Patent Publication Number: US-2012024647-A1

Title: Cylinder apparatus, method for manufacturing the same, and shock absorber

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a cylinder apparatus, a method for manufacturing the cylinder apparatus, and a shock absorber. 
     For example, Japanese Patent Public Disclosure Hei 5-263860 discloses a strut-type cylinder apparatus having an outer tube provided with a recess for preventing interference with other vehicle members disposed near the cylinder apparatus. 
     However, in a cylinder apparatus having an outer tube provided with, for example, a recess, the outer tube may be deformed due to a change in the residual stress at the recess formed on the outer tube. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a cylinder apparatus using an outer tube capable of being manufactured with improved precision. 
     To achieve the forgoing and other objects, the present invention provides a cylinder apparatus including a cylindrical outer tube having a recess recessed from a radial one side on a side surface thereof, and a residual stress adjustment portion on the side surface thereof at a position radially opposite from the recess. A residual stress value of the residual stress adjustment portion is closer to a residual stress value of the recess than a residual stress value of a portion surrounding the residual stress adjustment portion is to the residual stress value of the recess. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates application of crush processing to a base shell with use of a crush die according to an embodiment of the present invention, and is a cross-sectional view of the base shell taken along the axis of the base shell; 
         FIG. 2  is a cross-sectional view taken along line A-A shown in  FIG. 1 ; 
         FIG. 3  illustrates a first window or a second window formed at an upper die or a lower die; 
         FIG. 4  is a cross-sectional view of a surface including a recess and an impression formed on the base shell, taken along the axis of the base shell; 
         FIG. 5  is a perspective view illustrating the recess formed on the base shell; 
         FIG. 6  is a perspective view illustrating the impression of a bend prevention die formed on the base shell; and 
         FIG. 7  is a vertical cross-sectional view of a cylinder apparatus to which the present embodiment is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will be described with reference to the accompanying drawings. Here, first, an object of the present embodiment will be described in detail. 
     In a strut-type suspension, it is advantageous to reduce an offset amount between a strut axis and a load input axis as much as possible, in terms of improvement of a ride comfort of a vehicle such as an automobile. A reduction in an offset amount between a strut axis and a load input axis can be realized by erecting (vertically setting up) the strut axis, which, however, causes a problem about interference between a cylinder apparatus constituting the strut and a tire. Therefore, as a measure for preventing interference with a tire, there is known a cylinder apparatus having an outer tube, i.e., a member which will be herein referred to as “base shell”, at which a recess is formed for avoiding a contact with a tire. However, in a strut-type suspension, an inner tube, in which a piston of a twin-tube-type cylinder apparatus slides, tends to have an increased inner diameter for improving the damping force characteristic thereof. According to this increase, a space for a reservoir chamber defined between the base shell and the inner tube, i.e., a space between the base shell and the inner tube tends to be reduced, which leads to a demand for strict processing procession to the recess formed on the base shell. In the method of pressing the base shell with a die from the radially outer side in order to form the recess, the required high processing precision can be satisfied by inserting a mandrel in the inner circumference of the base shell, and then pressing the base shell with a large force from the radially outer direction. However, a change in the residual stress due to the recess when the recess is formed on the side surface of the base shell may cause the base shell to be warped (the central axis of the cylinder to be bent). With the aim of solving this problem, the following description of the present embodiment will introduce an example of the processing method for forming the recess to the outer tube with strict processing precision and an example of a cylinder apparatus using the thus-formed outer tube. 
     First,  FIG. 7  is a cross-sectional view illustrating an overall structure of a cylinder apparatus  100  employing the present embodiment. 
     The cylinder apparatus  100  is generally constituted by a base shell  1  serving as an outer tube, an inner tube  101  as an inner tube coaxially disposed in the base shell  1 , a piston  102  configured to slide in the inner tube, and a piston rod  103  having one end connected to the piston  102  and the other end protruding from one end of the base shell  1  by penetrating through a rod guide  105  having a seal  104 . Oil and gas are sealingly contained in the base shell  1  as operating fluid. 
     The operating fluid may be embodied by only gas such as air or nitrogen, or may be embodied by liquid such as water containing an additive. 
     A spring sheet  107  is fittedly attached around the outer circumference of the base shell  1  for mounting a spring for suspension between the cylinder apparatus  100  and a vehicle such as an automobile. A bracket  108  is attached to the lower end of the base shell  1  for attaching the cylinder apparatus  100  to a spindle of a wheel. 
     Damping valves  109  are disposed at the upper and lower end surfaces of the piston  102  for generating a damping force. 
     A bolt portion  110  is formed at the tip of the other end of the piston rod  103  for attaching the cylinder apparatus  100  to a vehicle body through a mount. 
     A recess  2  is formed on the outer circumference of the base shell  1  for preventing interference with a wheel when the cylinder apparatus  100  is mounted on the vehicle body. Further, an impression  21 , which serves as a residual stress adjustment portion of the present invention, is formed at a position radially opposite from the recess  2  of the base shell  1 . Apparently, this impression  21  cannot be distinguished from a surrounding portion  111  which surrounds the impression  21 , and a step is generated along the outer circumference of the impression  21  when a high pressure is applied thereto (a step is generated along a boundary between the impression  21  and the surrounding portion  111 ). However, the impression  21  is too inconspicuous to be noticed when the cylinder apparatus  100  is coated or painted. In other words, in the area of the impression (residual stress adjustment portion)  21 , the outer circumferential surface of the base shell  1  is substantially undeformed relative to the cylindrical portion surrounding the impression  21 . 
     Further, in the area of the impression (residual stress adjustment portion)  21 , the inner circumferential surface of the base shell  1  is substantially undeformed respective to the cylindrical portion surrounding the impression  21 . 
     In the present embodiment, the recess  2  is formed for the purpose of preventing interference with a wheel. However, the recess  2  may be formed at the same position or a different position for the purpose of avoiding a brake hose, a sensor, a suspension arm or a stabilizer. In this case, the shape of the recess  2  may vary according to an avoided member. 
     In the following, a description will be given of a method for processing a shock absorber (cylinder apparatus) constituting a strut of a strut-type suspension. More specifically, a description will be given of a method for forming the recess  2  at the base shell  1  of a twin-tube-type hydraulic shock absorber for preventing interference with a tire. 
     Referring to  FIGS. 1 and 2 , a description will be given of a crush die  3  which is used for crush processing to form the recess  2  at the base shell  1  of the cylinder apparatus  100 . The crush die  3  includes a holder die  4 , a mandrel  5 , a press die  6 , and a bend prevention die  7 . The holder die  4  is divided into an upper die  8  and a lower die  9 . The base shell  1  is held by the pair of dies  8  and  9  at a held portion  1   a  of the base shell  1 , i.e., the outer diameter (outer circumferential surface) of the substantially central portion of the base shell  1  in the axial direction thereof. Further, a holding surface  10  is formed on a matching surface  8   a  of the upper die  8 . The holding surface  10  has a shape of a half divided inner cylinder so as to correspond to the upper portion of the outer diameter of the held portion  1   a  of the base shell  1  shown in  FIGS. 1 and 2 . Further, a holding surface  11  is formed on a matching surface  9   a  of the lower die  9 . The holding surface  11  has a shape of a half divided inner cylinder so as to correspond to the lower portion of the outer circumference of the held portion  1   a  of the base shell  1  shown in  FIGS. 1 and 2 . 
     The holder die  4  is configured to hold the base shell  1  while in contact with the whole circumference of the outer diameter (outer circumferential surface) of the held portion  1   a  of the base shell  1  by tightening fastening bolts  12  to join the upper die  8  and the lower die  9  while the held portion  1   a  of the base shell  1  is sandwiched by the holding surface  10  of the upper die  8  and the holding surface  11  of the lower die  9 . Secure holding of the outer diameter of the base shell  1  can be realized by configuring the die  4  so that the die  4  has an appropriate clearance (space) between the matching surface  8   a  of the upper die  8  and the matching surface  9   a  of the lower die  9  while the die  4  is in a state before tightening the fastening bolts  12 . The drawings show a die for use in small production such as production of a prototype by way of example, and therefore the bolts  12  are used for fastening the upper die  8  and the lower die  9 . For mass production, the upper die  8  and the lower die  9  may be disposed to be movable toward or away from each other with use of, for example, a hydraulic pressure, and the tube may be held with aid of the hydraulic pressure. 
     As shown in  FIGS. 1 and 2 , the mandrel  5  is formed into a column shape, and has such a cross-sectional shape that the left side and the right side (left side and right side as viewed in  FIG. 2 ) of the upper portion thereof in the direction perpendicular to the axis, and a bottom  5   a  of the lower portion of the mandrel  5  can abut against the inner diameter (inner circumferential surface) of the held portion  1   a  of the base shell  1  held by the holder die  4 . Further, the mandrel  5  includes a die attachment portion  13  formed by cutting out, in the direction perpendicular to the axis, the upper portion of the center of the mandrel  5  in the axial direction thereof (left-right direction as viewed in  FIG. 1 ). An underside reception die  14  (flat surface of the mandrel  5 ) is attached to the die attachment portion  13  for receiving a load applied by the press die  6 . Further, the both ends of the mandrel  5  are supported by mandrel guides  15  disposed in the front and at the back (the left and right as viewed in  FIG. 1 ) of the lower die  9  of the holder die  4 . The mandrel guides  15  include a slide portion  15 A, and are disposed so as to be vertically movable relative to the lower die  9 . Stoppers  16  are attached to the both end surfaces of the mandrel  5  inserted through the base shell  1 , for preventing a movement of the mandrel  5  relative to the base shell  1  and the holder die  4 . Further, a clearance is provided at the left side and the right side as viewed in  FIG. 2  between the base shell  1  and the mandrel  5  for the purpose of facilitation of pullout of the mandrel  5  from the base shell  1 . 
     The holder die  4  includes a first rectangular window  17  (refer to  FIG. 3 ) having one side (upper side) open to the center of the top surface of the upper die  8 , and the other side (lower side) open to the holder surface  10  of the upper die  8 . The press die  6  is vertically slidably inserted in the first window  17  to face the underside reception die  14  via the side wall of the base shell  1 . Further, the holder die  4  includes a second rectangular window  18  having one side (upper side) open to the holding surface  11  of the lower die  9 , and the other side (lower side) open to the center of the bottom surface of the lower die  9 . A bend prevention die  7  is inserted in the second window  18  so as to face the press die  6  via the base shell  1  with the mandrel inserted therethrough, in such a manner that the bend prevention die  7  is vertically movable relative to the lower die  9 . Further, the lower die  9  is also borne by being vertically supported on a base  7   a  of the prevention die  7  by a spring  7   b.    
     The bend prevention die  7  includes the base  7   a  having a top surface which faces the bottom surface of the lower die  9  of the holder die  4  with a predetermined space therebetween. A guide post  19  is erected at each of the four corners of the base  7   a  of the bend prevention die  7 . The respective guide posts  19  are relatively movably inserted in respectively corresponding guide holes  20  formed at the lower die  9  of the holder die  4 . Due to this arrangement, the holder die  4  is supported in a floating manner so as to be vertically movable relative to the bend prevention die  7  while the base shell  1  shown in  FIGS. 1 and 2  is held by the holder die  4 . The crush die  3  may be configured such that a guide bush (sleeve) is provided at each of the guide holes  20 , and each of the guide posts  19  is inserted in each of the guide bushes. In this case, it becomes possible to reduce sliding resistance of the holder die  4  against the guide posts  19  to more smoothly move the holder die  4  in the vertical direction. 
     As shown in  FIG. 1 , a tapered portion  6 A is formed around the tip surface of the press die  6 . The crush die  3  is configured such that the substantial area (hereinafter referred to as “processing effective area of the press die  6 ”) of the molding surface of the press die  6  not including the tapered portion  6 A is substantially equal to the substantial area (hereafter referred to as “reception area of the bend prevention die  7 ”) of the reception surface of the bend prevention die  7 . Due to this arrangement, a processing force F (indicated by the down-pointing arrow in  FIG. 2 ) applied from the press die  6  acting on the base shell  1  becomes equal to a reactive force R (up-pointing arrow in  FIG. 2 ) applied from the bend prevention die  7 . Therefore, in the present embodiment, a substantially equal relationship is established between a residual stress generated at the recess  2  (refer to  FIG. 5 ) formed at the base shell  1  by the crush processing with use of the press die  6 , and a residual stress generated at an impression  21  (refer to  FIG. 6 ) of the bend prevention die  7  which is formed at the opposite side of the base shell  1  from the recess  2  (the position displaced from the recess  2  by an angle of 180° in the circumferential direction of the base shell  1 ) by the reactive force to the processing force received by the bend prevention die  7  during the crush processing. As a result, residual stresses generated at the both sides of the base shell  1  in the diametrical direction (vertical direction as viewed in  FIG. 4 ) of the base shell  1  have closer values, whereby a formation of a bend of the base shell  1  as shown in  FIG. 4  can be prevented. Typically, the impression  21  has a substantially identical shape to the shape of the recess  2 , as shown in  FIGS. 6 and 5 . For actual manufacturing of a product, an actual area and processing force F of the press die  6  is finely adjusted while a product test is conducted, thereby achieving a tube with little warp or deformation thereof. 
     The present embodiment functions as follows. First, the held portion  1   a  of the base shell  1  with the mandrel  5  inserted therethrough is sandwiched by the holding surface  10  of the upper die  8  and the holding surface  11  of the lower die  9 , and then the upper die  8  and the lower die  9  are joined together by tightening the fastening bolts  12 . As a result, the outer diameter (outer circumferential surface) of the held portion  1   a  of the base shell  1  is securely held by the holder die  4 . Attachment of the respective stoppers  16  at the both sides of the mandrel  5  supported by the pair of mandrel guides  15  enables proper positioning of the mandrel  5  relative to the holder die  4 . As shown in  FIG. 2 , when the base shell  1  is held by the holder die  4 , the press die  6  and the bend prevention die  7  vertically face each other through the base shell  1 , and the bend prevention die  7  abuts against the lower portion of the outer diameter of the held portion  1   a  of the base shell  1 . Further, the holder die  4  is supported by the bend prevention die  7  in a floating manner. 
     In this state, crush processing is locally applied to the upper portion of the outer diameter of the held portion  1   a  of the base shell  1  by pressing it by the press die  6  with the processing force F of 20 to 30 tons while the corresponding portion of the base shell  1  is received by a pressure reception portion formed on the flat surface of the underside reception die  14  of the mandrel  5 , thereby providing the base shell  1  with the recess  2  constituted by an impression of a rectangular flat surface as shown in  FIG. 5 . At the same time, the impression  21  of the bend prevention die  7  as shown in  FIG. 6  is formed at the portion opposite of the base shell  1  from the recess  2 , i.e., the portion against which the bend prevention die  7  abuts, due to the reactive force R to the processing force F of the press die  6  received by the bend prevention die  7 . In the present embodiment, the outer diameter (outer circumferential surface) of the base shell  1  is securely held by the holder die  4 , and the processing effective area of the press die  6  and the reception area of the bend prevention die  7  are sized so as to be equal to each other. In addition, the crush processing is performed as cold working. As a result, it is possible to achieve application of the reactive force R (R=20 to 30 tons), which has strength equal to the pressing force F applied by the press die  6 , from the bend prevention die  7  to the base shell  1 . 
     Therefore, the base shell  1  can be formed in such a manner that the residual stress generated at the recess  2  (refer to  FIG. 5 ) of the base shell  1  and the residual stress generated at the impression  21  (refer to  FIG. 6 ) formed opposite from the recess  2  become closer to each other. Further, in the present embodiment, the processing is performed in a cold condition, whereby it is highly unlikely to receive an influence of a stress change due to a heat during the processing. 
     Stress measurement was actually conducted, the result of which will be now described. The above-mentioned processing method was applied to an electric resistance welded tube as the base shell  1 , which is made of a carbon steel pipe for machine structural use (STKM13A) and has an outer diameter of 60 mm, a thickness of 3 mm and a length of 370 mm. After the processing, the axial stresses were measured by the X-ray diffraction method. As a result, the stress at the surrounding portion  111  around the impression  21  was 96 Mpa, and the stress at the recess  2  was −83 to 29 Mpa. On the other hand, the stress at the impression  21  (residual stress adjustment portion) was −16 to 32 Mpa. This result indicates that the stresses at the recess  2  and the impression  21  each were changed to a stress of the compression side, compared to the stress at the unprocessed portion (surrounding portion  111 ). Consequently, the residual stress generated at the impression  21  (refer to  FIG. 6 ) formed at the opposite side from the recess  2  became closer to the residua stress generated at the recess  2  (refer to  FIG. 5 ) of the base shell  1 , whereby the outer tube only had extremely little warp of 0.05 mm. 
     In this way, according to the present invention, it is possible to have closer values for the residual stresses generated at the both sides of the base shell  1  in the diametrical direction thereof (vertical direction as viewed in  FIG. 4 ), and thereby possible to effectively prevent the base shell  1  from bending as shown in  FIG. 4 . 
     The cylinder apparatus using the base shell  1  prepared in the above-mentioned manner is manufactured as follows. First, one end side of the base shell  1  is closed, for example, by welding a bottom cap thereto or by performing closing processing. The inner tube  102  is inserted in the base shell  1 . Then, after oil is supplied into the base shell  1  and the inner tube  102 , the piston rod assy is assembled by penetratingly inserting the piston rod  103  with the piston  102  attached thereto through the seal  104  and the rod guide  105 . This piston rod assy is inserted in the base shell  1  and the inner tube  102 , and the other end side of the base shell  1  is swaged, thereby completing manufacturing of a shock absorber for an automobile as the cylinder apparatus  100 . 
     The present embodiment provides the following advantageous effects. 
     According to the present embodiment, while the outer diameter (outer circumferential surface) of the base shell  1  with the mandrel  5  inserted therethrough is securely held by the holder die  4 , crush processing (cold working) is locally applied to the upper portion of the base shell  1  with use of the press die  6  to form the recess  2  at the base shell  1 , and at the same time, the impression  21  of the bend prevention die  7  is formed at the lower portion of the outer diameter of the base shell  1 , i.e., the opposite side from the recess  2  due to the reactive force to the processing force which is applied by the press die  6  and is received by the bend prevention die  7 . 
     Therefore, it can be prevented that the pressing force F applied by the press die  6  is consumed for, for example, elastic deformation of the base shell  1 , due to the secure holding of the outer diameter of the base shell  1  by the holder die  4 . In addition, setting the same width for the processing effective area of the press die  6  and the reception area of the bend prevention die  7  enables the processing force F applied by the press die  6  to the base shell  1  and the reactive force R from the bend prevention die  7  to become equal to each other. 
     This arrangement enables the residual stress generated at the recess  2  of the base shell  1  and the residual stress generated at the impression  21  formed at the opposite side from the recess  2  to become substantially equal to each other. As a result, the residual stresses generated at the both sides of the base shell  1  in the diametrical direction can have closer values, thereby effectively preventing the base shell  1  from bending. 
     In the present embodiment, the present invention is embodied as a processing method for forming a recess at a base shell by way of example. However, the present invention is not limited thereto. For example, the present invention may be applied as a processing method for providing a base shell with a spring sheet fixation portion or a protrusion required for mounting of a bracket to be formed on the base shell. In this case, a protrusion is formed at the underside reception die  14  of the mandrel  5 , and a recess is formed at the press die  6 . This embodiment can also bring about the same advantageous effects as the present embodiment. 
     It should be noted that the cylinder apparatus according to the present invention can be applied as a cylinder apparatus of an electromagnetic suspension equipped with a ball screw and a linear motor therein. According to the present embodiment, it is possible to provide a cylinder apparatus with use of an outer tube that can be manufactured with improved precision. 
     Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teaching and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention. 
     The present application claims priority to Japanese Patent Application No. 2010-171908 filed on Jul. 30, 2010. The entire disclosure of Japanese Patent Application No. 2010-171908 filed on Jul. 30, 2010 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.