Abstract:
A vibration and displacement damper includes a plunger piston ( 10 ) sliding with a slight clearance inside a guiding cylinder ( 20 ). The plunger piston includes over a part of its length a piston ring ( 12 ) sliding inside a main chamber ( 13 ) provided in the cylinder, the ring including calibrated through-orifices ( 14 ) for a high-viscosity fluid filling the chamber on both sides of the ring. The device is intended to damp vibrations and displacements, in particular of cable stays.

Description:
BACKGROUND OF THE INVENTION 
   The present invention basically relates to a vibration and displacement damper, more particularly one that is intended and is suitable for damping the vibrations of cable stays, for works such as suspension bridges, and for damping structures and buildings that are liable to become displaced and to develop resonant vibratory phenomena. 
   In order to damp vibrations and displacements, of this type, which are slight but highly repetitive, it is essential that the damper is able to withstand the forces to which it is subjected for very long periods of time, without receiving any maintenance, whilst at the same time displaying minimal variations in operation over the wide range of temperatures in which it is required to operate. 
   DESCRIPTION OF THE RELATED ART 
   U.S. Pat. No. 4,280,600 discloses a vibration damper in which the communication between a main chamber and secondary chambers is not controlled. 
   BE-458 209 discloses a vibration damper in which a main damping chamber is disposed in a loop, fluid being displaced by a blockage in the loop, which is equipped with a plurality of valves, the opening direction of which may vary, in which the damper frame is not cylindrical and the oil used is necessarily or low viscosity. 
   DE-2 623 622 discloses a vibration damper, which is incorporated into a suspended device intended for measuring the torque on industrial machines, which is not required to undergo significant variations in temperature, as is the case with cable stay dampers, and which uses a low-viscosity oil that varies considerably according to temperature. 
   DE-923 592 discloses a damper intended for vehicles and in which one of the secondary chambers is not inserted into a cylinder and the oil is necessarily of low viscosity. 
   JP-09 059 921 discloses the application of dampers for sustaining bridge cables. 
   SUMMARY OF THE INVENTION 
   The invention relates to a damper that comprises a plunger piston sliding on the inside of a guiding cylinder in a longitudinal direction, the plunger piston comprises over a part of its length a piston ring sliding with a slight clearance inside a main chamber provided in said cylinder, said ring comprises calibrated through-orifices for a high-viscosity fluid filling said chamber on both sides of said ring, said cylinder comprises, on either side of said main chamber in the longitudinal direction, two secondary chambers that the plunger piston enters, said secondary chambers are filled, at least in part, by said high-viscosity fluid and isolated, relative to the main chamber, from the entry of the high-viscosity fluid. A highly reliable and long-lasting damper may be obtained with a construction of this type, which does not require any form of seal. 
   Advantageously, the damper further comprises means for maintaining the two secondary chambers at substantially the same pressure. The fluid is thus prevented from becoming compressed in one of the chambers, such that the resistance to the, displacement of the piston relative to the cylinder is basically due to the shearing of the high-viscosity fluid in the main chamber, owing to the displacement of the ring. 
   In order to do this, according to a further advantageous, characteristic of the invention, one of the secondary chambers contains an air volume and is connected to the other secondary chamber, such that the high-viscosity fluid may circulate freely between these two secondary chambers. The air volume allows variations in the volume occupied by the fluid to be absorbed, without generating significant variations in pressure in the secondary chambers, whilst at the same time insulating the main chamber and the secondary chambers from the environment, the air being highly compressible. 
   Advantageously, in order to connect the two secondary chambers and to maintain them at substantially the same pressure, the damper comprises a duct, provided in the piston and opening into each of the secondary chambers. The fluid communication between the two secondary chambers is thus produced in a simple manner, and the piston is both light and very strong. 
   According to a further characteristic of the invention, the damper further comprises at least one through-path provided in the cylinder, in which means for limiting the passage of fluid from said main chamber toward the secondary chambers are inserted. In this way, it is ensured that the required quantity of damping fluid is maintained in the main chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be clarified by the following description, which refers to the accompanying drawings, in which: 
       FIG. 1  shows an application of a damping device for a cable stay used, for example, to support the deck of a bridge; 
       FIG. 2  is a longitudinal section of a damper constructed according to the invention; 
       FIG. 3  is a cross-section taken along the line III-III of  FIG. 2 ; 
       FIG. 4  is a longitudinal section of a further damper constructed according to the invention; 
       FIG. 5  is a cross-section taken along the line V-V of  FIG. 4 ; and 
       FIG. 6  is a longitudinal section of yet a further damper constructed according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows schematically part of the deck  1  of a bridge supported by a cable stay  2 , anchored to the deck at  3 . In order to damp the transverse vibrations to which the cable  2  is subjected as a result, for example, of wind and vehicles using the bridge, a damper, generally comprising an oleopneumatic cylinder, which is anchored at one of its ends, at  5 , to the cable  2  and at its other end, at  6 , to the deck  1 , has been provided at  4 . The stresses to which cylinders of this type are subjected are not generally very great, for example around 5 to 15 kN (kilonewtons), but the number of cycles is very high (between hundreds of thousands and several million per year of use), such that these devices require careful maintenance, in particular in respect of the seals and possible leakages. Furthermore, the characteristics of these devices vary considerably according to the temperature conditions to which they are subjected, such that in practice they have to be over-designed considerably in order to meet the requisite criteria in the least favourable conditions of use. 
   Reference will now be made to  FIGS. 1 and 2 , which show a damper constructed according to the invention. 
   The damper basically comprises a hollow plunger piston, designated in its entirety by the reference numeral  10 , and a guide cylinder, designated in its entirety by the reference numeral  20 , in which the plunger piston slides with a slight clearance in a substantially vertical longitudinal direction  30 . The guiding and sliding surfaces are located, in the embodiment shown, in the region of the cooperating surfaces, designated in  FIG. 2  by the reference numerals  7 ,  8 ,  9  and  11 . In order to assist sliding, the plunger piston  10  may, for example, be made from (surface-treated) chrome-plated steel, while the cylinder  20  may be produced with bronze guide bearings  7 ,  9  and  11 . Over an intermediate part of its length, the plunger piston  10  comprises a ring  12  (in the form of a thick disc), which slides with a slight clearance (in the region of the surface  8 ) inside a main chamber  13  provided in the cylinder  20  and completely filled with a high-viscosity damping fluid. The ring  12  of the piston comprises a specific number of calibrated orifices  14  (eight in the example shown, as may be seen from  FIG. 3 ), which allow the high-viscosity fluid to move from one side to the other of the ring  12  inside the chamber  13 , thus performing the damping function of the device. It will be noted that, owing to the slight clearance in the region of the surface  8 , the passage of fluid along this surface will generally be much less than the rate of passage through the calibrated orifices  14 . 
   In order to ensure long-term functioning of the damper, two secondary chambers,  15  and  16  respectively, which, in the embodiment shown, both communicate with the main chamber  13  via through-paths  17 ,  18 , to which non-return valves  19 ,  21  are fitted, are provided either side of the main chamber  13 . Jets or other similar means may be provided instead of the non-return valves  19 ,  21 . These means are intended to limit the leakage of fluid from the chamber  13  toward the chambers  15  and  16 , and to facilitate it from said secondary chambers toward the main chamber. 
   The piston  10  comprises a rod  26  having, in the longitudinal direction  30 , a fixing end  23  and a free end  28 , disposed inside the secondary chamber  15 . Said rod  26  is hollow over most of its length. It thus defines an internal conduit  27 , opening at the free end  28  into the secondary chamber  15 . The internal conduit communicates, near the fixing end  23  with the secondary chamber  16 , via orifices  29 . 
   The internal conduit  27  thus places the secondary chambers  15  and  16  in, fluid communication, such that they are at substantially the same pressure. 
   Furthermore, the secondary chamber  15  is completely filled with damping fluid, in contrast to the secondary chamber  16 , which contains an air volume  31  in its upper part. A bore  32 , delimited in particular by the guide bearing  11 , provided in the cylinder  20  and crossed by the rod  26 , has a first end  32   a  that is open to the exterior and a second end  32   b  opening into said air volume  31 , in which it is entirely enclosed. 
   Thus, even if the seal between the rod  26  and the bore  32  is not perfect, or if indeed there is no seal, there is no leakage of damping fluid. As shown, the bore  32  is the only opening made in the cylinder  20 , extending between the inside of the cylinder and the surrounding environment. 
   The device is, of course, completed by fixings at the two ends, one  22  provided on the cylinder  20  and the other  23  on the piston  10  respectively, in order to install the damper on the structure to be damped. 
   The embodiment shown in  FIGS. 4 and 5  differs from the embodiment shown in  FIGS. 2 and 3  only in that, as the damping device is intended to be fitted substantially horizontally, and not substantially vertically, as is the case with the embodiment of  FIGS. 2 and 3 , a complementary chamber  24 , which communicates with one of the secondary chambers  16  via at least two holes  25 , has also been provided. The construction is otherwise the same and will not be described any further, identical parts also being designated by identical reference numerals in  FIG. 2 and 4 . 
   The embodiment shown in  FIG. 6  shows three independent modifications that may be made to the embodiment shown in  FIGS. 4 and 5 , even though these modifications are not recommended. 
   The rod  26  of the piston  10  is solid, the damping fluid circulates between the secondary chambers  15  and  16  via an ancillary conduit  33 . 
   The damper is provided with non-return valves  19  only between one  15  of the secondary chambers and the main chamber  13 . 
   The secondary chamber  16  does not have a complementary chamber  24  and contains the air volume  31 . The important thing is that the air volume  31  is sufficiently large to absorb the differences in volume of damping fluid, but not too large, relative to the volume of the secondary chamber  16 , to prevent air from entering the main chamber  13 . 
   Tests have shown that dampers of this type operate in a highly satisfactory manner with viscous or very viscous fluids, the. viscosity having to be greater than or equal to 500 cSt and being able to go up to several million centistokes. For viscosities of this type, the variations in ambient temperature have little effect on damping efficiency, and virtually no fluid is lost over time. More precisely, even if the viscosity varies with temperature, it is still sufficiently high at the most elevated temperatures for the damping variations to be slight. 
   The absorbed stresses may range from 1 to 1000 kN and will generally be around 5 to 15 kN for a movement varying from 0 to ±100 mm. The dimensions of the damper remain relatively small; according to one embodiment, the damper had a length of approximately 30 cm with an external diameter of approximately 7 cm.