Patent Publication Number: US-10767722-B2

Title: Mount for subframe

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-232464 filed on Dec. 4, 2017, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a mount for a subframe enclosing a magnetorheological fluid (MRF), a fluid containing a magnetorheological compound (MRC), or other similar materials in a fluid tight manner and disposed on the subframe at a position where the subframe is supported by vehicle body. 
     Description of the Related Art 
     For example, Japanese Laid-Open Patent Publication No. 2006-077787 (hereinafter referred to as “JPA 2006-077787”) discloses a damper with variable damping force using a magnetorheological fluid of which viscosity changes according to effects of a magnetic field (FIG. 2 in JPA 2006-077787). 
     The damper with variable damping force encloses the magnetorheological fluid inside a cylinder and generates viscous drag or damping force by sliding a piston plate inside the cylinder. 
     The piston plate has orifices serving as paths of the magnetorheological fluid above and below the piston plate. 
     In addition, a coil is disposed adjacent to the orifices and is supplied with current from an external power source to generate magnetic flux crossing the orifices. 
     The magnetic flux increases local viscosity of the magnetorheological fluid passing through the orifices and thus increases the damping force against the movement of the piston plate. 
     In this manner, predetermined damping characteristics can be achieved in the axial (vertical) direction within an adjustment range by adjusting the strength of the magnetic field to be applied from the outside. 
     SUMMARY OF THE INVENTION 
     The above-described damper with variable damping force, however, can resist external forces only in the vertical direction. in a case of providing a mount at a position where the subframe is supported by a vehicle body, such a damper cannot be applied to the mount disposed on a subframe, on which, for example, a driving source of a vehicle is mounted, since external forces in the longitudinal and transverse directions of the vehicle, that is, in directions perpendicular to the axial direction (hereinafter referred to as “axis-perpendicular directions”) are applied to the mount in addition to the external forces in the axial (vertical) direction. 
     The present invention has been devised taking into consideration the aforementioned problems, and has the object of providing a mount for a subframe capable of exerting a variable damping force or a variable stiffness on external forces in the axial (vertical) direction and the axis-perpendicular directions (the longitudinal and transverse directions). 
     A mount for a subframe according to the present invention, having a cylindrical shape, containing magnetorheological fluid in a fluid tight manner, and disposed on the subframe at a position where the subframe is supported by a vehicle body, includes: 
     upper and lower fluid chambers; 
     a middle fluid chamber, including an axial path extending in a direction of an axis of the mount and an axis-perpendicular path extending in directions perpendicular to the axis, disposed between the upper and lower fluid chambers; and 
     a magnetic member; wherein: 
     one end of the axial path communicates with one of the upper and lower fluid chambers, another end of the axial path communicates with one end of the axis-perpendicular path, and another end of the axis-perpendicular path communicates with the other of the upper and lower fluid chambers; and 
     the magnetic member forms magnetic paths passing through the axial path of the middle fluid chamber in the directions perpendicular to the axis and passing through the axis-perpendicular path, in the direction of the axis when excitation current is applied to a coil wound about the axis. 
     According to the present invention, flows of the magnetorheological fluid are controlled to be stopped in the axial path and in the axis-perpendicular path inside the mount by the magnetic paths formed by applying the excitation current to the coil, and thus the elastic properties of the mount are adjusted such that the mount is hardened in the direction of the axis (vertical direction) and in the directions perpendicular to the axis (longitudinal and transverse directions). 
     As a result, a variable damping force can be exerted on external forces applied to the mount in the direction of the axis and in the directions perpendicular to the axis. 
     In addition, the magnetorheological fluid does not flow between the upper and lower fluid chambers without passing through the middle fluid chamber in which the magnetic paths are formed. Consequently, the elastic properties of the mount can be efficiently changed by changing the magnitude of the magnetic field of the magnetic paths in the middle fluid chamber. 
     Moreover, a mount for a subframe according to the present invention, having a cylindrical shape and disposed on the subframe at a position where the subframe is supported by a vehicle body, includes: 
     an inner cylinder including a hollow shaft portion for fastening the mount to the vehicle body; 
     an outer cylinder disposed to be coaxial with the inner cylinder; 
     a coil having a cylindrical shape secured adjacent to the inner cylinder; 
     first and second elastic members each having an annular shape, respectively disposed in upper and lower portions of the mount, and holding a magnetorheological fluid inside the mount in a fluid tight manner, wherein: 
     a first fluid chamber and a third fluid chamber accommodating the magnetorheological fluid are respectively disposed in the upper and lower portions inside the mount; 
     a second fluid chamber accommodating the magnetorheological fluid is disposed between the first fluid chamber and the third fluid chamber; 
     the second fluid chamber includes an axial path, extending in a direction of an axis of the mount and communicating with the first fluid chamber, and an axis-perpendicular path, extending in directions perpendicular to the axis and communicating with the axial path and the third fluid chamber; and 
     a first magnetic member is secured to an outer circumference of the inner cylinder and a second magnetic member is secured to an inner circumference of the outer cylinder such that magnetic paths passing through the axial path of the second fluid chamber in the directions perpendicular to the axis and passing through the axis-perpendicular path in the direction of the axis are formed when excitation current is applied to the coil. 
     According to the present invention, flows of the magnetorheological fluid are controlled to be stopped in the direction of the axis and in the directions perpendicular to the axis of the mount by applying the excitation current to the coil, and thus the elastic properties of the mount are adjusted such that the mount is hardened in the direction of the axis and in the directions perpendicular to the axis. 
     As a result, a variable damping force or a variable stiffness can be exerted on the external forces applied to the mount in the direction of the axis (vertical direction) and in the directions perpendicular to the axis (longitudinal and transverse directions). 
     In addition, the magnetorheological fluid does not flow between the first fluid chamber and the third fluid chamber without passing through the second fluid chamber in which the magnetic paths are formed. Consequently, the elastic properties of the mount can be efficiently changed by changing the magnitude of the magnetic field of the magnetic paths in the second fluid chamber. 
     In this case, the axial path and the axis-perpendicular path of the second fluid chamber form a crank-like shape when the mount is viewed in longitudinal section; 
     the magnetic paths in the directions perpendicular to the axis are formed radially in the directions perpendicular to the axis; and the magnetic paths in the direction of the axis are formed throughout the entire circumference of the axis. 
     According to this structure, the axial path and the axis-perpendicular path of the second fluid chamber through which the magnetorheological fluid passes are symmetrical with respect to the axis, and thus the elastic properties are adjusted to be uniform in the radial direction of the second fluid chamber. 
     Here, a volume of the second fluid chamber may be smaller than a volume of the first fluid chamber and than a volume of the third fluid chamber. 
     The second fluid chamber with a volume smaller than the volumes of the first fluid chamber and the third fluid chamber enables the formed magnetic paths to be compact, and thus the elastic properties can be changed while the power efficiency in forming the magnetic paths using the exciting coil is improved. 
     Moreover, a stiffness of one of the first and second elastic members, each having the annular shape, respectively disposed in the upper and lower portions of the mount, and holding the magnetorheological fluid inside the mount in a fluid tight manner, may be lower than a stiffness of the other. 
     That is, one of the elastic members with a stiffness lower than the stiffness of the other forms a diaphragm. When the fluid pressures in the fluid chambers are increased, the diaphragm expands to absorb the fluid pressures. This allows the stiffness of the mount to set to low in a state where no magnetic field is applied, and allows variable magnifications of the stiffness or the damping of the mount to be increased when a magnetic field is applied while the internal pressures in the fluid chambers, that is, the internal pressure in the mount is prevented from being increased. This prevents the mount from getting fatigued, leading to a longer life span of the mount. 
     Furthermore, a plurality of partition members may radially extend to partition the first fluid chamber and the third fluid chamber into sectors of a hollow cylinder. 
     The partition members limit the ranges of flows of the magnetorheological fluid in the directions around the axis in the first fluid chamber and the third fluid chamber and direct the flows of the magnetorheological fluid generated in response to inputs in the directions perpendicular to the axis toward the second fluid chamber. This enables the viscosity or the stiffness of the mount for the subframe to be changed. 
     According to the present invention, the flows of the magnetorheological fluid are controlled to be stopped in the axial path and in the axis-perpendicular path inside the mount by the magnetic paths formed by applying the excitation current to the coil, and thus the elastic properties of the mount are adjusted such that the mount is hardened in the direction of the axis (vertical direction) and in the directions perpendicular to the axis (longitudinal and transverse directions). 
     As a result, a variable damping force or a variable stiffness can be exerted on the external forces applied to the mount in the direction of the axis and in the directions perpendicular to the axis. 
     In addition, the magnetorheological fluid does not flow between the upper and lower fluid chambers without passing through the middle fluid chamber in which the magnetic paths are formed. Consequently, the elastic properties of the mount can be efficiently changed by changing the magnitude of the magnetic field of the magnetic paths in the middle fluid chamber. 
     The above and other objects features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of a vehicle to which a mount for a subframe according to the present invention is applied; 
         FIG. 2  is a partially omitted longitudinal sectional view illustrating how a mount for a subframe according to a first embodiment fastened to the subframe is mounted on a vehicle body (main frame); 
         FIG. 3  is a longitudinal sectional view illustrating components of the mount for the subframe according to the first embodiment alone; 
         FIG. 4A  is a distribution diagram of iron powder in a magnetorheological fluid containing structure in a state where no magnetic field is applied; 
         FIG. 4B  is a distribution diagram of the iron powder in the magnetorheological fluid containing structure when a magnetic field is applied; 
         FIG. 5  is a characteristic diagram illustrating the value of coil excitation current with respect to the yaw rate and the vehicle speed; 
         FIG. 6  is a longitudinal sectional view illustrating a magnetic field (magnetic paths) generated when an external force in the axial direction and an external force in a shear direction are applied to the mount for the subframe according to the first embodiment; 
         FIG. 7  is a cross-sectional view of the mount for the subframe according to the first embodiment taken along line VII-VII in  FIG. 6 ; 
         FIG. 8  is a longitudinal sectional view illustrating the structure and effects of a mount for a subframe according to a second embodiment; 
         FIG. 9A  is a cross-sectional view of a first fluid chamber of the mount for the subframe according to the second embodiment; 
         FIG. 9B  is a cross-sectional view of a second fluid chamber of the mount for the subframe according to the second embodiment; 
         FIG. 9C  is a cross-sectional view of a third fluid chamber of the mount for the subframe according to the second embodiment; 
         FIG. 10  is a longitudinal sectional view of the mount for the subframe according to the second embodiment in a state where no magnetic field is generated; 
         FIG. 11A  is a cross-sectional view of the first fluid chamber of the mount for the subframe according to the second embodiment in a state where no magnetic field is generated; 
         FIG. 11B  is a cross-sectional view of the second fluid chamber of the mount for the subframe according to the second embodiment in a state where no magnetic field is generated; 
         FIG. 11C  is a cross-sectional view of the third fluid chamber of the mount for the subframe according to the second embodiment in a state where no magnetic field is generated; 
         FIG. 12  is a longitudinal sectional view of a mount for a subframe according to a third embodiment in a state where no magnetic field is generated; 
         FIG. 13A  is a cross-sectional view of the first fluid chamber of the mount for the subframe according to the third embodiment in a state where no magnetic field is generated; 
         FIG. 13B  is a cross-sectional view of the second fluid chamber of the mount for the subframe according to the third embodiment in a state where no magnetic field is generated; 
         FIG. 13C  is a cross-sectional view of the third fluid chamber of the mount for the subframe according to the third embodiment in a state where no magnetic field is generated; 
         FIG. 14  is a longitudinal sectional view of a mount for a subframe according to a fourth embodiment in a state where no magnetic field is generated; 
         FIG. 15A  is a cross-sectional view of the first fluid chamber of the mount for the subframe according to the fourth embodiment in a state where no magnetic field is generated; 
         FIG. 15B  is a cross-sectional view of the second fluid chamber of the mount for the subframe according to the fourth embodiment in a state where no magnetic field is generated; 
         FIG. 15C  is a cross-sectional view of the third fluid chamber of the mount for the subframe according to the fourth embodiment when no magnetic field is generated; 
         FIG. 16  is a longitudinal sectional view illustrating the structure and effects of a mount for a subframe according to a fifth embodiment; and 
         FIG. 17  is a longitudinal sectional view illustrating the structure and effects of a mount for a subframe according to another example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a mount for a subframe according to the present invention will be described in detail below with reference to the accompanying drawings. 
     First Embodiment 
     [Structure] 
       FIG. 1  is a schematic plan view of a vehicle  10  to which a mount for a subframe according to the present invention is applied. 
     The vehicle  10  includes an approximately rectangular subframe  16  in the front part of a vehicle body (main frame)  12 . A component  14  including an internal combustion engine, an electric motor, a power generator, a differential gear, a fuel tank, and/or a transmission as appropriate is mounted on the subframe  16 . 
     The subframe  16  is provided, at the four corners, with mounts  18  for a subframe according to this (first) embodiment (hereinafter also referred to as “mounts”). 
     The subframe  16  is joined to the vehicle body (main frame)  12  via the mounts  18 . 
     The component  14  mounted on the subframe  16  is partially connected to front wheels W via an axle  20 . The front wheels W are steered wheels and are connected and suspended on the vehicle body (main frame)  12  and the subframe  16  by a suspension device (not illustrated). The front wheels W are connected to a steering wheel (not illustrated) via a rack mechanism and a steering shaft (both not illustrated). 
     The mounts  18  are connected with an electronic control unit (ECU)  24  serving as a controller and provided with coil excitation currents I by the ECU  24 . 
     The coil excitation currents I are controlled by the ECU  24  to have values according to the yaw rate YR obtained by a yaw rate sensor  26  and/or the vehicle speed Vv obtained by a vehicle speed sensor  28  such as a wheel speed sensor. The sensors are disposed adjacent to the center of gravity of the vehicle body  12 . 
       FIG. 2  is a partially omitted longitudinal sectional view illustrating how each mount  18  fastened to the subframe  16  by, for example, insertion is mounted on the vehicle body (main frame)  12 . 
     The mount  18  includes an outer cylinder  34  fitted in the subframe  16 , an inner cylinder (for ease of understanding, also referred to as “inner cylindrical magnetic core”)  40  composed of a magnetic body, and an internal mount structure  42  disposed between the inner cylinder  40  and the outer cylinder  34 . The inner cylinder  40  has a hollow shaft portion in which a bolt (through-bolt)  36  is fitted, and is fastened to the vehicle body (main frame)  12  by the bolt  36  and a nut  38 . The outer cylinder  34  is coaxially disposed on the radially outer side of the inner cylinder  40 . 
       FIG. 3  is an enlarged longitudinal sectional view illustrating components of the internal mount structure  42  of the mount  18  alone. 
     As illustrated in  FIG. 3 , the mount  18  is provided with a housing  48  including the inner cylinder  40  composed of a magnetic body for fastening the mount to the vehicle body  12 , the outer cylinder  34  fitted in the subframe  16 , a diaphragm  44  serving as a first elastic member having an annular shape and covering an upper portion of the mount  18  to hold magnetorheological fluid H, and a main rubber  46  serving as a second elastic member having an annular shape and covering a lower portion of the mount  18  to hold the magnetorheological fluid H. 
     The inner cylindrical magnetic core  40  includes a bolt hole  40   a  and an outer circumferential wall  40   b  serving as a hollow shaft portion for fastening the mount to the vehicle body (main frame)  12 . 
     An inner magnetic core  50  composed of a magnetic body is joined to the outer circumferential wall  40   b  of the inner cylindrical magnetic core  40 . 
     The inner magnetic core  50  includes an annular core portion  50   a , serving as a bottom portion, of which inner circumferential wall is joined to the outer circumferential wall  40   b  of the inner cylindrical magnetic core  40 , a cylindrical core portion  50   b  of which lower surface is joined to the upper surface of the annular core portion  50   a  adjacent to the outer circumference, and a brim-shaped core portion  50   c  having a cylindrical shape extending radially outward and joined to the upper surface of the cylindrical core portion  50   b.    
     The inner magnetic core  50  may be integrally molded. 
     A cylindrical exciting coil  52  is accommodated in a cylindrical space defined by the inner surface of the cylindrical core portion  50   b  and the outer circumferential wall  40   b  of the inner cylindrical magnetic core  40 . The exciting coil  52  is secured adjacent to the inner cylinder  40  and generates a magnetic field with a strength according to the coil excitation current I supplied by the ECU  24 . 
     An outer magnetic core  56  is joined to an upper portion of the outer cylinder  34 . 
     Specifically, the outer circumferential wall of a cylindrical core portion  56   a  of the outer magnetic core  56  is joined to the inner circumferential wall of the outer cylinder  34 . An annular core portion  56   b  is joined to the lower surface of the cylindrical core portion  56   a  such that part of the lower surface of the annular core portion  56   b  faces the upper surface of the brim-shaped core portion  50   c . The outer circumferential wall of the annular core portion  56   b  is joined to the inner circumferential wall of the outer cylinder  34 . 
     The outer magnetic core  56  may be integrally molded. 
     The inner space of the housing  48  of the mount  18  contains the magnetorheological fluid H such as magnetorheological fluid (MRF) or a fluid containing a magnetorheological compound (MRC) in a fluid tight manner. 
     In this case, a first fluid chamber  61  having a hollow cylindrical shape and accommodating the magnetorheological fluid H is defined in the upper portion of the mount  18  by the diaphragm  44  serving as the first elastic member having an annular shape, the cylindrical core portion  56   a  and the annular core portion  56   b  of the outer magnetic core  56 , and the outer circumferential wall  40   b  of the inner cylindrical magnetic core  40 . 
     In addition, a third fluid chamber  63  having a substantially hollow cylindrical shape and accommodating the magnetorheological fluid H is defined in the lower portion of the mount  18  by the main rubber  46  serving as the second elastic member having an annular (cylindrical) shape, the outer cylinder  34 , and the cylindrical core portion  50   b  and the brim-shaped core portion  50   c  of the inner magnetic core  50 . 
     A second fluid chamber  62  accommodating the magnetorheological fluid H is defined between the first fluid chamber  61  and the third fluid chamber  63  respectively defined in the upper and lower portions of the mount  18 . An upper portion of the second fluid chamber  62  communicates with the first fluid chamber  61 , and a lower portion communicates with the third fluid chamber  63 . 
     The second fluid chamber  62  includes an axial path  62   a  extending in a direction of the axis of the mount (hereinafter referred to as “axial direction”) and communicating with the first fluid chamber  61  and an axis-perpendicular path  62   b  extending in directions perpendicular to the axis (hereinafter referred to as “axis-perpendicular directions”) and communicating with the axial path  62   a  and the third fluid chamber  63 . 
     When the mount  18  is viewed in longitudinal section, the axial path  62   a  and the axis-perpendicular path  62   b  of the second fluid chamber  62  form a flange-like shape or a crank-like shape. 
     [Effects] 
     Next, the operational effects of the mount  18  enclosing the magnetorheological fluid H will be described. 
     [Description of Operational Effects of Magnetorheological Fluid Containing Structure with Basic Construction] 
       FIGS. 4A and 4B  are schematic distribution diagrams illustrating the operational effects of a magnetorheological fluid containing structure  100  with a basic construction. 
     First, before the operational effects of the mount  18  are described, the operational effects of the magnetorheological fluid containing structure  100  with a basic construction will be described with reference to  FIGS. 4A and 4B  for ease of understanding. 
       FIG. 4A  illustrates a state of the magnetorheological fluid containing structure  100  in a state where no magnetic field is applied. 
     In the magnetorheological fluid containing structure  100  illustrated in  FIG. 4A , for example, iron powder  104  serving as magnetic particles move freely in the magnetorheological fluid H in a path  102 . In this case, the viscosity of the magnetorheological fluid H acts as resistance in the direction of flow. 
     In a case where the magnetorheological fluid H is MRF, the magnetorheological fluid H functions as a fluid in which the iron powder  104  is dispersed. In a case where the magnetorheological fluid H is MRC, the magnetorheological fluid H functions as a thick, creamy compound, as is mayonnaise, in which the iron powder  104  is dispersed. 
       FIG. 4B  illustrates a state of the magnetorheological fluid containing structure  100  when a magnetic field is applied to generate a magnetic flux indicated by broken line arrows crossing the path  102 . 
     In the magnetorheological fluid containing structure  100 , to which the magnetic field is applied, illustrated in  FIG. 4B , the iron powder  104  forms valves along the magnetic field against the flow of the magnetorheological fluid H and functions as resistance, resulting in an increase in resistance in the direction of flow of the fluid. 
     In this manner, the apparent viscosity in the magnetorheological fluid containing structure  100  changes in proportion to the applied magnetic field. 
     Description of Operational Effects of Mount  18  for Subframe According to First Embodiment 
     Next, the operational effects of the mounts  18  for the subframe according to this embodiment, disposed on the subframe  16  at positions where the subframe  16  is supported by the vehicle body (main frame)  12  and containing the magnetorheological fluid H in a fluid tight manner as illustrated in  FIG. 2 , will be described. 
     As described above, the component  14  mounted on the subframe  16  includes an internal combustion engine, a differential gear, an electric motor, a fuel tank, and the like. The subframe  16  has mounting points (fastening positions) for a suspension system in addition to the component  14 , and is joined to the vehicle body (main frame)  12  via the mounts  18 . 
     As illustrated by example maps (characteristics)  201 ,  202 , and  203  in  FIG. 5 , the ECU  24  controls the coil excitation current I of the exciting coil  52  such that the coil excitation current I increases as the yaw rate YR obtained by the yaw rate sensor  26  increases and as the vehicle speed Vv obtained by the vehicle speed sensor  28  increases to increase the resilience of the mounts  18 . That is, the modulus of elasticity of the mounts  18  can be increased (changed). 
     Thus, for example, the coil excitation current I is set to zero or a small value to reduce the modulus of elasticity of the mounts  18  during traveling on a straight road or cruising on a freeway to prevent input of forced vibration from the internal combustion engine or the electric motor or input of vibration transmitted from the road surface to the vehicle body (main frame)  12  via the suspension. As a result, noise and vibration felt by occupants in the vehicle cabin are reduced and thus occupant comfort is improved. 
     On the other hand, the ECU  24  increases the coil excitation current I to harden (change the resilience of) the mounts  18  on a curve or a winding road. This improves the dynamic performance (turning performance) of the vehicle  10  and thus improves the controllability (handling performance) by the driver. 
       FIG. 6  illustrates the structure of the mount  18  and a magnetic field (magnetic flux), schematically illustrated by solid line arrows, generated by applying the coil excitation current I to the exciting coil  52  when an external force F 1  in the axial direction and an external force F 2  in a shear direction (axis-perpendicular direction) are applied to the mount  18 . 
     Note that broken line arrows in  FIG. 6  indicate directions in which the magnetorheological fluid H may move when the coil excitation current I is not applied. 
     Controlling the magnetic field by applying the coil excitation current I to the exciting coil  52  in response to the external force F 1  in the axial (vertical) direction and the external force F 2  in the axis-perpendicular direction (shear direction or the longitudinal or transverse direction of the vehicle) applied to the outer cylinder  34  of the mount  18  enables the resistance of the magnetorheological fluid H in the axial direction to be increased in the axial path  62   a  of the second fluid chamber  62 . 
     As illustrated in  FIG. 7  (cross-sectional view taken along line VII-VII in  FIG. 6 ), radial magnetic paths (magnetic flux) are generated in the axial path  62   a  of the second fluid chamber  62  as indicated by solid line arrows to control flows of the magnetorheological fluid H in directions around the axis indicated by broken line arrows to be stopped. 
     In addition, as illustrated in  FIG. 6 , the resistance of the magnetorheological fluid H in the axis-perpendicular path  62   b  of the second fluid chamber  62  is also increased. Consequently, the flows of the magnetorheological fluid H between the second fluid chamber  62  and the first fluid chamber  61  and between the second fluid chamber  62  and the third fluid chamber  63  are controlled to be stopped. 
     Thus, both the flow rate from the first fluid chamber  61  to the second fluid chamber  62  and the flow rate from the third fluid chamber  63  to the second fluid chamber  62  can be controlled in response to the external force F 1  serving as vibration input in the axial (vertical) direction to the outer cylinder  34  of the mounts  18  according to the first embodiment. In this manner, the stiffness of the mount  18  in the axial direction can be controlled in a wide range, and thus the transmission of the external force F 1  can be controlled. 
     On the other hand, in response to the external force F 2  applied in the shear direction (longitudinal or transverse direction), the flows of the magnetorheological fluid H in the directions around the axis cannot be eliminated or reduced in the axis-perpendicular path  62   b  of the second fluid chamber  62 , the first fluid chamber  61 , and the third fluid chamber  63  except the axial path  62   a  of the second fluid chamber  62 . Thus, the stiffness of the mount  18  according to the first embodiment can be controlled in a limited range. 
     Second Embodiment 
       FIG. 8  is a longitudinal sectional view illustrating the structure and effects of a mount  18 A for a subframe according to a second embodiment capable of eliminating or reducing flows in the directions around the axis in the first fluid chamber  61  and the third fluid chamber  63  in response to the external force F 2  applied in the shear direction (longitudinal or transverse direction). 
       FIGS. 9A, 9B, and 9C  are cross-sectional views of the first fluid chamber  61  (line IXA-IXA), the second fluid chamber  62  (line IXB-IXB), and the third fluid chamber  63  (IXC-IXC), respectively, of the mount  18 A for the subframe illustrated in  FIG. 8 . 
     The mount  18 A illustrated in  FIGS. 8 and 9A to 9C  includes a partition rubber plate  71  having an X shape when viewed in the transverse cross section and a partition rubber plate  73  having an X shape when viewed in the transverse cross section. The partition rubber plate  71  partitions the first fluid chamber  61  in a direction around the axis into four chamber sections, first fluid chamber sections  61   a ,  61   b ,  61   c , and  61   d , each having a shape of a sector of a hollow cylinder. The partition rubber plate  73  partitions the third fluid chamber  63  in the direction around the axis into four chamber sections, third fluid chamber sections  63   a ,  63   b ,  63   c , and  63   d , each having a shape of a sector of a hollow cylinder. 
     The upper partition rubber plate  71  is disposed between the lower surface of the diaphragm  44  and its upper surface of the annular core portion  56   b , and the thickness (length in the axial direction) extends vertically (see  FIG. 8 ). In addition, the lower partition rubber plate  73  is disposed between the lower surface of the brim-shaped core portion  50   c  and the upper surface of the main rubber  46 , and the thickness (length in the axial direction) extends vertically (see  FIG. 8 ). 
     The partition rubber plate  71  in the first fluid chamber  61  and the partition rubber plate  73  in the third fluid chamber  63  enable the flows of the magnetorheological fluid H around the axis to be eliminated or reduced in the first fluid chamber  61  and the third fluid chamber  63 . Furthermore, application of the magnetic field enables the flows of the magnetorheological fluid H around the axis to be eliminated or reduced in the second fluid chamber  62 . Thus, transmission of the external force F 2  applied in the shear direction (longitudinal or transverse direction) can be controlled. 
     Application of the magnetic field enables the flow rates of the magnetorheological fluid H from the first fluid chamber  61  to the second fluid chamber  62  and from the third fluid chamber  63  to the second fluid chamber  62  to be controlled, resulting in an efficient control of the stiffness of the mount  18 A in all the directions including the vertical, longitudinal, and transverse directions. 
       FIG. 10  is a longitudinal sectional view illustrating the flows of the magnetorheological fluid H indicated by solid line arrows in a state where the coil excitation current I is not applied to the exciting coil  52  of the mount  18 A for the subframe according to the second embodiment.  FIGS. 11A, 11B, and 11C  are cross-sectional views of the first fluid chamber  61  (line XIA-XIA), the second fluid chamber  62  (line XIB-XIB), and the third fluid chamber  63  (XIC-XIC), respectively, of the mount  18 A for the subframe illustrated in  FIG. 10  when the coil excitation current I is not applied to the exciting coil  52 , that is, in a state where no magnetic field is generated. 
     In this case, the magnetorheological fluid H can move freely from the first fluid chamber  61  to the second fluid chamber  62  and from the third fluid chamber  63  to the second fluid chamber  62 . As a result, the magnetorheological fluid H can move in the axial (vertical) direction between the first fluid chamber  61  and the third fluid chamber  63  and can move around the axis in the second fluid chamber  62  as illustrated in  FIG. 11B . In this manner, the stiffness of the mount  18 A can be reduced while the coil excitation current I is not applied. 
     Third Embodiment 
       FIG. 12  is a longitudinal sectional view illustrating the structure and effects of a mount  18 B for a subframe according to a third embodiment capable of eliminating or reducing flows in the directions around the axis in the first fluid chamber  61  and the third fluid chamber  63  in response to the external force F 2  applied in the shear direction (longitudinal or transverse direction). 
       FIGS. 13A, 13B, and 13C  are cross-sectional views of first fluid chamber sections  61   e ,  61   f  (line XIIIA-XIIIA), the second fluid chamber  62  (line XIIIB-XIIIB), and third fluid chamber sections  63   e ,  63   f  (XIIIC-XIIIC), respectively, of the mount  18 B for the subframe illustrated in  FIG. 12 . 
     The mount  18 B illustrated in  FIGS. 12 and 13A to 13C  includes partition rubber plates  71   a  having an I shape when viewed in the transverse cross section and partition rubber plates  73   a  having an I shape when viewed in the transverse cross section. The partition rubber plates  71   a  partition the first fluid chamber  61  in the direction around the axis into two chamber sections (halves), the first fluid chamber sections  61   e ,  61   f , each having a shape of a sector of a hollow cylinder. The partition rubber plates  73   a  partition the third fluid chamber  63  in the direction around the axis into two chamber sections (halves), the third fluid chamber sections  63   e ,  63   f , each having a shape of a sector of a hollow cylinder. 
     The partition rubber plates  71   a ,  71   a  are disposed between the lower surface of the diaphragm  44  and the upper surface of the annular core portion  56   b , and its thicknesses (lengths in the axial direction) extend vertically (see  FIG. 12 ). In addition, the partition rubber plates  73   a ,  73   a  are disposed between the lower surface of the brim-shaped core portion  50   c  and the upper surface of the main rubber  46 , and its thicknesses (lengths in the axial direction) extend vertically (see  FIG. 12 ). 
     The partition rubber plates  71   a ,  71   a  in the first fluid chamber  61  and the partition rubber plates  73   a ,  73   a  in the third fluid chamber  63  enable the flows of the magnetorheological fluid H around the axis to be eliminated or reduced in the first fluid chamber  61  and the third fluid chamber  63 . Furthermore, application of the magnetic field enables the flows of the magnetorheological fluid H around the axis to be eliminated or reduced in the second fluid chamber  62 . Thus, transmission of the external force F 2  applied in the shear direction (longitudinal or transverse direction) can be controlled. 
     Application of the magnetic field enables the flow rates of the magnetorheological fluid H from the first fluid chamber  61  to the second fluid chamber  62  and from the third fluid chamber  63  to the second fluid chamber  62  to be controlled, resulting in an efficient control of the stiffness of the mount  18 B in all the directions including the vertical, longitudinal, and transverse directions. 
     In the mount  18 B for the subframe illustrated in  FIGS. 12 and 13A to 13C , the flows of the magnetorheological fluid H in a state where the coil excitation current I is not applied to the exciting coil  52  are indicated by solid line arrows. 
     In this case, the magnetorheological fluid H can move freely from the first fluid chamber sections  61   e ,  61   f  to the second fluid chamber  62  and from the third fluid chamber sections  63   e ,  63   f  to the second fluid chamber  62 . As a result, the magnetorheological fluid H can move in the axial (vertical) direction between the first fluid chamber sections  61   e ,  61   f  and the third fluid chamber sections  63   e ,  63   f  and can move around the axis in the second fluid chamber  62  as illustrated in  FIG. 13B . In this manner, the stiffness of the mount  18 B can be kept low by not generating a magnetic field. 
     Fourth Embodiment 
       FIG. 14  is a longitudinal sectional view illustrating the structure and effects of a mount  18 C for a subframe according to a fourth embodiment capable of eliminating or reducing flows in the directions around the axis in the first fluid chamber  61  and the third fluid chamber  63  in response to the external force F 2  applied in the shear direction (longitudinal or transverse direction). 
       FIGS. 15A, 15B, and 15C  are cross-sectional views of first fluid chamber sections  61   g ,  61   h  (line XVA-XVA), the second fluid chamber  62  (line XVB-XVB), and third fluid chamber sections  63   i ,  63   j  (XVC-XVC), respectively, of the mount  18 C for the subframe illustrated in  FIG. 14 . 
     In the mount  18 C illustrated in  FIGS. 14 and 15A to 15C , a structure corresponding to the main rubber  46  (see  FIG. 12  and the like) evenly disposed in the lower portion of the mount in the above-described embodiments has different heights in the axial direction. 
     More specifically, the mount  18 C includes partition rubber plates  71   b  having an annular sector shape when viewed in the transverse cross section and partition rubber plates  73   b  having an annular sector shape when viewed in the transverse cross section. The partition rubber plates  71   b  partition the first fluid chamber  61  in the direction around the axis into two chamber sections, the first fluid chamber sections  61   g ,  61   h , each having a shape of a sector of a hollow cylinder. The partition rubber plates  73   b  partition the third fluid chamber  63  in the direction around the axis into two chamber sections, the third fluid chamber sections  63   i ,  63   j , each having a shape of a sector of a hollow cylinder. 
     The partition rubber plates  71   b ,  71   b  are disposed between the lower surface of the diaphragm  44  and the upper surface of the annular core portion  56   b , and its thicknesses (lengths in the axial direction) extend vertically. In addition, the partition rubber plates  73   b ,  73   b  are disposed between the lower surface of the brim-shaped core portion  50   c  and the upper surface of a main rubber  46 C of which thickness is reduced, and its thicknesses (lengths in the axial direction) of the partition rubber plates  73   b  extend vertically. 
     The partition rubber plates  71   b  in the first fluid chamber  61  and the partition rubber plates  73   b  in the third fluid chamber  63  enable the flows of the magnetorheological fluid H around the axis to be eliminated or reduced in the first fluid chamber  61  and the third fluid chamber  63 . Furthermore, application of the magnetic field enables the flows of the magnetorheological fluid H around the axis to be eliminated or reduced in the second fluid chamber  62 . Thus, transmission of the external force F 2  applied in the shear direction (longitudinal or transverse direction) can be controlled. 
     Application of the magnetic field enables the flow rates of the magnetorheological fluid H from the first fluid chamber  61  to the second fluid chamber  62  and from the third fluid chamber  63  to the second fluid chamber  62  to be controlled, resulting in an efficient control of the stiffness of the mount  18 C in all the directions including the vertical, longitudinal, and transverse directions. 
     In the mount  18 C for the subframe illustrated in  FIGS. 14 and 15A to 15C , the flows of the magnetorheological fluid H in a state where the coil excitation current I is not applied to the exciting coil  52  are indicated by solid line arrows. 
     In this case, the magnetorheological fluid H can move freely from the first fluid chamber sections  61   g ,  61   h  to the second fluid chamber  62  and from the third fluid chamber sections  63   i ,  63   j  to the second fluid chamber  62 . As a result, the magnetorheological fluid H can move in the axial (vertical) direction between the first fluid chamber sections  61   g ,  61   h  and the third fluid chamber sections  63   i ,  63   j  and can move around the axis in the second fluid chamber  62  as illustrated in  FIG. 15B . In this manner, the stiffness of the mount  18 C can be kept low by not generating a magnetic field. 
     Fifth Embodiment 
       FIG. 16  is a longitudinal sectional view illustrating the structure and effects of a mount  18 D for a subframe according to a fifth embodiment. 
     The mount  18 D includes an inner magnetic core  50 D, secured to the inner cylindrical magnetic core  40  and accommodating the exciting coil  52 , and an outer magnetic core  56 D, secured to the outer cylinder  34  and a main rubber  46 D, disposed upside down compared with the inner magnetic core  50  and the outer magnetic core  56  of the mount  18 A illustrated in  FIG. 8  ( FIG. 3 ), the mount  18 B illustrated in  FIG. 12 , and the mount  18 C illustrated in  FIG. 14 . 
     Similarly to the mounts  18 A to  18 C, the stiffness of the mount  18 D having the above-described structure can also be controlled in a wide range in response to the external force F 1  in the axial (vertical) direction and the external force F 2  in the shear direction (longitudinal or transverse direction) applied to the outer cylinder  34  of the mount  18 D according to how the magnetic field (magnetic flux) generated by the exciting coil  52  is distributed. 
     Another Example 
       FIG. 17  is a longitudinal sectional view illustrating the structure and effects of a mount  19  for a subframe according to another example. 
     In the mount  19 , an annular magnetic path plate  21  composed of a magnetic body and provided with a wedge-shaped (when viewed in longitudinal section) annular path around the circumference is disposed at the upper end of the inner cylinder (also referred to as “inner cylindrical magnetic core”)  40  composed of a magnetic body using an outer cylinder (also referred to as “outer cylindrical core”)  35  composed of a magnetic body. 
     An exciting coil  52 E is wound around the outer circumference of the inner cylinder  40  between the diaphragm  44  and an inner annular core portion  50 E composed of a magnetic body and secured to the inner cylinder  40 , and the coil excitation current I applied to the exciting coil  52 E forms a closed magnetic circuit serving as paths of magnetic flux using the inner cylindrical magnetic core  40 , the inner annular core portion  50 E, an outer magnetic core  56 E, the outer cylinder  35 , and the magnetic path plate  21 . This prevents the magnetorheological fluid H between a first fluid chamber  61 E and a third fluid chamber  63 E from flowing in a second fluid chamber  62 E in the axial direction, and thus the stiffness can be controlled in response to the external force F 1  applied in the axial direction. 
     CONCLUSION 
     As described above, the mounts  18  and  18 A to  18 D for the subframe according to the above-described embodiments are disposed on the subframe  16  at positions where the subframe  16  is supported by the vehicle body (main frame)  12 . The mounts  18  and  18 A to  18 D for the subframe have a cylindrical shape and contain the magnetorheological fluid H in a fluid tight manner. 
     The mounts  18  and  18 A to  18 D for the subframe each include the upper fluid chamber (first fluid chamber  61  or  61 D) and the lower fluid chamber (third fluid chamber  63  or  63 D). 
     In addition, the mounts  18  and  18 A to  18 D each include the middle fluid chamber (second fluid chamber  62  or  62 D) including the axial path  62   a  extending in the axial direction and the axis-perpendicular path  62   b  extending in the axis-perpendicular directions between the upper fluid chamber (first fluid chamber  61  or  61 D) and the lower fluid chamber (third fluid chamber  63  or  63 D). 
     One end of the axial path  62   a  communicates with one of the upper and lower fluid chambers, for example, the first fluid chamber  61 . Another end of the axial path  62   a  communicates with one end of the axis-perpendicular path  62   b . Another end of the axis-perpendicular path  62   b  communicates with the other of the upper and lower fluid chambers, for example, the third fluid chamber  63 . 
     Furthermore, magnetic members (for example, the inner cylindrical magnetic core  40 , the inner magnetic core  50 , and the outer magnetic core  56 ) are disposed such that magnetic paths (magnetic flux) passing through the middle fluid chamber, for example, through the axial path  62   a  of the second fluid chamber  62  in the axis-perpendicular directions and through the axis-perpendicular path  62   b  in the axial direction are produced when the coil excitation current I serving as the excitation current is applied to the exciting coil  52  serving as the coil wound about the axis of the mount. 
     In this manner, the flows of the magnetorheological fluid H inside the mounts  18  and  18 A to  18 D are controlled to be stopped in the axial direction and in the axis-perpendicular directions by applying the coil excitation current I to the exciting coil  52 , and thus the elastic properties of the mounts  18  and  18 A to  18 D are adjusted such that the mounts are hardened in the axial direction and in the axis-perpendicular directions. 
     As a result, a variable damping force can be exerted on the external force F 1  in the axial (vertical) direction and the external force F 2  in the axis-perpendicular direction (longitudinal or transverse direction) applied to the mounts  18  and  18 A to  18 D. 
     In addition, the magnetorheological fluid H does not flow between the upper and lower fluid chambers, for example, between the first fluid chamber  61  and the third fluid chamber  63 , without passing through the middle fluid chamber, for example, the second fluid chamber  62 , in which the magnetic paths are formed. Consequently, the elastic properties of the mounts  18  and  18 A to  18 D can be efficiently changed by changing the magnitude of the magnetic field of the magnetic paths in the middle fluid chamber, for example, the second fluid chamber  62 . 
     In this case, as illustrated in  FIG. 3  and the like, the axial path  62   a  and the axis-perpendicular path  62   b  of the second fluid chamber  62 , for example, form a crank-like shape when the mount  18  is viewed in longitudinal section. The magnetic paths in the axis-perpendicular directions are formed radially in the axis-perpendicular directions, and the magnetic paths in the axial direction are formed all around the axis in the entire circumference of the axis. 
     In this manner, the axial path  62   a  and the axis-perpendicular path  62   b  of the second fluid chamber  62  through which the magnetorheological fluid H passes are symmetrical with respect to the axis, and thus the elastic properties are adjusted to be uniform in the radial direction of the second fluid chamber  62 . 
     In the above-described embodiments, for example, the volume of the second fluid chamber  62  is smaller than the volumes of the first fluid chamber  61  and the third fluid chamber  63 , and the formed magnetic paths are compact accordingly. Thus, the elastic properties can be changed while the power efficiency in forming the magnetic paths using the exciting coil  52  is improved. 
     Furthermore, for example, the diaphragm  44  and the main rubber  46  respectively serving as the first and second elastic members having an annular shape are disposed in upper and lower portions of the mount  18  and hold the magnetorheological fluid H inside the mount  18  in a fluid tight manner. The stiffness of one of the diaphragm  44  and the main rubber  46  is lower than the stiffness of the other. In the above-described embodiments, the stiffness of the diaphragm  44  is lower than the stiffness of the main rubber  46 . 
     Since the stiffness of one of the upper and lower elastic members is lower than the other as described above, the diaphragm  44  expands to absorb the fluid pressures in the first fluid chamber  61  to the third fluid chamber  63  increased by forming the diaphragm  44  and thus prevents the internal pressures in the first fluid chamber  61  to the third fluid chamber  63 , that is, the internal pressure in the mount  18  from being increased. This prevents the mount  18  from getting fatigued, leading to a longer life span of the mount  18 . 
     Furthermore, the partition rubber plates  71  and  71   a  respectively illustrated in  FIGS. 9A and 13A  and the partition rubber plates  73  and  73   a  respectively illustrated in  FIGS. 9C and 13C  serving as the plurality of partition members radially extend to respectively partition the first fluid chamber  61  and the third fluid chamber  63  into sectors of a hollow cylinder when viewed in perspective (annular sectors when viewed in section). The partition members may be the partition rubber plates  71   b  and  73   b  each having a shape of a sector of a hollow cylinder as illustrated in  FIGS. 15A and 15C , respectively. 
     The partition rubber plates  71 ,  71   a , and  71   b  and the partition rubber plates  73 ,  73   a , and  73   b  serving as the partition members respectively reduce the ranges of flows of the magnetorheological fluid H in the directions around the axis in the first fluid chamber  61  and the third fluid chamber  63  and direct the flows of the magnetorheological fluid H generated in response to inputs in the axis-perpendicular directions toward the second fluid chamber  62 . This enables the viscosity or the stiffness of the mounts  18 A,  18 B and  18 C for the subframe to be changed. 
     The present invention is not limited to the above-described embodiments and may be applied to various configurations based on the disclosure of this application, for example, suspension bushes connecting suspension links in addition to the mounts  18  ( 18 A to  18 D) for the subframe. Moreover, a vehicle may be provided with a mode switch or the like for choosing to form or not to form magnetic paths to be bifunctional to allow a user to choose occupant comfort or steering stability. Furthermore, various structures can be employed based on the description of the specification as a matter of course. For example, occupant comfort is given a higher priority (lower stiffness, no magnetic paths are formed) during normal driving of a self-driving car or the like, and responsiveness is increased (higher stiffness, magnetic paths are formed) in case of emergency to improve the driving performance.