Digressive base valve for automotive damper

A base valve assembly fore regulating the flow of fluid through a twin tube fluid vehicle damper is provided. The base valve assembly has opposing compression and rebound surfaces. Inner and outer annular seats extend from the compression surface and define an annular channel therebetween. At least one fluid passageway connects the annular channel and the rebound surface for providing fluid communication therebetween. A connector secures a blow-off valve to the cylinder end. The blow-off valve has a hollow cylindrical portion with a flange extending transversely from an end thereof. The flange is adjacent the annular seats when the blow-off valve is in a closed position. The blow-off valve is movable from the closed position to an open position away from the compression surface. A helical spring is interposed between the connector and the blow-off valve for biasing the blow-off valve to the closed position. A low speed compression bypass valve is interposed between the annular channel and the blow-off valve for permitting fluid to exit the annular channel past the blow-off valve when the blow-off valve is in the closed position. The blow-off valve provides performance during compression of the damper.

TECHNICAL FIELD
 This invention relates to fluid dampers for vehicles, and more
 specifically, to a blow-off valve for use on a base valve assembly to
 provide digressive performance during compression of the damper.
 BACKGROUND OF THE INVENTION
 Fluid vehicle dampers, such as hydraulic shock absorbers and struts,
 provide a smooth ride by absorbing forces that are generated by an uneven
 road surface. Two common types of vehicle fluid dampers are mono-tube and
 twin tube shock absorbers, each of which have a cylinder and piston. Twin
 tube shock absorbers have a valve body located at an end of the piston,
 commonly referred to as a piston valve, and at a cylinder end, commonly
 referred to as a base valve. The piston valve moves toward the base valve
 during compression of the shock absorber and moves away from the base
 valve during rebound. The valve bodies divide the shock absorbers into
 several fluid chambers and regulate the flow of fluid from one chamber to
 another thereby achieving particular ride handling characteristics.
 Typically, each valve body has a compression and rebound valve assembly
 located on opposing surfaces of the valve body that regulate fluid flow
 during the compression and rebound strokes. By modifying the valve
 assemblies, the ride handling characteristics may be calibrated.
 It is desirable to have different rates of piston damping depending on
 vehicle ride condition. For example, during vehicle cornering maneuvers in
 which the piston undergoes low speed compression, it is desirable to have
 stiff ride handling characteristics. Conversely, when the vehicle travels
 over pot holes at relatively high vehicle speeds in which the piston
 undergoes high speed compression, it is desirable to have soft ride
 handling characteristics. The base valve assembly is the primary control
 of damping during compression. One way to best achieve different rates of
 damping is to provide a base valve assembly having digressive performance,
 that is, a base valve which provides generally independent and distinct
 damping rates during low and high speed piston compression. Prior art base
 valve assemblies have only provided low and high piston speed damping
 rates which are dependent on one another thereby compromising ride
 handling characteristics at low and high vehicle speeds. Therefore, what
 is needed is a base valve assembly that provides digressive performance.
 SUMMARY OF THE INVENTION
 The present invention provides a base valve assembly for regulating the
 flow of fluid through a twin-tube fluid vehicle damper. The base valve
 assembly has opposing compression and rebound surfaces. Inner and outer
 annular seats extend from the compression surface and define an annular
 channel therebetween. At least one fluid passageway connects the annular
 channel and the rebound surface for providing fluid communication
 therebetween. A connector secures a blow-off valve to the cylinder end.
 The blow-off valve has a hollow cylindrical portion with a flange
 extending transversely from an end thereof. The flange is adjacent the
 annular seats when the blow-off valve is in a closed position. The
 blow-off valve is movable from the closed position to an open position
 away from the compression surface. A helical spring is interposed between
 the connector and the blow-off valve for biasing the blow-off valve to the
 closed position. A low speed compression bypass valve is interposed
 between the annular channel and the blow-off valve for permitting fluid to
 exit the annular channel past the blow-off valve when the blow-off valve
 is in the closed position. The blow-off valve provides performance during
 compression of the damper.
 Accordingly, the present invention provides a base valve assembly that
 provides digressive performance with generally independent and distinct
 damping rates during low and high speed piston compression.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 A fluid vehicle damper or, more specifically, a twin tube shock absorber is
 generally shown at 10 in FIG. 1. The damper 10 has a reservoir tube 12 and
 a cylinder 14 disposed within the reservoir tube 12. A lower portion 16 of
 the reservoir tube 12 has a lower connection 18 for attachment to a
 suspension component (not shown). An upper portion 19 of the reservoir
 tube 12 slidably receives a piston rod 20 to which an upper connection 22
 is attached. The upper connection 22 is typically attached to a portion of
 the vehicle s frame (not shown). The suspension component (not shown)
 moves relative to the frame (not shown) as the vehicle travels over uneven
 road surfaces thereby moving the piston rod 20 within the cylinder 14.
 A piston valve assembly is schematically shown at 24 and is attached to the
 rod 20 opposite the upper connection 22. An interior wall 26 of the
 cylinder 14 slidably receives the piston valve assembly 24. A base valve
 assembly 29, is secured to an end 27 of the cylinder 14 and abuts the
 lower portion 16. The base valve assembly 29 separates the cylinder 14
 from the reservoir tube 12 and defines a first fluid chamber, or reservoir
 chamber 30, and a second fluid chamber, or compression chamber 32. The
 base valve assembly 29 has a base valve body 31 that includes a downwardly
 depending annular flange 33 with a plurality of notches 35 (best shown in
 FIG. 2) that permit fluid to flow uninhibited between the reservoir 30 and
 compression 32 chambers. The base valve body 29 is typically formed from
 powdered metal. The piston valve assembly 24 separates the compression
 chamber 32 from a rebound chamber 34. Hydraulic fluid in the chambers 30,
 32, 34 dampens forces as the fluid passes through the piston 24 and base
 29 valve assemblies and their associated valve assemblies, which are
 discussed in more detail below. The base valve assembly 29 primarily
 controls the compression characteristics of the damper 10, while the
 piston valve assembly 24 primarily controls the rebound characteristics of
 the damper 10.
 The valve body 31 includes an outer surface 38 adjacent to the interior
 wall 26 of the cylinder 14 and includes first 40 and second 42 opposing
 surfaces. The reservoir chamber 30 and compression chamber 32 are adjacent
 to the first 40 and second 42 surfaces, respectively. Referring to FIG. 2,
 the base valve assembly 29 includes a compression valve assembly 44 that
 regulates the flow of fluid from the compression chamber 32 to the
 reservoir chamber 30 during compression of the damper 10 and is the
 primary control of damping during compression. The base valve assembly 29
 also includes a rebound valve assembly 46 that regulates the flow of fluid
 from the reservoir chamber 30 to the compression chamber 32 during rebound
 of the damper 10. As is well known in the art, fluid flow in and out of
 reservoir chamber 30 is caused by the differing rates of volume change in
 compression chamber 32 and rebound chamber 34 as rod 20 moves in and out
 of rebound chamber 34.
 The compression valve assembly 44 includes a high speed bypass valve
 assembly 48 and low speed bypass valve assembly 50. The high speed bypass
 valve assembly 48 provides high speed damping and includes a blow-off
 valve 54, a spacer sleeve 56, and a spring 60, which are secured to the
 valve body 31 by a connector 58. The connector 58 may be a rivet as shown,
 a nut and bolt, or any suitable fastening device. The rivet 58 is received
 in a central bore 67 (shown in FIGS. 3-5) in the valve body 31. The high
 speed bypass valve assembly 48 is discussed in greater detail below. The
 low speed bypass valve assembly 50 has an orifice disc 52 which works in
 conjunction with the valve body 31 to provide low speed damping. The
 orifice disc 52 is typically made from a high strength spring steel.
 The rebound valve assembly 46 has a rebound disc 62 adjacent to the second
 surface 42. The rebound disc 62 is biased toward the second surface 42 by
 a coil spring 64 which is secured to the valve body 31 by a retainer 66
 and the rivet 58. The rivet 58 has an end with a cylindrical depression
 received within a central hole in the retainer 66 so that the end may be
 deformed to retain the valve assemblies 44, 46, 48, 50 onto the valve body
 31. The rebound valve assembly 46 permits fluid to flow from the reservoir
 chamber 30 to the compression chamber 32 at a predetermined rate.
 Referring now to FIGS. 3 and 4, the first 40 and second 42 surfaces of the
 valve body 31 have inner 70, 72 and outer 74, 76 annular seats,
 respectively, for sealing engagement with the discs 52, 62, respectively.
 The inner 70 and outer 74 annular seats extend from the first surface 40
 and define an annular channel 78 therebetween. At least one fluid
 passageway 80 connects the annular channel 78 and second surface 42 for
 providing fluid communication therebetween during compression of the
 damper 10. Preferably, the valve body 31 has a plurality of the fluid
 passageways 80 spaced radially about the bore 67. The fluid passageways 80
 may have a circular cross-section as shown, or any other suitable
 cross-section, such as an arcuate cross-section. It is to be understood
 that a different type of low speed bypass valve may be used other than the
 one depicted in the Figures. For example, coined slots or notches pressed
 into the outer annular seat 74 may be used to permit fluid to exit the
 annular channel 78 into the reservoir chamber 30.
 The valve body 31 also has rebound fluid passageways 82 connecting the
 first 40 and second 42 surfaces for providing fluid communication
 therebetween during rebound of the damper 10. The rebound passageways 82
 are spaced radially about the bore 67 and are interposed between the inner
 72 and outer 76 annular seats, which define a rebound annular channel 84.
 These passageways permit fluid to move from the reservoir chamber 30 back
 to the compression chamber 32 after compression, which is discussed in
 more detail below.
 Referring to FIG. 5, the high speed bypass valve assembly 48 is movable
 between an open position, which is in spaced relation from the first
 surface 40 and abuts the back end of the rivet 58, and a closed position
 adjacent to the annular seats 70, 74, as shown in the Figures. The
 blow-off valve 54 has a hollow cylindrical portion 88 with a flange 90
 extending transversely from an end 92 thereof. The blow-off valve 54 may
 be manufactured from steel, powdered metal or any other suitable material.
 The rivet 58 is partially disposed within the bore 67 and the cylindrical
 portion 88 for securing the blow-off valve 54 to the valve body 31.
 Specifically, an end 94 adjacent to the retainer 66 is deformed to secure
 the compression 44 and rebound 46 valve assemblies to the valve body 31.
 The spring 60 is interposed between the rivet 58 and the flange 90 for
 biasing the blow-off valve to the closed position. The cylindrical spacer
 sleeve 56 is disposed within the hollow cylindrical portion 88 and abuts a
 shoulder 96 on the rivet 58 and the first surface 40. The spacer sleeve 56
 provides a bearing surface for the blow-off valve 54 as it moves between
 the open and closed positions.
 As mentioned above, the spring 60 biases the blow-off valve 54 to the
 closed position. The spring 60 is installed between the rivet 58 and
 blow-off valve 54 with a preload sufficient to prevent the blow-off valve
 54 from moving to the open position during low speed piston compression.
 The spring 60 preferably has a low spring rate so that once high piston
 speed is reached the blow-off valve will move completely to the open
 position. A helical spring manufactured from a wire with a circular
 cross-section wound in a cylindrical shaped helix accomplishes these
 design objectives. Changing the preload and spring rate permits the
 damping characteristics at high speed to be easily calibrated for
 different vehicle applications.
 The orifice disc 52 is interposed between the annular seats 70, 74 and the
 high speed bypass valve assembly 48 for providing low speed bypass of
 fluid from the compression chamber 32 to the reservoir chamber 30 when the
 blow-off valve 54 is in the closed position. This is accomplished by at
 least one opening 100 in the orifice disc 52 which permits fluid to flow
 around the outer annular seat, as shown, or by coined slots, discussed
 above. Preferably, the orifice disc 52 abuts the annular seats 70, 74 and
 the flange 90 when the blow-off valve 54 is in the closed position, but
 additional discs may be arranged between the orifice disc 52 and the
 annular seats 70, 74.
 In operation, during low speed compression, such as a vehicle cornering
 maneuver, fluid flows from the compression chamber 32 through the fluid
 passageways 80, into the annular channel 78 and through either an orifice
 disc or coined slots (indicated by the arrow in FIG. 5) to the reservoir
 chamber 30. During low speed rebound, fluid returns to the compression
 chamber 32 through the rebound passageways 82 (indicated by the arrow in
 FIG. 5) by deflecting the rebound disc 62 and permitting fluid to flow
 past the outer annular seat 76. In operation, during high speed
 compression, such as travelling over a pot hole in a road at relatively
 high vehicle speeds, the spring 60 is compressed by the high forces of the
 fluid against the orifice disc 52, which moves the blow-off valve 54 to
 the open position. In the open position, the fluid may flow rapidly from
 the compression chamber 32 to the reservoir chamber 30 through the
 passageways 80. Similarly during high speed rebound, the coil spring 64
 becomes compressed by the high forces of the fluid against the rebound
 disc 62, thereby permitting a high rate of fluid flow from the reservoir
 chamber 30 to the compression chamber 32 through the rebound passageways
 82.
 The damping characteristic may be further calibrated to a particular
 vehicle application by changing the number of fluid passageways 80, the
 diameter of those passageways, and the preload spring 60, as shown by the
 charts in FIGS. 6 and 7. As demonstrated by FIG. 6, the greater the
 preload the more distinct the low and high piston speed damping rates
 become. Low speed bypass occurs at low piston speeds under 0.4 m/s. For
 119 and 149N of preload, the compression force during low speed bypass
 increases at a rapid rate. At about 0.4 m/s the high speed bypass valve
 opens and the compression force continues to increase at a rate of about
 one sixth of that during low speed bypass because the blow-off valve 54
 has opened and the fluid flow is less restricted. Because of the low
 spring rate, the blow-off valve 54 moves to the open position virtually
 instantaneously producing a sudden change in fluid flow rates.
 FIG. 7 depicts that the number of fluid passageways 80 has more of an
 impact on the high speed piston damping rate than the diameter of those
 passageways while the diameter has more of an impact on low speed piston
 damping rate.
 The invention has been described in an illustrative manner, and it is to be
 understood that the terminology that has been used is intended to be in
 the nature of words of description rather than of limitation. Obviously,
 many modifications and variations of the present invention are possible in
 light of the above teachings. It is, therefore, to be understood that
 within the scope of the appended claims the invention may be practiced
 otherwise than as specifically described.