FR2702257A1 - Device with radial and thrust bearings - Google Patents

Abstract

The invention relates to a device with radial and thrust bearings. t relates to a device which comprises a base (1), a spindle (2), a hub (7), and hydrodynamic radial and thrust bearings. A radial cylindrical member (4) fixed to the base and a radial sleeve (6) fixed to the hub constitute the radial bearing. The ends of the radial sleeve (6) and of the thrust plates (3) constitute the thrust bearings. The fluid for the thrust bearings is a gas. Chamfered parts of the radial member (4) and of the radial sleeve (6) form an air gap (13), and the fluid for the radial bearing is a lubricant liquid. A small hole (8) is formed in the radial member (4). Application to motors for driving hard disks.

Description

 The present invention relates to a bearing device which has a hydrodynamic liquid bearing as a radial bearing, and a hydrodynamic gas bearing as a thrust bearing, and which supports an object rotating at high speed. More specifically, the present invention relates to a bearing device which can be used with a spindle motor used for driving a hard disk drive member, a drive member of a laser printer, d a rotary drum device of a video system, etc., in which excellent rotational performance is required regardless of the position of the motor.

 Given the appearance of high storage capacities and the high speed rotation performance of recent hard drive drives, the spindle motors which drive them have been asked accordingly to have high performance. Thus, there has been demand for improved durability, cleanliness and high speed rotation performance and minimization of vibration during rotation, regardless of the position of the motor when in use, so that the motor is even better suited to such hard drive drives.

 Figure 8 is a sectional view showing the structure of a conventional spindle motor for a hard drive. In FIG. 8, a mount 31 (base) has a support shaft 32 (pin) mounted on its central part. An annular abutment plate 33 is fixed to the mount 31, and a radial cylindrical member 34 is fixed concentrically to the support shaft 32. Several stator windings 35, regularly spaced in circumferential direction, are fixed to the support shaft 32 above the radial cylindrical member 34. A support member 36 (hub) having a cap configuration is placed on the shaft 32 of support. The closure part placed at the upper end of the support member 36 is mounted with clearance on the upper end part of the shaft 32. The member 36 has an annular bearing member 37 (playing at the times the role of a radial and stop bearing) fixed to its lower end part. The annular member 37 has an L-shaped configuration in section. The lower end surface of the bearing member 37 faces the upper surface of the stop plate 33, while the inner peripheral surface of the annular member 37 faces the outer peripheral surface of the cylindrical member radial 34. The lower end surface of the bearing member 37 or the upper surface of the stop plate 33 has spiral grooves allowing the creation of dynamic pressure in the direction of the stop. The internal peripheral surface of the bearing member 37 or the external peripheral surface of the radial member 34 of the bearing has chevron grooves intended to create dynamic pressure in the radial direction.

 Several rotor magnets 38, regularly spaced in circumferential direction, are fixed to the internal periphery of the support member 36, opposite the stator windings 35. When the stator windings 35 successively receive an electric current, the support member 36, carrying the rotor magnets 38, begins to rotate and, consequently, a dynamic pneumatic pressure is created between the upper surface of the plate 33 and the lower end surface of the bearing member 37, and a dynamic pneumatic pressure is created in a similar manner between the outer peripheral surface of the radial bearing member 34 and the inner peripheral surface of the bearing member 37. upper surface of the plate 33 and the lower surface of the member 37 therefore constitute a thrust bearing, while the outer peripheral surface of the member 34 and the inner peripheral surface of the member 37 constitute a radial bearing. The support member 36 rotates while being supported by the thrust and radial bearings.

 However, the aforementioned spindle motor poses a problem since, when it works in a horizontal position (that is to say in the direction in which the direction of the gravity field is perpendicular to the motor shaft), a moment is created in the radial direction due to the action of the force of gravity on the rotor, so that the axis of the rotor inclines relative to the radial bearing and causes an increase in the imbalance of the radial magnetic force acting between the magnets 38 and the windings 35 and, in this state, the rotor member is brought into local contact with the bearing.

 In addition, since the stop plate and the radial bearing member are formed from separate magnets and are positioned independently on the frame, it is difficult to obtain the necessary perpendicularity between them at the time of mounting.

 In addition, when starting and stopping the spindle motor, the radial cylindrical member and the radial sleeve come into direct contact by sliding and wear is therefore caused during repeated starts and stops.

In addition, the number of revolutions of the scanning motors used in the drives of the laser printers increases from year to year, so that conventional bearings can hardly allow the high speeds of rotation of these motors.
In view of the problems posed by the prior art, the subject of the present invention is the production of a bearing device which is suitable for a spindle motor, which comprises hydrodynamic thrust and radial bearings intended to allow the spindle motor to have excellent rotational performance and rotate with minimal vibration regardless of the position of the motor when in use.

 The invention allows the solution of the aforementioned problems and to this end it relates to a bearing device, of the type which comprises a base, a spindle mounted on a central part of the base, a hub, and hydrodynamic radial and thrust bearings placed between the spindle and the hub, in which a radial cylindrical member is fixed to the base so that the spindle passes through a central part of this member, two stop plates are fixed to the two end surfaces of the radial cylindrical member and each have a central part through which the spindle passes, a radial sleeve is supported so that it can rotate at its internal periphery and at its two end surfaces by an external peripheral surface of the radial cylindrical member and of the surfaces opposite internal of the two stop plates respectively, this sleeve being fixed to the hub, the radial cylindrical member and the radial sleeve constitute the hydrodynamic bearing radia l, the two end parts of the radial sleeve or the opposite internal surfaces of the two stop plates have spiral grooves intended to create dynamic pressure, and the two end parts of the radial sleeve and the stop plates constitute the bearings hydrodynamic thrust bearings, and in which a fluid generating dynamic pressure, placed in the hydrodynamic thrust bearings, is a gas, the hydrodynamic radial bearing has a very small clearance, a chamfered part is formed at the outer periphery of each end of the radial cylindrical member, and another chamfered part is formed at the internal periphery of each end of the radial sleeve so that an air space is formed at each axial end of the radial hydrodynamic bearing, this space being surrounded by the part chamfered of the radial cylindrical member, the chamfered part of the radial sleeve and one of the stop plates, a fluid of the In order to create dynamic pressure in the radial hydrodynamic bearing is a lubricating liquid, and a small hole intended to collect the lubricating liquid is formed on an external peripheral surface of the radial cylindrical member, and the lubricating liquid is trapped in the clearance of the radial hydrodynamic bearing due to the presence of air spaces, and a change in the volume of the lubricating liquid at the time of rotation and stopping is absorbed by the small hole.

 In the bearing device having the aforementioned arrangement, the small hole, which is formed on the outer peripheral surface of the radial cylindrical member, may be a through hole which passes through the radial cylindrical member, or a hole which does not pass through 1 radial cylindrical member, its internal end being closed.

 The bearing device, which has the arrangement described above, may comprise several holes formed in the external peripheral surface of the radial cylindrical member at respective positions regularly distributed in the circumferential and / or axial direction.

 In the bearing device having the aforementioned arrangement, the lubricating liquid sealed in the radial hydrodynamic bearing can be a lubricating liquid having a low volatility and a dynamic viscosity of less than 10 cSt at 40 C.

 In the bearing device having the aforementioned arrangement, the lubricating liquid may contain an electrically conductive substance.

 In the bearing device having the aforementioned arrangement, the radial sleeve can be a rotary member or a fixed member.

 The bearing device having the aforementioned arrangement may include a device intended to create a magnetic attraction force in a stop direction relative to one of the stop plates, in the opposite direction to that of an applied force.

 In the bearing device having the aforementioned arrangement, the radial bearing can be arranged so that it supports a rotor, which is supported so that it rotates by the bearing device, at a small distance from the center of gravity of the rotor .

 In the bearing device according to the present invention, the hydrodynamic radial bearing is arranged in the form of a hydrodynamic bearing with a lubricating liquid, and the hydrodynamic thrust bearings have the form of hydrodynamic gas bearings as described above.

Thus, the force-absorbing capacity of the radial bearing is significantly increased compared to hydrodynamic gas bearings. In addition, the lubrication step at the start and stop is better so that it is possible to reduce the wear of the radial bearing.

 In addition, as the clearance of the radial bearing is tiny, the lubricating liquid can hardly penetrate elsewhere than in the hydrodynamic radial bearing under the action of its interfacial tension. In particular, as each air space, which is surrounded by a chamfered part of the radial cylindrical member, a chamfered part of the radial sleeve and a stop plate, is much wider than the sets of the radial and stop bearings, the interfacial tension of the lubricating liquid decreases in the air space, so that the lubricating liquid cannot penetrate into the clearance of the thrust bearing by passing through the air space. In other words, the air spaces keep the lubricating liquid in the clearance of the radial bearing, and it is therefore possible to prevent the entry of the lubricating liquid into an area other than the hydrodynamic radial bearing.

 Furthermore, even if the temperature of the lubricating liquid rises and increases in volume during the rotation of the rotor, the excess of the lubricating liquid is in fact absorbed in the small holes of the radial cylindrical member. During a suspension of operation, the temperature of the lubricating liquid decreases and its volume therefore decreases.

However, the lubricating liquid is transmitted through the small holes and thus compensates for the reduction in lubricating liquid from the bearing clearance. As the lubricating liquid is present at this time only in the radial hydrodynamic bearing as described above, it is possible to obtain an excellent lubrication condition for the radial hydrodynamic bearing.

 In addition, in the bearing device according to the present invention, no lubricating liquid is present in the hydrodynamic thrust bearings. As a result, losses in the bearings are minimal, and no lubricating liquid is disposed by centrifugal force during high speed rotation. It is thus possible to obtain a rotation at high speed with preservation of the necessary degree of cleanliness.

 In addition, thanks to the aforementioned arrangement of the bearing device according to the present invention, it is possible to easily achieve the necessary perpendicularity between the stop plate and the radial cylindrical member at the time of mounting by mutual clamping using for example a fixing nut, provided that the abutment plates are made so that their opposite internal surfaces have the necessary flatness and that each end surface and each external peripheral surface of the radial cylindrical member are mutually perpendicular.

 As the spaces between the opposite internal surfaces of the stop plates and the two end surfaces of the radial sleeve are determined by the difference in height of the radial cylindrical member and the radial sleeve, these members can be easily mounted with spaces given, provided that the radial cylindrical member and the radial sleeve, whose machining is easy, are favored with the appropriate height.

 As the radial sleeve acts as a bearing member for the radial bearings and as a stopper at the same time, the number of parts necessary for the formation of the entire bearing structure decreases, and the structure of the bearing devices is simplified .

 If the motor is in the form of an external rotor type motor and the bearing assembly is placed inside the stator core, the radial sleeve can be formed from a fixed member or from a rotary member. The arrangement of the center of gravity of the motor rotor at a predetermined distance from the bearing structure allows stable rotation of the rotor regardless of the attitude of the motor during use.

 When using a lubricating liquid with low volatility and a kinematic viscosity of less than 10 cSt at 40 OC, the lubricating liquid diffuses satisfactorily in the small holes or given parts of the sets of the bearings. Thus, the lubricating liquid cannot be dispersed as described above. In addition, losses in the bearings are minimized, and the durability of the bearing device is increased.

 If an electrically conductive substance is added to the lubricating liquid, the static electricity produced on a magnetic recording medium, for example in a hard disk drive, can be effectively transmitted to ground on the fix side. that is to say evacuated by the conductive function of the electrically conductive substance. It is thus possible to prevent the accumulation of static electricity between the magnetic recording medium and the head.

 The use of a device intended to produce a magnetic attraction force in the direction of abutment opposite to that of the applied force allows the reduction of the force applied in the abutment direction by the weight of the rotor when spindle motor operates in vertical position. It is thus possible to reduce the loss of torque in the thrust bearings when starting and stopping the engine and thus starting and stopping the rotation of the engine are facilitated. The wear of the bearing members is therefore reduced and the service life of the thrust bearings is increased. When the spindle motor works in a horizontal position, the rotor is pushed against a thrust bearing under the action of the device which produces the magnetic attraction force and the rotor therefore rotates on the base of the surface of this thrust bearing. It is therefore possible to suppress vibrations in the stop direction during rotation and to obtain a high degree of precision in rotation.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows of preferred embodiments, given with reference to the appended drawings in which
Figure I is a section showing the structure of an embodiment of a bearing device according to the present invention
Figures 2 (a) and 2 (b) are sections allowing the description of the operation in the case where neither a cylindrical radial member nor a radial sleeve has chamfered parts;
Figure 3 is a section showing the operation in the case where the radial cylindrical member and the radial sleeve have respective chamfered parts which form an air space between them, and a stop plate;
FIG. 4 represents spiral grooves intended to create hydrodynamic pressure and which are formed on a stop plate
Figure 5 is a section showing the arrangement of a spindle motor having the bearing device according to the invention;
Figure 6 is a section showing the arrangement of another spindle motor having the bearing device according to the invention;
FIGS. 7 (a) to 7 (f) are plan views and FIGS. 7 (a) to 7 (fl) are central sections respectively representing various shapes of small holes which are made in the radial cylindrical member; and
Figure 8 is a section showing the arrangement of a spindle motor comprising a conventional bearing device.

 Embodiments of the invention will now be described with reference to the accompanying drawings. Figure 1 is a schematic section showing the structure of an embodiment of the bearing device according to the invention. In FIG. 1, a base 1 carries a pin 2 mounted on its central part. A radial cylindrical member 4 is fixed to the spindle 2, the latter passing through the central part of this member. Two stop plates 33 are fixed to the two end surfaces of the member 4. The pin 2 passes through the central part of the upper and lower stop plates 3. A radial cylindrical sleeve 6 is fixed to the internal periphery of a hub 7. The radial sleeve 6 is supported at its internal peripheral surface and at its two end surfaces so that it can rotate by the external peripheral surface of the 'radial cylindrical member 4 and the opposite internal surfaces of the two stop plates 3 respectively.

The radial cylindrical member 4 and the radial sleeve 6 constitute a radial hydrodynamic bearing, while the two end portions of the radial sleeve 6 and the stop plates 3 constitute hydrodynamic stop bearings. The radial and thrust bearings constitute the bearing device of the present invention
The opposite internal surfaces of the two stop plates 3 have grooves intended to create dynamic pressure, that is to say spiral grooves 3a as shown in FIG. 4. The two end surfaces of the radial sleeve 6 are smooth . The arrangement can also be such that the two end surfaces of the radial sleeve 6 have grooves intended to create dynamic pressure, while the opposite internal surfaces of the stop plates 3 are smooth. The play existing between the external peripheral surface of the radial cylindrical member 4 and the internal peripheral surface of the radial sleeve 6 is tiny (from a few microns to a few tens of microns). The external peripheral surface of the radial cylindrical member 4 and the internal peripheral surface of the radial sleeve 6 have the shape of smooth surfaces.

As shown in FIG. 3, a chamfered part 4a is placed at the external periphery of each end of the radial cylindrical member 4, and another chamfered part 6a is formed at the internal periphery of each end of the radial sleeve 6, if although an air space 13 is formed at each axial end of the radial hydrodynamic bearing, which is surrounded by the chamfered part 4a, the chamfered part 6a and the surface of an abutment plate 3 which is turned towards these chamfered parts 4a and 6a. In addition, the external peripheral surface of the radial cylindrical member 4 has small holes 8 intended to collect a liquid. The fluid used for the creation of the dynamic pressure in the thrust bearings is a gas, for example air , while the fluid used to create the dynamic pressure in the radial bearing is a lubricating liquid
In the bearing device having the aforementioned arrangement, when the hub 7 is rotated by the application of a rotational force, a hydrodynamic pressure is created by the lubricating liquid between the internal peripheral surface of the radial sleeve 6 and the surface external device of the radial cylindrical member 4, and a hydrodynamic pressure is created in a similar manner by air or another gas between the two end surfaces of the radial sleeve 6 and the opposite internal surfaces of the stop plates 3. Thus, the radial sleeve 6 rotates while being supported by the radial cylindrical member 4 and the stop plates 3.

 The realization of the radial bearing, which comprises the cylindrical radial member 4 and the radial sleeve 6, in the form of a hydrodynamic bearing with a lubricating liquid, as described above, significantly increases the capacity for the collection of forces of the radial bearing with respect to the hydrodynamic gas bearings. In addition, the state of lubrication at the time of starting and stopping is improved so that it is possible to reduce the wear of the cylindrical member 4 and of the radial sleeve 6.

 In addition, since the clearance between the external peripheral surface of the radial cylindrical member 4 and the internal peripheral surface of the radial sleeve 6, that is to say the jtu of the radial bearing, is as small as a few microns to a few tens of microns, the lubricating liquid can hardly penetrate an area other than the sliding contact area between the outer peripheral surface of the radial cylindrical member 4 and the inner peripheral surface of the radial sleeve 6, that is to say the surface of the radial hydrodynamic bearing, under the action of the interfacial tension. In addition, like each air space 13, which is surrounded by a chamfered part of the radial cylindrical member 4, a chamfered part of the radial sleeve 6 and of a stop plate 3, is greater than the above clearance, the interfacial tension of the lubricating liquid weakens in the air space 13. Thus, the interfacial tension in the radial clearance is reduced by the air space 13, S i although the lubricating liquid cannot enter the stop set by passing through the air space.

 The action of the air space 13, disposed between the chamfered part 4a of the radial cylindrical member 4, the chamfered part 6a of the radial sleeve 6 and the surface of the stop plate 3 which is turned towards the chamfered parts 4a and 6a, is described in more detail below. If these chamfered parts are not present, a lubricating liquid Q, sealed in the clearance 14 between the external peripheral surface of the radial cylindrical member 4 and the internal peripheral surface of the radial sleeve 6, is only present in this clearance 14 in the initial state as indicated in FIG. 2 (a), and it does not penetrate into the clearance 11 formed between the end surface of the radial sleeve 6 and the surface of the thrust bearing 3 which is turned towards him. However, since the clearance 11 is tiny, the lubricating liquid Q penetrates into the clearance 11 by capillary action over time as indicated in FIG. 2 (b).

Consequently, the lubricating liquid Q undesirably constitutes a fluid which creates a dynamic pressure in the hydrodynamic thrust bearing. As a result, the losses in the bearing become greater than in the case of ur. hydrodynamic air bearing, so that it is difficult to obtain a rotation at high speed.

 On the contrary, if an air space 13 having, in section, a triangle configuration with an apex at the bottom, is formed between the chamfered part 4a of the member 4, the chamfered part 6a of the sleeve 6 and the surface of the stop plate 3 facing these chamfered parts 4a and 6a as shown in FIG. 3, as the air space 13 is much wider than the clearance 14, the interfacial tension decreases in the space 13 and the lubricating liquid Q retained in the space 14 of the radial hydrodynamic bearing does not enter the air space 13 having in section a triangle configuration with the point downwards.

Thus, it is not possible for the lubricating liquid Q to penetrate into the clearance 11 formed between the end surface of the radial sleeve 6 and the surface of the stop plate 3 which is turned towards it. In addition, as the clearance 14 of the radial bearing is tiny, the lubricating liquid Q is retained on the external peripheral surface of the radial cylindrical member 4 and the internal peripheral surface of the radial sleeve 6 under the action of the surface tension.

Consequently, the probability that the lubricating liquid Q leaves the set 14 is zero.

 Thanks to the presence of small holes 8 which are formed on the external peripheral surface of the radial cylindrical member 4, even if the lubricating liquid Q undergoes a rise in temperature and therefore an increase in volume during the rotation of the rotor, the excess lubricating liquid is effectively absorbed in the small holes 8. Conversely, during a suspension of operation, the lubricating liquid Q undergoes a reduction in temperature and its volume therefore decreases. In this case, however, the decrease in the lubricating liquid Q is compensated for by the lubricating liquid transmitted by the small holes 8. In this case, since the clearance 14 of the radial hydrodynamic bearing is tiny, no air bubble can be present between the small holes 8 and the clearance 14. Consequently, it is not possible that the lubricating liquid of the small holes 8 is separated from the lubricating liquid of the clearance 14 by air bubbles. The probability that the lubricating liquid cannot be absorbed by the small holes 8 or transmitted by them under the action of the higher tension forces of the lubricating liquid is zero. When the lubricating liquid is to be applied during the mounting operation, the clearance 14 of the radial hydrodynamic bearing can be uniformly filled with a suitable quantity of lubricating liquid by filling the small holes 8 of the radial cylindrical member 4 with a quantity sufficient lubricant before mounting the bearing device and then mounting the radial cylindrical member 4 in the radial sleeve 6. Thus, it is not necessary to adjust the amount of lubricant, which is very low.

The application of the lubricating liquid therefore becomes very easy.

 A liquid of low volatility and a kinematic viscosity of less than 10 cSt at 40 C. is used as the aforementioned lubricating liquid. If the kinematic viscosity of the lubricating liquid is greater than or equal to 10 cSt, losses in the bearings due to the rise the viscosity of the lubricating liquid increases so that it is difficult to obtain a rotation at high speed. A part of the lubricating liquid which has a kinematic viscosity of less than 10 cSt contains a volatile constituent of normal volume. However, if a lubricating liquid with such normal volatility is used in the bearing device according to the invention, the service life of the bearing is short since the bearing device cannot receive lubricant liquid from the outside, other than small holes 8. For this reason, according to the invention, a lubricating liquid of low volatility and kinematic viscosity less than 10 cSt at 40 OC is preferably used. It is also possible to use not only oils but also other substances which are used as solvents or general heating fluids insofar as their volatility is low and their kinematic viscosity at 40 ° C. is less than 10 cSt. In this embodiment, a hydrocarbon oil or a fluorocarbon heating fluid is used as the lubricating liquid.

 In the bearing device of this embodiment, the hydrodynamic radial bearing does not necessarily require grooves to create dynamic pressure. The grooves for creating dynamic pressure can be formed on the internal peripheral surface of the radial sleeve 6, the external peripheral surface of the cylindrical member 4 being smooth. Conversely, grooves for creating dynamic pressure can be formed on the external peripheral surface of the radial cylindrical member 4, the internal peripheral surface of the radial sleeve 6 being smooth. The materials used for the members constituting the bearings, that is to say the cylindrical radial member 4, the stop plates 3 and the radial sleeve 6, are not subject to particular restrictions. Any material that can be machined with a high degree of precision can be used. Examples of usable materials are metallic materials in general, ceramic materials, organic materials, etc.

 The addition of an electrically conductive substance to the aforementioned lubricating liquid gives the following advantageous effect. When a bearing device according to the invention is used for example in a spindle motor intended for driving a hard disk, mounted as shown in FIG. 5, the static electricity produced on a magnetic recording medium fixed directly or indirectly to the base 1 is effectively transmitted to the mass on the fixed side of a frame 10 via the base 1, the spindle 2, the cylindrical radial member 4, the lubricating liquid present in the clearance 14, the radial sleeve 6 and the hub 7. Thus, static electricity does not accumulate on the rotary members and in particular the magnetic recording medium. It is therefore possible to prevent the destruction of recorded information on the magnetic recording medium.

 The lubricating liquid can be made electrically conductive by adding an electrically conductive substance. Any type of material can be used as an electrically conductive substance. For example, at least one type of surfactant or polyoxyethylene builder or graphite powder is selected as the electrically conductive substance.

 In the aforementioned arrangement of the bearing device, at no time is lubricating liquid present in the clearances formed between the opposite internal surfaces of the stop plates 3 and the two end surfaces of the radial sleeve 6. Consequently, even if the radial sleeve 6, that is to say the rotor, rotates at high speed, no lubricating liquid is projected by the centrifugal force.

It is therefore possible to obtain a rotation at high speed.

 Thanks to the aforementioned arrangement of the bearing device, the necessary perpendicularity between the stop plates 3 and the radial cylindrical member 4 can be easily achieved by bringing the two end surfaces of the radial cylindrical member 4 into contact with the internal surfaces opposite of the stop plates 3 and by clamping with the aid of a fixing member 5 (nut), provided that the stop plates 3 are produced so that their opposite internal surfaces have the necessary flatness, and that the member cylindrical 4 is manufactured so that each end surface and its outer peripheral surface are mutually perpendicular. In addition, since the spaces between the opposite internal surfaces of the stop plates 3 and the two end surfaces of the radial sleeve 6 are determined by the difference in height of the radial cylindrical member 4 and the radial sleeve 6, these spaces can be easily determined at given values, provided that the radial cylindrical member 4 and the radial sleeve 6, the machining of which is easy, are produced with the appropriate height. In addition, since the radial sleeve 6 forms a bearing member for both the radial and thrust bearings, the number of parts necessary for the formation of the bearing device is reduced, and the structure is simplified.

 Although, in the bearing device described, the base 1, the spindle 2, the radial cylindrical member 4 and the stop plates 3 are fixed members, while the hub 7 and the radial sleeve 6 are rotary members, it should be noted that the above-mentioned fixed and rotary members can replace each other. Thus, the arrangement can be such that the base 1, the box 2, the radial cylindrical member 4 and the stop plates 3 are rotary members, while the hub 7 and the radial sleeve 6 are fixed members.

 Figure 5 is a sectional view showing the arrangement of a spindle motor which includes the bearing device of the invention. The bearing device intended for the spindle motor is produced in the same manner as that which is represented in FIG. 1. Thus, this device comprises a base 1, a spindle 2 mounted on the central part of the base 1, a cylindrical member radial 4 in the central part of which the pin 2 passes, a pair of stop plates 3 fixed to the two end surfaces of the radial cylindrical member 4 and of the central part of each of which passes the pin 2, and a radial sleeve 6 which is supported so that it can rotate at its internal peripheral surface and at its two end surfaces parallel to the external peripheral surface of the radial cylindrical member 4 and to the opposite internal surfaces of the two stop plates 3 respectively, this sleeve being fixed to the inner periphery of the hub 7. The radial cylindrical member 4 and the radial sleeve 6 constitutes a hydrodynamic radial bearing, while the two end parts of the man chon radial 6 and the stop plates 3 constitute hydrodynamic stop bearings. However, contrary to the arrangement shown in FIG. 1, the base 1, the spindle 2, the radial cylindrical member 4 and the stop plates 3 are rotary members.

 In addition, a chamfered part is formed at the external periphery of each end of the radial cylindrical member 4 and another chamfered part is formed at the internal periphery of each end of the radial sleeve 6 (see FIG. 3), so that an air space 13 is formed at each axial end of the radial hydrodynamic bearing, and is delimited between the chamfered parts and the surface of an abutment plate 3 turned towards these chamfered parts. In addition, small holes 8 are formed on the outer peripheral surface of the radial cylindrical member 4. The diameter of the small holes 8 is equal to 0.8 mm. A lubricating liquid is used as the fluid for creating pressure dynamic in the radial hydrodynamic bearing, whereas a gas, for example air, is used as fluid for the creation of a dynamic pressure in the hydrodynamic thrust bearings. Also at these locations, the bearing device represented in FIG. 5 is practically the same as that which is represented in FIG. 1. It should be noted that the reference 16 in FIG. 5 designates a nut used for fixing the plates 3 of stop and from the radial cylindrical member 4 to the spindle 2.

 A rear cylinder head 12 can be fixed to the external periphery of the base 1. In a variant, the cylinder head 12 can be formed in one piece with the base 1.

Several stator windings 9, regularly spaced in circumferential direction, (comprising a stator core), are fixed to the external periphery of the hub 7.

Several rotor magnets 15 regularly spaced in circumferential direction are fixed to the internal peripheral surface of the yoke 12 opposite the windings 9. The lower end of the hub 7 is fixed to the frame 10.

 In the spindle motor having the arrangement indicated in FIG. 5, as the stator windings 9 receive successively an electric current, the rotor of the motor, which comprises the yoke 12 having the magnets fixed to its internal peripheral surface, turns.

Thus, the rotor of the engine, which comprises the base 1 and the cylinder head 12, rotates under conditions such that the external peripheral surface of the radial cylindrical member 4, which is fixed to the spindle 2, is supported so that it can rotate by the internal peripheral surface of the radial sleeve 6, while the opposite internal surfaces of the stop plates 3, which are fixed at the two ends respectively of the radial cylindrical member 4, are supported by the two end surfaces of the radial sleeve 6 so that they can rotate.

 In the spindle motor having the aforementioned arrangement, the axial magnetic centers of the stator windings 9 and the rotor windings 15 are mutually offset by a distance d so that a magnetic attraction force acts in the opposite direction of abutment to that of the applied force. Thanks to this arrangement, when the spindle motor works in a vertical position, the force applied in the direction of abutment by the weight of the motor (comprising the yoke 12, the base 1, the spindle 2, the magnetic disc, etc.) can be reduced. Thus, it is possible to reduce the loss of torque in the thrust bearings at the time of starting and stopping the engine and consequently facilitating the starting and stopping of the rotation of the engine. The wear of the bearing members is therefore reduced and the service life of the bearings is increased. When the spindle motor works in a horizontal position, the rotor is pushed against a thrust bearing by the axis of the force generating device. offset magnetic attraction and therefore rotates on the base on the surface of this thrust bearing. As a result, it is possible to suppress vibrations in the stop direction during rotation and to obtain high rotational precision.

 In addition, by positioning the center of gravity g of the rotor at a predetermined instance of the bearing structure, the rotor can rotate stably regardless of the attitude of the motor when in use.

 Figure 6 is a sectional view showing the arrangement of another spindle motor which includes the bearing device according to the present invention. The bearing device of the spindle motor is produced in the same way as the bearing devices of FIGS. 1 and 5. Thus, the bearing device comprises a base 1, a spindle 2 mounted on the central part of the base 1, a radial cylindrical member 4 in the central part through which the spindle 2 passes, two stop plates 3 fixed to the two end surfaces of the radial cylindrical member 4 and in the central part of each of which passes the spindle 2, and a sleeve radial 6 is supported so that it can rotate at its internal peripheral surface and at its two end surfaces by the external peripheral surface of the radial cylindrical member 4 and the opposite internal surfaces of the two stop plates 3 respectively, the sleeve being fixed to the internal periphery of a hub 7. The radial cylindrical member 4 and the radial sleeve 6 constitute a radial hydrodynamic bearing, while the two end parts of the sleeve ra dial 6 and the stop plates 3 form the hydrodynamic stop bearings. However, unlike the arrangement shown in Figure 5, the base 1, the sleeve 2 and the radial cylindrical member 4 are in the form of fixed members (in the same manner as in the arrangement shown in Figure 1).

 In addition, a chamfered part is formed at the external periphery of each end of the radial cylindrical member 4, and another chamfered part is formed at the internal periphery of each end of the radial sleeve 6, with the formation of a space of air 13 at each axial end of the radial hydrodynamic bearing, this space being delimited between the chamfered parts and the surface of a first stop plate 3 which is turned towards these chamfered parts. In addition, small holes 8 with closed internal ends are made on the external peripheral surface of the radial cylindrical member 4. The diameter of the small holes 8 is 0.5 mm. A lubricating liquid is used as the fluid for creating a dynamic pressure in the radial hydrodynamic bearing, while a gas, such as air, is used as fluid for creating a dynamic pressure in the hydrodynamic thrust bearings. Also at these locations, the bearing device shown in FIG. 6 is practically the same as those in FIGS. 1 and 5. It should be noted that the reference 17 in FIG. 6 designates a bolt used for fixing the stop plates 3 and from the radial cylindrical member 4 to the spindle 2.

 Several stator windings 9 which are regularly spaced in the circumferential direction (comprising a stator core) are fixed to the external periphery of the base I. Several rotor magnets 15 which are regularly spaced in the circumferential direction are fixed to the internal peripheral surface of the base hub 7, opposite stator windings 9. The hub 7 thus constitutes a rear cylinder head.

 In the spindle motor having the arrangement of FIG. 6, when the stator windings 9 successively receive an electric current, the hub 7, to which the rotor magnets 15 are fixed to the internal peripheral surface, rotates. Thus, the hub 7 rotates in a state such that the internal peripheral surface of the radial sleeve 6 is rotatably supported by the external peripheral surface of the radial cylindrical member 4, which is fixed to the hub 2, while the two surfaces of end of the radial sleeve 6 are supported by the opposite internal surfaces of the stop plates 3, which are fixed to the two ends of the radial cylindrical member 4, while being able to rotate.

 In the spindle motor having the arrangement indicated in FIG. 6, as in the case of FIG. 5, the axial magnetic centers of the stator windings 9 and of the rotor windings 15 are mutually offset by a distance d so that a magnetic attraction force acts in the opposite direction to that of the applied force. In addition, the center of gravity G of the rotor (which includes the hub 7, the radial sleeve 6, the rotor magnets 15, the magnetic disc, etc.) is at a predetermined distance from the bearing structure. Thanks to the above arrangement, advantageous effects analogous to those of the spindle motor shown in FIG. 5 are obtained.

 Although, in the embodiments of Figures 1 and 5, the small holes 8 are through holes which pass through the radial cylindrical member 4, the small holes 8 are not necessarily limited to these through holes. The small holes 8 can be holes which do not pass through the cylindrical member 4, that is to say holes the internal ends of which are closed as shown in FIG. 6. Alternatively, the small holes 8 can be frustoconical. In addition, the small holes 8 can have various positions, as shown in Figures 7 (a) to 7 (f) and 7 (al) to 7 (fl). Thus, in FIGS. 7 (a) and 7 (al), two small through holes 8 are formed in the left and right parts of the radial cylindrical member 4 respectively. In FIGS. 7 (b) and 7 (bl), two small through holes 8 are formed in the right and left parts respectively of the radial cylindrical member 4 at respective positions which are axially offset from one another. In FIGS. 7 (c) and 7 (cl), two pairs of small through holes 8 are formed in the right and left parts respectively of a radial cylindrical member 4 with two axially distant positions. In FIGS. 7 (d) and 7 (dl), four small through holes 8 are formed in four parts spaced regularly in circumferential direction in the radial cylindrical member 4. In FIGS. 7 (e) and 7 (el), four small through holes 8 are formed in four regularly spaced portions in circumferential direction of the radial cylindrical member 4, at two axially spaced positions. In Figures 7 (f) and 7 (fl), three small through holes 8 are formed in regularly spaced parts in circumferential direction of the radial cylindrical member 4. It should be noted that FIGS. 7 (a) to 7 (f) are plan views and FIGS. 7 (a1) to 7 (fl) are views in section of the radial cylindrical member 4.

Possibility of industrial application
As described above, the invention relates to an excellent bearing device having the following advantages.

 (1) The bearing device has a radial hydrodynamic bearing in the form of a hydrodynamic bearing with a lubricating liquid and hydrodynamic thrust bearings formed by hydrodynamic gas bearings. Thus, the force-absorbing capacity of the radial bearing is significantly increased compared to hydrodynamic gas bearings. In addition, the state of lubrication at the time of starting and stopping is better, so that wear on the radial bearing can be reduced. In addition, since no lubricating liquid is present in the hydrodynamic thrust bearings, it is not possible for a lubricating liquid to be projected by centrifugal force during a rotation at high speed. Thus, the bearing device has excellent durability and excellent cleanliness and it gives rotational performance at high speed. The bearing device according to the invention is therefore suitable for a spindle motor to which these performances are required.

 (2) When mounting, it is easy to make the necessary perpendicularity between each stop plate and the radial cylindrical member and to obtain the necessary space between each stop plate and the end surface of the radial sleeve by tightening using, for example, a fixing nut, provided that the cylindrical radial member and the radial sleeve, the machining of which is easy, are produced at the appropriate height, and that the stop plates are produced so that their surfaces opposite internal have the desired flatness and that each end surface and each external peripheral surface of the radial cylindrical member are well perpendicular.

 (3) As the radial sleeve forms bearing members for both radial and thrust bearings, the number of elements necessary for the formation of the entire bearing structure is reduced and the structure of the device to be bearings is therefore simplified.

 (4) If the motor is in the form of an external rotor type motor and the bearing assembly is placed inside the stator core, the radial sleeve may be a fixed member or a rotary member. When the center of gravity of the motor rotor is placed at a predetermined distance from the bearing structure, the rotor can rotate stably regardless of the attitude of the motor when in use. It is therefore possible to make a device for bearings suitable for a spindle motor which must have stable rotational performance regardless of the attitude of the motor when used.

 (5) When using an electrically conductive liquid as a lubricating liquid, the static electricity produced on the magnetic recording medium, for example in a hard disk drive, can be effectively transmitted to the mass on the fixed side thanks to the conductive function of the electrically conductive liquid. It is therefore possible to avoid the accumulation of static electricity between the magnetic recording medium and the head. The bearing device is therefore suitable for a spindle motor used for driving hard disks or similar devices.

 (6) Thanks to the presence of a device creating a magnetic attraction force in the direction of abutment opposite to the applied force, it is possible to reduce the force applied in the direction of abutment by the weight of the rotor when the spindle motor works in a vertical position. It is therefore possible to reduce the loss of torque in the thrust bearings when starting and stopping the engine and consequently making it easier to start and stop the rotation of the engine.

 The wear of the bearing members is therefore reduced and the service life of the thrust bearings is increased. When the spindle motor works in a horizontal position, the rotor is pushed against a thrust bearing under the action of the device which creates the magnetic force of attraction, and consequently it rotates on the base of the surface of this thrust bearing . As a result, it is possible to suppress vibrations in the stop direction during rotation and to obtain a high degree of rotation precision.

Claims (10)

 1. A bearing device, of the type which comprises a base (1), a spindle (2) mounted on a central part of the base, a hub (7), and hydrodynamic radial and thrust bearings placed between the spindle and the hub, in which a radial cylindrical member (4) is fixed to the base so that the spindle passes through a central part of this member, two stop plates (3) are fixed to the two end surfaces of the cylindrical member radial and each have a central part through which the spindle passes, a radial sleeve (6) is supported so that it can rotate at its internal periphery and at its two end surfaces by an external peripheral surface of the cylindrical member radial and opposite internal surfaces of the two stop plates respectively, this sleeve being fixed to the hub, the radial cylindrical member (4) and the radial sleeve (6) constitute the radial hydrodynamic bearing, the two end portions of the radial sleeve where he opposite internal surfaces of the two stop plates have spiral grooves intended to create dynamic pressure, and the two end portions of the radial sleeve (6) and stop plates (3) constitute the hydrodynamic stop bearings, characterized in what  a fluid generating a dynamic pressure, placed in the hydrodynamic thrust bearings, is a gas,  the radial hydrodynamic bearing has a very small clearance,  a chamfered part (4a) is formed at the external periphery of each end of the radial cylindrical member (4), and another chamfered part (6a) is formed at the internal periphery of each end of the radial sleeve (6) so that an air space (13) is formed at each axial end of the radial hydrodynamic bearing, this space being surrounded by the chamfered part of the radial cylindrical member, the chamfered part of the radial sleeve and one of the stop plates,  a fluid intended to create dynamic pressure in the radial hydrodynamic bearing is a lubricating liquid, and a small hole (8) intended to collect the lubricating liquid is formed at an external peripheral surface of the radial cylindrical member (4), and  the lubricating liquid is enclosed in the clearance of the radial hydrodynamic bearing thanks to the presence of the air spaces (13), and a change in volume of the lubricating liquid at the time of rotation and of the stop is absorbed by the small hole.  2. A bearing device according to claim 1, characterized in that the small hole (8) which is formed on the external peripheral surface of the radial cylindrical member (4) is a through hole which passes through the radial cylindrical member.  3. A bearing device according to claim 1, characterized in that the small hole (8) which is formed on the external peripheral surface of the radial cylindrical member is a hole which does not pass through the radial cylindrical member (4) and which has a closed internal end.  4. A bearing device according to one of claims 2 and 3, characterized in that several small holes (8) are formed on the external peripheral surface of the radial cylindrical member (4) at respective locations regularly spaced in circumferential direction or axial.  5. A bearing device according to claim 1, characterized in that the lubricating liquid enclosed in the hydrodynamic radial bearing has a low volatility and a kinematic viscosity less than 10 cSt at 40 C.  6. A bearing device according to claim 5, characterized in that the lubricating liquid contains an electrically conductive substance.  7. A bearing device according to claim 1, characterized in that the radial sleeve (6) is a rotary member.  8. A bearing device according to claim 1, characterized in that the radial sleeve (6) is a fixed member.  9. A bearing device according to claim 8, characterized in that it comprises a device (9, 15) intended to create a magnetic force of attraction in a direction of abutment relative to one of the abutment plates, in opposite direction to an applied force.  10. A bearing device according to any one of claims 1 to 9, characterized in that the radial bearing is arranged so that it supports a rotor which is supported by the bearing device so that it can rotate, at a predetermined distance from the center of gravity of the rotor.