Abstract:
A clamp for measuring a ground loop current includes a first jaw secured to a fixed support and a second jaw pivotally mounted on the fixed support, and pivotable between a position closing the clamp and an open position, and an elastic element for returning the movable jaw to the closed position. The clamp includes a device scaling down the force required for opening the clamp, by application of the force to a trigger. The device is mounted between the elastic return element and the trigger, upon opening the clamp.

Description:
FIELD OF THE INVENTION 
     The invention relates to a clamp for measurement of a ground loop of the type comprising a jaw secured to a fixed support and a jaw pivotally mounted on the support, between a closure position of the clamp in which the end of the movable jaw is in pressed contact with the end of the fixed jaw and an opening position of the clamp, and an elastic return element of the movable jaw in its closure position, the movable jaw being formed by an arm of a lever with two arms, the other arm which forms the trigger for causing pivoting of the lever. 
     BACKGROUND 
     Ground loop measurement clamps of this type are already known. The general problem of these clamps is to have a pressure as large as possible for closing both of these clamps on each other in order to minimize the air gap between the contact faces of both jaws. In order to attain this goal, the clamps are designed so as to require a very large force for opening the jaws, which is very bothersome for the user. This force is located around 60 Newtons upon starting the opening and reaches 140 Newtons when maintaining the clamp in the completely open condition. Indeed, the more the clamp is opened, the more the force increases. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to overcome this drawback. 
     In order to achieve this object, the clamp according to the invention is characterized in that it comprises a device for scaling down the force for opening the clamp, by its application on the trigger, which is mounted between the return elastic element and the trigger, and adapted for reducing the opening force during the opening. 
     According to a feature of the invention, the scaling-down device comprises an arrangement of connecting rods, which is mounted between the movable end of the elastic element, the other end of which is connected to a joint axis fixedly mounted on the support, and the arm bearing the movable jaw ( 4 ) of the two-arm. According to another feature of the invention, the scaling-down device is adapted for reducing during pivoting of the clamp in its open position, the moment around the pivot axis of the lever, produced by the force opposing the opening of the clamp, under the effect of the return spring. 
     According to still another feature of the invention, the scaling-down device is adapted for reducing during pivoting of the clamp in its open position, the length of the arm of the moment of the opposing force around the pivot axis of the two-arm lever. 
     According to still another feature of the invention, the clamp comprises two connecting rods, one of which is interposed between the arm bearing the movable jaw and a joint axis at the movable end of the return element and a connecting rod which is interposed between the joint axis and the fixed joint axis so as to force, during the opening of the clamp, the axis to move on a circular arc around the fixed joint, that the force opposing the opening depends on the return force of the return element and on the ratio of the length of the arm of the moment produced by the return force around the fixed joint axis and on the length of the arm around the fixed axis of the moment of the opposing force, and in that the ratio of both arms decreases upon opening the clamp by the displacement of the movable joint axis on the circular arc. 
     According to still another feature of the invention, the moment of the application force around the pivot axis of the lever is equal to the moment of the opposing force around the axis. 
     According to still another feature of the invention, the arm of the moment of the opposing force around the pivot axis of the two-arm lever decreases during pivoting of the lever in the open position of the clamp. 
     According to another feature of the invention, the elastic element is a traction spring. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING FIGURES 
       The invention will be better understood, and other objects, features, details and advantages thereof will become more clearly apparent in the explanatory description which follows, made with reference to the appended drawings only given as an example illustrating an embodiment of the invention and wherein: 
         FIG. 1  is a perspective view of a ground loop measurement clamp of the type of the clamp according to the invention; 
         FIG. 2  is a schematic view of a ground loop measurement clamp according to the invention, showing this clamp in its closure position; 
         FIG. 3  shows the clamp according to  FIG. 2  in its open position; 
         FIGS. 4 and 5  are explanatory schematic views of the operation of the connecting rod device according to the invention, showing this device when the clamp is closed and open respectively. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG. 1 , a ground loop measurement clamp comprises a portion forming a casing  1 , to which is secured a portion  2  of forming the fixed jaw of the clamp and a movable jaw  4  between the closed position of the clamp illustrated in  FIGS. 1 and 2  and an open position according to  FIG. 3 , the movement of which is controlled by a member forming a trigger  5  which the user pushes into the casing  1  for opening the clamp. 
     Referring to  FIGS. 2 and 3 , it is seen that the trigger  5  forms an arm  5  of a two-arm lever  6 , which is pivotally mounted around an axis A. This axis is mounted on a fixed support of the casing. The movable jaw  4  is borne by the second arm  7  of the lever  6 . 
       FIGS. 2 and 3  show that the jaws  2  and  4  have the shape of circular arcs. In their position for closing the clamp, the end front faces  9 ,  10  of the jaws  2  and  4  are in pressed contact over the whole of the surface. 
     The mechanism for controlling the opening and the closing of the clamp essentially comprises a spring R and a device  16  formed and arranged so as to ensure scaling down of the forces for opening and closing the clamp  1 . The spring R is connected through one end to a fixed joint axis A 2 , i.e. fixedly mounted on a support of the casing. 
     The scaling-down device  16  comprises, in the illustrated example, a first curved connecting rod B 1  which is connected through one end  17  to a fixed axis of rotation A 1 , while its other end  18  is used as a movable joint axis A 3  bound to the spring R and to an end  20  of a second connecting rod B 2  of the scaling-down device. The other end  21  of the connecting rod B 2  which is a rectilinear connecting rod is connected to the joint axis A 4  provided on the arm  7  of the lever  6  which rotates around the fixed axis A and the other arm of which forms the control trigger  5 . The spring R, a helical spring, is oriented substantially rectangularly to the arm B 2  when the clamp is closed. It is actuated in traction during the opening of the clamp. 
     The operation of the scaling-down device will become apparent from the description of the clamp according to the invention which will be given hereafter. 
     When the user wishes to open the clamp by bringing it from its closure position of  FIG. 2  to the opening position according to  FIG. 3 , he applies a pressure force Fap on the trigger  5  which rotates the lever  6  in the direction D 1  around the fixed axis A, which causes displacement of the axis A 4  on the arm  7  of the lever in the direction of the arrow D 2 . The rectilinear connecting rod B 2  which is jointed to the axis A 4  causes displacement of the axis A 3  to which is jointed the spring R. But, the curved connecting rod B 1  forces displacement of the axis A 3  on a circular arc around the fixed axis A 1 . 
       FIGS. 4 and 5  explain the scaling-down effect produced by the scaling-down device  16 .  FIG. 4  shows this device formed by the two connecting rods B 1  and B 2  in the condition when the clamp is closed. The spring R, in this position of the device, exerts a force F 1  on the axis A 3 , which generates a moment around the fixed axis A 1  which is equal to the force F 1  multiplied by the torque arm d 1 . The spring also generates, due to the connecting rods B 1  and B 2 , a force F 2  which acts in the direction of the rectilinear connecting rod B 2 . This force causes occurrence of a moment M 2  which is equal to the product of this force F 2  and of the arm d 2 , also around the fixed axis A 1 . 
     Given that the axis A 1  is stationary, both moments should be equal. The relationships result therefrom:
 
 M   F1   =M   F2  
 
 F   1   ×d 1= F   2   ×d 2
 
     When the clamp is in its open position, illustrated in  FIG. 3 , with the device  16  according to  FIG. 5 , both moments act in the same way as in the closed position of the clamp, but the lengths of the arms d 1  and d 2  have changed and have become d 1 ′ and d 2 ′. The return force of the spring has become the force F 1 ′ and the force F 2  the force F 2 ′. But as before, it is possible to write:
 
 F   1   ′×d 1= F   2   ′×d 2′
 
     The forces F 2  and F 2 ′ opposing the opening of the clamp in the closed and open conditions respectively, may be determined by the following equations: 
     
       
         
           
             
               
                 F 
                 2 
               
               = 
               
                 
                   
                     F 
                     1 
                   
                   × 
                   
                     d 
                     1 
                   
                 
                 
                   d 
                   2 
                 
               
             
             ; 
           
         
       
       
         
           
             
               F 
               2 
               ′ 
             
             = 
             
               
                 
                   F 
                   1 
                   ′ 
                 
                 × 
                 
                   d 
                   1 
                   ′ 
                 
               
               
                 d 
                 2 
                 ′ 
               
             
           
         
       
     
     It is seen that the forces F 2  and F 2 ′ depend on the force F 1 , F 1 ′ of the spring R and on the ratio of the length of the arms 
                 d   1       d   2       ⁢           ⁢   and   ⁢           ⁢       d   1   ′       d   2   ′             
of the moments.
 
     The particularity of the invention notably lies in the discovery that by modifying the ratio of the arms of the moments during the opening of the clamp, the force opposing the opening may be caused to no longer essentially depend on the return spring R. 
     With reference to  FIGS. 4 and 5 , it is seen that the arm d 2  is twice or thrice shorter than the arm d 1 , while in the open condition of the clamp, the arm d 1 ′ is shorter than the arm d 2 ′ which has the consequence that the force F 2  for maintaining the clamp in the open condition is clearly smaller than the force F 2  for opening the clamp. 
     As regards the moments exerted by the forces F 2  and F 2 ′ around the axis A on the jaw arm, one obtains
 
 MA=F   2   ×d 3
 
 MA′=F   2   ′×d 3′
 
     d 3  and d 3 ′ being the arms of the moments around the axis A. 
     In order to open the clamp and to maintain it in the open condition, forces have to be applied to the trigger: 
     
       
         
           
             Fap 
             = 
             
               
                 M 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 A 
               
               da 
             
           
         
       
       
         
           
             Fap 
             = 
             
               
                 M 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   A 
                   ′ 
                 
               
               dA 
             
           
         
       
     
     dA being the length of the arm of the moments around the axis A, of the application forces Fap. 
     It is seen on  FIGS. 4 and 5  that the moment arm d 3 ′ of the open clamp is shorter than the arm d 3  of the closed clamp, which has the consequence that the force Fap to be applied on the trigger for maintaining the clamp open is considerably smaller than the force for opening it, because the moment MA′ is much smaller than the moment MA. 
     For obtaining this evolution of the arm ratios, the invention proposes as a non-exclusive example, the interposition between the return spring R and the movable jaw  4  of the scaling-down device  16  with two connecting rods as illustrated in the figures. In the illustrated example, by placing the rectilinear connecting rod B 2  close to the fixed joint axis A 1 , the arm d 2  of the moment generated by the force F 2  may be relatively short. But, by forcing the joint axis A 3  to move on a circular arc around the axis A 1 , by the connecting rod B 1 , the arm d 2  increases during the opening while the length of the arm d 1  decreases. Given that in the closed condition of the clamp, the angle between the axes of the connecting rods is relatively small, it is advantageous to give a curved shape to the connecting rod B 1  in order to facilitate the movement of the movable joint axis A 3  around the fixed axis A 1 , under the effect of the connecting rod d 2 . 
     In order to clearly show the particularity of the invention, allowing to obtain that the force applied to the trigger for maintaining the clamp open is considerably smaller than the applied force for opening it, a non-limiting example of a clamp according to the invention will be given here. 
     The scaling-down device of this clamp includes an arm B 2  with a length of 28 mm between the axes A 4  and A 3  and a short arm B 1 , the distance of which, i.e. the straight line, between the axes A 1  and A 3 , is 20 mm. In the closed condition of the clamp, the angle α between the arms B 2  and the straight line has a value of 18.36°. In the closed condition of the clamp, the distance between the axes A 4  and A 2  is 54.4 mm and the angle formed between the straight line connecting the axes A 4  and A 3  on the one hand and the straight line connecting the axes A 4  and A 2  on the other hand, is 58°. the distance between the axes A 4  and A 1  is 11 mm. 
     By thus producing the scaling-down device, it is obtained that in the closed condition of the clamp, the arm d 2  has a length of 6.3 mm and the arm d 1  the length of 19.11 mm. For a return force of the spring F 1 =24 N, a force F 2  of 72.81 N is obtained. Due to the positioning indicated above of the joint axes and of the connecting rods, in the open condition of the clamp, the arm d 2 ′ has a length of 15.08 mm, and the arm d 1 ′ a length of 13.19 mm. In this case, the force of the spring F 1 ′ is 46.56 N. A force F 2 ′ of 40.72 N results therefrom. The distance between the axes A 4  and A 2  is now 63 mm and the angle between the lines A 4 −A 2  and A 4 −A 3  is 65°. The distance between the axes A 4  and A 1  is 20 mm. 
     Further, for the arms d 3  a length of 21.68 mm for the closed clamp and 12.34 mm for the open clamp are obtained. The angle α has in the open condition of the clamp the value of 48.91°. 
     By using the equations indicated above, with an arm dA of 40.81 mm, the following values are obtained for the force Fap for application on the trigger for the opening and the force Fap′ for maintaining the open condition of the clamp. 
     
       
         
           
             Fap 
             = 
             
               
                 
                   M 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   A 
                 
                 dA 
               
               = 
               
                 
                   
                     F 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                     × 
                     d 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   dA 
                 
                 = 
                 
                   
                     
                       78.81 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       N 
                       × 
                       21.68 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                     
                       40.81 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                   
                   = 
                   
                     38.97 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     N 
                   
                 
               
             
           
         
       
       
         
           
             
               Fap 
               ′ 
             
             = 
             
               
                 
                   M 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     A 
                     ′ 
                   
                 
                 dA 
               
               = 
               
                 
                   
                     F 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       2 
                       ′ 
                     
                     × 
                     d 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       2 
                       ′ 
                     
                   
                   dA 
                 
                 = 
                 
                   
                     
                       40.72 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       N 
                       × 
                       12.34 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                     
                       40.81 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                   
                   = 
                   
                     12.31 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     N 
                   
                 
               
             
           
         
       
     
     It emerges from this practical example based on the principle of the configuration of the connecting rods illustrated in the figures, that the force of 72.81 N for starting the opening of the clamp is greater than the force of 40.72 N for maintaining the clamp open, while ensuring a pressure force for closing both jaws on each other, which has the result that the force of application on the trigger for maintaining the clamp open is much smaller (12.31 N) than the force for opening of the clamp (38.67 N). 
     Of course, multiple modifications may be made to the clamp as described and illustrated as an example in the figures. 
     It should be noted that the invention is not limited to ground loop measurement clamps but may be applied to any other clamps having the general structure of the described clamp.