JP3602325B2 - Dynamic pressure type porous oil-impregnated bearing - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a self-lubricating function by impregnating a porous body such as a sintered metal with a lubricating oil or a lubricating grease, and a lubricating oil film (a lubricating oil formed by a dynamic pressure action of a dynamic pressure groove) interposed in a bearing gap. For a hydrodynamic porous oil-impregnated bearing that supports a shaft in a non-contact manner with an oil film, high rotational accuracy is required at high speeds, especially for a polygon mirror motor (LBP) for a laser beam printer and a spindle motor (HDD) for a magnetic disk drive. It is suitable for a device such as a DVD-ROM or a device such as a DVD-ROM which is driven at a high speed due to a large unbalance load acting on the disk.
[0002]
[Prior art]
In such small spindle motors related to information equipment, further improvement in rotational performance and cost reduction are required, and as a means for this, replacing the bearing part of the spindle from a rolling bearing to a porous oil-impregnated bearing. Is being considered. However, since porous oil-impregnated bearings are a kind of perfect circular bearings, unstable vibrations tend to occur where the eccentricity of the shaft is small, and so-called whirling, which oscillates at half the rotational speed, is likely to occur. There is. Therefore, it has been attempted to provide a dynamic pressure groove such as a herringbone type or spiral type on the bearing surface, and to generate a lubricating oil film in the bearing gap by the action of the dynamic pressure groove accompanying the rotation of the shaft to support the shaft in a non-contact manner. (Dynamic pressure type porous oil-impregnated bearing).
[0003]
As a prior art in which a dynamic pressure groove is formed on a bearing surface of a porous oil-impregnated bearing, there is one disclosed in Japanese Utility Model Publication No. 63-19627. In the technology described in the same publication, a region where a dynamic pressure groove is formed on a bearing surface is subjected to surface blinding to seal the region where the dynamic pressure groove is formed.
[0004]
[Problems to be solved by the invention]
Usually, in order to secure the rotation accuracy of the shaft, a plurality of bearings, for example, two bearings are used in combination. Further, the bearing is often used by being press-fitted into the housing. Therefore, in order to ensure the coaxiality of the two bearings, a method of inserting the straightening pin into the housing and then press-fitting the two bearings simultaneously is adopted. However, in a bearing with a dynamic pressure groove provided on the bearing surface, if the straightening pin is used to forcibly correct the dynamic pressure groove, the dynamic pressure groove on the bearing surface will be crushed by the biting of the straightening pin, and a stable dynamic pressure effect will be obtained. Disappears. On the other hand, if the press-fitting operation is performed without using the straightening pins, the required coaxiality between the bearings cannot be obtained. Therefore, it can be said that the configuration described in Japanese Utility Model Publication No. 63-19627 is difficult to put into practical use.
[0005]
Japanese Patent Laid-Open No. 2-107705 discloses a configuration in which two bearing surfaces are formed so as to be separated from each other in the axial direction, and a region between the bearing surfaces is larger in diameter than the bearing surfaces. Although there is no practical problem, the dynamic pressure grooves are not formed on the bearing surface, so that unstable vibration such as whirl cannot be prevented.
[0006]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the related art, and can prevent unstable vibration such as whirl and eliminate inconvenience in assembling (collapse of dynamic pressure groove shape, deviation of coaxiality). It is an object of the present invention to provide a configuration in which
[0007]
[Means for Solving the Problems]
According to the present invention, a plurality of bearing surfaces opposed to an outer peripheral surface of a shaft to be supported via a bearing gap are formed on a bearing body made of a porous body at a distance in the axial direction, and at least one of the plurality of bearing surfaces is formed. An inclined dynamic pressure groove was formed on each of the two. The inner diameter of the region between the bearing surfaces is set to be larger than the inner diameter of the bearing surface. The apertures including the dynamic pressure grooves are substantially uniformly distributed on the bearing surface on which the dynamic pressure grooves are formed, and the surface area ratio of the apertures on the bearing surface with the dynamic pressure grooves is 2%. And the kinematic viscosity at 40 ° C. of the contained oil is 2 cSt or more, and the surface area ratio and the kinematic viscosity are expressed by the following formula:
(3/5) A-1 ≤ η ≤ (40/6) A + (20/3)
Here, A: surface area ratio of opening [%]
η; kinematic viscosity at 40 ° C. of oil [cSt]
In the bearing surface where the dynamic pressure groove is formed, the sliding surface of the shaft is floated and supported by the lubricating oil film of the oil interposed in the bearing gap, and through the opening of the bearing surface including the dynamic pressure groove. Oil circulates between the inside of the bearing body and the bearing gap.
[0008]
By forming a plurality of bearing surfaces on one bearing, it is possible to solve the problem of coaxiality when a plurality of bearings are incorporated as in the related art. That is, since a plurality of bearing surfaces are provided on one bearing, it is not necessary to secure coaxiality using a correction pin as in the related art, and the shape of the dynamic pressure groove by the correction pin does not collapse. . By forming an inclined dynamic pressure groove on at least one bearing surface, unstable vibration such as whirl can be effectively prevented.
[0009]
By providing a step at the boundary between the bearing surface and the region between the bearing surfaces, it is possible to effectively reduce the torque loss in the region between the bearing surfaces.
[0010]
By drawing the axial cross section of the region between the bearing surfaces with a curve that is continuous with the bearing surface, oil that has oozed from the opening in the region between the bearing surfaces flows in the axial direction along that region, and Since the oil is easily supplied to the surface, the effective use of oil and the formation of a lubricating oil film are promoted.
[0011]
The axial cross section of the region between the bearing surfaces can be described by an arc having the largest diameter at the center of the region. Oil that has oozed out of the surface openings in that region is likely to be supplied to the bearing surfaces on both sides.
[0012]
When the outer diameter of the outer diameter portion corresponding to at least one bearing surface is set smaller than the outer diameter of the outer diameter portion corresponding to the region between the bearing surfaces, when the bearing body is press-fitted and fixed to the housing, It is possible to prevent or reduce deformation of the bearing surface due to press-fitting.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0014]
FIG. 1 shows a state in which a porous oil-impregnated bearing 1 according to a first embodiment of the present invention is fixed to a housing 2. The porous oil-impregnated bearing 1 is composed of a bearing body 1a made of a porous body, and lubricating oil or lubricating grease impregnated in the bearing body 1a. The bearing main body 1a is made of, for example, a sintered metal mainly containing copper or iron, or both, and desirably contains 20 to 95% by weight of copper and has a density of 6.4 to 7.2 g / cm ^3. It is formed so that In addition, as the material of the bearing body 1a, a porous body having a large number of pores may be used by sintering or foaming cast iron, synthetic resin, ceramics, or the like.
[0015]
On the inner periphery of the bearing body 1a, a plurality of, for example, two bearing surfaces 1b opposed to an outer peripheral surface of a shaft to be supported via a bearing gap 4 (see FIG. 4) are formed in the axial direction so as to be separated from each other. , A plurality of dynamic pressure grooves 1c are respectively formed in the circumferential direction. The dynamic pressure groove 1c in this embodiment has a shape in which a groove region inclined to one side with respect to the axial direction and a groove region inclined to the other pair form a V-shaped continuous shape. The dynamic pressure groove 1c may be formed on at least one bearing surface 1b.
[0016]
The inner diameter D1 of the region 1d between the bearing surfaces 1b in the bearing body 1a is larger than the inner diameter of the bearing surface 1b (strictly speaking, the inner diameter of the region of the back 1e (see FIG. 4) between the dynamic pressure grooves 1c) D2. Is set. In this embodiment, the axial cross section of the region 1d is drawn as one circular arc that is continuous with the bearing surface 1b, and the largest diameter portion of the circular arc is located at the axial center of the region 1d. Note that a step may be provided at the boundary between the region 1d and the bearing surface 1b. In addition, the axial section of the region 1d can be drawn with a curve such as an ellipse, a parabola, or the like, in addition to a circular arc. You may draw by combining two curves of the same kind (for example, two arcs), combining two different kinds of curves (for example, a combination of an arc and a parabola), or combining a curve and a straight line. Further, the maximum diameter portion of the region 1d may be unevenly distributed on one bearing surface 1b side.
[0017]
In this embodiment, the outer diameter D3 of the outer diameter portion 1f corresponding to each of the two bearing surfaces 1b in the bearing body 1a is changed to the outer diameter size of the outer diameter portion 1g corresponding to the region 1d between the bearing surfaces 1b. It is set smaller than D4. In the case where the porous oil-impregnated bearing 1 is press-fitted and fixed to the inner periphery of the housing 2 in the manner shown in the figure, deformation due to press-fitting of the bearing surface 1b can be prevented or reduced, so that accuracy can be easily obtained. The fixing force is obtained by the press fit between the outer diameter portion 1g and the housing 2. The region 1d is formed to have a larger diameter than the bearing surface 1b, and is not directly involved in supporting the shaft. Therefore, even if a deformation corresponding to the press-in force occurs, the accuracy of the bearing is not affected. The dimensional difference between the outer diameter D3 of the outer diameter portion 1f and the outer diameter D4 of the outer diameter portion 1g (the dimensional difference before press-fitting) is the press-fitting allowance with the housing 2 (the press-fitting allowance of the outer diameter portion 1g). In consideration of the above, the outer diameter portion 1f is set so as not to be in contact with the inner periphery of the housing 2 or to have an interference that does not affect bearing accuracy. The outer diameter of only one of the two outer diameter portions 1f is set as described above, the inner diameter of the housing 2 corresponding to the other is increased, and the housing 2 is press-fitted and fixed to only the outer diameter portion 1g. You may.
[0018]
FIG. 2 shows the flow of oil in an axial section when the shaft 3 is supported by the porous oil-impregnated bearing 1 having the above-described configuration. With the rotation of the shaft 3, the oil impregnated in the bearing body 1a oozes into the bearing gap 4 from both sides in the axial direction of the bearing surface 1b and near the chamfer portion, and is further directed toward the axial center of the bearing gap 4 by the dynamic pressure groove. Pulled in. Due to the oil drawing action (dynamic pressure action), the pressure of the oil film interposed in the bearing gap 4 is increased, and the lubricating oil film 5 is formed. By the lubricating oil film 5 formed in the bearing gap 4, the shaft 3 is supported in a non-contact manner with respect to the bearing surface 1b without causing unstable vibration such as whirl. The oil that seeps into the bearing gap 4 is mainly a surface opening of the bearing surface 1b due to the pressure generated by the rotation of the shaft 3 (meaning a portion in which the pores of the porous body structure are opened on the outer surface). From the bearing body 1a, circulates inside the bearing body 1a, and oozes out again into the bearing gap 4 from the bearing surface 1b and the vicinity of the chamfer portion. In this way, while the oil impregnated in the bearing body 1a circulates between the bearing gap 4 and the bearing body 1a, the shaft 3 is continuously supported in a non-contact manner by the dynamic pressure effect as described above.
[0019]
Further, in the porous oil-impregnated bearing 1, the shaft 3 is supported in a non-contact manner by the two bearing surfaces 1b separated in the axial direction, so that the shaft 3 can be supported by one bearing with high precision. Further, a negative pressure is generated in a space formed between the region 1d between the bearing surfaces 1b and the outer peripheral surface of the shaft 3 due to the drawing action of the dynamic pressure groove 1c, and oil seeps from the surface opening in the region 1d. Since it is supplied to the bearing surface 1b, the formation of the lubricating oil film 5 in the bearing gap 4 is promoted, and the bearing force is increased. In particular, as in this embodiment, when the axial cross section of the region 1d is drawn by a continuous arc (or other curve) on the bearing surface 1b, the oil that has oozed from the surface opening of the region 1d is Since it flows in the axial direction along the region 1d and is effectively supplied to the bearing surface 1b, it leads to effective utilization of oil and promotion of formation of a lubricating oil film.
[0020]
By the way, in this embodiment, surface openings are distributed over the entire region of the bearing surface 1b including the region where the dynamic pressure groove 1c is formed. This is because the following problem occurs when the formation region of the dynamic pressure groove is sealed as described in Japanese Utility Model Publication No. 63-19627.
[0021]
{Circle around (1)} When the region where the dynamic pressure groove is formed is completely sealed, the circulation of oil, which is the greatest feature of the porous oil-impregnated bearing, is impeded in that region. Therefore, the oil once seeping into the bearing gap is pushed into the axial center of the bearing surface by the action of the dynamic pressure groove and stays there. Since a large shearing action is acting in the bearing gap, the oil remaining in the bearing gap due to the shearing force and frictional heat tends to be denatured, and the temperature tends to accelerate the oxidative deterioration. Therefore, the bearing life is shortened.
[0022]
{Circle around (2)} It is extremely difficult to seal the area where the dynamic pressure grooves are formed. Although the above-mentioned publication states that the hole can be sealed by plastic working, the groove depth of the dynamic pressure groove is usually on the order of μm, and the surface opening is not sealed by such compression molding. In addition, coating is mentioned as another means of plastic working, but the thickness of the coating film needs to be thinner than the groove depth, and it is extremely difficult to apply a coating film of several μm only to the inclined groove region. It is.
[0023]
As in this embodiment, the oil is circulated between the inside of the bearing main body 1a and the bearing gap 4 through the surface opening of the bearing surface 1b including the formation region of the dynamic pressure groove 1c. The above problems can be solved.
[0024]
In order to keep the circulation of the oil as described above appropriately, it is desirable that the surface openings are distributed almost uniformly in the region of the dynamic pressure groove 1c and the back 1e in the bearing surface 1b. When the ratio of the surface openings is small, the oil becomes difficult to move, and when it is large, the oil becomes easy to move. The viscosity of the impregnated oil is also related to the ease of movement of the oil. The lower the viscosity, the easier it is to move, and the higher the viscosity, the harder it is to move.
[0025]
When the surface porosity (meaning the area ratio of the surface porosity occupying a unit area of the outer surface) is large and the viscosity is low, the oil becomes extremely easy to move. The exuded oil is easily returned to the inside of the bearing main body, so that the dynamic pressure effect is reduced and high rotational accuracy cannot be maintained, and the shaft and the bearing surface may come into contact with each other. Conversely, when the surface porosity is small and the viscosity is high, the oil becomes extremely difficult to move, so the generated pressure increases, but proper circulation is hindered and the torque also increases. Oil degradation is promoted.
[0026]
Therefore, the surface porosity and the viscosity of the oil have an optimum range in which the formation of the lubricating oil film necessary for supporting the shaft in a non-contact manner can be ensured, and at the same time, the proper circulation of the oil can be ensured.
[0027]
In order to clarify this optimum range, an evaluation test was performed using an actual LBP motor. The actual motor used in the evaluation test had a shaft diameter of φ4, was mounted with a mirror, had a rotation speed of 10,000 rpm, and had an ambient temperature of 40 ° C.
[0028]
FIG. 5 shows the results of the evaluation test. In FIG. 5, “示 す” indicates that there was no problem in the durability test after continuous operation for 1000 hours. “Δ” indicates a problem such as an increase in shaft runout (5 μm or more), an increase in torque = a decrease in the number of revolutions (the number of revolutions does not increase until 10,000 rpm), abnormal noise, etc., during 500 to 1000 hours, and abnormal operation. Indicates that it is possible. “X” indicates that the above-mentioned trouble occurred by 500 hours.
[0029]
From the above evaluation experiments, the optimum range of the surface porosity and the viscosity of the oil (the range in which “x” does not exist) is the area defined by the solid line in FIG. 5, that is, the following conditions (1) Formation of dynamic pressure groove The surface porosity of the bearing surface including the region is 2% or more and 20% or less,
(2) The kinematic viscosity at 40 ° C. of the impregnated oil is 2 cSt or more,
{Circle around (3)} The surface porosity of the bearing surface and the kinematic viscosity of the oil at 40 ° C. are (3/5) A−1 ≦ η ≦ (40/6) A + (20/3)
Here, A; surface porosity [%]
η; kinematic viscosity at 40 ° C. of oil [cSt]
It can be understood that this is the case. By selecting the surface porosity and the viscosity of the oil within such a range, a sufficient lubricating oil film is formed to support the shaft in a non-contact manner, and at the same time, appropriate circulation of the oil is ensured. Rotation accuracy and long life can be achieved.
[0030]
The surface porosity of the bearing surface is desirably 2% or more and 15% or less.
[0031]
There is an optimal range for the ratio between the groove depth (h) of the dynamic pressure groove and the size of the bearing gap (radial gap: c), and it is considered that a sufficient dynamic pressure effect cannot be obtained outside this range. In order to clarify this optimum range, an evaluation test was performed by replacing the shaft of the actual LBP motor with a longer one so that the shaft runout could be measured. The rotation speed was 10000 rpm, the test atmosphere was room temperature and normal humidity, and the LBP actual motor was φ4 and no mirror was mounted. The shaft runout was measured by a non-contact displacement meter.
[0032]
Under the above conditions, the values of the shaft runout with respect to c / h (c: radial gap, h: groove depth) were plotted, and the results shown in FIG. 6 were obtained. According to FIG. 6, when c / h is in the range of 0.5 to 4.0, the shaft runout is 5 μm or less, but when it is less than 0.5 or larger than 4.0, it becomes 5 μm or more. Therefore, in order to maintain high accuracy, it is desirable that c / h be in the range of 0.5 to 4.0. Note that the size c (radial gap) of the bearing gap is preferably set within the range of c / R = 1/2000 to 1/400, where R is the radius of the shaft.
[0033]
FIG. 3 shows a porous oil-impregnated bearing 1 'according to a second embodiment of the present invention. The difference between the porous oil-impregnated bearing 1 'of this embodiment and the porous oil-impregnated bearing 1 of the first embodiment described above is the shape of the bearing surface 1b' and the shape of the region 1d 'between the bearing surfaces 1b'. . Other configurations are the same as those of the first embodiment, and corresponding members and portions are denoted by the same reference numerals, and redundant description will be omitted.
[0034]
The bearing surface 1b 'in the porous oil-impregnated bearing 1' of this embodiment has a first region m1 in which a plurality of hydrodynamic grooves 1c1 inclined to one side with respect to the axial direction are formed in a circumferential direction, and a first region m1. A second region m2 in which a plurality of dynamic pressure grooves 1c2 are formed in the circumferential direction and are spaced apart from each other in the axial direction and inclined in the other direction with respect to the axial direction, and are located between the first region m1 and the second region m2. And an annular smooth portion n. The back 1e1 of the first region m1 and the back 1e2 of the second region m2 are continuous with the smooth portion n. When relative rotation occurs between the bearing main body 1a 'and the shaft, the oil is drawn into the smooth portion n by the dynamic pressure grooves 1c1 and 1c2 that are formed in the first region m1 and the second region m2, respectively. Since oil is collected around the smooth portion n, the oil film pressure in the bearing gap is increased. Moreover, since the dynamic pressure grooves are not formed in the smooth portion n as compared with the bearing surface as shown in FIG. 1, the effect of forming the lubricating oil film in that portion is high, and in addition to the spine 1e1, 1e2, the smooth portion Since n is also a support surface for supporting the shaft, the support area is enlarged, and the bearing rigidity is increased.
[0035]
The axial section of the region 1d 'between the bearing surfaces 1b' is drawn as a straight line in the axial direction, and the boundary between the region 1d 'and the bearing surface 1b' is a step 1h. Incidentally, the axial section of the region 1d 'may be drawn by combining two straight lines inclined with respect to the axial direction (V-shape).
[0036]
As in the first embodiment, the inner diameter of the region 1d 'is set to be larger than the inner diameter of the bearing surface 1b', and the outer diameter of the outer diameter portion 1f corresponding to the bearing surface 1b 'is It is set smaller than the outer diameter of the outer diameter portion 1g corresponding to 1d '.
[0037]
【Example】
(1) Comparative test of press-fitting into housing Comparative example product: A configuration having one bearing surface in which a dynamic pressure groove is formed. Two test pieces each having an inner diameter of 3.006 mm before press-fitting were manufactured, and were press-fitted into the housing with an interference of 18 μm with the housing and a correction pin diameter of 3.000 mm.
[0038]
Example product: A configuration including two bearing surfaces on which dynamic pressure grooves are formed. The test piece was pressed into the housing under the same conditions as above.
[0039]
Test result: In each of the comparative examples, a part of the dynamic pressure groove on the bearing surface was crushed. A test was conducted by incorporating the motor into a motor. The results showed that the rotation was unstable and the shaft runout was lower than that of a normal circular bearing (a bearing having no dynamic pressure groove on the bearing surface). The reason why a part of the dynamic pressure groove was crushed is considered to be that the test piece has uneven thickness (the same applies to the bearing product), and thus the correcting force of the straightening pin strongly applied to a part of the dynamic pressure groove.
[0040]
On the other hand, in the example product, although the groove depth was reduced as a whole (decreased from 4 μm to 3.5 μm), the phenomenon of partially crushing was not observed. When the shaft runout was measured by incorporating the motor into a motor, it showed an excellent performance of 2 μm or less at 2000 to 15000 rpm.
(2) Rotational accuracy comparison test Comparative example: A configuration including two bearing surfaces on which no dynamic pressure grooves are formed.
[0041]
Example product: A configuration including two bearing surfaces on which dynamic pressure grooves are formed (the configuration in FIG. 1).
[0042]
Test results: The test results are shown in FIG. As shown in the figure, the product of the example exhibited superior performance as compared with the product of the comparative example (solid squares represent measurement data of the comparative product and comparative samples).
[0043]
【The invention's effect】
The present invention has the following effects.
[0044]
(1) Since a plurality of bearing surfaces are formed on one bearing, the problem of coaxiality when a plurality of bearings are incorporated as in the related art can be solved. That is, since a plurality of bearing surfaces are provided on one bearing, it is not necessary to secure coaxiality using a correction pin as in the related art, and the shape of the dynamic pressure groove by the correction pin does not collapse. . Further, since the inclined dynamic pressure grooves are formed on at least one bearing surface, unstable vibration such as whirl can be effectively prevented. In addition, on the bearing surface having the dynamic pressure groove, the distribution mode of the surface aperture, the surface aperture ratio, and the kinematic viscosity of the oil are optimally set, so that a sufficient lubricating oil film is formed to support the shaft in a non-contact manner. At the same time, appropriate circulation of oil is ensured, and high rotational accuracy and long life can be achieved.
[0045]
(2) By providing a step at the boundary between the bearing surface and the region between the bearing surfaces, it is possible to effectively reduce the torque loss in the region between the bearing surfaces.
[0046]
(3) By drawing the axial cross section of the region between the bearing surfaces with a curve that is continuous with the bearing surface, the oil that seeps out of the surface openings in the region between the bearing surfaces is axially drawn along the region. Since it flows and is easily supplied to the bearing surface, effective use of oil and formation of a lubricating oil film are promoted.
[0047]
(4) By drawing the axial cross section of the region between the bearing surfaces with an arc having the largest diameter at the center of the region, the oil oozing out from the surface opening in the region is effectively applied to the bearing surfaces on both sides. Can be supplied to
[0048]
(5) When the outer diameter of the outer diameter portion corresponding to the bearing surface is set smaller than the outer diameter of the outer diameter portion corresponding to the region between the bearing surfaces, the bearing body is press-fitted and fixed to the housing. It is possible to prevent or reduce deformation of the bearing surface due to press-fitting.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a first embodiment of the present invention.
FIG. 2 is a view schematically showing the flow of oil in an axial section when the shaft is supported by the porous oil-impregnated bearing of the embodiment.
FIG. 3 is a cross-sectional view showing a second embodiment of the present invention.
FIG. 4 is a diagram showing a partial cross section of a bearing surface.
FIG. 5 is a diagram showing the results of an evaluation test.
FIG. 6 is a diagram showing the results of an evaluation test.
FIG. 7 is a diagram showing the results of an evaluation test.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Porous oil-impregnated bearing 1a Bearing main body 1b Bearing surface 1c Dynamic pressure groove

Claims (7)

A plurality of bearing surfaces formed of a porous body and opposed to an outer peripheral surface of a shaft to be supported via a bearing gap are formed apart in the axial direction, and an inner diameter of a region between the bearing surfaces is an inner diameter of the bearing surface. A bearing body set larger than the lubricating oil or lubricating grease impregnated in the bearing body, and an inclined dynamic pressure groove formed on at least one of the plurality of bearing surfaces ,
On the bearing surface on which the dynamic pressure groove is formed, the apertures are substantially uniformly distributed including the dynamic pressure groove,
A surface area ratio of the opening portion on the bearing surface on which the dynamic pressure groove is formed is 2% or more and 15% or less;
A kinematic viscosity at 40 ° C. of the contained oil is 2 cSt or more;
The surface area ratio and the kinematic viscosity are represented by the following formula:
(3/5) A-1 ≤ η ≤ (40/6) A + (20/3)
Here, A: surface area ratio of opening [%]
η; kinematic viscosity at 40 ° C. of oil [cSt]
Satisfied,
On the bearing surface in which the dynamic pressure groove is formed, the sliding surface of the shaft is levitated and supported by a lubricating oil film of oil interposed in the bearing gap, and through an opening in the bearing surface including the dynamic pressure groove, A hydrodynamic porous oil-impregnated bearing that circulates oil between the inside of the bearing body and a bearing gap . 2. The hydrodynamic porous oil-impregnated bearing according to claim 1, wherein the bearing body is formed of a sintered metal. 3. The hydrodynamic porous oil-impregnated bearing according to claim 2, wherein the sintered metal contains copper or iron, or both of them as main components. 4. The hydrodynamic porous oil-impregnated bearing according to claim 1, wherein a boundary between the bearing surface and a region between the bearing surfaces is a step. 4. The hydrodynamic porous oil-impregnated bearing according to claim 1, wherein an axial cross section of a region between the bearing surfaces is drawn by a curve continuous with the bearing surface. 6. The hydrodynamic porous oil-impregnated bearing according to claim 5, wherein the curve is an arc having the largest diameter at the center of the region between the bearing surfaces. The outer diameter of an outer diameter portion corresponding to the at least one bearing surface is set smaller than the outer diameter of an outer diameter portion corresponding to a region between the bearing surfaces. 7. A dynamic pressure type porous oil-impregnated bearing according to item 5 or 6.

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