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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    A relevant subject matter is disclosed in a co-pending U.S. patent application (Attorney Docket No. US30359) filed on the same date and entitled “HINGE,” which is assigned to the same assignee as this patent application. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a hinge. 
         [0004]    2. Description of Related Art 
         [0005]    A foldable device, such as a notebook computer, or a foldable mobile phone, generally includes a base and a cover pivotably mounted to the base via a hinge. The hinge generally includes a pivot shaft, a first leaf, and a second leaf. The first leaf is mounted to the base, and the second leaf is mounted to the cover. A twisting force is required to overcome a friction between the second leaf and the pivot shaft to rotate the cover relative to the base. However, the friction between the second leaf and the pivot shaft remains the same when the device is unfolded. The twisting force required also remains the same, which gives a user an uneasy feeling. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an exploded, isometric view of a first exemplary embodiment of a hinge. 
           [0007]      FIG. 2  is an assembled, isometric view of the hinge of  FIG. 1 . 
           [0008]      FIG. 3  is similar to  FIG. 2 , but showing a different state. 
           [0009]      FIG. 4  is an exploded, isometric view of a second exemplary embodiment of a hinge. 
           [0010]      FIG. 5  is similar to  FIG. 4 , but viewed from another perspective. 
           [0011]      FIG. 6  is an assembled, isometric view of the hinge of  FIG. 4 . 
           [0012]      FIG. 7  is similar to  FIG. 6 , but showing a different state. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , a first exemplary embodiment of a hinge includes a pivot shaft  10 , a first leaf  20 , and a second leaf  30 . 
         [0014]    The pivot shaft  10  includes a first pivot pin  11 , a second pivot pin  12 , and a flange  13 . The flange  13  is positioned between the first pivot pin  11  and the second pivot pin  12 . A ledge  14  protrudes partially around the second pivot pin  12 , adjacent to the flange  13 . Two engaging surfaces  140  are formed on opposite ends of the ledge  14 . 
         [0015]    The first leaf  20  includes a mounting piece  22  defining two fixing holes  220  to fix the first leaf  20  to a base of a foldable device, such as a notebook computer. A side of the mounting piece  22  is curled to form a knuckle  24 . The first pivot pin  11  rotatably extends through the knuckle  24 . When the knuckle  24  rotates relative to the first pivot pin  11 , a friction between the first pivot pin  11  and the knuckle  24  is f 1 . 
         [0016]    The second leaf  30  includes a mounting piece  32  defining a plurality of fixing holes  320  to fix the second leaf  30  to a cover of the foldable device. A center of a side of the mounting piece  32  defines an incision  322 . The side is curled from two sides of the incision  322  in different directions, forming a first knuckle  34  and a second knuckle  36 . The first knuckle  34  is coaxial with the second knuckle  36 , and an inner diameter of the first knuckle  34  is equal to that of the second knuckle  36 . The second pivot pin  12  is rotatably received in the first knuckle  34  and the second knuckle  36 . When the second pivot pin  12  rotates relative to the first knuckle  34  and the second knuckle  36 , a friction between the second pivot pin  12  and the first knuckle  34  and the second knuckle  36  is f 2 . An arc-shaped tab  360  axially extends from one end of the second knuckle  36  away from the first knuckle  34 . The tab  360  is corresponding to the ledge  14 . Two engaging surfaces  364  are formed on opposite sides of the tab  360 . 
         [0017]    Referring to  FIG. 2 , during assembly, the knuckle  24  is knuckled on the first pivot pin  11 . The second leaf  30  is sleeved on the second pivot pin  12  such that the flange  13  locates between the knuckle  24  and the second knuckle  36 . The friction f 2  between the second pivot pin  12  and the first knuckle  34  and the second knuckle  36  of the second leaf  30  is smaller than the friction f 1  between the first pivot pin  11  and the knuckle  24 . In one exemplary embodiment, an inner diameter of the knuckle  24 , the first knuckle  34  and the second knuckle  36  are the same, while a diameter of the first pivot pin  11  is larger than that of the second pivot pin  12 , as a result, the friction f 1  is larger than the friction f 2 . 
         [0018]    In other exemplary embodiments, an inner wall of the knuckle  24  and a circumference of the first pivot pin  11  form friction pattern to increase the friction between the knuckle  24  and the first pivot pin  11 . Or the diameter of the first pivot pin  11  is equal to that of the second pivot pin  12 , the inner diameter of the knuckle  24  is reduced to make the friction f 1  larger than the friction f 2 . Or the diameter of the first pivot pin  11  is equal to that of the second pivot pin  12 , the inner diameters of the first knuckle  34  and the second knuckle  36  are increased to make the friction f 1  larger than the friction f 2 . 
         [0019]    Referring to the  FIG. 3 , to open the foldable device from a folded/closed state, initially, a smaller twisting force is required to overcome the friction f 2  between the first knuckle  34 , the second knuckle  36  of the second leaf  30  and the second pivot pin  12  to rotate the second leaf  30  relative to the pivot shaft  10 . This is because the friction f 2  between the second leaf  30  and the pivot shaft  10  is smaller than the friction f 1  between the first leaf  20  and the pivot shaft  10 . When one of the engaging surfaces  364  of the tab  360  engages with one of the engaging surfaces  140  of the ledge  14 , a larger twisting force is required to overcome the friction f 1  between the knuckle  24  and the first pivot pin  11  to simultaneously rotate the second leaf  30  and the pivot shaft  10  relative to the first leaf  20 . 
         [0020]    To close the foldable device from the unfolded/opened state, initially, a smaller twisting force is required to overcome the friction f 2  between the first knuckle  34 , the second knuckle  36  and the second pivot pin  12  to rotate the second leaf  30  relative to the pivot shaft  10 . When the other engaging surface  364  of the second leaf  30  engages with the other engaging surface  140  of the pivot shaft  10 , a larger twisting force is required to overcome the friction f 1  between the knuckle  24  and the first pivot pin  11  to simultaneously rotate the second leaf  30  and the pivot shaft  10  relative to the first leaf  20 . 
         [0021]    Referring to  FIG. 4 , a second exemplary embodiment of a hinge includes a pivot shaft  40 , a first leaf  50 , and a second leaf  60 . 
         [0022]    The pivot shaft  40  includes a first pivot pin  41 , a second pivot pin  42 , and a flange  46 . The flange  46  is positioned between the first pivot pin  41  and the second pivot pin  42 . An arc-shaped ledge  47  extends from a first end of the flange  46 , adjacent to the second pivot pin  42 . Two engaging surfaces  470  are formed on opposite ends of the ledge  47 . A wall  49  protrudes from a second end of the flange  46 , adjacent to the first pivot pin  41 . The wall  49  is arc-shaped. Two blocking surfaces  490  are formed on opposite ends of the wall  49 . 
         [0023]    Referring to  FIG. 5 , the first leaf  50  includes a mounting piece  52  defining two fixing holes  520  to fix the first leaf  50  to a base of a foldable device, such as a notebook computer. A side of the mounting piece  52  is curled to form a knuckle  54 . The first pivot pin  41  of the pivot shaft  40  rotatably extends through the knuckle  54 . When the first pivot pin  41  rotates relative to the knuckle  54 , a friction between the first pivot pin  41  and the knuckle  54  is f 3 . A positioning portion  540  axially extends from one end of knuckle  54 . The positioning portion  540  is corresponding to the wall  49 . Two positioning surfaces  542  are formed on opposite ends of the positioning portion  540 . 
         [0024]    The second leaf  60  includes a mounting piece  62  defining a plurality of fixing holes  620  to fix the second leaf  60  to a cover of the foldable device. A center of a side of the second leaf  60  defines an incision  622 . The side is bent and extends curled from two sides of the incision  622  in different directions, forming a first knuckle  64  and a second knuckle  66 . The first knuckle  64  is coaxial with the second knuckle  66 , and an inner diameter of the first knuckle  64  is equal to an inner diameter of the second knuckle  66 . When the second pivot pin  42  rotates relative to the first knuckle  64  and the second knuckle  66 , a friction between the second pivot pin  42  and the first knuckle  64 , the second knuckle  66  of the second leaf  60  is f 4 . An arc-shaped tab  660  axially extends from one end of the second knuckle  66  away from the first knuckle  64 . The tab  660  is corresponding to the ledge  47 . Two engaging surfaces  664  are formed on opposite ends of the tab  660 . 
         [0025]    Referring to  FIG. 6 , during assembly, the first pivot pin  41  extends through the knuckle  54 . The second leaf  60  is sleeved on the second pivot pin  42  such that the flange  46  locates between the knuckle  54  and the second knuckle  66 . The friction f 4  between the second pivot pin  42  and the first knuckle  64 , the second knuckle  66  of the second leaf  60  is smaller than the friction f 3  between the first pivot pin  41  and the knuckle  44 . In one exemplary embodiment, an inner diameter of the knuckle  54 , the first knuckle  64  and the second knuckle  66  are the same, while a diameter of the first pivot pin  41  is larger than that of the second pivot pin  42 , as a result, the friction f 3  is larger than the friction f 4 . 
         [0026]    In other exemplary embodiments, an inner wall of the knuckle  54  and a circumference of the first pivot pin  41  form friction pattern to increase the friction between the knuckle  54  and the first pivot pin  41 . Or the diameter of the first pivot pin  41  is equal to that of the second pivot pin  42 , the inner diameter of the knuckle  54  is reduced to make the friction f 3  larger than the friction f 4 . Or the diameter of the first pivot pin  41  is equal to that of the second pivot pin  42 , the inner diameters of the first knuckle  54  and the second knuckle  56  are increased to make the friction f 3  larger than the friction f 4 . 
         [0027]    Referring to the  FIG. 7 , to open the foldable device from a folded/closed state, initially, a smaller twisting force is required to overcome the friction f 4  between the first knuckle  64 , the second knuckle  66  of the second leaf  60  and the second pivot pin  42  to rotate the second leaf  60  relative to the pivot shaft  50 . This is because the friction f 4  between the second leaf  60  and the pivot shaft  40  is smaller than the f 3  between the first leaf  50  and the pivot shaft  40 . When one of the engaging surfaces  664  of the second leaf  60  engages with one of the engaging surfaces  470  of the pivot shaft  40 , a larger twisting force is required to overcome the friction f 3  between the knuckle  54  and the first pivot pin  41  to simultaneously rotate the second leaf  60  and the pivot shaft  40  relative to the first leaf  50 . When one of the blocking surfaces  490  engages with one of the positioning surfaces  542 , the first leaf  50  prevents the second leaf  60  and the pivot shaft  40  from simultaneously rotating relative to the first leaf  50 . 
         [0028]    To close the foldable device from the unfolded/opened state, initially, a smaller twisting force is required to overcome the friction f 4  between the first knuckle  64 , the second knuckle  66  of the second leaf  60  and the second pivot pin  42  to rotate the second leaf  60  relative to the pivot shaft  40 . When the other engaging surface  664  of the second leaf  60  engages the other engaging surface  470  of the pivot shaft  40 , a larger twisting force is required to overcome the friction f 3  between the knuckle  54  of the first leaf  50  and the first pivot pin  41  to simultaneously rotate the second leaf  60  and the pivot shaft  40  relative to the first leaf  50 . 
         [0029]    It is to be understood, however, that even though numerous characteristics and advantages of the present disclosure have been set forth in the foregoing description, together with details of the structure and function of the disclosure, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Summary:
A hinge includes a first leaf, a second leaf, and a pivot shaft. The pivot shaft includes a first pivot pin pivotably mounted to the first leaf and a second pivot pin pivotably mounted to the second leaf. A friction between the second leaf and the pivot shaft is smaller than a friction between the first leaf and the pivot shaft. The pivot shaft forms a ledge, and the second leaf correspondingly forms a tab to engage with the ledge. Upon the condition that the tab engages with the ledge such that the pivot shaft can rotate with the second leaf, a larger twisting force is needed for the second leaf, together with the pivot shaft, to rotate relative to the first leaf due to the larger friction between the first leaf and the pivot shaft.