CA1194993A - Disc mounting means - Google Patents

Abstract

Abstract of the Disclosure An apparatus is disclosed for centering one or more magnetic discs on a rotatable spindle. The apparatus includes a disc-carrying hub having a circumferential collar, a central housing supporting a centering ball, and a diaphragm joining the collar and housing, stiff in the radial direction but allowing axial movement of the centering element with respect to the collar. The hub is attracted to a rotatable spindle by an annular permanent magnet mounted on the spindle opposite a similarly sized armature mounted on the hub. A cup formed in the spindle at its center receives the centering ball and guides it to the center of spindle as the hub is attracted to the spindle. After the centering ball is seated in the center of the cup, the diaphragm flexes to allow continued axial movement of the collar to full engagement with the spindle. A second embodiment of the apparatus is a tool for centering a disc pack on a spindle, wherein shoulder screws extend through the tool and threadedly engage the spindle, to provide a hub-spindle attract-ive force in lieu of the magnet.

Description

This application is a division of ou] Canadian Patent Applicatio Serial No. 337,558 filed October 15, 1979.
This invention relates to memory devices including the discs and disc packs.
A flat and circular disc coated ~ith a magnetic reccrding material is a common type of memory device in computers. A single disc attached ~o a hub can be rotatably mounted on a spindle through tha hub, or a plurality of discs can be mounted on the spindle as a disc module or pack. In either case) a magnetic head is supported near the disc and is movable radially thereof.
Radial translation of the head, together with rotation of ~he disc, permits selective positioning of the head on the disc surface for reading or recording data In the case of a single disc mounted on a hub, it is often desirable to have interchangeability of single disc-hub assemblies on the same drive spindle. To insure convenience of changing discs, an annular permanent magnet mounted on the spindle is used to attract a correspondingly shaped and sized armature plate mounted to the aluminum hub carrying the disc. Attraction bet~een the magnet and armature plate maintains the hllb against the spindle.
In order to center the~hub on the spindleJ a female truncated cone is formed 2U i~ the prior art hub and adapted to center itself on a corresponding male truncated cone at the center of the spindle. A diaphragm bet~een the hub cone and periphery is sufficiently bendable so that ideally after the t~o cones are completely engaged, some continued flexure takes place permitting joining of ~he hub periphery and spindle periphery.
Due to the minute spacing bet~een adjacent data tracks o~ the disc recording surface, extremely accurate initial centering and repeatability of centering are vi-tal to proper recordlng and reading on the disc. Accuracy is impaired as the magnetic force can cause the disc to be pullecl to the spindle at the periphery before proper centering. This problem is recogni~ecl in United States Patent No. 3,706,085 to Mo~re~ ~ranted December 12, i~72 iil which the proffered solution is the replacement of the spindle permanent magnet with an electromagnet to be energized afte-r the centers are fit ~ogether. The premature off-center joinder can cause dents and nicks in the aluminum cast female cone of the hub due to sharp edges of the male cone and also from any foreign particles trapped between the two cones. Moreover, as damage could result from la even a minor mismatch in size, extremely accurate machining of the two cones is required.
A further impediment to accurate centering results from the manner in ~hich the aluminum hub is manufactured. Typically, this hub is die cast and then allo~ed to cool in ambient surroundings. The outer collar of ~he hub, ~eing significantly more massive than the diaphragm~ takes more time to cool~
As some shrinkage accompanies cooling, the continued shrinkage of the outer collar after the diaphragm has cooled can induce compressive and tensile stresses into the diaphragm. As force in the diaphragm must be dependent only upon diaphragm deflection for optimum centering accuracy, these residual 2Q stresses, uncertain in direction, can interfere with the elastic bending force.
In the case of a disc module mounted to the spindle, the problems inherent ~ith magnetic at~raction can be avoided if the module is mounted to the spindle at the manufacturing site, prior to the recording of servo tracts. If a problem develops in the field, however, the spindle and pack must be returned from the user as a unit for rework or replacement, at significant expense and lost timeO

- 2 --According to a brocld aspect of the invention there is provided apparatus for mounting a magnetic disc to a central hub, including; an annular hub having a circumferential collar including a flange positionable against an inner rim of said disc, and a step projec~ed axially from said flange a distance le.ss than the thickness of said disc; an annula:r clamping ring having a hub con-tact surface positionable agains* said step, a di5c contact surface positionable against said inner rim and generally radiall~ centered with respect to the con-tiguous area between said flange and disc, and means defining an annular groove in said clamping ring between said disc contact and hub contac* surfaces; and lQ means for fastening said clamping ring to said hub adapted to maintain said hub contact surface in engagement with said disc thereby causing said ring to deflect along said groove with said disc contact surface engaged with said disc.
A significant feature of the apparatus resides in the fact that a certain amount of connecting means deformation, herein diaphragm dcflection, is ~equired after the centering element is seated in the cup and before the collar is fully seated upon the collar-receiving surface of the spindle. In this manner, centering of the hub occurs subject only to minor frictional drag from hub tipping. Centering is complete before the occurrence of the substantial frictional force between the collar and spindle when ~ully engaged.
2Q In the preferred embodiment~ the centering element is a steel centering ball.

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having a llemispherical surface facing the spindle cup. The use of steel avolds the prior art problems of nicking and denting. Further as contact between the steel centering ball and spindle cup ta~es place over a much smaller contiguous area as compared to the prior art match:irlg cones, dust and other particles, rather than interfering wi~h centering~ ~re pulveri~ed under the vastly increased pressure. In this malmer the apparatus is self-cleaning.
The preferred em~odiments of the in~ention involve yet further improvements. For examples, the center of the centering elem~nt or ball can be positioned in axial alignment with the diaphragm. Consequently, forces directed through the -centering element in the Tadial direc~ion cannot produce bending moments upon the diaphra~n. Acting directly through the diaphragm, these forces more effectively overcome any friction between the collar and spindle. A further improvement relates to the method of manufacturing the hub. After the hub is cast, an annular groove is formed in the collar adjacent to the outer edge of the diaphragm. I~lis groove relieves stresses induced in the diaphragm by ur.even cooling, and further protects the diaphragm from potential stresses induced in further manufacturing processes.
The result is that flexure of the diaphragm depends almost entirely upon the forces induced by axial movement of the cente:ring element. Residual stresses do not materially interfere with the desired force-deflection re-lationship.
A fuxther improvement involves a clamping ring having therein an annular groove adjacent to the surface of the disc to be clamped against the hub. Due to the groove and positioning of the clamping ring disc-engaging surface, the tendency in the disc to deform from a planar axial configura-tion upon clamping is avoided.
Other features and advantages of the invention will become ~pparent upon reading the follo~ing detailed description and upon reference to the dra~irlgs in ~hich:
Figure 1 is a diagrammatical view of a prior art device for center~
ing a disc on a rotatable spindle;
Figure 2 is a digramma~ical view of the devicP of Figure 1 illustrat-ing inaccurate cente-ring;
Figure 3 is an elevational view of an apparatus according to the invention for centering a hub with respect to a spindle;
Figure 4 is an elevational sectional viel~ of a tooling and disc module attachable thereto for mounting to a rotatable spindle;
Figure 5 is an enlarged sec~ional view taken along the line 5-5 in Figure 4;
Figure 6 is a top view of the tool jig as viewed along the line 6-6 in Figure 4;
Figure 7 is a schematic view illustrating the progress o-f a center-ing element o the apparatus toward a centered po;ition; and ~ igure 8 is an enlarged partial view of Fi~ure 3, with parts removed to enhance clarity in illustration.
Turning to Figures 1 and 2 of the drawirlg, ~here is shown a prior ~o art device for rotatably driving a disc on a spindle assembly. A hub assembly carrying the disc includes an aluminum cast hub 10 upon which a disc 12 is fixed. The inner rim of disc 12 is seated against a collar 14 of hub 10 by an annular clamp 16 secured to the collar by a plurality of bolts 1~. An annular soft iron armature plate 19 is secured ~o the opposite surface of collar 14. A female hub centering cone 20 shaped as a truncated cone, is positioned in the center of hub lOr The female cone and collar 1 are joined by a flexible diaphragm 22.

3~3 ~ le spindle assembly includes an aluminum spindle 24 having mounted at its center a male steel centering cone 26~ Cone 26 is sh~ped for generall~
contiguous contact with female cone 20 of the hub. '~ne ~pindle assenibly further includes a perm~lent magnet 28 around the spindle periphery. Also at the periphery and surrounding the magnet is a sort iron pole piece 30.
Under ideal conditions, cones 20 and 26 are joined in face to face contact with one another. A slight amount of Flexure or bending in diaphragm 22 permits armature plate 19 to contact pole piece 30 around the en~ire circumference of the hub and spindle.
To overcome the force produced by bending of the diaphragm and in-sure the positive frictional contact necessary to impart rotation to the hub from the spindle, a strong magnetic force is necessary. One problem caused by the strength of the magnetic force is illustrated in Figure 2, where the left end of the hub as shown in Figure 2 has contacted the spindle prior to complete centering between cones 20 and 26. Female cone 20, being aluminum) is particularly vulnerable to nicking from the male cone as illus-trated herein at point 31. Further damage can result if any foreign part-icles are trapped between the two cones. Even thoughthere may be no damage, the fact that the cones contact one another over a relatively large area results in a contact pressure, over the contiguous area, insufficient to re-move or pulverize any dust particles trapped between the cones. Thus they remain to interfere with cen~ering accuracy.
Figure 3 illustrates an apparatus in accordance with the present invention to more accurately and reliably center a disc with respect to a rotatable spindle. A disc drive apparatus 32 includes a hub assembly having ~n aluminum die cast hub 34 on which is mounted a disc 36. Dis 36 is attach-ed to arigid outer rim or collar 3~ by a grooved clamping ring 40 attached to collar 3~ by a series of clamping bolts 42. ~l armature plate 44 of magnetiz-(:

able material is attached to collar 38. At the rotational center of hub 3 is an elongated steel centering peg 46 mourited within a housing 47. A
centering element or ball 48 has a hemispherical surface Whi.C}l is in contact with spindle centering cup 58 whenever hub 34 is positioned on the spindle.
A flexible diaphragm 50 joins to housing 47 and collar 38. ~iaphragm 50 is substantially rigid in the radial direction with respect to the hub. How-ever, it is bendable to allow centerlng peg 46, and thus centering ball 48, to move in either axial direction away rom a normal or unstressed position with respect to collar 38. An upright groove 52 is machined into the collar near the diaphragm and runs the circ~mferential length of the collar. An inner rim 54 adds structural stiffness near groove 52.
The spindle assembly includes a spindle 56 rotatable about a central axis and having at its center a centering cup 58. Cup 58 has a central base 60 adapted to cradle centering ball 48 when the hub and spindle are joined.
A permanent magnet 62 is attached at the periphery of spindle 56. A pole piece 64 of soft iron is attached adjacent to the magnet. A disc module or disc pack 66 is shown attached to spindle 56 by a plurality of module clamping bolts 67. The module includes first, second and third module discs, 68J 70 and 72 respectively. The module discs are contained between an outer flange 73 of a module hub 75, first and second spacers 74 and 76, and a module clamping ring 78 attached to the module hub by a plurality of clamping ring bolts 80. The module and spindle are rotatable with respect to a base 82.
An upper bearing assembly 84 and the lower bearing assembly S6 mount the spindle with respect to a spindle arbor 87. A motor 88 rotates the spindle via a belt 90 which drivably associates a spindle pulley 92 and a motor pulley 94. By selectively operating motor 88, the module discs can be placed in any desired rotational position while a read and record head positionable with respect to each disc, ls moved to the desired radial location. In this manner a desired portian of the disc ~ur;Eace area can be reached for reading or recording.
In Figure 3, hub 3~ is sho~n in its flllly mo~lted posit:ion. Center-ing ball 48 is fully cradled within cup 5~, and armatuIe pla~e 4~ is fully sea~ed against pole piece 64. In this position there is slight flexing of the diaphragm downward and radia]ly outward. A purpose of the flexure re~uirement is to insure that so long as the diaphragm is unstressed~ c0ntering ball 48 becomes fully seated in base 60 before armature plate '14 can become ully seated against pole piece 64. Hence, substantlal contact between the armature lQ plate and pole piece can not interfere with proper centering, for example by providing radial friction forces sufficient to resist movement of the hub to its centered position. It has been found that the preferred amount of diaphragm flexure in full engagement is that which displaces ball 48 .00~ inches axially from its zero force position with respect to collar 38. This varies with the geometry of the apparatus. The range of permitted displacement is between that crea~ing force sufficient to overcome such friction, and that creating a force as great as the magnetic attraction.
A notable feature of hub 34 is the positioning of centering ball 48 with respect to diaphragm 50. The center of the centering ball is axially 2~ ~ligned with the diaphragm. ~ecause the ball surface is hemispherical~ forces due to contact between the ball and cup 58, which forces act normal to the ball surface, are directed through the ball center. This is particularly important with respect to the radial components of these contact forces. These radial forces, ~horizontal as viewed in Figure 3), were they axially spaced from the plane of diaphragm 50, would create a bending moment about the diaphragm~ result-ing in a reduced radial stiffness between centering ball 48 and collar 38.
The reduced radial sti.ffness results in a higher degree of off center engagement.

Figures ~, 5 and 6 illustrate an alternative embodiment o~ the centering device, wherein the hub assembly is in the form of a tool jig 1030 A dust-cover disc 102 is attached to the tool jig at an outer rim 104 by a-plurality of bolts 106. A centering means lOS at the center of tool jig 1~0 includes a steel centering peg 108 press fit into a housing 110. A centsr-ing element or ball 112 prot.udes from the ~ool jig in an axially inward direction toward the spindle. A diaphragm 11~ connects the housing 110 with collar 104. Diaphragm 114 i~l the tool jig functions in a similar manner as did diaphragm 50 in ~he single disc hub. That is~ while diaphragm 114 is stiff in the radial direction, it is bendable to allow centering ball 112 to ve in either axial direction with respect to collar 104. A groove 116 is machined into collar 104 near the outer circumference of the diaphragm.
A first shoulder screw 120 and a second shoulder screw 121 are each extended through an oversized opening in a centering plate 122. A first compression spring 120A contained by a first washer 120B and a second compress-ion spring 121A contained by a second washer 121B, absorb, forces between ~heir respective shoulder screws and the centering plate to dampen its re-sponse to either screw. Plate 122 is pivotally connected to tool jig 100 by first and second pivotal dog set screws 124 and 125. Set screws 124 and 125 form a pivot axis normal to the tool jig axis of rotation. ~s perhaps best understood by viewing Figures 4, 5 and 6 in combination, a straight line passing through shoulder screws 120 and 121 would be perpendicular to the pivot axis formed by dog set screws 124 and 125~ and further would i~tersec~ the central axis. Shoulder screws 120 and 121 are equidistant from the pivot axis. Two plastic alignment pins 126 are positioned for generally aligning tool jig 100 with spindle 56. A series of tool jig bolts 128 are provided to connect the jig to module hub 75. First and second threaded opcnings 1~0 and 132, respectively are provided in spindle 56 for receiving shoulder screws 120 _9_ ,3 and 121.
In prac~ice, tool jig 100 is first mo~mted to disc module 66. The tool jig and disc module, as one assembly, are then subjected to the normal manufacturing processes of balancing~ writing of servo ~racks and error test-ing formatting. The assembly is then cantered on the spindle by tightening shoulder screws 120 and 121 into threaded openings 130 and 132 of spindle 56. The tightening of scrsws 120 and 121 creates an attractive force between tool jig 100 and spindle 56 which, with centering ball 112 in c~p 58, tends to draw ball 112 downward and toward the center of the cup. This arrang-ment insures that the resultant of the forces caused by theshoulderscrews C120,121), passes through the spindle rotational axis. This, in turn allows the tool jig 100 to center itself on the spindle centering cup 58 before frictional forces can be developed between the module hub 75 and spindle flange 136. I~hile in theory it would be possible to use a plurality of shoulder screws such as 120 and 121 without a centering plate such as 122, exactly simultane-ous tightening of all screws would be necessary, given the strict tolerance of centering required. It has been found that without centering plate 122, ~orces induced in the hub and centering peg from ~leven shoulder-screw tightening prevent accurate centering.
Tightening of screws 120 and 121 is continued until module hub 75 isbroughtinto full seating or face to face engagement with spindle ledge 136. At this point, the tool jig-module assembly and spindle are held together firmly and in centered relation.
With tool jig 100 centered and the sho~Ider screws completely tight-ened, clamping bolts 67 are tightened into threaded apertures in spindle 56. Module clamping bolts 67 are mounted in module 66 free of tool jig 100 and in oversized openings in hub 75 to allow lateral or radial movement relative to the module hub during centering. ~ith all module clamping bolts tightcned, -tool jig bolts 128 are released to separate jig 10û from module 66. This leaves the module connectcd to spindle 56 and exactly cen~er--ed.
A principal advantage of tool jig 100 is tha~. it enables on-site replacement of a moclule. The prior art requirecl shipmell~ of the module and the spindle as a unit bacl; to the manufacturing facility for rework or re-placement. Jig 100 enables servo tracks to be written with the module cen-tered on the tool. l'he tool-m~odule assembly is later centered Oll the spindle at the user's location and the tool alone removed. 'r]IUS, a significant amount 10 of tlme and labor is saved whenever a defective module must be replaced or repaired. Once removed from a centered module, the tool jig is ready or connection and processing with another module.
Figure 7 schematically shows the operation of the centering ball and cup. A centering peg 140, which could be part of either a hub' as in Figure ~ or a tool jig, as in Figure 4, is shown in broken lines at A and B, and in solid lines as centered at C within a centering cup 142. During centering, a centering ball 144 of peg 140 contacts the inside surace of cup 142, which is illustrated in an embodiment particularly advantageous in conjunction with the magnetic attraction between the hub and spindle. The in-20 side surface in part provides a base 146 adapted to cradle centering ball144 once -the ball has reached its centered position. Cup 142 further includes a guide surface 148 extending radially and axially outward from the base.
The base is inclined at an angle of 30 degrees from the central axis, while tlle guide surface is inclined at an angle of 60 degrees. The steeper slope of base 146 ensures positive cradling. The comparatively gen*le slope of the guide surface enables a wider capture range so that even approximate manual centering places ball 144 within the cup.
The sequence of hub centering is illustrated in steps showing the centering peg and ball in broken lines at A and B, and in solid lines in the cradled or cen~led position at C. The conditions necessary to initiate center-ing are that centering ball 144 be in contact with CUp 146J and i.:hat the attractive forcing means ~e.g., the magnet or shoulder screws) be operative in drawing the hub assembly axia]ly inward ~ ard spindle 143. Contact with the cup is insured by its size or ccapture range~ .Ind by ~olerances in the drive apparatus casing suff~ciently close to place ball 144 within the capture range upon manual, appro~imate centering. The forci~g means is present immediately upon manual positioning in ~:he case of the magnet; and with the shoulder screws, is created as they are threadedly turned into spindle 143.
The shape of cup 146 has been found advantageous, particularly in connection with a forcing means such as a magnet having a force proport-ional to the inverse square of the distance bet~een it and the armature plate. Base 146 is conically shaped with a slope of 30 degrees from the central axis, defining an overall angle of 60 degrees. The slope angle and overall angle of guide surface 148 measure 60 degrees and 120 degrees, respectively.
~ e advantage of the dual-slope surface of cup 146 can be understood from the sequence of centering. Initial ball contact with cup 146 is shown at position A, on guide surface 148 having a comparatively gradual slope.
Contact force, i.e., the force of the guide surface counter to gravity and the forcing means, acts perpendicular to the guide surface and thus acts through the center of ball 144. At this point the ball-centering radial component of the contact force (horizontal in Figure 7) is comparatively small. Ilowever, a small centering force is sufficient as the armature plate and magnet are sufficiently remote to avoid premature off-center engagement.
Even though the hub may tip and cause some armature-pole piece contact, no ~ (:
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material frictional drag is produced.
As the ball and peg reach position B, the ar~ature ancl magnet arenearer and the potential for ~rictional drag is signi~icantly increased.
Of course the magnetic attraction has similarly increased. More important, ho~ever, is that ball 144 at B contacts base 146, and the con~ac~ ~rce thus acts norma~ to the comparatively steep surface. The radial centering component is larger relative to the total contac~ force~ having increased from approximately half to approximatsly eighty-seven percent o ~he total.
rn short, by the time friction is potentially a problem, the centering force is strong enough to overcome it.
When centered at position C, contact forces are radially balanced to firmly cradle ball 144 within base 146. As ~mderstood from Figure 7~ the contiguous area is annular and narrow. The greatly reduced contiguous area as compared to the prior art cones, results in a greatly increased contact force per unit area. In fact, the pressure is such that foreign particles trapped between the steel ball and cup are pulverized and thus can not interfere with centering. In this manner the apparatus is self cleaning.
Figure ~ shows part of hub 34 to reveal groove 52 and rim 54 in greater detail. The purpose of groove 52 arises from the method of manufact--uring the hub, i.e., aluminum die casting. As the collar is quite massivecompared to the diaphragm, the diaphragm cools .nore rapidly. Shrinkage accompanies cooling, and hence the collar continues to shrink after the dia-phragm has substantially cooled. This introduces stresses, principally compressive into the diaphragm. Subsequent processing of the hub can intoduce additional stresses, all of which comb:ine to interfere with the de-sired force-deflection relationship in the diaphragm and thus interfere with centering accuracy.
~lub 34 is originally cast without groove 52. After the hub cools, however, a lathe and cutting tool is used to cut the groove into collar 38 near the circumference of diaphragm 50. The machining relieves the internal StreSSeS created by the uneven cooling, and further prevents the accumulation of internal stresses during subsequent processing of the hub. The result is that diaphragm 50 responds more predictably ~o the axially-~pplied forc-ing means and ball-cup contact force, which itself is axial once the ball is centered.
Sho~n in detail in Figure 8 is clamping ring 409 including an annular groove 150 between a hub contact surface 152 and a narrower disc contact sur-face 154. Disc 36 is positioned against a step 156 of the collar having a width slightly less than that of the disc thickness. Consequently with surface 154 contacting the disc, clamping ring 40 bends elastically as tighten-ing of bolt 42 brings surface 152 against the collar. The clamping force through surface 154 is generally centered with respect to clamping forces thTough an outer flange of the collar, an arrangement which maintains disc 36 in the desired axial plane.
Thus significant improvements in centering accuracy and repea~ability are achieved Usillg the apparatus disclosed. The ba l-cup interface directs the centering force through the ball center and thus to the collar directly through the diaphragm. Ball movement is thus translated substantially instantaneously to the collar, enabling it to move efficiently to the center-ed position. The flexing required of the diaphragm enables centering of the ball before any significant collar-spindle friction can develop. The groove machincd into the collar insures the desired force-deflection relation-ship in the diaphragm. Finally the steel cone cmd cup insure durability and enable self cleaning.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for mounting a magnetic disc to a central hub, including;
an annular hub having a circumferential collar including a flange positionable against an inner rim of said disc, and a step projected axially from said flange a distance less than the thickness of said disc;
an annular clamping ring having a hub contact surface positionable against said step, a disc contact surface positionable against said inner rim and generally radially centered with respect to the contiguous area between said flange and disc, and means defining an annular groove in said clamp-ing ring between said disc contact and hub contact surfaces; and means for fastening said clamping ring to said hub adapted to main-tain said hub contact surface in engagement with said disc thereby causing said ring to deflect along said groove with said disc contact surface engaged with said disc.