Method of manufacturing fluid dynamic bearing assembly

In a method of manufacturing a dynamic bearing assembly having a shaft and a sleeve whose inner circumferential surface faces an outer circumferential surface of the shaft, the shaft is inserted into a bearing hole from an upper end opening toward a bottom end opening of the bearing hole. Upon inserting the shaft into the bearing hole, fluid is fed from the bottom end opening toward the upper end opening of the bearing hole with a fluid feed apparatus.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to methods of manufacturing a fluid dynamic bearing assembly. More particularly, the present invention generally relates to a method of manufacturing a fluid dynamic bearing assembly to enable the bearing-assembly shaft to be inserted into the bearing hole in a sleeve without scratching the inner circumferential surface of the sleeve or the outer circumferential surface of the shaft.

2. Description of the Related Art

In manufacturing the bearing assembly including a shaft and a hollow sleeve having a bearing hole, axially penetrating the sleeve, the shaft needs to be inserted into the bearing hole from one end opening toward another end opening. To insert the shaft into the bearing hole, a conventional assembling machine of the bearing assembly, shown inFIG. 7, may be used.

As shown inFIG. 7, the conventional assembling machine includes a sleeve cradle101supporting the sleeve3, a vacuum line block102fixing the sleeve3and the sleeve cradle101by vacuuming up, a shaft insertion guide103having a guide hole103aleading the tip end of the shaft4into the bearing hole3aprovided on the sleeve3.

Moreover, the sleeve cradle101includes a plurality of suction holes101avacuuming up and fixing the sleeve3. The vacuum line block102includes a vacuum line102a. In addition, an airpool portion102is provided between the sleeve cradle101and the vacuum line block102. The vacuum line102is connected to a vacuum forming machine (not shown in Fig.) through a vacuum line105whose one end is connected to the vacuum line block102, another end is connected to the vacuum forming machine.

The conventional assembling machine with the configuration mentioned above vaccumizes the airpool104, the suction hole101a, the vacuum line102, and the vacuum line105, such that the sleeve3is aspirated and fixed to the sleeve cradle101. Subsequently, insertion guide103is adjusted so as to lead the shaft4into the guide hole103a. Under the condition, the shaft4is inserted into the bearing hole3athrough the guide hole103aof the shaft insertion guide103(seeFIG. 7A).

Upon inserting the shaft4, a position of the shaft insertion guide103is adjusted by an actuator, such as a motor, to align a center axis of the guide hole103aand a center axis of the bearing hole3a. In order to insert the shaft4into the bearing hole3awith the shaft insertion guide103, the center axes of the guide hole103aand the bearing hole3aneed to be precisely aligned. In other words, the shaft insertion guide103needs to be highly precisely adjusted.

On the other hand, there is a clearance between the shaft4and the bearing hole3a, and a tip end of the shaft4been inserted into the bearing hole3amay incline corresponding to the clearance. In order to prevent the shaft from inclining, the vibration along an X direction shown inFIG. 7bis applied to the vacuum line block102with an actuator such as a cylinder (not shown in Fig). With aligning the center axes of the shaft4and the bearing hole3aby applying vibration along the X direction in predetermined times, the shaft4is inserted into the bearing hole3a, and then the insertion the shaft4into the bearing hole3ais completed (FIG. 7C).

In the conventional manufacturing method, the outer circumferential surface of the shaft4and the inner circumferential surface of the sleeve3may contact each other, which may result in scratching the member with low hardness among the shaft4and the sleeves3. For instance, if the hardness of sleeve3is lower than the hardness of shaft4, the inner circumferential surface of the sleeve3is scratched. The sleeve3whose inner circumferential surface is scratched may influence the performance of the bearing assembly.

BRIEF SUMMARY OF THE INVENTION

A manufacturing method of a bearing assembly according to the preferred embodiment of the present invention is generally characterized by feeding fluid between an outer circumferential surface of the shaft and an inner circumferential surface of the bearing hole while inserting the shaft into the bearing hole. The fluid flows due to the static pressure difference between inside and outside of the bearing hole.

Under the condition, with the shaft being misaligned with the center axis of the bearing hole, a clearance between the outer circumferential surface of the shaft and the inner circumferential surface of the bearing hole (between a periphery of the opening of the bearing hole and a tip end of the shaft) becomes narrower at one side than another side. Therefore, at the one side, flow speed of the fluid becomes slower and it becomes faster on another side. Based on the Bernoulli's theorem, the pressure of the fluid becomes greater at one side than that at other side, such that the shaft is pushed toward the center axis of the sleeve to cancel the pressure difference. Therefore, with holding the shaft or sleeve softly enough to be moved by the pressure difference, the center axes of the shaft and the sleeve are automatically aligned.

As mentioned above, the center axes of the shaft and the bearing hole are aligned by the pressure difference of the fluid (i.e., self-alignment) when the shaft is inserted into the bearing hole, such that the uniform clearance between the outer circumferential surface of the shaft and inner circumferential surface of the bearing hole is maintained around the shaft. As a result, it is possible to start the shaft insertion into the bearing hole without contacting each other.

The fluid is fed toward the upper end opening of the bearing hole during the shaft insertion. The fluid with predetermined pressure existing between the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve prevent the shaft from contacting the sleeve. Therefore, the shaft may be inserted into the bearing hole without scratching the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve.

As described above, in the manufacturing method according to the preferred embodiment of the present invention, the shaft and the sleeve are appropriately self-aligned at the beginning of the shaft insertion into the bearing hole. Moreover, during the shaft insertion, the fluid with predetermined pressure is fed toward the upper end opening of the bearing hole. As a result, the fluid exists between the shaft and the sleeve, and prevents the outer circumferential surface of the shaft from contacting the inner circumferential surface of the bearing hole.

Therefore, the shaft may be inserted into the bearing hole without scratching the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve. In addition, production of the bearing assembly having predetermined performance may be facilitated.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments according to the present invention will be described below by referring toFIGS. 1 to 6. In the descriptions of the preferred embodiments of the present invention, words such as upper, bottom, left, and right for explaining positional relationships between respective members and directions merely indicate positional relationships and directions in the drawings. Such words do not indicate positional relationships and directions of the members mounted in an actual device.

FIG. 1shows a bearing assembly manufactured in accordance with one preferred embodiment of the present invention. An assembling machine of the bearing assembly in accordance with the preferred embodiment of the present invention is illustrated inFIGS. 2 to 5.

FIG. 1is a cross sectional view showing the bearing assembly manufactured in accordance with one preferred embodiment of the present invention. The bearing assembly in this preferred embodiment of the present invention is an oil dynamic bearing assembly2used for a spindle motor1driving a hard disk. The oil dynamic bearing assembly2includes a sleeve3, a shaft4, a thrust plate5, and a counterplate6.

For instance, the sleeve3is made of a copper metal array such as phosphor bronze and has a hollow cylinder shape having a bearing hole3awhose inner circumferential surface faces an outer circumferential surface of the shaft4. The shaft4is made of stainless steel. The shaft4is rotatably inserted into the bearing hole3a.

A dynamic pressure surface is provided on the outer circumferential surface of the shaft4and on the inner circumferential surface of the sleeve3. The dynamic pressure surfaces radially opposing each other with a gap maintained therebetween forms a radial dynamic bearing portion16. In this preferred embodiment, two radial dynamic bearing portions16are provided in an axially distanced manner. A clearance between the inner circumferential surface of the sleeve3and the outer circumferential surface of the shaft4at the radial dynamic bearing portion is about3pm to about6pm. At least each gap of the dynamic bearing portion16is filled with a lubricant fluid such as lubricant oil and ferrofluid. In addition, a radial dynamic pressure generating groove having such as a herringbone shape is formed at the dynamic bearing surface (not shown in Fig.).

The thrust plate5has a ring shape and is fixed to a bottom end portion of the shaft4by such as press fitting. The thrust plate5is arranged within a cylinder convex portion of the sleeve4caving inwardly from the bottom end surface of the sleeve4at around the center axis of the sleeve4. The counterplate6has a disk shape having larger diameter than that of the thrust plate5, and is arranged so as to adjoin and face the bottom end surface of the thrust plate5. The counterplate6is fixed to the sleeve3and occludes a bottom end opening of the sleeve3.

An upper surface of the thrust plate5and a counter surface of the sleeve3facing the upper surface of the thrust plate5include dynamic pressure surfaces. These dynamic pressure surfaces axially facing each other constitute a thrust dynamic bearing portion15a. Similarly, the dynamic pressure surfaces are provided on the bottom surface of the thrust plate5and a surface of the counterplate6facing the bottom surface of the thrust plate5. These dynamic pressure surfaces axially facing each other constitute a thrust dynamic bearing portion15b. At least each gap provided at the thrust dynamic bearing portion is filled with the lubricant fluid such as lubricant oil and ferrofluid.

A rotor hub8having a substantially cup shape is fixed to the upper tip end portion of the shaft4by such as press fitting. The rotor hub8includes a disk placing surface8a, and a recording disk is placed thereto. Moreover, an annular rotor magnet12is attached to the rotor hub8.

As explained above, the oil dynamic bearing assembly2is used for the spindle motor1to rotary drive a hard disk. The spindle motor1includes a fixed frame9. At a substantially middle portion of the fixed frame9, a bearing holder9ahaving a hollow cylinder body portion is arranged in a standing condition. The sleeve3is fixed to an inner side of the bearing holder9aby such as the press fitting, shrinkage fitting, and bonding. To an outer side of the bearing holder9a, a stator core10constituted by a laminated body of electrical steel is attached. The stator core10includes a plurality of teeth portion, and a coil11is wound around each of the teeth portion. The rotor magnet12and the coil11constitute an actuator portion of the spindle motor1.

FIG. 2is a cross sectional view showing an assembling machine of the bearing assembly according to one preferred embodiment of the present invention. An assembling machine19according to this preferred embodiment of the present invention assembles the bearing assembly2by inserting the shaft4from the upper end opening toward the bottom end opening of the bearing hole3aof the sleeve3. The assembling machine2includes a sleeve cradle21, fluid feed apparatus22, a positioning portion23, and a shaft support portion24. The sleeve cradle21supports the sleeve3so as to axially arrange the bearing hole3aof the sleeve3. The sleeve cradle21includes a sleeve support portion21aretaining the sleeve3, and an attaching portion21bbeing fixed to the fluid feed apparatus22.

The sleeve support portion21ahaving a hollow cylinder shape includes a sleeve insert hole21caxially penetrating the sleeve3along the center axis thereof. The sleeve insert hole21cincludes a step portion1dfor the axial positioning of the sleeve3upon inserting the sleeve3into the sleeve insert hole21c.An inner diameter of the sleeve insert hole21ais slightly larger than an outer diameter of the sleeve3. By virtue of the configuration, an inner circumferential surface of the sleeve insert hole21cand an outer circumferential surface of the sleeve3face each other with a gap radially maintained therebetween when the sleeve3is inserted and retained in the sleeve support portion21a.

The attaching portion21bhaving flange shape is formed at the bottom side of the sleeve cradle21. By fixing the attaching portion21bto a first fluid channel block27constituting the fluid feed apparatus22with a screw25, the sleeve cradle21is fixed to the fluid feed apparatus22.

The fluid feed apparatus22feeds fluid from the bottom end opening toward the upper end opening of the bearing hole3aof the sleeve retained on the sleeve cradle21.

The fluid feed apparatus22includes a pump26, the first fluid channel block27, a second fluid channel block28, a pipe29, and a coupling30. The pump26is used as a fluid pressurizing device to send the fluid with a predetermined pressure. The first fluid channel block27includes a fluid passing hole27aand a fluid passing hole27bwhich supply the fluid pumped out from the pump26to the bearing hole3aof the sleeve3retained on the sleeve cradle21. The second fluid channel block28includes a fluid pool (not shown in Figs.) provided between the fluid channel holes27aand27b. The piping29and coupling30connect the first fluid channel block27and the pump26.

In this preferred embodiment of the present invention, air is used as fluid. However, the fluid is not limited to the air, and any suitable substance such as gas other than air, water, and alcohol may be used as the fluid. For instance, upon using the liquid as a fluid, isopropyl alcohol may be suitable to the fluid in the view of its drying property and the cost. However, in this preferred embodiment of the present invention, in which the shaft4is inserted into the bearing hole3aby the self-weight of the shaft4as mentioned later, it is preferable to use air as the fluid.

The fluid channel hole27ais formed so that the fluid pumped out from the pump26inflows thereto through the pipe29and the coupling30, and then outflows to the fluid pool (not shown in Fig.) provided to the second fluid channel block28. Similarly, the fluid channel hole27bis formed so that the fluid outflowing from the fluid pool flows into the fluid channel hole27band outflows toward the bearing hole3a. Thus, a fluid channel20which connects the pump26and the bearing hole3ais constituted by the pipe29, the coupling30, and the fluid channel holes27aand27b.

An air outlet portion of the fluid channel20may be preferably connected to the bottom opening of the bearing hole in a coaxial manner so that the fluid smoothly flows into the bearing hole3a.

The first fluid block27includes a bypass passage27c. The bypass passage27cexhausts the fluid from the fluid channel20to maintain predetermined fluid pressure within the bearing hole3aupon inserting the shaft4into the bearing hole3aof the sleeve3.

The bypass passage27cincludes a first bypass passage27c1and a second bypass passage27c2.

The second bypass passage27c2has smaller diameter than the first bypass passage27c1and the fluid passage hole27, and connects the first bypass passage27c1and the fluid passing hole27. The first bypass passage27c1extends to an outside of the first fluid block27. Whereby, the bypass passage27cexhausting the fluid from the fluid channel hole27bis provided. The relation between fluid pressures in bearing hole3aand the second bypass passage hole27c2is described later.

The second fluid channel block28is fixed to the first fluid channel block27by a screw31. To facilitate the processing of the fluid channel holes27aand27b, the fluid channel block may be dividend into the first and second fluid channel blocks27and28, and then, these blocks27and28may be fixed each other by screw31after processing them. Alternatively, the fluid channel block may be solely constituted by the first fluid channel block27to which the fluid channel hole27aand27bare continuously formed.

The positioning portion23adjusts an incline of the center axis of the sleeve3retained to the sleeve cradle21. The positioning portion23includes a base32, an adjustment stand33, and a plurality of screws34connecting the adjustment stand33to the base32.

The shaft support portion24leads the shaft4into the bearing hole3a. The shaft support portion24includes a shaft support member35which retains the shaft4so as to lead the shaft4into the bearing hole3aand a vacuum pipe36which vacuums up and fixes the shaft4to the shaft support member35.

The shaft support member35includes a shaft supporting hole which retains the shaft4having the thrust plate5fixed thereto. The shaft support member35may move in axial (vertical) direction with the actuator such as the cylinder (not shown in Figs.). The vacuum pipe36is connected to a vacuuming unit (not shown in Figs) vacuuming up the shaft4to fix it to the shaft supporting member35.

Referring toFIGS. 2 to 5, the steps of inserting the shaft4into the bearing hole3aon the sleeve3from one end opening (the upper opening) toward another end opening (bottom end opening) are described in detail.

FIG. 3is a cross-sectional view of the assembling machine19shown inFIG. 2in the state of inserting the shaft4into the bearing hole3a.FIG. 4is a drawing explaining the principle of self-alignment of the shaft4upon the initial state of the shaft4insertion into the bearing hole3a.FIG. 5is drawing explaining the interaction between the shaft4and the bearing hole3aduring the shaft4insertion into the bearing hole3a.

Firstly, as shown inFIG. 2, the sleeve3is inserted into the sleeve insert hole21and retained with aligning the bearing hole3aof the sleeve3with the axial direction. The sleeve3is axially positioned with a step portion21d. The incline of the sleeve3against the central axis may be adjusted with the screw34. The sleeve3is retained with a gap radially maintained between the sleeve3and the inner surface of the sleeve insertion hole21c.The shaft4, to which the thrust plate5is fixed by, for example, press fitting, is vacuumed up and fixed to the shaft support portion24.

The fluid with the predetermined pressure is fed from the bottom end opening to the upper end opening of the bearing hole3avia the fluid channel20constituted by such as the fluid passing holes27aand27b.

Under the condition, an actuator (not shown in Figs) moves the shaft4into the downward direction, to begin the shaft4insertion into the bearing hole3a.

Upon beginning of the shaft4insertion into the bearing hole3, the center axis misalignment between the shaft4and the bearing hole3generates the flow resistance difference at the upper end opening of the bearing hole3as shown inFIG. 4A. For instance, when the center axis of the shaft4misaligns to the left side against that of the bearing hole3aas shown inFIG. 4A, a wide gap Y1and a narrow gap Y2are defined with the shaft4and the upper end opening of the bearing hole3a. The diameter difference of the gaps makes the amount of the fluid flowing into the gap different. More fluid flows into the wide gap Y1, and less fluid flows into the narrow gap Y2. In other words, the flow resistance at the narrow gap Y2is higher than that at the wide gap Y1.

The sleeve3is supported with a gap radially maintained between the sleeve3and the sleeve insert hole21c, such that the sleeve3may laterally move to cancel the flow resistance difference. In other words, the sleeve3may radially move so as to balance the flow resistance (amount of the fluid flowing around the shaft4) around the shaft4. As a result, the center axes of the shaft4and the bearing hole3aare self-aligned as shown inFIG. 4B.

When the center axes of the shaft4and the bearing hole3aare aligned, the shaft4is released from the shaft support portion by stopping the vacuuming up. The pressure of the fluid fed from the pump26is adjusted so that the shaft24enters into the bearing hole3aby its self-weight. Therefore, the shaft4released from the shaft support member24slowly enters into the bearing hole with being self-aligned (seeFIG. 3).

Upon the completion of the shaft4insertion into the bearing hole, the fluid feed from the pump26is stopped, and the sleeve3with the shaft4inserted therein is released from the sleeve cradle21. Then, the counterplate6is attached and fixed to the sleeve4.

During the shaft4insertion into the bearing hole3a, the pump26feeds the fluid with the predetermined pressure from the bottom end opening toward the upper end opening of the bearing hole3a. As a result, the fluid with the predetermined pressure exists within the gap defined with the outer circumferential surface of the shaft4and the inner circumferential surface of the sleeve3, such that the substantially uniform clearance is maintained therebetween.

By the way, as the shaft4is inserted into the bearing hole3a, the axial length of the portion where the outer circumferential surface of the shaft4faces the inner circumferential surface of the sleeve3becomes longer. In other words, the passage in which the fluid flows from the bottom end opening to the upper end opening becomes narrower as the shaft4is inserted into the bearing hole3a. As a result, the fluid resistance increases more as the shaft4is inserted into the bearing hole3a, therefore, the fluid pressure within the bearing hole3aincreases as well.

Once the pressure within the bearing hole exceeds the threshold point, the shaft4may be pushed into the opposing direction from the shaft insertion direction (upward direction in this preferred embodiment), which is so called “shaft jumping”. As a result of the shaft jumping, the inner circumferential surface of the sleeve3may varies and the shaft4and the sleeve3may contact.

In order to prevent the shaft jumping, the fluid is exhausted from the fluid passing hole27bvia the bypass passage27cbefore the fluid pressure exceeds the threshold point. The diameter of the second bypass passage hole27c2is smaller than that of the first bypass passage hole27c1and the fluid passage hole27b. The diameter of the second bypass passage hole27c2is determined depending on such as the weight of the shaft4, the fluid pressure, the diameters of the first bypass passage hole27c1or the fluid passage hole27b.

As mentioned above, in this preferred embodiment, the fluid feed apparatus22feeds the fluid from the bottom end opening toward the upper end opening of the bearing hole3ato which the shaft4is inserted. By use of the pressure of the fluid fed to the bearing hole3a, the shaft4is self-aligned with the bearing hole3aupon the beginning of the shaft4insertion into the bearing hole3a. Therefore the uniform clearance between the outer circumferential surface of the shaft4and the inner circumferential surface of the bearing hole3ais maintained. As a result, the shaft4insertion into the bearing hole3amay begin without contacting each other.

During the shaft4insertion process into the bearing hole3a, the fluid is fed toward the upper end opening of the bearing hole3a, which may prevent the shaft4from contacting the inner circumferential surface of the sleeve3. Therefore, the shaft4may be inserted into the bearing hole3awithout scratching the outer circumferential surface of the shaft4and the inner circumferential surface of the sleeve3.

In addition, by virtue of the bypass passage27c, the fluid pressure within the bearing hole3amay be maintained appropriately. Therefore, the occurrence of the shaft jumping may be inhibited, such that the shaft4may be inserted into the bearing hole3awithout scratching the outer circumferential surface of the shaft4and the inner circumferential surface of the sleeve3.

Moreover, in the assembling of the oil dynamic bearing assembly2, the shaft4is aligned with the bearing hole3aof the sleeve3and is inserted into the bearing hole3aby the self-weight of the shaft4.

Therefore, it is not necessary to use the driving source (actuator) to insert the shaft4into the bearing hole3a. Without applying the external force generated by the actuator, the self-alignment of the shaft4and the bearing hole3amay be facilitated, and the gap (clearance) between the outer circumferential surface of the shaft4and the inner circumferential surface of the sleeve3may be maintained uniformly. Using the actuator to insert the shaft into the bearing hole, it is necessary to determine how much force is applied to the shaft and how to hold the shaft to align the shaft and the bearing hole. However, in the preferred embodiment of the present invention, the self-alignment of the shaft and the bearing hole facilitated without considering above factors.

Moreover, by using air as the fluid, the assembling machine may be built at lower cost, comparing with the machine using other than air as the fluid. As a result, the bearing assembly may be manufactured at low cost.

With referring toFIG. 6, another preferred embodiment of present invention is described below.

The preferred embodiment of the present invention is applicable to the assembling machine inserting the shaft4, having the rotor hub8fixed thereto, into the bearing hole3a. In this case, however, the sleeve3with the shaft4inserted thereto can not be detached from the sleeve cradle21into the upward direction.

Consequently, in the assembling machine shown inFIG. 6, the sleeve cradle21is retained on the first fluid channel block27by a vacuuming portion. The vacuuming portion includes a vacuum forming portion40, a vacuum pipe41, and a vacuum pipe hole27eand a suction hole27fprovided on the first fluid channel block27.

By virtue of the configuration mentioned above, the sleeve cradle21may be detached by terminating the vacuum forming portion40. Therefore, the sleeve cradle21may be easily detached from the first fluid channel block27. Then, by vertically flipping the sleeve cradle21detached form the first fluid channel block27, the sleeve3may be detached from the sleeve cradle21.

Only selected embodiments have been chosen to illustrate the present invention. To those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for limiting the invention as defined by the appended claims and their equivalents.

For instance, upon the beginning of the insertion of the shaft4into the bearing hole3a, the sleeve3may be fixed and the shaft4may be laterally moved to align the center axes of them.

For instance, the shaft4may be inserted into the bearing hole3aby the self-weight of the sleeve3.

For instance, with horizontally aligning the center axes of the shaft4and the sleeve3, the shaft4may be inserted into the bearing hole3aby the actuator. In this case, it is necessary to determine how much force the actuator applies on shaft4to align the center axes of the shaft4and the bearing hole3aand how to hold the shaft4to insert the shaft4into the bearing hole3a.

In the preferred embodiments of the present invention, the bearing assembly is the oil dynamic bearing assembly2. However, the bearing assembly may be other than the oil dynamic bearing assembly, it may be such as gas dynamic bearing assembly.

The bearing assembly may be any bearing assembly as long as it includes the shaft and the hollowed sleeve having the bearing hole whose inner circumferential surface faces the outer circumferential surface of the shaft.