ELECTRICALLY POWERED APPARATUS

Abstract: An electrically powered apparatus is disclosed in which an axial end surface (9) of a rotor (10) and an axial end surface of a stator (8) are arranged to be sliding surfaces for effecting relative sliding motion therebetween, one of the sliding surfaces being provided with grooves adapted to generate fluid dynamic pressure upon relative rotation of said surfaces.

Description: The present invention relates to an electrically powered apparatus and in particular to an electrically powered apparatus having a flat structure comprising a rotor and a stator with a magnetic gap therebetween in the axial direction of the rotary axis of the rotor wherein no drive shaft for transmitting rotational power is required. Fig. ll is a vertical sectional view illustrating a brushless flat motor which is an example of an electrically powered apparatus according to prior art. A base 2l and a base plate 22 for attaching a stator coil are connected together by stator bolts 23 to provide a frame construction. A rotor 27 having a rotor magnet fixed thereto is secured to a motor shaft 26 rotatably supported by bearings 24 and 25 which are fixed to the base 2l and the base plate 22 respec­ tively. A stator coil 28 is secured to the base plate 22 in relation to the rotor 27 with the base plate 22 therebetween and secured to the base plate 22 are a tachogenerator 33 to be activated by the rotor 27 and a Hall element 29 adapted to control the rotational speed of the rotor. A construc­ tion similar to the above may also be applied to a sheet coil motor having a motor coil manufactured by a photo-­ etching method to allow the stator bolts to be made very thin. The electrically powered apparatus of the construc­ tion explained above may be utilized in many ways, for example, as a laser scanner with a polygon mirror fixed to the motor shaft 26, an optical disc driving means with an optical disc removably attached directly to the rotor 27 or a blower with the blades of a fan fixedly mounted to the rotor 27. In the electrically powered apparatus constructed as above, the clearance of the bearings 24 and 25 in the radial direction is so large that the rotor 27 may be vibrated, particularly where ball bearings are used for the bearings 24 and 25, and and vibration may thus be caused, such vibration causing problems in tape recorders and VTR which are susceptible to vibration. In addition, it is to be noted that there are no radial bearings available which display good resistance to corrosion without the use of sealing means and they thus cannot be used in a submerged condition. Such bearings also exhibit poor resistance characteristics under circumstances where dew is a frequent occurrence. It is further to be noted that in order for the motors to be utilized in electric appliances, uneven rota­ tion causes problems, and since the frictional resistance varies considerably in the case of radial bearings, uneven rotation may result possibly. For electrically powered apparatus of the sort described above, it is preferable in many cases that such apparatuses be short in the axial length, as well as being thin. However, the motor cannot be made thin unless the distance between the radial bearings 24 and 25 can be made short. Conversely if the distance between the radial bearings is made short, the radial load to be supported by the bearings will be increased and tilting possibility of the motor shaft will also be increased due to the clearance existing in the bearings. It is further to be noted that with the electrically powered apparatus of the type described above, the thrust load is always applied by the magnet of the rotor, and the metallic balls and the retainer of the ball bearings are subjected to permanent deformation if the apparatus is left unused for a long period of time, making it impossible for the apparatus to be started again. It is also to be noted that when the electrically powered apparatus is used for a high speed laser scanner which is adapted to reproduce an image of high resolution, the motor shaft 26 as well as the bearings 24 and 25 have to be machined extremely precisely, so as to minimize deflection of the plane of the polygon mirror, and at the same time they have to be assembled with the polygon mirror with high precision. An object of the present invention is to provide an electrically powered apparatus having a rotor and a stator arranged in such a manner as to provide a magnetic gap in the axial direction of the rotating axis of the rotor, characterized in possessing a simple construction and high durability. Another object of the present invention is to provide an electrically powered apparatus which will possibly minimize any vibration of the rotor and deflection of the plane of the rotor. A further object of the present inven­ tion is to provide an electrically powered apparatus which is efficient in respect of both durability and resistance against abrasion and which is capable of being applied in a wide range of environmental conditions including utilization within air, liquid, and so forth. The present invention provides an electrically powered apparatus in which a stator is arranged in such a manner as to oppositely face the end surface of a rotor in the axial direction of the rotary shaft, characterized in that a sliding suface on the rotary side is provided on the end surface of the rotor oppositely facing the stator, the front surface of the stator oppositely facing the sliding surface is formed as a bearing surface on the stationary side, the sliding surface on the rotary side and the bearing surface on the stationary side are respectively made of hard ceramic materials, either of the sliding or bearing surfaces on the rotary or stationary sides being formed as a smooth plane with the other of the surfaces being formed as a plane provided with grooves adapted to generate fluid dynamic pressure. The second mode of the present invention provides an electrically powered apparatus wherein the opposite end surfaces of the rotor in the axial direction of the rotary axis of the rotor are respectively formed to be rotary side sliding surfaces, a stator is arranged in such a manner as to oppositely face one of the sliding surfaces, and a rock­ able bearing surface is arranged in such a manner as to oppositely face the other of the sliding surfaces, charac­ terized in that the front surface oppositely facing the sliding surface on the rotary side is formed as a bearing surface on the stationary side, and the two rotary side sliding surfaces and the two bearing surfaces oppositely facing the sliding surfaces are respectively made of hard ceramic materials, either of the two surfaces constituting sliding portions formed at the opposite end surfaces of the rotor in the axial direction of the rotary axis being formed with grooves adapted to gener­ ate fluid dynamic pressure, the other of the surfaces being formed as a smooth plane. The third mode of the present invention further provides an electrically powered apparatus wherein the oppo­ site end surfaces of the rotor in the axial direction of the rotary axis are respectively formed to be the rotary side sliding surfaces, a stator is arranged in such a manner as to oppositely face one of the sliding surfaces on the rotary side and a rockable bearing surface is arranged in such a manner as to oppositely face the other of the rotary side sliding surfaces, characterized in that a rotationary shaft extending along the rotary axis of the rotor is fixedly mounted to the stator, and a through-­ hole is formed centrally of the rotor corresponding to the stationary shaft to form a radial bearing, and the surface of the stator oppositely facing the rotary side sliding surface is formed as a bearing surface on the stationary side, the two sliding surfaces on the rotary side and the two bearing surfaces oppositely facing the sliding surfaces being respectively made of hard ceramic materials, either of the two surfaces constituting the sliding portions formed at the opposite end surfaces of the rotor in the axial direction of the rotary axis being provided with grooves adapted to generate fluid dynamic pressure, and the other of the surfaces being formed to be a smooth plane. In the electrically powered apparatus according to the present invention, since the sliding portions defined between the rotor and the stator are respectively made of hard ceramic materials and either of the sliding portions is formed with grooves adapted to generate fluid dynamic pressure, dynamic pressure of the fluid is generated at the sliding portions through rotation of the rotor, and the rotor is caused to rotate in a floating condition, result­ ing in a fairly small sliding resistance. Further, the rotor may be prevented from deflection by the damping effect provided by the fluid film formed between the sliding portions. In the second mode of the present invention, deflec­ tion of the rotor may be further minimized because the opposite end surfaces of the rotor in the axial direction of the rotary axis are sustained respectively by the sliding portions having damping effects. Further, in the third mode of the present invention, wherein sliding portions adapted to sustain the radial load are defined between the through-hole provided centrally of the rotor and the stationary shaft, the sliding resistance is also small because a fluid film is formed there in the same manner as described above. The present invention will be further clarified if the ensuing description is reviewed in combination with the accompanying drawings, a brief explanation thereof being summarized below. Fig. l is a vertical sectional view showing an embodiment of the present invention; Fig. 2 is a partially enlarged view of Fig. l; Fig. 3 is a plan view of spiral grooves formed on a rotor surface shown in Fig. l; Fig. 4 is a vertical sectional view showing another embodiment of the present invention; Fig. 5 is a vertical sectional view showing a further embodiment of the present invention; Fig. 6 is a sectional view taken along the line A - A in Fig. 5; Fig. 7 is a vertical sectional view showing a still further embodiment of the present invention; Fig. 8 is a sectional view taken along the line B - B in Fig. 7; Fig. 9 and Fig. l0 show further embodiments of a portion of the apparatus shown in Fig. 7; and Fig. ll shows an apparatus according to a prior art. Embodiments of the present invention will now be explained by referring to the accompanying drawings. Fig. l is a vertical sectional view showing a brush­ less flat motor of a normal type wherein a rotary side sliding surface is formed at one of the end surfaces of a rotor l0 in the axial direction of the rotary axis. The rotor l0 consists of an iron yoke l of a disc-like construc­ tion having a circular flat surface, an annular permanent magnet 2 fitted in and secured to the yoke l and a plate l00 of ceramic material fixedly mounted to the yoke l and the permanent magnet 2 at the inner periphery of the permanent magnet 2. The permanent magnet 2 is magnetized with a plurality of magnetic poles which are arranged annually and equidistantly. Hard ceramic materials are used for the plate l00, such as fine and dense ceramic materials contain­ ing for example β-SiC, α-SiC, Si₃N₄ or the like. A surface 9 on the side of the rotor is a smooth plane formed with grooves adapted to generate fluid dynamic pressure. More specifically, the sliding surface 9 on the rotary side oppositely facing the stator is machined to have a flatness within a tolerance of l µm over the entire surface thereof and is provided with spiral grooves having a depth of 3 - 50 µm adapted to generate fluid dynamic pressure. At the position of the rotor l0 corresponding to the rotary axis is formed a semi-spherical recess l2. The stator side will next be described. A stator coil 4 is fixed to a stationary plate 6 which is in turn fixedly mounted on the stationary side member (not shown) by means of screws. On the stator coil 4 is fixedly mounted a stationary side bearing 3 which is made of a hard ceramic material. The stator coil 4 comprises a plurality of sheet coils bonded and fixed together. To the stator coil 4 are integrally fixed a Hall element 29 adapted to detect the magnetic poles of the permanent magnet 2 of the rotor l0 and a tachometer 33 adapted to detect the rotational speed of the rotor l0. The surface 8 of the stationary side bearing 3 fixed to the stator coil 4 corresponding to the sliding surface 9 at the rotary side of the rotor l0 is also machined to provide a smooth and non-undulating plane in a similar manner to the rotor. A semi-spherical recess ll is also formed at the position of the stator corresponding to the rotary axis of the rotor. A space defined by the two recesses ll and l2 accommodates a small ball 7 which is made of plastic material, metallic material or ceramic material and is adapted to sustain an small radial load. Fig. 2 is an enlarged view of the central portions of the rotary side sliding surface 9 and the stationary side bearing surface 8 shown in Fig. l. The space defined by the semi-spherical recesses ll and l2 formed respectively along the rotary axis l3 of the rotor is substantially spherical and the ball 7, which is smaller in diameter than the space, is accommodated therein. There is substantially no space left between rotary side sliding surface 9 of the plate l00 made of ceramic material and the stationary side bearing surface 8 of the stationary side bearing 3 when the rotor l0 is at rest or when they are in abutment with each other. However, when the rotor l0 starts to rotate, dynamic pressure of the fluid l4 is generated at the sliding portions causing the rotor l0 to float up. The floating height of the rotor l0 depends on various factors such as the viscosity of the fluid existing at the sliding portions, the rotational speed of the rotor l0, the thrust load imposed on the sliding portions, the configurations of the grooves adapted to generate fluid dynamic pressure, etc. In practice, the viscosity of the fluid has a particularly major influence. Fig. 3 shows the front view of the rotary side slid­ ing surface 9 in which spiral grooves l5 in one direction are formed on the circular plate l00 of ceramic material. the central portion l6 of the ceramic plate is recessed to the same depth as that of the grooves l5. The central portion l6 may alternatively be the same height as that of the smooth plane represented by the land portions l7 adja­ cent to the spiral grooves l5 or may be machined deeper than the depth of the spiral grooves l5. However, it is to be noted that the area of the land portions l7 which comprise the protruding portions on the rotary side sliding surface 9 is preferably minimized because they may otherwide require a larger starting torque in an electrically powered apparatus of this sort. In Fig. 3, the arrow A indicates the direction of rotation of the rotor l0. Since the rotary side sliding surface 9 is rotated together with the rotor l0, the fluid l4 is caused to flow in from the outer periphery of the rotor toward the central portion l6 by means of the spiral grooves l5. In the spiral grooves l5, a high fluid dynamic pressure is generated as the fluid moves from the outer peripheral end to the inner peripheral end and at the central portion l6 the fluid pressure is uniformly distri­ buted. The magnitude of the fluid dynamic pressure gener­ ated in the grooves is in proportion to the amount of force acting to depress the rotor l0 toward the stator l0l. The rotor l0 and the stator l0l may rotate with a small sliding resistance owing to the fluid film formed therebetween. The portions which protrude most from the rotary side sliding surface 9 are the land portions l7 and the undula­ tions of the respective land portions over the entire surface are within l µm. The protruding portions at the stationary side bearing surface 8 are the smooth bearing surfaces which have undulations restricted to within l µm over their entire surface area also. It is further to be noted that due to the short clearance between the rotary side sliding surface 9 and the stationary side bearing surface 8, the fluid pressure between the sliding portions defined between the surfaces has a damping effect which restricts abrupt variation in respect of the space defined at the sliding portions. Consequently, when the rotor l0 rotates, vibration of the rotor l0 in the axial direction of the rotary axis is limited to a small amount. As explained above, since the clearance at the sliding portions is so small and stable that the clearance between the permanent magnet 2 of the rotor l0 and the stator coil 4 is also made small, the magnetic field generated by the stator coil 4 can be effectively transmitted to the rotor l0. The stationary plate 6 opposite the coil 4 may be made of silicon steel in order to prevent leakage of the magnetic flux. It is to be understood here that since the hard ceramic material to be used for the bearing 3 on the sta­ tionary side which is arranged between the stator coil 4 and the permanent magnet 2 is to be inserted in the rotary magnetic field, it is preferable to have a large electric resistance in order to minimize the eddy current loss and for this reason Si₃N₄ or α-SiC with BeO added are prefer­ able for use among possible ceramic materials. With further reference to Fig. l, the stationary side bearing surface 8 may be used as the surface on which grooves adapted to generate fluid dynamic pressure are formed. In this case, the rotary side sliding surface 9 oppositely facing the bearing surface 8 will be left as a smooth plane surface. Fig. 4 is a sectional view illustrating another embodiment of the present invention which is capable of generating power at opposite ends thereof. The coil 4 on which sheet coils are laminated is fixed on the opposite surfaces by two stationary side bearing surfaces 3, 3 made of ceramic materials. The stationary side bearing surfaces 3, 3 have central bearing surfaces 8, 8 which are smooth planes. The tachometers 33 are intended from the central bearing surfaces 8 in such a manner as to avoid contact with the oppositely facing surfaces of the rotors l0. In Fig. 4, the position of the permanent magnet 2 attached to the underside of the rotor l0 is detected by the Hall element 29 and the lower rotor l0 is caused to rotate by energization of the certain coil 4 by the control device (not shown). The magnetic flux generated by the coil 4 at this instance will also appear at the upper side of the coil 4, which will also cause the upper rotor l0 to rotate. However, the upper and lower rotors l0 have poles of opposite polarity. As a result, the rotors l0, l0 will be attracted to each other and consequently the two rotors l0, 10 may be held by the stator without providing a special fixing means. It is to be noted that the small ball 7 substantially serves as a radial bearing. The small ball 7 displays the effect of preventing the rotros l0, l0 and the stator l0l from moving in the radial directions and also serves as a reference means for positioning. As the small ball men­ tioned above, an iron ball, a stainless steel ball or a ceramic ball, etc. may be employed and in addition, with regard to the configuration, various configurations such as a cylinder, a double cone or the like may be utilized. The main reason that hard ceramic materials are used for constituting the sliding portions according to the present invention is that the grooves adapted to generate fluid dynamic pressure may then be produced with high preci­ sion and the sliding portions may be given a configuration which is suitable for generating fluid dynamic pressure and this may be maintained when the fluid dynamic pressure has been actually generated, the sliding portions being suffi­ ciently durable to withstand the sliding abrasion between solid materials generated at the time when the electrically powered apparatus is started up or stopped if the load caused thereby is limited to a certain degree. With regard to the method of machining the grooves for generating the fluid dynamic pressure on the surface made of hard ceramic material, plastic maskings correspond­ ing to the land portions are applied to the surface made of ceramic material which is finished to be a smooth plane or plastic materials are applied to the portions corresponding to the land portions by means of a screen printing method or the like and then cured. Subsequently, particles of Al₂O₃, SiC or the like are injected on the exposed ceramic material portions by means of pressurized air in the form of a shot blast. After the shot blasting process, the plastic mate­ rial layer on the ceramic surface may be removed. The land portions may thus be restored to the state of the smooth plane while the grooves may be machined surfaces having an aventurine appearance. It is also to be understood that the grooves may be provided in a similar manner by the shot blasting method as explained above when the radial bearings which make use of the fluid dynamic pressure effect described below are to be provided. The various configurations of the electrically powered apparatus according to the present invention may be utilized in various application fields including a polygon mirror formed with a polyhederal mirror at the outer periph­ ery of a rotor, a pump with blades fixedly mounted directly on a rotor and a driving means for driving a magnetic disc or a photo disc which incorporates a means for detachably fixing the disc at the upper part of the surface of the rotor. Fig. 5 is a vertical sectional view showing the electrically powered apparatus according to the second mode of the present invention, this apparatus being used as the driving means of a polygon mirror. In Fig. 5, numeral 50 designates the support plate to be fastened to a stationary member (not shown). A cap 5l is arranged on the support plate 50 and screwed thereto for isolating the rotary members in the cap from the surrounding environment. A glass window 53 is provided on the cap at the position corresponding to a polygon mirror 52 inside the cap. To the support plate 50 is fixed a stator coil 54 and, on the upper surface of the coil 54, the stationary side bearing 55 is secured, the bearing 55 comprising a circular plate made of ceramic material having the front finished to a smooth plane 56. The smooth plane 56 serves as the bearing surface on the stationary side. At the position on the bearing surface 56 corresponding to the rotary axis of the polygon mirror 52 is formed a semi-­ spherical recess 57 and on the outside of the recess 57 are formed grooves adapted to generate fluid dynamic pressure. Above the stationary side bearing 55, a polygon mirror 52 is rotatably installed and above the polygon mirror 52 is provided a pressure plate 58. The polygon mirror 52 is so constructed that a perma­ nent magnet 6l and a mirror portion 62 are fixedly mounted between a lower sliding plate 59 made of a hard ceramic material disposed on the rotary side and facing the stator coil 54 and an upper sliding plate 60 made of a hard ceramic material disposed on the rotary side and facing the pressure plate 58. The front surfaces of the upper and lower sliding plates 59, 60 on the rotary side are similarly ground to form a smooth plane with little undulation and are respec­ tively provided with semi-spherical recesses 63, 64 at posi­ tions corresponding to the rotary axis of the polygon mirror 52. The pressure plate 58 is constructed of a back plate 65 made of iron, a rubber plate 66 and a bearing plate 67 made of a hard ceramic material, the three plates 65, 66, 67 being laminated and adhered together with the bearing plate 67 oppositely facing the polygon mirror 52. Further, the back plate 65 and the cap 5l are resili­ ently connected by means of a spring 68, so as to urge the pressure plate 58 against the polygon mirror 52. Inside the spring 68 is provided a solenoid 69 which when energized causes the pressure plate 58 to be attracted to the solenoid 69 whereby the force acting to press the back plate against the polygon mirror 52 may be mitigated or dissipated. The front surface (the lower side surface as viewed in the drawing) of the bearing plate 67 is formed as a smooth plane with little undulation and with grooves having a depth of from 3 - 50 µm adapted to generate fluid dynamic pressure, as well as having a semi-spherical recess 70 formed at a position corresponding to the rotary axis of the polygon mirror 52. The recesses 57, 63 and 64, 70 respectively face each other and the substantially spherical spaces defined by these recesses accommodate small balls 7l, 72 respectively made of hard materials (such as ceramic or metallic materials). Fig. 6 is a horizontal sectional view taken along the line A - A in Fig. 5. The permanent magnet 6l has a cir­ cular configuration and has eight magnetic poles magnetized in an annular arrangement. At the outer periphery of the permanent magnet, the mirror portion 62 is formed. Portions of the mirror portion 62 adapted to reflect laser beams are provided at the side portions 73 of the regular polygon of the mirror portion. The respective sides are finished to provide mirror surfaces each having a high reflection index. With respect to Fig. 5 and Fig. 6, when the polygon mirror 52 is rotated, the solenoid 69 is energized, the pressure plate 58 is slightly lifted up and then the stator coil 54 is connected to the power source. Since the circuit for detecting the rotational speed of the polygon mirror and the elements for detecting the magnetic poles are provided at the upper surface of the stator coil 54 in the same manner as the preceding embodiment, the most appropriate coil may be energized by the drive control apparatus (not shown) in accordance with the output signals provided by the circuit and elements, so that the polygon mirror 52 may be caused to rotate at a desired rotational speed. It is to be noted here that since the solenoid 69 will be de-energized before the rotational speed of the polygon mirror 52 reaches the rated speed, the polygon mirror 52 may be rotated under such conditions as the pressure plate 58 being depressed by the urging influence of the spring 68 and the fluid film being formed at the sliding portions between the opposite ends of the polygon mirror 52 in the axial direction of the rotary axis. In this embodiment, the damping effect provided by the fluid film formed between the opposite surfaces of the polygon mirror 52 will restrict deflection of the polygon mirror which is in this case the rotor in the direction of the rotary axis. Further since the mirror portion 62 of the polygon mirror 52 is of a regular polygonal configuration the sides of which are substantially in internal contact with the outer peripheral edges of the upper and lower sliding plates 59, 60, the mirror portion 62 it is not likely that the polygon mirror 52 will contaminated when handled. Fig. 7 and Fig. 8 illustrate the third mode of the present invention. Fig. 7 is a vertical sectional view of the means to drive the polygon mirror. Fig. 8 is a hori­ zontal sectional view taken along the line B - B in Fig. 7. In Fig. 7, those members which are common to those in Fig. 5 are denoted by the same reference numerals and similar members are denoted by corresponding numerals with l00 added thereto, respectively. Referring to Fig. 7, the cap 5l is screwed to the support plate 50 and the end portion 76 of the stationary shaft 74 made of a hard ceramic material is fitted in and secured to the central through-hole 75 of the support plate 50. On the outside of the stationary shaft 7l is disposed a stator coil 54 in an annular arrangement. The stator coil 54 is also secured to the support plate 50 by adhesive or a known fastening means. At the upper surface of the stator coil 54 or the surface facing the polygon mirror l52 is fixedly attached a bearing l55 at the rotary side of an annular disc made of a hard ceramic material. The surface of the rotary side bearing facing the polygon mirror l52 is finished to provide a smooth plane with little undulation and has grooves having a depth of 3 - 50 µm adapted to generate a fluid dynamic pressure formed thereon. Further, the surface of the stationary side bearing and the axis of the stationary shaft 74 are fixed in the direction normal to each other. A polygon mirror l52 is formed with sliding surfaces on the rotary side at the opposite end surfaces in the direction of the rotary axis. An upper rotary side sliding plate l60 and a lower rotary side sliding plate l59 are respectively made of hard ceramic materials. It is to be noted in particular that the lower rotary side sliding plate l59 is made integral with a rotary sleeve 79 of the polygon mirror l52. The inner circumferential surface of the rotary sleeve 79 is made to be a smooth cylindrical surface and maintained properly in the direction normal to the surface of the rotary side sliding plate l59 which faces the stator coil 54. The portion of the stationary shaft 74 correspond­ ing to the rotary sleeve 79 of the polygon mirror l52 is formed with grooves 80 adapted to generate a fluid dynamic pressure. As for the depth of the grooves 80 which are arranged in a herringbone configuration, 3 - l5 µm is suit­ able. The clearance between the stationary shaft 74 and the rotary sleeve 79 is preferably in the range of 2 - 3 µm at each side, respectively. A fluid film formed by the relative rotational movement between the stationary shaft 74 and the rotary sleeve 79 for sustaining the radial load is desirably generated so long as the sliding surfaces of the stationary shaft 74 and the rotary sleeve 79 are kept properly in parallel. However, in this embodiment since the length of the rotary sleeve 79 of the polygon mirror l52 (substantially equal to the thickness of the polygon mirror l52) is so short, the outer circumferential surface of the stationary shaft 74 and the inner circumferential surface of the rotary sleeve 79 may be worked more easily than in the prior art and therefore high machining preci­ sion may be attained. It is to be understood that the mirror portion 62, the upper rotary side sliding plate l60, the permanent magnet l6l and the lower rotary side sliding plate l59 (including the rotary sleeve 79) are integrally adhered together, and the upper rotary side sliding plate l60 is polished to attain good parallelism in relation to the upper and lower sliding plates on the rotary side. They are then adjusted for balancing. Fig. 8 is a sectional view taken along the line B - B in Fig. 7. The permanent magnet l6l has eight magnetic poles which are magnetized and arranged annularly and the outer peripheral edge of the mirror portion 62 has the configuration of a regular octagon. Fig. 9 illustrates a pattern of the stationary side sliding surface of the bearing l55 fixedly attached to the upper surface of the stator coil 54. The arrow 8l designates the direction of rotation of the polygon mirror l52 and numeral 82 designates a through-­ hole extending through the stationary shaft 74. On the front surface of the stationary side bearing l55 are alter­ natively formed around the entire circumference grooves 84 of spiral configuration indicated by a balck pattern for generating fluid dynamic pressure and land portions 84 of spiral configuration. The inner peripheral ends of the grooves are respectively in communication with each other through a groove 85 by virtue of which the pressure becomes uniform. On the inner side of the pressure equalizing groove 85 is formed a land 86. The land portions 84, 86 are located in the same plane. As a result, when the polygon mirror rotates in the direction indicated by the arrow 8l, the fluid (air) around the polygon mirror is subjected to the force directed from the outer periphery side to the inner periphery side of the spiral groove 83 and fluid dynamic pressure is generated at the sliding portions. Fig. l0 illustrates an example of grooves adapted to generate fluid dynamic pressure which is suitable for a polygon mirror adapted to be rotated at a high rotational speed, the grooves being formed on the rotary side sliding surface. The arrow 87 designates the direction of rotation of the polygon mirror. Centrally of the rotary side sliding plate 88 is formed a through-hole 89 through which the stationary shaft extends and the inner circumferential surface 90 of the through-hole 89 serves as a rotary sleeve. On the front surface of the rotary side sliding plate 88 are formed grooves 9l of a spiral configuration adapted to generate fluid dynamic pressure (designated by the black pattern) and land portions 92 adjacent to the grooves are finished to a smooth plane. A continuous land portion is also provided around the entire circumference of the outer peripheral edge 93 of the sliding plate 88 on the rotary side. When the polygon mirror rotates in the direction indicated by the arrow 87, centrifugal force generated by the spiral grooves 9l causes the fluid to flow from the inner periphery side of the grooves to the outer periphery side and the fluid pressure will be confined by the land portion 93 circumferentially extending around the outer periphery. As a result, larger fluid dynamic pressure will be generated toward the outer periphery side of the spiral grooves 9l. In a case where the pattern of the grooves for generating fluid dynamic pressure of the type shown in Fig. l0 is to be employed, the fluid therein will be caused to flow as a whole from the central portion to the outer periphery of the grooves. Accordingly, at the central portion of the pressure plate 58 and/or the central portion of the stator coil 54 is defined a passage for making the pressure uniform which is in communication with the atmos­ phere at the outer periphery of the grooves. Thus the fluid has to be replenished. According to the present invention, in an electri­ cally powered apparatus in which a stator is arranged to oppositely face the end surface of the rotor in the axial direction of the rotary axis, since a fluid dynamic pressure bearing in a flat plate form made of a hard ceramic material is provided at the sliding portions of the rotor and the stator, the following effects may be expected. 1. An electrically powered apparatus may be constructed which is compact and light in weight, and also having a long serviceable life. 2. Resistance to abrasion and corrosion at the thrust bearing part can be improved, so that maintenance is simple. 3. The electrically powered apparatus may be used not only in liquid but also in a gas or even in a slurry. 4. The frictional coefficient against sliding abrasion at the thrust bearing part is less than 0.00l and, thus, power loss at the bearing may be reduced to the minimum. 5. A flat motor may be provided. 6. Various kinds of electrically powered apparatuses of simple construction may be constituted by fixedly attaching the functional members such as blades, reflection mirror and the like to the rotor or making them integral with the rotor. In addition to the effects outlined above, the follow­ ing effect is also provided in the second mode. 7. Since the bearings for sustaining the fluid dynamic pressure made of hard ceramic materials are provided at the opposite ends of the rotor in the axial direction of the rotary axis, the rotor is caused to rotate while being subjected to the damping effect provided by both sliding surfaces and thus the degree of deflection of the rotor will be extremely small. As far as the third mode is concerned, the following effects are provided in addition to the above-mentioned effects l through 7. 8. Since the radial bearing constituted by the through-hole formed at the central portion of the rotor and the stationary shaft extending through the through-hole has a length in the direction of the rotary axis which is rather short, working accuracy may be enhanced and also a large fluid dynamic pressure may be obtained. 9. The radial bearing can be machined easily. l0. If the thrust bearings formed at the opposite ends of the rotor in the direction of the rotary axis are bearings for fluid dynamic pressure in which the fluid is forced from the outer periphery to the inner side, or are of the so-­ called "pump-in" type, the pressure of the fluid at the radial bearings may be increased, namely the fluid dynamic pressure effect of the radial bearings may be enhanced greatly. 11. If one of the thrust bearings formed at the opposite ends of the rotor in the direction of the rotary axis is of the above-mentioned "pump-in" type and the other is of the "pump-out" type, the fluid existing at the sliding portions will be caused to flow in one direction whereby cooling of the stator coil and the sliding portions may be effected. 12. If the electrically powered apparatus as described in the item l0 above is employed, the rotor may be accommodated in an enclosed container under reduced pressure and the polygon mirror's resistance against air may thus be reduced. As explained above, various effects may be provided according to the present invention. The present invention has been explained in detail by referring to particular embodiments. However, the present invention is not limited to those explained, and it may be changed or modified within the sprit and scope of the present invention which is defined in the Claims appended hereto.

Claims: 1. An electrically powered apparatus in which a stator is arranged to face an end surface of a rotor in the axial direction of a rotary axis, characterized in that a rotary side sliding surface is formed at the end surface of said rotor facing said stator, the surface of the stator facing said rotary side sliding surface is made as a stationary side bearing surface, said rotary side sliding surface and the stationary side bearing surface are respectively made of hard ceramic materials, either of said rotary side sliding surface or said stationary side bearing surface is formed to be a smooth plane, the other of said surfaces being formed as a plane provided with grooves adapted to generate fluid dynamic pressure. 2. An electrically powered apparatus as claimed in Claim l wherein recesses are formed in locations along the axis of rotation on said rotary side sliding surface and said stationary side bearing surface, and a hard material member is accommodated within the space defined by the two oppositely facing recesses provided respectively in said surfaces so as to support said rotary side sliding surface and the stationary side bearing surface to permit relative rotation therebetween. 3. An electrically powered apparatus as claimed in Claim l or 2 wherein the ceramic material used for said stationary side bearing surface is either α-SiC or Si₃N₄ containing β-SiC, BeO. 4. An electrically powered apparatus wherein the oppo­ site end surfaces of the rotor in the axial direction of the rotary axis are respectively formed to be rotary side slid­ ing surfaces , a stator is arranged to face one of said rotary side sliding surfaces and a rockable bearing surface is arranged to face the other of said rotary side sliding surfaces, characterized in that the surface of said stator arranged to face one of said rotary side sliding surfaces is formed to be the stationary side bearing surface, said two rotary sliding surfaces and said two bearing surfaces arranged to face said sliding surfaces are made respectively of hard ceramic materials, either one of the two surfaces constituting the sliding portions defined by the opposite end surfaces of the rotor in the axial direction of rotary axis being formed with grooves adapted to generate fluid dynamic pressure and the other of said surfaces being made to form a smooth plane. 5. An electrically powered apparatus as claimed in Claim 4 wherein recesses are formed in locations along the rotary axis of the respective surfaces of said stationary side bearing surface and said rotary side sliding surface which is arranged to face said bearing surface and a hard member adapted to support said stationary side bearing surface and the rotary side sliding surface so that they may be rotated relatively to each other is accommodated in the space defined by said two recesses which face each other. 6. An electrically powered apparatus as claimed in Claim 4 or 5 wherein the ceramic material used for said stationary side bearing surface is made either of α-SiC containing BeO or Si₃N₄ or β-SiC. 7. An electrically powered apparatus as claimed in any of Claims 4 to 6, wherein the outer peripheral surface of said rotor is provided with a polyhedral mirror. 8. An electrically powered apparatus wherein the oppo­ site end surfaces of the rotor in the axial direction of the rotary axis are respectively formed with the rotary side sliding surfaces, a stator is arranged to face one of said rotary side sliding surfaces and a rockable bearing surface is arranged to face the other of said rotary side sliding surfaces, characterized in that a stationary shaft extending along the rotary axis of said rotor is fixedly mounted to said stator and a through-­ hole is formed centrally in said rotor at a position corre­ sponding to said stationary shaft so that said hole is made as a radial bearing, the surface of said stator facing the rotary side sliding surface is made to be the stationary side bearing surface, said two rotary side sliding surfaces and two bearing surfaces arranged to face said sliding surfaces are respectively made of hard ceramic materials, and either one of the two surfaces constituting the sliding portions defined by the opposite end surfaces of the rotor in the axial direction of the rotary axis is formed with grooves adapted to generate fluid dynamic pressure and the other of said surfaces is made to be a smooth plane. 9. An electrically powered apparatus as claimed in Claim 8 wherein said stationary shaft and the radial bearing are respectively made of hard ceramic materials and either of said surfaces is formed with grooves adapted to generate fluid dynamic pressure. l0. An electrically powered apparatus as claimed in Claim 8 or 9 wherein said stationary shaft extends through the opposite ends of the through-hole of said rotor. 11. An electrically powered apparatus as claimed in any of Claims 8 to l0 wherein said hard ceramic material is either of SiC or Si₃N₄. 12. An electrically powered apparatus as claimed in any of Claims 8 to l0 wherein the outer peripheral surface of said rotor is provided with a polyhedral mirror. 13. An electrically powered apparatus as claimed in any of Claims 8 to l2 wherein an element adapted to detect the position of said rotor is fixed to said stationary side bearing surface. 14. An electrically powered apparatus as claimed in Claim l3 wherein a magnetic member is fixed at the position corresponding to said element adapted to detect the posi­ tion at the slidable surface on the rotary side facing said stationary side bearing surface and magnetized as a plural­ ity of magnet poles.

Inventors: CHIBA, KATSUTOSHI, KANAMORI, TOSHIYA, KIMURA, YOSHIKAZU, MIZOBUCHI, SHOTARO, SASAKI, KATSUMI
Applicants: EBARA CORPORATION
Application year: 1986
Main classification: H02K5
IPC codes: H02K29, G02B26, H02K5