Fluid-filled vibration damping device

A fluid-filled vibration damping device including a first mounting member, a second mounting member including a tubular portion having an end open toward the first mounting member, the first and second mounting members being spaced from, and opposed to, each other, an elastic rubber body which elastically connects the first and second mounting members to each other, and which fluid-tightly closes the open end of the tubular portion of the second mounting member and cooperates with the tubular portion to define a fluid chamber filled with a non-compressible fluid, and a working member which is supported by the first mounting member such that the working member extends, in the fluid chamber, in a direction substantially perpendicular to a central axis of the tubular portion of the second mounting member, and thereby divides the fluid chamber into two divided chambers which are located on opposite sides of the working member, respectively, and which are communicated with each other via a fluid-flow restricting passage defined by at least the working member, the elastic rubber body including at least one thin portion which is located on a side of one of the two divided chambers and which is thinner than a remaining portion of the rubber body so as to be more easily elastically deformable than the remaining portion.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a fluid-filled vibration damping device
 which exhibits a vibration damping effect based on flows of a
 non-compressible fluid enclosed therein, and which is particularly
 suitable for use as, e.g., automotive-vehicle engine mounts, body mounts,
 and differential mounts.
 2. Related Art Statement
 There is known a fluid-filled vibration damping device as a sort of
 vibration damping connecting or supporting device that is interposed
 between constituent members of a vibration transmitting system. The
 fluid-filled vibration damping device includes a first mounting member; a
 second mounting member including a tubular portion having an end open
 toward the first mounting member, the first and second mounting members
 being spaced from, and opposed to, each other; and an elastic rubber body
 which elastically connects the first and second mounting members to each
 other, and which fluid-tightly closes the open end of the tubular portion
 of the second mounting member and cooperates with the tubular portion to
 define a fluid chamber filled with a non-compressible fluid. When a
 vibrational load is applied to the vibration damping device, it exhibits a
 vibration damping effect based on flows of the fluid in the fluid chamber,
 in particular, resonance of the fluid.
 Meanwhile, there has been proposed another fluid-filled vibration damping
 device which includes, in addition to the above-indicated members, a
 working or umbrella-shaped member which is supported by the first mounting
 member such that the umbrella member extends, in the fluid chamber, in a
 direction substantially perpendicular to a central axis of the tubular
 portion of the second mounting member, and thereby divides the fluid
 chamber into two divided chambers which are located on opposite sides of
 the umbrella member, respectively, and which are communicated with each
 other via a fluid-flow restricting passage defined by at least the
 umbrella member.
 To the second fluid-filled vibration damping device including the umbrella
 member, a main vibrational load is applied in a direction in which the
 first and second mounting members are opposed to each other, i.e., a
 direction parallel to the central axis of the tubular portion of the
 second mounting member. Upon application of the main vibrational load to
 the vibration damping device, the umbrella member is reciprocatively moved
 in the fluid chamber, so that the fluid flows through the fluid-flow
 restricting passage. The vibration damping device can exhibit a vibration
 damping effect based on the flows of the fluid through the restricting
 passage, in particular, the resonance of the fluid.
 However, even in the second fluid-filled vibration damping device, the
 vibration damping effect based on the flows of the fluid through the
 restricting passage defined by the umbrella member is not satisfactory yet
 and, in some cases, the vibration damping device cannot exhibit a low
 dynamic spring effect to a desirable degree.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a
 fluid-filled vibration damping device which has an improved structure for
 exhibiting an excellent vibration damping effect based on flows of a
 non-compressible fluid enclosed in a fluid chamber.
 The present invention provides a fluid-filled vibration damping device
 which has one or more of the technical features that are described below
 in respective paragraphs given parenthesized sequential numbers (1) to
 (16). Any technical feature which includes another technical feature shall
 do so by referring, at the beginning, to the parenthesized sequential
 number given to that technical feature. Thus, two or more of the following
 technical features may be combined, if appropriate. Each technical feature
 may be accompanied by a supplemental explanation, as needed. However, the
 following technical features and the combinations thereof are just
 examples to which the present invention is by no means limited. Rather,
 the concept of the present invention should be construed based on the
 overall description of the specification and the drawings.
 (1) According to a first feature of the present invention, there is
 provided a fluid-filled vibration damping device comprising a first
 mounting member; a second mounting member including a tubular portion
 having an end open toward the first mounting member, the first and second
 mounting members being spaced from, and opposed to, each other; an elastic
 rubber body which elastically connects the first and second mounting
 members to each other, and which fluid-tightly closes the open end of the
 tubular portion of the second mounting member and cooperates with the
 tubular portion to define a fluid chamber filled with a noncompressible
 fluid; and a working member which is supported by the first mounting
 member such that the working member extends, in the fluid chamber, in a
 direction substantially perpendicular to a central axis of the tubular
 portion of the second mounting member, and thereby divides the fluid
 chamber into two divided chambers which are located on opposite sides of
 the working member, respectively, and which are communicated with each
 other via a fluid-flow restricting passage defined by at least the working
 member, the elastic rubber body including at least one thin portion which
 is located on a side of one of the two divided chambers and which is
 thinner than a remaining portion of the rubber body so as to be more
 easily elastically deformable than the remaining portion.
 In the fluid-filled vibration damping device according to the first feature
 (1), the elastic rubber body includes at least one more easily elastically
 deformable thin portion which is located on the side of one of the two
 divided chambers which are communicated with each other by the fluid-flow
 restricting passage. Accordingly, when a vibrational load is input to the
 vibration damping device and the working member is reciprocatively or
 periodically moved in the fluid chamber, the thin portion is more easily
 elastically deformed to permit a relative change between respective
 volumes of the two divided chambers. That is, the fluid is permitted to
 flow in more amount through the restricting passage. Therefore, the
 present vibration damping device can exhibit a higher vibration damping
 effect based on the flows of the fluid through the restricting passage.
 The fluid-flow restricting passage may be defined by, and between,
 respective opposed portions of an outer circumferential surface of the
 working member and an inner circumferential surface of the fluid chamber,
 or may additionally include at least one through-hole formed through the
 thickness of the working member. The shape or structure of the working
 member is by no means limited. For example, the working member may have a
 circular, an elliptical, or a polygonal outer periphery or contour.
 Otherwise, an outer circumferential surface of the working member may be
 covered with an elastic rubber layer. The means or structure used for
 attaching the working member to the first mounting member is by no means
 limited. For example, the first mounting member may include an integral
 support axial portion which projects from the remaining portion of the
 first member into the fluid chamber, and the working member may be fixed
 by, e.g., caulking to a free end portion of the axial portion that
 projects in the fluid chamber. The shape of the elastic rubber body is by
 no means limited, and may be determined depending upon, e.g., vibration
 damping characteristics the vibration damping device is required to have.
 For example, the elastic rubber body may have a thick disc-like shape, and
 the tubular portion of the second mounting member may be fixed to an outer
 circumferential surface of the disc-like rubber body. Alternatively, the
 rubber body may have a frustoconical shape or a thick-walled tapered
 tubular shape that projects from the tubular portion of the second
 mounting member in an outward direction away from the open end of the
 tubular portion. In the latter case, the first mounting member is fixed to
 an end surface of the small-diameter end portion of the tapered rubber
 body, and the tubular portion of the second mounting member is fixed to an
 outer circumferential surface of the large-diameter end portion of the
 tapered rubber body.
 (2) According to a second feature of the present invention that includes
 the first feature (1), the elastic rubber body includes a plurality of
 thin portions which are located on the side of the one divided chamber and
 each of which is thinner than the remaining portion of the rubber body so
 as to be more easily elastically deformable than the remaining portion,
 the thin portions being provided around the central axis of the tubular
 portion such that the thin portions are substantially equiangularly spaced
 from each other about the central axis.
 In the fluid-filled vibration damping device according to the second
 feature (2), the elastic rubber body includes a plurality of thin
 portions. Accordingly, the present vibration damping device exhibits a
 higher vibration damping effect based on the flows of the fluid through
 the fluid-flow restricting passage. In addition, since the plurality of
 thin portions are spaced from each other, the elastic rubber body enjoys a
 sufficiently high support spring strength. Moreover, since the plurality
 of thin portions are substantially equiangularly spaced from each other
 about the central axis of the tubular portion of the second mounting
 member, the flows of the fluid in the fluid chamber or through the
 restricting passage are stabilized when the working member is
 reciprocatively moved in the fluid chamber. In addition, the rubber body
 is effectively prevented from local concentration of stress and
 accordingly the elastic deformation of the rubber body is stabilized.
 Thus, the present vibration damping device can exhibit desirable vibration
 damping performance with higher stability.
 In the fluid-filled vibration damping device according to the second
 feature (2), it is preferred that the plurality of thin portions comprise
 four thin portions two of which are opposed to each other in one of two
 directions each of which is perpendicular to the central axis of the
 tubular portion of the second mounting member and which perpendicularly
 intersect each other on the central axis, and the other two of which are
 opposed to each other in the other of the two directions. In this case,
 the rubber body includes four thick portions which are alternate with the
 four thin portions about the central axis, that is, are angularly spaced
 from the corresponding thin portions by 45 degrees. Like the four thin
 portions, two of the four thick portions are opposed to each other in one
 of two directions each of which is perpendicular to the central axis and
 which perpendicularly intersect each other on the central axis, and the
 other two thick portions are opposed to each other in the other of the two
 directions. Therefore, the rubber body enjoys a sufficiently high spring
 hardness or stiffness in radial directions thereof perpendicular to the
 central axis.
 (3) According to a third feature of the present invention that includes the
 first or second feature (1) or (2), the elastic rubber body includes at
 least one concave portion opening in the fluid chamber, and at least one
 wall portion defining a bottom of the at least one concave portion, the at
 least one wall portion providing the at least one thin portion.
 In the fluid-filled vibration damping device according to the third feature
 (3), the thin portion of the elastic rubber body is elastically deformed,
 upon reception of a vibrational load, so that the fluid flows between the
 fluid chamber and the concave portion, e.g., a pocket-like void. Thus, the
 pocket void functions as a fluid passage, and the area of opening of the
 pocket void corresponds to the cross-section area of the fluid passage and
 the depth of the void corresponds to the length of the passage. The
 cross-section area and length of the pocket void can be adjusted as needed
 in consideration of the spring characteristic of the thin portion, so that
 the present vibration damping device may exhibit a more excellent
 vibration damping effect based on the flows of the fluid in the pocket
 void, in particular, the resonance of the fluid.
 In the case where the pocket void is utilized as a fluid passage and a
 vibration damping effect based on the flows of the fluid through the fluid
 passage is obtained, it is preferred that the resonance frequency of the
 fluid flowing through the fluid passage is tuned to be higher than that of
 the fluid flowing through the fluid-flow restricting passage defined by
 the working member. In the last case, the vibration damping device can
 exhibit, over a wider frequency range, an improved vibration damping
 effect owing to its low dynamic spring effect based on the fluid passage
 (or the pocket void) and the restricting passage.
 (4) According to a fourth feature of the present invention that includes
 the third feature (3), a dimension of the concave portion in a
 circumferential direction of the elastic rubber body increases in an
 outward direction perpendicular to a central axis of the rubber body that
 coincides with the central axis of the tubular portion of the second
 mounting member. In this case, the concave portion can have a sufficiently
 large area without lowering the spring hardness of the elastic rubber
 body.
 (5) According to a fifth feature of the present invention that includes the
 third or fourth feature (3) or (4), a ratio of an area of the concave
 portion that is projected in a direction parallel to a central axis of the
 elastic rubber body , to an area of the fluid chamber that is projected in
 the direction, falls within a range of 2 to 15%. In this case, the elastic
 rubber body is prevented from excessively large decrease of its
 durability, and the present vibration damping device exhibits an improve d
 vibration damping effect based on the thin portion of the rubber body. If
 the projected area is smaller than 2%, the vibration damping device cannot
 exhibit a sufficiently high vibration damping effect based on the thin
 portion of the rubber body. If the projected area is greater than 15%, the
 rubber body might suffer excessively shortened durability. Since the
 projected area falls within the range of 2 to 15%, the vibration damping
 device can more advantageously obtain a low dynamic spring effect based on
 the resonance of the fluid flowing through the concave portion, e.g., the
 pocket void.
 (6) According to a sixth feature of the present invention that includes any
 one of the first to fifth features (1) to (5), the vibration damping
 device further comprises a tubular rubber wall which extends from an outer
 peripheral portion of the elastic rubber body along an inner
 circumferential surface of the tubular portion of the second mounting
 member and which is formed integrally with the rubber body such that the
 rubber wall is fixed to the tubular portion and covers the inner
 circumferential surface of the tubular portion, the rubber wall including
 at least one portion which is not aligned with the at least one thin
 portion in a direction parallel to a central axis of the rubber body and
 which projects inward to a position inside an inner surface of the thin
 portion.
 In the fluid-filled vibration damping device according to the sixth feature
 (6), the tubular rubber wall extends from the outer peripheral portion of
 the elastic rubber body along the inner circumferential surface of the
 tubular portion of the second mounting member. The tubular rubber wall
 supports or reinforces the outer peripheral portion of the elastic rubber
 body, thereby reducing or minimizing the lowering of the spring hardness
 of the rubber body that results from the formation of the thin portion in
 the rubber body. In addition, the rubber wall includes at least one thin
 portion which is aligned with the at least one thin portion of the elastic
 rubber body. Therefore, the rubber body can have one or more sufficiently
 large thin portions, and the vibration damping device can exhibit an
 improved vibration damping characteristic based on the thin portion or
 portions of the rubber body.
 (7) According to a seventh feature of the present invention that includes
 the sixth feature (6), the vibration damping device further comprises a
 rigid support member which is fixed to the second mounting member such
 that the support member is held in contact with an end surface of the
 tubular rubber wall that is remote from the rubber body, so as to support
 the rubber wall. The rigid support member effectively helps the tubular
 rubber wall support and reinforce the elastic rubber body.
 (8) According to an eighth feature of the present invention that includes
 any one of the first to seventh features (1) to (7), the vibration damping
 device further comprises, in addition to the fluid chamber as a primary
 fluid chamber, an auxiliary fluid chamber which produces, upon application
 of a vibrational load to the vibration damping device, a pressure
 difference with respect to the primary fluid chamber, and a fluid-flow
 passage which communicates the primary and auxiliary fluid chambers with
 each other.
 The fluid-filled vibration damping device according to the eighth feature
 (8) can exhibit an excellent vibration damping effect based on the flows
 of the fluid through the fluid-flow passage between the primary and
 auxiliary fluid chambers, in particular, the resonance of the fluid. The
 fluid-flow passage may be tuned to a frequency range different from that
 to which the fluid-flow restricting passage defined by the working member
 is tuned. In the latter case, the vibration damping device can exhibit an
 excellent vibration damping effect in a wider frequency range. In order to
 obtain both a high vibration damping effect based on the fluid-flow
 passage and a high vibration damping effect based on the restricting
 passage defined by the working member, it is preferred that the fluid-flow
 passage is tuned to a frequency range lower than that to which the
 restricting passage is tuned.
 The fluid-flow passage may be one which always keeps the fluid
 communication between the primary and auxiliary fluid chambers, one which
 is provided with an opening and closing means such as a valve and which
 can be opened and closed by the means, or one which is provided with a
 partition member which is displaceable or deformable to permit the fluid
 to flow and which is controllable with respect to mount of displacement or
 amount of deformation for the purpose of controlling the amount of flow of
 the fluid. Otherwise, the fluid-flow passage may have any one of various
 known structures.
 It is preferred that the fluid-flow passage is formed to have such a
 structure which communicates the auxiliary chamber with the other of the
 two divided chambers of the fluid chamber that is opposite, with respect
 to the working member, to the one divided chamber located on the side of
 the elastic rubber body. In this case, the fluid flows in more amount
 through the fluid-flow restricting passage when the working member is
 reciprocatively moved in the fluid chamber.
 The auxiliary chamber may an equilibrium chamber whose wall is partly
 provided by a flexible sheet such a thin rubber sheet and whose volume is
 easily changeable.
 (9) According to a ninth feature of the present invention that includes the
 eighth feature (8), the vibration damping device further comprises a rigid
 partition member which is fixed to the tubular portion of the second
 mounting member, so that the primary fluid chamber is provided on one of
 opposite sides of the partition member and the auxiliary fluid chamber is
 provided on the other side of the partition member, the partition member
 defining the fluid-flow passage communicating the primary and auxiliary
 fluid chambers with each other. In this case, the primary and auxiliary
 fluid chambers and the fluid-flow passage enjoy respective simple
 structures.
 (10) According to a tenth feature of the present invention, there is
 provided a fluid-filled vibration damping device comprising a first
 mounting member; a second mounting member including a tubular portion
 having an end open toward the first mounting member, the first and second
 mounting members being spaced from, and opposed to, each other; and an
 elastic rubber body which elastically connects the first and second
 mounting members to each other, and which fluid-tightly closes the open
 end of the tubular portion of the second mounting member, and cooperates
 with the tubular portion to define a fluid chamber filled with a
 non-compressible fluid, the elastic rubber body including at least one
 concave portion opening in the fluid chamber, and at least one wall
 portion defining a bottom of the at least one concave portion, the at
 least one wall portion providing at least one thin portion which is
 thinner than a remaining portion of the rubber body so as to be more
 easily elastically deformable than the remaining portion.
 In the fluid-filled vibration damping device according to the tenth feature
 (10), the thin portion of the elastic rubber body is elastically deformed,
 upon reception of a vibrational load, so that the fluid flows in the
 concave portion, e.g., a pocket-like void. Therefore, the present
 vibration damping device can exhibit an excellent vibration damping effect
 based on the flows of the fluid through the pocket void, in particular,
 the resonance of the fluid. This advantage can be obtained with a simple
 structure of the damping device, without increasing the total number of
 parts needed to manufacture the damping device.
 (11) According to an eleventh feature of the present invention that
 includes the tenth feature (10), the elastic rubber body includes a
 plurality of concave portions opening in the fluid chamber, and a
 plurality of wall portions defining respective bottoms of the plurality of
 concave portions, the plurality of wall portions providing a plurality of
 thin portions each of which is thinner than the remaining portion of the
 rubber body so as to be more easily elastically deformable than the
 remaining portion, the thin portions being provided around the central
 axis of the tubular portion such that the thin portions are substantially
 equiangularly spaced from each other about the central axis.
 (12) According to a twelfth feature of the present invention that includes
 the tenth or eleventh feature (10) or (11), a dimension of the concave
 portion in a circumferential direction of the elastic rubber body
 increases in an outward direction perpendicular to a central axis of the
 rubber body that coincides with the central axis of the tubular portion of
 the second mounting member.
 (13) According to a thirteenth feature of the present invention that
 includes any one of the tenth to twelfth feature (10) to (12), a ratio of
 an area of the concave portion that is projected in a direction parallel
 to a central axis of the elastic rubber body, to an area of the fluid
 chamber that is projected in the direction, falls within a range of 2 to
 15%.
 (14) According to a fourteenth feature of the present invention that
 includes any one of the tenth to thirteenth features (10) to (13), the
 vibration damping device further comprises a tubular rubber wall which
 extends from an outer peripheral portion of the elastic rubber body along
 an inner circumferential surface of the tubular portion of the second
 mounting member and which is formed integrally with the rubber body such
 that the rubber wall is fixed to the tubular portion and covers the inner
 circumferential surface of the tubular portion, the rubber wall including
 at least one portion which is not aligned with the at least one thin
 portion in a direction parallel to a central axis of the rubber body and
 which projects inward to a position inside an inner surface of the thin
 portion.
 (15) According to a fifteenth feature of the present invention that
 includes any one of the tenth to fourteenth features (10) to (14), the
 vibration damping device further comprises, in addition to the fluid
 chamber as a primary fluid chamber, an auxiliary fluid chamber which
 produces, upon application of a vibrational load to the vibration damping
 device, a pressure difference with respect to the primary fluid chamber,
 and a fluid-flow passage which communicates the primary and auxiliary
 fluid chambers with each other.
 (16) According to a sixteenth feature of the present invention that
 includes the fifteenth feature (15), the vibration damping device further
 comprises a rigid partition member which is fixed to the tubular portion
 of the second mounting member, so that the primary fluid chamber is
 provided on one of opposite sides of the partition member and the
 auxiliary fluid chamber is provided on the other side of the partition
 member, the partition member defining the fluid-flow passage communicating
 the primary and auxiliary fluid chambers with each other.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIGS. 1 and 2 show an engine mount 10 for use in an automotive vehicle. The
 engine mount 10 includes a first mounting member 12 formed of metal, a
 second mounting member 14 formed of metal, and an elastic rubber body 15
 which elastically connects the first and second mounting members 12, 14 to
 each other. The first mounting member 12 is attached to the power unit
 (not shown) of the automotive vehicle, and the second mounting member 14
 is attached to the body (not shown) of the vehicle, so that the engine
 mount 10 supports the power unit on the body in a vibration damping
 fashion. In this state, a main vibrational load is input to the engine
 mount 10 in a direction in which the first and second mounting members 12,
 14 are opposed to each other, that is, in a vertical direction as seen in
 FIG. 1.
 More specifically described, the first mounting member 12 is provided by a
 rigid member which is formed of metal or the like and has a disc-like
 shape. A generally inverted-conical support member 20 formed of metal is
 fixed by welding to the first mounting member 12, so as to project axially
 downward from the same 12. A working member in the form of umbrella-shaped
 member 22 formed of metal is fixed by caulking to an axially lower end
 portion of the support member 20. The umbrella member 22 has a generally
 disc-like shape, and has a central through-hole which is formed through
 the thickness of a central portion of the member 22 and in which the lower
 end portion of the support member 20 is inserted to be fixed by caulking
 to the umbrella member 22. Thus, the umbrella member 22 extends along a
 plane perpendicular to a central axis of the support member 20. The first
 mounting member 12 has a central through-hole 16 which is formed through
 the thickness of a central portion of the member 12 and in which a fixing
 bolt 18 is pressfitted to project upward from the member 12, as seen in
 FIG. 1. With the fixing bolt 18, the first mounting member 12 is attached
 to the power unit of the vehicle.
 The second mounting member 14 is provided by a rigid member which is formed
 of metal or the like and which has a generally cylindrical shape having a
 large diameter. The second mounting member 14 is spaced from the first
 mounting member 12 in a direction parallel to a central axis of the member
 12. The second mounting member 12 includes an integral flange portion 24
 which extends radially outward from an upper one of axially opposite open
 ends of the member 12, and includes an integral engaging portion 26 which
 is somewhat bent radially inward from the lower open end of the member 12.
 The second mounting member 14 is press-fitted in a rigid bracket 28 having
 a thick-walled cylindrical shape having a large diameter, and is attached
 to the body of the vehicle with the bracket 28 being fixed to the body
 with fixing bolts or the like. The bracket 28 includes an integral flange
 portion 30 which extends radially outward from one of axially opposite
 ends of the bracket 28. Since the flange portion 24 of the second mounting
 member 14 is superposed on the flange portion 30 of the bracket 28, the
 engine mount 10 enjoys a high withstand-load strength with respect to the
 direction in which the load of the power unit is applied to the engine
 mount 10.
 The elastic rubber body 15 is interposed between the first and second
 mounting members 12, 14. The rubber body 15 has a generally thick-walled,
 tapered, cylindrical shape whose central portion is tapered in its axially
 upward direction. Thus, the rubber body 15 has a generally frustoconical,
 outer circumferential surface. The first mounting member 12 is vulcanized
 to a small-diameter, axially upper end surface of the rubber body 15, and
 an inner circumferential surface of the upper end portion of the second
 mounting member 14 is vulcanized to an outer circumferential surface of a
 large-diameter, axially lower end portion of the rubber body 15. The
 support member 20 welded to the first mounting member 12 extends through a
 central bore 32 of the rubber body 15 and is vulcanized to an inner
 surface of the rubber body 15 that defines the central bore 32. Thus, in
 the present embodiment, the first and second mounting members 12, 14 and
 the elastic rubber body 15 are manufactured in the form of an integrally
 vulcanized product 21 as shown in FIG. 3.
 The elastic rubber body 15 is continuous with its integral portion which
 extends over a substantially entire inner circumferential surface of the
 second mounting member 14. The integral portion includes a thick-walled
 tubular rubber wall 34 as a buffer rubber which covers the axially upper
 half portion of the inner surface of the second mounting member 14. The
 rubber body 15 and the rubber wall 34 cooperate with each other to define
 a generally cylindrical void 36 opening downward. The lower end portion of
 the support member 20 projects into the void 36 from the center of an
 upper surface (i.e., bottom surface) of the void 36. Thus, the umbrella
 member 22 supported by the support member 20 is located in the void 36.
 The above-indicated integral portion additionally includes a thin-walled
 seal rubber layer 38 which covers the axially lower half portion of the
 inner surface of the second mounting member 14. Thus, the elastic rubber
 body 15, the tubular rubber wall 34, and the seal rubber layer 38 are
 formed integrally with one another.
 Four concave portions in the form of four pocketlike voids 31 are formed in
 an inner portion of the tapered portion of the elastic rubber body 15 that
 is located between the first and second mounting members 12, 14. Each of
 the pocket voids 31 has a predetermined depth and opens downward in the
 cylindrical void 36. The rubber body 15 includes four thin-walled wall
 portions 33 providing respective bottoms of the four pocket voids 31.
 Thus, the wall portions 33 are thinner than the remaining, four thick
 walled portions of the rubber body 15. In the present embodiment, the wall
 portions 33 have a generally constant thickness equal to about one fourth
 to about one third of the thickness of the remaining portions of the
 rubber body 15.
 In the present embodiment, the four pocket voids 31 have an identical shape
 and are provided around the central axis of the elastic rubber body 15
 such that the four voids 31 are equiangularly spaced from one another
 about the central axis. Thus, the rubber body 15 has two pairs of pocket
 voids 31 one pair of which are opposed to each other in one of two
 directions each of which is perpendicular to the central axis of the
 rubber body 15 and which are perpendicular to each other and the other of
 which are opposed to each other in the other direction. Each of the pocket
 voids 31 has, in a radial direction of the rubber body 15, a dimension
 generally equal to the radius of the body 15 and has, in a circumferential
 direction of the body 15, a dimension which increases in a radially
 outward direction of the body 15. Thus, each pocket void 31 has a
 generally part-sectorial cross section. The area of the lower opening of
 each pocket void 31 that is projected in a direction (hereinafter,
 referred to as the "axial direction") parallel to the central axis of the
 rubber body 15, is equal to 2 to 15%, more preferably, 5 to 10%, of the
 area of the cylindrical void 36 that is projected in the axial direction.
 In the present embodiment, the radially outer edge of the lower opening of
 each of the pocket voids 31 is located radially outward of an inner
 circumferential surface of the tubular rubber wall 34. That is, the pocket
 voids 31 extend downward and reach the rubber wall 34, so that the rubber
 wall 34 has a shape as if an inner portion of the rubber wall 34 were cut
 away by the pocket voids 31. That is, the thickness of the rubber wall 34
 is smaller at respective portions thereof corresponding to the pocket
 voids 31, than at the remaining portions thereof. Those thick-walled
 portions of the rubber wall 34 reinforce the axially lower end portion
 (i.e., outer peripheral portion) of the rubber body 15.
 As described above, in the present embodiment, the elastic rubber body 15
 has the four pocket voids 31 which are equiangularly spaced from one
 another about the central axis of the body 15. Therefore, the rubber body
 15 has four thick-walled portions 39 which are free of the voids 31, are
 equiangularly spaced from one another about the central axis of the body
 15, and have respective angular phases different from those of the
 corresponding voids 31 by 45 degrees. Thus, the four thick-walled portions
 39 cooperate with one another to give, to the rubber body 15, a generally
 uniform spring hardness or stiffness in respective radial directions
 perpendicular to the central axis of the body 15. In a particular case
 where the engine mount 10 is installed on the automotive vehicle such that
 the four thick-walled portions 39 are oriented in forward and backward
 directions and leftward and rightward directions of the vehicle, in which
 considerably great vibrational loads are applied to the engine mount 10,
 the engine mount 10 advantageously enjoys a high spring hardness in its
 radial directions perpendicular to its central axis.
 A partition member 40 and a diaphragm 42 are inserted in the order of
 description through the axially lower open end of the second mounting
 member 14, and are assembled with the integrally vulcanized product 21.
 The partition member 40 is provided by a rigid member which is formed of
 synthetic resin, metal such as aluminum alloy, or the like and which has a
 generally disc-like shape. The diaphragm 42 is provided by a thin rubber
 sheet which is easily elastically deformable. A cylindrical fitting ring
 44 is vulcanized to an outer circumferential surface of the diaphragm 42.
 After the partition member 40 and the diaphragm 42 are inserted in the
 second mounting member 14 and are located in the axially lower half
 portion of the second mounting member 14 that is covered with the seal
 rubber layer 38, the second mounting member 14 is subjected to a
 diameter-reducing operation such as eight-die-using drawing. Thus, the
 partition member 40 and the diaphragm 42 (or the ring 44) are assembled
 with the vulcanized product 21.
 Accordingly, the axially lower open end of the second mounting member 14 is
 fluid-tightly closed by the diaphragm 42, so as to define, in the second
 mounting member 14, a fluid-filled space which is air-tightly isolated
 from the ambient air and is filled with a non-compressible fluid. The
 non-compressible fluid may be selected from among water, alkylene glycol,
 polyalkylene glycol, silicone oil, or a mixture of two or more of them.
 However, in order to obtain an excellent vibration-damping effect based on
 resonance of the fluid, it is preferred that the fluid has a low viscosity
 of not more than 0.1 Pa.multidot.s. Filling the space with the
 non-compressible fluid can be advantageously carried out by, for example,
 assembling the partition member 40 and the diaphragm 42 with the
 vulcanized product 21, in a tank filled with the fluid.
 The fluid-filled space is divided into upper and lower fluid-filled spaces
 by the partition member 40. The upper fluid-filled space that is partly
 defined by the elastic rubber body 15 provides a pressure receiving
 chamber 46 as a primary fluid chamber whose pressure changes upon
 application of a vibrational load to the engine mount 10. The lower
 fluid-filled space that is partly defined by the diaphragm 42 provides an
 equilibrium chamber 48 as an auxiliary fluid chamber whose volume easily
 changes because of elastic deformation of the diaphragm and thereby
 absorbs the change of pressure of the pressure receiving chamber 46. The
 partition member 40 is provided in the second mounting member 14 such that
 the partition member 40 is held in contact with a radially inner annular
 portion of the axially lower end surface of the tubular rubber wall 34.
 Thus, the partition member 40 supports the rubber wall 34, thereby helping
 the rubber wall 34 reinforce the rubber body 15. The lower open end of the
 cylindrical void 36 is fluid-tightly closed by the partition member 40, so
 as to define the pressure receiving chamber 46.
 The partition member 40 has a generally spiral groove 50 which is
 continuously formed in an outer circumferential surface of the member 40
 and opens in the outer surface. The spiral groove 50 is closed by the
 second mounting member 14 via the seal rubber layer 38, so as to define an
 orifice passage 52 which communicates the pressure receiving chamber 46
 and the equilibrium chamber 48 with each other. Upon application of a
 vibrational load to the engine mount 10, a pressure difference is produced
 between the two chambers 46, 48, so that the fluid flows through the
 orifice passage 52 and thereby exhibits a certain vibration damping
 effect. In the present embodiment, the cross-section area, length, etc. of
 the orifice passage 52 are so determined as to exhibit, based on the
 resonance of the fluid flowing through the passage 52, an excellent
 vibration damping effect against a low-frequency vibration such as engine
 shake.
 The partition member 40 has a generally cylindrical central hole 54 which
 opens in an upper surface thereof, and a flexible rubber plate 56 is
 accommodated in the central hole 54. The rubber plate 56 is assembled with
 the partition member 40 in such a manner that an outer peripheral portion
 of the plate 56 is fluid-tightly sandwiched by a bottom surface of the
 central hole 54 and an annular hold-down ring 58 fixedly fitted in an
 axially upper open end of the central hole 54. In this state, a central
 portion of the upper surface of the rubber plate 56 is exposed to the
 fluid present in the pressure receiving chamber 46 via a central aperture
 60 of the ring 58. Meanwhile, a central portion of the lower surface of
 the rubber plate 56 is exposed to the fluid present in the equilibrium
 chamber 48 via a plurality of communication holes 62 formed through the
 thickness of a bottom wall of the partition member 40 that defines the
 bottom surface of the central hole 54. Thus, the upper and lower surfaces
 of the rubber plate 56 receive the respective fluid pressures in the two
 chambers 46, 48, respectively. Therefore, upon application of a
 vibrational load to the engine mount 10, the rubber plate 56 is
 elastically deformed because of a pressure difference produced between the
 two chambers 46, 48. The elastic deformation of the rubber plate 56 causes
 flows of the fluid through the central aperture 60 of the ring 58 and the
 communication holes 62 of the partition member 40, so that the engine
 mount 10 exhibits, based on the resonance of the fluid and the pressure
 absorbing effect of the pressure receiving chamber 46, a low dynamic
 spring effect against input vibrations having frequencies in a
 predetermined range. In the present embodiment, the spring characteristic
 of the rubber plate 56 and the cross-section area, fluid-flow length, etc.
 of the fluid passages are so predetermined that the engine mount 10
 exhibits, based on flows of the fluid caused by the elastic deformation of
 the rubber plate 56, an excellent vibration damping effect against a
 medium- or high-frequency vibration such as idling vibration or low-speed
 booming noise. In addition, the amount of elastic deformation of the
 rubber plate 56 is limited by its elasticity and its contact with the
 bottom surface of the central hole 54. Therefore, when a lowfrequency,
 large-amplitude vibration such as engine shake is input to the engine
 mount 10, the amount of flow of the fluid caused by the elastic
 deformation of the rubber plate 56 remains small, whereas the flow of the
 fluid through the orifice passage 52 is permitted in sufficient amount.
 In the pressure receiving chamber 46 whose wall is partly provided by the
 elastic rubber body 15, the umbrella shaped member 22 extends along a
 plane perpendicular to the vibration-input direction in which a main
 vibrational load is input to the engine mount 10, that is, the direction
 parallel to the central axis of the engine mount 10 (i.e., the vertical
 direction as seen in FIG. 1). In the state in which the power unit is
 mounted on the engine mount 10, the rubber body 15 is deformed or
 compressed by the weight of the power unit, so that the umbrella member 22
 is moved downward from a state indicated in solid lines in FIG. 1, to a
 state indicated in phantom lines in which the umbrella member 22 is
 located in the center of the pressure receiving chamber 46. Thus, the
 umbrella member 22 divides the chamber 46 into upper and lower divided
 chambers 70, 72 located on opposite sides of the umbrella member 22 in the
 vibration input direction, i.e., the axial direction of the engine mount
 10. The upper and lower divided chambers 70, 72 are communicated with each
 other via an annular gap 64 which is defined by, and between, an outer
 circumferential surface 66 of the umbrella member 22 and an inner
 circumferential surface 68 of the tubular rubber wall 34 that partly
 defines the pressure receiving chamber 46. The outer surface 66 and the
 inner surface 68 are opposed to each other in all radial directions of the
 engine mount 10, and the annular gap 64 extends continuously all around in
 a circumferential direction of the umbrella member 22.
 In the present embodiment, the umbrella member 22 has a generally
 skirt-like or tapered shape which is tapered toward its central portion at
 which the umbrella member 22 is fixed by caulking t o the support member
 20. The tapered shape of the umbrella member 22 generally corresponds to
 the inner circumferential surface of the elastic rubber body 15 that
 partly defines the pressure receiving chamber 46. The umbrella member 22
 includes a cylindrical, axially lower end portion which extends downward
 from the outer periphery of the tapered portion thereof and whose outer
 circumferential surface 66 is cylindrical and is concentric with a central
 axis of the support member 22 (i.e., a central axis of the pressure
 receiving member 46). The inner circumferential surface 68 of the tubular
 rubber wall 34 that partly defines the chamber 46 in which the umbrella
 member 22 is provided, is generally cylindrical and is concentric with the
 central axis of the chamber 46. Therefore, the annular gap 64 between the
 respective opposed portions of the outer surface 66 and the inner surface
 68 extends continuously all around in the circumferential direction of the
 umbrella member 22, with a substantially constant dimension in all the
 radial directions of the same 22.
 Thus, the present engine mount 10 has the upper and lower divided chambers
 70, 72 which are divided by the umbrella member 22 and are communicated
 with each other via the annular gap 64. Upon application of a vibrational
 load to the engine mount 10 in the direction in which the first and second
 mounting members 12, 14 are opposed to each other, the umbrella member 22
 is reciprocatively moved in the pressure receiving chamber 46, so that the
 fluid flows between the upper and lower chambers 70, 72 via the annular
 gap 64. Thus, in the present embodiment, the annular gap 64 provides a
 fluid-flow restricting passage, and the engine mount 10 exhibits a
 vibration damping effect based on flows of the fluid through the
 restricting passage. The frequency range in which the mount 10 exhibits a
 low dynamic spring effect based on the resonance of the fluid flowing
 through the gap 64 can be tuned by adjusting the ratio of the
 cross-section area of the gap 64 to the length of the same 64 while
 taking, into account, the spring hardness of the rubber wall defining the
 chamber 46 and the density of the fluid enclosed in the chamber 46. As far
 as the present embodiment is concerned, it is preferred that the above
 indicated frequency range is higher (e.g., including high speed booming
 noise) than that against which the mount 10 exhibits a vibration damping
 effect based on flows of the fluid caused by the elastic deformation of
 the rubber plate 56. In this case, the engine mount 10 can exhibit an
 excellent vibration damping effect in a wide frequency range based on not
 only the orifice passage 52 and the rubber plate 56 but also the annular
 gap or restricting passage 64.
 When the umbrella member 22 is reciprocatively moved upon application of
 the vibrational load, the wall portions 33 that define the pocket voids 31
 and thereby partly define the upper divided chamber 70 can be elastically
 deformed considerably easily by the change of pressure of the fluid
 present in the chamber 70. Thus, the umbrella member 22 can be easily
 moved in the pressure receiving chamber 46, so that the relative change
 between the respective volumes of the upper and lower divided chambers 70,
 72 easily occurs. Consequently a sufficient amount of fluid flows through
 the annular gap 64. Thus, the engine mount 10 advantageously exhibits a
 low dynamic spring effect, and accordingly an excellent vibration damping
 effect, based on the resonance of the fluid flowing through the annular
 gap 64.
 In the present embodiment, the pressure receiving chamber 46 is
 communicated with the equilibrium chamber 48 via the orifice passage 52
 and the rubber plate 56, and the annular gap 64 is tuned to the frequency
 range higher than that to which the passage 52 and the plate 56 are tuned.
 Upon application of the high-frequency vibration to which the annular gap
 64 is tuned, the elastic deformation of the wall portions 33 defining the
 pocket voids 31 effectively causes the fluid to flow between the upper and
 lower divided chambers 70, 72, thereby reducing or preventing the flowing
 of the fluid away from the pressure receiving chamber 46 into the
 equilibrium chamber 48 via the orifice passage 52 and the rubber plate 56.
 Thus, the present engine amount 10 exhibits an excellent vibration damping
 effect against not only the low- or medium-frequency input vibration based
 on the respective operations of the orifice passage 52 and the rubber
 plate 56, but also the high-frequency vibration based on the resonance of
 the fluid flowing in sufficient amount through the annular gap 64.
 In the present embodiment, the elastic deformation of the wall portions 33
 of the elastic rubber body 15 causes the fluid to flow between the
 pressure receiving chamber 46 (or the upper divided chamber 70) and the
 pocket voids 31. The cross-sectional area and fluidflow length of each of
 the pocket voids 31 through which the fluid flows are so determined while
 taking into account the expansion spring hardness of the wall portions 33
 and the density of the fluid enclosed, so that the engine mount 10 can
 exhibit a low dynamic spring effect against a target vibration-frequency
 range based on the resonance of the fluid flowing through the voids 31.
 For example, in the case where the engine mount 10 is so tuned as to
 exhibit, in a substantially same vibration-frequency range, both the low
 dynamic spring effect based on the resonance of the fluid flowing through
 the pocket voids 31 and the low dynamic spring effect based on the
 resonance of the fluid flowing through the annular gap 64, the fluid flows
 in more amount through the annular gap 64, in that frequency range, so
 that the mount 10 exhibits a more excellent vibration damping effect in
 that frequency range.
 Alternatively, in the case where the engine mount 10 is so tuned as to
 exhibit the above-indicated two sorts o f low dynamic spring effects, in
 respective frequency ranges which differ from each other by several tens
 of hertz (iz) to several hundred hertz (Hz), the mount 10 exhibits a low
 dynamic spring effect based on the resonance of the overall fluid in a
 wide frequency range including the above-indicated two ranges, and
 accordingly exhibits an excellent vibration damping effect in that wide
 frequency range. In the last case, it is preferred that the frequency
 range to which the fluid flowing through the pocket voids 31 is tuned is
 higher than that to which the fluid flowing through the annular gap 64 is
 tuned.
 FIGS. 4 and 5 show an automotive-vehicle engine mount 80 as a second
 embodiment of the present invention. The second engine mount 80 is
 basically identical with the first engine mount 10 shown in FIGS. 1 to 3,
 but is different from the first mount 10 in that the second mount 80 does
 not have the umbrella-shaped member 22 in the pressure receiving chamber
 46, the chamber 46 is not divided by the umbrella member 22 into the two
 divided chambers 70, 72, and the annular gap or fluid-flow restricting
 passage 64 is not defined by the umbrella member 22 in the chamber 46. The
 same reference numerals as used for the first engine mount 10 as the first
 embodiment, shown in FIGS. 1 to 3, are used to designate the corresponding
 elements and parts of the second engine mount 80 as the second embodiment,
 shown in FIGS. 4 and 5, and the description thereof is omitted.
 The second engine mount 80 having the above-indicated structure exhibits,
 like the first engine mount 10, a vibration damping effect against the
 low-or medium-frequency input vibration based on the respective operations
 of the orifice passage 52 and the rubber plate 56. In addition, upon
 inputting of the high-frequency vibration to the engine mount 80, the wall
 portions 33 defining the pocket voids 31 are elastically deformed so that
 the fluid pressure of the pressure receiving chamber 46 is prevented from
 being excessively increased. Thus, the spring constant of the engine mount
 10 is prevented from being excessively increased, and the excellent
 vibration damping effect of the mount 10 is maintained.
 In the second engine mount 80, too, the pocket voids 31 function as fluid
 passages. Therefore, the cross-sectional area and fluid-flow length of
 each of the voids 31 are adjusted or tuned so that the engine mount 80
 exhibits, in a target vibration-frequency range, the low dynamic spring
 effect based on the resonance of the fluid flowing through the voids 31.
 In this case, the engine mount 80 can exhibit a more excellent vibration
 damping effect against input vibrations in a specific high-frequency
 range.
 FIG. 6 shows respective measured vibration damping characteristic values
 (i.e., absolute spring constant values) of the first and second engine
 mounts 10, 80 and a comparative example which is obtained without forming
 any pocket voids 31 in the elastic rubber body 15 of the first engine
 mount 10 but has the umbrella-shaped member 22 in the pressure receiving
 chamber 46.
 As is apparent from the results shown in FIG. 6, the second engine mount 80
 without the umbrella member 22 exhibits substantially the same degree of
 low dynamic spring effect as that of the comparative example with the
 umbrella member 22, and the second engine mount 80 can be tuned to exhibit
 the low dynamic spring effect in a frequency range higher than that in
 which the low low dynamic spring effect based on the umbrella member 22 is
 exhibited. In addition, the first engine mount 10 exhibits a low dynamic
 spring effect in a wider frequency range than that in which the
 comparative mount with the umbrella member 22 only or the second mount 80
 with the pocket voids 31 only does.
 While the present invention has been described in its preferred
 embodiments, the present invention may be otherwise embodied.
 For example, the cylindrical outer circumferential surface 66 of the
 umbrella member 22 or the cylindrical inner circumferential surface 68 of
 the tubular rubber wall 34 may be modified to have an elliptic or
 polygonal shape or other appropriate shape. In addition, the respective
 cylindrical surfaces 66, 68 of the umbrella member 22 and the rubber wall
 34 that are similar to each other may be modified to have non-similar
 surfaces, so that the annular gap or fluid-flow restricting passage 64
 defined between the two surfaces may be modified to have different radial
 dimensions in different radial directions of the umbrella member 22.
 The distance between the outer surface 66 of the umbrella member 22 and the
 inner surface 68 of the rubber wall 34 may be so determined that the two
 surfaces 66, 68 can be brought into abutting contact with each other to
 limit the amount of relative movement between the first and second
 mounting members 12, 14 in the direction perpendicular to the axial
 direction of the engine mount 10, 80. In this case, the umbrella member 22
 functions as a stopper.
 In each of the first and second engine mounts 10, 80, the orifice passage
 52 is employed to damp the lowfrequency vibration and the rubber plate 56
 is employed to damp the medium-frequency vibration. However, the orifice
 passage 52 and/or the rubber plate 56 may, or may not, be employed
 depending upon the vibration damping characteristics the mount 10, 80 is
 required to have. Thus, each of the orifice passage 52 and the rubber
 plate 56 is not essential to the concept of the present invention.
 The concept of the present invention is applicable to not only
 automotive-vehicle engine mounts but also automotive-vehicle body mounts,
 differential mounts, and suspension bushings, and additionally to various
 vibration damping devices which are employed in other structures than
 automotive vehicles.
 It is to be understood that the present invention may be embodied with
 other changes, modifications, and improvements that may occur to one
 skilled in the art without departing from the scope and spirit of the
 invention defined in the appended claims.