air-oil interface increases because volume variation of the oil due to the increase of the flying height is smaller than that due to the decrease of the height of the upper air-oil interface. Finally, we verified the proposed method by measuring the height of upper air-oil interface both in the non-operating and operating conditions.

Recent Technical Trends of Lubricating Oils for Fluid Dynamic Bearings in Hard Disk Drives

Recent Technical Trends of Lubricating Oils for Fluid Dynamic Bearings in Hard Disk Drives

An Efficient Iterative Approach for the Analysis of Thermal Instabilities in Rotating Machines

Most of the technological developments achieved in the turbomachinery field during the last years have been obtained through the introduction of fluid dynamic bearings, in particular tilting pad journal bearings (TPJBs). However, even those bearings can be affected by thermal instability phenomena as the Morton effect at high peripheral speeds. In this work, the authors propose a new iterative finite element method (FEM) approach for the analysis of those thermal–structural phenomena: the proposed model, based on the coupling between the rotor dynamic and the thermal behavior of the system, is able to accurately reproduce the onset of thermal instabilities. The authors developed two versions of the model, one in the frequency domain and the other in the time domain; both models are able to assure a good tradeoff between numerical efficiency and accuracy. The computational efficiency is critical when dealing with the typical long times of thermal instability. The research activity has been carried out in cooperation with General Electric Nuovo Pignone SPA, which provided both the technical and experimental data needed for the model development and validation.

Optimal design and experimental verification of fluid dynamic bearings with high load capacity applied to an integrated motor propulsor in unmanned underwater vehicles

We developed fluid dynamic bearings (FDBs) for application in an integrated motor propulsor (IMP) of an unmanned underwater vehicle. The structure of the FDBs is composed of plain and grooved journal and thrust bearings, and sea water is used as the fluid lubricant. The Reynolds equation and their perturbation equation were solved by applying the finite element method (FEM) to calculate static and dynamic coefficients. We developed optimal FDBs with high load capacity via an optimization problem of the multi-objective function of radial and axial load capacities and the constraint equations of friction, stiffness, and damping coefficients. Finally, we prototyped the optimal FDBs with high load capacity and verified the load capacity and friction torque of the FDBs via experiments.

Prediction of air-oil interface location in tied-shaft type fluid dynamic bearings

Tied-shaft type Fluid dynamic bearings (FDB) are widely used for small, high precision motor applications such as hard disk drives. This bearing contains two or more air-oil interfaces that cause the meniscus location to change as the bearing rotates from the stationary condition. To prevent oil shortage or leakage at the seal of the FDB, designing an internal geometry of the FDB that can maintain the airoil interface location at a suitable range is critical. This study proposes a system-level prediction model for the air-oil interface location in the taper seal element of the FDB in operating conditions. The FDB elements, grooves and a circulation hole are represented as pumps and flow resistance. With these representations, the FDB can be analyzed with a lumped parameter model. In addition, a hydrostatic equilibrium condition that incorporates capillary force at the meniscus in the taper seal element is used. The location of the meniscus is determined by the pressure difference between the upper and lower seal that is generated by the hydrodynamic effect inside the FDB in operating conditions. The characteristics of the FDB elements are obtained from CFD analysis and are used to solve five equations that predict the system-level performance characteristics: Upper/lower meniscus height, flow rate, pressure and flying height. With the proposed prediction model, the effects of the upper/lower chamfer location and the inclination angle of the circulation hole are investigated. These results confirm that an oil shortage occurs if the upper chamfer radius is below 1.5 mm, and the upper/lower chamfer radii have a bigger effect on the meniscus location than the inclination angle of the circulation hole.

Dynamic behavior of air–oil interface in fluid dynamic bearings with a double sealing structure in a 2.5′′ hard disk drive due to non-operating axial shock

We propose a numerical procedure to investigate the oil leakage of FDBs with a double sealing structure in a 2.5′′ HDD due to non-operating axial shock. We confirm the validity of the proposed procedure via a drop test to observe oil leakage of FDBs from the motion of rotor and stator under axial shock. We simulate the two-phase flow of air and oil in FDBs using a finite-volume method and a volume of fluid method to investigate the unsteady motion and break-up of the air–oil interfaces during and after axial shock. Furthermore, we analyze the axial shock-induced motion of air–oil interfaces due to magnitude of shock, and various design variables of the FDBs and the capillary seal.

Robust shaft design to compensate deformation in the hub press-fitting and disk clamping process of 2.5″ HDDs

We investigated deformation of the outer diameter of a shaft due to the hub press-fitting and disk clamping processes associated with a 2.5? hard disk drive. We propose a new robust shaft design to minimize the effect of deformation on the outer diameter of the shaft. We numerically show the effect of deformation on the shaft due to the pressure, stiffness, and damping coefficients of fluid dynamic bearings (FDBs), and the critical mass and excitation response of the rotor-bearing system. We also experimentally measured the axial non-repeatable runout and the amplitude at the half speed whirl frequency of FDBs with both conventional and proposed designs. Through these tests we confirm that the proposed design improves the static and dynamic performance of the FDBs and rotor-bearing system.

Prediction of the oil injection time of FDBs in an HDD spindle motor with a tied shaft by utilizing Kirchhoff’s pressure law

We propose a method to predict the oil injection time of fluid dynamic bearings (FDBs) with a tied shaft by applying Kirchhoff's pressure law. Since the oil is injected by capillary phenomenon, the volume flow rate can be calculated by utilizing Kirchhoff's pressure law. Then, we calculated the oil injection time of the FDBs with a tied shaft by dividing the volume flow rate by the clearance volumes of the journal bearing, the thrust bearing, and the recirculation channel (RC), respectively. We generated simulation models of the FDBs used in a 2.5? HDD spindle motor with a tied shaft. The total oil injection times of the FDBs with and without a RC were 0.302 and 0.335 s, respectively. Also, we verified the proposed method by measuring the oil injection time of FDBs with a RC. We applied the proposed method to predict and improve the oil injection time of the FDBs with a tied shaft due to the variation of major parameters affecting the oil injection time.

Control of Machining Profiles Based on Material Removal Capabilities in Cylindrical Blasting

Fluid dynamic bearings, which have an advantage in high-accuracy revolutions, are employed for spindle motors of hard disk drive, cooling fans for central processing unit, spindles of machine tool, etc. Micro herring-bone grooves are manufactured on cylindrical surface of the shafts in the bearing units, and they generate dynamic pressure during spins. Although the depth of the herring-bone grooves is generally constant, the shafts of which groove depth decreases from the ends of the groove array to its center improve the performance of bearing. The micro grooves are mainly manufactured by chemical etching, electrochemical machining, cutting and form rolling. However, the chemical etching and the electrochemical machining are difficult to control 3D machining profiles with high accuracy. The cutting and the form rolling have problems in tool lives and manufacturing costs. On the other hand, blasting is expected as one of micro fabrication methods. Therefore, we focused on blasting, and have investigated its material removal capabilities by fundamental tests. The purpose of this study is to control machining profiles in cylindrical blasting. The machining profiles of cylindrical workpieces in blasting were simulated based on the material removal capabilities obtained by the fundamental blasting tests. Then, the simulated profiles agreed well with the experimental ones by blasting under the same experimental condition.

Modified structure for reduction of cavitation at fluid dynamic bearing of HDD spindle motor under non-operating axial vibration

A hard disk drive (HDD) is subjected to the axial vibration while it is transported long distance or stored in vibrating environments. The cavitation may occur in the lubricant film of the spindle thrust bearing due to the axial vibration. The cavitation bubble may push the lubricating oil out of the bearing clearance so that it may be resulted in system failure of the HDD. The formation of cavitation bubble is studied for a specially designed fluid dynamic thrust bearing (FDTB) for the HDD. A novel design method for the FDTB is proposed, which suppresses cavitation effectively.

MINIATURE FLUID DYNAMIC BEARING WITH IMPROVED LOAD CAPACITY

In this paper a novel design is proposed to improve the load capacity of fluid dynamic bearing (FDB) for miniature spindle motors and small-form-factor data storage applications. In contrast to conventional miniature FDB with two sets of herringbone grooves on its inner surface, the proposed miniature FDB comprises another one set of herringbone grooves on its outer surface. The proposed miniature FDB is verified numerically utilizing commercial software Advanced Rotating Machinery Dynamics (ARMD). The simulation results show that compared to the conventional miniature FDB, the proposed miniature FDB can obviously improve the load capacity of the bearing system. Overall, the results presented in this study show that the proposed miniature FDB provides another solution for miniature spindle motor applications.

Fluid Dynamic Bearings: Modelling of Elastic Deformations

This article deals with a new approach for calculation of self-consistent pressure distribution and surface deflection for a lubricated journal bearing. This approach is based on the numerical solution of the 2-D Reynolds’ equation for the lubrication layer, numerical calculation of the surface deformations by the 3-D ANSYS package and Fourier series expansion for the compliance matrix. A simple analytical approximation is found for the obtained compliance matrix, which can be used for heavy loaded journal bearings. The compliance matrix is implemented into the iterative procedure for calculation of self-consistent pressure distribution and surface deflection in the contact zone. Results of calculations are presented for the particular journal bearing.

Effect of the cross-sectional shape of the recirculation channel on expulsion of air bubbles from FDBs used in HDD spindle motors

This paper experimentally and numerically investigated the behavior of air bubbles in the oil lubricant of operating fluid dynamic bearings (FDBs) with a cross-sectional recirculation channel (RC). Experiments were performed to visually observe the behavior of trapped air bubbles in operating FDBs with crescent and circular cross-sectional RCs, respectively. Furthermore, a numerical study was carried out to investigate the phenomenon of air bubble expulsion from the FDB. The flow field of a FDB was calculated by the Navier---Stokes equation, continuity equation and volume of fluid (VOF) method. Results indicated that a crescent cross-sectional RC generates a larger pressure difference between the upper and lower regions of the RC and a faster flow velocity along the RC than a circular cross-sectional RC, because the former has larger wall shear stress than the latter, mainly due to a longer perimeter. Finally, we designed a novel RC that efficiently expels air bubbles.

Stability analysis of a whirling rigid rotor supported by stationary grooved FDBs considering the five degrees of freedom of a general rotor-bearing system

This paper proposes a method to determine the stability of a whirling rotor supported by stationary grooved fluid dynamic bearings (FDBs), considering the five degrees of freedom of a general rotor-bearing system. Dynamic coefficients are calculated by using the finite element method and the perturbation method, and they are represented as periodic harmonic functions. Because of the periodic time-varying dynamic coefficients, the equations of motion of the rotor supported by FDBs can be represented as a parametrically excited system. The solution of the equations of motion can be assumed to be a Fourier series, allowing the equations of motion to be rewritten as simultaneous algebraic equations with respect to the Fourier coefficients. Hill's infinite determinant is calculated by using these algebraic equations to determine the stability. Increasing rotational speed increases $$K_{xx}$$Kxx and $$K_{{\theta_{x} \theta_{x} }}$$K?x?x, which decreases the stability of the stationary grooved FDBs; increasing whirl radius increases the stability of the FDBs because the resulting increases in the averages and variations of $$C_{xx}$$Cxx and $$C_{{\theta_{x} \theta_{x} }}$$C?x?x increase the stability faster than the corresponding increases of $$K_{xx}$$Kxx and $$K_{{\theta_{x} \theta_{x} }}$$K?x?x decrease the stability. The proposed method was verified by investigating the convergence and divergence of the whirl radius after the equations of motion were solved using the fourth-order Runge---Kutta method.

Monte Carlo simulation of the manufacturing tolerance in FDBs to identify the sensitive design variables affecting the performance of a disk-spindle system

This research proposes the Monte Carlo simulation for manufacturing tolerances in major design variables of fluid dynamic bearings (FDBs) to identify their sensitive design variables on the performance of FDBs and the dynamic performance of a disk-spindle system. Friction torque and critical mass were chosen as the performance indexes of FDBs and the disk-spindle system. The proposed method was applied to the FDBs of a 2.5? HDD, and the Monte Carlo simulation was performed for 16 major design variables of the FDBs. The clearance of the journal bearing was the most sensitive design variable of both the friction torque and critical mass. Additionally the groove to groove and ridge ratio and groove depth of the grooved journal bearing, which were manufactured by electrochemical machining, were also sensitive to determining the friction torque and critical mass of the FDBs, respectively. This research can be utilized to manage the manufacturing tolerance of FDBs to maintain consistent performance of FDBs and a disk-spindle system in an HDD.

Machining Characteristics in Cylindrical Blasting of Micro Grooves and Performance of Herring-Bone Bearing

The micro patterning on a cylindrical surface is conventionally carried out by electrochemical machining, chemical etching, cutting and so on. However, there exists some problems in machining dimensional errors and tool lives. On the other hand, blasting is expected as one of micro fabrication methods and progresses to apply for machining of hard and brittle materials. The purpose of this study is to develop the micro blasting technique for patterning of micro herring-bone grooves on spindle surfaces of the fluid dynamic bearings. Then, machining characteristics in blasting of rotating workpieces are experimentally investigated, analyzing stock removals, burr heights and groove shapes. In addition, micro herring-bone grooves are patterned on spindle surfaces by blasting and the performance of herring-bone bearings are evaluated. The herring-bone bearings with micro grooves patterned on a spindle surface by blasting have enough performance, compared with conventional ones.