The present invention relates to a sintered bearing, a fluid dynamic bearing device and a motor each comprising the sintered bearing, and to a method of manufacturing a sintered bearing.
A fluid dynamic bearing device is configured to relatively rotatably support a shaft member in a non-contact manner by a pressure generated by a fluid film (for example, an oil film) in a radial bearing gap defined between an outer peripheral surface of the shaft member and an inner peripheral surface of a bearing member. The fluid dynamic bearing device has advantages in high rotational accuracy and quietness. Thus, the fluid dynamic bearing device is preferably usable for a small-sized motor such as a spindle motor for information equipment (for example, magnetic disk drives such as an HDD, drives for optical discs such as a CD-ROM, a CD-R/RW, a DVD-ROM/RAM, and a Blu-ray Disc, and drives for magneto-optical disks such as an MD and an MO), a polygon scanner motor for a laser beam printer (LBP), a color wheel for a projector, and a fan motor to be used for a cooling fan of an electrical apparatus, and so on.
As the bearing member to be used for such a fluid dynamic bearing device, a sintered bearing made of sintered metal may be used. The sintered bearing is subjected to dimension accuracy correction processing called sizing, thereby obtaining extremely high dimension accuracy. Thus, a radial bearing gap between the sintered bearing and the shaft member is set with high accuracy, thereby being capable of obtaining high rotation accuracy. In order to actively increase the pressure of the oil film in the radial bearing gap, dynamic pressure generating grooves are often formed in the inner peripheral surface of the sintered bearing (see, for example, Patent Literatures 1 and 2).
The fluid dynamic bearing device as disclosed in Patent Literature 1 can be assembled by, for example, fixing the sintered bearing on an inner periphery of a housing, filling an oil, and then, inserting the straight shaft member along an inner periphery of the sintered bearing. In this case, when the shaft member is inserted along the inner periphery of the sintered bearing, the shaft member undesirably takes air so that air bubbles may be mixed in a lubricating oil in the bearing device. Further, during an operation of the fluid dynamic bearing device, due to local generation of a negative pressure in the lubricating oil in the bearing device, air bubbles maybe formed in the lubricating oil. When the air bubbles are formed in the lubricating oil in the bearing device, in particular, in the oil film in the radial bearing gap, the pressure of the oil film is reduced. Thus, sufficient rotation accuracy of the shaft member may not be obtained. Further, the inner peripheral surface of the sintered bearing may be damaged through contact with the outer peripheral surface of the shaft member, thereby degrading the durability.
In order to avoid such an inconvenience, a plurality of axial grooves are sometimes formed in an outer peripheral surface of the sintered bearing of the fluid dynamic bearing device (see, for example, Patent Literatures 1 and 2). When the shaft member is inserted along the inner periphery of the sintered bearing after the oil is filled, the air can be discharged to the outside through the axial grooves. Further, through the axial grooves, a space in the bearing device, which is liable to generate a negative pressure, can be communicated to a space in the vicinity of an opening of the bearing device. Thus, the space can be brought into a state of having a pressure close to an atmospheric pressure, to thereby prevent generation of the negative pressure.
Patent Literature 1: JP 3734981 B2
Patent Literature 2: JP 2012-177456 A
Incidentally, the sintered bearing is generally manufactured through a forming step of subjecting material powder to compression molding to form a compact, a sintering step of sintering the compact to form a sintered body, and a sizing step of recompressing the sintered body to correct dimension accuracy. The above-mentioned axial grooves each have a groove depth larger than that of each of the dynamic pressure generating grooves, and dimension accuracy is not required to be considerably high. Thus, the above-mentioned axial grooves are often formed in the forming step before the sintering. Meanwhile, the dynamic pressure generating grooves each have an extremely small groove depth (for example, 10 μm or smaller), and dimension accuracy is required to be high in order to efficiently cause a dynamic pressure generating action. Thus, the dynamic pressure generating grooves are often formed in the sizing step after the sintering. Specifically, forming patterns having a shape in conformity with the dynamic pressure generating grooves are arranged on an outer peripheral surface of a core rod. The sintered body is press-fitted to the inner periphery of the die under a state in which the core rod is inserted along the inner periphery of the sintered body. Thus, the sintered body is compressed from an outer periphery thereof to press the inner peripheral surface of the sintered body onto the forming patterns on the outer peripheral surface of the core rod. With this, the shape of the forming patterns is transferred to the inner peripheral surface of the sintered body, thereby forming the dynamic pressure generating grooves.
In the above-mentioned case, an outer peripheral surface of the sintered body to be subjected to the sizing step has the axial grooves formed in advance in the forming step. Therefore, when the sintered body is press-fitted to the inner periphery of the die, spaces are formed between the axial grooves of the sintered body and the inner peripheral surface of the die. In this case, in the region where the axial grooves are formed, a compression force from the die is not applied to the sintered body so that nonuniform stress distribution may occur in the sintered body. Thus, the inner peripheral surface of the sintered body is not uniformly pressed onto the core rod, thereby causing a fear in that dimension accuracy of the inner peripheral surface of the sintered bearing is not sufficiently increased. In particular, in a case where the dynamic pressure generating grooves are formed in the inner peripheral surface of the sintered body through die molding in the sizing step, when the inner peripheral surface of the sintered body is not uniformly pressed onto the forming patterns, variation in groove depth of the dynamic pressure generating grooves is increased, thereby causing a fear in that the effect of increasing the pressure of the fluid film in the radial bearing gap is reduced to lead to degradation of bearing rigidity.
In view of the above-mentioned circumstances, it is an object of the present invention to provide a sintered bearing having axial grooves in an outer peripheral surface thereof and dynamic pressure generating grooves in an inner peripheral surface thereof, in which variation in groove depth of the dynamic pressure generating grooves is suppressed to increase bearing rigidity.
As described above, when the sintered body having the plurality of axial grooves in the outer peripheral surface is subjected to the sizing, the compression force is not applied to the forming region for the axial grooves. Thus, the nonuniform stress distribution may occur in the sintered body, thereby forming waves in a circumferential direction on the inner peripheral surface of the sintered body. For example, as illustrated in
In view of the above, the number of the axial grooves to be formed in the outer peripheral surface of the sintered bearing is set equal to the number of the dynamic pressure generating grooves to be formed on the same circumference in the inner peripheral surface of the sintered bearing. Thus, a wavelength T of the waves on the inner peripheral surface of the sintered body and a pitch t of the dynamic pressure generating grooves become equal to each other so that the dynamic pressure generating grooves are formed at the same portions of the waves (see
The present invention has been made based on the above-mentioned idea. Specifically, according to one embodiment of the present invention, there is provided a sintered bearing having a plurality of axial grooves equiangularly arranged in an outer peripheral surface thereof and a plurality of dynamic pressure generating grooves equiangularly arranged in an inner peripheral surface thereof, wherein a number of the plurality of axial grooves is an integral multiple of a number of the plurality of dynamic pressure generating grooves on the same circumference.
Further, according to one embodiment of the present invention, there is provided a method of manufacturing a sintered bearing, comprising the steps of: subjecting material powder to compression molding to form a compact having a plurality of axial grooves equiangularly arranged in an outer peripheral surface thereof; sintering the compact to form a sintered body; and sizing by inserting a core rod having forming patterns on an outer peripheral surface thereof along an inner periphery of the sintered body, compressing the sintered body from an outer periphery thereof under a state in which the core rod is inserted along the inner periphery of the sintered body, and by pressing an inner peripheral surface of the sintered body onto the forming patterns on the outer peripheral surface of the core rod, thereby forming a plurality of dynamic pressure generating grooves equiangularly arranged in the inner peripheral surface of the sintered body, wherein a number of the plurality of axial grooves is an integral multiple of a number of the plurality of dynamic pressure generating grooves on the same circumference.
Further, during the sizing, a factor of leading to the nonuniform distribution of the stress in the sintered body includes the radial depth of each of the axial grooves other than the above-mentioned number of the axial grooves. When the radial depth of each of the axial grooves is excessively large, the waves on the inner peripheral surface of the sintered body become larger so that the variation in groove depth of the dynamic pressure generating grooves is increased. According to investigation conducted by the inventors of the present invention, it was verified that, in particular, when the radial depth of each of the axial grooves exceeds 20% of the radial thickness of the sintered bearing, the variation in groove depth of the dynamic pressure generating grooves is significantly increased. Thus, it is preferred that a radial depth of each of the plurality of axial grooves be equal to or smaller than 20% of a radial thickness of the sintered bearing.
The fluid dynamic bearing device comprising the above-mentioned sintered bearing, the shaft member inserted along the inner periphery of the sintered bearing, and the radial bearing portion configured to support the shaft member in a radial direction with the pressure of the fluid film generated in the radial bearing gap between the inner peripheral surface of the sintered bearing and the outer peripheral surface of the shaft member has high bearing rigidity and can obtain excellent rotation accuracy.
The fluid dynamic bearing device as described above can be appropriately incorporated into a motor comprising a stator coil and a rotor magnet.
As described above, according to the sintered bearing of the present invention, the variation in groove depth of the dynamic pressure generating grooves is suppressed, and hence the pressure of the fluid film in the radial bearing gap is efficiently increased, thereby being capable of increasing the bearing rigidity.
A spindle motor illustrated in
As illustrated in
The shaft member 2 is made of a metal material such as stainless steel, and comprises a shaft portion 2a and a flange portion 2b formed on a lower end of the shaft portion 2a. The shaft portion 2a has formed thereon a cylindrical outer peripheral surface 2a1 and a tapered surface 2a2 gradually reduced in diameter toward the upper side. An outer diameter of the shaft portion 2a is set to, for example, approximately from 1 mm to 4 mm.
The sintered bearing 8 is made of sintered metal and formed into a cylindrical shape. Specifically, the sintered bearing 8 is made of copper-based, iron-based, or copper-iron-based sintered metal. A plurality of dynamic pressure generating grooves arranged equiangularly are formed in an inner peripheral surface 8a of the sintered bearing 8. In this embodiment, radial bearing surfaces are formed at two positions vertically separated from each other, and the dynamic pressure generating grooves having a herringbone pattern as illustrated in
In this embodiment, the dynamic pressure generating grooves 8a11, 8a12, 8a21, and 8a22 (hereinafter referred to as “dynamic pressure generating grooves 8a11 and the like”) each comprising six grooves are formed (see
A plurality of axial grooves 8c1 are formed equiangularly in an outer peripheral surface 8c of the sintered bearing 8. The number of the plurality of axial grooves 8c1 is set to be an integral multiple of the number of the dynamic pressure generating grooves on the same circumference. In this embodiment, the number of the axial grooves 8c1 is six, and the number of any of the dynamic pressure generating grooves on the same circumference (that is, the number of each of the dynamic pressure generating grooves 8a11 and the like) is six (see
Dynamic pressure generating grooves 8b1 having a spiral pattern as illustrated in
The housing 7 has a cylindrical side 7a and a disk-like bottom 7b closing a lower opening of the side 7a. In the illustrated example, the side 7a and the bottom 7b are integrally formed through injection molding with resin. Pump-in type dynamic pressure generating grooves having a spiral pattern are formed in an upper end surface 7b1 of the bottom 7b as thrust dynamic pressure generating portions (not shown). The dynamic pressure generating grooves may be formed simultaneously with, for example, injection molding for the housing 7.
The sealing member 9 is made of resin or metal, and is formed into an annular shape. The sealing member 9 is fixed on an upper end of an inner peripheral surface 7a1 of the side 7a of the housing 7 (see
The lubricating oil as a lubricating fluid is injected into the fluid dynamic bearing device 1 comprising the above-mentioned components. In this manner, an internal space of the fluid dynamic bearing device 1 including internal pores of the sintered bearing 8 is filled with the lubricating oil, and an oil surface is always maintained within the seal space S. Grease or a magnetic fluid may be used as the lubricating fluid besides the lubricating oil.
When the shaft member 2 is rotated, a radial bearing gap is formed between the inner peripheral surface 8a of the sintered bearing 8 and the outer peripheral surface 2a1 of the shaft portion 2a. Further, a pressure of an oil film in the radial bearing gap is increased by the dynamic pressure generating grooves 8a11 and the like formed in the inner peripheral surface 8a of the sintered bearing 8. Owing to this dynamic pressure generating action, there are formed a first radial bearing portion R1 and a second radial bearing portion R2 configured to rotatably support the shaft member 2 in a non-contact manner.
At the same time, a thrust bearing gap is formed between an upper end surface 2b1 of the flange portion 2b and the lower end surface 8b of the sintered bearing 8, and a thrust bearing gap is formed between a lower end surface 2b2 of the flange portion 2b and the upper end surface 7b1 of the bottom 7b of the housing 7. Further, the pressure of the oil film in each of the thrust bearing gaps is increased by the dynamic pressure generating grooves 8b1 formed in the lower end surface 8b of the sintered bearing 8 and the dynamic pressure generating grooves formed in the upper end surface 7b1 of the bottom 7b of the housing 7. Thus, there are formed a first thrust bearing portion T1 and a second thrust bearing portion T2 configured to rotatably support the shaft member 2 in both thrust directions in a non-contact manner.
At this time, the dynamic pressure generating grooves 8b1 formed in the lower end surface 8b of the sintered bearing 8 and the dynamic pressure generating grooves formed in the upper end surface 7b1 of the bottom 7b of the housing 7 are both of the pump-in type. Thus, the lubricating oil, which is filled in a space on the radially outer side with respect to the thrust bearing gaps at the first and second thrust bearing portions T1, T2 (space between the outer peripheral surface of the flange portion 2b and the inner peripheral surface 7a1 of the housing 7), is drawn to the radially inner side. In this embodiment, the space is communicated to the seal space S through the axial grooves 8c1 of the outer peripheral surface 8c of the sintered bearing 8 and the annular groove 8d1 and the radial grooves 8d2 of the upper end surface 8d. Thus, the above-mentioned space is always maintained in a state having a pressure close to an atmospheric pressure, thereby being capable of preventing generation of a negative pressure in the space. In particular, in this embodiment, the dynamic pressure generating grooves 8a11 and 8a12 formed in the upper region in the inner peripheral surface 8a of the sintered bearing 8 have an asymmetric shape in the axial direction. Thus, along with the rotation of the shaft member 2, a pumping force of force-feeding downward the lubricating oil in the radial bearing gap is generated. With this, the lubricating oil circulates through a path in the order of the radial bearing gap, the thrust bearing gap at the first thrust bearing portion T1, the axial grooves 8c1, the annular groove 8d1 and the radial grooves 8d2, and the radial bearing gap. Thus, local generation of a negative pressure in the lubricating oil filled in the housing 7 can be reliably prevented.
Now, a method of manufacturing the above-mentioned sintered bearing 8 is described. The sintered bearing 8 is manufactured through a forming step, a sintering step, and a sizing step.
In the forming step, material powder obtained by mixing various kinds of powder is subjected to compression molding so as to be formed into a cylindrical shape, thereby obtaining a compact. The material powder contains, for example, copper powder, iron powder, tin powder, and graphite powder. Specifically, in the forming step, first, as illustrated in
Further, as illustrated in
In the sintering step, the compact 18 is sintered at a predetermined sintering temperature, thereby forming a sintered body 28. The sintering temperature is set to, for example, from 850° C. to 900° C., and is set to 870° C. in this embodiment.
In the sizing step, the sintered body 28 is corrected to a predetermined dimension accuracy with a sizing die 20, and the dynamic pressure generating grooves are formed in an inner peripheral surface 28a of the sintered body 28 through die molding. Specifically, as illustrated in
In this manner, the sintered bearing 8 is completed. The inner peripheral surface of the sintered body before being subjected to the sizing step may be subjected to pore-sealing treatment such as rotation sizing or shot blasting, as needed.
According to the above-mentioned manufacturing method, the following effects can be obtained. That is, an outer peripheral surface 28c of the sintered body 28 carried into the sizing die 20 has the plurality of axial grooves 8c1 formed in the forming step. Therefore, when the sintered body 28 is press-fitted to the inner periphery of the die 24 in the sizing step, as illustrated in
In this embodiment, as described above, the number of the axial grooves 8c1 to be formed in the compact 18 in the forming step is set equal to the number of the dynamic pressure generating grooves on the same circumference, which are to be formed in the sintered body 28 in the sizing step (the number of each of the dynamic pressure generating grooves 8a11, 8a12, 8a21, and 8a22 is six). With this, as illustrated in
The present invention is not limited to the embodiment described above. Now, description is made of other embodiments of the present invention. Portions having the same functions as those in the embodiment described above are denoted by the same reference symbols, and redundant description thereof is omitted.
In the above-mentioned embodiment, description is given of the case where the number of the axial grooves 8c1 to be formed in the outer peripheral surface 8c of the sintered bearing 8 is equal to the number of the dynamic pressure generating grooves 8a11 and the like on the same circumference, which are to be formed in the inner peripheral surface 8a. However, the present invention is not limited thereto, and it is all necessary that the number of the axial grooves 8c1 be an integral multiple of the number of the dynamic pressure generating grooves 8a11 and the like on the same circumference. For example, in the sintered bearing 8 as illustrated in
Further, in the above-mentioned embodiment, description is given of the case where the dynamic pressure generating grooves 8a11 and the like having a herringbone pattern are formed in the inner peripheral surface 8a of the sintered bearing 8. However, the present invention is not limited thereto, and there may be formed dynamic pressure generating grooves having a spiral pattern or dynamic pressure generating grooves having a stepped pattern extending along the axial direction. Any of the dynamic pressure generating grooves is formed by subjecting the cylindrical inner peripheral surface of the sintered body to sizing. Further, in the above-mentioned embodiment, the dynamic pressure generating grooves 8a11 and the like are formed at the two positions of the inner peripheral surface 8a of the sintered bearing 8, which are separated from each other in the axial direction. However, the dynamic pressure generating grooves 8a11 and the like may be formed continuously in the axial direction, or only one pair of dynamic pressure generating grooves having a herringbone pattern may be formed.
Further, in the above-mentioned embodiment, description is given of the case where the dynamic pressure generating grooves 8b1 having a spiral pattern are formed in the lower end surface 8b of the sintered bearing 8. However, the present invention is not limited thereto. Dynamic pressure generating grooves having another shape, such as a herringbone pattern or a stepped pattern, may be formed. Further, the lower end surface 8b of the sintered bearing 8 may be formed into a flat surface, and dynamic pressure generating grooves may be formed in the upper end surface 2b1 of the flange portion 2b of the shaft member 2, which is opposed to the lower end surface 8b of the sintered bearing 8.
Further, the above-mentioned sintered bearing 8 is applicable not only to the fluid dynamic bearing device of a shaft rotation type, which is configured to support rotation of the shaft member, but also to a fluid dynamic bearing device of a shaft fixing type, in which the shaft member is fixed and the sintered bearing is rotated, or a fluid dynamic bearing device in which both the shaft member and the sintered bearing are rotated.
Further, the above-mentioned fluid dynamic bearing device is applicable not only to a spindle motor for an HDD, but also to a spindle motor for other information equipment, a polygon scanner motor for a laser beam printer, a color wheel for a projector, or a fan motor for an electrical apparatus.
In order to verify effects of the present invention, regarding the sintered bearing having the dynamic pressure generating grooves 8a11 and the like as illustrated in
Further, in the above-mentioned comparative product, the variation in groove depth of the dynamic pressure generating grooves when the groove depths of the axial grooves were set to be different from each other were measured. As a result, as shown in
1 fluid dynamic bearing device
2 shaft member
7 housing
8 sintered bearing
8
a
11, 8a12, 8a21, 8a22 dynamic pressure generating groove
8
c
1 axial groove
9 sealing member
10 forming die
11 die
12 core rod
13 lower punch
14 upper punch
18 compact
20 sizing die
21 core rod
22 lower punch
23 upper punch
24 die
28 sintered body
R1, R2 radial bearing portion
T1, T2 thrust bearing portion
S seal space
Number | Date | Country | Kind |
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2014-047693 | Mar 2014 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2015/054310 | 2/17/2015 | WO | 00 |