WO2015050200A1

WO2015050200A1 - Sintered bearing and manufacturing process therefor

Info

Publication number: WO2015050200A1
Application number: PCT/JP2014/076399
Authority: WO
Inventors: 容敬伊藤; 山下　智典
Original assignee: Ntn株式会社; 容敬伊藤; 山下　智典
Priority date: 2013-10-03
Filing date: 2014-10-02
Publication date: 2015-04-09

Abstract

A sintered bearing (1) comprises iron, copper, a metal having a melting temperature lower than that of copper, and a solid lubricant as the main components. This sintered bearing (1) is composed of a surface layer (S1) and a base part (S2). The surface layer (S1) is formed mainly from flat copper powder particles which are arranged in such a state that the flat copper powder particles are thinner in the thickness direction of the surface layer (S1). In the base layer (S2), the iron structures (33) and the copper structures (31c) present in contact with the iron structures (33) are formed from a partial diffusion alloy powder in which copper powder is partially diffused in iron powder. Thus, an uncostly sintered bearing which combines high wear resistance of a bearing surface and high strength of a bearing can be provided.

Description

Sintered bearing and manufacturing method thereof

The present invention relates to a sintered bearing made of sintered metal and a method for manufacturing the same.

Sintered bearings are porous bodies having innumerable internal pores, and are usually used in a state in which internal pores are impregnated with a lubricating fluid (for example, lubricating oil). In this case, during relative rotation of the sintered bearing and the shaft inserted into the inner periphery thereof, the lubricating oil retained in the internal pores of the sintered bearing oozes into the inner peripheral surface (bearing surface) of the sintered bearing as the temperature rises. put out. The oozed lubricating oil forms an oil film in the bearing gap between the bearing surface of the sintered bearing and the outer peripheral surface of the shaft, and the shaft is supported so as to be relatively rotatable.

For example, in Patent Document 1 below, copper and iron-based sintered bearings mainly composed of iron and copper are coated with 10% by mass or more and less than 30% by mass of copper, and the particle size is 80%. A powdered and sintered copper-coated iron powder having a mesh or less is described.

Japanese Patent No. 3613569 Japanese Utility Model Publication No. 2-57251 JP 2012-26504 A

However, in the configuration of Patent Document 1, since the neck strength of the iron phase (iron structure) and the copper phase (copper structure) is low, there is a problem in that the bearing surface wears quickly.

It has been clarified that when the sintered bearing of Patent Document 1 is used for a vibration motor, the rotational fluctuation becomes large during long-term use. This is due to the early wear of the bearing surface due to insufficient neck strength between the iron and copper structures.

Also, it is known to use a slide bearing as a bearing that rotatably supports a motor shaft in an automobile starter (Patent Document 2). In an automobile starter, in order to obtain a large torque necessary for starting an engine, it is usual to reduce the motor output via a reduction gear having a large reduction ratio, for example, a planetary gear mechanism. It has already been proposed to press-fit a sintered bearing made of copper, iron, or copper iron to the inner periphery of the planetary gear that constitutes the planetary gear mechanism, and to support the planetary gear rotatably with respect to the shaft. (Patent Document 3).

By the way, since the starter is in a stopped state while the engine is rotating, there is no relative rotation between the shaft and the bearing. Therefore, it is unlikely that the bearing surface of the sintered bearing or the surface of the shaft facing the wear will wear during the rotation of the engine. However, according to the verification by the present inventors, it has been found that fretting wear occurs on the bearing surface and shaft surface of the sintered bearing when the engine is driven for a long time. This is because the vibration of the engine is transmitted to the starter during the rotation of the engine, the bearing surface and the surface of the shaft come into contact with each other due to this vibration, and this causes the bearing surface and the shaft surface to oxidize, This is thought to be because the surface texture is easily removed. In this case, if the neck strength between the iron structure and the copper structure is insufficient, the removal of the surface structure is promoted. Since this fretting wear becomes more prominent as the Fe content in the sintered bearing increases, this fretting wear is particularly problematic when using iron-based sintered bearings or copper-iron-based sintered bearings with a large amount of Fe. .

If a copper-based sintered bearing is used, oxidation is less likely to occur, so fretting wear can be prevented. However, copper-based sintered bearings tend to have insufficient bearing strength because copper itself is soft. Therefore, when the shaft contacts the bearing surface due to engine vibration, the bearing surface is deformed, or when the sintered bearing is press-fitted into the inner periphery of the housing, the effect of the reduced diameter deformation of the sintered bearing due to the press-fitting is Also, the accuracy of the bearing surface may be reduced.

Thus, copper-based sintered bearings are advantageous in terms of sliding characteristics such as initial conformability and quietness, but are difficult in terms of bearing strength. On the other hand, sintered iron bearings with high iron content and copper-iron alloys are advantageous in terms of bearing strength, but have difficulty in terms of sliding characteristics (as described above, depending on the operating conditions Ting wear is also a concern). Thus, it is difficult for existing sintered bearings to satisfy all of the sliding characteristics, bearing strength, wear resistance, etc., and these required characteristics are satisfied at a high level in order to expand the use of sintered bearings. It would be desirable to provide a sintered bearing.

SUMMARY OF THE INVENTION An object of the present invention is to provide a sintered bearing that has good sliding characteristics, can achieve both wear resistance and bearing strength of the bearing surface, and can reduce costs, and a method for manufacturing the same. To do.

In order to achieve the above object, a sintered bearing according to the present invention is a sintered bearing mainly composed of iron, copper, a metal having a lower melting point than copper, and a solid lubricant, and has an iron structure and a copper structure. Including a base part and a surface layer covering the surface of the base part, the surface layer being formed mainly of flat copper powder arranged so that the thickness direction thereof is thin, and the iron structure of the base layer and At least a part of the copper structure has an iron structure and a copper structure formed of a partially diffused alloy powder obtained by partially diffusing copper powder into iron powder.

扁 Flat copper powder has the property of adhering to the mold molding surface during molding of the raw powder, so that the compact after molding contains a large amount of copper in the surface layer. Therefore, a surface layer having a high copper content is formed in the sintered body after sintering (preferably a copper structure having an area ratio of 60% or more is formed on the surface of the surface layer). In this way, by increasing the copper content in the surface layer, it is possible to improve initial conformability and quietness, and in combination with the action of a solid lubricant such as graphite, the sliding characteristics are good. It will be something. In addition, since the aggression against the shaft is reduced, the durability life is improved. In addition, since a copper-rich bearing surface that is not easily oxidized is formed, fretting wear of the bearing surface can be prevented.

Since this sintered bearing contains a low melting point metal, the copper structure in contact with the iron structure of the base layer is basically a low melting point metal diffused into the copper powder. At the time of sintering, the low melting point metal wets the surface of copper and advances liquid phase sintering, so that the bonding force between the metal particles can be strengthened particularly in the base portion. In addition, since the base part is basically formed of partially diffused alloy powder in which part of copper powder is diffused into iron powder, the sintered copper structure (structure containing copper as a main component) and iron structure (iron) High neck strength can be obtained between the main components). From the above, it is possible to prevent the copper structure and the iron structure from falling off the bearing surface and to improve the wear resistance of the bearing surface. In addition, the bearing strength can be increased. Therefore, even when a sintered bearing is press-fitted and fixed to the inner periphery of the housing, the bearing surface is not deformed following the shape of the inner peripheral surface of the housing. Can be achieved. Further, since the foundation of the bearing surface is reinforced, deformation of the bearing surface when the shaft contacts the bearing surface due to vibration or the like can be suppressed. Accordingly, it is possible to provide a sintered bearing suitable for use in a starter for starting an engine (including a speed reducer incorporated in the starter) and a vibration motor used in a portable terminal or the like. . The proportion of copper in the partial diffusion alloy powder is preferably 10 wt% or more and 30 wt% or less.

When sintering a green compact containing flat copper powder and a low melting point metal, there is a concern that the flat copper powder may be spheroidized. In this invention, since the partial diffusion alloy powder which diffused a part of copper powder to iron powder is used, many copper powder exists around a low melting-point metal at the time of sintering. In this case, the low melting point metal melted as the temperature of sintering diffuses into the copper powder of the partial diffusion alloy powder prior to the flat copper powder, so the influence of the low melting point metal powder on the flat copper powder of the surface layer is affected. Can be suppressed. Therefore, spheroidization of the flat copper powder in the surface layer can be prevented, and the copper concentration on the surface of the surface layer can be increased.

In addition to forming the iron structure and copper structure of the base layer entirely with partially diffused alloy powder, the iron structure and copper structure of the base layer are either partially diffused alloy powder, simple iron powder and simple copper powder or It can be formed with both.

When using a combination of flat copper powder and a low melting point metal, the content of the low melting point metal relative to the flat copper powder should be less than 10 wt% in order to minimize the effect of spheroidization. Is the common technical knowledge so far. On the other hand, in this invention, since the spheroidization of the flat copper powder of the surface layer by the low melting point metal can be suppressed as described above, the content of the low melting point metal in the bearing can be increased. As the content of the low melting point metal increases in this way, the bonding force between the metal particles is further strengthened, which is effective in improving the bearing strength. Specifically, the low melting point metal can be contained in a weight ratio of 10 wt% to 30 wt% with respect to the flat copper powder.

The iron structure can be formed of a ferrite phase (only), or can be formed of a ferrite phase and a pearlite phase present at the grain boundary of the ferrite phase. In the former case, even when the base part containing a large amount of iron structure is exposed due to wear of the surface layer, the iron structure is mainly composed of a ferrite phase, so that the aggression against the shaft is weakened even if the copper content is small. Increase durability. In the latter case, since the hard pearlite phase supplements the wear resistance of the ferrite phase, the wear on the bearing surface can be suppressed. In the latter case, if the pearlite content is excessive, the aggression against the shaft increases and the shaft tends to wear. From this point of view, the pearlite phase is present at the grain boundaries of the ferrite phase (see FIG. 9). The sintered bearing is preferably impregnated with a lubricating oil having a kinematic viscosity of 30 mm ² / sec or more and 200 mm ² / sec or less.

The sintered bearing described above is a mixture of partially diffused alloy powder obtained by partially diffusing copper powder in iron powder, flat copper powder, metal powder having a melting point lower than that of copper, and solid lubricant powder. After the green compact is formed with the mixed powder, it can be manufactured by sintering the green compact at a temperature lower than the melting point of copper.

According to the present invention, it is possible to provide a low-cost sintered bearing having good sliding characteristics and having both bearing surface wear resistance and bearing strength.

It is sectional drawing of the sintered bearing concerning this invention. It is sectional drawing which simplifies and shows the typical structure of a starter. It is an enlarged view which shows a partial diffusion alloy powder typically. The upper part is a side view of the flat copper powder, and the lower part is a plan view of the flat copper powder. It is a side view which shows the flat copper powder and scaly graphite which mutually adhered. It is sectional drawing which shows the formation process of the green compact by a metal mold | die. FIG. 7 is an enlarged cross-sectional view of a region Q in FIG. 6. It is an enlarged view in the radial cross section of a sintered bearing (area | region P of FIG. 1). It is an enlarged view which shows the iron structure | tissue of FIG. 8, and its surrounding structure | tissue. It is an enlarged view explaining spheroidization of flat copper powder, and shows before sintering. It is an enlarged view explaining spheroidization of flat copper powder, and shows after sintering. It is an enlarged view which shows notionally the compact structure before sintering of this invention. It is sectional drawing which shows other embodiment of the sintered bearing concerning this invention. It is sectional drawing which shows other embodiment of the sintered bearing concerning this invention. It is a principal part schematic sectional drawing of a vibration motor. It is sectional drawing in the AA line shown in FIG. It is a microscope picture of the cross section containing a bearing surface. It is a figure which shows a part of powder compact notionally. It is a microscope picture of the cross section containing the bearing surface of the sintered bearing concerning a prior art.

Hereinafter, a first embodiment of the present invention will be described with reference to the accompanying drawings.

As shown in FIG. 1, the sintered bearing 1 is formed in a cylindrical shape having a bearing surface 1a on the inner periphery. The sintered bearing 1 of this embodiment is used by impregnating lubricating oil in the internal pores of a porous sintered body (also called a sintered oil-impregnated bearing). When the shaft 2 made of stainless steel or the like is inserted into the inner periphery of the sintered bearing 1 and the shaft is rotated in this state, or the bearing 1 is rotated, the lubricating oil retained in the countless holes of the sintered bearing 1 Oozes out to the bearing surface 1a as the temperature rises. The oil that has oozed out forms an oil film in the bearing gap between the outer peripheral surface of the shaft and the bearing surface 1 a, and the shaft 2 is supported by the bearing 1 so as to be relatively rotatable.

FIG. 2 shows a simplified representative configuration of a starter ST used for starting an automobile engine. As shown in FIG. 2, the starter ST includes a housing 3, a motor unit 4 having a motor shaft 2a, a reduction gear 5 having an output shaft 2b, an overrunning clutch 6 having an output shaft 2c, a pinion gear 7, a shift lever. 8 and the electromagnetic switch 9 are main components. The shift lever 8 is rotatable around a fulcrum O, and the tip thereof is disposed behind the overrunning clutch 6 (input side). The overrunning clutch 6 is a one-way clutch, and an output shaft 2b of the reduction gear 5 is connected to an input side of the overrunning clutch 6 via a spline or the like so as to be slidable in the axial direction. A pinion gear 7 is attached to the output shaft 2 c of the overrunning clutch 6, and the overrunning clutch 6 can move in the axial direction integrally with the output shaft 2 c and the pinion gear 7.

When the ignition is turned on, the motor unit 4 is driven, and the torque of the motor shaft 2a is transmitted to the pinion gear 7 via the speed reduction device 5 and the overrunning clutch 6. Further, the electromagnetic switch 9 is turned on and a rotational force in the direction of the arrow in the figure is applied to the shift lever 8, so that the overrunning clutch 6 and the pinion gear 7 move forward together. As a result, the pinion gear 7 meshes with the ring gear 10 coupled to the crankshaft, the torque of the motor unit 4 is transmitted to the crankshaft, and the engine is started. After the engine is started, the electromagnetic switch 9 is turned off, the overrunning clutch 6 and the pinion gear 7 are retracted, and the pinion gear 7 is separated from the ring gear 10. Since the engine torque immediately after engine startup is interrupted by the overrunning clutch 8, it is not transmitted to the motor unit 4.

The sintered bearing 1 of the present invention is press-fitted and fixed to the inner periphery of the housing 3 or the like of the starter ST described above and supports the various shafts 2 (2a to 2c) in the starter ST (in FIG. 2, the motor shaft 2a and The case where the output shaft 2c of the overrunning clutch 6 is supported by the sintered bearing 1 is illustrated). Although detailed illustration is omitted, the sintered bearing 1 can also be used for supporting the gear of the reduction gear 5. For example, when the speed reducer 5 is constituted by a planetary gear mechanism, the planetary gear is rotatably supported with respect to the shaft by press-fitting the sintered bearing 1 of the present invention into the inner periphery of the planetary gear rotating with respect to the shaft. can do.

The sintered bearing 1 described above is formed by filling raw material powder mixed with various powders into a mold, compressing this to form a green compact, and then sintering the green compact.

The raw material powder is a mixed powder mainly composed of partially diffused alloy powder, flat copper powder, low melting point metal powder, and solid lubricant powder. To this mixed powder, various molding aids, for example, a lubricant (metal soap or the like) for improving releasability are added as necessary. Hereinafter, the raw material powder and the manufacturing procedure will be described in detail for the first embodiment of the sintered bearing 1.

[Partial diffusion alloy powder]
As the partial diffusion alloy powder, as shown in FIG. 3, Fe—Cu partial diffusion alloy powder 11 in which a large number of copper powders 13 are partially diffused on the surface of iron powder 12 is used. The diffusion part of the partial diffusion alloy powder 11 forms an Fe—Cu alloy, and as shown in the enlarged partial view in FIG. 3, the alloy part is formed by bonding iron atoms 12a and copper atoms 13a to each other. It has a crystal structure. As the partial diffusion alloy powder 11, one having an average particle diameter of 75 μm to 212 μm is preferably used.

As the iron powder 12 constituting the partial diffusion alloy powder 11, known iron powders such as reduced iron powder and atomized iron powder can be used, but reduced iron powder is used in this embodiment. The reduced iron powder has an irregular shape that approximates a spherical shape and has a sponge shape (porous shape) having internal pores, and is also referred to as sponge iron powder. The iron powder 12 used preferably has an average particle size of 45 μm to 150 μm, and more preferably an average particle size of 63 μm to 106 μm.

The average particle size is determined by irradiating a particle group with laser light and calculating the particle size distribution by calculating from the intensity distribution pattern of diffraction / scattered light emitted from the particle group. (The average particle size of each powder described below can also be measured by the same method).

Further, as the copper powder 13 constituting the partial diffusion alloy powder 11, a widely used irregular shape or dendritic copper powder can be widely used. For example, electrolytic copper powder, atomized copper powder, or the like is used. In the present embodiment, an atomized copper powder having a large number of irregularities on the surface, an irregular shape that approximates a spherical shape as a whole particle, and excellent in formability is used. The copper powder 13 to be used has a smaller particle diameter than the iron powder 12, and specifically, one having an average particle diameter of 5 μm to 45 μm is used. The proportion of Cu in the partial diffusion alloy powder 11 is 10 to 30 wt% (preferably 22 to 26 wt%).

[Flat copper powder]
The flat copper powder is flattened by stamping raw material copper powder made of water atomized powder or the like. As the flat copper powder, one having a length L of 20 μm to 80 μm and a thickness t of 0.5 μm to 1.5 μm (aspect ratio L / t = 13.3 to 160) is mainly used. Here, “length” and “thickness” refer to the geometric maximum dimension of each flat copper powder 3 as shown in FIG. The apparent density of the flat copper powder is 1.0 g / cm ³ or less. If the flat copper powder has the above size and apparent density, the adhesion of the flat copper powder to the mold forming surface is increased, so that a large amount of flat copper powder can be attached to the mold forming surface.

[Fluid lubricant]
In order to attach the flat copper powder to the molding surface, a fluid lubricant is previously attached to the flat copper powder. This fluid lubricant only needs to be attached to the flat copper powder before filling the raw material powder into the mold, preferably before mixing the raw material powder, more preferably to the raw material copper powder at the stage of crushing the raw material copper powder. Let The fluid lubricant may be attached to the flat copper powder by means such as supplying the fluid lubricant to the flat copper powder and stirring it after mixing and before mixing with other raw material powders. In order to secure the adhesion amount of the flat copper powder on the molding surface, the blending ratio of the fluid lubricant to the flat copper powder should be 0.1% by weight or more, and aggregation due to the adhesion of the flat copper powders In order to prevent this, the blending ratio is 0.8% by weight or less. Desirably, the lower limit of the blending ratio is 0.2% by weight or more, and the upper limit is 0.7% by weight. As the fluid lubricant, fatty acids, particularly linear saturated fatty acids are preferred. This type of fatty acid is represented by the general formula C _n-1 H _2n-1 COOH. This fatty acid has a Cn in the range of 12 to 22, and for example, stearic acid can be used as a specific example.

[Low melting point metal powder]
The low melting point metal powder is a metal powder having a melting point lower than that of copper. In the present invention, a metal powder having a melting point of 700 ° C. or lower, for example, a powder of tin, zinc, phosphorus or the like is used. Of these, tin is preferred because it causes less transpiration during sintering. The average particle diameter of the low-melting metal powder is preferably 5 μm to 45 μm, and is preferably smaller than the average particle diameter of the partial diffusion alloy powder 11. These low melting point metal powders have high wettability with respect to copper. By blending the low melting point metal powder with the raw material powder, the low melting point metal powder is first melted at the time of sintering to wet the surface of the copper powder and diffused into the copper to melt the copper. Liquid phase sintering proceeds by the molten alloy of copper and low melting point metal, and the bond strength between iron particles, between iron particles and copper particles, and between copper particles is strengthened.

[Solid lubricant powder]
The solid lubricant powder is added to reduce friction at the time of metal contact due to sliding with the shaft 2, and for example, graphite is used. At this time, as the graphite powder, it is desirable to use scaly graphite powder so that adhesion to the flat copper powder can be obtained. As solid lubricant powder, molybdenum disulfide powder can be used in addition to graphite powder. Molybdenum disulfide powder has a layered crystal structure and peels into layers, and thus adheres to flat copper powder in the same manner as scale graphite.

[Combination ratio]
In the raw material powder blended with the above powders, 75 to 90 wt% of partially diffused alloy powder, 8 to 20 wt% of flat copper powder, 0.8 to 6.0 wt% of low melting point metal powder (for example, tin powder), solid lubrication It is preferable to mix 0.5 to 2.0 wt% of agent powder (eg, graphite powder). The reason for this blending ratio is as follows.

In the present invention, as will be described later, flat copper powder is adhered to the mold in layers when the raw powder is filled into the mold. If the blending ratio of flat copper in the raw material powder is less than 8% by weight, the amount of flat copper adhering to the mold becomes insufficient, and the effect of the present invention cannot be expected. Moreover, the adhesion amount of the flat copper powder to the mold is saturated at about 20 wt%, and even if the blending amount is further increased, the cost increase due to the use of the high-cost flat copper powder becomes a problem. If the ratio of the low melting point metal powder is less than 0.8 wt%, the strength of the bearing cannot be ensured, and if it exceeds 6.0 wt%, the influence of spheroidizing the flat copper powder cannot be ignored. Further, if the ratio of the solid lubricant powder is less than 0.5% by weight, the effect of reducing friction on the bearing surface cannot be obtained, and if it exceeds 2.0% by weight, the strength is reduced.

[mixture]
It is desirable to mix the powders described above in two steps. First, as primary mixing, scaly graphite powder and flat copper powder to which a fluid lubricant is previously attached are mixed with a known mixer. Next, as the secondary mixing, the partial diffusion alloy powder and the low melting point metal powder are added to and mixed with the primary mixed powder, and further, the graphite powder is added and mixed as necessary. Flat copper powder has a low apparent density among various raw material powders, so it is difficult to disperse uniformly in the raw material powder, but it is premixed with flat copper powder and graphite powder that have the same apparent density in the primary mixing. As shown in FIG. 5, the flat copper powder 15 and the graphite powder 14 adhere to each other and overlap each other due to the fluid lubricant or the like attached to the flat copper powder, and the apparent density of the flat copper powder increases. Therefore, it becomes possible to uniformly disperse the flat copper powder in the raw material powder during the secondary mixing. If a lubricant is added separately during the primary mixing, the adhesion between the flat copper powder and the graphite powder is further promoted, so that the flat copper powder can be more uniformly dispersed during the secondary mixing. As the lubricant to be added, a powdery lubricant can be used in addition to the same or different fluid lubricant as the fluid lubricant. For example, the above-mentioned forming aid such as metal soap is generally powdery and has a certain degree of adhesion, which can be promoted by adhesion of flat copper powder and graphite powder.

Since the adhesion state of the flat copper powder 15 and the scaly graphite powder 14 shown in FIG. 5 is maintained to some extent even after the secondary mixing, when the raw material powder is filled in the mold, the flat copper powder is put on the mold surface. A lot of graphite powder will adhere.

[Molding]
The raw material powder after the secondary mixing is supplied to the mold 20 of the molding machine. As shown in FIG. 6, the mold 20 includes a core 21, a die 22, an upper punch 23, and a lower punch 24, and a raw material powder is filled in a cavity partitioned by these. When the upper and

lower punches

23 and 24 are brought close to each other and the raw material powder is compressed, the raw material powder is formed by the molding surface formed by the outer peripheral surface of the core 21, the inner peripheral surface of the die 22, the end surface of the upper punch 23, and the end surface of the lower punch 24. A cylindrical green compact 25 is obtained by molding.

Among the metal powders in the raw powder, the flat copper powder has the smallest apparent density. Further, the flat copper powder is a foil having the length L and the thickness t, and the area of the wide surface per unit weight is large. Therefore, the flat copper powder 15 is easily affected by the adhesion force of the fluid lubricant adhered to the surface thereof, and further by the Coulomb force, etc. After filling the raw material powder into the mold 20, FIG. 7 (in FIG. 6) As shown in an enlarged view of the region Q), the flat copper powder 15 has a layer state in which a wide surface is directed to the molding surface 20a of the mold 20 and a plurality of layers (about 1 to 3 layers) overlap. And adheres to the entire area of the molding surface 20a. At this time, scaly graphite adhering to the flat copper powder 15 also adheres to the flat copper powder 15 and adheres to the molding surface 20a of the mold (illustration of graphite is omitted in FIG. 7). On the other hand, in the inner region (region on the cavity center side) of the layered structure of the flat copper 15, the dispersion state of the partial diffusion alloy powder 11, the flat copper powder 15, the low melting point metal powder 16, and the graphite powder is uniform throughout. It has become. The green compact 25 after the molding holds the distribution state of each powder almost as it is.

[Sintering]
Thereafter, the green compact 25 is sintered in a sintering furnace. In the present embodiment, the sintering conditions are determined so that the iron structure becomes a two-phase structure of a ferrite phase and a pearlite phase. In this way, if the iron structure is a two-phase structure of ferrite phase and pearlite phase, the hard pearlite phase contributes to the improvement of wear resistance and suppresses the wear of the bearing surface under high surface pressure, thereby improving the bearing life. Can be made.

When carbon diffuses, the abundance of pearlite (γFe) becomes excessive, and when the proportion is equal to or higher than that of ferrite (αFe), the aggression of the pearlite against the shaft is remarkably increased and the shaft is easily worn. In order to prevent this, the pearlite phase (γFe) is suppressed to the extent that it exists (is scattered) at the grain boundary of the ferrite phase (αFe) (see FIG. 9). The “grain boundary” here means both the grain boundary formed between the powder particles and the crystal grain boundary 18 formed in the powder particle. In this way, when the iron structure is formed of a two-phase structure of a ferrite phase (αFe) and a pearlite phase (γFe), the proportion of the ferrite phase (αFe) and the pearlite phase (γFe) in the iron structure depends on the base portion S2 described later. It is desirable that the area ratios in the arbitrary cross sections are about 80 to 95% and 5 to 20% (αFe: γFe = 80 to 95%: 5 to 20%), respectively. Thereby, it is possible to achieve both suppression of wear of the shaft 2 and improvement of wear resistance of the bearing surface 1a.

The growth rate of pearlite mainly depends on the sintering temperature. Therefore, in order to allow the pearlite phase to exist at the grain boundary of the ferrite phase in the above-described manner, the sintering temperature (furnace atmosphere temperature) is about 820 ° C. to 900 ° C., and the gas containing carbon as the furnace atmosphere, for example, Sintering using natural gas or endothermic gas (RX gas). Thereby, carbon contained in the gas diffuses into iron during sintering, and a pearlite phase (γFe) can be formed. Sintering at a temperature exceeding 900 ° C. is not preferable because carbon in the graphite powder reacts with iron and the pearlite phase increases more than necessary. With the sintering, the fluid lubricant, other lubricants, and various molding aids burn inside the sintered body or vaporize from inside the sintered body.

By passing through the sintering step described above, a porous sintered body can be obtained. The sintered body 1 (sintered oil-impregnated bearing) shown in FIG. 1 is completed by sizing the sintered body and further impregnating it with lubricating oil or liquid grease by a method such as vacuum impregnation. The lubricating oil impregnated in the sintered body is held not only in the pores formed between the particles of the sintered structure, but also in the pores of the reduced iron powder of the partial diffusion alloy powder. As the lubricating oil impregnated into the sintered body, one having a kinematic viscosity of 30 mm ² / sec or more and 200 mm ² / sec or less is preferable. In addition, depending on a use, the impregnation process of lubricating oil can be abbreviate | omitted and it can also be set as the sintered bearing 1 used under oil-free.

FIG. 8 schematically shows the microstructure near the surface of the sintered bearing 1 (region P in FIG. 1) that has undergone the above manufacturing steps.

As shown in FIG. 8, in the sintered bearing 1 of the present invention, the green compact 25 is formed in a state where the flat copper powder 15 is adhered in a layered manner to the mold forming surface 20a (see FIG. 7). Since the powder 15 is sintered, a surface layer S1 having a higher copper concentration than the others is formed on the entire surface including the bearing surface 1a of the bearing 1. In addition, the wide surface of the flat copper powder 15 may have adhered to the molding surface 20a, so that most of the copper structure 31a of the surface layer S1 has a flat shape in which the thickness direction of the surface layer S1 is thinned. The thickness of the surface layer S1 corresponds to the thickness of the flat copper powder layer adhering to the mold forming surface 20a in layers, and is about 1 μm to 6 μm. The surface of the surface layer S1 is mainly composed of free graphite 32 (shown in black) in addition to the copper structure 31a, and the remainder is a pore opening or an iron structure described later. In this, the area of the copper structure 31a is the largest, specifically, 60% or more of the surface becomes the copper structure 31a.

On the other hand, two types of copper structures (31b, 31c), iron structure 33, free graphite 32, and pores are formed in the inner base part S2 covered with the surface layer S1. One copper structure 31b (first copper structure) is formed from the flat copper powder 15 contained in the green compact 25, and has a flat shape corresponding to the flat copper powder. . The other copper structure 31 c (second copper structure) is formed by diffusing a low-melting-point metal into the copper powder 13 constituting the partial diffusion alloy powder 11, and is formed in contact with the iron structure 33. . As will be described later, the second copper structure 31c plays a role of increasing the bonding force between the particles.

FIG. 9 shows an enlarged view of the sintered iron structure 33 and its surrounding structure shown in FIG. As shown in FIG. 9, tin as a low melting point metal is first melted during sintering and diffused into copper powder 13 constituting partial diffusion alloy powder 11 (see FIG. 3), and bronze phase 16 (Cu—Sn). ). Liquid phase sintering proceeds by this bronze layer 16, and iron particles, iron particles and copper particles, or copper particles are firmly bonded. Further, among the individual partial diffusion alloy powders 11, the molten tin is diffused into the part where the part of the copper powder 13 is diffused to form the Fe—Cu alloy, and the Fe—Cu—Sn alloy (alloy phase 17 ) Is formed. A combination of the bronze layer 16 and the alloy phase 17 becomes the second copper structure 31c. As described above, since the second copper structure 31 c is partially diffused in the iron structure 33, high neck strength can be obtained between the second copper structure 31 c and the iron structure 33. In FIG. 9, the ferrite phase (αFe), the pearlite phase (γFe), and the like are represented by shades of color. Specifically, the colors are darkened in the order of ferrite phase (αFe) → bronze phase 16 → alloy phase 17 (Fe—Cu—Sn alloy) → pearlite phase (γFe).

When normal iron powder 19 is used instead of the partial diffusion alloy powder 11, a part of the low melting point metal powder 16 exists between the flat copper powder 15 and the normal iron powder 19 as shown in FIG. Will do. When sintered in this state, the flat copper powder 15 is drawn into the low melting point metal powder 16 by the surface tension of the molten low melting point metal powder 16 and rounds around the low melting point metal powder 16 as a core. Cause problems. If the spheroidization of the flat copper powder 15 is allowed to stand, the area of the copper structure 31a (see FIG. 9) in the surface layer S1 is reduced, which greatly affects the slidability of the bearing surface 1a.

On the other hand, in the present invention, as shown in FIG. 11, the partial diffusion alloy powder 11 in which substantially the entire circumference of the iron powder 12 is covered with the copper powder 13 is used as the raw material powder. A large number of copper powders 13 are present in the vicinity of. In this case, the low melting point metal powder 16 melted with the sintering diffuses into the copper powder 13 of the partial diffusion alloy powder 11 before the flat copper powder 15. In particular, at the initial stage of sintering, the fluid lubricant remains on the surface of the flat copper powder 15, and this phenomenon is promoted. Thereby, the influence which the low melting metal powder 16 has on the flat copper powder 15 of the surface layer S1 can be suppressed (even if the low melting metal powder 16 exists directly under the flat copper powder 15, the flat copper powder The surface tension acting on 15 is reduced). Therefore, the spheroidization of the flat copper powder 15 in the surface layer can be suppressed, the ratio of the copper structure on the bearing surface including the bearing surface 1a can be increased, and good sliding characteristics can be obtained. In order to take advantage of the above characteristics, it is preferable not to add as much iron powder as possible to the raw material powder. That is, it is preferable that all the iron structures 33 are derived from the partially diffused alloy powder.

Thus, in the present invention, since the spheroidization of the flat copper powder 15 in the surface layer S1 can be avoided, the blending ratio of the low melting point metal powder 16 in the bearing can be increased. That is, in the conventional technical common sense, in order to suppress the influence of the spheroidization of the flat copper powder 15, the blending ratio (weight ratio) of the low melting point metal to the flat copper powder 15 should be suppressed to less than 10 wt%. According to the present invention, this ratio can be increased to 10 wt% to 30 wt%. By increasing the blending ratio of the low melting point metal as described above, the effect of promoting the bonding between the metal particles by the liquid phase sintering is further enhanced. Therefore, the strength of the sintered bearing 1 is increased.

From the above configuration, the area ratio of the copper structure to the iron structure can be 60% or more over the entire surface of the surface layer S1 including the bearing surface 1a, and the copper-rich bearing surface 1a that is not easily oxidized is stably obtained. be able to. Even if the surface layer S1 is worn, the copper structure 31c derived from the copper powder 13 adhered to the partial diffusion alloy powder 11 appears on the bearing surface 1a. Therefore, even when the sintered bearing 1 is used for the starter ST, it is possible to prevent fretting wear of the bearing surface 1a. In addition, the sliding characteristics of the bearing surface 1a including initial conformability and quietness can be improved.

On the other hand, the base portion S2 inside the surface layer S1 has a hard structure with a small amount of copper and a large amount of iron compared to the surface phase S1. Specifically, the content of Fe is the maximum in the base portion S2, and the content of Cu is 20 to 40 wt%. As described above, since the iron content increases in the base portion S2 that occupies most of the bearing 1, the amount of copper used in the entire bearing 1 can be reduced, and cost reduction can be achieved. Further, since the iron content is large, the strength of the entire bearing can be increased.

In particular, in the present invention, a predetermined amount of a metal having a melting point lower than that of copper is blended, and the bonding force between metal particles (between iron particles, between iron particles and copper particles, or between copper particles) is improved by liquid phase sintering. In addition, high neck strength can be obtained between the copper structure 31c and the iron structure 33 derived from the partial diffusion alloy powder 11. From the above, it is possible to prevent the copper structure and the iron structure from falling off the bearing surface 1a, and to improve the wear resistance of the bearing surface. In addition, the bearing strength can be increased, and specifically, it is possible to achieve a crushing strength (300 MPa or more) that is twice or more that of an existing copper-iron-based sintered body. Therefore, even when the sintered bearing 1 is press-fitted and fixed to the inner periphery of the housing 3 as shown in FIG. 2, the bearing surface 1a is not deformed following the shape of the inner peripheral surface of the housing 3, and the bearing surface even after the mounting. The roundness and cylindricity of 1a can be stably maintained. Therefore, after press-fitting and fixing the sintered bearing 1 to the inner periphery of the housing 3, a desired roundness (for example, sizing) is additionally performed without finishing processing (for example, sizing) for finishing the bearing surface 1a to an appropriate shape and accuracy. For example, a roundness of 3 μm or less can be ensured. Further, deformation of the bearing surface 1a can be prevented even when the shaft 2 contacts the bearing surface 1a due to engine vibration.

In addition, free graphite is deposited on the entire surface including the bearing surface 1a, and the scaly graphite is attached to the mold forming surface 61 in a form accompanying the flat copper powder 3. Therefore, the graphite in the surface layer S1 The content rate becomes larger than the content rate of graphite in the base portion S2. Therefore, the bearing surface 1a can be reduced in friction, and the durability of the bearing 1 can be increased.

In the first embodiment described above, the iron structure is a two-layer structure of a ferrite phase and a pearlite phase, but the pearlite phase (γFe) is a hard structure (HV300 or higher) and has a strong attacking property against the counterpart material. For this reason, depending on the use conditions of the bearing, there is a possibility that the wear of the shaft 2 may proceed. In order to prevent this, the entire iron structure 33 can be formed of a ferrite phase (αFe).

Thus, in order to form all of the iron structure 33 with the ferrite phase, the sintering atmosphere is a gas atmosphere (hydrogen gas, nitrogen gas, argon gas, etc.) not containing carbon or a vacuum. By these measures, the raw material powder does not react with carbon and iron, and therefore the iron structure after sintering is all soft (HV200 or less) and a ferrite phase (αFe). With such a configuration, even if the surface layer S1 is worn and the iron structure 33 of the base portion S2 appears on the surface, the bearing surface 1a can be softened and the aggressiveness against the shaft 2 can be weakened. . Other configurations, such as the composition of the raw material powder and the manufacturing procedure, are the same as those in the first embodiment, and a duplicate description is omitted.

As shown in FIG. 12, tapered surfaces 1b1 and 1b2 whose opening side has a large diameter may be formed on both axial sides of the cylindrical bearing surface 1a of the sintered bearing 1 having the surface layer S1 and the base portion S2. it can. By forming the tapered surfaces 1b1 and 1b2 at both ends in the axial direction of the sintered bearing 1 in this way, even when the shaft 2 is bent, the outer peripheral surface of the shaft 2 is locally applied to the end of the sintered bearing 1. It is possible to prevent contact, and it is possible to prevent local wear of the bearing surface 1a due to stress concentration, a decrease in bearing strength, and the generation of abnormal noise.

In order to obtain the above effect, the ratio X (X = X) of the maximum value γ of the radial drop amount with respect to the axial lengths b1 and b2 (both not including the chamfers at the axial ends) of the tapered surfaces 1b1 and 1b2. γ / b1 or X = γ / b2) is in the range of 1.75 × 10 ⁻³ ≦ X ≦ 5.2 × 10 ⁻² (in the range where the inclination angle of the tapered surface is 0.1 ° to 3 °) ) Is preferable. In this case, the ratio of the sum of the axial lengths of the two tapered surfaces 1b1 and 1b2 to the total axial length a of the sintered bearing 1 is set in a range of 0.2 ≦ (b1 + b2) /a≦0.8. Is preferred. The sintered bearing 1 shown in FIG. 12 can be used, for example, for a power window drive mechanism or a power seat drive mechanism of an automobile.

As shown in FIG. 13, a tapered surface 1b1 having a large diameter on the opening side can be formed only on one side in the axial direction of the cylindrical bearing surface 1a of the sintered bearing 1, and this is also shown in FIG. The same effect as the embodiment can be obtained. Similarly to the embodiment shown in FIG. 12, the ratio X (X = γ / b1) of the maximum value γ of the radial drop amount with respect to the tapered surface 1b1 is 1.75 × 10 ⁻³ ≦ X ≦ 5.2 × 10 ^{−. It} is preferable to set within the range of ² (in the range where the inclination angle of the tapered surface is 0.1 ° to 3 °). In this case, the ratio of the axial length of the tapered surface 1b1 to the total axial length a of the sintered bearing 1 is preferably set in the range of 0.2 ≦ b / a ≦ 0.8. The sintered bearing 1 shown in FIG. 13 can be used, for example, for a power window drive mechanism or a power seat drive mechanism of an automobile.

The sintered bearing 1 shown in FIG. 1 can be used in a vibration motor that functions as a vibrator for notifying incoming calls or emails in mobile terminals such as mobile phones and smartphones. As shown in FIG. 14, the vibration motor is configured by rotating a weight (eccentric weight) W attached to one end of the shaft 3 as shown in FIG. It is the structure which generates a vibration in the whole portable terminal. FIG. 1 conceptually shows a main part of a vibration motor 1 when two sintered bearings 1 (101, 102) are used. In the illustrated example, a shaft 2 protruded on both sides in the axial direction of the motor unit 4. Both sides are supported by the sintered bearing 1 (101, 102) so as to be rotatable. The sintered bearing 101 on the weight W side is disposed between the weight W and the motor unit 4. The sintered bearing 101 on the weight W side is thicker than the sintered bearing 102 on the opposite side of the weight W. And it is formed in a large diameter. Each of the two sintered bearings 1 has a bearing surface 1a on the inner periphery, and is fixed to the inner periphery of the housing 3 made of, for example, a metal material by means such as press fitting.

In this vibration motor, the shaft 2 is driven at a rotational speed of 10,000 rpm or more. When the shaft 2 rotates, the shaft 2 rotates under the influence of the weight W while swinging along the entire surface of the bearing surface 1a. In a sintered bearing for normal use, the shaft 2 rotates while maintaining an eccentric state in the direction of gravity. However, in the sintered bearing 1 for a vibration motor, as shown in FIG. The shaft 2 rotates in a state where the center Oa is decentered not only in the direction of gravity but also in all directions.

As described above, in the bearing for a vibration motor, the shaft 2 swings around the entire bearing surface, and the bearing surface is frequently hit by the shaft due to an unbalanced load (the shaft frequently comes into sliding contact with the bearing surface). The bearing surface is more easily worn than a sintered bearing for normal use. Further, when the sintered bearing is press-fitted into the inner periphery of the housing 3, if the bearing surface is slightly deformed following the shape of the inner peripheral surface of the housing, the rotational accuracy of the shaft 2 is greatly affected. These problems can be solved by using the sintered bearing 1 of the present invention in a vibration motor.

In the sintered bearing 1 for a vibration motor, it is preferable to use a powder having an average particle size of 145 mesh or less (average particle size of 106 μm or less) as the partial diffusion alloy powder. As a result, the porous structure of the bearing can be made uniform to prevent the formation of rough air holes, so that the bearing 1 can be densified to obtain the crushing strength and wear resistance that can withstand use as a vibration motor bearing. It becomes possible. In order to prevent the powder filling property in the compacting process from being lowered, the proportion of the partial diffusion alloy powder having an average particle size of 350 mesh (average particle size 45 μm) or less is preferably less than 25% by mass. Further, for example, as the sintered bearing 1 for the vibration motor, the sintered bearing 1 described in any one of FIG. 12 and FIG. 13 may be used. In this case, the ratio X of the maximum value δ of the radial drop amount to the axial length of the tapered surface in both figures can be set in the same range as described above.

So far, the explanation has been given of the case where all of the iron structure and copper structure of the sintered bearing 1 are formed only by the partial diffusion alloy powder, but either one or both of the simple iron powder and the simple copper powder are added to the raw material powder. And a part of iron structure and copper structure in base part S2 can also be formed with a simple iron powder or a simple copper powder. In this case, in order to ensure the minimum wear resistance, strength, and sliding characteristics, the proportion of the partial diffusion alloy powder in the raw material powder is preferably 50% by mass or more. In this case, the raw material powder includes 8 to 20 wt% of flat copper powder, 0.8 to 6.0 wt% of low melting point metal powder (eg, tin powder), and 0.5 to 6.0 wt. Of solid lubricant powder (eg, graphite powder). 2.0 wt% is blended, and the balance is made into simple iron powder or simple copper powder (or both simple powders).

In such a configuration, the bearing amount is maintained while maintaining the wear resistance, high strength, and good sliding characteristics obtained by using the partial diffusion alloy powder by changing the blending amount of the single iron powder and the single copper powder. The characteristics can be adjusted. For example, by adding simple iron powder, it is possible to increase the wear resistance and strength of the bearing while reducing the cost by reducing the amount of partially diffused alloy powder, and by adding simple copper powder, the sliding characteristics are further improved. can do. As a result, the development cost of sintered bearings suitable for various applications can be reduced, and it is possible to cope with the production of various types of sintered bearings in small quantities.

[Second Embodiment]
In the sintered bearing 1 of the first embodiment described above, the copper structure of the bearing surface 1a is formed of flat copper powder, but the copper structure of the bearing surface 1a is copper powder contained in the partial diffusion alloy powder. It can also be formed. Hereinafter, the details of the sintered bearing 1 will be described as a second embodiment, taking as an example the case of use in a vibration motor (FIG. 14).

The raw material powder in the second embodiment is a mixture in which a partial diffusion alloy powder, a low melting point metal powder, a solid lubricant powder, and an additive powder composed of either one or both of a simple iron powder and a simple copper powder are mixed. It becomes powder. The mass ratio of each powder in the raw material powder is the largest for the partially diffused alloy powder. Various molding lubricants (for example, a lubricant for improving releasability) may be added to the raw material powder as necessary.

As the partial diffusion alloy powder 11 (see FIG. 3), the proportion of particles having an average particle size of 145 mesh or less (average particle size of 106 μm or less) and an average particle size of 350 mesh (average particle size of 45 μm) or less is less than 25 mass as described above. It is preferable to use the above.

As the iron powder 12 constituting the partially diffused alloy powder 11, known iron powder such as reduced iron powder and atomized iron powder can be used. In this embodiment, reduced iron powder is used. The iron powder 12 used preferably has an average particle size of 20 μm to 106 μm, and more preferably an average particle size of 38 μm to 75 μm.

Moreover, as the copper powder 13 which comprises the partial diffusion alloy powder 11, the irregular shape and dendritic copper powder which are generally used can be widely used, for example, electrolytic copper powder, atomized copper powder, etc. are used. In the present embodiment, an atomized copper powder having a large number of irregularities on the surface, an irregular shape that approximates a spherical shape as a whole particle, and excellent in formability is used. The copper powder 13 to be used has a smaller particle diameter than the iron powder 12, and specifically, an average particle diameter of 5 μm to 20 μm (preferably less than 20 μm) is used. The proportion of Cu in the partial diffusion alloy powder 11 is 10 to 30% by mass (preferably 22 to 26% by mass).

As the low melting point metal powder, a metal powder having a melting point of 700 ° C. or less, for example, a powder of tin, zinc, phosphorus or the like is used. In the present embodiment, among these, tin powder that can easily diffuse into copper and iron and can be used as a single powder, particularly atomized tin powder, is used. As the tin powder (atomized tin powder), those having an average particle diameter of 5 to 63 μm are preferably used, and those having an average particle diameter of 20 to 45 μm are more preferably used.

As the solid lubricant, one or more powders such as graphite and molybdenum disulfide can be used. In the present embodiment, graphite powder, particularly scaly graphite powder is used in consideration of cost.

The soot-added powder is composed of one or both of simple iron powder and simple copper powder. As the simple iron powder, either reduced iron powder or atomized iron powder can be used, and the iron powder to be used is selected according to the application of the bearing. In addition, the mixture of reduced iron powder and atomized iron powder can also be used as a single-piece iron powder. Moreover, both the electrolytic copper powder and the atomized copper powder can be used as the single copper powder, and the copper powder to be used is selected according to the application of the bearing. In addition, the mixture of electrolytic copper powder and atomized copper powder can also be used as a simple substance copper powder. The average particle size of the simple iron powder and the simple copper powder can be widely selected according to the application of the bearing. For example, as the simple iron powder, the average particle size is in the range of 45 to 200 μm (preferably 100 to 150 μm), A single copper powder having an average particle diameter of 45 to 150 μm (preferably 80 to 125 μm) can be used.

Thereafter, the raw material powder is compression-molded using a molding die set in a die set of a cam-type molding press, for example, to form a compact. As schematically shown in FIG. 17, in the green compact 25, the partial diffusion alloy powder 11, the tin powder 16, the graphite powder (not shown) and the additive powder are uniformly dispersed. Since the partial diffusion alloy powder 11 used in the present embodiment uses reduced iron powder as the iron powder 12, the powder is softer than the partial diffusion alloy powder using atomized iron powder, and the compression moldability is improved. Excellent. Therefore, the strength of the green compact 25 can be increased even at a low density, and chipping or cracking of the green compact 25 can be prevented.

Next, the green compact 25 is sintered to obtain a sintered body. The sintering conditions are such that graphite (graphite powder) does not react with iron (no carbon diffusion occurs). In the iron-carbon equilibrium state, there is a transformation point at 723 ° C., and beyond this, the reaction between iron and carbon is initiated and a pearlite phase (γFe) is generated in the iron structure. After that, the reaction between carbon (graphite) and iron begins, and a pearlite phase (γFe) is generated. Since the pearlite phase (γFe) has high hardness (HV300 or higher) and is highly aggressive against the mating material, if the pearlite phase (γFe) is excessively present in the iron structure of the sintered bearing 4, the wear of the shaft 3 may be advanced. There is. In a general sintered bearing manufacturing process, green compacts are heated in an atmosphere of endothermic gas (RX gas), which is a mixture of liquefied petroleum gas such as butane and propane and air, and pyrolyzed with Ni catalyst.・ It is often sintered. However, in the endothermic gas, carbon may diffuse and the surface of the green compact may be cured, and the same problem as described above is likely to occur.

From the above viewpoint, the green compact 25 is heated at 900 ° C. or lower, specifically 800 ° C. (preferably 820 ° C.) or higher and 880 ° C. or lower (low temperature sintering). The sintering atmosphere is a gas atmosphere containing no carbon (hydrogen gas, nitrogen gas, argon gas, etc.) or a vacuum. Under such sintering conditions, the reaction between carbon and iron does not occur in the raw material powder, and therefore the iron structure after sintering becomes a soft ferrite phase (HV200 or less). In the case where various molding lubricants such as a fluid lubricant are included in the raw material powder, the molding lubricant volatilizes with sintering.

As shown in FIG. 9, the pig iron structure can be a two-phase structure of ferrite phase αFe and pearlite phase γFe. Thereby, the pearlite phase γFe harder than the ferrite phase αFe contributes to the improvement of the wear resistance of the bearing surface, and the wear of the bearing surface under high surface pressure can be suppressed to improve the bearing life. However, if the pearlite phase γFe is present in an excessive proportion and becomes equal to the ferrite phase αFe, the aggressiveness of the pearlite against the shaft 3 increases and the shaft 3 is likely to wear. In order to prevent this, as shown in FIG. 7, the pearlite phase γFe is suppressed to the extent that it exists (is scattered) at the grain boundary of the ferrite phase αFe. The term “grain boundary” as used herein means both a grain boundary formed between powder particles and a crystal grain boundary formed in the powder particle. When the iron structure is formed of a two-phase structure of a ferrite phase αFe and a pearlite phase γFe, the ratio of the ferrite phase αFe and the pearlite phase γFe in the iron structure is an area ratio in an arbitrary cross section of the sintered body. % And 5 to 20% (αFe: γFe = 80 to 95%: 5 to 20%) are desirable. Thereby, it is possible to achieve both suppression of wear of the shaft 2 and improvement of wear resistance of the bearing surface 1a.

The amount of precipitation of the pearlite phase γFe mainly depends on the sintering temperature and the atmospheric gas. Therefore, in order to allow the pearlite phase γFe to be present at the grain boundary of the ferrite phase αFe in the above-described manner, the sintering temperature is raised to about 820 ° C. to 900 ° C., and the gas containing carbon as the furnace atmosphere, such as natural gas, Sintering is performed using an endothermic gas (RX gas). As a result, carbon contained in the gas diffuses into iron during sintering, and pearlite phase γFe can be formed. As described above, when the green compact 25 is sintered at a temperature exceeding 900 ° C., the carbon in the graphite powder reacts with iron to form a pearlite phase γFe. Sintering is preferred.

After sintering, sizing the sintered body, finishing the sintered body into a finished shape and dimensions, and then impregnating the internal pores of this sintered body with a lubricating oil by a method such as vacuum impregnation, as shown in FIG. The sintered bearing 1 is completed. Lubricating oil impregnated in the internal pores of the sintered body is low viscosity, specifically, kinematic viscosity at 40 ° C. is 10 to 50 mm ² / s (for example, synthetic hydrocarbon lubricating oil). This is to suppress the increase in rotational torque while ensuring the rigidity of the oil film formed in the bearing gap. The internal pores of the sintered body may be impregnated with grease based on a lubricating oil having a kinematic viscosity at 40 ° C. of 10 to 50 mm ² / s. Further, sizing is sufficient if necessary, and it is not always necessary. In addition, depending on the application, the step of impregnating the lubricating oil can be omitted, and a sintered bearing used without oil supply can be obtained.

If the sintering temperature of the green compact 25 is 900 ° C. or lower, which is much lower than the melting point of copper (1083 ° C.), the partial diffusion alloy powder 11 contained in the green compact 25 is obtained. The constituent copper powder 13 does not melt, and therefore copper does not diffuse into iron (iron structure) with sintering. Therefore, an appropriate amount of copper structure is formed on the surface (bearing surface 1a) of the sintered body. Moreover, free graphite is also exposed on the surface of the sintered body. Therefore, it is possible to obtain a sintered bearing 1 having good initial conformability with the shaft 2 and a small friction coefficient of the bearing surface 1a.

In the fired sintered body, an iron structure mainly composed of iron and a copper structure mainly composed of copper are formed. Although most of the iron structure and copper structure of the sintered body are formed of the partial diffusion alloy powder 11, in the partial diffusion alloy powder, a part of the copper powder is diffused into the iron powder. High neck strength can be obtained between the copper structure and the copper structure. Further, at the time of sintering, the tin powder 16 in the green compact 25 melts and wets the surface of the copper powder 13 constituting the partial diffusion alloy powder 11. Along with this, liquid phase sintering proceeds between tin (Sn) and copper (Cu), and as shown in FIG. 9, the iron structure and copper structure of adjacent partial diffusion alloy powders 11, or between copper structures A bronze phase (Cu-Sn) 16 is formed. In addition, in each of the partial diffusion alloy powders 11, molten Sn is diffused and Fe—Cu is diffused in a part where a part of the copper powder 13 is diffused on the surface of the iron powder 12 to form an Fe—Cu alloy. Since the Sn alloy (alloy phase) 17 is formed, the neck strength between the iron structure and the copper structure is further increased. Therefore, a high crushing strength, specifically, a crushing strength of 300 MPa or more can be obtained even at the low temperature sintering as described above. Further, the bearing surface 1a can be hardened to improve the wear resistance of the bearing surface 1a.

Moreover, in 2nd embodiment, the additive powder which consists of a single-piece | unit iron powder and a single-piece | unit copper powder is mix | blended with raw material powder. Therefore, by changing the blending amount of single iron powder and single copper powder, bearing characteristics are maintained while maintaining the wear resistance, high strength, and good sliding characteristics obtained by using the partial diffusion alloy powder. It becomes possible to adjust. For example, if a single iron powder is used as the additive powder, the wear resistance and strength of the bearing can be further increased, and if a single copper powder is used as the additive powder, the sliding characteristics can be further improved. Therefore, it is possible to reduce the development cost of the sintered bearing suitable for each application, and it is possible to cope with the production of various types of sintered bearings in small quantities. For example, the diffusion amount of the copper powder in the partial diffusion alloy powder is limited to about 30% by mass. Therefore, when the copper structure is formed only with the partial diffusion alloy powder, it becomes difficult to increase the ratio of copper in the bearing further. . On the other hand, the ratio of the copper in a bearing can be made larger than 30 mass% by mix | blending single-piece | unit copper powder as an additional powder.

If the blending ratio of the partial diffusion alloy powder in the raw material powder is too small, the merit of using the partial diffusion alloy powder is diminished and it becomes difficult to satisfy the wear resistance, strength, and sliding characteristics. Therefore, the blending ratio of the partial diffusion alloy powder in the raw material powder is preferably 50% by mass or more (desirably 75% by mass or more). Further, if the amount of the solid lubricant powder is too small, the sliding characteristics will be impaired, and if it is too large, the crushing strength will be lowered. Therefore, the blending ratio in the raw material powder is set to 0.3 to 1.5 mass%. If the amount of the low melting point metal powder is small, the progress of liquid phase sintering becomes insufficient and the strength is lowered. If the amount of the low melting point metal powder is large, the mechanical strength of the sintered body increases, but there is a problem that the number of rough air holes increases. Therefore, the blending ratio of the low melting point metal powder should be about 10% by mass of the total mass of the copper powder in the raw material powder (the sum of the copper powder in the partial diffusion alloy powder and the single copper powder added as the additive powder). preferable. Specifically, the blending ratio of the low melting point metal in the raw material powder is set to 0.5 to 5.0 mass%. The balance of the raw material powder consists of additive powder and inevitable impurities. The blending ratio of the additive powder is preferably at least 1.0% by mass or more of the raw material powder, considering the merit of blending it.

In addition, in order to satisfy the sliding characteristics required for the bearing surface 1a, the ratio of copper in the sintered bearing 1 is at least 10% by mass (preferably 15% by mass or more).

Further, by using a powder having an average particle size of 145 mesh or less (average particle size of 106 μm or less) as the partial diffusion alloy powder 11, the porous structure of the sintered body can be made uniform to prevent the formation of rough atmospheric pores. . Therefore, the density of the sintered body can be increased to further improve the crushing strength and the wear resistance of the bearing surface 1a.

As described above, the sintered body of the present embodiment has a crushing strength of 300 MPa or more, and the value of this crushing strength is more than twice that of an existing copper-iron-based sintered body. is there. Moreover, the density of the sintered body of the present embodiment is 6.8 ± 0.3 g / cm ³ , which is higher than the density of the existing iron-copper-based sintered body (about 6.6 g / cm ³ ). Even existing iron-copper-based sintered bodies can be densified by high compression in the green compact molding process, but this will prevent the internal fluid lubricant from burning during sintering. Because of gasification, the pores in the surface layer portion become coarse. In the present invention, it is not necessary to perform high compression at the time of forming the green compact, and such a problem can be prevented.

一方で While increasing the density of the sintered body in this way, the oil content can be increased to 15 vol% or more, and the oil content comparable to that of existing iron-copper sintered bearings can be ensured. This is mainly due to the use of reduced iron powder having a spongy shape and excellent oil retention as the iron powder 12 constituting the partial diffusion alloy powder 11. In this case, the lubricating oil impregnated in the sintered body is reduced not only in the pores formed between the particles of the sintered structure, but also reduced iron powder (in addition to the reduced iron powder constituting the partial diffusion alloy powder, as additive powder) When iron powder is used, the reduced iron powder is also contained).

Rough air holes are likely to occur particularly in the surface layer portion of the sintered body (region from the sintered body surface to a depth of 100 μm). However, if the sintered body is obtained as described above, the surface layer is formed as described above. It is possible to increase the density of the surface layer portion by preventing the generation of rough atmospheric holes in the portion. Specifically, the porosity of the surface layer portion can be 5 to 20%. This porosity can be obtained, for example, by image analysis of the area ratio of the pores in an arbitrary cross section of the sintered body.

By increasing the density of the surface layer portion in this way, the surface area ratio of the bearing surface 1a is also reduced. Specifically, the surface area ratio of the bearing surface 1a is set within a range of 5% to 20%. can do. When the surface area ratio is less than 5%, it becomes difficult to exude a necessary and sufficient amount of lubricating oil into the bearing gap (insufficient oil film forming ability), and a merit as a sintered bearing can be obtained. Can not.

In addition, as the raw material powder for obtaining this sintered body, since the main raw material is partially diffused alloy powder 11 in which copper powder 13 is partially diffused on the surface of iron powder 12, existing iron copper is used. It is possible to prevent the segregation of copper, which is a problem in the sintered sintered bearing. Further, with this sintered body, the mechanical strength can be improved without using expensive metal powder such as Ni or Mo, so that the cost of the sintered bearing 4 can be reduced.

As described above, since the sintered bearing 1 according to the second embodiment has high crushing strength (crushing strength of 300 MPa or more), even when press-fitted and fixed to the inner periphery of the housing 3 as shown in FIG. The bearing surface 1a is not deformed following the shape of the inner peripheral surface of the housing 3, and the roundness, cylindricity, and the like of the bearing surface 1a can be stably maintained even after the mounting. Therefore, after press-fitting and fixing the sintered bearing 1 to the inner periphery of the housing 3, a desired roundness (for example, sizing) is additionally performed without finishing processing (for example, sizing) for finishing the bearing surface 1a to an appropriate shape and accuracy. For example, a roundness of 3 μm or less can be ensured. Further, if the sintered bearing 1 has a crushing strength of 300 MPa or more, a vibration motor incorporating this sintered bearing 4 (and thus a portable terminal equipped with this vibration motor) will drop, etc., thereby causing a bearing surface 1a. Even when a large impact load is applied, the deformation of the bearing surface 1a is prevented as much as possible. Further, since the bearing surface 1a is hardened and has high wear resistance, even if the shaft 2 swings around the entire surface of the bearing surface 1a or the shaft 2 frequently collides with the bearing surface 1a, the bearing surface 1a. Wear and damage can be suppressed. Therefore, according to the present invention, the sintered bearing 1 suitable for supporting the vibration motor can be provided at low cost.

顕微鏡 Here, for reference, a micrograph of a surface layer portion of a sintered bearing (hereinafter referred to as “copper-coated iron powder bearing”) according to the technical means described in Patent Document 1 is shown in FIG. Comparing FIG. 18 with a micrograph (see FIG. 16) of the surface layer portion of the sintered bearing 1 according to the second embodiment, the sintered bearing 1 according to the second embodiment is a copper-coated iron powder bearing. In comparison, it is understood that the porous structure of the surface layer is uniform and dense. Actually, the porosity of the surface layer portion of the sintered bearing 1 according to the second embodiment was 13.6%, whereas the porosity of the surface layer portion of the copper-coated iron powder bearing was about 25.5%. there were. As a factor causing such a difference, in the copper-coated iron powder, only the copper film is in close contact with the iron powder, and the neck strength between the iron phase and the copper phase is insufficient.

The configuration and operation of the second embodiment described above are summarized as follows.

A sintered bearing having a bearing surface on the inner periphery that forms a bearing gap between the shaft to be supported, a partial diffusion alloy powder obtained by partially diffusing copper powder into iron powder, a low melting point metal powder, It is characterized by comprising a sintered body obtained by molding and sintering a raw material powder containing a solid lubricant powder and an additive powder composed of one or both of simple iron powder and simple copper powder.

In the partially diffused alloy powder, since a part of the copper powder is diffused into the iron powder, a higher neck strength is obtained between the sintered iron structure and the copper structure than when the copper-coated iron powder is used. Moreover, the low melting point metal powder contained in the green compact is melted by sintering after the raw material powder is molded (compressed). Since the low melting point metal has high wettability with respect to copper, the iron structure and the copper structure of adjacent partial diffusion alloy powders, or the copper structures can be firmly bonded by liquid phase sintering. Furthermore, among the individual partial diffusion alloy powders, the molten low melting point metal diffuses into the part where a part of the copper powder is diffused on the surface of the iron powder and the Fe—Cu alloy is formed. The neck strength between the structure and the copper structure can be further increased. From these facts, it becomes possible to obtain a high-strength sintered bearing having excellent bearing surface wear resistance even at low-temperature sintering. In addition, since the partial diffusion alloy powder contains a considerable amount of copper powder, a large amount of copper structure can be formed on the bearing surface, and therefore good sliding characteristics (low torque, initial conformability, quietness, etc.) ) Can be obtained.

In addition, since the additive powder consisting of one or both of single iron powder and single copper powder is blended into the raw material powder, high wear resistance can be achieved by changing the blending amount of single iron powder and single copper powder. It is possible to adjust the bearing characteristics according to the application while satisfying the properties and strength and the good sliding characteristics. For example, if single iron powder is added, wear resistance and bearing strength can be further increased, and if single copper powder is added, sliding characteristics can be improved. In order to ensure the minimum wear resistance, strength, and sliding characteristics, the proportion of the partial diffusion alloy powder in the raw material powder is preferably 50 wt% or more.

もの This sintered bearing preferably has a crushing strength of 300 MPa or more. By using the partial diffusion alloy powder as a main raw material, it is easy to ensure the crushing strength.

In order to obtain the sintered bearing (sintered body) described above, as the partial diffusion alloy powder included in the raw material powder, copper powder having an average particle size of 5 μm or more and less than 20 μm partially diffuses on the surface of the iron powder, and in the alloy powder It is preferable to use a material containing 10 to 30% by mass of Cu.

If the raw material powder contains a partially diffused alloy powder having a large particle size exceeding the average particle size of 106 μm, rough air holes are likely to be formed inside the sintered body, and as a result, the required bearing surface wear resistance is required. It has been found that there are cases where it is not possible to ensure properties and crushing strength. Accordingly, it is preferable to use a partially diffused alloy powder having an average particle size of 145 mesh or less (average particle size of 106 μm or less). By using such an alloy powder, it is possible to stably obtain a sintered body in which the sintered metal structure (porous structure) is made uniform and the generation of rough atmospheric pores in the metal structure is suppressed. it can. Thereby, it becomes possible to stably obtain a sintered bearing in which the wear resistance of the bearing surface and the crushing strength of the bearing are further improved.

In this sintered bearing, tin powder can be used as the low melting point metal powder, and graphite powder can be used as the solid lubricant powder.

By making the iron structure of the sintered body mainly composed of a soft ferrite phase, the aggressiveness of the bearing surface against the shaft can be weakened, and the shaft wear can be suppressed. An iron structure mainly composed of a ferrite phase can be obtained, for example, by sintering a green compact at a temperature of 900 ° C. or less at which iron and graphite do not react.

The iron structure mainly composed of a ferrite phase includes an iron structure in which a pearlite phase harder than a ferrite phase is present at the grain boundary of the ferrite phase in addition to a structure in which all of the ferrite phase is a ferrite phase. Thus, by forming a pearlite phase at the grain boundary of the ferrite phase, it is possible to improve the wear resistance of the bearing surface as compared with the case where the iron structure is composed only of the ferrite phase. In order to achieve both the suppression of shaft wear and the improvement of wear resistance of the bearing surface, the proportions of ferrite phase (αFe) and pearlite phase (γFe) in the iron structure are 80 to 95% and 5 to 20%, respectively. (ΑFe: γFe = 80 to 95%: 5 to 20%) is preferable. In addition, said ratio can be calculated | required by the area ratio of each of the ferrite phase and the pearlite phase in the arbitrary cross sections of a sintered compact, for example.

Reduced iron powder can be used as the iron powder constituting the partially diffused alloy powder (Fe—Cu partially diffused alloy powder). As the iron powder, for example, atomized iron powder can be used in addition to the reduced iron powder. However, since the reduced iron powder has a spongy shape (porous shape) having internal pores, it is compared with the atomized iron powder. The powder is soft and excellent in compression moldability. Therefore, the green compact strength can be increased even at low density, and chipping and cracking of the green compact can be prevented. Moreover, since reduced iron powder makes a spongy shape as described above, it also has an advantage of superior oil retention as compared with atomized iron powder.

において In the above configuration, the porosity of the surface layer portion, particularly the porosity of the surface layer portion including the bearing surface is preferably 5 to 20%. Here, the surface layer portion is a region from the surface to a depth of 100 μm.

The sintered body (internal pores) can be impregnated with a lubricating oil, and a lubricating oil having a kinematic viscosity at 40 ° C. in the range of 10 to 50 mm ² / s is preferably used. This is to suppress the increase in rotational torque while ensuring the rigidity of the oil film formed in the bearing gap. The oil impregnated in the sintered body may be a liquid grease based on an oil (lubricating oil) having a kinematic viscosity at 40 ° C. in the range of 10 to 50 mm ² / s.

使用 Use of the above-mentioned sintered bearing as a bearing for supporting the shaft of the vibration motor improves the wear resistance and strength of the bearing surface, so that rotation fluctuations can be prevented. Further, since the sintered body has a crushing strength of 300 MPa or more, deformation of the bearing surface during press-fitting and deformation of the bearing surface due to an impact load can be prevented as much as possible.

In the above description, the case where the present invention is applied to a perfect circle bearing having a perfect circle shape on the bearing surface 1a is illustrated. However, the present invention is not limited to a perfect circle bearing, and the outer circumference of the bearing surface 1a and the shaft 2 is exemplified. The present invention can be similarly applied to a fluid dynamic pressure bearing in which a dynamic pressure generating portion such as a herringbone groove or a spiral groove is provided on the surface. Moreover, although the case where the axis | shaft 2 was rotated was demonstrated in this embodiment, it can be used also for the use which rotates the bearing 1 conversely. Furthermore, although the vibration motor etc. which are used for the starter for vehicles, a portable terminal, etc. were illustrated as a use, the use of the sintered bearing 1 concerning this invention is not limited to these, It applies widely also to other uses other than illustrated Is possible.

When the green compact 25 is compression-molded, a so-called warm molding method in which the green compact 25 is compression-molded in a state where at least one of the molding die 20 and the raw material powder is heated, A die lubrication molding method may be employed in which the green compact 25 is compression molded in a state where a lubricant is applied to the molding surface. By adopting such a method, the green compact 25 can be molded with higher accuracy.

DESCRIPTION OF SYMBOLS 1 Bearing 1a Bearing surface 2 Shaft 3 Housing 4 Motor 11 Partially diffused alloy powder 12 Iron powder 13 Copper powder 14 Graphite powder 15 Flat copper powder 16 Bronze layer 17 Alloy phase 25 Compact 31a Surface layer copper structure 31b One copper structure 31c Second copper structure 32 of base part Graphite (solid lubricant)
33 Iron structure S1 Surface layer S2 Base part ST Starter αFe Ferrite phase γFe Pearlite phase M Motor part W Weight

Claims

A sintered bearing mainly composed of iron, copper, a metal having a lower melting point than copper, and a solid lubricant,
Having a base part including an iron structure and a copper structure, and a surface layer covering the surface of the base part, the surface layer being formed mainly of flat copper powder arranged so that its thickness direction is thin, and A sintered bearing characterized in that at least a part of the iron structure and copper structure of the base layer is formed of a partially diffused alloy powder obtained by partially diffusing copper powder into iron powder.
The sintered bearing according to claim 1, wherein a copper structure having an area ratio of 60% or more is formed on the surface of the surface layer.
The sintered bearing according to claim 1 or 2, wherein the iron structure and copper structure of the base layer are all formed of partially diffused alloy powder.
The sintered bearing according to claim 1 or 2, wherein the iron structure and the copper structure of the base layer are formed of a partial diffusion alloy powder and one or both of a single iron powder and a single copper powder.
The sintered bearing according to any one of claims 1 to 4, wherein a copper structure in contact with the iron structure of the base layer is obtained by diffusing a low melting point metal into copper powder.
The sintered bearing according to any one of claims 1 to 5, wherein the low melting point metal is contained in a weight ratio of 10 wt% to 30 wt% with respect to the flat copper powder.
The sintered bearing according to any one of claims 1 to 6, comprising graphite as a solid lubricant.
The sintered bearing according to any one of claims 1 to 7, wherein the iron structure is formed of a ferrite phase.
The sintered bearing according to any one of claims 1 to 7, wherein the iron structure is formed of a ferrite phase and a pearlite phase existing at a grain boundary of the ferrite phase.
The sintered bearing according to any one of claims 1 to 9, wherein the proportion of copper in the partial diffusion alloy powder is 10 wt% or more and 30 wt% or less.
The sintered bearing according to any one of claims 1 to 10, impregnated with a lubricating oil having a kinematic viscosity of 30 mm 2 / sec or more and 200 mm 2 / sec or less.
The sintered bearing according to any one of claims 1 to 11, which is used for a starter for starting an engine.
The sintered bearing according to any one of claims 1 to 11, which is used for a vibration motor.
A partially diffused alloy powder obtained by partially diffusing copper powder into iron powder, flat copper powder, metal powder having a melting point lower than copper, and solid lubricant powder were mixed, and a green compact was formed from this mixed powder. Thereafter, the green compact is sintered at a temperature lower than the melting point of copper.