Method of making a spindle motor to allow magnetic alignment between the magnet and the stator in the motor

A spindle motor in a disc drive has a rotor assembly rotatably mounted to a base. A stator is mounted to the base and a magnet is mounted to the rotor assembly. The magnet or the stator is allowed to move in an axial direction relative to the other under the influence of magnetic interaction between the magnet and the stator. The magnet and stator are then rigidly fixed relative to one another in the axial direction.

BACKGROUND OF THE INVENTION 
The present invention relates to disc drives. More specifically, the 
present invention relates to a spindle motor in a disc drive and a method 
of making the motor. 
A typical disc drive includes one or more magnetic discs mounted for 
rotation on a hub or spindle. A typical disc drive also includes one or 
more transducers supported by a hydrodynamic air bearing which flies above 
each magnetic disc. The transducers and the hydrodynamic air bearing are 
collectively referred to as a data head. A drive controller is 
conventionally used for controlling the disc drive system based on 
commands received from a host system. The drive controller controls the 
disc drive to retrieve information from the magnetic discs and to store 
information on the magnetic discs. 
An electromechanical actuator operates within a negative feedback, 
closed-loop servo system. The actuator moves the data head radially over 
the disc surface for track seek operations and holds the transducer 
directly over a track on the disc surface for track following operations. 
Information is typically stored on the magnetic discs by providing a write 
signal to the data head to encode flux reversals on the surface of the 
magnetic disc representing the data to be stored. In retrieving data from 
the disc, the drive controller controls the electromechanical actuator so 
that the data head flies above the magnetic disc, sensing the flux 
reversals on the magnetic disc, and generating a read signal based on 
those flux reversals. The read signal is then decoded by the drive 
controller to recover the data represented by flux reversals stored on a 
magnetic disc, and consequently represented in the read signal provided by 
the data head. 
Accurate positioning of the data head over a track on the disc is of great 
importance in writing data to the disc and reading data from the disc. 
In current generation disc drive products, the most commonly used type of 
actuator is a rotary moving coil actuator. The discs themselves are 
typically mounted in a "stack" on the hub structure of a brushless DC 
spindle motor. The rotational speed of the spindle motor is precisely 
controlled by motor drive circuitry which controls both the timing and the 
power of commutation signals directed to the stator windings of the motor. 
The hub structure is rotatably coupled to a shaft via a set of bearings. 
The hub includes a sleeve or back iron portion which carries a magnet. 
Interaction between the magnet and the stator causes controlled rotation 
of the hub about the shaft. 
Motor acoustic levels are related to the degree of alignment between the 
stator and the magnet, both axially (along the axis of rotation of the 
hub) and radially. Alignment of the magnet and stator in both the axial 
and radial directions is difficult to control during volume production of 
disc drive motors due to tolerances incurred in producing the magnet and 
stator parts. 
In addition, alignment in the axial direction is difficult because the 
"magnetic center" of the hub does not necessarily correspond to the 
"geometric center". In other words, a certain imbalance in the flux field 
can be caused by the configuration of the back iron. This results in the 
magnetic center of the magnet relative to the stator being slightly 
different than the precise geometric center of the two parts. 
Currently, the magnet and stator are axially aligned using nominal 
dimensions. However, the tolerances on the parts allow up to 0.3 mm of 
misalignment between the two parts. Further, even if the two parts are 
perfectly geometrically aligned, they may well not be magnetically 
aligned. Both of these misalignments can be problematic during operation 
of the drive motor. 
SUMMARY OF THE INVENTION 
A spindle motor in a disc drive has a rotor assembly rotatably mounted to a 
base. A stator is mounted to the base and a magnet is mounted to the rotor 
assembly. The magnet or the stator is allowed to move axially relative to 
the other under the influence of magnetic interaction between the magnet 
and the stator. The magnet and stator are then rigidly fixed relative to 
one another in the axial direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a side sectional view of a disc drive motor 10 according to the 
present invention. Drive motor 10 includes a base 12 and a shaft 14 
fixedly attached to base 12. An axis of rotation 16 is generally defined 
by the longitudinal axis of shaft 14. Motor 10 also includes rotor 
assembly 17. Rotor assembly 17 includes bearings 18 and 20, bearing holder 
22, hub 30 and magnet 
Bearings 18 and 20 are disposed about shaft and are supported by bearing 
holder 22. Labyrinth seal 24 and ferro-fluid seal 26 are disposed 
generally at opposite axial ends of bearing holder 22 and seal a bearing 
compartment 28 which holds bearings 18 and 20. Hub 30 is fixedly coupled 
to bearing holder 22 and has a flange 32. Flange 32 supports magnetic 
discs (not shown) for rotation about axis 16. 
Bearing holder 22 includes a back iron or sleeve portion 34 which is 
generally cylindrical in shape and is preferably formed integrally with 
bearing holder 22. Seal holder 36 is formed of a non-magnetic material and 
is mounted to an inner surface of sleeve 34 to support ferro-fluid seal 
26. 
Sleeve 34 has an outer periphery defined by a generally cylindrical surface 
38. Magnet 40 is coupled to outer surface 38 of sleeve 34. Magnet 40 is a 
generally annular-shaped magnet disposed about outer surface 38. 
Stator windings 42 are rigidly attached to base 12 and are selectively 
provided with commutation signals. Interaction between the magnetic field 
generated by stator 42 and magnet 40 causes controlled rotation of rotor 
assembly 17 about shaft 14. 
It is desirable, in assembling motor 10, to have magnet 40 axially aligned 
(i.e., in a direction parallel to axis 16) with stator 42. It is 
preferable to have the axial alignment correspond to magnetic axial 
alignment, rather than simply geometric axial alignment. However, due to 
dimensional tolerances in manufacturing the parts of stator 42 and magnet 
40, misalignment of as much as 0.3 mm can occur in motor 10. Further, even 
if magnet 40 is coupled to sleeve 34 at the precise geometric center of 
stator 42, this may not result in optimal motor performance. If sleeve 34 
is formed of magnetic material, an imbalance results in the flux field 
which can cause a shift in the magnetic center between magnet 40 and 
stator 42 from the geometric center. 
With the present invention, this problem is substantially overcome. Magnet 
40 is initially movably coupled to sleeve 34 so it can magnetically align 
itself with stator 42. Once aligned, magnet 40 is rigidly connected to 
sleeve 34. 
In the preferred embodiment, magnet 40 is assembled onto sleeve 34 
utilizing an application of a slow curing adhesive 44 between magnet 40 
and sleeve 34. Thus, after initially being mounted on sleeve 34, magnet 40 
is movable in the axial direction, until the adhesive layer 44 cures. 
While magnet 40 is still movable, rotor assembly 17 is mounted onto base 
12, and magnet 40 is brought into a position proximate stator 42. Magnetic 
interaction between stator 42 and magnet 40 induces a force on magnet 40 
tending to center magnet 40 axially relative to stator 42. Since magnet 40 
is movable in the axial direction (because the adhesive 44 bonding magnet 
40 to sleeve 34 has not cured) magnet 40 is free to move in the axial 
direction to a position which is magnetically aligned with stator 42. Once 
in this position, the adhesive 44 is allowed to cure thus rigidly holding 
magnet 40 in its magnetically centered position. This produces enhanced 
performance of motor 10. In the preferred embodiment, the adhesive 44 is a 
commercially available adhesive identified by the trade name DP-460 
manufactured by Minnesota Mining and Manufacturing of St. Paul, Minn., or 
any other suitable adhesive. 
FIG. 1A shows a second embodiment of a motor according to the present 
invention. The motor shown in FIG. 1A is the same as motor 10 shown in 
FIG. 1, except that slow curing adhesive 44 is applied between stator 42 
and base 12. Further, magnet 40 is rigidly attached to sleeve 34. 
Therefore, when rotor assembly 17 is mounted onto base 12, stator 42 is 
movable relative to base 12, and relative to rotor assembly 17. Magnetic 
interaction between stator 42 and magnet 40 induces a force on stator 42 
tending to center stator 42 axially relative to magnet 40. Since stator 42 
is movable in the axial direction (because the adhesive 44 bonding stator 
42 to base 12 has not cured) stator 42 is free to move in the axial 
direction to a position which is magnetically aligned with magnet 40. Once 
in this position, the adhesive 44 is allowed to cure thus rigidly holding 
stator 42 in its magnetically centered position. 
FIG. 2 shows a third embodiment of a motor 50 according to the present 
invention. Motor 50 is similar to motor 10 shown in FIG. 1, and similar 
parts are correspondingly numbered. However, bearing holder 22 shown in 
FIG. 1 is formed of a magnetic steel material. Since back iron or sleeve 
34 is integrally formed with bearing holder 22, sleeve 34 is also formed 
of magnetic steel material. An overhang portion 52 of sleeve 34 shown in 
FIG. 1 extends radially outwardly from axis 16 over magnet 40. Magnet 40 
may be attracted to overhang portion 52 or other areas on magnetic bearing 
holder 22 or sleeve 34 which are undesirable. In order to overcome this 
problem, motor 50 incorporates a number of differences over motor 10 shown 
in FIG. 1. 
First, motor 50 includes a bearing holder 52 which is formed of 
non-magnetic steel material. Any suitable non-magnetic steel material can 
be used. Thus, sleeve 58 (preferably integrally formed with bearing holder 
52) is also formed of non-magnetic material). 
In addition, rather than having magnet 40 simply being adhered to sleeve 
58, a back iron annulus 54 is formed of magnetic steel material. Back iron 
54 has an outer diameter which closely approximates the inner diameter of 
magnet 40. Magnet 40 and back iron 54 are rigidly attached to one another 
to form a magnet assembly 56. Magnet assembly 56 is then assembled onto 
sleeve 58 in a manner similar to that in which magnet 40 is assembled onto 
sleeve 34 in FIG. 1. In other words, a slow curing adhesive layer 44 is 
applied to the junction between the inner diameter of back iron 54 and the 
outer diameter of sleeve 58. The magnet assembly 56 is then placed 
proximate stator 42 and allowed to magnetically align itself axially with 
stator 42. Once this alignment has taken place, motor 50 is held in a 
position so that magnet assembly 56 does not move until the slow curing 
adhesive 44 cures to rigidly attach magnet assembly 56 to sleeve 58. 
By providing bearing holder 52 of non-magnetic steel, another advantage is 
achieved. In motor 10 shown in FIG. 1, ferro-fluid seal 26 is coupled to 
bearing holder 22 through seal holder 36. Seal holder 36 is formed of 
non-magnetic material to isolate the magnets in ferro-fluid seal 26 from 
sleeve 34. This allows the magnets to operate so as to generate flux lines 
which hold the ferro-fluid in place. However, since bearing holder 52 in 
the embodiment shown in FIG. 2 is formed of non-magnetic material, the 
magnets in ferro-fluid seal 26 are directly connected to bearing holder 
52. This eliminates the need for bearing holder 36 which, in turn, 
eliminates assembly steps in assembling motor 50. 
In should also be noted that, in the preferred embodiment, radial alignment 
of magnet 40 with respect to rotor assembly 17 and stator 42 is achieved 
using the magnet assembly system shown in FIG. 2. The machining of annular 
back irons 54 can be done with a greater degree of precision than the 
forming of magnets 40. Therefore, annular back irons 54 are preferably 
made with a plurality of different outside diameters, but with a constant 
inside diameter. Magnets 40 are then preferably sorted according to their 
inside diameter dimensions. As stated previously in the specification, 
this dimension is variable because of the manufacturing tolerances for 
magnet 40. These tolerances are typically quite loose tolerances relative 
to tolerances on the other parts in the motor. 
After the magnets 40 are sorted, each magnet is assembled onto an 
appropriately sized annular back iron 54. This results in a reduction in 
any radial misalignment which may occur due to manufacturing tolerances of 
the parts. By reducing radial misalignment, the weight distribution (or 
balance) of the rotor assembly 17 is improved. This leads to enhanced 
motor performance. 
FIG. 3 shows a fourth embodiment of a motor 70 according to the present 
invention. Whereas motors 10 and 50 in FIGS. 1 and 2, respectively, are 
fixed shaft motors, motor 70 in FIG. 3 is formed in a rotating shaft 
design. Motor 70 includes a hub assembly 72 and bearing holder assembly 
74. In the preferred embodiment, hub assembly 72 includes hub sleeve 76 
and annular magnet 78. Annular magnet 78 is fixedly attached to an inner 
peripheral surface of hub sleeve 76 by a slow curing adhesive layer 80. 
Adhesive layer 80 is preferably formed of the same adhesive used in the 
embodiments shown in FIGS. 1 and 2. Hub sleeve 76 preferably has an 
integrally formed annular flange 82. Annular flange 82 provides a support 
for the magnetic discs which are disposed about hub sleeve 76 for rotation 
about axis of rotation 16. 
Also, in the preferred embodiment, hub sleeve 76 has an integrally formed 
shaft portion 84. Shaft portion 84, along with the remainder of hub 
assembly 72, is supported for rotation relative to bearing holder assembly 
74 by a set of bearings 86 and 88. 
Bearing holder assembly 74 includes a generally cylindrical sleeve portion 
90 which is fixedly coupled to a base (not shown). Sleeve 90 includes a 
pair of annular notches 92 and 94 which are disposed in sleeve 90 to 
support bearings 86 and 88. 
FIG. 3 also shows that stator assembly 95 is disposed about an outer 
periphery of sleeve 90. Stator assembly 95 includes laminated core 96 and 
coil 98. Interaction between the magnetic field generated by stator 
assembly 95 and magnet 78 causes controlled rotation of hub assembly 72 
about axis 16. 
As with the embodiments shown in FIGS. 1 and 2, hub sleeve 76 is preferably 
assembled over the outer radial periphery of magnet 78 utilizing an 
application of slow curing adhesive 80 between magnet 78 and hub sleeve 
76. Thus, after initially being mounted within hub sleeve 76, magnet 78 is 
movable in the axial direction, until adhesive layer 80 cures. While 
magnet 78 is still movable, hub assembly 72 is mounted into bearing holder 
assembly 74, and magnet 78 is brought into a position proximate stator 
assembly 95. Magnetic interaction between stator assembly 95 and magnet 78 
induces a force on magnet 78 tending to center magnet 78 axially relative 
to stator assembly 95. Since magnet 78 is movable in the axial direction 
(because the adhesive layer 80 bonding magnet 78 to hub 76 has not cured) 
magnet 78 is free to move in the axial direction to a position which is 
magnetically aligned with stator assembly 95. Once in this position, the 
adhesive forming adhesive layer 80 is allowed to cure thus rigidly holding 
magnet 78 in its magnetically centered position. 
It should also be noted that a magnet assembly, such as magnet assembly 56 
shown in FIG. 2, could also be used with the embodiment shown in FIG. 3. 
In such an embodiment, a back iron 100 shown in phantom in FIG. 3 is 
rigidly attached to magnet 78 to form a magnet assembly. Back iron 100 is 
formed of magnetic steel material and hub sleeve 76 is formed of 
non-magnetic material. The magnet assembly is inserted within hub sleeve 
76 and bonded thereto by the slow curing adhesive described above. This 
allows the magnet assembly to magnetically center itself before being 
rigidly affixed to hub sleeve 76. The same type of sorting as that 
described with reference to FIG. 2 (i.e., sorting according to size of 
back iron 100 and magnet 78) is also preferably performed to reduce radial 
misalignments. 
FIG. 3A shows a fifth embodiment of the present invention. Motor 70 shown 
in FIG. 3A is the same as motor 70 shown in FIG. 3, except that adhesive 
layer 80 is applied between stator assembly 95 and sleeve 90. This allows 
stator assembly 95 to center itself with respect to magnet 78 during 
assembly. 
FIG. 4 is a side sectional view of a sixth embodiment according to the 
present invention. Motor 101 of FIG. 4 is similar to the motors in FIGS. 1 
and 2 in that it is a fixed shaft design. However, FIG. 4 is also similar 
to FIG. 3 in that a hub sleeve carries the annular magnet on its inner 
peripheral surface. 
FIG. 4 includes rotor assembly 102 and stator assembly 104. Rotor assembly 
102 includes hub sleeve 106, bearing holder sleeve 108 and annular magnet 
110. Hub sleeve 106 has an annular flange 112 which supports magnetic 
discs mounted about hub sleeve 106. Rotor assembly 102 and the discs 
mounted on flange 112 are mounted for rotation about axis of rotation 16. 
Bearing holder sleeve 108 is rigidly attached to hub sleeve 106. Annular 
magnet 110 is fixedly attached to an inner peripheral surface of bearing 
holder sleeve 108 by a slow curing adhesive layer 114. Adhesive layer 114 
is preferably formed of the same adhesive used in the embodiments shown in 
FIGS. 1-3. Stator assembly 104 includes shaft 116, the center axial 
portion of which generally defines axis 16. Shaft 116 has bearings 118 and 
120 mounted about the periphery of shaft 116 so that hub assembly 102 is 
rotatable relative to shaft 116. 
Stator assembly 104 further includes laminated core 122 and stator windings 
124. Interaction between the magnetic field generated by stator assembly 
104 and magnet 110 causes controlled rotation of rotor assembly 102 about 
axis 16. 
As with the embodiments shown in FIGS. 1-3, hub sleeve 106 and bearing 
holder sleeve 108 are preferably assembled over the outer radial periphery 
of magnet 110 utilizing an application of slow curing adhesive 114 between 
magnet 110 and bearing holder sleeve 108. Thus, after initially being 
mounted within hub sleeve 106, magnet 110 is movable in the axial 
direction, until adhesive layer 114 cures. While magnet 110 is still 
movable, hub assembly 102 is mounted over stator assembly 104, and magnet 
110 is brought into a position proximate stator assembly 104. Magnetic 
interaction between stator assembly 104 and magnet 110 induces a force on 
magnet 110 tending to center magnet 110 axially relative to stator 
assembly 104. Since magnet 110 is movable in the axial direction (because 
the adhesive layer 114 bonding magnet 110 to bearing holder sleeve 108 has 
not cured) magnet 110 is free to move in the axial direction to a position 
which is magnetically aligned with stator assembly 104. Once in this 
position, the adhesive forming adhesive layer 114 is allowed to cure thus 
rigidly holding magnet 110 in its magnetically centered position. 
It should also be noted that a magnet assembly, such as magnet assembly 56 
shown in FIG. 2, can also be used with the embodiment shown in FIG. 4. In 
such an embodiment, a back iron 126 (shown in phantom in FIG. 4) is 
rigidly attached to magnet 110 to form a magnet assembly 128. Back iron 
126 is preferably formed of magnetic steel material, and bearing holder 
sleeve 108 is formed, in this preferred embodiment, of non-magnetic 
material. The magnet assembly 128 is inserted within the hub sleeve 106 
and bonded thereto by the slow curing adhesive described above. This 
allows the magnet assembly 128 to magnetically center itself before being 
rigidly affixed to hub sleeve 106. The same type of sorting according to 
size of back iron 100 and magnet 78 as described with reference to FIG. 2 
is also preferably performed to reduce radial misalignments in the motor. 
FIG. 4A shows a seventh embodiment of a motor according to the present 
invention. Motor 101 in FIG. 4A is similar to motor 101 in FIG. 4, except 
that adhesive layer 114 is applied between stator assembly 104 and the 
shaft defining axis of rotation 16. Thus, stator assembly 104 centers 
itself axially relative to magnet 110 during assembly of motor 101. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention.