Introduction

This invention relates to bearings for damping relative motion between a structure and a foundation or another part of the structure. More particularly it relates to the design of sliding bearings which have constant outer dimensions but in which the damping forces and the elastic forces may be variable with respect to one another.

Background

Bearings are known for seismically isolating structures during earthquakes. Such a bearing is the subject of US 6,385,918 and earlier patents mentioned in the specification of that patent.

In WO 2004/079113 we describe a self-centring sliding bearing. This bearing damps relative horizontal motion between upper and lower seats. The relative horizontal motion may be caused by earthquakes, wind loads or by other forces.

While the seismic isolation provided by such bearings is excellent, because of their structure the freedom to vary the size and proportion of the damping and elastic components is subject to compromise. Elastomeric bearings (with or without lead cores) have an inherent limitation. For some applications where they must accommodate large displacements together with low shear stiffness the shear moduli of the rubber may be too low for satisfactory manufacture.

It would be desirable to be able to design a bearing where the damping force and the elastic force can be variable with respect to one another, but where the overall dimensions of the bearing does not have to be changed to accommodate this.

It is an object of this invention to go some way towards overcoming the above disadvantage or achieving the above desideratum or at least to offer the public a useful choice. Summary of Invention

In one embodiment, the invention consists in a bearing for damping relative motion between a structure and a foundation, or between one part of a structure and another, the bearing comprising,

an upper seat and a lower seat,

a slider between the upper and lower seats, optionally secured to one or other of these seats, having a peripheral portion extending radialy outwardly beyond the area of the slider which is in contact with one or both of the upper and lower seats,

an annular elastic diaphragm secured at its outer edge at or adjacent the outer periphery of the upper or lower seat and at its inner edge to the outer periphery of the slider,

whereby in use relative horizontal movement between the upper and lower seats is damped by friction between the slider and the seat to which it is not secured, and the upper and lower seats are restored to a centred position by elastic force imparted by the diaphragm.

In one embodiment the slider is secured to the bottom seat.

In another embodiment the slider is not secured to either seat.

In one embodiment the peripheral portion of the slider is a radially outwardly projecting collar.

Iri one embodiment there are provided two annular elastic diaphragms, one secured at its outer periphery at or adjacent the outer periphery of the upper seat and the other secured at its outer edge at or adjacent the outer periphery of the lower seat.

In one embodiment the diaphragm or each diaphragm is constructed of a continuous piece of vulcanised rubber. In another embodiment the diaphragm or each diaphragm is constructed of a net-like structure of elastic material.

In another embodiment the diaphragm or each diaphragm is constructed of strips of elasticised material.

In one embodiment the diaphragm or diaphragms are secured at its or their inner edge to the outer edge of the peripheral portion of the slider by a retaining band.

In a second aspect the invention is a method for designing a bearing, as defined above, having constant external dimensions for use in different applications comprising the steps:

a) selecting the outer dimensions of the bearing having upper and lower seats of a predetermined cross-sectional area,

b) selecting a cross-sectional area for the slider face or faces contacting the lower and/or upper faces of the upper and/or lower seats,

c) determining the elastic force required to restore the slider to be centred based on the weight of the structure to be supported by the bearing, and

d) selecting a diaphragm or pair of diaphragms capable of providing that force.

In one embodiment the force to be imparted by the diaphragm is selected by varying the inner diameter with respect to the outer diameter of the diaphragm.

In another aspect the force to be imparted by the diaphragm is selected by varying the thickness in an axial direction of the diaphragm.

In another embodiment the force to be imparted by the diaphragm is selected by varying the outer diameter of the peripheral portion of the slider.

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 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.

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

Brief Description of the Drawings

In the accompanying drawings:

Figure 1 is a cross-sectional view of a sliding bearing in which the elastic force to be provided is not independently variable with respect to the damping force.

Figures IA and IB are the views of figure 1 with the upper seat partially and fully displaced relative to the bottom seat.

Figure 2 is a cross-sectional view of a first embodiment of a bearing according to the invention having one diaphragm and having its slider secured to the lower seat.

Figures 2A and 2B are also cross-sectional views of the embodiment shown in figure 2, with the upper seat in a partially and fully displaced relative to the bottom seat.

Figure 3 is a cross-sectional view of a second embodiment of the invention- having two- diaphragms and a slider free to move with respect to both seats.

Figure 3A is a cross-sectional view of the bearing shown in figure 3 with the upper seat displaced relative to the lower seat.

Figure 3B is a cross-sectional view of the second embodiment of the invention in which the inner portions of the diaphragms are held in place against the outer edge of the peripheral portion of the slider by a retaining band. Figure 4 is a plan view of a bearing as illustrated in figures 2 to 2A with a diaphragm of an alternative construction.

Figure 5 is the sectional view V-V shown in figure 4 of one alternative diaphragm cross- section.

Figure 6 is the sectional view VI-VI shown in figure 4 of another alternative diaphragm cross-section.

Figure 7 is a plan view of a bearing of the construction shown in figures 2 to 2A with a diaphragm formed by four segments.

Figure 8 is the sectional view VIII-VIII shown in figure 7 with diaphragm segments of first cross-sections.

Figure 9 is the sectional view IX-IX shown in figure 7 with diaphragm segments of alternative cross-sections.

Detailed Description of the Invention

The invention is 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 the specification reference is made to a slider 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 its face makes frictional contact with the slider to dampen motion.

In the control bearing illustrated in figures 1, Ia and Ib a structure 12 is supported on a column 16 by a bearing having an upper seat 10, a lower seat 14 and a slider 20 there¬ between. 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 is a diaphragm 24. It is folded at its outer periphery 28 over a rim 30 secured to the outer edge. In one embodiment this is done by screws (not shown) passing through the periphery 28 and into rim 30. In another alternative, where the diaphragm is rubber, the inside edge of periphery 28 is vulcanised to the outer edge of rim 30. The radially inside edge 26 of diaphragm 24 is fitted in a snug fit over the outside of slider 20.

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 co-efficient of friction of surface 18 and the weight which is supported by slider 20. The elastic restoring forces to restore upper seat 10 to its centred position illustrated in figure 1 is determined by the elastic force provided by diaphragm 24.

When the upper and lower seats are displaced relative to one another (as would happen in an earthquake) as illustrated in figure IA, movement of the upper seat 10 away from its centred position stretches one side of the diaphragm 24 while the other side goes slack and folds over. When the upper seat 10 has reached its maximum displacement as shown in figure IB the diaphragm 24 is stretched to a maximum and thereby imparts a maximum restoring force to return the bearing to the centred position shown in figure 1.

The relative damping and restoring forces of the control bearing in figure 1 are inter- dependent. The dimensions of the annular diaphragm" 24 are" determined by its inner diameter (ID) and its outer diameter (OD) and to a certain extent its thickness. If the bearing design requires a greater damping force, then the diameter of sliding surface 22 and the slider 20 must be increased. This increase will at the same time increase the inner diameter of diaphragm 24 thus reducing the cross-sectional area and restoring force of the diaphragm 24. This means that the designer either has to increase the outside diameter of the diaphragm, use a diaphragm which is thicker in the axial direction or use a diaphragm with a higher modulus of elasticity than the one used with the original slider. The freedom to design a bearing with the same external dimensions for use in different applications is limited. This can lead to increased production costs because each bearing needs to be custom built rather than mass produced.

In the embodiment illustrated in figure 2 a bearing according to the invention has an upper seat 10, a lower seat 14 and a slider 34.' The lower seat 14 sits on a column 16. The upper seat 10 supports a structure 12. The lower face of seat 10 is lined with stainless steel 18. There is a sliding surface 22 on the top of slider 34. Slider 34, in contrast to slider 20 shown in figure 1 has an extension 36 projecting radially outwardly. A diaphragm 32 extends radially outwardly from the outer edge extension 36 to its folded edge 28 where it is secured over a rim 30 as in the embodiment of figure 1.

The thickness in an axial direction of diaphragm 32 may be the same from the outer edge of extension 36 to its periphery 28. Its axial thickness may be inversely proportional to its radius from the outer edge of extension 36 to its periphery 28. The diaphragm 32 can also be made of laminated annular discs of rubber. The diaphragm may have variable upper, lower or both surfaces.

Other possible embodiments of a diaphragm useful in the bearing are described with reference to figures 4 to 9 below.

Rim 30 in the embodiments in figures 2 and 3 has a square cross-section. It may be circular, elliptical or rectangular in section as well. Rim 30 in figure 2 extends below face 18 of upper seat 10. It can also be flush with that face, or recessed to allow slider 34 to travel past it in another alternative.

As is illustrated in figures 2 A and 2B when the upper seat 10 is displaced relative to the lower seat 14 one side of the diaphragm 32 is stretched so as to impart a maximum force while another side is in a slack position. Once the horizontal force displacing the upper seat has been damped and has stopped then the elastic force of diaphragm 32 restores the seats 10 and 14 to the centred position shown in. figure 2.

Referring to figure 2 the diaphragm 32 has an outer diameter (OD) which is determined by the outer dimensions of the bearing. The diameter of the sliding surface 22 (ID1), and therefore the damping force imparted by slider 34, in this embodiment remains constant. The inside diameter of the diaphragm 32 (ID2) is determined by the outer diameter of extension 36, not by that of sliding surface 22. Thus a bearing designer has the freedom to vary the elastic force by varying the inside diameter of the rubber diaphragm 32, while retaining a constant damping force provided by surface 22 of the slider. The inside diameter (ID2) of diaphragm 32 is greater than the inside diameter (IDi) of diaphragm 24 shown in Figure 1. The size of diaphragm 32 is therefore less than that of diaphragm 24 and it will impart a lesser elastic force. The external diameter of slider 34 and the inside diameter of diaphragm 32 can be varied between ID2 and IDi in this embodiment.

If the design requires an increased damping force while keeping the elastic force constant then the diameter of the contacting surface 22 can be increased while the inside diameter of the diaphragm (ID2) can be left constant by keeping the external diameter (ID2) of the extension 36 of the slider 34 the same.

A second embodiment of a bearing according to the invention is illustrated in figure 3. The bearing illustrated has an upper seat 10 supporting a structure 12 and a lower seat 14 resting on a column 16. Between the two seats is a slider 38 which slides relative both to the upper seat 10 and the lower seat 14. Slider 38 has a peripheral extension 40. In this embodiment there are two diaphragms 44 and 46 which are snugly fitted at their inner diameters over extension 40 and at their outer diameters 43 and 28 over rims 42 and 30 respectively.

In the second embodiment of the invention illustrated in Figure 3B a retaining band 45 is provided to secure the inner edges of diaphragms 44 and 46 to the outer edge of the peripheral extension 40 of sliding member 38. The provision of the retaining band 45 makes it even less likely that diaphragms 44 and 46 would slip off peripheral extension 40 than' would be the- case without them. 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.

The advantage of a two-way slider is that both seats 10 and 14 are being displaced laterally in opposite directions to one another. This means that a bearing of a pre-determined width can be displaced twice as far as is the case when only one seat is displaced relative to the other seat. The embodiment of figure 3, by the provision of the extension 40, is able to have the same design freedom as described with relation to the embodiment of figure 2.

Because the diaphragms 46 and 44 are symmetrically disposed about slider 38 to ensure that it is restored to a centred position after the horizontal forces have acted upon the structure 12 or the column 16.

The sliders 34 and 38 and extensions 36 and 40 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.

In the description of the embodiments of the invention shown in figures 2 and 3 the diaphragms 32, 44 and 46 are represented as a single annulus of elastic material. A useful type of material can be vulcanised rubber.

In the embodiment shown in figure 4 the diaphragm 32 is of a continuous construction. Its thickness in an axial direction is variable from its inner to its outer diameter.

In the embodiment illustrated in figure 5 the thickness profile begins with a gradually increasing portion 51, a middle portion 48 which has the thickest part at its centre and an outer portion 50.

In the embodiment illustrated in figure 6 the inner and outer portions 57 and 56 respectively increase and decrease in thicknesses as they approach their starting and end points respectively. The thickness of the remaining thickness profile of the diaphragm is of a wave : form with a low point 54 and a high point 52.

In the embodiment illustrated in figures 7 to 9 the diaphragm is in the form of two pairs of segments 58, 58 and 59, 59. In the embodiment illustrated in figure 8 the main body portions 60 of segments 58 are thicker than the main body portions 62 of segments 59. Similarly in the alternative profiles illustrated in figure 9 the main body portions 64 and 66 have undulating thicknesses and are of different thicknesses. They may be all of uniform thickness as well. The different thicknesses of pairs of segments allows for enhanced elastic centring forces in different directions where the application calls for this distinction. There may be more or fewer segments than the four illustrated in figure 7.

Although the diaphragms of the embodiments described are of vulcanised rubber, any other suitable elastomeric material may be used. It may be of continuous, segmented or mesh construction to meet the requirements of the design application.

It is not the intention to limit the scope of the invention to the abovementioned examples only. As would be appreciated by a skilled person in the art, many variations are possible without departing from the scope of the invention as defined in the appended claims.