Abstract:
A two-stage piston has a twin piston design where one piston produces a high damping force and the second piston produces a low damping force. At specified positions along the pressure tube, by-pass passages are formed to allow fluid flow around the high damping force piston to provide a low damping force or a soft ride. In the areas of the pressure tube where there are no by-pass passages, the high damping force piston provides a high damping force or a firm ride. In the area of the pressure tube where there are no by-pass passages, the high damping force piston and the low damping force piston both contribute to the high damping force.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a hydraulic damper or shock absorber for use in a suspension system such as the suspension systems used for automotive cars and trucks. More particularly, the present invention relates to a hydraulic damper or shock absorber which has a two-stage damping characteristic where a relatively low level damping is provided over a specified portion of the stroke of the hydraulic damper or shock absorber and a relatively high level of damping is provided outside of the specified portion of the stroke of the hydraulic damper or shock absorber. 
     BACKGROUND OF THE INVENTION 
     A conventional prior art mono-tube hydraulic damper or shock absorber comprises a cylinder defining a working chamber having a piston slidably engaging the cylinder within the working chamber. The piston thus divides the working chamber into an upper working chamber and a lower working chamber. A piston rod is connected to the piston and it extends out through one end of the cylinder. A first valving system is incorporated into the piston for generating a damping force during the extension stroke of the piston and a second valving system is incorporated into the piston for generating a damping force during the compression stroke of the piston. In a dual tube hydraulic damper or shock absorber, a reservoir tube surrounds the pressure tube to define a reserve chamber. A base valve assembly controls fluid flow between the working chamber and the reserve chamber. A first valving system is incorporated into the piston for generating a damping force during the extension stroke and a second valving system is incorporated into the base valve assembly for generating a damping force during the compression stroke of the piston. The piston includes a valve system to regulate the pressure drop across the piston for fluid flow during the compression stroke and the base valve assembly includes a check valve for fluid flow during the extension stroke. 
     Various types of damping force generating devices have been developed to generate a variety of desired damping forces in relation to various operating characteristics such as the speed and/or the displacement of the piston within the cylinder. These multi-force damping force generating devices have been developed to provide a relatively small or low damping force during the normal running of the vehicle and a relatively large or high damping force during maneuvers requiring extensions or large suspension movements. The normal running of the vehicle is accompanied by relatively small or fine vibrations of the unsprung mass of the vehicle and thus, the need for a soft ride or low damping characteristic of the suspension system to isolate the sprung mass from these vibrations. During a turning or braking maneuver, as an example, the sprung mass of the vehicle will attempt to undergo a relatively slow and/or large vibration which then requires a firm ride or high damping characteristics of the suspension system to support the sprung mass and provide a stable handling characteristic for the vehicle. Thus, these multi-force damping force generating devices offer the advantage of a smooth steady ride by eliminating the transmission of the high frequency/small amplitude vibrations between the unsprung mass and the sprung mass while still providing the necessary high damping or firm ride for the suspension during vehicle maneuvers causing larger excitations of the sprung mass to provide stability to the sprung mass. 
     The continued development of hydraulic dampers includes the development of multi-force damping force generating devices which are simpler to manufacture, can be manufactured at a lower cost and which improve the desired force generating characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention provides the art with a multi-stage hydraulic damper or shock absorber that provides different levels of damping based upon the position of the piston with respect to the pressure tube of the damper. The multi-stage damping is provided by the incorporation of twin pistons and the incorporation of a plurality of by-pass notches formed into the pressure tube wall. The plurality of notches allow fluid flow around one of the two pistons but not around both of them. Thus, when the by-passed piston is in engagement with one or more of the bypass notches, a relatively low damping force is generated. When the by-passed piston is not in engagement with any of the by-pass notches, a relatively high damping force is generated. By appropriately positioning the plurality of by-pass notches within the pressure tube, a relatively low damping force can be generated at typical vehicle heights while still allowing the hydraulic damper or shock absorber to generate a relatively high damping force when the shock absorber travels outside the typical vehicle height. 
     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 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is an illustration of an automobile using shock absorbers incorporating the multi-force damping force generating device in accordance with the present invention; 
     FIG. 2 is a cross-sectional side view of a dual tube shock absorber incorporating the multi-force damping force generating device in accordance with the present invention; 
     FIG. 3 is an enlarged cross-sectional view of the pressure tube illustrated in FIG. 2; 
     FIG. 4 is an enlarged cross-sectional view illustrating the fluid flow with respect to one of the two pistons illustrated in FIG. 2; 
     FIG. 5 is an enlarged cross-sectional view illustrating the fluid flow with respect to the other of the two pistons illustrated in FIG. 2; and 
     FIG. 6 is a cross-sectional side view of a mono-tube shock absorber incorporating the multi-force damping force generating device in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 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 having shock absorbers which include the multi-force damping force generating device in accordance with the present invention and which is designated generally by the reference numeral  10 . Vehicle  10  includes a rear suspension  12 , a front suspension  14  and a body  16 . Rear suspension  12  includes a rear axle assembly (not shown) adapted to operatively support a pair of rear wheels  18 . The rear axle assembly is operatively connected to body  16  by means of a pair of rear shock absorbers  20  and a pair of rear helical coil springs  22 . Similarly, front suspension  14  includes a front axle assembly (not shown) adapted to operatively support a pair of front wheels  24 . The front axle assembly is operatively connected to body  16  by a pair of front shock absorbers  26  and by a pair of front helical coil springs  28 . Shock absorbers  20  and  26  serve to dampen the relative motion of the unsprung portion (i.e., front and rear suspensions  12  and  14 , respectively) from the sprung portion (i.e., body  16 ) of vehicle  10 . While vehicle  10  has been depicted as a passenger car having front and rear axle assemblies, shock absorbers  20  and  26  may be used with other types of vehicles or in other types of applications including, but not limited to, vehicles incorporating independent or non-independent front and rear suspension assemblies. Further, the term “shock absorber” as used herein is meant to refer to dampers in general and thus will include McPherson struts. 
     Referring now to FIG. 2, rear shock absorber  20  is shown in greater detail. While FIG. 2 shows only rear shock absorber  20 , it is to be understood that front shock absorber  26  also includes the multi-force damping force generating device in accordance with the present invention which is described below for rear shock absorber  20 . Front shock absorber  26  only differs from rear shock absorber  20  in the way in which it is adapted to be connected to the sprung and the unsprung portions of vehicle  10 . Shock absorber  20  comprises a pressure tube  30 , a piston assembly  32 , a piston rod  34 , a reservoir tube  36  and a base valve assembly  38 . 
     Pressure tube  30  defines a working chamber  42 . Piston assembly  32  is slidably disposed within pressure tube  30  and it divides working chamber  42  into an upper working chamber  44  and a lower working chamber  46 . Pressure tube  30  defines a plurality of by-pass indentations  48  that are formed into pressure tube  30  for example in a spiral formation as shown in FIGS. 2 and 3. By-pass indentations are specifically located along the length of pressure tube  30  to define an area of relatively low damping force generating for a soft ride as is discussed in detail below. 
     Piston rod  34  is attached to piston assembly  32  and it extends through upper working chamber  44  and through an upper end cap  50  which closes the upper end of both pressure tube  30  and reservoir tube  36 . A sealing system  52  seals the interface between upper end cap  50 , pressure tube  30 , reservoir tube  36  and piston rod  34 . The end of piston rod  34  opposite to piston assembly  32  is adapted, in the preferred embodiment, to be secured to the sprung portion of vehicle  10 . Valving within piston assembly  32  creates a damping force by controlling the movement of fluid between upper working chamber  44  and lower working chamber  46  during an extension stroke of piston assembly  32  with respect to pressure tube  30 . Because piston rod  34  extends only through upper working chamber  44  and not lower working chamber  46 , movement of piston assembly  32  with respect to pressure tube  30  causes a difference in the amount of fluid displaced in upper working chamber  44  from the amount of fluid displaced in lower working chamber  46 . This difference in the amount of fluid displaced is known as the “rod volume” and it flows through base valve assembly  38 . Valving in base valve assembly  38  creates a damping force by controlling the movement of fluid between lower working chamber  46  and a reservoir chamber  54  defined between pressure tube  30  and reservoir tube  36  during a compression stroke of piston assembly  32  with respect to pressure tube  30 . While shock absorber  20  is being illustrated as a dual tube shock absorber having base valve assembly  38 , it is within the scope of the present invention to utilize pressure tube  30  and piston assembly  32  in a mono-tube designed shock absorber as shown in FIG.  6  and detailed below. 
     Reservoir tube  36  surrounds pressure tube  30  to define reserve chamber  54  located between tubes  30  and  36 . The bottom end of reservoir tube  36  is closed by an end cap  56  which is adapted, in the preferred embodiment, to be connected to the unsprung portion of vehicle  10 . The upper end of reservoir tube  36  is attached to upper end cap  50 . Base valve assembly  38  is disposed between lower working chamber  46  and reservoir chamber  54  to control the flow of fluid between the two chambers. When shock absorber  20  extends in length (extension or rebound), an additional volume of fluid is needed in lower working chamber  46  due to the “rod volume” concept. Thus, fluid will flow from reservoir chamber  54  to lower working chamber  46  through base valve assembly  38 . This fluid flow will not create a damping force. The damping force in an extension stroke is created by valving in piston assembly  32 . When shock absorber  20  compresses in length (compression), replacement fluid for upper working chamber  44  flows through piston assembly  32 . This fluid flow does not create a damping force. An excess amount of fluid must be removed from lower working chamber  46  due to the “rod volume” concept. Thus, fluid flow will flow from lower working chamber.  46  to reservoir chamber  54  through valving in base valve assembly  38  to create a damping force during the compression stroke. 
     The present invention is directed to a unique piston assembly  32  which works in conjunction with pressure tube  30  and its by-pass passages  48  to provide a multi-force damping force device which alters the size of the damping force generated based upon the position of the piston with respect to pressure tube  30  and by-pass notches  48 . 
     Referring now to FIGS. 2,  4  and  5 , piston assembly  32  comprises a low force piston assembly  60 , a spacer  62 , a high force piston assembly  64  and a retaining nut  66 . Piston rod  34  defines a reduced diameter section  68  which defines a shoulder  70 . Lower force piston assembly  60  is disposed adjacent shoulder  70 , spacer  62  is disposed adjacent lower force piston assembly  60  and high force piston assembly  64  is disposed adjacent spacer  62  as shown in FIG.  2 . Retaining nut  66  is threadingly received by piston rod  34  to secure low force piston assembly  60 , spacer  62  and high force piston assembly  64  to piston rod  34 . 
     Low force piston assembly  60  comprises a piston body  72 , a seal  74  (the seal-function can optionally be performed by the piston body), a compression valve assembly  76  and an extension valve assembly  78 . Piston body  72  is slidingly received within pressure tube  30  with seal  74  being disposed between piston body  72  and pressure tube  30 . Seal  74  permits sliding movement of piston body  72  with respect to pressure tube  30  without generating undue frictional forces as well as sealing upper working chamber  44  from lower working chamber  46 . Compression valve assembly  76  comprises a spring seat  80 , a valve plate  82  and a biasing member or spring  84 . Spring seat  80  abuts shoulder  70 . Valve plate  82  abuts piston body  72  to control fluid flow through a plurality of compression passages  86  extending through piston body  72 . Biasing member  84  is disposed between spring seat  80  and valve plate  82  to bias valve plate  82  against piston body  72  to close compression passages  86 . During a compression stroke, fluid pressure builds up in compression passages  86  until the load exerted by biasing member  84  is overcome. This lifts valve plate  82  from piston body  72  to allow fluid flow through passages  86 . The strength of biasing member  84  is chosen to generate a relatively low damping load during a compression stroke. During a compression stroke, base valve assembly  38  generates the damping force. Extension valve assembly  78  comprises a spring seat  88 , a valve plate  90  and a biasing member or spring  92 . Valve plate  90  abuts piston body  72  to control fluid flow through a plurality of extension passages  94  extending through piston body  72 . Biasing member  92  is disposed between high force piston assembly  64  and spring seat  88  to bias spring seat  88  against valve plate  90  and thus, valve plate  90  against piston body  72  to close compression passages  94 . During an extension stroke, fluid pressure builds up in extension passages  94  until the load exerted by biasing member  92  is overcome. This lifts valve plate  90  from piston body  72  to allow fluid flow through passages  94 . The strength of biasing member  92  is chosen to provide a relatively low damping force characteristic or a soft ride characteristic for shock absorber  20  during the extension stroke. Extension valve assembly  78  also prohibits fluid flow through passages  94  during a compression stroke of shock absorber  20 . 
     High force piston assembly  64  comprises a piston body  102 , a seal  104 , a compression valve assembly  106  and an extension valve assembly  108 . Piston body  102  is slidingly received within pressure tube  30  with seal  104  being disposed between piston body  102  and pressure tube  30 . Seal  104  permits sliding movement of piston body  102  with respect to pressure tube  30  without generating undue frictional forces as well as sealing upper working chamber  44  from lower working chamber  46 . Compression valve assembly  106  comprises a spring seat  110 , a valve plate  112  and a biasing member or spring  114 . Spring seat  110  abuts spacer  62 . Valve plate  112  abuts piston body  102  to control fluid flow through a plurality of compression passages  116  extending through piston body  102 . Biasing member  114  is disposed between spring seat  110  and valve plate  112  to bias valve plate  112  against piston body  102  to close compression passages  116 . During a compression stroke, fluid pressure builds up in compression passages  116  until the load exerted by biasing member  114  is overcome. This lifts valve plate  112  from piston body  102  to allow fluid flow through passages  116 . The strength of biasing member  114  is chosen to provide a relatively high damping force during the compression stroke. Compression valve assembly  106  is designed to prohibit fluid flow through passages  116  during an extension stroke. During a compression stroke, base valve assembly  38  generates the damping force. Extension valve assembly  108  comprises a spring seat  118 , a valve plate  120  and a biasing member or spring  122 . Valve plate  120  abuts piston body  102  to control fluid flow through a plurality of extension passages  124  extending through piston body  102 . Biasing member  122  is disposed between retaining nut  66  and spring seat  118  to bias spring seat  118  against valve plate  120  and thus valve plate  120  against piston body  102  to close compression passages  124 . During an extension stroke, fluid pressure builds up in extension passages  124  until the load exerted by biasing member  122  is overcome. This lifts valve plate  120  from piston body  102  to allow fluid flow through passages  124 . The strength of biasing member  122  is chosen to provide a relatively high damping force characteristic or a firm ride characteristic for shock absorber  20  during the extension stroke. Extension valve assembly  108  also prohibits fluid flow through passages  124  during a compression stroke of shock absorber  20 . 
     FIG. 4 illustrates the relationship between low force piston assembly  60  and the plurality of by-pass indentations  48 . Seal  74  is designed to be axially longer than the axial length of each individual indentation  48 . Thus, when low force piston assembly  60  moves over the plurality of by-pass indentations  48 , there is no change to the fluid flow with respect to low force piston assembly  60 . 
     FIG. 5 illustrates the relationship between high force piston assembly  64  and the plurality of by-pass indentations  48 . Seal  104  is designed to be axially shorter than the axial length of each individual indentation  48 . Thus, when high force piston assembly  64  moves over the plurality of by-pass indentations  48 , fluid flow by-passes high force piston assembly  64  to remove the effect of extension valve assembly  108  and compression valve assembly  106 . The travel of high force piston assembly  64  within the area of the plurality of by-pass indentations  48  essentially converts the twin piston design into a single piston design by allowing the fluid flow to by-pass piston assembly  64  but not bypass low force piston assembly  60 . 
     During an extension stroke of shock absorber  20 , with high force piston assembly  64  being located within the area of indentations  48  as shown in FIG. 5, the damping force is created only by extension valve assembly  76  of low force piston assembly  60 . Because biasing member  92  is designed to provide a relatively low damping force, shock absorber  20  provides a soft ride. The effect of high force piston assembly  64  is negated by the plurality of indentations  48  which allow fluid flow around high force piston assembly  64 . When high force piston assembly  64  moves into an area of pressure tube  30  which does not include the plurality of indentations  48  as shown in FIG. 4, the flow of fluid around piston assembly  64  is prohibited. The damping force for shock absorber  20  is created by extension valve assembly  78  and extension valve assembly  108  in series. Because biasing member  122  of extension valve assembly  108  is designed to provide a relatively high damping force, shock absorber  20  will provide a firm ride. 
     During a compression stroke of shock absorber  20 , with high force piston assembly  64  being located within the area of indentations  48  as shown in FIG. 5, the damping force is created only by compression valve assembly  76  of low force piston assembly  60 . Because biasing member  84  is designed to provide a relatively low damping force, shock absorber  20  provides a soft ride. The effect of high force piston assembly  64  is negated by the plural indentations  48  which allow fluid flow around high force piston assembly  64 . When high force piston assembly  64  moves into an area of pressure tube  30  which does not include the plurality of indentations  48  as shown in FIG. 4, the flow around piston assembly  64  is prohibited. The damping force for shock absorber  20  is created by compression valve assembly  76  and compression valve assembly  106  in series. Because biasing member  114  on compression valve assembly  106  is designed to provide a relatively high damping force, shock absorber  20  will provide a firm ride. 
     Referring now to FIG. 6, a rear shock absorber  220  according to another embodiment of the present invention is illustrated. Similar to shock absorber  20 , shock absorber  220  can replace either rear shock absorber  20  or front shock absorber  26 . Shock absorber  220  comprises pressure tube  30 , a piston assembly  232  and piston rod  34 . 
     Piston assembly  232  is the same as piston assembly  32  except that compression valve assembly  276  replaces compression valve assembly  76  and compression valve assembly  306  replaces compression valve assembly  106 . Compression valve assembly  276  comprises spring seat  80 , valve plate  82  and a biasing member or spring  284 . Compression valve assembly  276  and biasing member  284  are incorporated to provide the damping force during the compression stroke. Compression valve assembly  306  comprises spring seat  110 , valve plate  112  and biasing member or spring  314 . Compression valve assembly  306  and biasing member  314  are incorporated to provide the damping force during the compression stroke. Thus, a two level damping force is generated by compression valve assembly  276  and compression valve assembly  306  in a similar manner to that described above for extension valve assembly  78  and extension valve assembly  108 , respectively. 
     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.