DE102009042521A1 - Fluid dynamic bearing system for use in spindle motor of hard disk drive utilized for rotary drive of magnetic storage disk, has bearing groove structures or pump structures pumping bearing fluid in direction of opening of channel - Google Patents

Abstract

The system has fluid dynamic radial bearings (18, 22) and a fluid dynamic axial bearing (28), which are arranged along a bearing space (14). A recirculation channel (32) axially runs and comprises an opening in a section of the bearing space close to an opened end and another opening in a section of the bearing space close to a closed end. Bearing groove structures or pump structures are provided between the openings of the recirculation channel. The groove structures or pump structures pump bearing fluid in a direction of one of the openings of the channel. An independent claim is also included for a spindle motor comprising a fluid dynamic bearing system.

Description

Field of the invention

The invention relates to a fluid dynamic bearing system, which comprises at least one radial bearing and at least one axial bearing, according to the features of the preamble of claim 1. Such fluid dynamic bearings are used for rotary mounting of motors, such as spindle motors, in turn, for driving disk drives, fans or serve similar.

State of the art

Fluid dynamic bearings, as used in spindle motors, usually comprise at least two relatively rotatable bearing components, the bearing surfaces between one associated with a bearing fluid, for. B. air or bearing oil, filled bearing gap form. In a known manner, the bearing surfaces assigned and acting on the bearing fluid surface structures are provided. In fluid dynamic bearings, the surface structures in the form of depressions or elevations are usually applied to one or both of the opposing bearing surfaces. These arranged on corresponding bearing surfaces of the bearing partner surface structures serve as storage and / or pumping structures that generate a hydrodynamic pressure with relative rotation of the bearing components within the bearing gap. In radial bearings, for example, sinusoidal, parabolic or herringbone groove-like surface structures are used, which are arranged distributed perpendicular to the axis of rotation of the bearing components over the circumference of at least one bearing component. Axial bearings, for example, spiral groove-shaped surface structures are used, which are usually arranged perpendicular to a rotation axis. In a fluid dynamic bearing of a spindle motor for driving hard disk drives according to a known type, a shaft is rotatably mounted in a bearing bore of a bearing bush. The diameter of the bore is slightly larger than the diameter of the shaft, so that between the surfaces of the bearing bush and the shaft remains filled with a bearing fluid bearing gap. The mutually facing surfaces of the shaft and / or the bearing bush have pressure-generating bearing groove structures as part of at least one fluid dynamic radial bearing. A free end of the shaft is connected to a hub, the lower surface forms a fluid dynamic thrust bearing together with an end face of the bearing bush. For this purpose, one of the mutually facing surfaces of the hub or the bearing bush is provided with pressure generating Lagerrillenstrukturen.

At the lower, closed shaft end a stopper ring is preferably integrally formed on the shaft or connected to the shaft. A recirculation channel connects this bearing area with the radially outer area of the thrust bearing gap.

A known design includes fluid dynamic bearing with one side open bearing gap, that is, the bearing bush of the bearing is sealed at one end. Due to the different pumping effects of the bearing groove structures, it can happen in extreme operating cases of the bearing that a negative pressure (negative pressure) arises in the region of the closed end of the bearing within the bearing gap. A negative pressure facilitates the outgassing of dissolved air in the bearing fluid, which then combines in the bearing gap to larger air bubbles. Such air bubbles can affect the safe operation of the bearing and should therefore be avoided.

Fluid dynamic bearing systems for use in spindle motors must be constructed in such a way that as far as possible no bearing fluid can escape from the bearing gap to other areas of the spindle motor. On the one hand, bearing fluid emerging from the bearing gap reduces the service life of the bearing system, for example since there is a risk of dry running of the bearing, and secondly, bearing fluid that escapes contaminates other components of the spindle motor. The leakage of bearing fluid from the bearing gap is therefore prevented by appropriate sealing arrangements. Preferably, so-called capillary seals are used, which adjoin the open end of the bearing gap and prevent leakage of the bearing fluid from the bearing gap. The bearing fluid is retained in the capillary seal by capillary forces.

Capillary seals have the disadvantage that the sealing gap expands relatively strong starting from the bearing gap, so that in particular the shock resistance of the bearing suffers. Furthermore, a capillary seal forms a relatively large surface area between liquid and surrounding air, so that bearing fluid can easily evaporate from the sealing gap. Furthermore, the sloping surfaces which limit the conical sealing gap can only be produced with a high mechanical outlay.

Disclosure of the invention

It is the object of the invention to provide a fluid dynamic bearing system of the type mentioned, in which the occurrence of a negative pressure in the region of the closed end of the bearing is reduced or avoided. Furthermore, the storage system compared to known storage systems a longer life, improved Shock resistance and optionally have an improved retention capacity for the bearing fluid in the bearing gap.

This object is achieved by a storage system according to the features of claim 1.

Preferred embodiments and further advantageous features of the invention are specified in the dependent claims.

The invention relates to a fluid dynamic bearing system with at least one fixed bearing component and at least one movable bearing component, which are rotatable relative to each other about a common axis of rotation and form between bearing surfaces associated with a bearing fluid filled bearing gap. The bearing gap has a closed and an open end, wherein along the bearing gap at least one fluid dynamic radial bearing and at least one fluid dynamic thrust bearing are arranged, wherein the open end of the bearing gap is sealed by a sealing gap. There is provided a substantially axially extending recirculation passage having a first orifice in a portion of the bearing gap near the open end and a second orifice in a portion of the bearing gap near the closed end. According to the invention, bearing groove structures or pump structures are provided between the first mouth of the recirculation channel and the sealing gap, which pump the bearing fluid in the direction of the first orifice.

According to the invention, the first mouth of the recirculation channel is not in the region of the sealing gap, where ambient pressure prevails, but bearing groove structures or pump structures are provided beyond the first mouth of the recirculation channel, which pump the bearing fluid in the direction of the first orifice. As a result, there is a positive pressure in the region of the first orifice of the recirculation channel, which also bears in the recirculation channel and at the second orifice of the recirculation channel. The second mouth of the recirculation channel is in the region of the closed end of the bearing. So there is always a positive pressure, so that a formation of a negative pressure in this area is safely avoided.

In a preferred embodiment of the invention, the fixed component comprises a bearing bush with a bearing bore, in which a shaft, which is part of the movable bearing component, is rotatably received. At the free end of the shaft, a hub is arranged, which has a cylindrical projection, which surrounds the bearing bush to form the sealing gap partially.

The sealing gap is arranged between a radially outwardly directed lateral surface of the stationary bearing component and an opposite, radially inwardly directed lateral surface of the movable bearing component and at least partially filled with bearing fluid.

In a preferred embodiment of the invention, along a section of the sealing gap, a pumping seal with pumping structures is arranged, which pumps the bearing fluid in the direction of the first mouth of the recirculation channel. The pumping seal according to the invention comprises pumping structures, which are arranged on the lateral surface of the fixed and / or the lateral surface of the movable bearing component. Preferably, the pumping structures extend over the entire axial length of the sealing gap. It is important that the lateral surfaces of the sealing gap in the region of the pumping structures are preferably cylindrical, so that the sealing gap in this region has a constant width of, for example, 50 to 100 micrometers. A capillary seal can be connected to the pumping seal in a section of the sealing gap.

In known manner, the fluid dynamic radial bearing pressure generating Lagerrillenstrukturen which are arranged on the surface of the bearing bore and / or on the surface of the shaft. The fluid-dynamic thrust bearing is formed by an end face of the bearing bush and one of these opposite surface of the hub, wherein one or both of these surfaces are provided with pressure-generating bearing groove structures. The bearing groove structures of the axial bearing are at least partially disposed beyond the first mouth of the recirculation channel and pump the bearing fluid in the direction of this first orifice.

The sealing of the bearing gap by a pumping seal according to the invention has significant advantages. For example, it increases the shock resistance of the bearing, since the bearing gap of the pump seal is much narrower compared to a capillary seal and thus the capillary forces that hold back the bearing fluid, are substantially larger. Another advantage is that the evaporation of the bearing fluid from the relatively narrow sealing gap is substantially reduced compared to a capillary seal. This increases the life of the bearing as less bearing fluid is lost through evaporation. In addition, a pumping seal promotes the escape of air bubbles from the bearing gap, as the pumping seal creates a pressure gradient in the direction of the bearing gap, which presses the bearing fluid inwards, but allows the air bubbles to escape to the outside. Also, the filling of the bearing gap and the sealing gap with bearing fluid is facilitated because the sealing gap in principle can be completely filled with bearing fluid. The machining of the lateral surfaces of the bearing components, which limit the sealing gap, is also simpler than with a conical capillary seal. The lateral surfaces can be produced cylindrically with high accuracy in a simple manner, with the pump structures subsequently being applied, for example, by means of an ECM method.

According to a first embodiment of the invention, the first mouth of the recirculation channel between the thrust bearing and the sealing gap is arranged. The second mouth of the recirculation passage is disposed in a gap between a stopper ring and a cover which closes the bearing down. In this embodiment, the recirculation passage is arranged in the bearing bush.

In another embodiment of the invention, an axially extending recirculation channel is arranged in the shaft. The first orifice of the recirculation passage is disposed in the region of the bearing gap between the radial bearing and the thrust bearing, while the second orifice is arranged in a gap between a stopper ring arranged on the shaft and a cover. The advantage of a recirculation channel in the shaft is that it can simply be drilled axially into the shaft. A provided in the bearing bush recirculation channel must be drilled substantially obliquely to the axis of rotation, which means a higher cost and in particular makes a clean deburring of the bore ends necessary. A recirculation channel in the shaft is said to be relatively easy to manufacture and can then be connected via a transverse bore with any areas of the camp. Only after the introduction of the recirculation channel and the connecting holes then the shaft can be finished machined and ground.

A recirculation channel in the shaft also makes it possible to reduce the outer diameter of the bearing bush, since no additional space for a recirculation channel must be provided. The reduction of the outer diameter of the bearing bush makes it possible to build a much smaller bearing and also increases the rigidity of the bearing. In particular, the ratio between the outer diameter of the shaft and the outer diameter of the bearing bush can be greater than or equal to 0.45.

More particularly, the invention relates to a fluid dynamic bearing system for a spindle motor such as may be used to drive hard disk drives.

The invention will now be described with reference to preferred embodiments with reference to the drawings described below. This results in further features, advantages and applications of the invention.

Brief description of the drawings

1 shows a longitudinal section through a spindle motor with a first embodiment of a fluid dynamic bearing according to the invention.

2 shows a longitudinal section through a spindle motor with a second embodiment of a fluid dynamic bearing according to the invention.

3 shows an enlarged view of a bearing bush of the bearing in a opposite 1 modified embodiment.

4 shows a spindle motor with fluid dynamic bearing, similar to the camp according to 2 is constructed

Description of preferred embodiments of the invention

The 1 shows a section through a spindle motor with a first embodiment of a fluid dynamic bearing system according to the invention. The spindle motor includes a fixed bushing 10 having a central bore and forming the fixed component of the storage system. Into the bore of the bearing bush 10 is a wave 12 used, whose diameter is slightly smaller than the diameter of the bore. Between the surfaces of the bearing bush 10 and the wave 12 there remains a bearing gap 14 , The opposite surfaces of the shaft 12 and the location socket 10 form two fluid dynamic radial bearings 18 . 22 out, by means of which the shaft 12 around a rotation axis 16 rotatable in the bearing bush 10 is stored. The radial bearings 18 . 22 are through storage structures 20 . 24 marked on the surface of the shaft 12 and / or the bearing bush 10 are applied. The bearing gap 14 is filled with a suitable bearing fluid, such as a bearing oil. The storage structures 20 . 24 practice with rotation of the shaft 12 a pumping action on the in the bearing gap 14 between wave 12 and bearing bush 10 located bearing fluid, so that the radial bearings 18 . 22 become sustainable. At the bottom of the shaft 12 is a one-piece with the shaft or a separately formed stopper ring 13 arranged, which has an enlarged outer diameter compared to the shaft diameter. The stopper ring prevents the shaft from falling out 12 from the bushing 10 , The bearing is on this side of the bearing bush 10 through a cover plate 34 locked.

A free end of the wave 12 is with a hub 26 connected to a cylindrical approach 38 which partially surrounds the bearing bush. A lower, flat surface of the hub 26 forms together with an end face of the bearing bush 10 a fluid dynamic thrust bearing 28 out. Here, the end face of the bearing bush 10 and / or the opposite surface of the hub 26 with storage structures 30 provided during rotation of the shaft 12 a pumping action on the in the bearing gap 14 between the hub 26 and the end face of the bearing bush 10 located bearing fluid exerts, so that the thrust bearing 28 becomes sustainable. In the bearing bush, a recirculation channel 32 be provided, the one at the outer edge of the thrust bearing 28 located section of the storage gap 14 with a portion of the bearing gap surrounding the stopper ring 14 interconnects and supports a circulation of the bearing fluid in the camp.

The bearing bush 10 is in a base plate 36 arranged the spindle motor. The hub 26 has on its outer circumference on a peripheral edge. The bearing bush 10 surrounding is a stator assembly 40 on the base plate 36 arranged, which consists of a ferromagnetic laminated stator core and corresponding stator windings. This stator arrangement 40 is of an annular rotor magnet 42 surrounded, which on the inner circumference of the circumferential edge of the hub 26 is attached. Shown is an external rotor motor. Alternatively, of course, an internal rotor motor can be used. Below the rotor magnet 42 is a ferromagnetic metal ring 44 arranged, which attracts the rotor magnet, causing a downward to the base plate 36 directed force results. This force is used for the axial preload of the bearing system.

The bearing gap 14 includes an axial section extending along the shaft 10 and the radial bearing 18 . 22 extends, and a radial portion extending along the end face of the bearing bush 10 and the thrust bearing 28 extends. At the radially outer end of its radial portion of the bearing gap is 14 in a gap with a larger gap distance above, to which a. seal gap 46 followed. The gap initially extends from the bearing gap 14 radially outwardly and merges into an axial section extending along the outer circumference of the bushing 10 between the bearing bush 10 and the cylindrical approach 38 the hub 26 extends and the sealing gap 46 forms. With a diameter of the bearing bush 10 of a few millimeters, the width of the sealing gap 46 typically 50-100 microns, preferably 75 microns.

The outer lateral surface 52 the bearing bush 10 as well as the inner lateral surface 54 of the cylindrical approach 38 the hub 26 are initially cylindrical and form the boundary of the sealing gap 46 which widens slightly conically in the direction of its opening. Thus, the sealing gap runs 46 essentially parallel to the axis of rotation 16 and has a width starting from the bearing gap. Such lateral surfaces, which are the boundaries of the sealing gap 46 form, can be processed very precisely and, above all, with a low surface roughness and set the bearing fluid to a low frictional resistance.

The sealing gap 46 or the outer lateral surface 52 the bearing bush or the inner lateral surface 54 of the cylindrical approach 38 the hub 26 has pump structures according to the invention 51 on. These pump structures 51 form a pumping seal 50 in the cylindrical section of the sealing gap 46 is arranged. The pump structures 51 practice with rotation of the lateral surfaces 52 . 54 a pumping action on the bearing fluid in the direction of the bearing gap 14 , so in the interior of the camp. This prevents bearing fluid from the bearing gap 14 can escape. The pump seal 50 can be additionally supported by a capillary seal 48 , which adhere to the pumping seal 50 connects and in the conically widening area of the sealing gap 46 is arranged.

2 shows a second embodiment of a bearing system according to the invention for the rotational mounting of a spindle motor. The spindle motor off 2 is off the spindle motor 1 very similar.

The bearing comprises a bearing bush 110 in which a wave 112 is rotatably mounted. Between the outer diameter of the shaft 112 and the bearing bore of the bearing bush 110 there remains a bearing gap 114 , The lower end of the bearing is covered by a cover 134 locked. The shaft has a stopper ring on this side 113 on, which is arranged in a corresponding recess of the bearing bush. There are two radial bearings 118 and 122 provided, respectively, by radial bearing structures 120 . 124 Marked are. An axial bearing 128 with corresponding thrust bearing structures 130 is formed by a front side of the bearing bush 110 and a lower planar surface of a hub 126 , which is formed substantially pot-shaped. The bearing gap extends along the axial bearing 128 and goes over into a sealing gap 146 that is between an outer surface 152 the bearing bush 110 and an inner circumferential surface 154 a cylindrical approach 138 the hub 126 is provided. The bushing is in a base plate 136 held. The base plate carries a stator assembly 140 that is a rotor magnet 142 opposite, with the two components 140 . 142 form the electromagnetic drive system of the spindle motor. A metal ring 144 , which is below the rotor magnet 142 is arranged, provides an axial preload of the thrust bearing 128 ,

The sealing gap 146 includes a pumping seal 150 the, as related to 1 described, corresponding pump structures 151 comprising, preferably on the outer circumferential surface 152 the bearing bush 110 are arranged. The pump structures 151 but also on the inner surface 154 of the cylindrical approach 138 be arranged. At the end of the sealing gap 146 is additionally a capillary seal 148 arranged.

In a further aspect of the invention, the shaft captures a central bore that defines a recirculation channel 132 represents. The recirculation channel 132 opens on the one hand in a gap area between the front side of the stopper ring 113 and the opposite surface of the cover plate 134 and on the other hand is a transverse bore 133 with the bearing gap in the area between the radial bearing 118 and the thrust bearing 128 connected.

In 3 is a view of one opposite 1 modified bearing bush 10 given in detail. The bearing bush 10 comprises a bearing bore, on whose inner surfaces the bearing structures 20 and 24 the two radial bearings 18 . 22 are arranged. On the outer lateral surface 52 that the sealing gap 46 limited, are the pumping structures 51 the pump seal 50 arranged. 3 shows a partial section, ie the bearing bore is shown in section, while the pump structures 51 to represent a supervision. The pump structures 51 are over the entire circumference of the bearing bush 10 arranged. The lateral surface 52 is cylindrical, so that the sealing gap 46 has a constant width over its entire length.

4 shows a spindle motor with fluid dynamic bearing, similar to the camp according to 2 is constructed. Identical components are provided with the same reference numerals. The structure and function of the spindle motor were already related to 2 described.

In this spindle motor, the fluid dynamic bearing system has no pumping seal with pump structures, as in 2 is described. The sealing gap 146 extends between the outer circumference of the bearing bush 110 and the inner circumference of the neck 138 the hub 126 , wherein the lateral surfaces 152 . 154 widen conically and form a conical capillary seal. The first mouth of the recirculation channel 132 . 133 is radially inside the thrust bearing 128 arranged, whose spiral bearing groove structures 130 a pumping action radially inward on the bearing fluid in the bearing gap 114 exercise. This prevails at the first mouth of the recirculation channel 132 . 133 always a positive pressure via the recirculation channel 132 . 133 is passed to the closed end of the camp.

LIST OF REFERENCE NUMBERS

10, 110

bearing bush

12, 112

wave

13, 113

stopper ring

14, 114

bearing gap

16, 116

axis of rotation

18, 118

radial bearings

20, 120

surface structures

22, 122

radial bearings

24, 124

surface structures

26, 126

hub

28, 128

thrust

30, 130

warehouse structures

32, 132

recirculation

133

cross hole

34, 134

cover

36, 136

baseplate

38, 138

cylindrical approach (hub)

40, 140

stator

42, 142

rotor magnet

44, 144

metal ring

46, 146

seal gap

48, 148

capillary

50, 150

pump seal

51, 151

pumping structures

52, 152

Lateral surface (bearing bush)

54, 154

Lateral surface (approach)

Claims (19)

Fluid dynamic storage system with at least one fixed ( 10 ; 110 ) Bearing component and at least one movable bearing component ( 12 . 26 ; 112 . 126 ) relative to each other about a common axis of rotation ( 16 ; 116 ) are rotatable and between bearing surfaces associated with a bearing fluid filled with a bearing gap ( 14 ; 114 ) having a closed and an open end, wherein along the bearing gap at least one fluid dynamic radial bearing ( 18 ; 22 ; 118 . 122 ) and at least one fluid dynamic thrust bearing ( 28 ; 128 ) are arranged, and wherein the open end of the bearing gap by a sealing gap ( 46 ; 146 ) is sealed, wherein a substantially axially extending recirculation channel ( 32 ; 132 ) having a first orifice in a portion of the bearing gap near the open end and a second orifice in a portion of the bearing gap (US Pat. 14 ; 114 ) near the closed end, characterized in that bearing groove structures or pumping structures are provided between the first mouth of the recirculation channel and the sealing gap, which pump the bearing fluid in the direction of the first mouth of the recirculation channel. Fluid dynamic bearing system according to claim 1, characterized in that the fixed Component a bearing bush ( 10 ; 110 ) comprising a bearing bore. Fluid dynamic bearing system according to one of claims 1 or 2, characterized in that the movable bearing component rotatably mounted in the bearing bore shaft ( 12 ; 112 ) whose free end is connected to a hub which has a cylindrical projection ( 38 ; 138 ), which the bushing ( 10 ) forming the sealing gap ( 46 ; 146 ) partially surrounds. Fluid dynamic bearing system according to one of claims 1 to 3, characterized in that the sealing gap ( 46 ; 146 ) between a radially outwardly directed lateral surface ( 52 ; 152 ) of the fixed bearing component ( 10 ; 110 ) and one of these opposite, radially inwardly directed lateral surface ( 54 ; 154 ) of the movable bearing component is arranged and at least partially filled with bearing fluid. Fluid dynamic bearing system according to one of claims 1 to 4, characterized in that along a portion of the sealing gap ( 46 ; 146 ) a pumping seal ( 50 ; 150 ) with pump structures ( 51 ; 151 ) is arranged, which pump the bearing fluid in the direction of the first mouth of the recirculation channel. Fluid dynamic bearing system according to one of claims 1 to 5, characterized in that the pump structures ( 51 ; 151 ) on the lateral surface ( 52 ; 152 ) of the fixed and / or the lateral surface ( 54 ; 154 ) of the movable bearing component are arranged. Fluid dynamic bearing system according to one of claims 1 to 6, characterized in that the pump structures ( 51 ; 151 ) over the entire axial length of the sealing gap ( 46 . 146 ). Fluid dynamic bearing system according to one of claims 1 to 7, characterized in that the lateral surfaces ( 52 . 152 ; 54 . 154 ) of the sealing gap ( 46 ; 146 ) are cylindrical. Fluid dynamic bearing system according to one of claims 1 to 8, characterized in that a portion of the sealing gap ( 46 ; 146 ) a capillary seal ( 48 ; 148 ) trains. Fluid dynamic bearing system according to one of claims 1 to 9, characterized in that on the surface of the bearing bore and / or on the surface of the shaft ( 12 ; 112 ) Pressure producing bearing structures ( 20 ; 24 ; 120 . 124 ) are formed as part of at least one fluid dynamic radial bearing ( 18 ; 22 ; 118 . 122 ). Fluid dynamic bearing system according to one of claims 1 to 10, characterized in that on an end face of the bearing bush ( 10 ; 110 ) and / or one of this face opposite surface of the hub ( 26 ; 126 ) Pressure producing bearing structures ( 30 ; 130 ) are formed as part of a fluid dynamic thrust bearing ( 28 ; 128 ). Fluid dynamic bearing system according to one of claims 1 to 11, characterized in that the axial bearing structures ( 30 ; 130 ) at least partially beyond the first orifice of the recirculation channel recirculation channel ( 32 ; 132 ) are arranged and pump the bearing fluid in the direction of the first mouth. Fluid dynamic storage system according to one of claims 1 to 12, characterized in that the recirculation channel ( 32 ) is arranged in the bearing bush. Fluid dynamic bearing system according to claim 13, characterized in that the first mouth of the recirculation channel between the thrust bearing ( 28 ) and the sealing gap ( 46 ) and the second orifice in a portion of the storage gap surrounding the stopper ring (FIG. 14 ) is arranged. Fluid dynamic storage system according to one of claims 1 to 12, characterized in that the recirculation channel ( 132 ) is arranged in the shaft. Fluid dynamic bearing system according to claim 15, characterized in that the first mouth of the recirculation channel ( 132 ) in the bearing gap between the radial bearing ( 118 ) and the thrust bearing ( 128 ) and the second orifice in a gap between a stopper ring ( 113 ) and a cover ( 134 ). Fluid dynamic bearing system according to one of claims 1 to 16, characterized in that the ratio between the outer diameter of the shaft ( 112 ) and the outer diameter of the bearing bush ( 110 ) is greater than or equal to 0.45. Spindle motor with a fluid dynamic bearing system according to claims 1 to 17, with a base plate ( 36 ; 136 ) for receiving the stationary bearing component ( 10 ; 110 ) of the storage system and an electro-magnetic drive system ( 40 ; 42 ; 140 ; 142 ) for driving the movable bearing component ( 12 ; 26 ; 112 ; 126 ). Hard disk drive with a spindle motor according to claim 18 for the rotary drive of at least one magnetic storage disk, and a writing and reading device for writing and Reading data to or from the magnetic disk.

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