JP2015092097A - Sintered bearing and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost sintered bearing which is made compatible in the friction resistance of a bearing face and bearing rigidity.SOLUTION: A sintered bearing 1 uses iron, copper, metal which is lower than copper in a melting point, and a solid lubricant as main components. The sintered bearing 1 is composed of a surface layer S1 and a base part S2. The surface layer S1 is formed of flat copper powder arranged so as to be thin in its thickness direction as a main part, and an iron copper composition 33 and a copper composition 31c contacting with the iron composition out of the base layer S2 are formed of partially-diffused alloy powder which is obtained by partially diffusing the copper powder into iron powder.

Description

  The present invention relates to a sintered bearing made of a 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.

  When the sintered bearing of patent document 1 was used for a vibration motor, it became clear that rotation fluctuation became large at the time of 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.

  It is also 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 that 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, a copper-based sintered bearing is advantageous in terms of sliding characteristics such as initial conformability and quietness, but has difficulty 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 portion and a surface layer covering the surface of the base portion, wherein the surface layer is mainly formed of flat copper powder arranged so that the thickness direction thereof is thin, and the base layer is formed of iron powder. It has an iron structure and a copper structure formed of partially diffused alloy powder obtained by partially diffusing copper powder.

  The flat copper powder has the property of adhering to the mold forming surface when the raw material powder is molded. 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). 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 the green compact containing the flat copper powder and the low melting point metal is sintered, there is a concern about the spheroidization of the flat copper powder. 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 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, FIG.10 (a) shows before sintering and FIG.10 (b) 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.

  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 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 configuration of a typical 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 2 a 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 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 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 a 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, 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 as shown in FIG. 3. 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 arranged by binding 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 the present 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).

  Moreover, as the copper powder 13 which comprises the partial diffusion alloy powder 11, the irregular-shaped and dendritic copper powder currently used widely can be used widely, 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, one having an average particle diameter of 5 μm to 45 μm is used. In addition, the ratio of Cu in the partial diffusion alloy powder 11 shall be 10-30 wt% (preferably 22-26 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. As this fatty acid, Cn is 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 containing the above powders, 75 to 90 wt% of the partial diffusion alloy powder, 8 to 20 wt% of the flat copper powder, 0.8 to 6.0 wt% of the low melting point metal powder (for example, tin powder), solid lubrication It is preferable to mix 0.5 to 2.0 wt% of the agent powder (for example, graphite powder). The reason for this blending ratio is as follows.

  In the present invention, as will be described later, the 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 material powder, the apparent density of the flat copper powder is the smallest. 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 area Q), the flat copper powder 15 has a layered state in which the 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 is diffused, the existence ratio of pearlite (γFe) becomes excessive, and when the ratio is equal to or higher than that of ferrite (αFe), the aggressiveness 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 ratio in the arbitrary cross section is 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 for the pearlite phase to be present 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, those having a kinematic viscosity of 30 mm ² / sec or more and 200 mm ² / sec or less are 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 process.

  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, the inner base portion S2 covered with the surface layer S1 is formed with two types of copper structures (31b, 31c), iron structure 33, free graphite 32, and pores. 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 is 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 the copper powder 13 constituting the 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 this 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 copper content and a large iron content 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.

[Other forms of metal structure]
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.) that does not contain 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.

[Other configurations of sintered bearings]
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, reduction in bearing strength, generation of abnormal noise, and the like.

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. Similar 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.

[Other uses of 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 or an incoming mail in a mobile terminal such as a mobile phone or a smartphone. 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.

[Other Embodiments]
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, it is preferable that the proportion of the partially diffused alloy powder in the raw material powder is 50% by mass or more. In this case, the raw material powder is 8 to 20 wt% of flat copper powder, 0.8 to 6.0 wt% of low melting point metal powder (for example, tin powder), and 0.5 to 6.0 of solid lubricant powder (for example, 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.

  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

Claims (14)

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 the base layer has an iron structure and a copper structure formed of 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 the copper structure of the base layer are all formed of partially diffused alloy powder.   3. The sintered bearing according to claim 1, 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 the 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 a ratio of copper in the partial diffusion alloy powder is 10 wt% or more and 30 wt% or less. The sintered bearing according to claim 1, 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 in a starter for starting an engine.   The sintered bearing according to any one of claims 1 to 11, which is used in 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.