WO2009139645A1

WO2009139645A1 - A self-centring sliding bearing

Info

Publication number: WO2009139645A1
Application number: PCT/NZ2009/000043
Authority: WO
Inventors: Christopher Ross Gannon; William Henry Robinson
Original assignee: Robinson Seismic Ip Limited
Priority date: 2008-05-14
Filing date: 2009-03-26
Publication date: 2009-11-19

Abstract

A bearing assembly has an upper bearing seat (210), a lower bearing seat (214) and a sliding load bearing member (221). Friction between the sliding load bearing member and said upper and/or lower bearing seat damps relative horizontal movement between said upper bearing seat and said lower bearing seat. A plurality of elastic cord portions (224a, 224b) are co-operable with the sliding load bearing member and the upper and/or lower bearing seat to centre the bearing assembly.

Description

A SELF-CENTRING SLIDING BEARING

TECHNICAL FIELD

This invention relates to sliding beatings. More particularly it relates to sliding bearings with elastic self-centring. In a preferred embodiment sliding bearings according to the invention may be used in seismic isolation, but they may be used in other applications to dampen relative movement between an item to be supported and a support surface.

BACKGROUND ART

In the field of seismic isolation the use of sliding bearings is well known. One known type of sliding bearing is a bearing assembly having upper and lower bearing seats and a load bearing sliding member between the seats, the member being able to slide relative to both seats. Examples of such bearing assemblies are in US 4,320,549; US 5,597,239, US 6,021,992, and US 6,126,136.

In another type of sliding bearing the sliding member is fixed to one or other upper or lower bearing seat. In such an embodiment the sliding member is may be a pillar projecting from the bearing seat to which it is affixed. It is usually the upper seat which is movable relative to the slider member. Examples of this type of sliding bearing are found in US 4,644,714; US 5,867,951; US 6,289,640; the embodiments shown in each of Figures 4 to 6 in US 6,021,992; and the embodiments shown in Figures 4 and 5 of US 6,126,136.

Some of the above mentioned sliding bearings have a curved bearing seat surface and a corresponding curved surface on the sliding member which provide a form of passive self- centring of the sliding member and the bearing seats. None of either types of sliding bearings mentioned above have elastic self-cen tiring.

"Self-centring" is, for the purposes of this specification, urging the sliding load bearing member and the upper and lower bearing seats to remain in or return to substantially symmetrical alignment with the longitudinal axis passing through the upper and lower bearing seats and the sliding load bearing member perpendicular to a horizontal plane. An advantage of elastic self-centring is that it provides a means to control the elastic shear stiffness of the bearing to ensure that the isolated item has a natural period which exceeds the period of the seismic event or other horizontal forces which the bearing assembly is designed to damp so as to enhance the effectiveness of the seismic isolation.

Another advantage, particularly when the sliding load bearing member is movable with respect to both the upper and lower bearing seats, is that a bearing assembly may be constructed of a reduced cross sectional area in comparison with a bearing assembly without elastic self-centring.

Our patent application PCT/NZ2004/000045 (published as WO 2004/079113) describes a bearing that uses diaphragm(s) and/ or a sleeve to provide elastic self-centring. Our patent application PCT/NZ2005/000232 (published as WO 2006/028391) describes a modified bearing that uses diaphragm(s) to provide elastic self-centring. While these bearings provide good seismic isolation for large structures such as buildings, they require large loadings to obtain useful displacement so are not well suited for supporting smaller, lighter structures such as electronics equipment.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents or such sources of information is not to be construed as an admission that such documents or such sources of information, in any jurisdiction, are prior art or form part of the common general knowledge in the art.

It is an object of at least preferred embodiments of the invention to go some way towards addressing the issues outline above, or at least to offer the public a useful choice.

SUMMARY OF THE INVENTION

The term "comprising" as used in this specification means "consisting at least in part of. When interpreting each statement in this specification and claims that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" ate to be interpreted in the same manner.

In accordance with a first aspect of the present invention, there is provided a bearing assembly comprising: an upper bearing seat, a lower bearing seat and a sliding load bearing member therebetween, said sliding load bearing member optionally being fixed to one or other of said upper and lower bearing seats, friction between said sliding load bearing member and said upper or lower bearing seat, or between said sliding load bearing member and said upper and lower bearing seats, in use, damping relative horizontal movement between said upper bearing seat and said lower bearing seat; and either: said assembly, when said sliding load bearing member is fixed to one or other of said upper and lower bearing seats, further comprising a plurality of elastic cord portions co-operable with the sliding load bearing member and the seat to which the sliding load bearing member is not fixed to urge said seat to which said sliding load bearing member is not fixed to return to or remain in a centred position relative to said sliding load bearing member and the seat to which said sliding load bearing member is fixed; or said assembly, when said sliding load bearing member is not fixed to either of said upper and lower bearing seats, further comprising a plurality of elastic cord portions co- operable with the sliding load bearing member and the upper bearing seat, and a plurality of elastic cord portions co-operable with the sliding load bearing member and the lower bearing seat to urge said seats to return to or remain in a centred position relative to said sliding load bearing member.

As used herein, an "elastic cord portion" is a flexible elongate member that is capable of returning to its initial form or state after being stretched in its elongate direction.

Generally, the cord portion's resistance to deformation will be substantially greater in the elongate direction of the cord portion than in the transverse direction of the cord portion. The cord portion will generally have a length substantially greater than its cross-sectional dimension. The cord portions could be any suitable cross-sectional shape, such as circular, elliptical, flat, rectangular, or square for example.

In one preferred embodiment, said sliding load bearing member is fixed to one of the upper and lower bearing seats, and is slidable relative to the other of the upper and lower bearing seats. In an alternative embodiment, said sliding load bearing member is not fixed to either of said upper or lower bearing seats, and is slidable relative to both bearing seats. This embodiment is preferred, as it provides greater displacement for a given bearing size.

Preferably, said elastic cord portions are connected to the upper and/or lower bearing seat(s) at or adjacent a periphery of the upper and/or lower bearing seat(s). Preferably, said elastic cord portions are connected to the sliding load bearing member at or adjacent a periphery of the sliding load bearing member. The sliding load bearing member may be substantially cylindrical in cross-section, or any other suitable shape. The sliding load bearing member may comprise a rigid peripheral portion extending radially outwardly beyond the area of the sliding load bearing member that is in contact with one or both of the upper and lower seats, and the elastic cord portions may be connected to the peripheral portion of the sliding load bearing member. In one embodiment, the rigid peripheral portion comprises a disc. In another embodiment, the sliding load bearing member may comprise a hub and a plurality of spokes.

In the embodiment in which the sliding load bearing member is not fixed to either of said upper and lower bearing seats, the elastic cord portions that are co-operable with the upper bearing seat may be formed separately from the elastic cord portions that are co-operable with the lower bearing seat. That is, there may be a plurality of elastic cord portions co- operable with the upper bearing seat that are formed from one or more elastic cords, and a plurality of elastic cord portions co-operable with the lower bearing seat that are separately formed from one or more elastic cords. Alternatively, at least some of the elastic cord portions that are co-operable with the upper bearing seat may be formed integrally with at least some of the elastic cord portions that are co-operable with the lower bearing seat.

Alternatively, there could be a single elastic cord that provides all the elastic cord portions, which may alternate from the sliding load bearing member to the upper and lower bearing seats. Preferably, in the embodiment in which the sliding load bearing member is not fixed to either of the upper bearing seat and the lower bearing seat, at least three, and preferably four or more cord portions are co-operable with the sliding load bearing member and the upper bearing seat. When four cord portions are used, preferably two of the cord portions extend transversely relative to the other two of the cord portions. Preferably, at least three, and more preferably four or more cord portions are co-operable with the sliding load bearing member and the lower bearing seat. When four cord portions are used, preferably two of the cord portions extend transversely relative to the other two of the cord portions. Additional cord portions may be provided between some or all of the cord portions. The number of cord portions in one direction may differ from the number cord portions in another direction, to vary the cord portion densities and thereby the elastic restoring force in the different directions.

When the sliding load bearing member is fixed to one or other of the upper and lower bearing seats, the elastic cord portions that are co-operable with the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed, may be formed from a single elastic cord or may be formed from a plurality of elastic cords.

Preferably, in the embodiment in which the sliding load bearing member is fixed to one of the upper bearing seat and the lower bearing seat, at least three, and preferably four or more cord portions are co-operable with the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed. When four cord portions are used, preferably two of the cord portions extend transversely relative to the other two of the cord portions. Additional cord portions may be provided between some or all of the cord portions. The number of cord portions in one direction may differ from the number of cord portions in another direction, to vary the cord portion densities and thereby the elastic restoring force in die different directions.

The elastic cord portions may be single strand, in which there is a single length of material in the cord, or alternatively may be multi-strand, in which there are multiple lengths of material that may be twisted together. The cord portions may be made from any suitable material, such as extruded rubber or a different elastomeric material for example. Differing cord portions may be made of differing materials, to provide variations in elasticity. Either the single strand or multi-strand cord portions may optionally comprise a covering layer, such as of nylon for example. That covering layer will provide resistance to dust. A multi- strand cord is preferred, as if one of the strands was to fail then the remainder of the cord would still provide sufficient elasticity and strength. The cord(s) may be of the type known as bungy cords. The strands of the cord portions could be any suitable cross-sectional shape, such as circular, elliptical, flat, rectangular, or square for example.

Preferably, the cord portions are capable of stretching to at least twice their undeformed length.

The cord portions preferably extend generally outwardly from the sliding load bearing member to the upper bearing seat and/ or lower bearing seat. That is, the cord portions preferably extend generally radially outwardly from the sliding load bearing member, at least when the upper and lower bearing seats and sliding load bearing member are in a centered position. It will be appreciated that the sliding load bearing member and bearing seats need not be circular in plan view, and such variants are intended to be encompassed by the "radially" language. Preferably, said sliding load bearing member is of regular geometrical shape in cross-section. In a preferred form, the sliding load bearing member is substantially circular in plan view, so it has a substantially cylindrical configuration. In an alternative form, the sliding load bearing member may be generally elliptical. In such a configuration, if substantially circular upper and lower bearing seats are provided and the cord portions are connected at or adjacent the periphery of the bearing seats, the cord portions extending from the ends of the sliding load bearing member will be shorter than the cord portions extending from the sides of the sliding load bearing member, and the elastic restoring force in one direction will thereby differ from the elastic restoring force in the transverse direction. Rather than being elliptical, the sliding load bearing member could be any other suitable shape that is elongated in one direction relative to the other; for example, rectangular.

Means may be provided between the radially extending cord portions to maintain spacing between adjacent cord portions. In an alternative form, the sliding load bearing member may be substantially circular in cross section or any other shape with relatively even dimensions in transverse directions, but the bearing seats may be generally elliptical in plan view. That will again provide cord portions of differing lengths, and the elastic restoring force in one direction will thereby differ from the elastic restoring force in the transverse direction. Rather than being elliptical, the bearing surfaces could each be any other suitable shape that is elongated in one direction relative to the other, for example, rectangular.

The cord portions may all be substantially the same length, cross section, elasticity, and density (number of radially extending cord portions in a given angular spacing around the sliding load bearing member) That will result in substantially the same elastic restoring force in all directions around the sliding load bearing member. Alternatively, at least some of the cord portions may vary. In a preferred embodiment, the cord portions extending in first and second generally opposite directions from the sliding load bearing member may differ from the cord portions extending in third and fourth generally opposite directions that are transverse to the first and second directions, in one or more of length, cross section, elasticity, or density That will provide one elastic restoring force in the first and second directions, and a differing elastic restoring force in the third and fourth directions By using cord portions for elastic self-centring, the restoring forces can be readily varied in different diiections by varying the cord portion lengths, cross sections, and/or densities

In a preferred embodiment, the elastic restoring force in one direction differs from the elastic restoring force in a second geneially perpendicular direction That can be particularly useful for seismic isolation and support of rack equipment where a large displacement (and relatively low elastic restoring force) may be desired for one direction, and a small displacement (and lelatively high elastic restoring force) may be desired for the generally perpendicular direction That also may be suitable for paiticular seismic sites wheie a greater elastic restoring force may be lequaed in one direction than in a generally perpendiculai diiection By way of example only, the elastic restoring force in one direction may be twice the elastic restoring foice of the geneially perpendiculai diiection That can be achieved by having the cord portions in said one direction approximately half the length of the cord portions in said generally perpendicular direction, which iesults in the cord portions in said one direction having twice the cord stiffness and thereby providing twice the elastic restoring force of the cord portions in the generally perpendicular direction.

There may be one, two, three, or more elastic cord portions extending in each direction. That is, there may be clusters of cord portions extending in each direction. The number of cord portions in the different directions may vary.

The bearing assembly is preferably suitable for seismic isolation and support, and a satisfactory amount of possible displacement, of loads of about 10 tonnes or less in most cases. The bearing assembly is preferably suitable for supporting computers or electronics equipment for example.

The bearing surfaces of said lower and upper bearing seats and the sliding load bearing member may be substantially flat. Alternatively, one or both of the bearing surfaces of said upper or lower bearing seats may be curved and the corresponding bearing surface(s) of said sliding load bearing member may be curved to cooperate therewith. By providing one or more curved bearing surfaces, gravity can assist with the centring of the device following a displacement.

At least one, and in some embodiments all, cord portion(s) may be pre-tensioned so that when the bearing assembly is in its centred position, the cord portion(s) will have some tension. In some embodiments, at least one cord portion, and preferably a plurality of cord portions, is/are in a slackened configuration when the bearing assembly is in the centred position. Those cord portions will contribute to the elastic restoring force once they become tensioned. Therefore, they will contribute to the elastic restoring force after the cord portions that are pre-tensioned. As the pre-tensioned cord portions are stretched, their cross-sections will reduce in size, and therefore the elastic restoring force provided by those cord portions will reduce. The cord portions in the bearing are preferably configured so that the cord portion(s) that are not pretensioned will begin to engage and contribute to the elastic restoring force when the elastic restoring force from the pretensioned cord portions reduces. The cross-sections of at least some of the cord portions may be substantially constant along the lengths of the cord portions. Alternatively, the cross-sections of at least some of the cord portions may vary along the lengths of the cord portions. By way of example only, the cross section of at least some of the cord portions may taper such that the cross section of the cord portion closer to the sliding load bearing member is larger than the cross section of the cord portion closer to the periphery of the bearing seat.

In accordance with a second aspect of the present invention, there is provided a method for seismically isolating an item, comprising: providing a bearing assembly as outlined in relation to the first aspect above, and installing the bearing assembly between the item and a support to seismically isolate the item.

Generally, a plurality of bearing assemblies will be installed between the item and the support, to seismically isolate the item.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

The invention consists in the foregoing and also envisages constructions of which the following gives examples only.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure Ia is a sectional view of one embodiment of the invention in which a sliding load bearing member is fixed to the lower bearing seat and elastic self-centring is provided by a plurality of elastic cord portions;

Figure Ib and Ic show the embodiment of Figure Ia partially and fully displaced in the course of an earthquake;

Figure 2a is a sectional view of another embodiment of the invention in which the sliding load bearing member is fixed to the lower bearing seat and comprises a projection, and elastic self-centring is provided by a plurality of elastic cord portions;

Figure 2b and 2c show the embodiment of Figure 2a partially and fully displaced in the course of an earthquake;

Figure 3a and 3b are sectional views of another embodiment of the invention in which the sliding load bearing member is movable relative to both the upper and lower bearing seats and elastic cord portions provide elastic self-centring;

Figure 4a is a sectional view of another embodiment of the invention in which the sliding load bearing member is movable relative to both the upper and lower bearing seats and elastic cord portions provide elastic self-centring, and in which the sliding load bearing member comprises a projection;

Figures 4b shows the embodiment of Figure 4a partially displaced in the course of an earthquake; Figure 5 is a sectional view of another embodiment of the invention that uses a retaining band to connect the elastic cord portions to the slider;

Figure 6 is a sectional view of an embodiment of the invention similar to that shown in Figure Ia but with the bearing face of the upper bearing seat being curved;

Figure 7 is a sectional view of a bearing assembly similar to that shown in Figure 3a but with the bearing faces of both the upper and lower bearing seats being curved

Figure 8 is a plan view of any one of the above embodiments, showing example cord portion layouts;

Figure 9 is a side perspective view of a bearing of the type shown in Figure 3a, showing the cord portion attachment; Figure 10 is a cross-sectional detail view of part of one alternative form of the sliding load bearing member for use in any of the above embodiments; Figures 11, 12, and 13 show hysteresis loops for trials for a bearing with stainless steel bearing surfaces and a PTFE slider under vertical loads of 15, 35, and 55 kg respectively; and

Figures 14, 15, and 16 show hysteresis loops for trials for a bearing with stainless steel bearing surfaces and a nylon slider under vertical loads of 15, 35, and 55 kg respectively.

DETAILED DESCRIPTION OF THE INVENTION

The invention is generally described with respect to a bearing having a circular cross- section. While this cross-section is convenient for most applications of the invention any other cross-section may be used and should be considered within the scope of the invention defined. For example, the bearing may elliptical, square or rectangular in section.

In tlie specification reference is made to a slider or sliding load bearing member fixed to a bottom seat. This description is intended to include bearings in which the slider is effectively an extension from a lower foundation or a structure part without a seat in the sense that the slider is an integral part of the lower foundation or structure part.

In the first preferred embodiment bearing illustrated in Figures Ia, Ib and Ic an item 12 to be supported is supported on a support 16 by a bearing assembly having an upper bearing seat 10, a lower bearing seat 14 and a sliding load bearing member 20 therebetween. In this embodiment the slider 20 is anchored to the lower face 14. On the lower face of seat 10 is a stainless steel surface 18. On the upper face of slider 20 is a layer of material which has a relatively low coefficient of friction and is resistant to wear forming a sliding surface 22. Polytetraflouroethylene (PTFE) is a suitable material.

Connecting the outer periphery of slider 20 to the outer edge of upper seat 10 are a plurality of elastic cord portions 24 that extend radially outwardly from the slider. The cord portions are folded at their outer ends 28 over a rim 30 secured to the outer edge. In one embodiment this is done by clips or hooks (not shown) that capture the outer ends of the cord portions. In another alternative, where the elastic cord portions are rubber, the inner edge of the outer end 28 of each elastic cord portion 24 is vulcanised to the outer edge of rim 30. The inner ends of the elastic cord portions are connected to the outer periphery of the slider in any suitable way, such as those mentioned above.

The elastic cord portions for this and the following embodiments, may be single strand, in which there is a single length of material in the cord, or alternatively may be multi-strand, in which there are multiple lengths of material that may be twisted together. The cord portions could be any suitable cross-sectional shape, such as circular, elliptical, flat, rectangular, or square for example.

The cord portions may be made from any suitable material, such as extruded rubber or a different elastomeric material for example. Either the single strand or multi-strand cord portions may comprise a covering layer, such as of nylon for example. That covering layer will provide resistance to dust. A multi-strand cord is preferred, as if one of the strands was to fail then the remainder of the cord would still provide sufficient elasticity and strength. The cords may be of the type known as bungy cords. The strands of the cord portions could be any suitable cross-sectional shape, such as circular, elliptical, flat, rectangular, or square for example.

One example preferred form of cord comprises a core of rubber threads with one or more outer layers of braided high tenacity multifilament polyester yarn with good UV characteristics. The braiding process allows for varying patterns of colours in the outer layer for aesthetic purposes. For example, mixtures of colours could be provided in strips, spirals, or dashes.

The cord portions will be selected so they are capable of stretching to at least twice their undeformed length.

The force damping relative motion between upper seat 10 and lower seat 14 is determined by the area of sliding surface 22 and its coefficient of friction, the coefficient of friction of surface 18 and the weight which is supported by slider 20. The elastic restoring force to restore upper seat 10 to its centred position illustrated in Figure Ia is determined by the elastic force provided by the elastic cord portions 24. When the upper and lower seats are displaced relative to one another (as would happen in an earthquake) as illustrated in Figure Ib, movement of the upper seat 10 away from its centred position stretches the elastic cord portions extending generally from one side of the slider, while the elastic cord portions extending generally from the opposite side of the slider will go slack and fold over. When the upper seat 10 has reached its maximum displacement as shown in Figure Ic the elastic cord portions on one side are stretched to a maximum and thereby impart a maximum restoring force to return the bearing to the centred position shown in Figure Ia.

In an alternative embodiment, the slider may be fixed to, or integral with, the upper seat and slidable relative to a lower seat.

Figures 2, 3, 4, 5, 6, and 7 show alternative preferred embodiments of the invention. Unless described below, the features and functionality should be considered the same as for earlier embodiments, and like numerals are used to indicate like parts, with die addition of 100 for each embodiment.

In the second preferred embodiment illustrated in Figure 2a the bearing assembly has an upper seat 110, a lower seat 114 and a sliding load-bearing member 120. The lower seat 114 sits on a support 116. The upper seat 110 supports an item 112 to be supported. The lower face of seat 110 is lined with stainless steel 118. There is a sliding surface 122 on the top of slider 120. Slider 120, in contrast to slider 20 shown in Figure Ia has a central part 120a and an extension 120b projecting radially outwardly therefrom. Connecting the outer periphery 132 of slider projection 120b to the outer edge of upper seat 110 are a plurality of elastic cord portions 124 that extend radially outwardly from the projection 120b, and are secured over a rim 130 as in the embodiment of Figure Ia. The central part 120a of the slider may have any of the possible shapes described herein in relation to sliders without projections.

Rim 30, 130 in the embodiments in Figures Ia and 2a has a square cross-section. It may alternatively be circular, elliptical or rectangular in section for example. Rim 130 in Figure 2a extends below face 118 of upper seat 110. It can also be flush with that face, or recessed to allow slider 120 to travel past it in another alternative. As is illustrated in Figures 2b and 2c when the upper seat 110 is displaced relative to the lower seat 114, the elastic cord portions extending generally from one side of the slider are stretched, while the elastic cord portions extending generally from the opposite side of the slider 120 will go slack and fold over. Once the horizontal force displacing the upper seat has been damped and has stopped then the elastic force of elastic cord portions 124 restores the seats 110 and 114 to the centred position shown in Figure 2a.

Referring to Figure 2a the length of each elastic cord portion is determined by an outer dimension (OD) of the bearing upper seat, and the outer dimension ID₂ of the extension 120b. The diameter of the sliding surface 122 (ID₁), and therefore the damping force imparted by slider 120, in this embodiment remains constant. Thus a bearing designer has the freedom to vary the elastic force by varying the length of the elastic cord portions, while retaining a constant damping force provided by surface 122 of the slider. The elastic cord portions of this embodiment will have a greater stiffness and will therefore impart a greater elastic force than in the embodiment of Figure Ia, as the cord portions 124 are shorter than in the Figure Ia embodiment. The external diameter of slider 120 can be varied between ID₂ and ID₁ in this embodiment.

A third embodiment of a bearing according to the invention is illustrated in Figure 3a. In the embodiment illustrated in Figure 3a upper and lower bearing seats 210 and 214 are of similar construction to the seats in Figure Ia. The difference is that lower bearing seat 214 has a continuous flat load bearing surface. Between the bearing seats is a sliding load bearing member 220. In a preferred embodiment this sliding load bearing member 220 is a cylinder made of PTFE. It is able to move horizontally relative to both the upper bearing seat 210 and the lower bearing seat 214.

In this embodiment a plurality of elastic cord portions 224a, 224b extend radially outwardly from the slider and are connected to the upper and lower seats, by any suitable means such as those described above. The elastic cord portions are co-operable with the sliding load bearing member 220 and the upper bearing seat 210 and lower bearing seat 214 to urge said seats to return to or remain in a centred position relative to said sliding load bearing member. In the embodiment illustrated in Figure 3b the elastic self-centring force from the elastic cord portions extending generally from opposite sides of the slider will urge the sliding load bearing member 220 and the bearing seats 210 and 214 to a centred position after a displacement caused by an earthquake. As can be seen in Figure 3b, when the upper 210 and lower 214 bearing seats and sliding load bearing member 220 are not in a centered position, the upper left and lower right cord portions are stretched, and the upper right and lower left cord portions are slackened. That is, some diagonally opposite cord portions are stretched, and other diagonally opposite cord portions are slackened.

In the embodiment shown in Figure 4a, the bearing has an upper seat 310 supporting an item 312 to be supported and a lower seat 314 resting on a support 316. Between the two seats is a sliding load bearing member 320 which slides relative both to the upper seat 310 and the lower seat 314. Slider 320 has a central portion 320a and a rigid peripheral extension 320b. In this embodiment elastic cord portions 324a, 324b are connected to the periphery of the extension 320b and to rims 330a, 330b respectively.

In the embodiment of the invention illustrated in Figure 5 a retaining band 445 is provided to secure the inner ends of the elastic cord portions 424a, 424b to the outer edge of the peripheral extension 420b of sliding load bearing member 420. This is just one means by which a single cord can be configured to provide multiple cord portions. The retaining band 45 may be any type of retaining band, such as a wire rope, a clamp or a strap with a tightening mechanism. In a similar manner, retaining bands could be used to connect the outer ends of the elastic cord portions to the upper and lower bearing seats. Additionally, one or more retaining bands could be used for any of the above described embodiments.

The advantage of a double acting slider such as that shown in Figures 3, 4, and 5 is that both bearing seats are being displaced laterally in opposite directions to one another. This means that a bearing of a pre-determined width can be displaced approximately twice as far as is the case when only one seat is displaced relative to the other seat.

The central parts 120a, 320a, 420a of sliders 120, 320, 420, and extensions 120b, 320b, 420b may be constructed as a single component made of, for example, PTFE, or of a laminated construction with a seat contacting surface chosen to have the desired friction damping properties.

The embodiment illustrated in Figure 6 is similar in construction to that illustrated in Figure Ia. It consists of a lower bearing seat 514 from which projects a sliding load bearing member 520 having a PTFE load bearing surface 522 at its upper end. Between the bearing member 520 and the PTFE surface 522 is a flexing portion 523. The flexing portion enables an angular change of the PTFE surface 522 relative to member 520, so the PTFE surface can follow the curve of the upper bearing seat 510. In the assembly of Figure 6 the bearing face of the upper bearing seat 510 is spherical rather than flat. The load bearing surface 522 of the sliding load bearing member 520 has a convex spherical curve which corresponds to the concave spherical curve of the load bearing surface of upper bearing seat 510.

The embodiment illustrated in Figure 7 is similar in construction to that illustrated in

Figure 3a. However, as with the embodiment in Figure 6 the load bearing surface of the upper bearing seat 610 is spherical as is the load bearing surface of the lower bearing seat 614. The sliding load bearing member 620 has hemispherical load bearing end surfaces 620', 620" of shapes which corresponds to the inner surfaces of the upper and lower bearing seats 610, 614.

Figure 8 shows example configurations for the elastic cord portions in the above bearings. Figure 8 is a plan view, in which the outer circle represents both the upper 10, 110, 210, 310, 410, 510, 610 and lower 14, 114, 214, 314, 414, 514, 614 bearing seats. The cord portions and sliding load bearing member are shown in solid lines, so the cord portion connections to the upper and lower bearing seats are visible. However, it will be appreciated that in the actual bearings, the cord portions and sliding load bearing member will not be visible from above when the bearing is in centred position.

As Figure 8 shows example configurations for the elastic cord portions, the cord portions are only shown for small sectors of the bearing. However, it should be appreciated that the chosen elastic cord portion configuration will generally continue around the bearing. In the embodiments in which the sliding load beating membet 220, 320, 420, 620 is not fixed to either of said upper and lower bearing seats, the elastic cord portions 224a, 324a, 424a, 624a that are co-operable with the upper bearing seat may be formed separately from the elastic cord portions 224b, 324b, 424b, 624b that are co-operable with the lower bearing seat, as indicated in the right side of Figure 8. That is, there may be a plurality of elastic cord portions co-operable with the sliding load bearing member and upper bearing seat that are formed from one (or more) elastic cord(s), and a plurality of elastic cord portions co-operable with the sliding load bearing member and lower bearing seat that are separately formed from one (or more) elastic cord(s). In the configuration of the right side of Figure 8, every second cord portion may extend between the upper bearing seat and the sliding load bearing member, with the other elastic cord portions extending between the lower bearing seat and the sliding load bearing member. However, that configuration could vary.

Alternatively, at least some of the elastic cord portions 224a, 324a, 424a, 624a that are co- operable with the upper bearing seat may be formed integrally with at least some of the elastic cord portions 224b, 324b, 424b, 624b that are co-operable with the lower bearing seat, as indicated in the left side of Figure 8. Alternatively, there could be a single elastic cord that provides all the elastic cord portions, which may alternate from the sliding load bearing member to the upper and lower bearing seats. In the configuration of the left side of Figure 8, every second pair of elastic cord portions may extend between the upper bearing seat and the sliding load bearing member, with the other pairs of elastic cord portions extending between the lower bearing seat and the sliding load bearing member. However, that configuration could vary.

Preferably, in the embodiments in which the sliding load bearing member is not fixed to either of the upper bearing seat and the lower bearing seat, at least three, and more preferably at least four cord portions 224a, 324a, 424a, 624a are co-operable with the sliding load bearing member and the upper bearing seat. When four cord portions are provided, two of the cord portions extend transversely, and preferably perpendicularly, relative to the other two of the cord portions. At least three, and more preferably at least four cord portions 224b, 324b, 424b, 624b are co-operable with the sliding load bearing member and the lower bearing seat. When four cord portions are provided, two of the cord portions extend transversely, and preferably perpendicularly, relative to the other two of the cord portions. Additional cord portions may be provided between some or all of the cord portions. The number of cord portions in one direction (eg A-A in Figure 8) may differ from the number of cord portions in another direction (eg B-B in Figure 8), to vary the cord portion densities and thereby the elastic restoring force in the different directions.

In the embodiments in which the sliding load bearing member is fixed to one or other of the upper and lower bearing seats, the elastic cord portions 24, 124, 524 that are co- operable with the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed, may be formed from a single elastic cord (as indicated on the left side of Figure 8) or may be formed from a plurality of elastic cords (as indicated on the right side of Figure 8).

Preferably, in the embodiment in which the sliding load bearing member is fixed to one of the upper bearing seat and the lower bearing seat, at least three, and more preferably at least four cord portions 24, 124, 524 are co-operable with the sliding load bearing member 20, 120, 520 and the bearing seat to which the sliding load bearing member is not fixed. When four cord portions are provided, two of the cord portions extend transversely, and preferably perpendicularly, relative to the other two of the cord portions. Additional cord portions may be provided between some or all of the cord portions. The number of cord portions in one direction (eg A-A in Figure 8) may differ from the number of cord portions in another direction (eg B-B in Figure 8), to vary the cord portion densities and thereby the elastic restoring forces or spring rates in the different directions.

In either type of embodiment, the number of elastic cord portions may be varied depending on the desired properties. For example, there may be four, eight, ten, twelve, or any other suitable multiple of cord portions, for example twenty, thirty, forty, fifty, sixty, seventy, eighty, ninety, or one hundred cord portions. The number of cord portions will preferably be minimised when designing and building the bearing, to minimise material cost.

In the embodiments having a projection 120b, 320b, 420b from the slider, the elastic cord portions will be connected to the projection 120b, 320b, 420b. Figure 9 shows a perspective view of one particularly preferred cord attachment arrangement. This is shown with reference to an embodiment sitnilar to Figure 3, but could be applied to any of the different embodiments. In this figure, one side of the upper bearing seat 210 has been lifted away from the lower bearing seat 214. A single cord 224a' forms all of the cord portions 224a that are co-operable with the upper bearing seat 210 and the sliding load bearing member 220. Another single cord 224b' forms all of the cord portions 224b that are co-operable with the lower bearing seat 214 and the sliding load bearing member 220.

The upper bearing seat 210 is provided with a plurality of angularly spaced connecting portions 210a at or adjacent the periphery of the upper bearing seat 210. The lower bearing seat 214 is provided with a plurality of angularly spaced connecting portions 214a at or adjacent the periphery of the lower bearing seat 214. The upper cord 224a' extends, in a repeating pattern, from an annular cavity 221 in the sliding load bearing member 220 through a connecting portion 210a, back to the cavity 221, and out to the next connecting portion 210a. The lower cord 224b' extends, in a repeating pattern, from the annular cavity 221 in the sliding load bearing member 220 through a connecting portion 214a, back to the cavity 20a, and out to the next connecting portion 14a.

The cavity 221 is provided with angularly spaced upstands 221a that capture parts of the cords in the cavity 221. The receiving portions 210a, 214a will be provided with similar features.

In this and the above embodiments, there may be may be one, two, three, or more elastic cord portions extending in each direction. That is, there may be clusters of cord portions extending in each direction. The number of cord portions in the different directions may vary. In the embodiment of Figure 9, two cord portions that are substantially parallel to each other extend in each direction.

The cord portions may all be substantially the same length, cross section, and density

(number of radially extending cord portions in a given angular spacing around the sliding load bearing member). That will result in substantially the same spring rate in all directions around the sliding load bearing member. The configuration of the cord portions - namely the length, cross section, and density of the cord portions - will be selected to provide a desired elastic restoring force for an installation site and application.

At least some of the cord portions may vary to provide different properties in different directions. In a preferred embodiment, the cord portions extending in first and second generally opposite directions (eg A-A of Figure 8) from the sliding load bearing member may differ from the cord portions extending in third and fourth generally opposite directions (eg B-B in Figure 8) that are transverse to the first and second directions, in one or more of length, cross section, or density. That will provide one elastic restoring force in the first and second directions, and a differing elastic restoring force in the third and fourth directions. By using cord portions for elastic self-centring, the elastic restoring forces can be readily varied in different directions by varying the cord portion lengths, cross sections, elasticity, and/ or densities.

In a preferred embodiment, the elastic restoring force in one direction differs from the elastic restoring force in a second generally perpendicular direction. That can be particularly useful for seismic isolation and support of rack equipment where a large displacement (and relatively low elastic restoring force) may be desired for one direction, and a small displacement (and relatively high elastic restoring force) may be desired for the generally perpendicular direction. That also may be suitable for particular seismic sites where greater elastic restoring force in one direction may be required than in a generally perpendicular direction. By way of example only, the elastic restoring force in one direction may be twice the elastic restoring force of the generally perpendicular direction.

The bearing seats or the sliding load bearing member may be of a shape that is elongate in one direction relative to the other transverse direction, to provide shorter cord portions in the one direction and longer cord portions in the transverse direction. By way of example only, the bearing seats or sliding load bearing member may be elliptical or rectangular.

Means may be provided between the radially extending cord portions to maintain spacing between adjacent cord portions. As shown in Figure 10, rather than being solid, the sliding load bearing member in any of the above embodiments may be an annulus 24 with a central web 26, preferably of stainless steel. As illustrated in detail in Figure 10 in die recesses 31 defined below and above web 26 within annulus 24 there is a laminated construction. This consists of a rubber layer 28 secured to the web 26 inside of the annulus 24. A second layer 30, preferably of stainless steel with a recess in its lower face is affixed to the rubber layer 28. The lower bearing seat contacting surface is disc shaped PTFE insert 32. The same laminated structure is provided above web 26. Thus the load bearing surfaces of the sliding load bearing member which contact the faces of the upper bearing seat 10 and the lower bearing seat 12 are of each of PTFE. The sliding load bearing member therefore has alternating layers of more rigid and more resilient materials.

In this and the other embodiments, the sliding load bearing member is preferably configured to slide as a single unit relative to the upper and/ or lower bearing seats.

One of the applications of the bearing assembly is as a support for seismic isolation. Seismic isolation is the technique whereby the natural period of oscillation of the structure is increased to a value beyond that of the main period of the earthquake together with an optimum value of damping. Optimum values of these two factors enable a reduction in the acceleration transmitted to the structure by a factor of at least two.

The bearing assemblies of preferred embodiments of this invention are compact self contained units which can be designed to maximise the effectiveness of seismic isolation. They are particularly suitable for seismic isolation and support, and provide a desirable amount of possible displacement, of loads of about 10 tonnes or less. The bearing assemblies are suitable for supporting computers or electronics equipment for example.

The bearing assembly can also be readily modified to provide desired properties, without needing to mould new diaphragms for each application, by varying one or more of cord portion length, density, cross-section, and elasticity. Therefore, the bearing assembly provides an inexpensive way of tuning performance for particular applications. The bearing assembly can also be readily tuned to provide differing performance in differing directions, without requiring new components to be moulded for each application. Experimental Results

A prototype bearing was constructed and tested. The bearing is shown in Figure 9. The bearing had upper and lower bearing seats of a stainless steel material. Two different types of sliding load bearing members were tested; one was polytetrafluoroethylene (PTFE or TEFLON) and the other nylon. The dimensions of the bearings are as follows:

Sliding load bearing member diameter: 70 mm;

Upper and lower bearing seat diameter: 650 mm;

Cord portion dimensions: 270 mm by 5 mm diameter;

Number of cord portions: 16 cord portions extending from sliding load bearing member to upper bearing seat, and 16 cord portions extending from sliding load bearing member to lower bearing seat.

The cord portions were made of industrial shock or bungy cord. The cords comprised a core of rubber threads with one or more outer layers of braided high tenacity multifilament polyester yarn.

The bearings were tested under vertical loads of 15 kg, 35 kg, and 55 kg. The hysteresis loops for the trials are shown in Figures 11 to 16. The vertical axis represents the elastic restoring force, and the horizontal load represents the displacement.

The bearings show good damping performance and displacement under the tested loads. In the tested bearings, the cord portions were pre-tensioned so that when the bearings were at rest in their centred positions, the cord portions had some tension. It can be seen that as the pre-tensioned cord portions were stretched, their cross-sections reduced in size, and therefore their stiffnesses and the elastic restoring force provided by those cord portions reduced. That explains the relatively vertical centre part of the hysteresis loops and the relatively horizontal side parts of the hysteresis loops. In some embodiments, and for some applications, it may be desirable to have a more constant restoration provided. The bearing may be provided -with at least one cord portion, and preferably a plurality of cord portions, that is/are in a slackened configuration when the bearing is in the relaxed centred position. Those cord portions will contribute to the elastic restoring force once they become tensioned. Therefore, they -will contribute to the elastic restoring force after the cord portions that are pre-tensioned. The cord portions in the bearing are preferably configured so that the cord portion(s) that are not pretensioned will begin to engage and contribute to the elastic restoring force when the elastic restoring force from the pretensioned cord portions reduces.

Any suitable configuration of slackened and pre-tensioned cord portions could be provided. By way of example only, there may be an even distribution of slackened and pretensioned cord portions, with the cord portions alternating around the bearing. Alternatively, there could, for example, be a greater number of pre-tensioned cord portions than slackened cord portions. The configuration could be repetitive around the bearing, or could vary.

The above describes preferred embodiments of the present invention only, and modifications can be made thereto without departing from the scope of the present invention. Example modifications are listed in the "Summary of the Invention" section.

Claims

CLAIMS:

1. A bearing assembly comprising: an upper bearing seat, a lower bearing seat and a sliding load bearing member therebetween, said sliding load bearing member optionally being fixed to one or other of said upper and lower bearing seats, friction between said sliding load bearing member and said upper or lower bearing seat, or between said sliding load bearing member and said upper and lower bearing seats, in use, damping relative horizontal movement between said upper bearing seat and said lower bearing seat; and either: said assembly, when said sliding load bearing member is fixed to one or other of said upper and lower bearing seats, further comprising a plurality of elastic cord portions co-operable with the sliding load bearing member and the seat to which the sliding load bearing member is not fixed to urge said seat to which said sliding load bearing member is not fixed to return to or remain in a centred position relative to said sliding load bearing member and the seat to which said sliding load bearing member is fixed; or said assembly, when said sliding load bearing member is not fixed to either of said upper and lower bearing seats, further comprising a plurality of elastic cord portions co- operable with the sliding load bearing member and the upper bearing seat, and a plurality of elastic cord portions co-operable with the sliding load bearing member and the lower bearing seat to urge said seats to return to or remain in a centred position relative to said sliding load bearing member.

2. A bearing assembly as claimed in claim 1 , wherein said elastic cord portions are connected to the upper and/or lower bearing seat(s) at or adjacent a periphery of the upper and/or lower bearing seat(s).

3. A bearing assembly as claimed in claim 1 or 2, wherein said elastic cord portions are connected to the sliding load bearing member at or adjacent a periphery of the sliding load bearing member.

4. A bearing assembly as claimed in claim 3, wherein the sliding load bearing member comprises a rigid peripheral portion extending radially outwardly beyond the area of the sliding load bearing member that is in contact with one or both of the upper and lower seats, and the elastic cord portions are connected to the peripheral portion of the sliding load bearing member.

5. A bearing assembly as claimed in any one of claims 1 to 4, wherein said sliding load bearing member is fixed to one of the upper and lower bearing seats, and is sϋdable relative to the other of the upper and lower bearing seats.

6. A bearing assembly as claimed in claim 5, wherein the elastic cord portions that are co-operable with the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed, are formed from a single elastic cord.

7. A bearing assembly as claimed in claim 5, wherein the elastic cord portions that are co-operable with the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed, are formed from a plurality of elastic cords.

8. A bearing assembly as claimed in any one of claims 5 to 7, wherein at least three cord portions are co-operable with, the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed.

9. A bearing assembly as claimed in claim 8, wherein four or more cord portions are co-operable with the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed.

10. A bearing assembly as claimed in claim 9, wherein four cord portions are co- operable with the sliding load bearing member and the bearing seat to which the sliding load bearing member is not fixed, and wherein two of the cord portions extend transversely relative to the other two of the cord portions.

11. A bearing assembly as claimed in any one of claims 1 to 4, wherein said sliding load bearing member is slidable relative to both the upper and lower bearing seats.

12. A bearing assembly as claimed in claim 11, wherein the elastic cord portions that are co-operable with the upper bearing seat are formed separately from the elastic cord portions that are co-operable with the lower bearing seat.

13. A bearing assembly as claimed in claim 11, wherein at least some of the elastic cord portions that are co-operable with the upper bearing seat are formed integrally with at least some of the elastic cord portions that are co-operable with the lower bearing seat.

14. A bearing assembly as claimed in claim 13, wherein a single elastic cord provides all the elastic cord portions.

15. A bearing assembly as claimed in any one of claims 11 to 14, wherein at least three cord portions are co-operable with the sliding load bearing member and the upper bearing seat, and at least three cord portions are co-operable with the sliding load bearing member and the lower bearing seat.

16. A bearing assembly as claimed in claim 15, wherein four or more cord portions are co-operable with the sliding load bearing member and the upper bearing seat, and four or more cord portions are co-operable with the sliding load bearing member and the lower bearing seat.

17. A bearing assembly as claimed in claim 16, wherein four cord portions are co- operable with the sliding load bearing member and the upper bearing seat, and four cord portions are co-operable with the sliding load bearing member and the lower bearing seat, and wherein two of the cord portions that are co-operable with the sliding load member and the upper bearing seat extend transversely relative to the other two of the cord portions that are co-operable with the sliding load bearing member and the upper bearing seat, and wherein two of the cord portions that are co-operable with the sliding load member and the lower bearing seat extend transversely relative to the other two of the cord portions that are co-operable with the sliding load bearing member and the lower bearing seat.

18. A bearing assembly as claimed in any one of claims 1 to 17, wherein the cord portions extend generally radially outwardly from the sliding load bearing member, at least when the upper and lower bearing seats and sliding load bearing member are in a centered position.

19. A bearing assembly as claimed in any one of claims 1 to 18, wherein there are clusters of cord portions extending in each direction.

20. A bearing assembly as claimed in any one of claims 1 to 19, wherein the cord portions extending in first and second generally opposite directions from the sliding load bearing member differ from the cord portions extending in third and fourth generally opposite directions that are transverse to the first and second directions, in one or mote of length, cross section, elasticity, or density, to provide one elastic restoring force in the first and second directions, and a differing elastic restoring force in the third and fourth directions.

21. A bearing assembly as claimed in any one of claims 1 to 20, wherein at least one cord portion is pre-tensioned so that when the bearing assembly is in its centred position, the cord portion(s) will have some tension.

22. A bearing assembly as claimed in claim 21, wherein at least one cord portion is in a slackened configuration when the bearing assembly is in the centred position.

23. A bearing assembly as claimed in claim 22, wherein the cord portions are configured so that the cord portion(s) that are not pretensioned will begin to engage and contribute to the elastic restoring force when the elastic restoring force from the pretensioned cord portion(s) reduces as they stretch.

24. A bearing assembly as claimed in any one of claims 1 to 23, wherein the cord portions are capable of stretching to at least twice their undeformed length.

25. A bearing assembly as claimed in any one of claims 1 to 24, wherein the bearing assembly is suitable for seismic isolation and support, and a satisfactory amount of possible displacement, of loads of about 10 tonnes or less.

26. A method for seismically isolating an item, comprising: providing a bearing assembly as claimed in any one of claims 1 to 25; and installing the bearing assembly between the item and a support to seismically isolate the item.