Abstract:
A frequency-dependent damper incorporates a compensated piston assembly to reduce the amount of static push-out force. The piston rod is a hollow rod with a compensator being disposed within the hollow portion of the piston rod. The piston rod is a hollow rod with a compensator being disposed within the hollow portion of the piston rod. The compensator is attached to the pressure tube of the damper such that the compensator slides within the piston rod during stroking of the shock absorber. The compensated piston assembly reduces the difference in cross-sectional area between the upper and lower surfaces of the piston.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to dampers or shock absorbers adapted for use in a suspension system such as the suspension system used for automotive vehicles. More particularly, the present invention relates to a shock absorber that utilizes a gas as the damping medium and that includes a compensated piston rod to reduce the static push-out force on the piston rod.  
       BACKGROUND OF THE INVENTION  
       [0002]     Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations that occur during driving. To absorb these unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber and the pressure tube is normally attached to the unsprung portion of the vehicle. The piston is normally attached to the sprung portion of the vehicle through a piston rod that extends through the pressure tube. The piston divides the pressure tube into an upper working chamber and a lower working chamber, both of which are typically filled with a hydraulic liquid. Because the piston is able, through valving, to limit the flow of the hydraulic liquid between the upper and lower working chambers when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force that counteracts the vibration that would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle. In a dual tube shock absorber, a fluid reservoir or reserve chamber is defined between the pressure tube and a reserve tube. A base valve assembly is disposed between the lower working chamber and the reserve chamber to also produce a damping force that counteracts the vibrations that would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle.  
         [0003]     Shock absorbers filled with hydraulic liquid have met with continuous success throughout the automotive industry. While meeting with success in the automotive industry, hydraulic liquid filled shock absorbers are not without their problems. One problem with these prior art shock absorbers is that they are not sensitive to the frequency of the vibrations. Complex systems have been developed to modify these liquid filled shock absorbers to provide a shock absorber that is relatively soft for high frequency vibrations while being relatively stiff for low frequency vibrations. Other problems associated with the prior art hydraulic liquid filled shock absorbers include the variability in their damping forces due to temperature changes of the hydraulic liquid. As the temperature of the hydraulic liquid changes, the viscosity of the liquid also changes, which significantly affects the damping force characteristics of the liquid and, thus, the shock absorber. In addition, any aeration of the hydraulic liquid during operation of the shock absorber adversely affects the operation of the shock absorber due to the introduction of a compressible gas into a non-compressible liquid. Finally, the hydraulic liquid adds to the weight of the shock absorber, as well as presenting environmental concerns regarding the use and disposal of a hydraulic liquid.  
         [0004]     In an effort to overcome the problems associated with shock absorbers that utilize hydraulic liquid as the damping medium, shock absorbers that utilize a gas as the damping medium having been developed. The use of a gas, preferably air, as the damping medium produces a frequency dependent damper or shock absorber that is significantly less sensitive to temperature when compared to hydraulic liquid dampers, is not adversely affected by aeration over time, is lower in weight and, especially when the gas is air, is environmentally friendly due to the elimination of the hydraulic oil.  
         [0005]     While gas shock absorbers have resolved some of the issues that relate to hydraulic liquid shock absorbers, they are not without their own problems. One problem associated with gas shock absorber is a relatively high static push-out force that reacts against the piston, tending to extend the shock absorber. This static load is caused by the high pressure gas within the shock absorber in conjunction with the fact that the piston rod is located on only one side of the piston.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides the art with a gas-filled shock absorber that incorporates a unique compensated piston rod assembly design that significantly reduces the static push-out force for the gas-filled shock absorber. The piston rod assembly incorporates a hollow piston rod that includes a compensator that reduces the difference between the cross-sectional area on the upper side of the piston and the cross-sectional area on the lower side of the piston.  
         [0007]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0009]      FIG. 1  is an illustration of an automobile incorporating the gas-filled shock absorber that incorporates the unique compensated piston assembly design in accordance with the present invention;  
         [0010]      FIG. 2  is a side view, partially in cross-section, of the gas-filled shock absorber that incorporates the unique compensated piston assembly design in accordance with the present invention;  
         [0011]      FIG. 3  is an enlarged cross-sectional view of the piston assembly illustrated in  FIG. 2 ;  
         [0012]      FIG. 4  is an enlarged cross-sectional view of the compensator illustrated in  FIG. 2 ; and  
         [0013]      FIG. 5  is a view similar to  FIG. 3 , but illustrating a compensated piston assembly in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0015]     Referring now to the drawings in which like reference numerals designate like or corresponding parts throughout the several views, there is shown in  FIG. 1 a  vehicle incorporating a suspension system having the gas-filled shock absorbers, which incorporate the compensated piston assembly design in accordance with the present invention and which is designated generally by the reference numeral  10 . Vehicle  10  includes a rear suspension system  12 , a front suspension system  14  and a body  16 . Rear suspension system  12  includes a pair of independent suspensions adapted to operatively support a pair of rear wheels  18 . Each rear independent suspension is attached to body  16  by means of a shock absorber  20  and a helical coil spring  22 . Similarly, front suspension system  14  includes a pair of independent suspensions adapted to operatively support a pair of front wheels  24 . Each independent front suspension is attached to body  16  by means of a shock absorber  26  and a helical coil spring  28 . Rear shock absorbers  20  and front shock absorbers  26  serve to dampen the relative movement of the unsprung portion (i.e., front and rear suspension systems  12  and  14 , respectively) of vehicle  10  with respect to the sprung portion (i.e., body  16 ) of vehicle  10 . While vehicle  10  has been depicted as a passenger vehicle having independent front and rear suspensions, shock absorbers  20  and  26  may be incorporated into other types of vehicles having other types of suspensions and springs or into other types of applications, including, but not limited to, vehicles incorporating air springs, leaf springs, non-independent front and/or non-independent rear suspension systems. One of the unique features of the present invention is that if it is combined with an air spring, the air spring and the shock absorber can communicate with each other or the air spring and the shock absorber can be separate units. Further, the term “shock absorber” as used herein is meant to refer to dampers in general and thus will include MacPherson struts, spring seat units, as well as other shock absorber designs known in the art.  
         [0016]     Referring now to  FIG. 2 , front shock absorber  26  is shown in greater detail. While  FIG. 2  shows only front shock absorber  26 , it is to be understood that rear shock absorber  20  is or can be designed to incorporate the compensated piston assembly design in accordance with the present invention. Rear shock absorber  20  would only differ from front shock absorber  26  in the way it is adapted to be connected to the sprung and unsprung portions of vehicle  10  and in the dimensions of the various components. Shock absorber  26  comprises a pressure tube  30 , a compensated piston assembly  32 , a piston rod  34  and a rod guide assembly  36 .  
         [0017]     Pressure tube  30  defines a working chamber  42 . Working chamber  42  is filled with a gas, preferably air, at a specified pressure to act as the damping medium. Compensated piston assembly  32  is slidably disposed within working chamber  42  and divides working chamber  42  into an upper working chamber  44  and a lower working chamber  46 . A seal assembly  48  is disposed between piston assembly  32  and pressure tube  30  to permit sliding movement of piston assembly  32  with respect to pressure tube  30  without generating undue frictional forces as well as sealing upper working chamber  44  from lower working chamber  46 . Piston rod  34  is attached to piston assembly  32  and extends through upper working chamber  44  and through rod guide assembly  36 , which closes the upper end of pressure tube  30 . The end of piston rod  34  opposite to piston assembly  32  is adapted to be secured to the sprung portion of vehicle  10 . The end of pressure tube  30  opposite to rod guide assembly  36  is closed by an end cap  50  and end cap  50  is adapted to be connected to the unsprung portion of vehicle  10 . While piston rod  34  is shown adapted for being connected to the sprung portion of vehicle  10  and end cap  50  is adapted for being connected to the sprung portion of vehicle  10 , due to the use of a gas as the pressure medium, it is within the scope of the present invention to have piston rod  34  adapted to attach to the unsprung portion of vehicle  10  and end cap  50  adapted to attach to the sprung portion of vehicle  10  if desired.  
         [0018]     Referring now to  FIGS. 2-4 , compensated piston assembly  32  comprises a piston body  52 , a compression valve assembly  54 , a rebound or extension valve assembly  56 , a compensator  58  and a connecting rod  60 . Piston body  52  is attached to piston rod  34  by welding, by a threaded connection or by other means known in the art.  
         [0019]     Seal assembly  48  comprises a pair of annular seals  62  located between piston body  52  and pressure tube  30 . Seal assembly  48  is held in position by a plurality of grooves  64  formed in piston body  52 . Seal assembly  48  permits sliding movement of piston body  52  with respect to pressure tube  30  without generating unique frictional forces as well as providing a seal between upper working chamber  44  and lower working chamber  46 . This dual roll played by seal assembly  48  is extremely important for pneumatic shock absorber  26  due to the high pressures generated in working chambers  44  and  46  and the continued need for limiting the sliding forces generated between piston assembly  32  and pressure tube  30 .  
         [0020]     Piston body  52  defines one or more compression passages  70  and one or more extension passages  72 . During a compression movement of shock absorber  26 , gas flows between lower working chamber  46  and upper working chamber  44  through passages  70  as described below. During an extension movement of shock absorber  26 , gas flows between upper working chamber  44  and lower working chamber  46  through passages  72  as described below.  
         [0021]     Compression valve assembly  54  comprises a stop  74 , a pair of annular seals  76  and a valve plate  78 . Valve plate  78  is normally positioned against annular seals  76  to normally close the plurality of compression passages  70 . During a compression stroke of shock absorber  26 , the gas in lower working chamber  46  is compressed including the gas located within the plurality of compression passages  70 . The compressed gas located within compression passages  70  exerts a force on valve plate  78 , which will remain seated, closing passages  70  until the force created by the gas pressure exceeds the bending stiffness of valve plate  78 . When the load produced by the gas pressure exceeds the bending stiffness of valve plate  78 , valve plate  78  will deflect away from seals  76  to allow gas flow from lower working chamber  46  to upper working chamber  44  through passages  70 .  
         [0022]     Extension valve assembly  56  comprises a valve stop  84 , a pair of annular seals  86  and a valve plate  88 . Valve plate  88  is normally positioned against seals  86  to normally close the plurality of extension passages  72 . During an extension stroke of shock absorber  26 , the gas in upper working chamber  44  is compressed including the gas located within the plurality of extension passages  72 . The compressed gas located within extension passages  72  exerts a force on valve plate  88 , which will remain seated, closing passages  72  until the force created by the gas pressure exceeds the bending stiffness of valve plate  88 . When the load produced by the gas pressure exceeds the bending stiffness of valve plate  88 , valve plate  88  will deflect away from seals  86  to allow gas flow from upper working chamber  44  to lower working chamber  46  through passages  72 .  
         [0023]     Rod guide assembly  36  provides sealing for hollow piston rod  34  and pressure tube  30 . Rod guide assembly  36  comprises a main housing  90 , an outer seal assembly  92  and an inner seal assembly  94 . Main housing  90  is pressfit or otherwise secured to pressure tube  30 . Outer seal assembly  92  includes a pair of seals  96  disposed between pressure tube  30  and main housing  90 . Hollow piston rod  34  is slidingly received within main housing  90 ; and inner seal assembly  94  includes a pair of seals  98  disposed between piston rod  34  and main housing  90 . Inner seal assembly  94  permits sliding movement of piston rod  34  with respect to rod guide assembly  36  without generating undue frictional forces as well as sealing upper working chamber  44  from the environment surrounding shock absorber  26 .  
         [0024]     Compensator  58  is slidingly received within a cavity  100  defined by hollow piston rod  34 . A seal  102  is disposed between piston rod  34  and compensator  58 . Seal  102  permits sliding movement of piston rod  34  with respect to compensator  58  without generating undue frictional forces, as well as sealing lower working chamber  46  from the environment surrounding shock absorber  26 . A vent hole  104  establishes communication between the portion of cavity  100  located above compensator  58  and the environment surrounding shock absorber  26 . While the present invention is illustrated with vent hole  104 , vent hole  104  is optional and can be deleted if desired.  
         [0025]     Connecting rod  60  extends between end cap  50  and compensator  58  to maintain the position of compensator  58  with respect to pressure tube  30 . Connecting rod  60  is attached to end cap  50  using an attachment  106 . Attachment  106  is a flexible attachment that provides an improved alignment of piston rod  34  with compensator  58 .  
         [0026]     In a typical solid piston rod design for a gas shock absorber, the pressure of the gas within pressure tube reacts against the upper side of the piston and against the lower side of the piston. In a static condition, the gas pressure in the upper working chamber and the lower working chamber is generally equal. This creates a push-out force that attempts to extend the shock absorber. This creation of the push-out force is caused by the area of the piston open to the lower chamber being larger than the area of the piston open to the upper working chamber due to the piston rod being located in only the upper working chamber.  
         [0027]     The present invention significantly reduces this difference in the area exposed to the lower and upper working chambers  44  and  46 , respectively, by incorporating compensated piston assembly  32 . In compensated piston assembly  32 , the difference in area exposed to the gas pressure is reduced to the cross-sectional area of hollow piston rod  34 , and not cavity  100 , thus significantly reducing the static push-out force.  
         [0028]     Referring now to  FIG. 5 , a compensated piston assembly  32 ′ is illustrated. Compensated piston assembly  32 ′ is designed to be a direct replacement for compensated piston assembly  32 . Compensated piston assembly  32 ′ comprises a piston body  52 ′ and at least one tunable restriction  70 ′. Tunable restriction  70 ′ replaces passages  70  and  72  of piston assembly  32  and provides communication between upper working chamber  44  and lower working chamber  46 . The damping characteristics for a shock absorber  26  that incorporates piston assembly  32 ′ is controlled by the size of tunable restriction  70 ′. The function, operation and advantages listed above for shock absorber  26  utilizing piston assembly  32  are the same for shock absorber  26  when piston assembly  32 ′ replaces piston assembly  32 .  
         [0029]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.