Abstract:
A shock 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 the 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 the 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 dual valving systems can be incorporated into the piston for a compression stroke, be incorporated into the piston for an extension stroke, or two dual valving systems can be incorporated into the piston for compression and extension strokes.

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 (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 automobile 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. When a full displacement piston valving system is used, the fluid reservoir is in direct communication with the lower portion of the working chamber defined by the pressure tube (the area below the piston). All damping forces produced by the shock absorber are the result of piston valving when a full displacement valving system is used. The greater the degree to which the flow of fluid within the shock absorber is restricted by the piston, 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 for the main springs of the vehicle as well as the spring constant for the seat and 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 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 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 of 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 of the shape of the fixed orifice bleed circuit force velocity characteristic is totally different in configuration 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 would also increase in order 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 orifice 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 valving or high speed valving circuit. The smooth transition between these two circuits helps to reduce and/or eliminate any harshness during the 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 or the secondary valving system. The secondary valving system of the present invention includes a first plurality of discs secured to the piston to close the mid/high speed extension and compression fluid passages extending through the piston. The first plurality of discs deflect due to a pressure differential to open the mid/high speed extension or compression fluid passages during the second stage valving. The low speed variable orifice bleed circuit of the present invention includes a second plurality of discs secured to the piston but separate from the first plurality of discs. The second plurality of discs close the low speed extension and compression fluid passages extending through the piston. The second plurality of discs also deflect due to a pressure differential to open the low speed extension or compression 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 damping forces created by the shock absorber during both an extension stroke and a compression stroke of the shock absorber and thus improve the vehicle handling without creating harshness. 
     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 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 exposed perspective view of the piston assembly shown in FIG. 3; and 
     FIG. 5 is an enlarged side elevational view, partially in cross-section, of a piston assembly incorporating an independent variable bleed orifice 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  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 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  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 away in 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 fitting  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 undue frictional 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 displaced is known as the “rod volume” and it flows through base fitting  40 . While shock absorber  26  is being illustrated as a dual tube shock absorber having base fitting  40 , it is within the scope of the present invention to utilize piston assembly  32  in a mono-tube designed shock absorber if desired. 
     Reservoir tube  36  surrounds pressure tube  30  to define a reserve 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 upper end cap  50 . Base fitting  40  is disposed between lower working chamber  46  and reserve chamber  54  to allow 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, fluid will flow from reserve chamber  54  to lower working chamber  46  through base fitting  40 . When shock absorber  26  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 reserve chamber  54  through base fitting  40 . 
     The present invention is directed to a unique full flow piston assembly  32  which includes variable bleed orifice valving for both rebound and compression 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 both a compression movement and a rebound movement of shock absorber  26 . The damping characteristics for both rebound (extension) and compression for shock absorber  26  are determined by piston assembly  32  thus eliminating the need for a base valve assembly. 
     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 fluid passages  68 . Compression valve assembly  62  is disposed on the upper side of piston  60  adjacent a shoulder  70  defined by piston rod  34 . Compression valve assembly  62  comprises a piston plate  72 , a plurality of mid/high speed valve discs  74 , a bleed valve body  76 , a bleed valve disc  78 , a bleed washer  80 , and a bleed check plate  82 . Piston plate  72  is disposed adjacent piston  60  and it defines a plurality of compression passages  84  that are in registry with the plurality of compression fluid passages  66  and  84 . Bleed valve body  76  defines a plurality of compression bleed passages  86  which are also in fluid communication with the plurality of compression fluid passages  66 . 
     Valve discs  74  are sandwiched between a shoulder  88  on piston plate  72  and an annular surface  90  on bleed valve body  76  to close the plurality of compression passages  84  and thus the plurality of compression fluid passages  66 . Bleed valve disc  78  is located adjacent bleed valve body  76  to close the plurality of bleed passages  86 . Bleed washer  80  is disposed between bleed valve disc  78  and bleed check plate  82 . Bleed check plate  82  is located adjacent shoulder  70  on piston rod  34 . A retaining nut  92  is assembled to the end of piston rod  34 . Nut  92  maintains the assembly of compression valve assembly  62 , piston  60  and rebound valve assembly  64  as shown in FIG.  3 . 
     During a compression stroke for shock absorber  26 , fluid pressure increases in lower working chamber  46  and fluid pressure decreases in upper working chamber  44 . The increase in fluid pressure in lower working chamber  46  is transferred through passages  66  and  84  to exert a load on mid/high speed discs  74  and through passages  86  to exert a load on bleed valve disc  78 . Bleed valve disc  78  is designed to deflect at a lower load than discs  74  and thus will deflect first to allow fluid flow between lower working chamber  46  and upper working chamber  44  during low speed movements of piston  60  when relatively low pressure differentials across disc  78  exist. As the pressure differentials across disc  78  continue to increase, disc  78  will deflect an additional amount to increase the fluid flow between lower working chamber  46  and upper working chamber  44 . The amount of deflection and thus the metering for the fluid flow is controlled by the thickness of bleed washer  80 . Eventually, as the speed of movement of piston  60  increases, the bleed flow of fluid will reach a saturation point due to bleed washer  80  and the pressure differential across mid/high speed valve discs  74  (which is the same pressure differential across disc  78 ) will increase and exert a sufficient load against valve discs  74  to cause deflection of valve discs  74  to allow additional flow of fluid between lower working chamber  46  and upper working chamber  44 . The transition between the fluid flow past disc  78  and the fluid flow past discs  74  can be controlled by the design of bleed valve body  76 , bleed valve disc  78 , bleed washer  80  and bleed check plate  82 . Factors that will affect the shape of the transition curve include, but are not limited to, the diameter of bleed valve body  76 , the size of passages  86 , the thickness, size and stiffness of bleed valve disc  78 , the diameter and thickness of bleed washer  80  and the size of bleed check plate  82 . All of the factors which control the shape of the transition curve are independent of the design for piston plate  72  and the plurality of mid/high speed valve discs  74 . 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 valving systems. Even though bleed valve body  76  interfaces between the low speed valving and the mid/high speed valving, the independence between these two valving systems is maintained since the low speed valving system is affected by the design of the upper surface of bleed valve body  76  while the mid/high speed valving system is affected by the design of the lower surface of bleed valve body  76 . 
     Rebound valve assembly  64  is disposed on the lower side of piston  60  adjacent retaining nut  92 . Rebound valve assembly  64  comprises a second piston plate  102 , a second plurality of mid/high speed valve discs  104 , a second bleed valve body  106 , a second bleed valve disk  108 , a second bleed washer  110 , and a second bleed check plate  112 . Piston plate  102  is disposed adjacent piston  60  and it defines a plurality of rebound passages  114  that are in registry with the plurality of rebound fluid passages  68 . Bleed valve body  106  defines a plurality of rebound bleed passages  116  which are also in fluid communication with the plurality of rebound fluid passages  68  and  114 . 
     Valve discs  104  are sandwiched between a shoulder  118  on piston plate  102  and an annular surface  120  on bleed valve body  106  to close the plurality of rebound passages  114  and thus the plurality of rebound fluid passages  68 . Bleed valve disc  108  is located adjacent bleed valve body  106  to close the plurality of bleed passages  116 . Bleed washer  110  is disposed between bleed valve disc  108  and bleed check plate  112 . Bleed check plate  112  is located adjacent retaining nut  92  which is assembled to the end of piston rod  34 . Nut  92  maintains the assembly of compression valve assembly  62 , piston  60  and rebound valve assembly  64  as shown in FIG.  3 . 
     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 passages  68  and  114  to exert a load on mid/high speed discs  104  and through passages  116  to exert a load on bleed valve disc  108 . Bleed valve disc  108  is designed to deflect at a lower load than discs  104  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  108  exist. As the pressure differentials across disc  108  continues to increase, disc  108  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 thickness of bleed washer  110 . Eventually, as the speed of movement of piston  60  increases, the bleed flow of fluid will reach a saturation point due to bleed washer  110  and the pressure differential across mid/high speed valve discs  104  (which is the same pressure differential across disc  108 ) will increase and exert a sufficient load against valve discs  104  to cause deflection of valve discs  104  to allow additional flow of fluid between upper working chamber  44  and lower working chamber  46 . The transition between the fluid flow past disc  108  and the fluid flow past discs  104  can be controlled by the design of bleed valve body  106 , bleed valve disc  108 , bleed washer  110  and bleed check plate  112 . Factors that will affect the shape of the transition curve include but are not limited to the diameter of bleed valve body  106 , the size of passages  116 , the thickness, size and stiffness of bleed valve disc  108 , the diameter and thickness of bleed washer  110  and the size of bleed check plate  112 . All of the factors which control the shape of the transition curve are independent of the design for piston plate  102  and the plurality of mid/high speed valve discs  104 . 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 valving systems. Even though bleed valve body  106  interfaces between the low speed valving and the mid/high speed valving, the independence between these two valving systems is maintained since the low speed valving system is affected by the design of the lower surface of bleed valve body  106  while the mid/high speed valving system is affected by the design of the upper surface of bleed valve body  106 . 
     Referring now to FIG. 5, a piston assembly  32 ′ in accordance with another embodiment of the present invention is disclosed. 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 fluid 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 plurality of mid/high speed valve discs  74 ′, a bleed valve body  76 ′, a bleed valve disc  78 ′ and a bleed washer  80 ′. 
     Valve discs  74 ′ are sandwiched between a shoulder  88 ′ on piston  60 ′ and an annular surface  90 ′ on bleed valve body  76 ′ to close the plurality of compression fluid passages  66 ′. Bleed valve body  76 ′ defines a plurality of compression bleed passages  86 ′ which are in communication with the plurality of compression fluid passages  66 ′. Bleed valve disc  78 ′ is located adjacent bleed valve body  76 ′ and adjacent bleed washer  80 ′ to close the plurality of bleed passages  86 ′ and  66 ′. Retaining nut  92  maintains the assembly of compression valve assembly  62 ′, piston  60 ′ and rebound valve assembly  64 ′ as shown in FIG.  6 . 
     During a compression stroke for shock absorber  20 , the operation and function for compression valve assembly  62 ′ is the same as that described above for compression valve assembly  62 . 
     Rebound valve assembly  64 ′ is disposed on the lower side of piston  60 ′ adjacent retaining nut  92 . Rebound valve assembly  64 ′ comprises a plurality of mid/high speed valve discs  104 ′, a bleed valve body  106 ′, a bleed valve disc  108 ′ and a bleed washer  110 ′. 
     Valve discs  104 ′ are sandwiched between a shoulder  1   18 ′ on piston  60 ′ and an annular surface  120 ′ on bleed valve body  106 ′ to close the plurality of rebound fluid passages  68 ′. Bleed valve body  106 ′ defines a plurality of rebound bleed passages  116 ′ which are in communication with the plurality of rebound fluid passages  68 ′. Bleed valve disc  108 ′ is located adjacent bleed valve body  106 ′ and adjacent bleed washer  110 ′ to close the plurality of bleed passages  116 ′ and  68 ′. Retaining nut  92  maintains the assembly of compression valve assembly  62 ′, piston  60 ′ and rebound valve assembly  64 ′ as shown in FIG.  6 . 
     During a rebound stroke for shock absorber  20 , the operation and function for rebound valve assembly  64 ′ is the same as that described above for rebound valve assembly  64 . 
     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.