Patent Application
20160215820
The present invention relates to a sliding member having a sliding surface slidable relative to another member, and to a method of manufacturing the sliding member.
A bearing to be used at, for example, a joint portion of an arm of construction machinery is required to have excellent wear resistance because remarkably large surface pressure is applied to a bearing surface of the bearing. As the bearing of this type, for example, there are known a bearing obtained by cutting casting alloy and a bearing obtained by embedding particles of graphite in a sliding surface in a spotted manner. However, both of the bearings have a problem of high manufacturing cost. Accordingly, instead of those bearings, a sintered bearing made of sintered metal excellent in moldability is used. For example, in Patent Literature 1, there is disclosed, as a bearing for construction machinery, a sintered bearing that is obtained by dispersing copper in iron-carbon-based alloy containing martensitic structure. This sintered bearing is obtained by quenching (oil-quenching, for example) an entire sintered compact after sintering, and finishing the sintered compact by cutting and grinding inner and outer peripheral surfaces and end surfaces thereof into predetermined dimensions.
In addition, in Patent Literature 2, there is disclosed a method of molding a green compact having a double-layer structure, which is formed into a sintered compact including portions different from each other in materials so that those portions exert functions different from each other. Specifically, an inner peripheral surface side and an outer peripheral surface side of the sintered compact are different from each other in materials. More specifically, an outer peripheral surface side of the green compact is made of a first powder having high strength, whereas an inner peripheral surface side thereof is made of a second powder excellent in low-friction property, and then this green compact is sintered.
Patent Literature 1: JP 2003-222133 A
Patent Literature 2: JP 2005-95979 A
In the double-layer-structure sintered compact disclosed in Patent Literature 2, in order to reduce a friction coefficient of the inner peripheral surface, a copper-rich layer needs to be formed on the inner peripheral surface of the sintered compact. Meanwhile, in order to secure high strength on the outer peripheral surface side of the double-layer-structure sintered compact, specifically, strength as high as strength that is necessary for the bearing disclosed in Patent Literature 1, which is arranged at the joint portion of the arm of the construction machinery, the outer peripheral surface side of the sintered compact needs to be formed of structure containing iron and carbon as main components (pearlite structure). In this case, in order to form the pearlite structure, the green compact is sintered at a temperature of 1,130° C. or more.
However, when sintering is performed at the temperature higher than the melting point of copper (1,083° C.) in this manner, copper contained in the copper-rich layer on the inner peripheral surface is completely molten. The molten copper is drawn into a radially outer layer having a low copper concentration, and hence copper structure is not sufficiently formed on the inner peripheral surface through the sintering. Meanwhile, when the sintering temperature is merely reduced, strength that is necessary on the outer peripheral surface side of the sintered compact cannot be secured. As a result, with this configuration, an intended object of the double-layer-structure sintered compact cannot be achieved.
In view of the circumstances, the present invention has an object to provide a sliding member that allows a sliding surface to be enhanced in sliding property and durability while securing strength of a sintered compact, and to provide a method of manufacturing the sliding member.
In order to achieve the above-mentioned object, according to one embodiment of the present invention, there is provided a sliding member made of a sintered compact to have a sliding surface slidable relative to another member, the sliding member comprising: a sliding layer that contains Fe containing an alloying element, Cu, and C as main components; and a base layer that is sintered together with the sliding layer while being in contact with the sliding layer, and contains Fe, Cu, a low-melting-point metal having a melting point lower than a melting point of the Cu, and C as main components, the sliding layer having the sliding surface. This sliding member can be used, for example, as a bearing at a joint portion of an arm of construction machinery.
When Cu and the low-melting-point metal are contained in the base layer, at the time of sintering, first, the low-melting-point metal contained in the base layer is molten. A melt of the low-melting-point metal is dispersed deeply into Fe particles by capillary action. Further, the melt of the low-melting-point metal wets surfaces of Cu particles. Thus, Cu is molten at a temperature lower than its melting point, and Cu and the low-melting-point metal thus molten penetrate the Fe particles, and disperse to the inside of the Fe particles. With this, the Fe particles are firmly bound to each other, and strength of the base layer is increased. Thus, even when the sintering temperature is reduced, strength of the bearing can be secured. When the sintering temperature is set to a temperature lower than the melting point of Cu, the Cu particles contained in the sliding layer is maintained in a solid form without being molten through the sintering. Thus, the Cu particles contained in the sliding layer are not drawn into the base layer, and hence a target amount of Cu structure can be distributed in the sliding surface. In this manner, both a sliding property of the sliding surface and strength of the sintered compact can be secured.
In the sliding member, when the alloying element contained in the sliding layer comprises a quenching-property enhancing element (at least one kind selected from among Ni, Mo, Mn, and Cr), without performing heat treatment such as carburizing and quenching, martensitic transformation and bainitic transformation can be caused at least in a part of Fe structure in the sliding layer through a cooling process after the sintering (sinter hardening). With this, the sliding layer having the sliding surface is increased in hardness, and hence the sliding surface can be enhanced in wear resistance. Further, the strength of the base layer is also increased through the penetration and the dispersion of the Cu particles and particles of the low-melting-point metal into the Fe particles in the base layer, and hence strength of the entire sintered compact is increased. Thus, the sintered compact can be used as a sliding member to which an impact load is frequently applied under large surface pressure, such as a bearing to be used at the joint portion of the arm of the construction machinery.
Meanwhile, basically, the base layer, which accounts for a large proportion of the sintered compact, does not contain the above-mentioned alloying element. Thus, also after the cooling, a large proportion of the base layer is not subjected to the sinter hardening. Therefore, the martensitic transformation and the bainitic transformation do not occur in Fe structure in the base layer. When the quenching-property enhancing element is mixed only into the sliding layer in this manner, a use amount of the expensive alloying element is reduced, and hence cost reduction can be achieved. Further, the base layer is softer than the sliding layer. Thus, dimensional correction of the sintered compact can be performed in sizing (step of reshaping the sintered compact through compression in the die assembly). In the configuration of Patent Literature 1, the entire sintered compact is hardened through oil quenching after the sintering, and hence the dimensional correction of the sintered compact needs to be performed through machining such as cutting and grinding. However, the dimensional correction of the sliding member of the present invention can be performed through sizing. Thus, the machining need not be performed. Further, the quenching step after the sintering need not be performed. In this manner, the quenching step and the machining step after the sintering can be omitted. Thus, further cost reduction of the sliding member can be achieved in comparison with the invention disclosed in Patent Literature 1.
It is preferred that phosphorus be used as the low-melting-point metal contained in the base layer. Further, it is preferred that a concentration of the low-melting-point metal in the base layer be set within a range of from 0.1 wt % to 0.6 wt %.
When a concentration of Cu in the sliding layer is set to 10 wt % or more and 30 wt % or less, cost increase due to excessive use of copper can be prevented while securing the sliding property of the sliding surface. In order to bind the Fe particles in the base layer to each other, Cu needs to be contained also in the base layer. At this time, when a concentration of Cu in the base layer is set lower than the concentration of Cu in the sliding layer, a use amount of expensive copper can be reduced. With this, cost reduction can be achieved.
When the sintered compact is obtained through sintering at a temperature that is lower than the melting point of Cu and higher than a starting temperature of a reaction between Fe and C, copper contained in the sliding layer can be maintained in a solid form without being completely molten through the sintering. With this, a sliding property of the sliding layer can be enhanced. Further, through the reaction between Fe and C, a hard pearlite phase (partially, ferrite phase) is formed in the Fe structure, and hence the strength of the base layer can be secured.
A cross-section of the sliding layer orthogonal to an axial direction of the sliding member may be formed into a partially cylindrical shape with ends. In this case, the sliding layer that needs to be a copper-rich layer is formed not over an entire circumferential region of the sintered compact but only in a partial circumferential region thereof. With this, in comparison with a case where the sliding surface is formed over the entire circumferential region of the sintered compact, the use amount of expensive copper can be reduced. Thus, cost reduction of the sintered bearing can be achieved. Note that, when the sliding member is used as a bearing, a shaft is unlikely to slide relative to an entire periphery of an inner peripheral surface of the sliding member. In the sliding member, by influence of gravity, a region sliding relative to the shaft is limited to the partial circumferential region in many cases. Thus, prior to fixing the sliding member to a housing, an orientation or a posture of the sliding member is adjusted so that the sliding surface of the sliding layer is positioned at the limited partial region. With this, the shaft can be stably supported by the sliding surface.
The above-mentioned sliding member can be obtained by preparing a first powder that contains Fe, Cu, a metal having a melting point lower than a melting point of the Cu, and C as main components, preparing a second powder that contains Fe containing an alloying element, Cu, and C as main components, arranging a partition member in a die assembly to form a first cavity and a second cavity, filling the first powder into the first cavity, filling the second powder into the second cavity, compressing, under a state in which the partition member is removed, the first powder and the second powder in the die assembly simultaneously to form a green compact, sintering the green compact to form the base layer having a composition corresponding to the first powder and the sliding layer having a composition corresponding to the second powder, and performing sizing and oil impregnation of a sintered compact obtained after the sintering.
At the time of compressing the first powder and the second powder simultaneously to mold the green compact, when an apparent density of the first powder and an apparent density of the second powder are significantly different from each other, the green compact cannot be properly molded. Meanwhile, when a thickness of the base layer is set larger than a thickness of the sliding layer, and when the apparent density of the first powder is set lower than the apparent density of the second powder, the green compact can be molded. In other words, even when the apparent density of the first powder and the apparent density of the second powder are somewhat different from each other, the green compact can be easily molded.
As described above, according to the one embodiment of the present invention, the sliding surface can be enhanced in sliding property and durability while securing the strength of the sintered compact.
Now, with reference to the drawings, description is made of a sintered bearing according to embodiments of the present invention, which is taken as an example of a sliding member of the present invention.
The sintered bearing according to the present invention is suited to use at a joint portion configured to couple arms (comprising a boom and a bucket) of construction machinery such as a hydraulic excavator and a bulldozer.
As illustrated in
When being used at the joint portion, the sintered bearing 1 is formed so as to have, for example, an inner diameter of from 30 mm to 100 mm and a radial thickness of from 5 mm to 50 mm. A radial thickness of the sliding layer 2 is set to from approximately 1% to approximately 20% (preferably from approximately 2% to approximately 10%) of the radial thickness of the sintered bearing 1, and an actual thickness dimension thereof is set, for example, to from approximately 0.3 mm to approximately 2 mm. When the sliding layer 2 is extremely thin, a filling property of a raw-material powder at the time of molding is deteriorated and an allowable wear limit is reduced. When the sliding layer 2 is extremely thick, use amounts of elements for enhancing a quenching property and copper, which are described later, are increased to cause cost increase.
As a lubricating material, a lubricating oil such as a mineral oil and a synthetic oil is impregnated into fine pores of the porous sintered bearing 1. At the time of relative rotation between the first arm 6 and the second arm 7, the lubricating oil retained in the inner pores of the sintered bearing 1 seeps out of surface pores of the inner peripheral surface 1a of the sintered bearing 1, and a film of the lubricating oil is formed between the inner peripheral surface 1a and the shaft 4. With this, wear of the inner peripheral surface 1a is suppressed or prevented. An oil-impregnation rate of the entire sintered bearing 1 is set, for example, to from 10 vol % to 25 vol %, preferably from 15 vol % to 25 vol %. This is because, when the oil-impregnation rate falls below 10 vol %, a desired lubricating property cannot be stably maintained or exerted over a long time period, and when the oil-impregnation rate exceeds 25 vol %, a proportion of the inner pores is increased. In this manner, mechanical strength necessary for the sintered bearing 1 may not be secured.
Further, when a viscosity of the lubricating oil to be impregnated into the sintered bearing 1 is extremely low, the lubricating oil is liable to leak out to an outside of the bearing. In addition, the oil film having predetermined strength cannot be formed between the inner peripheral surface 1a and the shaft 4, with the result that the inner peripheral surface 1a is liable to be worn. Meanwhile, when the viscosity of the lubricating oil is extremely high, a seeping amount of the lubricating oil through the surface pores of the inner peripheral surface 1a is insufficient, with the result that the wear of the inner peripheral surface 1a may be accelerated. From such viewpoints, it is preferred that the lubricating oil have, at a temperature of 40° C., a kinematic viscosity of 5 mm2/s or more and 600 mm2/s or less, more preferably 50 mm2/s or more and 550 mm2/s or less, still more preferably 100 mm2/s or more and 500 mm2/s or less.
Note that, as the lubricating material to be impregnated into the inner pores of the sintered bearing 1, a liquid grease may be selected instead of the lubricating oil described above. The liquid grease to be used may be obtained, for example, by using, as a base oil, the lubricating oil having the kinematic viscosity within the above-mentioned ranges at the temperature of 40° C., and adding a soap-based thickener such as lithium soap, or a non-soap-based thickener such as urea to the base oil.
The sintered bearing 1 of the present invention has a double-layer structure comprising the sliding layer 2 and the base layer 3 that are different from each other in metal composition. The sintered bearing 1 having the double-layer structure is manufactured by sequentially performing a compression-molding step, a sintering step, a sizing step, and an oil-impregnating step described below.
In the compression-molding step, there is employed a so-called two-color molding method in which a material for the sliding layer 2 and a material for the base layer 3 are fed into the same die assembly (die) to simultaneously mold the sliding layer 2 and the base layer 3. The two-color molding comprises forming two cavities on a radially outer side and a radially inner side in the die assembly, and filling respective powders into the cavities. The two-color molding is performed by using, for example, a die assembly illustrated in
First, as illustrated in
Next, as illustrated in
Next, as illustrated in
Under a state in which the partition member 14 is removed from the die assembly in this manner, the upper punch 16 is lowered as illustrated in
Note that, the first powder M1 corresponding to the base layer 3 contains an iron powder, a copper powder, and a graphite powder as main components, and a low-melting-point metal as the balance. As the iron powder, there may be used, for example, a reduced iron powder and an atomized iron powder. In particular, it is preferred to use a porous reduced iron powder excellent in oil impregnation property. As the copper powder, there may be used an electrolytic copper powder and an atomized copper powder. In particular, it is more preferred to use the electrolytic copper powder. This is because particles thereof have a branched shape as a whole, and hence strength of the green compact can be increased, and copper particles are easily dispersed among Fe particles at the time of sintering. Further, as the low-melting-point metal, there may be used, for example, metals having a melting point lower than that of copper, specifically, metals such as tin (Sn), zinc (Zn), and phosphorus (P) each having a melting point of 700° C. or less. Such low-melting-point metals can be added, for example, by adding a powder of one kind to a powder mixture, or by using a powder that is alloyed with iron. Among those low-melting-point metals, particles of phosphorus are easily dispersed among particles of iron and can be dispersed deeply into the particles of iron. In addition, the particles of phosphorus also promote the dispersion of the particles of copper. In other words, phosphorus is compatible with both iron and copper. Thus, it is preferred that phosphorus be used as the low-melting-point metal. Specifically, when an iron-phosphorus alloy powder (Fe3P) is mixed with the copper powder and the graphite powder, not only an advantage of ease of mixing and molding the first powder M1, but also an advantage of higher safety can be obtained.
It is preferred that mixing amounts of the powders in the first powder M1 be set so that the first powder M1 contains, for example, the copper powder of from 2 wt % to 5 wt %, the graphite powder of from 0.5 wt % to 0.8 wt %, and an alloy steel powder of iron and the low-melting-point metal as the balance. In this case, a ratio of the low-melting-point metal in the first powder M1 is set to from 0.1 wt % to 0.6 wt % (preferably from 0.3 wt % to 0.5 wt %). The copper powder functions as a binder for binding the particles of the iron powder to each other. When a mixing amount of the copper powder is extremely small, strength of the base layer 3 is reduced. When the mixing amount of the copper powder is extremely large, dispersion of carbon is hindered to reduce strength and hardness of the sintered compact. For those reasons, the above-mentioned ranges are employed. The low-melting-point metal is mixed so as to be dispersed among the iron particles and to promote the dispersion of the copper particles among the iron particles, thereby increasing strength of the sintered compact. When a mixing amount of the low-melting-point metal is extremely small, such effects cannot be sufficiently exerted. When the mixing amount of the low-melting-point metal is extremely large, the low-melting-point metal is segregated to embrittle the sintered compact, resulting in reduction of the strength. For those reasons, the above-mentioned ranges are employed. Further, the graphite powder is mixed so as to react iron and carbon to each other at the time of sintering, thereby forming a hard pearlite phase. When a mixing amount of the graphite powder is small, the strength of the base layer cannot be secured. When the mixing amount of the graphite powder is extremely large, iron is transformed into fragile cementite structure, resulting in reduction of the strength. For those reasons, the above-mentioned ranges are employed.
Meanwhile, the second powder M2 corresponding to the sliding layer 2 is obtained by mixing the iron powder containing alloying elements (alloy steel powder), the copper powder, and the graphite powder with each other. As the alloying elements, quenching-property enhancing elements, specifically, any one kind or two or more kinds selected from among Ni, Mo, Mn, and Cr are used. In the present invention, Ni and Mo are selected, and hence an alloy steel powder of Ni, Mo, and iron (Fe—Ni—Mo-based alloy steel powder) is used. The quenching-property enhancing elements are added so as to cause martensitic transformation and bainitic transformation as described later for sinter hardening. Ni and Mo are preferred due to their significantly excellent effects of enhancing quenching properties. It is preferred that a completely alloyed powder be used as the alloy steel powder in the second powder M2. Although it is preferred that the electrolytic copper powder be used as the copper powder, the atomized copper powder may be used.
It is preferred that mixing amounts of the powders in the second powder M2 be set so that the second powder M2 contains the copper powder of from 10 wt % to 30 wt % (preferably from 15 wt % to 20 wt %), the graphite powder of from 0.2 wt % to 1.0 wt % (preferably from 0.3 wt % to 0.8 wt %), and the alloy steel powder as the balance. Further, a kind and an amount of the alloy steel powder are selected so that the second powder M2 contains Ni at a ratio of from 1.0 wt % to 4.0 wt % (preferably from 1.5 wt % to 3.5 wt %), and Mo at a ratio of from 0.5 wt % to 2.0 wt % (preferably from 0.5 wt % to 1.5 wt %). Mixing amounts of Ni and Mo are set based on moldability and the effects of enhancing quenching properties. When the mixing amount of copper is extremely small, a sliding property of the sliding surface 2a is deteriorated. When the mixing amount of copper is extremely large, the bearing surface is extremely softened to cause problems with wear resistance. For those reasons, the above-mentioned ranges are employed. The graphite powder in the second powder M2 is mixed so as to react iron and carbon to each other at the time of sintering, thereby mainly forming a martensite phase and a bainite phase, and to cause the second powder M2 to function as a solid lubricant. An upper limit and a lower limit of a mixing ratio thereof are set for the same reason that the mixing ratio of the graphite powder in the first powder M1 is set.
Apparent densities of the first powder M1 corresponding to the base layer 3 and the second powder M2 corresponding to the sliding layer 2 are each from 1.0 g/cm3 to 4.0 g/cm3. Both the powders are inevitably different from each other in apparent density due to the difference in composition therebetween. As a result, at the time of molding the first powder M1 and the second powder M2 simultaneously in the compression-molding step, due to the difference in apparent density, the green compact M may be collapsed, for example, and hence may be difficult to mold. However, as in this embodiment, the thickness of the sliding layer 2 is sufficiently smaller than a thickness of the base layer 3 (as described above, thickness of the sliding layer 2 is from 1% to 20%, preferably from 2% to 10% of the thickness of the sintered bearing), and the apparent density of the first powder M1 is lower than the apparent density of the second powder M2. In such settings, when a difference in density therebetween is 0.5 g/cm3 or less, the green compact M can be molded even when the first powder M1 and the second powder M2 are molded simultaneously. Thus, it is preferred that the apparent density of the first powder M1 be set lower than the apparent density of the second powder M2, and that the difference in density therebetween be suppressed to 0.5 g/cm3 or less.
The green compact M obtained through the compression-molding step described above is sintered in the sintering step. With this, a sintered compact M′ is obtained (see
The sintered compact M′ obtained through the sintering step is shifted to the sizing step so as to be subjected to dimensional correction. In this embodiment, as illustrated in
At the time of sintering in the sintering step illustrated in
Further, the quenching-property enhancing elements such as Ni and Mo are contained in the sliding layer 2. Thus, without additionally performing heat treatment such as carburizing and quenching, hardness can be increased by causing the martensitic transformation and the bainitic transformation in the Fe structure of the sliding layer 2 (sinter hardening) during passage through the cooling zone 20b of the continuous sintering furnace 20 illustrated in
Meanwhile, the quenching-property enhancing elements are not added to the base layer 3 that accounts for a large proportion of the sintered compact M′. Thus, use amounts of those expensive elements in the entire bearing are reduced, and hence cost reduction of the bearing can be achieved. Further, in the base layer 3, the sinter hardening is not performed, and hence the martensitic transformation and the bainitic transformation are not caused. Thus, the base layer 3 is softer than the sliding layer 2. Therefore, the dimensional correction of the sintered compact M′ can be performed in the sizing step. In the configuration of Patent Literature 1 described above, the entire sintered compact is hardened through oil quenching after sintering, and hence the dimensional correction of the sintered compact needs to be performed through machining such as cutting and grinding. However, the dimensional correction of the sintered compact M′ of the present invention can be performed through the sizing. Thus, post-processing through machining need not be performed. Further, even when the quenching after the sintering is not performed, necessary and sufficient strength (specifically, radial crushing strength of 500 MPa or more) can be secured. In this manner, the quenching step and the machining step after the sintering can be omitted. Thus, further cost reduction of the sintered bearing 1 can be achieved in comparison with the invention disclosed in Patent Literature 1.
For reference, the surface (bearing surface A) of the sliding layer 2 after the sizing has a hardness of 85 or more, preferably 90 or more, more preferably 95 or more on the Rockwell F-scale (HRF) specified in “JIS Z2245:2011.” Further, the surface of the base layer 3 after the sizing has a hardness of from approximately 55 to approximately 85 on the Rockwell F-scale.
The entire graphite in the base layer 3 basically has been decomposed into carbon through the sintering, and has reacted to Fe. Meanwhile, the graphite in the sliding layer 2 partially remains as particles after the sintering. This is because a content of copper is higher in the sliding layer 2 than in the base layer 3, and hence the particles of copper cover parts of surfaces of the particles of iron, with the result that Fe and C are difficult to react to each other. In this manner, a larger amount of particles of graphite exist in the sliding layer 2 than in the base layer 3. Thus, the particles of graphite can be used as a solid lubricant, and hence the sliding property of the sliding surface A can be enhanced.
Note that, the quenching-property enhancing elements (in this embodiment, Ni and Mo) are not contained in the first powder M1 corresponding to the base layer 3. Thus, theoretically, those elements are not contained in the base layer 3. However, actually, as shown in
Similarly, the low-melting-point metal (in this embodiment, phosphorus) is not contained in the second powder M2 corresponding to the sliding layer 2. Thus, theoretically, the low-melting-point metal is not contained in the sliding layer 3. For the same reason as described above, a concentration gradient of the low-melting-point metal is present at the interface between the sliding layer 2 and the base layer 3. In the sliding layer 2, the low-melting-point metal is not contained in a region sufficiently spaced apart from the base layer 3, such as a surface on an opposite side with respect to the base layer 3 (in this embodiment, sliding surface A of the sliding layer 2).
In the sintered bearing 1 manufactured by the procedure described above, microstructure of the sliding layer 2 is schematically illustrated in
As illustrated in
Further, as illustrated in
In the case described above, the inner peripheral surface of the sliding layer 2 is formed so as to serve as the sliding surface A. However, the present invention is not limited thereto. For example, as illustrated in
Next, description is made of a second embodiment of the present invention.
Similarly to
In this embodiment, a cross-section of the sliding layer 2 orthogonal to the axial direction (horizontal cross-section) is formed into a partially cylindrical shape with ends (semi-cylindrical shape). Further, a horizontal cross-section of the sliding surface A on an inner periphery of the sliding layer 2 is similarly formed into a partially-cylindrical surface shape with ends (in the illustrated example, semi-cylindrical-surface shape). The base layer 3 integrally comprises a thin portion 31 having a small radial thickness, and a thick portion 32 having a large radial thickness, and a circumferential region out of the sliding layer 2 corresponds to the thick portion 32. The inner peripheral surface 1a of the sintered bearing 1 is formed by the sliding surface A of the sliding layer 2 and an inner peripheral surface of the thick portion 32 of the base layer 3, and the outer peripheral surface of the base layer 3 corresponds to the outer peripheral surface 1b (mounting surface B) of the sintered bearing 1. As described above, at this joint portion, the pin 4 sinks by gravity, and slides relative to the lower region of the inner peripheral surface 1a of the sintered bearing 1. Thus, the sintered bearing 1 is inserted into the insertion hole 7a of the second arm 7 in such an orientation or a posture that the sliding surface A of the sliding layer 2 covers a lower region of the pin 4. With this, the pin 4 can be stably supported over a long time period by the sliding layer 2.
According to the configuration of this embodiment, the sliding layer 2 containing not only a high content of copper but also a high content of the quenching-property enhancing elements is formed not over an entire periphery of the inner peripheral surface 1a of the sintered bearing 1 but only in the partial circumferential region. Thus, the use amounts of expensive copper and the expensive quenching-property enhancing elements are reduced, and hence cost reduction of the sintered bearing 1 can be achieved.
Now, description is made of a third embodiment of the present invention.
In the second embodiment described above, the sintered bearing 1 comprises the sliding layer 2 and the base layer 3 having the compositions different from each other. However, as illustrated in
Next, description is made of a fourth embodiment of the present invention.
The sintered bearing 1 integrally comprises an intermediate portion 301, and a pair of surface layer portions 201 and 202 arranged on both sides in a radial direction with respect to the intermediate portion 301. The surface layer portion 201 on the radially inner side and the intermediate portion 301, and the intermediate portion 301 and the surface layer portion 202 on the radially outer side are bound to each other through sintering of the green compact.
The intermediate portion 301 is formed of the base layer 3 described above. Further, the surface layer portion 201 on the radially inner side is formed of the sliding layer 2 described above. Similarly to the surface layer portion 201 on the radially inner side, the surface layer portion 202 on the radially outer side is made of, for example, cylindrical sintered metal containing not only Fe as a main component, but also Cu, the alloying elements for enhancing a quenching property (in this case, Ni and Mo), and C. Concentrations (mixing ratio) of the elements of the surface layer portion 202 on the radially outer side may be set equal or unequal to concentrations of the elements of the surface layer portion 201 on the radially inner side. Note that, the sliding surface A slides relative to the shaft 4, and hence it is preferred that the surface layer portion 201 on the radially inner side be made of a sintered metal that is superior in sliding property to the surface layer portion 202 on the radially outer side. Thus, it is preferred that a concentration of Cu of the surface layer portion 201 on the radially inner side be set higher than a concentration of Cu of the surface layer portion 202 on the radially outer side.
When the surface layer portion 201 on the radially inner side and the surface layer portion 202 on the radially outer side contain the quenching-property enhancing elements, the martensitic transformation and the bainitic transformation (sinter hardening) are caused in the Fe structure contained in both the surface layer portions 201 and 202 by the sintering, and the surface layer portions 201 and 202 are increased in hardness. As a result, the sliding surface A and the mounting surface B that have high hardness and excellent wear resistance can be formed. Further, when the concentration of Cu of the surface layer portion 201 on the radially inner side is set higher than the concentration of Cu of the surface layer portion 202 on the radially outer side, the sliding layer A can be formed so as to have excellent sliding property. Thus, wear of the sliding surface A as a result of repetitive sliding relative to the shaft 4 can be suppressed.
It is preferred that a density of the intermediate portion 301 be set lower at least than a density of the surface layer portion 201 on the radially inner side of both the surface layer portions 201 and 202. When the density of the intermediate portion 301 is set lower than that of the surface layer portion 201 on the radially inner side, the lubricant retained in the intermediate portion 301 can be supplied to the surface layer portion 201 on the radially inner side by a capillary force. With this, an ample amount of the lubricant can be interposed between the sliding surface A and an outer peripheral surface of the shaft 4 so that wear of the sliding surface A can be effectively suppressed or prevented. In order to obtain the intermediate portion 301 having the density lower than that of the surface layer portion 201 on the radially inner side, it is appropriate, for example, to mold the intermediate portion 301 by using a powder having an average particle diameter (specifically, average particle diameter of the Fe powder being a main component powder) larger than that of a powder for molding the surface layer portion 201 on the radially inner side (when necessary, also than that of a powder for molding the surface layer portion 202 on the radially outer side). Note that, as long as the intermediate portion 301 basically has a function of an oil supply layer for supplying the lubricant to the surface layer portion 201 on the radially inner side, levels of a mechanical strength, wear resistance, a sliding property, and the like of the intermediate portion 301 need not be as high as those necessary for the surface layer portion 201 on the radially inner side. Thus, a content of Cu of the intermediate portion 301 need not be set unnecessarily high as long as the particles of Fe can be bound to each other with requisite minimum binding strength.
Further, the intermediate portion 301 does not contain the alloying elements for enhancing a quenching property, and hence the sinter hardening does not occur at the time of sintering. Thus, the intermediate portion 301 can be formed so as to be softer than the surface layer portion 201 on the radially inner side and the surface layer portion 202 on the radially outer side. In this case, the intermediate portion 301 can be utilized as a deformation absorbing portion of the sintered bearing 1. Thus, even when the sintered bearing 1 is press-fitted and fixed to the hole portion 7a of the arm, accuracies in shape and dimension of the sliding surface A are less liable to be adversely affected, and the shaft 4 is supported with high accuracy.
In the compression-molding step for manufacturing the sintered bearing 1 having the triple-layer structure described above, multi-color molding similar to the two-color molding described above is employed. Specifically, a plurality of cavities (three in this case) formed in isolation from each other in the radial direction within a single die assembly are respectively filled with the first powder M1 as a powder for forming the surface layer portion 202 on the radially outer side, the second powder M2 as a powder for forming the intermediate portion 301, and a third powder M3 as a powder for forming the surface layer portion 201 on the radially inner side. Next, the powders M1 to M3 are released from the mutually isolated state, and then the powders M1 to M3 are compressed simultaneously in the axial direction. In this manner, the green compact M is molded. This multi-color molding can be performed by using, for example, a molding die assembly apparatus 10 illustrated in
The molding die assembly apparatus 10 comprises the die 11 configured to mold a radially outer surface of the green compact M, the core pin 12 arranged along the inner periphery of the die 11 and configured to mold the inner peripheral surface of the green compact M, first to third lower punches 13a to 13c and first and second partition members 14a and 14b arranged between the die 11 and the core pin 12 and configured to mold one end surface of the green compact M, and an upper punch 15 configured to mold another end surface of the green compact M. The lower punches 13a to 13c and the partition members 14a and 14b can be raised and lowered independently of each other.
In the molding die assembly apparatus 10 configured as described above, first, the second and third lower punches 13b and 13c and both the partition members 14a and 14b are located at the upper end position. In this state, the first lower punch 13a is lowered to the lower end. With this, the inner peripheral surface of the die 11, an outer peripheral surface of the first partition member 14a, and an upper end surface of the first lower punch 13a forma first cavity 16. The first powder M1 is filled into the first cavity 16. Next, as illustrated in
The first powder M1 has a metal composition corresponding to that of the surface layer portion 202 on the radially outer side, and the second powder M2 has a metal composition corresponding to that of the intermediate portion 301. Note that, in order to form the intermediate portion 301 having the density lower than that of the surface layer portion 201 on the radially inner side and that of the surface layer portion 202 on the radially outer side, the powder having the particle diameter larger than that of the Fe powder being a main component powder in the first powder M1, and than that of the Fe powder being a main component powder in the third powder M3 described later is used as the Fe powder being a main component powder in the second powder M2.
Then, as illustrated in
Next, as illustrated in
Then, after a surplus of the powders M2 and M2 overflowing the cavity 19 is removed, as illustrated in
The green compact M obtained through the compression-molding step described above is heated and sintered under predetermined conditions in the sintering step. With this, the sintered compact M′ (see
The sintered compact M′ obtained through the sintering step is finished so as to have a predetermined shape and dimensions in the sizing step. The sizing of the sintered compact M′ is performed by so-called sizing in which, as illustrated, for example, in
Now, description is made of a fifth embodiment of the present invention.
In the case described in the fourth embodiment, the three metal layers bound to each other through the sintering (surface layer portion 201 on the radially inner side, intermediate portion 301, and surface layer portion 202 on the radially outer side) are laminated in the radial direction. However, as in the example illustrated in
In this sintered bearing 1, an inner peripheral surface 201a of the one surface layer portion 201, and an inner peripheral surface 202a of the another surface layer portion 202 are each formed so as to have a relatively small diameter, thereby forming the sliding surface A for supporting the shaft 4. An inner peripheral surface 301a of the intermediate portion 301 is formed so as to have a relatively large diameter, thereby forming a so-called intermediate relief portion 28. With this, in comparison with a case where the inner peripheral surfaces 201a, 202a, and 301a are formed so as to have the same diameter such that the entire inner peripheral surface of the sintered bearing 1 serves as the sliding surface A, a rotational torque of the shaft 4 can be reduced, and the shaft 4 can be supported with high accuracy.
In
Respectively between the one surface layer portion 202 and the intermediate portion 301 and between the intermediate portion 301 and the another surface layer portion 202, the cylindrical concentration gradient layers R are formed. In each of the concentration gradient layers R, the concentration gradient of the quenching-property enhancing elements (Ni and Mo) is present. The portions 201, 202, and 301 are bound to each other substantially through intermediation of those concentration gradient layers R.
Note that, at the time of supplying the powders M1 to M3 into the molding die assembly apparatus 30, the lower punch 13 may be lowered in three phases so as to sequentially form cavities, and the first to third powders M1 to M3 may be filled sequentially into those cavities.
After the powders M1 to M3 are filled into the cavity in this manner, the upper punch 15 is lowered so as to compress the powders M1 to M3 in the axial direction. With this, the green compact is molded. After the green compact is molded, the lower punch 13 is raised, and the green compact is taken out of the molding die assembly apparatus 10. Then, the green compact thus taken out is shifted to the sintering step. Next, a sintered compact obtained therethrough is shifted sequentially to the sizing step and the oil-impregnating step. In this manner, the sintered bearing 1 is obtained.
The above description exemplifies the sintered bearings 1 having the two metal layers (sliding layer 2 and base layer 3) or the three metal layers (two surface layer portions 201 and 202, and intermediate portion 301) laminated in the radial direction or the axial direction. However, the present invention is suitably applicable also to a sintered bearing having four or more metal layers laminated in the radial direction or the axial direction.
Further, the forms of the sintered compact M′ and the sliding surface A are arbitrary, and the present invention is applicable to other sliding members such as a spherical bush and a flat pad-like member (such as boom pad). As for the former, the sliding surface A is formed into a spherical surface shape. As for the latter, the sliding surface A is formed into a flat surface shape. One or a plurality of recessed portions (having a groove shape, for example) may be formed in the sliding surface A so as to be utilized as a lubricant storage.
Further, the above description exemplifies a case where the interface between the sliding layer 2 and the base layer 3 assumes a cylindrical surface shape, but the present invention is not limited thereto. A cross-section of the interface orthogonal to an axis may be formed into a non-circular shape (for example, polygonal shape or spline shape) (not shown). Thus, the binding strength between the sliding layer 2 and the base layer 3 is further increased. The shape of the interface is formed in conformity with a shape of the partition member 14 (see
Further, the above-mentioned embodiments exemplify a case where the sintered bearing 1 is applied to construction machinery, but the present invention is not limited thereto. The sliding member of the present invention may be suitably applied to such various applications that high surface pressure is applied to the sliding surface.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-186993 | Sep 2013 | JP | national |
| 2014-020343 | Feb 2014 | JP | national |
| 2014-153710 | Jul 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/073317 | 9/4/2014 | WO | 00 |
