Abstract:
A fluid filled vibration damping device including: an elastic body elastically connecting a first and a second mounting member and partially defining a pressure receiving chamber filled with a non-compressible fluid and undergoing fluid pressure variation upon application of vibrational load between the first and second mounting members; a flexible diaphragm partially defining an equilibrium chamber filled with the non-compressible fluid and whose volume is variable; and an orifice passage permitting a fluid communication between the pressure-receiving chamber and the equilibrium chamber. The flexible diaphragm is constituted by a rubber elastic layer having an annular thick-walled portion and a central thick-walled portion situated at a substantially central portion of an inner area surrounded by the annular thick-walled portion, while being independent of the annular thick-walled portion.

Description:
INCORPORATED BY REFERENCE 
   The disclosure of Japanese Patent Application No. 2003-024383 filed on Jan. 31, 2003 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to fluid filled vibration damping devices exhibiting damping effect on the basis of flows of the non-compressible fluid sealed therein, and more particularly to a fluid filled vibration damping device of novel construction, which may be usable as an engine mount or other mounts for use in automotive vehicles. 
   2. Description of the Related Art 
   A fluid filled vibration damping device is known as one type of a vibration-damping coupling or mount adapted to be installed between two members of a vibration systems so as to elastically connect or support the two members in a vibration-damping fashion. A typical fluid filled vibration-damping device includes, as shown in JP-A-2001-59540 for example, a rubber elastic body elastically connecting a first and second mounting member fixable to one and the other member of the vibration system, respectively, a pressure receiving chamber partially defined by the rubber elastic body and filled with a non compressible fluid, an equilibrium chamber partially defined by a flexible layer and filled with the non-compressible fluid, and an orifice passage for permitting a fluid communication between the pressure-receiving chamber and the equilibrium chamber. Upon application of a vibrational load between the first and second mounting members, a fluid pressure in the pressure receiving chamber varies due to the elastic deformation of the elastic body, while a change in volume of the equilibrium chamber is permitted due to elastic displacement or deformation of the flexible layer, whereby the fluid is forced to flow through the orifice passage between the pressure receiving chamber and the equilibrium chamber. Such a conventional fluid filled vibration damping device is able to exhibit excellent vibration damping effect on the basis of resonance or flows of the fluid through the orifice passage, which effect is so superior that a vibration damping device simply relying on a rubber elastic body cannot achieve it. For the above-described advantage, the conventional fluid filled vibration damping device has been attempted to be used as an engine mount, a body mount, or other mounts for automotive vehicles, for example. 
   In the conventional vibration damping device, the flexible layer is generally formed of a thin rubber elastic layer. In order to provide a sufficient amount of fluid flows through the orifice passage upon application of vibration, and in order to realize an excellent damping performance of the device, important is to permit sufficient amount of volumetric change in the equilibrium chamber by sufficiently increasing a permissible amount of distending deformation of the flexible layer. 
   However, if the permissible amount of distending deformation of the flexible layer is excessively large, the flexible layer excessively distending outwardly would be brought into contact with a bracket or other members, when these members are disposed in the vicinity of the flexible layer. This may possibly cause a problem of deterioration in durability of the flexible layer. 
   To enhance the durability of the flexible layer, it may be proposed to increase the wall thickness of the overall flexible layer. However, this measure makes it difficult for the flexible layer to permit a sufficient permissible amount of distending deformation thereof, possibly leading to decrease in the amount of fluid flows through the orifice passage, whereby the damping performance with the help of the orifice passage is considerably decreased. 
   Further, in order to enhance durability of the flexible layer, it may be proposed to partially increase the wall thickness of the flexible layer, at an interference area where the flexible layer is brought into contact with the other member(s). In this measure, although the flexible layer has its wall thickness enlarged in the interference area, the interference between the flexible layer and the other member(s) is not avoidable, and the interference area of the flexible layer is finally brought into contact with the other member(s). Conversely, this may cause a relatively large impact noise in comparison with the case where the flexible layer has no thick walled portion. Therefore, the conventionally proposed measures have not been appropriate to solve the aforesaid conventionally experienced problems. 
   SUMMARY OF THE INVENTION 
   It is therefore one object of this invention to provide a fluid filled vibration damping device of novel construction, which is capable of ensuring a sufficient permissible amount of distending deformation of a flexible layer, and eliminating and minimizing problems of impact noise and low durability of the flexible layer due to its contact or interference with other components, as well. 
   The above and/or optional objects of this invention may be attained according to at least one of the following modes of the invention. Each of these modes of the invention is numbered like the appended claims and depending from the other mode or modes, where appropriate, to indicate possible combinations of elements or technical features of the invention. It is to be understood that the principle of the invention is not limited to these modes of the invention and combinations of the technical features, but may otherwise be recognized based on the teachings of the present invention disclosed in the entire specification and drawings or that may be recognized by those skilled in the art in the light of the present disclosure in its entirety. 
   A first mode of this invention provides a fluid filled vibration damping device for connecting two members in a vibration damping fashion, including: a first mounting member fixable to one of the two members; a second mounting member fixable to an other of the two members; an elastic body elastically connecting the first and second mounting members and partially defining a pressure receiving chamber filled with a non-compressible fluid and undergoing fluid pressure variation upon application of vibrational load between the first and second mounting members; a flexible diaphragm partially defining an equilibrium chamber filled with the non-compressible fluid and whose volume is variable; and an orifice passage permitting a fluid communication between the pressure-receiving chamber and the equilibrium chamber, wherein the flexible diaphragm being constituted by a rubber elastic layer and having an annular thick-walled portion and a central thick-walled portion situated at a substantially central portion of an inner area surrounded by the annular thick-walled portion, while being independent of the annular thick-walled portion. 
   In a fluid filled vibration damping device constructed according to the first mode of the invention, if the rubber elastic layer undergoes distending deformation, the rubber elastic layer is subjected to tensile deformation so as to expand in a direction along its surface. Since the wall thickness of the annular thick-walled portion is made larger than that of the other part of the elastic body, the annular thick walled portion can generate a restricting force responsive to the tensile deformation in its circumferential direction. With this arrangement, even when a fluid pressure in the equilibrium chamber is entirely evenly exerted on the rubber elastic layer, an amount of deformation of the thick-walled portion may be entirely limited. 
   With this regards, an amount of elastic deformation of the rubber elastic layer is not limited entirely, but limited at a portion where the annular thick-walled portion is formed. In addition, the inner area of the thick-walled portion is formed with a reduced thickness, whereby the inner area permits easily its elastic deformation in the distending direction thereof, as well. Therefore, the annular thick-walled portion makes it possible, without restricting unnecessarily wide area of the rubber elastic layer, to advantageously limit an amount of its distending deformation at the portion where the annular thick-walled portion is formed, while effectively permitting a sufficient amount of volumetric change in the equilibrium chamber based on the elastic deformation of the rubber elastic layer. 
   Moreover, the inner area surrounded by the annular thick walled portion is formed with the central thick walled portion situated at a central portion thereof that distends most outwardly. Therefore, if the central portion of the inner area comes into contact with another member, desired durability of the rubber elastic layer can be advantageously ensured by the central thick walled portion. In addition, since the periphery of the inner area is substantially restricted by the annular thick walled portion, an outward elastic deformation of the inner area is thus restricted. Therefore, even in the case where the central thick walled portion comes into contact with another member, is effectively restricted an abutment force of the central thick walled portion against the other member, effectively eliminating or reducing generation of impact noises or other defects. 
   The configurations and the wall thickness dimensions of the annular thick walled portion and the central thick walled portion are not particularly limited, but may be suitably determined taken into account a material or required properties of the rubber elastic layer, a clearance between thereof and other members, or the like. For instance, the annular thick walled portion and the central thick walled portion may be equal to, or alternatively varied from each other in terms of their wall thickness dimension. The annular thick walled portion has a wall thickness thicker than do an inner and outer circumferential side portions thereof. The annular thick walled portion and the central thick walled portion may provide its wall thickness on either or both sides of the rubber elastic layer. Further, on the rubber elastic layer partially defining the equilibrium chamber, may be provided one or more of the annular thick walled portion and the central thick walled portion. 
   A second mode of the invention provides a fluid filled vibration damping device according to the first mode, further comprising a mounting bracket provided to at least one of the first and second mounting members, and the annular thick walled portion and the central thick walled portion are formed at a portion of the rubber elastic layer situated opposite to the mounting bracket. 
   According to this mode of the invention, in the case where the rubber elastic layer comes into contact or interference with a bracket for mounting the first mounting member on one of the two member to be connected in a vibration damping manner, for example, the rubber elastic layer can be improved in its durability, while advantageously obtaining a sufficient amount of volumetric change in the equilibrium chamber, and a resultant amount of flow of the fluid flowing through the orifice passage. This arrangement enables to employ a mounting bracket having a likelihood of an interference with the rubber elastic layer, and makes it possible to decrease a space for installation of a fluid filled vibration-damping device with bracket. Also, this arrangement enhances a degree of freedom in designing the fluid filled vibration-damping device including the mounting bracket. 
   A third mode of the present invention provides a fluid filled vibration damping device according to the first or second mode, wherein the annular thick-walled portion has an inner and an outer circumferential edge of a smooth curvature configuration with no apparent corner on an entire circumference thereof. This arrangement makes it possible to ease stress concentration generated at thin walled portions adjacent to the inner and outer circumferential edges of the annular thick walled portion, thereby enhancing durability of the rubber elastic layer. 
   A fourth mode of the present invention provides a fluid filled vibration damping device according to any one of the first through third modes of the invention, wherein the annular thick-walled portion is partially bonded to the first or second mounting members. While the elastic deformation of the rubber elastic layer is consistently restricted by the first or second mounting member at its bonding portion to the mounting member, the bonding portion of the rubber elastic layer is constituted by the annular thick walled portion and accordingly has a sufficiently large wall thickness. This further enhances durability of the bonding portion of the rubber elastic layer, and the rubber elastic layer it self. 
   A fifth mode of the present invention provides a fluid filled vibration damping device according to any one of the first through fourth modes of the invention, wherein the rubber elastic layer has a curved slack portion, and at least the inner area situated inside the annular thick-walled portion, is formed at an area flatter than the curved slack portion. According to this arrangement, a portion of the rubber elastic layer where the wall thickness is varied by the annular thick-walled portion and the central thick walled portion can be entirely formed on the flatter area as much as possible. This makes it possible to avoid that thick walled portions of the rubber elastic layer promotes stress concentration due to local bending or the like, resulting in further enhanced durability of the rubber elastic layer. 
   A sixth mode of the present invention provides a fluid filled vibration damping device according to any one of the first through fifth modes of the invention, wherein the first mounting member is bonded to a central portion of the elastic body and the second mounting member is bonded to an outer circumferential portion of the second mounting member so that the elastic body elastically connects the first and second mounting members, and the pressure receiving chamber is disposed on an inside of the elastic body, while the rubber elastic layer is disposed surrounding an outside surface of the elastic body such that a central portion thereof is bonded to the first mounting member and an outer peripheral portion thereof is bonded to the second mounting member, to provide the equilibrium chamber on an outside of the elastic body. 
   According to this mode, the pressure receiving chamber and the equilibrium chamber are formed on the opposite sides of the elastic body that is elastically connecting the first and second mounting members, making it possible to minimize entire size of the vibration damping device, especially in an axial direction of the device in which a primary vibrational load is applied to the device. Although, in this case, the rubber elastic layer disposed surrounding the rubber elastic body is likely to come into contact or interference with other members, the present invention can effectively ensure durability of the rubber elastic layer as described above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The forgoing and/or other objects features and advantages of the invention will become more apparent from the following description of a preferred embodiment with reference to the accompanying drawings in which like reference numerals designate like elements and wherein: 
       FIG. 1  is an elevational view in axial or vertical cross section of a fluid filled vibration damping device in the form of an engine mount for use in an automotive vehicle, which is constructed according to one preferred embodiment of the invention, and which corresponds to a cross sectional view taken along line  1 — 1  of  FIG. 3 ; 
       FIG. 2  is an elevational view in axial or vertical cross section of the engine mount of  FIG. 1 , taken along line  2 — 2  of  FIG. 3 ; 
       FIG. 3  is a top plane view of a first integral vulcanization product of the engine mount of  FIG. 1 ; 
       FIG. 4  is a top plane view of a second integral vulcanization product of the engine mount of  FIG. 1 ; 
       FIG. 5  is a front elevational view of the engine mount of  FIG. 1 , as seen in a direction indicated by arrows  5 — 5  of  FIG. 4 ; and 
       FIG. 6  is a vertical cross sectional view of the engine mount of  FIG. 1  where the engine mount is installed in position with its flexible layer distending outwardly. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring first to  FIGS. 1–4 , shown is a fluid filled vibration damping device in the form of an engine mount  10  constructed according to a first embodiment of the present invention. The engine mount  10  includes a first mounting member  12  and a second mounting member  14  which are both made of metal, and are elastically connected to each other via a rubber elastic body  16  interposed therebetween. With the first mounting member  12  fixed to a power unit (not shown) of the vehicle, and the second mounting member  14  fixed to a body (not shown) of the vehicle, the engine mount  10  can support the power unit on the body of the vehicle in a vibration damping fashion. With the engine mount  10  installed in position as described above, a vibrational load to be damped is primarily applied between the first and second mounting members  12 ,  14  in an approximately axial direction of the engine mount  10 , i.e., the vertical direction as seen in  FIG. 1 . In the following description, the vertical direction is basically equal to the vertical direction as seen in  FIG. 1 . 
   The first mounting member  12  includes an elastic-body-side inner member  18  and a diaphragm-side inner member  20 , while the second mounting member  14  includes an elastic-body-side outer sleeve member  22  and a diaphragm-side outer sleeve member  24 . The elastic-body-side inner member  18  and the elastic-body-side outer sleeve member  22  are bonded to the rubber elastic  16  by an integral vulcanization molding of a rubber material with the two members  18 ,  22 , thereby providing a first integral vulcanization product  28 . Likewise, the diaphragm-side inner member  20  and the diaphragm-side outer sleeve member  24  are bonded to a flexible layer constituted by a rubber elastic layer in the form of a flexible diaphragm  30  by integral vulcanization molding of a rubber material with the two members  20 ,  24 , thereby providing a second integral vulcanization product  32 . These first and second integral vulcanization products  28 ,  32  are mutually assembled. 
   Described in detail, the elastic-body-side inner member  18  of the first integral vulcanization product  28  has an approximately inverted truncated conical shape in its entirety. An upper end face (large diameter end face) of the elastic-body-side inner member  18  has a fitting recess  34  open therein, and a tapped hole  38  is open in a bottom face of the fitting recess  34 . 
   The elastic-body-side outer sleeve member  22  includes a cylindrical wall portion  40  of substantially large-diameter cylindrical configuration, and a flange portion  42  integrally formed at an axially lower end portion of the cylindrical wall portion  40 , and extending diametrically outwardly. An axially upper end portion of the cylindrical wall portion  40  provides a tapered cylindrical portion  44  whose diameter gradually increases as its goes axially upward. The elastic-body-side outer sleeve member  22  of this configuration provides a circumferential groove  45  open in an outer circumferential surface thereof and extending circumferentially with a circumferential length slightly smaller than a circumference thereof. The elastic-body-side inner member  18  is disposed upward of and concentrically with the elastic-body-side outer sleeve member  22  with an axial spacing therebetween, such that an outer circumferential surface of tapered configuration of the elastic-body-side inner sleeve member  18  and an inner circumferential surface of the tapered cylindrical portion  44  of the elastic-body-side outer sleeve member  22  are mutually opposed to each other with a spacing therebetween. The elastic body  16  is disposed in between and elastically connecting the outer circumferential surface of the elastic-body-side inner sleeve member  18  and the inner circumferential surface of the tapered cylindrical portion  14 . 
   The elastic body  16  has a large-diameter truncated conical shape in its entirety. In a small diameter or central portion of the elastic body  16 , the elastic-body-side inner member  18  is embedded in a coaxial relationship with the elastic body  16 , by the aforesaid integral vulcanization molding. In a large diameter portion of the elastic body  16 , the tapered cylindrical portion  44  of the elastic-body-side outer sleeve member  22  is bonded to an outer circumferential surface of the elastic body  16  by the aforesaid integral vulcanization molding. Thus, the elastic body  16  is equipped with the elastic-body-side inner member  18  and the elastic-body-side outer sleeve member  22 , providing the first integral vulcanization product  28 . 
   On the other hand, the diaphragm-side inner member  20  of the second integral vulcanization product  32  has a thick-walled disk shape. The diaphragm-side inner member  20  has a fitting projection  46  projecting outward from its lower face, and a through hole  52  extending through the fitting projection  46 . The diaphragm-side inner member  20  further includes an integrally formed mounting plate portion  58  projecting outward from its upper face. The mounting plate portion  58  is of a rectangular plate configuration, and has a bolt hole  59  extending through its central portion. The diaphragm-side inner member  20  has a bolt head housing recess  60  open in its upper end face and disposed in coaxial relationship with the through hole  52 . The bolt head housing recess  60  has a diameter so as not to reach the mounting plate portion  58 . 
   The diaphragm-side outer sleeve member  24  has a thick-walled large-diameter cylindrical shape in its entirety, and has a mounting plate portion  62  integrally formed at its axially upper open end portion extending diametrically outwardly. The mounting plate portion  62  has a plurality of through holes into which fixing bolts  64  secured press fit, respectively. The diaphragm-side outer sleeve member  24  also has a flange portion  66  integrally formed at its axially lower open-end portion extending diametrically outwardly. Integrally formed at an outer peripheral edge of the flange portion  66  is an annular caulking lip  68  projecting axially downward. 
   The diaphragm-side inner member  20  is disposed upward of and concentrically with the diaphragm-side outer sleeve member  24  with an axial spacing therebetween, and is elastically connected with the diaphragm-side outer sleeve member  24  by a diaphragm  30 . 
   The diaphragm  30  is a thin rubber layer of approximately annular configuration, and extends circumferentially with a curve cross section as to provide a large slack to permit an easy elastic deformation thereof. An inner peripheral edge of the diaphragm  30  is bonded to an outer peripheral edge of the diaphragm-side inner member  20  by the aforesaid integral vulcanization molding, and an outer peripheral edge of the diaphragm  30  is bonded to the axially upper open end portion of the diaphragm-side outer sleeve member  24  by the afore said integral vulcanization molding. Thus, the diaphragm  30  is equipped with the diaphragm-side inner member  20  and the diaphragm-side outer sleeve member  24 , providing the second integral vulcanization product  32 . 
   In the present embodiment, a cross sectional shape of the diaphragm  30  is changed at one circumferential position so as to provide an intended abutment portion  67  where a relatively small amount of outward distending deformation thereof is permitted. More specifically, the diaphragm  30  of annular configuration basically has a cross sectional shape extending diametrically so as to connect the diaphragm-side inner member  20  and the diaphragm-side outer member  24  with an outwardly curved arcuate configuration. The diaphragm  30  has a curved slack portion. However, at the circumferential position where is formed the intended abutment portion  67 , the cross sectional shape of the diaphragm  30  is an approximately flat plate somewhat curved inwardly, as shown in  FIG. 1 . Namely, the intended abutment portion  67  is formed flatter than does the curved slack portion. 
   As is well apparent from  FIG. 5 , the intended abutment portion  67  has an annular thick walled portion  69  at its outside peripheral portion. In the present embodiment, each of an inner and outer circumferential edge of the annular thick walled portion  69  is of substantially rectangular configuration with rounded corners, and no apparent corner is formed on its entire circumference thereof, thus eliminating or minimizing a problem of local stress concentration during displacement of the diaphragm  30 . 
   The intended abutment portion  67  has an inner area  71  surrounded by and situated on the inside of the annular thick-walled portion  69 . The inner area  71  has a wall thickness dimension smaller than that of the annular thick-walled portion  69 . Namely, in the present embodiment, the diaphragm  30  measures a substantially same wall thickness dimension at the inside and outside of the annular thick walled portion  69 . In the substantially central portion of the inner area  71 , a central thick walled portion  73  is formed. The central thick-walled portion  73  has a wall thickness dimension larger than that of the other part of the inner area  71 , and as large as that of the annular thick-walled portion  69 . The central thick-walled portion  73  is surrounded by the thin-walled inner area  71  over its entire circumference, so that the thick-walled portion  73  is situated separately from the annular thick walled portion  69 . With this regards, the central thick walled portion  73  as well as the annular thick-walled portion  69  may be made thick on either side of the diaphragm  30 . 
   As shown in  FIG. 5 , the annular thick-walled portion  69  partially extends beyond the intended abutment portion at its outer circumferential edge, but is held within the area of the substantially planar intended abutment portion  67  at its inner circumferential edge that partially defines the inner area  71 . The outer circumferential edge of the annular thick-walled portion  69  is partially extend to a portion at which the diaphragm  30  is bonded to the diaphragm-side outer sleeve member  24 . 
   The second integral vulcanization product  32  is superposed on and assembled with the first integral vulcanization product  28  such that the diaphragm-side inner member  20  is affixed to the elastic-body-side inner member  18 , while the diaphragm-side outer sleeve member  24  is affixed to the elastic-body-side outer sleeve member  22 . With the first and second vulcanization products  28 ,  32  assembled together, the diaphragm  30  is situated outward of the elastic body  16  with a spacing therebetween, while covering an entire outer circumferential surface of the elastic body  16 . 
   Namely, the diaphragm-side inner member  20  is directly superposed on the upper surface of the elastic-body-side inner member  18  with its fitting projection  46  secured press fit into the fitting recess  34  of the elastic-body-side inner member  18 . With this mating state, the diaphragm-side inner member  20  and the elastic-body-side inner member  18  are mutually positioned in a coaxial fashion. On an outer and inner circumferential surface of the fitting projection  46  and the fitting recess  34 , are provided engaging portions  50 ,  36 , respectively (see  FIG. 2 ). By means of mutual engagement of the engaging portions  50 ,  36 , the diaphragm-side inner member  20  and the elastic body-side inner member  18  are mutually positioned in a circumferential direction as well, whereby the through hole  52  of the diaphragm-side inner member  20  and the tapped hole  38  of the elastic-body-side inner member  18  are in alignment with each other. 
   With the elastic-body-side inner member  18  and the diaphragm-side inner member  20  assembled with each other as shown in  FIGS. 1 and 2 , a connecting bolt  70  is inserted through the through hole  52  and threaded and tightened into the tapped hole  38 . With the elastic-body-side inner member  18  and the diaphragm-side inner member  20  connected together by means of the connecting bolt  70 , is provided the first mounting member  12 . 
   On the other hand, the diaphragm-side outer sleeve member  24  is assembled from the axially upper side with the elastic-body-side outer sleeve member  22 , so as to be disposed about the elastic-body-side outer sleeve member  22 . At the lower side of the elastic-body-side outer sleeve member  22 , the flange portion  42  is held in contact at its peripheral portion with the flange portion  66  of the diaphragm-side outer sleeve member  24  in the axial direction. At the upper side, an open peripheral portion of the tapered cylindrical portion  44  is held against the inner circumferential surface of the diaphragm-side outer sleeve member  24  in the diametric direction. With this mating state, the caulking lip  68  of the diaphragm-side outer sleeve member  24  is caulked against the peripheral portion of the flange portion  42  of the elastic-body-side outer sleeve member  22 , whereby the elastic-body-side outer sleeve member  22  and the diaphragm-side outer sleeve member  24  are mutually fastened together. In addition, the upper and lower end of the elastic-body-side outer sleeve member  22  are held against the diaphragm-side outer sleeve member  24  with sealing rubber layers integrally formed with the elastic body  16  and the diaphragm  30  compressed therebetween, respectively, so as to provide a fluid-tight sealing therebetween. 
   With the elastic-body-side outer sleeve member  22  assembled with the diaphragm-side outer sleeve member  24  as described above, the opening of the circumferential groove  45  is fluid-tightly closed by the diaphragm-side outer sleeve member  24 . Thus, there is formed an annular fluid passage  72  continuously extending in the circumferential direction between the cylindrical wall portion  40  of the elastic-body-side outer sleeve member  22  and the diaphragm-side outer sleeve member  24 , with a given circumferential length, or over an entire circumference of the cylindrical wall portion  40 . On the lower side of the elastic-body-side outer cylindrical member  22 , is disposed a large diameter disk shaped lid member  26  of metal that is held in contact with the lower end faces of the elastic body  16  and the flange portion  42  of the elastic-body-side outer sleeve member  22 . An peripheral edge portion of the lid member  26  is fixed, together with the flange portion  42  of the elastic-body-side outer sleeve member  22 , against to the flange portion  66  of the diaphragm-side outer sleeve member  24  by caulking the caulking lip  68  against the lid member  26 . With this arrangement, an axially lower open-end portion of the elastic-body-side outer sleeve member  22  is fluid-tightly closed by the lid member  26 . A fluid-tight sealing at an interface between the elastic-body-side outer sleeve member  22  and the lid member  26  is provided by means of a sealing rubber integrally formed with the elastic body  16 . 
   The thus mutually assembled diaphragm-side outer sleeve member  24 , the elastic-body-side outer sleeve member  22 , and the lid member  26  are fastened together by caulking, to thereby provide the second mounting member  14  that is elastically connected to the first mounting member via the elastic body  16 . 
   With the lower open end of the second mounting member  14  fluid-tightly closed with the lid member  26 , a pressure-receiving chamber  76  filled with a non-compressible fluid is formed between the elastic body  16  and the lid member  26 . The pressure-receiving chamber  76  is partially defined by the elastic body  16 , and undergoes fluid pressure variation due to elastic deformation of the elastic body during input of vibrational load between the first mounting member  12  and the second mounting member  14 . 
   Further, with the elastic body  16  and the diaphragm  30  are bonded to the first and second mounting members  12 ,  14  at their inner circumferential edge portions and outer circumferential edge portions, respectively, an equilibrium chamber  78  filled with the non-compressible fluid is formed between the elastic body  16  and the diaphragm  30 . Namely, the equilibrium chamber  78  is partially defined by the diaphragm  30  easily deformable, so as to permit a volumetric change on the basis of elastic deformation of the diaphragm  30 . Generally, a non-compressible fluid filling the pressure-receiving chamber  76  and the equilibrium chamber  78  is preferably a low viscous fluid whose viscosity is 0.1 Pa.s or lower, for permitting the engine mount  10  to exhibit a high damping effect at a required frequency range on the basis of resonance of the non-compressible fluid flowing through an orifice passage  50  that will be described later. 
   The aforesaid annular fluid passage  72  formed within the second mounting member  14 , is connected at its opposite ends to the pressure receiving chamber  76  on the lower side of the elastic body  16  and equilibrium chamber  78  on the upper side of the elastic body  16  through communication holes  82 ,  84 , thereby providing an orifice passage  80  with a given length for permitting, which permits a fluid communication between the pressure receiving chamber  76  and the equilibrium chamber  78 . As well known in the art, the fluid is forced to flow through the orifice passage  80  on the basis of relative fluid pressure variation caused between the pressure receiving chamber  76  and the equilibrium chamber  78  during input of vibrational load. Thus, the engine mount  10  can exhibit excellent damping effect with respect to the input vibrational load on the basis of resonance of the fluid flowing through the orifice passage  80 . The damping performance of the engine mount  10  on the basis of the flows of the fluid through the orifice passage  80  can be adjusted in terms of a frequency characteristic, by only tuning a ratio of the cross sectional area to the length of the orifice passage  80 . 
   The engine mount  10  of construction as discussed above is installed between a power unit and body of the vehicle (not shown) such that the mounting plate portion  58  of the first mounting member  12  is fixed to the power unit of the vehicle, and the mounting plate portion  62  of the second mounting member  14  is superposed on and fastened by means of the connecting bolt  70  to the body. Thus, the engine mount  10  can elastically mount the power unit on the body in a vibration isolation fashion. 
   As shown in  FIG. 6 , the first mounting member  12  is fixed to the power unit via a mounting bracket  74 . More specifically, one end of the bracket  74  is fastened to the mounting plate portion  58  of the diaphragm-side inner member  20  by means of a mounting bolt  75 , whereby the bracket  74  is fixed to the inner mounting member  12  so as to extend outward from the first mounting member  12  in a direction orthogonal to a center axis of the engine mount  10 . While not illustrated, the other end of the bracket  74  is fastened to the power unit of the vehicle by means of a mounting bolt extending through a through hole formed through an appropriate portion of the bracket  74 . 
   With the engine mount  10  installed in position as described above, the diaphragm  30  is disposed about the elastic body  16  covering the outside of the elastic body  16 , and is located below the bracket  74  at one circumferential position thereof. One circumferential portion of the diaphragm  30 , which is situated opposite to the bracket  74 , is formed as the intended abutment portion  67 . 
   With this state, the engine mount  10  is able to exhibit high damping effect on the basis of resonance of the fluid flowing through the orifice passage  80  due to a relative fluid pressure variation caused between the pressure-receiving chamber  76  and the equilibrium chamber  78 , with respect to vibration applied between the first and second mounting members  12 ,  14 . 
   When an internal fluid pressure change induced in the pressure receiving chamber  76  is transmitted to the equilibrium chamber  78  through the orifice passage  80 , the diaphragm  30  undergoes distending/contracting deformation in association with change in an internal fluid pressure of the equilibrium chamber  78 . While the diaphragm  30  is situated opposite to the bracket  74  at the intended abutment portion  67 , the intended abutment portion  67  is restricted in an amount of outward distending deformation thereof. This arrangement makes it possible to prevent or moderate an interference or a contact between the diaphragm  30  and the bracket  74 , when the diaphragm undergoes outward distending deformation in association with the fluid pressure increase in the equilibrium chamber  78 . 
   Moreover, the intended abutment portion  67  has a specific structure in which an annular thick walled portion  69 , the inner area  71  and the central thick-walled portion  73  are combined together as described above, providing durability of the intended abutment portion  67  considerably advantageously. Described in detail, the presence of the annular thick-walled portion  67  is substantially identical with a provision of an annular reinforcing member integrally formed on the intended abutment portion  67 . During outward distending deformation of the diaphragm  30 , every area of the diaphragm  30  is subjected to tensile force in all directions along a surface of the diaphragm  30 . However, the area where the annular thick-walled portion  69  is formed, has a high spring stiffness in the circumferential direction, and accordingly is restricted in its elastic deformation. This arrangement makes it possible to disperse substantially evenly stress or deformation over the diaphragm  30  in the circumferential direction, while effectively limiting the amount of distending deformation of the intended abutment portion  67 . Thus, the interference between the diaphragm  30  and the bracket  74  can be effectively prevented or minimized. 
   Since the inner area  71  situated inward of the annular thick-walled portion  69  has a thin wall thickness, a required amount of distending deformation of the diaphragm  30  can be allowed in this inner area  71 , making it possible to eliminate a likelihood of a less amount of volumetric change of the equilibrium chamber  78  caused by an excess limitation of an amount of distending deformation of the intended abutment portion  67 . Even if the intended abutment portion  67  undergoes excess distending deformation, and results in an interference or contact with the bracket  67 , the central thick-walled portion  73  provided at the central portion of the inner area  71  where is expected the largest amount of outward distending deformation, may be brought into abutting contact with the bracket  74 , thus providing durability of the abutment portion (central thick-walled portion  73 ) of the inner area  71  in an effective manner. 
   Namely, if the overall intended abutment portion  67  is formed with a large thickness, without providing the thin-walled inner area  71 , the equilibrium chamber  78  may suffer from difficulty in permitting a required amount of volumetric change. It might be considerable that this problem may be solved by minimizing the size of the intended abutment portion  67 , for example. However, the smaller the size of the intended abutment portion  67 , the larger the stress concentration during its deformation, resulting in inevitable deterioration of the intended abutment portion  67 . On the other hand, if the annular thick-walled portion  69  is employed together with the thin-walled inner area  71 , according to the present embodiment, it becomes possible to obtain a sufficient amount of volumetric change of the equilibrium chamber  78 , as well as to limit an amount of distending deformation of the intended abutment portion  67 , while dispersing stress over a wide area of the intended abutment portion. Therefore, a sufficient amount of volumetric change of the equilibrium chamber  78  as well as a limitation of the distending displacement of the intended abutment portion  67  can be realized in a sophisticated and compassable manner. 
   While the presently preferred embodiment of this invention has been described in detail for the illustrative purpose only, it is to be understood that the present invention is not limited to the details of the illustrated embodiment. It is also to be understood that the present invention may be embodied with various changes, modifications and improvements which may occur to those skilled in the art, without departing from the spirit and scope of the invention, although detail description of these modifications is omitted. 
   In the illustrated embodiment, the one intended abutment portion  67  is formed at the one circumferential portion of the diaphragm  30 . It may be possible to provide a plurality of intended abutment portions, each consisting of the thick-walled portion  69 , the inner area  71  and/or the central thick-walled portion  73 , at respective circumferential positions where the diaphragm  30  is opposite to other members such as a bracket, and is expected to come into contact or interference with these members. 
   While the diaphragm  30  is disposed about the elastic body  16  so as to surround the outside of the elastic body  16 , the principle of the present invention includes a variety of disposing positions and mounting structures of a diaphragm, but not limited to the illustrated ones. For instance, the principle of the present invention may be applicable to a fluid filled elastic mount as disclosed in JP-2000-274480 wherein the second mounting member has a cylindrical configuration, and one open end of the second mounting member is fluid-tightly closed by the elastic body, and the other open end of the second mounting member is fluid-tightly closed by the flexible diaphragm, while a pacing between the elastic body and the diaphragm is divided by a partition member supported by the second mounting member into the pressure receiving chamber partially defined by the elastic body and the equilibrium chamber partially defined by the equilibrium chamber. In this mount, an intended abutment portion of illustrated specific structure may be formed on a central portion or other suitable portion on the diaphragm. 
   In the illustrated embodiment, the present invention is applied to a vibration damping device of passive type that exhibits damping effect with the help of pressure variation in a fluid sealed therein and flows of the fluid, which are passively caused due to input vibration, for the illustrative purpose only. It should be appreciated that the principle of the present invention may also be applicable to a vibration damping device of active type, as disclosed in the aforesaid citation 1, for example, in which an actuator is employed to actively control a fluid pressure variation of non-compressible fluid sealed therein in order to change damping characteristics of the device, or alternatively to compensate or minimize vibration. 
   Additionally, while the engine mount for automotive vehicles has been described as one preferred embodiment of the invention, the principle of the present invention may be equally applicable to a body mount, a member mount or other mounts for automotive vehicles, or a variety of vibration damping devices for other than automotive vehicles. 
   It is also to be understood that the present invention may be embodied with various other changes, modifications and improvements, which may occur to those skilled in the art, without departing from the spirit and scope of the invention defined in the following claims.