WO2003087636A1

WO2003087636A1 - Magnetic seal and bearing arrangement

Info

Publication number: WO2003087636A1
Application number: PCT/GB2003/001533
Authority: WO
Inventors: Doug Brooks
Original assignee: Advanced Fluid Systems Limited
Priority date: 2002-04-12
Filing date: 2003-04-10
Publication date: 2003-10-23
Also published as: AU2003224260A1; GB0208556D0

Abstract

The present invention relates to a magnetic seal and bearing arrangement which comprises a shaft (2) rotatable within a housing (12), a magnetic fluid-based seal assembly (10) arranged to form at least one seal of magnetic fluid between the shaft (2) and the housing (12), and a first air bearing (8a) which in use radially supports the shaft (2) in order to minimise unwanted radial motion of the shaft (2). The arrangement preferably includes a second air bearing (8b, 36, 38) which axially supports the shaft.

Description

MAGNETIC SEAL AND BEARING ARRANGEMENT

The present invention relates to a magnetic seal and bearing arrangement, particularly, but not exclusively to an arrangement which is subject in use to a pressure differential across the seal.

The use of a magnetic seal around the rotating shaft of a machine tool (often referred to as a "feedthrough"; or a "drivethru" where the seal/shaft arrangement includes an integral motor and speed sensor) offers a number of advantages over conventional sealing arrangements. Specifically the seal does not wear and therefore does not generate contaminating particles and the seal is extremely effective from indexing to very high speeds. These benefits are particularly desirable where the working environment must be kept free from contamination and/or the working environment is maintained under vacuum. Such feedthroughs/drivethrus are commonly used in the semiconductor processing and optical deposition industries.

In a conventional magnetic seal arrangement, the required gap between rotating and stationary elements (e.g. a shaft within a housing) is maintained by roller bearings; either deep groove radial or angular contact bearings. However, bearings with rolling elements are limited in their control of unwanted axial and radial motions of the shaft ("run-out"). Such motions can be the source of poor product yield in many . semiconductor processing and optical deposition environments. It is an object of the present invention to provide an improved magnetic seal and bearing arrangement which obviates or mitigates one or more of the disadvantages of the known magnetic seal and bearing arrangements.

According to the present invention there is provided a magnetic seal and bearing arrangement comprising:-

(i) a shaft rotatable within a housing,

(ii) a magnetic fluid-based seal assembly arranged to form a seal of magnetic fluid between the shaft and the housing, and

(iii) a first air bearing which in use radially supports the shaft whereby to minimise unwanted radial motion of the shaft.

As an alternative to roller bearings, air bearings have been developed for ultra high precision machining operations. However, to the best of the inventor's knowledge, air bearings have never been proposed for use with a magnetic fluid-seal assembly.

Preferably, said arrangement also comprises a second air bearing perpendicularly disposed to the first air bearing which in use axially supports the shaft whereby to minimise unwanted axial movement of the shaft.

Preferably, said arrangement comprises a motor and shaft-rotation speed sensor mounted within the housing (such arrangement commonly referred to as a "Drivethru").

The present invention also resides in a machine tool incorporating the seal and bearing arrangement of the present invention. An embodiment of the invention will now be described with reference to the accompanying drawings in which:-

Figure 1 is a longitudinal section through a magnetic seal and air-bearing arrangement in accordance with the present invention,

Figure 2 is a cross section through the arrangement of figure 1 , and

Figures 3 and 4 are detail views of different regions of Figure 1.

Referring to the drawings a drivethru generally comprises a rotatable shaft 2, a motor 4 (servomotor), a speed sensor 6 (encoder), an air-bearing component 8 and a magnetic seal assembly 10, all contained within a non-magnetic two-part housing 12. A first end 14 of the housing 12 is adapted to be secured to a vacuum chamber or other working environment (not shown). The end of the shaft 2 extending out of the housing 12 is fitted with a work tool (not shown).

The magnetic seal assembly 10 is located at a first end region of the housing 12 and comprises a plurality (e.g. 20) permanent disc magnets 16, in this case neodymium iron boron magnets arranged in a circle and sandwiched between a pair of annular, magnetically permeable (in this case stainless steel) pole pieces 18 which are mounted around a first region 2a of the shaft 2 and which focus the magnetic field. The shaft 2 is also magnetically permeable and provides the return circuit for the magnetic field. Each of the pole pieces 18 has an annular stepped region on its inner surface, such that a first inner surface region 18a of the pole piece 18 is spaced from but in close proximity to the shaft 2 whereas a second inner surface region 18b is spaced further from the shaft 2. In addition, the first inner surface region 18a of each pole piece 18 is provided with annular castellations 20 (Figure 3). The outer surface region of each pole piece 18 is provided with a pair of mutually interconnected annular cooling fluid channels 22. The pole pieces 18 are sealed within the housing 12 by means of O-ring sealing elements 24. A first end cap 26 is threadingly engaged with the first end 14 of the housing 12 and serves to locate the magnetic seal assembly 10 within the housing 12. A synthetic hydrocarbon based magnetic fluid is retained between the pole pieces 18 and the shaft 2 (described in more detail below).

At an end of the shaft 2 remote from the magnetic seal assembly 10, the motor 4 is mounted to the shaft 2 and is electrically connected to the encoder 6. The encoder 6 comprises a read head 6a (fixed in the housing 12) and an encoder disc 6b (mounted for rotation with the shaft 2). The shaft 2 has a region 2b of relatively narrow diameter thereby defining with the housing 12 a chamber 27 in which the motor 4 is accommodated, and an even narrower end region 2c defining with the housing 12 a chamber 28 in which the encoder 6 is accommodated. The encoder 6 is unconventional in that it is not provided with its own bearings, but instead relies on an air bearing (described below) to correctly position the read head 6a axially and radially relative to the encoder disc 6b. The second end 30 of the housing 12 is sealed by a second endcap 32, a connector 34 being provided therein for an electrical supply (not shown).

The air bearing component 8 is in the form of a tubular sleeve 8a having a collar 8b at its end closest to the magnetic seal assembly 10 and is a close fit around the shaft 2 (thereby providing a radial air bearing in use) and extends towards the second end 30 of the housing 12. The air-bearing collar 8b is seated on an annular abutment surface 12a provided on the. housing 12. The shaft 2 is also provided with a collar 36 which is closely spaced from the air-bearing collar 8b on the opposite side of the air- bearing collar 8b to the abutment surface 12a (thereby providing a first part of an axial-air bearing in use). An annular spacer 38 is located on an opposite side of the shaft collar 36 to the air-bearing collar 8b (thereby providing a second part of the axial air bearing in use), the air bearing collar 8b therefore being generally located in the annular space between the spacer 38 and the air-bearing collar 8b. The spacer 38 is fixed in position by an annular retaining plate 40 which is bolted in position (not shown). The retaining plate 40 is provided with a small annular flange 41 on its inner side. The shaft 2 is also provided with an annular flange 42, the two flanges 41 ,42 being closely spaced whereby to form a single convolution labyrinthine seal 44 between the air bearings and the magnetic seal assembly 10. It will be-understood that in other embodiments multiple flanges may be provided to form a multiply- convoluted labyrinthine seal.

In order to assist cooling of magnetic fluid, the arrangement includes a cooling fluid circuit. A cooling fluid inlet drilling 46 is provided along the length of the housing 12 and is in communication with an annular chamber 48 around the pole pieces 18, which is in turn in communication with the channels 22 in the pole pieces 18. The cooling fluid circuit is completed by a cooling fluid outlet drilling 50 which runs through the housing 12 parallel to the inlet drilling 46 a small angular spacing therefrom.

In order to provide pressurised air for the air bearings, the arrangement also includes an air flow circuit. An air inlet drilling 52 is provided through the housing 12 and a pair of parallel air exhaust drillings 54 are provided along the length of the shaft 2. The upstream opening 56 of each exhaust drilling 54 is perpendicular to the shaft 2 and is located between the shaft collar 36 and the shaft flange 42 and faces the annular spacer 38 adjacent the air-bearing collar 8b. The downstream end of each exhaust drilling 54 opens into the motor chamber 27. A series of radial drillings (not shown) are provided from the inlet drilling 52 through the housing 12 and the air-bearing tubular sleeve 8a such that the inlet drilling 52 is in communication with the clearances between the air bearing sleeve 8a and the shaft 2, the spacer 38 and the shaft collar 36, and the shaft collar 36 and the air-bearing collar 8b. The clearances are in communication with the upstream openings 56 of the exhaust drillings 54.

Seals (such as nitrile rubber or Viton [TM] O-rings) 58 are provided between stationary components where necessary to prevent unwanted flow of air or cooling fluid during operation of the drivethru.

In use, the shaft 2 is rotated by the motor 4 and the speed of the shaft 2 is monitored and accurately controlled by the encoder 6. An annular magnetic seal is formed between the shaft 2 and the pole pieces 18. The stepped region in the pole pieces 18 focuses the magnetic flux so that the magnetic fluid is drawn into and maintained in the space between the shaft 2 and the first regions 18a of the pole pieces 18, thereby resulting in two distinct annular regions of magnetic fluid. The two regions of magnetic fluid, which can each be regarded as an hermetic liquid O-ring seal, will resist any attempt to displace the magnetic fluid and so the seals will maintain a pressure differential. During operation it will be understood that viscous shearing of the magnetic fluid can generate significant heat which must be dissipated in order to minimise differential thermal expansion in the drivethru which might affect optimum operation of the air bearings. This is a particular problem when air bearings are employed since they permit very high rotation speeds. Heat generation can be reduced by using a magnetic fluid of low viscosity, but additional cooling is still required. Cooling is effected by passing water through the cooling circuit. Water enters through the cooling fluid inlet, passes around the outside of the pole pieces 18 and into the channels 22 in the pole pieces 18 and returns through the cooling fluid outlet drilling 50. Dissipation of heat from the magnetic fluid through the pole pieces 18 is aided by the castellations 20 on the pole pieces 18 which provide a large surface area (Figure 3).

At the same time, compressed air (about 90 psi) is fed into the air flow circuit. Pressurised air passing between the air-bearing sleeve 8a and the rotating shaft 2 via the inlet drilling 52 and the radial drillings (not shown) serves as a radial air bearing for the shaft 2. Pressurised air passing between the air-bearing collar 8b and the shaft collar 36 and between the shaft collar 36 and the spacer 38 serves as an axial air bearing which overcomes the thrust load due to atmospheric pressure acting over the area of the magnetic fluid seal. The air bearings are low friction non- contacting bearings which can support high radial and axial loads. Thus, it will be understood that accurate positioning of the shaft 2 with minimal runout is maintained by the radial and axial air bearings. Exhaust air is vented through the exhaust drillings 54 at a reduced pressure. Due to the large cross-sectional area of the upstream openings 56 relative to the labyrinthine seal 44, air preferably exits through the outlet drillings 54. It will be understood that the above-described embodiment may be modified in a number of ways. The following is a non-exhaustive list of features, any one or more of which may be applied to the above described embodiment. It will also be understood that the following features have general applicability to embodiments of the invention other than that described above:-

1. One or both pole pieces may have more than one region which focuses the magnetic field, thereby forming more than two magnetic fluid sealing regions. It will be understood that the more seals that are formed, the greater the pressure differential that the seal withstand will be, but the less compact the design will be.

2. The shaft may be shaped in the region of the pole pieces to focus the magnetic field. This may be instead of or in addition to the shaping of the pole pieces.

3. The labyrinthine seal between the air bearings and the magnetic seal can be omitted (particularly if there are a large number of magnetic fluid sealing regions) or substituted by a different seal design. Where the seal is omitted, a cut-out mechanism may be provided to protect over- pressurisation of the magnetic seal. For example, a biased (e.g. spring- loaded) sealing element for isolating the air bearings from the magnetic seal can be provided in the pressurised air flow, the sealing element being held in a non-sealing position against the bias during normal operation by virtue of the pressure differential across the sealing element, but biased into a sealing position if the pressure differential is reduced (for example if the air exhaust becomes blocked). 4. Other permanent magnets may be used including Ferrite, Almco and Alcomax or other rare earth magnets such as samarium cobalt. The magnets may be sintered or bonded. Electromagnets can also be used.

5. Elastomeric O-rings are permeable and outgas when used in vacuum systems. Thus, particularly where the O-rings are likely to be subject to vacuum, they may be replaced by metal C-rings or O-rings. In this context it should be noted that one of the pole pieces will always be on the working environment side of the magnetic seal. As an alternative to metal ring seals, it is possible to weld or otherwise permanently mount that pole piece in the housing.

6. The pole pieces may be zinc-plated to improve durability and prevent corrosion.

7. The magnetic fluid may be water-based or based on mineral oil, silicone oil, diesters, or perfluorinated polyether oils.

8. The axial air-bearing is optional, particularly where there is no pressure differential across the magnetic seal.

9. In non-vacuum environments, air bearings may be provided on both sides of the magnetic seal.

10. The air bearing may be sandwiched between a pair of magnetic seals. 1 1. Additional cooling (liquid or air) may be provided for the air bearings.

12. The cooling circuit may be used as a heating circuit. This is particularly important in cold operating environments e.g. aircraft at altitude.

Claims

1. A magnetic seal and bearing arrangement comprising:- (i) a shaft rotatable within a housing,

(ii) a magnetic fluid-based seal assembly arranged to form at least one seal of magnetic fluid between the shaft and the housing, and (iii) a first air bearing which in use radially supports the shaft whereby to minimise unwanted radial motion of the shaft.

2. An arrangement as claimed in claim 1 , additionally comprising a second air bearing perpendicularly disposed to the first air bearing which in use axially supports the shaft whereby to minimise unwanted axial movement of the shaft.

3. An arrangement as claimed in claim 1 or 2 additionally comprising a motor and shaft-rotation speed sensor mounted within the housing.

4. An arrangement as claimed in claim 3, wherein the speed sensor comprises a non-rotating first element mounted in the housing and a second element mounted for rotation with the shaft.

5. An arrangement as claimed in claim 4, wherein the correct relative axial and radial positioning of the first and second elements of the sensor maintained by the second air bearing when present.

6. An arrangement as claimed in any preceding claim, wherein a labyrinthine seal is provided between the magnetic seal assembly and the first air bearing and the second air bearing when present.

7. An arrangement as claimed in any one of claims 1 to 5, additionally comprising a cut-out mechanism.

8. An arrangement as claimed in claim 7, wherein the cut-out mechanism comprises a biased sealing element for isolating the air bearings from the magnetic seal, said sealing element being positioned in the pressurised air flow of the first air bearing and held in a non-sealing position against the bias during normal operation by virtue of the pressure differential across the sealing element, and biased into a sealing position if the pressure differential is reduced

9. An arrangement as claimed in any preceding claim, wherein the magnetic seal assembly comprises a plurality of pole pieces, said pole pieces being made from neodymium-iron-boron, ferrite, Almco, Alcomax or samarium cobalt.

10. An arrangement as claimed in claim 9, wherein the pole pieces are zinc plated.

1 1. An arrangement as claimed in any preceding claim, wherein a second magnetic sealing assembly is provided, the first air bearing being located between the first and second magnetic sealing assemblies.

12. An arrangement as claimed in any preceding claim, wherein drillings are provided in the housing to form a fluid flow passage which in use serves as a heating or cooling fluid circuit.

13. An arrangement as claimed in claim 12, wherein the fluid flow passage is in communication with the magnetic seal assembly.

14. - A machine tool incorporating the seal and bearing arrangement of any one of claims 1 to 13.