Disk drive with disk stack assembly

Abstract: A disk drive has a disk stack assembly comprising rigid magnetic disks 8, and disk spacer rings 13 with an annular groove 22 in the outer cylindrical surface. A concentrated axial load is applied to the stack by support surfaces 11, 16 at each axial end to retain the disks in position. The concentrated load is only partially distributed over the radial extent of the spacers at the axial ends of the stack to resist coning or cupping of the end disks, while the force distribution on the inner disks approaches a uniform loading to retain their undistorted, planar orientation. Thus the grooved spacers reduce distortion, and additionally also provide a surface which can be used to engage and manipulate the disk assembly during manufacture of the drive.

Description: This invention pertains to disk drives and more particularly to a disk stack assembly structure for such devices. Background of the invention A conventional rigid magnetic disk data storage device such as that described in EP-A-0166118 has a disk stack assembly with a series of disks mounted for rotation on a hub. The disks are separated by spacers and retained on the hub by a clamp. To achieve the highest possible volumetric density of data in such devices, the greatest areal storage density on the disk surfaces and the maximum number of disks are used. The spacing between disks is reduced to the minimum required for the functioning of the transducer head, its suspension and the actuator arm on which the head-suspension assembly is mounted. A further step is to reduce the thickness of the disks. Having optimized the above parameters, the ability of the disk stack clamping to maintain the disks flat, parallel and free of deformation becomes critical. For a given clamping force on the disk stack, concentrated loads will tend to cause more severe disk deformation than distributed loads. One typical distortion mode is coning (or cupping) of the disks. Distortion has been observed on industry standard thickness disks and is more severe still on reduced thickness disks. The coning of the disks is detrimental because it affects the fly height of the heads. With the higher bit densities and lower fly heights required by high density thin disks, ensuring the flatness of the disks in the disk stack assembly and solving the problems of disk distortion due to concentrated disk stack clamping forces and the low residual clamp loads of a single spring element system is becoming increasingly important. The use of a flexible clamp ring to reduce disk distortion is disclosed in US-A-3587073, but relatively little attention appears to have been paid to the design of the spacers in relation to this problem. US-A-4819105 discloses a disk drive with U-shaped spacers, with the legs of the U parallel to the disk axis, but this is to reduce the mass of the spacers and to help airflow through the disk stack. US-A-3587073 has a groove on the inner radial surface of each of the spacers. JP-A-58-155519 describes a disk drive in which the spacer has an outer circumference tilted to the centre shaft of the spindle. The shape of the spacer helps to prevent airflow separating from the surface of the spacer and causing turbulence over the disk. Disclosure of the Invention Accordingly, the invention provides for a disk drive including a disk stack assembly with: at least two axially aligned disks; an annular spacer separating said at least two axially aligned disks, said spacer adjoining the inner diameter of said disks and having an annular groove; and clamping means maintaining a compressive retaining force on said at least two disks and said spacer; said disk drive being characterised in that: said annular groove extends radially inward from the outer surface of said spacer and has an approximately U-shaped cross-section in the plane containing both the disk stack axial and radial directions, with the legs of the U-shape extending radially outwards. The width and depth of the annular groove imparting the U-shaped cross-section to the spacer depend on factors such as disk thickness, load application point and clamping force. The groove allows the spacer to behave similarly to a cantilever spring when a load is applied adjacent the outer diameter of the spacer. In the disk stack assembly, a concentrated load is applied to the top disk and the bottom disk. The grooved spacer, by acting like a spring, causes this applied concentrated load to become a distributed load more rapidly than if the spacer did not possess the groove. The distributed load is highest at the outside diameter and decreases toward the inside diameter of the spacer ring. This distribution of the concentrated load will counterbalance the natural deflection of the disk, permitting the top and bottom disks to be flat when clamped in the disk stack assembly. Preferably the clamping means comprises a hub that passes through an axial hole in the disks and spacer, and a clamp ring concentric with the disks having a thermal shrink fit connection with the hub. A first support surface on a flange extending from the hub and a second support surface on the clamp ring maintain the compressive retaining force on the disks and spacer therebetween. Preferably the inner radial diameter of the first and second support surfaces is aligned with the outer radial diameter of the spacer inside the groove. It is further preferred that there is a gap between the outer radial surface of the hub and the inner radial surface of the disks and of the spacer, so that the hub outer surface does not contact the disks or the spacer during thermal expansion or contraction. The invention is of benefit both in disk drives that have only one pair of disks and a single spacer, and also in those that have three or more disks in the disk stack assembly, each separated by an annular spacer. In the latter case it is preferred that there is an annular groove extending radially inwards from the outer surface of every annular spacer. This leads to the concentrated load tending to distribute itself further as it is transferred to the central disks and spacers in the stack, becoming nearly a uniformly distributed load. This load does not deflect the spacer as much as a concentrated load because more oi the load is acting on a part of the spacer that is not a spring element. Thus, the central disks in the disk stack assembly remain flat. When several disks and spacers are present however, it is the spacers between the two disks at each axial end of the disk stack that have the greatest role in transforming the load on the disks from a concentrated load into a distributed load. It may therefore sometimes be possible to just have annular grooves in these end spacers, rather than in every spacer, and still eliminate distortion effectively. Clearly this depends on exactly how much distortion the system can tolerate. Brief description of the drawings Fig. 1 is a radial sectional view of a magnetic disk stack assembly from a disk drive according to the invention; and Fig. 2 is an exploded radial sectional view of the disk stack assembly of Fig. 1 showing the clamping load distribution within the clamped components of the disk stack. Detailed Description A disk stack assembly from a magnetic disk drive is illustrated in Fig. 1 as a three disk stack mounted on a hub 6. The hub 6 includes a space 7 concentrically within the stack of disks 8 that contains the spindle motor (not shown). Thus, the hub 6 not only supports the disks 8, but also functions as the housing for the spindle motor rotor. The hub 6 has a lower flange 10 which presents an annular support surface 11 (Fig. 2) upon which the lowermost disk 8 of the stack is supported. Between each adjoining pair of disks is a spacer ring 13 and at the top of the stack a clamp ring 15 presents a second, downwardly facing support surface 16. The stack of disks 8 and spacer rings 13 are clamped and held captive between the hub annular support surface 11 and the clamp ring annular support surface 16. The clamp ring 15 has an inner diameter 18 that is slightly smaller than the outer diameter 17 of the top of hub 6 and is assembled to the hub by being heated to expand the inner diameter 18, being placed about the hub outer surface 17 while simultaneously applying an axial force to the stack, and then being allowed to cool to establish an interference fit with the hub outer surface that clamps the disks 8 and spacers 13 in position, maintaining the desired axial retaining force upon the disk stack (this clamping technique is taught in EPA 0166118). It will also be noted that a clearance is maintained between the inner diameter 20 of the disks, the inner diameter 21 of the spacers and the outer substantially cylindrical wall surface 17 of the hub 6. This avoids the imposition of forces and dislocations that might occur due to physical contact between such parts during temperature changes. The disk spacers 13 are provided with an annular groove 22, centrally positioned with respect to the outer cylindrical surface 23 and radially inwardly extending therefrom. The width and depth of the groove 22 are tailored to the disk thickness, axial load application location and clamping force. When a load is applied near the outer diameter of spacer 13, the groove 22 permits the spacer to function in a manner similar to a cantilever spring. With the disk stack in the assembled condition, a concentrated load is applied to the disk stack between the hub flange annular support surface 11 and the clamp ring annular support surface 16. The grooved spacer 13, functioning like a spring, causes the concentrated load to become a distributed load more rapidly than would be possible if a solid ring shaped spacer were used. The distributed load is highest at the spacer ring outer diameter 23 and decreases toward the spacer inner diameter 21 to cause the concentrated load to counterbalance the natural deflection of the disk 8 at each axial end of the stack. The top and bottom disks of the stack thereby remain flat and parallel to the other disks of the stack, overcoming the coning or cupping distortion common to the clamping of axially aligned clamped disks. Although the embodiment of the invention shown in Figs. 1 and 2 has for simplicity only three disks, the technique can also be applied to much larger disk stacks, with 8, 10 or 12 disks for example, with close spacing between disks and reduced thickness disk media. The concentrated load tends to be further distributed as it is transferred to disks at the centre of the stack to approach a uniformly distributed load. This more uniformly distributed load does not deflect the spacer as much as the concentrated load, since more of the load is acting on a part of the spacer that is not functioning as a spring element. Thus disks in the centre of the stack maintain their flat planar configuration. The load distribution is illustrated by the position and length of the arrows in Fig. 2. The support surfaces 11, 16 of the hub 6 and the clamp ring 15 respectively impart a concentrated load largely at the outside diameters of the support surfaces. The grooved spacer rings 13 cause the concentrated load at the axial ends of the stack to be distributed from the outer radius to the inner radius of the spacer through the spring like function of the annular spacer projections adjoining the groove. As the axial compressive load is transferred to the disks beyond those at the axial ends of the stack, the loading approaches a uniform distribution from the outside diameter to the inside diameter of the spacer ring 13. Thus the use of the grooved spacers overcomes the tendency of the disks at the axial ends of the stack to be distorted through coning or cupping and does not distort the disks at the centre of the stack, maintaining their flat planar disposition. In the embodiment shown in Figs. 1 and 2, the depth of the groove 22 is 1 millimetre and the radial width of the clamp ring and hub annular support surfaces is 1 millimetre. Thus, the groove and the support surfaces are aligned and coextensive; however, this common dimension is exemplary only and would not necessarily be applicable to other combinations of disk thickness, load magnitude and location of application. The grooved spacer rings 13 also provide a cylindrical surface 23 with a locating groove 22 that can be used for automated handling and manipulation of the disk stack assembly during manufacture of the head-disk assembly.

Claims: A disk drive including a disk stack assembly with: at least two axially aligned disks (8); an annular spacer (13) separating said at least two axially aligned disks, said spacer adjoining the inner diameter of said disks and having an annular groove (22); and clamping means (6, 15) maintaining a compressive retaining force on said at least two disks and said spacer; said disk drive being characterised in that: said annular groove (22) extends radially inward from the outer surface (23) of said spacer and has an approximately U-shaped cross-section in the plane containing both the disk stack axial and radial directions, with the legs of the U-shape extending radially outwards. A disk drive as claimed in claim 1, wherein said clamping means comprises a hub (6) that passes through an axial hole in said spacer and each of said axially aligned disks, said hub including a flange (10) that presents a first support surface (11) for said disks. A disk drive as claimed in claim 2, wherein the inner radial surfaces (20, 21) of said axially aligned disks and spacer are separated from the outer radial surface (17) of said hub, so that said hub outer surface does not contact said disks or said spacers during thermal expansion or contraction. A disk drive as claimed in claim 2 or claim 3, wherein said clamping means further comprises a clamp ring (15) presenting a second support surface (16) for said disks, said clamp ring having a thermal shrink fit connection around said hub such that said first and second support surfaces maintain said compressive retaining force on said disks and said spacer. A disk drive as claimed in claim 4, wherein said annular groove has a depth such that the outer radial diameter of said spacer inside said annular groove is no larger than the inner radial diameter of said first and second support surfaces. A disk drive has claimed in claim 5, wherein the inner radial diameter of said first and second support surfaces is axially aligned with the outer radial diameter of said spacer inside said annular groove. A disk drive as claimed in any preceding claim, having at least three axially aligned disks, wherein annular spacers are alternated with said axially aligned disks such that there is an annular spacer between each adjoining pair of disks, with an annular groove extending radially inwards in at least the annular spacers that separate the last two disks at each axial end of the disk stack assembly. A disk drive as claimed in claim 7, wherein there is an annular groove extending radially inwards from the outer surface of every annular spacer. The use, in a disk drive including a disk stack assembly with: at least two axially aligned disks (8); an annular spacer (13) separating said at least two axially aligned disks, said spacer adjoining the inner diameter of said disks and having an annular groove (22) extending radially inward from the outer surface (23) of said spacer; and clamping means (6, 15) maintaining a compressive retaining force on said at least two disks and said spacer; of the spacer with the annular groove extending radially inward from its outer surface to distribute the compressive retaining force from the clamping means from a concentrated load to a more uniform load, thereby reducing the tendency of the disks to distort. The use of a disk drive as claimed in claim 9, wherein said clamping means comprises a hub (6) that passes through an axial hole in said spacer and each of said axially aligned disks, said hub including a flange (10) that presents a first support surface (11) for said disks. The use of a disk drive as claimed in claim 10, wherein the inner radial surfaces (20, 21) of said axially aligned disks and spacer are separated from the outer radial surface (17) of said hub, so that said hub outer surface does not contact said disks or said spacers during thermal expansion or contraction. The use of a disk drive as claimed in claim 10 or claim 11, wherein said clamping means further comprises a clamp ring (15) presenting a second support surface (16) for said disks, said clamp ring having a thermal shrink fit connection around said hub such that said first and second support surfaces maintain said compressive retaining force on said disks and said spacer. The use of a disk drive as claimed in claim 12, wherein said annular groove has a depth such that the outer radial diameter of said spacer inside said annular groove is no larger than the inner radial diameter of said first and second support surfaces. The use of a disk drive as claimed in claim 13, wherein the inner radial diameter of said first and second support surfaces is axially aligned with the outer radial diameter of said spacer inside said annular groove. The use of a disk drive as claimed in any of claims 9 to 14, wherein said annular spacer has an approximately U-shaped cross-section in the plane containing both the disk stack axial and radial directions, with the legs of the U-shape extending radially outwards. The use of a disk drive as claimed in any of claims 9 to 15, having at least three axially aligned disks, wherein annular spacers are alternated with said axially aligned disks such that there is an annular spacer between each adjoining pair of disks, with an annular groove extending radially inwards in at least the annular spacers that separate the last two disks at each axial end of the disk stack assembly. The use of a disk drive as claimed in claim 16, wherein there is an annular groove extending radially inwards from the outer surface of every annular spacer.

Inventors: BROOKS PETER EVERETT, JAN KIMBERLY GRAF, BROOKS, PETER EVERETT, JAN, KIMBERLY GRAF
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B17
IPC codes: G11B17, G11B23