JP7686527B2 - Spindle motor and manufacturing method thereof - Google Patents

Description

The present invention relates to spindle motors used in, for example, hard disk drives, and in particular to technology for attaching the shaft to a base with high precision.

Since the spindle motors of recent recording disk devices are required to have high rotational accuracy, the rotating part rotates relative to the fixed part via a fluid dynamic bearing part. The fluid dynamic bearing part has a structure in which the shaft inserted into the bearing member faces the inner circumferential surface of the bearing member across a small gap, and the small gap is filled with lubricating oil. In addition, dynamic pressure generating grooves are formed on at least one of the inner circumferential surface of the bearing member and the shaft. When the bearing member and the shaft rotate relative to each other, dynamic pressure is generated and the bearing member and the shaft are put into a non-contact state, allowing the rotating part of the spindle motor to rotate with high accuracy.

Here, the fluid dynamic bearing may have a shaft fixed to the rotating part and rotate, or a shaft fixed to the fixed part and the bearing member rotates. Taking a hard disk drive as an example of a recording disk device, there may be a rotating shaft type motor in which the shaft is fixed to a rotor hub (rotating part) that carries a magnetic disk, and a fixed shaft type motor in which the shaft is fixed to a base (fixed part) that holds a stator. It is also known that the shaft can be fixed by a combination of press fitting and bonding.

For example, Patent Document 1 discloses a spindle motor in a recording disk device that rotates optical disks such as CDs and DVDs on a turntable, the spindle motor including a turntable made of a deformable iron-based material, a turntable inner diameter portion provided in the center of the turntable, having a center hole and a coupling portion having a groove or embossed shape on its inner circumferential surface, and a rotating shaft that is inserted into the center hole of the turntable inner diameter portion and comes into contact with the coupling portion to perform axial rotation. The inner diameter of the coupling portion is characterized by being the same as or smaller than the outer diameter of the rotating shaft.

According to the description of the embodiment in Patent Document 1, the rotating shaft 130 is inserted into the central through-hole of the turntable inner diameter portion 120, and when the rotating shaft 130 is inserted, it comes into contact with the coupling portion 140 formed in the turntable inner diameter portion 120. When the rotating shaft 130 is inserted, the contact with the coupling portion 140 causes local deformation in the coupling portion 140, and the rotating shaft 130 is fixedly inserted into the center of the turntable inner diameter portion 120. The coupling portion 140 of the turntable inner diameter portion 120 may have a groove or embossed shape. In this case, after the rotating shaft 130 is inserted into the turntable inner diameter portion 120, the gap may be fixed with a bonding agent.

JP 2013-25856 A

In the above-mentioned shaft fixing method, the shaft and the shaft insertion hole are fitted together in a tight fit that causes local deformation, so a large press-in force is required, and galling is likely to occur when the shaft is pressed in. In other words, if the central axis of the shaft is tilted or eccentric even slightly with respect to the central axis of the shaft insertion hole when the shaft is inserted, the tip of the shaft will bite into the groove or embossed shape of the insertion hole, damaging the inner surface. The shaft galling creates resistance to the press-in, but since the press-in force is large, the shaft may be inserted overcoming this resistance. In this case, the shaft is installed with an installation error due to the tilt, and the rotation accuracy of the rotating part (turntable or rotor hub) on which the recording disk is placed is deteriorated. For example, the repeated runout synchronized with the rotation of the rotating part, so-called synchronous runout (RRO), is deteriorated due to the installation error of the shaft. On the other hand, in the case of a fixed shaft type motor, the shaft is press-fitted and fixed to the base, and the rotating part is fixed to the bearing member side, but the parallelism of the disk mounting surface of the rotating part relative to the base surface is deteriorated due to the installation error of the shaft.

However, if the press-fit allowance is reduced to prevent seizing, the shaft will not be secured firmly enough. If the amount of adhesive is increased by increasing the gap between the shaft and the inner surface of the insertion hole to compensate for the lack of securing strength, as in Patent Document 1, this is not desirable in the case of hard disk drives, as it increases the amount of outgassing that is generated from the adhesive and contaminates the hard disk.

The present invention was made in consideration of the above circumstances, and aims to provide a spindle motor in which the shaft can be attached with high precision without compromising the fixing strength of the shaft.

The present invention relates to a spindle motor in which a rotating part rotates relative to a fixed part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled in a minute gap between a shaft and a bearing member , the rotating part or the fixed part has a shaft insertion hole, the shaft is made of a material harder than the rotating part or the fixed part which has the shaft insertion hole, the shaft is pressed into a shaft insertion hole provided in either the rotating part or the fixed part, the inner surface of the shaft insertion hole includes a first inner circumferential surface which is a portion that comes into contact with the shaft, the outer circumferential surface of the shaft includes a first outer circumferential surface which is a portion that comes into contact with the first inner circumferential surface, and the arithmetic mean roughness of the first inner circumferential surface is greater than the arithmetic mean roughness of the first outer circumferential surface.

The present invention also relates to a method for manufacturing a spindle motor in which a rotating part rotates relative to a fixed part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled in a minute gap between the outer circumferential surface of the shaft and the inner circumferential surface of a bearing member into which the shaft is inserted, and includes the steps of: preparing the rotating part, the fixed part, and a shaft made of a material harder than the rotating part and the fixed part; providing a shaft insertion hole in the rotating part or the fixed part whose inner circumferential surface has an arithmetic mean roughness greater than the arithmetic mean roughness of the outer circumferential surface of the shaft; applying a lubricating liquid to at least one of the inner circumferential surface of the shaft insertion hole and the outer circumferential surface of the shaft; and press-fitting the shaft into the shaft insertion hole after applying the lubricating liquid.

The present invention provides a spindle motor that allows the shaft to be attached with high precision without compromising the shaft fixing strength.

1 is a cross-sectional view showing a spindle motor according to a first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a spindle motor according to a second embodiment of the present invention. FIG. 11 is a cross-sectional view showing a spindle motor according to a third embodiment of the present invention. FIG. 2 is a plan view of a hard disk drive showing locations where parallelism is measured in an embodiment of the present invention. FIG. 5 is a cross-sectional view of FIG. FIG. 4 is a rear view of the hard disk drive showing a reference surface when measuring parallelism in the embodiment of the present invention.

1. First embodiment Fig. 1 shows a first embodiment of the present invention, which is an example in which the present invention is applied to a rotating shaft type spindle motor used in a hard disk drive. A spindle motor 10 includes a base 11, and a cylindrical portion 11a extending upward is formed on the base 11. A stator core 12 is fixed to the outer periphery of the cylindrical portion 11a. The stator core 12 is made by laminating a plurality of thin, annular soft magnetic material sheets (e.g., electromagnetic steel sheets) in the axial direction, and includes a plurality of pole teeth protruding radially outward. The plurality of pole teeth are provided at equal intervals along the circumferential direction, and a coil 13 is wound around each of them.

A sleeve (fixed part, bearing member) 14 is fixed to the inner circumference of the cylindrical part 11a by means of press-fitting, gluing or the like, and a shaft 20 is inserted into the bearing hole 14a of the sleeve 14 via a small gap. A large diameter part 14b, which is larger in diameter than the bearing hole 14a, is formed at the lower end side of the bearing hole 14a, and a ring-shaped flange part 21 is arranged on the large diameter part 14b, which is fixed to the outer circumference of the lower end of the shaft 20 by press-fitting. The lower side of the large diameter part 14b is closed by a counter plate 22 fixed to the lower end opening of the sleeve 14. As a result, a small gap is formed between the shaft 20 and the sleeve 14, with only the upper end side being open. The small gap is continuously filled with lubricating oil O.

A minute gap is formed between the outer peripheral surface of the shaft 20 or the inner peripheral surface of the sleeve 14, and dynamic pressure grooves are formed at multiple locations (two locations in this example) spaced apart in the axial direction on either of the opposing surfaces. A fluid dynamic pressure radial bearing is formed by filling the minute gap with lubricating oil. A minute gap is also formed between the lower and upper surfaces of the flange portion 21 and the end surface of the sleeve 14 facing the flange portion 21 and the counter plate 22, and dynamic pressure grooves are formed on either of the opposing surfaces. A fluid dynamic pressure thrust bearing is formed by filling the minute gap with lubricating oil. When the shaft 20 rotates, the fluid dynamic pressure radial bearing 14c generates dynamic pressure that supports the shaft 20 in the radial direction, and the fluid dynamic pressure thrust bearing generates dynamic pressure that floats the shaft 20 in the axial direction, so that the shaft 20 and the flange portion 21 are in a non-contact state with the sleeve 14.

The shaft 20 is made of martensitic stainless steel (e.g., SUS420J2) whose hardness has been increased by heat treatment. The outer circumferential surface of the shaft 20 is polished to an arithmetic mean roughness of Ra 0.03.

The rotor hub (rotating part) 30 is fixed to the upper end of the shaft 20 by press fitting. The rotor hub 30 is made of aluminum or an aluminum alloy. Therefore, the hardness of the rotor hub 30 is lower than that of the shaft 20. The rotor hub 30 may be made of ferritic stainless steel such as SUS430. The rotor hub 30 is provided with a shaft insertion hole 30a for press fitting the shaft 20.

The inner peripheral surface of the shaft insertion hole 30a is finished by cutting into a smooth cylindrical surface with a larger arithmetic mean roughness than the outer peripheral surface of the shaft 20. In this embodiment, the outer peripheral surface of the shaft 20 has an arithmetic mean roughness of Ra 0.03, while the inner peripheral surface of the shaft insertion hole 30a has an arithmetic mean roughness of Ra 0.2. The arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a is not limited to this, and a different combination of arithmetic mean roughness may be used as long as the surface is rougher than the outer peripheral surface of the shaft 20 and mounting accuracy and fixing strength are obtained.

For example, the arithmetic mean roughness of the outer peripheral surface of the shaft 20 may be Ra0.05, and the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a may be Ra0.1. However, when considering processing costs, it is preferable to set the arithmetic mean roughness of the outer peripheral surface of the shaft 20 in the range of Ra0.025 to Ra0.2, and the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a in the range of Ra0.1 to Ra0.8. However, the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a must be greater than the arithmetic mean roughness of the outer peripheral surface of the shaft.

Here, the arithmetic mean roughness of at least the portion (first inner surface) of the inner surface of the shaft insertion hole 30a that contacts the outer surface of the shaft 20 after press-fitting should be greater than the arithmetic mean roughness of the portion (first outer surface) of the outer surface of the shaft 20 that contacts the first inner surface. In other words, even when the shaft 20 is press-fitted, the arithmetic mean roughness of the portion (e.g., chamfered portion and countersunk portion) of the inner surface of the shaft insertion hole 30a that does not contact the outer surface of the shaft 20 is not particularly limited.

When the shaft 20 is pressed in, a lubricating liquid may be applied to the outer peripheral surface of the shaft 20 and/or the inner peripheral surface of the shaft insertion hole 30a to facilitate the press-in. As the lubricating liquid, a liquid adhesive, a lubricating oil, or isopropyl alcohol (IPA) can be used. Since the press-in is an interference fit, the applied lubricating liquid is pushed out into the shaft. If the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a is the same as that of the outer peripheral surface of the shaft 20 (e.g., 0.03) or less, the surface of the shaft insertion hole 30a is too smooth and almost no lubricating liquid remains in the press-in portion, so that galling may occur and the installation accuracy of the shaft 20 may deteriorate. However, according to the present invention, the inner peripheral surface of the shaft insertion hole 30a is rougher than the outer peripheral surface of the shaft 20, so that the lubricating liquid remains in the pressed-in portion and the lubricating effect between the inner peripheral surface of the shaft insertion hole 30a and the outer peripheral surface of the shaft 20 can be exhibited. Therefore, galling is prevented and high installation accuracy can be obtained.

However, in order to achieve the same effect, the surface roughness of the outer circumferential surface of the shaft 20 cannot be made greater than the surface roughness of the inner circumferential surface of the shaft insertion hole 30a. As mentioned above, the shaft 20 is harder than the rotor hub 30, so if the outer circumferential surface of the shaft 20 is rough, the shaft insertion hole 30a of the rotor hub 30, which has a low hardness, may be scraped off by contact. In addition, it is not preferable to form macroscopic irregularities that make the shaft insertion hole 30a of the rotor hub 30 less smooth and form a lubricant liquid reservoir. The macroscopic irregularities referred to here mean, for example, spiral grooves, circumferential grooves, and uneven patterns formed in the shaft insertion hole 30a. Such macroscopic irregularities make it difficult to obtain the inner diameter dimension by polishing, impairing the inner diameter dimensional accuracy of the shaft insertion hole 30a, and ultimately impairing the installation accuracy and fixing strength of the shaft 20. Furthermore, if there are macroscopic irregularities, the amount of adhesive or lubricant remaining will be too much, increasing the outgassing beyond the allowable range, which is not preferable.

Therefore, it is preferable that the inner circumferential surface of the shaft insertion hole 30a is a smooth cylindrical surface without macroscopic irregularities at the press-fit portion. This provides an excellent lubricating effect to prevent galling. In this embodiment, an acrylic anaerobic adhesive is used as the lubricating liquid. When an anaerobic adhesive is used, the adhesive remaining between the shaft 20 and the inner circumferential surface of the shaft insertion hole 30a is isolated from the air and completely hardens, which is preferable because it suppresses the generation of outgassing and increases the fixing strength of the shaft 20 due to the adhesive effect. Also, a thermosetting epoxy adhesive can be used instead of the anaerobic adhesive.

A flat disk mounting portion 31 is formed on the outer periphery of the rotor hub 30. Multiple hard disks are stacked on the disk mounting portion 31 with spacers between them. A cylindrical portion 32 extending downward is formed on the outer periphery of the disk mounting portion 31, and a ring-shaped rotor magnet 34 is fixed to the inner periphery of the cylindrical portion 32 via a yoke 33. The rotor magnet 34 is magnetized so that adjacent portions along the circumferential direction, SNSN..., have alternately different polarities. The inner periphery of the rotor magnet 34 faces the outer periphery of the pole teeth of the stator core 12 with a gap therebetween. Then, by supplying a driving current to the coil 13, a driving force that rotates the rotor magnet 34 is generated, and the rotor hub 30 rotates relative to the base 11 around the shaft 20.

In the spindle motor 10 configured as described above, the arithmetic mean roughness of the first inner circumferential surface of the shaft insertion hole 30a that contacts the shaft 20 is greater than the arithmetic mean roughness of the first outer circumferential surface of the shaft 20 that contacts the first inner circumferential surface. Therefore, when the shaft 20 is pressed into the shaft insertion hole 30a, the tip of the convex portion of the minute irregularities on the inner circumferential surface of the shaft insertion hole 30a is pressed by the shaft 20 and plastically deformed, and the shaft insertion hole 30a expands in diameter and the press-in resistance is reduced. As a result, galling is less likely to occur when the shaft 20 is pressed in, the installation accuracy of the rotor hub 30 can be improved, and the press-in allowance can be increased, thereby increasing the fixing strength of the rotor hub 30.

On the other hand, if the installation accuracy is poor when the shaft 20 is inserted into the shaft insertion hole 30a of the rotor hub 30, the perpendicularity of the shaft 20 relative to the rotor hub 30 will deteriorate. This will result in a worsening of the repeated runout of the rotor hub 30 rotating around the shaft 20, known as synchronous runout (RRO). In this regard, in the spindle motor 10 configured as described above, the installation accuracy of the shaft 20 is improved, improving the RRO of the rotor hub 30.

2 is a cross-sectional view showing a spindle motor 100 according to a second embodiment of the present invention. The spindle motor 100 includes a base (fixed portion) 110. The base 110 is made of aluminum or an aluminum alloy, and includes a shaft insertion hole 112.

The shaft 220, which is generally cylindrical, is fixed in the shaft insertion hole 112 by press fitting. The shaft 220 is made of martensitic stainless steel (e.g., SUS420J2) whose hardness has been increased by heat treatment. Therefore, the shaft 220 is harder than the base 110.

The outer peripheral surface of the shaft 220 is finished by polishing to an arithmetic mean roughness Ra of 0.2. On the other hand, the inner peripheral surface of the shaft insertion hole 112 is finished by cutting to a smooth cylindrical surface having a larger arithmetic mean roughness than the outer peripheral surface of the shaft 220. In this second embodiment, the outer peripheral surface of the shaft 220 has an arithmetic mean roughness Ra of 0.2, while the inner peripheral surface of the shaft insertion hole 112 has an arithmetic mean roughness Ra of 0.8.

A conical bearing portion 214 is formed at the upper end of the shaft 220. A conical bearing member 220a is fixed to the axial middle portion of the shaft 220 by means of press fitting, adhesive, or other means. The conical bearing member 220a is made of SUS303, an austenitic stainless steel.

The base 110 has a cylindrical portion 114 extending upward. A stator core 120 similar to that of the first embodiment is fixed to the outer periphery of the cylindrical portion 114, and a coil 130 is wound around each of the multiple pole teeth of the stator core 120.

A sleeve (bearing member) 370 is disposed on the outside of the shaft 220. Micro-gaps are formed between the conical bearing portion 214 of the shaft 220, the axial middle portion of the shaft 220, and the outer peripheral surface of the conical bearing member 220a and the inner peripheral surface of the sleeve 370, and dynamic pressure grooves are formed on one of the opposing surfaces, and lubricating oil is continuously supplied to the micro-gaps to form a fluid dynamic pressure bearing.

The rotor hub (rotating part) 360 is fixed to the outer periphery of the sleeve 370 by means of press-fitting, adhesive, or other means. A flat disk mounting part 361 is formed on the outer periphery of the rotor hub 360. A cylindrical part 362 extending downward is formed on the outer periphery of the disk mounting part 361, and an annular rotor magnet 330 is fixed to the inner periphery of the cylindrical part 362 via a yoke 320. The inner periphery of the rotor magnet 330 faces the outer periphery of the pole teeth of the stator core 120 with a gap therebetween.

In the spindle motor 100 configured as described above, the arithmetic mean roughness of the outer peripheral surface of the shaft 220 is Ra0.2, and the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 112 of the base 110 is Ra0.8. Therefore, when the shaft 220 is pressed into the shaft insertion hole 112, the tip of the convex part of the minute irregularities on the inner peripheral surface of the shaft insertion hole 112 is pressed by the shaft 220 and plastically deformed, and the shaft insertion hole 112 expands in diameter and the press-in resistance is reduced. As a result, galling is less likely to occur when the shaft 220 is pressed in, the attachment accuracy of the rotor hub 360 can be improved, and the press-in allowance can be increased, thereby increasing the fixing strength of the rotor hub 360.

3. Third embodiment FIG. 3 is a cross-sectional view showing a spindle motor 500 of a third embodiment. The spindle motor 500 includes a base 510. The base 510 is made of aluminum or an aluminum alloy, and includes a through hole 512 and a cylindrical portion 513 extending upwardly from the through hole 512. A stator core 540 similar to that of the first embodiment is fixed to the outer periphery of the cylindrical portion 513, and a coil 550 is wound around each of the multiple pole teeth of the stator core 540. A cup member (fixing portion) 520 is fixed in the through hole 512 by press-fitting and bonding. The cup member 520 is made of ferritic stainless steel such as SUS430, and has a shaft insertion hole 521 formed in the center thereof and a cylindrical portion 522 extending upwardly formed at the upper edge thereof.

A cylindrical shaft 530 is fixed in the shaft insertion hole 521 by press fitting. The shaft 530 is made of martensitic stainless steel (e.g., SUS420J2) whose hardness has been increased by heat treatment. Therefore, the shaft 530 has a higher hardness than the cup member 520. A large diameter portion 531 that is larger in diameter than the other portions is formed at the upper end of the shaft 530.

The outer peripheral surfaces of the shaft 530 and the large diameter portion 531 are polished to an arithmetic mean roughness of Ra 0.03. On the other hand, the inner peripheral surface of the shaft insertion hole 521 is cut to a smooth cylindrical surface having a larger arithmetic mean roughness than the outer peripheral surfaces of the shaft 530 and the large diameter portion 531. In this third embodiment, the outer peripheral surfaces of 530 and the large diameter portion 531 have an arithmetic mean roughness of Ra 0.03, while the inner peripheral surface of the shaft insertion hole 521 has an arithmetic mean roughness of Ra 0.1.

A rotor hub 560 is disposed on the outside of the shaft 530. A through hole 561 is formed in the center of the rotor hub 560, and a minute gap is formed between the inner peripheral surface of the through hole 561 and the outer peripheral surface of the shaft 530. Dynamic pressure grooves are formed in the upper and lower parts of the inner peripheral surface of the through hole 561, which is divided into approximately three equal parts in the vertical direction, or on the outer peripheral surface of the shaft 530 facing these parts, and the minute gap is filled with lubricating oil to form a fluid dynamic pressure radial bearing 532.

Furthermore, a cylindrical portion 562 extending upward is formed on the upper end surface of the rotor hub 560, and the large diameter portion 531 of the shaft 530 is housed in the cylindrical portion 562. A minute gap is formed between the inner peripheral surface of the cylindrical portion 562 and the outer peripheral surface of the large diameter portion 531. A pumping seal portion 533 consisting of a spiral groove is formed on the inner peripheral surface of the cylindrical portion 562 or the outer peripheral surface of the large diameter portion 531, and is designed to return lubricating oil to the fluid dynamic radial bearing 532 side as the rotor hub 560 rotates.

A tapered section 563 is formed at the lower end of the rotor hub 560, the diameter of which increases as it extends downward. A gap 523 with a wedge-shaped cross section is formed between the outer circumferential surface of the tapered section 563 and the cylindrical section 522 of the cup member 520, and serves as an oil reservoir. A minute gap is also formed between the lower end surface of the rotor hub 560 and the bottom surface of the cup member 520. A dynamic pressure groove is formed on the lower end surface of the rotor hub 560 or the bottom surface of the cup member, and the minute gap is filled with lubricating oil to form a fluid dynamic pressure thrust bearing 524.

The rotor hub 560 has a circulation passage 564 that communicates with the lower end surface of the tapered portion 563 and the upper end surface facing the large diameter portion 531 of the shaft 530. The circulation passage 564 is filled with lubricating oil, and when the rotor hub 560 rotates, centrifugal force acts on the lubricating oil, which moves the lubricating oil downward and supplies it to the fluid dynamic thrust bearing 524. The lubricating oil is also supplied to the fluid dynamic radial bearing 532 and then returned to the circulation passage 564. In this way, the lubricating oil circulates due to the pumping action generated by the rotation of the rotor hub 560.

A cylindrical portion 565 extending downward is formed on the outer periphery of the rotor hub 560, and an annular rotor magnet 570 is fixed to the inner periphery of the cylindrical portion 565. The inner periphery of the rotor magnet 570 faces the outer periphery of the pole teeth of the stator core 540 with a gap between them. A flange-shaped disk mounting portion 566 is formed on the lower edge of the cylindrical portion 565.

In the spindle motor 500 having the above configuration, the arithmetic mean roughness of the outer peripheral surface of the shaft 530 is Ra0.03, and the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 521 of the cup member 520 is Ra0.1. Therefore, when the shaft 530 is pressed into the shaft insertion hole 521, the tip of the convex part of the minute irregularities on the inner peripheral surface of the shaft insertion hole 521 is pressed by the shaft 530 and plastically deformed, and the shaft insertion hole 521 expands in diameter and the press-in resistance is reduced. As a result, galling is less likely to occur when the shaft 530 is pressed in, the installation accuracy of the rotor hub 560 can be improved, and the press-in allowance can be increased, thereby increasing the fixing strength of the rotor hub 560.

4. Modifications The present invention is not limited to the above-described embodiment, and various modifications are possible as follows.
i) In the above embodiment, the present invention is applied to a spindle motor equipped with a hard disk, but it can also be applied to spindle motors equipped with other magnetic disks or optical disks.

ii) In the second embodiment shown in FIG. 2, the rotor hub 360 and the sleeve 370 are configured as separate bodies, but they may be formed as an integral unit.
iii) A fluid dynamic thrust bearing is not necessarily required.

The effects of the present invention will be described in detail with reference to specific examples. Example 1 is a spindle motor 10 shown in Fig. 1 (arithmetic mean roughness of the outer circumferential surface of the shaft: Ra 0.03, arithmetic mean roughness of the inner circumferential surface of the shaft insertion hole: Ra 0.2), and Example 2 is a spindle motor in which the arithmetic mean roughness of the outer circumferential surface of the shaft is changed to Ra 0.05.

The spindle motor 100 shown in FIG. 2 (arithmetic mean roughness of the shaft outer peripheral surface: Ra 0.2, arithmetic mean roughness of the shaft insertion hole inner peripheral surface: Ra 0.8) was used as Example 3. The spindle motor 500 shown in FIG. 3 (arithmetic mean roughness of the shaft outer peripheral surface: Ra 0.03, arithmetic mean roughness of the shaft insertion hole inner peripheral surface: Ra 0.1) was used as Example 4, and the arithmetic mean roughness of the shaft outer peripheral surface was changed to Ra 0.1, and the arithmetic mean roughness of the shaft insertion hole inner peripheral surface was changed to Ra 0.5 was used as Example 5.

In the spindle motor 100 shown in FIG. 1, the arithmetic mean roughness of the shaft outer surface was Ra0.05 and the arithmetic mean roughness of the shaft insertion hole inner surface was Ra0.04, which was comparative example 1, and the arithmetic mean roughness of the shaft outer surface was Ra0.04 and the arithmetic mean roughness of the shaft insertion hole inner surface was Ra0.03, which was comparative example 2. In addition, in the spindle motor 10 shown in FIG. 2, the arithmetic mean roughness of the shaft outer surface was Ra0.2 and the arithmetic mean roughness of the shaft insertion hole inner surface was Ra0.1, which was comparative example 3.

2. RRO Test The RRO runout during rotation was measured and the RRO defect rate was compared for the shaft rotating spindle motors of Examples 1, 2, and 5 and Comparative Examples 1 and 2. This test was conducted to confirm the effect on the defect rate when a rotor hub with a large surface roughness of the shaft insertion hole and a rotor hub with a small surface roughness of the shaft insertion hole were combined in a spindle motor of the same structure.

The RRO test was performed as follows. 100 spindle motors with different surface roughness combinations were rotated at 7200 rpm, and the RRO runout of the disk mounting surface 31 of the rotor hub 30 was confirmed and the failure rate was examined. Specifically, the axial displacement was measured using a non-contact capacitance probe P, and the amplitude was taken as the RRO runout. Next, spindle motors with RRO runout measurements of 8 μm or more were considered defective, and the failure rate (%) = 100 × (number of defects/number of tests). The results of the failure rate (%) obtained in this way are shown in Table 1. As shown in Examples 1, 2, and 5 and Comparative Examples 1 and 2, it was confirmed that in axial rotation type spindle motors, the RRO failure rate was significantly reduced when the inner peripheral surface roughness of the hub was greater than the surface roughness of the shaft.

3. Parallelism test The parallelism was measured by using a reference plane A defined by three measurement points A1 to A3 on the lower surface of the base plate shown in Figure 6 as the measurement reference plane, bringing a height measuring probe into contact with four points H1 to H4 on the disk mounting surface 31 (361) shown in Figure 4 while the plate is stationary to measure the height, and calculating the parallelism from the measured value. The parallelism was calculated using the following formula 1.

Next, spindle motors with a parallelism measurement value of 30 μm or more were deemed defective, and the defect rate (%) was set to 100×(defective number/test number). If the parallelism is 30 μm or more, there is a possibility that the hard disk mounted on the disk mounting surface 361 may come into contact with the base 510. The defect rate (%) thus obtained is also shown in Table 1. As shown by Examples 3 and 4 and Comparative Example 3, it was confirmed that in the fixed-shaft spindle motor 100, the parallelism defect rate was significantly reduced by a combination in which the roughness of the inner peripheral surface of the shaft insertion hole 112 was greater than the surface roughness of the shaft 220.

The present invention can be used in electronic devices such as spindle motors and hard disk drives that are equipped with magnetic disks and optical disks.

10,100,500...spindle motor, 11,110,510...base (fixed portion), 11a,114,513...cylindrical portion, 12,120,540...stator core, 13,130,550...coil, 14,370...sleeve (fixed portion, bearing member), 14a...bearing hole, 14b...large diameter portion, 14c,532,...fluid dynamic radial bearing, 20,220,530...shaft, 21...flange portion, 22...counter plate, 30,360,560...rotor hub (rotating portion), 30a,112,521...shaft insertion hole, 31, 361, 566...disk mounting portion, 32, 362, 565...cylindrical portion, 33, 320...yoke, 34, 330, 570...rotor magnet, 214...conical bearing portion, 220a...conical bearing member, 512...through hole, 520...cup member (fixed portion), 522...cylindrical portion, 523...gap, 524...fluid dynamic thrust bearing, 531...large diameter portion, 533...pumping seal portion, 561...through hole, 562...cylindrical portion, 563...tapered portion, 564...circulation passage, A...measurement reference surface, O...lubricating oil, P...probe.

Claims (10)

In a spindle motor, a rotating part rotates relative to a fixed part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled in a minute gap between a shaft and a bearing member,
The rotating part or the fixed part has a shaft insertion hole,
the shaft is made of a material harder than the rotating part or the fixed part having the shaft insertion hole,
The shaft is press-fitted into a shaft insertion hole provided in either the rotating part or the fixed part,
an inner circumferential surface of the shaft insertion hole includes a first inner circumferential surface that is a portion that contacts the shaft,
the outer circumferential surface of the shaft includes a first outer circumferential surface that is a portion in contact with the first inner circumferential surface,
A spindle motor, wherein the arithmetic mean roughness of the first inner circumferential surface is greater than the arithmetic mean roughness of the first outer circumferential surface. The spindle motor of claim 1, wherein the first inner peripheral surface is a smooth cylindrical surface. The spindle motor according to claim 1 or 2, wherein at least one of a lubricating oil component, an isopropyl alcohol component, and a hardened adhesive remains between the first inner peripheral surface of the shaft insertion hole and the first outer peripheral surface of the shaft. A spindle motor according to any one of claims 1 to 3, in which the shaft is press-fitted into the rotating part, the bearing member is fixed to the fixed part, and the shaft rotates together with the rotating part. A spindle motor according to any one of claims 1 to 3, wherein the shaft is press-fitted into the fixed portion, the bearing member is fixed to the rotating portion, and the bearing member rotates together with the rotating portion. A spindle motor according to any one of claims 1 to 5 for rotating a rotating part,
the rotating portion includes a disk mounting portion,
A recording disk device including a recording disk that is attached to the disk placement portion and rotates. 7. The recording disk device according to claim 6, wherein the recording disk is a magnetic disk. The recording disk device according to claim 6, wherein the recording disk is an optical disk. A method for manufacturing a spindle motor in which a rotating part rotates relative to a fixed part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled in a minute gap between an outer circumferential surface of a shaft and an inner circumferential surface of a bearing member into which the shaft is inserted, comprising the steps of:
preparing the rotating part, the fixed part, and a shaft made of a material harder than the rotating part and the fixed part;
providing a shaft insertion hole in the rotating portion or the fixed portion, the inner circumferential surface of which has an arithmetic mean roughness greater than the arithmetic mean roughness of the outer circumferential surface of the shaft;
applying a lubricating liquid to at least one of an inner circumferential surface of the shaft insertion hole and an outer circumferential surface of the shaft;
a step of press-fitting the shaft into the shaft insertion hole after applying the lubricating liquid;
A method for manufacturing a spindle motor comprising the steps of: The method for manufacturing a spindle motor according to claim 9, characterized in that the lubricating liquid applied to at least one of the inner circumferential surface of the shaft insertion hole and the outer circumferential surface of the shaft is an adhesive, a lubricating oil, or isopropyl alcohol.

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