The invention relates to a bearing as stated in the introductory part of claim 1 with permanent magnetic elements, particularly to take axial forces, e.g. in a hubless propeller for ships, a turbine of an electric generator or a pump.

Background It is desirable to use bearings with permanent magnetic elements or passive magnetic bearings (PM bearings) for various purposes for which such bearings have not previously been suitable. An example is thrusters for ships. Water lubricated slide bearings have been proposed for such propellers, which can endure the axial forces during operation. It is desirable to design a bearing with permanent magnets, which can be used alone or combined with a water lubricated slide bearing for this and similar purposes, in which the bearing is submersed in a liquid.

A passive magnetic bearing with permanent magnets (PM) is known, primarily in connection with active magnets, mostly used in small applications with electrical machines and high rotational speed and normally in radial direction. Magnetic bearings are also used in connection with flywheels and rotating energy storing devices. This presents other problems than in the present application, wherein the rotational speed is relatively low and the area available for the magnets are relatively large.

The publication WO99/37912 describes a corresponding bearing with advantages and disadvantages. This document relates to a device with pulling and actively controlled electro magnets.

The publication WOO 1/84693 describes a passive magnetic axial/radial bearing with statically and rotating PM arranged with alternating polarities. In this case, a ferromagnetic element is rotating with the rotor.

US patent specification 5,894,181 describes a passive magnetic axial/radial bearing with axially rotating PM, wherein the bearing is a compromise between axial and radial rigidity balancing each other. Objects The main object of the invention is to create a bearing which primarily is handling axial forces. The bearing should provide an optimum of repulsive power and rigidity, which counteract each other. The bearing should be adaptable to different tolerance requirements.

The Invention The invention is stated in claim 1. A larger number of grooves and magnet sections will provide a more rigid bearing than one groove and one magnet ring.

The bearing is a passive magnetic bearing which e.g. can be used for a rotating part of machinery with no axle. The bearing has a stationary and a rotating part. The magnets are arranged as rings, and are arranged mutually repulsive. Normally pulling magnets are used because the attraction between unequal poles is larger than the repulsive force between poles at the same distance. Magnets with axial magnetizing in the same direction are arranged in concentrically grooves in a ferromagnetic material, for alternating N/S polarity, which provides a compressing of the flux and thus larger repulsive magnetic force. This will provide a larger rigidity of the bearing. The magnets to be placed in the ferromagnetic material are calculated in regard of width of the groove, space and depth, relative to the specification of the magnets, to provide optimum repulsion and rigidity. The design and the arrangement of the magnets of the bearing are essential to the force to be accommodated by the bearing, as well as the weight and length of the bearing. One of the advantage of the bearing is that the gap can be made optimal, by the use of FEM analysis relatively to the medium concerned and the load, to reduce viscous losses. A further advantage of permanent magnets relatively to other bearing materials in the proposed area of use, is the low costs of permanent magnets relatively to composite materials used for water lubricated bearings.

The difference to prior art from WO99/37912, WO01/84693 and US 5,894,181 is among other the passive system of the present invention, based on permanent magnets with repulsive polarity, which are embedded in grooves in a ferromagnetic material generating repulsive forces over the gap and thus compressing the flux density. The invention provides a higher rigidity of the bearing. Further details of the invention are stated in the description of an example.

Example The invention is described below with reference to the drawings, in which Figure 1 shows a schematically section through a first embodiment of an annular bearing according to the invention, Figure 2 shows a plane view of an annular magnet loop of the bearing of figure 1, Figure 3-5 show sections through three further alternatives of annular bearings according to the invention, Figure 6 shows a perspective view of a partly sectioned electrically powered propeller with circumferential drive, with a bearing according to the invention, while Figure 7 shows a force distance diagram of an embodiment of a bearing according to the invention, this bearing been shown in section above the diagram.

The bearings of the Figures 1 and 3 - 5 are shown with open gap a, which will be remarkably reduced when loaded.

Figures 1 and 2 shows a bearing 11 with two bearing rings 12, 13, where the lower bearing ring 12 is fixed with a vertical axis 14. The upper bearing ring 13 will thus carry the load. Each bearing ring 12, 13 comprises a ring 15 of soft iron with an annular groove 16 with rectangular section, which is providing a base or yoke 17. In this annular groove 16 a series of magnet segments 18, in the example sixteen, are embedded, e.g. by gluing. The magnet segments 18 may be suitable permanent magnets, e.g. of sintered neodymium-iron-boron alloy or samarium-cobalt alloy.

A basic parameter for dimensioning the width of the axial flanges 19 and 20 of soft iron, provided by the annular groove 16, is considered to be a closely below saturation state.

Figure 3 shows a section through an alternative embodiment, in which two bearing rings 22, 23 each has two grooves 24, 25 for reception of segments 26 of permanent magnets like at the grooves 16 of the embodiment in Figures 1 and 2. Thus three flanges or webs 27, 28, 29 of magnetic conducting soft iron are provided. The load carrying capacity will increase with reduction of the width of the magnetic conductive material, down to a limit whereat the material is saturated.

Figure 4 shows a further embodiment with two bearing rings 30, 31, each with three parallel annular grooves 32, 33, 34. In addition to an inner web 35, and two intermediate webs 36, 37 all ending in then plane of the magnet segments, the upper bearing ring 31 has an external flange 38 extending down external to the lower bearing ring 30, to provide an air gap 39 which is the gap of the bearing at nominal load. This bearing will have a certain radial rigidity.

Figure 5 shows two parallel series of permanent magnets, with a lower bearing ring 40 and an upper bearing ring 41. The lower bearing ring 40 has a frustro conical bearing face with two annular grooves 42, 43, with magnet segments as described for Figure 3. The upper bearing ring 41 provides a mating face facing downward and inward and having two annular grooves 45, 46 with magnet segments 47. This embodiment creates a bearing with radial stability and rigidity.

Figure 6 shows an embodiment of a bearing according to the invention for journaling an electrically powered propeller 48. The propeller 48 is surrounded by a cylindrically tubular housing 49 with an upward protruding, centrally located connecting piece 50, which gives access for electrical cables and for mounting to a ship with a mounting flange 51.

At each end of the housing 49, an annular groove 52 is provided adjacent to the propeller 48. At the bottom of the annular groove 52, a bearing ring 53 according to the invention is arranged, e.g. as shown in Figure 3. The bearing ring 53 has a collar 54 of soft iron, with two magnet rings 55, 56 provided of magnet segments in annular grooves. Symmetrically to the bearing ring 53 is an identical outer bearing ring 57. The bearing ring 53 is secured to the propeller 48 with screws at the bottom of the annular groove 52.

A corresponding bearing ring is arranged at the other end of the tubular housing 49. Each of said two bearing rings is kept mounted by a locking ring 58 which is forcing the outer bearing ring 57 against a recess 59 in the annular groove 52. The locking rings 58 are mounted with series of securing bolts around the circumference. The bearing rings may be covered by a coating preventing corrosion when the bearing is used submerged in water.

In an example, the invention was designed for a 10OkW thruster propeller installed on a ship. The requirement for the bearing was the ability to take a propulsive force of 15000 kp, with a distance of 2 millimetre between the bearing faces. The dimensions of the bearing were:

Inner diameter 0, 60 meters Outer diameter; 0,70 metres Maximum height 0,70 metres Average height 0,05 metre Average circumference 2,04 metre

Permanent magnets of a neodymium alloy were used.

Modifications Multiple rows of more magnets glued end to end in the annular groove than shown may be used. The annular magnets can be assembled of fewer or more segments then shown in the examples. The webs or teeth between and on the side of the magnet segments of the examples are shown with an end flush with the magnet rings. These webs and end flanges can be protruding slightly from the permanent magnets, to take up shocks and to compensate for inaccuracy in the assembly. These can then preferably be provided as a covering of a composite material.

In Figure 7 a force distance diagram is shown for an embodiment of a bearing according to the invention shown in section above the diagram. The distance is stated in millimetre and the force in kilopond (1 kp = 9,8 Newton). A double acting, biased thrust bearing with a first pair of cooperating bearing elements 61, 62 and a second pair of cooperating bearing elements 63, 64 connected to the same machinery is shown. The bearing elements 62, 63 will thus be rotating together. With an axial displacement of the rotating bearing elements 62, 63 to the left of the Figure of 1,3 millimetre, the bearing force of the left part of the bearing will increase to 2250 kp, while the counter force for the right part of the bearing will b reduced to 1100 kp with a distance of 3,3 millimetre, The bearing will then be in equilibrium with an axial balance F of l l50 kp.