Abstract:
Provided is a valve structure of a shock absorber which is capable of controlling respective damping forces according to a frequency in compression and rebound motions of a piston valve, thereby satisfying both the ride comfort and the control stability. The valve structure of the shock absorber, which has a cylinder filled with a working fluid and a piston rod having one end located inside the cylinder and the other end extending outward from the cylinder, includes: a main piston valve assembly installed at one end of the piston rod and configured to operate in a state that the inside of the cylinder is divided into an upper chamber and a lower chamber, and generate a damping force varying according to a moving speed; and a frequency unit configured to move together with the main piston valve assembly and generate a damping force varying according to a frequency. The frequency unit includes: a hollow housing mounted at a lower end of the piston rod such that the housing is disposed under the main piston valve assembly; and a free piston disposed to be vertically movable within the housing.

Description:
CROSS-REFERENCE(S) TO RELATED APPLICATION 
     This application is a Continuation of U.S. application Ser. No. 13/554,434, entitled “Valve Structure of Shock Absorber,” filed Jul. 20, 2012, which claims priority of Korean Patent Application No. 10-2011-0072634, filed on Jul. 21, 2011, in the Korean Intellectual Property Office, both of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a valve structure of a shock absorber, and more particularly, to a valve structure of a shock absorber which is capable of controlling respective damping forces at a small amplitude and a large amplitude in compression and rebound motions of a piston valve, thereby satisfying both the ride comfort and the control stability. 
     Description of the Related Art 
     In general, a suspension is installed in a vehicle to dampen a shock or vibration transferred from a road surface to an axle during driving. As one example of such a suspension, a shock absorber has been used. 
     A shock absorber operates according to a vibration of a vehicle caused by a state of a road surface. In this case, a damping force generated in the shock absorber varies according to an operating speed of the shock absorber, that is, a fast or slow operating speed thereof. 
     A vehicle ride comfort and a steering stability may be controlled according to how to adjust a characteristic of a damping force generated in a shock absorber. Therefore, in designing a vehicle, it is very important to adjust a characteristic of a damping force of a shock absorber. 
     A conventional piston valve is designed to have a constant damping characteristic at a high speed, a middle speed, and a low speed due to the use of a single flow passage. Therefore, when intending to improve a ride comfort by reducing a low-speed damping force, middle-speed and high-speed damping forces may also be affected. In addition, a conventional shock absorber has a configuration in which a damping force varies according to a change in a speed of a piston, regardless of a frequency or a stroke. In the case of the damping force varying according to only the change in the speed of the piston, the same damping force is generated even in various states of the road surface. Therefore, it is difficult to satisfy both the ride comfort and the steering stability. 
     Accordingly, there is a need for continuously conducting research and development on a valve structure of a shock absorber which can vary a damping force according to various road conditions, such as a frequency and a stroke, thereby satisfying both the vehicle ride comfort and the steering stability. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is directed to provide a valve structure of a shock absorber, which includes a main piston valve configured to generate a damping force varying according to a moving speed of a piston, and a frequency unit configured to generate a damping force varying according to a frequency, thereby satisfying both the vehicle ride comfort and the control stability. 
     According to another embodiment of the present invention, a valve structure of a shock absorber, which has a cylinder filled with a working fluid and a piston rod having one end located inside the cylinder and the other end extending outward from the cylinder, includes: a main piston valve assembly installed at one end of the piston rod and configured to operate in a state that the inside of the cylinder is divided into an upper chamber and a lower chamber, and generate a damping force varying according to a moving speed; and a frequency unit configured to move together with the main piston valve assembly and generate a damping force varying according to a frequency, wherein the frequency unit includes: a hollow housing mounted at a lower end of the piston rod such that the housing is disposed under the main piston valve assembly; and a free piston disposed to be vertically movable within the housing. 
     The frequency unit may include an auxiliary valve assembly mounted at a lower end of the housing. 
     A flow of a working fluid pressurizing the free piston and a flow of a working fluid passing through the free piston and flowing to an opposite side of the free piston may be formed as a single flow. 
     An inner space of the housing may be partitioned into an upper space and a lower space by the free piston. 
     The upper space may communicate with the upper chamber through a connection passage formed inside the piston rod, and the lower space may communicate with the lower chamber through the auxiliary valve assembly amounted at the lower end of the housing. 
     The free piston may have a through-hole that is opened during a low-frequency compression to allow the working fluid to flow from the lower space to the upper space, and when no external force is applied, the through-hole may maintain a state closed by a valve body. 
     A lip portion made of a rubber may be integrally formed on an outer circumferential surface of the free piston, and the lip portion may closely contact an inner surface of the housing. 
     A stepped portion limiting the movement of the free piston may be formed on an inner surface of the upper space of the housing. A plurality of groove portions may be formed on an inner surface of the lower space of the housing. An intermediate portion having an internal diameter substantially equal to an external diameter of the free piston may be formed between the stepped portion and the groove portion. 
     The frequency unit may include an inner tube installed inside the housing to open or close a flow passage in cooperation with the free piston. 
     The inner tube may include at least one of a convex portion, a concave portion, a hole, or a cut-out portion, such that a passage communicating the upper chamber with the lower chamber within the cylinder is opened or closed according to a vertical movement of the free piston within the housing. 
     The inner tube may include at least one upper concave portion, which is concavely formed on an inner surface of the inner tube, and at least one lower concave portion, which is not connected to the upper concave portion and is formed in a straight line with the upper concave portion, and when no external force is applied, the free piston may be located between the upper concave portion and the lower concave portion. 
     The inner tube may include a ring-shaped concave portion that is concavely formed on the inner surface in a ring shape, and when no external force is applied, the free piston may be located at a position where the ring-shaped concave portion is formed. 
     The free piston may be supported by an upper elastic member and a lower elastic member, such that the free piston moves vertically within the inner space of the housing according to a frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a valve structure of a shock absorber according to the present invention. 
         FIG. 2  is a cross-sectional view showing main parts for describing a fluid flow through a valve structure of a shock absorber at a high frequency according to a first embodiment of the present invention. 
         FIG. 3  is a cross-sectional view showing main parts for describing a fluid flow through a valve structure of a shock absorber at a low frequency according to a first embodiment of the present invention. 
         FIG. 4  is a cross-sectional view showing main parts for describing a fluid flow through a valve structure of a shock absorber at a low frequency according to a second embodiment of the present invention. 
         FIG. 5  is a cross-sectional view showing main parts for describing a fluid flow through a valve structure of a shock absorber at a high frequency according to a second embodiment of the present invention. 
         FIGS. 6A to 6D  are perspective views of inner tubes having various shapes according to the present invention. 
         FIG. 7  is a cross-sectional view showing a valve structure of a shock absorber according to a third embodiment of the present invention. 
         FIG. 8  is a cross-sectional view showing main parts for describing a fluid flow through a valve structure of a shock absorber in a low-frequency compression mode according to a third embodiment of the present invention. 
         FIG. 9  is a cross-sectional view showing main parts for describing a fluid flow through a valve structure of a shock absorber in a low-frequency rebound mode according to a third embodiment of the present invention. 
     
    
    
     
       
         
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
               
                 &lt;Description of Reference Numerals&gt; 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10: cylinder 
                 11: upper chamber 
               
               
                 12: lower chamber 
                 20: piston rod 
               
               
                 21: connection passage 
                 30: main piston valve assembly 
               
               
                 31: main piston body 
                 32: main compression passage 
               
               
                 33: main rebound passage 
                 35: main compression valve unit 
               
               
                 37: main rebound valve unit 
                 39: band 
               
               
                 100, 200, 300: frequency unit 
                 110, 210, 310: housing 
               
               
                 120, 220, 320: free piston 
                 130, 230, 330: inner tube 
               
               
                 131: upper concave portion 
                 132: lower concave portion 
               
             
          
           
               
                 140, 240, 340: auxiliary valve assembly 
               
               
                 141, 241, 341: auxiliary valve body 
               
               
                 142, 242, 342: auxiliary compression passage 
               
               
                 143, 243, 343: auxiliary rebound passage 
               
               
                 145, 245, 345: auxiliary compression valve unit 
               
               
                 147, 247, 347: auxiliary rebound valve unit 
               
             
          
           
               
                 157, 257, 357: upper spring 
                 158, 258, 358: lower spring 
               
             
          
           
               
                 231: ring-shaped concave portion 
               
               
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, valve structures of shock absorbers according to preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     As shown in  FIG. 1 , a shock absorber having a valve structure according to the present invention includes an approximately cylindrical cylinder  10  filled with a working fluid such as oil, and a piston rod  20  having one end located inside the cylinder  10  and the other end extending outward from the cylinder  10 . 
     The valve structure of the shock absorber according to the present invention includes a main piston valve assembly  30  and a frequency unit  100 . The main piston valve assembly  30  is installed in one end of the piston rod  20 . The main piston valve assembly  30  operates in a state that the inside of the cylinder  10  is divided into an upper chamber  11  and a lower chamber  12 , and generates a damping force varying according to a moving speed. The frequency unit  100  moves together with the main piston valve assembly  30 , and generates a damping force varying according to a frequency. 
     The main piston valve assembly  30  and the frequency unit  100  are successively installed at an end of the piston rod  20 . The other end of the piston rod  20  is slidable along and liquid-tightly passes through a rod guide and an oil seal, and extends outward from the cylinder  10 . 
     The main piston valve assembly  30  may include a main piston body  31 , a main compression valve unit  35 , and a main rebound valve unit  37 . The main piston body  31  has at least one main compression passage  32 , through which a working fluid passes during compression of the shock absorber, and at least one main rebound passage  33 , through which a working fluid passes during rebound of the shock absorber. The main compression valve unit  35  is disposed above the main piston body  31  to generate a damping force against a pressure of the working fluid passing through the main compression passage  32 . The main rebound valve unit  37  is disposed under the main piston body  31  to generate a damping force against a pressure of the working fluid passing through the main rebound passage  33 . 
     In addition, a Teflon band  39  may be installed on the outer circumferential surface of the main piston body  31  in order for close contact with the inner circumferential surface of the cylinder  10  and prevention of friction. 
     The frequency unit  100  according to the first embodiment includes a hollow housing  110 , a free piston  120 , and an auxiliary valve assembly  140 . The inside of the housing  110  is empty, and the housing  110  is mounted at a lower end of the piston rod  20  such that it is disposed under the main piston valve assembly  30 . The free piston  120  opens or closes a flow passage while moving within the housing  110 . The auxiliary valve assembly  140  is mounted at a lower end of the housing  110 . 
     The auxiliary valve assembly  140  includes an auxiliary valve body  141 , an auxiliary compression valve unit  145 , and an auxiliary rebound valve unit  147 . The auxiliary valve body  141  has at least one auxiliary compression passage  142 , through which the working fluid passes during compression of the shock absorber, and at least one auxiliary rebound passage  143 , through which the working fluid passes during rebound of the shock absorber. The auxiliary compression valve unit  145  is disposed above the auxiliary valve body  141  to generate a damping force against a pressure of the working fluid passing through the auxiliary compression passage  142 . The auxiliary rebound valve unit  147  is disposed under the auxiliary valve body  141  to generate a damping force against a pressure of the working fluid passing through the auxiliary rebound passage  143 . A fixing member  144 , including a rivet, a bolt, and a nut, is installed in the middle of the auxiliary valve body  141 , such that the auxiliary compression valve unit  145  and the auxiliary rebound valve unit  147  are disposed above and under the auxiliary valve body  141 . 
     The auxiliary valve body  141  of the auxiliary valve assembly  140  is fixed under the main piston valve assembly  30  by the housing  110 . The inner space of the housing  110 , in particular, an upper space  111  above the free piston  120 , may communicate with the upper chamber  11  through a connection passage  21  formed inside the piston rod  20 . The inner space of the housing  110  may be partitioned into the upper space  111  and the lower space  112  by the free piston  120 . 
     The free piston  120  is installed to move vertically within the inner space of the housing  110  according to a frequency (amplitude). The free piston  120  is supported within the inner space of the housing  110  by an upper spring  157  as an upper elastic member and a lower spring  158  as a lower elastic member. The upper elastic member and the lower elastic member may be any one selected from a spring, a disk, and a clip. The upper elastic member and the lower elastic member may be any member that can support the free piston  120  by elasticity. The upper spring  157  and the lower spring  158  as the elastic members may be different in shape or modulus of elasticity, and various modifications may be made in design. In the case in which cone-type coil springs are used as the upper spring  157  and the lower spring  158 , it is advantageous to improving a ride comfort and securing an additional free length. 
     A mount portion may be formed on the top surface of the free piston  120  such that the lower end of the upper spring  157  is mounted thereon. A mount portion may be formed on the bottom surface of the free piston  120  such that the upper end of the lower spring  158  is mounted thereon. The lower end of the lower spring  158  is mounted on the fixing member  144  of the auxiliary valve assembly  140 . As in the case of the main piston valve assembly, a Teflon band  129  may be attached to the outer circumferential surface of the free piston  120 . 
     According to the present invention, an inner tube  130 , in which a convex portion, a concave portion, a hole, or a cut-out portion is formed, may be inserted into the housing  110  such that the passage communicating the upper chamber  11  with the lower chamber  12  within the cylinder  10  is opened or closed according to the vertical movement of the free piston  120  within the housing  110 . 
     According to the first embodiment of the present invention, the inner tube  130  inserted into the housing  110  includes at least one upper concave portion  131 , which is formed concavely on the inner surface of the inner tube  130 , and at least one lower concave portion  132 , which is not connected to the upper concave portion  131  but is formed in a straight line with the upper concave portion  131 . When no external force is applied, the free piston  120  is disposed between the upper concave portion  131 and the lower concave portion  132 . That is, when no external force is applied, the free piston  120  is maintained at a height where the concave portion is not formed, and does not allow the flow of the working fluid between the upper chamber  11  and the lower chamber  12 . To this end, an internal diameter of the inner tube  130  in a region where the concave portion is not formed is substantially equal to an external diameter of the free piston  120 . 
     According to the first embodiment of the present invention, when the passage between the upper space  111  and the lower space  112  is opened, the working fluid pressurizing the free piston  120  flows through this passage. In other words, according to the first embodiment of the present invention, since the working fluid pressurizing the free piston  120  flows through the passage to an opposite side of the free piston  120 , the flow of the working fluid pressurizing the free piston  120  and the flow of the working fluid passing through the free piston and flowing to the opposite side are formed as a single flow, not separate flows. 
     Hereinafter, the operation of the valve structure according to the first embodiment of the present invention will be described with reference to  FIGS. 2 and 3 . 
       FIG. 2  shows a position of the free piston  120  at a high frequency (that is, a small amplitude), and  FIG. 3  shows a position of the free piston  120  at a low frequency (that is, a large amplitude). When the external force, such as the inertia and the pressure of the working fluid, is applied, the free piston  120  may move while compressing the upper spring  157 or the lower spring  158 . That is, when the external force applied to the free piston  120  is strong enough to compress the upper spring  157  or the lower spring  158 , the free piston  120  moves upward or downward. 
       FIG. 2  shows a state in which the external force applied to the free piston  120  is not strong enough to compress the upper spring  157  or the lower spring  158  because the movement amplitude of the piston rod of the shock absorber is small and the frequency thereof is high. In a state that the free piston  120  does not move, the outer surface of the free piston  120  is in contact with the inner surface of the inner tube  130  all over the entire periphery. Therefore, the flow of the working fluid is impossible. In this case, the working fluid of the upper chamber  11  may flow to the connection passage  21  formed inside the piston rod  20  and the upper space  11 , that is, the space above the free piston  120  among the inner spaces of the housing  110 , but a more flow is impossible by the free piston  120 . 
     As such, at the high frequency and the small amplitude, the working fluid can mainly flow through the main piston valve assembly  30 . Therefore, the damping force is mainly obtained by the main piston valve assembly  30 . 
       FIG. 3  shows a state in which the external force applied to the free piston  120  is strong enough to compress the upper spring  157  or the lower spring  158  because the movement amplitude of the piston rod of the shock absorber is large and the frequency thereof is low. In this case, the working fluid of the upper chamber  11  may flow to the lower chamber  12  through the connection passage  21  formed inside the piston rod  20 , the lower concave portion  132  formed on the inner surface of the inner tube  130 , and the auxiliary valve assembly  140 . The working fluid may also flow from the lower chamber  12  to the upper chamber  11 . That is, the working fluid of the lower chamber  12  may flow to the upper chamber  11  through the auxiliary valve assembly  140 , the lower concave portion  132  formed on the inner surface of the inner tube  130 , and the connection passage  21  formed inside the piston rod  20 . 
     Although only the state of the rebound stroke is shown in  FIG. 3 , the free piston  120  moves upward and the working fluid can flow through the upper concave portion  131 , even when the external force applied to the free piston  120  is strong enough to compress the upper spring  157  because the movement amplitude of the piston rod of the shock absorber is large and the frequency thereof is low during the compression stroke. 
     As such, at the low frequency and the large amplitude, the damping force can be obtained by the main piston valve assembly  30  and the auxiliary valve assembly  140 . 
     Hereinafter, a valve structure according to a second embodiment of the present invention will be described with reference to  FIGS. 4 and 5 . Since the valve structure according to the second embodiment is different in the frequency unit from the valve structure according to the first embodiment, a description will focus on the difference therebetween. 
     The frequency unit  200  according to the second embodiment includes a hollow housing  210 , a free piston  220 , and an auxiliary valve assembly  240 . The inside of the housing  210  is empty, and the housing  210  is mounted at a lower end of the piston rod  20  such that it is disposed under the main piston valve assembly  30 . The free piston  220  opens or closes a flow passage while moving within the housing  210 . The auxiliary valve assembly  240  is mounted at a lower end of the housing  210 . 
     The auxiliary valve assembly  240  includes an auxiliary valve body  241 , an auxiliary compression valve unit  245 , and an auxiliary rebound valve unit  247 . The auxiliary valve body  241  has at least one auxiliary compression passage  242 , through which a working fluid passes during compression of the shock absorber, and at least one auxiliary rebound passage  243 , through which a working fluid passes during rebound of the shock absorber. The auxiliary compression valve unit  245  is disposed above the auxiliary valve body  241  to generate a damping force against a pressure of the working fluid passing through the auxiliary compression passage  242 . The auxiliary rebound valve unit  247  is disposed under the auxiliary valve body  241  to generate a damping force against a pressure of the working fluid passing through the auxiliary rebound passage  243 . A fixing member  244 , including a rivet, a bolt, and a nut, is installed in the middle of the auxiliary valve body  241 , such that the auxiliary compression valve unit  245  and the auxiliary rebound valve unit  243  are disposed above and under the auxiliary valve body  241 . 
     The auxiliary valve body  241  of the auxiliary valve assembly  240  is fixed under the main piston valve assembly  30  by the housing  210 . The inner space of the housing  210 , in particular, an upper space  211  above the free piston  220 , may communicate with the upper chamber  11  through a connection passage  21  formed inside the piston rod  20 . The inner space of the housing  210  may be partitioned into the upper space  211  and the lower space  212  by the free piston  220 . 
     The free piston  220  is installed to move vertically within the inner space of the housing  210  according to a frequency (amplitude). The free piston  220  is supported within the inner space of the housing  210  by an upper spring  257  as an upper elastic member and a lower spring  258  as a lower elastic member. The upper elastic member and the lower elastic member may be any one selected from a spring, a disk, and a clip. The upper elastic member and the lower elastic member may be any member that can support the free piston  220  by elasticity. The upper spring  257  and the lower spring  258  as the elastic members may be different in shape or modulus of elasticity, and various modifications may be made in design. In the case in which cone-type coil springs are used as the upper spring  257  and the lower spring  258 , it is advantageous to improving a ride comfort and securing an additional free length. 
     A mount portion may be formed on the top surface of the free piston  220  such that the lower end of the upper spring  257  is mounted thereon. A mount portion may be formed on the bottom surface of the free piston  220  such that the upper end of the lower spring  258  is mounted thereon. The lower end of the lower spring  258  is mounted on the fixing member  244  of the auxiliary valve assembly  240 . As in the case of the main piston valve assembly, a Teflon band  229  may be attached to the outer circumferential surface of the free piston  220 . 
     According to the present invention, an inner tube  230 , in which a convex portion, a concave portion, a hole, or a cut-out portion is formed, may be inserted into the housing  210  such that the passage communicating the upper chamber  11  with the lower chamber  12  within the cylinder  10  is opened or closed according to the vertical movement of the free piston  220  within the housing  210 . 
     According to the second embodiment, the inner tube  230  inserted into the housing  210  has a ring-shaped concave portion  231  that is concavely formed in a ring shape on the inner surface thereof. When no external force is applied, the free piston  220  is disposed at a position where the ring-shaped concave portion  231  is formed. That is, when no external force is applied, the free piston  220  is maintained at a height where the concave portion is formed, and allows the flow of the working fluid between the upper chamber  11  and the lower chamber  12 . On the other hand, when the external force is applied to move the free piston  220  vertically by more than a predetermined distance and thus the free piston  220  gets out of the region where the ring-shaped concave portion  231  is formed, the flow passage of the working fluid between the upper chamber  11  and the lower chamber  12  is closed by the free piston  220 . To this end, an internal diameter of the inner tube  230  in a region where the concave portion is not formed is substantially equal to an external diameter of the free piston  220 . 
     Hereinafter, the operation of the valve structure according to the second embodiment of the present invention will be described with reference to  FIGS. 4 and 5 . 
       FIG. 4  shows a position of the free piston  220  at a low frequency (that is, a large amplitude), and  FIG. 5  shows a position of the free piston  220  at a high frequency (that is, a small amplitude). When the external force, such as the inertia and the pressure of the working fluid, is applied, the free piston  220  may move while compressing the upper spring  257  or the lower spring  258 . That is, when the external force applied to the free piston  220  is strong enough to compress the upper spring  257  or the lower spring  258 , the free piston  220  moves upward or downward. 
       FIG. 4  shows a state in which the external force applied to the free piston  220  is strong enough to compress the upper spring  257  or the lower spring  258  because the movement amplitude of the piston rod of the shock absorber is large and the frequency thereof is low. In a state that the free piston  220  moves, the outer surface of the free piston  220  is in contact with the inner surface of the inner tube  230  all over the entire periphery. Therefore, the flow of the working fluid is impossible. In this case, the working fluid of the upper chamber  11  may flow to the connection passage  21  formed inside the piston rod  20  and the upper space  211 , that is, the space above the free piston  220  among the inner spaces of the housing  210 , but a more flow is impossible by the free piston  220 . 
     Although only the state of the rebound stroke is shown in  FIG. 4 , the free piston  220  moves upward and the flow of the working fluid is impossible, even when the external force applied to the free piston  220  is strong enough to compress the upper spring  257  because the movement amplitude of the piston rod of the shock absorber is large and the frequency thereof is low during the compression stroke. 
     As such, at the low frequency and the large amplitude, the working fluid can mainly flow through the main piston valve assembly  30 . Therefore, the damping force is mainly obtained by the main piston valve assembly  40 . 
       FIG. 5  shows a state in which the external force applied to the free piston  220  is not strong enough to compress the upper spring  257  or the lower spring  258  because the movement amplitude of the piston rod of the shock absorber is small and the frequency thereof is high. In this case, the working fluid of the upper chamber  11  may flow to the lower chamber  12  through the connection passage  21  formed inside the piston rod  20 , the ring-shaped concave portion  232  formed on the inner surface of the inner tube  230 , and the auxiliary valve assembly  240 . The working fluid may also flow from the lower chamber  12  to the upper chamber  11 . That is, the working fluid of the lower chamber  12  may flow to the upper chamber  11  through the auxiliary valve assembly  240 , the ring-shaped concave portion  232  formed on the inner surface of the inner tube  230 , and the connection passage  21  formed inside the piston rod  20 . As such, at the high frequency and the small amplitude, the damping force can be obtained by the main piston valve assembly  30  and the auxiliary valve assembly  240 . 
     Inner tubes having various shapes are shown in  FIGS. 6A to 6D .  FIG. 6A  is a perspective view of the inner tube  130  applied to the first embodiment of the present invention. In the example of  FIG. 6A , upper and lower concave portions are formed by pressurizing a cylindrical tube by a press or the like. An example in which an inner surface is processed in a circumferential direction is shown in  FIG. 6B . If necessary, a plurality of holes may be formed in the cylindrical tube as shown in  FIG. 6C , or the inner tube may be manufactured by forming cut-out portions in upper and lower sides as shown in  FIG. 6D . 
     Hereinafter, a valve structure according to a third embodiment of the present invention will be described with reference to  FIGS. 7 and 9 . Since the valve structure according to the third embodiment is different in the frequency unit from the valve structure according to the first embodiment, a description will focus on the difference therebetween. 
     The frequency unit  300  according to the third embodiment includes a hollow housing  310 , a free piston  320 , and an auxiliary valve assembly  340 . The inside of the housing  310  is empty, and the housing  310  is mounted at a lower end of the piston rod  20  such that it is disposed under the main piston valve assembly  30   a . The free piston  320  opens or closes a flow passage while moving within the housing  310 . The auxiliary valve assembly  340  is mounted at a lower end of the housing  310 . 
     Although the main piston valve assembly  30   a  of  FIG. 7  is shown as having a different configuration from the main piston valve assembly  30  of  FIG. 1 , the configurations of the main piston valve assemblies  30  and  30   a  are merely exemplary and the present invention is not limited by the configurations of the main piston valve assemblies. 
     The auxiliary valve assembly  340  includes an auxiliary valve body  341 , an auxiliary compression valve unit  345 , and an auxiliary rebound valve unit  347 . The auxiliary valve body  341  has at least one auxiliary compression passage  342 , through which a working fluid passes during compression of the shock absorber, and at least one auxiliary rebound passage  343 , through which a working fluid passes during rebound of the shock absorber. The auxiliary compression valve unit  345  is disposed above the auxiliary valve body  341  to generate a damping force against a pressure of the working fluid passing through the auxiliary compression passage  342 . The auxiliary rebound valve unit  347  is disposed under the auxiliary valve body  341  to generate a damping force against a pressure of the working fluid passing through the auxiliary rebound passage  343 . A fixing member  344 , including a rivet, a bolt, and a nut, is installed in the middle of the auxiliary valve body  341 , such that the auxiliary compression valve unit  345  and the auxiliary rebound valve unit  347  are disposed above and under the auxiliary valve body  341 . 
     Although the auxiliary valve assembly  340  of  FIG. 7  is shown as having a different configuration from the auxiliary valve assembly  140  of  FIG. 1 , the configurations of the auxiliary valve assemblies  140  and  340  are merely exemplary. 
     The auxiliary valve body  341  of the auxiliary valve assembly  340  is fixed under the main piston valve assembly  30   a  by the housing  310 . The inner space of the housing  310 , in particular, an upper space  311  above the free piston  320 , may communicate with the upper chamber  11  through a connection passage  21  formed inside the piston rod  20 . A lower space  312  under the free piston  320  may communicate with the lower chamber  12  through the auxiliary valve assembly  340 . The inner space of the housing  310  may be partitioned into the upper space  311  and the lower space  312  by the free piston  320 . 
     The free piston  320  is installed to move vertically within the inner space of the housing  310  according to a frequency (amplitude). The free piston  320  is supported within the inner space of the housing  310  by an upper spring  357  as an upper elastic member and a lower spring  358  as a lower elastic member. The upper elastic member and the lower elastic member may be any one selected from a spring, a disk, and a clip. The upper elastic member and the lower elastic member may be any member that can support the free piston  320  by elasticity. The upper spring  357  and the lower spring  358  as the elastic members may be different in shape or modulus of elasticity, and various modifications may be made in design. In the case in which cone-type coil springs are used as the upper spring  357  and the lower spring  358 , it is advantageous to improving a ride comfort and securing an additional free length. 
     The free piston  320  has a through-hole  325  that is opened during a low-frequency compression to allow the working fluid to flow from the lower space  312  to the upper space  311 . If no external force is applied, the through-hole  325  maintains a state closed by a valve body  326 . The valve body  326  is stacked on the upper surface of the free piston  320 . The lower end of the upper spring  357  is mounted on the valve body  326 . Accordingly, the valve body  326  is pressurized toward the free piston  320 . A mount portion may be formed on the bottom surface of the free piston  320  such that the upper end of the lower spring  358  is mounted thereon. The lower end of the lower spring  358  is mounted on the fixing member  344  of the auxiliary valve assembly  340 . 
     As in the case of the first and second embodiments, a Teflon band may be attached to the outer circumferential surface of the free piston  320 . On the other hand, in the third embodiment, a lip portion  329  made of a rubber may be integrally formed. The lip portion  329  may closely contact the inner surface of the housing  310  and perform a sealing function. 
     According to the third embodiment, instead of inserting the separate inner tube into the housing  310 , a stepped portion  313  and a plurality of groove portions  314  may be directly formed on the inner surface of the housing  310  when needed. Accordingly, as the free piston  320  moves vertically within the housing  310 , the passage communicating the upper chamber  11  with the lower chamber  12  within the cylinder  10  may be opened or closed. 
     According to the third embodiment, the stepped portion  313  limiting the movement of the free piston  320  is formed on the inner surface of the upper space  311  of the housing  310 . The plurality of groove portions  314  are formed on the inner surface of the lower space  312  of the housing  310 . An intermediate portion  315  is formed between the stepped portion  313  and the groove portion  314  on the inner surface of the housing  310 . The intermediate portion  315  has an internal diameter substantially equal to an external diameter of the free piston  320 , more specifically, an external diameter of the lip portion  329  integrally formed at a circumferential edge of the free piston  320 . When no external force is applied, the free piston  320  is disposed at the intermediate portion  315  of the housing  310 . 
     When no external force is applied, the free piston  320  is disposed at the intermediate portion  315 . Accordingly, the free piston  320  does not allow the flow of the working fluid between the upper chamber  11  and the lower chamber  12 . On the other hand, when the external force is applied to move the free piston  320  downward by more than a predetermined distance and thus the free piston  320  gets out of the intermediate portion  315 , the working fluid may flow through the groove portions  314 . In addition, when the external force is applied to move upward the valve body  326  stacked on the upper surface of the free piston  320  while compressing the upper spring  357 , the through-hole  325  is opened to allow the flow of the working fluid. 
     According to the third embodiment of the present invention, when the passage between the upper space  311  and the lower space  312  is opened, the working fluid pressurizing the free piston  320  flows through this passage. 
     In other words, according to the third embodiment of the present invention, since the working fluid pressurizing the free piston  320  flows through the passage to an opposite side of the free piston  320 , the flow of the working fluid pressurizing the free piston  320  and the flow of the working fluid passing through the free piston  320  and flowing to the opposite side of the free piston  320  are formed as a single flow, not separate flows. 
     Hereinafter, the operation of the valve structure according to the third embodiment of the present invention will be described with reference to  FIGS. 7 and 9 . 
       FIG. 7  shows a position of the free piston  320  in an initial state in which no external force is applied.  FIG. 8  shows a position of the free piston  320  during a low-frequency (that is, large-amplitude) compression, and  FIG. 9  shows a position of the free piston  320  during a low-frequency (that is, large-amplitude) rebound. When the external force, such as the inertia and the pressure of the working fluid, is applied, the free piston  320  may move while compressing the upper spring  357  or the lower spring  358 . That is, when the external force applied to the free piston  320  is strong enough to compress the upper spring  357  or the lower spring  358 , the free piston  320  moves upward or downward. 
       FIG. 7  shows a state in which the external force applied to the free piston  320  is not strong enough to compress the upper spring  357  or the lower spring  358  because the movement amplitude of the piston rod of the shock absorber is small and the frequency thereof is high. In a state that the free piston  320  is located at the intermediate portion  315 , the outer surface of the free piston  320  is in contact with the intermediate portion  315  of the inner surface of the housing. Therefore, the flow of the working fluid between the upper space  311  and the lower space  312  is impossible. 
       FIG. 8  shows a state in which the external force applied to the free piston  320  is strong enough to compress the upper spring  357  because the downward-movement amplitude of the piston rod of the shock absorber is large and the frequency thereof is low. When the free piston  320  moving while compressing the upper spring  257  comes into contact with the stepped portion  313 , further movement of the free piston  320  is limited. In this case, when the external force is continuously applied, the valve body  326  closing the through-hole  325  moves while further compressing the upper spring  357 . Accordingly, the through-hole  325  is opened, and the working fluid may flow from the lower space  312  to the upper space  311 . 
       FIG. 9  shows a state in which the external force applied to the free piston  320  is strong enough to compress the upper spring  358  because the upward-movement amplitude of the piston rod of the shock absorber is large and the frequency thereof is low. When the free piston  320  moving while compressing the lower spring  358  moves down to a region where the groove portion  314  is formed, the passage allowing the flow of the working fluid is opened and thus the working fluid can flow to the lower space  312 . As described above, the present invention can provide a valve structure of a shock absorber, which includes a main piston valve configured to generate a damping force varying according to a moving speed of a piston, and a frequency unit configured to generate a damping force varying according to a frequency. 
     Therefore, the valve structure of the shock absorber according to the present invention can satisfy both the vehicle ride comfort and the steering stability. 
     While the valve structure of the shock absorber according to the present invention has been described with reference to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.