Patent Publication Number: US-6904683-B2

Title: Nail clippers

Description:
This application claims the benefit of International Patent Application No. PCT/JP01/09925 filed on Nov. 13, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to nail clippers. 
     A conventional nail clipper, for example, as shown in Japanese Unexamined Utility Model Publication No. Sho 61-82502 has a supporting shaft which is notched to form a bearing hole, and a connecting pin attached to a lever is inserted to the bearing hole. In this nail clipper, although the lever can be incorporated into the supporting shaft easily by virtue of the bearing hole opening to the periphery of the shaft, this configuration per se involves problems that the connecting pin of the lever easily slips off the bearing hole of the supporting shaft and that the strength of the portion of the supporting shaft around the bearing hole is lowered. In addition, it is troublesome to form a supporting shaft having such a shape of bearing hole as described above. 
     It is an objective of the present invention to provide nail clippers, capable of facilitating incorporation of a lever, ensuring bearing of the lever and enhancing the strength of the supporting section. It is another objective of the present invention to provide a nail clipper whose supporting shaft can be molded easily. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to attain the objectives as described above, the nail clipper according to one aspect of the present invention contains a pair of blade bodies each having a cutting edge, a lever to be operated for bringing the cutting edges of the blade bodies into press contact with each other and a supporting shaft for linking the lever to the blade bodies. The cutting edges of the blade bodies are adapted to be brought into press contact with each other by pressing the blade bodies with the lever against resilience of the blade bodies. The nail clipper is provided with a boss formed either on the lever or on the supporting shaft so as to allow the supporting shaft to pivotally support the lever, and a groove formed on the rest of the supporting shaft and the lever, with which the boss is engaged. The groove has a mouth portion through which the boss is engaged and disengaged, a guiding portion capable of pivotally supporting the boss, and a connecting portion connecting the mouth portion and the guiding portion to each other. Further, the groove has a pair of side wall surfaces opposing each other in the mouth portion, connecting portion and guiding portion and also a bottom surface connecting the side wall surfaces to each other. 
     The nail clipper according to another embodiment contains an upper blade body having an upper cutting edge; a lower blade body having a lower cutting edge opposed to the upper cutting edge; a supporting shaft inserted to the blade bodies near the cutting edges respectively, a lower end portion of the supporting shaft being engaged with the lower blade body to be locked therewith, while an upper end portion of the supporting shaft protrudes upward through the upper blade body; and a lever having a first supporting section; the first supporting section being linked on the upper blade body to a second supporting section provided at an upper end portion of the supporting shaft; wherein the cutting edges of the blade bodies are brought into press contact with each other by pressing the upper blade body with the lever against resilience of the upper and lower blade bodies. In this nail clipper, either the first supporting section of the lever or the second supporting section of the supporting shaft is a boss, while the other supporting section is a groove, with which the boss is engaged. The groove contains a mouth portion through which the boss is engaged and disengaged, a guiding portion capable of pivotally supporting the boss, and a connecting portion connecting the mouth portion and the guiding portion to each other. The groove has a pair of side wall surfaces opposing each other in the mouth portion, connecting portion and guiding portion and also a bottom surface connecting the side wall surfaces to each other. 
     The nail clipper according another embodiment contains an upper blade body having an upper cutting edge; a lower blade body having a lower cutting edge opposed to the upper cutting edge; a supporting shaft inserted to the blade bodies near the cutting edges respectively, a lower end portion of the supporting shaft being engaged with the lower blade body, while an upper end portion of the supporting shaft protrudes upward through the upper blade body; a lever having a pair of supporting arms; the upper end portion of the supporting shaft being inserted on the upper blade body to a bearing recess defined between the supporting arms of the lever; a first supporting section provided in the supporting arms; and a second supporting section provided at the upper end portion of the supporting shaft; the first and second supporting sections being linked to each other; wherein the upper cutting edge and the lower cutting edge are brought into press contact with each other by pressing the upper blade body with the lever against resilience of the upper and lower blade bodies. In this nail clipper, either the first supporting section of the lever or the second supporting section of the supporting shaft is a pair of bosses opposing each other, and the other supporting section is a pair of grooves, with which the bosses are engaged respectively. Each groove has a mouth portion through which the boss is engaged and disengaged, a guiding portion capable of pivotally supporting the boss, and a connecting portion connecting the mouth portion and the guiding portion to each other. Each groove has a pair of side wall surfaces opposing each other in the mouth portion, connecting portion and guiding portion and also a bottom surface connecting the side wall surfaces to each other. 
     The nail clipper according to another embodiment contains an upper blade body having an upper cutting edge; a lower blade body having a lower cutting edge opposed to the upper cutting edge; a supporting shaft inserted to the blade bodies near the cutting edges respectively, a lower end portion of the supporting shaft being engaged with the lower blade body to be locked therewith, while an upper end portion of the supporting shaft protrudes upward through the upper blade body; and a lever having a pair of supporting arms; the upper end portion of the supporting shaft being inserted on the upper blade body to a bearing recess defined between the supporting arms of the lever; a first supporting section provided in the supporting arms; and a second supporting section provided at the upper end portion of the supporting shaft; the first and second supporting sections being linked to each other; wherein the upper cutting edge and the lower cutting edge are brought into press contact with each other by pressing the upper blade body with the lever against resilience of the upper and lower blade bodies. In this nail clipper, the first supporting section of the lever is a pair of bosses protruding from the supporting arms respectively to oppose each other in the bearing recess of the lever; whereas the second supporting section of the supporting shaft is a groove formed continuously fully along the circumference thereof, with which the bosses are engaged. Each groove has a mouth portion through which the bosses are engaged and disengaged and a guiding portion capable of pivotally supporting the bosses. 
     The nail clipper according to another embodiment contains an upper blade body having an upper cutting edge; a lower blade body having a lower cutting edge opposed to the upper cutting edge; a supporting shaft inserted to the blade bodies near the cutting edges respectively, a lower end portion of the supporting shaft being engaged with the lower blade body to be locked therewith, while an upper end portion of the supporting shaft protrudes upward through the upper blade body; a first supporting section provided in a lever; and a second supporting section provided at the upper end portion of the supporting shaft, the first and second supporting sections being linked to each other; wherein the upper cutting edge and the lower cutting edge are brought into press contact with each other by pressing the upper blade body with the lever against resilience of the upper and lower blade bodies. In this nail clipper, the supporting shaft is molded with a synthetic resin material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG.  1 ( a ) is a front view of the nail clipper according to a first embodiment, showing a state where the nail clipper is at rest; FIG.  1 ( b ) is a perspective view showing the supporting shaft of the nail clipper; and FIG.  1 ( c ) is a perspective view showing the lever of the nail clipper; 
       FIG.  2 ( a ) is a front view of the supporting shaft of the nail clipper; FIG.  2 ( b ) is a left side view of the supporting shaft shown in FIG.  2 ( a ); and FIG.  2 ( c ) is a bottom view of the supporting shaft shown in FIG.  2 ( a ); 
       FIG.  3 ( a ) is a cross-sectional view taken along the line  3   a—   3   a  in FIG.  2 ( a ); FIG.  3 ( b ) is a cross-sectional view taken along the line  3   b—   3   b  in FIG.  2 ( a ); and FIG.  3 ( c ) is a cross-sectional view taken along the line  3   c—   3   c  in FIG.  2 ( a ); 
       FIG.  4 ( a ) is a partial plan view of the lever of the nail clipper; and FIG.  4 ( b ) is a left side view of the lever shown in FIG.  4 ( a ); 
       FIGS.  5 ( a ),  5 ( b ) and  5 ( c ) are partial cross-sectional views showing process steps of engaging the pair of bosses of the lever with the grooves of the supporting shaft; 
       FIG.  6 ( a ) is a cross-sectional view taken along the line  6   a—   6   a  in FIG.  5 ( b ); and FIG.  6 ( b ) is a cross-sectional view taken along the line  6   b—   6   b  in FIG.  5 ( c ); 
       FIG.  7 ( a ) is a front view of the nail clipper according to the first embodiment showing a state where the nail clipper is ready for use; and FIG.  7 ( b ) is a front view thereof showing a state where the nail clipper is in action; 
       FIG.  8 ( a ) is a front view showing the support shaft in the nail clipper according to another embodiment; FIG.  8 ( b ) is a left side view of the supporting shaft shown in FIG.  8 ( a ); FIG.  8 ( c ) is a cross-sectional view taken along the liner  8   c — 8   c  in FIG.  8 ( a ); and FIG.  8 ( d ) is a cross-sectional view taken along the line  8   d—   8   d  in FIG.  8 ( a ); 
       FIG.  9 ( a ) is a cross-sectional view showing a transient step of forming bosses in the lever of the nail clipper according to another embodiment; and FIG.  9 ( b ) is a cross-sectional view showing the bosses; and 
       FIG.  10 ( a ) is a front view showing the groove of the supporting shaft in the nail clipper according to another embodiment; and FIG.  10 ( b ) is a cross-sectional view taken along the line  10   b—   10   b  in FIG.  10 ( a ). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The nail clipper according to a first embodiment of the present invention will be described below referring to  FIGS. 1  to  7 . 
     Scheme of Nail Clipper 
     As shown in FIG.  1 ( a ), the nail clipper has an upper blade body  1  and a lower blade body  2  each having a plate form, which are fixed to each other at proximal portions  1   a  and  2   a . The upper blade body  1  and the lower blade body  2  have resilience and can be moved closer to and away from each other. The upper blade body  1  and the lower blade body  2  have cutting edges  3  and  4  formed at the distal ends thereof respectively to oppose each other. The upper cutting edge  3  and the lower cutting edge  4  are normally spaced away from each other by the resilience of the blade bodies  1  and  2 . As shown in FIGS.  5 ( a ),  5 ( b ) and  5 ( c ), circular bearing holes  5  and  6  are defined through the upper blade body  1  and the lower blade body  2 , and they are located near the cutting edges  3  and  4 , respectively. 
     A supporting shaft  7  shown in FIG.  1 ( b ) penetrates the upper blade body  1  through the bearing hole  5  thereof and the lower blade body  2  through the bearing hole  6  thereof. This supporting shaft  7  contains a large-diameter head  8  formed at the lower end thereof and a small-diameter shank  10  extended from the head  8  upward through a step  9 . The head  8  is locked by the rim of the bearing hole  6  on the lower side of the lower blade body  2 . An upper end portion  11  of the shank  10  protrudes through the bearing hole  5  to the upper side of the upper blade body  1 . 
     A lever  12  shown in FIG.  1 ( c ) is placed on the upper side of the upper blade body  1 . The lever  12  is forked at the distal portion to form a pair of supporting arms  13  and also a bearing recess  14  between these two supporting arms  13 . The upper end portion  11  of the supporting shaft  7  is inserted to this bearing recess  14 . Each supporting arm  13  of the lever  12  and the upper end portion  11  of the supporting shaft  7  are linked to each other in a supporting section S. 
     Materials of Supporting Shaft  7  and Lever  12   
     Both the supporting shaft  7  and the lever  12  are integrally molded respectively with a synthetic resin material. The synthetic resin material is based on polyamide, polypropylene or aromatic nylon and can contain a predetermined filler. As the filler, a glass fiber, a mineral fiber or a carbon fiber can be used singly, or a combination of at least two of the glass fiber, mineral fiber and carbon fiber may be used. Provided that the content (wt %) of such fillers is α, the value α is defined in this embodiment as 15%≦α≦65%. However, it is preferably 40%≦α≦60%. 
     The following characteristics can be obtained by utilizing such synthetic resins. 
     (1) Synthetic resins have high strength and high rigidity after wetting or at high temperatures, high impact strength and high elongation at break as rigid materials and excellent fatigue strength, so that the shaft  7  and the lever  12  come to have high strength and rigidity. 
     (2) Synthetic resins have good fluidity and provide good appearance, since they can be subjected to injection molding at a mold temperature of 80 to 110° C. and since dies, screws and cylinders do not wear much; and 
     (3) Synthetic resins do not wear much when they are brought into contact with metallic materials to improve wear resistance of the shaft  7  and of the lever  12 . 
     Linking Structure of Supporting Shaft  7  and Lever  12   
     As shown in FIG.  1 ( c ), in the bearing recess  14  of the lever  12 , a boss  15  (supporting section S) is formed on each supporting arm  13  such that the boss formed on one supporting arm  13  opposes the counterpart formed on the other supporting arm  13 . Both the head  8  and the shank  10  of the supporting shaft  7  are formed to have circular cross sections respectively. The shank  10  has on each side of the circumference thereof a pair of grooves  16  (supporting section S) at 180° intervals to oppose the bosses  15  respectively, and the bosses  15  are engaged with the opposing grooves  16  respectively. 
     As shown in FIGS.  1 ( b ),  2 ( a ) to  2 ( c ) and FIGS.  3 ( a ) to  3 ( c ), each groove  16  contains a mouth portion  17  engageable with and disengageable from the boss  15 , a guiding portion  18  capable of pivotally supporting the boss  15 , and a connecting portion  19  connecting the mouth portion  17  and the guiding portion  18  to each other. Each groove  16  has a pair of opposing side wall surfaces  20  in the mouth portion  17 , connecting portion  19  and guiding portion  18 ; a bottom surface  21  connecting the pair of side wall surfaces  20  to each other in the mouth portion  17 , connecting portion  19  and guiding portion  18 ; and an opening  22  opposing the bottom surface  21 . The pair of grooves  16  are separated from each other by a common bottom plate  23  throughout the mouth portion  17 , connecting portion  19  and guiding portion  18 . The bottom surfaces  21  of the grooves  16  are located on each side of the bottom plate  23  in the mouth portion  17 , connecting portion  19  and guiding portion  18 . 
     The center line between the pair of side wall surfaces  20  in each groove  16  extends throughout the mouth portion  17 , connecting portion  19  and guiding portion  18 . The center line contains a first portion  17   a  running through the mouth portion  17 , a second portion  19   a  running through the connecting portion  19  and a third portion  18   a  running through the guiding portion  18 . The first portion  17   a  extends from the mouth portion  17  to the connecting portion  19  orthogonally to the axial direction  7   a  of the supporting shaft  7 . The third portion  18   a  extends from the connecting portion  19  to the guiding portion  18  along the axial direction  7   a  of the supporting shaft  7 . The second portion  19   a  is arcuated to connect the first portion  17   a  and the third portion  18   a  to each other. 
     The pair of side wall surfaces  20  in each groove  16  are inclined to be away from each other toward the opening  22 . Thus, the distance W between the side wall surfaces  20  in each groove  16  is designed to be greater on the opening ( 22 ) side than on the bottom surface ( 21 ) side. 
     Provided that the thickness of the bottom plate  23  or the distance between the bottom surfaces  21  of the grooves  16  is T; the distance between the opening  22  of the grooves  16  or the outside diameter of the shank  10  of the supporting shaft  7  is D 10 ; and that the value T (thickness)/D 10  (outside diameter) is A, the value A is set within the range of 0≦A≦0.5. More preferably, the value A is set within the range of 0.2≦A≦0.4. In each groove  16  of this embodiment, the bottom plate  23  is formed throughout the mouth portion  17 , connecting portion  19  and guiding portion  18 . However, for example, the bottom plate  23  may be formed in the mouth portion  17  and a part of the connecting portion  19 , and the bottom plate  23  may be omitted in the rest of the connecting portion  19  and in the guiding portion  18  to allow the grooves  16  to communicate with each other. In the case where the bottom plate  23  is omitted, T=0, and hence A=0. 
     Provided that the distance between the side wall surfaces  20  in the guiding portion  18  of each groove  16  and the outside diameter of the shank  10  of the supporting shaft  7  are W and D 10  respectively and that the value W (distance)/D 10  (outside diameter) is B, the value B is set within the range of 0.15≦B≦0.8 in this embodiment. The value B is more preferably set within the range of 0.4≦B≦0.7. 
     In the supporting shaft  7 , the outside diameter D 8  of the head  8  is designed to be greater than the outside diameter D 10  of the shank  10 . Provided that the length of the head  8  along the axial direction  7   a  and the length of the shank  10  along the axial direction  7   a  are H and L respectively and that the value H (width)/L (length) is C, the value C is set within the range of 0.05≦C≦0.3 in this embodiment. The value C is more preferably set within the range of 0.1≦C≦0.2. 
     Further, in the supporting shaft  7 , the step  9  present between the head  8  and the shank  10  has a reinforcing portion  24  having an arcuate cross section formed fully along the outer circumference. Provided that the radius of the reinforcing portion  24  is r, the value r is set within the range of 0.1 mm≦r≦3 mm. The value r is more preferably set within the range of 0.2 mm≦r≦1.5 mm. 
     Referring to the thickness T of the bottom plate  23  present between the grooves  16 , each bottom surface  21  is sloped or stepped from the mouth portion  17  toward the guiding portion  18  so that the thickness T is greater on the mouth portion ( 17 ) side than on the guiding portion ( 18 ) side. In other words, each groove  16  is designed to have such a depth as is slightly shallower on the mouth portion ( 17 ) side than on the guiding portion ( 18 ) side. Meanwhile, the distance W between the side wall surfaces  20  in each groove  16  is designed to be greater on the mouth portion ( 17 ) side than on the connecting portion ( 19 ) side or the pivotal guiding portion ( 18 ) side. Further, the supporting shaft  7  has a locking pin hole  25  formed in the head  8  thereof. 
     Next, the bosses  15  shown in FIG.  1 ( c ) and FIGS.  4 ( a ) and  4 ( b ) will be described. Each boss  15  has a peripheral surface  28  formed to extend from a proximal portion  26  to a distal portion  27  thereof, and an end face  29  provided at the distal portion  27 . In each boss  15 , the cross section taken orthogonal to the axial direction  15   a  has a circular shape. The peripheral surface  28  is tilted with respect to the axial direction  15   a . The peripheral surface  28  of each boss  15  is designed to have an outside diameter D 26  at the proximal portion  26 , which is greater than the outside diameter D 27  of the same at the distal portion  27 . Thus, a cross-sectional area of each boss  15  taken orthogonal to the axial direction  15   a  reduces gradually from the proximal portion ( 26 ) side toward the distal portion ( 27 ) side. 
     In each boss  15 , provided that the outside diameter of the peripheral surface  28  at the proximal portion  26  and the length from the proximal portion  26  to the distal portion  27  in the axial direction  15   a  are D 26  and M respectively, and that the value D 26  (outside diameter)/M (length) is E, the value E is set within the range of 1≦E≦3 in this embodiment. The value E is more preferably set within the range of 1.5≦E≦2.5. 
     In the bosses  15 , provided that the sum of the length M in the axial direction  15   a  from the proximal portion  26  to the distal portion  27  in one boss  15  and the length M in the axial direction  15   a  from the proximal portion  26  to the distal portion  27  in the other boss  15  is N (N=M+M), and the dimension of a clearance  30  secured between the distal portions  27  of the bosses  15  (i.e., the clearance between the end faces  29 ) is P, and that the value N (sum of lengths)/P (clearance) is F, the value F is set within the range of 1≦F≦3 in this embodiment. The value F is more preferably set within the range of 1.3≦F≦2.3. 
     Next, the procedures of coupling the bosses  15  with the grooves  16  respectively and supporting state between them will be described. 
     As shown in FIG.  5 ( a ), in the state where the supporting shaft  7  penetrates through the upper and lower blade bodies  1  and  2 , a locking pin (not shown) is inserted to the locking pin hole  25  formed at the head  8  of the supporting shaft  7  to achieve positioning of the supporting shaft  7 , and also the upper blade body  1  is pushed down so that the cutting edges  3  and  4  of the blade bodies  1  and  2  approach each other. In this state, the supporting arms  13  of the lever  12  are applied onto the upper side of the upper blade body  1 , and the bosses  15  of the supporting arms  13  are moved, on the upper side of the upper blade body  1 , toward the grooves  16  formed at the upper end portion  11  of the shank  10  in the supporting shaft  7 . 
     Next, as shown in FIGS.  5 ( b ) and  6 ( a ), each boss  15  is engaged with the mouth portion  17  of the opposed groove  16  while the upper end portion  11  of the shank  10  is inserted to the bearing recess  14  defined between the supporting arms  13 . At the initial stage of engagement, each boss  15  approaches the side wall surfaces  20  of the groove  16  such that the peripheral surface  28  of the boss  15  slides along the side wall surfaces  20  of the groove  16  and that the end face  29  of the boss  15  slides along the bottom surface  21  of the groove  16 . 
     Further, as shown in FIGS.  5 ( c ) and  6 ( b ), each boss  15  is slipped from the mouth portion  17  of the groove  16  into the guiding portion  18  through the connecting portion  19  in each groove. In the state where the boss  15  is fully engaged with the groove  16 , the boss  15  is brought into pressure contact with the side wall surfaces  20  of the guiding portion  18  under the resilience of the upper and lower blade bodies  1  and  2  so as to regulate the boss  15  not to shift toward the connecting portion  19  of the groove  16 . 
     Example of How the Nail Clipper is Used 
     In the state where the nail clipper is at rest as shown in FIG.  1 ( a ), the lever  12  is turned over to rest on the upper side of the upper blade body  1 , and the upper and lower blade bodies  1  and  2  are spaced vertically from each other under the resilience thereof. In this state, the resilience of the upper blade body  1  is borne by the bosses  15  and the grooves  16  formed in the shank  10  of the supporting shaft  7  through the supporting arms  13  of the lever  12 ; whereas the resilience of the lower blade body  2  is borne by the head  8  of the supporting shaft  7 , maintaining the upper cutting edge  3  and the lower cutting edge  4  to be spaced away from each other. 
     Next, the lever  12  is pivoted on the upper side of the upper blade body  1  by 180° together with the supporting shaft  7  and then inverted to be positioned over the upper blade body  1  so as to form an upward slope with respect to the blade body  1 , as shown in FIG.  7 ( a ) illustrating the state where the nail clipper is ready for use. Then, the upper blade body  1  is pressed with the lever  12  against the resilience of the upper and lower blade bodies  1  and  2 , and the upper cutting edge  3  and the lower cutting edge  4  are brought closer and abutted against each other, as shown in FIG.  7 ( b ) illustrating the state where the nail clipper is in action. 
     In any of the states described above, the bosses  15  of the lever  12  move within the guiding portions  18  of the respective grooves  16  defined in the supporting shaft  7  following the movement of the lever  12 . 
     Other Embodiments 
     Another Embodiment Shown in  FIG. 8   
     In the first embodiment shown in FIGS.  2 ( a ) and  2 ( b ), each groove  16  has the mouth portion  17  only on one side of the axial line  7   a  of the supporting shaft  7 . However, in the embodiment shown in FIGS.  8 ( a ) and  8 ( b ), each groove  16  has a pair of mouth portions  17  on each side of the axial line  7   a  of the supporting shaft  7 . 
     Another Embodiment Shown in  FIG. 9   
     In the first embodiment shown in FIG.  1 ( c ), the entire lever  12  including the bosses  15  is molded with a synthetic resin material. However, in the embodiment shown in FIG.  9 ( a ), the main body of the lever  12  is molded with a synthetic resin material, and a metallic boss-forming material  31  is inserted to the supporting arms  13  to be integrated with the lever  12 , and then the boss-forming material  31  is cut to form a pair of bosses  15 . 
     Another Embodiment Shown in  FIG. 10   
     In the first embodiment, the grooves  16  are defined on the periphery of the shank  10  of the supporting shaft  7  to oppose diametrically each other. In place of this constitution, a groove  16  is formed fully along the circumference of the shank  10  of the supporting shaft  7 . This groove  16  has an upper edge and a lower edge, and the upper edge has a wavy convexoconcavity  32 . Further, a guiding portion  18  also serving as a mouth portion  17  is provided between each concavity  32   a  of the upper edge and the lower edge. The boss  15  in each supporting arm  13  of the lever  12  is introduced through this mouth portion  17  into the guiding portion  18 . 
     Although not illustrated, the groove formed fully along the circumference of the shank  10  of the supporting shaft  7  may have the following constitution. A crosswise groove is formed on the shank  10  of the supporting shaft  7  in the circumferential direction. While the crosswise groove has an upper edge and a lower edge intersecting with the axial direction  7   a , a plurality of vertical grooves are formed to extend parallelwise from the upper edge in the axial direction. The crosswise groove serves both as a mouth portion and as a connecting portion. Each vertical groove serves both as a connecting portion and as a guiding portion. The bosses  15  in the supporting arms  13  of the lever  12  are introduced from the mouth portion into the guiding portions through the connecting portions, respectively. 
     In any of the above embodiments, the supporting shaft  7  is provided with a groove or grooves  16 , whereas each supporting arm  13  of the lever  12  is provided with a boss  15 . Although not illustrated, in place of this configuration, a boss may be formed on the supporting shaft  7 , and a groove may be formed on each supporting arm  13  of the lever  12 . 
     Although not shown, the configuration of the boss  15  in each supporting arm  13  of the lever  12  may be modified. While the bosses  15  in the foregoing embodiments have a truncated cone shape, they may have, for example, a columnar or semispherical shape. Further, each boss  15  may have an elliptical cross section in place of the circular cross section.