Abstract:
A two-stage shock absorber has a pressure tube within which a valve assembly is slidably disposed. A piston rod is attached to the valve assembly and extends out of the pressure tube. A ring is slidably disposed within the pressure tube and engages the valve assembly. After a specified amount of movement of the valve assembly with respect to the pressure tube in an extension movement of the shock absorber, the sleeve engages a metered slot and reduces 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 monotube and double tube shock absorbers. 
     Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which illustrate the best mode presently contemplated for carrying out the present invention: 
     FIG. 1 is cross-sectional side view of a monotube 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. 2 but illustrating a piston valve assembly in accordance with another embodiment of the present invention; 
     FIG. 9 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; and 
     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. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 two-stage monotube 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 monotube 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 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 . Piston body  40  defines a plurality of compression flow passages  54  and a plurality of rebound flow passages  56 . 
     Compression valve assembly  42  comprises a plurality of compression valve plates  58  and a compression support plate  60  . Valve plates  58  are disposed adjacent to piston body  40  to cover the plurality of compression flow passages  54 . Support plate  60  is disposed between valve plates  58  and shoulder  50  to hold valve plates  58  against piston body  40  to close passages  54 . During a compression stroke of shock absorber  10 , fluid pressure builds up in lower working chamber  22  until the fluid pressure applied to valve plates  58  through passages  54  overcomes the bending load of valve plates  58 . Valve plates  58  elastically deflect around the outer edge of support plate  60  to allow fluid to flow from lower working chamber  22  to upper working chamber  20  as shown by arrows  62  in FIGS. 5 and 6. 
     Rebound valve assembly  44  comprises a plurality of valve plates  66 , a rebound support plate  68  and a piston nut  70 . Valve plates  66  are disposed adjacent to piston body  40  to cover the plurality of rebound flow passages  56 . Support plate  68  is disposed between piston nut  70  and valve plates  66 . Piston nut  70  is threaded onto end  52  of piston rod  18  to retain support plate  68  and hold valve plates  66  against piston body  40  to close passages  56 . 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  66  through passages  56  overcomes the bending load of valve plates  66 . Valve plates  66  elastically deflect around the outer edge of support plate  68  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 a chamber  84  defined by piston rod  18  and piston nut  70 . Metering slot  76  includes a bore  86  extending radially through piston nut  70  and a tapered slot  88  extending axially along the outer surface of piston nut  70 . Sliding sleeve  78  is slidingly received within pressure tube  14  and slidingly received on piston nut  70  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 nut  70  due to the friction with pressure tube  14  and does not restrict fluid flow through passage  74  and slot  76 . Fluid flow from upper chamber  20  of pressure tube  14  into lower chamber  22  of pressure tube  14  occurs through two generally parallel paths. The first path is numbered  72  and extends from upper chamber  20  of pressure tube  14  through passages  56  unseating valve plates  66  from piston body  40  to enter lower 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  and enters chamber  84 . Fluid flows from chamber  84  through metering slot  76  to also enter lower 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 bore  86  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 chamber  20  of pressure tube  14  into lower 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 chamber  20  of pressure tube  14  through passages  56  unseating valve plates  66  from piston body  40  to enter lower chamber  22  of pressure tube  14 . Simultaneously, fluid flow through second flow path  92  by leaving upper working chamber  20  through passage  74  and enters chamber  84 . Fluid flows from chamber  84  through bore  86  and through tapered slot  88  to also enter 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 slot  88  and the design of 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 bore  86  and slot  88 . Fluid flow from upper chamber  20  of pressure tube  14  into lower chamber  22  of pressure tube  14  occurs through only one path which is path  72 . As stated above, path  72  extends from upper chamber  20  of pressure tube  14  through passages  56  unseating valve plates  66  from piston body  40  to enter lower 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  62  and  94  depicting the fluid flow. During small amplitudes of compression, sliding sleeve  78  will move only a small amount with respect to piston nut  70  due to the friction with pressure tube  14 . Fluid flow from lower chamber  22  of pressure tube  14  into upper chamber  20  of pressure tube  14  occurs through two generally parallel paths. The first path is numbered  62  and extends from lower chamber  22  of pressure tube  14  through passages  54  unseating valve plate  58  from piston body  40  to enter upper chamber  20  of pressure tube  14 . Simultaneously, fluid flows through a second flow path as depicted by arrows  94 . Fluid flow leaves lower chamber  22  through metering slot  76  into chamber  84  and through passage  74  to enter upper chamber  20  of pressure tube  14 . 
     A large amplitude compression of shock absorber  10  is illustrated in FIG. 6 with arrows  62  and  94  depicting fluid flow. During large amplitudes of compression, sliding sleeve  78  remains in position due to friction and valve plates  66  contact sliding sleeve  78 . Fluid flow from the lower chamber  22  of pressure tube  14  into upper 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 valve assembly in accordance with another embodiment of the present invention is illustrated and indicated generally by the reference numeral  116 . Piston valve assembly  116  is the same as piston valve assembly  16  except for the incorporation of a check valve assembly  190 . Check valve assembly  190  comprises a check ball  192  and a biasing spring  194 . Biasing spring  194  urges check ball  192  against a valve seat  196  formed at the lower end of axial passage  82 . Check valve assembly  190  functions during a compression stroke to prohibit fluid flow through passage  74 . Thus, only the fluid flow depicted by arrows  62  is allowed. Fluid flow depicted by arrows  94  in FIGS. 5 and 6 is prohibited. Thus, the stroke dependency and fluid flow through passage  74  are ineffective during the compression stroke with check valve assembly  190  opening in the extension or rebound stroke. 
     Referring now to FIG. 9, a piston valve assembly in accordance with another embodiment of the present invention is illustrated and indicated generally by the reference numeral  216 . Piston valve assembly  216  is the same as piston valve assembly  16  except for the incorporation of a check valve assembly  290 . Check valve assembly  290  comprises check ball  192  and biasing spring  194 . Biasing spring  194  urges check ball  192  against a valve seat  296  formed at the upper end of an axial passage  298  extending through the bottom of piston nut  70 . Check valve assembly  290  functions during a compression stroke to limit fluid flow through passage  74  to the fluid flow through metering slot  76  until a prespecified fluid pressure is reached in lower working chamber  22 . Thus, until the pre-specified pressure is reached, the fluid flow depicted by arrows  62  and  94  is allowed. Fluid flow depicted by arrows  94  is limited to the flow through metering slot  76 . Once the pre-specified pressure in lower working chamber  22  is achieved, fluid flows through passage  298  thus increasing the fluid flow through passage  74 , thus increasing the fluid flow depicted by arrows  94 . 
     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 and is attached to piston rod  18 . Piston valve assembly  316  comprises a piston body  340 , a compression valve assembly  342 , an extension or rebound valve assembly  344  and sliding valve assembly  46 . Piston body  340  is located on reduced diameter section  48  with compression valve assembly  342  being located between piston body  340  and shoulder  50  and with rebound assembly  344  being located between piston body  340  and threaded end  52  of piston rod  18 . Piston body  340  defines a plurality of compression flow passages  354  and a plurality of rebound flow passages  356 . 
     Compression valve assembly  342  comprises a compression valve plate  358 , a compression support plate  360  and a compression valve spring  361 . Valve plate  358  is disposed adjacent to piston body  340  to cover the plurality of compression flow passages  354 . Support plate  360  is disposed adjacent to shoulder  50  and valve spring  361  is disposed between support plate  360  and valve plate  358  to bias valve plate  358  against piston body  340  to close passages  354 . 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  358  through passages  354  overcomes the load being exerted by valve spring  361  opening passages  354  to allow fluid to flow from lower working chamber  22  to upper working chamber  20  as shown by arrows  62  in FIG.  10 . 
     Rebound valve assembly  344  comprises a plurality of valve plates  366 , a rebound support plate  368  and piston nut  70 . Valve plates  366  are disposed adjacent to piston body  340  to cover the plurality of rebound flow passages  356 . Support plate  368  is disposed between piston nut  70  and valve plates  366 . Piston nut  70  is threaded onto end  52  of piston rod  18  to retain support plate  368  and hold valve plates  366  against piston body  340  to close passages  356 . 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  as shown by arrows  72  in FIG.  10 . 
     The operation and function of sliding valve assembly  46  in conjunction with compression valve assembly  342  and rebound assembly  344  is the same as that described above for valve assemblies  46 ,  42  and  44 . In addition, it is within the scope of the present invention to incorporate check valve assembly  190  shown in FIG. 8 or check valve assembly  290  shown in FIG. 9 into piston valve assembly  316  if desired. 
     While the above detailed description describes the preferred embodiment of the present invention, it should be understood that the present invention is susceptible to modification, variation and alteration without deviating from the scope and fair meaning of the subjoined claims.