JP6741730B2 - Sintered bearing and manufacturing method thereof - Google Patents

Description

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

The sintered bearing is a porous body having innumerable internal pores and is usually used in a state where the 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 in the inner circumference of the sintered bearing, the lubricating oil retained in the internal pores of the sintered bearing seeps out to the inner peripheral surface (bearing surface) of the sintered bearing as the temperature rises. put out. The lubricating oil thus exuded forms an oil film in the bearing gap between the bearing surface of the sintered bearing and the outer peripheral surface of the shaft, so that the shaft is supported so as to be relatively rotatable.

For example, in Patent Document 1 below, as a copper-iron sintered bearing containing iron and copper as main components, 10 mass% or more and less than 30 mass% of copper is coated with respect to iron powder, and the particle size is 80. It is described that a copper-coated iron powder having a mesh or less is pressed and sintered.

Japanese Patent No. 3613569 Japanese Utility Model Laid-Open 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 is worn early.

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

Further, it is known to use a slide bearing as a bearing that rotatably supports a motor shaft in a starter for an automobile (Patent Document 2). In a starter for an automobile, in order to obtain a large torque necessary for starting the engine, it is customary to decelerate the motor output through 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 such as a copper-based, iron-based, or copper-iron-based bearing on the inner circumference of a planetary gear that constitutes this planetary gear mechanism, and rotatably support the planetary gear with respect to the shaft. (Patent Document 3).

By the way, since the starter is stopped while the engine is rotating, relative rotation does not occur 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 it will be worn during the rotation of the engine. However, according to the verification by the present inventors, it was found that fretting wear occurs on the bearing surface of the sintered bearing and the surface of the shaft when the engine is driven for a long time. This is because the vibration of the engine is transmitted to the starter while the engine is rotating, and this vibration causes the bearing surface and the surface of the shaft to come into contact with each other to cause a slight slip, which triggers oxidation of the bearing surface and the surface of the shaft. It is considered that 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. This fretting wear becomes more remarkable as the content of Fe in the sintered bearing increases, and thus becomes a problem particularly when an iron-based sintered bearing or a copper-iron-based sintered bearing having a large amount of Fe is used. ..

When 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 lack bearing strength because copper itself is soft. Therefore, when the shaft comes into contact with the bearing surface due to engine vibration, the bearing surface is deformed, or when the sintered bearing is press-fitted into the inner circumference of the housing, the effect of shrinking deformation of the sintered bearing due to press-fitting is There is a risk that the accuracy of the bearing surface will be reduced as it reaches the surface.

As described above, the copper-based sintered bearing is advantageous in terms of sliding properties such as initial conformability and quietness, but is difficult in terms of bearing strength. On the other hand, an iron-based or copper-iron-based sintered bearing containing a large amount of Fe is advantageous in terms of bearing strength, but it is difficult in terms of sliding characteristics. Worrying wear). As described above, 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 applications of sintered bearings. It is desired to provide a sintered bearing that can be used.

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

In order to achieve the above object, the sintered bearing according to the present invention is a sintered bearing containing iron, copper, a metal having a melting point lower than that of copper, and a solid lubricant as a main component, and has an iron structure and a copper structure. It has a base portion including a surface layer covering the surface of the base portion, the surface layer is formed mainly of flat copper powder arranged so that its thickness direction becomes thin, and the base portion is iron powder. It has an iron structure and a copper structure formed of a partially diffused alloy powder obtained by partially diffusing copper powder.

The flat copper powder has a property of adhering to the molding surface of the mold during the molding of the raw material powder, and therefore the green 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). By increasing the content of copper in the surface layer in this way, it is possible to improve the initial conformability and quietness, and in combination with the action of solid lubricants such as graphite, good sliding characteristics are obtained. It will be In addition, since the aggressiveness to the shaft is also reduced, the durable life is improved. In addition, since a copper-rich bearing surface that is difficult to oxidize 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 portion is basically a copper powder in which the low melting point metal is diffused. At the time of sintering, the low melting point metal wets the surface of copper to promote liquid phase sintering, so that the bonding force between the metal particles can be strengthened particularly in the base portion. Further, since the base portion is basically formed of a partially diffused alloy powder obtained by diffusing a part of copper powder in iron powder, the copper structure (structure containing copper as a main component) and the iron structure (iron A high neck strength can be obtained between the structures mainly composed of. From the above, it is possible to prevent the copper structure and the iron structure from falling off from the bearing surface and improve the wear resistance of the bearing surface. In addition, the bearing strength can be increased. Therefore, even if the sintered bearing is press-fitted and fixed to the inner circumference of the housing, the bearing surface does not deform following the shape of the inner circumference of the housing, and the bearing surface has high accuracy. Can be promoted. Further, since the base of the bearing surface is strengthened, it is possible to suppress deformation of the bearing surface when the shaft comes into contact with the bearing surface due to vibration or the like. Therefore, it is possible to provide a sintered bearing suitable for use in a starter for starting an engine (including a reduction gear incorporated in the starter) and for use in a vibration motor used in a mobile 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 the compact powder containing the flat copper powder and the low melting point metal is sintered, the flat copper powder may be spheroidized. In the present invention, since the partially diffused alloy powder obtained by diffusing a part of copper powder in iron powder is used, a large number of copper powder exists around the low melting point metal during sintering. In this case, the melting point of the low-melting-point metal that has been melted as the temperature rises in the sintering diffuses into the copper powder of the partially-diffused alloy powder before the flattened copper powder. Can be suppressed. Therefore, the flattened copper powder in the surface layer can be prevented from being spheroidized, and the copper concentration on the surface of the surface layer can be increased.

The iron structure and the copper structure of the base part are all formed of the partial diffusion alloy powder, and the iron structure and the copper structure of the base layer are the partial diffusion alloy powder and one of the single iron powder and the single copper powder, or It can be formed with both.

When using the flat copper powder and the low melting point metal in combination, the content of the low melting point metal in the flat copper powder should be smaller than 10 wt% in order to minimize the influence of spheroidization. Is common technical knowledge so far. On the other hand, in the present invention, as described above, it is possible to suppress the spheroidizing of the flat copper powder of the surface layer due to the low melting point metal, so that the content of the low melting point metal in the bearing can be increased. By increasing the content of the low melting point metal in this way, the binding 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 an amount of 10 wt% or more and 30 wt% or less with respect to the weight of the flat copper powder.

The iron structure can be formed by the ferrite phase (only), or can be formed by the ferrite phase and the pearlite phase existing at the grain boundary of the ferrite phase. In the former case, even when the base portion containing a large amount of iron structure is exposed due to wear of the surface layer, the iron structure mainly consists of the ferrite phase, so even if the copper content is low, the aggression to the shaft is weakened. It is possible and the durability is increased. In the latter case, the hard pearlite phase supplements the wear resistance of the ferrite phase, so that wear of the bearing surface can be suppressed. In the latter case, if the proportion of pearlite present is excessive, the aggression of the shaft is increased and the shaft is easily worn. From this point of view, the pearlite phase exists to the extent that it exists (is scattered) at the grain boundaries of the ferrite phase (see FIG. 9). It is preferable to impregnate the sintered bearing 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 a partially diffused alloy powder obtained by partially diffusing copper powder in iron powder, a flat copper powder, a metal powder having a melting point lower than copper, and a solid lubricant powder. It can be manufactured by forming a green compact with the mixed powder and then 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 that has good sliding characteristics and has both wear resistance of the bearing surface 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 stage is a side view of the flat copper powder, and the lower stage is a plan view of the flat copper powder. It is a side view which shows the flat copper powder and the scaly graphite which adhered to each other. It is sectional drawing which shows the molding process of the green compact by a metal mold. It is an expanded sectional view of the area|region Q in FIG. It is an enlarged view in a radial cross section of a sintered bearing (region P in FIG. 1). FIG. 9 is an enlarged view showing the iron structure of FIG. 8 and its peripheral structure. FIG. 10A is an enlarged view for explaining the spheroidizing of the flat copper powder, and FIG. 10A shows before sintering and FIG. 10B shows after sintering. It is an enlarged view which shows notionally the green 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.

Hereinafter, embodiments 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 circumference. The sintered bearing 1 of this embodiment is used by impregnating internal pores of a porous sintered body with lubricating oil (also referred to as a sintered oil-impregnated bearing). When the shaft 2 made of stainless steel or the like is inserted into the inner circumference of the sintered bearing 1 and the shaft is rotated or the bearing 1 is rotated in that state, the lubricating oil retained in the numerous holes of the sintered bearing 1 Exudes on the bearing surface 1a as the temperature rises. This leached lubricating oil forms an oil film in the bearing gap between the outer peripheral surface of the shaft and the bearing surface 1a, and the shaft 2 is supported by the bearing 1 so as to be relatively rotatable.

FIG. 2 shows a simplified typical 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 speed reducer 5 having an output shaft 2b, an overrunning clutch 6 having an output shaft 2c, a pinion gear 7, and a shift lever. 8 and the electromagnetic switch 9 as main constituent elements. The shift lever 8 is rotatable around a fulcrum O, and its tip is arranged behind (on the input side) the overrunning clutch 6. The overrunning clutch 6 is a one-way clutch, and the output shaft 2b of the speed reducer 5 is axially slidably coupled to the input side of the clutch via a spline or the like. A pinion gear 7 is attached to the output shaft 2c of the overrunning clutch 6, and the overrunning clutch 6 is movable in the axial direction integrally with the output shaft 2c 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 reduction gear 5 and the overrunning clutch 6. Further, the electromagnetic switch 9 is turned on, the rotational force in the direction of the arrow in the drawing is given to the shift lever 8, and the overrunning clutch 6 and the pinion gear 7 advance integrally. 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 starts. After the engine is started, the electromagnetic switch 9 is turned off, the overrunning clutch 6 and the pinion gear 7 retract, and the pinion gear 7 separates from the ring gear 10. The engine torque immediately after the engine is started is cut off by the overrunning clutch 6 and is not transmitted to the motor unit 4.

The sintered bearing 1 of the present invention is press-fitted and fixed to the inner circumference of the housing 3 or the like of the starter ST described above, and supports 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 not shown in detail, the sintered bearing 1 can also be used for supporting the gears of the speed reducer 5. For example, when the reduction gear 5 is composed of a planetary gear mechanism, the sintered bearing 1 of the present invention is press-fitted into the inner circumference of the planetary gear that rotates with respect to the shaft, so that the planetary gear is rotatably supported with respect to the shaft. can do.

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

The raw material powder is a mixed powder containing a partial diffusion alloy powder, a flat copper powder, a low melting point metal powder, and a solid lubricant powder as main components. If necessary, various molding aids such as a lubricant (metal soap or the like) for improving releasability are added to the mixed powder. Hereinafter, the raw material powder and the manufacturing procedure of the first embodiment of the sintered bearing 1 will be described in detail.

[Partially diffused 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 the iron powder 12 is used. The diffusion portion of the partially diffused alloy powder 11 forms a Fe—Cu alloy, and as shown in a partially enlarged view of FIG. 3, iron atoms 12a and copper atoms 13a are bonded to each other and arranged in the alloy portion. Has a crystal structure. As the partial diffusion alloy powder 11, it is preferable to use one having an average particle diameter of 75 μm to 212 μm.

Known iron powder such as reduced iron powder and atomized iron powder can be used as the iron powder 12 constituting the above-mentioned partial diffusion alloy powder 11, but in the present embodiment, reduced iron powder is used. The reduced iron powder is also called sponge iron powder because it has an irregular shape similar to a sphere and is spongy (porous) having internal pores. The iron powder 12 used preferably has an average particle size of 45 μm to 150 μm, and more preferably has 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 from the intensity distribution pattern of the diffracted/scattered light emitted from the particle group. It can be measured by using SALD31000 manufactured by Shimadzu Corporation (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, widely used irregularly shaped or dendritic copper powder can be widely used, and for example, electrolytic copper powder, atomized copper powder and the like are used. In this embodiment, atomized copper powder having a large number of irregularities on the surface and an irregular shape that is close to a spherical shape as a whole particle and excellent in formability is used. The copper powder 13 used has a particle size smaller than that of the iron powder 12, and more specifically has an average particle size of 5 μm or more and 45 μm or less. 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 such as water atomized powder. The flat copper powder 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. The "length" and "thickness" referred to here are the maximum geometrical dimensions of each flat copper powder 15 as shown in FIG. The apparent density of the flat copper powder is 1.0 g/cm ³ or less. With the flat copper powder having the above size and apparent density, the adhesion of the flat copper powder to the die molding surface is increased, so that a large amount of the flat copper powder can be attached to the die molding surface.

[Fluid lubricant]
In order to attach the flat copper powder to the die molding surface, a fluid lubricant is previously attached to the flat copper powder. This fluid lubricant may be adhered to the flat copper powder before filling the raw material powder into the mold, preferably before mixing the raw material powder, and more preferably at the stage of grinding the raw material copper powder, to the raw material copper powder. Let The fluid lubricant may be applied to the flat copper powder by a means such as supplying the flat copper powder with a fluid lubricant and stirring the mixture until it is mixed with other raw material powder after crushing. In order to secure the adhesion amount of the flat copper powder on the die molding surface, the mixing ratio of the fluid lubricant to the flat copper powder is 0.1% by weight or more, and the aggregation due to the adhesion of the flat copper powder to each other is made. In order to prevent this, the compounding ratio is 0.8% by weight or less. Desirably, the lower limit of the mixing ratio is 0.2% by weight or more and the upper limit is 0.7% by weight. As the fluid lubricant, a fatty acid, particularly a straight chain saturated fatty acid is preferable. This type of fatty acid is represented by the general formula of Cn _{- 1H2n-} 1COOH. This fatty acid has a Cn in the range of 12 to 22, and as a specific example, stearic acid can be used.

[Low melting metal powder]
The low melting point metal powder is a metal powder having a melting point lower than that of copper, and in the present invention, a metal powder having a melting point of 700° C. or lower, for example, powder of tin, zinc, phosphorus or the like is used. Among these, tin is preferable because it has less evaporation during sintering. The low melting point metal powder has an average particle size of 5 μm to 45 μm, and is preferably smaller than the average particle size of the partial diffusion alloy powder 11. These low melting point metal powders have high wettability with copper. By mixing the low-melting-point metal powder with the raw material powder, the low-melting-point metal powder first melts at the time of sintering to wet the surface of the copper powder and diffuse into the copper to melt the copper. Liquid phase sintering proceeds due to 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 graphite, for example, 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 the solid lubricant powder, molybdenum disulfide powder can be used in addition to graphite powder. Since the molybdenum disulfide powder has a layered crystal structure and is exfoliated in a layered form, the adhesion to the flat copper powder can be obtained like the scaly graphite.

[Mixing ratio]
In the raw material powder containing the above powders, the partial diffusion alloy powder is 75 to 90 wt%, the flat copper powder is 8 to 20 wt%, the low melting point metal powder (for example, tin powder) is 0.8 to 6.0 wt%, and the solid lubrication is used. It is preferable to add 0.5 to 2.0 wt% of agent powder (for example, graphite powder). The reason for setting this mixing ratio is as follows.

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

[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 has been previously attached are mixed with a known mixer. Then, as the secondary mixing, the partial diffusion alloy powder and the low melting point metal powder are added to the primary mixed powder and mixed, and further, 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 uniformly disperse it in the raw material powders, but the apparent density in the primary mixing is the same level as the flat copper powder and the graphite powder in advance. Then, due to the fluid lubricant or the like attached to the flat copper powder, the flat copper powder 15 and the graphite powder 14 are attached to each other and overlap each other as shown in FIG. 5, and the apparent density of the flat copper powder is increased. Therefore, the flat copper powder can be uniformly dispersed in the raw material powder during the secondary mixing. If a lubricant is separately added during the primary mixing, the adhesion of the flat copper powder and the graphite powder is further promoted, so that the flat copper powder can be dispersed more uniformly during the secondary mixing. As the lubricant added here, in addition to the same or different kind of fluid lubricant as the above-mentioned fluid lubricant, powdery one can be used. For example, the above-mentioned forming aids such as metal soaps, which are generally powdery, have a certain degree of adhesion, so that they can be promoted by the adhesion of flat copper powder and graphite powder.

The state of adhesion between 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. Therefore, when the raw material powder was filled in the mold, the flat copper powder and the flat copper powder were formed 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 is composed of 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 defined by these. When the raw material powder is compressed by bringing the upper and lower punches 23 and 24 close to each other, the raw material powder is formed by the molding surface including 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. Molded to obtain a cylindrical green compact 25.

Among the metal powders of the raw material powder, the flat copper powder has the smallest apparent density. The flat copper powder is a foil having the length L and the thickness t, and has a large area of the wide surface per unit weight. Therefore, the flat copper powder 15 is easily affected by the adhesive force of the fluid lubricant adhering to the surface thereof, further the Coulomb force, etc. (Enlarged view of area Q), the flat copper powder 15 has a wide surface facing the molding surface 20a of the mold 20, and a plurality of layers (about 1 to 3 layers) are stacked. Then, it adheres to the entire molding surface 20a. At this time, the scaly graphite attached to the flat copper powder 15 also attaches to the flat copper powder 15 and attaches to the molding surface 20a of the mold (graphite is not shown in FIG. 7). On the other hand, in the region inside the layered structure of the flat copper 15 (the region on the center side of the cavity), the partial diffusion alloy powder 11, the flat copper powder 15, the low melting point metal powder 16, and the graphite powder are uniformly dispersed throughout. It has become. The green compact 25 after molding holds the distribution state of each powder as it is.

[Sintering]
Then, the green compact 25 is sintered in a sintering furnace. In the present embodiment, the sintering conditions are determined so that the iron structure has a two-phase structure of a ferrite phase and a pearlite phase. In this way, if the iron structure has 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 to improve the bearing life. Can be made.

Due to the diffusion of carbon, the abundance ratio of pearlite (γFe) becomes excessive, and when the ratio is equal to or higher than that of ferrite (αFe), the aggression of the pearlite on the shaft remarkably increases 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 boundaries of the ferrite phase (αFe) (see FIG. 9). The term “grain boundary” as used herein means both the grain boundaries formed between the powder particles and the crystal grain boundaries 18 formed in the powder particles. When the iron structure is formed of a two-phase structure of the ferrite phase (αFe) and the pearlite phase (γFe) as described above, the ratio of the ferrite phase (αFe) and the pearlite phase (γFe) in the iron structure is determined by 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. This makes it 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 mode, the sintering temperature (in-furnace atmosphere temperature) is set to about 820° C. to 900° C., and a gas containing carbon as the in-furnace atmosphere, for example, Sintering is performed using natural gas or endothermic gas (RX gas). As a result, during sintering, carbon contained in the gas diffuses into iron and a pearlite phase (γFe) can be formed. It should be noted that sintering at a temperature exceeding 900° C. is not preferable because the carbon in the graphite powder reacts with iron and the pearlite phase increases more than necessary. With sintering, the fluid lubricant, other lubricants, and various molding aids are burned inside the sintered body or vaporized from the inside of the sintered body.

A porous sintered body is obtained through the above-described sintering process. The sintered body is sized and further impregnated with lubricating oil or liquid grease by a technique such as vacuum impregnation, whereby the sintered bearing 1 (sintered oil-impregnated bearing) shown in FIG. 1 is completed. The lubricating oil impregnated in the sintered body is retained 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 partially diffused alloy powder. The lubricating oil with which the sintered body is impregnated preferably has a kinematic viscosity of 30 mm ² /sec or more and 200 mm ² /sec or less. Depending on the application, the lubricating oil impregnation step may be omitted and the sintered bearing 1 may be used without oil supply.

The microstructure in the vicinity of the surface (region P in FIG. 1) of the sintered bearing 1 that has undergone the above manufacturing steps is schematically shown in FIG.

As shown in FIG. 8, in the sintered bearing 1 of the present invention, the green compact 25 is molded in a state where the flat copper powder 15 is adhered in layers on the die molding surface 20a (see FIG. 7). Due to the fact that the powder 15 is sintered, the surface layer S1 having a higher copper concentration than the other is formed on the entire surface of the bearing 1 including the bearing surface 1a. Moreover, since the wide surface of the flat copper powder 15 was attached to the molding surface 20a, 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 reduced. The thickness of the surface layer S1 corresponds to the thickness of the flat copper powder layer layered on the die molding surface 20a, and is approximately 1 μm to 6 μm. The surface of the surface layer S1 is formed mainly of free graphite 32 (shown by black coating) in addition to the copper structure 31a, and the rest is pore openings and an iron structure described later. Among them, the area of the copper structure 31a is the largest, and specifically, 60% or more of the surface is the copper structure 31a.

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

FIG. 9 is an enlarged view of the iron structure 33 after sintering shown in FIG. 8 and its surrounding structure. As shown in FIG. 9, tin as a low melting point metal is first melted at the time of sintering and diffused into the copper powder 13 that constitutes the partially diffused alloy powder 11 (see FIG. 3), and the bronze phase 16 (Cu-Sn). ) Is formed. Liquid phase sintering proceeds by the bronze phase 16, and the iron particles, the iron particles and the copper particles, or the copper particles are firmly bonded to each other. In addition, in each of the partially diffused alloy powders 11, the molten tin also diffuses into a portion in which a part of the copper powder 13 diffuses to form the Fe—Cu alloy, and the Fe—Cu—Sn alloy (alloy phase 17 ) Is formed. The combination of the bronze phase 16 and the alloy phase 17 becomes the second copper structure 31c. As described above, since a part of the second copper structure 31c is diffused in the iron structure 33, high neck strength can be obtained between the second copper structure 31c and the iron structure 33. In FIG. 9, the ferrite phase (αFe), the pearlite phase (γFe), etc. are represented by the shade of color. Specifically, the color is darkened in the order of ferrite phase (αFe) → bronze phase 16 → alloy phase 17 (Fe-Cu-Sn alloy) → pearlite phase (γFe).

When the 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 be done. When sintered in this state, the flat copper powder 15 is drawn into the low melting metal powder 16 by the surface tension of the melted low melting metal powder 16, and rounds with the low melting metal powder 16 as a nucleus, so-called spherical copper powder 15. Raises the problem of commutation. If the flattened copper powder 15 is left spheroidized, 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, since 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, the low melting point metal powder 16 A large number of copper powders 13 are present around the area. In this case, the low-melting-point metal powder 16 melted by 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, since the fluid lubricant remains on the surface of the flat copper powder 15, this phenomenon is promoted. As a result, the influence of the low melting point metal powder 16 on the flat copper powder 15 of the surface layer S1 can be suppressed (even if the low melting point metal powder 16 exists directly below the flat copper powder 15, the flat copper powder 15). 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 proportion 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 make full use of the above characteristics, it is preferable to add as little iron powder as possible to the raw material powder. That is, it is preferable that the iron structure 33 is entirely derived from the partial diffusion alloy powder.

In this way, according to the present invention, the spheroidizing of the flat copper powder 15 in the surface layer S1 can be avoided, so that the blending ratio of the low melting point metal powder 16 in the bearing can be increased. That is, according to the technical common knowledge so far, in order to suppress the influence of the spheroidizing of the flat copper powder 15, the compounding 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 mixing ratio of the low-melting point metal in this way, the effect of promoting the bonding between the metal particles by liquid phase sintering is further enhanced, which is effective in increasing the strength of the sintered bearing 1.

With the above configuration, the area ratio of the copper structure to the iron structure can be set to 60% or more on the entire surface of the surface layer S1 including the bearing surface 1a, and the copper-rich bearing surface 1a that is hard to be oxidized is stably obtained. be able to. Further, even if the surface layer S1 is worn, the copper structure 31c derived from the copper powder 13 attached 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. Further, the sliding characteristics of the bearing surface 1a including the initial conformability and quietness can be improved.

On the other hand, the base portion S2 inside the surface layer S1 has a hard structure in which the content of copper is smaller and the content of iron is larger than that of the surface layer S1. Specifically, the Fe content is maximum in the base portion S2, and the Cu content is 20 to 40 wt %. Since the iron content is large in the base portion S2 that occupies most of the bearing 1 in this manner, 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 liquid phase sintering improves the bonding force between metal particles (between iron particles, iron particles and copper particles, or copper particles). Moreover, a high neck strength can be obtained between the copper structure 31c derived from the partial diffusion alloy powder 11 and the iron structure 33. From the above, it is possible to prevent the copper structure and the iron structure from falling off from the bearing surface 1a and improve the wear resistance of the bearing surface. Further, the bearing strength can be increased, and more specifically, it is possible to achieve a radial crushing strength (300 MPa or more) that is at least twice that of the 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 does not deform following the shape of the inner peripheral surface of the housing 3, and the bearing surface 1a does not deform after mounting. The roundness and cylindricity of 1a can be stably maintained. Therefore, after the sintered bearing 1 is press-fitted and fixed to the inner periphery of the housing 3, the desired roundness (for example, sizing) is not additionally executed for finishing the bearing surface 1a into an appropriate shape and precision. For example, a roundness of 3 μm or less) can be secured. Further, it is possible to prevent the deformation of the bearing surface 1a even when the shaft 2 comes into contact with the bearing surface 1a due to the vibration of the engine.

In addition, since free graphite is deposited on the entire surface including the bearing surface 1a, and scaly graphite is attached to the mold molding surface 20a in a form accompanying the flat copper powder 3, the graphite of the surface layer S1 The content rate becomes higher 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.

[Other forms of metal structure]
In the first embodiment described above, the iron structure has a two-layer structure of a ferrite phase and a pearlite phase. However, the pearlite phase (γFe) has a hard structure (HV300 or more) and has a strong aggression property against the mating material. Therefore, depending on the usage conditions of the bearing, the wear of the shaft 2 may progress. In order to prevent this, the entire iron structure 33 can be formed of a ferrite phase (αFe).

Since the entire iron structure 33 is formed in the ferrite phase in this manner, the sintering atmosphere is a gas atmosphere containing no carbon (hydrogen gas, nitrogen gas, argon gas, etc.) or a vacuum. By these measures, the reaction between carbon and iron does not occur in the raw material powder, and therefore, the iron structure after sintering becomes a soft (HV200 or less) ferrite phase (αFe). With such a configuration, even if the surface layer S1 is abraded and the iron structure 33 of the base portion S2 appears on the surface, the bearing surface 1a can be softened and the aggressiveness to 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 thus duplicated description will be omitted.

[Other configurations of sintered bearings]
As shown in FIG. 12, tapered surfaces 1b1 and 1b2 having a large diameter on the opening side may be formed on both sides in the axial direction 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 axial ends of the sintered bearing 1 in this way, the outer peripheral surface of the shaft 2 locally contacts the end of the sintered bearing 1 even when the shaft 2 is bent. It is possible to prevent contact, and it is possible to prevent local wear of the bearing surface 1a due to stress concentration, deterioration of bearing strength, occurrence of abnormal noise, and the like.

In order to obtain the above effects, the ratio X (X=X=X) of the maximum value γ of the radial drop amount with respect to the axial lengths b1 and b2 of both tapered surfaces 1b1 and 1b2 (neither includes the chamfer at the axial end). 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 tapered surfaces 1b1 and 1b2 to the axial total length a of the sintered bearing 1 is set in the range of 0.2≦(b1+b2)/a≦0.8. Is preferred. The sintered bearing 1 shown in FIG. 12 can be used, for example, in 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, which is also shown in FIG. It is possible to obtain the same effect as that of the embodiment. Similar to the embodiment shown in FIG. 12, the ratio X (X=γ/b1) of the maximum value γ of the radial drop amount to the tapered surface 1b1 is 1.75×10 ⁻³ ≦X≦5.2×10 ^{− It} is preferable to set it in the range of ² (the range in which 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 axial total 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, in a power window drive mechanism or a power seat drive mechanism of an automobile.

[Other uses for sintered bearings]
The sintered bearing 1 shown in FIG. 1 can be used for a vibration motor that functions as a vibrator for notifying an incoming call, receiving an email, or the like in a mobile terminal such as a mobile phone or a smartphone. In this vibration motor, as shown in FIG. 14, a weight (eccentric weight) W attached to one end of the shaft 2 is rotated by a motor unit 4 to cause vibration to the housing 3 of the vibration motor and further to the entire mobile terminal. It is configured to generate. FIG. 14 conceptually shows the main part of the vibration motor when two sintered bearings 1 (101, 102) are used. In the illustrated example, the shaft 2 of the motor 2 is projected to both sides in the axial direction. Both sides are rotatably supported by sintered bearings 1 (101, 102). The weight W side sintered bearing 101 is disposed between the weight W and the motor unit 4, and the weight W side sintered bearing 101 is thicker than the weight W opposite side sintered bearing 102. Moreover, it is formed in a large diameter. Each of the two sintered bearings 1 has a bearing surface 1a on its inner circumference and is fixed to the inner circumference of a housing 3 made of, for example, a metal material by means of press fitting or the like.

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

As described above, in the bearing for the vibration motor, the shaft 2 swirls over the entire bearing surface, and the bearing surface is frequently hit by the shaft due to an unbalanced load (the shaft frequently slides against the bearing surface). The bearing surface is more likely to be worn than the sintered bearing for normal use. Further, when the sintered bearing is press-fitted into the inner circumference of the housing 3, even if the bearing surface is slightly deformed following the shape of the inner circumferential 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 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 and the formation of coarse air holes can be prevented, so that the bearing 1 can be densified, and radial crushing strength and wear resistance that can be used as a bearing for a vibration motor can be obtained. It becomes possible. In order to prevent the powder filling property from being lowered in the powder compacting step, 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 a vibration motor, the sintered bearing 1 shown in either FIG. 12 or FIG. 13 may be used. In this case, the ratio X of the maximum value γ of the radial drop amount to the axial length of the tapered surface in both figures can be set in the same range as above.

[Other Embodiments]
Up to now, the case where all of the iron structure and the copper structure of the sintered bearing 1 are formed by only the partial diffusion alloy powder has been described, but either one or both of the simple iron powder and the simple copper powder is added to the raw material powder. However, it is also possible to form part of the iron structure or the copper structure in the base portion S2 with simple iron powder or simple copper powder. In this case, in order to secure the minimum wear resistance, strength, and sliding characteristics, it is preferable that the ratio of the partial diffusion alloy powder in the raw material powder is 50% by mass or more. In this case, as the raw material powder, 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 0.5% of solid lubricant powder (eg, graphite powder) are used. 2.0 wt% is blended, and the balance is a single iron powder or a single copper powder (or both single powders).

In such a configuration, the wear resistance, high strength, and good sliding characteristics obtained by using the partial diffusion alloy powder are maintained by changing the blending amount of the single iron powder or the single copper powder, and the bearing It becomes possible to adjust the characteristics. For example, adding elemental iron powder can improve the wear resistance and strength of the bearing while reducing the cost by reducing the amount of partially diffused alloy powder, and adding elemental copper powder further improves the sliding characteristics. can do. Therefore, it is possible to reduce the development cost of the sintered bearing suitable for various applications, and it is possible to support the production of a large variety of sintered bearings in small quantities.

In the above description, the case where the present invention is applied to a perfect circular bearing in which the bearing surface 1a has a perfect circular shape is illustrated, but the present invention is not limited to the perfect circular bearing, and the outer circumferences of the bearing surface 1a and the shaft 2 are illustrated. The present invention can be similarly applied to a fluid dynamic bearing having a dynamic pressure generating portion such as a herringbone groove or a spiral groove on its surface. Further, in the present embodiment, the case where the shaft 2 is rotated has been described, but conversely, it can be used for the purpose of rotating the bearing 1. Furthermore, as an application, a vibration motor or the like used for an automobile starter or a mobile terminal is illustrated, but the application of the sintered bearing 1 according to the present invention is not limited to these, and is widely applied to other applications other than those illustrated. It is possible to

When the green compact 25 is compression-molded, 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 adopted in which the green compact 25 is compression molded with a lubricant applied to the molding surface. If such a method is adopted, the green compact 25 can be molded with higher accuracy.

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 Powder compact 31a Surface layer copper structure 31b Base part No. 1 1st copper structure 31c 2nd copper structure 32 of base part Graphite (solid lubricant)
33 Iron structure S1 Surface layer S2 Base part ST Starter

Claims (20)

A sintered bearing containing iron, copper, a metal having a melting point lower than that of copper, and a solid lubricant as a main component,
A base portion including an iron structure and a copper structure, and a surface layer that covers the surface of the base portion, the surface layer is formed mainly of flat copper powder arranged so that its thickness direction becomes thin, and The base layer diffuses a part of each of the plurality of copper powders having a particle size smaller than that of the iron powder into the iron powder, and an iron structure formed of a partially diffused alloy powder in which an Fe-Cu alloy is formed in the diffused portion, and It has a copper tissue, the low-melting metal than the copper, the 10 wt% or more by weight relative to the flat copper powder contains less 30 wt%,
Forming a copper structure of 60% or more in area ratio on the surface of the surface layer,
A sintered bearing, wherein the proportion of copper in the partially diffused alloy powder is 10 wt% or more and 30 wt% or less. The sintered bearing according to claim 1, wherein the iron structure and the copper structure of the base portion are all formed of the partially diffused alloy powder. The sintered bearing according to claim 1, wherein the iron structure and the copper structure of the base portion are formed of the partial diffusion alloy powder and either one or both of a simple iron powder and a simple copper powder. The sintered bearing according to any one of claims 1 to 3, wherein the copper structure in contact with the iron structure of the base portion is copper powder in which a metal having a melting point lower than that of the copper is diffused. The sintered bearing according to any one of claims 1 to 4, which contains graphite as the solid lubricant. The sintered bearing according to claim 1, wherein the iron structure is formed of a ferrite phase. The sintered bearing according to claim 1, wherein the iron structure is formed of a ferrite phase and a pearlite phase existing in a grain boundary of the ferrite phase. The sintered bearing according to claim 1, wherein a lubricating oil having a kinematic viscosity of 30 mm ² /sec or more and 200 mm ² /sec or less is impregnated. The sintered bearing according to any one of claims 1 to 8, which is used as a starter for starting an engine. The sintered bearing according to claim 1, which is used for a vibration motor. A method of manufacturing a sintered bearing having a base portion including an iron structure and a copper structure, and a surface layer covering a surface of the base portion,
A part of a plurality of copper powders each having a smaller particle size than the iron powder is diffused into the iron powder, and a partially diffused alloy powder in which a Fe-Cu alloy is formed in this diffusion portion, a flat copper powder, and a lower copper Metal powder having a melting point and solid lubricant powder are mixed, a green compact is molded with this mixed powder, and then the green compact is sintered at a temperature lower than the melting point of copper,
A metal having a melting point lower than that of copper is contained in the flat copper powder in a weight ratio of 10 wt% or more and 30 wt% or less,
Forming a copper structure of 60% or more in area ratio on the surface of the surface layer,
The method for producing a sintered bearing, wherein the proportion of copper in the partially diffused alloy powder is 10 wt% or more and 30 wt% or less. The method for manufacturing a sintered bearing according to claim 11, wherein the iron structure and the copper structure of the base portion are all formed of the partially diffused alloy powder. The sintered bearing according to claim 11, wherein the iron structure and the copper structure of the base portion are formed of the partial diffusion alloy powder and either one or both of a simple iron powder and a simple copper powder. Method. The method for manufacturing a sintered bearing according to any one of claims 11 to 13, wherein the copper structure in contact with the iron structure of the base portion is copper powder in which a metal having a melting point lower than that of copper is diffused. The method for manufacturing a sintered bearing according to any one of claims 11 to 14 , which contains graphite as the solid lubricant. Method for producing a sintered bearing according to the iron tissue to any one of claims 11 to 15 formed in the ferrite phase. The iron structure, and the ferrite phase, the production method of the sintered bearing according to any one of claims 11 to 16 which is formed in the pearlite phase present in the grain boundary of the ferrite phase. Kinematic viscosity of 30 mm ² / sec or more, the production method of the sintered bearing according to any one of 200 mm ² / sec or less of the lubricating oil according to claim impregnated with 11-17. The sintered bearing according to any one of claims 1 to 10, wherein the metal having a melting point lower than that of copper is any one of tin, zinc, and phosphorus. The low melting point metal powder than copper, tin, zinc, method for producing a sintered bearing according to any one of claims 11 to 18 is any one of phosphorus.