GB2146079A - Bearing assembly with integrated ferrofluid seal - Google Patents

Abstract

A bearing assembly suitable for use in supporting a shaft (12) of a disc drive comprises inner and outer races (26, 24) having ball bearings (30) therebetween. A ferrofluid seal is integrated with this bearing assembly and comprises an annular magnet (34) and a pole piece (38) extending into close non-contacting relationship with the moving race. An annular splash guard (42) extends from the stationary race towards this moving race at an axial location between the ball bearings (30) and the magnet (34), the pole piece (38) being located on the side of the magnet remote from the splashguard. <IMAGE>

Description

SPECIFICATION Bearing assembly with integrated ferrofluid seal Bearing assemblies are employed to provide a sliding or rotating movement and typically provide support and permit rotation of a rotatable shaft element with a minimum of friction. For example, bearing assemblies, such as ball- or roller-bearing assemblies, are employed to support the spindledisc-drive shaft employed to carry a computer magnetic disc which is read by a memory head.

The computer disc and memory head must be protected from an outside environment, and typically a ferrofluid exclusion seal is employed, and such ferrofluid exclusion seal is employed, and such ferrofluid exclusion seals are employed as a separate component. While the ferrofluid exclusion seal used, for example, with a rotating shaft such as a computer-disc-drive shaft or spindle, is satisfactory, having the seal apparatus as a separate component, is not always satisfactory, particularly where space requirements are at a minimum.

Ferrofluid seal apparatuses, such as multiplestage seal apparatuses, have been employed in conjunction with ball bearings to support a shaft, such as, for example, described in U.S-A3,620,584. Further, ball-bearing assemblies have been employed using a magnetic ball bearing, in order to retain ferrofluid lubricants in place, such as set forth in U.S.-A- 3,977,739.

In our earlier U.S. patent application Serial No.

450, 339, filed December 16, 1982 (now published as U.S.-A- 4,407, 508) we describe a single-polepiece seal apparatus having a single magnetically permeable pole piece, an annular permanent magnet, means to secure the pole piece and the magnet together, and with one end of the pole piece extending into a close relationship with the surface of the shaft to be sealed, to define a radial gap, and ferrofluid disposed in the radial gap, to provide an exclusion seal or a pressure-type seal. The magnetic flux path of the single-pole-piece seal extends through the permanent magnet, the single pole piece, the ferrofluid in the radial gap, the shaft element to be sealed and an air gap between the shaft element and the other end of the permanent magnet, with typically the radial gap being shorter or less in gap length than the air gap.The singlepole-piece ferrofluid seal apparatus is particularly useful where space is at a premium, and where the shaft comprises a small-diameter, high-speed shaft element, and with the magnetic flux concentrated in the radial gap, the magnetic flux density in the air gap being quite small.

Therefore, it is desirable to provide a bearing assembly having an integrated single-pole-piece ferrofluid exclusion seal and which is compact in design and is particularly useful where space requirements are important.

The invention relates to a bearing assembly containing an integrated ferrofluid seal, and more particularly the invention concerns a ball-bearing assembly containing an integrated, single-polepiece ferrofluid exclusion seal incorporated therein of a unique and compact design, and to a method of operating such bearing assembly and to the computer-disc-drive system in which such bearing assembly is incorporated.

It has been discovered that a bearing assembly, particularly a ball-bearing assembly that incorporates an integral single-stage or single-pole-piece ferrofluid seal therein, provides for a bearing assembly and an integrated seal which has a small axial length, compared with the employment of a bearing assembly and seal as separate components, particularly where space is at a premium, such as in a bearing assembly and seal employed in a computer-memory disc-drive spindle system. It has been found that the integration, in the manner described and claimed, of a ferrofluid single-polepiece seal into a bearing assembly allows for a significant and substantial reduction in overall axial length, which makes the bearing assembly especially attractive, with shaft elements and spindles, where a compact design is essential.The ferrofluid exclusion seal replaces the separate-component ferrofluid seal or the general elastomeric-type seal used in bearing assemblies. The elastomeric-type seals tend to generate heat, particles, and particularly provide for an ineffective sealing arrangement with time.

The bearing assembly of the invention is particularly useful with computer-disc-drive spindles, and wherein a ferrofluid exclusion seal is required, in order to protect the memory head from particulate or other contamination. However, it is recognized that the bearing assembly may be employed in a variety of other applications, such as in electric motors which are required to be sealed, for exampie, in the textile industry from fiber or particle contamination, or for use in a stepper motor to seal and protect the ball bearing from a hostile environment.The invention will be described, for the purpose of illustration only, employing an exclusion-type single-pole-piece seal: however, it is recognized that pressure-type-capacity ferrofluid with separate ferrofluid O-ring seals, also may be incorporated in the bearing assembly such as by employing two or more edges at the end of the single pole piece or grooves in the shaft element to be sealed.

The bearing assembly of the invention comprises a bearing assembly typically a ball-bearing assembly, which includes inner and outer races, to define a raceway, and contains therein a plurality of roller elements, such as ball-bearing elements within a cage or keeper, which in combination provide for the relative rotation of the inner and outer races relative to each other, with the inner and outer races adapted to surround one race to be secured to a movable mechanism, particularly a rotatable shaft, such as the shaft of a disc drive or motor shaft.The ferrofluid seal assembly, which is integral with the bearing assembly, comprises a ferrofluid seal apparatus positioned within the raceway and closely adjacent to the roller elements, such as the ball-bearing elements, and typically, as in its use with computer-disc drives, is positioned toward the environment to be protected; that is, the memory-head area in the computer-disc-drive application.

The single-pole-piece ferrofluid seal comprises a single, magnetically permeable pole piece, an an nular permanent magnet usually directly secured to one side of the annular pole piece, the pole piece extending generally perpendicular to the in ner and outer races and generally across the raceway, one end of which extends into a close, noncontacting relationship with the inner surface of the inner or outer raceway to define a radial gap. A sealing amount of a ferrofluid is placed in the radial gaps for example, about 2-6 mils, (0.05 to 0.15 mm) to provide for an exclusion seal, with the ferrofluid held in place by a magnetic flux passing through the radial gap.

The integral ferrofluid seal also includes a nonmagnetically permeable housing, such as an alu- minum housing, which is positioned such as to divert magnetic flux of the permanent magnet toward the inner or outer, rather than the other, race: that is, toward the race surface in which the ferrofluid radial sealing gap is formed. Where the ferrofluid seal apparatus is placed in a ball-bearing assembly for computer-disc-drive spindle, then the nonmagnetic housing typically is positioned so as to divert magnetic flux toward the inner, rather than the outer, race.The nonmagnetic housing is essential to the operation of the integral seal, and generally comprises in section an L-shaped, annular housing of a nonmagnetic material, such as aluminium or plastics material, which insulates the one end of the pole piece which does not form the radial gap, and also insulates the one end of the permanant magnet from the other inner or outer race surface, and generally extends along a portion or usually a substantial portion of the one side of the annular permanent magnet between that side of the magnet and the roller elements of the bearing assembly and toward the other end of the magnet where the air gap is formed. By this arrangement, the roller elements, such as the ball bearings, which are typically composed of steel or magnetically permeable material, are protected from any magnetic flux from the adjacent permanent magnet.Typically the bearing assembly; that is, the ball bearings and the inner and outer races, are of a magnetically permeable material. It is desirable to prevent magnetic flux from going through the inner and outer races and the ball bearings, so as to prevent additional stresses on the bearing assembly. Thus, the employment of a nonmagnetic housing and the use, for example, of a low-energy permanent magnet are desirable, in order not to disturb the bearing-assembly materials or operation by external magnetic forces or stress.

Thus, for example, in one embodiment, the permanent magnet employed should be a low-energy permanent magnet, such as a magnet composed of a ceramic or polymeric material, such as a rigidtype polymeric material containing a combination of a nylon resin with magnetic particles, or an elastomeric-type magnet which is more flexible and which contains elastomeric materials, such as rub ber containing magnetic particles. Such low-energy magnets generally may have a flux energy of about 1.1 million gauss-oersteds to about 1.6 mil lion gauss-oersteds. Typically, such magnets pro vide for a flux density ranging from about 2000 to 5000 gauss in the air gap formed in the integrated, single-pole-piece ferrofluid seal apparatus.

While the bearing assembly with the integrated ferrofluid seal is both compact and unique in design, it has a smaller axial length, compared with a separately composed, two-pole-piece ferrofluid seal. The ferrofluids employed also may be se lected so that the ferrofluids are compatible with the grease or lubricant material employed in the roller elements of the bearing. Generally, grease is placed inside the race of the bearing between the surface of the roller elements and the raceway, to provide for low friction operation of the bearing assembly. Generally, such grease is a hydrocar bon-based grease, and, therefore, it is desirable to employ a ferrofluid having the same or similar and at least compatible carrier liquids, such as a hydrocarbon-carrier liquid based ferrofluid.It has been found that, if the ferrofluid is not wholly compatible then the physical forces involved in the bearing assembly may throw out the grease employed, to contact the closely adjacent ferrofluid in the radial gap or the seal where it may destabilize the ferrofluid, or the ferrofluid can absorb the grease and be affected in its colloidal stability, thereby shortening the life of the ferrofluid seal. Thus it is desirable to provide for a ferrofluid seal material which is compatible with the grease employed in the bearing assembly, and generally both are hydrocarbon-based.While the magnetic forces may alter the viscosity and magnetization of ferrofluid, the selection of a compatible ferrofluid is important, to ensure further long-term life for the integrated seal, particularly where the ferrofluid seal is closely adjacent to the roller elements on the side of the environment which is to be protected.

In another embodiment, the bearing assembly also may include a magnetically permeable; for example, steel, splash guard extending generally across the raceway and generally adjacent the nonmagnetic housing, to define a small radial air gap. It has been discovered that the splash guard increases the pressure capacity of the integrated, single-stage, single-pole-piece ferrofluid seal by a factor of about two. The splash guard reduces any splash and intermixes or contacts between the ferrofluid and the lubricant of the roller elements.

However, the splash guard also, importantly, aids in completing the magnetic flux circuit, by extending beyond the one end of the permanent magnet and toward the inner surface of the outer or preferably inner race; that is, the same surface of the race where the ferrofluid is held in the radial gap, to aid in completing the magnetic circuit.

In a further embodiment, the bearing assembly may include both a dynamic and a static ferrofluid seal; that is, a dynamic ferrofluid exclusion seal, with the surface of the moving member, typically the inner race, and a static ferrofluid exclusion seal with the surface of the nonmoving member, typi cally the outer race. The dynamic-state ferrofluid seal can be formed using the same permanent magnet and by forming the ferrofluid seal at each end of the annular single pole piece. In such an arrangement the nonmagnetic housing is partially removed to permit the static seal end or surface of the pole piece to extend into a close, noncontacting relationship with the magnetically permeable, nonmoving or static: that is, internal, surface of the outer race to form a small radial gap, with ferrofluid retained in the radial gap.

The invention will be described for the purpose of illustration only with reference to the accompanying drawings, wherein: Figure 1 is a schematic, perspective view of a computer-disc-drive system incorporating the bearing assembly with an integrated, single-stage ferrofluid seal of the invention; Figure 2 is an enlarged, fragmentary, sectional view of one side of the bearing assembly of Figure 1 with the integrated, single-pole-piece ferrofluid seal apparatus; and Figure 3 is an enlarged, fragmentary, sectional view of a dynamic-static ferrofluid seal in the bearing assembly of Figure 1.

Figure 1 shows a computer-disc-drive-spindle system 10, which includes the bearing assembly with the integrated ferrofluid seal of the invention.

The system 10 includes a computer-disc-drive-spindie shaft 12 driven at high speed; for example, 2400 to 6000 rpms, by a motor (not shown), usually by a belt and pulley 14. An air-bearing memory-disc pack 20, which rides on an air film, is positioned adjacent the top of the spindle shaft 12 by the disc 22 in a clean magnetic-disc area or chamber 46 maintained in a clean environment by filtered air or the use of an inert gas, such as helium. The spindle 12 is within a housing 44 and is sup supported by upper and lower bearing assemblies 16 and 18. Bearing assembly 16 may comprise a known bearing, while bearing assembly 18 includes a ferrofluid exclusion seal, this bearing being described in more detail in Figure 2.

Figure 2 illustrates the left-hand section of the upper bearing assembly 18 which shows a magnetically permeable inner race 26 and an outer race 24 which define therebetween an annular raceway 28, which raceway contains a plurality of uniformly space, magnetically permeable ball bearings 30 within a ball-bearing cage or keeper 32 on either side. A single-stage, single-pole-piece ferrofluid seal is disposed and positioned adjacent the raceway and directly downstream and toward the magnetic disc area 46 to be protected. The bearing assembly 18 optionally contains a magnetically permeable splash guard or shield 42 which generally extends across the raceway 28 toward the inner race 26, to form a small radial air gap.The guard 42 prevents contamination of the ferrofluid by the grease employed in the ball bearings and raceway and aids in completion of the magnetic flux circuit with the outer surface of the inner race 26. The single-stage seal shown comprises an annular permanent magnet 34 composed of a lowflux magnet, such as a ceramic permanent magnet or a Plastiform (trademark) permanent magnet 34, with the magnet surrounded at the one axial end, on one circumferential side of the magnet and on one circumferential edge of the pole piece 38 with a nonmagnetic, L-section housing 36 composed of aluminium or plastics.The pole piece 38 comprises a magnetically permeable material, such as steel, and is a single pole piece which extends at one radial circumferential edge into a close, noncontacting relationship with the surface of the inner race 26 secured to spindle shaft 12, to form a radial gap therebetween, in which radial gap the magnetic flux is concentrated, and which contains a sealing amount of a hydrocarbon-based ferrofluid 40 which is compatible with the grease employed in the raceway 28. As described, the ferrofluid exclusion seal is placed only on the memory disc area 46 side, although it is recognized that the seal, if desired in certain applications, where hostile environments are on both sides, may be placed on both sides, or a pressure-type seal may be employed on one or both sides if required.Also, the air gap between the edge of the L-section housing 36 and the surface of the inner, race 26 may be eliminated, and the splash guard 42 may extend along or adjacent the surface of the magnet 34.

Figure 3 shows the bearing assembly of Figure 1 which includes a static ferrofluid seal. By removing a portion of the nonmagnetic housing 36 and extending the other circumferential edge of the single pole piece 38 into a close relationship with the inner surface of the outer race 24, to form a small radial gap therewith, which retains a sealing amount of ferrofluid 48 in the radial gap, to form a static ferrofluid exclusion seal. As also illustrated in this embodiment, the one side of the housing 36 has been removed and replaced with the splash guard 42.

The ball-bearing assembly described, with the integrated, uniquely designed single-stage ferrofluid seal, is of a very compact, small-length design. The integration of the ferrofluid seal into the ball-bearing assembly permits significant reduction in overall length over the employment of separate components. The splash guard and magnetic spacer 42 is above the cage 32 defined by the ball bearing 30, to aid in completing the magnetic flux circuit, and to allow higher pressure capacities in the single-stage ferrofluid seal design. As described, low-energy magnets are employed for the seal, to minimize the effect of a strong magnetic flux on the closely adjacent ball bearings. The radial gap is quite small, typically from 2 to 6 mils, (0.05 to 0.15 mm) and generally is smaller than the air gap which exists between the circumferential edge of the magnet and the inner surface of the inner race.

Claims (5)

1. A bearing assembly adapted to surround a shaft (12) and having an inner race (26) and a spaced-apart outer race (24) to define a raceway (28) therebetween and outer elements (30) retained in the raceway (28) to provide for rotating of the inner and outer races relative to one another, characterised in that a ferrofluid seal (40) is provided in the raceway generally adjacent the roller elements and has a single pole piece (38) having one edge which extends into a close, noncontacting relationship with the inner surface of the inner or outer race to define a small radial gap, an annular permanent magnet (34) is in a close magnetic flux relationship with the pole piece, ferrofluid (40) retained in the radial gap by magnetic flux of the permanent magnet provides a ferrofluid exclusion seal with the surface of the inner or outer race, and a nonmagnetically permeable housing (36) is provided to divert magnetic flux of the permanent magnet from the roller elements (30), the magnetic flux circuit passing through the pole piece (38), the permanent magnet (34) the ferrofluid (40) in the radial gap, the inner or outer race (26) and an air gap between the permanent magnet and the surface of the inner or outer race.

2. A bearing assembly according to claim 1, which includes a magnetically permeable splash guard (42) extending across the raceway (28) between the ferrofluid seal and the roller elements and toward one surface of the inner or outer raceway to form a radial air gap.

3. A bearing assembly according to claim 1 or 2, characterised in that the nonmagnetic housing extends across at least one surface of the permanent magnet.

4. A bearing assembly according to any preceding claim, which includes a dynamic ferrofluid seal (40) and a static ferrofluid seal (48) the annular pole piece forming at the circumferential edge a dynamic ferrofluid seal with the surface of the moving race element (26) and the other circumferential edge of the pole piece extending into close noncontacting relationship with the surface of the other, nonmoving, race element (24) to form a radial gap therewith, ferrofluid (48) being retained in the radial gap to form a static ferrofluid seal with the stationary race surface (24).

5. A bearing assembly according to any preceding claim, characterised by being included in a computer-disc-drive system which comprises a rotatable disc-drive-spindle shaft (12) extending through the bearing assembly (18) and secured to the inner race (26); a memory-reading head (20) in a protected environment; a computer disc (22) on the shaft to be read by the reading head in the environment; and means to rotate the shaft, the ferrofluid seal having the radial gap formed with the surface of the inner race and positioned on the side of the roller elements nearer the memoryreading head.