GB2093930A - Improved bearing particularly adapted to devices for extracting energy from waves on a fluid - Google Patents

Abstract

A bearing (e.g. for use on a Salter duck) which comprises two surfaces (27, 32) between which relative movement occurs and between which a liquid film exists, said surfaces being held apart by magnetic repulsion between magnets (25), resiliently supported from one member (2) of the bearing via liquid-filled hollow bodies (22), and magnets (30) supported by the other member (1) of the bearing. <IMAGE>

Description

SPECIFICATION Improved bearing particularly adapted to devices for extracting energy from waves on a liquid This invention relates to an improved bearing which has particular, but not exclusive, application to devices for extracting energy from waves on a liquid, and more particularly but not exclusively to the Salter Duck, the subject of British Patent No. 1,482,085 (United States Patent No. 3,928,967) which is incorporated by reference herein.

One form of bearing suitable for use in a marine environment comprises a plurality of extensible hollow bodies disposed in the gap between relatively turnable members, each body being attached to one member, filled with water and supporting a bearing surface for engagement with the other member. The hollow bodies have restricted outlets for water fiow. This bearing is described in UK Patent Application No. 2062130.

According to one aspect of the present invention there is provided, a bearing for supporting a first member on a second member while accommodating relative movement between said members, said bearing being accommodated in a gap between the said members and including a thin film of liquid disposed between confronting surfaces between which the relative movement occurs, one of said surfaces being mounted on one of the members for resilient movement towards and away from the said one member and said one confronting surface being urged away from the other confronting surface in the direction to increase the thickness of the liquid film by passive magnetic repulsion.

Normally the film of liquid is an annular film between circular cylindrical surfaces and the magnetic repulsion is'generated between concentric rings of plate magnets.

Preferably the resilient movement of said one confronting surface is effected by supporting said one confronting surface from said one member by a resilient structure defining a plurality of adjacent hollow bodies having liquid inlets/outlets in said one confronting surface which communicate with said film of liquid.

According to a further aspect of the invention, there is provided a bearing for location in a liquidfilled gap defined between confronting surfaces of two members capable of relative reciprocal movement, which comprises a plurality of liquidfilled hollow bodies located in the gap which are mounted on one of said members and are extensible/compressible in the transverse direction between the members across the gap and have restricted inlets/outlets for liquid flow to/from the interior of the hollow bodies to accommodate changes in the dimension of the gap in the transverse direction thereof, a plurality of first magnets supported from said one member by the hollow bodies and a plurality of second magnets supported by the other of said members to closely confront the first magnets and magnetically repel the latter.

Suitably the hollow bodies are disposed in a close-packed array in the said gap so that there is no possibility for unrestricted flow of liquid within the gap, the only possibility being for liquid flow into and out of the individual hollow bodies as the gap shape varies under loads.

The hollow bodies may have concertina-type walls and can be of any desired cross-section (e.g.

rectangular, circular or hexagonal). To facilitate close packing, helical corrugations can be provided in each body, adjacent bodies having the corrugations of opposite hand.

Suitably the restricted inlets/outlets to the hollow bodies are provided in a membrane which is disposed closely adjacent to said other member, and is used to support said first magnets.

Desirably, where the bearing acts in the annular gap between a central circular cylindrical support and a surrounding member with a circular cylindrical bore therein, the hollow bodies are secured within the bore directly, or indirectly via a resilient layer, to the surrounding member so that their extensible direction is radial of the gap and are secured to a flexible cylindrical membrane which closely surrounds the support.The second magnets can then be formed as rings around the outer surface of the cylindrical support and the first magnets can be formed into rings embedded in the flexible cylindrical membrane closely surrounding the outer surface of said cylindrical support, said membrane being apertured between the rings to provide inlet/outlet ports to individual ones of a close-packed array of extensible/compressible hollow bodies interposed in the annular space between said membrane and the outer wall of the cylindrical bore formed in said surrounding member.

In a typical case, using the bearing in a Salter Duck, each ring of magnets comprises an array of individual plate magnets arranged end-to-end, the north pole of each magnet in a ring facing in the same radial direction as every other magnet in that ring. Each magnet in each ring of magnets on the cylindrical support can have the same pole facing towards the gap, but alternatively, each magnet in each alternate ring of magnets on the cylindrical support has its north pole facing towards the gap and each magnet in all the other rings of magnets on the cylindrical support has its south pole facing towards the gap.

Preferably, the first and second magnets are ceramic magnets (e.g. anisotropic barium or strontium ferrite magnets) since these have high coercivity.

The clearance between the membrane and the support preferably does not exceed 10 mm (i.e.

from 0 to 10 mm) the hole(s) opening from this clearance into each hollow body being of crosssectional area which is less than 25% of the cross-sectional area of the respective hollow body. Preferably each hollow body has a radial dimension which is less than its transverse dimension.

To prevent axial displacement of the flexible cylindrical membrane along the cylindrical support, and thereby ensure axial alignment of said rings of first and second magnets, locating means can be provided on the support which coact with rollers supported by the said membrane.

Thus according to a still futher aspect of the present invention there is provided a bearing for supporting a relatively turnable first member from a surrounded second member in a marine environment, which bearing is located in an annular gap between said members, the bearing comprising a plurality of extensible hollow bodies disposed in the gap, each of said bodies being attached to one of said members, being adapted to fill with water in use via at least one restricted inlet/outlet for water flow and supporting a membrane closely surrounding the second member, between which membrane and the second member the relative turning movements occur, said membrane being supported in spaced relation to said second member by opposed magnets which repel each other.

The invention also extends to a device for extracting energy from waves arising on a body of water which includes a bearing as claimed herein located between two members moved one relative to the other by the waves, and to electrical power generated by such a device.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a section of a Salter duck device for extracting energy from water waves.

Figure 2 is a schematic sectional view through one embodiment of bearing according to the invention used in the device of Figure 1, Figure 3 is a representation of the clearance versus force curves which apply in a bearing such as those shown in Figure 2 and Figure 4, and Figure 4 is a schematic isometric cross-section through a second embodiment of bearing according to the invention.

Referring to Figure 1 , the device partially submerged in a body of water, comprises a cylindrical spine or back bone 1, a Salter duck 2 and a bearing 3 interposed in the annular gap between the spine 1 and the cylindrical bore 4 of the duck 2.

In known manner, the duck 2 rocks to and fro on the spine 1 extracting energy from the waves on the body of water, the extracted energy being usefully used (e.g. to generate electrical power).

Figure 2 shows a small part of the bearing 3.

The spine 1 of cast concrete is fitted with circumferential rings of magnetic tiles 5 embedded in the concrete and enclosed by thin bands 6 of cupro-nickel shim or some other smooth marine non-fouling material.

Outside this on the duck 2 are bands of thin perforated rubber sheet 7 with mesh reinforcement having enough compliance to conform to the distortions and large scale irregularities of the spine 1 but being stiff on a local scale.

Bonded to this rubber sheet 7 is a second ring of magnets 8 with poles opposing those in the spine opposite. In Figure 2, the polarities of the rings of magnets 5 and 8 alternate in the axial direction along the spine 1, but all the magnets 5 can be similarly poled in each ring, provided there is magnetic repulsion between opposite rings of magnets 5 and 8.

Outside the second ring of magnets 8 is a layer of flexible hollow bodies or cells 9 in the form of an array of close-packed concertinas. Water can flow in or out of them through holes 10 in the rubber sheet 7 but circumferential and axial flow outside the sheet 7 is prevented by the concertina walls of the hollow bodies 9. A liquid film 11 exists in the gap between the sheet 7 and the bands 6.

Both magnet gap and hollow bodies will have spring characteristics which must be carefully chosen to provide the desired performance for the bearing 3. These are shown together in Figure 3.

The load that can be taken by the bearing 3 is the sum of the local pressure plus the spring characteristics of the magnets and hollow bodies.

As there is an easy flow path from the film 11 through the holes 10 in the rubber sheet 7, there will not be any pressure differences across the magnets 5/8. This means that any force on the duck 2 which tends to change the magnet clearance will make a corresponding change to the deflection of the concertina walls of the hollow bodies 9. Point A marks the unloaded working point of the bearing. This is chosen from knowledge of the static part of the load between the duck 2 and the spine 1 caused by mooring forces or ballasting errors. It amounts to about 0.1 5 psi (100 N/m2). The opposing force on the far side of the duck 2 is also taken into account.

If the load is increased, the magnets sustain most of the rise up to point C with little change in the volume of the hollow bodies 9. But above point C the volumes of the hollow bodies reduce and the water has to be expelled. It can choose to flow circumferentially or axially in the film 11. The distances are long, and because the adjacent hollow bodies are behaving in an identical fashion, the pressure gradients are very small. But more importantiy the clearance is closing fast, and impedance to flow depends on the inverse cube of clearance.

Finally at point D the rubber sheet 7 touches, the cupro-nickel shim 6. This is the first solid-tosolid contact. The force of contact cannot be higher than the force needed to deflect the walls of the hollow bodies, which still have plenty of water to discharge but now very little clearance to discharge it through. The deflection characteristic curve for the hollow bodies is very flat at this point D.

A bearing such as that shown in Figure 2 can withstand very large forces for times of several hundred seconds. The wave loads expected on a Salter duck will always reverse within eight seconds. It can withstand smaller direct forces indefinitely. It can tolerate distortions in geometry provided that they are not abrupt, and material can be distributed round the duck 2 to exploit the uneven load pattern. Solid-to-solid contacts will be rare events and the contact pressures will be much smaller than the bearing pressures which themselves are small.

To ensure the bearing has axial stability, means (not shown) must be provided to withstand the axial forces. Wheels can be mounted on the duck 2 to roll on the spine 1 and prevent axial displacement between the rings of magnets 5 and 8.

Deleterious marine fouling in the bearing can be prevented by the introduction of toxins or predators into the bearing. A target life in excess of 25 years can be obtained with a bearing of the kind described above.

Figure 4 shows a modified bearing which also relies on passive magnetic repulsion to augment the performance of a squeeze-film bearing.

The bearing shown in Figure 4 is designed so as to provide a very small, but finite, clearance between the outer surface of the spine 1 and the inner annular surface of the duck 2, while still allowing for quite large relative displacements and/or stress-induced deformations of both the duck and the spine. This is accomplished by bonding to the surface of the duck a compressible 'slubber' layer 20 which occupies almost the entire available volume between the duck and the spine.

The slubber bearing comprises a resilient layer 21 with significant strength in compression (i.e.

radial loading) but little in shear. Secured to the layer 21 is a honeycomb structure 22 defining a plurality of hollow cells 23. The lower surface of the cell layer is bonded to a semi-stiff sandwich 24 in which are embedded a large number of small ceramic magnets 25. The sandwich 24 includes two elastomeric sheets 26, 27, each about 1 mm thick with rings of rubber-elastic material 28 bonded between the sheets 26, 27 and the rings of magnets 25. The surface of the spine 1 is overlaid with a similar magnetic sandwich 29 in which magnets 30 are laid endto-end, forming closed circumferential rings whose polarity is such that they present a continuous north (or south) pole on their radially outer face. Further rings of non-magnetic material 31 are located between the rings of magnets 30.

The rings of magnets 30 are aligned with the magnets 25 in the lower slubber layer which also form circumferential rings, but need not be continuous since the back of the duck 2 will be subject to smaller forces than the "beak", and in back area, the slubber magnets 25 can be separated by discrete circumferential gaps. The duck and spine magnetic rings are arranged in mutual repulsion.

The outer sheet 32 of the sandwich 29 can consist of sheets c. 2 m wide (axial dimension) applied from a roll, around the circumferential length of the bearing. Each sheet can be pre tensioned and held at both edges by rails (not shown) attached to, and forming closed rings round, the spine 1. Rollers attached to the sandwich 24, at intervals therealong can engage the rails to maintain close axial alignment of the magnetic rings and ensure axial stability of the bearing.

In each flexible cell 23 will be one or more holes 33, which allow water free access into or out of the cell. In the gap between the sandwiches 24 and 29, water will be free to flow axially or circumferentially around the spine; inside each cell 23 it will be trapped, unable to flow in any direction other than out of the cell.

The spring characteristics of the repelling magnets and the compressible cells will be chosen to complement one another again as shown in Figure 3.

The load that can be sustained by the bearing will simply be the sum of the local pressure, plus the forces exerted by the magnets and cell walls; the freedom of flow in and out of the cells ensures no pressure gradients are set up between the water inside, and that outside in the bearing clearance.

The small initial force increase from A to B in Figure 3 will give rise to a limited, but significant compression of the layer 21. At B this layer is fully compressed, and this marks the unloaded working point of the bearing, chosen from a knowledge of the mooring forces and ballasting errors. This "built-in squeeze" is to allow the magnet clearance to remain relatively small when subjected to negative loads, i.e. on the side of the duck diametrically opposite to the point of loading.

As the load increases, magnetic repulsion absorbs most of the rise from B to C with the gap closing accordingly; so far there is little change in the cells volumes. Above the point C the structure 22 begins to compress; the volume of the cells 23 reduces and water is expelled through the holes 33. Once released, the water may flow circumferentially or axially round the spine-the distances in either case are long compared with the clearance, and pressure gradients between adjacent cells are small. Flow is further impeded by the narrowing of the gap, and as volume flow rate is proportional to clearance cubed, this effect becomes increasingly important.

At D the two sandwiches 24 and 29 are about to contact, with the force limited to that required to deflect the structure 22; the cells 23 still have plenty of water left to discharge, but the clearance into which they can discharge is again infinitesimally small. The spring characteristics of the hollow bodies give a very flat curve at this point.

When two permanent magnets are arranged in repulsion, each comes under the influence of a demagnetising field, this being the vector sum of the magnet's own self-demagnetising field and the field created by the presence of the other. The property which determines the field strength around (and within) a permanent magnet, and the attractive or repulsive force it can exert, is its rhagnetisation J. This vector quantity, also known as the pole strength per unit area, is not constant, but varies with the demagnetising field experienced by the magnet. To further complicate matters, removal of the externally applied field in general restores the value of J to less than its former magnitude, depending on the hysteresis properties of the particular material used. In other words, permanent magnets in repulsion tend to permanently weaken each other.

For this and other reasons, the magnets 25, 30 are preferably anisotropic barium or strontium ferrite magnets, which belong to the class of "hard" magnetic materials that have relatively low values of J, but can withstand extremely strong demagnetising fields with little ioss of performance.

The magnets are required to have the spring characteristics shown in the left-hand-graph in Figure 3, the desirable parameters being a high maximum force value combined with a sharp decline over the region CB-the latter feature is necessary when taking into account the behaviour at the opposite side of the spine. The magnetic forces must not cancel each other out.

Opposing magnets with square cross-sections of the same dimensions give the best force per unit weight at contact; with one magnet assumed infinitely long. The repulsive force is then proportional to the length of the opposing magnet. Furthermore, the force per unit weight varies inversely as the linear dimensions.

The bearing described uses relatively small magnets of square cross-section, thus giving both a high force to weight ratio at small clearances and a rapid decrease in repulsion as the clearances become large compared with the magnetic dimensions. This allows the best use to be made of magnetic material. The decision whether to opt for a homopolar (all spine magnets similarly poled) or a heteropolar arrangement (in which alternate rings are oppositely poled) depends on the relative proportions of the axial pitch and the magnetic dimensions. Both arrangements alone can be used, or a combination of both arrangements along the length of the bearing.

The method of attachment of the ferrite magnetic blocks 30 to the spine 1 is of some importance and deserves further comment. These magnets must exhibit no tendency to work free during the lifetime of the bearing.

The (duck) magnets 25, sandwiched between the two elastomeric sheets 26, 27 at the base of the slubber layer, are well secure and Figure 4 shows a similar sandwich 29-without any holes through it--overlaid on the spine.

Another arrangement is to use spine magnets with a shallow channel along the length of their upper surface and to tightly wind a thin belt made of synthetic fabric around the spine to lie in the aligned channels and thereby restrain the magnets 30 in place.

Cement can then be cast between the rings of magnets 30 and an elastomeric layer wrapped around them to complete the spine side of the bearing.

Claims (23)

Claims

1. A bearing for supporting a first member on a second member while accommodating relative movement between said members, said bearing being accommodated in a gap between the said members and including a thin film of liquid disposed between confronting surfaces between which the relative movement occurs, one of said surfaces being mounted on one of the members for resilient movement towards and away from the said one member and said one confronting surface being urged away from the other confronting surface in the direction to increase the thickness of the liquid film by passive magnetic repulsion.

2. A bearing as claimed in claim 1, in which said film of liquid is an annular film between circular cylindrical surfaces and said magnetic repulsion is generated between concentric rings of plate magnets.

3. A bearing as claimed in claim 1 or claim 2, in which said resilient movement of said one confronting surface is effected by supporting said one confronting surface from said one member by a resilient structure defining a plurality of adjacent hollow bodies having liquid inlets/outlets in said one confronting surface which communicate with said film of liquid.

4. A bearing for location in a liquid-filled gap defined between confronting surfaces of two members capable of relative reciprocal movement, comprising a plurality of liquid-filled hollow bodies located in the gap which are mounted on one of said members and are extensible/compressible in the transverse direction between the members across the gap and have restricted inlets/outlets for liquid flow to/from the interior of the hollow bodies to accommodate changes in the dimension of the gap in the transverse direction thereof, a plurality of first magnets supported from said one member by the hollow bodies and a plurality of second magnets supported by the other of said members to closely confront the first magnets and magnetically repel the latter.

5. A bearing as claimed in claim 4, in which the hollow bodies are disposed in a close-packed array within the gap.

6. A bearing as claimed in claim 4 or claim 5, in which the restricted inlets/outlets to the hollow bodies are provided in a membrane which is disposed closely adjacent to said other member.

7. A bearing as claimed in claim 6, in which said membrane supports said first magnets.

8. A bearing as claimed in any one of claims 4 to 7, in which said confronting surfaces are substantially coaxial circular cylindrical surfaces with an annular gap therebetween.

9. A bearing as claimed in claim 8, in which said second magnets are formed as rings around the outer surface of said other member which forms a cylindrical support within a bore of said one member.

10. A bearing as claimed in claim 9, in which said first magnets are formed into rings embedded in a flexible cylindrical membrane closely surrounding the outer surface of said cylindrical support, said membrane being apertured between the rings to provide inlet/outlet ports to individual ones of a closepacked array of extensible/compressible hollow bodies interposed in the gap between said membrane and the outer wall of a cylindrical bore formed in said one member.

11. A bearing as claimed in claim 9 or claim 10, in which the or each ring comprises an array of individual magnets arranged end to end, the north pole of each magnet in a ring facing in the same radial direction as every other magnet in that ring.

12. A bearing as claimed in any of claims 9, 10 or 11, in which each magnet in each ring of magnets on the cylindrical support has the same pole facing towards the gap.

13. A bearing as claimed in any of claims 9, 10 or 11, in which each magnet in each alternate ring of magnets on the cylindrical support has its north pole facing towards the gap and each magnet in all the other rings of magnets on the cylindrical support has its south pole facing towards the gap.

14. A bearing as claimed in any of claims 4 to 13, in which the first and second magnets are anisotropic barium or strontium ferrite magnets.

1 5. A bearing as claimed in any of claims 12, 13 or 14 when dependent on claim 10, in which means is provided on the cylindrical support to prevent axial displacement of the flexible cylindrical membrane along the cylindrical support and thereby ensure axial alignment of said rings of first and second magnets.

1 6. A bearing as claimed in claim 6 or any claim dependent thereon, in which the clearance between the membrane and said other member is not more than 10 mm.

1 7. A bearing as claimed in any one of claims 4 to 16, in which the inlet/outlet to each hollow body has a cross-sectional area which is less than one quarter of the cross-sectional area of the respective hollow body.

18. A bearing as claimed in any one of claims 4 to 17, in which each hollow body has a radial dimension which is less than its transverse dimension.

1 9. A bearing for supporting a relatively turnable first member from a surrounded second member in a marine environment, which bearing is located in an annular gap between said members, the bearing comprising a plurality of extensible hollow bodies disposed in the gap, each of said bodies being attached to one of said members, being adapted to fill with water in use via at least one restricted inlet/outlet for water flow and supporting a membrane closely surrounding the second member, between which membrane, and the second member the relative turning movements occur, said membrane being supported in spaced relation to said second member by opposed magnets which repel each other.

20. A bearing as claimed in claim 19, in which said first member is a Salter duck.

21. A bearing substantially as hereinbefore described with reference to, and as illustrated in Figure 2 or Figure 4 of the accompanying drawings.

22. A device for extracting energy from waves arising on a body of water which comprises a bearing as claimed in any preceding claim located between two members moved one relative to the other by the waves.

23. Electrical power generated by a device as claimed in claim 22.