Abstract:
An adjustable vibration damper, having a displacer in a cylinder, a damping medium exerting a damping force via an adjustable damping valve. The adjustable damping valve is activated by a control pressure of a pneumatic spring A safety device is provided which, in the event of a control pressure reduced as a result of damage to the pneumatic spring, provides a sufficiently high damping force of the vibration damper. At least two pneumatic springs are connected to the safety device, which is constructed from a pneumatic circuit functioning as a pressure balance between the pneumatic springs which releases the higher control pressure of a pneumatic spring in the pneumatic circuit. The adjustable vibration dampers which belong to the pneumatic springs are connected via at least one pressure outlet connection of the pneumatic circuit and are activated by the highest control pressure of the pneumatic springs involved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an adjustable vibration damper. 
     2. Discussion of the Prior Art 
     In adjustable vibration dampers which are used together with pneumatic springs, there is an increasing tendency to utilize the pressure in the pneumatic spring of the vehicle as a control signal for an adjustable damping valve on or in the vibration damper. The essential advantage of these damping valves controllable by air pressure is the simple and therefore also cost-effective design. Particularly in the commercial vehicle sector, the aim is to achieve simple and robust solutions, since the chassis components are exposed to very high loads on off-highway journeys. 
     Due to the high loads, pneumatic springs may become leaky because of the embrittlement of the pneumatic spring bellows. It must also be taken into account that a pneumatic spring may burst. For the activation of the pneumatic spring, there is the important difference as to whether a low control pressure is present because of a low vehicle load or because of a leak. 
     For this type of pneumatic spring failure, German reference DE 41 05 771 A1 discloses a solution in which a pneumatic control pressure is used for an adjustable damping valve. A relatively complicated damping valve is employed, in which a control piston is subjected on one side to a control pressure and on the other side to a reference pressure. The atmosphere serves as reference pressure. As soon as the control pressure falls short of a particular level, the reference pressure presses the control piston into an operating position which is linked to a higher damping force for the vibration damper. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an adjustable vibration damper which, when the control pressure fails, can continue to build up a sufficient damping force. 
     The object is achieved, according to the invention, in that at least two pneumatic springs are connected to the safety device, which is constructed from a pneumatic circuit functioning as a pressure balance between the pneumatic springs which releases the higher control pressure of a pneumatic spring in the pneumatic circuit. The adjustable vibration dampers which belong to the pneumatic springs are connected via at least one pressure outlet connection of the pneumatic circuit and are activated by means of the higher control pressure of the pneumatic springs involved. 
     The pneumatic circuit represents an interrogation device. It is assumed that the higher control pressure is, under all circumstances, on the safe side of the operation of the pneumatic spring. An intact system can always generate a higher pressure than a defective pneumatic spring. The faulty control pressure is consequently filtered out. 
     The simplest variant is distinguished in that the pneumatic circuit consists of a shuttle valve which has connections to the pneumatic springs. A shuttle valve known per se may be used. This affords the advantage that two pneumatic springs are protected by means of one shuttle valve. 
     There may be provision for using the vibration damper in a vehicle axle, the pneumatic springs of a vehicle axle being connected to the pneumatic circuit. This design makes it possible, within limits, to have a beneficial influence on the rolling behavior of a vehicle since all the vibration dampers of a vehicle axle are activated by means of the higher control pressure of the pneumatic spring on the outside of a bend. 
     Alternatively, the vibration damper may be used in a vehicle having a plurality of pneumatically suspended axles, the pneumatic springs of different axles being connected to the pneumatic circuit. The background to this is the philosophy that one pneumatic spring has burst because of an excessive axle load. The remaining pneumatic spring is consequently subjected to even higher load on this axle, so that the probability of further failure increases. If, however, the control pressure of a pneumatic spring of another axle is used, the adjustable valve of the vibration damper can continue to be operated with a greater degree of safety. 
     There is also the possibility of using the vibration damper in a vehicle having a plurality of pneumatically suspended axles, the pneumatic springs of a number of axles being connected, in each case per axle, and the pneumatic springs of different axles being connected to the pneumatic circuit. In this case, the pneumatic springs are always compared with one another in pairs and the pneumatic spring released at the time is compared with another released pneumatic spring. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an adjustable vibration damper as an individual component; 
     FIG. 2 shows an exemplary embodiment of the adjustable valve on the vibration damper; 
     FIG. 3 shows a safety device for two adjustable vibration dampers; and 
     FIG. 4 shows a multiaxle pneumatically suspended vehicle with a safety device for all the adjustable valves of the vibration dampers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a vibration damper  1  with a cylinder  3 , in which a piston rod  5  is arranged so as to be axially moveable. The cylinder  3  is closed off downward by means of a bottom  7 . The piston rod  5  is led out of the upper end of the cylinder through a guiding and sealing unit  9 . A piston unit  11  having a piston valve arrangement  13  is fastened to a piston rod  5  within the cylinder  3 . The bottom of the cylinder  3  is provided with a bottom valve arrangement  15 . The cylinder  3  is encased by a container tube  17 . An annular space  19 , which constitutes a compensating chamber, is formed between the container tube  17  and the cylinder  1 . The space within the cylinder  1  is subdivided by means of the piston unit  11  into a first working chamber  21   a  and a second working chamber  21   b . The working chambers  21   a ,  21   b  are filled with pressure liquid. The compensating chamber  19  is filled to the level  19   a  with liquid and above it with gas. Within the compensating chamber  19 , a first line section, specifically a high-pressure subsection  23   a , is formed by an intermediate tube  23  and is connected via a bore  25  of the cylinder  1  to the second working chamber  21   b . Connected to this high-pressure subsection is a valve  27  which is mounted laterally on the container tube  17  and reacts as a function of pressure. A second line section, specifically a low-pressure subsection, leads (not illustrated) from the valve  27  to the compensating space  19 . 
     When the piston rod  5  is extended upward out of the cylinder  3 , the upper working chamber  21   b  is reduced in size. An excess pressure is formed in the upper working chamber  21   b  and can be bled into the lower working chamber  21   a  by means of the piston valve arrangement  13 , as long as the valve  27  reacting as a function of pressure is closed. When the valve  27  reacting as a function of pressure is opened, liquid simultaneously flows from the upper working chamber  21   b  through the high-pressure subsection  23  and the valve  27  reacting as a function of pressure into the compensating chamber  19 . The damping characteristic of the vibration damper during the extension of the piston rod  5  is therefore dependent on whether the valve  27  reacting as a function of pressure is open or closed. 
     When the piston rod  5  is retracted into the cylinder  3 , an excess pressure is formed in the lower working chamber  21   a . Liquid can pass from the lower working chamber  21   a  through the piston valve arrangement  13  upward into the upper working chamber  21   b . The liquid displaced by means of the increasing piston rod volume within the cylinder  3  is expelled through the bottom valve arrangement  15  into the compensating chamber  19 . Since the throughflow resistance of the piston valve arrangement  13  is lower than the throughflow resistance of the bottom valve arrangement  15 , a rising pressure likewise occurs in the upper working chamber  21   b . With the valve  27  reacting as a function of pressure being open, this rising pressure can, in turn, flow through the high-pressure subsection  23   a  over into the compensating space  19 . This means that, with the valve  27  reacting as a function of pressure being open, the shock absorber has a softer characteristic, even during retraction, and a harder characteristic when the valve  27  reacting as a function of pressure is closed, in exactly the same way as when the piston rod  5  is extended. It should be noted that the direction of flow through the high-pressure subsection  23   a  of the bypass is always the same, irrespective of whether the piston rod  5  is retracted or extended. 
     FIG. 2 shows an embodiment of the pressure-dependent valve  27  as an individual component. Mounted within a tubular connection piece  29  arranged on the outside of the container tube  17  is a pot-shaped insert  31  which has a connection  33  to the high-pressure subsection  23   a  (see also FIG. 1) of the vibration damper  1 . A valve surface  35  and at least one outflow orifice  37  to the compensation space  19  are worked in on the bottom of the pot-shaped insert  31 . 
     In this operating position, a valve body  39  is prestressed on the valve surface. The valve body  39  is guided radially in a central stepped orifice  41  of a pressure intensifier  43 , a valve body seal  39   a  separating the valve body front side from the valve body rear side. The valve body  39  has adjoining it a prolongation  45  which passes completely through the pressure intensifier  43  and ends in a cover  49  in a pressure connection orifice  47 . The pressure connection orifice  47  is connected to a pneumatic spring not illustrated (see FIG. 3 or  4 ). 
     The pressure in the pressure connection orifice  47 , referred to below as control pressure, acts on the pressure intensifier  43 , which is guided in an axially floating manner in the pot-shaped insert  31 . The pressure intensifier is formed by a disk which carries a seal  51  at its outside diameter. For this purpose, the inside diameter of the tubular insert is fashioned, starting from the cover  49  as far as a first supporting surface  53 , as a guide surface  55 . 
     A first spring element  59 , which consists preferably of layered flat disks, is arranged on the first supporting surface  53 , radially on the outside, and on a shoulder  57  on the underside of the pressure intensifier  43 . A second spring element  61  is braced between a further shoulder  63  of the pressure intensifier  43  and a second supporting surface  65 . The second supporting surface  65  is provided by a tension ring  67 , of which the outside diameter relative to the guide surface  55  is dimensioned in such a way that there is a press fit between these surfaces. 
     Starting from the seal  51 , the guide surface  55  forms, with the top side of the pressure intensifier  43  and the cover  49 , a pressure space  69 , in which the control pressure prevails. The control medium does not pass directly from the pressure connection orifice  47  into the pressure space  69  since the prolongation  45  is sealed off on the outside relative to the pressure connection orifice  47 . The prolongation  45  has a central throttling inflow duct  71  which reaches approximately as far as the valve body. The pressure medium can flow from there through the slight gap between the pressure intensifier  43  and the prolongation  45  as far as a threaded connection which forms a setting device  73 . The threaded connection constitutes a continuation of the throttling inflow duct  71 . 
     So that the setting device  73  or the threaded connection operates without any play, a prestressing spring  75  is arranged between the prolongation  45  and the top side of the pressure intensifier  43  and prestresses the two components  45 ,  43  of the threaded connection in such a way that the same thread flanks are always in engagement. A supporting disk  77  serves as an abutment for the prestressing spring  75 . 
     A cover seal  81  is intended to seal off the pressure space  69  and the compensating space  19  relative to the surroundings in the region of the valve  27 . A retaining ring  83  holds the cover  49  in the closed position, the cover  49  being rotatable so that a supply line, not illustrated, can be oriented into a desired position relative to the valve  27 . 
     While the damper is in operation, damping liquid is displaced via the high-pressure subsection  23   a  into the connection  33  of the valve  27 . Depending on the desired damping force characteristic, the damping medium must, if appropriate, pass through a prethrottle  85 , before it strikes an onflow surface  87  of the valve body  39 . The pressure on the onflow surface  87 , multiplied by its area, constitutes an opening force which takes effect on the valve body  39  and acts counter to the resulting closing force consisting of the control pressure in the pressure space  69  on the pressure intensifier  43  and the resulting spring force of the two spring elements  59 ;  61 . When the opening force is greater than the closing force, the valve body  39  lifts off or an already opened valve body, which in the pressureless state already allows a valve passage cross section, lifts off further. The damping medium can flow out via the outflow orifices  37  into the compensating space  19  from a low-pressure space  89  between the bottom of the pot-shaped insert  31  and the underside of the pressure intensifier  43 . Due to the pressure drop of the damping medium at the valve  27 , a damping force is established which can be set by means of the magnitude of the control pressure. 
     It should also be mentioned that, when there is no piston movement taking place, the instantaneous pressure in the vibration damper acts on the surface consisting of the size of the underside of the pressure intensifier, plus the onflow surface  87  of the valve body  39 , and constitutes an opening force. Particularly in vibration dampers with gas pressurization in the compensation space, this fact must be taken into account in setting the spring elements, since, in the event of heating, for example due to damping operation, gas pressurization may rise, with the result that, by virtue of the greater opening force, the valve body sets a larger valve passage cross section and the damping force falls correspondingly. 
     This design of the vibration damper and of the adjustable valve connected to it is to be seen as an example. It is intended merely to illustrate the operation of a pneumatically activated valve. In principle, the adjustable valve may also be designed differently. A piston arrangement is also possible. 
     FIG. 3 shows a circuit diagram which contains two pneumatic springs  91  with two adjustable vibration dampers  1 . The vibration dampers  1  may be arranged on one vehicle axle, but also on different vehicle axles. The illustration of the compressed air supply device has been dispensed with for the sake of clarity. 
     The design of a pneumatic spring  91  is presumed to be known. Reference is made, for example, to German reference DE 32 46 962 A1. It is also possible, however, for the pneumatic spring and the vibration damper to form a structural unit, as is already known from German reference DE 21 18 080 A1. 
     Connected in each case to the pneumatic spring is a control line  93   a ;  93   b , that transmits the air pressure p 1 ; p 2  to a connection S 1 ; S 2  of a safety device  95 . This safety device is a shuttle valve which releases the higher of the two control pressures p 1 ; p 2 , in that a valve body  99  held in an initial position by springs  97   a ;  97   b  opens a pressure outlet connection  101  to the vibration dampers  1 . The safety device may also be followed by a throttle  103  which filters out pneumatically an undesirably high pressure change frequency. 
     While the pneumatic springs  91  are in operation, they are kept constantly under pressure by the pressure supply device. If it is assumed that both pneumatic springs are supplied uniformly with compressed air, the safety device  95  or the valve body  99  assumes the switching position illustrated, in which both control connections s 1 ; s 2  are released and the control pressure of both pneumatic springs  91  acts on the adjustable valve  27  of the vibration dampers  1 . 
     As soon as a control line  93   a ;  93   b  and also the pneumatic spring  91  has a defect linked to a pressure loss at the control connection s 1 ; s 2 , the valve body  99  of the safety device  95  is brought, by the higher pressure within the intact pneumatic spring, into a switching position in which the control connection having the pressure loss is shut off. Both vibration dampers are consequently supplied with a control pressure by the intact pneumatic spring. 
     FIG. 4 shows a variation of the version according to FIG.  3 . In this case, the safety device consists of 3 shuttle valves  95   a ;  95   b ;  95   c  which are interlinked to form a logic circuit. Basically, according to the principle of FIG. 3, two pneumatic springs  91  are connected to one shuttle valve  95   a ;  95   b  via control lines  93   a ;  93   b ;  93   c ;  93   d . These shuttle valves  95   a ;  95   b  are, in turn, connected to a central shuttle valve  95   c  by means of further control lines  93   e ;  93   f , so that, for example, the pneumatic springs of a first vehicle axle are compared with pneumatic springs of a second vehicle axle on the principle of the pressure balance, the higher pressure of the control connections always prevailing at the pressure outlet connection  101   c  and activating all the vibration dampers  1 . 
     The invention is not limited by the embodiments described above which are presented as examples only but can be modified in various ways within the scope of protection defined by the appended patent claims.