Abstract:
There is disclosed a device comprising two wire rope isolators for isolating an object from external motions. 
     The wire rope isolators are arranged such that, if one of the two wire rope isolators is loaded in compression, then the other wire rope isolator is loaded in tension.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national phase filing under 35 U.S.C. §371 of PCT/EP2011/058675 filed May 26, 2011, which application claims priority to and the benefit of European Application No. 10164781.6, filed on Jun. 2, 2010. All of the aforementioned applications are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a device for isolating an object from external motions. For example, the invention is particularly applicable to mechanical isolators against shocks and vibrations. 
     BACKGROUND 
     Absorbing shocks and vibrations is a typical problem encountered in mechanics related domains. Most usual solutions are based on combining a spring, for example a helical or a disc-shaped belleville spring, with a unidirectional damper, for example a viscous liquid damper or a rubbery/elastomeric damper. A major disadvantage of these isolators is that they involve complex constructions, especially to ensure liquid or air sealing. In addition, they often involve hitting between elements. Moreover, their properties may depend on the ambient temperature. 
     Wire rope isolators (WRI&#39;s) constitute another common type of mechanical isolators against shocks and vibrations, including for example polycal WRI&#39;s, helical WRI&#39;s, ring-type WRI&#39;s, straight cable WRI&#39;s and other special WRI&#39;s. The  FIG. 1  illustrates a helical WRI of the prior art, along with its main loading directions. The exemplary helical WRI comprises two retainer bar assemblies arranged parallel with an x-axis, each retainer bar assembly comprising holes. The two retainer bar assemblies are bound one to the other by a single cable, the cable bent between the bar assemblies, passing through their holes and generally clamped by each of the retainer bar assemblies using screws. The direction along the x-axis is called the shear direction, the direction along the y-axis is called the roll direction and the direction along the z-axis is called the tension-compression direction. For polycal WRI&#39;s, the difference between the roll and the shear directions is less obvious. For ring-type WRI&#39;s, the roll and the shear directions are equivalent and best known as the radial direction. 
     A major disadvantage of WRI&#39;s is that they are omnidirectional dampers with directionally dependent stiffness and damping properties, which results in the circumstance that the tuning of an application based on WRI&#39;s is difficult. 
     Yet another disadvantage of WRI&#39;s is that the maximum attainable distance from a single interface plane with a given size to the elastic centre of an optimised set-up of spring-damper elements will be smaller for a set-up with omnidirectional springs as compared to an optimised set-up with unidirectional springs. That is, if only a single interface plane is available, then achieving balance of an isolated object requires more space using a set-up with omnidirectional springs. This drawback of the WRI&#39;s will be further explicated in the following, as well as how it may be overcome by the present invention. 
     Yet another disadvantage of WRI&#39;s is that the size of an omnidirectional WRI in the tension-compression direction generally becomes significantly larger due to repeated force-deflection cycling in the tension-compression direction and even due to repeated force-deflection cycling in the roll direction. This effect is believed to be caused by plastic deformation of the wires of the steel cable. Because the external load levels and the accompanying material stress levels are much higher upon tension than compression, the plastic deformation tends to increase the size of the WRI in the tension direction. A consequence of this phenomenon is that the average gravity loaded position of an object isolated with omnidirectional WRI&#39;s, relative to its direct surroundings, is not constant, but changing with repeated loading during the lifetime of the WRI&#39;s. In addition, depending on the location and orientation of the WRI&#39;s, the average orientation of the isolated object may be affected. Moreover, due to the increase of size in the tension direction, the amount of travel available for shock isolation in the tension direction becomes smaller than initial, resulting in higher maximum residual accelerations. 
     In an attempt to overcome some of the aforementioned drawbacks, the U.S. Pat. No. 5,482,259 discloses a unidirectional damper to be used as a pipe restraint, which makes use of the shear direction of a single helical WRI. A major disadvantage of a unidirectional damper according to the U.S. Pat. No. 5,482,259 is that it is hardly applicable to practical shock damping, as shock damping requires a rather high (initial) stiffness, in order to limit the displacements due to gravity and dynamic excitations with low frequency content. In fact, the mass of a unidirectional damper according to U.S. Pat. No. 5,482,259, which would be required to achieve a stiffness value suitable for practical shock damping, would be very large. 
     SUMMARY 
     The present invention aims at providing a solution that may be used to overcome at least some of the technical problems described above. In particular, it aims to provide a device with high (initial) stiffness over own mass ratio, which may thus be suitable for shock damping. At its most general, the invention proposes a device comprising two wire rope isolators for isolating an object from external motions. The wire rope isolators are arranged such that, if one of the two wire rope isolators is loaded in compression, then the other wire rope isolator is loaded in tension. 
     Advantageously, each of the two wire rope isolators comprising two retainer bars flexibly connected one to the other by at least one cable, the device may comprise linear guiding means allowing only for a relative displacement of some of the retainer bars with respect to the other retainer bars in a single direction, said direction being the common tension-compression direction of the two wire rope isolators. 
     In a preferred embodiment, the device comprising an upper wire rope isolator and a lower wire rope isolator, each wire rope isolator comprising an upper retainer bar and a lower retainer bar, the upper retainer bar of the upper wire rope isolator may be rigidly connected to the lower retainer bar of the lower wire rope isolator by virtue of bridging means. The lower retainer bar of the upper wire rope isolator may be rigidly connected to the upper retainer bar of the lower wire rope isolator, said rigidly connected retainer bars forming a central retainer bar assembly between the two wire rope isolators. 
     Preferably, the bridging means may include at least one tube, of which one of the ends may be rigidly connected to the upper retainer bar of the upper wire rope isolator and of which other end may be rigidly connected to the lower retainer bar of the lower wire rope isolator. 
     Preferably, the linear guiding means may comprise a hollow connecting rod rigidly connected to the central retainer bar assembly and arranged substantially parallel with the common tension-compression direction of the two wire rope isolators. The linear guiding means may also comprise a guiding pin rigidly connected to the lower retainer bar of the lower wire rope isolator and arranged substantially parallel with the common tension-compression direction of the two wire rope isolators. The linear guiding means may also comprise a guide bush rigidly connected to the upper retainer bar of the upper wire rope isolator. The hollow connecting rod may slide through the guide bush and over the guiding pin. 
     For example, the hollow connecting rod may comprise, at its end opposite to the guiding pin, an upper universal revolute joint, and the guiding pin may comprise, at its end opposite to the hollow connecting rod, a lower universal revolute joint. The two universal revolute joints may be spherical bearings. 
     Advantageously, at least one of the two wire rope isolators may be a polycal wire rope isolator or a helical wire rope isolator or a ring-type wire rope isolator. 
     Preferably, the device may comprise means to limit rotation around the common tension-compression direction of the two wire rope isolators. For example, these means may comprise a pin extending through one of the spherical bearings, one of the retainer bars comprising two protrusions arranged so as to slide over the cylindrical surface of the pin when the isolated object moves. In another embodiment, the means to limit rotation may comprise one or two cables rigidly connected to one of the retainer bars at one of their ends, the cables being rigidly connected to the surroundings of the object at their other ends. 
     Thus, a major advantage provided by a device according to the present invention is that it is provides a simple, reliable and easy-to-tune unidirectional damper, with more linear and substantially temperature-independent force-deflection properties and anti-bottoming feature. 
     Another advantage provided by a damper according to the present invention is that, despite plastic deformation occurs during use in each of its two WRI&#39;s, similarly as in a single WRI, their total external dimension in the tension-compression direction is constrained by the bridging means. Thus, an internal compression load develops by repeated loading during the lifetime of the damper. Because the total dynamic loads on and the resulting plastic deformation in each of the two WRI&#39;s is approximately equal, the average position of the central retainer bar assembly and thus the external dimension of two assembled WRI&#39;s is not significantly affected. Thus, the position and orientation of an object isolated with the damper is not affected by repeated loading as much as for an object isolated with common omnidirectional WRI&#39;s. Also the available shock travel in both loading directions remains approximately equal to the initial value, resulting in similar maximum residual accelerations for similar shocks during the lifetime of the damper. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting examples of the invention are described below with reference to the accompanying drawings in which: 
         FIG. 1  schematically illustrates a helical Wipe Rope Isolator according to the prior art; 
         FIG. 2  schematically illustrates by a cross section an exemplary embodiment of a unidirectional damper according to the invention; 
         FIGS. 3 a  and 3 b    schematically illustrate by perspective views another exemplary embodiment of a unidirectional damper according to the invention; 
         FIGS. 4 a  and 4 b    schematically illustrate by perspective views yet another exemplary embodiment of an unidirectional damper according to the invention; 
         FIG. 5  schematically illustrates graphically the efficiency of an exemplary unidirectional damper according to the invention; 
         FIGS. 6 a , 6 b , 6 c    and  7  schematically illustrate further advantages of using unidirectional dampers according to the present invention; 
         FIGS. 8 and 9  schematically illustrate by perspective views exemplary mountings to the surroundings of an unidirectional damper according to the invention including means to limit the rotation of the damper around the common tension-compression direction. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  schematically illustrates by a cross section an exemplary embodiment of a unidirectional damper according to the invention. The exemplary embodiment may comprise two similar WRI&#39;s, for example two standard polycal WRI&#39;s. An upper polycal WRI may comprise an upper retainer bar  1  and a lower retainer bar  2 , the upper retainer bar  1  and the lower retainer bar  2  being flexibly connected by cable loops  3  and  4 . A lower polycal WRI may comprise a lower retainer bar  6  and an upper retainer bar  5 , the upper retainer bar  5  and the lower retainer bar  6  being flexibly connected by cable loops  7  and  8 . The two adjoining retainer bars  2  and  5  of the upper and the lower WRI respectively, may be rigidly connected. In another embodiment, the two retainer bars  2  and  5  may be replaced by a same and single integrated retainer bar. In the present embodiment, the two retainer bars  2  and  5  may be rigidly connected to a hollow connecting rod  9 . The hollow connecting rod  9  may slide over a central guiding pin  10  as well as slide through a guide bush  12 . The sliding interfaces may be realized by loose fits. The central guiding pin  10  may be rigidly connected to the lower retainer bar  6 . The guide bush  12  may be rigidly connected to the upper retainer bar  1 . It is important noting that the hollow connecting rod  9  and the guiding pin  10  are arranged parallel with the tension-compression direction of the two WRI&#39;s. A rigid connection may bridge between the upper and lower retainer bars  1  and  6 , for example two hollow tubes  13  and  14  mounted to the upper and lower retainer bars  1  and  6  by bolts or nuts. Universal revolute joints may be arranged at both mounting interfaces, for example implemented by spherical bearings  15  and  16 , so as to obtain a unidirectional mode of working whilst allowing relative rotations to the surroundings to which the damper is connected. The guiding pin  10 , the hollow connecting rod  9  and the guide bush  12  form a linear guiding system, which ensures a relative rectilinear motion between the mounting interfaces of the damper. It also enables a sufficiently high bending stiffness and sufficiently high resonance frequencies of the bending modes of the damper, as well as prevents buckling during compression of the damper. 
     During displacements of the hollow connecting rod  9  over the central guiding pin  10  and through the guide bush  12 , the upper and the lower WRI&#39;s are used in their compression and tension directions only, these directions providing the highest stiffness, thus resulting in an unidirectional combination with the highest possible stiffness over own mass ratio. In addition, the compression of one WRI is accompanied with the tension of the other WRI. Actually, their output reaction forces are summed, as they are mechanically placed parallel. Hereby, the stiffening behaviour of the tension direction of one WRI is partly compensated by the softening behaviour of the compression direction of the other WRI. This results in a more linear force-deflection characteristic. The maximum displacement of the combination equals the maximum displacement of each individual WRI. 
     Moreover, at maximum displacements of the hollow connecting rod  9  over the central guiding pin  10  and through the guide bush  12 , the cable loops of the WRI loaded in tension tend to straighten, by which the force increases more rapidly, resulting in a stiffening force-deflection behaviour and in an effective anti-bottoming measure. 
       FIGS. 3 a  and 3 b    schematically illustrate by perspective views another exemplary embodiment of a unidirectional damper according to the invention, based on commercially available polycal WRI&#39;s. The extended position of the damper is illustrated by  FIG. 3 a   , while its compressed position is illustrated by  FIG. 3 b   . For the sake of clarity, references have been inserted only in  FIG. 3 a   . The damper may comprise an upper polycal WRI, said upper polycal WRI comprising an outer part  37  and an inner part  38  of an upper retainer bar, as well as an inner part  39  and an outer part  40  of a lower retainer bar, said upper and lower retainer bars being connected by four upper cables  49   a ,  49   b ,  49   c  and  49   d . The damper may also comprise a lower polycal WRI, said lower polycal comprising an outer part  41  and an inner part  42  of an upper retainer bar, as well as an inner part  45  and an outer part  46  of a lower retainer bar, said upper and lower retainer bars being connected by four lower cables  50   a ,  50   b ,  50   c  and  50   d . The outer part  40  of the lower retainer bar of the upper polycal WRI may be rigidly connected to the outer part  41  of the upper retainer bar of the lower polycal WRI by virtue of a stud and of screws, which are not visible on  FIGS. 3 a  and 3 b   . The damper may comprise a connecting rod  32  with an upper rod end  31 , the connecting rod  32  sliding through a guide bush  34  and over a guiding pin  43  with a lower rod end  48 . The guide bush  34  may be rigidly connected to an upper flange  33 . The upper flange  33  may be rigidly connected to two tubes  44  and  44  by virtue of two upper bolts  35  and  35 ′ respectively. The two tubes  44  and  44 ′ may be rigidly connected to a lower flange  47  by virtue of two bolts respectively, which are not visible on  FIGS. 3 a  and 3 b   . The outer part  37  of the upper retainer bar of the upper polycal WRI may be rigidly connected to the upper flange  33  by virtue of two upper screws  36  and  36 ′, while the outer part  46  of the lower retainer bar of the lower polycal WRI may be rigidly connected to the lower flange  47  by virtue of two screws, which are not visible on  FIGS. 3 a    and  3   b.    
     It is worth noting that, in order to achieve a design with less parts and a smaller mass, the design may be further optimised, especially by modifying the central retainer bar assembly between the two polycal WRI&#39;s and by replacing the outer part of the upper retainer bar of the upper WRI and the adjoining flange with a single part as well as by replacing the outer part of the lower retainer bar of the lower WRI and the adjoining flange with a single part. 
       FIGS. 4 a  and 4 b    schematically illustrate by perspective views yet another exemplary embodiment of a unidirectional damper according to the invention, in which two sets of two helically wound cables are clamped by three retainer bar assemblies. The extended position of the damper is illustrated by  FIG. 4 a   , while its compressed position is illustrated by  FIG. 4 b   . For the sake of clarity, references have been inserted only in  FIG. 4 a   . The damper may comprise an upper WRI comprising an outer part  63  and an inner part  66  of an upper retainer bar, as well as an upper part  68  and a central part  69  of a middle retainer bar, said upper and middle retainer bar being connected by two upper cables  77   a  and  77   b . The outer part  63  and the inner part  66  of the upper retainer bar may be rigidly connected by virtue of eight screws  79   a  to  79   h , the screws  79   b  to  79   h  not being visible on  FIGS. 4 a  and 4 b   . The damper may also comprise a lower WRI comprising an outer part  75  and an inner part  74  of an lower retainer bar, as well as a lower part  70  and the central part  69  of the middle retainer bar, said lower and middle retainer bar being connected by two lower cables  78   a  and  78   b . The outer part  75  and the inner part  74  of the lower retainer bar may be rigidly connected by virtue of eight screws  81   a  to  81   h , the screw  81   e  not being visible on  FIGS. 4 a  and 4 b   . The upper part  68 , the central part  69  and the lower part  70  of the middle retainer bar may be rigidly connected by virtue of ten screws  80   a  to  80   j , the screws  80   d ,  80   e ,  80   f ,  80   g ,  80   h ,  80   i  and  80   j  not being visible on  FIGS. 4 a  and 4 b   . Alternatively, in order to assist more easy mounting of the unidirectional damper assembly, only the central part  69  and the lower part  70  of the middle retainer bar may be rigidly connected by the virtue of ten (shorter) screws  80   a  to  80   j , the screws  80   d ,  80   e ,  80   f ,  80   g ,  80   h ,  80   i  and  80   j  not being visible on  FIGS. 4 a  and 4 b   , while the upper part  68  and the central part  69  of the middle retainer bar may be rigidly connected by the virtue of ten screws  80   k  to  80   t , none of which is visible on  FIGS. 4 a  and 4 b   . The damper may comprise a connecting rod  62  with an upper spherical bearing  61 , the connecting rod  62  sliding through a guide bush, which is not visible on  FIGS. 4 a  and 4 b   , and over a guiding pin  71 . The inner part  66  of the upper retainer bar of the upper WRI may be rigidly connected to two tubes  72  and  72 , the tube  72  not being visible on  FIG. 4 a    however being visible on  FIG. 4 b   , by virtue of bolts  65  and  65 ′ respectively, the bolt  65 ′ not being visible on  FIGS. 4 a  and 4 b   . The two tubes  72  and  72 ′ may be rigidly connected to the outer part  75  of the lower retainer bar of the lower WRI by virtue of two bolts respectively, which are not visible on  FIGS. 4 a  and 4 b   . The outer part  75  of the lower retainer bar of the lower WRI may comprise a lower spherical bearing  76 . 
     It is important understanding that, despite the preceding exemplary embodiments of the invention are based on polycal and helical WRI&#39;s, other types of WRI&#39;s may also be put to use without departing from the scope of the present invention. It is also worth noting that the exemplary embodiment with helical WRI&#39;s may have a higher stiffness and may thus be suitable for higher loads than the embodiment with polycal WRI&#39;s, because more cable loops are available and because the maximum cable diameter of commercially available helical WRI&#39;s is larger than that of commercially available polycal WRI&#39;s. Anyway, the type and the class of WRI&#39;s should be chosen on the basis of the stiffness and maximum displacement requirements. Then, the guiding and bridging means should be physically combined by virtue of dedicated WRI retainer bar assemblies. For the cables, best austenitic stainless steel should be used, similar as the steel used in common WRI&#39;s. For the screws and bolts, also austenitic stainless steel should be preferred. For the other parts, a high strength aluminium alloy should preferably be used, for example a precipitation hardening copper containing aluminium alloy. For parts of the linear guidance system, such as the guide bush, the guiding pin and the connecting rod, ferritic, martensitic or austenitic stainless steel may optionally be used in order to reduce friction and wear of the sliding interfaces. 
       FIG. 5  shows force-deflection curves in a system of axes where the horizontal axis represents a displacement in millimeters (mm) and the vertical axis represents a force in kilo-Newton (kN). A force-deflection curve  1  has been actually measured in the tension compression direction of a real single polycal WRI for a displacement ranging from −60 to +60 mm, after previous repeated loading with smaller displacement ranges. A curve  2  has been obtained by reflection of the curve  1  using the origin (0 mm, 0 N) as the point of reflection. The curve  2  is theoretically valid for a polycal WRI operating in the opposite direction. A curve  3  has been obtained by summation of the curve  1  and the curve  2 . The curve  3  is theoretically valid for a hypothetical unidirectional combination of two polycal WRI&#39;s. A curve  4  has been actually measured in the tension compression direction of a real prototype of a damper according to the present invention. 
     It is important noticing that the average load of curve  1  at 0 mm displacement is not 0 N, but about −2 kN. This is due to the plastic deformation in the polycal stainless steel cables, which occurred during the previous applied repeated force deflection cycling. At 0 mm relative displacement, which corresponds to the original size of the polycal WRI in tension direction, a compressive load developed. The unloaded size of the polycal WRI is about 20 mm larger than original. 
     The curve  3  is a theoretical approximation of the behaviour of a unidirectional damper according to the invention, incorporating the force-deflection behaviour of the two polycal WRI&#39;s only. It appears that the measured force-deflection behaviour of the prototype of the unidirectional damper represented by the curve  4  has the same shape, but is significantly less stiff. This is due to the limited stiffness of the other parts used for construction of the prototype. Especially the separate upper and lower flanges  33  and  47  illustrated by  FIGS. 3 a  and 3 b    show appreciable bending. By integrating these flanges with parts of the retainer bars a more stiff construction is obtained. This has been realised in the design of the unidirectional damper illustrated by  FIGS. 4 a    and  4   b.    
     In view of the preceding, it is clear that the present invention enables straightforward balancing of isolated objects. 
     In addition,  FIGS. 6 a , 6 b  and 6 c    illustrate another major advantage of an unidirectional damper according to the invention as compared to omnidirectional WRI&#39;s using 2-dimensional sketches. 
       FIG. 6 a    illustrates an object  90  having a squared section of a length a. The object  90  is isolated by virtue of a mechanical isolation set-up comprising two unidirectional dampers  91  and  92  according to the invention. The dampers  91  and  92  are arranged with a distance b between their lower ends and with their main axes making angles of 45° with the vertical direction, such that the elastic centre EC a  of the isolation set-up is at the same location as the centre of gravity CoG of the object  90 , EC and CoG standing at a same height h ECa . 
       FIG. 6 b    illustrates the same object  90  isolated by virtue of a mechanical isolation set-up comprising two common polycal or helical WRI&#39;s  93  and  94  arranged with a distance c between their lower ends and with their tension-compression direction parallel to the vertical direction, such that the elastic centre EC b  of the isolation set-up is at the same location as the centre of gravity CoG of the object  90 , EC and CoG standing at a same height h ECb . 
       FIG. 6 c    illustrates the same object  90  isolated by use of another mechanical isolation set-up comprising the two same WRI&#39;s  93  and  94  arranged with a distance d between their lower ends and with their tension-compression directions making angles of 45° with the vertical direction, such that the elastic centre EC c  of the isolation set-up is at the same location as the centre of gravity CoG of the object  90 , EC and CoG standing at a same height h ECc . 
     For each of these three set-ups, the elastic centre coincides with the centre of gravity of the object  90 , resulting in a balanced set-up. A balanced set-up means that the that rotations occurring due to translational accelerations are minimized. Vice versa, translations occurring due to rotational accelerations are minimized. Also in this case, the natural vibration modes of the isolated object  90  are uncoupled into pure translation and rotation modes. 
     With regard to  FIGS. 6 a , 6 b  and 6 c   , the following relations between the distances a, b, c and d and between the heights h ECa , h ECb  and h ECc  are valid:
 
b&gt;d&gt;c&gt;a
 
h ECa &gt;h ECc &gt;h ECb  
 
     The advantage concerns the location of the elastic centre of a mechanical isolation set-up if only a single interface plane to the surroundings is available. The distance from the interface plane to the elastic centre can be larger if unidirectional dampers are employed. For a balanced set-up, the centre of gravity of the mechanically isolated object and the elastic centre have to coincide. A balanced set-up means that rotations occurring due to translational accelerations are minimized. Also in this case, the natural vibration modes of the isolated object are uncoupled into pure translation and rotation modes. Thus, by use of unidirectional dampers according to the invention, higher balanced isolated objects may be achieved. 
     It is also worth noting that the two unidirectional dampers according to the invention of  FIG. 6 a   , which include four common WRI&#39;s in total, theoretically have twice the mass loading capacity of a single common WRI, because two mutually perpendicular directions are supported. For easy and fair comparison, fair meaning that the stiffnesses in vertical direction, the mass loading capacities in vertical direction as well as the frequencies of the resonance mode in vertical direction of the set-ups in  FIGS. 6 a  to 6 c    are approximately equal, one common omnidirectional WRI is assumed to be arranged at each corner of the object  90  for the set-ups of  FIGS. 6 b  and 6 c   . Also omnidirectional WRI&#39;s can be placed slanted, for example by 45° as depicted on  FIG. 6 c   . However their transverse stiffness, which is usually 20 to 50% of the tension-compression stiffness, still limits the attainable height of the elastic centre. Usually the mass load capacity of WRI&#39;s in the 45° roll-tension-compression direction is presented by WRI suppliers as equal to that of the tension-compression direction. Actually it is smaller, because the load capacity of the roll direction is smaller. Because in a unidrectional damper according to the invention only the stiffness in tension-compression direction is employed and because additional parts are needed, the total effective stiffness over own mass ratio of a complete set-up is smaller and thus worse than for a set-up with common omnidirectional WRI&#39;s. However, by use of unidirectional dampers according to the invention, higher balanced isolated objects may be achieved if only a single interface plane to the surroundings is available. 
     The aforementioned advantage concerning the attainable height of the balanced isolated objects is further illustrated by  FIG. 7 , which illustrates again the object  90  mechanically isolated by virtue of the same three mechanical isolator set-ups as those illustrated by  FIGS. 6 a , 6 b    and  6   c.    
     For the set-up with the unidirectional dampers  91  and  92  according to the invention, the elastic centre is at the height of the crossing of the lines through the main axes of the unidirectional dampers  91  and  92 . For the set-up with the common WRI&#39;s  93  and  94  arranged slanted by 45°, the elastic centre is at a location depending on the relative stiffness in tension-compression and roll (or shear) directions of the common WRI&#39;s  93  and  94 . For the set-up with the common WRI&#39;s  93  and  94  arranged vertically, the elastic centre is at the same height as the height of the geometric centre of the common WRI&#39;s  93  and  94 . 
     Obviously, for the set-up with the unidirectional dampers  91  and  92  according to the invention, the distance from either their upper or lower interface plane to the horizontal plane passing through the centre of gravity is larger than the corresponding distances for the other set-ups with common WRI&#39;s  93  and  94 , i.e. the following relations are valid:
 
h1L=h ECa &gt;h2L=h ECc &gt;h3L=h ECb  
 
h1U&gt;h2U&gt;h3U
 
     This is yet another advantage if a balanced suspension set-up is required, while only a single interface plane is available for mounting the set-ups and when the distance between the centre of gravity of the object  90  and this interface plane is relatively large, i.e. about one half of the main horizontal dimensions of the object  90 . 
     An unidirectional damper according to the invention may be connected to relatively stable surroundings by virtue of mounting feet as illustrated by  FIGS. 8 and 9 , which show similar exemplary unidirectional dampers as those illustrated by  FIGS. 4 a  and 4 b   . A rigid connection between a lower foot  100  and the inner part of the lower spherical bearing  76  is realized by a lower pin  101  and a rigid connection between an upper foot  102  and the inner part of the upper spherical bearing  61  is realized by an upper pin  103 . After mounting of both the upper and lower feet  100  and  102  to the surroundings respectively, rotation of a unidirectional damper according to  FIGS. 4 a  and 4 b    around it&#39;s main axis remains possible, until for example the outer part  75  of the lower retainer bar of the lower WRI hits against the lower foot  100 . To prevent such hitting, additional rotation limiting means may be added. 
       FIG. 8  illustrates exemplary rotation limiting means, which may comprise two additional protrusions  104   a  and  104   b  of the outer part  75  of the lower retainer bar of the lower WRI. The additional protrusions  104   a  and  104   b  may be realized by separate parts attached to the outer part  75  or preferably as machined features of the outer part  75  itself. The protrusions  104   a  and  104   b  may slide over the cylindrical surface of the lower pin  101 , which may advantageously extend beyond the lower foot  101 . The contact of each of the protrusions  104   a  and  104   b  with lower pin  101  may be constituted by a small loose fit or show limited preload generated by the elastic properties and particular dimensioning of the adjoining parts. 
       FIG. 9  illustrates other exemplary rotation limiting means, which may comprise at least one of two or possibly two stainless steel anti-rotation cables  110   a  and  110   b , with a relatively small diameter, flexibly connecting the lower retainer bar of the lower WRI to the surroundings. The connection of the cable  110   a  to the surroundings may be realized by clamping the cable  110   a  between two blocks  111   a  and  112   a , the blocks  111   a  and  112   a  being hold tightly together with bolts  113   a  and  113   b . Similarly, the connection of the cable  110   b  to the surroundings may be realized by clamping the cable  110   b  between two blocks  111   b  and  112   b , the blocks  111   b  and  112   b  being hold tightly together with bolts  113   c  and  113   d.