Abstract:
A dual tube shock absorber includes a spool valve located between the upper working chamber and the reserve chamber. The spool valve moves with the piston rod to open and close a flow path between the upper working chamber and the reserve chamber. This provides a low damping characteristic for small movements of the piston rod which changes to a high damping characteristic for larger movement of the piston rod.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to automotive dampers or shock absorbers which receive mechanical shock. More particularly, the present invention relates to a unique hydraulic valve assembly which allows greater tunability of the shock absorber, especially in the mode of low hydraulic fluid flow. 
     BACKGROUND OF THE INVENTION 
     Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which 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, with the piston being connected to the sprung portion of the automobile through a piston rod and the pressure tube being connected to the unsprung portion of the automobile. Because the piston is able, through valving, to limit the flow of damping fluid between opposite sides of the piston, when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the unwanted vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile. In a dual tube shock absorber, a fluid reservoir is defined between the pressure tube and the reservoir tube. A base valve can be located between the lower working chamber (the area below the piston) and the reservoir to limit the flow of fluid between the lower working chamber and the reservoir to produce a damping force which also counteracts the unwanted vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile. The greater the degree to which the flow of fluid within the shock absorber is restricted by the piston and/or the base valving, the greater the damping forces which are generated by the shock absorber. Thus, a highly restricted flow of fluid would produce a firm ride while a less restricted flow of fluid would produce a soft ride. 
     In selecting the amount of damping that a shock absorber is to provide, at least three vehicle performance characteristics are considered. These three characteristics are ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main springs of the vehicle as well as the spring constant of the seat, tires and the damping coefficient of the shock absorber. For optimum ride comfort, a relatively low damping force or a soft ride is preferred. 
     Vehicle handling is related to the variation in the vehicle&#39;s attitude (i.e., roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces, or a firm ride, are required to avoid excessively rapid variations in the vehicle&#39;s attitude during cornering, acceleration and deceleration. 
     Finally, road holding ability is generally a function of the amount of contact between the tires and the ground. To optimize road handling ability, large damping forces, or a firm ride, are required when driving on irregular surfaces to prevent loss of contact between the wheel and the ground for excessive periods of time. 
     Various types of shock absorbers have been developed with multi-force damping force generating devices to generate the desired damping forces in relation to the various vehicle performance characteristics. Shock absorbers have been developed to provide different damping characteristics depending on the distance or the speed at which the piston moves within the pressure tube. Because of the exponential relation between pressure drop and flow rate, it is a difficult task to obtain a damping force at relatively low piston velocities, particularly at velocities near zero. 
     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 piston that is frictionally held in place on the piston rod inside of the pressure cylinder. While the shock absorber undergoes a small stroke, the sliding sleeve moves with the piston rod and the fluid flows through a separate flow path to provide a soft damping. When the shock absorber undergoes a large stroke, the sliding sleeve moves against a stop to close off the flow path which in turn provides a firm damping. 
     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 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 side view illustrating the upper piston of the shock absorber shown in  FIG. 1 ; 
         FIG. 4  is a perspective view illustrating the upper piston of the shock absorber shown in  FIG. 1 ; 
         FIG. 5  is a cross-sectional side view illustrating a shock absorber in accordance with another embodiment of the present invention; and 
         FIG. 6  is an enlarged cross-sectional side view illustrating an upper piston 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 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 shock absorbers in accordance with the present invention 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  has a transversely extending rear axle assembly (not shown) adapted to operatively support the vehicle&#39;s rear wheels  18 . The rear axle assembly is operatively connected to body  16  by means of a pair of shock absorbers  20  and a pair of helical coil springs  22 . Similarly, front suspension  14  includes a transversely extending front axle assembly (not shown) to operatively support the vehicle&#39;s front wheels  24 . The front axle assembly is operatively connected to body  16  by means of a second pair of shock absorbers  26  and by a pair of 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) and 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 front and/or independent rear suspension systems. 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 , shock absorber  20  is shown in greater detail. While  FIG. 2  shows only shock absorber  20 , it is to be understood that shock absorber  26  also includes the valving in accordance with the present invention described below for shock absorber  20 . Shock absorber  26  differs from shock absorber  20  in the way in which it is adapted to be connected to the sprung and unsprung portions of vehicle  10 . Shock absorber  20  comprises a pressure tube  30 , a piston  32 , a piston rod  34 , a reservoir tube  36  and a base valve assembly  40 . 
     Pressure tube  30  defines a working chamber  42 . Piston  32  is slidably disposed within pressure tube  30  and divides working chamber  42  into an upper working chamber  44  and a lower working chamber  46 . A seal  48  is disposed between piston  32  and pressure tube  30  to permit sliding movement of piston  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  32  and extends through upper working chamber  44  and through a rod guide  50  which closes the upper end of both pressure tube  30  and reservoir tube  36 . A sealing system  52  seals the interface between rod guide  50 , pressure tube  30 , reservoir tube  36  and piston rod  34 . The end of piston rod  34  opposite to piston  32  is adapted in the preferred embodiment, to be secured to the sprung portion of vehicle  10 . Valving in piston  32  controls the movement of fluid between upper working chamber  44  and lower working chamber  46  during movement of piston  32  within pressure tube  30 . Because piston rod  34  extends only through upper working chamber  44  and not lower working chamber  46 , movement of piston  32  with respect to pressure tube  30  causes a difference in the amount of fluid displaced in upper working chamber  44  than 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  40 . 
     Reservoir tube  36  surrounds pressure tube  30  to define a reservoir chamber  54  located between the tubes. 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 rod guide  50 . Base valve assembly  40  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 (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  40 . When shock absorber  20  compresses in length (compression), an excess volume of fluid must be removed from lower working chamber  46  due to the “rod volume” concept. Thus, fluid will flow from lower working chamber  46  to reservoir chamber  54  through base valve assembly  40 . 
     The present invention is directed towards a micro amplitude damping valve assembly  100  which provides reliable small amplitude damping characteristics for shock absorber  20 . 
     Referring now to  FIG. 3 , damping valve assembly  100  comprises a spool valve  102 , a plurality of holes  104  extending through an upper end of pressure tube  30 , and a retainer  106 . Spool valve  102  is dimensioned to have a tight slip fit with respect to the inner diameter of the pressure tube  30 . The inner diameter of spool valve  102  is dimensioned to have a loose fit with respect to piston rod  34 . The inner diameter of spool valve  102  includes a friction control device  108  which provides a greater amount of friction between piston rod  34  and spool valve  102  than the amount of friction developed between spool valve  102  and pressure tube  30 . This will result in spool valve  102  following the axial motion of piston rod  34  for small displacements. For larger displacements, spool valve  102  will be constrained by rod guide  50  in the rebound direction and by retainer  106  in the compression direction. Retainer  106  can comprise a snap ring disposed within pressure tube  30  or a precisely placed indentation  110  formed into pressure tube  30 . 
     Spool valve  102  defines a plurality of axial holes  112  which ensure a near zero pressure drop across spool valve  102  for axial motion of spool valve  102 . Concentric grooves  114  evenly placed on the outside circumferences of spool valve  102  are provided to increase the localized pressure drop for axial flow within the region between the outside diameter of spool valve  102  and the inside diameter of pressure tube  30 . Additionally, grooves  114  serve to allow equalization of pressure around the outside circumference thereby causing spool valve  102  to center itself with respect to pressure tube  30 . A plurality of radial holes  116  extend from outside circumference of spool valve  102  and open into respective axial holes  112  but radial holes  116  do not extend entirely through spool valve  102 . When spool valve  102  is in its mean position, radial holes  116  align with and are in communication with the plurality of holes  104  extending through the upper end of pressure tube  30 . A groove  118  is provided at the center of radial holes  116  to eliminate the need to orient radial holes  116  and the plurality of holes  104 . 
     Thus, for near micro amplitude axial movements of piston rod  34  (&lt;1 mm) hydraulic fluid is allowed to pass between upper working chamber  44  to reservoir chamber  54  of shock absorber  20 . This is the condition for very low amplitude motion of piston rod  34  in both the compression and rebound directions. For larger amplitude motions of piston rod  34 , radial ports  116  misalign with holes  104  and flow is prevented from passing between upper working chamber  44  and reservoir chamber  54 . During the larger amplitude motions of piston rod  34 , spool valve  102  will contact rod guide  50  during a rebound stroke and spool valve  102  will contact retainer  106  during a compression stroke. Thus, both rod guide  50  and retainer  106  operate as positive stops for spool valve  102 . 
     An additional embodiment which has the capability to produce low amplitude shock performance is illustrated in  FIG. 5 .  FIG. 5  illustrates shock absorber  200 . Shock absorber  200  is a mono-tube design and comprises a piston rod assembly  212  and a pressure tube  214 . Piston rod assembly  212  includes a piston valve assembly  216  and a piston rod  218 . Valve assembly  216  divides pressure tube  214  into an upper working chamber  220  and a lower working chamber  222 . Piston rod  218  extends out of pressure tube  214 , for attachment of one of the sprung or unsprung mass of the vehicle. Pressure tube  214  is filled with fluid and attaches to the other sprung or unsprung masses of the vehicle. Thus, suspension movements of the vehicle will cause extension or compression movement of piston rod assembly  212  with respect to pressure tube  214  and these movements will be dampened due to the restricted fluid flow between chambers  220  and  222  through piston valve assembly  216 . 
     Piston rod  218  emanates axially from both ends of valve assembly  216 . Seals are required at each end of shock absorber  200  and clearance for the motion of piston rod  218  is required at both ends of shock absorber  200 . The key operating characteristics of the through rod shock is a lack of change in volume for working chambers  220  and  222  with any displacement or the elimination of the “rod volume” concept. 
     Therefore, a simple position sensitive bypass of valve assembly  216  would suffice to relieve low amplitude pressure within the working chambers  220  and  222 . The design would employ a flow path which bypasses the piston for low amplitude movements of piston rod  218  while closing the flow path for larger movements of piston rod  218 . The flow path could be placed in piston rod  218 , it could be external to the working chamber such as an additional concentric tube or it could be an indentation  224  in pressure tube  214 . This will provide the necessary relief of pressure to achieve desirable low amplitude performance. 
     Referring now to  FIG. 6 , a micro amplitude damping valve assembly  300  in accordance with another embodiment of the present invention is illustrated. Valve assembly  300  is a replacement for valve assembly and is shown incorporated into shock absorber  20 . 
     Damping valve assembly  300  comprises a spool valve  302 , a plurality of holes  304  extending through a lower rod guide  306 . Lower rod guide  306  is secured to rod guide  50  by a press fit or by other means known in the art. Once assembled, lower rod guide  306  is held in position by the press fit as well as by pressure tube  30  which bears against base valve assembly  40  which in turn bears against end cap  56 . 
     Spool valve  302  is dimensioned to have a tight slip fit with respect to the inner diameter of lower rod guide  306 . The inner diameter of spool valve  302  is dimensioned to have a loose fit with respect to piston rod  34 . The inner diameter of spool valve  302  includes a friction control device  308  which provides a greater amount of friction between piston rod  34  and spool valve  302  than the amount of friction developed between spool valve  302  and lower rod guide  306 . This will result in spool valve  302  following the axial motion of piston rod  34  for small displacements. For larger displacements, spool valve  302  will be constrained by rod guide  50  in the rebound direction and by lower rod guide  306  in the compression direction. 
     Spool valve  302  defines a plurality of axial holes  312  which ensure a near zero pressure drop across spool valve  302  for axial motion of spool valve  302 . Concentric grooves  314  evenly placed on the outside circumferences of spool valve  302  are provided to increase the localized pressure drop for axial flow within the region between the outside diameter of spool valve  302  and the inside diameter of lower rod guide  306 . Additionally, grooves  314  serve to allow equalization of pressure around the outside circumference thereby causing spool valve  302  to center itself with respect to lower rod guide  306 . A plurality of radial holes  316  extend from outside circumference of spool valve  302  and extend entirely through spool valve  302 . When spool valve  302  is in its mean position, radial holes  316  and a groove  318  align with and are in communication with the plurality of holes  304  extending through the upper end of lower rod guide  306 . Groove  318  is provided at the center of radial holes  316  to eliminate the need to orient radial holes  316  and the plurality of holes  304 . 
     Thus, for near micro amplitude axial movements of piston rod  34  (&lt;1 mm) hydraulic fluid is allowed to pass between upper working chamber  44  to reservoir chamber  54  of shock absorber  20 . This is the condition for very low amplitude motion of piston rod  34  in both the compression and rebound directions. For larger amplitude motions of piston rod  34 , radial groove  318  misaligns with holes  304  and flow is prevented from passing between upper working chamber  44  and reservoir chamber  54 . During the larger amplitude motions of piston rod  34 , spool valve  302  will contact rod guide  50  during a rebound stroke and spool valve  302  will contact lower rod guide  306  during a compression stroke. Thus, both rod guide  50  and lower rod guide  306  operate as positive stops for spool valve  102 . 
     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.