Abstract:
A single absorber includes a valve assembly with a low speed valving system and a high speed valving system. Both systems control fluid flow through the respective valve assembly for fluid flow in the same direction. The low speed valving system is independently tunable in order to provide low speed damping to improve both vehicle control and handling. The independent tuning of the low speed valving system allows the optimization of the low speed valving system in relation to the high speed valving system as well as independent tuning of the high speed valving system in relation to the low speed valving system. The independent tuning of the two systems allow the achievement of a smooth transition between the two systems. The two valving systems can be incorporated into a piston assembly for an extension stroke, a base valve assembly for a compression stroke or both.

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 speed or low hydraulic fluid flow. 
     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 (wheels) of the automobile. A piston is located within a working chamber defined by a pressure tube of the shock absorber, with the piston being connected to the sprung portion of the automobile through a piston rod. The pressure tube is connected to the unsprung portion of the vehicle by one of the methods known in the art. 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 damps 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 reserve tube. A base valve can be located between the lower portion of the working chamber (the area below the piston) and the reservoir to limit the flow of fluid between the lower working chamber and the reservoir. When both piston valving and a base valve are utilized, the piston valving produces a damping force which counteracts the unwanted vibrations during an extension stroke of the shock absorber and the base valve produces a damping force which counteracts the unwanted vibrations during a compression stroke of the shock absorber. The greater degree to which the flow of fluid within the shock absorber is restricted by the piston valving and the base valve, 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 restrictive 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 for the main springs of the vehicle as well as the spring constant for the seat and tire 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 handling 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 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 upon the speed or acceleration of the piston 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. Low speed damping force is important to vehicle handling since most vehicle handling events are controlled by low speed vehicle body velocities. 
     Various prior art systems for tuning shock absorbers during low speed movement of the piston create a fixed low speed bleed orifice which provides a bleed passage which is always open across the piston. This bleed orifice can be created by utilizing orifice notches positioned either on the flexible disc adjacent to the sealing land or by utilizing orifice notches directly in the sealing land itself. The limitations of these designs is that because the orifice is constant in cross-sectional area, the created damping force is not a function of the internal pressures of the shock absorber. In order to obtain the low speed control utilizing these open orifice notches, the orifice notches have to be small enough to create a restriction at relatively low velocities. When this is accomplished, the low speed fluid circuit of the valving system will operate over a very small range in velocity. Therefore, the secondary or high-speed stage valving is activated at a lower velocity than is desired. Activation of the secondary valving at relatively low velocities creates harshness because the shape of the fixed orifice bleed circuit force velocity characteristic is totally different than the shape of the high speed circuit. 
     Prior art attempts at overcoming the problems of fixed orifice bleed valving and thus eliminate harshness during low speed piston movements have included the incorporation of a variable orifice bleed valving circuit. As the velocity of the piston increases, the flow area of the variable orifice also increases to smooth the transition to the secondary valving. These prior art variable orifice bleed valving circuits are typically located at the outer periphery of the flexible valve disc and thus they are dependent on the diameter of the disc to determine the rate at which the flow area increases. As the diameter of the flexible disc increases, it becomes more difficult to control the rate at which the flow area of the orifice increases. Since the flow area is increased by the deflection of the variable orifice bleed disc, a small deflection in a large diameter variable orifice bleed disc provides a rapid increase in the flow area of the bleed orifice. This rapid increase in the flow area complicates the tuning between the low speed valving circuit and the secondary or high-speed valving circuit. 
     Still other prior art systems have developed variable bleed valving circuits which are integrated with the mid/high-speed valving systems. The integration of the low speed circuit with the mid/high speed circuit creates a system where the tuning of the low speed circuit affects the mid/high-speed circuit and the tuning of the mid/high-speed circuit affects the low speed circuit. 
     The continued development of shock absorbers includes the development of a valving system which can provide a smooth transition between a low speed valving circuit and the secondary or high speed valving circuit. The smooth transition between these two circuits helps to reduce and/or eliminate any harshness during transition. In addition to the smooth transition, the development of these systems has also been directed towards the separation of these two circuits in order to be able to independently tune each of these circuits. 
     SUMMARY OF THE INVENTION 
     The present invention provides the art with a method for independently tuning damping forces at low piston velocities in order to improve the handling characteristics of the vehicle without creating harshness. The present invention provides a low speed variable orifice bleed circuit which is separate from the mid/high-speed circuit of the secondary valving system. The secondary valving system of the present invention includes a first disc secured to the piston to close the mid/high-speed extension passages extending through the piston. The first disc deflects due to a pressure differential to open the mid/high-speed extension fluid passages during the second storage valving. The low speed variable orifice bleed circuit of the present invention includes a plurality of discs secured to the piston but separate from the first disc. The second plurality of discs close the low speed extension fluid passages extending through the piston. The second plurality of discs also deflect due to a pressure differential to open the low speed extension fluid passages during the initial stage valving. The separation of these two valving systems allows the designer to separately optimize the tuning of each valving system to optimize the tuning of each varying system to optimize the damping forces created by the shock absorber during an extension stroke and thus improve the vehicle handling without creating harshness. A similar dual valving system can be incorporated into the base valve of the present invention to optimize the damping forces created during a compression stroke. 
     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 an illustration of an automobile using the variable bleed orifice in accordance with the present invention; 
     FIG. 2 is a side view, partially in cross-section of a shock absorber incorporating the independent variable bleed orifice in both the piston valving system and the base valving system in accordance with the present invention; 
     FIG. 3 is an enlarged side elevational view, partially in cross-section, of the piston assembly for the shock absorber shown in FIG. 2; 
     FIG. 4 is an exploded perspective view of the piston assembly shown in FIG. 3; and 
     FIG. 5 is an enlarged side elevational view, partially in cross-section, of the base valve assembly for the shock absorber shown in FIG.  1 . 
    
    
     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 vehicle incorporating a suspension system having the independent variable bleed orifice 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 traversely extending rear axle assembly (not shown) adapted to support the vehicles rear wheels  18 . The rear axle is operatively connected to body  16  by a pair of shock absorbers  20  and a pair of helical coil springs  22 . Similarly, front suspension  14  includes a traversely 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  26  is shown in greater detail. While FIG. 2 shows only shock absorber  26 , it is to be understood that shock absorber  20  also includes the variable bleed orifice valving in accordance with the present invention which is described below for shock absorber  26 . Shock absorber  20  differs from shock absorber  26  in the way which it is adapted to be connected to the sprung and unsprung portions of vehicle  10 . Shock absorber  26  comprises a pressure tube  30 , a piston assembly  32 , a piston rod  34 , a reservoir tube  36  and a base valve assembly  40 . 
     Pressure tube  30  defines a working chamber  42 . Piston assembly  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 assembly  32  and pressure tube  30  to permit sliding movement of piston assembly  32  with respect to pressure tube  30  without generating unique functional forces as well as sealing upper working chamber  44  from lower working chamber  46 . Piston rod  34  is attached to piston assembly  32  and extends through upper working chamber  44  and through 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 in piston assembly  32  controls the movement of fluid between upper working chamber  44  and lower working chamber  46  during movement of piston assembly  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 assembly  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 displaces is known as the “rod volume” and it flows through base valve assembly  40 . 
     Reservoir tube  36  surrounds pressure tube  30  to define a reserve chamber  54  located between the tubes. The bottom 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  40  is disposed between lower working chamber  46  and reserve chamber  54  to control the flow of fluid between the two chambers. When shock absorber  26  extends in length (rebound), an additional volume of fluid is needed in lower working chamber  46  due to the “rod volume” concept. Thus, the fluid will flow from reserve chamber  54  to lower working chamber  46  through base valve assembly  40 . When shock absorber  26  compresses in length (compression), an excess 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 reserve chamber  54  through base valve assembly  40 . 
     The present invention is directed to a unique piston assembly  32  and base valve assembly  40  each of which includes variable bleed orifice valving for rebound or extension strokes which is independent of the mid/high-speed valving. Piston assembly  32  provides an independent tunable smooth transition between the low speed valving and the mid/high speed valving in a rebound movement of shock absorber  26 . The damping characteristics for a compression movement are determined by base valve assembly  40  as detailed below. 
     Referring now to FIGS. 3 and 4, piston assembly  32  comprises a piston  60 , a compression valve assembly  62  and a rebound valve assembly  64 . Piston  60  is secured to piston rod  34  and it defines a plurality of compression fluid passages  66  and a plurality of rebound passages  68 . 
     Compression valve assembly  62  is disposed on the upper side of piston  60  adjacent shoulder  70  defined by piston rod  34 . Compression valve assembly  62  comprises a support washer  72 , an intake spring  74  and an intake valve  76 . Support washer  72  is disposed adjacent shoulder  70  with intake spring  74  being disposed adjacent support washer  72  and intake valve  76  being disposed between intake spring  74  and piston  60 . Intake spring  74  is a star shaped flat metal spring which provides support for intake valve  76  as well as providing flow paths for the fluid within upper chamber  44  to flow into rebound passages  68 . Intake valve  76  covers the plurality of compression fluid passages  66  and it defines a flow passage  78  for providing fluid flow from upper working chamber  44  to rebound passages  68 . During a compression stroke for shock absorber  10 , fluid pressure increases in lower working chamber  46  and decreases in upper working chamber  44 . The increase in fluid pressure in lower working chamber  46  is transferred through compression fluid passages  66  to exert a load on intake valve  76 . As fluid pressure increases in lower working chamber  46  and the pressure differential across intake valve  76  increases, intake valve  76  will deflect intake spring  74  to allow fluid to flow between lower working chamber  46  and upper working chamber  44 . Compression valve assembly  62  does not determine the damping characteristics for shock absorber  10  during a compression stroke. Base valve assembly  40  performs this function. Compression valve assembly  62  functions as a one-way valve to replace fluid within upper working chamber  44  during a compression stroke and to close rebound passages  68  during a rebound stroke. 
     Rebound valve assembly  64  is disposed on the lower side of piston  60 . A retaining nut  80  is threaded onto piston rod  34  to retain the assembly of piston assembly  32  and piston rod  34 . Rebound valve assembly  64  comprises a high speed valve disc  82 , a ported plate  84 , a bleed disc  86  and a low speed valve disc  88 . High speed valve disc  82  is disposed adjacent piston  60  and it closes the plurality of rebound passages  68 . High speed valve disc  82  defines a central aperture  90  which includes a plurality of tabs  92 . Tabs  92  center high speed valve disc  82  on piston rod  34  while still allowing fluid flow through central aperture  90 . Ported plate  84  is disposed adjacent high speed valve disc  82  and it defines a contoured surface  94  which controls the flexing of high speed valve disc  82 . Ported plate  84  defines a central aperture  96  which includes a plurality of tabs  98 . Tabs  98  center ported plate  84  on piston rod  34  while still allowing fluid flow through central aperture  96  around piston rod  34 . 
     Bleed disc  86  is disposed adjacent ported plate  84  and with ported plate  84  defines a closed low speed pressure chamber  100 . Bleed disc  86  defines a central aperture  102  and a plurality of bleed slots  104  extending radially outward from aperture  102 . Bleed slots  104  define a fluid flow path such that fluid in upper working chamber  44  is in communication with low speed pressure chamber  100  through intake spring  74 , flow passage  78  in intake valve  76 , extension passages  68 , aperture  90 , aperture  96  and slots  104 . Low speed valve disc  88  is disposed adjacent bleed disc  86  and it defines a central aperture  106 . Low speed valve disc  88  closes bleed slots  104  and thus seals low speed pressure chamber  100 . Retaining nut  80  is disposed adjacent low speed valve disc  88  and it secures piston assembly  32  to piston rod  34 . 
     During a rebound stroke for shock absorber  26 , fluid pressure decreases in lower working chamber  46  and fluid pressure increases in upper working chamber  44 . The increase in fluid pressure in upper working chamber  44  is transferred through intake spring  74 , flow passage  78  in intake valve  76 , through passages  68  to exert a load on high speed valve disc  82 . The increase in fluid pressure is also transferred through aperture  90 , aperture  96  and slots  104  into chamber  100  where it exerts a load on low speed valve disc  88 . Low speed valve disc  88  is designed to deflect at a lower load than high speed valve disc  82  and thus will deflect first to allow fluid flow between upper working chamber  44  and lower working chamber  46  during low speed movements of piston  60  when relatively low pressure differentials across disc  88  exist. In addition, the low speed pressure area of disc  88  defined by chamber  100  is greater than the high speed pressure area of disc  82  defined by passages  68 . This larger pressure area allows rebound valve assembly  64  to produce a soft blow-off characteristic. This feature is beneficial to vehicle  10  since low speed control force improves vehicle handling and the soft blow-off reduces harshness experienced by the vehicle passengers. 
     As the pressure differentials across low speed valve disc  88  continues to increase, disc  88  will deflect an additional amount to increase the fluid flow between upper working chamber  44  and lower working chamber  46 . The amount of deflection and thus the metering for the fluid flow is controlled by the size of bleed slots  104 . Eventually, as the speed of movement of piston  60  increases, the bleed flow of fluid will reach a saturation point due to bleed slots  104  and the pressure differential across high speed valve disc  82  (which is the same pressure differential across disc  88 ) will increase and exert a sufficient load against high speed valve disc  82  to cause deflection of high speed valve disc  82  to allow additional flow of fluid between upper working chamber  44  and lower working chamber  46 . The transition between the fluid flow past disc  88  and the fluid flow past disc  82  can be controlled by the design of ported plate  84 , bleed disc  86  and low speed valve disc  88 . Factors that will affect the shape of the transition curve include but are not limited to the diameter of ported plate  84 , the size of bleed slots  104 , the diameter and thickness of bleed disc  86  and the diameter and size of low speed valve disc  88 . All of the factors which control the shape of the transition curve are independent of the design of piston  60  and high speed valve disc  82 . Thus, the tuning of the transition between low speed valving and mid/high speed valving is independent from the mid/high speed valving, thus allowing the independent tuning of both valve systems. 
     Referring now to FIG. 5, base valve assembly  40  is illustrated. Base valve assembly  40  is disposed between lower working chamber  46  and reserve chamber  54 . Thus, base valve assembly provides damping characteristics for shock absorber  10  during a compression stroke in a manner identical to that described above for rebound valve assembly  64  during a rebound stroke. Base valve assembly  40  comprises a valve body  160 , a threaded retainer  134 , nut  80 , high speed valve disc  82 , ported plate  84 , bleed disc  86  and low speed valve disc  88 . 
     Valve body  160  is identical to piston  60  except that it is adapted to be secured to pressure tube  30  instead of piston rod  34 . In addition, the plurality of compression fluid passages  66  in piston  60  become the plurality of rebound passages  166  in valve body  160  and the plurality of rebound passages  68  in piston  60  become the plurality of compression passages  168  in valve body  160 . Threaded retainer  134  is identical to the end of piston rod  34  in that it defines shoulder  70  and threadingly accepts nut  80  to keep the components of bleed valve assembly  40  together. 
     The function and operation of base valve assembly  40  is the same as that described above for piston assembly  32  except that base valve assembly  40  creates a damping load during a compression stroke and has a check valve for replacing fluid in lower working chamber  46  in an extension stroke. Base valve assembly  40  operates to create a damping force during a compression stroke due to its positioning between lower working chamber  46  and reserve chamber  54 . The features and advantages described above for piston assembly  32  in an extension stroke apply equally well for base valve assembly  40  during a compression stroke. 
     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.