Composite bearing structure

Abstract: Disclosed herein is a composite bearing structure which can withstand high speed rotation. The composite bearing structure comprises first, second and third bearing means. The first bearing means, which is adapted to support radial impact force being applied to a rotator during rotation, is formed by an inner ring (1) and an outer ring (2) of silicon nitride ceramic sintered bodies. The second bearing means, which is adapted to support an axial load being applied to the rotator while maintaining a prescribed clearance between the same and the rotator, is formed by two permanent magnets (12, 13) being thrust-directionally opposite to each other. The third bearing means, which is adapted to maintain radial rotational accuracy of the rotator, is formed by a radial dynamic pressure producing groove (5) being provided on a cylindrical surface of the inner ring (1).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a bearing structure in general, and more specifically, it relates to a composite bearing structure for supporting a rotator which has a prescribed load and rotates at a high speed. Description of the Background Art In general, a member forming a rotary part which is provided in a laser printer, a facsimile, a bar code reader or the like must be capable of withstanding high speed rotation. With increase in the printing speed, for example, a rotator of a polygon mirror which is provided in a laser printer must rotate at a speed of at least 20000 r.p.m. In a sliding portion of such a rotator, a ball bearing is generally applied to a bearing, for example. So far as such a conventional ball bearing is employed, however, the rotator can merely withstand a rotating speed of about 16000 r.p.m. at the maximum, due to problems of burning, wear and the like. JP-A-2 173 610 (1990) proposes an air bearing (dynamic pressure gas bearing) of ceramics such as SiC or Si 3 N 4 , which is employed for a rotary sliding portion in order to drive a polygon mirror of a laser printer at a high speed. Upon rotation of a rotator which is supported by such an air bearing, air is forcibly introduced into at least a clearance between a radial bearing member or a thrust bearing member and the rotator through a groove. Thus, the air pressure in the clearance is so increased that the rotator rotates at a high speed through the air bearing which is provided between the members. In order to implement such high speed rotation, rotational accuracy is maintained during high speed rotation through the air bearing, which is also adapted to support a thrust-directional load being applied to the rotator. The radial bearing member and the rotator, which are formed by ceramics, can withstand sliding between the members in a low speed region upon starting or stopping of the rotator. When a conventional radial bearing member of ceramics such as Si 3 N 4 or the like is employed at a high rotating speed exceeding 5000 r.p.m., however, an impactive knock wear phenomenon results from contact between the bearing members. Namely, such radial impact force being applied to the rotator during high speed rotation causes sliding between the members in a high speed region, and it is difficult for a conventional ceramic sintered body to withstand such high speed sliding. When the radial or thrust bearing member is formed by an air bearing member which is prepared from a ceramic sintered body, therefore, it is difficult to support radial impact force being suddenly applied to the rotator during high speed rotation, although rotational accuracy is maintained and a thrust-directional load being applied to the rotator is supported. When the air bearing member is formed by ceramics, further, such a ceramic member must be worked/assembled in high accuracy, and hence the manufacturing cost is increased. In addition, the air bearing member must be used under clean environment, to be protected against dust. Thus, the structure related to the air bearing member is so complicated that a closed container may be required to contain the same, and the space therefor is disadvantageously increased. In order to solve such a problem, a bush type slide bearing member of ceramics may be applied to a bearing for high speed rotation. Fig. 16 is a longitudinal sectional view schematically showing the structure of a conventional bush type slide bearing member of ceramics. This bush type slide bearing member is formed by two thrust slide bearing members 81 and 82, and a radial slide bearing member 83 and a rotator 84, which are paired with each other. Such a bush type slide bearing member can withstand sliding between the members in a low speed region upon starting or stopping. Similarly to the aforementioned air bearing member of ceramics, however, an impactive knock wear phenomenon is caused by contact between the bearing members in sliding following high speed rotation at a speed exceeding 5000 r.p.m. Therefore, it is difficult for the conventional bush type slide bearing member to withstand such sliding, and hence frictional resistance is increased by roughened sliding surfaces. This tendency is particularly remarkable with respect to a thrust-directional load. With respect to the same load, thrust-directional frictional resistance is larger by 5 to 10 times as compared with radial frictional resistance. Also when a bush type slide bearing member of ceramics is employed as a bearing for high speed rotation, therefore, it is difficult to support not only sudden radial impact force during rotation but also a thrust-directional load which is applied to a rotator. In EP-A-0,401,761 a bearing structure is disclosed including a dynamic pressure gas bearing with spiral grooves in both radial and thrust directions. The bearing is made of ceramic material such as silicon carbide, silicon nitride or aluminium oxide sintered material. Not all ceramic materials contribute to an improvement of the sliding characteristics of bearings such as wear resistance and impact resistance in the same manner. Ceramic materials such as silicon carbide or aluminium oxide have a high hardness but low strength and fracture toughness, and thus the material is hard and fragile. In DE-A-3,733,117 a bearing structure is disclosed utilizing a combined structure of a magnetic bearing in thrust direction and an air bearing with dynamic pressure generating grooves in radial direction. The air bearing structure includes a sleeve member and a fixed shaft which are covered with ceramic material such as TiN or SiN. The dynamic pressure portions of the air bearing are covered with a ceramic film made of SiC. The coating with a ceramic film usually provides thermal stress on the dynamic pressure portions because of a mismatch between the coating film and the bearing member being the substrate. In EP-A-0,178,169 a rolling element bearing having members exhibiting excellent mechanical strength and resistance is shown. The members comprise silicon nitride powder having good mechanical properties, but it is not disclosed in EP-A-0,178,169 to manufacture a silicon nitride sintered body with a special size of crystal grains or with special diameter of the pores. The object of the present invention is to provide a journal bearing which is superior in wear resistance and impact resistance. Still another object of the present invention is to suppress increase of the frictional resistance caused in a bearing member in accordance with a thrust-directional load which is applied to a rotator during high speed rotation. A further object of the present invention is to provide a bearing structure which can maintain rotational accuracy in high speed rotation for a long time. A composite bearing structure according to the present invention as defined in claim 1, which is adapted to support a rotator having a prescribed load and rotating at a high speed, comprises journal bearing means and thrust bearing means. The journal bearing means includes ceramic sintered bodies for supporting radial impact force which is applied to the rotator during rotation and maintaining radial rotational accuracy of the rotator. The thrust bearing means supports an axial load which is applied to the rotator while maintaining a prescribed clearance between the same and the rotator. Even if sudden radial impact force is applied to the rotator which rotates at a high speed, the ceramic sintered bodies forming the journal bearing means can support the impact force and withstand sliding in a high speed region. Further, the thrust bearing means supports an axial (thrust-directional) load which is applied to the rotator, while maintaining a clearance between the same and the rotator. Even if the rotator rotates at a high speed, therefore, it is possible to reduce the increase rate of frictional resistance which is caused with respect to the thrust-directional load. In addition, a magnetic bearing can maintain radial rotational accuracy of the rotator which rotates at a high speed. Thus, it is possible to maintain the rotational accuracy, which is required for a precision instrument, also after driving for a long time. According to the present invention, as hereinabove described, the composite bearing structure comprises the bearing means, whereby no wear phenomenon is caused by sudden impact force during high speed rotation and increase of frictional resistance by a thrust-directional load being applied to the rotator is suppressed, while rotational accuracy can be maintained also in rotation for a long time. According to the present invention, therefore, it is possible to provide a composite bearing structure which is suitable for supporting a rotator rotating at a high speed. According to the present invention, the ceramic sintered body forming the journal bearing means includes a silicon nitride ceramic sintered body. The silicon nitride ceramic sintered body contains crystal grains having linear density of at least 35 grains per 30 µm in length, with a boundary phase volume ratio of not more than 15 percent by volume. Further, the silicon nitride sintered body contains pores of not more than 20 µm in maximum diameter, in a content of not more than 3 percent. Such a silicon nitride sintered body has mechanical properties of JIS three-point bending strength of at least 80 kg/mm 2 , preferably at least 100 kg/mm 2 , and fracture toughness of at least 5 MPa·m 1/2 . The silicon nitride sintered body forming the journal bearing means contains crystal grains having linear density of at least a constant value, while the boundary phase volume rate and the porosity thereof are suppressed below constant values. Thus, a bearing member is formed by a silicon nitride sintered body which contains fine crystal grains. Consequently, the surface of the silicon nitride sintered body forming the bearing member is not chipped by falling of crystal grains or the like, but exhibits excellent chipping resistance. Thus, the bearing member formed by the aforementioned silicon nitride sintered body has excellent wear resistance also against high speed sliding accompanied by a knock wear phenomenon. Therefore, sudden radial impact force being applied to the rotator during high speed rotation is supported by the silicon nitride sintered body, which exhibits excellent wear resistance against resulting high speed sliding. In a preferred aspect of the present invention, the thrust bearing means includes dynamic pressure gas bearing means which supports a load by a gas pressure being produced by rotation of the rotator. The thrust bearing means alternatively includes magnetic bearing means which supports the load through magnetic attraction or repulsion. Each of the dynamic pressure gas bearing means and the magnetic bearing means includes a rotating member which rotates with the rotator, and a fixed member which is opposed to the rotating member while maintaining an axial clearance between the same and the rotating member. In the dynamic gas bearing means, the rotating member and the fixed member include ceramic sintered bodies, while one of the rotating and fixed members is provided with a groove for introducing a gas into the clearance for producing a gas pressure. In the magnetic bearing means, on the other hand, the rotating and fixed members include magnets for generating magnetic attraction or repulsion in the clearance. The dynamic gas bearing means can support or reduce a thrust-directional load, which most heavily contributes to increase of frictional resistance in the bearing member, through the pressure of the gas being introduced into the clearance from the groove. Alternatively, the magnetic bearing means can support or reduce the thrust-directional load through magnetic repulsion or attraction. Thus, it is possible to extremely suppress increase of a driving torque with respect to increase in the rotating speed. Particularly when the thrust bearing means is formed by the magnetic bearing means, it is possible to more extremely suppress such increase of the driving torque with respect to increase in the rotating speed as compared with the dynamic gas bearing means. According to a preferred aspect of the present invention, the journal bearing means includes a ceramic sintered body which maintains rotational accuracy while maintaining a prescribed clearance between the same and the rotator. In this case, it is in principle possible to improve rotational accuracy by minimizing the clearance. According to a more preferred aspect of the present invention, the journal bearing means includes dynamic gas bearing means which maintains rotational accuracy by a gas pressure being produced by rotation of the rotator. The dynamic gas bearing means includes a rotating member which rotates with the rotator, and a fixed member which is opposed to the rotating member while maintaining a radial clearance. The rotating member and the fixed member include ceramic sintered bodies, while either one of the rotating and fixed members is provided with a groove for introducing a gas into the clearance for producing a gas pressure. When the journal bearing means is thus prepared from dynamic pressure gas bearing means, it is possible to highly maintain the rotational accuracy in high rotation for a long time, even if the clearance between the rotating and fixed members is relatively increased. In a further preferred aspect of the present invention, the journal bearing means includes magnetic bearing means which maintains rotational accuracy through magnetic attraction or repulsion, while maintaining a prescribed clearance between the same and the rotator. This magnetic bearing means includes a rotating member which rotates with the rotator, and a fixed member which is opposed to the rotating member while maintaining a radial clearance. The rotating member and the fixed member include magnets for generating magnetic attraction or repulsion in the clearance. The rotating and fixed members are formed by ceramic sintered bodies, and the magnets are embedded in these ceramic sintered bodies. Also when the journal bearing means is prepared from magnetic bearing means, it is possible to highly maintain the rotational accuracy in high speed rotation for a long time, even if the clearance between the rotating and fixed members is relatively increased. As hereinabove described, the present invention can provide various composite bearing structures which are suitable for supporting rotators rotating at high speeds. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a conceptual diagram schematically showing a composite bearing member according to Structural Example A of the present invention; Fig. 2(A) is a conceptual diagram schematically showing a composite bearing member according to comparative structural example B, and Fig. 2(B) is an enlarged view showing permanent magnets employed therein; Figs. 3(A) and 3(B) are conceptual diagrams showing two composite bearing members according to Structural Example C of the present invention, respectively; Fig. 4 is a conceptual diagram schematically showing a composite bearing member according to Structural Example D of the present invention; Fig. 5 is a conceptual diagram schematically showing a composite bearing member according to Structural Example E of the present invention; Figs. 6(A) and 6(B) are conceptual diagrams schematically showing two composite bearing members according to Structural Example F of the present invention; Figs. 7(A) and 7(B) are conceptual diagrams schematically showing two composite bearing members according to Structural Example G of the present invention; Fig. 8 is a conceptual diagram schematically showing a composite bearing member according to comparative structural example H; Figs. 9(A) and 9(B) are graphs showing relations between wear rates of sintered bodies forming radial impact resistant members and P·V values in relation to a composite bearing structure according to the present invention and conventional and comparative examples; Fig. 10 schematically illustrates the structure of a testing machine employed for evaluating bearing performance of the composite bearing member according to Structural Example G of the present invention; Fig. 11 schematically illustrates the structure of a testing machine employed for evaluating bearing performance of the composite bearing member according to Structural Example D of the present invention; Fig. 12 schematically illustrates the structure of a testing machine employed for evaluating bearing performance of comparative structural example H in relation to the present invention; Fig. 13 is a graph showing relations between rotating speeds and driving torques as to performance of Structural Examples G and D and comparative structural example H; Fig. 14 is a block diagram schematically showing an exemplary rotator for a polygon mirror to which the inventive composite bearing structure is applied; Fig. 15 is a block diagram schematically showing another exemplary rotator for a polygon mirror to which the inventive composite bearing structure is applied; and Fig. 16 is a longitudinal sectional view schematically showing the structure of a conventional bush type slide bearing member. DESCRIPTION OF THE PREFERRED EMBODIMENTS Table 1 shows various Structural Examples A and C to G of composite bearing members according to the present invention, which are formed by radial bearing means and thrust bearing means, and comparative structural examples B and H. Figs. 1 to 8 schematically illustrate Structural Example A, comparative structural example B, Structural Examples C to G, and comparative structural example H respectively. Referring to Table 1 and Figs. 1 to 8, Structural Examples and comparative examples are now described in due order. Structural Example A Fig. 1 is a conceptual diagram schematically showing a composite bearing structure according to an embodiment of the present invention. Referring to Fig. 1, the composite bearing structure comprises an inner ring 1, an outer ring 2, a lower thrust plate 3 and an upper thrust plate 4. These members 1 to 4 are formed by silicon nitride ceramic sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of encircling the outer peripheral surface of a prescribed shaft, and a plurality of V-shaped radial dynamic pressure producing grooves 5 are formed in the cylindrical surface of the inner ring 1. The lower and upper thrust plates 3 and 4 are provided to be in contact with both end surfaces of the inner ring 1. The outer ring 2, also having a cylindrical shape, is so provided as to define prescribed clearances between the same and the cylindrical surface of the inner ring 1 as well as the inner side surfaces of the lower and upper thrust plates 3 and 4. A plurality of thrust dynamic pressure producing grooves 6 are provided in opposite surfaces of the outer ring 2 and the lower thrust plate 3. These thrust dynamic pressure producing grooves 6, which are formed in both of the outer ring 2 and the lower thrust plate 3 in this embodiment, may alternatively be formed only in one of these members. In the composite bearing member having the aforementioned structure, it is assumed that the outer ring 2 rotates with a rotator while the inner ring 1 and the thrust plates 3 and 4 are fixed to a prescribed shaft. When the outer ring 2 rotates with the rotator, air is introduced into the clearances between the outer ring 2 and the inner ring 1 as well as the lower thrust plate 3 along the radial dynamic pressure producing grooves 5 and the thrust dynamic pressure producing grooves 6, to maintain the clearances constant. Thus, an air bearing is defined. The outer ring 2 can rotate with the rotator at a high speed by air pressures being produced in the clearances, without coming into contact with the inner ring 1 and the thrust plates 3 and 4. According to Structural Example A, both of radial and thrust bearing members are formed by ceramic air bearing members. Therefore, radial rotational accuracy is maintained by the radial air bearing member, while a thrust-directional load is supported by the thrust air bearing member. Further, the ceramic sintered bodies forming the air bearing members are prepared from silicon nitride sintered bodies, which support sudden radial impact force during high speed rotation. Comparative Structural Example B Fig. 2(A) is a conceptual diagram schematically showing a composite bearing structure according to comparative example B for the present invention, and Fig. 2(B) is an enlarged view showing permanent magnets employed in this bearing structure. Referring to Fig. 2(A), the composite bearing member according to comparative example B comprises an inner ring 1, an outer ring 2, a lower thrust plate 3 and an upper thrust plate 4. These members 1 to 4 have base materials of silicon nitride ceramic sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of encircling the outer peripheral surface of a prescribed shaft. The thrust plates 3 and 4 are provided to be in contact with both end surfaces of the inner ring 1. The outer ring 2 is so provided as to maintain prescribed clearances between the same and the cylindrical surface of the inner ring 1 as well as the inner side surfaces of the thrust plates 3 and 4. In this comparative example B, ring-type permanent magnets 7, 8, 9 and 10 are arranged on opposite cylindrical surfaces of the inner and outer rings 1 and 2. Further, spiral thrust dynamic pressure producing grooves 6 are formed in opposite surfaces of the outer ring 2 and the lower thrust plate 3. When the outer ring 2 rotates with a rotator in this composite bearing structure, air is introduced into the clearance between the outer ring 2 and the lower thrust plate 3 along the thrust dynamic pressure producing grooves 6, to maintain the clearance constant. Thus, an air bearing is defined. On the other hand, the permanent magnets 7 (9) and 8 (10) of the inner and outer rings 1 and 2 are so arranged that the same poles are opposite to each other as shown in Fig. 2(B), thereby maintaining the clearance between the inner and outer rings 1 and 2 constant by repulsion. As hereinabove described, a thrust-directional load being applied during high speed rotation is supported by an air bearing member while radial rotational accuracy is maintained by a magnetic bearing member in comparative structural example B. When sudden radial impact force is applied to the rotator during high speed rotation, however, the opposite permanent magnets 7 (9) and 8 (10) inevitably come into contact with each other. Thus, this structure is weak against radial impact force. Structural Example C Figs. 3(A) and 3(B) are conceptual diagrams schematically showing composite bearing structures according to another embodiment of the present invention. Referring to Fig. 3(A), the composite bearing member comprises an inner ring 1, an outer ring 2 and thrust plates 3 and 4. These members 1 to 4 are mainly formed by silicon nitride ceramic sintered bodies. The thrust plates 3 and 4 are provided to be in contact with both end surfaces of the inner ring 1. The outer ring 2 is so provided as to define prescribed clearances between the same and a cylindrical surface of the inner ring 1 as well as inner side surfaces of the thrust plates 3 and 4. In this Structural Example, permanent magnets 7 to 10 are provided on opposite cylindrical surfaces of the inner and outer rings 1 and 2. Further, a plurality of spiral thrust dynamic pressure producing grooves 6 are formed in opposite surfaces of the outer ring 2 and the lower thrust plate 3. The clearance between the inner and outer rings 1 and 2 is defined by a first clearance between the ceramic sintered bodies forming these members, and a second clearance between the permanent magnets 7 (9) and 8 (10). The second clearance is larger than the first clearance. When the outer ring 2 rotates with a rotator in the composite bearing member having the aforementioned structure, air is introduced into the clearance between the outer ring 2 and the lower thrust plate 3, to maintain the clearance constant. Thus, a thrust air bearing is defined. In the radial direction, on the other hand, repulsion generated between the permanent magnets 7 (9) and 8 (10) maintains a constant clearance, thereby maintaining rotational accuracy during high speed rotation. Even if sudden radial impact force is applied to the rotator during such rotation, the inner and outer rings 1 and 2 come into contact with each other not between the permanent magnets 7 to 10 but between the silicon nitride sintered bodies having high strength. Thus, no impactive knock wear phenomenon is caused by contact of the inner and outer rings 1 and 2 in sliding following the high speed rotation. Thus, Structural Example C provides a composite bearing structure, which can not only maintain radial rotational accuracy and support the thrust-directional load but also withstand sudden radial impact force. Fig. 3(B) shows another type of Structural Example C. This structure is different from that shown in Fig. 3(A) in a point that a permanent magnet 11 is embedded in an inner ring 1, while permanent magnets 8 and 10 are provided on an outer peripheral cylindrical surface of an outer ring 2. In this case, the permanent magnets 8 (10) and 11 generate repulsion through ceramic sintered bodies forming the inner and outer rings 1 and 2 respectively. In this case, a clearance between the inner and outer rings 1 and 2 is set in consideration of the repulsion between the permanent magnets 8 (10) and 11 required for maintaining prescribed rotational accuracy. Structural Example D Fig. 4 is a conceptual diagram schematically showing a composite bearing structure according to still another embodiment of the present invention. Referring to Fig. 4, the composite bearing member comprises an inner ring 1, an outer ring 2 and thrust plates 3 and 4. These members 1 to 4 are formed by silicon nitride ceramic sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of encircling the outer peripheral surface of a prescribed shaft. The thrust plates 3 and 4 are provided to be in contact with both end surfaces of the inner ring 1. The outer ring 2 is so provided as to maintain prescribed clearances between the same and the thrust plates 3 and 4 as well as the inner ring 1 respectively. Spiral thrust dynamic pressure producing grooves 6 are provided in opposite surfaces of the outer ring 2 and the lower thrust plate 3. The clearance between the inner and outer rings 1 and 2 is set at a small value, to be capable of maintaining prescribed rotational accuracy during high speed rotation. Thus, a thrust-directional load is supported by an air bearing member defined in the aforementioned manner, while radial rotational accuracy is maintained by the small clearance between the inner and outer rings 1 and 2. Further, sudden radial impact force being applied during high speed rotation is supported by the silicon nitride ceramic sintered bodies having high strength forming the inner and outer rings 1 and 2. Structural Example E Fig. 5 is a conceptual diagram schematically showing a composite bearing member according to a further embodiment of the present invention. Referring to Fig. 5, the composite bearing member comprises an inner ring 1, an outer ring 2, a lower thrust plate 13 and an upper thrust plate 4. The inner and outer rings 1 and 2 and the upper thrust plate 4 are mainly formed by silicon nitride ceramic sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of encircling the outer peripheral surface of a prescribed shaft. A plurality of V-shaped radial dynamic pressure producing grooves 5 are formed in the cylindrical surface of the inner ring 1. The lower thrust plate 13 is made of a permanent magnet, while the outer ring 2 is provided with a ring-shaped permanent magnet 12 on a surface being opposite to the lower thrust plate 13. When the outer ring 2 rotates with a rotator in the composite bearing member having the aforementioned structure, air is introduced into a clearance between the inner and outer rings 1 and 2 along the radial dynamic pressure producing grooves 5, to maintain the clearance constant. Thus, an air bearing member is so defined as to maintain radial rotational accuracy. Further, the permanent magnet 12 of the outer ring 2 and the lower thrust plate (permanent magnet) 13 generate repulsion, to maintain a prescribed clearance between the outer ring 2 and the lower thrust plate 13. Thus, a thrust-directional load is supported during high speed rotation. In addition, the inner and outer rings 1 and 2 are formed by silicon nitride ceramic sintered bodies having high strength, whereby no knock wear phenomenon is caused even if the inner ring 1 comes into contact with the outer ring 2 due to application of sudden impact force during high speed rotation. Thus, Structural Example E provides a composite bearing member, which can not only maintain rotational accuracy and support a thrust-directional load during high speed rotation, but also withstand impact force during the high speed rotation. Structural Example F Figs. 6(A) and 6(B) are conceptual diagrams schematically showing structures of composite bearing members according to a further embodiment of the present invention. Referring to Fig. 6(A), the composite bearing member comprises an inner ring 1, an outer ring 2 and thrust plates 3 and 4. These members 1 to 4 are mainly formed by silicon nitride ceramic sintered bodies. This Example is different from Structural Example C shown in Fig. 3(A) in the structure of a thrust-directional bearing member. Permanent magnets 15 and 16 are provided on opposite surfaces of the outer ring 2 and the lower thrust plate 3. The permanent magnet 15 is also opposed to a permanent magnet 14 which is provided on the inner ring 1. Thus, a thrust-directional load is also supported by a magnetic bearing member. According to this Structural Example F, the magnetic bearing member maintains radial rotational accuracy and supports the thrust-directional load, while the ceramic sintered bodies having high strength support radial impact force. Fig. 6(B) shows another type of Structural Example F. This structure is different from Structural Example C shown in Fig. 3(B) in a point that a thrust-directional load is not supported by an air bearing member, but by a magnetic bearing member. Namely, a permanent magnet 17 which is provided on a lower thrust plate 3 is opposed to a permanent magnet 10 which is provided on an outer ring 2. Structural Example G Figs. 7(A) and 7(B) are conceptual diagrams schematically showing structures of composite bearing members according to a further embodiment of the present invention. Referring to Fig. 7(A), the composite bearing member comprises an inner ring 1, an outer ring 2, a lower thrust plate 13 and an upper thrust plate 4. The inner and outer rings 1 and 2 and the upper thrust plate 4 are mainly formed by silicon nitride ceramic sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of encircling the outer peripheral surface of a prescribed shaft. The thrust plates 13 and 4 are provided to be in contact with both end surfaces of the inner ring 1. The lower thrust plate 13 is made of a permanent magnet. The outer ring 2 is provided with a permanent magnet 12 on a surface being opposite to the lower thrust plate 13. A clearance between the inner and outer rings 1 and 2 is set at a small value, to be capable of maintaining prescribed rotational accuracy during high speed rotation. Thus, a thrust-directional load is supported by repulsion generated by the permanent magnet 12 and the lower thrust plate (permanent magnet) 13, while radial impact force is supported by the silicon nitride sintered bodies having high strength forming the inner and outer rings 1 and 2. Referring to Fig. 7(B), on the other hand, the composite bearing member comprises an upper thrust plate 19 of a permanent magnet and an outer ring 2 which is provided with a permanent magnet 18 on a surface being opposite thereto, to be capable of maintaining prescribed rotational accuracy during high speed rotation, even if a clearance between an inner ring 1 and the outer ring 2 is set at a larger value as compared with the composite bearing member shown in Fig. 7(A). This is because upper and lower portions of the outer ring 2 are held by magnetic bearings, which suppress inclination of the outer ring 2 during high speed rotation. Comparative Structural Example H Fig. 8 is a conceptual diagram schematically showing the structure of a composite bearing member according to comparative example H for the present invention. Referring to Fig. 8, the composite bearing member comprises an inner ring 1, an outer ring 2 and thrust plates 3 and 4. These members 1 to 4 are formed by silicon nitride ceramic sintered bodies. The inner ring 1 has a cylindrical shape, to be capable of encircling the outer peripheral surface of a prescribed shaft. The thrust plates 3 and 4 are provided to be in contact with both end surfaces of the inner ring 1. The outer ring 2 is so provided as to maintain prescribed clearances between the same and the inner ring 1 as well as the thrust plates 3 and 4. According to the composite bearing member having the aforementioned structure, the clearance between the inner and outer rings 1 and 2 is set at a small value, to maintain radial rotational accuracy during high speed rotation. Further, sudden impact force during high speed rotation is supported by the silicon nitride ceramic sintered bodies having high strength forming the inner and outer rings 1 and 2. However, it is difficult to support a thrust-directional load by the thrust plates 3 and 4 during high speed rotation. As to support structures for thrust-directional loads, the magnetic bearing members according to Structural Examples E to G can be simplified in structure as compared with the air bearings according to Structural Examples A, C and D and comparative structural example B. In order to support a thrust-directional load by an air bearing member, a constant area is required for ensuring lift. Further, an air bearing member cannot generate lift unless the rotating speed exceeds a constant value, while a magnetic bearing member can generate lift immediately upon starting of rotation. Means for improving radial rotational accuracy is effectively formed by a radial air bearing member (Structural Example A or E), or a combination with a magnetic bearing member (comparative structural example B, or Structural Example C or F). Also when a bush type slide bearing member is employed, it is possible to improve rotational accuracy by setting a clearance (gap) between inner and outer rings at a small value (Structural Example D or G, or comparative structural example H). Except for comparative structural example B employing no ceramic sintered body for the radial bearing member, every one of the aforementioned structures can satisfy radial impact resistance, and causes no burning even if impact force is applied during high speed rotation and the inner and outer rings come into contact with each other. Particularly when silicon nitride ceramic sintered bodies having high strength are employed, it is possible to maintain excellent bearing characteristics since no chipping is caused. In relation to a material for a radial impact resistant member of the inventive composite bearing structure, a method of preparing a silicon nitride sintered body is now described. Raw material powder of Si 3 N 4 was prepared to have a mean particle diameter of 0.3 µm, particle size distribution of 3ρ = 0.20 µm, an α crystallization rate of 96.5 percent and an oxygen content of 1.4 percent by weight. 92 percent by weight of this Si 3 N 4 raw material powder was wet-blended with 4 percent by weight of Y 2 O 3 powder of 0.8 µm in mean particle diameter, 3 percent by weight of Aℓ 2 O 3 powder of 0.5 µm in mean particle diameter and 1 percent by weight of AℓN powder of 1.0 µm in mean particle diameter in ethanol for 100 hours with a ball mill. Thereafter the mixed powder was dried and subjected to CIP (cold isostatic pressing) molding under a pressure of 5000 kg/cm 2 . The as-formed compact was held under a nitrogen gas atmosphere of 1 atm. at a temperature of 1600°C for 4 hours. Then, the compact was sintered at a temperature of 1750°C for 6 hours, to obtain a sintered body. Thereafter this sintered body was subjected to HIP (hot isostatic pressing) in a gaseous nitrogen atmosphere of 1000 atm. at a temperature of 1700°C for 2 hours. The as-obtained silicon nitride sintered body exhibited a mean major axis particle diameter of not more than 5 µm, a crystal grain aspect ratio of at least 4, and the maximum major axis particle diameter of not more than 15 µm. As to mechanical properties, this sintered body exhibited JIS three-point bending strength of at least 80 kg/mm 2 and fracture toughness of at least 5 MPa·m 1/2 . Test pieces of samples a to e were cut out from a sintered body, to be subjected to evaluation of wear resistance along an Amsler type wear resistance test. Fig. 9(A) is a graph showing relations between products (P·V) of loads being applied to the samples a to e and rotating speeds, and wear rates. Referring to Fig. 9(A), curves a to e show results of measurement of the following samples: Inventive Sample a : A silicon nitride sintered body obtained by the aforementioned method, with linear density of 40 crystal grains per 30 µm in length, a boundary phase volume ratio of 8 percent by volume, porosity of 0.05 percent and the maximum pore diameter of 8 µm. Comparative Sample b : A silicon nitride sintered body obtained by the aforementioned method, with linear density of 30 crystal grains per 30 µm in length, a boundary phase volume ratio of 16 percent by volume, porosity of 3.2 percent and the maximum pore diameter of 22 µm. Conventional Samples c , d and e : Silicon nitride sintered bodies described in Yogyo Kyokai-Shi, 1985, vol. 93, pp. 73 to 80 (Fig. 3, in particular). As to the Amsler wear resistance test, two ring-type samples of 16 mm by 30 mm by 8 mm were fixed to a rotary shaft of a testing machine so that circumferential surfaces thereof were correctly in contact with each other, and a prescribed load was applied to drive the rotary shaft at a prescribed rotating speed thereby performing a sliding test of about 100000 revolutions. Thereafter degrees of weight reduction of the two samples were measured. As clearly understood from Fig. 9(A), the inventive sintered body has remarkably excellent wear resistance. Fig. 9(B) shows the P·V values (kg·m/s) appearing in Fig. 9(A) in terms of the unit areas of the sliding surfaces. Consider that a sintered body prepared in the aforementioned manner is exposed to sliding during high speed rotation of a rotating polygon mirror for a laser printer, for example. The polygon mirror is in the form of a polyhedron (hexa- to icosahedron) which is received in a circumscribing cylinder of about 30 to 150 mm, with a thickness of about 10 mm and weight in a range of about 20 to 500 g. Assuming that a bearing member for supporting the polygon mirror is 10 mm in diameter and the polygon mirror rotates at a speed of 2000 to 100000 r.p.m., its circumferential speed is 100 to 5200 cm/s. Assuming that the bearing area is 3.14 cm 2 , the P·V value (kg/cm·s) is 1 to 828 kg/cm·s. Refer to Fig. 9(B) as to the P·V values in this range. The inventive sintered body exhibits a low wear rate with respect to a wide range of P·V values of 1 to 600 kg/cm·s, and can sufficiently withstand sliding during high speed rotation of the polygon mirror. In the aforementioned example, it is assumed that the sintered body regularly slides during high speed rotation. Also in this case, the inventive sintered body exhibits a low wear rate with respect to a wide range of P·V values. Even if sudden impact force is applied to cause sliding following high speed rotation, therefore, the inventive sintered body can sufficiently withstand such sliding. The aforementioned Structural Examples G and D and comparative structural example H were subjected to evaluation of bearing performance, with prescribed testing machines. Fig. 10 schematically illustrates a testing machine which was employed for evaluating bearing performance of Structural Example G. An inner ring 50 of the aforementioned silicon nitride sintered body was prepared with an inner diameter of 10 mm, to rotate with a rotator 200. An outer ring 100 of the aforementioned silicon nitride sintered body was prepared to define a prescribed clearance between the same and the inner ring 50. A magnetic bearing 500 formed by repulsion of a ferrite magnet was mounted on an upper portion of the rotator 200. Fig. 11 schematically illustrates a testing machine which was employed for evaluating bearing performance of Structural Example D. An inner ring 1000 of the aforementioned silicon nitride sintered body was prepared with an inner diameter of 10 mm, to rotate with a rotator 200. An outer ring 1100 of the aforementioned silicon nitride sintered body was prepared to define a prescribed clearance between the same and the inner ring 1000. A thrust plate 900 of a silicon nitride ceramic sintered body was provided under the outer ring 1100. A spiral dynamic pressure producing groove 1200 was formed in a surface of the inner ring 1000 being opposite to the thrust plate 900. Fig. 12 schematically illustrates a testing machine which was employed for evaluating bearing performance of comparative structural example H. An inner ring 700 of the aforementioned silicon nitride sintered body was prepared with an inner diameter of 10 mm, to rotate with a rotator 200. An outer ring 800 of the aforementioned silicon nitride sintered body was prepared to define a prescribed clearance between the same and the inner ring 700. A thrust plate 600 of a silicon nitride ceramic sintered body was provided under the outer ring 800, with a prescribed clearance between the former and the latter. Using the aforementioned testing machines for evaluating bearing performance, numbers of revolutions of the rotators 200 were increased to set values by motors 300, and then maintained at the set values for 10 minutes. Thereafter driving torques were measured with torque meters 400. Fig. 13 is a graph showing relations between the numbers of revolutions (r.p.m.) (or circumferential speeds (m/sec) in the inner peripheral surfaces of the inner rings) of the rotators supported by Structural Examples G and D and comparative structural example H and driving torques (g·cm), on the basis of results of the aforementioned measurement. Black circles, triangles and white circles show the results of Structural Example G and D and comparative example H respectively. As clearly understood from Fig. 13, Structural Example G has the minimum ratio of the increase rate of the number of revolutions (circumferential speed) to that of the driving torque, while Structural Example D has an intermediate ratio and comparative structural example H has the maximum ratio. As understood from the aforementioned results of measurement, it is possible to extremely suppress increase of the driving torque with respect to increase of the rotating speed by employing a magnetic bearing member as a thrust-directional bearing member in the inventive composite bearing structure. In other words, it is most preferable to support a thrust-directional load, which most heavily contributes to increase in the frictional resistance of the bearing member, by a magnetic bearing member. The inventive composite bearing structure can be applied to a rotating polygon mirror for a laser printer, for example. Such a rotating polygon mirror for a laser printer is now described. Fig. 14 schematically illustrates an exemplary rotating polygon mirror 30 which is employed for a laser printer. Referring to Fig. 14, a frame 20a of a motor 20 is provided with a spindle 21. A cylindrical ring 22 of the aforementioned silicon nitride sintered body is mounted on the outer periphery of the spindle 21. Further, flat thrust plates 23 and 24 of the aforementioned silicon nitride sintered bodies (or permanent magnets) are mounted on base and forward ends of the spindle 21 respectively, to be located on upper and lower sides of the inner ring 22. An outer ring 25, which serves as a rotator forming a pivotal portion of the rotating polygon mirror 30, is also formed by the aforementioned silicon nitride sintered body in the form of a ring. Thus, the members 22, 23, 24 and 25 define a composite bearing structure corresponding to Structural Example A, C, D, E, F or G. An aluminum reflecting frame 27 is mounted on the outer periphery of the outer ring 25. prescribed clearances are defined between the inner and outer rings 22 and 25 as well as between the thrust plates 23 and 24 and the outer ring 25. A plurality of mirror surfaces 27a are formed on the outer periphery of the reflecting frame 27, to define a polyhedron such as an octahedron, for example, about the inner ring 22. On a bottom wall surface of the frame 20a, on the other hand, a plurality of coils 28 are arranged along a circumference which is concentric with the spindle 21. A plurality of permanent magnets 29 are arranged on the inner side surface of the reflecting frame 27, to be opposed to the coils 28. The outer ring 25 and the reflecting frame 27 define a rotor of the driving motor 20. Tables 2, 3 and 4 show results of performance evaluation in relation to Structural Examples E, F and G being applied to the rotating polygon mirror shown in Fig. 14. Referring to Tables 2 to 4, stationary currents indicate current values of the driving motor measured when the polygon mirror reached stationary rotation, i.e., values corresponding to driving torques. Inclination angles of the mirror surfaces were measured in order to evaluate degrees of vibration of the polygon mirror during rotation. Surface inclination angles appearing in Tables 2 to 4 indicate values of maximum vibration widths of all mirror surfaces. Referring to Structural Example E, as shown in Fig. 5, the inner ring 1 has an outer diameter of 20 mm and a length of 30 mm, while the clearance between the inner and outer rings 1 and 2 is 60 µm. Referring to Structural Example F, as shown in Fig. 6(A), the inner ring 1 has an outer diameter of 20 mm in the portion formed by the silicon nitride ceramic sintered body, and a length of 30 mm, while the clearance between the inner and outer rings 1 and 2 is 40 µm. Referring to Structural Example G, as shown in Fig. 7(A), the inner ring 1 has an outer diameter of 20 mm and a length of 30 mm, while the clearance between the inner and outer rings 1 and 2 is 2.5 µm. Number of Revolutions (r.p.m) Stationary Current (A) Surface Inclination Angle (Sec.) Initial Value After Driving for 10,000h. 5,000 0.61 5.2 5.2 10,000 1.03 5.5 5.5 20,000 2.10 5.8 5.8 30,000 3.05 6.0 6.0 50,000 4.85 10.5 10.9 70,000 6.67 14.7 15.5 100,000 9.30 20.1 21.6 Number of Revolutions (r.p.m) Stationary Current (A) Surface Inclination Angle (Sec.) Initial Value After Driving for 10,000h. 5,000 0.42 3.1 3.1 10,000 0.71 3.8 3.8 20,000 1.40 4.7 4.7 30,000 2.05 6.1 6.1 50,000 3.28 10.3 10.8 70,000 4.48 14.1 14.6 100,000 6.25 18.5 19.2 Number of Revolutions (r.p.m.) Stationary Current (A) Surface Inclination Angle (Sec.) Initial Value After Driving for 10,000h. 5,000 0.41 2.9 3.1 10,000 0.69 3.5 3.8 20,000 1.38 4.9 5.4 30,000 2.02 6.2 6.8 50,000 3.26 9.8 11.7 70,000 4.46 11.4 13.8 100,000 6.20 15.3 19.3 In every one of Structural Examples E, F and G, the surface inclination angle of the mirror is suppressed in a range of one minute after driving at a number of revolutions of 100000 r.p.m. for 10000 hours. It is understood from the aforementioned results that a polygon mirror rotator employing the inventive composite bearing member exhibits excellent rotational accuracy, even if high speed rotation is carried out for a long time. Referring to Tables 2 to 4, the bearing structures of Structural Examples E, F and G attained surface inclination angles of substantially similar degrees. When Structural Example G (Table 4) is employed, however, it is necessary to set the clearance between the inner and outer rings at a small value, as hereinabove described. When Structural Example E or F (Table 2 or 3) is employed, on the other hand, it is possible to set the clearance between the inner and outer rings at a relatively large value, as hereinabove described. Fig. 15 schematically illustrates another exemplary polygon mirror rotator, to which the inventive composite bearing structure is applied. Referring to Fig. 15, a case 51 is provided on its axis with a rotary shaft 52, which is rotatably supported by a composite bearing member 53 formed by any one of Structural Examples A, C, D, E, F and G. This composite bearing member 53 is formed by an inner ring 60, thrust plates 61 and 62 and an outer ring 63. A rotor 54 is mounted on the outer ring 63 of the composite bearing member 53. The case 51 is further provided with a window 57, while a polygon mirror 56 is mounted on an upper portion of the rotary shaft 52, to be opposed to the window 57. Although the inventive composite bearing member is applied to a bearing member for a rotating polygon mirror, the present invention is not restricted to this. The composite bearing member according to the present invention is also applicable to a bearing for a spindle motor for rotating a hard disk and photomagnetic disk a turbine bearing for a supercharger which rotates at a high speed of 80000 to 150000 r.p.m., a bearing for a turbine or a compressor which rotates at a high speed of 20000 to 30000 r.p.m., a high speed rotation bearing for a rocket engine turbopump, a bearing employed for a machine tool such as a CNC ultraprecise lathe, an ultraprecise lathe for working a cylinder or an ultraprecise surface grinding machine, or the like, for example. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A composite bearing structure for supporting a rotator having a prescribed load and rotating at a high speed, said composite bearing structure comprising: a journal bearing means (1, 2) including ceramic sintered bodies provided on fixed and rotating members for supporting radial impact force and maintaining radial rotational accuracy, whereby the journal bearing means is either constituted by a slide bearing or a dynamic pressure gas bearing or a magnetic bearing with a ceramic bush, and a thrust bearing means (2, 12, 3, 13) which is either constituted by a dynamic gas bearing or a magnetic bearing, characterized in that at least the two bodies of the fixed and rotating members of the journal bearing means (1, 2) are both made of silicon nitride containing chrystal grains having linear density of at least 35 grains/30 µm in length with a boundary phase volume ratio of not more than 15% of volume as well as pores of not more than 20 µm in maximum diameter in a content of not more than 3%. A composite bearing structure in accordance with claim 1, characterized in that said thrust bearing means is constituted by a dynamic pressure gas bearing including a rotating member (2) rotating with said rotator and a fixed member (3) being opposed to that rotating member while axially maintaining a prescribed clearance. A composite bearing structure in accordance with claim 2, characterized in that said rotating member (2) and said fixed member (3) include ceramic sintered bodies, grooves (6) being formed in either one of said rotating and fixed members for introducing a gas into that clearance in order to produce a gas pressure. A composite bearing structure in accordance with one of claims 1 to 3, characterized in that said journal bearing means is constituted by a dynamic pressure gas bearing including a rotating member (2) rotating with said rotator and a fixed member (1) being opposed to said rotating member while radially maintaining a prescribed clearance. A composite bearing structure in accordance with claim 4, characterized in that said rotating member (2) and said fixed member (1) include ceramic sintered bodies, either one of said rotating and fixed members being provided with a groove (5) for introducing a gas into said clearance in order to produce a gas pressure. A composite bearing structure in accordance with one of claims 1 to 3, characterized in that said journal bearing means is constituted by a slide bearing including a ceramic sintered body for maintaining said rotational accuracy while maintaining a prescribed clearance between the same and said rotator. A composite bearing structure in accordance with claim 1, characterized in that said thrust bearing means is constituted by a magnetic bearing including a rotating member (2, 12) rotating with said rotator and a fixed member (3, 13) being opposed to said rotating member while axially maintaining a prescribed clearance. A composite bearing structure in accordance with claim 7, characterized in that said rotating member and said fixed member include magnets (12, 13, 10, 17, 15, 16) for generating magnetic attraction or repulsion in said clearance. A composite bearing structure in accordance with claims 7 or 8, characterized in that said journal bearing means is constituted by a dynamic pressure gas bearing including a rotating member (2) rotating with said rotator and a fixed member (1) being opposed to said rotating member while radially maintaining a prescribed clearance. A composite bearing structure in accordance with claim 9, characterized in that said rotating member (2) and said fixed member (1) include ceramic sintered bodies, either one of said rotating and fixed members being provided with a groove (5) for introducing a gas into said clearance in order to produce a gas pressure. A composite bearing structure in accordance with claim 7 or 8, characterized in that said journal bearing means is constituted by a slide bearing including a ceramic sintered body for maintaining said rotational accuracy while maintaining a prescribed clearance between the same and said rotator. A composite bearing structure in accordance with claims 1 to 3, characterized in that said journal bearing means is constituted by a magnetic bearing with a ceramic bush including a rotating member (2) with said rotator and a fixed member (1) being opposed to said rotating member while radially maintaining a prescribed clearance. A composite bearing structure in accordance with claim 12, characterized in that said rotating member (2) and said fixed member (1) include magnets (7, 8, 9, 10, 11) for generating magnetic attraction or repulsion in said clearance. A composite bearing structure in accordance with claim 13, characterized in that said rotating and fixed members are formed by said ceramic sintered bodies, said magnets (11) being embedded in said ceramic sintered bodies. A composite bearing structure in accordance with claims 7 or 8, characterized in that said journal bearing means is constituted by a magnetic bearing with a ceramic bush including a rotating member (2) with said rotator and a fixed member (1) being opposed to said rotating member while radially maintaining a prescribed clearance. A composite bearing structure in accordance with claim 15, characterized in that said rotating member (2) and said fixed member (1) include magnets (7, 8, 10, 11, 14, 15) for generating magnetic attraction or repulsion in said clearance. A composite bearing structure in accordance with claim 16, characterized in that said rotating and fixed members are formed by said ceramic sintered bodies, said magnets (11) being embedded in said ceramic sintered bodies.

Inventors: HIGUCHI MATSUO, KATAYAMA TETSUYA, KOMURA OSAMU, MATSUNUMA KENJI, MIYAKE MASAYA, YAMAKAWA AKIRA, YASUOKA NORIO, HIGUCHI, MATSUO, KATAYAMA, TETSUYA, KOMURA, OSAMU, MATSUNUMA, KENJI, MIYAKE, MASAYA, YAMAKAWA, AKIRA, YASUOKA, NORIO, Higuchi, Matsuo, c/o Itami Works of, Katayama, Tetsuya, c/o Itami Works of, KOMURA, OSAMU, C/O ITAMI WORKS OF, MATSUNUMA, KENJI, C/O ITAMI WORKS OF, MIYAKE, MASAYA, C/O ITAMI WORKS OF, YAMAKAWA, AKIRA, C/O ITAMI WORKS OF, Yasuoka, Norio, c/o Itami Works of
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: F16C17
IPC codes: F16C39, F16C33