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This is a nationalization of PCT/NZ04/000045 filed 5 Mar. 2004 and published in English. 
   FIELD OF THE INVENTION 
   This invention relates to sliding bearings. 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 a structure and another structure or ground supporting the first structure. 
   BACKGROUND OF THE INVENTION 
   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 U.S. Pat. No. 4,320,549; U.S. Pat. No. 5,597,239, U.S. Pat. No. 6,021,992, and U.S. Pat. No. 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 U.S. Pat. No. 4,644,714; U.S. Pat. No. 5,867,951; U.S. Pat. No. 6,289,640; the embodiments shown in each of FIGS. 4 to 6 in U.S. Pat. No. 6,021,992; and the embodiments shown in FIGS. 4 and 5 of U.S. Pat. No. 6,126,136. 
   Some of the above mentioned sliding bearings have a curved bearing seat surface and a corresponding curved surface on the sliding element which provide a form of passive self-centring of the sliding element and the bearing seats. None of either types of sliding bearings mentioned above have elastic self-centring. 
   “Self-centring” is, for the purposes of this specification, urging the sliding element 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 element 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 structure 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 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. The sliding member in  FIGS. 2 ,  3  and  7  is at rest at the midpoint between the upper and lower seats. 
   SUMMARY OF THE INVENTION 
   It is an object of this invention to go some way towards achieving these desiderata or at least to offer the public a useful choice. 
   Accordingly, the invention may be said broadly to consist in a bearing assembly comprising:
         an upper bearing seat, a lower bearing seat and a sliding load bearing member there between, the sliding member optionally being fixed to one or other of the upper and lower bearing seats, friction between the sliding member and the upper or lower bearing seats, or between the sliding member and the upper and lower bearing seats, in use, damping relative horizontal movement between the upper bearing seat and the lower bearing seat, the assembly, when the sliding member is fixed to one or other of the upper or lower bearing seats further comprising an elastic sleeve surrounding the outer peripheries of the upper and lower seats co-operable with the upper or lower bearing seats to urge the seat to which the sliding member is not fixed to return to or remain in a centred position relative to the sliding member and the seat to which the sliding member is fixed.       

   In another embodiment the invention may be said broadly to consist in a bearing assembly comprising:
         an upper bearing seat, a lower bearing seat and a sliding load bearing member therebetween, the sliding member optionally being fixed to one or other of the upper and lower bearing seats, friction between the sliding member and the upper or lower bearing seat, or between the sliding member and the upper and lower bearing seats, in use, damping relative horizontal movement between the upper bearing seat and the lower bearing seat,   the assembly further comprising a diaphragm, the sliding member being located at or near or joined to the centre of the diaphragm, the periphery of the diaphragm being joined to or adjacent to the periphery of one or both of the upper and lower bearing seats co-operable with the sliding means and one or other or both of the upper and lower bearing seats to urge the sliding means to return to or remain in a centered position.       

   In one embodiment the sliding member is not fixed to either of the upper or lower bearing seats. 
   In another embodiment, wherein the sliding member is not fixed to either the upper or lower bearing seats, the self-centring means comprises two diaphragms. 
   In another embodiment the elastic self-centring means includes both a sleeve over the outer periphery of the upper and lower bearing seats and one or two diaphragms. 
   Preferably the diaphragm or the two diaphragms comprises or comprise vulcanized rubber. 
   The invention also consists in a bearing assembly comprising:
         an upper bearing seat, a lower bearing seat and a sliding load bearing member there-between, the sliding member being slideable relative to each of the upper and lower bearing seats, friction between said sliding member and the upper and lower bearing seats, in use, damping relative horizontal movement between the upper bearing seat and the lower bearing seat,   the assembly further comprising an elastic self-centring means comprising a sleeve over the outer periphery of and co-operable with the upper and lower bearing seats to urge the seats to return to or remain in a centered position relative to the sliding member and a rigid member extending peripherally outwardly from the slider to cooperate with the sleeve to centre the slider between the upper and lower seats.       

   In one alternative said rigid member is affixed to the elastic sleeve and abuts the sliding member. 
   In one embodiment the rigid member is a disc. 
   In another embodiment the rigid member is a hub and a plurality of spokes. 
   Alternatively the sliding member is substantially cylindrical in shape and the bearing surfaces of the lower and upper bearing seats are substantially flat. 
   Preferably the sliding member is of regular geometrical shape in cross-section. 
   Alternatively one or other of the bearing surfaces of the upper or lower bearing seats is curved and the corresponding bearing surface of the sliding member is curved to cooperate therewith. 
   Preferably the diaphragm is made of vulcanized rubber. 
   Preferably the sleeve is made of vulcanized rubber or other appropriate elastic material. 
   The invention may also be said broadly to consist in a method for seismically isolating a structure which comprises installing a bearing assembly as herein above defined between the structure and a foundation. 
   In one alternative the foundation is another structure. 
   This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by having reference to the accompanying drawings wherein: 
       FIG. 1  is a sectional view of one embodiment of the invention in which a sliding element is fixed to the lower bearing seat and elastic self-centring is provided by both a diaphragm and a sleeve. 
       FIG. 1   a  shows the embodiment of  FIG. 1  displaced in the course of an earthquake. 
       FIG. 1   b  shows a variation of the embodiment shown in  FIG. 1  where there is only a diaphragm providing elastic self-centring. 
       FIG. 1   c  shows a variation of the embodiment shown in  FIG. 1  where there is only a sleeve providing elastic self-centring. 
       FIGS. 2 and 2   a  are sectional views of another embodiment of the invention in which the sliding element is movable relative to both the upper and lower bearing seats and two diaphragms and a peripheral sleeve providing elastic self-centring means. 
       FIG. 3  is a sectional view of a further embodiment of the invention in which elastic self-centring means is provided by a peripheral sleeve and a sliding member with a rigid peripheral projection extending to the rubber sleeve and beyond the peripheries of the upper and lower bearing seats. 
       FIG. 4  is a sectional view of an alternative to the embodiment in  FIG. 3  in which the rigid projection from the sliding member does not extend beyond the periphery of the upper and lower bearing seats. 
       FIG. 4   a  shows the embodiment in  FIG. 4  in use with the lower bearing seat moved horizontally relative to the upper bearing seat. 
       FIG. 5  is the detail shown in the circle V in each of  FIGS. 3 and 4 . 
       FIG. 6  is a sectional view of an embodiment of the invention similar to that shown in  FIG. 1  but with the bearing face of the upper bearing seat being curved. 
       FIG. 7  is a sectional view of a bearing assembly similar to that shown in  FIG. 2  but with the bearing faces of the upper and lower bearing seats being curved. 
       FIG. 8  is a plan view of a further embodiment of a bearing according to the invention. 
       FIG. 9  is a side sectional view shown by the section line VIII-VIII in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Construction of First Embodiment 
   A bearing assembly according to a first embodiment of the invention is illustrated in  FIG. 1 . This embodiment has a lower bearing seat  12 , preferably made of stainless steel, from which projects a sliding member  14 . There is a layer of polytetrafluoroethylene (PTFE) or other suitable sliding material  15  on the load bearing upper face of sliding member  14 . 
   The upper bearing seat  10  is also made of stainless steel. Its face is substantially flat and rests on the PTFE layer  15  of sliding member  14 . 
   Bearing seats  10  and  12  may be of any regular geometrical shape in cross-section. In one preferred embodiment they are circular in cross-section. 
   Surrounding the outer periphery of upper bearing seat  10  and lower bearing seat  12  is a sleeve  18 , preferably of vulcanized rubber. 
   Also provided is a diaphragm  16  made of vulcanized rubber. In the embodiment illustrated the diaphragm  16  has a central hole of diameter slightly smaller of that sliding member  14  so as to be able to slide over and remain in place on sliding member  14 . The outer periphery of diaphragm  16  is fitted within a recess  17  on the outer face of bearing seat  10  by sleeve  18 . However, it may be clamped into place by a metal ring or by other means known to those skilled in the art. 
   In the embodiments illustrated in  FIGS. 1 and 1   a  the elastic self-centring forces are provide by a combination of sleeve  18  and diaphragm  16 . However, self-centring can be achieved by a sleeve alone or a diaphragm alone. In the embodiment shown in  FIG. 1   b  the self-centring means is a diaphragm  16 . In  FIG. 1   c  it is a sleeve  18 . These are exemplary of alternatives to the embodiments shown in  FIGS. 2 ,  6  and  7  as well. 
   Sleeve  18  may contain annular reinforcing rings of stiffing material embedded into the rubber of the sleeve. These serve to stabilize the sleeves during large displacement by spreading the displacements more equally. 
   Construction of Second Embodiment 
   The construction of a second embodiment of the invention is illustrated in  FIG. 2 . In the embodiment illustrated in  FIG. 2  upper and lower bearing seats  10  and  12  are of similar construction to the seats in  FIG. 1 . The difference is that lower bearing seat  12  has a continuous flat load bearing surface. Between the bearing seats is a sliding member  20 . In a preferred embodiment this sliding member  20  is a cylinder made of PTFE. It is able to move horizontally relative to both the upper bearing seat  10  and the lower bearing seat  12 . 
   In this embodiment there are a pair of rubber diaphragms  16  and  22 , each having a central hole through which the sliding member  20  is fitted in a snug fit. The peripheries of diaphragms  16  and  22  are held in recesses at the outer peripheries of bearing seats  10  and  12  by a rubber sleeve  18  as with the embodiment illustrated in  FIG. 1 . 
   Construction of Third Embodiment 
   A third embodiment is illustrated in  FIG. 3 . In this embodiment the sliding member is an annulus  24  having a central web  26 , preferably of stainless steel. As illustrated in detail in  FIG. 5  in the 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 element in the embodiment in  FIG. 3  which contact the faces of the upper bearing seat  10  and the lower bearing seat  12  are of each of PTFE. 
   There is also provided projecting outwardly from the sliding element in the assembly of  FIG. 3  a disc  34 . The outer periphery of disc  34  extends outwardly beyond the outer peripheries of upper bearing seat  10  and lower bearing seat  12 . A rubber sleeve  18  extends over the peripheral edge of disc  34  as well as around the peripheral edges of upper bearing seat  10  and lower bearing seat  12 . 
   Construction of Fourth Embodiment 
   The embodiment illustrated in  FIG. 4  is substantially the same as that in  FIG. 3  except that the outer periphery of disc  34  lies substantially in vertical registry with the outer peripheries of upper bearing seat  10  and lower bearing seat  12  respectively. This is in contrast to the disc  34  in the embodiment in  FIG. 3  which extends peripherally beyond the peripheries of seats  10  and  12 . 
   Disc  34  serves as a rigid connection between sleeve  18  and the sliding member. The invention contemplates other mechanical equivalents. Instead of a solid disc  34 , a perforated disc may be used. It would also be possible to have spokes extending outwardly from annulus  24 . It is equally contemplated that a disc  34  may be attached to the inner surface of sleeve  18  and not attached to the slider. In such an embodiment perforated discs or spokes with inner and outer annular rims could also be employed for the same purpose. 
   Construction of Fifth Embodiment 
   The embodiment illustrated in  FIG. 6  is substantially the same as that in  FIG. 1 . It consists of a lower bearing seat  36  from which projects a sliding member  40  having a PTFE load bearing surface  39  at its upper end. In the assembly of  FIG. 6  the bearing face of the upper bearing seat  38  is spherical rather than flat. The load bearing surface  39  of the sliding member  40  has a convex spherical curve which corresponds to the concave spherical curve of the load bearing surface of upper bearing seat  38 . 
   The diaphragm  16  and the sleeve  18  are of the same material and construction of those described in the embodiment illustrated in  FIG. 1 . 
   Construction of Sixth Embodiment 
   The embodiment illustrated in  FIG. 7  is similar in construction to that illustrated in  FIG. 2 . However, as with the embodiment in  FIG. 6  the load bearing surface of the upper bearing seat  38  is spherical as is the load bearing surface of the lower bearing seat  44 . The sliding member  42  has hemispherical load bearing end surfaces  43  of shape which corresponds to the inner surface of the upper and lower bearing seats  38  and  44 . 
   Diaphragms  16  and  22  and sleeve  18  illustrated in  FIG. 7  are of the same materials and construction as the corresponding diaphragms and sleeve described in relation to  FIG. 2 . 
   Construction of Seventh Embodiment 
   In the embodiment illustrated in  FIGS. 8 and 9  the bearing has an upper plate  60  on which a structure may rest and a lower plate  62  which may rest on a foundation or further structure. The inward faces  61  and  63  of the plates  60  and  62  are coated with stainless steel. 
   The sliding member  64  consists of an opposed pair of annulus halves  70  similar to the annulus illustrated in  FIGS. 3 to 5 . As with the previous construction in a recess in each annulus half there is inserted, progressing outwardly, three layers. The innermost layer  72  is of rubber. The next layer  74  is of steel and the outer face  76  is of PTFE. 
   The self-centring for this bearing is provided by upper diaphragm  66  and lower diaphragm  68  which are fitted over the sliding member  64  in much the same manner as the diaphragms  16  and  22  in  FIG. 2 . 
   The outer periphery  82  of upper diaphragm  66  is fitted over a rim  80 . There are provided a set of four bolts  78  as illustrated in  FIG. 11  which secure the diaphragm edge  82  to rim  80  and rim  80  to upper plate  60 . Similarly a set of four bolts  78  secures diaphragm edge  84  to rim  86  and rim  86  to lower plate  62 . 
   Bolts (not illustrated) passed through holes in plates  60  and  62  may be threaded into nuts  88  and  89  in order to secure a structure to other plate  60  and to secure lower plate  62  to a foundation or a further structure. 
   Operation of First Embodiment 
   The embodiment in  FIG. 1  is illustrated in operation in  FIG. 1   a . An external force, such as an earthquake, has moved lower bearing seat  12  to the position illustrated. This relative horizontal movement between the upper bearing seat  10  and the lower bearing seat  12  is damped by the friction between the upper surface  15  of sliding member  14  and the inner surface of bearing seat  10 . 
   It will be seen that sleeve  18  has been stretched both on the right and left sides of the bearing assembly. The elasticity in the sleeve  18  will urge the support bearing seat  10  to return to the rest position shown in  FIG. 1 . Similarly the left hand portion of diaphragm  16  is stretched while the right hand portion is slack. While the relative movement between the upper and lower bearing seats is being damped by the friction between the sliding element  14  and the upper bearing seat  10 , both the sleeve  18  and the diaphragm  16  will urge the sliding member  14  and the upper valve seat  10  to the centred position illustrated in  FIG. 1 . 
   Although the embodiment illustrated in  FIG. 1  has both a diaphragm  16  and a sleeve  18  other embodiments within the scope of the invention can include an assembly which has only a diaphragm  16  and another assembly which has only an elastic sleeve  18 . 
   Operation of Second Embodiment 
   In the embodiment illustrated in  FIG. 2   a  the elastic self-centring force from both the elastic sleeve  18  and the pairs of diaphragms  16  and  22  will urge the sliding member  20  and the bearing seats  10  and  12  to a centred position. The left side of diaphragm  22  is slack and the right side is stretched in  FIG. 2   a . Diaphragm  16  is stretched and slack in the same manner as is illustrated in  FIG. 1   a.    
   Operation of Third and Fourth Embodiments 
   Referring to  FIG. 4   a , an earthquake force has displaced the lower bearing seat  12  to the right. Frictional forces between the load bearing faces of sliding member  24  and the load bearing faces of seats  10  and  12  will damp the relative movements between the seats. Elastic sleeve  18  will urge both the upper and lower bearing seats and the disc  34  into a centered position. 
   Operation of Fifth and Sixth Embodiments 
   In the embodiments illustrated in  FIGS. 6 and 7  the curved surfaces of the bearing seats add additional passive centring forces to the elastic self-centring provided by the diaphragms  16  and  22  and the sleeve  18 . 
   Operation of Seventh Embodiment 
   The embodiment illustrated in  FIGS. 8 and 9  operates in the manner of the second embodiment illustrated in  FIGS. 2 and 2   a.    
   ADVANTAGES 
   One advantage provided by elastic self-centring of a seismic sliding bearing is that it provides a means for controlling the period of the isolated structure so that the period of the isolated structure exceeds the period of the earthquake. In seismic isolation this is better known as period shift. The concept is more full described in “Introduction to Seismic Isolation”, Skinner et al., John Wiley &amp; Sons, (1993), pages 4 to 7. 
   Another advantage is that it minimizes the cross sectional area occupied by the bearing assembly. The advantages of the bearing assembly illustrated in  FIGS. 2 ,  4 , and  7  that they are double acting. That is, the top and the bottom seats  10  and  12  move in opposite directions relative to the sliding member thereby reducing the required size of the sliding surface of the bearing seats by a factor of two. 
   The total horizontal force required to operate the bearing assembly F(horizontal) is given by the sum of the force to overcome the friction, F(μ), the force to deform the rubber diaphragm, F(m), plus the forces required to deform the rubber sleeve, F(w). The forces to deform the rubber are mainly elastic in nature. 
   Thus:
 
 F (horizontal)= F (μ)+ F ( m )+ F ( w )
 
Where
 
 F (μ)=μ F (vertical)
 
 F ( m )≈[α· E (rubber)·τ( m )] x  
 
 F ( w )≈[α· E (rubber)+β· G (rubber)]·[ A ( w )/ h ( w )] x  
 
Where
         μ=the coefficient of friction between the two sliding surfaces
 
 F (vertical)=(total mass)· g  
   t(m)=thickness of the diaphragm (see  FIG. 1 )
 
x=horizontal displacement of the top seat relative to the bottom seat,
   where
           x=0 when the seats are centred.   α=a geometric term for the diaphragm   β=a geometric term for the sleeve   E(rubber)=Young&#39;s modulus for the rubber diaphragm   G(rubber)=the shear modulus of the rubber sleeve   A(w)=the cross sectional area of the sleeve   h(w)=the height of the sleeve (see  FIG. 1 )   
               

   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 a 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 assembly of this invention is a compact self contained unit which can be designed to maximise the effectiveness of seismic isolation.

Summary:
A bearing assembly having upper and lower bearing seats and a sliding load bearing member between the seats. The sliding member is fitted with an elastic self-centering element. The assembly in operation damps relative horizontal movement between the upper and lower seats, the self-centering element returning the sliding member to a centered position at rest. Typically a structure rests upon and is secured to the upper seat and the lower seat rests upon or is fixed to a foundation. The relative horizontal movement may be caused by earthquakes, wind loads or the like.