Abstract:
A spring retainer formed from a titanium alloy and comprising a cylinder and a brim formed integrally with the cylinder is disclosed. The brim has on a top thereof a slope formed such that a thickness of the brim decreases radially outwardly.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an improvement in a valve spring retainer for an engine and a method for manufacturing the retainer.  
       BACKGROUND OF THE INVENTION  
       [0002]     A valve spring retainer for engine acts to receive an end of a valve spring, and retain intake/exhaust valves and a cotter fitted in the valves, which are located inside the retainer, so that the valve spring does not unfasten during operation of the valves.  
         [0003]     JP-A-7-180013, for example, proposes a valve spring retainer for engine using a Ti-6Al-4V-based Ti alloy as a material. The valve spring retainer is now described with reference to  FIG. 11  hereof.  
         [0004]     As shown in  FIG. 11 , the valve spring retainer for engine  200  has a through-hole  201  through which valves aligned along an axis pass, and receiving parts  202 ,  203  for supporting the valve spring at a periphery of the through-hole.  
         [0005]     JP-A-7-180013 discloses a technique for hot-forming the Ti-6Al-4V-based Ti alloy at 300 to 800° C. into a certain shape.  
         [0006]     Furthermore, JP-A-7-180013 discloses a technique for cold-forming a Ti-4Al-22V-based Ti alloy at about 200° C. into a certain shape.  
         [0007]     However, the Ti-6Al-4V-based α/β Ti-alloy has the problem that since it must be formed by hot forging, finishing is necessary to improve dimension accuracy or obtain a smooth surface, which increases production cost due to increase in number of steps and the like.  
         [0008]     On the other hand, the Ti-4Al-22V-based Ti alloy is a β-type Ti alloy, which is formed by cold rolling, and therefore a product with excellent shape is obtained. However, the β-type Ti alloy has the problem that the material is expensive, and that the life of the metal mold is short because of high deformation resistance in forging, resulting in increase in cost of the valve spring retainer for engine.  
       SUMMARY OF THE INVENTION  
       [0009]     According to an aspect of the present invention, there is provided a spring retainer for retaining one end of a valve spring for biasing intake/ exhaust valves to a closed position, which comprises a cylinder and a brim formed integrally with the cylinder and extending radially outwardly from an upper end of the cylinder, wherein the spring retainer comprises a titanium alloy formed on a top of the brim by cold forging at least upon finishing, and a slope made by compressing the brim obliquely downwardly during the cold forging such that the brim has a thickness decreasing in a radially outward direction.  
         [0010]     In this way, the brim top is compressed obliquely downwardly during the cold forging so that the thickness of the brim decreases in a radially outward direction. As a result, even if an α-type titanium alloy having large deformation anisotropy is used as a material, when the retainer is formed by cold forging, shrinkage, which tends to be generated at the brim base where a lower surface of the brim intersects with an outer peripheral surface of the cylinder, can be suppressed, and anisotropy of the outer diameter can be also suppressed. Consequently, it becomes possible to mass-produce valve spring retainers made of titanium alloy, which are high in strength and inexpensive.  
         [0011]     Preferably, the titanium alloy comprises an α-type titanium alloy containing 0.5 to 1.5 mass percent of iron and 0.2 to 0.5 mass percent of oxygen in addition to titanium, and contains other inevitable impurities. Accordingly, the cost of the alloy material becomes less than that of the Ti-6Al-4V alloy or the Ti-4Al-22V alloy.  
         [0012]     Desirably, the titanium alloy comprises an α-type titanium alloy containing 0.5 to 1.5 mass percent of iron, 0.2 to 0.5 mass percent of oxygen, and 0.01 to 0.06 mass percent of nitrogen in addition to titanium, and contains other inevitable impurities. Therefore, material cost in the alloy can be reduced compared with the Ti-6Al-4V alloy or the Ti-4Al-22V alloy, and this alloy containing N can have greater strength than a α-type titanium alloy without N.  
         [0013]     In a preferred form, the slope formed on the brim runs radially outwardly from a position only a distance t away from an outer peripheral surface of the cylinder. The distance may be set on the basis of (0.395D−0.5d)≦t≦(0.453D−0.5d), where D is an outer diameter of the brim and d is an outer diameter of the cylinder. Accordingly, t is set to fall within the range in which the anisotropy of deformation can be securely suppressed.  
         [0014]     Preferably, the brim has a distal end of a width set to fall within a range of 41% to 70% of a maximum thickness of the brim. When it is less than 41%, a defective shape appears at the edge of the brim, and when it exceeds 70%, the effect of suppressing anisotropy of the slope becomes insufficient. Thus, in each case, the spring retainer has a bad shape. For these reasons, the thickness of the edge of the brim is set to 41% to 70% of the maximum thickness of the brim, and thereby a spring retainer having an excellent shape can be obtained.  
         [0015]     It is desirable that the brim has a relief portion of a constant thickness formed at an outer edge or peripheral end thereof. With the relief portion, the deformation anisotropy is sufficiently suppressed, and therefore uniformity of the outer diameter of the brim is improved.  
         [0016]     Desirably, the relief portion has a width set to be at most 30% of a length of the slope formed on the brim. With this, the deformation anisotropy can be sufficiently suppressed, and the uniformity of the outer diameter can be improved with certainty without reducing the degree of freedom in the shape of the slope formed on the brim.  
         [0017]     According to another aspect of the present invention, there is provided a method for manufacturing a spring retainer which comprises a cylinder and a brim projecting outwardly therefrom and is designed for supporting one end of a valve spring with an outer peripheral surface of the cylinder and a lower surface of the brim, the method comprising the steps of cutting a titanium blank in the form of a wire or rod to obtain a forming material, upsetting the forming material placed in a recess of a die of a metal mold by a bottom surface of a punch, punching a hole in the upset piece after the upsetting, obtaining a preform by cold forging the punched piece placed in the recess of the die using a punch, and forming the brim such that a thickness of the brim decreases in a radially outward direction by cold forging the preform placed in the recess of the die using the punch.  
         [0018]     In this way, the spring retainer is manufactured through at least five stages: cutting the wire rod or the stick of titanium to make the forming material in the first step, upsetting the forming material in the second step, punching a hole in the material in the third step, cold-forging it into the preform in the fourth step, and then cold-forging the piece to obtain the spring retainer as a fifth step.  
         [0019]     Since cold forging is used in the method, a finished forming product of the spring retainer can be completed without post-machining such as grinding after final forging.  
         [0020]     Moreover, the brim is compressed obliquely downward in cold forging so that the thickness of the brim is decreased in a radially outward direction, so that material flow at the brim can be made uniform, and thus a spring retainer having high uniformity of outer diameter can be obtained. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0022]      FIG. 1  is a sectional view showing an example of a valve operating mechanism including a spring retainer;  
         [0023]      FIG. 2A  to  FIG. 2C  are views showing processes from cutting of a titanium rod to hole punching during cold forming;  
         [0024]      FIG. 3A  and  FIG. 3B  are views showing steps from a preform of the spring retainer to finish forming;  
         [0025]      FIG. 4  is a sectional view of a type-A spring retainer considered in the invention;  
         [0026]      FIG. 5  is a plane view of  FIG. 4 ;  
         [0027]      FIG. 6  is a sectional view of a type-B spring retainer considered in the invention;  
         [0028]      FIG. 7  is a sectional view of a type-C spring retainer considered in the invention;  
         [0029]      FIG. 8  is a sectional view of a type-D spring retainer considered in the invention;  
         [0030]      FIG. 9  is a sectional view of a spring retainer according to the invention;  
         [0031]      FIG. 10A  and  FIG. 10B  are views showing a shape of a punch according to the invention; and  
         [0032]      FIG. 11  is a sectional view showing a spring retainer in the related art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     A valve operating mechanism  10  shown in  FIG. 1  comprises a valve seat  13  for receiving a valve head  12  of an intake valve (or exhaust valve)  11 ; a valve stem  14  extending upward from the valve head  12 ; a valve guide  15  for guiding the valve stem  14 ; a valve spring  16  for biasing the intake valve  11  to the closed position; a spring retainer  20  for retaining one end  17  of the valve spring  16 ; a cotter  22  that is located inside the spring retainer  20  and fits in an upper recess  21  of the valve stem  14 ; an inner shim  23  provided at the upper end of the valve stem  14 ; a lifter  24  that covers the valve stem  14 , valve spring  16 , spring retainer  20 , cotter  22  and inner shim  23 ; and a cam shaft  25  having a cam  26  which contacts with the lifter  24 .  
         [0034]     Reference numeral  27  indicates a piston, and  28  indicates a cylinder head.  
         [0035]     Steps from cutting of a titanium rod to cold punching are described according to  FIG. 2A ,  FIG. 2B , and  FIG. 2C .  
         [0036]     First, as shown in  FIG. 2A , a titanium rod  32  is carried on a base  31  of a shearing apparatus  30 , and then the titanium rod  32  is cut out by a cutter  33  to obtain a forming material  34  as shown by arrow P.  
         [0037]     In  FIG. 2B , the forming material  34  is placed in a recess  37  of a die  36  of a metal mold  35 , and then a bottom surface  39  of a punch  38  is moved as shown by arrow Q to upset the forming material  34  in the recess.  
         [0038]     In  FIG. 2C , the obtained upset piece  43  is set in a recess  42  of a die  41 . Then, the upset piece  43  is subjected to punching by moving the punching tool  44  as shown by arrow U.  
         [0039]      FIG. 3A  and  FIG. 3B  show steps from the perform to finish forming of a spring retainer.  
         [0040]     In  FIG. 3A , a punched piece  47  is placed in a recess  46  of a die  45 , and then a punch  48  is moved as shown by arrow V to cold-forge the punched piece  47  into the preform.  
         [0041]     In  FIG. 3B , the preform  52  is placed in a recess  51  of a die  49 , and then a punch  53  is lowered as shown by arrow W to cold-forge the preform into the main spring retainer form or the finished form.  
         [0042]     Each of the steps can be successively carried out using forging equipment such as a header machine, or can be carried out separately.  
         [0043]      FIG. 4  shows a type-A spring retainer  55 .  
         [0044]     The type-A spring retainer  55  is a component having a body  56  which comprises a cylinder  57 , a brim  58  provided outside the cylinder, and a top  59  of the brim  58  which is beaten flat out to the outer peripheral end or edge  61  of the brim  58 . Reference numeral  62  indicates a through-hole, and  63  indicates the brim base. The brim base  63  resides where a lower surface of the brim  58  intersects with an outer peripheral surface of the cylinder  57 . The brim base  63  has a round shape. The spring retainer  55  shown in  FIG. 4  is here referred to as configuration A.  
         [0045]     As shown in  FIG. 5 , in the type-A spring retainer  55  used as a final product, the brim  58  preferably has a circular profile. However, when the brim  58  is subjected to stretch forming by cold forging, sometimes it has a distorted circular profile. Thus, to quantify the level of the distortion, oblateness of the outer diameter is obtained from the equation oblateness={(D 1 −D 2 )/D 1 }×100, where the maximum diameter is D 1  and the minimum diameter is D 2 . Of course, the smaller the oblateness of the outer diameter, the better.  
         [0046]     A stick of (Ti-1Fe-0.3O)(oxygen of 0.3 mass percent) was used as starting material. The stick was subjected to cutting, and then subjected to upsetting, punching, preforming (cold forging), and main-forming (cold forging) as described, so that the type-A spring retainer  55  shown in  FIG. 4  was obtained. The oblateness of the completed product was 8.9%. It is said that the allowable oblateness of the outer diameter is at most 1.0%; therefore, a retainer having oblateness of 8.9% could never be used.  
         [0047]     Realizing that anisotropy is significant in the inexpensive titanium alloy Ti-1Fe-0.3O, the inventors investigated measures to deal with this anisotropy. One of several ideas considered was that when the brim  58  shown in  FIG. 4  was expanded radially outward and fluidized, if the flow was appropriately controlled, the oblateness may possibly be improved. That is, it was considered that if flow at regions where the brim easily expands was suppressed, and flow at those regions was directed to regions where the brim does not readily expand, the outer diameter could be made uniform. Based on the idea, the following configuration B was determined.  
         [0048]      FIG. 6  shows a type-B spring retainer  64 .  
         [0049]     The type-B spring retainer  64  is a component in which a body  65  has a cylinder  66 ; a brim  68  is provided outside of the outer peripheral surface  67  of the cylinder  66 ; and assuming that an extension line S extended upward from the outer peripheral surface  67  is the reference, and a position on a top of the body  65  that is displaced only −t toward the central axis of the cylinder  66  from the extension line S is made the compression starting position  69 , a slope  71  inclined downward toward the edge  72  of the brim  68  with the compression starting point  69  as a starting point of the slope is provided. This shall be referred to as configuration B. A reference numeral  73  indicates a through-hole, while  74  indicates a brim base. The brim base  74  resides where the lower surface of the brim  68  intersects with the outer peripheral surface  67  of the cylinder  66 . The brim base  74  has a round shape.  
         [0050]     Table 1 shows results of the investigation on the oblateness of the outer diameters of the brims of the configurations A and B.  
                               TABLE 1                       Test           Oblateness of outer   Roof shape of       No.   Configuration   t (mm)   diameter of brim (%)   brim base                   1   A   —   8.9   Excellent       2   B   −0.3   2.9   Shrinkage       3   B   −0.7   2.8   Shrinkage       4   B   −1.0   3.8   Shrinkage                 t: distance from the edge of the cylinder to the taper starting point            Oblateness of outer diameter of brim (%): {(D1 − D2)/D1} × 100, less than 1.0% is acceptable            D1, D2: outer diameter of brim            —: no data because of no taper             
 
         [0051]     The sign t indicates distance from the outer edge of the cylinder to a taper starting point (hereinafter, referred to as compression starting position).  
         [0052]     In test  1  which was a case of the configuration A and no taper, the oblateness of the outer diameter of the brim was 8.9%.  
         [0053]     In tests  2  to  4 , a configuration was B, and when t was −0.3 mm, 0.7 mm and −1.0 mm respectively, the oblateness of the outer diameters of the brims was 2.9%, 2.8% and 3.8%, respectively. In addition, shrinkage was evaluated by examination of shapes of the brim bases  74  (see  FIG. 6 ).  
         [0054]     In this way, the oblateness of the outer diameter of the brim was significantly improved by using the configuration B having the obliquely downward taper in the brim  68 ; however, it still did not reached an acceptability criterion of less than 1.0%.  
         [0055]     The inventors posited that the slope  71  was excessively long in the configuration B, and as a result the oblateness was not as improved as expected and shrinkage was generated. If this is true, it is effective to investigate an intermediate configuration between the configurations A and B.  
         [0056]      FIG. 7  is a sectional view of a type-C spring retainer  75 .  
         [0057]     The type-C spring retainer  75  has a body  76  comprising a cylinder  77  and a brim  79  provided outside of the outer peripheral surface  78  of the cylinder  77 ; and the intersection between the extension line S extended upward from the outer cylinder side  78  and the body upper surface  76  is made the compression starting position  81 . A slope  82  inclined downward toward the edge  83  of the brim  79  starting at the compression starting point  81  is provided. This is here called configuration C. Reference numeral  84  indicates the brim base. The brim base  84  reside in a position where the lower surface of the brim  79  intersects with the outer peripheral surface  78  of the cylinder  77 . The brim base  84  has a round shape.  
         [0058]      FIG. 8  shows a type-D spring retainer  85 .  
         [0059]     The type-D spring retainer  85  is a component in which a body  86  has a cylinder  87 ; a brim  89  is provided outside of the outer peripheral surface  88  of the cylinder  87 ; and using the extension line S extended upward from the outer cylindrical side  88  as a reference, the position on a top of the body  86  which is located outside of the extension line S, displaced only +t toward the brim  89  from the extension line S, is made the compression starting position  91 , a slope  92  inclined downward toward the outer edge  93  of the brim  89  with the compression starting point  91  as a starting point is provided. This here called configuration D. A reference numeral  94  is the brim base. The brim base  94  resides in a position where a lower surface of the brim  89  intersects with the outer peripheral surface  88  of the cylinder  87 . The brim base  94  has a round shape.  
         [0060]     Results of comparison between the type-C spring retainer and the type-D spring retainer are shown in Table 2.  
                                                     TABLE 2                       Test           Oblateness of outer   Roof shape of       No.   Configuration   t (mm)   diameter of brim (%)   brim base                                5   C   0   2.5   Some                       shrinkage       6   D   0.7   0.8   Excellent                 t: distance from the outer peripheral surface of the cylinder to the taper starting point             
 
         [0061]     The sign t indicates distance from the outer circumference of the cylinder to the compression starting point.  
         [0062]     In the test  5 , which was a case where the configuration was C and t was 0 mm, although the oblateness of the outer diameter of the brim was improved to 2.5%, shrinkage was generated in the shape of the brim base  84 .  
         [0063]     In the test  6 , which was a case that the configuration was D and t was 0.7 mm, the oblateness of the outer diameter of the brim was remarkably improved to 0.8%, in addition, the brim base  84  had excellent shape, with no shrinkage.  
         [0064]     Consequently, the configuration D was determined to be used as the configuration of the spring retainer according to the invention.  
         [0065]     As described above, the invention comprises a spring retainer for retaining one end of the valve spring for biasing the intake/exhaust valves to the closed position, and which has a brim that is stretch-formed from a cylinder, and receives the valve spring at the outer peripheral surface of the cylinder and the lower surface of the brim, wherein the spring retainer comprises a titanium alloy, and at least the finishing step is performed by cold forging, and the brim is compressed obliquely downward in the cold forging so that the thickness of the brim is decreased toward the outside in the radial direction.  
         [0066]     As above, since it was found that the configuration D was preferable, next an additional experiment was conducted to find a preferable value of the distance t. Contents and results of the experiment are shown in Table 3.  
                                                             TABLE 3                                   Oblateness of   R shape           Test           outer diameter   of brim       No.   Configuration   t (mm)   of brim (%)   base   Evaluation                                5   C   0   2.5   Some   X                       shrinkage       7   D   0.3   0.9   Excellent   ◯       8   D   0.5   0.9   Excellent   ◯       9   D   1.5   0.9   Excellent   ◯       10   D   2.0   3.3   Excellent   X                 t: distance from the outer circumference of the cylinder to the taper starting point            oblateness of outer diameter of the brim (%): {(D1 − D2)/D1} × 100, less than 1.0% is acceptable            D1, D2: outer diameters of the brim             
 
         [0067]     The configurations C and D were tested.  
         [0068]     In the test  5 , which was a case that the configuration was C and t was 0 mm, the oblateness of the outer diameter of the brim was 2.5%, and some shrinkage was generated in the shape of the brim base  84 . Therefore, it was evaluated to be bad (hereinafter, abbreviated as x).  
         [0069]     In the tests  7  to  9 , which were cases that the configuration was C and when t was 0.3 mm, 0.5 mm and 1.5 mm respectively, the oblateness of the outer diameters of the brims was 0.9%, 0.9% and 0.9% respectively. Therefore, they were evaluated to be excellent (hereinafter, abbreviated as O).  
         [0070]     In the test  10 , which was a case that t was 2.0 mm, the oblateness of the outer diameter of the brim was 3.3% larger than the 1.0% acceptability criterion. Therefore, it was evaluated to be x.  
         [0071]     Thus, t was determined to be 0.3 mm to 1.5 mm.  
         [0072]     Next, conversion of the range of t 0.3 mm≦t≦1.5 mm into a general numerical formula for various spring retainers was attempted. In order to generalize the range of t with a numerical formula, reference was made to  FIG. 9 .  
         [0073]     A spring retainer  100  shown in  FIG. 9  has a body  101  comprising a first cylinder  109  having a large diameter, a second cylinder  114  having a small diameter, and a brim  108  projecting radially outward from the outer peripheral surface  111  of the first cylinder  109 .  
         [0074]     Using the extension line S extended upward from the outer peripheral surface  111  of the first cylinder  109  as a reference, a position on a top  102  of the body  101  which is displaced by a distance t from the extension line S radially outward on the brim  101  is a compression starting point  103 .  
         [0075]     The top of the brim  108  is formed as a slope  104  inclined downward toward the outer edge  106  of the outer peripheral surface portion  105  of the brim  108 , the slope starting from the compression starting point  103 .  
         [0076]     A lower surface  107  of the brim  108  and the outer peripheral surface  111  of the first cylinder  109  support one end of an outer spring (not shown).  
         [0077]     A lower surface  112  of the first cylinder  109  connected to the above outer peripheral surface  111 , and the outer peripheral surface  113  of the second cylinder  114  connected to the lower surface  112  support one end of an inner spring (not shown).  
         [0078]     A reference numeral  115  indicates a brim base. The brim base  115  resides at a position where the lower surface  107  of the brim  108  intersects with the outer peripheral surface  111  of the first cylinder  109 . The brim base  108  has a round shape.  
         [0079]     Here, the outer diameter of the brim is D, the distance between radially opposing compression starting points  103  is T, the outer diameter of the first cylinder is d, and the distance from the outer peripheral surface  111  of the first cylinder  109  to the compression starting point  103  is t. T is set in proportion to D.  
         [0080]     Hereinafter, basic equations of D, T, d and t are shown in equation (1) and equation (2). 
 
(D−T)/D   (1) 
 
 T=d+ 2 t    (2) 
 
         [0081]     The equation (2) is substituted into the equation (1), thereby equation (3) is derived. 
 
(D−(d+2t))/D   (3) 
 
         [0082]     Here, assuming that D=21 mm and d=16 mm, these values are substituted into the equation (3) along with a maximum value of t of 1.5 mm or a minimum value of t of 0.3 mm, so that equation (4) and equation (5) are obtained. 
 
( D −( d+ 2 t ))/ D =(21−(16+2×1.5))/21=0.095   (4) 
 
( D −( d+ 2 t ))/ D =(21−(16+2×0.3))/21=0.21   (5) 
 
         [0083]     The equation (4) is transformed to obtain equation (6) expressing the maximum value of t in terms of D and d. 
 
 D −( d+ 2 t )=0.095 D  
 
−( d+ 2 t )=0.095 D−D=− 0.905 D  
 
2 t= 0.905 D−d  
 
 t= 0.453 D− 0.5 d    (6) 
 
         [0084]     The equation (5) is transformed to obtain equation (7) expressing the minimum value of t in terms of D and d. 
 
 D −( d+ 2 t )=0.21 D  
 
−( d+ 2 t )=0.21 D−D=− 0.79 D  
 
2 t= 0.79 D−d  
 
 t= 0.395 D− 0.5 d    (7) 
 
         [0085]     Since the equation (6) expresses the maximum value of t, and the equation (7) expresses the minimum value of t, the range of t is generalized by the numeral formula (8): 
 
(0.395 D− 0.5 d )≦ t ≦(0.453 D− 0.5 d )   (8) 
 
         [0086]     When D=21 mm and d=16 mm are substituted into the equation (8), t is given as approximately 0.3 mm≦t≦1.5 mm.  
         [0087]     The above is summarized as follows: the slope formed on the brim is to start at a position separated from the outer peripheral surface of the cylinder by distance t in the radially outward direction, and the distance t is determined so that (0.395D−0.5d)≦t≦(0.453D−0.5d), where the outer circumference of the brim is D, and the outer circumference of the cylinder is d.  
         [0088]     Next, detailed investigation was conducted to determine the type of the titanium alloy used for the spring retainer. As the titanium alloy to be used, an α-type titanium alloy, which can be cold-forged and which contains a small amount of iron (hereinafter, referred to as Fe), oxygen (hereinafter, referred to as O) in addition to titanium (hereinafter, referred to as Ti), was investigated.  
         [0089]     Table 4 shows results of the investigation for determining the amount of Fe in the Ti—Fe—O-based titanium alloy.  
                                                                             TABLE 4                                               Oblateness                               of outer           Con-       Alloy composition   Pres-   diameter       Test   figu-   t   (mass percent)   ence   of   evalu-            No.   ration   (mm)   Fe   O   Ti   of crack   brim (%)   ation               11   D   1.0   0.3   0.4   remainder   none   2.6   X       12   D   1.0   0.5   0.4   remainder   none   0.8   ◯       13   D   1.0   1.0   0.4   remainder   none   0.9   ◯       14   D   1.0   1.0   0.4   remainder   none   0.8   ◯       15   D   1.0   1.0   0.4   remainder   present   —   X                 —: no data            t: distance from the outer peripheral surface of the cylinder to the taper starting point            oblateness of outer diameter of brim (%): {(D1 − D2)/D1} × 100; less than 1.0% is acceptable            D1, D2: outer diameter of brim             
 
         [0090]     In any of them, the configuration is D, and t, which is the distance from the outer peripheral surface of the cylinder to the compression starting point, is 1.0 mm.  
         [0091]     In the test  11  which was a titanium alloy containing Fe 0.3 mass percent, O 0.4 mass percent, the remainder being Ti, while cracks were not present after cold forging, the oblateness of the outer diameter of the brim was 2.6%, beyond the acceptability criterion of less than 1.0%. Therefore, it was evaluated as x.  
         [0092]     In the tests  12  to  14  which were titanium alloys whose Fe content was 0.5 mass percent, 1.0 mass percent, and 1.5 mass percent respectively, and with O 0.4 mass percent the remainder being Ti, cracks were not present after cold forging, and the oblateness of the outer diameter of the brim was 0.8%, 0.9% and 0.8% respectively, within the acceptability criterion of less than 1.0%. Therefore, they were evaluated as O.  
         [0093]     In the test  15  which was a titanium alloy containing Fe 1.7 mass percent, O 0.4 mass percent, the remainder being Ti, cracks were present after cold forging. Therefore, it was evaluated as x.  
         [0094]     Thus, the amount of Fe in the Ti—Fe—O-based titanium alloy was determined to be 0.5 mass percent to 1.5 mass percent.  
         [0095]     Similarly, Table 5 shows results of the investigation for determining the amount of O in the Ti—Fe—O-based titanium alloy.  
                                                                             TABLE 5                               Con-       Alloy composition   Pres-   Strength           Test   figu-   t   (mass percent)   ence   of simple   evalu-            No.   ration   (mm)   Fe   O   Ti   of crack   alloy   ation               16   D   1.0   1.0   0.1   remainder   none   X   X       17   D   1.0   1.0   0.2   remainder   none   ◯   ◯       18   D   1.0   1.0   0.3   remainder   none   ◯   ◯       19   D   1.0   1.0   0.5   remainder   none   ◯   ◯       20   D   1.0   1.0   0.6   remainder   present   —   X                 —: no data            t: distance from the outer peripheral surface of the cylinder to the taper starting point             
 
         [0096]     In all of these, the configuration is D, and t, which is the distance from the outer peripheral surface of the cylinder to the compression starting point, is 1.0 mm.  
         [0097]     In the test  16  which was a titanium alloy containing Fe 1.0 mass percent, O 0.1 mass percent, the remainder being Ti, while cracks were not present after cold forging, the strength of simple alloy was weak. Therefore, it was evaluated as x.  
         [0098]     In the tests  17  to  19  which were titanium alloys containing Fe 1.0 mass percent, and whose O content was 0.2 mass percent, 0.3 mass percent, and 0.5 mass percent respectively, the remainder being Ti, cracks were not present after cold forging, and the strength of simple alloy was strong. Therefore, they were evaluated as O.  
         [0099]     In the test  20  which was a titanium alloy containing Fe 1.0 mass percent, O 0.6 mass percent, the remainder being Ti, cracks were present after cold forging. Therefore, it was evaluated as x.  
         [0100]     Consequently, the amount of O in the Ti—Fe—O-based titanium alloy is determined to be 0.2 mass percent to 0.5 mass percent.  
         [0101]     The results of Table 4 and Table 5 are summarized in that the titanium alloy comprises α-type titanium alloy containing 0.5 to 1.5 mass percent of iron, and 0.2 to 0.5 mass percent of oxygen in addition to titanium, and also contains inevitable impurities.  
         [0102]     Generally, in the titanium alloy used for the spring retainer, N is sometimes added to make a Ti—Fe—O—N-based titanium alloy, for the purpose of increasing strength of the Ti—Fe—O-based titanium alloy. Thus, investigation for determining the amount of N in the Ti—Fe—O—N-based titanium alloy was conducted.  
         [0103]     Table 6 shows results of the investigation for determining the amount of N.  
                                                                                 TABLE 6                                       Alloy composition       Strength           Test       t   (mass percent)   Presence   of simple            No.   Configuration   (mm)   Fe   O   N   Ti   of crack   alloy   evaluation               21   D   1.0   1.0   0.3   0.01   remainder   none   ◯   ◯       22   D   1.0   1.0   0.3   0.03   remainder   none   ◯   ◯       23   D   1.0   1.0   0.3   0.06   remainder   none   ◯   ◯       24   D   1.0   1.0   0.3   0.08   remainder   present   —   X                 —: no data            t: distance from the outer circumference of the cylinder to the taper starting point             
 
         [0104]     In all of these, the configuration is D, the distance t from the outer peripheral surface of the cylinder to the compression starting point is 1.0 mm, and Fe 1.0 mass percent and O 0.3 mass percent are contained.  
         [0105]     In the tests  21  to  23  which were titanium alloys with N content of 0.01 mass percent, 0.03 mass percent, and 0.06 mass percent respectively, the remainder being Ti, cracks were not present after cold forging and the strength of simple alloy was strong. Therefore, they were evaluated as O.  
         [0106]     In the test  24  which was a titanium alloy containing N 0.08 mass percent, cracks were present after cold forging. Therefore, it was evaluated as x.  
         [0107]     Thus, the amount of N in the Ti—Fe—O—N-based titanium alloy was determined to be 0.01 mass percent to 0.06 mass percent.  
         [0108]     Summarizing the results of Table 4, Table 5 and Table 6, the titanium alloy is a α-type titanium alloy containing 0.5 to 1.5 mass percent of iron, 0.2 to 0.5 mass percent of oxygen, and 0.01 to 0.06 mass percent of nitrogen in addition to titanium and other inevitable impurities.  
         [0109]     From the above investigations, it was determined that in the completed spring retainer, at least the finishing step was carried out by cold forging, the brim having a constant thickness was compressed in the cold forging such that the thickness was decreased toward the outside in the radial direction, and an inexpensive titanium alloy was used.  
         [0110]     In  FIG. 9 , thickness of the central base portion of the brim  108  is here called “maximum thickness e of the brim”, and thickness of the outer peripheral edge  106  that is the edge of the brim  108  is here called “edge thickness f of the brim”. A relation between the base having the maximum thickness and the edge having the minimum thickness is an important factor for determining a section profile of the brim  108 . Thus, the relation between the maximum thickness e and the edge thickness f was investigated. Contents and results of the investigation are shown in Table 7. Since the following tests were carried out after performing tests  25  to  29  described later, they were given test numbers  30  to  35 .  
                                           TABLE 7                                       Edge                               Maximum   thick-           Con-       thickness   ness f   (f/e) ×       E-       Test   figu-   t   e of brim   of brim   100       valu-       No.   ration   (mm)   (mm)   (mm)   (%)   Formability   ation                   30   D   1.0   1.7   0.6   35   Bad shape   X       31   D   1.0   1.7   0.7   41   Excellent   ◯       32   D   1.0   1.7   0.8   47   Excellent   ◯       33   D   1.0   1.7   1.0   59   Excellent   ◯       34   D   1.0   1.7   1.2   70   Excellent   ◯       35   D   1.0   1.7   1.5   88   Bad shape   X                  
 
         [0111]     In all of these, the configuration is D, the distance t from the outer peripheral surface of the cylinder to the compression starting point is 1.0 mm, and the maximum thickness e of the brim is 1.7 mm.  
         [0112]     The test  30  is a case where the edge thickness f is 0.6 mm, and f/e is 35%. In the forming test, material was insufficiently filled into clearance for forming the edge, and accordingly the shape of the brim was defective. Therefore, it was evaluated as x.  
         [0113]     In the tests  31  to  34  which were cases where the edge thickness f of the brims was 0.7 mm, 0.8 mm, 1.0 mm and 1.2 mm, and f/e was 41%, 47%, 59% and 70%, respectively, results of the forming tests were excellent. Therefore, they were evaluated as O.  
         [0114]     The test  35  is a case where the edge thickness f of the brim is 1.5 mm, and f/e is 88%. In the forming test of it, the effect of suppressing the anisotropy of the deformation by the slope was insufficient, accordingly a bad shape appeared in relief. Therefore, it was evaluated as x.  
         [0115]     From the above results, when thickness of the base of the brim  108  is the “maximum thickness e of the brim”, and thickness of the edge of the brim  108  (outer peripheral edge  106 ) is the “edge thickness f of the brim”, it is important that the edge thickness f of the brim is set to 41% to 70% of the maximum thickness e of the brim, and the shape of the brim can be made excellent by setting the thickness f within this range.  
         [0116]     As shown in  FIG. 3B , a punch  53  having a sharp edge is used to realize the invention. However, the more the edge of the punch is sharpened, the shorter its useful life (the number of shots), which affects productivity. Thus, the shape of the punch is here investigated.  
         [0117]      FIG. 10A  and  FIG. 10B  show shapes of punches according to the invention.  
         [0118]     As shown in  FIG. 1A , a slope  122  of a brim  121  of a spring retainer  120  is machined using an inclined portion  123  of a punch  53 , and then the punch  53  is drawn apart from the spring retainer  120  as shown by an arrow Y  
         [0119]     In this case, although the slope  122  can be formed on the brim  121  of the spring retainer  120 , the tip of the inclined portion  123  of the punch  53  becomes more round as the punch is repeatedly used, which may cause decrease in the useful life of the metal mold. Thus, the punch was improved in the following way.  
         [0120]      FIG. 10B  shows a condition where relief portion  127 , provided on brim  125  of a spring retainer  134 , along with slope  126 , is machined using an inclined portion  129  and a horizontal surface  131  formed on a punch  128 , and then the punch  128  is drawn away from the spring retainer  124  as shown in an arrow Z.  
         [0121]     In  FIG. 10B , e is a maximum thickness of the brim  125 , k is a thickness of the relief portion  127 , and h is a width of the relief portion  127 .  
         [0122]     In this way, since the relief portion  127  having a constant thickness is formed on the outer edge of the brim  125 , the edge of the punch  128  is not deformed regardless of how long it is used. As a result, uniformity of the diameter of the brim  125  is improved.  
         [0123]     From the above, it was found that accuracy of the outer circumference of the brim  125  was improved by forming the relief portion  127 , and subsequently additional tests were conducted to find the optimum thickness k of the relief  127 . Contents and results of the tests are shown in Table 8.  
                                           TABLE 8                                   Maximum   Thick-                       Con-       thickness   ness k   (k/e) ×       E-       Test   figu-   t   e of brim   of relief   100       valu-       No.   ration   (mm)   (mm)   (mm)   (%)   Formability   ation                   25   D   1.0   1.7   0.6   35   Bad shape   X                               of relief       26   D   1.0   1.7   0.8   47   Excellent   ◯       27   D   1.0   1.7   1.0   59   Excellent   ◯       28   D   1.0   1.7   1.2   70   Excellent   ◯       29   D   1.0   1.7   1.5   88   Bad shape   X                               in relief                 t: distance from the outer circumference of the cylinder to the taper starting point             
 
         [0124]     In all of these, the configuration is D, the distance t from the outer peripheral surface of the cylinder to the taper starting position is 1.0 mm, and the thickness of the brim is 1.7 mm.  
         [0125]     The test  25  is the case where the thickness k of the relief is 0.6 mm, and ratio of the thickness k of the relief to the maximum thickness e of the brim is 35%. In this forming test, since material did not uniformly enter the clearance for forming the relief, the resulting relief had a bad shape. Therefore, it was evaluated as x.  
         [0126]     In the tests  26  to  28 , the thickness k of the relief was 0.8 mm, 1.0 mm and 1.2 mm respectively, and ratio of the thickness k of the relief to the maximum thickness e of the brim was 47%, 59% and 70% respectively, and results of the forming tests were excellent. Therefore, they were evaluated as O.  
         [0127]     While not shown in Table 8, the oblateness of the outer diameter of the brim was 0.5% in the tests  26  to  28 .  
         [0128]     The test  29  is a case where the thickness k of the relief  127  is 1.5 mm, and the ratio of the thickness k of the relief to the maximum thickness e of the brim is 88%. In the forming test, suppression of the anisotropy of the deformation by the slope was insufficient, and the resulting relief had a bad shape. Therefore, it was evaluated as x.  
         [0129]     From the above results, the thickness k of the relief  127  needs to be set to 47 to 70% of the maximum thickness e of the brim  125 .  
         [0130]     Since the width h of the relief  127  was set to be a size that does not exceed 30% of length of the slope formed on the brim  125 , the uniformity of the outer diameter can be improved with certainty.  
         [0131]     That is, the brim includes the relief having a constant thickness formed on the outer peripheral edge.  
         [0132]     Furthermore, the width of the relief is set to be at most 30% of the length of the slope formed on the brim.  
         [0133]     Moreover, the brim is compressed obliquely downward in the cold forging so that thickness is decreased toward the outside in the radial direction. As a result, even if the α-type titanium alloy having large deformation anisotropy is used for the material, the shrinkage that tends to be generated at the brim base after forming by cold forging can be suppressed, and the anisotropy of the outer diameter can be also suppressed.  
         [0134]     The type of the engine to which the spring retainers  100 ,  124  of the invention are applied is not particularly limited, as long as the engine has an intake valve and exhaust valve.  
         [0135]     Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.