Abstract:
A screw comprising a first member having a plurality of alternating first thread ridges and first thread grooves, a ridge width of the first thread ridges being larger than a groove width of the first thread grooves at a measurement diameter of the first member. The measurement diameter is determined by calculating a pitch diameter of a hypothetical member having a plurality of hypothetical thread grooves and hypothetical thread ridges, with the hypothetical thread grooves being identical to the first thread grooves, and with the hypothetical thread ridges and the hypothetical thread grooves having identical, but opposite, profiles, and with the measurement diameter being identical to the pitch diameter of the hypothetical member.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a screw used in an accumulator and the like. In particular, it relates to a pitch diameter displaced screw in which, between an external screw and an internal screw that engage with each other, the strength of a weaker screw is improved. 
   Conventionally, for the accumulator and the like, there is used a triangular screw, for example, a metric screw and an inverse buttress screw. As shown in  FIG. 4 , in a standard triangular screw, a sectional shape of a thread ridge is formed in an approximately regular triangle. Further, as shown in  FIG. 5 , in a standard inverse buttress screw, flank inclination angles of its thread ridge are formed in an inverse manner to that of a thread ridge of a buttress screw, i.e., an inclination angle of a pressure flank is formed greater than that of a clearance flank, as described in Japanese unexamined utility model publication No. Hei 4-77017 (1992-77017).) 
   In  FIGS. 4 and 5 , numeral  1  denotes an internal screw meshing with an external screw  2 , and  4  denotes a thread ridge of the internal screw  1  having a pressure flank and a clearance flank. Numeral  6  denotes a pitch diameter of the internal screw  1 , i.e., a diameter of such an imaginary cylinder that a width P 1  of the thread ridge  4  becomes equal to a width P 2  of a thread groove  8 . Numeral  9  denotes an inner diameter of the internal screw  1 , and numeral  11  denotes a thread groove of the external screw  2 , meshing with the thread ridge  4  of the internal thread. Numeral  13  denotes an outer diameter of a thread ridge  14  of the external screw  2 , numeral  16  denotes a pitch diameter of the external screw  2 , and A 2  denotes a load applying direction. 
   Incidentally, a pitch P is equal to a total of the width P 1  of the thread ridge  4  and the width P 2  of the thread groove  8 . Further, each of the widths P 1 , P 2  is a half of the pitch P, i.e., P/ 2 . 
   In the conventional screw, as to the mutually meshing external screw  2  and internal screw  1 , since the pitch diameters  6 ,  16  are located in the center of meshing heights of the thread ridges  4 ,  14 , the width P 1  of the thread ridge  4  of the internal screw  1  becomes equal to the width P 2  of the thread ridge  14  of the external screw  2 , meshing with the thread groove  8  of the internal screw. When a load in the arrow A 2  direction is exerted on the external screw  2  in this state, a compressive force is applied to the external screw  2  while a tensile force is applied to the internal screw  1 , and both forces are equal. Incidentally, at this time, a compressive stress concentrates on thread bottom  10  of the external screw  2 , and a tensile stress concentrates on thread bottom  5  of the internal screw  1 . 
   However, since a screw has a property that it is resistant to the compressive force but weak at the tensile force, in a case where the force of the same magnitude is applied to both thread ridges  4 ,  14 , a screw to which the tensile force is applied, i.e., the internal screw  1 , is the first to suffer a breakage from the thread bottom  5 . Namely, the internal screw  1  is smaller in its fracture strength than the external screw  2 , and weaker. 
   SUMMARY OF THE INVENTION 
   In view of the above circumstances, an object of the present invention is to improve the fracture strength of the weaker screw. According to the present invention, a width of a thread ridge in a position of a standard pitch diameter is formed greater than a width of a thread groove, thereby displacing a pitch diameter. Further, according to the present invention, with respect to a weaker screw between an internal screw and an external screw which mesh with each other, a width of a thread ridge in a position of its standard pitch diameter is formed greater than a width of a thread groove, thereby displacing a pitch diameter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a main part enlarged longitudinal sectional view showing a first embodiment of the present invention; 
       FIG. 2  is a longitudinal sectional view showing the first embodiment of the present invention; 
       FIG. 3  is a main part enlarged longitudinal sectional view showing a second embodiment of the present invention; 
       FIG. 4  is a longitudinal sectional view showing a prior art example (standard metric screw); and 
       FIG. 5  is a longitudinal sectional view showing a prior art example (standard inverse buttress screw). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The first embodiment of the present invention will be explained with  FIGS. 1 and 2 . An accumulator ACC has a bladder  19  inside a container main body  18 . The bladder  19  is a pleated bladder and provided with creases so as to be folded in a predetermined shape. 
   A flange part  20  of the bladder  19  is fixed by a lid  21  in an upper part  18   a  of the container main body  18 . The lid  21  is provided with an intake and exhaust cylinder  22  communicating with an inside of the bladder  19 , and an external screw  25  meshing with an internal screw  23  of the container main body  18 . Both of the screws  23 ,  25  are typically formed from the same material, but can be formed from different materials. 
   Each of the screws  23 ,  25  correspond to a triangular screw in a state that the position of a pitch diameter (hereinafter referred to as the standard pitch diameter) of the standard triangular thread (metric screw) shown in  FIG. 4  is shifted. Namely, a position of a pitch diameter  26  of the internal screw  23  is such a position that a width P 3  ( FIG. 1 ) of a thread ridge  27  in a position of the standard pitch diameter  6  is made greater at a predetermined ratio than a width P 4  of a thread groove  29 . Further, a position of a pitch diameter  30  of the external screw  25  is such a position that the width P 3  of a thread groove  32  in a position of the standard pitch diameter  16  is made greater at the above predetermined ratio than the width P 4  of a thread ridge  33 . 
   As this predetermined ratio P 3 :P 4 , there is adopted 1.25:0.75. However, this ratio is suitably selected as occasion demands within a range of P 3 &gt;P 4 , and it is possible to adopt, for example, from about 1.2:0.8 to about 1.5:0.5. 
   Incidentally, numeral  35  ( FIG. 1 ) denotes an inner diameter of the internal screw, numeral  36  denotes an outer diameter of the external screw, P denotes a pitch, P 5  denotes a width of the thread ridge of the internal screw in a position of the displaced pitch diameter, P 6  ( FIG. 1 ) denotes a width of the thread groove of the internal screw in the position of the displaced pitch diameter, and  18   c  ( FIG. 2 ) denotes an axis of the accumulator ACC. 
   A bottom part  18   b  of the container main body  18  is provided with a through-hole  38 , and an intake and exhaust cylinder  39  is attached to the through-hole  38  through an O-ring. A flange part  40  of the intake and exhaust cylinder  39  is pressure-contacted to a receiving part  42  of the through-hole  38 . 
   The intake and exhaust cylinder  39  includes a poppet valve  45  with a cushion cup  43  is supported so as to be capable of sliding. The intake and exhaust cylinder  39  is fixed to the container main body  18  by a threaded nut  46 . 
   Next, an operation of this embodiment will be explained. 
   The accumulator ACC is connected to a hydraulic circuit (not shown in the drawing) through the intake and exhaust cylinder  39 . If a hydraulic pressure of the hydraulic circuit changes and a pressure in the container main body  18  ( FIG. 2 ) is increased, the lid  21  is pressed in the arrow A 2  direction, and a pressure flank  49  ( FIG. 1 ) of the thread ridge  33  presses a pressure flank  50  of the thread ridge  27 . For this reason, a tensile load concentrates on a thread bottom  29 B of the internal screw, and the compressive stress is generated in a thread bottom  32 B of the external screw. However, since the thickness of the thread ridge  27  of the internal screw is greater than that of the thread ridge  33  of the external thread, i.e., since the internal screw  23  is stronger in its fracture strength than the external screw  25 , the tensile stress generated in the thread bottom  29 B of the internal screw  23  becomes extremely smaller than the prior art. Accordingly, it is possible to prevent the internal screw  23 , having a property of weakness against the tensile force, from being fractured. 
   The second embodiment of the present invention will be explained with  FIG. 3 , and the same reference numerals as that shown in  FIGS. 1 and 2  are the same in their names and functions. In  FIG. 3 ,  59 B denotes a thread bottom of the internal screw, numeral  62  denotes an inner diameter of the internal screw, numeral  63  denotes an outer diameter of the external screw, numeral  65  denotes a thread groove of the external screw, and  65 B denotes a thread bottom of the external screw. 
   A difference between this embodiment and the first embodiment is a point that, as the screw, an inverse buttress screw is used in place of the triangular screw. 
   As to a thread ridge  53  of an internal screw  52  of this inverse buttress screw  51 , the inclination angle of the pressure flank and that of the clearance flank of a normal buttress internal screw are made inverse, i.e., the inclination angle of a pressure flank  54  is made greater than that of a clearance flank  55 . For example, the inclination angle of the pressure flank  54  is formed at 45 degrees, and that of the clearance flank  55  at 15 degrees. 
   Also, as to a thread ridge  57  of an external screw  56  of the above screw  51 , similarly to the above internal screw ridge  53 , sizes of the inclination angles of its flanks are made inverse to those of the inclination angles of the flanks of a standard inverse buttress external screw. 
   This screw  51  corresponds to an inverse buttress screw in a state that the position of a pitch diameter (hereinafter referred to as the standard pitch diameter)  6  of such a standard inverse buttress screw as shown in  FIG. 5  is shifted to a predetermined position. Namely, a position of a pitch diameter  58  of the internal screw  52  of the inverse buttress screw  51  is such a position that the width P 3  of the internal thread ridge  53  in the position of the standard pitch diameter  6  is made greater at a predetermined ratio than the width P 4  of a thread groove  59 . 
   This predetermined ratio P 5 :P 6  is similar to the predetermined rate P 3 :P 4  of the first embodiment. 
   The present invention is not limited to the above embodiments. For example, the present invention can be applied to a screw other than the triangular screw and the inverse buttress screw, and also it can be applied not only to the internal screw but also to the external screw. 
   EXPERIMENTAL EXAMPLES 
   By using the above first and second embodiments and an accumulator (made by Nippon Accumulator Co., Ltd., Type N 210-1D) of a prior art example, the inventor analyzed a tensile stress value in a first thread valley bottom of the internal screw by a finite-element method. The following results were obtained. Incidentally, the “first thread valley bottom” means a first thread valley bottom of a base portion of the internal screw, which initially undergoes a load of the external screw.
         Metric internal thread of the prior art example (Refer to  FIG. 4 ): 780 N/mm 2      Metric internal thread of the present invention (Refer to  FIG. 1 ): 570 N/mm 2      Inverse buttress internal thread of the prior art example (Refer to  FIG. 5 ): 592 N/mm 2      Inverse buttress internal thread of the present invention (Refer to  FIG. 3 ): 490 N/mm 2          

   As apparent from the above, as to the metric internal thread, the stress value according to one example of the present invention is reduced by 27% than the prior art example. As to the inverse buttress internal thread, the stress value according to one example of the present invention is reduced by 17% than the prior art example. As a result, it is understood that, according to the present invention, the fracture strength of the internal thread is extremely improved in comparison with the prior art examples.