JPH10210700A - Bearing for motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent flow-outs of lubricant from a sintered oil-retaining bearing for bearing a rotary shaft in the inner circumferential surface in the radial direction, even in the case of precessional motion of the rotary shaft and to prevent generation of uneven rotation or metallic contact in a motor having a thrust bearing for bearing thrust load of a rotor on the end face thereof, disposed radially outside of the rotary shaft. SOLUTION: A semi-solid grease 20 for blocking flow-outs of lubricant from a sintered oil retaining bearing is applied into a gap between the inner circumferential surface of the sintered oil-retaining bearing 15 and the outer circumferential surface of a rotary shaft 7, so that a fibrous material in the grease captures the lubricant to prevent the flow-out thereof. A plurality of grease sumps are provided in the inner circumferential surface of the sintered oil-retaining bearing 15, in order to hold the grease, thus preventing flow-out of lubricant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing device for a motor provided with a thrust bearing which is disposed radially outward of a rotary shaft and supports a thrust load of a rotor at an end face. .

[0002]

2. Description of the Related Art In order to support a thrust load of a rotor of a motor, one end of a rotating shaft is generally brought into contact with a thrust bearing. In some cases, a motor provided with a thrust bearing that is disposed radially outward of the rotary shaft and that supports the thrust load of the rotor at the end face is used. FIG. 6 (a)
(B) shows an example of a motor having such a surface sliding bearing.

In FIG. 6A, one end of a sintered oil-impregnated bearing 2 is fitted to a hole formed in the center of a motor substrate 1 made of a magnetic material such as an iron plate, and is fixed by a fixing means such as caulking. The sintered oil-impregnated bearing 2 is fixed to the substrate 1.
On the substrate 1, the sintered oil-impregnated bearing 2 projecting above the substrate 1 is provided.
A thrust bearing 6 composed of a ball bearing is disposed on the outer peripheral side of the thrust bearing. Further, a plate-shaped core holder 3 is fixed to the outer peripheral side of the thrust bearing 6, and the inner peripheral edge of the stator core 4 is mounted on the outer peripheral upper surface of the core holder 3.
The stator core 4 is fixed on the substrate 1 by screwing a set screw passing through the substrate 1 and the core holder 3 into the stator core 4. The stator core 4 is formed by laminating a plurality of core elements, and has a plurality of salient poles radially, and a drive coil 5 is wound around each salient pole, as is well known.

A rotating shaft 7 is provided on the inner peripheral side of the sintered oil-impregnated bearing 2.
The rotary shaft 7 is radially supported on the inner peripheral surface of the sintered oil-impregnated bearing 2. The lubricating oil is impregnated in the sintered oil-impregnated bearing 2, and the lubricating oil is interposed in the gap between the inner peripheral surface of the sintered oil-impregnated bearing 2 and the outer peripheral surface of the rotating shaft 7, so that the rotating shaft 7 rotates smoothly. It is supposed to. The center hole of a flat cup-shaped rotor case 8 is press-fitted into the outer periphery of the rotating shaft 7 protruding from the upper end surface of the sintered oil-impregnated bearing 2, and the rotating shaft 7 and the rotor case 8 are integrally connected. The rotor case 8 covers a stator of the motor including the stator core 4, and a drive magnet 12 is fixed to an inner surface of an outer peripheral wall of the rotor case 8. The inner peripheral surface of the drive magnet 12 faces the distal end surface of each of the salient poles, which is the outer peripheral surface of the stator core 4, with an appropriate gap.

This motor is a motor for driving a disk, and a hub base 9 on which a disk hub is mounted is integrally formed in a rotor case 8 by outsert molding of resin or the like. A part of the resin forming the hub base 9 passes through the hole of the rotor case 8 and reaches the back side of the rotor case 8, that is, the lower side in FIG. 6 (a). They are fixed together. The thrust ring 10 is mounted on a ball bearing constituting the thrust bearing 6. A magnetic attractive force is generated between the drive magnet 12 and the substrate 1, and a thrust load of the rotor due to the magnetic attractive force is supported by the thrust bearing 6 via the thrust ring 10. The thrust bearing 6 allows the rotation of the rotor while supporting the thrust load of the rotor. A drive pin 1 for rotating the disk by engaging with an engagement hole of a disk hub mounted on the hub table 9 is provided on the hub table 9.
1 is provided. As is well known, the driving magnet 12
By controlling the energization of the drive coil 5 of each salient pole in accordance with the rotational position of the rotor, the rotor having the drive magnet 12, the rotor case 8, the hub 9 and the drive pin 11 is driven to rotate, The disk mounted on the hub base 9 can be driven to rotate.

[0006] The example shown in FIG.
This is the same as the example shown in (a), except that the sintered oil-impregnated bearing 15 that supports the rotating shaft 7 in the radial direction also serves as the thrust bearing of the rotor. More specifically, the sintered oil-impregnated bearing 15 fitted and fixed in the hole of the substrate 1 protrudes relatively large radially outward on the substrate 1, and rotates on the inner peripheral surface 15 a of the sintered oil-impregnated bearing 15. The shaft 7 is supported in the radial direction, and the outer peripheral edge upper end surface 15 of the sintered oil-impregnated bearing 15 is supported.
When the thrust ring 10 slides on b, the thrust load of the rotor is supported. The other configuration is the same as the example shown in FIG.

[0007]

As described above, in a motor having a surface sliding bearing type thrust bearing, the thrust bearing supports the thrust load of the rotor radially outside the rotary shaft. The rotor rotates in a state in which the thrust load of the rotor is always applied to the entire circumference, that is, specifically, in a state in which the entire circumference of the thrust ring 10 is always in sliding contact with the upper end surface 15a of the thrust bearing 6 or the sintered oil-impregnated bearing 15. . Therefore, assuming that the perpendicularity between the sliding surface of the rotor with respect to the thrust bearing and the center axis of the rotating shaft 7 does not accurately appear, the entire rotor follows the sliding surface with the thrust bearing in FIG. Although rotating in a horizontal plane, the rotating shaft 7 rotates while drawing a conical shape called a rubbing motion. Therefore, in order to eliminate the sliding motion of the rotating shaft 7, the perpendicularity between the sliding surface of the rotor with respect to the thrust bearing and the central axis of the rotating shaft 7 may be improved. Each part accuracy and assembly accuracy must be strictly controlled, which causes a rise in cost. In particular, it is difficult for the flat motor as shown in FIG.

[0008] The sliding motion of the rotating shaft 7 does not directly adversely affect signal recording and signal reading on the disk. However, since the sliding motion of the rotating shaft 7 is performed with the center of the lower end of the rotating shaft 7 in FIG. 6 as a fulcrum, the outer peripheral edge of the lower end of the rotating shaft 7 is attached to the inner peripheral surface of the sintered oil-impregnated bearing. Hit as if kneading. Moreover,
In the flat type motor, since a strong magnet material is used as the drive magnet 12 to make the motor thinner,
The magnetic attraction between the drive magnet 12 and the substrate 1 is large,
The thrust load of the rotor is also large. As a result, the outer peripheral edge of the lower end of the rotary shaft 7 due to the so-called rubbing motion is more strongly contacted with the inner peripheral surface of the sintered oil-impregnated bearing, and the resistance to rotation is increased, and rotation unevenness is likely to occur.

FIG. 7 schematically shows the bearing portion of the example shown in FIG. 6B, in which the size of the gap between the inner peripheral surface of the sintered oil-impregnated bearing 15 and the outer peripheral surface of the rotary shaft 7 is emphasized. As shown in FIG. 7, the gap between the inner peripheral surface of the sintered oil-impregnated bearing 15 and the outer peripheral surface of the rotating shaft 7 is
The lubricating oil 13 impregnated in the sintered oil-impregnated bearing 15 is interposed. The rotational unevenness is not so large while the sintered oil-impregnated bearing is sufficiently impregnated with the lubricating oil. However, when the rotating shaft 7 performs the rubbing motion, the lubricating oil 13 impregnated in the sintered oil-impregnated bearing is short-lived. And the rotating shaft 7 comes into metal contact with the sintered oil-impregnated bearing, causing uneven rotation. In FIGS. 6 (a), 6 (b) and FIG. 7, reference numeral 13a denotes lubricating oil which has been expelled from the sintered oil-impregnated bearing and adhered in a droplet form to the lower surface of the sintered oil-impregnated bearing as described above. I have. When the lubricating oil is supplied between the rotating shaft 7 and the sintered oil-impregnated bearing when the rotational unevenness occurs as described above, the rotational unevenness is temporarily improved. This causes rotation unevenness again. When the rotating shaft 7 comes into metal contact with the sintered oil-impregnated bearing in this way, burning may occur in severe cases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and has a thrust bearing which is disposed radially outward of a rotary shaft and which supports a thrust load of a rotor at an end face. In a motor, the perpendicularity between the sliding surface of the rotor with respect to the radial bearing and the rotation center axis of the rotation shaft is not exactly set, and even if the rotation shaft makes a sliding motion, the rotation shaft is radially fixed on the inner circumferential surface. It is an object of the present invention to provide a motor bearing device that can prevent lubricating oil from flowing out of a sintered oil-impregnated bearing that is supported in a direction, thereby eliminating occurrence of rotation unevenness and occurrence of metal contact.

[0011]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is directed to a sintered oil-impregnated bearing that radially supports a rotating shaft on an inner peripheral surface, A thrust bearing disposed on the outside and supporting a thrust load of the rotor at an end face thereof, wherein a sintered bearing is disposed in a gap between an inner peripheral face of the sintered oil-impregnated bearing and an outer peripheral face of the rotary shaft. By applying semi-solid grease that suppresses the outflow of lubricating oil impregnated in oil-impregnated bearings, the fibrous material contained in the grease accumulates lubricating oil and prevents lubricating oil from flowing out. And

A second aspect of the present invention is characterized in that a plurality of grease reservoirs are provided on the inner peripheral surface of the sintered oil-impregnated bearing, and grease is retained in the grease reservoir to prevent outflow of lubricating oil. I do.

According to a third aspect of the present invention, the lubricating oil and the grease are made hard to flow out by projecting one end of the inner peripheral surface of the sintered oil-impregnated bearing in the thrust direction from one end of the rotary shaft. Features.

[0014] The invention described in claim 4 is characterized in that the grease contains the same base oil as the base oil contained in the lubricating oil impregnated in the sintered oil-impregnated bearing.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a motor bearing device according to the present invention will be described below with reference to FIGS. The present invention provides a sintered oil-impregnated bearing that radially supports a rotating shaft on an inner peripheral surface, as shown in the example of FIGS. 6A and 6B, and is disposed radially outward from the rotating shaft. The motor is provided with a thrust bearing that supports the thrust load of the rotor at the end face, and the bearing portion of such a motor has structural features. The configuration other than the bearing portion is shown in FIG. Since the configuration may be the same as the example shown in b), the configuration of the bearing portion of the motor will be mainly described here.
The same components as those in the example shown in FIG. 6B are denoted by the same reference numerals.

In FIG. 1, a rotary shaft 7 forming a part of a motor rotor is mounted on the inner peripheral side of the sintered oil-impregnated bearing 15. There is a gap between the outer peripheral surface of the rotating shaft 7 and the inner periphery of the sintered oil-impregnated bearing 15, and semi-solid (gel-like or jelly-like) grease 20 is applied to this gap. Although the above gap is actually a slight gap, it is drawn as an extremely large gap in the figure for easy understanding. The lower end surface of the sintered oil-impregnated bearing 15 and the lower end surface of the rotating shaft 7 are substantially flush with each other. The sintered oil-impregnated bearing 15 has a myriad of minute pores, and the porous oil is impregnated with lubricating oil.

The lubricating oil is generally composed of a base oil composed of an α-olefin or ester synthetic oil or a paraffin or naphthenic mineral oil, an antioxidant, a rust inhibitor, an antiwear agent, a metal deactivator. And the like. On the other hand, the grease 20 comprises a base oil of the same type as the base oil of the lubricating oil, and a metal salt such as a fatty acid or naphthenic acid, and an additive containing an alkali metal salt such as lithium soap and sodium soap, It is in the form of a semi-liquid, semi-solid gel or jelly. The alkali metal salt such as the lithium soap and the sodium soap contained in the grease 20 is a chain compound and has a fibrous property, so that the fibrous material can be held by holding oil.

Accordingly, in the example shown in FIG. 1, the grease 20 applied to the gap between the inner peripheral surface of the sintered oil-impregnated bearing 15 and the outer peripheral surface of the rotary shaft 7 is impregnated in the sintered oil-impregnated bearing 15. The lubricating oil is retained, and the lubricating oil is prevented from flowing out of the sintered oil-impregnated bearing 15. Even if the rotating shaft 7 makes a rubbing movement and the outer peripheral edge of the lower end of the rotating shaft 7 strongly contacts the inner peripheral surface of the sintered oil-impregnated bearing 15, the grease 20 flows out of the lubricating oil of the sintered oil-impregnated bearing 15. To prevent uneven rotation of the rotating shaft 7 and the rotor integrated with the rotating shaft 7 and the occurrence of metal contact due to lack of oil. In the example shown in FIG. 1, the lower end surface of the sintered oil-impregnated bearing 15 and the lower end surface of the rotary shaft 7 are substantially flush with each other. Therefore, as shown in FIG. The protruding portion 20a that protrudes from the lower end surface of the rotating shaft 7 in the form of a droplet may be generated. However, even if the protruding portion 20a is generated, the grease 20 has a fibrous property as described above and is hard to spill. Therefore, the grease 20 itself does not flow out. As described above, since the base oil of the grease 20 is of the same type as the base oil of the lubricating oil impregnated in the sintered oil-impregnated bearing 15, the grease 20 itself also exerts a lubricating effect, causing uneven rotation and metal contact. Prevent the occurrence of such.

The following table shows that a sintered oil-impregnated bearing 15 as shown in FIG. 6 (b) radially supports the rotating shaft 7 and that the end surface of the motor constitutes a thrust bearing. General ordinary product, ie, sintered oil-impregnated bearing 1
5 which is simply impregnated with lubricating oil, and a gel-like material which prevents the lubricating oil impregnated in the sintered oil-impregnated bearing from flowing out into the gap between the inner peripheral surface of the sintered oil-impregnated bearing and the outer peripheral surface of the rotating shaft. Alternatively, the rotation unevenness and the measured current value are compared with the example of FIG. 1 in which jelly-like grease is applied. Rotation unevenness (%) Current value (mA) Normal product Initial stage 0.12 42.6 (Conventional) After test 0.41 47.2 Grease coated product Initial stage 0.12 42.6 (Invention) After test 0. 12 41.5

As can be seen from the above table, a sintered oil-impregnated bearing 15 that radially supports the rotating shaft 7 on the inner peripheral surface;
In a motor bearing device having a thrust bearing disposed radially outside the rotating shaft 7 and supporting a thrust load of the rotor at an end face, an inner peripheral surface of the sintered oil-impregnated bearing 15 and an outer periphery of the rotating shaft 7 are provided. According to the embodiment shown in FIG. 1 in which a gel or jelly-like grease 20 for suppressing the outflow of the lubricating oil impregnated in the sintered oil-impregnated bearing 15 is applied in the gap between the lubricating oil and the oil-impregnated bearing 15, Outflow can be suppressed, and deterioration of rotation unevenness of the motor and an increase in current value can be suppressed. Further, there is no metal contact, and seizure between the sintered oil-impregnated bearing 15 and the rotating shaft 7 can be prevented.

Next, various modifications of the present invention will be described. If a grease reservoir is provided on the inner peripheral surface of the sintered oil-impregnated bearing and the grease is retained in the grease reservoir, it is more effective to suppress the lubricating oil from flowing out. FIG. 2 shows an example in which a grease reservoir is provided. The inner peripheral portion of the lower end of the sintered oil-impregnated bearing 15 is formed in a conical shape and widened, and the conical portion is used as a grease reservoir 15d.
The grease 20 is applied and held in the range of the length L in the axial direction. In the gap between the inner peripheral surface of the sintered oil-impregnated bearing 15 and the outer peripheral surface of the rotary shaft 7, lubricating oil 13 leached from the sintered oil-impregnated bearing 15 is interposed. Since there is a grease 20 that holds the oil 13 and suppresses the lubricating oil 13 from flowing out of the sintered oil-impregnated bearing 15, as in the embodiment shown in FIG. Deterioration and increase in current value can be suppressed. Further, there is no metal contact, and seizure between the sintered oil-impregnated bearing 15 and the rotating shaft 7 can be prevented. The above dimension L
Is desirably 0.5 mm or more.

FIG. 3 shows another embodiment of the grease reservoir. In FIG. 3, a plurality of groove-shaped grease reservoirs 15e are formed at equal intervals in the circumferential direction on the inner peripheral surface of the sintered oil-impregnated bearing 15, and each grease reservoir 15e holds a grease 20. The grease reservoir 15e has a groove shape elongated in the center axis direction. For example, when the inner diameter of the sintered oil-impregnated bearing 15 is 4.0 mm and the outer diameter of the rotating shaft 7 is 3.99 mm, each grease reservoir 15e is formed.
e has a circumferential width W of 0.5 mm or more and a depth of 0.01 m.
m or more. In the example of FIG. 3, six grease reservoirs 15e are formed, but the number of grease reservoirs 15e is arbitrary. However, it is desirable that there be a plurality of grease reservoirs 15e so that the grease 20 is evenly distributed in the circumferential direction.

FIGS. 4A, 4B and 4C show various examples of the formation position of the grease reservoir 15e. 4A shows a grease reservoir 15e formed over the entire range of the sintered oil-impregnated bearing 15 in the central axis direction. FIG. 4B shows a grease reservoir 15e formed in the upper half of the sintered oil-impregnated bearing 15 in the central axis direction. FIG. 4C shows a sintered oil-impregnated bearing 15 in which a grease reservoir 15e is formed in the lower half in the central axis direction. FIG.
In any of the examples (a), (b) and (c), the grease reservoir 15
e, the grease 20 is retained, and the lubricating oil impregnated in the sintered oil-impregnated bearing 15 is retained by the grease 20 to prevent the lubricating oil from flowing out of the sintered oil-impregnated bearing 15. As in the embodiments described above, it is possible to suppress the deterioration of the rotation unevenness of the motor and the increase in the current value. Further, there is no metal contact, and seizure between the sintered oil-impregnated bearing 15 and the rotating shaft 7 can be prevented.

FIG. 5 shows still another embodiment, in which the lower end of the rotary shaft 7 is retracted from the lower end of the sintered oil-impregnated bearing 15 by a dimension S. In other words, the lower end of the sintered oil-impregnated bearing 15 is rotated. A circular space 15 f having a dimension S in the center axis direction is formed at the lower end of the inner periphery of the sintered oil-impregnated bearing 15 by projecting from the lower end of the shaft 7 by the dimension S. Although grease may be held in this space 15f, in the example shown in FIG. 5, grease 20 is held in the gap between the inner peripheral surface of sintered oil-impregnated bearing 15 and the outer peripheral surface of rotating shaft 7. The space 15f is desirably 0.5 mm or more. This embodiment also has the same effects as those of the embodiments described above.

Incidentally, there are techniques described in several gazettes as a conventional technique related to the present invention. Japanese Patent Application Laid-Open No. 6-346923 is one of them, in which lubricating oil has good fluidity and lubricity, but is liable to be scattered and evaporated by centrifugal force generated by rotation of a rotating shaft. Due to the disadvantages, liquid grease, which has both good fluidity and lubricity, which is the advantage of lubricating oil, and retention and non-evaporation, which is the advantage of grease, is sintered in place of the above lubricating oil. It is made by impregnating an oil-impregnated bearing. Therefore, as in the invention of the present application, gel or jelly-like grease that suppresses the outflow of the lubricating oil impregnated in the sintered oil-impregnated bearing into the gap between the inner peripheral surface of the sintered oil-impregnated bearing and the outer peripheral surface of the rotating shaft. The technical idea is different from that in which a plurality of grease reservoirs are provided on the inner peripheral surface of the sintered oil-impregnated bearing.

As an example of a motor having a thrust bearing for supporting a thrust load of a rotor, see Japanese Patent Application Laid-Open No. 6-20 / 1994.
No. 7619 and Japanese Utility Model Laid-Open No. 5-8033. In these examples, the tip of the rotating shaft is brought into contact with the thrust bearing to support the thrust load at the tip of the rotating shaft.Since the thrust bearing is provided on the inner diameter portion of the radial bearing, the tip of the rotating shaft is Is merely set back from the bearing end face. Further, as in the present invention, a sintered oil-impregnated bearing that radially supports a rotating shaft on an inner peripheral surface, and a thrust bearing disposed radially outside of the rotating shaft and supporting a thrust load of a rotor at an end surface Since the motor is not premised on the structure of the present invention, there is little outflow of lubricating oil from the end face of the sintered oil-impregnated bearing due to its structure, and the technical problem to be solved by the present invention does not occur.

[0027]

According to the first aspect of the present invention, a sintered oil-impregnated bearing for radially supporting the rotating shaft on the inner peripheral surface, and a sintered oil-impregnated bearing disposed radially outside of the rotating shaft and having an end surface of the rotor. In a bearing device for a motor having a thrust bearing for supporting a thrust load, the lubricating oil impregnated in the sintered oil-impregnated bearing flows into a gap between the inner peripheral surface of the sintered oil-impregnated bearing and the outer peripheral surface of the rotating shaft. The semi-solid grease is applied to suppress the lubrication, and this grease can hold the lubricating oil impregnated in the sintered oil-impregnated bearing to prevent the lubricating oil from flowing out. Can be suppressed. Further, there is no metal contact, and seizure between the sintered oil-impregnated bearing and the rotating shaft can be prevented.

According to the second aspect of the present invention, since a plurality of grease reservoirs are provided on the inner peripheral surface of the sintered oil-impregnated bearing and the grease is retained in the grease reservoir, the grease is effectively contained in the sintered oil-impregnated bearing. Thus, the effect obtained by the first aspect of the present invention can be further enhanced.

According to the third aspect of the present invention, since one end of the inner peripheral surface of the sintered oil-impregnated bearing protrudes in the thrust direction from one end of the rotary shaft, one end of the inner peripheral surface of the sintered oil-impregnated bearing is provided. The space formed between the part and one end of the rotating shaft can be used as a grease reservoir, and the effect obtained by the invention of claim 1 can be further enhanced.

According to the invention described in claim 4, grease is:
Since the same base oil as the base oil contained in the lubricating oil impregnated in the sintered oil-impregnated bearing is included, the grease itself also exerts a lubricating effect, and is effective in preventing the occurrence of uneven rotation and metal contact. It is.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a motor bearing device according to the present invention.

FIG. 2 is a cross-sectional view showing another embodiment of the motor bearing device according to the present invention.

FIG. 3 is a sectional view showing still another embodiment of the bearing device for a motor according to the present invention.

FIG. 4 is a development view showing various examples of a grease reservoir forming position in the embodiment.

FIG. 5 is a sectional view showing still another embodiment of the bearing device for a motor according to the present invention.

FIG. 6 is a sectional view showing an example of a conventional motor.

FIG. 7 is a cross-sectional view illustrating an example of a conventional motor bearing device.

[Description of Signs] 7 Rotating shaft 13 Lubricating oil 15 Sintered oil-impregnated bearing 15d Grease reservoir 15e Grease reservoir 20 Grease reservoir

Claims (4)

[Claims] 1. A sintered oil-impregnated bearing that radially supports a rotating shaft on an inner peripheral surface, and a thrust bearing that is disposed radially outside of the rotating shaft and supports a thrust load of a rotor at an end surface. In a motor bearing device, semi-solid grease for suppressing the outflow of lubricating oil impregnated in the sintered oil-impregnated bearing is applied to the gap between the inner peripheral surface of the sintered oil-impregnated bearing and the outer peripheral surface of the rotating shaft. A bearing device for a motor. 2. The bearing device for a motor according to claim 1, wherein a plurality of grease reservoirs are provided on an inner peripheral surface of the sintered oil-impregnated bearing, and the grease reservoir holds grease. 3. The bearing device for a motor according to claim 1, wherein one end of the inner peripheral surface of the sintered oil-impregnated bearing protrudes in the thrust direction from one end of the rotary shaft. 4. The motor bearing device according to claim 1, wherein the grease contains the same base oil as the base oil contained in the lubricating oil impregnated in the sintered oil-impregnated bearing. .