FIELD OF THE INVENTION  
         [0001]    The present invention relates to a push switch used in input operation sections of electronic apparatuses.  
         BACKGROUND OF THE INVENTION  
         [0002]    Push switches used in input operation sections of electronic apparatuses are demanded to have small sizes and excellent durability, and are further desired to operate by an operating force predetermined to prevent malfunction due to accidental touch on the operation sections and to generate a proper click feel. Therefore, the push switches often includes elastic members made of rubber and having conical shapes.  
           [0003]    A conventional push switch disclosed in Japanese Patent Laid-Open Publication No.10-92260 and Japanese Patent Laid-Open Publication No.11-265634 will be explained. FIG. 7 is a front sectional view of the conventional push switch, and FIG. 8 is a perspective exploded view of the switch. A contact substrate  1  includes a peripheral fixed contact  2 , a central fixed contact  3 , and connection terminals  2 A and  3 A connected with the contacts  2  and  3 , respectively. A movable contact  4  of elastic metal thin plate is provided over the contact substrate  1  and includes a ring  4 A and a tongue  4 B. The ring  4 A is mounted on the peripheral fixed contact  2 . The tongue  4 B projects toward the center of the ring from an inner periphery of the ring and is folded upward. The contact substrate  1  and the movable contact element  4  provides a switch contact section.  
           [0004]    A rubber elastic element  5  is placed on the movable contact element  4 . The elastic element  5  includes a central columnar portion  5 A, a conical portion  5 B, a ring portion  5 C, and a protrusion  5 D. The conical portion  5 B having a predetermined thickness flares linearly downward obliquely from a junction  5 E at the outer circumference of the columnar portion  5 A. The ring portion  5 C has a lower end mounted on the annular portion  4 A of the movable contact element  4 . The protrusion  5 D having a diameter smaller than that of the columnar portion  5 A faces the tongue  4 B of the movable contact element  4  provided at the lower end of the columnar portion  5 A. The conical portion  5 B is hollow and has a truncated cone shape.  
           [0005]    A push button  6  made of rigid resin as an operation section is located on the top of the columnar portion  5 A of the elastic element  5 . A case  7  is fixed by a pawl  1 A of the contact substrate  1  so as to surround the periphery of the conical portion  5 B. The button  6  is supported by a guide groove  7 A in the case  7  so as to be movable up and down.  
           [0006]    [0006]FIG. 9 is a front sectional view of the conventional push switch which is operating.  
           [0007]    In this push switch, when the top of the push button  6  is pressed by a pressing force F, the columnar portion  5 A and protrusion  5 D of the elastic element  5  are pushed downward with the button  6 . Then, as shown in FIG. 9, the conical portion  5 B elastically deforms outward by a predetermined stroke with a click feel, and the leading end of the protrusion  5 D pushes the tongue  4 B of the movable contact  4  to have the contact  4  contact the central fixed contact  3 . This contacting allows the peripheral fixed contact  2  to be connected with the central fixed contact  3 , and makes the contacts  2  and  3  to output a signal transmitted to a circuit of the electronic apparatus through the connection terminals  2 A and  3 A.  
           [0008]    Then, when the pressing force applied to the button  6 , i.e., the columnar portion  5 A of the elastic element  5  is released, the conical portion  5 B restores its original truncated conical shape with its own elastic restoring force, so that the button  6  is pushed upward with the columnar portion  5 A. Simultaneously to this, the tongue  4 B of the movable contact element  4  restores upward to its original shape, and is departed from the central fixed contact  3 .  
           [0009]    In the conventional push switch, after tens of thousand times of operations at a temperature extremely lower or higher than a room temperature, cracks may be generated in the junction  5 E linked to the conical portion  5 B linearly flaring from the outer circumference of the columnar portion  5 A of the elastic element  5 .  
           [0010]    If the rubber forming the elastic element  5  has a hardness reduced by having the composition of the rubber vary, a durability of the rubber against deterioration of the rubber is improved, as disclosed in Japanese Utility Model Laid-Open Publication No.6-56929  
           [0011]    If the rubber forming the elastic element  5  has a small hardness, however, the conical portion  5 B starts deforming with a small force, and thus, an operating force of the push switch becomes smaller.  
         SUMMARY OF THE INVENTION  
         [0012]    A push switch includes a switch contact section including first and second contacts, and an elastic element for connecting the first and second contacts by pushing the first contact. The elastic element includes a columnar portion for connecting the first and second contacts by pushing the first contact, a hollow conical portion extending from a junction positioned at an end of the columnar portion, the conical portion having a truncated conical shape, and a thick portion provided at a whole circumstance of the junction. In the thick portion, a distance from an intersection where an extension line an outer circumference of the columnar portion and an extension line of an outer circumference of the conical portion cross to an intersection where an outer circumference of the junction and a bisector of a crossing angle formed by the extension line of the outer circumference of the columnar portion and the extension line of the outer circumference of the conical portion ranges from 0.08 times to 0.14 times of a thickness of the conical portion.  
           [0013]    This push switch operates with a click feel at a predetermined force, and operates stably by a great number of times even at a severe temperature apart from a room temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a front sectional view of a push switch according to an exemplary embodiment of the present invention.  
         [0015]    [0015]FIG. 2 is an enlarged sectional view of an elastic element of the push switch of the embodiment.  
         [0016]    [0016]FIG. 3 is a front sectional view of the push switch of the embodiment.  
         [0017]    [0017]FIG. 4A shows a distribution of a stress during an operation of pressing an elastic element of a conventional push switch.  
         [0018]    [0018]FIG. 4B shows a distribution of a stress during an operation of pressing the elastic element of the push switch of the embodiment.  
         [0019]    [0019]FIG. 5A shows dimensions of the elastic element of the push switch of the embodiment.  
         [0020]    [0020]FIG. 5B is a partially enlarged view of the elastic element shown in FIG. 5A.  
         [0021]    [0021]FIG. 6 is a front sectional view of another push switch according to the embodiment.  
         [0022]    [0022]FIG. 7 is a front sectional view of the conventional push switch.  
         [0023]    [0023]FIG. 8 is a perspective exploded view of the conventional push switch.  
         [0024]    [0024]FIG. 9 is a front sectional view of the conventional push switch. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    [0025]FIG. 1 is a front sectional view of a push switch according to an exemplary embodiment of the present invention. The switch has a basic structure and overall dimensions identical to those of a conventional push switch shown in FIG. 7. Therefore, the same parts as of the conventional switch are denoted by the same reference numerals, and detailed description is omitted.  
         [0026]    A contact substrate  1  includes a peripheral fixed contact  2 , a central fixed contact  3 , and connection terminals  2 A and  3 A connected to the contacts  2  and  3 , respectively. A movable contact element  4  made of elastic thin metal plate includes a ring  4 A and a tongue  4 B and is put on a contact substrate  1 , thus providing a switch contact section. An elastic element  11  made of rubber is put on the contact substrate  1 . The elastic element  11  includes a columnar portion  11 A, a hollow conical portion  11 B, a ring portion  11 C at the lower end of the conical portion  11 B, and a protrusion  11 D provided at the lower end of the columnar portion  11 A. The conical portion  11 B has a truncated conical shape flaring linearly downward obliquely from a junction  11 E at the outer circumference of the columnar portion  1 A. The protrusion  11 D has a diameter smaller than that of the columnar portion  11 A. The protrusion  11 D faces the tongue  4 B of the movable contact  4 . The conical portion  11 B is hollow and has a truncated conical shape.  
         [0027]    This push switch, similarly to the conventional push switch, is pressed, and the protrusion  11 D pushes the tongue  4 B to have the tongue  4 B contact the central fixed contact  3 .  
         [0028]    A push button  6  made of rigid resin is placed on the top of the columnar portion  11 A of the elastic element  11  as an operation section. This push button  6 , similarly to the conventional push switch shown in FIG. 7, is supported by a guide groove  7 A in a case  7  fixed on the contact substrate  1  so as to be movable up and down.  
         [0029]    [0029]FIG. 2 is an enlarged sectional view of the elastic element  11 . The elastic element  11  further includes a thick portion  12 A surrounded by an arc  12  having a predetermined radius, as shown in FIG. 2, along the entire periphery of the junction  11 E, the border between the conical portion  11 B and columnar portion  11 A. The hardness of the rubber forming the elastic element  11  is smaller than that of the elastic element  5  of the conventional push switch.  
         [0030]    [0030]FIG. 3 is a front sectional view of the push switch of the embodiment during an pressing operation. In the push switch of the embodiment having the elastic element  11 , when the top of the push button  6  is pressed by a pressing force F 2 , the columnar portion  11 A and protrusion  11 D of the elastic element  11  are pushed downward with the push button  6 . Then, as shown in FIG. 3, the conical portion  11 B elastically deforms by a predetermined stroke outward with a click feel. The protrusion  11 D pushes the tongue  4 B of the movable contact element  4  to have the tongue  4 B contact the central fixed contact  3 , and the peripheral fixed contact  2  is connected to the central fixed contact  3 , thus transmitting a signal to a circuit of an electronic apparatus through the connection terminals  2 A and  3 A.  
         [0031]    [0031]FIG. 4A shows a distribution of a stress during a pressing operation applied to the conventional elastic element  5 . FIG. 4B shows a distribution of a stress during the pressing operation applied to the elastic element  11  of the embodiment. In the elastic element  5 , a stress concentrates at a portion which extremely deforms as shown by a mesh near the junction  5 E. By contrast, in the elastic element  11  of the embodiment having the thick portion  12 A at the junction  11 E, a mesh deforms moderately near the junction  11 E, and the stress is alleviated more than the conventional elastic element  5 .  
         [0032]    Results of measuring durability of the elastic element  11  having the junction  11 E including the thick portion  12 A of various sizes made of rubber materials having various hardnesses.  
         [0033]    [0033]FIG. 5A shows dimensions of the elastic element  11  used for the measurement. The elastic element  11  is made of silicone rubber of type A durometer hardness of HA75 as measured in JIS K6253 durometer hardness test specified in JIS K 6249 (hereinafter called JIS hardness HA 75). A diameter D of the columnar portion  11 A is 2.2 mm. The conical portion  11 B flares downward obliquely from the outer periphery of the columnar portion  11 A by an angle of θ=25°, has a wall thickness of T=0.45 mm and a stroke S of 1.3 mm.  
         [0034]    The elastic element  11  having the thick portion  12 A of various radiuses R of curvature and the maximum thickness t, as shown in FIG. 5A, was tested at a temperature of 70° C. by deforming the conical portion  11 B of the push switch with a click feel. Table 1 shows operating forces, click feel, and a number of times of operation of the elastic element  11 . Ten samples for each of eight radiuses were prepared, and the operation force is an average of respective operation forces the ten samples for each radius.  
         [0035]    [0035]FIG. 5B is a partially enlarged view of the portion  5 B of the elastic element  11  shown in FIG. 5A. As shown in FIG. 5B, the maximum thickness “t” of the thick portion  12 A is defined as follows. In the thick portion  12 A of the radius R of curvature, a bisector L 3  of an angle formed by an extension line L 1  of an outer side of the columnar portion  11 A and an extension line L 2  of an outer side of the conical portion  11 B crossing at an intersection P 0  is defined. The distance from the intersection P 0  to the intersection P 1  where the bisector L 3  and an outer circumference of the thick portion  12 A cross is defined as the maximum thickness “t”. Table 1 also shows the ratio t/T of the thickness “t” to a wall thickness “T” (=0.45 mm) of the conical portion  11 B. The thick portion  12 A is provided around the entire circumstance of the junction  11 E, and therefore, the intersections P 0  and P 1  are actually intersecting lines.  
                                   TABLE 1                       Radius R of   Maximum               Durability       Curvature   Thickness t       Operation   Click   (Number       (mm)   (mm)   t/T   Force (N)   Feel   of Times)                   0.0   0.000   0.000   3.0   A   30,000-50,000       1.0   0.024   0.053   3.2   A   50,000-150,000       1.5   0.037   0.082   3.4   A   &gt;130,000       2.0   0.049   0.109   2.5   A   &gt;200,000       2.5   0.061   0.136   3.6   A   &gt;200,000       3.0   0.073   0.162   3.8   B   &gt;200,000       4.0   0.097   0.216   4.0   C   &gt;200,000       5.0   0.121   0.269   4.2   D   &gt;200,000                  
 
         [0036]    The classification “A” for the click feel denotes the clearest and most excellent click feel. The classifications “B” and “C” denote a weak click feel. The classification “D” denotes substantially no click feel.  
         [0037]    As shown in Table 1, according to an increase of the radius R of curvature of the thick portion  12 A, the durability of the elastic element  11  is improved, but the operating force of the push switch increases, and the click feel becomes weak gradually. According to Table 1, in the elastic element  11  having the thick portion  12 A of the radius R of curvature ranging from 1.5 mm to 2.5 mm provides the durability more than 100,000 times, a clear click feel, and the operating force larger than that of the elastic element without the thick portion  12 A by 10% to 20%.  
         [0038]    The radius R of curvature ranging from 1.5 mm to 2.5 mm provides the ratio, t/T, ranging from 0.082 to 0.136.  
         [0039]    That is, in order to obtain the durability exceeding 100,000 times and the clear click feel, the radius R of curvature of the thick portion  12 A is determined so that the maximum thickness t of the thick portion  12 A ranges from 0.08 to 0.15 times of the wall thickness of the conical portion  11 B.  
         [0040]    In this conditions, however, the operating force of the push switch is large. Hence, elastic elements  11  made of silicone rubber materials having various hardnesses and including the thick portion  12 A having a radius R of curvature of 2 mm, which is the median of the conditions, were tested at a temperature of 70° C. to measure an operating force and durability shown in Table 2. Ten samples for each hardness were prepared, and the operation force shown is an average of respective operation forces of the ten samples  
                                   TABLE 2                                   JIS                       Hardness   Operating   Click   Durability           HA   Force (N)   Feel   (Number of Times)                           75   3.5   A   &gt;200.000           70   3.0   A   &gt;200.000           65   2.6   A   &gt;200.000                      
 
         [0041]    The classification “A of the click feel denotes the clearest and the most excellent click feel.  
         [0042]    As shown in Table 2, the elastic element  11  of silicone rubber of JIS hardness HA 70 having the thick portion  12  of the radius R of curvature of 2 mm operates by the same operating force as the elastic element  11  of silicone rubber of JIS hardness HA 75 without the thick portion  12 , and generates the click feel similar to that of the elastic element of JIS hardness HA 75, and an enhanced and stabilized durability.  
         [0043]    The junction  11 E between a linear portion of the side surface of the conical portion  11 B and the outer circumference of the columnar portion  11 A is chamfered to form a thick portion having the thickness t on the bisector, from the side view, of the angle is 0.08 times to 0.14 times of the thickness T of the conical portion  11 B. The dimensional range of the elastic element  11  expected to provide such effect is estimated from experiments conducted for the elastic elements  11  of similar shape and dimension. The estimated ranges are shown as follows: the diameter D of columnar portion  11 A ranges from 2 mm to 5 mm; an inclination angle θ of the conical portion  11 B ranges from 200 to 400; the thickness T of the conical portion  11 B ranges from 0.3 mm to 0.6 mm; and the stroke S ranging from 0.5 mm to 2.5 mm.  
         [0044]    In this case, the junction  11 E of the elastic element  11  has the thick portion  12 A having an arc section on its side. Even if the side of the thick portion  12 A is linear, the substantially same effect as the thick portion  12 A having the same maximum thickness t is obtained.  
         [0045]    Thus, the thick portion  12 A at the junction  11 E at the border between the outer circumference of the columnar portion  11 A of the elastic element  11  and the conical portion  11 B flaring linearly allows the stress concentrating in this area to be reduced. The thick portion  12 A increases the operating force for manipulating the columnar portion  11 A, allows the hardness of the rubber for the elastic element  11  to be small, and improves the durability of the element  11 . Therefore, even if being used at a temperature extremely depart from the room temperature, the push switch operates stably more than 100,000 times.  
         [0046]    In the above explanation, the columnar part  11 A of the elastic element  11  is pressed with the push button  6  supported by the case  7  to have the button  6  movable up and down. This structure allows the columnar portion  11 A of the elastic element  11  to be pushed down stably and not to be inclined during the pressing operation. As a result, the stress generated in the elastic element  11  is distributed uniformly around the columnar portion  11 A and does not concentrates in a specific direction, so that the number of times of operation until occurrence of crack in the elastic element  11  can be extended.  
         [0047]    If a hinged operation button is used in an electronic apparatus to press the button stably, the push button  6  may be omitted, and the top of the columnar portion  11 A of the elastic element  11  may be directly pushed by the operation button of the electronic apparatus.  
         [0048]    [0048]FIG. 6 is a front sectional view of another push switch of the embodiment. In the switch contact section of the push switch of the embodiment, the protrusion  11 D of the elastic element  11  pushes the tongue  4 B of the movable contact  4  onto the contact substrate  1 , and the tongue  4 B contacts and departs from the central fixed contact  3 . The switch contact section may include, as shown in FIG. 6, a conductive portion  16  provided at a leading end of a protrusion  15 B at the lower end of a columnar portion  15 A of an elastic element  15  and fixed contacts  14 A and  14 B provided independently on a contact substrate  13 . This push switch includes the switch contact section having a simple structure and operates stably.
A push switch includes a switch contact section including first and second contacts, and an elastic element for connecting the first and second contacts by pushing the first contact. The elastic element includes a columnar portion for connecting the first and second contacts by pushing the first contact, a hollow conical portion extending from a junction positioned at an end of the columnar portion, the conical portion having a truncated conical shape, and a thick portion provided at a whole circumstance of the junction.