WO1993026006A1

WO1993026006A1 - Disk centering device

Info

Publication number: WO1993026006A1
Application number: PCT/US1993/004835
Authority: WO
Inventors: Thom S. Stalcup; Tave J. Fruge
Original assignee: Digital Equipment Corporation
Priority date: 1992-06-08
Filing date: 1993-05-21
Publication date: 1993-12-23

Abstract

A unitary centering device made of a pliant material is interposed between a cylindrical spindle and a plurality of annular disks in a disk drive. The centering device is corrugated to provide spaced-apart radial spring forces to hold the disks concentrically on the spindle during the assembly process. The corrugated shape also ensures that an air gap exists between the disks and the spindle, so that temperature cycling during operation does not result in radial disk shifting.

Description

Disk Centering Device

Field of the Invention

The invention relates to the manufacture of disk storage devices used in data processing systems.

Background of the Invention The heart of a disk drive is known as a spindle-disk assembly, which consists of several spaced-apart annular disks stacked upon a cylindrical spindle. During operation of the disk drive, the spindle-disk assembly rotates at high speed. When data is to be stored on a disk, a non-rotating magnetic head impresses a magnetic pattern which represents the data onto a circular track on the surface of the disk as it spins. During a subsequent data retrieval, the head is positioned over the same track on the disk to detect the impressed magnetic pattern.

To achieve high data storage density, modern disk drives have a large number of narrow, closely-spaced tracks for storing data on the surfaces of the disks. Due to the close spacing of the tracks, the mechanical tolerances of the spindle-disk assembly are very critical. If a disk is unduly eccentric with respect to the spindle, excessive tracking errors will result during operation. Therefore it is desirable for the disks to be mounted concentrically on the spindle, and for them to remain concentric throughout the operational life of the disk drive.

Disks can become eccentric with respect to the spindle as a result of thermally-induced radial shifting. As the temperature changes, the aluminum disks expand and contract differently than does the steel spindle. If the inner edge of a disk is in contact with the spindle when the temperature changes from some initial value to a new value, the disk may be forced to move radially to accommodate this differential expansion. As a result, when the temperature returns to the initial value, the disk is no longer in the same position relative to the spindle.

The problems of eccentricity and thermally-induced radial shifting are known to the art, and there are several different methods employed to overcome them. One approach is to reduce the tolerances between the spindle and the inner edge of the disk, so that the disk fits tightly on the spindle. While this approach achieves good initial concentricity, it renders assembly difficult and . actually increases the sensitivity of the assembly to thermally-induced radial shifting. Another approach is to employ very precise tooling in the assembly process so that the disk is placed precisely concentric with the spindle, but does not touch it. This approach also achieves good concentricity, but the required tooling is very expensive.

Yet another approach to the problem is described in U.S. Patent No. 4,754,351 issued Jun. 28, 1988 to Wright, entitled "Method and Apparatus for Controlling Radial Disk Displacement in Winchester Disk Drives". Using that method, multiple spaced-apart strips of a low-modulus plastic film such as Teflon™ are attached longitudinally to the spindle. The plastic strips provide a set of contact points which prevent the disk media from contacting the spindle, and thus reduce radial shifting of the disk. However, the strips do transmit forces between the spindle and the disk during thermal changes which could result in radial shifting of the disk. This method also requires preassembly of the plastic strips to the spindle prior to mounting the disks. Because it is time consuming and labor intensive, it is an expensive manufacturing step. Additionally, the disks may still require an additional balancing step after installation on the spindle.

In addition to the above-described methods for overcoming disk eccentricity and radial shifting, there are other known devices for installation between loosely fitting components to hold them rigidly spaced apart. An example of such a device is a rigid metallic ring which exerts constant high pressure between concentrically- mounted components such as, for example, a bearing and a rotating actuator. Because of their rigidity and ability to transmit large forces radially, such devices could contribute to thermally-induced radial disk shifting if used in a spindle-disk assembly.

It is desirable to have an easily-assembled spindle- disk assembly which exhibits excellent concentricity and resists thermally-induced radial disk shifting.

Summary of the Invention The invention in its broad form resides in a method of assembling a disc drive, as recited in claim 1. The invention also consists in a spindle disk assembly as broadly recited in claim 9.

Described herein is a process and apparatus for holding an annular disk concentrically on a cylindrical spindle while the disk and spindle are being assembled. In one broad sense, the invention includes a pliant unitary centering device used on a spindle and effective to provide spaced-apart radial resilient spring forces between the spindle and the inner edge of an annular disk placed over the centering device on the spindle. During assembly, the centering device is placed circumferentially on the spindle. Then the radial extent of the centering device is reduced to be less than the radius of the central opening of the annular disk. The disk is then placed on the spindle encircling the centering device, and the centering device is allowed to expand to contact the inner edge of the disk. The expansion of the centering device creates spaced-apart radial resilient spring.forces between the disk and the spindle sufficient to hold the disk concentrically on the spindle. There is no need to secure the centering device to the spindle prior to assembly. Prior to installation of the disk, the centering device is normally held in place by a tool. After the tool is removed, the spindle and disk compressively hold the centering device in place. Furthermore, because the centering device is free to expand throughout the annular gap, it provides uniform spaced-apart radial spring forces, resulting in excellent centering of the disk on the spindle. As a result, there is no need for a subsequent manufacturing step to balance the spindle-disk assembly.

Because the disk remains separated from the spindle by the annular air gap, the disk does not urge against the spindle during thermal expansion and contraction. The force that the pliant centering device transmits between the spindle and disk during thermal expansion or contraction is insufficient to cause radial disk shifting. Since radial disk shifting is eliminated, the disk drive achieves accurate tracking over a wide temperature range.

Brief Description of the Drawing A more detailed understanding of the invention may be had from the following description of a prefered embodiment, given by way of example and to be understood in conjunction with the acompanying drawing wherein: is an exploded view of a spindle-disk assembly in a disk drive embodying the invention; is a perspective view of a centering device used in the spindle-disk assembly of Figure 1; is a top view of the spindle-disk assembly of Figure 1; is a perspective view showing two retainers holding the centering device of Figure 2 during its installation in the spindle-disk assembly of Figure 1/ and is a perspective view showing three retainers holding the centering device of Figure 2 during its installation in the spindle-disk assembly of Figure 1.

Description of an Illustrative Embodiment

Figure 1 is an exploded view of a spindle-disk assembly for a 63mm (2-1/2") hard disk drive. Figure 1 shows a spindle motor or spindle 100 having a cylindrical outer surface. The spindle 100 has a flange 102 at one end, and a threaded collar 103 at the opposite end. A centering device 104 circumscribes the spindle 100.

Annular magnetic disks 105 and ring-shaped spacers 106 are alternately stacked on the spindle 100 surrounding the centering device 104. A clamp ring 107 is mounted against the outer magnetic disk 105. A clamp nut 108 attaches to the spindle collar 103, and acts with clamp ring 107 and the spindle flange 102 to compressively hold the assembly together and to transfer torque from the spindle 100 to the disks 105 during operation. After assembly, the spindle-disk assembly of Figure 1 is included in a disk drive by steps which are not described herein.

The centering device 104 operates during the assembly process to provide spaced-apart radial spring forces sufficient to hold the disks 105 and spacer 106 concentrically on the spindle 100. In addition, the centering device 104 is sufficiently pliant to allow differential thermal expansion of the disks 105 and spindle 100 during operation without resultant radial shifting of the disks 105. In practice, a corrugated centering device 104 has proven very effective. Figures 2 and 3 show in greater detail the centering device 104 and its relation to the spindle 100 and disks 105 of Figure 1. The centering device 104 is a unitary member effective to provide spaced-apart radial resilient spring forces between the spindle 100 and the disks 105. In the illustrated embodiment, both the corrugated shape of the centering device 104 and the material from which it is made contribute to the creation of these forces.

The centering device 104 consists of a pliant, moderately-elastic polymer film. Several types of plastic material may be used, such as polyester, polyimide, poloycarbonate, and fluorocarbon materials. In practice a polyester material known by the trade name Mylar® has been particularly suitable. The polymer film preferably holds a corrugated shape throughout the process of assembling the spindle-disk assembly, and also relaxes over time so that the stress that the centering device 104 places on the media and spindle ultimately diminishes. Additionally, the polymer film should be available in a suitable thickness so that it can be interposed between the spindle 100 and the disk 105 while maintaining a slight annular air gap.

The centering device 104 in Figure 2 has thickness T and width w„. The width W. is equal to the height of the spindle 100 along its longitudinal axis, which in the illustrated embodiment is approximately 9.5mm (0.375"). The width w. should not exceed the longitudinal width of the spindle 100, so that the centering device 104 is not deformed by the clamp ring 107 of Figure 1 when the clamp nut 108 is tightened during assembly. The centering device 104 is corrugated to form alternating transverse ridges 200 and valleys 201 along its corrugated length i__β. There is a corrugation cycle between adjacent valleys 201 spanning a corrugation period p and having a corrugation amplitude A. The corrugation of the centering device 104 may be accomplished, for example, by an embossing tool having a pair of opposing gears through which a rectangular, planar piece of film material is fed. There are several corrugation shapes which may be employed, such as triangular, sinusoidal, or rectangular. However, a smooth corrugation shape, such as the illustrated undulating shape, is preferred. When the centering device 104 has a smooth corrugation shape, it resists cracking and provides a smooth surface over which to slide the disks 105 and spacers 106 during assembly. While the alternating ridges 200 and valleys 201 are shown lying transversely in Figure 2, there may be other arrangements that may be employed as well. In particular, it is possible to create a diamond-shaped pattern by having two sets of ridges and valleys running at oblique angles with respect to each other. However, the transverse configuration shown in Figure 2 has been found to provide better support for the disks 105 of Figure 1 than that provided by a diamond-shaped pattern of ridges and valleys.

Figure 3 shows the centering device 104 installed in an annular gap 300 which exists between the spindle 100 and both the disks 105 and the spacers 106 (not shown in Figure 3) . The width W of the annular gap 300 is the difference between the radius of the spindle 100 and the radius of the circular inside edges of both the disks 105 and the spacers 106. In the embodiment of Figure 3, the radius of the spindle 100 is in the range of 9.863 to 9.872mm ( 0.3883" to 0.3887"), while the radius of the circular inside edges of the disks 105 and spacers 106 is in the range of 9.989 to 9.999mm (0.3933" to 0.3937"). Therefore the width W-. of the annular gap 300 is in the range of 0.1168 to 0.1371mm (0.0046" to 0.0054").

The thickness T of the centering device 104 is preferably less than the width W_g of the annular gap 300 so that there remains an air gap when the centering device 104 is present in the gap 300. However, the thickness T must also be large enough so that the centering device 104 is sufficiently strong and stiff to hold the disks 105 and spacers 106 concentric with the spindle 100 during assembly. The optimum value for the thickness T therefore depends upon the material and shape used for the centering device 104, as well as the width -- of the annular gap 300. In the illustrated embodiment, the thickness T is in the range of 0.0457 to 0.0558mm (0.0018" to 0.0022").

The corrugation period P of the centering device 104 should be small relative to the circumference of the spindle 100 so that there are numerous corrugation cycles around the spindle 100. This provides for uniform pressure throughout the annular gap 300 when the centering device 104 is installed between the spindle 100 and the disks 105 and spacers 106. While eight cycles are shown in the simplified embodiment of Figures 2 and 3, there are typically many more. In practice a corrugation period or pitch P of approximately 2.54mm (0.1") has been effective when used with a spindle 100 having a circumference of approximately 60.96mm (2.4") . Such a centering device 104 has approximately 24 corrugation cycles rather than the eight shown in Figures 2 and 3.

The relaxed corrugation amplitude A should be chosen to be greater than the width W-- of the annular gap 300, so that when the centering device 104 is compressively held within the gap 300 it provides sufficient radial spring forces to hold the disks 105 and spacers 106 concentrically with the spindle 100 during assembly. In the illustrated embodiment, the relaxed corrugation amplitude A is in the range 0.254 to 0.381mm of (0.010" to. 0.015").

The centering device 104 generates spaced-apart radial spring forces in the range of 13.79 to 62.05 kPa (2 to 5 p.s.i.) when installed in the annular gap 300. When fully compressed, the centering device 104 generates forces in the range of 55.16 to 62.05kPa 8 to 9 p.s.i. These forces are large enough to move the disks 105 and spacers 106 to a concentric position on the spindle 100 during assembly, and small enough to allow the spindle 100 and the disks 105 to expand and contract without inducing radial disk shifting. The centering device 104 has an uncorrugated length i-_u, not shown in Figure 2, which is the length of the strip of film material before being corrugated. Due to the corrugation, the corrugated length L_β is less than the uncorrugated length 1^. The relationship between the lengths I-,, and L₀ is a function of the circumference of the spindle 100, the width W_g of the gap 300, and the corrugation pattern, among other things. The uncorrugated length I-,, is preferably sufficient to allow the centering device 104 to substantially circumscribe the spindle 100 when the centering device 104 is compressed within the annular gap 300. In the illustrated embodiment, the uncorrugated length ,. is approximately 61.21mm (2.41"), and the corrugated length L_α is approximately 55.88mm (2.20"). While in Figure 3 there is a slight gap between the ends of corrugated length L_c, it may be advantageous in other embodiments for the ends to meet or even to overlap slightly. After the centering device 104 has been fabricated, it is used in the process of assembling the spindle-disk assembly. The corrugation of the centering device 104 is preferably done shortly before the beginning of the assembly process, so that the centering device 104 has a predictable form and spring constant during assembly.

Figure 4 illustrates a method of holding the centering device 104 in place during assembly of the spindle-disk assembly of Figure 1. The spindle 100 is mounted on a stand, which is not shown in Figure 4. The centering device 104 is placed on the spindle 100, and two semicircular retainers 401 are used to radially compress the centering device 104. Compressing the centering device 104 reduces its radial extent so that the disks 105 and spacers 106 of Figure 1 fit over the centering device 104. It should be noted that the retainers 401 are so operated during assembly, that there is enough room to install at least the first disk 105 while the retainers

401 are holding the centering device 104 radially pressed.

After the centering device 104 has been compressed by the retainers 401, the first disk 105 is placed on the spindle 100 over the centering device 104. At this point, the retainers 401 are removed to make room for the remaining disks 105 and spacers 106. When the retainers 401 are removed, the centering device 104 expands to contact the inner edge of the previously-installed disk 105, which acts to hold the centering device 104 in place. Then the spacers 106 and remaining disks 105 are alternately stacked on the spindle 100 over the centering device 104 and pushed toward the flange 102.

It should be noted that the centering device 104 is compressively held in place by the spindle 100 and the disks 105 and spacers 106. There is no need to attach the centering device 104 to the spindle 100 prior to the assembly step shown in Figure .

After all of the disks 105 and spacers 106 have been installed, the clamp ring 107 and clamp nut 108 are installed to compressively hold the stack of disks 105 and spacers 106 against the flange 102 of the spindle 100. A small air gap is maintained between the disks 105 and the spindle 100, so that subsequent temperature cycling tends not to result in radial displacement of the disks 105. Over time, the centering device 104 relaxes, so that the stress that it applies to the spindle-disk assembly diminishes.

Figure 5 illustrates an alternative method of holding the centering device 104 in place during assembly of the spindle-disk assembly of Figure 1. In Figure 5, three curved retainers 501 are used to compressively hold the centering device 104 instead of the two semicircular retainers 401 of Figure 4. The retainers 501 shown in Figure 5 each cover approximately 120 degrees, or one- third, of the circumference of the spindle 100; however, other configurations are possible. The use of three retainers 501 may be advantageous to avoid pinching the centering device 104 between the ends of adjacent retainers 501 during assembly.

While the invention has been described as it relates to a specific embodiment, there are other possible embodiments. Although a multiple-disk assembly has been described, the invention may be used with a spindle-disk assembly having only a single disk. Also, the disks may be mounted on any generally cylindrical spindle, whether or not it has an integral motor. The centering device may be formed in a variety of ways from a number of pliant materials. These and other variations fall within the scope of the invention.

Claims

Claims WHAT IS CLAIMED IS:

1. A method of assembling a disk drive which comprises a spindle and at least one disk with an inner periphery, comprising the steps of: placing a unitary radially resilient annular centering device circumferentially around a cylindrical spindle; resiliently reducing the radial extent of said centering device to less than a radius of said inner periphery of a disk to be mounted on said spindle; placing said disk on said spindle such that the inner periphery of said disk encircles said resilient annular centering device; and allowing said centering device to its normal state to support to expand to hold said disk concentrically on said spindle.

2. A method of assembling a disk drive according to claim 1, wherein said reducing step is accomplished by compressing said centering device with two opposed substantially semicircular retainers.

3. A method of assembling a disk drive having a according to claim 1, wherein said reducing step is accomplished by compressing said centering device with three arcuate retainers.

4. A method of assembling a disk drive according to claim 1, further comprising the step of corrugating a piece of pliant resilient material to form said unitary radially resilient annular centering device.

5. A method of assembling a disk drive according to claim 4, wherein said corrugating step comprises the step of feeding pliant material through an embossing tool having a pair of opposed gears.

6. A method of assembling a disk drive according to claim 1, further comprising the step of installing a clamp ring and a clamp nut on said spindle to compressively hold siad disk on said spindle.

7. A method of assembling a disk drive having a plurality of annular disks and interposed annular spacers mounted on a cylindrical spindle having a flnage at one end, comprising the steps of: placing a unitary centering device circumferentially around said spindle; radially compressing said centering device with a retaining tool to reduce the radial extent of said centering device to less than the inner radius of an annular disk to be mounted on said spindle; placing a first one of said disks on said spindle, such that said first one of said disks encircles said centering device to subsequently retain said centering device upon removal of said retaining tool; removing said retaining tool, thereby allowing said centering device to expand to the inner edge of said first one of said disks; alternately pushing said spacers and the remaining ones of said disks onto said spindle; and installing a clamp ring and a clamp nut on said spindle, thereby compressively holding said disks and said spacers in a stack against said flange on said spindle.

8. A spindle-disk assembly for a disk drive, comprising: a cylindrical spindle; an annular disk mounted on said spindle and separated therefrom by an annular gap; and a unitary radially resilient centering device disposed within said annular gap to provide spaced-apart radial spring forces to hold said disk concentrically on said spindle .

9. A spindle-disk assembly for a disk drive, comprising: a cylindrical spindle having a flange at one end; a plurality of annular disks and interposed annular spacers mounted on said spindle and separated therefrom by an annular gap; a unitary centering device disposed within said annular gap providing spaced-apart radial spring forces to hold said disks and said spacers concentrically on said spindle; and clamping means for compressively holding said disks and said spacers in a stack against said flange on said spindle.

10. A spindle disk assembly as recited in claim 9 wherein said unitary radially resilient centering device comprises polymer film material having a thickness range of 0.0457mm to 0.0558mm, with a corrugation pitch of 0.254 mm to 0.381 mm.

11. A spindle disk assembly as recited in claim 10 wherein said unitary radially resilient centering device exerts a radial pressure of 55.16 to 62.05kPa.