CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation-in-part application of U.S. patent application Ser. No. 13/469,469 filed on May 11, 2012, of which the entire disclosure is incorporated herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a locking pliers, more particularly to a locking pliers that can be held firmly in a closed and locked position without the continuous application of force by a user while tightly gripping a workpiece. 
         [0004]    2. Description of the Related Art 
         [0005]    Referring to  FIG. 1 , a conventional locking pliers  10  disclosed in U.S. Pat. No. 7,861,622 B2 is shown to include a fixed handle  11  supporting a fixed jaw  12 , a movable jaw  14  pivotally connected to the fixed handle  11  by means of a first pivot pin  13 , a movable handle  16  pivotally connected to the movable jaw  14  by means of a second pivot pin  15 , and a toggle-link locking mechanism  17  disposed to lock the movable jaw in a closed, locked position. The fixed handle  11  has an oval slotted aperture  111  such that the first pivot pin  13  is movable in the aperture  111  during use of the pliers  10 . A long axis of the aperture  111  is arranged at an angle a with respect to a line extending through the center of the closed jaws  12  and  14 . The angle a is approximately  15 . The locking mechanism  17  has a link  173  pivotally connected to the movable handle  16  and in sliding and pivoting contact with an adjustment screw  171 , and a biasing member  172  extending between the movable jaw  14  and the fixed handle  11  to bias the jaws  12  and  14  away from each other. 
         [0006]    Referring to  FIGS. 2 and 3 , in operation, when the pliers is first locked onto a workpiece  20  and a turning force is applied to the pliers, the movable jaw  14  rotates clockwise around the second pivot pin  15  toward the fixed jaw  12  to allow a rearward movement of the first pivot pin  13  in the aperture  111 . The movable jaw  14  is also moved rearward and toward the fixed jaw  12 , as indicated by arrows  21  and  22 , so as to increase the gripping force on the workpiece  20 . Since the first pivot pin  13  is moved to a rear end of the aperture  111  when the jaws  12  and  14  are operated to the closed, locked position, the design of the aperture  111  is critical. However, the aperture  111  does not prevent a problem that a sliding engagement of the first pivot pin  13  with a wall defining the aperture  111  may be unstable. Therefore, the gripping force on the workpiece  20  is adversely affected. In addition, the aperture  111  has to be made precisely. If the axis of the aperture  111  suffers deviations from the angle a, the first pivot pin  13  can not move stably or even move rearward in the aperture  111 . 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide a locking pliers which can exert a firm and stable gripping force onto a workpiece while being held firmly in a closed and locked position without continuous application of force by a user. 
         [0008]    According to this invention, the locking pliers includes a fixed handle. A fixed jaw is connected securely to the fixed handle and includes a first clamping surface. A movable jaw cooperates with the fixed jaw and includes a second clamping surface and movable and fixed pivoting regions. A movable handle is pivotally connected to the movable pivoting region about a pivotal axis. A movable pivot unit includes a pivot pin engaging with the fixed handle and the fixed pivoting region and includes a pivot hole disposed in one of the fixed handle and the fixed pivoting region and configured and dimensioned to loosely receive the pivot pin. The pivot hole has a first diametrical size and the pivot pin having a second diametrical size not greater than 0.8 times the first diametrical size. A biasing member engages with the fixed handle and the movable jaw, with the fixed handle and the movable jaw being subject to a tensional force of the biasing member. 
         [0009]    Upon changing the locking pliers from initial, locking, to tightening positions thereof, the pivot pin has an annular trajectory in the pivot hole, and the movable jaw is pivoted toward closer to the fixed jaw, and the pivot pin is in abutting engagement with trailing, middle and leading regions defined from a contour of the pivot hole, respectively. 
         [0010]    Other objectives, advantages, and new features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a partially sectioned side view of a conventional locking pliers disclosed in U.S. Pat. No. 7,861,622 B2 in a closed and locked position; 
           [0012]      FIG. 2  is a partially sectioned side view of the conventional locking pliers in its closed, locked position on a workpiece; 
           [0013]      FIG. 3  is a partially sectioned side view of the conventional locking pliers in its closed, locked position on a workpiece with a turning force applied to the pliers; 
           [0014]      FIG. 4  is a perspective view of the preferred embodiment of a locking pliers according to this invention; 
           [0015]      FIG. 5  is an exploded perspective view of the preferred embodiment of a locking pliers according to this invention; 
           [0016]      FIG. 6  is a partially sectioned side view of the preferred embodiment in an opened state; 
           [0017]      FIG. 7  is a partially sectioned side view of the preferred embodiment in an initially operating state; 
           [0018]      FIG. 8  is a partially sectioned enlarged side view of  FIG. 7 . 
           [0019]      FIG. 9  is a partially sectioned enlarged side view of the preferred embodiment in a locking state; 
           [0020]      FIG. 10  is a partially sectioned enlarged side view of the preferred embodiment in a tightening state; 
           [0021]      FIG. 11  is a partially sectioned enlarged side view of the preferred embodiment in a gripping state where a workpiece is gripped thereon; 
           [0022]      FIG. 12  is a partially sectioned enlarged side view of the preferred embodiment in a locking state on the workpiece; 
           [0023]      FIG. 13  is a partially sectioned enlarged side view of the preferred embodiment in a tightening state on the workpiece; and 
           [0024]      FIG. 14  is a fragmentary sectional view of a movable pivot unit of the preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring to  FIGS. 4 through 6 , the preferred embodiment of a locking pliers according to the present invention is shown to comprise a fixed handle  30 , a fixed jaw  40 , a movable jaw  50 , a movable handle  70 , a movable pivot unit, a biasing member  95 , and a lock unit  90 . 
         [0026]    The fixed handle  30  has front and rear handle portions  31  and  32  opposite to each other in a longitudinal direction X. A pivot hole  33  and a screw hole  34  are formed in the front handle portion  31  and the rear handle portion  32 , respectively. In this embodiment, the front handle portion  31  is in the form of a grooved plate which includes an upper plate  311  and a pair of side plates  312  extending from the upper plate  311  in a transverse direction Z, which is transverse to the longitudinal direction X, to cooperatively define a recess  313 . The pivot and screw holes  33  and  34  are communicated with the recess  313 . A tab  314  extends from the upper plate  311  and into the recess  313 . 
         [0027]    The fixed jaw  40  has a first connected portion  41  which is connected securely to the front handle portion  31 , and a first working portion  42  which extends forward in the longitudinal direction X from the first connected portion  41  and terminates at a first nose end  44  having a flat surface. The first working portion  42  has a fixed clamping surface  43  that is disposed immediately rearward in the longitudinal direction X from the first nose end  44  and that faces in the transverse direction Z and that is formed with gripping teeth  46  extending from the first nose end  44  along a concaved contour  45 . 
         [0028]    The movable jaw  50  has a second connected portion  55  which extends in the transverse direction Z to terminate at fixed and movable pivoting regions  51  and  54 , and a second working portion  53  which extends forward in the longitudinal direction X from the second connected portion  55  and terminates at a second nose end  57  having a flat surface. The second working portion  53  has a movable clamping surface  56  that is disposed immediately rearward in the longitudinal direction X from the second nose end  57  and that confronts the fixed clamping surface  43  and that is formed with gripping teeth  59  extending rearward from the second nose end  57  along a concaved contour  58 . 
         [0029]    The movable handle  70  has a front pivot portion  74  which is pivotally connected to the movable pivoting region  54  by means of a pivotal pin  80  about a pivotal axis, which has a fixed position, in an axial direction Y that is transverse to the longitudinal and transverse directions X and Z, and a rear actuating portion  76  which extends rearward from the front pivot portion  74  in the longitudinal direction X and terminates at a movable handle end  77  that is manually operable. In this embodiment, the movable handle  70  is in the form of a grooved plate which includes a base seat  71  and a pair of wings  72  that extend from the base seat  71  in the transverse direction Z and that are spaced apart from each other in the axial direction Y to cooperatively define a concavity  73 . 
         [0030]    Referring to  FIGS. 6 and 14 , a movable pivot unit includes a pivot pin  60  and a pivot hole  52 . The pivot pin  60  is disposed on the front handle portion  31  through the pivot hole  33  and extends along a pivot axis parallel to the pivotal axis of the pivotal pin  80 . The pivot pin  60  has a circular contour. The pivot hole  52  is disposed in the fixed pivoting region  51  of the movable jaw  50 . Alternatively, the pivot pin  60  may be disposed on the fixed pivoting region  51  of the movable jaw  50  while the pivot hole  52  is disposed in the front handle portion  31 . The pivot hole  52  has an inner periphery defining an inner tubular surface  521 . The inner tubular surface  521  has a circular contour. A central axis I extends through a center of the pivot hole  52 . The inner tubular surface  521  of the pivot hole  52  includes the contour thereof defining a first contour section and a second contour section. The first and second contour sections delimit the inner periphery of the pivot hole  52 . The first contour section is opposite the second contour section. Leading, middle and trailing regions  524 ,  522 , and  525  extend one after another and are angularly displaced from one another about the central axis I and are defined on the first contour section of the inner tubular surface  521 . The first contour section of the inner tubular surface  521  includes the leading, middle, and trailing regions  524 ,  522 , and  525  sequentially in a counterclockwise direction. The pivot hole  52  has a diameter larger than that of the pivot pin  60 . The pivot hole  52  has a diametrical size D. The pivot pin  60  has a diametrical size 0.8 D. The pivot pin  60  has a diametrical size not greater than 0.8 D is also within the scope of the invention. In this embodiment, a subtended angle θ by the first contour section at the center of the pivot hole  52  is ranging from 180 degrees to 270 degrees. Accordingly, a subtended angle by the second contour section at the center of the pivot hole  52  may be ranging from 90 degrees to 180 degrees. The pivot pin  60  is angularly displaceable relative to and is able to be in continuous contact with the first contour section of inner tubular surface  521  upon changing locking pliers among initial, locking, and tightening positions thereof, where the pivot pin  60  is in abutting engagement with the trailing, middle and leading regions  525 ,  522 , and  524 , respectively, and wherein a center of the pivot pin  60  is displaceable in III, II, and I quadrants, respectively, wherein the pivot pin  60  is displaceable in an annular trajectory as it moves on the trailing, middle, and leading regions sequentially  525 ,  522 , and  524  or in reverse, and where the movable clamping surface  56  is close, closer and closest to the fixed clamping surface  43 , respectively, as will be described hereinafter. Because the pivot pin  60  moves in an annular trajectory, it is subject to a horizontal component force and a vertical component force. The vertical component force helps maintain the pivot pin  60  in a continuous contact with the first contour section of the inner tubular surface  521  of the pivot hole  52  when the pivot pin  60  moves on the trailing, middle and leading regions  525 ,  522 , and  524 . 
         [0031]    The biasing member  95  is connected to the tab  314  and the second connected portion  55  to bias the movable jaw  50 . Therefore, the fixed handle  30  and the movable jaw  50  are subject to a tensional force of the biasing member  95 . The biasing member  95  is released from tension when the locking pliers is in the initial position thereof. In this embodiment, the biasing member  95  is a tension spring. 
         [0032]    The lock unit  90  is a toggle locking assembly which includes a linkage interconnecting the fixed and movable handles  30  and  70 . The linkage includes a first linkage  91  and a second linkage  93 . The first linkage  91  is disposed in the concavity  73  and has a first proximate end  913  pivotally connected to the rear actuating portion  76  of the movable handle  70  by a first linking pin  92  about a first linking axis and a first distal end  914 . In this embodiment, the first linkage  91  is a grooved plate and has a lever wall  910  that extends rearward in the longitudinal direction X and terminates at a power end  911  and a pair of side walls  912  that extend in the transverse direction Z from the lever wall  910 . The second linkage  93  has a second proximate end  931  which is pivotally connected to the first distal end  914  by a second linking pin  94  about a second linking axis, a second distal end  932  that is disposed in the recess  313  and slidably and retainingly engaged with the rear handle portion  32  and that is contact with an adjustment screw  96  which is threadedly engaged with the screw hole  34 , and a protrusion  933  which extends rearward to serve as a stop. The adjustment screw  96  can be operated to adjust a separating distance between the fixed and movable jaws  40  and  50 . Accordingly, the protrusion  933  abuts against and stops on the base seat  71  when the locking pliers is in the tightening position thereof. 
         [0033]    In this embodiment, each of the pivot pin  60 , the pivotal pin  80 , and the first and second linking pins  92  and  94  is a rivet. 
         [0034]    During operation, as shown in  FIG. 6 , when the locking pliers is in the initial position thereof, the movable handle  70  is disposed away from the fixed handle  30 , the power end  911  of the first linkage  91  is turned by the second linkage  93  about the first linking axis to be spaced apart from the base seat  71  in the transverse direction Z, and the pivot pin  60  is in abutting engagement with the trailing region  525  and is spaced apart from the leading region  524  by a clearance  100 . In addition, the pivot pin  60  is disengaged from the second contour section of the inner tubular surface  521  of the pivot hole  52 . 
         [0035]    As shown in  FIGS. 6 ,  7 , and  8 , when a manual turning force is applied to move the movable handle  70  toward the fixed handle  30  to permit the power end  911  to contact with the base seat  71 , the movable jaw  50  is turned toward the fixed jaw  40 . While the movable jaw  50  is turned toward the fixed jaw  40 , the pivot pin  60  is disengaged from the second contour section of the inner tubular surface  521  of the pivot hole  52 . 
         [0036]    As shown in  FIGS. 7 and 9 , the manual turning force is continued to apply to the movable handle  70 . Consequently, the second linkage  93  is turned clockwise about the second linking axis of the second linking pin  94 . Hence, the movable jaw  50  is turned clockwise as indicated by the arrow  110  about the pivotal axis of the pivotal pin  80  against the biasing action of the biasing member  95  and the locking pliers is in the locking position thereof when the second nose end  57  abuts against the first nose end  44 , and when the pivot pin  60  is in abutting engagement with the middle region  522 , and when the clearance  100  is disposed lower than the pivot pin  60  and between the pivot pin  60  and the inner tubular surface  521 . The pivot pin  60  is also disengaged from the second contour section of the inner tubular surface  521  of the pivot hole  52  when the locking pliers is in the locking position thereof. In addition, when the movable jaw  50  is displaced from a position when the locking pliers is in initial position thereof to another positon when the locking pliers is in the locking position thereof, the lever wall  910  causes the second proximate end  931  to displace to a first over-center position in which the pivotal axis of the pivotal pin  80 , the second linking axis of the second linking pin  94 , and the second distal end  932  are in a near straight line with the first linking axis of the first linking pin  92 , thereby holding the movable jaw  50  in a locking position thereof. 
         [0037]    As shown in  FIGS. 9 and 10 , when the manual turning force is continued to apply to the movable handle end  77  toward the rear handle portion  32  to permit the protrusion  933  to contact with the base seat  71 , the movable jaw  50  is forced to turn clockwise about the pivotal axis of the pivotal pin  80  and is subject to a force component in a direction  112  to move forward and another force component in a direction  114  to bring the flat surfaces of the first and second nose ends  44 ,  57  into fully abutment with each other. The locking pliers is in the tightening position thereof when the first and second nose ends  44  and  57  abut against one another, and when the pivot pin  60  is permitted to be in abutting engagement with the leading region  524  and is spaced apart from the trailing region  525 , and when the clearance  100  is disposed in front of the pivot pin  60  and between the pivot pin  60  and the inner tubular surface  521 . The pivot pin  60  is also disengaged from the second contour section of the inner tubular surface  521  of the pivot hole  52  when the locking pliers is in the tightening position thereof. In addition, when the movable jaw  50  is displaced from the position when the locking pliers is in locking position thereof to a positon when the locking pliers is in the tightening position thereof, the second proximate end  931  is caused by the wall lever  910  to turn about the second linking axis of the second linking pin  94  so as to be shifted to a second over-center position in which the pivotal axis of the pivotal pin  80 , the second linking axis of the second linking pin  94 , and the second distal end  932  are in a substantially straight line with the first linking axis of the first linking pin  92 , thereby holding the movable jaw  50  in a tightening position thereof. 
         [0038]    Therefore, once the manual turning force on the movable handle  70  is eliminated after continuing on turning the movable jaw  50  clockwise about the pivotal axis of the pivotal pin  80 , a friction force generated as a result of the abutting engagement between the pivot pin  60  and the leading region  524  counteracts the biasing action of the biasing member  95  to prevent the movable clamping surface  58  from moving away when the locking pliers is in the tightening position thereof. As a result, the gripping force between the fixed and movable clamping surfaces  43  and  56  is firm and stable during the operation of the locking pliers to the tightening position thereof. 
         [0039]    When it is desired to release the movable jaw  50  from the tightening position thereof, an opposite manual turning force is applied to the movable handle  70  away from the rear handle portion  32  so as to permit the movable jaw  50  back to the initial position by the biasing action of the biasing member  95 . 
         [0040]    In use, referring to  FIGS. 11 to 13 , a workpiece  120  is shown to be gripped by the clamping surfaces  43  and  56 . The workpiece  120  is gripped firmly with the concaved gripping teeth  46  and  59 . Firstly, the movable jaw  50  is opened relative to the fixed jaw  40  so the workpiece  120  can be placed therebetween. Subsequently, the movable handle  70  is turned toward the rear handle portion  32  to turn the movable jaw  50  toward the fixed jaw  40 , thereby permitting the clamping surfaces  43  and  56  to lock on the workpiece  120 . Finally, the manual turning force is continued to apply to the movable handle  70 , and when the locking pliers is in the tightening position thereof, the movable jaw  50  is tightly gripped by the gripping teeth  46  and  59  of the respective clamping surfaces  43  and  56 . 
         [0041]    As illustrated, since the pivot pin  60  is kept in abutting engagement with the inner tubular surface  521  during the operation of the locking pliers, and since the movable jaw  50  is moved forward and toward the fixed jaw  40  when the manual turning force is further applied to the movable handle  70  to displace the movable jaw  50  from the position when the locking pliers is in the locking position thereof to the position when the locking pliers is in the tightening position thereof, the gripping force is increased and stable. 
         [0042]    While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretations and equivalent arrangements.
A locking pliers locking pliers includes a fixed handle. A fixed jaw is connected to the fixed handle. A movable jaw cooperates with the fixed jaw and includes movable and fixed pivoting regions. A movable handle is pivotally connected to the movable pivoting region. A movable pivot unit includes a pivot pin engaging with the fixed handle and the fixed pivoting region, and a pivot hole disposed in one of the fixed handle and the fixed pivoting region and configured and dimensioned to loosely receive the pivot pin. The pivot hole has a first diametrical size and the pivot pin having a second diametrical size not greater than 0.8 times the first diametrical size. A biasing member engages with the fixed handle and the movable jaw.