GB2302921A - Labyrinth and magnetic seal - Google Patents

Abstract

The seal is made up of a combination of two components. A labyrinth arrangement, and a magnetic circuit arrangement. The labyrinth arrangement having a gap 7 can, of itself, form a barrier to ingress of particulates or fluids. It can also be packed with grease to enhance this capability. The magnetic circuit is designed to entrap a magnetic fluid and has pole-pieces 2 and a magnetically permeable sleeve and provides a virtually hermetic rotating seal. This part of the seal relies on: the ability of this trapped fluid to withstand a small pressure differential and to be impervious to all gases including helium, and, the buoyancy effect induced by the magnetic fluid on non-magnetic particles, effectively expelling them from penetrating the magnetic field.

Description

SEAL BACKGROUND In many cases the problem of preventing ingress of contaminants into sensitive areas, such as roller bearings, has been adequately overcome. However, in extreme environments, typified by conveyor rollers and mining equipment, roller bearings are subject to premature failure due to ingress of contaminants.

A combined magnetic fluid and labyrinth seal has been developed that offers near total protection of bearings from particulate contamination. It is therefore, ideally suited to extreme environments where spray, wash down products, and particulates abound.

TECHNICAL FEATURES Labyrinth seals are one of the most common forms of preventing ingress of contaminants into bearings. They range from simple shields to complex multi-path components. They can be unobstructed or filled with a substance, typically a grease. They are a reasonably inexpensive and effective seal for many light duty applications.

Magnetic fluid seals have been developed for use in vacuum systems where they perform the function of providing a virtually hermetic rotating seal. These products rely on the entrapment of magnetic fluid and its ability to withstand a small pressure differential when trapped. The resulting seal is able to withstand low pressures and is impervious to all gases including Helium. This is the basic mechanism used in existing magnetic fluid sealing applications.

Whilst the pressure withstanding capability is a useful addition to the seal described in this invention it is not the underlying principle. The magnetic fluid characteristic that is used to create the expulsion seal is the magnetic buoyancy force.

This is a force that is produced on any non-magnetic particles by a magnetised magnetic fluid. Any non-magnetic particle trying to penetrate a magnetised magnetic fluid is expelled from the fluid. This principle is used for the magnetic seal described in this invention.

By combining these two separate and distinct features a seal has been invented that benefits from considerable synergy between the two techniques. The resulting seal has superior performance and offers almost total exclusion of gases, particulates, and wash down products in the most arduous of environments.

Figure 1 shows a cross section of a typical toothed magnetic arrangement.

Figure 2 shows a cross section of a typical multi-stage labyrinth seal.

Figure 3 shows a cross section of a preferred embodiment of the combined seal.

Figure 4 shows a cross section of an alternative embodiment of the combined seal.

These figures will now be described in detail commencing with figure 1. Item 1 is a source of magneto motive force (mmf), which in the preferred embodiment is a permanent magnetic with north and south poles as indicated. It can be made from any known magnetic material such as Ferrite, Alnico, Alcomax, or any of the rare earth magnets such as Samarium Cobalt or Neodymium Iron Boron. The magnet can be of sintered type or bonded type. Alternatively the mmf can be generated by an electro magnetic coil.

The mmf is directed and focused by pole-pieces, item 2, which are required to be magnetically permeable. Typical materials are steel and magnetic stainless steel. If of a corrodible material some form of protective plating, for example zinc plating, is required.

Item 3, which is commonly referred to as a shaft, provides the return circuit for the magnetic field and also needs to be magnetically permeable. To further focus the magnetic field, the surface geometry of the shaft, item 3, can be modified. In the embodiment shown this is through a set of castellation, or teeth, which cause an intense focusing of the magnetic field. Both the pressure capability and the buoyancy force are a direct function of the magnetic field intensity and it is judicious to keep this an elevated level. The profile of the toothed structure can be rectangular as shown, triangular, or any other bounded surface. One useful embodiment is a toothed structure defined by at least one pair of interweaved, clockwise and anti-clockwise, helical grooves. The number of teeth is a function of the desired performance characteristics and the available installation length.One tooth is the minium. There is no maximum. Typically two to twenty teeth are employed.

Item 4, is the magnetic fluid held in place by the focused magnetic field. There are a number of types of magnetic fluid based on water, mineral oils, silicone oils, diesters, and perfluorinated polyethers (PFPE) oils. The nature and type of magnetic fluid is not relevant to this invention and any magnetic fluid will suffice. The choice of the most appropriate depends on the application.

Figure 2 depicts a typical labyrinth seal. Item 5 is a structure that provides one side, item 6 is a structure that provides the other side. Conventionally one of the structures is stationary whilst the other revolves. Direction of rotation is unimportant and it is clear that the two structures could rotate, either in the same direction or in opposite directions. Item 7 is a filling medium, such as grease, which further restricts the passage of contaminants. The nature and type of grease is unimportant to the invention as any grease can be used, the choice being dependent on the application in question. For example, in very wet conditions, brine spray for example, a grease especially developed for such an environment would be selected.

Two labyrinth paths are depicted in figure 2. The performance of the seal is improved as the number of labyrinths are increased. The minium requirement is for one, and there is no maximum. With multiple labyrinths the spacing between the structures can be the same or varied through the entire network of labyrinths.

Figure 3 depicts a preferred embodiment of the combined seal in which the magnetic seal and the labyrinth seal are radially orientated. The figure numbers identify items shown in figures 1 and 2. Item 1, is the magnet, which in this embodiment comprises a number of Samarium Cobalt disc magnets. The number depending on the space available and the required strength of the focused magnetic field. Typically four to twenty are employed, although their is no upper limit. Item 2 are the pole-pieces and in this embodiment the toothed structure is manufactured as part of the pole pieces. For ease of manufacture these can be a multi-part assembly made from simple steel pressings of the appropriate internal and external diameters and widths. Item 3 is a magnetically permeable sleeve that provides the return path for the magnetic field.It is supported on item 6, which in this embodiment is manufactured from a low coefficient of friction material, such as acetal. The use of such a material reduced the problems associated with contact of the rotating and non-rotating parts. Typically, in conveyor applications, body item 8 rotates and a flanged collar, item 6, is stationary.

The labyrinth seal is achieved using a shield, item 5, in conjunction with item 6.

Item 5 also confers rigidity, strength, and serves to define an external face of the combined seal. The gap in the labyrinth, item 7, is filled with appropriate grease. Possible contact points, 10 and 11, are lubricated to reduce friction and in the case of the contact point at 10 to provide an additional labyrinthine structure. It is a preferred engineering technique to fabricate the labyrinth within the structure of existing parts as shown in figure 3.

The assembly is positioned on a stationary shaft and held by means of O-rings, item 9. Additional O-rings, item 12, locate the body, item 8, relative to a conveyor roller, (not shown), fitted with a roller or ball bearing to be protected by the seal. These O-rings also prevent contamination across the location points and their inherent friction prevents relative rotation of the body, item 8, to the roller or of the flanged collar, item 6, to the conveyor roller shaft. The nature and type of these O-rings are unimportant to the invention.

Axial and radial location of the assembly is achieved by appropriate location against the bearing that the seal is protecting. It is therefore necessary to ensure that the maximum radial motion within the bearing can be accommodated by the gap where the magnetic fluid, item 4, is held. It has been found that a gap of 0.25 mm is adequate.

Closure of this gap is not possible owing to the fit at points 10 and 11.

Assembly on, for example a conveyor roller, is simply by means of a push fit. The O-rings allow for the assembly to be self centering on the shaft and the conveyor roller body. The body, item 8, and the flanged collar, item 6, can be of any material providing the requirements of the magnetic circuit is achieved. That is, the surrounding structure is non-magnetic and a return magnetic path is available. Consequently alloys, plastics, and any non-ferrous material is suitable for the body.

Figure 4 depicts a preferred embodiment of the combined seal in which the magnetic seal and the labyrinth seal are axially orientated. Item 1 is the magnet which is supported in a non-magnetic body, item 6. Item 6 is so formed as to provide one part of the labyrinth adopting the engineering principle of achieving maximum benefit from minimum number of parts. . It is required to be non-magnetic to ensure the mmf is directed along the toothed structure. Item 5, a double flanged collar, defines the teeth and other part of the labyrinth. It is manufactured from two halves to aid assembly. The magnetic fluid, item 4, is located between and around the teeth. Item 7, is the restricted path of the labyrinth. As in the previous embodiment, items 9 and 12, are O-rings that locate the seal assembly to stationary shaft and the conveyor roller body respectively.

Embodiments can be provided that are combinations of axial and radial arrangements. The magnetic fluid seal can be axial and the labyrinth radial and vica versa. Additionally, a labyrinth can be integrated into the seal at either end of the magnetic seal for additional protection.

The main objective of this invention is the prevention of contamination of bearings and it is therefore convenient to make the internal diameter and the external diameter compatible with standard bearings arrangements.

Both the body, item 8, and/or the shaft, item 6, may be part of an existing component and the combined seal may be built into equipment as opposed to being a self-contained unit. All that is necessary is that adherence to the requirement of the magnetic circuit is achieved. That is the surrounding structure is non-magnetic and a return magnetic path is available.

Depending on the number of teeth in the magnetic seal, the seal will be able to withstand a certain level of pressure. This capability is not a prerequisite to the invention but it does confer advantages. Many conveyor rollers are sealed units and, during thermal cycling, pressure gradients can be developed across the bearing and any installed conventional seal. Typical pressure differentials are in the order of 0.5 bar. If the pressure gradient is positive, grease can be forced out of the bearing and out of any conventional labyrinth seal. Under a negative pressure gradient contaminants can be sucked into the bearing. The construction of the magnetic seal is preferably such that the pressure withstanding capability is in the order of 0.75 bar which prevents both of these eventualities.

Claims (16)

1 A combination of a labyrinth seal and a magnetic fluid seal for use in the protection of a bearing.

2 A combination as claimed in claim 1, wherein both seals are provided between relatively rotatable parts which are spaced apart predominantly radially with respect to the axis of rotation.

3 A combination as claimed in claim 1, wherein both seals are provided between relatively rotatable parts which are spaced apart predominately in the axial direction relative to the axis of rotation.

4 A combination as claimed in claim 1, wherein either seal is provided between relatively rotatable parts which are spaced apart predominately radially or predominantly axially relative to the axis of rotation.

5 A combination as claimed in any preceding claim, wherein the magnetic seal is constructed to withstand a pressure differential up to 0.75 bar.

6 A combination as claimed in any preceding claim, wherein the labyrinth of the labyrinth seal is empty or grease-filled.

7 A combination as claimed in any preceding claim, wherein the labyrinthine structures are an integrated feature of other structural components.

8 A combination as claimed in any preceding claim, wherein the magnetic seal structures are an integrated feature of other structural components.

9 A combination as claimed in any preceding claim, wherein both the labyrinthine seal and the magnetic seal structures are derived from existing structural components.

10 A combination as claimed in any preceding claim, wherein self-centering means are provided for mounting the combination on a shaft so that the combination is centred on the axis of the shaft.

11 A combination as claimed in any preceding claim, wherein self-centering means are provided for mounting the combination in a body so that the combination is centred on a predetermined axis of the body.

12 A combination as claimed in any preceding claim, wherein of construction and size such as to be compatible with a standard roller bearing.

13 A combination as claimed in any preceding claim, comprising a seal member which defines part of the labyrinth seal and part of the magnetic fluid seal.

14 A combination as claimed in any preceding claim 10, wherein said seal member is apertured to receive a shaft fitted with the bearing to be protected by the combination.

15 A combination as claimed in claim 1, substantially as herein described with reference to figure 3 or figure 4 of the accompanying drawings.

16 A bearing when fitted with a combination as claimed in any preceding claim.