Overview Of Hard Disk Drives

LITERATURE REVIEW

The suspension manufacturing process is an important research area. So, this chapter presents:

An introduction physical components in HDD

An introduction component of suspension

Finite Element Method

Principle types of bending

Bending Methods

Theoretical Bending Models

Design of Experiments

Experimental Analysis

2.1 Physical component in HDD

The computer hard disk drives (HDD) are used to store a large quantity of information for retrieval as and when required. Figure 2.1 shows the main components of Hard disk drives. A Hard disk drive consists of three important mechanical components such as a fly head mechanism, a head positioning mechanism, and a disk spindle mechanism. However, Read/Write operations on a magnetic disk are performed by the rotational motion of the recording and the radial motion of the recording head using the swing arm actuator. [6].

Figure 2.1: Components of hard disk drive assembly.

Table 2.1: Function for component of Hard disk drives

Components of hard disk

drive

Functions

HEAD ARM

The magnetic element reads/writes magnetic signals

SLIDER

The strategically shaped block of graphite that carries the head

READ/WRITE HEAD

It is integrated with slider, which is mounted on the head of suspension. This device used to record information on the disk or read information from it.

SUSPENSION

The head is attached to this components and it is the "carrier" for the Slider that allows the head to "fly" over the disk surface.

VCM

The VCM or actuator is the device used to position the head arm to different tracks on the surface of the platter.

DISK/MEDIA/PLATTERS

A thin round aluminum or glass substrate that is coated with a magnetic material. The head writes and reads to/from the disk.

SPINDLE/MOTOR

Disks are stacked along the length of the spindle and the motor spins the disks at a consistent RPM (Revolutions Per Minute).

2.2 Component of suspension

The suspension interconnects carries signals from the write driver to the write head and from the read head to the preamplifier. Therefore, the components of Hard disk drive are composed of Load beam, Mount plate, and Flat flexure circuit as shown in Figure 2.2 [7].

Figure 2.2: The component of the suspension.

2.2.1 Suspension Functions

All magnetic Hard disk drive require suspensions to hold/support the Read/Write head and provide precise preload angle (GL) [8].

Figure 2.3: Suspension Functions.

A suspension assembly holds recording heads over the spinning disk surface but instead "fly� at a precise very small height above the disk surface. It is the suspension that allows the head to "fly" less than 0.01 micrometer above the disk surface that is rotating continuously at speeds range between 5,400 to 20,000 rpm as shown in Figure 2.4.

Human hair~50 mm Dia.

Fly height <0.01mm

Dust particle ~5 mm Dia.

Figure 2.4: The precise very small height above the disk surface.

1. The "flying" height concern, the ability of the head to read or write data, while

insufficient "flying" height causes the head to scratch the disk surface and destroy the magnetic coating and the data on the disk. The head is able to stay precisely at the correct "flying" height because of the equilibrium of the upward force of the air driven under the head and the downward force controlled by the suspension assembly as shown in Figure 2.5.

Preload angle

>DGL

<DGLL

Figure 2.5: Force balance on suspension operation.

Spring rate is both the output of individual standard Gram load and the displacement, because the individual standard Gram load and displacement are directly affected the spring rate whether low or high Spring rate. The range of changing <DGL has low spring rate. On other hand, the range of changing >DGL has high spring rate. However, low or high spring rate will be displacement Dz in z direction, if we consider low spring rate better than high spring rate.

2. The suspension must also hold the head at the correct angles in two axes PSA and RSA simultaneously.

3. The suspension must also optimize resonance for provide less off-track movement during seek.

4. Others critical parameters as customer requirement.

2.3 Finite Element Method

2.3.1 Introduction

Considered to the direct methods, the finite element technique further offers the idea

of discretization, this time of the very structure or solid under investigation. This allows broadening the class of problems amenable to solution so as to include those dealing directly with modern technology. On the other hand, a sufficiently fine mesh and/or high order of approximation within elements ensure that the error is kept reasonably small. This technique requires the processing of extensive data and may efficiently be implemented with the help of computers only.

2.3.2 Fundamental Concepts

FEM cuts a structure into several elements (pieces of the structure). Then it reconnects

elements at “nodes� as if nodes were pins or drops of glue that hold elements together.

This process results in a set of simultaneous algebraic equations.

Figure 2.6: Many engineering phenomena can be expressed by “governing equations� and “boundary conditions�.

Figure 2.7: FEM Resolution way; FEM approximates problem equations to a set of algebraic equations.

K = Property

U = Behavior

F = Action

Figure 2.8: Behavior is the unknown parameter of the problem.

2.3.4 Basic steps in the Finite Element Method

The basic steps involved in and finite element analysis consist as shown in Figure 2.9 [9].

Post-processing Phases

Solution Phase

Preprocessing Phase

Figure 2.9: Basic steps in the Finite element method.

2.3.4-1 Preprocessing phase

1. Create and discretize the solution domain into finite element; that is, subdivide the problem into nodes and elements.

2. Assume a shape function to represent the physical behavior of an element; that is an approximate continuous function is assumed to represent the solution of an element.

3. Develop equation for an element.

4. Assemble the element to present the entire problem. Construct the global stiffness matrix.

5. Apply boundary conditions, initial conditions, and loading.

2.3.4-2 Solution phase

Solve a set of linear or nonlinear algebraic equations simultaneously to obtain nodal results, such as displacement values at different nodes or temperature values at different nodes in a heat transfer problem.

2.3.4-3 Post-processing phase

Obtain other important information. At this point, we may interest in values of principal stresses.

2.4 Principle types of Bending

2.4.1 Introduction

Sheet-metal parts are typically produced by a sequence of bending operations. The bending process starts with a flat part and ends up with a three-dimensional object of interconnected planes. Sheet metal bending and metal forming is performed with the use of a metal bending machine called press brake, using various tool and holding resources. Tools consist of dies and punches of different shape and length. There is also a need of grippers that hold the part during operations.

The workpiece to be metal formed is placed in the machine and the shoes are raised with elevation levers. The work is then slid into place, and the shoes lowered back down onto the piece to be formed. Metal bending occurs by applying force to the bending lever on the bending brake as shown in Figure 2.10. Adjustment bolts determine the radius of the metal bend. Minimum bend radius is a function of the ductility and thickness of the material being worked. It is needful to bend the work slightly beyond the desired angle because the sheet metals will springback. Sheet metal bending is carried out in metal fabrication shops [10].

(a) (b) (c) (d)

Figure 2.10: Sheet metal bending process. (a): position part on the die; (b): position punch

on the part; (c): perform bending; (d): take out the part.

2.4.2 Design parameters

The problems associated with forming high strength steel often are created during the

design stage, when products are designed in such a way that they cannot be manufactured using conventional stamping methods. Or the product requires numerous or special offline operations, such as annealing or normalizing. Further, few of the most important part characteristics that need to be addressed during the design stage [10].

2.4.3 Bending Radius and Bending Angle

Bending dies should be designed so as to avoid sharp bent edges. The inside bending

radius ri1,mm depends on the sheet metal thickness s [mm] and should be selected to

be as large as possible, because sharp bent edges may lead to material failure. On principle, the bending radius should assume the values recommended by DIN6953. When bending sheet metal, particular attention should also be paid to the rolling direction for bending is transverse to the direction of the sheet metal, r1, min [10].

Figure 2.11: Elastic recovery after Bending.

Remember that a small profile radius not only may cause an embrittlement fracture, but

also can reduce the metal flow inward. Metal that is not in radial compression has very

little resistance to flow. For example, the side walls of a deep-draw box have very little

resistance to flow and are not as sensitive to the limiting draw ratio; however, the corners of the box are in radial compression. This radial compression causes a resistance to flow. Increasing the radius size reduces the amount of compression, resulting in increased metal flow inward [10].

2.4.4 Springback

When designing a bending die, it is necessary to consider springback that occurs after unloading. The material has a tendency to partially return to its original shape because

of the elastic recovery of the material as shown in Figure 2.11. This is influenced not only by the tensile and yield strengths, but also by thickness, bend radius and bend angle. Springback occurs with all types of forming by bending, when bending in presses, folding, roll forming and roll bending.

As a result of springback, the bending die angle a does not correspond precisely to the

angle desired at the workpiece a2. The angle ratio is the so-called springback factor kR,

which depends on the material characteristics and the ratio between the bending radiuses

and sheet metal thickness (r/s) [10].

2.5 Bending Methods

There are so many types of bending operation. Most common methods for bending metal sheet are: Wiping Die Bending, Double Die Bending, and Roll Bending [11].

2.5.1 Wiping Die Bending

Wiping die bending is also known as flanging. One edge of the sheet is bent to 90 while the other end is restrained by the material itself and by the force of blank-holder and pad. The flange length can be easily changed and the bend angle can be controlled by the stroke position of the punch.

Figure 2.12: Wiping Die Bending.

2.5.2 Double Die Bending

Double die bending can be seen as two wiping operations acting on the work piece one after another. Double bending can enhance strain hardening to reduce springback.

Figure 2.13: Double Die Bending.

2.5.3 Roll Bending

The operations described in this section use rolls to form sheet metal. Roll bending is an operation in which large sheet-metal parts are formed into curved sections by means of rolls. As the sheet passes between the rolls, the rolls are brought toward each other to a configuration that achieves the desired radius of curvature on the work. A related operation is roll straightening in which nonflat sheets are straightened by passing them between a series of rolls. The rolls subject the work to a sequence of decreasing small bends in opposite directions, thus causing it to be straight at the exit [12].

Figure 2.14: Roll Bending.

2.6 Theoretical Bending Models

Bending along a straight line is the most common of all sheet forming processes; it can

be done in various ways such as forming along the complete bend in a die, or by wiping,

folding or flanging in special machines, or sliding the sheet over a radius in a die. A very large amount of sheet is roll formed where it is bent progressively under shaped rolls. Failure by splitting during a bending process is usually limited to high-strength, less

ductile sheet and a more common cause of unsatisfactory bending is lack of dimensional

control in terms of springback [15].

2.6.1 Variables in bending a continuous sheet

To consider a unit width of a continuous sheet in which a cylindrical bent region of radius of curvature Ď? is flanked by flat sheet as shown in Figure 2.15. The bend angle is θ, and a moment per unit width M, and a tension (force per unit width) T are applied. We note that the tension T is applied at the middle surface of the sheet. The units of M are [force] [length]/ [length] and of T [force]/ [length].

Figure 2.15: A unit length of a continuous strip bent along a line.

2.6.2 Geometry and strain in bending

In bending a thin sheet to a bend radius more than three or four times the sheet thickness,

it may be assumed that a plane normal section in the sheet will remain plane and normal

and converge on the centre of curvature as shown in Figure 2.16.

Figure 2.16: Deformation of longitudinal fibers in bending and tension.

In general, a line CDO at the middle surface may change its length to CD if, for example,

the sheet is stretched during bending; i.e. the original length lO becomes.

A line AB0 at a distance y from the middle surface will deform to a length.

The axial strain of AB is

where εa is the strain at the middle surface or the membrane strain and εb is the bending

strain. Where the radius of curvature is large compared with the thickness, the bending

strain can be approximated as,

The strain distribution is approximately linear as shown in Figure 2.17.

Figure 2.17: Assumed strain distribution in bending.

2.6.3 The pure bending calculation of beam

The pure bending operation mention as below section, which is calculated for the theoretical bending moment [13].The simple case of pure bending is examined that the possessing a vertical axis of symmetry, subjected to equal and opposite and couples as shown in Figure 2.18.

Figure 2.18: Pure bending.

The stress is needed to assured that is consistent with the boundary conditions at ends. These condition are required the results of the internal forces be zero. Therefore, the bending moments about the neutral axis equal the applied moment, :

,

An expression for normal stress can be written as follows.

This is familiarly elastic flexure formula applicable to straight beams applicable.

Since, given section, I and M are constant; the maximum stress is obtained by taking:

Where S is the elastic section modulus, these formula is widely employed in practice because if its simplicity and facilitate its use also. This,, is regularly used as a measure of the bending strength of materials.

2.6.4 Plastic deformation

Unlike elastic deformation, during which, for example, a rod under a tensile load returns to its initial length as long as a defined value (elastic limit of the material) is not exceeded, a workpiece which is plastically deformed retains its shape permanently [14].

Figure 2.19: Tension test bar change in length under stress.

2.6.5 Deformation resistance

The resistance to be overcome during a deformation is composed of the flow stress and the

friction resistances in the tool, which are brought together under the term “resistance to flow� [14].

Kf=Flow stress which is direct variation as follows material, type of force, true strain, strain rate, and Temperature.

Kr=Friction between tools with material which is direct variation as follows lubricate, surface by surface between tools and material, Temperature, Specify compression stress between tools work piece.

Ki= the geometric of forming area, from stress of forming area and relations between Kf and Kr.

2.6.6 Choice of material model for forming

In the experiment, the material model for the bending have as actual stress-stain curve. In general material will have an elastic, plastic strain-hardening behavior [15]. Figure 2.20 shows that several example of material behavior are given in many cases. The magnitude of the strain is depending on the bend ratio, this is defined as the ratio of the radius of curvature to sheet thickness,.

Figure 2.20: Material models for bending. (a) An actual stress–strain curve. (b) An elastic, perfectly plastic model. (c) A rigid, perfectly plastic model. (d) A strain-hardening plastic model.

2.6.6-1 Strain-hardening model

The preload forming process deforms the stainless steel material into some angles, which use a strain-hardening plastic model [15]. Other material of preload forming is not formed by forming tools, which is only assembled with the stainless steel. So, we will describe its behavior as shown in Figure 2.20(d), the strain-hardening model is large the strain. The elastic strains can be neglected, and the low strain hardening model is used, the model expresses as following.

Where is strength coefficient

Table 2.2 Typical values of strength coefficient K and strain hardening exponent n for selected metals.

Material

Strength Coefficient, K

Strain Hardening Exponent, n

MPa

lb/in2

Aluminum, pure, appealed

175

25,000

0.20

Aluminum, alloy, appealeda

240

35,000

0.15

Aluminum, alloy, heat treated

400

60,000

0.10

Copper, pure, annealed

300

45,000

0.50

Copper alloy,brassa

700

100,000

0.35

Steel, low C, appealeda

500

75,000

0.25

Steel, high C, appealeda

850

125,000

0.15

Steel, alloy, appealeda

700

100,000

0.15

Steel, stainless, austenitic, appealeda

1200

175,000

0.40

2.6.6-2 Bending without tension

The workpiece is bent by a pure moment without any tension or compression being applied, the neutral axis will be at the mid-surface [15]. The pure bending process is examined here for elastic bending and strain-hardening as the following.

2.6.6-3 Elastic bending

The linear elastic is bent of sheet shows the material models in the Figure 2.21. The material model [15] of elastic bending as shown in Figure 2.21(a) where the yield stresses-

is S. The relationship of stress–strain relation is given as . The distribution shown in Figure 2.21(b), the distribution of stress in Figure 2.21(c).

Figure 2.21: Linear elastic bending of sheet showing the material model (a), the strain distribution (b), and the stress distribution (c).

The stress at a distance y from the neutral axis, is

The moment at the section for elastic spring back, is

Figure 2.22: Moment curvature diagram for elastic bending.

Where is the second moment of area for a unit width of sheet, and is the curvature. The limit of elastic bending is when the outer fibre at reaches the plane strain yield stress S. The limiting elastic moment is given by

The curvature at this moment is;

From Figures 2.22 shown that the moment, curvature diagram are within this elastic range, which is a linear.

2.6.7 Elastic unloading and spring back

If a sheet is bent by a moment to a particular curvature, as shown in Figure 2.23, and the moment then released [15]. There will be a change in curvature and bend angle. The length of the mid-surface is

This will remain unchanged during unloading as the stress and strain at the middle surface are zero. From this, we obtain

in which = constant, we obtain

Figure 2.23: Unloading a sheet that has been bent by a moment without tension.

2.7 Design of Experiments

In engineering, experimentation shows an important role in new product design, manufacturing process development, and process improvement. The object in many cases may be to develop a robust design.

In this study, design of experiments (DOE) will be used to optimize composite design tools to minimize variation in forming process. Usually, experiments are performed to gain insight about a process so that conclusions and decisions can be made to develop design tools, and mathematics model. Statistically designed experiments make it possible to test several process key parameter input variables (KPIV) simultaneously in order to assess the effect of each on the process key parameter output variables (KPOV). At composite manufacturing, processes are geared towards factors significant and as-significant production runs. This approach is known as “sequential experimentation.

In general, experiments are used to study the performance of design. The design can be represented by the model as shown in Figure 2.24. We can usually visualize the process as a combination of machines, methods, people, and other resources that transforms some input into an output that has one or more observable responses. Some of the process variables x1, x2 and so on are controllable, whereas other variables z1, z2 and so on are uncontrollable.

Inputs (KPIV)

X1, X2, X3,

X4…Xn

Outputs

(KPIV)

Y

Design forming Process

Figure 2.24: General mathematical model of process and design.

The process variation or error, e, will be some function of many component errors.

e = e1 + e2 + e3 + … en

The central limit theorem states that the distribution of the linear function of errors will tend to normality almost irrespective of the individual distributions. The error in an experiment or process can arise in an additive manner from several independent sources; consequently, the normal distribution becomes a plausible model for the combined experimental for process error.

2.8 Experimental Analysis

In this section, we will focus on the HDD suspension and the researches that relate for background of this thesis, these topics are discussed follows.

Esat et al. (2002) Finite element analysis of spring in bending of aluminum sheets [2]. In manufacturing industry, the bending operation involves with springback. Therefore, it is a practical problem to predict the final geometry of the workpiece after elastic springback recovery and also the design a appropriate tools in tools for compensate for springback , So, this research uses the commercial software for finite for finite element method (FEM) to analyze plastic strains and the equivalent von misses stresses are presented. The results of FEA are comparing with the empirical data, which is a good agreement of its result. The numerical method analyze and design bending dies, punch, and others parameter. Because of it uses very short period of time, the FEA is possible to obtain suitable dies that compensate for springback. On the other hand, the manufacturing process are trial and error procedures that a long time and the result wastage of material and effort.

Chou,I.N & Hung, C. (1997). Finte element analysis and optimization on springback reduction [3].Several springback reduction techniques used in the U-channel bending processes were analyzed with the finite element method, which included are bottoming, pinching die and spanking and movement techniques. The relationship between the amount of springback and the forming parameters in each technique was first established through finite element simulations, and then the optimization analysis was coupled with the finite element analysis to find the optimum forming parameters for each springback reduction technique.

Siwakorn Srisawat (2008). He study simulation model for reducing residual stress in HDD suspension during forming process [16]. One component of the HDD suspension is the flat TG, which is formed into a curved TG during the TG forming process. This process leaves residual stress in the TG walls. This residual stress could affect the head gimbal assembly at factories and also the response of the HDD operation. A simulation model in finite element analysis is proposed in this paper to address two aims. The first is improved prediction of elastic springback as forming TG is an elastic-plastic recovery phenomenon during unloading that leads to springback. The second aim is to reduce the residual stress that occurs in TG during its forming process.

Kazan Recep al. (2002) Prediction of springback in wipe-bending process of sheet metal using neural network [17]. The wipe-bending is one of processes the most frequently used in the sheet metal product industry. Furthermore, the springback of sheet metal, which is defined as elastic recovery of the part during unloading, should be taken into consideration so as to produce bent sheet metal parts within acceptable tolerance limits. Springback is affected by factors such as sheet thickness, tooling geometry, lubrication conditions, and material properties and processing parameters. In this paper, the prediction model of springback in wipe-bending process was developed using artificial neural network (ANN) approach. Here, several numerical simulations using finite element method (FEM) were performed to teaching data of network. The learned network is numerically tested and can be easily implemented springback prediction for new cases.

Tekiner Zafer. (2004) the examination of springback of sheet metal with several thicknesses and properties in bending dies [18]. The bending die has an importance in the sheet metals product industry. However, the springback of the sheet metal may be taken into consideration in the bending die design for an experimental study. There are two types of the bending die have been conducted to study that is V-bending die and this is a subdivided into corner bending in the air. The springback must be known in order to produce bent sheet metal part within the acceptable tolerance limits. In this research carried out on the determination of amount springback of bent products. The modular V-bend die many induces the several sheet metal with the bending angles for the amount of springback. The amount of springback results are in line with the result of pervious researches.

Ekaratch et al.(2008) Vibration analysis of suspension in HDD with FEM [19] the vibration of slider or Read/Write head is one of the major concerns for hard disk drive manufacture. R/W head is an important part that performs read/write data on the media. The air pressure, caused by air flow through the gap between R/W head and media, sustain the flight height of slider above the media, called air bearing. In general, the gap between the head and the media is very small. Thus, if the vibration level of the Head-Suspension is too high, the head and the media could come into contact and damage both components. The main objective of is work is to study the mechanical vibration of suspension that holds R/W head. The Finite Element model of suspension is conducted having air bearing stiffness between the Head and the media as a combination of linear springs. The natural frequencies and the corresponding mode shapes of the combined system are employed to use in conjunction with mechanical troubles and cost occurred for testing physical models.

Kajonsak et al. (2009) He had studied of HGA behaviors after mounted with Shipping Comb [20]. In Head Stack Assembly process, a shipping comb is mounted to HAS adjacent the HGA area. This is to prevent the vibration of the sliders, attached at the end of suspension, when joining HAS to the other parts or conduct the measurements, storage and transportation. This work is aimed to study the behaviors of HGA when shipping comb. Having various dimensions and shapes, is mounted on using Finite Element analysis for analysis and comparison Gram load value. The components of Shipping Comb are 5 parts Swage shuttle fixture, fixture, Spreader pins, Load cell and Gripper.