The dampers of the invention may be used in large structures such as bridges or buildings to reduce the effects of motion induced during earthquakes or from strong winds. They may also be used to damp motion of large or small moving objects. They may be used to damp motion in industrial machinery or engines or the like or from domestic appliances such as washing machines for example, or in any other application where it is desired to damp any motion, vibrations or similar. They may be used to damp displacement arising from thermal expansion. The extrusion dampers of the invention have various applications.

BACKGROUND OF THE INVENTION Devices known as extrusion dampers which employ elastic or plastic deformation of certain materials to absorb energy are well known. US Patent No. 3,833,093 describes a form of extrusion damper consisting of an energy absorber material confined between an elongate outer jacket which is typically a cylinder, and a shaft which moves longitudinally within the jacket. The absorber material is typically lead while the jacket and shaft are typically formed of steel. Opposite ends of the

jacket and shaft are connected between two members in a structure which are expected to move relative to one another during an earthquake or other induced motion. A general discussion of these and related devices is given in "An Introduction to Seismic Isolation", R I Skinner, H Robinson and G H McVerry, Wiley, 1993.

Lead is the preferred deformable energy absorbing material for several reasons. First it yields at a room temperature shear stress of around 10.5MPa which is low compared with other metals and equivalent plastic materials. Second it restores its mechanical properties through recrystallisation and associated processes relatively rapidly following yield deformations, which provides outstanding resistance to work hardening under cyclic shear at ordinary temperatures. Third lead is readily available at the purity required to exhibit these properties.

In practice such dampers are individually designed to protect a particular structure against damage by damping certain motions imparted to it. Their behaviour is quite closely approximated by that of an ideal Coulomb damper in having a force-displacement hysteresis loop which is nearly rectangular and practically rate independent over a wide range of frequencies. Research into performance of these devices is ongoing.

SUMMARY OF THE INVENTION It is an object of the present invention to provide for improved performance of such dampers for seismic isolation and other applications.

The invention broadly comprises an damper for interposing between two members to damp motion which may be induced between the two, comprising an elongate outer jacket means to be connected to one of said members, a body of plastically deformable energy absorbingmaterial contained within said jacket means, and a shaft means to be connected to the other of said members and having a shape which varies over the length of the shaft and which is forced through the energy absorbing material during said induced motion.

In one form of the damper of the invention the shaft comprises a multiple number of parts of lesser diameter alternating between a multiple number of parts of greater diameter.

In another form of the damper of the invention the shaft has a substantially constant cross-section which is periodically displaced away from a central axis of the shaft over at least a part of the length of the shaft so that the shaft is sinusoidally shaped or of other "wave" shape of regular or irregular amplitude or frequency over the length of the shaft.

In another form of damper of the invention the shaft has an elliptical cross-section which rotates periodically about a central axis of the shaft over at least a part of the length of the shaft so that the shaft is spirally or helically shaped.

The shaft may contact the energy absorbing material directly, or alternatively through a series of adjacent parallel plates embedded within the energy absorbing material and encircling the shaft which are forced to move relative to one another across the direction of motion of the shaft and through the energy absorbing material as the shaft moves.

Preferably the body of energy absorbing material is subjected to approximately hydrostatic pressure at least approaching the shear yield stress of the material. Preferably the hydrostatic pressure applied to the core exceeds the shear yield stress of the energy absorbing material. Preferably the hydrostatic pressure is 5MPa or more and most preferably in the range 10-lOOMPa.

Preferably the energy absorbing material is lead, but other energy absorbing materials which may be used include alloys of lead, aluminium at elevated temperature e.g. about 200°C, tin, zinc, brass, iron, super plastic alloys, or any other material having a low rate of work hardening, including also densely packed granular materials such as steel shot, glass beads.

alumina, silica, silicon carbide or any other very hard granular material.

Dampers of the invention may be used in seismic isolation applications to damp seismic motion in large structures such as bridges or buildings or motion from very strong wind buffeting or similar. They may also be used in any other application where it is desired to damp any motion, vibrations, or similar. For example, dampers of the invention may be used to damp motion of engines or other industrial machinery. In domestic applications, dampers of the invention may be used in washing machines or spin dryers or dish washers to isolate vibrations. Small size dampers of the invention may be used as "microisolators" for sensitive electronic equipment such as the mechanism of . a video recorder etc or in other similar applications. Numerous applications of the extrusion dampers of the invention are envisaged and the invention is not limited only to seismic isolation dampers.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred dampers of the invention will be described by way of example with reference to the following drawings, wherein:

Fig. 1 is a perspective view of one form of damper of the invention.

Fig. 2 shows the damper of Fig. 1 in longitudinal cross-section.

Fig. 3 shows another form of damper of the invention similar to that of Figs 1 and 2 in longitudinal cross-section.

Fig. 4 shows another form of damper of the invention similar to that of Figs 1 and 2 in longitudinal cross-section.

Fig. 5 shows a further form of damper of the invention again similar to that of Figs 1 and 2 in longitudinal cross- section.

Fig. 6 shows another form of damper of the invention in longitudinal cross-section.

Fig. 7 shows another form of damper of the invention similar to that of Fig. 6 in longitudinal cross-section,

Fig. 8 shows another form of damper of the invention similar to that of Fig. 6 in longitudinal cross-section,

Figs 9 and 10 are Mohr circle constructions which will be referred to further in description of the extrusion dampers of the invention,

Fig. 11 is a graph of shaft displacement against time graphically illustrating testing applied to an extrusion damper of the type shown in Figs 1 to 2,

Fig. 12 is a graph of load resistance exhibited against time for an extrusion damper of the type shown in Figs 1 and 2 subjected to the displacement cycling of Fig. 11, and

Fig. 13 is a graph of load resistance exhibited during the test cycling of Fig. 11 against displacement showing successive hysteretic leafs for successive cycles.

DESCRIPTION OF PREFERRED FORMS The dampers shown in Figs 1 to 5 each comprise an outer jacket 1 which is typically formed of steel and may be cylindrical as shown but could be of other cross-sectional shapes such as oval for example. A shaft 2 is able to move longitudinally through the jacket 1 in the direction of arrow A in each case. It is not necessary for the shaft.to be positioned centrally within the outer jacket 1, but it could instead be off¬ set somewhat. The shaft is also typically formed of steel.

In accordance with the invention the shaft 2 has a shape which varies over the length of the shaft. In the damper of Fig. 2 the shaft is sinusoidally shaped i.e. the shaft has a substantially constant cross-section which is periodically displaced away from a central axis of the shaft. The cross-

section shape of the shaft at any point is preferably square or rectangular but could be of other shapes such as circular or elliptical for example.

In the damper of Figs 1 to 5, a series of adjacent parallel plates 4 encircle the shaft. Each plate 4 may be circular in overall shape with a square or rectangular hole through the centre through which the shaft passes in the case of a shaft with a square or rectangular cross-sectional shape. The plates could alternatively be oval or of any other overall shape with circular or oval holes for a circular or oval shaft, for example. The plates 4 may be formed of steel or alternatively glass or other metal. Preferably an O-ring (not shown) is provided between each adjacent plate 4. The O-ring is located in a circular groove in the face of one plate and contacts the smooth opposite face of the adjacent plate so that the O-ring seals between the two as the plates slide relative to one another. The space between the outer jacket 1 and the plates 4 surrounding the shaft is filled with a plastically deformable energy absorbing material such as lead 3 so that the plates are embedded within the lead. The plate 4 at either end of the damper slides against an end plate la forming part of the outer jacket. The end plates la at either end of the outer jacket 1 are fixed in position by bolts 6 as shown.

Fig. 1 shows the damper from the exterior while Figs 2 to 5 show the damper in longitudinal cross-section showing

variations in construction which will be referred to further later.

In use the outer jacket 1 of the damper is coupled to one member of a building or other structure through a suitable mechanical coupling, and the shaft 2 is coupled to another member, through the end 2b of the shaft, which may move relative to the first in an induced motion. During such motion such as an earthquake in a seismic isolation application, the shaft 2 is forced through the damper. As the sinusoidally shaped shaft moves through the series of plates 4 embedded within the lead 3, each plate 4 will be caused to move relative to one another across the direction of motion of the shaft and through the lead. As the shaft moves the stack of plates will be forced to oscillate back and forth across the lead. This creates a damping effect by conversion of kinetic energy to plastic deformational energy and heat, and further heat during re-crystallisation and other spontaneous recovery processes.

Fig. 3 shows a damper similar to that of Fig. 2 except that the amplitude of the sinusoidally shaped shaft 2 is greater towards the ends of the shaft i.e. the displacement of the cross- section of the shaft away from the central axis of the shaft increases from the centre of the shaft towards either end. With this damper, the load resistance exhibited by the damper is greater towards either end of the movement of the shaft. Conversely, the load resistance against movement of the shaft is

of the cross-section of the shaft 2 away from the central axis is greater towards the ends of the shaft. This will have substantially the same effect of relatively increasing the load resistance provided by the damper at either end of the movement of the shaft, or alternatively the shaft can be designed to lower the force threshold required to initiate movement of the shaft. In this figure and all the drawings the amplitude of the wave shape of the shaft is exaggerated for illustration.

It is also possible that in another damper of the invention the shaft may combine both increasing amplitude and increasing frequency of displacement of the cross-section of the shaft away from the central axis towards the ends of the shaft i.e. combine the characteristics of the shafts of Figs 3 and 4.

In the damper of Fig. 5 the shaft 2 is not a rectangular, wave shaped shaft but is formed with a spiral or helical convolution i.e. the shaft has an elliptical cross- section which rotates periodically about a central axis of the shaft. Again, as the shaft 3 is forced to move by induced

motion, it will in turn cause the plates 4 to oscillate within the lead 3 creating a damping effect.

With the dampers as described incorporating the series of plates 4, the shaft 2 may be readily removed and replaced with a shaft with different characteristics. For example an engineer may specify a particular shaft design "off the shelf" for specific desired damping characteristics. As the shaft does not directly contact the lead 3, longer shaft life is also expected.

While in the damper of Figs 2 to 5 the shaft 2 contacts the lead 3 through the series of adjacent parallel plates 4 embedded within the lead, this is not essential. Wave shaped or spiral shaped or other shaped shafts as described may contact the lead directly i.e. without any plates 4 or equivalent embedded in the lead 3 through which the shaft passes. Fig. 6 shows another damper of the invention having an irregularly shaped shaft 2 which in this case has a varying diameter over the length of the shaft as shown. In the damper of Fig. 6 the shaft 2 contacts the lead 3 directly. The lead completely fills the interior of the outer jacket 1 surrounding the shaft 2. Expanding end seals 8 such as chevron seals are provided at either end of the outer jacket 1 which are retained in position by the end plates la of the outer jacket. Again, during motion such as an earthquake in a seismic isolation application, the shaft 2 is forced through the lead 3 which creates a damping effect.

The shaft 2 could also be formed as a double start or twin counter rotating ellipse or with triangular or groove-like variations. It is desirable however that the shaft is shaped so as to maintain a constant shaft volume within the lead at any time so that the lead is effectively moved within the damper as the shaft moves rather than the damper attempting to compress the lead.

In dampers of the invention the energy absorbing material such as lead may optionally be prestressed under an approximately hydrostatic pressure at least approaching and preferably exceeding the shear yield stress of the energy absorbing material so that the material will always be in compression. With lead pressures of 5MPa or more, typically lOMPa to 30MPa, but up to lOOMPa or more have been found effective.

The effect of a hydrostatic pressure may be explained briefly by way of the Mohr circle constructions shown in Figs 9 and 10, which enables the properly tensor description of stress to be represented in two dimensions. A hydrostatic pressure applied to a body is then defined as one third the sum of the three principal stresses which act upon it. In Fig. 9 the hydrostatic pressure is 0, and the principal tensile stress σ _x , the principal compressive stress σ _y and the maximum shear stress σ' _xy are all equal in magnitude. In Fig. 10 a hydrostatic pressure p equal to the shear stress σ'^ has been applied. The

maximum tensile stress is then 0 so that the body is always under compression. Therefore the body cannot fail in tension.

Various alternative arrangements for applying hydrostatic pressure to the lead (or other energy absorbing material) are possible. A pad or disc of elastomeric material such as rubber or a spring steel disc may be provided within the outer casing at one or both ends of the absorber material 3. The end caps la of the outer jacket may be designed so that when the end caps are fitted and bolted fully home they directly contact the lead and apply the desired hydrostatic pressure to the lead. Screw on end caps or multiple screw in plugs or hydraulically driven plugs which contact the lead at various points and apply pressure may alternatively be used. In Fig. 8 the absorber material is entirely encased in a layer of elastomeric material such as reinforced rubber to apply hydrostatic pressure.

In Fig. 7 a sleeve 10 with a circular weave of carbon fibre, bronze or an elastomeric material such as reinforced rubber surrounds the shaft 2. This sleeve 10 is desirably lubricated between the exterior of the sleeve and the lead 3. In Fig. 8 a similar casing 11 completely encloses the absorber material.

In each case the energy absorbing material 3 may be cast directly into place within the outer jacket 1 around the shaft 2. In the embodiments of Figs 7 and 8 the elastomeric

sleeve 10 or casing 11 may be stretched around the lead body after casting around the shaft 2 and the shaft and lead then press fitted into the outer casing 1.

As stated the damper may be cylindrical or alternatively oval, square, rectangular or any other desired shape in overall cross-sectional shape.

The shaft and also other parts of the damper may optionally be coated with teflon, porcelain, titanium nitride, a hard ceramic material, glass or similar.

Preferably during assembly of the dampers the component parts are coated with a high temperature/pressure grease or other lubricant.

The following description of testing of extrusion dampers of the invention further illustrates the invention:

TEST 1

A damper was constructed comprising a cylindrical outer jacket of steel of internal diameter 150 mm and interior length 312 mm. The shaft was a sinusoidally shaped shaft of square cross-section 33mm x 35mm. Each sinusoidal undulation had a wavelength of 42.85mm and a wave amplitude of ±2mm which was constant over the shaft length. The shaft passed through the centre of 31 circular steel plates of 102.5mm diameter and 10mm

thickness. Lead of 99.9% purity was cast into place between the outer jacket and the plates surrounding the shaft within the outer jacket. The shaft and all plates were coated with a high pressure lubricant prior to assembly. When the end cap of the outer jacket was bolted home the lead was subjected to approximately 30MPa hydrostatic pressure. An Instron test machine subjected the damper to cycles of shaft movement of displacement ±195mm with a maximum cross head speed of 200 mm/minute and a maximum force of 250 kN. The total device length was less than the stroke length of the shaft. The results were recorded directly on a chart recorder connected to the Inston and by a data logger. Fig. 11 shows shaft displacement against time. Fig. 12 shows the load resistance exhibited by the damper, against time. Fig. 13 shows the load resistance exhibited by the damper against displacement, showing successive hysteretic loops for successive cycles. After extended testing the damping force and energy absorbed per cycle were still within 20% of the starting values. At the completion of extended testing the damper was removed from the test rig and disassembled. The lead core was visually inspected and found to be in good condition.

The foregoing describes the invention including various preferred forms thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated herein as defined in the claims.