Patent Publication Number: US-2022230662-A1

Title: Disk device with damper attached to arm of actuator assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/180,533, filed on Feb. 19, 2021, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-155669, filed on Sep. 16, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a disk drive. 
     BACKGROUND 
     As a disk drive, for example, a hard disk drive (HDD) comprises a magnetic disk installed in a housing, a spindle motor which supports and drives to rotate the magnetic disk, a head actuator which supports the magnetic head, a voice coil motor which drives the head actuator and the like. The head actuator comprises an actuator block including a plurality of arms and a suspension assembly(, which may be referred to as a head gimbal assembly (HGA)) attached to each arm to support the magnetic head. 
     Recently, as the storage capacity of the HDD increases, the number of magnetic disks installed is increasing accordingly. In order to deal with a number of magnetic disks, the so-called split actuator has been proposed, in which a head actuator is split into a plurality of, for example, two head actuators each independently rotatable and the two head actuators are disposed in a multilayered fashion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a hard disk drive (HDD) according to a first embodiment diagram, when a top cover is removed. 
         FIG. 2  is a perspective view showing an actuator assembly and a wiring substrate unit of the HDD. 
         FIG. 3  is a cross-sectional view of actuator assemblies which is in a state. 
         FIG. 4  is a cross-sectional view showing a part of the arm of the actuator assembly and a damper. 
         FIG. 5  is a cross-sectional view schematically showing an actuator assembly of a HDD according to a second embodiment. 
         FIG. 6  is a plan view schematically showing arms of the actuator assembly in the second embodiment. 
         FIG. 7  is a cross-sectional view schematically showing an actuator assembly of a HDD according to a third embodiment. 
         FIG. 8  is a cross-sectional view schematically an actuator assembly of a HDD according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments will be described hereinafter with reference to the accompanying drawings. 
     In general, according to one embodiment, a disk device comprises a plurality of disk-shaped recording media each including a recording layer and an actuator assembly comprising an actuator block rotatably supported around a rotation shaft, a plurality of arms extending from the actuator block, and suspension assemblies respectively attached to the arms and supporting the respective magnetic heads. Of the plurality of arms, at least one arm has vibration characteristics different from those of the other arms. 
     The disclosure is merely an example, and proper changes in keeping with the spirit of the invention, which are easily conceivable by a person of ordinary skill in the art, come within the scope of the invention as a matter of course. In addition, in some cases, in order to make the description clearer, the widths, thicknesses, shapes and the like, of the respective parts are illustrated schematically in the drawings, rather than as an accurate representation of what is implemented. However, such schematic illustration is merely exemplary, and in no way restricts the interpretation of the invention. In addition, in the specification and drawings, the same elements as those described in connection with preceding drawings are denoted by like reference numbers, and detailed description thereof is omitted unless necessary. 
     First Embodiment 
     As a disk drive, a hard disk drive (HDD) according to a first embodiment will be described in detail. 
       FIG. 1  is an exploded perspective view of the HDD according to the first embodiment, when a top cover thereof is removed. The HDD comprises a flat and substantially rectangular housing  10 . The housing  10  includes a rectangular box-shaped base  12  with an upper opening, and a top cover  14 . The base  12  includes a rectangular bottom wall  12   a  opposing the top cover  14  with an interval therebetween, and side walls  12   b  set to stand along circumferential edges of the bottom wall  12   a , which are formed to be integrated as one body from, for example, aluminum. The top cover  14  is formed into a rectangular plate shape of, for example, stainless steel. The top cover  14  is screwed to the side walls  12   b  of the base  12  by a plurality of screws  13  so as to close the upper opening of the base  12 . 
     In the housing  10  are provided a plurality of, for example, six magnetic disks  18  as recording media and a spindle motor  19  as a drive unit which supports and rotates the magnetic disks  18 . The spindle motor  19  is disposed on the bottom wall  12   a . Each magnetic disk  18  is formed to have, for example, a diameter of 96 mm (approximately, 3.5 inches), and comprises a magnetic recording layer(s) on upper and/or lower surfaces. The magnetic disks  18  are engaged with a hub (not shown) of the spindle motor  19  so as to be coaxial with each other and are clamped by a clamp spring  20  so as to be fixed to the hub. For example, the six magnetic disks  18  are placed parallel to each other in a multilayered manner with intervals therebetween. Further, the magnetic disks  18  are supported so as to be located parallel to the bottom wall  12   a  of the base  12 . The magnetic disks  18  are rotated at a predetermined number of revolutions by the spindle motor  19 . 
     Note that the number of magnetic disks  18  is not limited to six, but it may be increased or decreased. 
     The housing  10  includes therein a plurality of magnetic heads  17  which performs recording and reproduction of data with respective to the magnetic disks  18 , respectively and a head actuator assembly(, which may be referred to as a head actuator) which supports the magnetic heads  17  movably with respect to the respective magnetic disks  18 . In this embodiment, the head actuator assembly is configured as split actuator assembly divided into a plurality of actuator assemblies, that is, for example, a first actuator assembly  22 A and a second actuator assembly  22 B. The first and second actuator assemblies  22 A and  22 B are rotatably supported around a common support shaft (a rotation shaft)  26  standing on the bottom wall  12   a  of the base  12 . 
     In the housing  10  are provided a voice coil motor (VCM)  24  which pivots and positions the first and second actuator assemblies  22 A and  22 B, a ramp load mechanism  25  which holds the magnetic heads  17  in an unload position spaced away from a respective magnetic disk  18  when the magnetic heads  17  move to an outermost circumference of the magnetic disk  18 , and a wiring substrate unit (FPC unit)  21 , on which electronic components such as conversion connectors are mounted. 
     On an outer surface of the bottom wall  12   a , a printed circuit board (not shown) is fixed by screwing. The printed circuit board constitutes a controller, and the controller controls operation of the spindle motor  19  and controls operation of the VCM  24  and the magnetic heads  17  via the wiring substrate unit  21 . 
       FIG. 2  is a perspective diagram showing the split actuator assemblies and the wiring substrate unit, and  FIG. 3  is a cross-sectional view of the split actuator assemblies in order. 
     As shown in  FIGS. 2 and 3 , the split actuator assembly includes the first actuator assembly  22 A and the second actuator assembly  22 B. The first and second actuator assemblies  22 A and  22 B are disposed in a multilayered manner one above another, and are provided to be rotatable independently from each other around the common support shaft  26  standing on the bottom wall  12   a  of the base  12 . The first actuator assembly  22 A and the second actuator assembly  22 B are configured to have structures substantially identical to each other. For example, the upper actuator assembly is referred to the first actuator assembly  22 A, and the lower actuator assembly is the second actuator assembly  22 B. 
     The first actuator assembly  22 A comprises an actuator block (a first actuator block)  29 , four arms  30  extending from the actuator block  29 , head suspension assemblies(, which may be referred to as head gimbal assemblies (HGAs))  32  respectively attached to the arms  30 , and magnetic heads  17  respectively supported by the head suspension assemblies. The actuator block  29  comprises an inner hole  31 , to which a bearing unit (unit bearing)  50  is mounted. The actuator block  29  is supported rotatably on the support shaft  26  by the bearing unit  50 . 
     In this embodiment, the actuator block  29  and the four arms  30  are formed to be integrated as one body from aluminum or the like, and constitute a so-called E block. The arms  30  are each formed into, for example, a slender flat plate shape, and extend from the actuator block  29  in a direction normal to the support shaft  26 . The four arms  30  are provided parallel to each other with intervals respectively therebetween each other. In this embodiment, the four arms  30  are formed to have dimensions identical to each other and shapes identical to each other. 
     The first actuator assembly  22 A includes a support frame  34  extending from the actuator block  29  in a direction opposite to the arms  30 . A voice coil  36  is supported by the support frame  34 . As shown in  FIGS. 1 and 2 , the voice coil  36  is located between a pair of yokes  38  installed in the base  12  and it constitutes the VCM  24  together with the yokes  38  and a magnet  39  secured to one of the yokes  38 . 
     As shown in  FIGS. 2 and 3 , the first actuator assembly  22 A comprises six head suspension assemblies  32 , and the head suspension assemblies  32  are respectively attached to extending ends of the respective arms  30 . The head suspension assemblies  32  include up-head suspension assemblies which support the respective magnetic head  17  upward and down-head suspension assemblies which support the respective magnetic head  17  downward. The up-head and down-head suspension assemblies can be formed from head suspension assemblies of the same structure by placing them in different directions up and down. In this embodiment, in the first actuator assembly  22 A, a down-head suspension assembly is attached to the uppermost arm  30 , and an up-head suspension assembly is provided to the lowermost arm  30  ( 30   a ), and two head suspension assemblies of an up-head suspension assembly and a down-head suspension assembly are attached to each of the other two arms  30 . 
     Six head suspension assemblies  32  extend from the four arms  30  and are disposed substantially parallel to each other with regular intervals therebetween respectively each other. Two magnetic heads  17  supported by a pair of a down-head suspension assembly  32  and an up-head suspension assembly  32  are located to face each other with a predetermined interval therebetween. These magnetic heads  17  are located to oppose respective surfaces of the corresponding magnetic disk  18 . 
     As illustrated schematically in  FIG. 2 , the suspension assemblies  32  each comprise a slender plate spring-shaped suspension (a base plate and a load beam) and a slender belt-shaped flexure (a wiring member)  74 . A distal end-side portion of the flexure  74  is attached on surfaces of the load beam and the base plate, and a proximal end-side portion of the flexure  74  extend to a proximal end of the arm  30  along the arm  30 . The magnetic head  17  is mounted on a gimbal portion (an elastic support portion) (not shown) provided at the distal end portion of the flexure  74 . Wiring lines of the flexure  74  are electrically connected to a read element, a write element, a heater and other members of the magnetic head  17 . 
     The proximal end-side portion of the flexure  74  is joined to a connection portion (a wiring substrate)  46  of the flexible printed-circuit board (FPC), mounted on a mount surface of the actuator block  29 . 
     The second actuator assembly  22 B has a structure substantially identical to that of the first actuator assembly  22 A. That is, as shown in  FIGS. 2 and 3 , the second actuator assembly  22 B comprises an actuator block (a second actuator block)  29  in which a bearing unit is built, four arms  30  extending from the actuator block  29 , six head suspension assemblies  32  respectively attached to the arms  30 , magnetic heads  17  mounted on the respective head suspension assemblies and a support frame  34  which supports the voice coil  36 . 
     The actuator block  29  is supported rotatably by the support shaft  26  via the bearing unit. The actuator block (the second actuator block)  29  is supported on a proximal end portion (a half portion on a bottom wall  12   a  side) of the support shaft  26 , and is coaxially placed below the first actuator block  29 . The actuator block (the second actuator block)  29  opposes the first actuator block  29  with a slight gap therebetween. 
     The actuator block  29  and the four arms  30  are formed to be integrated as one body from aluminum or the like, and constitutes the so-called E block. The arms  30  are each formed into, for example, a slender flat plate shape, and extend from the actuator block  29  in a direction normal to the support shaft  26 . The four arms  30  are provided parallel to each other with intervals respectively therebetween. In this embodiment, the four arms  30  are formed to have dimensions identical to and shapes identical to those of the arms  30  of the first actuator assembly  22 A. 
     A lowermost arm  30   a  of the first actuator assembly  22 A and an uppermost arm  30   b  of the second actuator assembly  22 B are located most adjacent to a boundary between the first actuator assembly  22 A and the second actuator assembly  22 B. The lowermost arm  30   a  and the uppermost arm  30   b  are disposed substantially parallel to each other with a predetermined interval therebetween. 
     The VCM  24  which drives the first actuator assembly  22 A and the VCM  24  which drives the second actuator assembly  22 B are provided independent from each other. With this structure, the first actuator assembly  22 A and the second actuator assembly  22 B can be driven (rotated) independent from each other around the support shaft  26 . 
     As shown in  FIGS. 2 and 3 , in the first actuator assembly  22 A and the second actuator assembly  22 B, a damper  52  is attached to each arm  30 . In the first actuator assembly  22 A, dampers  52  are attached respectively to upper surfaces (upper surfaces facing a top cover  14  side) of the uppermost, second and third arms  30 . A damper  52   a  is attached on a lower surface (a surface on a boundary side) of the lowermost arm  30   a.    
     In the second actuator assembly  22 B, a damper  52   b  is attached on an upper surface (a surface on a boundary side) of the uppermost arm  30   b , and dampers  52  are respectively attached on lower surfaces (lower surfaces facing the bottom wall  12   a  side) of the second, third and lowermost arms  30 . 
       FIG. 4  is a cross-sectional view of an example of the damper. 
     As shown, each of the dampers  52 , for example, the damper  52   b  has a double-layered structure of a viscoelastic layer V 1  and a constraint layer C 1 . The viscoelastic layer V 1  is made of a viscoelastic material, and the constraint layer C 1  is made of, for example, a material having a rigidity higher than that of the viscoelastic layer, that is, stainless steel. The viscoelastic layer V 1  and the constraint layer C 1  are formed into planar shapes substantially identical to each other and for example, they are formed into planar shapes substantially the same as the planar shape of the arms  30 . The damper  52  covers the surface of the respective arm  30  while the viscoelastic layer V 1  is attached on the surface of the arm  30 . When an arm  30  is deformed by vibration, the viscoelastic layer V 1  between the arm  30  and the constraint layer C 1  is warped, thereby creating a vibration damping effect. Usually, as the thickness of the damper is greater or the plane area of the damper is greater, the damping effect is enhanced. 
     As described above, in the actuator assemblies configured as described above, at least one arm  30  has vibration characteristics different from those of the other arms  30 . According to this embodiment, as shown in  FIG. 3 , in the first actuator assembly  22 A, the damper  52   a  attached to the lowermost arm  30   a  (the arm most adjacent to the second actuator assembly  22 B) is formed thicker than the other dampers  52  respectively attached on the other arms  30 . For example, when the thickness of the viscoelastic layer V 1  of the dampers  52  is 50 μm and the thickness of the constraint layers C 1  is 50 μm, the thickness of the viscoelastic layer V 1  of the damper  52   a  is set to 80 to 100 μm, and the thickness of constraint layers C 1  is 50 μm. Therefore, the damper  52   a  can exhibits a vibration damping effect higher than that of the other dampers  52 . Thus, the lowermost arm  30   a  on which the damper  52   a  is attached, that is, the arm  30   a  adjacent to the boundary between the actuator assemblies  22 A and  22 B has vibration characteristics different from those of the other arms  30 . Since the vibration damping effect is higher in the damper  52   a  than in the other dampers  52 , and therefore the arm  30   a  can reduce the generated vibration as compared to the other arms  30 . 
     Note that in order to enhance the vibration damping characteristics of the damper  52   a , the constraint layer C 1  thereof may be formed thicker than the constraint layers of the other dampers  52  in place of thickening the viscoelastic layer V 1 . Alternatively, both the thickness of the viscoelastic layer V 1  and the thickness of the constraint layer C 1  of the damper  52   a  may be set greater than the thickness of the other dampers  52 . Further, by using a material of the viscoelastic layer V 1  or the constraint layer C 1  of the damper  52   a , different from that of the material of the damper  52 , the vibration damping effect can be further enhanced. 
     In the second actuator assembly  22 B, the damper  52   b  attached to the uppermost arm  30   b  (the arm most adjacent to the first actuator assembly  22 A) is formed thicker than the other dampers  52  respectively attached onto the other arms  30 . For example, when the thickness of the viscoelastic layer V 1  of the dampers  52  is 50 μm and the thickness of the constraint layers C 1  is 50 μm, the thickness of the viscoelastic layer V 1  of the damper  52   b  is set to 80 to 100 μm, and the thickness of constraint layers C 1  is 50 μm. Therefore, the damper  52   b  can exhibits a vibration damping effect higher than that of the other dampers  52 . Thus, the uppermost arm  30   b  on which the damper  52   b  is attached, that is, the arm  30   b  adjacent to the boundary between the actuator assemblies  22 A and  22 B has vibration characteristics different from those of the other arms  30 . Since the vibration damping effect is higher in the damper  52   b  than in the other dampers  52 , and therefore the arm  30   b  can reduce the generated vibration as compared to the other arms  30 . 
     As shown in  FIG. 2 , the FPC unit  21  includes a first FPC unit  21   a  connected to the first actuator assembly  22 A and a second FPC unit  21   b  connected to the second actuator assembly  22 B. 
     The first FPC unit  21   a  includes a substantially rectangular base portion  42   a , a belt-like relay portion  44   a  extending from one side edge of the base portion  42   a , and a junction (a first wiring substrate)  46   a  continuously provided onto a distal end of the relay portion  44   a , which are integrated as one body. The base portion  42   a , the relay portion  44   a  and the junction  46   a  are formed from a flexible printed-circuit board (FPC). The flexible printed-circuit board includes an insulator layer such as of polyimide or the like, a conducting layer formed on the insulating layer, which forms wiring lines, contact pads and the like and a protective layer which covers the conducting layer. 
     On the base portion  42   a , electronic components including a conversion connector  47   a  and a plurality of capacitors (not shown) and the like are mounted and are electrically connected to the wiring lines of the FPC. To the base portion  42   a , a metal band  45   a  is attached, which functions as a reinforcing plate. The metal band  45   a  and the base portion  42   a  are each bent into substantially an L shape. The base portion  42   a  is disposed on the bottom wall  12   a  of the base  12 . The relay portion  44   a  extends from a side edge of the base portion  42   a  towards the first actuator assembly  22 A. The junction  46   a  provided in the extending end of the relay portion  44   a  is attached onto one side surface (installation surface) of the first actuator block  29 , and further fixed by screwing onto the installation surface by a fixing screw. 
     A connecting end portion of each flexure  74  is disposed to overlay on the junction  46   a  and is electrically and mechanically joined to the junction  46   a . Thus, the six magnetic heads  17  of the first actuator assembly  22 A are electrically connected to the base portion  42   a  via the wiring lines of the flexure  74 , the junction  46   a  of the first FPC unit  21   a  and the relay portion  44   a , respectively. Further, the base portion  42   a  is electrically connected to the printed circuit board on a bottom surface side of the housing  10  via the conversion connector  47   a.    
     Similarly, the second FPC unit  21   b  includes a substantially rectangular base portion  42   b , a belt-like relay portion  44   b  extending from one side edge of the base portion  42   b , and a junction (not shown) continuously provided onto a distal end of the relay portion  44   b , which are integrated as one body. The base portion  42   b , the relay portion  44   b  and the junction are formed from a flexible printed-circuit board (FPC). 
     On the base portion  42   b , electronic components including the conversion connector  47   b , capacitors (not shown) and the like are mounted and are electrically connected to the wiring lines of the FPC. The base portion  42   b  is disposed to be adjacent to and in order with the base portion  42   a  of the first FPC unit  21   a  and is installed on the bottom wall  12   a  of the base  12 . The relay portion  44   b  extends from a side edge of the base portion  42   b  towards the second actuator assembly  22 B. The junction formed at the extending end of the relay portion  44   b  is attached onto one side surface (installation surface) of the second actuator block  29 , and further fixed by screwing to the installation surface with fixing screws. 
     The connecting end portion of each flexure  74  is disposed to be overlaid on the junction so as to be electrically and mechanically joined to the junction. Thus, the six magnetic heads  17  of the second actuator assembly  22 B are electrically connected to the base portion  42   b  via the wiring lines of the flexure  74 , the junction of the second FPC unit  21   a  and the relay portion  44   b , respectively. Furthermore, the base portion  42   b  is electrically connected to the printed circuit board on a bottom surface side of the housing  10  via the conversion connector  47   b.    
     In the split actuator assembly configured as described above, the vibration of the first actuator assembly  22 A and the vibration of the second actuator assembly  22 B may interfere with each other via the support shaft  26 . When such an interaction occurs, the vibrational response in the vicinity of an axial central portion of the support shaft  26  increases. 
     To avoid this, in the HDD of this embodiment, thick dampers  52   a  and  52   b  are attached to the vicinity of the axial center of the support shaft  26 , more specifically, on the arm  30   a  and the arm  30   b  located near the boundary between the first actuator assembly  22 A and the second actuator assembly  22 B. With this structure, if vibration occurs near the axial central portion of the support shaft  26 , the vibration of the arms  30   a  and  30   b  can be effectively reduced. Thus, the contact between the arms  30   a  and  30   b  and the respective magnetic disk  18  can be prevented, making it possible to improve the reliability. 
     Between the arm  30   a  and arm  30   b , a magnetic disk  18  is not located, and therefore even if the thickness of the dampers  52   a  and  52   b  is increased, the dampers do not approach a magnetic disk. With this structure, the contact between the dampers  52   a  and  52   b  and the respective magnetic disk  18  can be prevented, thereby making it possible to improve the reliability. 
     As described above, according to the first embodiment, the vibration of the arms of the head actuators can be restrained, and thus a disk drive with an improved reliability cab be obtained. 
     Next, HDDs according to other embodiments will be described. In the other embodiments to be described below, portions equivalent to those of the first embodiment are denoted by the same reference numbers and detailed explanation is omitted or simplified, such explanation being mainly given to portions different from those of the first embodiment. 
     Second Embodiment 
       FIG. 5  is a cross-sectional view schematically showing an actuator assembly of an HDD according to a second embodiment, and  FIG. 6  is a plan view schematically showing arms of the actuator assembly in the second embodiment. 
     As shown in  FIG. 5 , according to the second embodiment, dampers  52  respectively attached to four arms  30  and  30   a  of a first actuator assembly  22 A are all formed to have the same thickness. Dampers  52  attached to four arms  30  and  30   b  of a second actuator assembly  22 B are all formed to have the same thickness. 
     In the actuator assembly, at least one arm  30  has vibration characteristics different from those of the other arms  30 . According to this embodiment, in the first actuator assembly  22 A, a lowermost arm  30   a  is formed into a shape different from that of the other three arms  30 .  FIG. 6 , in part (a), shows a planar shape of the three arms  30  on an upper side. Each of these arms  30  is formed into a slender flat plate shape and comprises a plurality of through holes including a first through hole  33 .  FIG. 6 , in part (b), shows a planar shape of the lowermost arm  30   a . The arm  30   a  is formed into a slender flat plate shape, and comprises a plurality of through holes including the first through hole  33 . Here, the first through hole  33  of the arm  30   a  is greater in open area than the first through holes  33  of the other arms  30 . That is, the arm  30   a  has a plane area less than that of the other arms  30 , and a mass less than that of the other arms  30 . Thus, the lowermost arm  30   a  has vibration characteristics different from those of the other arms  30 . The arm  30   a  is formed to exhibit a characteristic frequency different from that of the other arms  30 . 
     In the second actuator assembly  22 B, an uppermost arm  30   b  is formed into a shape different from that of the other three arms  30 .  FIG. 6 , in part (a), shows a planar shape of the three arms  30  on a lower side. Each of these arms  30  is formed into a slender flat plate shape and comprises a plurality of through holes including a first through hole  33 .  FIG. 6 , in part (b), shows a planar shape of the uppermost arm  30   b . The arm  30   b  is formed into a slender flat plate shape, and comprises a plurality of through holes including the first through hole  33 . Here, the first through hole  33  of the arm  30   b  is greater in open area than the first through holes  33  of the other arms  30 . That is, the arm  30   b  has a plane area less than that of the other arms  30 , and a mass less than that of the other arms  30 . Thus, the lowermost arm  30   b  has vibration characteristics different from those of the other arms  30 . The arm  30   b  is formed to exhibit a characteristic frequency different from that of the other arms  30 . 
     In the second embodiment, the other configuration of the HDD is the same as that of the HDD according to the first embodiment previously described. 
     According to the actuator assembly of the HDD configured as described above, if vibration occurs near the axial central portion of the support shaft  26 , the vibration can be attenuated by adjusting the characteristic frequency of each of the arms  30   a  and  30   b  located near the axial center of the support shaft  26 , that is, near the boundary between the first actuator assembly  22 A and the second actuator assembly  22 B. Thus, the contact between the arms  30   a  and  30   b  and the respective magnetic disk  18  can be prevented, making it possible to improve the reliability. 
     Note that the arms are not limited to the above-described configuration that they are different in the shape and size of the first through holes, but may be of such a configuration that the arms are different in outer shape and thickness. 
     Third Embodiment 
       FIG. 7  is a cross sectional view schematically showing an actuator assembly of an HDD according to a third embodiment. 
     For increasing the vibration damping characteristics of a damper, the plane area of the damper can be set greater than that of the other dampers in place of increasing the thickness of the damper. In the third embodiment, the dampers provided in at least one arm are formed to have a plane area greater than the plane areas of the dampers of the other arms. 
     As shown in  FIG. 7 , the HDD of the third embodiment includes an odd number of, for example, five magnetic disks  18 . The split actuator assembly includes a first actuator assembly  22 A and a second actuator assembly  22 B. The first and second actuator assemblies  22 A and  22 B are disposed in a multilayered manner one above another, and are provided to be rotatable independently from each other around the common support shaft  26  standing on the bottom wall  12   a  of the base  12 . The first actuator assembly  22 A and the second actuator assembly  22 B are configured to have structures substantially identical to each other. 
     The first actuator assembly  22 A disposed on an upper side comprises an actuator block (a first actuator block)  29 , three arms  30  extending from the actuator block  29 , head suspension assemblies  32  respectively attached to the arms  30 , and magnetic heads  17  respectively supported by the head suspension assemblies. 
     The actuator block  29  and the three arms  30  are formed to be integrated as one body from aluminum or the like, and constitute the so-called E block. The arms  30  are each formed into, for example, a slender flat plate shape, and extend from the actuator block  29  along a direction normal to the support shaft  26 . The three arms  30  are provided parallel to each other with intervals respectively therebetween each other. In this embodiment, the three arms  30  are formed to have dimensions identical to each other and shapes identical to each other. 
     In this embodiment, in the first actuator assembly  22 A, a down-head suspension assembly is attached to the uppermost arm  30 , and two head suspension assemblies, namely, an up-head suspension assembly and a down-head suspension assembly are attached to each of the middle arms  30  and the lowermost arm  30   a.    
     The second actuator assembly  22 B disposed on a lower side comprises an actuator block (a second actuator block)  29 , three arms  30  extending from the actuator block  29 , five head suspension assemblies  32  respectively attached to the arms  30 , magnetic heads  17  mounted on the respective head suspension assemblies and a support frame  34  which supports the voice coil. 
     The actuator block  29  is supported pivotably by the support shaft  26  via a bearing unit. The actuator block (the second actuator block)  29  is supported by a proximal end portion (a half portion on a bottom wall  12   a  side) of the support shaft  26 , and is coaxially placed below the first actuator block  29 . The actuator block (the second actuator block)  29  opposes the first actuator block  29  with a slight gap therebetween. 
     The actuator block  29  and the three arms  30  are formed to be integrated as one body from aluminum or the like, and constitutes the so-called E block. The arms  30  are each formed into, for example, a slender flat plate shape, and extend from the actuator block  29  in a direction normal to the support shaft  26 . The three arms  30  are provided parallel to each other with intervals respectively therebetween. In this embodiment, the three arms  30  are formed to have dimensions identical to and shapes identical to those of the arms  30  of the first actuator assembly  22 A. 
     A lowermost arm  30   a  of the first actuator assembly  22 A and an uppermost arm  30   b  of the second actuator assembly  22 B are located most adjacent to a boundary between the first actuator assembly  22 A and the second actuator assembly  22 B. The lowermost arm  30   a  and the uppermost arm  30   b  are disposed substantially parallel to each other with a predetermined interval therebetween. 
     In this embodiment, in the second actuator assembly  22 B, an up-head suspension assembly is attached to the lowermost arm  30 , and two head suspension assemblies, namely, an up-head suspension assembly and a down-head suspension assembly are attached to each of the middle arms  30  and the uppermost arm  30   b.    
     In the first actuator assembly  22 A and the second actuator assembly  22 B, ten head suspension assemblies  32  extend respectively from six arms  30  and are placed substantially parallel to each other with predetermined intervals respectively therebetween. Two magnetic heads  17  supported by a pair of a down-head suspension assembly  32  and an up-head suspension assembly  32  are located to face each other with a predetermined interval therebetween. These magnetic heads  17  are located to oppose respective surfaces of the corresponding magnetic disk  18 . 
     When there are an odd number of magnetic disks  18  are installed, the magnetic disk  18  located exactly in the middle along their stacking direction is placed between the lowermost arm  30   a  of the first actuator assembly  22 A and the uppermost arm  30   b  of the second actuator assembly  22 B. Therefore, the magnetic head  17  of the down-head suspension assembly  32  attached to the lowermost arm  30   a  and the magnetic head  17  of the up-head suspension assembly  32  attached to the uppermost arm  30   b  are located to oppose respective surfaces of the central magnetic disk  18 . 
     The first actuator assembly  22 A and the second actuator assembly  22 B can be independently driven (pivoted) around the support shaft  26 . 
     In the first actuator assembly  22 A and the second actuator assembly  22 B, a damper  52  is attached to each arm  30 . In the first actuator assembly  22 A, dampers  52  are attached respectively to upper surfaces (upper surfaces facing a top cover  14  side) of two arms, that is, the uppermost and the middle arms  30 . A damper  52   a  is attached on a lower surface (a surface on a boundary side) of the lowermost arm  30   a.    
     In the second actuator assembly  22 B, a damper  52   b  is attached on an upper surface (a surface on a boundary side) of the uppermost arm  30   b , and dampers  52  are respectively attached on lower surfaces (lower surfaces facing the bottom wall  12   a  side) of two arms, the middle and the lowermost arms  30 . 
     In the actuator assemblies configured as above, at least one arm  30  has vibration characteristics different from those of the other arms  30 . According to this embodiment, in the first actuator assembly  22 A, the damper  52   a  attached to the lowermost arm  30   a  (the arm most adjacent to the second actuator assembly  22 B) is formed to have a plane area greater than the other dampers  52  respectively attached on the other arms  30 . For example, the damper  52   a  is formed to have a length along the extending direction of the arm  30 , greater than the length of the other dampers  52 , and a plane area greater than that of the other dampers  52 . Note that the dampers  52  and  52   a  are set to have the same thickness. 
     The damper  52   a , with its larger plane area, can exhibit a vibration damping effect higher than that of the other dampers  52 . Thus, the lowermost arm  30   a  on which the damper  52   a  is attached has vibration characteristics different from those of the other arms  30 . The damper  52   a  has a vibration damping effect higher than that of the other dampers  52 , and thus the arm  30   a  exhibits can be effectively reduced as compared to the other arms  30 . 
     In the first actuator assembly  22 A, the damper  52   b  attached to the uppermost arm  30   b  (the arm most adjacent to the first actuator assembly  22 A) is formed to have a plane area greater than the other dampers  52  respectively attached on the other arms  30 . For example, the damper  52   b  is formed to have a length along the extending direction of the arm  30 , greater than the length of the other dampers  52 , and a plane area greater than that of the other dampers  52 . Note that the dampers  52  and  52   b  are set to have the same thickness. 
     The damper  52   a , with its larger plane area, can exhibit a vibration damping effect higher than that of the other dampers  52 . Thus, the uppermost arm  30   b  on which the damper  52   b  is attached has vibration characteristics different from those of the other arms  30 . The damper  52   b  has a vibration damping effect higher than that of the other dampers  52 , and thus the arm  30   a  exhibits can be effectively reduced as compared to the other arms  30 . 
     In the third embodiment, the other configurations of the actuator assembly and the HDD are the same as those of the HDD according to the first embodiment previously described. 
     According to the actuator assembly of the HDD configured as described above, if vibration occurs near the axial central portion of the support shaft  26 , the vibration of each of the arms  30   a  and  30   b  located near the axial center of the support shaft  26 , that is, near the boundary between the first actuator assembly  22 A and the second actuator assembly  22 B can be attenuated. Thus, the contact between the arms  30   a  and  30   b  and the respective magnetic disk  18  can be prevented, making it possible to improve the reliability. 
     When there are an odd number of magnetic disks  18  are installed, the magnetic disk  18  located exactly in the middle is placed between the arm  30   a  and the arm  30   b . In this case, the dampers  52   a  and  52   b  attached to the arms  30   a  and  30   b  approach close to the respective magnetic disk  18 , thus making it difficult to increase the thickness of the dampers  52   a  and  52   b . On the other hand, in this embodiment, the plane area of the dampers  52   a  and  52   b  can be set greater in place of increasing the thickness of the dampers. Thus, the vibration damping effect can be improved while preventing contact between the dampers  52   a ,  52   b  and the respective magnetic disk  18 . 
     As described above, according to the third embodiment, the vibration of the arms of the head actuators can be restrained, and thus a disk drive with an improved reliability cab be obtained. 
     Fourth Embodiment 
       FIG. 8  is a cross-sectional view schematically showing an actuator assembly of an HDD according to a fourth embodiment. 
     As shown, according to the fourth embodiment, an actuator assembly  22  of the HDD is configured as a single actuator assembly. The actuator assembly  22  comprises an actuator block  29  rotatably supported by a support shaft  26  via a bearing unit (not shown), seven arms  30  extending from the actuator block  29 , head suspension assemblies  32  attached to the respective arms  30 , and magnetic heads  17  supported by the respective head suspension assemblies. 
     The actuator block  29  and the seven arms  30  are formed to be integrated as one body from aluminum or the like, and constitute the so-called E block. The arms  30  are each formed into, for example, a slender flat plate shape, and extend from the actuator block  29  along a direction normal to the support shaft  26 . The seven arms  30  are provided parallel to each other with intervals respectively therebetween each other. In this embodiment, the seven arms  30  are formed to have dimensions identical to each other and shapes identical to each other. 
     The actuator assembly  22  comprises a support frame  34  extending from the actuator block  29  in a direction opposite to the arms  30 . A voice coil (not shown) which constitutes the VCM is supported on the support frame  34 . 
     The actuator assembly  22  comprises twelve head suspension assemblies  32 , and the head suspension assemblies  32  are attached respectively to extending ends of the arms  30 . In the actuator assembly  22 , a down-head suspension assembly  32  is attached to an uppermost arm  30 , and an up-head suspension assembly  32  is provided at a lowermost arm  30 , and two head suspension assemblies, namely, the up-head suspension assembly  32  and the down-head suspension assembly  32  are attached to each of the other five arms  30 . 
     Two magnetic heads  17  supported by a pair of a down-head suspension assembly  32  and an up-head suspension assembly  32  are located to face each other with a predetermined interval therebetween. The magnetic heads  17  are located to oppose respective surface of the corresponding magnetic disk  18 . 
     In the actuator assembly  22  configured as described above, at least one arm  30  has vibration characteristics different from those of the other arms  30 . According to this embodiment, in the actuator assembly, a damper  52  is attached on an upper surface (a surface facing the top cover  14 ) of the uppermost arm, and a damper  52  is attached on a lower surface (a surface facing the bottom wall  12   a  side) of the lowermost arm  30 . 
     As in the first embodiment described above, the dampers  52  have a double-layered structure of a viscoelastic layer and a constraint layer. The viscoelastic layer is made of a viscoelastic material, and the constraint layer is made of, for example, a material having a rigidity higher than that of the viscoelastic layer, that is, stainless steel. The viscoelastic layer and the constraint layer are formed into planar shapes substantially identical to each other and for example, they are formed into planar shapes substantially the same as the planar shape of the arms  30 . The damper  52  covers the surface of the respective arm  30  while the viscoelastic layer is attached on the surface of the arm  30 . When an arm  30  is deformed by vibration, the viscoelastic layer between the arm  30  and the constraint layer is warped, thereby creating a vibration damping effect. 
     As described above, the dampers  52  is attached respectively on the uppermost arm  30  and the lowermost arm  30 , these arms have vibration characteristics different from those of the other arms  30 . That is, due to the vibration damping effect of the dampers  52 , the uppermost and lowermost arms  30  attenuate the vibration occurred, as compared to the other arms  30 . 
     When an external impact or the like applied on the HDD, the vibration of the uppermost arm and the lowermost arm of the actuator assembly  22  can be suppressed, thereby making it possible to prevent collision between the respective arm  30  and the top cover  14 , collision between the respective arm  30  and the bottom wall  12   a  and contact between the arm  30  and the magnetic disk  18 . As described above, in the third embodiment as well, the vibration of the arm of the actuator assembly is restrained, and thus a disk drive with improved reliability can be obtained. 
     Note that the structure in which the arms have different vibration characteristics is not limited to the case where dampers are attached, but such a configuration will do that arms are formed to have a shape different from the shape of the other arms. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 
     The head actuator assembly can be divided into not only two, that is, the first and second actuator assemblies, but also three or more. The number of magnetic disks is not limited to six, but it may be five or less, or seven or more. The number of arms, that of head suspension assemblies, and the number of magnetic heads may be increased and decreased according to the number of magnetic disks installed. 
     The arms of different vibration characteristics are not limited to the uppermost and lowermost arms, but any other arms can be selected. The material, shape, size and the like of the elements which constitute the disk drive are not limited those of the embodiments, but they may be changed in various ways as needed.