Patent Publication Number: US-7221537-B2

Title: Plastic spacer and disk clamp assembly

Description:
RELATED APPLICATIONS 
     This application is a continuation of copending U.S. patent application Ser. No. 10/652,059, filed Aug. 29, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to disk drive components, and more particularly, this invention relates to a composite disk clamp that provides high rigidity and integral material matching at the disk-clamp interface. 
     BACKGROUND OF THE INVENTION 
     A typical disk drive storage system includes one or more magnetic disks which are mounted for co-rotation on a hub or spindle. A typical disk drive also includes a transducer supported by a hydrodynamic bearing which flies above each magnetic disk. The transducer and the hydrodynamic bearing are sometimes collectively referred to as a data head or a product head. A drive controller is conventionally used for controlling the disk drive based on commands received from a host system. The drive controller controls the disk drive to retrieve information from the magnetic disks and to store information on the magnetic disks. An electromechanical actuator operates within a negative feedback, closed-loop servo system to move the data head radially or linearly over the disk surface for track seek operations and holds the transducer directly above a desired track or cylinder on the disk surface for track following operations. 
     Typically the magnetic disks  2  also comprise servo sectors  18  which are recorded at a regular interval and interleaved with the data sectors  12 , as shown in  FIG. 1 . A servo sector, as shown in  FIG. 2 , typically comprises a preamble  20  and sync mark  22  for synchronizing to the servo sector; a servo data field  24  comprising coarse position information, such as a Gray coded track address, used to determine the radial location of the head with respect to the plurality of tracks; and a plurality of servo bursts  26  recorded at precise intervals and offsets from the track centerlines which provide fine head position information. When writing or reading data, a servo controller performs a “seek” operation to position the head over a desired track; as the head traverses radially over the recording surface, the Gray coded track addresses in the servo data field  24  provide coarse position information for the head with respect to the current and target track. When the head reaches the target track, the servo controller performs a tracking operation wherein the servo bursts  26  provide fine position information used to maintain the head over the centerline of the track as the digital data is being written to or read from the recording surface. 
     To ensure that the head remains properly aligned with the data tracks, the disks must be securely attached to the spindle. Current practice is to separate the disks in the stack with spacer rings, and position a spacer ring on top of the disk/spacer stack. Then a top ring, called a clamp, with several apertures is placed over the top spacer ring. The disks are bolted to the spindle via bolts extending through the apertures in the top clamp. Great pressure must be exerted by the bolts on the top clamp in order to prevent slippage of the disks in the event that the drive is bumped or uneven thermal expansion that breaks the frictional coupling, because once the disks slip, the drive loses its servo and the data is lost. 
     Disks are typically formed from aluminum or glass. Aluminum is more easily deformed, so any external stress can cause deformations to the disk. Glass, too, will deform under uneven stress patterns. 
     A major drawback of the current practice is that when the bolts are tightened, the top clamp and spacer become deformed due to the uneven pressures exerted by the individual bolts. The deformation translates out to the disk, creating an uneven “wavy” disk surface, which is most prominent at the inner diameter of the disk. Any unevenness (waviness) on the disk surface compounds the tendency to lose the servo, especially near the inner diameter zone closest to the spacer ring. 
     Further, it has been found that stresses induced on the top disk in the stack transfer down and propagate into some or all of the remaining disks in the stack. Thus, it would be desirable to reduce uneven stresses at the top disk so that the remaining disks remain flat. 
     Another issue encountered in the prior art is the high cost of assembling the drives. Each spacer must be placed in the drive and then the top clamp added and bolted down. This process is time consuming. To reduce assembly costs, it would be desirable to couple the top clamp and top spacer ring together so that they can be placed in the drive at the same time. This would save a processing step in that only one piece (top clamp-spacer composite) need be handled instead of two parts (top clamp and spacer ring individually). 
     The cost savings obtainable by using a composite structure would be increased in new high capacity drives which require only a few disks as opposed to several. For example, in a drive with five disks, five parts must be handled: the top clamp-spacer composite and four more spacer rings. In a drive with only two disks, only two parts are handled: the top clamp-spacer composite and one spacer. 
     Additional cost savings would be realized during manufacture of the top clamp and top spacer ring themselves, as it would no longer be necessary to machine two surfaces in such a way to match flatness. 
     SUMMARY OF THE INVENTION 
     An assembly for coupling a disk to a spindle includes an annular spacer ring adapted to engage the disk and an annular clamp. The spacer ring is constructed at least in part of a plastic material. The clamp is constructed of a material having a Young&#39;s modulus greater than that of the spacer ring. The assembly is particularly suited to coupling disks made in part of a plastic material that is the same or different from the plastic in the spacer ring. 
     Preferably, the spacer ring has a density gradient that decreases from a clamp end of the spacer ring to a disk end of the spacer ring. In one embodiment, the spacer ring has material strengthening elements embedded therein, such as beads having a Young&#39;s modulus greater than a hardness of the plastic material. 
     In another embodiment, the clamp and/or spacer ring have protrusions extending therefrom for mating with the other piece. The protrusions may taper to a point, have a generally rectangular cross section, may taper apart (i.e., dovetail) towards free ends thereof, may have bulbous portions towards free ends thereof, etc. and combinations thereof. 
     The clamp may or may not include a plastic material. Preferably, the clamp has a hardness at least as hard as a primary material of the disk. 
     The spacer ring and clamp may be installed in the disk drive as separate units. The spacer ring and clamp may also be fixedly coupled together, such as by mechanical bonding, an adhesive, and a coupling at a molecular level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings. 
         FIG. 1  shows a typical format for of a disk surface comprising a plurality of radially spaced, concentric data tracks partitioned into a number of data sectors and embedded servo sectors for positioning the heads over the disk surfaces while seeking and tracking. 
         FIG. 2  shows a typical format of an embedded servo sector. 
         FIG. 3  is a schematic and simplified vertical sectional view of a rigid magnetic disk drive unit embodying the present invention. 
         FIG. 4  is a top plan view of the structure shown in  FIG. 3 . 
         FIG. 5  is a perspective view of a composite spacer according to one embodiment. 
         FIG. 6A  is an exaggerated cross-sectional view of a composite ring with upper and lower layers coupled together via a series of ridges and troughs. 
         FIG. 6B  is an exaggerated cross-sectional view of a composite ring with upper and lower layers coupled together via a series of dovetail protrusions and receiving areas. 
         FIG. 6C  is an exaggerated cross-sectional view of a composite ring with upper and lower layers coupled together via a series of bulbous protrusions and receiving areas. 
         FIG. 7  is a side view of the first layer of  FIG. 6A  taken along lines  7 — 7  of  FIG. 6A . 
         FIG. 8  is a cross-sectional view of a clamp/spacer ring assembly according to one embodiment. 
         FIG. 9  is a cross-sectional view of a clamp/spacer ring assembly according to another embodiment. 
         FIG. 10  is a cross-sectional view of a clamp/spacer ring assembly according to yet another embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The following description is the best embodiment presently contemplated for carrying out the present invention. This description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. 
     Referring now to the drawings wherein like reference numerals designate like or similar parts throughout the several views there is illustrated in  FIG. 3  a cross-sectional diagram of parts of a data storage disk drive system  30  including a rigid magnetic disk drive unit generally designated as  32  and a control unit generally designated as  34 . While a magnetic disk drive unit is illustrated, it should be understood that other mechanically moving memory configurations may be used. Unit  32  is illustrated in simplified form sufficient for an understanding of the present invention because the utility of the present invention is not limited to the details of a particular drive unit construction. After data storage disk drive system  30  is completely assembled, servo information used to write and read data is written using the disk drive system  30 . 
     Referring now to  FIGS. 3 and 4  of the drawing, disk drive unit  32  includes a stack  36  of disks  38  having at least one magnetic surface  40 . The disks  38  are mounted in parallel for simultaneous rotation on and by an integrated spindle and motor assembly  46 . The disks  38  are separated by spacers  33  and are coupled to the spindle at the top by a composite clamp ring  70  which is pressed onto the stack of disks  38  by bolts  35 . 
     Data information on each disk  38  are read and/or written to by a corresponding transducer head  48  movable across the disk surface  40 . In a disk drive using a dedicated or hybrid servo, one of the disk surfaces  40 ′ stores servo information used to locate information and data on the other disk surfaces  40 . 
     Transducer heads  48  are mounted on flexure springs  50  carried by arms  52  ganged together for simultaneous pivotal movement about a support spindle  54 . One of the arms  52  includes an extension  56  driven in a pivotal motion by a head drive motor  58 . Although several drive arrangements are commonly used, the motor  58  can include a voice coil motor  60  cooperating with a magnet and core assembly (not seen) operatively controlled for moving the transducer heads  48  in synchronism in a radial direction in order to position the heads in registration with data information tracks or data cylinders  62  to be followed and access particular data sectors  64 . Although a rotary actuator is shown, it should be understood that a disk drive with a linear actuator can be used. Data storage disk drive system  30  is a modular unit including a housing  66 . The various components of the disk drive system  30  are controlled in operation by signals generated by control unit  34  such as motor control signals on line  46 A and position control signals on line  58 A. 
     Numerous data information tracks  62  are arrayed in a concentric pattern in the magnetic medium of each disk surface  40  of data disks  38 . A data cylinder includes a set of corresponding data information tracks  62  for the data surfaces  40  in the data storage disk drive system  30 . Data information tracks  62  include a plurality of segments or data sectors  64  each for containing a predefined size of individual groups of data records which are saved for later retrieval and updates. The data information tracks  62  are disposed at predetermined positions relative to servo information, such as a servo reference index. In  FIG. 4  one sector  64  is illustrated as SECTOR O with a fixed index or mark INDEX for properly locating the first data sector. The location of each next sector  64  is identified by a sector identification (SID) pulse read by transducer heads  48  from surfaces  40 ,  40 ′. 
       FIG. 5  illustrates a composite top clamp ring  70  according to one embodiment. The composite ring  70  includes a stiff annular upper layer  72  that provides rigidity to the clamping structure  70  by providing a more even distribution of stresses on the disks below from the clamping bolts coupled to the composite ring  70  through the apertures  76 , which in turn reduces deformation of the disk(s). An annular lower layer  74  is fixedly coupled to the upper layer  72 . The lower layer  74  may be of secondary stiffness and is preferably made of a material similar to the disk, e.g., aluminum or ceramic. The highly rigid upper layer  72  of the composite ring  70  reduces deformation caused by the clamping stresses, yet the composite ring  70  provides high mechanical stability due to integral material matching for the clamping interface. The material-matching at the ring-disk interface allows the lower layer  74  and the disk to expand and contract together under temperature variations. (Using a hard material for the lower layer  74  would cause the bottom layer to expand out of phase with the disk.) 
     The upper layer  72  of the composite ring is preferably made of a material with a high hardness and high modulus so that it is less susceptible to bending and/or cracking under the stress of the clamping forces. Preferably, the upper layer  72  has a material hardness and/or modulus that is at least as hard as the disk itself. Thus, if an aluminum disk is used in the drive, the stiffening layer should have a hardness at least about the same as the aluminum material used to form the disk. Illustrative materials from which the upper layer  72  can be formed are nickel, titanium, chrome, stainless steel, materials treated (e.g., doped) for stiffness and hardness, silicon nitride, aluminum nitride, alloys, composites, etc. 
     The lower layer  74  is preferably constructed of a material which is of a similar or about the same coefficient of thermal expansion as that of the primary material of the disk, i.e., disk substrate of glass, aluminum, etc. The thermal conductivity parameter shown in Table 1 (below) is important but to a lesser extent than the coefficient of thermal expansion and the Young&#39;s modulus of the material. The thermal conductivity should be similar to the disk since heat from the motor does not build up in the lower layer  74  but can transmit to the disk uniformly. 
     The following table lists several materials from which the composite ring  70  can be constructed, and their properties. Note that the modulus is a measure of the load a material can handle before it starts to deform. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Thermal 
                 Coefficient of 
               
               
                   
                 Hardness 
                 Modulus 
                 Conductivity 
                 Thermal Expansion 
               
               
                 Material 
                 (kg/mm 2 ) 
                 (GPa) 
                 (W/m-K) 
                 (10 −6 /C.) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Aluminum 
                 27 
                 70 
                 221 
                 25 
               
               
                 Chrome 
                 125 
                 26 
                 14 
                 6 
               
               
                 Titanium 
                 65 
                 110 
                 2 
                 8.5 
               
               
                 Nickel 
                 210 
                 200 
                 60 
                 13 
               
               
                 Glass 
                 185 
                 63 
                 1 
                 4 
               
               
                 Stainless 
                 160 
                 205 
                 16 
                 12 
               
               
                 Steel 
               
               
                   
               
            
           
         
       
     
     An illustrative range of materials usable in the composite ring  70  have a hardness of about 20 to about 250 kg/mm 2 , a modulus of about 60 to about 300 GPa and a thermal expansion between 1 and 25 (10 −6 /C). Note that glass and aluminum have about the similar Young&#39;s modulus, but the aluminum has about 6 times the coefficient of thermal expansion as glass. Therefore, an aluminum spacer is preferred for use with aluminum disks, while a ceramic spacer is preferred for use with glass disks. 
     The following table illustrates exemplary Young&#39;s modulus ratios for the upper and lower layers of the composite ring  70 . The modulus ratio is important as a measure of how well the composite ring  70  will provide the desired properties. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Modulus Ratio of 
               
               
                   
                 Two Materials: 
               
               
                 Material Combination 
                 first + second layer 
               
               
                   
               
             
            
               
                 Stainless Steel Clamp with Aluminum spacer 
                 205/70 = 3.0 
               
               
                 Stainless Steel Clamp with Glass spacer 
                 205/63 = 3.3 
               
               
                 Titanium Clamp with Aluminum spacer 
                 110/70 = 1.6 
               
               
                 Titanium Clamp with Glass spacer 
                 110/63 = 1.8 
               
               
                   
               
            
           
         
       
     
     A preferred modulus ratio range can be shown by the following: 1.0&lt;Modulus Ratio&lt;5.0. However, this range may not be all inclusive and would allow some combinations outside of this range, especially using nonmetallic disks and clamps, i.e. plastic and silicon disks with plastic and silicon lower layers  74 . When glass disks are used, an illustrative modulus ratio of about 3.3 is provided by a stainless steel upper layer  72  and a ceramic lower layer. 
     A middle layer of the composite ring, positioned between the upper and lower layers  72 ,  74 , can be added to achieve the desired overall modulus ratio. The middle layer can be constructed of another material such as one or more of stainless steel, chrome, nickel, etc. and composites and alloys thereof. 
     The layers of the composite ring  70  can be coupled together using any suitable process. Several techniques to perform such bonding are described below. Three particular techniques include mechanical bonding, adhesive chemical bonding, and bonding at the molecular level. 
     Mechanical bonding of the layers can be achieved by protrusions and corresponding receiving areas such as ridges/textured lines and coincident troughs. The mechanical coupling encourages the various layers to expand and contract together, thereby maintaining the proper alignment. 
       FIG. 6A  is an exaggerated view of the composite ring  70  with the upper and lower layers  72 ,  74  coupled together via a series of ridges  82  and troughs  84 .  FIG. 7  shows the ridge structure on the bottom of the upper layer  72  which mates with a trough structure on the lower layer  74  to create the composite ring. The ridges  82  mate into the troughs  84 , providing mechanical bonding and strength between the layers. With this type of mating for the composite ring  70 , high structural integrity and superior bonding is maintained for a system that is exposed to the high stress of clamping a multi-disk pack. 
     Other variations to create the mechanical coupling can include random or nonrandom fingerlike perturbations and recesses, teeth (see  FIG. 9 ), etc. To enhance the coupling, “dovetail” protrusions, bulbous protrusions, etc. can be used to form semi-permanent or permanent coupling. Note  FIGS. 6B and 6C . 
     Chemical bonding can also be used individually, or in combination with protrusions/receiving portions as described in the immediately preceding paragraphs. Adhesives such as silicon-based adhesives can be used to create the chemical bond. In a similar manner, bonding at the molecular level can be achieved by sintering, welding, ultrasonic welding, etc. 
     Because the two layers  72 ,  74  are coupled together, a processing step during drive assembly is saved in that only one piece (clamp-spacer composite) need be handled instead of two parts (clamp and spacer). Additional cost savings are realized during manufacture of the annular layers  72 ,  74  themselves, as it is longer necessary to machine two surfaces to obtain a hardness match. 
     A clamp/spacer ring assembly according to another embodiment is constructed at least in part of plastic. Such a ring assembly  90 , particularly suitable for use in a drive having disks themselves constructed at least in part of a plastic material, is shown in  FIG. 8 . The ring assembly  90  according to one embodiment includes a stiff annular clamp  92 , followed by an annular plastic spacer ring  94 . The clamp  92  is preferably constructed of materials described above with respect to the first layer of the composite ring. The spacer ring  94  is preferably constructed of any plastic material having properties similar to that of the plastic in the disks, and may be formed or molded as a single layer, a composite structure, etc. Illustrative plastic materials include polyolefins, polyethylenes, polycarbonates, polystyrenes, polyvinyl chlorides, polymers, resins, etc. In the embodiment shown in  FIG. 8 , the spacer ring  94  has glass beads  96  (or other material strengthening elements) of any shape embedded into the plastic matrix in a non-uniform manner. Preferably, the material strengthening elements are harder than the material from which the spacer ring  94  is constructed. This allows a gradient of density from high to low (high closest to the clamp  92 ) which will allow a gradient of thermal expansion to be made more uniform from the stiff clamp  92  to the lower portion of the plastic spacer ring  94 . Without this gradient the clamp  92 /spacer ring  94  combination would suffer from thermal expansion properties as well as modulus problems. The plastic lower layer closest to the clamp  92  would have a high density of materials, allowing for a thermal expansion and modulus closest to the clamp  92 . The portion of the plastic ring closest to the plastic disk would then have material properties very similar to that of the plastic disk being clamped. Preferably, some type of mechanical mating (e.g., see  FIGS. 9 and 10 ) is implemented to integrally couple the layers to prevent slipping of the layers, particularly where dissimilar materials are used. 
     Another way to obtain a non-uniform plastic structure is with teeth  102 , as shown in  FIG. 9 . The teeth  102  are preferably extensions from the clamp  92  which extend into the plastic matrix. The teeth  102  may be annularly aligned, segmented, randomly placed, etc. This will increase the density and modulus of the plastic structure and will give it similar material properties as explained in the previous paragraph. Note also that the teeth  102  and ridges  104  may also extend from the plastic ring  94  into the clamp  92 . 
     Yet another way to obtain a non-uniform plastic structure is with a ridge/trough  104 ,  106  combination, as shown in  FIG. 10 . The teeth  104  are preferably extensions from the clamp  92  which extend into the plastic matrix. The ridges  104  may be annularly aligned, segmented, randomly placed, etc. This will increase the density and modulus of the plastic structure and will give it similar material properties as explained in the previous paragraphs. Note also that the ridges  104  may also extend from the plastic ring into the clamp  92 . 
     The clamp  92  and plastic spacer ring  94  of the ring assemblies shown in  FIGS. 8–10  can be installed individually in the disk drive. The mechanical coupling causes the layers to expand and contract together in a uniform fashion, thereby preventing the disk from going off-center. Alternatively, the clamp  92  and plastic spacer ring  94  can be fixedly coupled together to form a composite structure similar to that described above with respect to  FIG. 5 . 
     Thus, the composite structure described herein will reduce the parts count and the cost associated with an individual clamp  92  and spacer ring  94 . The composite material choices will also allow better thermal expansion, stiffness and modulus which is mated and matched perfectly to the top disk of the stack. Under torquing requirements the stress is spread out evenly over the top disk so as to minimize tangential and radial curvature effects. 
     Composite structures that have a blend of coupling morphologies are also to be considered as a composite structure that are mechanically bonded to one another. The set of mechanical couplings indicated in the individual  FIGS. 6A ,  6 B and  6 C can be blended into one as a combination, i.e. A+B, A+C, B+C. Also, for the plastic clamp/ring structure shown in  FIGS. 9 and 10 , these can also be combined, i.e.  9 + 10 . 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.