Abstract:
A shock absorber includes a valve assembly with a low speed oil flow circuit and a mid/high speed oil flow circuit. Both circuits control fluid flow through the valve assembly in the same direction. The low speed oil flow circuit is tunable in order to provide low speed damping to improve both the vehicle control and handling. The tuning of the low speed oil flow circuit is accomplished by supporting a low speed valve disc on a chordal edge at a position radially inward of its outer edge to control the rate at which the low speed oil flow circuit opens. The valve assembly of the present invention can be located within either a base valve assembly or a piston valve assembly.

Description:
FIELD OF THE INVENTION  
         [0001]    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 and the smooth transition between the bleed and blow-off stages of the valving.  
         BACKGROUND OF THE INVENTION  
         [0002]    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 a 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.  
           [0003]    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.  
           [0004]    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.  
           [0005]    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.  
           [0006]    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 at which the piston moves within the pressure tube. Because there is an exponential relationship 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, while still maintaining acceptable damping forces at the relatively high speed piston velocities. Low speed damping force is important to vehicle handling since most vehicle handling events are controlled by relatively low speed vehicle body velocities which thus lead to relatively low speed piston velocities.  
           [0007]    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 utilizing orifice notches directly in the sealing land itself. 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 piston velocities creates harshness because the shape of the low speed fixed orifice bleed circuit force velocity characteristic is totally different in configuration than the shape of the high-speed circuit. As the shock absorber transitions between these two circuits, harshness can be sensed in the vehicle.  
           [0008]    Continued development of shock absorbers include 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 will help to reduce and/or eliminate any harshness experienced during the transition.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides the art with a method for tuning damping forces during the transition between low speed piston velocities and high speed piston velocities in order to improve the ride and handling characteristics of the vehicle without creating harshness. The present invention provides the art with a variable orifice bleed circuit which is incorporated into the secondary valving system. The secondary valving system includes a plurality of discs secured to the piston to close the fluid passages extending through the piston. The plurality of discs deflect due to a pressure differential to open the fluid passages during the second stage valving. The variable orifice bleed circuit of the present invention incorporates a clipped valve disc directly adjacent the main valve disc which rests directly on the piston. This clipped disc allows an outer circumferential portion of the main valve disc in contact with the piston to deflect prior to the deflection of the entire stack of valve discs to provide the variable orifice bleed circuit. The clipped disc has a crescent shaped portion which is removed to control the deflection of the main valve disc.  
           [0010]    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  
       [0011]    In the drawings which illustrate the best mode presently contemplated for carrying out the present invention:  
         [0012]    [0012]FIG. 1 is an illustration of an automobile using the variable bleed orifice in accordance with the present invention;  
         [0013]    [0013]FIG. 2 is a side view, partially in cross-section of a shock absorber incorporating the variable bleed orifice in accordance with the present invention;  
         [0014]    [0014]FIG. 3 is an enlarged side elevational view, partially in cross-section, of the base valve assembly for the shock absorber shown in FIG. 2;  
         [0015]    [0015]FIG. 4 is an exposed perspective view of the base valve assembly shown in FIG. 3; and  
         [0016]    [0016]FIG. 5 is an enlarged side elevational view, partially in cross-section of a piston assembly incorporating the variable bleed orifice in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    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 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.  
         [0018]    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 variable bleed orifice 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 .  
         [0019]    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 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  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 .  
         [0020]    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 upper end cap  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 .  
         [0021]    The present invention is directed to a unique base valve assembly  40  which includes variable bleed orifice valving in accordance with the present invention. Base valve assembly  40  provides a tunable smooth transition between the low speed compression valving and the second stage of compression valving which comes into play during mid and high speed piston movements.  
         [0022]    Referring now to FIG. 3, base valve assembly  40  comprises a valve body  60 , a rebound valve assembly  62 , a compression valve assembly  64 , a retaining bolt  66  and a retaining nut  68 . Valve body  60  is secured to pressure tube  30  and end cap  56  by press fitting or by other methods known well in the art. End cap  56  is secured to reservoir  36  and it defines a plurality of fluid passages  70  which allow communication between reservoir chamber  54  and base valve assembly  40 . Valve body  60  defines a plurality of rebound fluid passages  72 , a plurality of compression fluid passages  74  and a central bore  78 . Retaining bolt  66  extends through central bore  78  and threadingly engages retaining nut  68  to secure both rebound valve assembly  62  and compression valve assembly  64  to valve body  60 .  
         [0023]    Referring now to FIGS. 3 and 4, rebound valve assembly  62  comprises a valve disc  80  and a valve spring  82 . Valve disc  80  is an annular member which defines an internal bore  84  for allowing fluid flow to compression fluid passages  74  as described below. Valve disc  80  is biased against the upper surface of valve body  60  by valve spring  82  which is located between valve disc  80  and retaining nut  68 . Valve disc  80  closes the plurality of rebound passages  72 . During a rebound or extension stroke of shock absorber  20 , fluid pressure decreases in lower working chamber  46  until the fluid pressure within reservoir chamber  54  and within passages  72  is capable of overcoming the biasing of valve spring  82 . When the biasing force of valve spring  82  is exceeded by fluid pressure acting against valve disc  80 , valve disc  80  is moved away from valve body  60  to allow fluid flow from reservoir chamber  54  to lower working chamber  46 .  
         [0024]    Compression valve assembly  64  comprises a variable orifice bleed disc  90  and a supporting disc  92 . Discs  90  and  92  are stacked together and located adjacent to valve body  60  with variable orifice bleed disc  90  abutting valve body  60  and supporting disc  92  abutting variable orifice bleed disc  90 . Discs  90  and  92  are held in position by being sandwiched between a shoulder  96  located on retaining bolt  66  and the lower surface of valve body  60 . Retaining bolt  66  is secured to valve body  60  by retaining nut  68 .  
         [0025]    Each variable orifice bleed disc  90  is an annular disc defining a central bore  100  through which bolt  66  extends. Disc  90  is sized to close the plurality of compression passages  74  but allow fluid access to the plurality of rebound passages  72 . When fluid pressure builds up in passages  74 , disc  90  will flex at its outer circumference to allow for the bleed flow of fluid past disc  90  and will flex along with supporting disc  92  to allow for the bleed-off flow of fluid past disc  90 .  
         [0026]    Supporting disc  92  is an annular disc defining a centrally located bore  110  through which bolt  66  extends. The outer periphery of supporting disc  92  includes a cut away section  112  formed by clipping supporting disc  92  along a chord  114  of the circle defined by the outer periphery of supporting disc  92 . The size of the clipped portion which is defined by the length of chord  116  determines the transitional curve between the low speed circuit and the high speed circuit for shock absorber  20 .  
         [0027]    During a compression stroke, fluid pressure builds up in lower working chamber  46  and fluid pressure decreases in upper working chamber  44 . The increase in pressure in lower working chamber  46  causes a pressure imbalance between lower working chamber  46  and reservoir chamber  54  and compression passages  74 . This pressure imbalance within passages  74  will react against variable orifice bleed disc  90  causing disc  90  to deflect along chord  116  to allow fluid flow past disc  90 . The pressure difference between lower working chamber  46  and reservoir chamber  54  required to cause deflection of disc  90  along chord  116  will be determined by the bending stiffness of disc  90  and the radial positioning of support for disc  90  by chord  116  of supporting disc  92 . As the fluid pressure difference continues to increase, disc  90  will deflect more allowing additional flow of fluid past disc  90 . The shape of the pressure differential vs. flow curve will be determined by the size of cut away section  112  of supporting disc  92 . As the fluid pressure differential continues to increase, the load exerted on variable orifice bleed disc  90  will be transferred to supporting disc  92  to eventually cause the deflection of disc  94  allowing for full flow of fluid through compression valve assembly  64 .  
         [0028]    Thus, the present invention provides increased low speed damping force which enhances vehicle control by having variable bleed disc  90  deflect along chord  116 . Vehicle handling is improved as a result of transferring the inertia of the vehicle to a force applying the tire to the road. The amount of force transferred by shock absorber  20  can be tuned to meet specific vehicle performance criteria. Prior art designs cannot transfer vehicle body inertia to the tire because a damping force is not created at low velocities due to the fixed bleed orifices or notches. The present invention is a distinct advantage over the prior art systems since the low speed valving is a tunable feature.  
         [0029]    While the present invention has been illustrated in conjunction with base valve assembly  40 , it is within the scope of the present invention, as shown in FIG. 5, to incorporate valve assembly  64  on either side of piston  32  if desired.  
         [0030]    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.