Abstract:
Damping device for damping motion of movable furniture parts. A piston mounted in housing in longitudinally displaceable manner which, due lo displacement, forms two variable-size working chambers on opposite sides with damping medium. Via overflow-connection, throttled transfer of damping medium is possible between two working chambers. Piston plunger, fed selectively, is connected on one side of pislon. Via the outer end of piston plunger, the motion of the furniture part to be damped is transferred to the piston. On side of piston, an elongated adjustment shaft is connected to the piston plunger. Free end of plunger is fed in sealed manner out of front face lying opposite front face that is sealed on the piston plunger side. The end of adjustment shaft engages with a passage duct connecting two working chambers and forms a variable-sized throttle cross section in the passage duct, making adjusting motion of shaft relative to piston.

Description:
This application is a 371 application of PCT/EP2007/008461 filed Sep. 28, 2007, which claims priority to the German application 20 2006 015 656.9 filed Oct. 12, 2006 and German Application 20 2007 001 571.2 filed Feb. 3, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a damping device, in particular for the damping of the movement of movable furniture parts of pieces of furniture, with a piston which is mounted in a housing so as to be longitudinally displaceable and which as a function of its displaced position forms on opposing sides two working chambers which are variable in size and are filled with a fluid damping medium, enabling by means of at least one overflow connection a throttle overflow o the damping medium between the two working chambers, wherein a piston rod which extends in a sealed manner out of the associated front end of the housing is connected on one side of the piston and the movement of the furniture part to be damped is transferred to the piston by way of the outer end of the piston rod remote from the piston. 
     2. Description of Related Art 
     In furniture construction such damping or also decelerating devices serve for example to avoid or certainly largely to reduce the stresses and noises occurring during the rapid vigorous closing of doors or closing of drawers of cabinets as the movable furniture part mounted on the carcass is decelerated jerkily. Damping devices which operate with viscous fluids, such as for example silicone oil, as damping medium (e.g. DE 103 00 73 A1) have the advantage over devices operating with gaseous damping medium that these fluid damping media are practically incompressible, so that they do not have any resilient properties which would seek to move the movable furniture part back somewhat out of the actual closed position after the closed position is reached. When such dampers are used the problem arises that the damping forces to be generated for damping of the movable furniture part are also dependent upon the mass of the respective furniture part and the speed of closing it, so that for different damping tasks different dampers are necessary which are adapted according to the required damping characteristic. 
     SUMMARY OF THE INVENTION 
     By contrast the object of the invention is to create a damping device for furniture parts which can be adjusted to different damping characteristics without conversion or replacement of individual parts. In this case the dimensions of the housing of the damping device should at least not be increased but should preferably be reduced by comparison with the damping devices known from the prior art. 
     Starting from a device of the type referred to in the introduction this object is achieved according to the invention in that on the side of the piston lying opposite the working chamber on the piston rod side there is connected an elongate adjusting shaft with a diameter substantially corresponding to the diameter of the piston rod, the free end of the adjusting shaft also extending out of the housing out of the end face lying opposite the end face closed on the piston rod side, and that the end of the adjusting shaft connected to the piston engages in a through channel connecting the two working chambers and is constructed in such a way that it forms a throttle cross-section which is variable in size in the through channel due to adjusting movements of the shaft relative to the piston. Due to the diameter of the adjusting shaft having the same dimensions as the diameter of the piston rod the total volume of the two working chambers remains the same during the displacement of the piston in the housing, so that there is no necessity for a volume equalisation device which increases the external dimensions of the damper housing and which receives or feeds back the difference in the fluid damping medium forced out of the working chambers or flowing into them as the piston is displaced. In this respect the damping device according to the invention corresponds functionally to damping devices with piston/cylinder dampers operating with a piston rod running through, i.e. extending out of the end faces of the housing and sealed at both ends. In addition, due to the arrangement of the adjusting shaft in the working chamber remote from the piston rod as an actuating shaft for a throttle means the desired alteration of the damping characteristic of the damping device from the exterior is made possible, i.e. without opening of the housing. 
     In a preferred alternative embodiment of the invention the construction is such that in the end region of the adjusting shaft on the piston side a threaded portion is provided which is enlarged in diameter by comparison with the shaft diameter and engages in a portion of the through channel provided with a complementary matching thread, wherein at least one groove-like recess which forms a passage from the working chamber into the through channel is then provided in the threaded portion of the adjusting shaft and forms a part of the throttle channel connecting the two working chambers. A lug which tapers conically in the direction of its free end then projects from the end face of the threaded portion of the adjusting shaft remote from the working chamber into a portion of the through channel which is reduced in diameter relative to the threaded portion. By alteration of the depth to which the threaded portion of the adjusting shaft is screwed into the complementary matching thread of the through channel, the throttle cross-section can then be enlarged or reduced smoothly, so that a smooth change in the damping characteristic of the damping device is possible 
     In this case a handle is advantageously provided on the outer free end of the adjusting shaft, the handle being constructed for example as a rotary knob which has a greater diameter than the shaft diameter of the adjusting shaft and is advantageously provided on its circumferential surface with grooves or knurlings. 
     In a preferred embodiment of the invention the longitudinal central axis of the piston rod is aligned with the longitudinal central axis of the housing. 
     On the other hand, the adjusting shaft and the through channel aligned therewith in the piston are—unlike hydraulic damping devices with a piston rod running through which extends out of the end faces of the housing on both sides—offset laterally and are disposed parallel to the longitudinal central axis of the housing. 
     In this case it is recommended that the housing and the piston disposed so as to be longitudinally displaceable—unlike a circular cross-section—are constructed in such a way that they have a cross-section which is oval or is formed by two parallel straight boundaries which are closed at their ends by circular arcs. Due to this “compressed” cross-sectional shape of the housing of the damping device it is possible to dispose the damping device also in those regions of a piece of furniture in which there is only a small spacing between the furniture parts which are moved relative to one another. By way of example reference is made here to the spacing between the outer face of a drawer side wall and the associated inner face of the carcass of the drawer cabinet. 
     In an advantageous alternative embodiment of the invention the construction may be such that in a region of the piston which is offset with respect to the region of the through channel receiving the adjusting shaft in relation to the longitudinal central axis of the housing, preferably diametrically opposed, a second through channel is provided which connects the two working chambers and via which damping medium can flow between the working chambers. 
     Then a shaft is advantageously disposed in the second through channel so as to be longitudinally movable by a predetermined stroke, wherein at least in a part-region the cross-sectional surface of the shaft is smaller than the clear cross-sectional surface of the second through channel in order to form an overflow passage between the working chambers. 
     For presetting different damping characteristics with opposing stroke movements of the piston it may then be advantageous. On one of the free ends of the shaft retained in the second through channel a valve disc may be provided which has a larger cross-section at least in some areas than the cross-section of the second through channel and which, in the case of a higher pressure in the adjoining working chamber than at the free end of the opposing second working chamber, is pressed onto the facing end wall of the piston and closes the overflow passage at least partially and then prevents overflow of damping medium between the working chambers and acts functionally as a non-return valve. 
     A pin, of which the opposing free ends projecting from the shaft engage in slot-like slits in the piston which extend in the longitudinal direction of the shaft, passes through the end of the shaft lying opposite the end provided with the valve disc. Thus within the scope of the stroke predetermined by the valve disc on the one hand and one end of the slot-like slit on the other hand the shaft retained in the second passage is retained so as to be longitudinally movable but undetachable in the piston. 
     If the damping device according to the invention is to produce the higher damping effect when the piston rod is inserted into the housing, the design is such that the valve disc is provided on the free end of the shaft which is retained in the second through channel and is directed into the working chamber through which the adjusting shaft passes. The pressure in the damping medium which rises during insertion of the piston rod in the second working chamber then forcibly presses the valve disc onto the facing end face of the piston, whereby the overflow cross-section formed between the shaft and the through channel is closed partially or completely—depending upon the design of the valve disc. 
     In a further embodiment of the invention which is optimised in an advantageous manner with regard to its external dimensions the design may be such that, with the longitudinal central axis of the piston rod offset laterally with respect to the longitudinal central axis of the adjusting shaft, the piston rod is retained in a through opening in the piston in such a way that its free end within the housing still projects over the adjoining lateral region of the end face of the piston into the associated working chamber, that an overflow passage is formed in the region of the through opening in the piston through which the piston rod passes, and that on the end of the piston rod projecting into the appertaining working chamber a valve disc is retained so as to be movable between a position in which it rests on the piston end face and closes the overflow passage and a position in which it is lifted off from the piston end face and connects the overflow passage to the adjoining working chamber. Thus in this case the non-return function of the shaft which in the first embodiment is disposed in a second passage with a valve disc is realised directly in the region of the end of the piston rod within the housing, so that the space which is necessary for the second passage and inevitably enlarges the dimensions of the piston. 
     In this case the design is advantageously such that the through opening in the piston has a circular cross-section which substantially corresponds to the diameter of the piston rod and is provided in a part-region with a radial expansion of the diameter which produces a through groove forming an overflow passage in the through opening. 
     In a variant of the previously described embodiment of the damping device the adjusting shaft can be disposed in the through channel of the piston so as to be rotatable and longitudinally movable by a predetermined stroke and can be retained so as to be biased resiliently into an end position in which a throttle collar which projects radially from the circumferential surface of the adjusting shaft in the working chamber on the adjusting shaft side rests on the piston end face, wherein the throttle collar is then provided with a plurality of cut-outs which are offset with respect to one another in the circumferential direction with differing passage cross-sections and which can be selectively aligned, by rotation of the adjusting shaft in the through channel, with an overflow passage provided in the piston and connecting the working chambers. By rotation of the adjusting shaft in such a way that cut-outs with differing passage cross-sections come into alignment with the overflow passage the throttle effect and thus the damping characteristic of the damping device can then be altered in a number of stages which corresponds to the number of cut-outs. 
     In this case the overflow passage is preferably formed by a groove-like recess in the wall of the through channel which has a complementary diameter to the adjusting shaft and passes through the piston. 
     The cut-outs in the throttle collar are preferably formed by cut-outs of differing radial extent which open in the circumferential surface of the throttle collar. 
     The selected throttle stage can be fixed in that a projection which corresponds in width to the cut-outs in the throttle collar projects from the end face of the piston facing the throttle collar, engages in one of the cut-outs in the position in which it is forced resiliently into the contact position of the throttle collar on the piston end face, and locks the adjusting shaft against rotation, wherein this locking position can be produced by longitudinal displacement of the adjusting shaft against the resilient spring bias out of the engaged position of the projection with the cut-out and the adjusting shaft is then rotatable into a different throttle position. 
     The overflow of the damping medium from the working chamber through which the adjusting shaft passes into the working chamber through which the piston rod passes can also be made possible in that the piston rod is retained in the piston so that it is longitudinally movable in a through opening by a predetermined stroke, that at least one overflow passage is constructed in the region of the through opening through which the piston rod passes, and that the piston rod is provided in the working chamber through which it passes with a valve collar in the shape of an annular disc which is adjustable during the longitudinal displacement of the piston rod within the scope of the predetermined stroke between a position in which it rests on the associated piston end surface and thereby closes the overflow passage and a position in which it is lifted off from the piston end face and frees the overflow passage. Thus in this embodiment the valve collar—which is preferably formed integrally on the piston rod—replaces the valve disc which is disposed so as to be longitudinally movable on the free end of the piston rod in the working chamber on the adjusting shaft side. 
    
    
     
       DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention is explained in greater detail in the following description of three embodiments in conjunction with the drawings, in which: 
         FIG. 1  shows an isometric view of a first embodiment of a damping device according to the invention in which the piston rod is completely inserted into the housing; 
         FIG. 2  shows a side view of the damping device viewed in the direction of the arrow  2  in  FIG. 1 ; 
         FIG. 3  shows a view of the damping device viewed in the direction of the arrow  3  in  FIG. 2 ; 
         FIG. 4  shows a sectional view in the direction of the arrows  4 - 4  in  FIG. 3 ; 
         FIG. 5  shows an isometric representation of the damping device, corresponding to the illustration in  FIG. 1 , in the position in which the piston rod is completely withdrawn from the housing; 
         FIG. 6  shows a side view in the direction of the arrow  6  in  FIG. 5 ; 
         FIG. 7  shows a view in the direction of the arrow  7  in  FIG. 6 ; 
         FIG. 8  shows a sectional view in the direction of the arrows  8 - 8  in  FIG. 7 ; 
         FIG. 8   a  shows a sectional view in the direction of the arrows  8   a - 8   a  in  FIG. 8 ; 
         FIG. 9  shows an exploded representation of the damping device according to the invention in an isometric representation of its components; 
         FIG. 10  shows a side view of the second embodiment of a damping device according to the invention; 
         FIG. 11  shows a view in the direction of the arrow  11  in  FIG. 10 ; 
         FIG. 12  shows a side view in the direction of the arrows  12 - 12  in  FIG. 11 ; 
         FIG. 12   a  shows a sectional view in the direction of the arrows  12   a - 12   a  in  FIG. 12 ; 
         FIG. 13  shows a side view of an assembly comprising the piston, the piston rod and the adjusting shaft of a third embodiment of a damping device according to the invention; 
         FIG. 14  shows a view in the direction of the arrow  14  in  FIG. 13 ; 
         FIG. 14   a  shows a sectional view in the direction of the arrows  14   a - 14   a  in  FIG. 14  on an enlarged scale; 
         FIG. 15  shows a sectional view in the direction of the arrows  15 - 15  in  FIG. 14 ; 
         FIG. 16  shows an isometric representation of the assembly of the third embodiment shown in  FIGS. 13 to 15 ; 
         FIG. 17  shows a side view of the adjusting shaft of the assembly shown in  FIGS. 13 to 16 ; 
         FIG. 18  shows a view of the adjusting shaft viewed in the direction of the arrow  18  in  FIG. 17 ; 
         FIG. 19  shows a corresponding view of the piston of the assembly in the viewing direction of  FIG. 14 ; 
         FIG. 20  shows a view of the underside of the piston viewed in the direction of the arrow  20  in  FIG. 19 ; 
         FIG. 21  shows a sectional view in the direction of the arrows  21 - 21  in  FIG. 19 ; and 
         FIG. 22  shows an isometric view of the piston shown in  FIGS. 19 to 21  viewed in an oblique direction with respect to its underside. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The first embodiment of the damping device according to the invention which is illustrated in  FIGS. 1 to 8   a  and is designated as a whole by  10  has a housing  12  which in the specially illustrated case—unlike known damping devices with a hollow cylindrical housing which is therefore also denoted briefly as a rule as a “cylinder”—has a cross-section formed by two parallel straight boundary edges connected at their ends by semi-circular boundary lines. Accordingly in the hollow interior of this housing  12  a piston  14  with a cross-section complementary to the interior of the housing is inserted so as to be movable, and with a sealing ring  18  inserted into a circumferential groove  16  in the outer piston wall this piston seals the circumferential slit-shaped intermediate space between the outer face of the piston and the inner face of the housing. 
     On its lower end in the drawings the housing  12  is closed by an integral end wall  20  and on its upper end it is closed by a separate housing cover  22  fixed on the housing  12 , whereby in the interior of the housing working chambers  24 ,  26  which are filled with a fluid damping medium are produced respectively on opposing sides of the end faces of the piston  14 . 
     A sealed piston rod  28  passing through the housing cover  22  and rigidly connected to the piston  14  at its end within the housing transfers to the piston  14  movements introduced by the movable furniture part to be damped into its free outer end, so that the piston is displaced thereby in the housing  12 , whereby the volumes of the working chambers  24 ,  26  are each increased or decreased as a function of the direction of movement of the piston  14 . According to the change in the volumes of the working chambers the damping medium enclosed in the housing must be capable of passing from the decreasing working chamber into the opposing increasing working chamber. For this purpose through channels are formed in the piston  14 , the design of these through channels being explained in greater detail below. 
     The sealing of the piston rod  28  in the housing cover is performed by a sealing ring  32 , which is inserted into a stepped through opening  30  in the housing cover  22  and in the illustrated case is constructed as an O-ring, which is retained by an annular disc  34 . A toothed disc  36  which digs into the wall of the opening  30  with its resilient outwardly biased teeth fixes the disc  34  and thus the sealing ring  32  in the housing cover  22 . 
     The working chamber  24  lying opposite the working chamber  26  through which the piston rod  28  passes has passing through it an adjusting shaft  38  which is laterally offset with respect to the piston rod  28  projecting centrally from the piston  14 , the adjusting shaft for its part being passed in a similar manner to the piston rod  28  through a stepped opening  30  in the housing end wall  20 . Also in this case the sealing is again performed by a sealing ring  32  constructed as an O-ring which is retained by an annular disc  34  and an adjoining toothed disc  36  in the opening  30  and is sealed on the circumferential surface of the adjusting shaft  38 . On the end on the piston side the adjusting shaft  38  is provided with a threaded portion  40  which is somewhat enlarged relative to the shaft diameter and which engages in a portion of the through channel  42  which is provided with a complementary matching thread and passes through the piston  14  so as to be laterally offset with respect to the piston rod  28 . An elongate groove-like recess  44  which forms a passage from the working chamber  24  to the through channel  42  is provided in the threaded portion  40  of the adjusting shaft  38 . A lug  46  which tapers conically in the direction of its free end projects from the end of the threaded portion  38  on the carcass side into a portion of the through channel  42  which is reduced in diameter relative to the threaded portion. By rotation of the adjusting shaft  38  the threaded portion  40  is screwed—depending upon the direction of rotation—into or out of the through channel  42 , whereby the annular gap formed between the conical lug  46  and the through channel  42  is decreased or increased in size. This annular gap, through which the fluid medium overflowing out of one working chamber  24 ,  26  into the respective other working chamber  26 ,  24  must pass during the piston movement, forms a throttle gap which is the cause of the magnitude of the damping or decelerating force counteracting the piston movement. Thus it is clear that by rotation of the adjusting shaft  38  a change in the passage cross-section of the throttle gap and thus in the damping characteristic of the damping device is possible. 
     In order to simplify delicate rotation of the adjusting shaft  38 , of which the diameter corresponds to the diameter of the piston rod  28 , a handle  48  in the form of a rotary knob of enlarged diameter provided with grooves on its outer circumferential surface is fixed on the outer free end of the adjusting shaft  38 . 
     A second through channel  50 , which connects the two working chambers and in which an elongate shaft  52  is retained so as to be longitudinally movably by a predetermined stroke, is provided in a region of the piston  14  which with respect to the longitudinal central axis of the housing lies diametrically opposite the region of the through channel  42  which receives the adjusting shaft  38 . The second through channel  50  and the shaft  52  have differing cross-sectional dimensions. As can be seen in  FIG. 8   a , in order to form an overflow passage between the working chambers  24 ,  26  the shaft  52  is provided in opposing regions with flat areas  51  over the damping medium can flow through. 
     On the lower end of the shaft  52  facing the working chamber  24  there is provided a valve disc  54  which has a larger diameter than the cross-section of the second through channel  50  and which when an overpressure prevails in the working chamber  24  is pressed onto the end face of the piston  14  and then closes the overflow passage between the second through channel  50  and the shaft  52 . 
     On the end opposite the end provided with the valve disc, the shaft  52  has a pin  56  passing through it of which the end regions projecting from the shaft  52  at both ends engage in lateral slits  58  in the piston  14 . It can be seen that the shaft  52  is retained undetachably in the piston  14  by the valve disc  54  on the one hand and the pin  56  on the other hand, whereby the possible longitudinal movement of the shaft  52  in the through channel  50  is predetermined by the longitudinal extent of the slits  58 . 
       FIGS. 10 to 12   a  show an embodiment of a damping device  10  according to the invention in which the external dimensions are further reduced by comparison with the embodiment described above in connection with  FIGS. 1 to 8  and which largely corresponds to the embodiment described above in its basic construction and operation. Therefore in order to avoid unnecessary repetitions only the modifications which have been made are described below, whilst reference may be made to the preceding description for the conforming features of both embodiments, since component parts in both embodiments which correspond functionally are assigned the same reference numerals in the appertaining drawings. 
     The significant difference from the first embodiment is that the effect of a non-return valve, produced by the shaft  52  which is disposed so as to be longitudinally movable in the second through channel  50  with a valve disc  54  provided on its free end, is achieved in the second embodiment in alignment with the longitudinal central axis of the piston rod  28 . for this purpose the fixing hole receiving the end of the piston rod  28  on the housing side is constructed as a through opening  50 ′ passing completely through the piston. The lengthened piston rod  28  passes completely through the through opening  50 ′ and projects somewhat into the lower working chamber  24 . A valve disc  54  constructed as an annular disc is then again disposed movably on the projecting end of the piston rod  28 . The travel of this valve disc  54  is delimited by a retaining head of enlarged diameter forced onto the free end of the piston rod. 
     The piston rod  28  is fixed in the piston  14  by a pin  60  which is inserted into a transverse bore in the piston  14  passing tangentially through the through opening  50 ′ and which in the region of the through opening  50 ′ engages in a corresponding tangentially extending rounded recess in the piston rod  28 . The overflow passage through which the damping medium can pass from the working chamber  26  into the working chamber  24  when the valve disc  54  is lifted off from the lower end face of the piston is formed by a longitudinal groove  51 ′ shown in  FIGS. 12 and 12   a  in the wall of the through opening  50 ′ also shaped with a complementary cross-section relative to the piston rod  28 . Due to the integration into the lengthened piston rod of the functional components which bring about the non-return function, the piston can have a reduced width in the transverse direction and thus also the housing  12  of the damper has a correspondingly reduced width. 
     A third embodiment, which differs from the previously described second embodiment in that the alteration of the damping characteristic can be set in predetermined stages rather than progressively, is described below in connection with  FIGS. 13 to 22 . 
     Since the modifications which have been made relate exclusively to the assembly formed by the piston  14 , the piston rod  28  and the adjusting shaft  38 , i.e. the housing is unchanged relative to the housing  12  of the second embodiment, only the changes which have been made to the said assembly are described below, and functionally similar components of this assembly are also assigned the same reference numerals as have been used in the embodiments described above, so that it is sufficient for only the modifications which have been made to be described below. The said assembly is illustrated in  FIGS. 13 to 16  in different views or sections, whilst  FIGS. 17 and 18  show the adjusting shaft  38  and  FIGS. 19 to 22  show the piston in each case separately in various views or sectional representations. 
     The end portion  38   b  of the adjusting shaft  38  on the piston side which is reduced in diameter relative to the portion  38   a  passing through the working chamber  24  is rotatable in the passage  42  of the piston  14  and retained so as to be longitudinally movable by a predetermined stroke. In the transition from the portion  38   a  with the larger diameter to the portion  38   b  with the smaller diameter a radially projecting throttle collar  46 ′ is provided which in one end position of the longitudinal travel of the adjusting shaft  38  rests against the recessed end face of the piston  14  facing the working chamber  24 . In this position resting on the piston the adjusting shaft is retained by a helical spring  62  which is pre-assembled under compression pretension and supported on the base of a recess provided in the opposing piston end face, the other end of the helical spring being supported on a lock washer  64  mounted on the free end of the adjusting shaft  38  lying in the working chamber  26  and facing away from the piston. 
     The throttle collar  46 ′ is provided with a plurality of radial cut-outs  66  which are offset with respect to one another in the circumferential direction and have the same width but different radial extent and which can be selectively aligned with an overflow passage  42 ′ connecting the working chambers  24 ,  26  by rotation of the adjusting shaft  38 . In this case the overflow passage  42 ′ is formed by a groove-like recess passing through the piston  14  in the wall of the portion of the adjusting shaft  38   b  which passes through with a complementary diameter to the through channel  42 . 
     A projection  70  which corresponds in width to the cut-outs  66  in the throttle collar  46  projects from the recessed end face of the piston facing the throttle collar  46 ′, wherein in the position in which it is forced resiliently into the abutment position of the throttle collar  46 ′ on the end face of the piston the projection engages in one of the cut-outs  66  and locks the adjusting shaft  38  against rotation. The cut-out  66  located in the locking position can be lifted out of the engaged position with the associated cut-out by longitudinal displacement of the adjusting shaft  38  against the resilient bias of the helical spring  62 , whereby the adjusting shaft is then rotatable in the through channel  42  and thus another cut-out  66  of differing passage cross-section provided in the throttle collar  46 ′ can be aligned with the overflow passage  42 ′. 
     A further modification relative to the second embodiment resides in the fact that the piston rod  28  is retained in the through opening  50 ′ in the piston  14  so as to be longitudinally movable by a predetermined stroke. In the region of the through opening  50 ′ through which the piston rod  28  passes, at least one overflow passage  51 ′ is constructed and the piston rod is provided in the working chamber  26  through which it passes with a valve collar  54 ′ in the shape of an annular disc which during the longitudinal displacement of the piston rod  28  in the context of the predetermined stroke is adjustable between a position in which it rests on the associated piston end surface and thus closes the overflow passage  51 ′ and a position in which it is lifted off from the piston end face and frees the overflow passage. The longitudinally movable fixing of the piston rod  28  in the piston  14  again takes place by means of a pin  60  which is inserted into a transverse bore in the piston  14  passing tangentially through the through opening  50 ′ and which in the region of the through opening  50 ′ engages in an associated tangentially extending recess  55 , the longitudinal extent of which parallel to the longitudinal central axis of the piston rod  28  is dimensioned to correspond to the predetermined stroke. 
     In the illustrated embodiment the valve collar  54 ′ is formed integrally on the piston rod  28 . 
     It can be seen that within the scope of the idea underlying the invention modifications and variants of the described embodiments are possible. Thus for example it can be readily seen that the strong damping effect which in the described embodiments is achieved during insertion of the piston rod  28  into the housing  12  and can be varied by rotation of the adjusting shaft  38  can be converted to a damping effect in the opposite direction, i.e. when the piston rod  28  is withdrawn from the housing, if the function of the non-return valve is replaced by arrangement of a valve disc on the opposing upper face of the piston, i.e. the face directed towards the working chamber  26 .