JPS6124585B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for sealing a rotating shaft with a magnetic fluid.

Ferrofluidic sealing devices are well known in which a single or multistage ferrofluid O-ring seal is placed around the shaft to seal a rotating shaft (e.g.
(See U.S. Pat. No. 3,620,584, which describes a multi-stage magnetic fluid rotating shaft seal).

To protect the environment on one side of the shaft from contamination of the environment on the other side of the shaft, single or multi-stage ferrofluids are used as hermetic seals. Ferrofluidic type hermetic seals are useful, particularly with respect to computer disk drive spindles, to prevent contaminants in the external environment from reaching the memory disk area.

One standard ferrofluidic seal currently used in the computer field consists of an annular permanent magnet formed around a spindle shaft and sandwiched between two identical pole piece elements. be. The magnetic pole piece element is in contact with one and the other magnetic pole end of the permanent magnet so that the magnetic flux is continuous. For example, the inner peripheral surface of the pole piece element has a distance of about 0.05 to
It is in a non-contact relationship with the shaft or spindle so as to form a small gap of 0.25 mm (2-10 mils). A ferrofluid is disposed, and upon insertion of a magnetically permeable shaft or spindle, the ferrofluid is magnetically retained within the gap, forming a single or multi-stage O-ring, which causes the ferrofluid to flow around the shaft. helps form a hermetic seal.

Although various types of magnetic materials can be used to form permanent magnets, the materials typically have a
It is a sintered or bonded ceramic material having a longitudinal thickness of 3.8 mm (approximately 80 to 150 mils). The pole piece elements are made of magnetic stainless steel (e.g.
400 series) and is made of a magnetically permeable material such as
It has a thickness range of 0.6 to 2 mm (approximately 25 to 80 mils). Standard hermetic seals may be provided as described above or placed within a non-magnetic housing such as aluminum or stainless steel (e.g. 300 series) by gluing or bayonet assembly techniques, depending on the user's requirements. provided.

The hermetic seal is formed by placing precisely the right amount of magnetic fluid within the annular gap region between the inner peripheral surface of the pole piece and the spindle shaft. Typically, ferrofluids include a low vapor pressure carrier such as fluorocarbons, polyphenyl ethers, hydrocarbons, diester liquids, and similar low vapor pressure liquids, thereby allowing the ferrofluid to form an O-ring seal. Provides a hermetic seal with very low mass loss and long operating life. For example, standard ferrofluidic seals typically run up to 4.6 cm at computer disk driven spindle speeds of 3600 rpm.
(1.8 inches) shaft diameter will last for several years under moderate temperature conditions. The viscosity and saturation magnetization of the ferrofluids used vary, but these are typically between 20 and 500 cp and
It is in the range of 100-400 Gauss.

Particularly under high temperature conditions, e.g. high ambient temperatures above 50°C, spindle speeds above 3600 rpm and large shaft diameters or a combination of these conditions.
It is desirable to extend the useful operating life of ferrofluidic seals.

The present invention relates to a long-life magnetic fluid rotary shaft sealing method and a sealing device of this type. More particularly, the present invention relates to ferrofluidic seals that are particularly useful for use in and long-term sealing of computer disk drive spindles.

It has been found that there are two basic design considerations in standard ferrofluidic seals. One is magnetism, which determines seal pressure, and the other is heat generation, which determines seal life.

Generally, the total pressure capacity of current ferrofluidic seals ranges from about 30 to 60 inches of water, which is divided approximately equally between the two pole pieces. The pressure conditions required in a typical disk drive application are only about 13 cm (5 cm) of water.
inches) and therefore the seal has a large safety margin with respect to pressure. In fact, even one ferrofluid O-ring seal is more than sufficient to produce the required pressure capacity. However, in current standard designs, two pole pieces are used so that the closed magnetic path or flux path is completely completed.

It is known that heat is generated by viscous shear of the ferrofluid between the outer circumferential surface of the rotating spindle shaft and the inner circumferential surface of the fixed pole piece, resulting in a temperature gradient across the ferrofluid O-ring seal. Some of this heat escapes by conduction through the pole pieces and spindle shaft. The temperature of the ferrofluid during operation therefore depends on the capacity of its sealing material and structure to dissipate heat or act as a heat sink, and this in turn determines the evaporation rate of the ferrofluid and hence the lifetime of the seal. Determine.

When both gap regions are filled with ferrofluid, the operating fluid temperature is higher than when only one stage is filled with ferrofluid and the other stage has an air gap. This is because each gap region filled with ferrofluid acts as an independent heat source, and since there are two heat sources in a local area, only one stage can control the temperature of the seal structure. arises from raising it to a higher value than if it were filled with.

Unlike the fact that seal pressure is doubled when both stages are filled with ferrofluid compared to when only one stage is filled with ferrofluid, seal life is This is increased by filling only one gap region with magnetic fluid, rather than the gap region. The ideal situation would therefore be if only one pole piece was activated with magnetic fluid. The second pole piece, operating with an air gap, is used only to complete the magnetic circuit, ie to form a closed magnetic path. The air gap helps move air out of the cavity between the pole pieces. However, current seal setting techniques hinder achieving this goal. This is because the ferrofluid is injected into the magnet region and causes the ferrofluid to migrate to both gap regions upon insertion of the spindle shaft.

The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a magnetic fluid seal device that is easy to assemble and can continue to work for a long period of time.

According to the present invention, the above-mentioned object is configured to circumferentially surround a part of a rotary shaft to be sealed made of a high-speed magnetic material, and opposite opposite ends located at both ends in the axial direction of the rotary shaft. a permanent magnet whose polarized magnetic pole portions extend toward the circumferential surface of the rotating shaft to form a gap therebetween so as to cooperate with the magnetically permeable rotating shaft to form a closed magnetic flux path; a magnetic structure and a magnetic fluid initially filling each gap to form an O-ring seal at each of the two gaps, the two gaps being connected to each other by the rotation of the rotating shaft; When the magnetic fluid is filled in one of the two gaps, the magnetic fluid filled in the other gap is evaporated and disappears within a shorter period of time than the magnetic fluid filled in the other gap. This is achieved by a magnetic fluid seal device configured as follows.

In a preferred embodiment of the present invention, the permanent magnet structure is configured to surround the part of the rotating shaft, and has permanent magnet magnetic pole parts of opposite polarity at both ends with respect to the axial direction of the rotating shaft. One end is in contact with one of the two permanent magnet magnetic pole parts, and the other end is in contact with the circumferential surface of the rotating shaft to form the gap between the permanent magnet and the circumferential surface of the rotating shaft. A first magnetically permeable magnetic pole piece extends toward the rotating shaft, one end side is in contact with the other of the two permanent magnet magnetic pole sections, and the other end extends toward the circumferential surface of the rotating shaft. and a second magnetically permeable magnetic pole piece, preferably, the permanent magnet and the first and second magnetic pole pieces are
When the two gaps are filled with magnetic fluid, the magnetic fluid is configured to cooperate with the circumferential surface of the rotating shaft to form an airtight air cavity around the rotating shaft between the first and second magnetic pole pieces. ing.

In a magnetic fluid rotary shaft sealing method according to a preferred embodiment of the present invention, a rotary shaft to be sealed is surrounded by a permanent magnet, and first and second magnets are arranged to guide magnetic flux generated from both ends of the permanent magnet to the surface of the shaft. magnetically permeable pole pieces are disposed about the shaft and at opposite ends of the permanent magnet to form an air cavity between the first and second pole pieces, and at least two magnetically permeable pole pieces are arranged between the first and second pole pieces and the surface of the shaft. one end of each pole piece is extended in non-contacting relation to the surface of the shaft to define a radial gap of the shaft, forming at least two magnetic O-ring seals around the shaft; In the method for sealing a rotating shaft in which a magnetic fluid is placed in the gap, the magnetic fluid in the gap of the first magnetic pole piece is first eliminated by evaporation, and then the magnetic fluid in the gap of the second magnetic pole piece is removed. To extend the life of a seal by sealing the shaft.

Further, a preferred embodiment of the present invention provides a magnetic fluid rotary shaft seal device having a long seal life.
A ring-shaped permanent magnet that surrounds the rotating shaft to be sealed and has magnetic poles of opposite polarity at both ends, and a seal that guides the magnetic flux generated from both ends of the permanent magnet to the surface of the rotating shaft. first and second magnetically permeable magnetic pole pieces disposed at opposite ends of a permanent magnet surrounding a rotating shaft to be rotated and defining an air cavity therebetween; One end of each pole piece extends in close, non-contacting relation to the surface of the shaft to define at least two radial gaps, and one end of each pole piece extends in close, non-contacting relation to the surface of the shaft to define at least two radial gaps. In order to form O-ring seals,
A magnetic fluid is held within the gap,
The first and second magnetically permeable magnetic poles are configured such that during rotation of the shaft, the magnetic fluid in the gap of the first magnetic pole piece disappears first by evaporation, and the magnetic fluid in the gap of the second magnetic pole piece remains. It consists of a piece.

The seal life of a magnetic fluid rotary seal arrangement can be extended, for example, by the use of larger than normal, and preferably unequal, pole piece widths. Current standard ferrofluidic seals, whether single-stage or multi-stage, are made with pole pieces of the same width (axial thickness of the shaft), for example, about 0.8-1.1 mm (30-45 mils). It has been found that thicker width pole pieces result in longer life. This is a result of the larger volume of ferrofluid in the formed gap, increasing the evaporation time, and the larger cross-sectional area from which heat is extracted from the ferrofluid. Thickness approximately 1.3~2mm (50~80mil)
Optimal pole piece thicknesses of 100 or more provide a lifetime increase of as much as 90%.

In particular, hermetic seals with unequal pole piece widths (axial thicknesses) have been found to be particularly advantageous in extending seal life.

Standard two-stage ferrofluidic bipolar seals have the disadvantage that the life of the seal is substantially equal to the life of a single-stage seal. Additionally, the presence of ferrofluid in the second stage makes it two independent heat generators, so the lifespan of each stage is worse than if the second stage had no ferrofluid, i.e. with an air gap. .

The assembly approach for setting up the seal does not allow for easy filling of only one stage with ferrofluid. It has been found that fluid migration to one stage can be eliminated while maintaining fluid in the other stage. For seals with two unequal pole shoe widths, the narrower pole shoe seal will fail sooner, causing the thicker pole shoe seal to operate as a single stage seal for the remainder of the seal's life. The magnetic circuit configuration for the thicker pole piece is more than sufficient to meet certain seal pressure requirements required.

In forming a seal, such as a seal in a computer, ferrofluid is injected into the magnet region of the unequal pole piece seal prior to installation in the computer disk drive. Upon insertion of the spindle shaft, this fluid is withdrawn and distributed unevenly into two stages, generally proportional to the width (axial thickness) of the gap under each end of the pole piece. The experiment was performed using a shaft diameter of 4.6 cm (1.8 inch),
In this experiment, conducted at an operating speed of 3600 rpm and a radial gap of approximately 0.15 mm (6 mils),
It has been shown that the temperature of the ferrofluid for the thinner pole piece is higher than for the thicker pole piece.
This difference becomes larger as the fluid viscosity of the magnetic fluid becomes higher. The width of the thinner pole piece is usually approx.
0.6 mm (25 mils), but essentially determined by mechanical strength considerations, e.g.
(20 to 40 mils). The thicker pole piece should have a minimum thickness of approximately 1.3 mm (50 mils). The life of this seal, determined by fluid dissipation in the thicker pole piece, is 25% to 100% longer than that of a standard seal. Also, in the case of unequal pole shoe seals, the ferrofluid O-ring seal under the thicker pole shoe lasts five times longer than the seal under the thinner pole shoe and has a narrower or standard pole shoe element. To extend the seal life beyond that of the magnetic fluid seal.

Although the following description will be made in conjunction with particularly preferred embodiments for illustrative purposes, those skilled in the art will recognize that various changes and modifications may be made without departing from the scope of the claims.

FIG. 1 shows an extended life ferrofluid seal device 10 that includes a permanent magnet ring 14 disposed within a non-magnetic housing 12, such as aluminum or non-magnetic stainless steel. and includes pole pieces 16 and 18 in a sanderch configuration in contact with opposite sides of the magnet 14 and adjacent the opposing poles, with an annular sealed air cavity 22 formed therebetween. Magnet 14 and pole pieces 16 and 18 within housing 12 are disposed about a magnetically permeable shaft 26, such as a computer disk drive spindle. One end of each pole piece 16 and 18 extends into close, non-contacting relation to the surface of shaft 26, typically about 0.05 to 0.15 mm (2 to 6 mils) or more, such as about 0.3 to 6 mils or more. 0.75mm (12
forming first and second gaps of limited radial thickness (~30 mils).

A ferrofluid 24, such as a diester ferrofluid having a viscosity of 50 to 500 cp and a magnetic saturation of 100 to 450 gauss, is held within the gap 20 at the end of each pole piece and rotates as the shaft 26 rotates. Two O-ring seals 30 and 32 (indicated by parallel dotted lines) are formed on the surface. The closed magnetic (flux) path formed is illustrated by dotted line 34. The housing 12 may be an optional member, for example, approximately 1.3~
Pole piece 18 made of a flat sheet of 0.5 mm (50-20 mil) aluminum thermally conductive non-magnetic material.
It has an extension 36 extending to one end to conduct heat away from the magnetic fluid 24 in the gap 20 below the pole piece 18.

Pole piece 18 is larger than pole piece 16, e.g., about 0.6 to 1 mm (25 to 40 mils), e.g., about 1.3 to 40 mils.
has an axial thickness or width of the shaft of 2 mm (50 to 80 mils), so that the area of O-ring seal 32 below pole piece 18 is wide;
If the gaps under 6 and 18 are equal, the amount of ferrofluid 24 in gap 20 is greater under this pole piece 18. Although the illustrated sealing arrangement provides a long seal life, the ferrofluid under the pole piece 16 is first lost by evaporation and is removed from the cavity 2.
2 is open, the magnetic fluid 24 under the pole piece 18
A single-stage seal is formed, thereby creating a long-life seal. The optional use of thermally conductive extensions 36 and the use of large gap widths further extend seal life.

FIG. 2 shows the state of the sealing device 10 after the magnetic fluid 24 in the gap 20 below the pole piece 16 has disappeared; the seal is converted into a single-stage seal by this disappearance, and the seal is converted into a single-stage seal under the wide pole piece 18. ferrofluid helps extend seal life.

2 of the same axial thickness e.g. approximately 1 mm (40 mils)
Experiments were conducted on a standard hermetic seal with two pole pieces and a hermetic seal with one pole piece having an axial thickness of approximately 0.6 mm (25 mils) and the other pole piece having an axial thickness of approximately 1.4 mm (55 mils). I did it. Experiments were performed using a hydrocarbon-based ferrofluid with a viscosity of 50 cp and a magnetic saturation of 200 Gauss, with a radial gap of the seal of approximately 0.15 mm (6
A 4.6 cm (1.8 inch) diameter computer disk drive shaft was operated at 3600 rpm. Experimental data shows that standard seals fail in about 180 hours, whereas O-ring seals under thin pole pieces fail in 155 hours, whereas O-ring seals under approximately 1.4 mm (55 mil) thick pole pieces fail in 155 hours. showed that it was impaired after 265 hours.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of the magnetic fluid seal of the present invention at the start of operation, and FIG. 2 is a cross-sectional view showing the state of the magnetic fluid under one magnetic pole piece of the seal shown in FIG. 1 after evaporation. . 10...Sealing device, 12...Housing, 14...
Permanent magnet, 16, 18... Magnetic pole piece, 20... Gap, 22... Cavity, 24... Magnetic fluid, 30,
32...O-ring seal, 34...Magnetic circuit.

Claims (1)

[Claims] 1. The rotary shaft is made of a highly permeable material and is configured to circumferentially surround a part of the rotary shaft to be sealed, and oppositely polarized rotary shafts are located at both ends of the rotary shaft in the axial direction. A permanent magnet structure in which the magnetic pole portion extends toward the circumferential surface of the rotating shaft to form a gap therebetween so as to cooperate with the magnetically permeable rotating shaft to form a closed magnetic flux path. a magnetic fluid initially filling each gap to form an o-ring seal at each of said two gaps, said two gaps being separated by said rotary shaft being rotated; When the magnetic fluid fills one of the two gaps, the magnetic fluid fills the other gap of the two gaps and disappears by evaporation within a shorter period of time. A magnetic fluid seal device consisting of. 2. The permanent magnet structure is configured to surround the part of the rotating shaft, and includes a permanent magnet having permanent magnet magnetic pole parts of opposite polarity at both ends with respect to the axial direction of the rotating shaft, and two permanent magnets. A first member whose one end side is in contact with one of the permanent magnet magnetic pole portions of the magnetic pole portions, and whose other end extends toward the circumferential surface of the rotating shaft to form the gap between the first and second permanent magnet magnetic pole portions. a magnetically permeable magnetic pole piece, and a second magnetically permeable magnetic pole, one end of which is in contact with the other of the two permanent magnet magnetic pole sections, and the other end of which extends toward the circumferential surface of the rotating shaft. 2. A device according to claim 1, comprising a piece. 3. When the two gaps are filled with magnetic fluid, the permanent magnet and the first and second magnetic pole pieces cooperate with the circumferential surface of the rotating shaft to generate a magnetic field between the first and second magnetic pole pieces. 3. The apparatus of claim 2, wherein the apparatus is configured to form a gas-tight air cavity around the rotating shaft. 4. Claim 2 or 3: The amount of magnetic fluid held in the gap between the first magnetic pole pieces is smaller than the amount of magnetic fluid held in the gap between the second magnetic pole pieces.
Equipment described in Section. 5. The device according to claim 2, wherein the thickness of the first magnetic pole piece in the axial direction of the shaft is smaller than the thickness of the second magnetic pole piece. 6 The thickness of the first magnetic pole piece is approximately 0.5 to 1 mm (approximately 20
5. The apparatus of claim 5, wherein the filtrate is in the range of 40 mils). 7 The thickness of the second magnetic pole piece is approximately 1.3 to 2 mm (approximately 50
80 mils). 8. Claim 2, wherein the permanent magnet structure includes a non-magnetic thermally conductive member in thermal conductive relationship with the second magnetic pole piece to reduce the temperature of the magnetic fluid in the gap between the second magnetic pole pieces. 8. The device according to any one of items 7 to 7. 9. The thermally conductive member is a sheet-like member that is in contact with the outer wall of the second magnetic pole piece, and one end of which extends close to one end of the second magnetic pole piece and the magnetic fluid in the gap therebetween. Apparatus according to claim 8.