Seismic isolator

A seismic isolator comprises: a friction device having upper friction plate and a lower friction plate, a friction plane of the upper friction plate and the lower friction plate having a characteristic of Coulomb friction; and horizontally placed springs which reduce a relative displacement and a residual displacement. The upper friction plate includes being any one selected from the group consisting of a polyacetal resin containing lubricating oil, a bronze or brass alloy with solid lubricant buried therein, a sintered alloy with solid lubricant dispersed therein, a porous structure of cast iron with lubricant oil contained therein and a steel backing sintered layer with solid lubricant and oil impregnated therein. Furthermore, the lower friction plate includes being any one surface-treated material selected from the group consisting of a hard chromium-plated plate and a hard nickel-plated plate. A pre-tension is introduced into the horizontally placed springs, the tension being applied to the springs within an area of tension.

FIELD OF THE INVENTION 
1. Background of the Invention 
The present invention relates to a seismic isolator which reduces a 
response acceleration arising from an earthquake and restrains a response 
displacement as well as a residual displacement under a desired value. 
2. Description of the Prior Art 
In order to protect a structure or equipment (hereinafter referred to as 
"Objective" when abbreviated) from damage due to an earthquake, a seismic 
isolator is usually made use of. The seismic isolator is installed between 
the Objective and a foundation or a floor slab. Through the seismic 
isolator, a response acceleration arising from the earthquake is reduced 
to be transmitted to the Objective. As a device built in sucha seismic 
isolator, a damping rubber, a spring, a damper or a friction plate has 
been customarily used. Furthermore, a combination of an elastic material 
and a damping rubber or viscous fluid, or a combination of a spring and a 
friction plate or a damper has been often used. 
When springs are used, instead of friction plates for a seismic isolator, a 
pre-tension or a pre-compression is introduced into the springs so that an 
Objective is always fixed to the foundation or the floor slab. Therefore, 
when an earthquake occurs, the seismmic isolator does not operate until 
the inputted earthquake force, which is transferred to the Objective, 
exceeds the force of the pre-tension or pre-compression. The damping 
effect depends on the springs or dampers, not on the friction. 
A seismic isolator, having only friction plates, starts its operation, when 
the friction plates begin to slide. When a seismic isolator is practically 
used, in addition to friction plates, the seismic isolator has a device of 
a damping rubber or a spring which works before the friction plates slide, 
and the seismic isolator operates even upon a small earthquake. Those 
prior art seismic isolators have problems, described herein-below, to be 
overcome. 
A prior art seismic isolator, which has a slightly long natural period by 
means of arrangement of stiffness of an elastic rubber support so as to 
reduce a response acceleration, produces a large relative displacement 
between the Objective and the foundation or the floor slab even if a small 
earthquake occurs. Another prior art seismic isolator having a combination 
of an elastic rubber support and viscous fluid produces a relative 
displacement between the Objective and the foundation or the floor slab 
even on the small earthquake as well. The damping depends on the rubber or 
the viscous fluid. Therefore, not only the maintenance and inspection of 
the rubber or the viscous fluid but also the preventive measures for 
deterioration of those materials and for possible fire are required. 
In a further prior art seismic isolator having friction plates, the 
friction plates do not slide, due to their own friction force, in response 
to a small earthquake, but it is possible that a residual displacement 
becomes large when a strong earthquake occurs. A seismic isolator used in 
practice has a device comprised of a damping rubber of a spring placed 
below or on its friction plates, and therefore, the total displacement 
would become fairly large and, at the same time, a large residual 
displacement would remain because the shearing force of the rubber is 
produced when the friction plates begin to slide. It is also difficult to 
restore the residual displacement to the initial position of the 
Objective. When prior art friction plates are used, the friction force 
depends on a relative velocity between the friction plates. Consequently, 
not only the operating acceleration but also the damping capability are 
unstable. 
Computer processing units are usually guaranteed to maintain their function 
against a response acceleration of about 250 gal or less, but more than 
250 is not always guaranteed. The response acceleration of the structure 
or the computer equipment varies every moment during the earthquake. 
Therefore, in case that a prior art friction material is used in a seismic 
isolator, the displacement from the initial position of the Objective, 
which is produced during the operation, or the residual displacement 
therefrom has a tendency to increase because the friction force is greatly 
affected by the feature of the friction material. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a seismic isolator which 
not only reduces a response acceleration, transferred to a structure or 
equipment therein, arising from an earthquake but also restrains a 
response displacement and a residual displacement under a desired value, 
and which has a stable operating characteristic in the response 
acceleration, regardless of the acceleration arising from the earthquake 
being large or small. 
To attain the object in accordance with the present invention, a seismic 
isolator is provided, comprising: 
a friction device having an upper friction plate and a lower friction 
plate, a friction plane of the upper friction plate and the lower friction 
plate having a characteristic of Coulomb friction; and 
horizontally placed springs which reduce a relative displacement and a 
residual displacement. 
The above and other objects and advantages of the present invention will 
become apparent from the detailed description to follow, taken in 
conjunction with the appended drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
To overcome those problems in the prior art seismic isolators, a seismic 
isolator of the present invention comprises a friction device having an 
upper friction plate and a lower friction plate, a friction plane of the 
upper friction plate and the lower friction plate having Coulomb friction; 
and horizontally placed springs into which an initial tension being 
introduced. 
As the upper friction plate, any one material selected from the group 
consisting of a polyacetal resin containing lubricating oil, a bronze or 
brass alloy with solid lubricant buried therein, a sintered alloy with 
solid lubricant dispersed therein, a porous structure of cast iron with 
lubricant oil contained therein and a steel backing and sintered layer 
with solid lubricant dispersed and oil impregnated therein is used, and as 
the lower friction plate any one surface-treated material selected from 
the group consisting of a hard chromium-plated plate or a hard 
nickel-plated plate is used. 
Coulomb friction has a feature in that static coefficient of friction is 
equal to dynamic one and that the dynamic one is hardly affected by a 
relative velocity. Consequently, a seismic isolator having a friction 
device with a characteristic of the Coulomb friction and having a 
combination of springs and ball bearings provides the following 
advantages: 
(a) Since the capability of damping energy is subject to the feature of a 
friction material to be used, a response acceleration can be easily made 
stable and reduced. 
(b) The reduction of the acceleration in operation can be attained by means 
of substituting some pairs of the friction plates for such ball bearings 
as having a small coefficient of friction; and 
(c) By means of making use of the work of a spring system incorporated in 
the seismic isolator, the relative displacement can be restored and the 
residual displacement can also be within a predetermined range. 
Now, with specific reference to the appended drawings, a preferred 
embodiment of the present invention will be described. FIG. 1 
schematically shows an embodiment of a seismic isolator of the present 
invention, (a) of FIG. 1 being an elevation view of the embodiment, and 
(b) of FIG. 1 a plan view thereof. 
Referential numeral 11 denotes a foundation or a floor slab, 12 an 
Objective (a structure or equipment), 13 a seismic isolator, 14 a cover 
plate, 15 slide blocks, 16 a height adjuster with a ball joint 18, 17 an 
upper friction plate, 19 a slide plate, 20 a lower friction plate, 21 a 
damping rubber sheet for damping vibrations, 22 a beam element, 23 a bolt 
connection, 24 a pin joint, 25 a spring system, 26 springs horizontally 
placed, 27 a wire element, 28 a length adjuster, 29 pulleys, 30 a damper 
system, 31 a damper, 32 a stopper, 33 a displacement revision jig and 34 
ball bearings. 
As shown in FIG. 1, seismic isolator 13 is installed between foundation or 
floor slab 11 and Objective 12. A friction plane is formed by an upper 
friction plate 17 and lower friction plate 20, the upper friction plate 
being arranged below ball joint 18 and the lower friction plate being 
placed on slide plate 19 which is fixed, together with damping rubber 
sheet 21, on the foundation or the floor slab. The upper friction plate is 
made of a polyacetal resin containing lubricating oil and the lower 
friction plate is made of a hard chromium-plated stainless steel sheet. 
Furthermore, height adjuster 16 with ball joint 18 is guided and inserted 
vertically by and between side blocks 15 so as to adjust a height level of 
friction plate 17. The height adjuster 16 follows the slope deflection of 
the slide plate 19 at the time of setting or replacing the friction 
plates. On blocks 15, cover plate 14 is placed and Object 13 is laid on 
the blocks. The blocks are also connected to beam element 22 by means of 
bolt connection 23. 
Now, the work of a seismic isolator, which has the constitution as 
mentioned above, will be described herein-below. 
When an earthquake acceleration is inputted to seismic isolator 13, the 
friction plane of upper friction plate 17 and lower friction plate 20 
causes Coulomb friction, because a pair of those friction plates have a 
characteristic of the Coulomb friction. When the force inpputed to 
Objective 13 by the earthquake is smaller than the Coulomb friction force, 
no relative displacement is produced between friction plates 17 and 20. If 
the force inputted to the Objective becomes equal to or greater than the 
Coulomb friction force, a relative displacement is produced between the 
friction plates, and a spring force corresponding to the relative 
displacement is developed to work on spring system 25 as a restoring 
force. Damping rubber sheet 21, which is inserted fixedly between slide 
plate 19 and foundation or floor slab 11, is used in damping high 
frequency components of vibrations arising out of the earthquake. 
The stiffness of bolt connection 23 connecting beam element 22 to slide 
blocks 15 is controlled so as for the Objective to follow a deflection of 
foundation or floor slab 11. Stopper 32 takes a roll of stopping an 
excessive displacement. For the stopper, material, such as steel material, 
a vibration damping rubber and a visco-elastic material, having a large 
energy absorption capability is preferably used. 
Spring system 25 comprising springs 26, wire element 27 and a length 
adjuster 28 is looped through pulley 29 to be connected to pin joint 24 
which is fixed to beam element 22. Length adjuster 28 controls a length of 
wire element 27 to introduce pre-tension into springs 26. Since pulley 29 
is fixed to foundation or floor slab 11, spring system 25 lies between 
beam element 22 and foundation or floor slab 11. An initial pre-tension 
force is introduced, by means of length adjuster 28, into spring system 
25. As shown in FIG. 2, springs 26 are always used within an area where a 
tension varies so as to set the natural period of seismic isolator 13 as 
an optimum value. When damper 31 is provided with spring system 25 so as 
to increase the damping capability of the seismic isolator, a part of 
springs 26 is replaced by damper 31 as the case may be. As shown in FIG. 
1, at least one pair of the spring system or the damping system is 
installed in a diagonal direction, and thus, a seismic isolator which is 
workable in two horizontal directions can be easily obtained, one of the 
two directions crossing at right angles to the other. Furthermore, it is 
preferable to set displacement revision jig 33 around foundation or slab 
floor 11 so as to amend a position of a residual displacement. 
In addition, in order to further reduce a response acceleration in 
operation, it is also preferable that another device for rolling friction 
is arranged so as to operate in combination with the Coulomb friction 
function of upper friction plate 17 and lower friction plate 20. This 
device has simply the ball bearings, instead of the two friction plates 17 
and 20 forming the friction device. 
Parts for a seismic isolator are easy to be standardized into an individual 
unit, the seismic isolator is light in weight and low in level height and 
therefore, it is easy and simple for construction work. 
Moreover, it is of low construction cost and free from maintenance. 
It should be noted that a plurality of seismic isolators such as the 
embodiment shown in FIG. 1 can be combined, as one unit, into a large 
scale seismic isolator. 
In the seismic isolator of the present invention, an operating response 
acceleration arising from an earthquake excitation, a maximum displacement 
and a residual displacement can be put within a desired level by means of 
selecting a coefficient of friction or spring stiffness of material to be 
used because the Coulomb friction can be applied to the selected material. 
Furthermore, the response results become stable and reproducible. Even if a 
residual displacement exists, the displacement can be revised easily to 
the original position because the coefficient of friction is fairly small. 
While the preferred embodiment of the present invention has been described, 
it is to be understood that modifications will be apparent to those 
skilled in the art without departing from the spirit of the invention. The 
scope of the invention, therefore, is to be determined solely from the 
following claims.