Abstract:
A hydraulic damper ( 2 ) includes at least one spring valve assembly ( 28   b,    28   c ) having a body ( 231, 261 ) provided with through flow channels ( 282 ), at least one deflectable disc ( 281 ) covering these through flow channels ( 282 ), and a supporting member ( 285, 285   c ) fixed to an axial member for clamping the at least one disc ( 281 ) at the inner circumferential part thereof. A spring seat ( 283   b,    283   c ) is disposed around the supporting member ( 285, 285   c ) and abuts the at least one deflectable disc ( 281 ) in at least one radial position ( 2831, 2832 ) at the outer circumferential part thereof. A spring ( 284 ) is preloaded between the spring seat ( 283   b,    283   c ) and the supporting member ( 285, 2851   c ). The spring seat ( 283   b,    283   c ) includes at least one axial projection ( 2833   b,    2833   c ) perimetrically engaging the at least one disc ( 281 ), and a circular gap ( 286 ) is provided between the spring seat ( 283   b,    283   c ) and the supporting member ( 285, 2851   c ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of PCT International Application Serial No. PCT/CN2013/078807 filed on Jul. 4, 2013 and entitled a “Hydraulic Suspension Damper with a Spring Valve Assembly”, the entire disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a hydraulic damper, in particular a motor vehicle suspension damper, comprising a tube filed with working liquid inside of which a slidable piston body attached to a piston rod led outside the damper is disposed, wherein the flow of the working liquid is controlled within said tube during the rebound and the compression stroke of the piston body by at least one spring valve assembly provided with an axial member and rebound and compression valves surrounding the axial member. Said at least one spring valve assembly comprises
         a body provided with through flow channels,   at least one deflectable disc covering these through flow channels,   a supporting member fixed to said axial member,   a spring seat disposed around said supporting member and abutting said at least one disc in at least one radial position at the outer circumferential part thereof, and   a spring preloaded between the spring seat and the supporting member.       

     BACKGROUND OF THE INVENTION 
     Valve assembly of this kind, where the spring seat is disposed slidably around the supporting member, is known from the state of art (cf. for example Polish patent applications P. 358597, P. 358598, P. 358599 and European patent applications EP 1925845 A1, EP 2233775). For its efficient operation it is absolutely essential to ensure substantially frictionless sliding (axial) movement of the spring seat with respect to the supporting member. 
     Various factors contribute to this problem including eccentricity and radial dimensional tolerances of particular elements of the valve assembly, possible buckling of the spring and distribution of the axial pressure exerted by the spring over the perimeter of the spring seat. 
     Influence of friction on operation of a valve assembly of this kind may be observed by measuring the force reaction of a damper in dependence to the velocity of the piston rod (a force to velocity characteristic). As it appears a substantial hysteresis occurs. 
     In other words forces measured while the piston rod accelerates to trigger opening of the spring valve assembly and deflection of discs are higher than forces measured while the piston rod decelerates to close the spring valve assembly and discs move to their neutral position. A difference exists therefore between damping forces generated by the damper for the same velocity of the piston rod but measured during its acceleration or deceleration and it is desirable to limit this difference as much as possible in order to improve the damper performance, repeatability coefficient and also the comfort for the passengers of the vehicle. 
     Patent specification GB 699896 discloses a hydraulic shock absorber, where a spring seat has a form of an abutment ring having an outwardly extending flange which rests upon the outer deflectable disc adjacent its outer edge. The outer deflectable disc is provided with an orifice covered by the inner deflectable disc and a clearance exists between the abutment ring and a cylindrical member that allows fluid communication through said orifice between the flow channels and the chamber of the shock absorber once the discs are deflected. 
     Patent specification U.S. Pat. No. 4,096,928 discloses a valve assembly for a shock absorber, where a spring seat has a form of a valve cage having a cylindrical portion surrounding the resilient valve disc for a centered guiding thereof. The cylindrical portion has an edge face which serves as an abutment for cooperating with the closing disc. 
     It has been the object of the present invention to provide a hydraulic damper having an improved sensitivity of operation in a result of minimizing the friction between cooperating elements of the valve assembly. 
     Another object of the present invention is to provide a hydraulic damper that would enable to achieve comparable working characteristic of all dampers in the line production within a large range of dimensional tolerances of the damper components in order to minimize the production specific losses, decrease the costs of production and increase the result repeatability coefficient. 
     SUMMARY OF THE INVENTION 
     In order to accomplish the aforementioned and other objects, in a damper of the kind mentioned in the outset, according to the present invention the spring seat of said at least one spring valve assembly comprises at least one axial projection perimetrically engaging said at least one disc and a circular gap is provided between said spring seat and said supporting member. 
     Such a construction enables for centering of the spring seat only with relation to the upper deflectable disc of the spring valve assembly, so that no guidance of the seat is required for its axial movement and no sliding problems occur. 
     Preferably said spring seat comprises one axial projection or alternatively a number of axial projections distributed, preferably equiangularly, over the outer perimeter thereof. 
     Preferably axially outer diameter of said at least one axial projection is larger than its axially inner diameter to ensure axial guidance of the spring seat over said at least one disc during assembly of the hydraulic damper. 
     Preferably said at least one valve assembly is a part of a piston valve assembly. 
     Alternatively or additionally (in case of a twin-tube damper) said at least one valve assembly may form a part of a base valve assembly. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The exemplary embodiments of the present invention are presented below in connection with the attached drawings on which: 
         FIG. 1  schematically illustrates a front right motor vehicle suspension; 
         FIG. 2  is a schematic cross-sectional view of a fragment of a hydraulic damper according to the present invention provided with two embodiments of a valve assembly; 
         FIG. 3  is a schematic cross-sectional view of a fragment of a piston assembly known from the state of art; 
         FIG. 4  is a schematic cross-sectional view of a fragment of a piston assembly shown in  FIG. 1 ; 
         FIG. 5  and  FIG. 6  show two exemplary spring seats that may be employed in a damper according to the present invention; 
         FIG. 7  is a schematic cross-sectional view of a fragment of a base valve assembly shown in  FIG. 2 , 
         FIG. 8  is a force to velocity characteristic illustrating the testing procedure carried out for a damper made according to the principles of the present invention, and 
         FIG. 9  is a detailed view of the hysteresis area A of the characteristic shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates a fragment of an exemplary vehicle suspension  1  attached to a vehicle chassis  11  by means of a top mount  12  and a number of screws  13  disposed on the periphery of the upper surface of the top mount  12 . The top mount  12  is connected to a coil spring  14  and a rod  21  of a mono- or twin-tube hydraulic damper  2 . Inside a tube of the damper  2  a piston assembly attached to the rod  21  led outside the tube is slidably disposed. At the other end the damper tube is connected to the steering knuckle  15  or a swing arm supporting the vehicle wheel. 
     A hydraulic damper  2  shown in part in  FIG. 2  is an example of a twin-tube damper that may be employed in a vehicle suspension  1  presented in  FIG. 1 . It comprises a movable piston assembly  23  making a sliding fit with the inner surface  221  of the tube  22  and dividing the tube  22  into a rebound chamber  24  (above the piston assembly) and a compression chamber  25  (below the piston assembly). 
     At one end the piston rod  21  passes through and is secured to the piston assembly  23 . The other end of the piston rod  21  is led axially outside the damper  2  through a sealed rod guide (not shown). At the compression end, the tube  22  is closed by a base valve assembly  26 . 
     Valve assemblies  27  and  28   b  of the piston assembly  23  control the flow of working liquid passing between the rebound chamber  24  and the compression chamber  25  while the piston is in movement. Similarly, valve assemblies  29  and  28   c  control the flow of working liquid passing between the compression chamber  25  and an additional compensation chamber  30  located between the tube  22  and the second outer tube  32  of the damper. 
     Valve assembly  28   b  of the piston assembly  23  has been illustrated in  FIG. 4  in comparison to the valve assembly  28   a  known from the state of art illustrated in  FIG. 3 . Above and below, reference numerals of elements performing the same or similar functions remain the same, wherein suffixes (b-c) have been added to distinguish particular embodiments of the invention, where appropriate. 
     As shown, axial member in a form of partially threaded projection  211  of the piston rod  21  supports the compression valve assembly  27 , a piston body  231  and the rebound valve assembly  28   b.    
     The compression valve assembly  27  comprises a number of discs  271  deflectably covering the compression through channels (not shown) in the piston body  23 . Similarly, the stack of deflectable discs  281  covers the rebound through channels  282  in the piston body  231 . All elements of the piston assembly  23  are clamped at the inner circumferential portions thereof by a supporting member in a form of a shoulder nut  285  screwed on the external thread provided on a part of the projection  211  with a predetermined torque. Supporting member  285  clamps at least one disc  271  of the valve assembly  27  at the inner circumferential part thereof. 
     Deflectable discs  281  are additionally preloaded by a spring  284  compressed between the shoulder of the nut  285  and a spring seat  283   a  or  283   b  disposed to displace along the longitudinal axis of the damper  2  in order to angularly equalize possible variations in axial pressure of the spring over its perimeter transferred to the top disc  281  surface. Furthermore, each spring seat  283   a  and  283   b  is provided with two circumferential projections  2831  and  2832  to control deflection of the discs under the pressure of the working liquid flowing through the channels  282  during rebound stroke of the damper  2 . 
     Number, shape, diameter and thickness of discs  281 , number and diameter of the channels  282  and preload of the spring  284 , among others, constitute the parameters that are used to adjust damping forces. 
     As shown the spring seat  283   a  known from the state of art is disposed to slide over the surface of the shoulder nut  285  and in its sliding movement is guided by the surface of the shoulder nut  285 . Therefore this sliding movement is prone to dry friction that is influenced by various factors, such as possible buckling of the spring that generates bending moment over the spring seat  283   a  so it may grind over the surface of the shoulder nut  285 . Dimensional tolerance of the particular elements of the assembly also influences the operation of a valve assembly after it is manufactured. 
     If for example a nominal diameter of the supporting member  285  is d 1  and nominal diameter of the spring seat  283   a  is d 2  then to ensure the sliding movement between any pair of these components taken out of the production batch:
 
 d   1 /2+Δ d   1 /2&lt; d   2 /2− Δd   2 /2
 
where Δd 1  and Δd 2  are respectively upper deviation of the supporting member  285  diameter and lower deviation of the spring seat  283   a  diameter. From the inequality above it follows that a gap will usually exist between the spring seat  283   a  and the shoulder nut  285  at least in some angular areas due to mutual eccentricity of these elements. This gap will obviously negatively influence sliding between these elements.
 
     Contact between the spring seat  283   a  and the shoulder nut  285  is obviously necessary for the sliding guidance of the spring seat  283   a  over the shoulder nut  285  but at the same time, due to the unpredictable (inevitable axial shift of these elements) locations of these areas of contact and the contact itself, aforementioned friction problems occur. 
     As shown in  FIG. 4  the spring seat  283   b  of the present invention is centered in relation to the top disc  281  by perimetrically engaging the seat  283   b  by means of an axial projection  2833   b . In this embodiment, the seat  285   b  is provided with one projection  2833   b  extending over the whole perimeter of the seat  283   b  (cf.  FIG. 5 ) and abutting the perimetric edge of the top disc  281 . 
     As shown outer diameter of the axial projection  2833   b  is slightly larger than its axially inner diameter, do that the seat  285   b  simply draws over the top disc  281  during assembly of the piston assembly  23 . 
     A circular gap  286  is provided between the inner surface of the sleeve-like part of the seat  283   b  and the external surface of the shoulder nut  285 . In a result, the seat  283   b  is guided solely by means of the top disc  281  in such a manner that the seat  283   b  does not contact the shoulder nut  285  at all during its axial movement. The sleeve-like part of the seat  283   b  serves only for positioning (centering) the spring  284  relative to the seat  283   b  and for guiding the spring  284 . In other words the gap  286  serves to avoid contact of the seat  283   b  and the shoulder nut  285  and is not an obstacle for the axial movement of these two components that is required in the assembly  28   a  known from the state of art (cf.  FIG. 3 ). 
       FIG. 6  presents an alternative embodiment of the spring seat  283   c  provided with six axial projections  2833   c  equiangularly distributed over the outer perimeter of the seat  283   c . The seat  283   c  may be successfully employed in the valve assembly  28   b  in place of the seat  283   b  of  FIG. 5 . 
       FIG. 7  presents a detailed view of the base valve assembly  26  of the hydraulic damper  2  shown in part in  FIG. 2 . Axial partially threaded projection  311  of a bolt  31  of the base valve assembly  26  supports the compression valve assembly  28   c  and the rebound valve assembly  29  on a body  261  of the base valve assembly  26 . The deflectable discs of the valve assemblies  28   c  and  29  are clamped on the projection  311  at the inner circumferential portions thereof by a nut  285   c  screwed on the external thread provided on part of the projection  311  with a predetermined torque. Deflectable discs of the compression valve assembly  28   c  are preloaded by a spring  284  compressed between a supporting member  2851   c  and the spring seat  283   c  of  FIG. 6 . The supporting member in a form of a clamp nut  2851   c  is clamped on the nut  285   c  forming at least one indent in at least one circular undercut of the nut  285   c.    
     Similarly as in the rebound valve assembly  28   b  of  FIG. 4 , between the inner surface of the sleeve-like part of the seat  283   c  and the outer surface of the external surface of the nut  285   c  a circular gap is provided, wherein the seat  283   c  is centered in relation to the disc of the valve assembly  28   c  by means of the axial projections  2833   c  perimetrically engaging the disc. 
     As the seat  283   c  is not guided by the nut  285   c , it may be substantially shortened to a length sufficient only for reliable catching the uppermost one or two coils of the spring  284  thus enabling for appropriate positioning of the spring  284  relative to the seat  283   c . In comparison with the embodiment of  FIG. 4 , such a construction is less sensitive to possible axial distortions of the seat  283   c . In order to compensate shortening of the spring seat  283   c , the supporting member  2851   c  is provided with a lengthened section guiding the spring  284  and providing an improved stabilization and guidance for the lower section of the spring  284 . 
     In order to measure the influence of valve assembly of the present invention on the damper performance the inventors conducted the following experiment. 
     Eight springs have been used of various deflection angle defined as an angle between bottom and top surfaces of the spring so that a perfect spring would have a zero angle. Springs  1 - 3  were three different but typical springs used in valve assemblies of this kind; springs  4 - 8  were additionally deformed in order to artificially increase this naturally present deflection angle. Three spring seats have been used: two spring seats according to the present invention (Spring seat No.  1 , cf.  283   b  on  FIG. 4  and  FIG. 5  and Spring seat No.  2  cf.  283   c  on  FIG. 6 ) and a standard spring seat (Spring seat No.  3 , cf.  283   a  on  FIG. 3 ). 
     Springs and seats were mounted on a piston assembly of a twin-tube damper as shown in  FIG. 2  wherein each spring was tested with each spring seat so that 24 experiments have been performed. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Maximum values of force hysteresis 
               
             
          
           
               
                 Spring 
                 Observed “force gap” D 
               
             
          
           
               
                   
                 Deflection 
                   
                   
                 Seat No 3 
               
               
                 No. 
                 angle [deg] 
                 Seat No 1 
                 Seat No 2 
                 (comparative) 
               
               
                   
               
             
          
           
               
                 1 
                 0.49 
                 0.0311 
                 0.0182 
                 0.1048 
               
               
                 2 
                 0.68 
                 0.0283 
                 0.0192 
                 0.0997 
               
               
                 3 
                 0.38 
                 0.0266 
                 0.0153 
                 0.1069 
               
               
                 4 
                 5.96 
                 0.0334 
                 0.0199 
                 0.1253 
               
               
                 5 
                 1.21 
                 0.0266 
                 0.0217 
                 0.1059 
               
               
                 6 
                 0.76 
                 0.0159 
                 0.0215 
                 0.1453 
               
               
                 7 
                 1.54 
                 0.028 
                 0.018 
                 0.173 
               
               
                 8 
                 1.66 
                 0.0264 
                 0.037 
                 0.0805 
               
             
          
           
               
                 AVERAGE: 
                 0.027 
                 0.021 
                 0.118 
               
               
                   
               
             
          
         
       
     
     Testing procedure involved measuring damping force as a piston rod velocity input sinusoidal function and subsequently determining the maximum measured difference D (“force gap”) during rebound stroke between damping forces observed for the same velocity value during acceleration and deceleration of the piston rod. 
     Results of the testing procedure are listed in Table 1 and illustrated in  FIGS. 8, 9  showing a force to velocity characteristic observed for a standard spring seat  283   a  shown on  FIG. 3 . 
     As shown, the valve assembly with a spring seat of the present invention allowed on average for 77% (Spring seat No.  1 ) and 82% reduction (Spring seat No.  2 ) of the measured “force gap” with respect to the typical spring seat known from the prior art. 
     The above embodiments of the present invention are merely exemplary. The figures are not necessarily to scale, and some features may be exaggerated or minimized. These and other factors however should not be considered as limiting the spirit of the invention, the intended scope of protection of which is indicated in appended claims.