Motor with bearing device

Abstract: A bearing apparatus (10) comprising a rotating shaft (1) formed of a magnetic material, bearing means (2, G7) for rotatably supporting the rotating shaft (1), a cylindrical-shaped bearing support for supporting the bearing means in a cylinder, a magnetic fluid (11) filled in the cylinder of the bearing support, and a magnetic seal (2) disposed on the opening side of the bearing support away from the bearing means and for constituting a magnetic circuit through the bearing means, and means for preventing said magnetic fluid from flowing out from said bearing means to the opening side of said bearing support and further to the outside.

Description: BACKGROUND OF THE INVENTION This invention relates to a motor comprising a bearing device as described in the preamble portion of claim 1, and a laser beam printer being equipped with such a motor. Such a motor is known from GB-A-2 147 059. Such motors are used as drive motors for laser beam printers, and as motors for magnetic disk equipments, VTR and the like, of which motors are required having a rotation of high accuracy with less shaft whirling. High speed and rotation of high accuracy with less shaft whirling (hereinbelow referred to as "high accuracy rotation") are required for drive motors of the above type in the aspect of highly minute images and highly densed memory. In particular, a clean bearing apparatus adapted for "high accuracy rotation" and free from oil contamination is desired in laser beam printers and magnetic disk equipments. With respect to such demands, problems relating to rotational accuracy and to contamination of main machines are directly related to performances of a bearing apparatus. In this respect, conventional ball bearings have limitations on high speed and the accuracy of rotation resulting from the accuracy of working in rolling members and inner and outer races. For this reason, fluid-lubricated plain bearings have been employed as bearings which are effective in high speed rotation and enabling "high accuracy rotation", and various improvements have been made on such plain bearings for use in motors of the above type. In a plain bearing, a fluid lubricant film formed on sliding surfaces upon rotation of a shaft maintains a shaft and a bearing in non-contact condition to rotatably support the shaft, and oil or gas is used as a lubricant. Unlike a gas bearing, a plain bearing making use of a lubricating oil is not expected to involve low torque, but provides an oil film of high rigidity which attains "high accuracy rotation" with less shaft whirling. Accordingly, such plain bearing can be designed to be of a small diameter as compared with a gas bearing, thus enabling realizing a compact motor. However, in a plain bearing making use of a lubricating oil, oil leakage is always problematic, and dispersion of oil during high speed rotation poses a problem in the practical use of polygon mirror drive motors and magnetic disk spindle drive motors. To cope with this problem, there has been proposed a magnetic fluid bearing for use in drive motors of the above type, which bearing makes use of a permanent magnet and a magnetic fluid on the basis of the knowledge that the magnetic fluid has a sealing function and provides lubrication. The magnetic fluid is formed by treating magnetic powders with a surface active agent and dispersing the same in a base oil. There are two types of magnetic fluid bearings classified in a basic construction. One type of magnetic fluid bearing is one which retains a magnetic fluid on sliding bearing surfaces by magnetizing the same by means of a cylindrical-shaped permanent magnet, as disclosed in Japanese Patent Unexamined Publications Nos. 55-139559 and 59-147117. The other type of magnetic fluid bearing is one which has a permanent magnet arranged at an end of the bearing and in which the permanent magnet and a permeable rotating shaft constitute a magnetic fluid sealing to have a magnetic fluid filled in a bearing section for lubrication, as disclosed in Japanese Patent Unexamined Publications Nos. 61-270520 and 60-88223 (equivalent to GB-A-2 147 059). These two types of magnetic fluid bearings are intended for the prevention of dispersion of a magnetic fluid by magnetizing and providing the same with a sealing function, and are designed to increase magnetic flux density for sealing. In these prior bearings, a magnetic fluid is disadvantageously dispersed due to increased centrifugal forces upon rotation of high speeds if an amount of magnetic fluid as retained is slightly more than as required. Even if an amount of magnetic fluid as retained is appropriate, the magnetic fluid tends to be dispersed in the range of high rotational speeds due to cubical expansion produced by temperature rise since the magnetic fluid has a considerably large, thermal expansion coefficient as compared with constituent materials of the bearing, as described in the monthly magazine "Tribology" page 20, March 1988. In addition, there is a possibility of deterioration of a magnetic fluid when used at high temperatures. That is, as described in "Tribology" page 15, a surface active agent tends to separate from the magnetic powders when exposed to high temperatures, so that the magnetic powders will cohere to mar dispersion. In particular, in the case of a plain bearing, an eccentric load produced by centrifugal forces are applied on sliding surfaces to increase viscous shearing stresses on the lubricant film, unlike a magnetic fluid sealing. In addition, as rotational speeds are increased, a shear velocity becomes high to increase the density of generation of heat due to viscous friction, so that the surface active agent is separated from the magnetic powders to cause deterioration of the bearing performance. Therefore, thermal measures such as cooling are necessary in case a magnetic fluid is used as a sealing or lubricating fluid in high speeds. GB-A-2 147 059 discloses a motor comprising a bearing device which comprises a rotating shaft formed of a permeable material, radial bearing means for rotatably supporting the rotating shaft, a housing formed of a non-magnetizable material having a bearing support for supporting the radial bearing means and a bottom portion, thrust bearing means provided at the bottom portion, a magnetic fluid filled in the radial bearing means so as to wet sliding surfaces of the radial bearing means, a magnetic fluid seal disposed on the opening side of the housing, provided with a spacing from the radial bearing means and constituting a magnetic fluid circuit through the rotating shaft. SUMMARY OF THE INVENTION It is the object of the invention to provide a motor with a bearing device and a laser beam printer incorporating such a motor, respectively, which are effective in rotation with high speed and high accuracy and which are improved in durability and sealing performance of their bearings. To that end, this object is solved with a motor and a laser beam printer according to claim 1 and 2, respectively. DESCRIPTION OF THE DRAWINGS Fig. 1 is a longitudinal cross-sectional view of a first embodiment of a bearing device; Fig. 2 is a cross-sectional view taken along the line I-I of Fig. 1; Fig. 3 is a cross-sectional view showing flow of a fluid lubricant in a radial bearing section in the first embodiment of the bearing device; Fig. 4 is a transverse cross-sectional view showing the relationship between a rotating shaft and a bearing, and flow of a fluid lubricant; Fig. 5 is a cross-sectional view showing the construction of a thrust bearing and flow of a fluid lubricant in the first embodiment of the bearing device; Fig. 6 is a longitudinal cross-sectional view of a magnetic fluid sealing in the first embodiment of the bearing device; Fig. 7 is a longitudinal cross-sectional view of a thrust bearing in a second embodiment of the bearing device; Fig. 8 is a longitudinal cross-sectional view of a magnetic fluid sealing in a third embodiment of the bearing device; Fig. 9 is a longitudinal cross-sectional view showing a portion of a magnetic fluid circulating mechanism in a fourth embodiment of the bearing device; Fig. 10 is a perspective view of a radial bearing in the magnetic fluid circulating mechanism shown in Fig. 9; Fig. 11 is a longitudinal cross-sectional view of a fifth embodiment of a bearing device; and Fig. 12 is a longitudinal cross-sectional view of a polygon mirror drive motor according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, there is shown a first embodiment of a bearing device 10, including a bearing housing 14, a magnetic fluid sealing section 2, a first radial bearing 6, a second radial bearing 7, a thrust bearing 8 and a fluid lubricant 11 which fills the bearing apparatus up to an end b of the first radial bearing 6 full. The magnetic fluid sealing section 2 is constituted by a permanent magnet 3, a pole piece 4, a cover 5 and a magnetic fluid 12, said permanent magnet, pole piece and cover being coaxial with the bearing housing 14. The magnetic fluid sealing section 2 is securedly mounted on an open end of the bearing housing 14 to be spaced away from the first radial bearing 6 with a space e defined therebetween. The fluid lubricant for the first and second radial bearings and the thrust bearing may be the same as the magnetic fluid 12 for the magnetic fluid sealing section 2 or a magnetic fluid of a different composition. Passages a , b , c and d for the fluid lubricant are defined by the bearing housing 14, the first and second radial bearings 6, 7 and a rotating shaft 1. In Fig. 2, there are shown three axial passages c disposed around the radial bearing 6 or 7. However, one passage c may suffice. Instead of forming such passage c by cutting the outer periphery of the radial bearing, it may be constituted by a groove or grooves formed in the bearing housing 14. Referring now to Fig. 3, there are shown flow of the fluid lubricant 11 circulating about the radial bearing 6 in the first embodiment of the bearing device 10. When the fluid lubricant 11 revolves upon rotation of the rotating shaft 1, the axial length of the passage b on one end of the radial bearing 6 becomes smaller than that of the passage d on the other end of the radial bearing 6. Accordingly, centrifugal forces applied to the fluid lubricant are larger in the passage d than in the passage b , so that the fluid lubricant 11 flows in a direction shown by an arrow in Fig. 3. As the rotating shaft 1 rotates with a small amount of eccentricity S relative to the radial bearing 6, a bearing clearance on the end surface of the radial bearing 6, that is, in the passage b is divergent in shape on the lefthand side of a point P where the fluid lubricant flows axially. Also, as the bearing clearance is convergent in shape on the righthand side of the point P in a clockwise direction, the fluid lubricant tends to flow axially toward the passage b but it generally flows in the passage a in a direction from the passage b to the passage d since the centrifugal forces are large in the passage d as compared with the forces in the passage b . Such flow is realized by the provision of the passages a , b , c and d . Accordingly, the fluid lubricant having been subjected to viscous shearing in the passage a to generate heat flows to the end surface of the radial bearing and circulates while discharging heat to the bearing housing 14 to be cooled. The fluid lubricant is not subjected to repeated viscous shearing in the bearing clearance or the passage a , so that a surface active agent will not separate from the magnetic powders, which are treated with the agent, for thermal reasons. Referring now to Fig. 5, there is a circulating flow of the fluid lubricant in the thrust bearing 8 of the first embodiment of the bearing device. The thrust bearing 8 is formed at its central portion with a hole 29 and in the peripheral portion thereof with a plurality of holes 30. The central portion of the thrust bearing 8 faces the end surface of the rotating shaft 1. As the rotating shaft 1 rotates, a lubricant film f is formed on the sliding surface in the central portion of the thrust bearing 8, and the fluid lubricant flows radially outwardly. The fluid lubricant 11 circulates through the holes 29 and 30 in a direction shown by an arrow in Fig. 5. Thus the fluid lubricant 11 is not subjected to repeated viscous shearing in an area between the end surface of the rotating shaft 1 and the central portion of the thrust bearing 8, but it circulates in the above-mentioned manner to improve lubrication on the sliding surface or the central portion of the thrust bearing 8. Without such hole 29, the sliding surface of the rotating shaft 1 is not supplied with any fluid lubricant, so that rotation of high speeds will cause poor lubrication to be resulted in seizure and wear. In Fig. 6, there is shown a magnetic fluid sealing 2. A magnetic circuit m shown by dotted lines is formed by the permanent magnet 3 which is magnetized to be axially oriented, the pole piece 4 closely contacted by the permanent magnet, and the permeable rotating shaft 1. Magnetic flux density is increased in an area between the pole piece 4 and the rotating shaft 1, in which area a magnetic fluid 12 is retained to provide a magnetic fluid sealing. Also, a non-magnetizable disk or cover 5 is provided so that the magnetic fluid 12 will not flow out of the bearing housing 14 even when it is dispersed out of the magnetic fluid sealing section. The cover 5 is non-magnetizable not to define any magnetic circuit in a gap g between it and the rotating shaft 1, so that even if the magnetic fluid 12 retained at the inner periphery of the pole piece 4 is dispersed, it will be caught by the sealing portion of the pole piece 4 to maintain a regenerative function of the magnetic fluid sealing. Accordingly, if the gap g between the rotating shaft 1 and the cover 5 is set as small as possible, reduction of the magnetic fluid 12 caused by vaporization can be suppressed to eliminate environmental contamination produced by oil vapor even in long term use. In the present bearing device, it is possible to suppress reduction of the magnetic fluid caused by vaporization by setting the size of the gap g equivalent to the bearing clearance, that is, the passage a or smaller than the gap around the magnetic fluid sealing section, that is, the gap between the rotating shaft and the pole piece. Since there is a possibility of injuring the rotating shaft due to oscillations and the like when the gap g is set equivalent to the bearing clearance, such injury to the rotating shaft can be prevented by employing a lubricating copper alloy softer than the material of the rotating shaft or a sliding material such as Teflon and nylon to form the cover. In the present bearing device, cubical expansion of the fluid lubricant 11 produced when it is subjected to thermal expansion caused by generation of heat due to viscous friction is accommodated by the space e . Thus the pressure within the space e is somewhat increased because a compressive gas is contained in the space e . However, such pressure is not raised to affect the sealing performance, so that dispersion of the magnetic fluid is avoided unlike the prior art. In addition, the provision of such space e enables making use of a lubricating oil for the magnetic fluid 12 and adjusting an amount of the fluid lubricant 11 in the permissible range determined by the space e without having to strictly control an amount of the magnetic fluid 12 retained by the gap g . This constitution is advantageous in terms of cost in the field of mass-produced motors of this type. Referring to Fig. 7, there is partly shown a second embodiment of the bearing device, in which a hole 30a constituting a circulating mechanism of a thrust bearing 8a is formed in a bearing housing 14a. Referring to Fig. 8, there is shown a magnetic fluid sealing section 2b. In the Figure, the same elements as those shown in Fig. 6 are designated by the same reference numerals. In this embodiment, a permanent magnet 3b is U-shaped in an axial cross-section, and is dispensed with any pole piece. The magnetic fluid sealing section functions in a manner equivalent to that shown in Fig. 6 while it is superior to the latter in that it has a less number of elements than the latter. Referring now to Figs. 9 and 10, there is shown a fourth embodiment of the bearing device, in which a groove (16 or 17) is formed in the rotating shaft 1 or a radial bearing (6c or 7c) to promote the circulating function of a circulating mechanism. Referring to Fig. 11, there is shown a fifth embodiment of the bearing device 10d, in which a rotating shaft 1d is rotatably supported by a single radial bearing 6d and a disk-shaped thrust bearing 8d. The rotating shaft 1d securedly mounts thereon a rotor 15 for a polygon mirror, disk, cylinder and the like, so that when the rotor 15 rotates, an air flow in a direction shown by an arrow is produced due to an action of fan between the cover 5 and the rotor 15. Without the cover 5, such air flow would tend to vaporize the magnetic fluid 12. However, such reduction of the magnetic fluid 12 caused by vaporization can be suppressed by making the gap g between the rotating shaft 1d and the cover 5 small. Fig. 12 shows a polygon mirror drive motor for laser printers according to the invention. The motor comprises a motor rotor 117 formed of a permanent magnet and secured to a rotor 115, a motor stator 118 secured to a motor housing 119, a polygon mirror 120 securedly mounted on the rotor 115 by a mirror retainer 121, and a rotating shaft 101 press fitted into the rotor 115 and rotatably supported by a bearing device. The bearing device comprises a magnetic fluid sealing 102, radial bearings 106, 107, a thrust bearing 108 and a bearing housing 114. A predetermined amount of magnetic fluid 111 is filled in the bearing apparatus as a fluid lubricant to submerge the end surface of the upper radial bearing 106. A body of magnetic fluid 112 is provided by lifting the rotating shaft 101 having been inserted into the bearing housing so as to cause a portion of the magnetic fluid 111 adhered to the rotating shaft 101 to be retained in a gap between the magnetic fluid sealing 102 and the rotating shaft 101. The polygon mirror drive motor thus constituted operates in the following manner. When the rotor is driven at high speeds, the magnetic fluid 111 circulates in a flow passage on the sliding surfaces of the radial bearings 106, 107 and the thrust bearing 108 to ensure lubrication and cooling for stable rotation in a long term. Even when the rotation of the rotor causes a temperature rise in the magnetic fluid 111 and hence a cubical expansion thereof, the magnetic fluid 111 will not overflow or disperse to contaminate the polygon mirror since the radial bearing 106 and the magnetic fluid sealing 102 are spaced away from each other to provide a space e . In the bearing apparatus the magnetic fluid 111 performs circulative flow to be improved in cooling, by which a surface active agent will not separate from magnetic powders treated therewith. Thus the properties of the magnetic fluid 111 is maintained stable over a long period of time. Accordingly, when the bearing device is applied on a polygon mirror drive motor, a rotor is rotatably supported by an oil film of high rigidity in a sliding bearing, so that shaft whirling is all eliminated and rotation of high accuracy is maintained while a polygon mirror is free from contamination due to dispersion of a fluid lubricant. In this manner, the polygon mirror drive motor in accordance with the invention can be made reliable. A magnetic fluid having a high saturation magnetization (that is, a large proportion of magnetic powders in a base oil) and a high viscosity is retained in a magnetic fluid sealing section, and a fluid lubricant of low viscosity or a magnetic fluid is filled in a bearing section, whereby bearing loss is reduced and cubical expansion is suppressed by a lowering of temperature rise to improve reliability of a magnetic fluid sealing. In addition, a circulating mechanism making use of rotation of a rotating shaft cooperates with a space between a magnetic fluid sealing and a bearing section to contribute to improvement of sealing performance and prevent deterioration of performance of a magnetic fluid sealing. In the invention, the provision of a space between a bearing section and a magnetic fluid sealing section eliminates the need of strictly adjusting an amount of fluid lubricant filled into the bearing section to be very effective in reduction of cost in mass production. In addition, a circulating mechanism for the magnetic fluid lubricant can prevent deterioration of performance of the magnetic fluid due to temperature rise. Thus a bearing device having durability and high reliability can be provided. In a magnetic fluid sealing section, a non-magnetizable disk is provided on an open end of a bearing housing, and a gap defined between the disk and a rotating shaft is set to have the same size as that of a bearing clearance, whereby reduction of a magnetic fluid due to vaporization can be suppressed in the bearing device to make the magnetic fluid durable. As described above, the bearing device is applied on a polygon mirror drive motor and a spindle drive motor for magnetic disk equipments, which motors are required of "high accuracy rotation" and cleanliness, whereby it is possible to realize durability, high reliability and reduction in production cost.

Claims: A motor comprising a bearing device (10) which comprises a rotating shaft (1) formed of a permeable material, radial bearing means (6, 7) for rotatably supporting the rotating shaft (1), a housing (14) formed of a non-magnetizable material having a bearing support for supporting the radial bearing means (6, 7) and a bottom portion, thrust bearing means provided at the bottom portion, a magnetic fluid (11) filled in the radial bearing means so as to wet sliding surfaces of the radial bearing means (6, 7), a magnetic fluid seal (2) disposed on the opening side of the housing (14), provided with a spacing from the radial bearing means (6, 7) and constituting a magnetic fluid circuit (12) through the rotating shaft (1) characterized in that a space is defined between the magnetic fluid seal (2) and a liquid surface of the magnetic fluid (11). A laser beam printer comprising a motor according to claim 1, of which a rotor mounts thereon a polygon mirror, said polygon mirror rotating to scan a laser beam.

Inventors: HIROYAMA HIROO, KAWAIKE KAZUHIKO, NII KATSUTOSHI, UNO SATOSHI, HIROYAMA, HIROO, KAWAIKE, KAZUHIKO, NII, KATSUTOSHI, UNO, SATOSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: F16C33
IPC codes: F16J15, F16C33