Abstract:
A two-stage shock absorber has a pressure tube within which a piston assembly is slidably disposed. A piston rod is attached to the piston assembly and extends out of the pressure tube. A sleeve is slidably disposed within the pressure tube and engages the piston rod. After a specified amount of movement of the piston assembly with respect to the pressure tube in an extension movement of the shock absorber, the sleeve engages a plurality of spirally positioned bores on the piston rod and reduces the fluid flow through the valve assembly to progressively switch the shock absorber from soft damping to firm damping. In another embodiment the sleeve engages a spiral groove of variable depth on the piston rod to reduce the fluid flow through the valve assembly to progressively switch the shock absorber from soft damping to firm damping. In yet another embodiment of the present invention, the sleeve engages a tapered slot of variable cross-section to reduce the fluid flow through the valve assembly to progressively switch the shock absorber from soft damping to firm damping.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a hydraulic damper or shock absorber adapted for use in a suspension system such as the systems used for automotive vehicles. More particularly, the present invention relates to a hydraulic damper having a two-stage damping characteristic where a relatively low level damping is provided for small amplitudes of movement and a relatively high level of damping is provided for large amplitudes of movement. 
     BACKGROUND OF THE INVENTION 
     A conventional prior art hydraulic damper or shock absorber comprises a cylinder defining a working chamber having a piston slidably disposed in the working chamber with the piston separating the interior of the cylinder into an upper and a lower working chamber. A piston rod is connected to the piston and extends out of one end of the cylinder. A first valving system is incorporated for generating damping force during the extension stroke of the hydraulic damper and a second valving system is incorporated for generating damping force during the compression stroke of the hydraulic damper. 
     Various types of damping force generating devices have been developed to generate desired damping forces in relation to 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 extended suspension movements. The normal running of the vehicle is accompanied by small or fine vibrations of the un-sprung 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 stable handling characteristics to the vehicle. Thus, these multi-force damping force generating devices offer the advantage of a smooth steady state ride by eliminating the high frequency/small excitations from the sprung mass while still providing the necessary damping or firm ride for the suspension system during vehicle maneuvers causing larger excitations of 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 damping which varies according to the stroke amplitude. Soft damping is provided for small strokes and firm damping is provided for large strokes. The variable damping is provided by a sliding sleeve that is frictionally held in place in the pressure cylinder. When the shock absorber undergoes a small stroke, the sliding sleeve remains inactive and the fluid flows through two separate flow paths to provide a soft damping. When the shock absorber undergoes a large stroke, the sliding sleeve moves to progressively close off one of the two flow paths which in turn provides a firm damping. Various design iterations are disclosed for both mono-tube and double tube shock absorbers. 
     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 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; 
         FIG. 2  is an enlarged cross-sectional side view illustrating the piston assembly of the shock absorber shown in  FIG. 1  during a small extension stroke of the shock absorber; 
         FIG. 3  is an enlarged cross-sectional side view illustrating the piston assembly of the shock absorber shown in  FIG. 1  during a larger extension stroke of the shock absorber; 
         FIG. 4  is an enlarged cross-sectional side view illustrating the piston assembly of the shock absorber shown in  FIG. 1  during an even larger extension stroke of the shock absorber; 
         FIG. 5  is an enlarged cross-sectional side view illustrating the piston assembly of the shock absorber shown in  FIG. 1  during a small compression stroke of the shock absorber; 
         FIG. 6  is an enlarged cross-sectional side view illustrating the piston assembly of the shock absorber shown in  FIG. 1  during a large compression stroke of the shock absorber; 
         FIG. 7  is an enlarged view of the metering slot shown in  FIGS. 1-6 ; 
         FIG. 8  is an enlarged cross-sectional side view similar to  FIG. 7  but illustrating a metering system in accordance with another embodiment of the present invention; 
         FIG. 9  is an enlarged cross-sectional side view similar to  FIG. 8  but illustrating a metering system in accordance with another embodiment of the present invention; 
         FIG. 10  is an enlarged cross-sectional side view similar to  FIG. 2  but illustrating a piston valve assembly in accordance with another embodiment of the present invention; 
         FIG. 11  is an enlarged view of a sleeve incorporating a metering system in accordance with another embodiment of the present invention; 
         FIG. 12  is an enlarged view of a sleeve incorporating a metering system in accordance with another embodiment of 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 the like reference numerals designate like or corresponding parts throughout the several views, there is shown in  FIG. 1  a two-stage mono-tube shock absorber which incorporates the multi-force damping force generating device in accordance with the present invention and which is designated generally by the reference numeral  10 . Shock absorber  10  is a mono-tube design and comprises a piston rod assembly  12  and a pressure tube  14 . Piston rod assembly  12  includes a piston valve assembly  16  and a piston rod  18 . Valve assembly  16  divides pressure tube  14  into an upper working chamber  20  and a lower working chamber  22 . Piston rod  18  extends out of pressure tube  14  and includes a fitting  24  for attachment to one of the sprung or unsprung mass of the vehicle. Pressure tube  14  is filled with fluid and includes a fitting  26  for attachment to the other of the sprung or unsprung masses of the vehicle. Thus, suspension movements of the vehicle will cause extension or compression movement of piston rod assembly  12  with respect to pressure tube  14  and these movements will be dampened due to the restricted fluid flow between working chambers  20  and  22  through piston valve assembly  16 . 
     Referring now to  FIG. 2 , piston valve assembly  16  is attached to piston rod  18  and comprises a piston body  40 , a compression valve assembly  42 , an extension or rebound valve assembly  44  and a sliding valve assembly  46 . Piston rod  18  includes a reduced diameter section  48  located on the end of piston rod  18  disposed within pressure tube  14  to form a shoulder  50  for mounting the remaining components of piston valve assembly  16 . Piston body  40  is located on reduced diameter section  48  with compression valve assembly  42  being located between piston body  40  and shoulder  50  and with rebound valve assembly  44  being located between piston body  40  and a threaded end  52  of piston rod  18 . A retaining nut  54  maintains the assembly of these components. Piston body  40  defines a plurality of compression flow passages  56  and a plurality of rebound flow passages  58 . 
     Compression valve assembly  42  comprises a compression valve plate  60 , a compression support plate  62  and a compression spring  64 . Valve plate  60  is disposed adjacent to piston body  40  to cover the plurality of compression flow passages  56 . Support plate  62  is disposed adjacent shoulder  50  and compression spring  64  is disposed between valve plate  60  and support plate  62  to hold valve plate  60  against piston body  40  to close passages  56 . During a compression stroke of shock absorber  10 , fluid pressure builds up in lower working chamber  22  until the fluid pressure applied to valve plate  60  overcomes the load exerted on valve plate  60  by compression spring  64 . Compression spring  64  will compress to allow compression support plate  62  to unseat from piston body  40  to allow fluid flow from lower working chamber  22  to upper working chamber  20  through compression flow passages  56  as shown by the arrows  34  in  FIGS. 5 and 6 . 
     Rebound valve assembly  44  comprises a plurality of valve plates  68 , a rebound support plate  70  and piston nut  54 . Valve plates  68  are disposed adjacent to piston body  40  to cover the plurality of rebound flow passages  58 . Support plate  70  is disposed between piston nut  54  and valve plates  68 . Piston nut  54  is threaded onto end  52  of piston rod  18  to retain support plate  70  and hold valve plates  68  against piston body  40  to close passages  58 . During an extension stroke of shock absorber  10 , fluid pressure builds up in upper working chamber  20  until the fluid pressure applied to valve plates  68  through passages  58  overcomes the bending load of valve plates  68 . Valve plates  68  elastically deflect around the outer edge of support plate  70  to allow fluid to flow from upper working chamber  20  to lower working chamber  22  as shown by arrows  72  in  FIGS. 2-4 . 
     Sliding valve assembly  46  comprises a flow passage  74 , a metering slot  76  and a sliding sleeve  78 . Flow passage  74  extends through piston rod  18  and includes a radial passage  80  and an axial passage  82  which opens into lower working chamber  22 . Metering slot  76  includes a tapered slot  88  extending axially along the outer surface of piston rod  18 . Sliding sleeve  78  is slidingly received within pressure tube  14  and slidingly received on piston rod  18  to provide the multi-stage damping characteristics for shock absorber  10 . 
       FIGS. 2 through 6  illustrate the various damping characteristics provided for by piston rod assembly  12  of shock absorber  10 .  FIG. 2  illustrates a small amplitude extension,  FIG. 3  illustrates a larger amplitude extension,  FIG. 4  illustrates an even larger amplitude extension,  FIG. 5  illustrates a small amplitude compression and  FIG. 6  illustrates a large amplitude compression for shock absorber  10 . 
     A small amplitude extension of shock absorber  10  is illustrated in  FIG. 2  with arrows  72  and  92  depicting the fluid flow. During small amplitudes of extension, sliding sleeve  78  will only move a small amount with respect to piston rod  18  due to the friction with pressure tube  14  and it does not restrict fluid flow through passage  74  and metering slot  76 . Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  occurs through two generally parallel paths. The first path is numbered  72  and extends from upper working chamber  20  of pressure tube  14  through passages  58  unseating valve plates  68  from piston body  40  to enter lower working chamber  22  of pressure tube  14 . Simultaneously, fluid flows through the second flow path as depicted by arrows  92 . Fluid flow leaves upper working chamber  20  through passage  74  metering slot  76  and through to also enter lower working chamber  22  of pressure tube  14 . These dual parallel flow paths  72  and  92 , will thus provide a relatively soft ride for small movements of shock absorber  10 . 
     A larger amplitude extension of shock absorber  10  is illustrated in  FIG. 3  with arrows  72  and  92  depicting fluid flow. During the larger amplitudes of extension, sliding sleeve  78  will move enough to cover a portion of passage  74  and possibly a portion of tapered slot  88  due to the friction with pressure tube  14  and will begin progressively closing fluid passage  74 . As shown in  FIGS. 3 and 7 , tapered slot  88  of metering slot  76  permits a gradual or progressive closing of fluid passage  74  which provides the advantage of the major reduction or elimination of the switching noise typical with a dual-stage damping device. Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  still occurs through two generally parallel paths but the second path is progressively being closed off as a function of the amplitude of the stroke. The shape of tapered slot  88  thus provides the shock absorber designer the option of defining the curve between the soft damping characteristics of shock absorber  10  and the firm damping characteristics of shock absorber  10  and no longer requires him to accept a step function. First path  72  extends from upper working chamber  20  of pressure tube  14  through passage  58  unseating valve plates  66  from piston body  40  to enter lower working chamber  22  of pressure tube  14 . Simultaneously, fluid flow through second flow path  92  by leaving upper working chamber  20  through metering slot  76  and through passage  74  to also enter lower working chamber  22  of pressure tube  14 . The amount of fluid flowing through second flow path  92  will be determined by the position of sliding sleeve  78  with respect to tapered slot  88  and the design of tapered slot  88 . 
     An even larger amplitude extension of shock absorber  10  is illustrated in  FIG. 4  with arrows  72  depicting fluid flow. During large amplitudes of extension, sliding sleeve  78  remains in position due to friction and entirely covers passage  74  and tapered slot  88 . Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  occurs through only one path which is path  72 . As stated above, path  72  extends from upper working chamber  20  of pressure tube  14  through passages  58  unseating valve plates  66  from piston body  40  to enter lower working chamber  22  of pressure tube  14 . Flow path  92 , shown in  FIGS. 2 and 3 , is blocked due to the position of sliding sleeve  78 . The single flow path will thus provide a relatively firm ride for larger movements of shock absorber  10 . 
     A small amplitude compression of shock absorber  10  is illustrated in  FIG. 5  with arrows  34  and  94  depicting the fluid flow. During small amplitudes of compression, sliding sleeve  78  will move only a small amount with respect to piston rod  18  due to the friction with pressure tube  14 . Fluid flow from lower working chamber  22  of pressure tube  14  into upper working chamber  20  of pressure tube  14  occurs through two generally parallel paths. The first path is numbered  34  and extends from lower working chamber  22  of pressure tube  14  through passages  56  unseating valve plate  60  from piston body  40  to enter upper working chamber  20  of pressure tube  14 . Simultaneously, fluid flows through a second flow path as depicted by arrows  94 . Fluid flow leaves lower working chamber  22  through passage  74  and through metering slot  76  to enter upper working chamber  20  of pressure tube  14 . 
     A large amplitude compression of shock absorber  10  is illustrated in  FIG. 6  with arrows  34  and  94  depicting fluid flow. During large amplitudes of compression, sliding sleeve  78  remains in position due to friction and a retaining ring  96  contacts sliding sleeve  78 . Fluid flow from the lower working chamber  22  of pressure tube  14  into upper working chamber  20  of pressure tube  14  occurs through the same two flow paths described above for small compression movement soft shock absorber  10  as shown in  FIG. 5 . The multi-force damping characteristics for shock absorber  10  of this embodiment only effect extension movement of shock absorber  10  and not compression movements. 
     Referring now to  FIG. 8 , a piston rod  118  in accordance with another embodiment of the present invention is illustrated. Piston rod  118  is designed to replace piston rod  18  in shock absorber  10  and thus the discussion above of shock absorber  10  also applies to piston rod  118 . The difference between piston rod  118  and piston rod  18  is in the manner that fluid flows through passage  74 . 
     Piston rod  118  defines a series of bores  186  extending radially through piston rod  118  to open into passage  74 . The series of bores  186  are positioned or created in a helical pattern which extends axially along piston rod  118 . Sliding sleeve  78  is slidingly received within pressure tube  14  and slidingly received on piston rod  118 , similar to piston rod  18 , to provide the multi-stage damping characteristics for shock absorber  10 . 
     During small amplitudes extensions of shock absorber  10 , sliding sleeve  78  will move only a small amount with respect to piston rod  118  due to the friction with pressure tube  14  and thus it does not restrict fluid flow through passage  74  and all of bores  186 . The fluid flow is similar to that shown in  FIG. 2  for piston rod  18 . 
     During larger amplitude extensions of shock absorber  10 , sliding sleeve  78  will move enough to cover one or more of bores  186  due to the friction with pressure tube  14  and it will progressively close more and more of bores  186  as it moves axially along piston rod  118 . Similar to that shown in  FIG. 3 , the helical series of spaced bores  186  will permit a gradual closing of the entire passage  74  which provides the advantage of the major reduction or elimination of the switching noise which occurs between soft and firm damping characteristics in a dual-stage damping device. Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  still occurs through the two generally parallel flow paths shown by arrows  72  and  92  but the second flow path shown by arrow  92  is progressively being closed off as a function of the amplitude of the stroke. The variable helical pattern of bores  186  thus provides the shock absorber designer the option of defining the curve between the soft damping characteristics of shock absorber  10  and the firm damping characteristics of shock absorber  10  and no longer requires him to accept a step function. The first path shown by arrow  72  extends from upper working chamber  20  of pressure tube  14  through passages  58  unseating valve plates  66  from piston body  40  to enter lower working chamber  22  of pressure tube  14 . Simultaneously, fluid flows through the second flow path shown by arrow  92  by leaving upper working chamber  20  through one or more of bores  186  and through passage  74  to also enter lower working chamber  22  of pressure tube  14 . The amount of fluid flowing through the second flow path shown by arrow  92  will be determined by the position of sliding sleeve  78  and the number of bores  186  which sliding sleeve  78  covers. 
     During even larger amplitude extensions of shock absorber  10 , sliding sleeve  78  will move enough to cover all of bores  186 . Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  occurs only through the first flow path depicted by arrow  72 . This single flow path will thus provide a relatively firm ride. The fluid flow is similar to that shown in  FIG. 4  for piston rod  18 . 
     Small amplitude compression and large amplitude compression of shock absorber  10  is the same as that illustrated above in  FIGS. 5 and 6  for piston rod  18 , respectively. During all compression strokes for shock absorber  10 , all bores  186  are open providing for the dual path fluid flow depicted by arrows  34  and  94 . 
     Referring now to  FIG. 9 , a piston rod  218  in accordance with another embodiment of the present invention is illustrated. Piston rod  218  is designed to replace piston rod  18  in shock absorber  10  and thus the discussion above of shock absorber  10  also applies to piston rod  218 . The difference between piston rod  218  and piston rod  18  is in the manner that the fluid flows through passage  74 . 
     Piston rod  218  defines a helical groove  188  extending axially along the outer surface of piston rod  218 . Helical groove  188  has a depth that varies continuously over the length of helical groove  188 . The depth of helical groove  188  is at its maximum value adjacent passage  74  and at its minimum value at its opposing terminal end. Sliding sleeve  78  is slidingly received within pressure tube  14  and slidingly received on piston rod  218 , similar to piston rod  18 , to provide the multi-stage damping characteristics for shock absorber  10 . 
     During small amplitude extensions of shock absorber  10 , sliding sleeve  78  will only move a small amount with respect to piston rod  218  due to the friction with pressure tube  14  and thus does not restrict fluid flow through groove  188  and passage  74 . The fluid flow is similar to that shown in  FIG. 2  for piston rod  18 . 
     During larger amplitude extensions of shock absorber  10 , sliding sleeve  78  will move enough to cover a portion of groove  188 . The movement of sliding sleeve  78  with respect to piston rod  218  will cover more and more of groove  188 . Fluid flow will flow from upper working chamber  20  though groove  188  through passage  74  and into lower working chamber  22 . The continuously varying depth of groove  188  will permit a gradual closing of the entire passage  74  which provides the advantage of the major reduction or elimination of the switching noise typical with a dual-stage damping device. Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  sill occurs through the two generally parallel paths depicted by arrows  72  and  92  but the second path depicted by arrow  92  is progressively being closed off as a function of the amplitude of the stroke. The variable depth of groove  188  thus provides the shock absorber designer the option of defining the curve between the soft damping characteristics of shock absorber  10  and the firm damping characteristics of shock absorber  10  and no longer requires him to accept a step function. The fluid flow is similar to that shown in  FIG. 3  for piston rod  18 . 
     Even larger amplitude extensions of shock absorber  10  causes sliding sleeve  78  to cover all of groove  188  to close fluid passage  74 . Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  occurs only through the path depicted by arrows  72 . This single flow path will thus provide a relatively firm ride. The fluid flow is similar to that shown in  FIG. 4  for piston rod  18 . 
     Small amplitude compression and large amplitude compression of shock absorber  10  is similar to that illustrated above in  FIGS. 5 and 6 , respectively, for piston rod  18 . During compression strokes for shock absorber  10 , groove  188  is open providing for the dual path fluid flow as depicted by arrows  34  and  94 . The fluid flow is the same as that shown in  FIGS. 5 and 6  for piston rod  18 . 
     Referring now to  FIG. 10 , a piston valve assembly in accordance with another embodiment of the present invention is illustrated and indicated generally by the reference numeral  316 . Piston valve assembly  316  is designed for a dual tube shock absorber  310  and is attached to a piston rod  318 . As is well known in the art, a dual tube shock absorber includes reservoir tube  320  surrounding pressure tube  14  to form a reservoir chamber  322 . A base valve assembly (not shown) is disposed between lower working chamber  22  and reservoir chamber  322 . Piston valve assembly  316  comprises a piston body  340 , a compression check valve assembly  342 , an extension or rebound valve assembly  344  and a sliding valve assembly  346 . Piston body  340  is located on a reduced diameter section  348  with compression check valve assembly  342  being located between piston body  340  and a shoulder  350  and with rebound valve assembly  344  being located between piston body  340  and a threaded end  352  of piston rod  318 . Retaining nut  54  maintains the assembly of these components. Piston body  340  defines a plurality of compression flow passages  356  and a plurality of rebound flow passages  358 . 
     Compression check valve assembly  342  comprises a compression valve plate  360 , a compression support plate  362  and a compression valve spring  364 . Valve plate  360  is disposed adjacent to piston body  340  to cover the plurality of compression flow passages  356 . Support plate  362  is disposed adjacent to shoulder  350  and valve spring  364  is disposed between support plate  362  and valve plate  360  to hold valve plate  360  against piston body  340  to close passages  356 . During a compression stroke of the shock absorber, fluid pressure builds up in lower working chamber  22  until the fluid pressure applied to valve plate  360  through passages  356  overcomes the load being exerted by valve spring  364  opening passages  356  to allow fluid to flow from lower working chamber  22  to upper working chamber  20 . 
     The flow of fluid through compression check valve assembly does not create a damping load for dual tube shock absorber  310 , it is designed to replace hydraulic fluid within upper working chamber  20  due to the movement of piston valve assembly  316 . The damping characteristics for dual tube shock absorber  310  is provided by a compression valve assembly (not shown) located in the base valve assembly of shock absorber  310  as is well known in the art. 
     Rebound valve assembly  344  comprises a plurality of valve plates  366 , a rebound support plate  368  and piston nut  54 . Valve plates  366  are disposed adjacent to piston body  340  to cover the plurality of rebound flow passages  358 . Support plate  368  is disposed between piston nut  54  and valve plates  366 . Piston nut  54  is threaded onto end  352  of piston rod  318  to retain support plate  368  and hold valve plates  366  against piston body  340  to close passages  358 . During an extension stroke of the shock absorber, fluid pressure builds up in upper working chamber  20  until the fluid pressure applied to valve plates  366  overcomes the bending load of valve plates  366 . Valve plates  366  elastically deflect around the outer edge of support plate  368  to allow fluid to flow from upper working chamber  20  to lower working chamber  22 . 
     Rebound valve assembly  344  provides the damping characteristics for shock absorber  310  during an extension stroke. As is well known in the art, a rebound check valve assembly (not shown) is located in the base valve assembly of shock absorber  310  to replace hydraulic fluid within lower working chamber  22  during an extension stroke. 
     Sliding valve assembly  346  is illustrated in conjunction with dual tube shock absorber  310 . It is within the scope of the present invention to replace sliding valve assembly  46  with sliding valve assembly  346  for shock absorber  10 . Sliding valve assembly  346  comprises a flow passage  374 , a collar  376  and sliding sleeve  78 . Flow passage  374  extends through piston rod  18  and includes a radial passage  380  and an axial passage  382 . Radial passage  380  opens into a groove  384  formed in piston rod  318  and axial passage  382  opens into lower working chamber  22 . Collar  376  is located on a reduced diameter section  386  of piston rod  318 . Collar  376  defines a tapered slot  388  and a bore  390 . Bore  390  is aligned with groove  384  of piston rod  318  such that bore  390  is always in communication with flow passage  374  through groove  384 . Sliding sleeve  78  is slidingly received within pressure tube  14  and slidingly received on collar  376  to provide the multiple stage damping characteristics for shock absorber  310 . 
     The various damping characteristics for shock absorber  310  are similar to those shown in  FIGS. 2 through 6  for shock absorber  10  except that collar  376  defines tapered slot  388  for shock absorber  310  whereas piston rod  18  defined tapered slot  88  for shock absorber  10 . By utilizing collar  376 , it simplifies the manufacturing of the tapered slot, it allows for the use of a common piston rod for multiple applications and it allows for changing the design for the fluid flow system. 
     Similar to  FIG. 2 , during small amplitude extension movements, sliding sleeve  78  will only move a small amount with respect to collar  376  due to the friction with pressure tube  14  and thus does not restrict fluid flow through passage  374  and bore  390 . Fluid flow from upper working chamber  20  into lower working chamber  22  occurs through two paths. The first path extends from upper working chamber  20  through passages  358  unseating valve plates  366  from piston body  340  to enter lower working chamber  22 . Simultaneously, fluid flows through the second flow path which extends from upper working chamber  20 , through bore  390 , through groove  384  and through passage  374  to also enter lower working chamber  22 . These dual parallel flow paths will provide a relatively soft ride for small movements of shock absorber  310 . 
     Similar to  FIG. 3 , during larger amplitudes of extension, sliding sleeve  78  will move enough with respect to collar  376  to cover a portion of bore  390  and possibly a portion of tapered slot  388  due to the friction with pressure tube  14 . This movement will progressively close fluid passage  374  due to tapered slot  388 . Tapered slot  388  permits a gradual or progressive closing of fluid passage  374  which provides the advantage of a major reduction or elimination of the switching noise typical with a dual-stage damping device. Fluid flow from upper working chamber  20  into lower working chamber  22  occurs through the same two paths, but the second flow path is progressively being closed off as a function of the amplitude of the extension stroke. The shape of tapered slot  388  thus provides the shock absorber designer the option of defining the curve between the soft damping characteristics and the firm damping characteristics for shock absorber  310  and no longer requires him to accept a step function. The first path extends from upper working chamber  20  through passages  358  unseating valve plates  366  from piston body  340  to enter lower working chamber  22 . Simultaneously, fluid flows through the second flow path which extends from upper working chamber  20 , through bore  390 , through groove  384  and through passage  374  to also enter lower working chamber  22 . The amount of fluid flowing through the second flow path will be determined by the position of sliding sleeve  78 . 
     Similar to  FIG. 4 , during large amplitudes of extension, sliding sleeve  78  remains in its position due to friction with pressure tube  14  and sliding sleeve  78  entirely covers bore  390  and tapered slot  388 . This closes the second fluid path and fluid flow between upper working chamber  20  and lower working chamber  22  will occur only through the first fluid path. Fluid will flow from upper working chamber  20 , through passages  358  unseating valve plates  366  from piston body  340  to enter lower working chamber  22 . The flow of fluid through only this single flow path will provide a relatively firm damping characteristic for shock absorber  310 . 
     Similar to  FIG. 5 , during small amplitude compression movements sliding sleeve  78  will move only a small amount with respect to collar  376  due to friction with pressure tube  14 . Fluid flow between lower working chamber  22  and upper working chamber  20  occurs through two generally parallel flow paths. The first flow path extends from lower working chamber  22 , through passages  356  unseating valve plate  360  from piston body  340  to enter upper working chamber  20 . Simultaneously, fluid flows through a second flow path. Fluid flows from lower working chamber  22 , through passage  74 , through groove  384 , through bore  390  and into upper working chamber  20 . 
     Similar to  FIG. 6 , during large amplitudes of compression, sliding sleeve  78  remains in position due to friction with pressure tube  14  and retainer  96  contacts sliding sleeve  78 . Fluid flow between lower working chamber  22  to upper working chamber  20  occurs through the same two flow paths described above for small compression movements. The multi-force damping characteristics for shock absorber  310  only effect extension movement of shock absorber  310 . 
     Referring now to  FIG. 11 , a collar  476  in accordance with another embodiment of the present invention is illustrated. Collar  476  is designed to replace collar  376  and thus the discussion above regarding shock absorber  310  also applies to sleeve  476 . The difference between collar  376  and collar  476  is that bore  390  and tapered slot  388  have been replaced with a series of bores  486  extending radially through collar  476  to open into groove  384  and thus passage  374 . The series of bores  486  are positioned or created in a helical pattern which extends axially along collar  476 . The axial length of groove  384  has to be large enough so that all of bores  486  communicate with groove  384 . Sliding sleeve  78  is slidingly received within pressure tube  14  and slidingly received on collar  476 , similar to collar  376 , to provide multi-stage damping characteristics for shock absorber  310 . 
     During small amplitudes extensions of shock absorber  310 , sliding sleeve  78  will move only a small amount with respect to collar  476  due to the friction with pressure tube  14  and thus it does not restrict fluid flow through passage  374 , groove  384  and all of bores  486 . The fluid flow is similar to that shown in  FIG. 2  for piston rod  18 . 
     During larger amplitude extensions of shock absorber  310 , sliding sleeve  78  will move enough to cover one or more of bores  486  due to the friction with pressure tube  14  and it will progressively close more and more of bores  486  as it moves axially along collar  476 . Similar to that shown in  FIG. 3 , the helical series of spaced bores  486  will permit a gradual closing of the entire passage  374  which provides the advantage of the major reduction or elimination of the switching noise which occurs between soft and firm damping characteristics in a dual-stage damping device. Fluid flow from upper working chamber  20  of pressure tube  14  into lower working chamber  22  of pressure tube  14  still occurs through the two generally parallel flow paths but the second flow is progressively being closed off as a function of the amplitude of the stroke. The variable helical pattern of bores  486  thus provides the shock absorber designer the option of defining the curve between the soft damping characteristics of shock absorber  310  and the firm damping characteristics of shock absorber  310  and no longer requires him to accept a step function. The first path extends from upper working chamber  20  through passages  358  unseating valve plates  366  from piston body  340  to enter lower working chamber  22 . Simultaneously, fluid flows through the second flow path by leaving upper working chamber  20  through one or more of bores  486 , through groove  384  and through passage  374  to also enter lower working chamber  22 . The amount of fluid flowing through the second flow path will be determined by the position of sliding sleeve  78  and the number of bores  486  which sliding sleeve  78  covers. 
     During even larger amplitude extensions of shock absorber  310 , sliding sleeve  378  will move enough to cover all of bores  486 . Fluid flow from upper working chamber  20  into lower working chamber  22  of pressure tube  14  occurs only through the first flow path. This single flow path will thus provide a relatively firm ride. The fluid flow is similar to that shown in  FIG. 4  for piston rod  18 . 
     Small amplitude compression and large amplitude compression of shock absorber  10  is the same as that illustrated above in  FIGS. 5 and 6  for piston rod  18 , respectively. During all compression strokes for shock absorber  10 , all bores  486  are open providing for the dual path fluid flow. 
     Referring now to  FIG. 12 , a collar  576  in accordance with another embodiment of the present invention is illustrated. Collar  576  is also designed to replace collar  376  and thus the discussion above regarding shock absorber  310  also applies to collar  576 . The difference between collar  376  and collar  576  is that bore  390  and tapered slot  388  have been replaced with a helical groove  588  and a bore  590 . Helical groove  588  extend axially along the outer surface of collar  576 . Helical groove  588  has a depth that varies continuously over the length of helical groove  588 . The depth of helical groove  588  is at its maximum valve adjacent bore  590  which provides communication between groove  588  and groove  384  and thus passage  374 . Sliding sleeve  78  is slidingly received within pressure tube  14  and slidingly received on collar  576 , similar to collar  376 , to provide the multi-stage damping characteristics for shock absorber  310 . 
     During small amplitude extensions of shock absorber  310 , sliding sleeve  78  will only move a small amount with respect to collar  586  due to the friction with pressure tube  14  and thus does not restrict fluid flow through groove  588 , bore  590 , groove  384  and passage  374 . The fluid flow is similar to that shown in  FIG. 2  for piston rod  18 . 
     During larger amplitude extensions of shock absorber  310 , sliding sleeve  78  will move enough to cover a portion of bore  590  and possibly a portion of groove  588 . The movement of sliding sleeve  78  with respect to collar  576  will cover more and more of groove  588 . Fluid flow will flow from upper working chamber  20  though groove  588 , bore  490 , groove  384  and through passage  374  and into lower working chamber  22 . The continuously varying depth of groove  588  will permit a gradual closing of the entire passage  374  which provides the advantage of the major reduction or elimination of the switching noise typical with a dual-stage damping device. Fluid flow from upper working chamber  20  into lower working chamber  22  still occurs through the two generally parallel paths but the second path is progressively being closed off as a function of the amplitude of the stroke. The variable depth of groove  588  thus provides the shock absorber designer the option of defining the curve between the soft damping characteristics of shock absorber  310  and the firm damping characteristics of shock absorber  310  and no longer requires him to accept a step function. The fluid flow is similar to that shown in  FIG. 3  for piston rod  18 . 
     Even larger amplitude extensions of shock absorber  310  causes sliding sleeve  78  to cover all of groove  588  to close fluid passage  374 . Fluid flow from upper working chamber  20  into lower working chamber  22  occurs only through the first fluid path. This single flow path will thus provide a relatively firm ride. The fluid flow is similar to that shown in  FIG. 4  for piston rod  18 . 
     Small amplitude compression and large amplitude compression of shock absorber  310  is similar to that illustrated above in  FIGS. 5 and 6 , respectively, for piston rod  18 . During compression strokes for shock absorber  310 , groove  588  and bore  590  are open providing for the dual path fluid. The fluid flow is the same as that shown in  FIGS. 5 and 6  for piston rod  18 . 
     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.