Abstract:
The present invention is directed to an improved hand tool for using one hand to grasp a workpiece between a first and a second jaw of the hand tool and adjust the force applied to the workpiece. The hand tool includes a first arm and a second arm, the second arm is operably linked to the first arm so as to cause relative motion of the first and second jaws upon motion of the arms. The hand tool further includes a control arm having a first end pivotably linked to the first arm and a second end pivotably linked to the second arm at a moveable pivot location. The control arm is configured to control relative motion of the first and second arms into and out of an overcenter lock position. Movement of a force adjustor located on the second arm in a first direction moves the moveable pivot location so as to increase the level of force required on the second arm to move the control arm into the overcenter lock position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/761,522, filed Jan. 24, 2006; and is a continuation-in-part of application Ser. No. 10/463,843, filed Jun. 18, 2003, now U.S. Pat. No. 7,100,479; which is a continuation of application Ser. No. 09/942,095, filed Aug. 28, 2001, now U.S. Pat. No. 6,748,829; which is a continuation of application Ser. No. 09/594,191, filed Jun. 14, 2000, now U.S. Pat. No. 6,279,431; which in turn is a continuation-in-part of application Ser. No. 09/334,055, filed Jun. 15, 1999, now U.S. Pat. No. 6,212,978. This application claims priority to the aforementioned applications, the disclosures of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to pliers, and, more particularly, to a self-adjusting pliers that grips workpieces of various sizes without manual adjustment.  
       BACKGROUND OF THE INVENTION  
       [0003]     The traditional version of a pliers includes two elongated members joined at a pivot pin. One end of each elongated member forms a jaw, and the other end forms a handle. Workpieces of different sizes are grasped in different manners, due to the constant geometry of the elongated members and the jaws. Some adjustability may be achieved by providing a slotted receiver in one of the handles, so that the handle with the pivot pin may be moved between different positions in the slot to provide adjustability for gripping objects of different sizes.  
         [0004]     U.S. Pat. No. 4,651,598 provides an improved pliers whose jaws are self adjusting according to the size of the workpiece. Commercial versions of this pliers are useful, but have significant drawbacks. Perhaps the most significant problem with the pliers made according to the &#39;598 patent is that the jaws move slightly relative to each other in an end-to-end manner as they are clamped down onto a workpiece. The surfaces of soft workpieces such as brass or copper may be marred as a result. The clamping force applied by these pliers depends upon the size of the workpiece being grasped.  
         [0005]     Another problem with the pliers of the &#39;598 patent is that they do not lock to the workpiece, an important convenience in some uses of pliers. Overcenter locking pliers are described in a series of patents such as U.S. Pat. No. 4,541,312. Conventional overcenter locking pliers provide adjustability in the size of the workpiece that may be gripped through a screw adjustment to the pivoting position of the control arm, but this adjustability is not automatic in the sense of the pliers of the &#39;598 patent.  
         [0006]     Other types of locking pliers such as the AutoLock™ pliers combine the self-adjusting feature with an overcenter locking mechanism. This pliers can be inconvenient to use for some sizes of workpieces, suffers from some of the problems of the pliers of the &#39;598 patent, does not achieve a large gripping force, and may unexpectedly unlock when large objects are being gripped. Additionally, as with some other pliers, two hands are required for its operation.  
         [0007]     There is a need for a self-adjusting pliers which does not experience shifting of the jaw position as the object is grasped, which may be operated with one hand, and which may be provided in a locking version. There also exists a need for a self-adjusting pliers where one hand may operate and adjust the force that is applied to a workpiece grasped between a first and second jaw. The present invention fulfills these needs.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a self-adjusting pliers wherein the jaws automatically adjust to various sizes of workpieces. There is no end-to-end relative movement of the jaws as they grasp the workpiece, so that there can be no surface marring of the type observed with the pliers of the &#39;598 patent. The clamping force is substantially constant regardless of the size of the workpiece, but is adjustable in some versions of the pliers. The clamping force against the workpiece is multiplied several times by the mechanism, leading to a much higher maximum available clamping force than possible with conventional pliers. The pliers may be provided with no locking or with releasable overcenter locking, or with the ability to switch between the two. The self-adjusting pliers is preferably operable with one hand.  
         [0009]     In accordance with the invention, a self-adjusting pliers is operable to grasp a workpiece between first and second jaw. The pliers includes an upper arm having a first end and a second end. The first jaw may be further defined as an upper jaw the upper jaw being at the first end of the upper arm. A jaw arm has a first end and a second end. The second end of the jaw arm is pivotably connected to the upper arm at a main pivot adjacent to the second end of the upper arm, so that the first end of the jaw arm is movable in a circular arc relative to the main pivot. The second jaw may be further defined as a lower jaw, the lower jaw being located at the first end of the jaw arm in movable facing relation to the upper jaw as the jaw arm pivots about the main pivot, so that the workpiece may be grasped between the upper jaw and the lower jaw. An engagement mechanism releasably engages the jaw arm to the upper arm at an engagement position responsive to a movement of the jaw arm relative to the upper arm and responsive to a size of the workpiece grasped between the upper jaw and the lower jaw. Further gross rotation of the jaw arm relative to the upper arm is thereby prevented until the engagement to the workpiece is released. The upper jaw and the lower jaw are each preferably of a multilayer metallic construction.  
         [0010]     Preferably, there is a support integral with, and extending from the upper arm toward and past the jaw arm. The support includes a support engagement curved in a circular arc centered about the main pivot. The support engagement desirably includes an engagement slot or channel in the support, and a restraining plate to restrain, guide, position, and align some of the components of the engagement mechanism. There is additionally a lower arm that is linked to the jaw arm at a location adjacent to the lower jaw, but that is not integral with the jaw arm. A control arm has a first end and a second end. The first end of the control arm is pivotably connected to the jaw arm at an upper control-arm pivot pin adjacent to the second end of the jaw arm. The second end of the control arm is pivotably connected to the lower, arm at a lower control-arm pivot pin at a location along the length of the lower arm. A lower-arm spring biases the lower arm so as to resist rotation of the lower arm about the upper control-arm pivot pin.  
         [0011]     The engagement mechanism desirably includes a shifter and a pawl that is pivotably supported on the shifter. The shifter is operable to engage the pawl to the upper arm, and specifically to the downwardly extending support, at the engagement position responsive to the movement of the jaw arm relative to the upper arm and responsive to the size of the workpiece grasped between the upper jaw and the lower jaw. The shifter transmits a locking and engaging force applied through the lower arm to the lower jaw and also engages the pawl to the support engagement slot responsive to the movement of the jaw arm relative to the upper arm and responsive to the size of the workpiece grasped between the upper jaw and the lower jaw. The shifter is pivotable relative to the jaw arm and is rotatable relative to the lower arm, and the pawl is pivotably supported on the shifter.  
         [0012]     The engagement mechanism releasably engages the jaw arm to the upper arm. There may also be a locking mechanism that releasably locks the jaw arm to the upper arm, and specifically to the downwardly extending support, at the engagement position. Some versions of the pliers are controllably alterable between the releasable-engagement type and the releasable engagement-and-lock type by the operation of a locking engagement control. In one design, a locking-engagement control of the locking mechanism interferes with a rotation of the control arm about the upper control-arm pivot pin in the releasable-engagement embodiment, and the locking engagement control does not interfere with a rotation of the control arm about-the upper control-arm pivot pin in the releasable engagement-and-lock embodiment.  
         [0013]     In one form, the pliers includes a releasable overcenter lock for the jaws. In this version, there is a downwardly extending lobe on the control arm. A release arm is pivotably connected to the lower arm and has a release pad disposed to contact the lobe of the control arm when the release arm is pivoted. In operation, the control arm moves to an overcenter position when the clamping force is fully applied. This overcenter position may be released to unlock the jaws from the workpiece either by pulling the handles apart, or by manually pivoting the release arm. The overcenter locking is readily released by pulling the upper arm and the lower arm apart when the clamping force is small, but is more conveniently released by operating the release arm when the clamping force is large.  
         [0014]     In another version, the pliers is controllably switchable between a nonlocking function and a locking function. An overcenter lock switch mechanism in the lower handle is movable between a first position whereat the overcenter lock switch mechanism does not prevent pivoting movement of the lower arm relative to the control arm prior to reaching an overcenter lock, and a second position whereat the overcenter lock switch mechanism does prevent pivoting movement of the lower arm relative to the control arm prior to reaching an overcenter lock. The movement of the locking switch mechanism to the second position prevents the pivoting movement of the lower arm and the control arm to an overcenter locking position, and thereby prevents this overcenter locking function. Thus, there may be nonlocking-only, locking-only, or switchable embodiments of the pliers that may be switched between the nonlocking and locking forms.  
         [0015]     The maximum magnitude of the clamping force applied to the workpiece may be much larger than possible with conventional pliers, due to a force multiplication effect present in the mechanism. The length of the arms, the angle between the control arm and the lower arm, the relative location of the shifter pivot points, and the movement of the shifter relative to the jaw mechanism all contribute to a leveraged multiplication of the force applied though the handles. The multiplication factors are established by the structural geometry built into the pliers.  
         [0016]     The pliers may be provided with control over the clamping force applied to the workpiece through the jaws. A manual force adjuster acting on the control arm is provided at a location adjacent to the second end of the upper arm. The manual force adjuster is operable to move the upper control-arm pivot pin along the jaw arm. This movement of the pivot point of the first end of the control arm changes its angle and position relative to the lower arm and to the jaw arm, with the result that the maximum clamping force applied through the jaws is controllably variable. It is preferred to combine the features of both the manual force adjuster and the releasable overcenter lock in a single pliers, when either feature is provided. In other embodiments, the manual force adjuster may be associated with the lower arm rather than the upper arm. In such case, the manual force adjuster may be operable to move a lower control arm pivot-pin along the lower jaw arm. Accordingly, the movement of the pivot point changes the control arm angle and position relative to the upper arm and the jaw arm, with the same resulting clamp force variability.  
         [0017]     With respect to certain embodiments, in operation, with the jaws separated and not contacting the workpiece, the jaw arm, the lower arm, the control arm, and the engagement mechanism initially rotate relative to the upper arm as an interconnected unit about the main pivot. An anti-squat mechanism aids in maintaining the fixed geometrical relationship of these elements during the initial rotation. A main spring reacts between this interconnected unit and the upper arm, and specifically between the jaw arm and the upper arm. The main spring weakly biases the interconnected unit away from the upper arm to initially keep the jaws separated. The hand force applied by the user through the upper arm and the lower arm overcomes this biasing to move the jaws toward contact with the workpiece. When the jaws contact the workpiece, the shifter begins to rotate to apply the hand force of the user to the workpiece as the clamping force. As the contact pressure increases further, the force multiplication effect comes into play to produce a clamping force that is greater than the user would otherwise produce. The workpiece is thereby clamped between the jaws with a maximum clamping force that is controllable through the force adjuster. Release of the hand force by the user reverses the process. If the pliers is the locking embodiment or the switchable embodiment operated in the locking mode, the lock automatically engages to hold the workpiece securely even though the user relaxes the force applied through the upper arm and the lower arm. The locking may be unlocked by operating the release arm.  
         [0018]     The mechanism of the invention is operable to move the lower jaw upwardly along the downwardly extending guide until the lower jaw contacts the workpiece, and to then engage the jaw arm to the upper arm and to transfer a clamping force to the lower jaw. The clamping mechanism is thus self-adjusting to accommodate any size workpiece that will fit between the jaws. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.  
         [0019]     In a first aspect of the present invention a hand tool is operable to grasp a workpiece between a first jaw and a second jaw. The hand tool comprises a first arm, a second arm operably linked to the first arm so as to cause relative motion of the first and second jaws upon motion of the arms, a control arm having a first end pivotably linked to the first arm and a second end pivotably linked to the second arm at a moveable pivot location, the second end of the control arm being closer to the jaws than the first end, the control arm configured to control relative motion of the first and second arms into and out of an overcenter lock position, and a force adjustor associated with the second arm so that movement of the force adjustor in a first direction moves the moveable pivot location so as to increase the level of force required on the second arm to move the control arm into the overcenter lock position.  
         [0020]     In accordance with the first aspect of the present invention, the force adjustor may move in a second direction allowing the moveable pivot location to move so as to decrease the level of force required on the second arm to move the control arm into the overcenter lock position. The force adjustor may include a knob and a threaded shaft extending outwardly from a bottom surface of the knob. The knob of the force adjustor may be knurled.  
         [0021]     In accordance with the first aspect of the present invention, the second arm may have a pivot block associated therewith, the pivot block having a fixed relation with the second arm. The pivot block may be internally threaded with a thread pattern matching that of the threaded shaft of the force adjustment mechanism. The second arm may further include a pin slot for securing a pivot pin connected to the second end of the control arm.  
         [0022]     In accordance with the first aspect of the present invention, movement of the force adjustor in the first direction forces the end of the threaded shaft against the second end of the control arm causing the pivot pin to slide along the pin slot of the second arm in substantially the same first direction. The pivot pin may be movably positioned near the geometric midpoint of the second arm.  
         [0023]     In accordance with the first aspect of the present invention, movement of the force adjustor in the first and second direction are substantially opposite directions in the same plane. The force adjustor may be located adjacent a lower arm pad associated with the second arm.  
         [0024]     In accordance with the first aspect of the present invention, the hand tool may include a jaw arm having a first end and a second end, the first end integral with the second jaw and the second end pivotably coupled to the upper arm. The second arm may include a spring, the spring connected between a projection on the second arm and an intermediate location on the jaw arm. When the control arm is not in the overcenter lock position, movement of the force adjustor in the first direction may cause the spring to expand thereby increasing the distance between the jaw arm and the second arm.  
         [0025]     In a second aspect of the present invention the hand tool is operable to grasp a workpiece between a first jaw and a second jaw. The hand tool comprises a first arm, a second arm operably linked to the first arm so as to cause relative motion of the first and second jaws upon motion of the arms, a control arm having a first end pivotably linked to the first arm and a second end pivotably linked to the second arm at a moveable pivot location, the second end of the control arm being closer to the jaws than the first end, and a force adjustor associated with the second arm so that movement of the force adjustor in a first or second direction moves the moveable pivot location in the first and second direction.  
         [0026]     A third aspect of the present invention is a method of grasping a workpiece between a first jaw and a second jaw of a hand tool and adjusting the force applied to the workpiece. The method comprises gripping the hand tool with one hand, the hand tool having a first arm, a second arm operably linked to the first arm so as to cause relative motion of the first and second jaws upon motion of the arms, a control arm having a first end pivotably linked to the first arm and a second end pivotably linked to the second arm at a moveable pivot location, the second end of the control arm being closer to the jaws than the first end, the control arm configured to control relative motion of the first and second arms into and out of an overcenter lock position, and a force adjustor associated with the second arm, grasping a workpiece between the first jaw and second jaw by reducing the distance of the second arm in relation to the first arm, and moving the force adjustor in a first direction thereby moving the moveable pivot location so as to increase the level of force required on the second arm to move the control arm into the overcenter lock position. Alternatively, a user may first grasp a workpiece between a first and second jaw of the hand tool causing the control arm to reach an overcenter lock position, release the hand tool from the overcenter lock position, move the force adjustor in a first or second direction, and re-grasp the workpiece with an adjusted force.  
         [0027]     In accordance with the third aspect the method may include grasping the workpiece so as to cause the control arm to move into the overcenter lock position.  
         [0028]     In accordance with the third aspect the method may include moving the force adjustor in a second direction thus allowing the moveable pivot location to move so as to decrease the level of force required on the second arm to move the control arm into the overcenter lock position.  
         [0029]     In accordance with the third aspect the method may include grasping the workpiece so as to cause the control arm to move into the overcenter lock position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is an elevational view of a pliers in accordance with certain embodiments of the present invention;  
         [0031]      FIG. 2  is a schematic perspective view of the pliers of  FIG. 1 , with portions of the external structure removed;  
         [0032]      FIG. 3  is a schematic perspective view of the pliers of  FIG. 1 , with additional portions of the external structure removed;  
         [0033]      FIG. 4  is a schematic perspective view of the pliers of  FIG. 1 , with further portions of the external structure removed;  
         [0034]      FIG. 5  is a detail perspective view near the second end of the upper arm of the pliers of  FIG. 1 ;  
         [0035]      FIG. 6  is a detail perspective view in the region of the shifter of the pliers of  FIG. 1 ;  
         [0036]      FIG. 7  is an elevational view of a pliers in accordance with further embodiments of the present invention;  
         [0037]      FIG. 8  is a detail perspective view of a portion of the lower arm near the lower jaw of the pliers of  FIG. 7 ;  
         [0038]      FIG. 9  is a schematic perspective view of the pliers of  FIG. 7  with portions of the external structure removed; and  
         [0039]      FIG. 10  is a schematic perspective view of the pliers of  FIG. 7  with portions of the external structure removed, and the pliers in the closed position. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]      FIGS. 1-6  illustrate a self-adjusting pliers  20  according to certain embodiments of the present invention.  FIG. 1  is an elevational view, and  FIGS. 2-4  show the same pliers  20  with portions of the structure progressively removed to illustrate the internal structure and mechanics.  FIGS. 5-6  are details.  FIGS. 7 and 8 , which relate to further embodiments of the present invention, will be discussed in detail following the discussion of the embodiments of  FIGS. 1-6 .  
         [0041]     “Up” and “down” reference directions are indicated on several of the figures and apply to all of the embodiments. In the figures, rivets that are present to hold the structure together are not shown because their heads tend to obscure the views of the relevant structure. The appropriate rivet holes are visible.  
         [0042]     As illustrated in  FIG. 1 , the self-adjusting pliers  20  is a hand tool that is operable to grasp a workpiece  22  between an upper jaw  24  and a lower jaw  26 . An upper arm  28  has a first end  30  and a second end  32 . The upper jaw  24  is at the first end  30  of the upper arm  28 , and is integral with the remainder of the upper arm  28  in the depicted embodiment.  
         [0043]     As best seen in  FIG. 3 , a jaw arm  34  has a first end  36  and a second end  38 . The second end  38  of the jaw arm  34  is pivotable relative to the upper arm  28  on a main pivot  40  adjacent to the second end  32  of the upper arm  28 . The main pivot  40  is a segment of a circle defined on a pivot block  82  that is fixedly supported between the sides of the upper arm  28 . The first end  36  of the jaw arm  34  is therefore movable in a circular arc relative to the center defined by the main pivot  40 . The upper arm  28  is a generally U-shaped channel over most of its length with the opening of the U facing downwardly, so that the jaw arm  34  may be received between the sides of the upper arm  28  as the jaw arm  34  pivots. The lower jaw  26  is at the first end  36  of the jaw arm  34  in movable facing relation to the upper jaw  24 . As the jaw arm  34  pivots about the main pivot  40 , reducing the distance between the jaws  24  and  26 , the workpiece  22  is grasped between the upper jaw  24  and the lower jaw  26 . As seen in  FIGS. 2-4 , in the preferred embodiment the upper jaw  24  and the lower jaw  26  are each preferably of a multilayer metallic construction. That is, each of the jaws  24  and  26  is made by stacking appropriately shaped thin metallic plates, and attaching them together with rivets extending through transverse rivet holes  42  in the jaws  24  and  26 . Similarly, in this embodiment the arms are made of overlying plates. In other embodiments, the jaws may be made of solid, non-laminated metal, and some of the arms may be made as a single piece of metal formed into a U-shaped channel, as appropriate.  
         [0044]     A support  44  is integral with and extends downwardly from the upper arm  28  toward and past the jaw arm  34 . The support  44  includes a support engagement  46  therein, curved in a circular arc centered about the center of the main pivot  40 . The support engagement  46  is preferably a support engagement slot  48 . The support engagement slot  48  desirably includes small support engagement teeth  50  along a side  51  of the slot  48  nearest the jaws  24  and  26 .  
         [0045]     A lower arm  52  is linked to the jaw arm  34  at a location adjacent to the lower jaw  26 . The lower arm  52  is not integral with the jaw arm  34 . The lower arm  52  extends generally parallel to the upper arm  28 . The upper arm  28  and the lower arm  52  are grasped by the hand of the user of the pliers  20 , and an upper arm pad  54  and a lower arm pad  56  are provided in their outwardly facing surfaces to facilitate this grasping and aid in the user positioning the grasping hand correctly. The upper arm  28  and the lower arm  52  thereby serve as the handles grasped by the user of the pliers  20 .  
         [0046]     A control arm  58  has a first end  60  and a second end  62 . The first end  60  of the control arm  58  is pivotably connected to the jaw arm  34  at an upper control-arm pivot pin  64  adjacent to the second end  38  of the jaw arm  34 . The upper control-arm pivot pin  64  extends between the sides of the jaw arm  34 . The second end  62  of the control arm  58  is pivotably connected to the lower arm  52  at a lower control-arm pivot point  66  that is positioned at a location, in this case an intermediate location, along the length of the lower arm  52 .  
         [0047]     A lower-arm spring  68  biases the lower arm  52  so as to resist rotation of the lower arm  52  about the upper control arm pivot point  64 . In the illustrated embodiment, the lower-arm spring  68  is a coil spring connected between a projection  70  on the lower arm  52  and an intermediate location  72  on the jaw arm  34 .  
         [0048]     In operation, the jaw arm  34 , the lower arm  52 , the control arm  58 , and an engagement mechanism initially rotate relative to the upper arm  28  as an interconnected unit  73  about the main pivot  40 . A main spring  74 , illustrated as a main leaf spring, reacts between this interconnected unit  73  and the upper arm  28 , and specifically between the jaw arm  34  and the upper arm  28 . The main leaf spring  74  biases the interconnected unit  73  away from the upper arm  28 , so that the jaws  24  and  26  are normally spread apart to receive the workpiece  22  therebetween. The squeezing hand force of the user grasping the upper arm  28  through the upper arm pad  54 , and the lower arm  52  through the lower arm pad  56 , overcomes this biasing force of the main leaf spring  74  to achieve the initial contact and initial grasping of the workpiece  22  between the jaws  24  and  26 .  
         [0049]     In the preferred form of the pliers  20 , the upper control-arm pivot pin  64  is selectively movable generally (but not precisely) parallel to a line extending between the first end  30  and the second end  32  of the upper arm  28 . This movement serves to adjust the maximum clamping force exerted by the jaws  24  and  26  on the workpiece  22 , when the workpiece  22  is clamped between the jaws  24  and  26 , by changing the geometry of the linkage between the jaw arm  34 , the lower arm  52 , and the control arm  58 . The movement and adjustability are achieved by slidably supporting the upper control arm pivot pin  64  in a pin slot  80  in the jaw arm  34 .  
         [0050]     As best seen in  FIG. 5 , a force adjuster  84  extends from the second end  32  of the upper arm  28 . The force adjuster  84  is a knob, preferably a knurled knob, accessible to the fingers of the user of the pliers and having an integral threaded shaft  85  that extends through and is threadably engaged to the pivot block  82 . An end of the threaded shaft  85  remote from the force adjuster  84  has a dome shape that is forced against the upper control-arm pivot pin  64 . When the force adjuster  84  is turned, the shaft  85  drives the upper control-arm pivot pin  64  along the pin slot  80 , in a direction generally (but not exactly) parallel to the line extending between the first end  30  and the second end  32  of the upper arm  28 .  
         [0051]     An engagement mechanism  86  releasably engages the jaw arm  34  to the upper arm  28 , and specifically to the support engagement  46  of the support  44 . The releasable engagement is made at an engagement position responsive to a movement of the jaw arm  34  relative to the upper arm  28  and responsive to a size of the workpiece  22  grasped between the upper jaw  24  and the lower jaw  26 . (As will be discussed, the preferred engagement mechanism  86  includes a shifter and a pawl, and their related structure.) This engagement prevents further gross rotation of the jaw arm  34  and the remainder of the interconnected unit  73  relative to the upper arm  28  when the workpiece  22  is so grasped with the clamping force determined by the position of the upper control-arm pivot pin  64  in the pin slot  80 , although there is a further minor rotation of the jaw arm  34 . That is, when the jaws  24  and  26  are separated further than the size of the workpiece  22 , the force of the hand of the user on the pads  54  and  56  causes the jaws  24  and  26  to close to contact the workpiece  22  by the rotation of the interconnected unit  73  relative to the upper arm  28  about the main pivot  40 . When the jaws  24  and  26  contact the workpiece  22  and as there is an initial application of a small clamping force to the workpiece  22 , the engagement mechanism  86  automatically operates to engage the jaw arm  34  and the interconnected unit  73  to the support  44  and thence to the upper arm  28 , so that there is no further gross rotation of the interconnected unit  73 . The pliers  20  is thereby automatically adjustable to the size of the workpiece  22  being grasped.  
         [0052]     The engagement mechanism  86  includes a pivotably supported pawl  88 . The pawl  88  rides on the jaw arm  34  in the support engagement slot  48  in facing relation to the support engagement teeth  50 . The pawl  88  has pawl teeth  90  thereon. Prior to engagement, the pawl  88  is separated from a side  51  of the support engagement slot  48  that is nearest the jaws  24  and  26 . During engagement, the pawl  88  is moved into contact with the side  51  so that the pawl teeth  90  mesh with the support engagement teeth  50  to prevent further upward gross motion of the jaw arm  34 . A restraining plate  140  overlies a portion of the pawl  88 , holds the pawl on its pawl pivot pin  93 , and serves to align and guide the movement of the pawl  88 .  
         [0053]     The engagement mechanism  86  also includes the shifter  92 . The shifter  92 , shown in detail in  FIG. 6 , transfers the force applied to the lower arm  52  by the hand of the user, from the lower arm  52  to the lower jaw  26 . Additionally, the shifter  92  pivotably supports the pawl  88  on the pawl pivot pin  93  that extends through the shifter  92  and the pawl  88 , activates the pawl  88 , and engages the pawl  88  to the support  44  of the upper arm  28  when the workpiece  22  is contacted by the jaws  24  and  26 . This engagement is responsive to the movement of the jaw arm  34  relative to the upper arm  28  and responsive to the size of the workpiece  22  grasped between the upper jaw  24  and the lower jaw  26 .  
         [0054]     The shifter  92  is in the form of a thin plate that transfers force. The shifter  92  has three pivot points, including the pawl pivot pin  93 , a pinned pivot point  94 , and a contact face  98  thereon arranged in a triangular pattern. The pawl pivot pin  93  becomes a pivot point after the pawl  88  is engaged to the support  44 , but not prior to that engagement. The pivot point  94  is pivotably connected by a pin to the lower arm  52  at a shifter pin pivot  100 . The contact face  98  pivots and slides against, but is not pinned to, the jaw arm  34  at a contact face  104 . The pawl  88  is pivotably connected to the central portion of the shifter  92  at the pawl pivot pin  93 . (The pawl  88  is not shown in  FIGS. 4 and 6 , because it would obscure the view of the shifter  92 , but it is shown in  FIG. 3 .) The shifter  92  thereby provides the force transfer between the lower arm  52 , the pawl  88 , and the lower jaw  26 . That is, the lower jaw  26  is not integral with the lower arm  52 , but instead is linked to it by a linkage provided by the shifter  92 , in this embodiment.  
         [0055]     In operation, starting with the jaws  24  and  26  at their greatest separation, the user grasps the upper arm  28  and the lower arm  52  and moves them toward each other. The interconnected unit  73  rotates relative to the upper arm  28  as a rigid interconnected structure around the main pivot  40 . The geometric relationships of the elements of the interconnected unit  73 , including the jaw arm  34 , the lower arm  52 , the control arm  58 , and the engagement mechanism  86 , is kept rigid by means of an anti-squat mechanism  120  during this initial rotation. The anti-squat mechanism  120  includes the contact face  96  of the shifter  92 , and the contact face  102  of the lower jaw  26 . An anti-squat spring  122 , illustrated as an anti-squat leaf spring, reacting against an upper surface  126  of the shifter  92 , holds the contact faces  96  and  102  in contact during this period of rotation of the interconnected unit  73 . By keeping the contact faces of  102  and  96  in contact until the lower jaw  26  and the upper jaw face  24  contact the work piece  22 , the antisquat mechanism  120  keeps the interconnected unit  73  geometrically rigid until the jaws  24  and  26  touch and begin to apply force to the work piece  22 , and additionally prevents the rotation of the shifter  92 .  
         [0056]     After the jaws  24  and  26  have contacted the workpiece  22  and begun to apply a contact force into the workpiece  22 , the contact face  96  lifts up and away from the contact face  102  that is part of the lower jaw  26 , against the biasing force of the antisquat leaf spring  122 . The shifter  92  rotates clockwise (in the view of the drawings) about the pivot established between the contact surface  98  and the contact face  104 . The pawl  88  rotates clockwise about the pawl pivot pin  93  and moves toward the lower jaw  26  to engage the pawl teeth  90  to the support engagement teeth  50 . This engagement of the pawl teeth  90  to the support engagement teeth  50  halts further gross rotation and motion of the interconnected unit  73 .  
         [0057]     For most applications, it is desirable that the contacting force of the jaws  24  and  26  to the workpiece  22  be large in order to ensure that the workpiece is firmly held. To accomplish that result, the shifter achieves a force-multiplier effect wherein the contact force applied to the workpiece  22  is significantly greater than the force produced by the grasping action of the hand of the user. With the illustrated design, the force multiplier is on the order of about 3-4 when friction and other effects are considered, although higher force multipliers are possible in other designs. The force multiplication arises as follows. Once the pawl teeth  90  are engaged to the support engagement teeth  50 , the rotational pivot point of the shifter  92  is transferred from the contact face  98  of the shifter  92  to the pawl pivot pin  93 . The contact face  98  rides on the inclined contact face  104 . The shifter  92  continues to rotate about the pivot pin  93  as the lower arm  52  is moved toward the upper arm  28 , producing a further minor rotation of the jaw arm  34 . The hand force of the user moving over a longer distance is transferred into the lower jaw  26 , which moves a shorter distance but with greater contact force applied to the workpiece  22 , than the hand force of the user. The force multiplication is achieved because the contact faces  98  and  104  act as an inclined plane as the shifter  92  rotates. The difference in the length of the lever arm between the locations  93 - 98  and  93 - 94  also contributes to the force multiplication.  
         [0058]     The release of the force on the lower arm  52  reverses this process, causes the shifter  92  to rotate counterclockwise, disengages the pawl teeth  90  from the engagement teeth  50 , allows the lower jaw  26  to move downwardly, and disengages the jaws  24  and  26  from the workpiece  22 .  
         [0059]     In the use of the pliers  20  just discussed, the jaws  24  and  26  engage and hold the workpiece  22  such that release of the pressure applied to the upper arm  28  and the lower arm  52  immediately releases the workpiece  22 . In another embodiment, the jaws  24 ,  26  may be engaged to the workpiece  22  and releasably locked to the workpiece  22  by a locking mechanism  150 , which in this case is an overcenter locking mechanism.  
         [0060]     The overcenter locking mechanism  150  with its associated release are conveniently provided by placement of an unlocking lobe  106  on the lower side of the control arm  58 . A release arm  108  is pivotably connected to the lower arm  52  and accessible to the hand of the user of the pliers  20  at the end of the lower arm  52  remote from the shifter  92 . A release pad  110  on the upper side of the release arm  108  is disposed to contact the unlocking lobe  106  when the release arm  108  is rotated. In operation, the lower control arm pivot point  66  moves to an overcenter position relative to the upper control-arm pivot pin  64  and the pivot pin  94 , when the lower arm  52  is moved upwardly to the limit of its travel established by the operation of the engagement mechanism  86 . Stated alternatively, when the lower arm  52  is fully open (moved to its downward limit of travel) as in  FIG. 1 , the lower control arm pivot point  66  lies below a straight line drawn between the upper contact-arm pivot pin  64  and the pivot point  94 . As the lower arm  52  is moved upwardly, the lower control arm pivot point  66  moves closer to a straight-line relationship between the pivot pins  64  and  94 , and eventually crosses over that straight line to lie above the straight line drawn between the pivot pins  64  and  94 . This is the overcenter lock position. To release the pliers  20  from this overcenter lock position, the release arm  108  is operated to rotate the release pad  110  upwardly against the unlocking lobe  106 , and thereby force the lower arm  52  downwardly and out of the overcenter relationship.  
         [0061]     This type of overcenter locking capability may be provided instead of or in addition to the engaging-but-nonlocking embodiment described previously. The embodiment of  FIGS. 1-4  allows the pliers  20  to be selectively shifted between the non-locking version and the locking/release version. A locking engagement control  112  includes an overcenter lock selector  114 . The overcenter lock selector  114  selectively moves the release arm  108  in a track  124  to a position wherein an overcenter blocking pad  111  on the release arm  108  contacts the unlocking lobe  106  to block the movement of the control arm  58  that is required to reach the overcenter locked position. In this position, the pliers  20  functions to grasp the workpiece  22  between the jaws  24  and  26 , but does not lock the jaws  24  and  26  against the workpiece  22 . When the force is released from the arms  28  and  52 , the workpiece  22  is released. On the other hand, when the overcenter lock selector  114  is repositioned to move the release arm  108  in the track  124  so that the overcenter blocking pad  111  does not block the movement of the control arm  58  that is required to reach the overcenter locked position, the force on the arms  28  and  52  causes the jaws  24  and  26  first to grasp and, then with continued force, to lock onto the workpiece  22 . Release of the force on the arms  28  and  52  does not itself cause the jaws  24  and  26  to release from the workpiece  22 . Instead, the release arm  108  is pivoted to contact the unlocking lobe  106  and push the lower arm  52  away from the control arm  58 . The contacting force applied by the jaws  24  and  26  to the workpiece  22  is released, and the disengagement of the jaws  24  and  26  from the workpiece proceeds. The ability to readily switch between nonlocking and locking pliers is an important advantage of one embodiment of the present approach.  
         [0062]     For either the engaging-only or the engaging-and-locking embodiments, it is often helpful to know whether the maximum permissible clamping force, as determined by the position of the upper contact-arm pivot pin  64 , has been applied through the jaws  24  and  26  to the workpiece  22 . In the presently preferred approach, a force indicator window  130  is provided through each of the sides of the lower arm  52 . When the control arm  58  has been sufficiently rotated to correspond to the maximum permissible clamping force, a force indicator  132  is visible through the force indicator window  130 . The force indicator  132  is preferably a region of contrasting color on a projection on the side of the control arm  58 , for example, a yellow force indicator  132  on a black metallic control arm  58 . If the control arm  58  is only partially rotated toward the position associated with less than the maximum contact force on the workpiece  22 , the force indicator  132  is not visible thorough the force indicator window  130 . If the control arm  58  is fully rotated to the position associated with the maximum contact force on the workpiece  22 , the force indicator  132  is visible through the force indicator window  130 , giving an indication of this force status to the user of the pliers  20 .  
         [0063]     In accordance with further embodiments, pliers  20 ′ may be configured to include a force adjuster  84 ′ in a relocated position, such that the force adjuster may be manipulated by the thumb and fore-finger of the hand in which the user is operating the pliers, such that use of the pliers is truly a one-handed operation.  
         [0064]     As illustrated in  FIGS. 7 through 10 , the self-adjusting pliers  20 ′ is a hand tool that is operable to grasp a workpiece  22 ′ between an upper jaw  24 ′ and a lower jaw  26 ′. An upper arm  28 ′ has a first end  30 ′ and a second end  32 ′. The upper jaw  24 ′ is at the first end  30 ′ of the upper arm  28 ′, and is integral with the remainder of the upper arm  28 ′ in the depicted embodiment.  
         [0065]     As best seen in  FIG. 7 , a jaw arm  34 ′ has a first end  36 ′ and a second end  38 ′. The second end  38 ′ of the jaw arm  34 ′ is pivotable relative to the upper arm  28 ′ on a main pivot point  40 ′, which operates much like the main pivot  40  of the previous embodiments. The first end  36 ′ of the jaw arm  34 ′ is therefore movable in a circular arc relative to a center defined by the main pivot point  40 ′. The upper arm  28 ′ is a generally U-shaped channel over most of its length with the opening of the U facing downwardly, so that the jaw arm  34 ′ may be received between the sides of the upper arm  28 ′ as the jaw arm  34 ′ pivots. The lower jaw  26 ′ is at the first end  36 ′ of the jaw arm  34 ′ in movable facing relation to the upper jaw  24 ′. As the jaw arm  34 ′ pivots about the main pivot point  40 ′, reducing the distance between the jaws  24 ′ and  26 ′, the workpiece  22 ′ is grasped between the upper jaw  24 ′ and the lower jaw  26 ′. As previously discussed, in the preferred embodiments, the upper jaw  24 ′ and the lower jaw  26 ′ are each preferably of a multilayer metallic construction. That is, each of the jaws  24 ′ and  26 ′ is made by stacking appropriately shaped thin metallic plates, and attaching them together with rivets extending through transverse rivet holes  42 ′ in the jaws  24 ′ and  26 ′. Similarly, in this embodiment the arms are made of overlying plates. In other embodiments, the jaws may be made of solid, non-laminated metal, and some of the arms may be made as a single piece of metal formed into a U-shaped channel, as appropriate.  
         [0066]     A lower arm  52 ′ is linked to the shifter  92 ′ at a location adjacent to the lower jaw  26 ′ with a ball and socket joint forming a pivot point  94 ′. The lower arm  52 ′ is not integral with the jaw arm  34 ′. The lower arm  52 ′ extends generally parallel to the upper arm  28 ′. The upper arm  28 ′ and the lower arm  52 ′ are grasped by the hand of the user of the pliers  20 ′, and an upper arm pad  54 ′ and a lower arm pad  56 ′ are provided in their outwardly facing surfaces to facilitate this grasping and aid in the user positioning the grasping hand correctly. The upper arm  28 ′ and the lower arm  52 ′ thereby serve as the handles grasped by the user of the pliers  20 ′.  
         [0067]     A control arm  58 ′ has a first end  60 ′ and a second end  62 ′, and may have a lobes, such as lobe  106 ′, as previously discussed with respect to other embodiments for relation with the release arm  108 ′. The first end  60 ′ of the control arm  58 ′ forms the ball of a ball and socket joint and is pivotably connected to the jaw arm  34 ′ at a socket forming an upper control-arm pivot point  64 ′ adjacent to the second end  38 ′ of the jaw arm  34 ′ about which the control arm  58 ′ may rotate. The second end  62 ′ of the control arm  58 ′ is pivotably connected to the lower arm  52 ′ at the control-arm pivot pin  66 ′ that is positioned near the geometric midpoint of the lower arm  52 ′. The second end  62 ′ is closer to the jaws  24 ′,  26 ′ then the first end  60 ′ of the control arm  58 ′.  
         [0068]     A lower-arm spring  68 ′ spans between the jaw arm  34 ′ and the lower arm  52 ′. In the illustrated embodiment, the lower-arm spring  68 ′ is a coil spring connected between a projection  70 ′ on the lower arm  52 ′ and an intermediate location  72 ′ on the jaw arm  34 ′. The lower-arm spring  68 ′ resets the shifter  92 ′ to place the contact face  96 ′ directly adjacent the contact face  102 ′.  
         [0069]     In the preferred form of the pliers  20 ′ in accordance with these further embodiments, the control-arm pivot pin  66 ′ is selectively movable generally along a portion of the length of the lower arm  52 ′, within slot  80 ′. This movement serves to adjust the maximum clamping force exerted by the jaws  24 ′ and  26 ′ on the workpiece  22 ′, when the workpiece  22 ′ is clamped between the jaws  24 ′ and  26 ′, by changing the geometry of the linkage between the jaw arm  34 ′, the lower arm  52 ′, and the control arm  58 ′. The movement and adjustability are achieved by slidably supporting the control arm pivot pin  66 ′ in a pin slot  80 ′ in the lower arm  52 ′.  
         [0070]     As best seen in  FIG. 8 , a force adjustment mechanism  86 ′ comprises a force adjuster  84 ′ extending from a point near the projection  70 ′ of the lower arm  52 ′ toward the release arm  108 ′. The force adjuster  84 ′ may be a knob, preferably a knurled knob, accessible to the thumb and fore-finger of the same hand in which a user is grasping the pliers and having an integral and externally threaded shaft  85 ′ that extends through and is threadably engaged to the pivot block  82 ′. It will be appreciated that the knurled knob extends beyond the limits of the lower arm  52 ′ so it can be easily manipulated. The force adjustment mechanism also includes the pivot block  82 ′ which is internally threaded with a thread pattern matching that of the shaft  85 ′. The pivot block  82 ′ is in fixed relation with the lower arm  52 ′, such that the pivot block  82 ′ can not move relative to the lower arm  52 ′. The end of the threaded shaft  85 ′ remote from the force adjuster  84 ′ is forced against the second end  62 ′ of the control arm  58 ′ as the shaft is rotated in a first direction, to slide the control-arm pivot pin  66 ′ along the pin slot  80 ′. It will be appreciated that when the force adjuster  84 ′ is turned in the first direction, the shaft  85 ′ drives the control-arm pivot pin  66 ′ along the pin slot  80 ′, in a direction toward the release arm  108 ′, while rotation in a second direction permits the spring  68 ′ to pull the control-arm pivot pin  66 ′ toward the lower jaw  26 ′.  
         [0071]     With the exception of the relocating of the force adjuster  84 ′, it is to be understood that operation of the pliers  20 ′ is substantially as previously discussed with relation to other embodiments, without major modifications unrelated to the force adjuster  84 ′ relocation.  
         [0072]     In that regard, in order to use the pliers  20 ′, a user may grasp the pliers  20 ′ with one hand, for example the right hand, about the upper arm  28 ′ and lower arm  52 ′. For comfort, the pliers  20 ′ may be provided with the upper arm pad  54 ′ and the lower arm pad  56 ′, which are typically formed from plastic and may be molded to ergonomically fit the human hand.  
         [0073]     The user may then manipulate the pliers  20 ′ and/or workpiece  22 ′ such that the workpiece  22 ′ is moved within the jaws  24 ′,  26 ′ of the pliers  20 ′. Squeezing of the lower arm  52 ′ and upper arm  28 ′ will force the lower jaw  26 ′ upward toward the upper jaw  24 ′. As previously discussed, and as shown in  FIG. 9 , the lower jaw  26 ′, jaw arm  34 ′, lower arm  52 ′, and an engagement mechanism are at this time in locked geometric relation by virtue of the anti-squat mechanism  120 ′, where the contact face  96 ′ of the shifter  92 ′ and the contact face  102 ′ of the lower jaw  26 ′ remain in contact by force of a biasing mechanism, typically in the form of a leaf spring (for example, leaf spring  122  of FIG.  6 ), and form an interconnected unit  73 ′. During this period of contact, rotation of the shifter  92 ′ about pin  93 ′ is prevented.  
         [0074]     After the jaws  24 ′ and  26 ′ have contacted the workpiece  22 ′ and begun to apply a contact force into the workpiece  22 ′, the contact face  96 ′ lifts up and away from the contact face  102 ′ that is part of the lower jaw  26 ′, against the biasing force of the antisquat leaf spring (shown in  FIG. 6 ). The shifter  92 ′ rotates clockwise (in the view of the drawings) about the pivot established between the contact surface  98 ′ of the shifter  92 ′ and the contact face  104 ′ of the lower jaw  26 ′. The pawl  88 ′ rotates clockwise about the pawl pivot pin  93 ′ and moves toward the lower jaw  26 ′ to engage the pawl teeth  90 ′ to the support engagement teeth  50 ′. This engagement of the pawl teeth  90 ′ to the support engagement teeth  50 ′ halts further gross rotation and motion of the interconnected unit  73 ′.  
         [0075]     For most applications, it is desirable that the contacting force of the jaws  24 ′ and  26 ′ to the workpiece  22 ′ be large in order to ensure that the workpiece is firmly held. To accomplish that result, the shifter  92 ′ achieves a force-multiplier effect wherein the contact force applied to the workpiece  22 ′ is significantly greater than the force produced by the grasping action of the hand of the user. The force multiplication arises as follows. After the upper arm  28 ′ and the lower arm  52 ′ are brought together and the workpiece  22 ′ is contacted, the shifter  92 ′ begins to rotate. The pawl  88 ′ is connected with the shifter  92 ′, and rotates with the shifter  92 ′. Once the pawl teeth  90 ′ are engaged to the support engagement teeth  50 ′, the rotational pivot point of the shifter  92 ′ is transferred from the contact face  98 ′ of the shifter  92 ′ to the pawl pivot pin  93 ′ as the contact face  96 ′ of the shifter slides off the contact face  102 ′ of the lower jaw  26 ′. The contact face  98 ′ of the shifter  92 ′ rides on the inclined contact face  104 ′ of the lower jaw  26 ′. The shifter  92 ′ continues to rotate about the pivot pin  93 ′ as the lower arm  52 ′ is moved toward the upper arm  28 ′, producing a further minor rotation of the jaw arm  34 ′. The hand force of the user moving over a longer distance is transferred into the lower jaw  26 ′, which moves a shorter distance but with greater contact force applied to the workpiece  22 ′, than the hand force of the user. The force multiplication is achieved because the contact faces  98 ′ and  104 ′ act as an inclined plane as the shifter  92 ′ rotates. The difference in the length of the lever arm between the locations  93 ′- 98 ′ and  93 ′- 94 ′ also contributes to the force multiplication.  
         [0076]     The release of the force on the lower arm  52 ′ reverses this process through action of the lower-arm spring, causing the shifter  92 ′ to rotate counterclockwise, disengaging the pawl teeth  90 ′ from the engagement teeth  50 ′, allowing the lower jaw  26 ′ to move downwardly, and disengaging the jaws  24 ′ and  26 ′ from the workpiece  22 ′.  
         [0077]     In the use of the pliers  20 ′ just discussed, the jaws  24 ′ and  26 ′ engage and hold the workpiece  22 ′ such that release of the pressure applied to the upper arm  28 ′ and the lower arm  52 ′ immediately releases the workpiece  22 ′. In another embodiment, the jaws  24 ′,  26 ′ may be engaged to the workpiece  22 ′ and releasably locked to the workpiece  22 ′ by a locking mechanism  150 ′, which in this case is an overcenter locking mechanism.  
         [0078]     The overcenter locking mechanism  150 ′ with its associated release are conveniently provided by placement of an unlocking lobe  106 ′ on the lower side of the control arm  58 ′. A release arm  108 ′ is pivotably connected to the lower arm  52 ′ and accessible to the hand of the user of the pliers  20 ′ at the end of the lower arm  52 ′ remote from the shifter  92 ′. A release pad  110 ′ on the upper side of the release arm  108 ′ is disposed to contact the unlocking lobe  106 ′ when the release arm  108 ′ is rotated, such as shown in  FIG. 10  ( FIG. 10  depicts the release arm  108 ′ in several different orientations). In operation, the lower control arm pivot pin  66 ′ moves to an overcenter position relative to the upper control-arm pivot point  64 ′ and the pivot point  94 ′ formed by a ball and socket joint between the lower arm  52 ′ and the shifter  92 ′, when the lower arm  52  is moved upwardly to the limit of its travel established by the operation of the force adjustment mechanism  86 ′. Stated alternatively, when the lower arm  52 ′ is fully open (moved to its downward limit of travel) as in  FIG. 7 , the lower control arm pivot pin  66 ′ lies below a straight line drawn between the main pivot point  40 ′, upper contact-arm pivot point  64 ′, and the pivot point  94 ′. As the lower arm  52 ′ is moved upwardly, the lower control arm pivot pin  66 ′ moves closer to a straight-line relationship between the pivot points  64 ′ and  94 ′, and main pivot point  40 ′, and eventually crosses over that straight line to lie above the straight line drawn between the pivot points  64 ′,  94 ′ and main pivot point  40 ′. This is the overcenter lock position. Typically, the pliers  20 ′ permit approximately two degrees of angulation past center, which is sufficient to lock the pliers  20 ′. To release the pliers  20 ′ from this overcenter lock position, the release arm  108 ′ is operated to rotate the release pad  110 ′ upwardly against the unlocking lobe  106 ′, and thereby force the lower arm  52 ′ downwardly and out of the overcenter relationship.  
         [0079]     This type of overcenter locking capability may be provided instead of or in addition to the engaging-but-nonlocking embodiment described previously. The embodiment of  FIGS. 7-10  allows the pliers  20 ′ to be selectively shifted between the non-locking version and the locking/release version. A locking engagement control  112 ′ includes an overcenter lock selector  114 ′. The overcenter lock selector  114 ′ selectively moves the release arm  108 ′ in a track  124 ′ to a position wherein an overcenter blocking pad  111 ′ on the release arm  108 ′ contacts the unlocking lobe  106 ′ to block the movement of the control arm  58 ′ that is required to reach the overcenter locked position. In this position, the pliers  20 ′ functions to grasp the workpiece  22 ′ between the jaws  24 ′ and  26 ′, but does not lock the jaws  24 ′ and  26 ′ against the workpiece  22 ′. When the force is released from the arms  28 ′ and  52 ′, the workpiece  22 ′ is automatically released. On the other hand, when the overcenter lock selector  114 ′ is repositioned to move the release arm  108 ′ in the track  124 ′ so that the overcenter blocking pad  111 ′ does not block the movement of the control arm  58 ′ that is required to reach the overcenter locked position, the force on the arms  28 ′ and  52 ′ causes the jaws  24 ′ and  26 ′ first to grasp and, then with continued force, to lock onto the workpiece  22 ′ in the overcenter position. Release of the force on the arms  28 ′ and  52 ′ does not itself cause the jaws  24 ′ and  26 ′ to release the workpiece  22 . Instead, the release arm  108 ′ is pivoted to contact the unlocking lobe  106 ′ and push the lower arm  52 ′ away from the control arm  58 ′. The contacting force applied by the jaws  24 ′ and  26 ′ to the workpiece  22 ′ is released, and the disengagement of the jaws  24 ′ and  26 ′ from the workpiece proceeds. The ability to readily switch between nonlocking and locking pliers is an important advantage of one embodiment of the present approach.  
         [0080]     Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.