Abstract:
A gripping mechanism includes:
       a plurality of chuck claws that, when having come close to each other, generate a gripping force on a gripped body;   a chuck body that holds the plurality of chuck claws on a common planar surface, and moves them on the planar surface; and   a plurality of chuck plates that, when each of the plurality of chuck claws grips the gripped body, are interposed between each of the plurality of chuck claws and the gripped body, wherein   a thermal expansion coefficient α l  of the plurality of chuck claws, a thermal expansion coefficient α 2  of the plurality of chuck plates and a thermal expansion coefficient α W  of the gripped body has a relationship indicated by Equation 1:       
 
       α W &lt;α 1 &lt;α 2    (Equation 1)

Description:
[0001]    The contents of the following Japanese patent application are incorporated herein by reference NO. 2015-187824 filed on Sep. 25, 2015. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a gripping mechanism. 
         [0004]    2. Related Art 
         [0005]    The gripping force exerted on a gripped body by a gripping mechanism including a chuck lowers in some cases due to temperature changes. To cope with this, a collet chuck-type gripping structure designed to suppress lowering of the gripping force due to the restoring force of a spring has been proposed (see Patent Document 1, for example). 
         [0006]    Patent Document 1: Japanese Patent Application Publication No. 2008-178956 
         [0007]    Japanese Industrial Standard defines [B 6151-1993] as a scroll gear-type gripping mechanism having another structure. The gripping force of a scroll gear-type chuck also lowers in some cases due to temperature changes. However, the structure described in the above-mentioned document cannot be applied to a scroll gear-type chuck. 
       SUMMARY 
       [0008]    An aspect of the present invention provides a gripping mechanism including: 
         [0009]    a plurality of chuck claws that, when having come close to each other, generate a gripping force on a gripped body; 
         [0010]    a chuck body that holds the plurality of chuck claws on a common planar surface, and moves them on the planar surface; and 
         [0011]    a plurality of chuck plates that, when each of the plurality of chuck claws grips the gripped body, are interposed between each of the plurality of chuck claws and the gripped body, wherein 
         [0012]    a thermal expansion coefficient α 1  of the plurality of chuck claws, a thermal expansion coefficient α 2  of the plurality of chuck plates and a thermal expansion coefficient α W  of the gripped body has a relationship indicated by Equation 1: 
         [0000]      α W &lt;α 1 &lt;α 2    (Equation 1)
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is an outline sectional view of a gripping mechanism  101 . 
           [0014]      FIG. 2  is an outline sectional view of a gripping mechanism  102 . 
           [0015]      FIG. 3  is a graph showing a relationship between an outer diameter D w  of a gripped body  150  and a thickness d 0  of a chuck plate  140 . 
       
    
    
       [0016]    The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. 
       DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0017]    Hereinafter, (some) embodiment(s) of the present invention will be described. The embodiment(s) do(es) not limit the invention according to the claims, and all the combinations of the features described in the embodiment(s) are not necessarily essential to means provided by aspects of the invention. 
         [0018]      FIG. 1  is a schematic outline sectional view of a gripping mechanism  101 . The gripping mechanism  101  is a scroll chuck having one-piece type jaws  130 , and has a chuck body  110 , the jaws  130  and a chuck plate  140 . Also, the illustrated gripping mechanism  101  is gripping a gripped body  150 . The gripped body  150  is for example a glass cylindrical bar that forms an end portion of an optical fiber base material. 
         [0019]    The chuck body  110  has a disk-like outer shape. Also, the chuck body  110  is coupled, by a fixation bolt  162  inserted at its center in the thickness direction, to a rotation axis  160  of a rotary driving mechanism. 
         [0020]    Also, the chuck body  110  has a scroll gear (not illustrated) built-in as a driving mechanism. The scroll gear has a spiral groove that is open toward one surface of the chuck body  110 . This groove meshes with tooth flanks  134  formed on end portions of the jaws  130  that are described below. Accordingly, if a driving force is supplied from the outside of the chuck body  110  to rotate the scroll gear, a driving force moving in a radial direction of the chuck body  110  is transmitted to the jaws  130 . 
         [0021]    Furthermore, the chuck body  110  holds a plurality of the jaws  130  on the above-mentioned one surface. Each of the plurality of jaws  130  is disposed at a constant interval in the circumferential direction on a circular surface of the chuck body  110 . If two or more jaws  130  are provided, the gripping mechanism  101  can grip the gripped body  150 , but in cases for example where an end portion of a cylindrical bar is to be gripped, three or more jaws  130  are provided. 
         [0022]    Each of the plurality of jaws  130  has a shape protruding from the chuck body  110 , and has a high strength and rigidity. Also, each of the jaws  130  is held relative to the chuck body  110  such that the jaw  130  can move in a radial direction of the chuck body  110 . Furthermore, each of the plurality of jaws  130  has, at an end portion thereof on the chuck body  110  side, the tooth flanks  134  that mesh with the scroll gear. 
         [0023]    If the scroll gear is rotated in the chuck body  110 , the plurality of jaws  130  moves along the front surface of the chuck body  110  while maintaining a positional relationship where they are arranged concentrically with the circle formed by one surface of the chuck body  110 . In other words, if the scroll gear does not rotate in the chuck body  110 , the jaws  130  meshing with the scroll gear are fixed relative to the chuck body  110 . 
         [0024]    Accordingly, if the plurality of jaws  130  have moved and have come close to each other, the gripped body  150  is gripped between the jaws  130  whose intervals have become narrow. Also, if the plurality of jaws  130  have moved and have become distanced from each other, the gripped body  150  that had been gripped by the plurality of jaws  130  is released. 
         [0025]    In the gripping mechanism  101 , the chuck plate  140  is attached to a surface, of each of the plurality of jaws  130 , facing the gripped body  150 . Each of the chuck plates  140  is fixed, by a screw  142 , relative to the jaw  130 . Thereby, if the gripped body  150  is gripped in the gripping mechanism  101 , the chuck plate  140  is sandwiched between each of the plurality of jaws  130  and the gripped body  150 . Accordingly, by replacing chuck plates  140 , the surfaces of the jaws  130  to contact the gripped body  150  can be renewed. 
         [0026]    Japanese Industrial Standard [B 6151-1993] about a scroll chuck as the gripping mechanism  101  defines that jaws  130  should have a hardness of 55 HRC or higher. Accordingly, the jaws  130  of the gripping mechanism  101  are formed with a material that has a strength defined by the standard, for example, steel for general structures, carbon steel for mechanical structures, alloy steel for mechanical structures or the like as its material. The thermal expansion coefficients of steel materials such as steel for general structures, carbon steel for mechanical structures, or alloy steel for mechanical structures are 10 to 13×10 −6 /K. 
         [0027]    In the gripping mechanism  101 , members other than the jaws  130  such as the chuck body  110  including the scroll gear are also often formed with steel castings, forgings or the like. These materials also have thermal expansion coefficients that are similar to those of steel for general structures, carbon steel for mechanical structures, alloy steel for mechanical structures or the like. Accordingly, an assembly including the chuck body  110  and the jaws  130  has a generally uniform thermal expansion coefficient as a whole. 
         [0028]    If a temperature change occurs in a state where the gripped body  150  formed with a material having a low thermal expansion coefficient such as synthetic quarts or an invar material is gripped by a scroll chuck formed with a material like those mentioned above, the gripping force exerted on the gripped body  150  by the gripping mechanism  101  lowers in some cases due to the difference in the thermal expansion coefficients of the gripping mechanism  101  and the gripped body  150 . If the gripping force is insufficient in the gripping mechanism  101 , there is a risk of the gripped body  150  falling, and also there is a concern about damages to the gripped body  150  itself and facilities including the gripping mechanism. 
         [0029]    For example, in a step of manufacturing a glass base material for optical fiber, a glass particle-containing flame is impinged upon in a state where a dummy rod formed with synthetic quarts or the like is gripped by the gripping mechanism  101 . For this reason, the temperature of the gripping mechanism  101  also rises due to heat transmitted through the gripped body  150 , but because the thermal expansion coefficient of the gripping mechanism  101  is significantly different from the thermal expansion coefficient of a glass base material, there is a possibility of the force to sandwich the gripped body  150  lowering due to widening intervals of the jaws  130  in the gripping mechanism  101 . 
         [0030]    Here, a case where a temperature change has occurred in a state where the gripping mechanism  101  is gripping the gripped body  150  having a diameter D w  is considered. It is assumed that a pair of the chuck plates  140  having the respective thicknesses of D 0  sandwiches the gripped body  150 , and each of the chuck plates  140  is attached to the jaws  130  that are disposed at positions that are symmetric about the center of the chuck body  110  facing each other. The thermal expansion coefficient of the jaws  130  is α 1 , the thermal expansion coefficient of the chuck plates  140  is α 2 , and the thermal expansion coefficient of the gripped body  150  is α W . The thermal expansion coefficient α 0  of the chuck body  110  is assumed to be equal to the thermal expansion coefficient α 1  of the jaws  130 . 
         [0031]    An initial interval d 1  between the pair of chuck plates  140  gripping the gripped body  150  is equal to the diameter D W  of the gripped body  150 . Accordingly, about the mutually facing jaws  130 , an interval d 2  of surfaces  136  that support the rear surfaces of the chuck plates  140  can be expressed by Equation 2: 
         [0000]        d   2 =(2 d   0   +D   W )   (Equation 2)
 
         [0032]    Next, if the temperature of the gripping mechanism  101  gripping the gripped body  150  changes, and for example, the temperature rises, the entire assembly formed by the chuck body  110  and the jaws  130  expands with heat, and the interval d 2  of the surfaces  136  supporting the rear surfaces of the chuck plate  140  changes. A change amount Δd 2  per unit temperature change amount of the interval d 2  is expressed by Equation 3: 
         [0000]      Δ d   2 =α 1 (2 d   0   +D   w )   (Equation 3)
 
         [0033]    Also, about each chuck plate  140 , a change amount Δd 0  of the thickness d 0  per unit temperature change amount can be expressed by Equation 4: 
         [0000]      Δ d   0 =α 2   d   0    (Equation 4)
 
         [0034]    Furthermore, because each of the chuck plates  140  is attached to a jaw  130 , about the interval d 1  of the chuck plates  140  sandwiching the gripped body  150  in the gripping mechanism  101 , a change amount Δd 1  per unit temperature change amount can be expressed by Equation 5: 
         [0000]      Δ d   1 =2 d   0 (α 1 −α 2 )+α 1   d   1    (Equation 5)
 
         [0035]    On the other hand, about the outer diameter D W  of the gripped body  150  having the thermal expansion coefficient α W , an outer diameter change amount ΔD W  per unit temperature change can be expressed by Equation 6: 
         [0000]      ΔD W =α W   D   w    (Equation 6)
 
         [0036]    Here, as indicated by Equation 7, if the outer diameter change amount D W  of the gripped body  150  indicated by Equation  6  is larger than the change amount Δd 1  of the interval d 1  of the chuck plates  140 , the force of the chuck plates  140  in the gripping mechanism  101  to fasten the gripped body  150  does not lower even when a temperature change occurs. 
         [0000]      2 d   0 (α 1 −α 2 )+α 1   D   w α w   D   w    (Equation 7)
 
         [0037]    Equation 7 can be modified into Equation 8 paying attention to the thickness d 0  of the chuck plates  140 . 
         [0000]        d   0   ≦{D   W (α W −α 1 )}/{2(α 1 −α 2 )}  (Equation 8)
 
         [0038]    Accordingly, the gripping mechanism  101  whose gripping force does not lower due to temperature changes can be formed by selecting a material and the thickness d 0  of the chuck plates  140  in the gripping mechanism  101  so as to satisfy Equation 8: 
         [0039]    However, the interval d 2  of the surfaces  136  of the jaws  130  is equal to or larger than the sum (2d 0 ) of the thicknesses of the two chuck plates  140 . Accordingly, in the gripping mechanism  101 , in order for Equation 8 to hold, it is premised that the thermal expansion coefficient α 1  of the jaws  130 , the thermal expansion coefficient α 2  of the chuck plates  140  and the thermal expansion coefficient α W  of the gripped body  150  have the relationship indicated by Equation 1: 
         [0000]      α W &lt;α 1 &lt;α 2    (Equation 1)
 
         [0040]    When the condition of Equation 8 are to be met, the larger the absolute value of the difference between the thermal expansion coefficient α 1  of the jaws  130  and the thermal expansion coefficient α 2  of the chuck plates  140 , the larger the thickness d 0  of the chuck plates  140 . Also, the range of the outer diameter D W  of the gripped body  150  at which the gripped body  150  can be gripped without lowering of the fastening force widens. 
         [0041]    More specifically, taking into consideration a thermal expansion coefficient of a metallic material that can be used industrially as a material of the jaws  130 , if the thermal expansion coefficient α 1  of the jaws  130  is within the range of 0.5 to 19.0×10 −6 /K, the thermal expansion coefficient α 2  of the chuck plates  140  is preferably within the range of 16.0 to 24.0×10 −6 /K. 
         [0042]    Furthermore, it is assumed in the above-mentioned explanation that the thermal expansion coefficient α 0  of the chuck body  110  including a driving mechanism such as a scroll gear is equal to the thermal expansion coefficient α 1  of the jaws  130 . However, the thermal expansion coefficients α 0  and α 1  may not be the same. 
         [0043]    However, if the thermal expansion coefficient α 0  of the chuck body  110  is lower than the thermal expansion coefficient of the jaws  130 , a change in the interval d 1  due to a temperature change is suppressed. Accordingly, a chuck plate  140  whose thickness d 0  is smaller or whose thermal expansion coefficient α 1  , is lower can be used. Accordingly, the thermal expansion coefficient α 0  of the chuck body  110  and the thermal expansion coefficient α 1  of the jaws  130  preferably are in a relationship that meets Equation 9: 
         [0000]      α 0 ≦α 1    (Equation 9)
 
         [0044]      FIG. 2  is an outline sectional view illustrating the structure of a gripping mechanism  102  comprising two-piece type jaws  130  that are different from one-piece type jaws of the gripping mechanism  101  illustrated in  FIG. 1 . The gripping mechanism  102  has the same structure as the gripping mechanism  101  illustrated in  FIG. 1  except for the points explained below. Common elements are provided with the same reference numbers, and redundant explanation is omitted. 
         [0045]    The gripping mechanism  102  has a structure different from that of the gripping mechanism  101  in that each of the plurality of jaws  130  is respectively attached to the chuck body  110  via a base portion  120 . The base portion  120  has the tooth flanks  134  that mesh with a scroll gear built into the chuck body  110 , and is disposed on one circular surface of the chuck body  110 . 
         [0046]    The chuck body  110  in the gripping mechanism  102  holds a plurality of the base portions  120  on one surface. Each of the plurality of base portions  120  is disposed at a constant interval in the circumferential direction on the circular surface of the chuck body  110 . Also, the jaws  130  are held relative to the chuck body  110  such that they can move in a radial direction of the chuck body  110 . 
         [0047]    Each of the plurality of jaws  130  has a shape protruding from one surface of the chuck body  110 , and is respectively fixed to any of the plurality of base portions  120  by a screw  132 . Thereby, if the scroll gear is rotated in the chuck body  110 , the plurality of jaws  130  moves in a radial direction of the chuck body  110  together with the base portions  120  while maintaining a positional relationship where they are arranged concentrically with the circle formed by one surface of the chuck body  110 . Also, if the scroll gear does not rotate in the chuck body  110 , the jaws  130  are fixed relative to the chuck body  110  via the base portions  120  meshing with the scroll gear. 
         [0048]    Accordingly, similar to the gripping mechanism  101 , if the plurality of jaws  130  have moved and have come close to each other, the gripped body  150  is gripped between the jaws  130  whose intervals have become narrow. Also, if the plurality of jaws  130  have moved and have become distanced from each other, the gripped body  150  that had been gripped by the plurality of jaws  130  is released. Similar to the gripping mechanism  101 , each of the plurality of jaws  130  sandwiches the gripped body  150  via the chuck plate  140  fixed by the screws  142 . 
         [0049]    The base portions  120  in the gripping mechanism  102  having the above-mentioned structure have, for example, the same thermal expansion coefficient α 0  as the chuck body  110 . Accordingly, the thermal expansion coefficient of the entire assembly including the chuck body  110 , the base portions  120  and the jaws  130  becomes generally uniform. As a result, the gripping mechanism  102  also satisfies the same conditions as the gripping mechanism  101  does, and lowering of the gripping force due to temperature changes can be prevented. 
         [0050]    Most materials exhibit changes in their thermal expansion coefficients depending on temperature. Accordingly, a series of the above-mentioned conditions about thermal expansion coefficient is desirably satisfied within a temperature range at which the gripping mechanisms  101  and  102  are used. As a result, for example, if the gripping mechanisms  101  and  102  are to be used in a sintering device that sinters optical fiber base materials, the above-mentioned conditions about thermal expansion coefficient are preferably satisfied within a temperature range of room temperature (for example, 0 to 30° C.) or higher and 250° C. or lower. 
       EXPERIMENTAL EXAMPLE 
       [0051]    As materials to form the chuck body  110 , the base portions  120  and jaws  130  in the gripping mechanisms  101  and  102 , the materials shown in Table 1 were prepared. Also, as a material to form the chuck plates in the gripping mechanisms  101  and  102 , the materials shown in Table 2 were prepared. The thermal expansion coefficients within the temperature range of 250° C. or lower are shown together. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Thermal Expansion Coefficients 
               
               
                   
                 Jaw Materials 
                 (α 1 ) [×10 −6 /K] 
               
               
                   
                   
               
             
             
               
                   
                 Steel for general structures 
                 10-12 
               
               
                   
                 Carbon steel for mechanical 
                 10-12 
               
               
                   
                 structures 
               
               
                   
                 Alloy steel for mechanical 
                 10-12 
               
               
                   
                 structures 
               
               
                   
                 Martensite stainless 
                  9-11 
               
               
                   
                 Ferrite stainless 
                  9-11 
               
               
                   
                 Invar alloy 
                 0.5-3   
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Thermal Expansion Coefficients 
               
               
                   
                 Chuck Plate Materials 
                 (α 2 ) [×10 −6 /K] 
               
               
                   
                   
               
             
             
               
                   
                 aluminum alloy 
                 22-24 
               
               
                   
                 austenite stainless 
                 16-18 
               
               
                   
                   
               
             
          
         
       
     
         [0052]    First, as a material to form the chuck body  110 , the base portions  120  and the jaws  130 , carbon steel for mechanical structures, S45C (α 1 : 12.1×10 −6 /K), was selected from among the materials shown in Table 1. Also, as a material of the chuck plates  140 , aluminum alloy, A5052 (α 2 : 23.8×10 −6 /K), was selected from among the materials shown in Table 2. Furthermore, in order to evaluate changes in the gripping force, synthetic quarts (α w : 0.6×10 −6 /K) having an outer diameter D w  of 80 mm was prepared. 
         [0053]    By using the thermal expansion coefficients of the selected materials, the thickness d 0  of the chuck plates  140  that satisfies Equation 8 if the gripped body  150  is gripped was calculated, and the relationship between the thickness d 0  of the chuck plates  140  and the outer diameter of the gripped body  150  was plotted in a graph. The graph is shown in  FIG. 3 . 
         [0054]    As can be known from  FIG. 3 , if the outer diameter of the gripped body  150  is 80 mm, the thickness d 0  of the chuck plates  140  that does not cause lowering of the gripping force for the gripped body  150  due to temperature changes can be known to be 39.3 mm or larger. Accordingly, a chuck plate  140  having a thickness of 40 mm was manufactured, and mounted on the gripping mechanisms  101  and  102 , and changes in the gripping force due to temperature changes were evaluated. 
         [0055]    First, in a state where both the entire gripping mechanism  101  manufactured with the above-mentioned materials and dimensions, and the cylindrical gripped body  150  formed with synthetic quarts and having an outer diameter of 80 mm are at room temperature (298 K), the gripping mechanism  101  was caused to grip the gripped body  150 . The gripping force exerted on the gripped body by the gripping mechanism  101  was set to a force that does not allow rotation of the gripped body  150  even if it was attempted to rotate the gripped body  150  in its circumferential direction by a manual force. 
         [0056]    Next, the gripping mechanism  101  and the gripped body  150  are entirely heated to 489 K. Although in this state, it was attempted to rotate the gripped body  150  by a method which is the same as that for the case of room temperature, the gripped body  150  was gripped firmly by the gripping mechanism  101 , and the gripped body  150  could not be rotated. In evaluating the force to grip the gripped body  150 , the gripped body  150  was held by hands wearing, on them, leather-made gloves which were the same for both the case of room temperature and the case of high temperature, and it was attempted to rotate the gripped body  150 . 
         [0057]    As can be seen, the gripping force exerted by the gripping mechanisms  101  and  102  on the gripped body  150  does not lower even if temperature changes. Accordingly, the gripped body  150  can be gripped stably even in an environment or use that accompanies rapid temperature changes. 
         [0058]    While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. 
         [0059]    The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order. 
       EXPLANATION OF REFERENCE SYMBOLS 
       [0060]      101 ,  102 : gripping mechanism;  110 : chuck body;  120 : base portion;  130 : jaw;  132 ,  142 : screw;  134 : tooth flank;  136 : surface;  140 : chuck plate;  150 : gripped body;  160 : rotation axis;  162 : fixation bolt