Patent Publication Number: US-2017350466-A1

Title: Shock absorber

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2016 209 824.1, filed Jun. 3, 2016, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
     FIELD OF THE INVENTION 
     The invention concerns a shock absorber. 
     BACKGROUND OF THE INVENTION 
     A shock absorber is known from DE 10 2010 029 180 A1. 
     SUMMARY OF THE INVENTION 
     The problem which the present invention proposes to solve is to improve a shock absorber so that its damping action can be adjusted in an uncomplicated, especially an automated, and robust manner. 
     This problem is solved according to the invention by a shock absorber comprising a housing having a work space, damping fluid present in the work space, a piston unit arranged in the work space with a piston rod having a longitudinal axis, a piston secured to the piston rod, dividing the work space into a first partial work space and a second partial work space and a flow channel connecting the first partial work space and the second partial work space, and an adjustment unit for adjusting the damping force of the shock absorber with an adjustment element for adjusting the effective flow cross section area of the flow channel and an adjustment actuator for the automated adjusting of an arrangement of the adjustment element and the piston rod, wherein the adjustment unit is arranged inside the shock absorber. 
     According to the invention, it has been discovered that an adjustment unit for adjusting a damping force of a shock absorber is arranged inside the shock absorber. The adjustment unit in particular is arranged in a housing of the shock absorber. Such a shock absorber has an especially compact form. The shock absorber, especially the adjustment unit, has an enhanced functional integration. The shock absorber according to the invention can be used, for example, to move a vehicle seat, especially a seat in a lorry or in a passenger car. A separate electromechanical coupling of the shock absorber to an externally arranged adjustment unit is unnecessary. This results in a reduced footprint. Force transmission elements situated outside the shock absorber between the adjustment unit and the shock absorber are unnecessary. The housing comprises a work space in which damping fluid is present. In the work space there is arranged a piston unit with a piston rod having a longitudinal axis, and a piston secured to the piston rod, dividing the work space into a first partial work space and a second partial work space. The adjustment unit can also be arranged inside the piston rod. A displacement of the piston rod along the longitudinal axis relative to the housing can result in the adjustment unit situated inside the piston rod being arranged outside the housing at least for a portion and/or at least temporarily. Such an arrangement of the adjustment unit is to be understood as being inside the shock absorber in the sense of the invention. 
     The first partial work space and the second partial work space are interconnected by a flow channel. The adjustment unit comprises an adjustment element, with which an effective flow cross section area of the flow channel can be adjusted. For an automated adjustment, an adjustment actuator is used that adjusts an arrangement of the adjustment element and the piston rod. The shock absorber according to the invention has the advantage, in particular compared to a shock absorber governed by a magnetic valve, of a simplified, uncomplicated design. The shock absorber according to the invention is compact and space-saving and resistant to failure. The shock absorber according to the invention enables an adjustment within a time interval of less than 1 second, especially less than 500 ms, particularly less than 300 ms. Another major advantage of the shock absorber according to the invention is that a permanent energization, i.e., a permanent electrical power supply, is not necessary. 
     A control, which stands in signal communication with the adjustment unit for the specific adjusting of the effective flow cross section area of the flow channel, simplifies the automated adjusting of the damping force. In particular, it is possible by means of a specific control signal which is transmitted from the control unit to the adjustment unit to accomplish a specific adjusting of the adjustment element in relation to the piston rod. 
     An adjustability, in which the adjustment element and the piston rod are rotatable relative to each other in regard to the longitudinal axis, enables an advantageous direct adjusting of the flow cross section area of the flow channel. 
     An embodiment of the shock absorber, in which the adjustment element is rotatable in regard to the longitudinal axis, enables an uncomplicated design. In particular, the adjustment element is arranged rotatable in the piston rod. The adjustment element in particular can be driven in rotation with regard to the piston rod and especially in regard to the housing. 
     The embodiment of the adjustment element, in which the adjustment element is designed as a shaft shoulder, which has a step, in particular multiple steps, along the longitudinal axis, wherein at least one step of the shaft shoulder has a non-round cross section area oriented perpendicular to the longitudinal axis, enables a specific, in particular a stepwise covering of a plurality of through-openings which form the flow channel. Stepwise means the shaft shoulder has different lengths in a direction parallel to the longitudinal axis, so that flow openings which are formed in a lateral wall of the piston rod are covered or opened up, depending on the relative rotary position of the adjustment element with respect to the piston rod. 
     The embodiment of the shock absorber, in which the adjustment element is axially movable with regard to the longitudinal axis, enables an uncomplicated design of the adjustment element. The damping action is adjusted by means of an axial displacement of the adjustment element. By means of a kinematic device, in particular a rotary drive movement of the adjustment actuator is converted into an axial displacement of the adjustment element. The adjustment element in particular can move relative to the piston rod, especially under driving action. 
     Advantageous embodiments of the adjustment element, in which the adjustment element is designed as a spindle nut or in which the adjustment unit is designed as a needle valve, enable an uncomplicated implementing of the axial adjustability of the adjustment element. 
     The integrated arrangement, in which the adjustment unit is arranged integrated in the piston rod, is especially space-saving. In particular, it has been discovered that the piston rod can be effectively utilized in that a volume situated therein can be utilized as structural space for the adjustment unit. In particular, it is not necessary to create additional structural space inside or outside the shock absorber in order to accommodate the adjustment unit. In this embodiment, a design of a shock absorber with enhanced functionality and the same structural size is possible. 
     A shock absorber, in which the piston rod is tubular in design, enables an effective integration of the adjustment unit. Tubular means that the piston rod is hollow. It is advantageous for the outer contour of the piston rod to be circular. It is advantageous for the inner contour of the piston rod to be circular. In particular, the inner contour can also have a different shape, which in particular is non-round with respect to the longitudinal axis. 
     A position recognition unit, which stands in signal communication in particular with the adjustment unit, especially with the adjustment actuator, simplifies the adjustment of the damping action, especially given a separation of the shock absorber from the power supply. The position recognition unit stands in signal communication in particular with the adjustment unit. The position recognition unit stands in signal communication in particular with the control unit. 
     A reference element for defining a reference position of the adjustment element simplifies the immediate position recognition, especially by means of an encoder. The reference element for example may be a reference mark or a multiturn encoder. In particular, the reference element is designed as a mechanical reference, especially as an end stop element. When the adjustment element comes up against the reference element, this results in a significant rise in the motor current of the adjustment actuator. The control unit can detect this significant rise in the motor current as the reaching of the reference element. 
     A shock absorber, in which the adjustment actuator is designed as a rotary drive, especially an electric motor, enables an uncomplicated providing of an adjustment actuator. 
     A sealing element for sealing off a cavity of the piston rod ensures that damping fluid does not escape unintentionally from the work space of the shock absorber across the piston rod. The sealing element is designed, for example, as an O-ring and it seals the adjustment unit radially at an inner cylinder surface of the piston rod. The position of the sealing element along the longitudinal axis can be established in various ways. For example, it is conceivable to seal the adjustment element in the piston rod. The adjustment element in particular is arranged facing the work space of the shock absorber. It is also possible to place the sealing element on a top side of the adjustment actuator facing away from the work space. In such an embodiment, in particular the adjustment element and the adjustment actuator are surrounded by the damping fluid, especially damping oil. 
     Further advantageous embodiments, additional features and details of the invention will emerge from the following description of exemplary embodiments with the aid of the drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a perspective representation of a longitudinal section through a shock absorber according to a first exemplary embodiment, 
         FIG. 2  shows an enlarged partial view of a first arrangement of an adjustment unit per  FIG. 1 , 
         FIG. 3  shows a view corresponding to  FIG. 2  in a second arrangement of the adjustment unit, 
         FIG. 4  shows a longitudinal section through a piston rod with integrated adjustment unit of a shock absorber according to a second exemplary embodiment in a first arrangement, 
         FIG. 5  shows a representation corresponding to  FIG. 4  with the adjustment unit in a second arrangement, 
         FIG. 6  shows a schematic representation of an adjustment kinematics according to a third exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A shock absorber  1  shown in  FIGS. 1 to 3  is a fluid shock absorber. The design and function of such a fluid shock absorber are known from DE 10 2010 029 180 A1, to which reference is made here. 
     The shock absorber  1  comprises a housing  4  with a first housing end  2  and a second housing end  16 . The housing  4  is closed at the first housing end  2  by a guiding and sealing unit  3 . At the second housing end  16  the housing  4  comprises a second fastening element  15 . The housing  4  encloses a work space  5  and an equalizing space  6 . The housing  4  has a longitudinal axis  7 . In particular, it is configured rotationally symmetrical to the longitudinal axis  7  at least for a portion. The housing  4  may be of double-wall design. In particular, it comprises an inner housing  8  and an outer housing  9 . The outer housing  9  surrounds the inner housing  8 . The outer housing  9  in particular can be arranged concentrically to the inner housing  8 . Thus, the equalizing space  6  is formed as an annular cylindrical cavity. 
     In an alternative variant, not represented in the figures, the outer housing  9  may also be arranged offset from the inner housing  8 , so that the equalizing space  6  has a variable, that is, a nonconstant width along its circumference. In this case, the equalizing space  6  can be designed topologically contractibly, in particular. 
     The work space  5  is filled with a damping fluid  10 . The damping fluid  10  is in particular a hydraulic oil. The equalizing space  6  is partly filled with the damping fluid  10 . The rest of the equalizing space  6  is filled with gas, especially air. 
     The guiding and sealing unit  3  comprises a first seal element  41 , which lies tightly against the piston rod  12 . For its securing on the piston rod  12 , the first seal element  41  has an annular groove  42 , in which a clamping ring  43  is arranged. Furthermore, the guiding and sealing unit  3  comprises a supporting element  44  braced against the outer housing  9 . The supporting element  44  is tightly mounted by means of a sealing ring  45  against the outer housing  9 . It has a central blind hole  46 . The guiding and sealing unit  3  has a central bore  47 . The bore  47  in particular is arranged concentrically to the longitudinal axis  7 . The piston rod  12  is led through the bore  47 . 
     Furthermore, the shock absorber  1  comprises a piston unit  11  with a piston rod  12  and a piston  13 . The piston  13  is secured to the piston rod  12  and led movably in the inner housing  8  along the longitudinal axis  7 . The piston rod  12  is led out from the housing  4 , sealed by the guiding and sealing unit  3 . At its end opposite the piston  13 , the piston rod  12  is connected to a first fastening element  14 . 
     The piston  13  divides the work space  5  into a first partial work space  17  facing the first housing end  2  with a first work space end  18  and a second partial work space  19  facing the second housing end  16  with a second work space end  20 . 
     At the first work space end  18  there is arranged a first closing element  21 . The first closing element  21  is arranged in the inner housing  8 . The first closing element  21  can be inserted in particular into the inner housing  8 , preferably by press fit or by screwing. It is sealed off against the inner housing  8  by means of a sealing ring  22 . The first closing element  21  is fashioned as a single piece with the supporting element  44 . Thus, it is likewise a component of the guiding and sealing unit  3 . In principle, however, it is also conceivable to design the first closing element  21  and the supporting element  44  as separate parts. For further details regarding embodiments of the first closing element  21 , refer to DE 10 2005 023 756 A1. 
     The first closing element comprises a first equalizing channel, which forms a flow connection between the first partial work space  17  and the equalizing space  6 . 
     At the second work space end  20  there is arranged a second closing element  24 . The second closing element  24  is arranged in the inner housing  8 . The second closing element  24  can in particular be inserted into the inner housing  8 , preferably by press fit. Furthermore, the inner housing  8  can lie against the outer housing  9  in regions at its circumference in the region of the second work space end  20 . 
     The second closing element  24  comprises a second equalizing channel, which forms a flow connection between the second partial work space  19  and the equalizing space  6 . 
     The second closing element  24  is axially braced by a stop shoulder  114  at the end face against the inner housing  8  and radially braced by an insertion collar  115  against the inner cylinder envelope surface of the inner housing  8 . The insertion collar  115  is radially pretensioned by means of a radial spring element  116  in regard to the longitudinal axis  7 , the pretensioning being applied axially by means of a clamping nut  117  on a clamping bolt  118 . The inner housing  8  is pressed radially outward against an inner side of the outer housing  9  and thereby held in the region where the insertion collar  115  lies against the inner cylinder envelope surface of the inner housing  8 . 
     For a proper functioning of the shock absorber  1 , the work space  5  should always be entirely filled with damping fluid  10 . This can be achieved by a suitable design and arrangement of the second equalizing channel as well as a quantity of damping fluid  10  which is adapted to the volume of the work space  5  and the configuration of the equalizing space  6 . In particular, the shock absorber  1  has a preferred installation position, such that the extending direction  40  is directed opposite the force of gravity. The proper functioning of the shock absorber  1  can then be ensured up to an angle of rotation of at least 77° from the preferred installation position. 
     In the second equalizing channel there is provided an equalizing valve  31 . Regarding the design of the equalizing valve  31 , reference is made to DE 10 2010 029 180 A1 and corresponding US 2011/0284333 A1, the entire contents of which are hereby incorporated by reference. 
     The equalizing valve  31  is designed as a self-acting valve. It can be designed as a one-way valve. In particular, it is designed so as to allow a flow from the equalizing space  6  through the second equalizing channel into the second partial work space  19 . In other words, the equalizing valve  31  is designed so that it opens upon movement of the piston  13  in an extending direction  40  parallel to the direction of the longitudinal axis  7 . 
     In the exemplary embodiment shown in  FIGS. 1 to 3 , the equalizing valve  31  is designed so as to allow a bidirectional flow through the second equalizing channel. It is thus designed as a two-way valve. In particular, the equalizing valve  31  can have characteristics which allow a bidirectional flow between the second partial work space  19  and the equalizing space  6  regardless of the position of the valve disc. 
     In general, it is provided that the equalizing valve  31  forms an overload protection element, which ensures an open state of the second equalizing channel upon exceeding a predetermined limit force in the direction of the longitudinal axis  7  on the piston rod  12 . The activation characteristic of this overload protection can be achieved in a simple manner by suitable selection and dimensioning of a valve helical spring and a valve plate spring. 
     An alternative design of the equalizing valve  31  is conceivable. For further details about the equalizing valve  31 , refer to DE 10 2005 023 756 A1, especially paragraph [0022]. 
     The piston rod  12  is multipart, in particular two-part. It comprises an outer, tubular shaped piston rod sleeve  48 . 
     The piston rod sleeve  48  can be connected to the first fastening element  14 . The first fastening element  14  has a snug fit  66  arranged concentrically to the longitudinal axis  7 , by which the first fastening element is shoved onto an outer side of the piston rod  12 , especially the piston rod sleeve  48 . The first fastening element  14  is connected to the piston rod  12  by a weld  67 . 
     The first fastening element  14  can alternatively have an internal thread, by means of which the first fastening element  14  is screwed onto a corresponding external thread on the piston rod sleeve  48 . 
     In one end region  69 , an adjustment unit  101  has an adjustment element  102  with a recess  70 . The recess  70  is fashioned as a circle segment in the direction perpendicular to the longitudinal axis  7 . It has a centre angle b. The centre angle is at least 15°, especially at least 30°, especially at least 45°, especially at least 60°, especially at least 90°. It can also be 120° in particular. The upper limit for the centre angle is at most 270°, especially at most 180°. In principle, a circle sector shape of the recess  70  is also possible. 
     The recess  70  is part of a flow channel  71 , which forms a flow connection between the partial work spaces  17 ,  19 . In addition to the recess  70 , the flow channel  71  comprises several bores  72  in the piston rod sleeve  48 . In other words, the bores  72  together with the recess  70  form the flow channel  71 . The flow channel  71  is thus arranged in the piston rod  12 . 
     At least one bore  72  is provided in the piston rod sleeve  48 . In the exemplary embodiment shown in  FIGS. 1 to 3 , the piston rod sleeve  48  has three bores  72 . It may also have four, five, or more bores  72 . The bores  72  are offset to one another in the circumferential direction. The bores  72  all have the same size. However, bores  72  of different size are also conceivable. 
     Alternatively to several discrete bores  72 , the piston rod sleeve  48  can also have an elongated flow opening. The flow opening extends preferably in the circumferential direction. It covers an angle range which is at most as large as the centre angle of the recess  70 . 
     The bores  72  are selectively closable by means of the end region  69 . Thus, the end region  69  forms the adjustment element  102  by means of which the effective flow cross section of the flow channel  71  can be adjusted. The adjustment element for the adjusting of the effective flow cross section of the flow channel  71  is thus arranged inside the piston rod  12 , especially inside the piston rod sleeve  48 . 
     By means of the adjustment element  102 , the flow channel  71  can be closed in particular to interrupt the flow connection between the partial work spaces  17 ,  19 . In this way, the shock absorber  1  can be blocked. 
     Several discrete bores  72  enable several different discrete damping settings of the shock absorber  1 . The shock absorber  1  can thus have a stepwise damping characteristic. By an advantageous arrangement of the bores  72 , it is possible to achieve a continuously adjustable damping behaviour of the shock absorber  1 , for example by arranging the bores  72  to overlap at least partly along an activation direction of the adjustment element  102 . The adjusting direction of the adjustment element  102  is oriented axially and/or tangentially to the longitudinal axis  7 . Likewise, an elongated opening in the piston rod sleeve  48  enables a continuously adjustable damping behaviour of the shock absorber  1 . 
     The adjustment element  102  is movable, especially rotatable, with respect to the piston rod sleeve  48 . 
     The piston rod sleeve  48  has a reduced outer diameter at a first piston rod end  50  arranged in the inner housing  8 , forming a piston rod end stop  51 . On the piston rod sleeve  48  in the region of the first piston rod end  50  there are arranged, starting from the piston rod end stop  51 , a first spacer washer  52 , a first closure element  53 , especially in the form of a plate spring, a piston washer  54 , a second closure element  55 , especially in the form of a plate spring, a second spacer washer  56  and a securing nut  57 . The securing nut  57  is screwed onto a piston rod thread and secures the piston  13  on the piston rod  12 . The piston  13  is formed by the first closure element  53 , the piston washer  54 , the second closure element  55  and a piston seal  58 . The piston seal  58  is formed as a ring and arranged in an annular groove  59  in the piston washer  54 . The piston seal  58  thus seals the piston washer  54  off against the inner housing  8 . 
     Several flow channels  60  are provided in the piston washer  54 . The flow channels  60  form a flow connection between the first partial work space  17  and the second partial work space  19 . The closure elements  53 ,  55  each interact with at least one of the flow channels  60 . They may also interact with several of the flow channels  60 . In particular, they may act as a valve element and influence the efficient flow cross section of the flow channels  60  in dependence on a movement direction and/or movement velocity of the piston  13  in regard to the extending direction  40 . In particular, they may be designed such that only a unidirectional flow through the flow channels  60  is possible. In this case, the closure elements  53 ,  55  form a one-way valve. The closure elements  53 ,  55  can also be designed in particular so as to open when a certain limit force is exceeded. In this case, they form an overload protection. 
     An alternative design of the piston  13  is conceivable. In this regard, as well as for further details regarding the flow channel  60  and the closure elements  53 ,  55 , refer to the specification of DE 10 2005 023 756 A1, especially paragraph [0023] et seq. In particular, it is also possible to design the piston  13  tight, that is, without flow channels  60 . In this case, the partial work spaces  17 ,  19  are separated in fluid-tight manner by the piston  13 . The flow channel  71  in the piston rod  12  in this case forms the sole direct flow connection between the partial work spaces  17 ,  19 . 
     The adjustment unit  101  serves to adjust the damping force of the shock absorber  1 . The adjustment unit  101  comprises the adjustment element  102 . The adjustment element  102  is designed as a stepped shaft shoulder. The adjustment element  102  for example can be designed with a non-round end region, as is known per DE 10 2010 029 180 A1. The adjustment element  102  is sealed by means of an O-ring as sealing element  103  against an inner cylinder envelope surface of the hollow piston rod  12 . According to one exemplary embodiment not shown, the sealing element can also be arranged along the longitudinal axis  7  between the motor  105  and the position recognition unit  107 . In this case, the motor  105 , the transmission  104  and the adjustment element  102  would be exposed to the damping fluid, especially the damping oil. On the other hand, it would be sufficient to ensure the seal with respect to the position recognition unit  107  and to ensure the wiring which connects the adjustment unit  101  to a control unit. 
     The adjustment element  102  is coupled across a transmission  104  to an electric motor  105 . The electric motor  105  provides a rotary movement, which is transmitted across the transmission  104  to the adjustment element  102 . The adjustment element  102  is arranged in the cavity  106  of the piston rod  12 , able to rotate with respect to the longitudinal axis  7 . 
     The axial displacement of the adjustment element  102  is realized for example by means of an electric drive, especially by means of an electric motor. In addition or alternatively, hydraulic and/or pneumatic drives can be used to provide the axial drive movement. The implementing of the rotary drive movement can be implemented for example by means of a spindle drive. Alternatively, linear adjustment actuators can be used, such as a linear motor, an electromagnet, a pneumatic cylinder and/or a hydraulic cylinder. 
     At one end of the motor  105  facing away from the work space  5  there is arranged a position recognition unit  107  in the form of an encoder. The position recognition unit  107  serves to recognize the incremental relative position of the adjustment unit  101 , especially the adjustment element  102 . 
     In particular, a reference element  108  is provided. According to the exemplary embodiment shown, the reference element  108  is designed as a radial pin protruding inwardly into the cavity  106  and arranged on an inner surface of the piston rod  12 . The reference element  108  is a mechanical reference end stop element for the adjustment element  102 . 
     Instead of the radial pin  108 , other reference elements can be used. For example, a reference mark can be provided, which is optically recognized, for example. The reference mark can also be magnetically designed, corresponding to the adjustment element. It is important that the angle of rotation by which the adjustment element  102  can turn about the longitudinal axis  7  is less than 360° thanks to the reference element. This enables a clear-cut coordinating of the absolute position of the adjustment element  102  through the position recognition unit  107 . 
     The tubular piston rod  12  has several flow openings  72  spaced apart from each other. The flow openings  72  completely pierce the cylinder envelope wall of the piston rod  12 . The flow openings  72  form a fluid connection of the cavity  106  to the upper, first partial work space  17 . In the arrangement of the adjustment element  102  shown in  FIG. 2 , the flow openings  72  are clear. In this arrangement, a flow connection is provided from the first partial work space  17  through the cavity  106  in the piston rod  12  into the second partial work space  19 . 
     By a rotating of the adjustment element  102  relative to the piston rod  12 , the flow openings  72  are covered in succession. For this, it is advantageous for the flow openings  72  to be arranged at a distance from each other along an outer circumferential direction about the longitudinal axis  7 . This means that the individual flow openings  72  are spaced apart from each other in terms of a rotary angle position about the longitudinal axis  7 . 
     Upon activation of the adjustment actuator  105 , the rotary motion is transmitted across the transmission  104  to the adjustment element  102 . If a rotary motion is sufficient, the adjustment element  102  as shown in  FIG. 3  will come to lie in an arrangement in which the flow openings  72  are closed. In this arrangement, there is no fluid connection between the first partial work space  17  and the second partial work space  19  via the cavity  106  of the piston rod  12 . The fluid connection is interrupted in this arrangement. 
     Depending on the rotary position of the adjustment element  102  with respect to the piston rod  12 , more or fewer flow openings  72  or more of less of an overall flow area of the flow channel are opened up. Depending on the size of the effective flow cross section area of the flow channel, the damping action or the damping force of the shock absorber  1  changes. The smaller the effective flow cross section area, the greater the damping force of the shock absorber  1 . 
     In addition or alternatively, a potentiometer can be provided between the transmission  104  and the adjustment element  102  in order to detect directly the absolute position of the adjustment element  102 . 
     The reference travel enables the restoring of an absolute position after a power supply interruption or after an unforeseen loss of position in the control unit. For this, the adjustment element  102  is moved to a reference position, which is defined by the reference element  108 . 
     In the following, the function of the shock absorber  1  shall be described. In the adjusting position of the adjustment element represented in  FIGS. 1 and 2 , the flow channel  71  in the piston rod  12  is opened to the utmost. Upon movement of the piston  13  against the extending direction  40 , the damping fluid  10  can thus flow from the second partial work space  19  through the flow channel  71  in the piston rod  12  into the second partial work space  17 . Furthermore, the damping fluid  10  displaced by the additional volume of the piston rod  12  from the work space  5  can flow through the first equalizing channel into the equalizing space  6 . 
     It is provided that the second equalizing channel is closed as much as possible during small forces directed against the extending direction  40  on the piston rod  12 , especially during small velocities of the piston  13  against the extending direction  40 . In the case of an equalizing valve  31  which enables a bidirectional flow through the second equalizing channel, the equalizing valve  31  is not fully closed. On account of the characteristics, a bidirectional flow through the second equalizing channel is always possible. In theory, however, it is also possible to design the equalizing valve  31  as a one-way valve, which is in a closed position during small forces on the piston rod  12  directed against the extending direction  40 . The response behaviour of the equalizing valve  31  is determined by a suitable choice and setup of the valve helical spring and the valve plate spring. 
     Accordingly, the flow channel  60  in the piston  13  can be closed by the first and/or the second closure element  53 ,  55  during low velocities of the piston  13 . 
     Upon movement of the piston  13  in the extending direction  40 , the damping fluid  10  can flow from the first partial work space  17  through the flow channel  71  in the piston rod  12  into the second partial work space  19 . Furthermore, the equalizing valve  31  opens and enables a flow of damping fluid  10  from the equalizing space  6  through the second equalizing channel into the second partial work space  19 . In this way, it is ensured that the work space  5  is always entirely filled with damping fluid  10  apart from the volume displaced by the piston mechanism  11 . 
     The equalizing valve  31  in the second closing element and/or the closure elements  53 ,  55  in the piston  13  may be designed such that, upon movement of the piston rod  12  in the extending direction  40 , a flow of damping fluid  10  through the second equalizing channel in the second closing element  24  and/or the flow channel  60  in the piston  13  occurs only during a large extending velocity of the piston rod  12  or a large force on it in the extending direction  40 . 
     The flow channel  60  in the piston  13  and/or the equalizing valve  31  in the second closing element thus act as an overload protection, which is triggered by large velocities and/or forces on the piston rod  12  and thereby prevents a damaging of the shock absorber  1 . Of course, the damping behaviour of the shock absorber  1  can be influenced as needed by suitable selection of the closure elements  53 ,  55  of the flow channel  60  and/or the valve elements of the equalizing valve  31 . 
     By a rotating of the adjustment element  102  about the longitudinal axis  7 , the bores  72  of the flow channel  71  in the piston rod  12  can be closed. In this way, the effective flow cross section of the flow channel  71  in the piston rod  12  is decreased, in particular is closed, in particular is closed entirely. A flow of damping fluid  10  from the first partial work space  17  through the flow channel  71  into the second partial work space  19  or vice versa is then no longer possible. 
     Insofar as the equalizing valve  31  prevents a flow of damping fluid  10  from the second partial work space  19  into the equalizing space  6 , the piston rod  12  in this position of the adjustment element  102  is blocked against displacement opposite the extending direction  40  on account of the totally closed volume of the second partial work space  19 . 
     However, if the force on the piston rod  12  in the direction opposite the extending direction  40  exceeds a predetermined limit force, the overload protection is activated and the flow channel  60  in the piston  13  and/or the second equalizing channel in the second closing element is opened. 
     Since the equalizing valve  31  in the second closing element opens upon movement of the piston  13  in the extending direction  40  in order to allow a flow of damping fluid  10  from the equalizing space  6  into the second partial work space  19 , and the first equalizing channel in the first closing element  21  is closed, the shock absorber  1  is not blocked entirely against a movement of the piston rod  12  in the extending direction  40  even in the case of a closed flow channel  71  in the piston rod  12 . However, it has a maximum hard damping, since the damping fluid  10  cannot flow through the flow channel  71  in the piston rod  12  from the first partial work space  17  into the second partial work space  19 , but rather flows from the first partial work space  17  into the equalizing space  6  and from the equalizing space  6  through the second equalizing channel into the second partial work space  19 . Thus, in this case the damping characteristic is determined by the equalizing channels and also in particular by the equalizing valve  31 . 
     In an alternative embodiment it can be provided that the closure elements  53 ,  55  are designed such that the flow channels  60  in the piston  13  open or close depending on the velocity of movement of the piston rod  12  in the extending direction  40 . In this way, a velocity-dependent damping characteristic can be achieved. For details in this regard, reference is made to DE 10 2005 023 756 A1, paragraph [0028] et seq. 
     In the case of an interruption in the power supply, the position recognition unit  107  ensures a resetting of the position recognition by means of a reference travel. For this, a reference mark is used in particular, which may be designed for example as an end stop element. It is ensured that the position of the adjustment element can be clearly and easily determined and established after an unforeseen interruption in its displacement. 
     In the following, making reference to  FIGS. 4 and 5 , a second exemplary embodiment of the invention shall be described. Identical parts are given the same reference numbers as in the first exemplary embodiment, to whose description reference is hereby made. Structurally different, yet functionally identical parts are given the same reference numbers with a suffixed “a”. 
     In the shock absorber  1   a , the adjustment element  102   a  is designed as a spindle nut, which can be driven by a corresponding displacement spindle  110 . The displacement spindle forms a force transmission device, which is coupled directly to the transmission  104 . A rotary movement of the displacement spindle  110  produces—depending on the direction of rotation about the longitudinal axis  7 —a displacement of the adjustment element  102   a  along the longitudinal axis  7  in or opposite the extending direction  40 . 
     In the arrangement shown in  FIG. 4 , some of the flow openings  72  are clear. In the arrangement shown in  FIG. 5 , in which the spindle nut  102   a  is arranged further downward, facing the work space  5  of the shock absorber, flow openings  72  are covered. This arrangement corresponds to the arrangement per  FIG. 3  of the first exemplary embodiment. 
     In another embodiment, not shown, the adjustment unit can be designed as a needle valve with a needle movable along the longitudinal axis  7 , which can dip into a corresponding valve opening. Depending on the depth of insertion of the valve needle into the valve opening, the effective flow cross section area is changed. It is advantageous for the valve needle and/or the valve opening to have a conical profile at least for a portion along the longitudinal axis  7 , so that with increasing depth of insertion of the valve needle into the valve opening the effective flow cross section area is reduced. 
     In the following, a third exemplary embodiment of the invention shall be described with the aid of  FIG. 6 . Identical parts are given the same reference numbers as in the first two exemplary embodiments, to whose description reference is hereby made. Structurally different, yet functionally identical parts are given the same reference numbers with a suffixed “b”. 
     As in the second exemplary embodiment, a kinematic element  110   b  is provided, which converts a rotary drive movement into an axial displacement of the adjustment element  102   b.    
     The flow openings  72  are arranged at an axial distance from each other especially along the longitudinal axis  7 . By an axial displacement of the adjustment element  102   a , successively more or fewer flow openings  72  can be covered or opened up. 
     It is conceivable to provide only a single flow opening, for example in the form of an elongated hole, which can be continuously covered or opened up by means of the adjustment element  102   a . This enables a continuous adjusting of the damping force of the shock absorber  1   a.    
     The flow openings  72  are arranged on the piston rod  12  above a radial piston rod end stop  51 . The flow openings  72  are coordinated with the first partial work space  17 . 
     The kinematic element  110   b  is designed as a motion thread, with an outer sleeve  111 , having a helical slit guide  112 . In the slit guide  112  there is arranged a cross-bolt  113 , which is oriented transversely to the longitudinal axis  7 . The cross-bolt  113  is connected firm against rotation to an inner displacement element, not otherwise shown. Upon rotary drive movement of the adjustment actuator and the adjustment element, the latter is force-guided by means of the cross-bolt  113  in the guide slit  112  and moved upward or downward along the longitudinal axis  7 , depending on the direction of rotation.