Patent Publication Number: US-7216570-B2

Title: Switchable self-adjusting pliers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/463,843, filed Jun. 18, 2003, now U.S. Pat. No. 7,100,479 which claims the benefit of U.S. Provisional Application No. 60/390,007 filed Jun. 18, 2002 and which is a continuation-in-part of U.S. application Ser. No. 09/942,095, filed Aug. 28, 2001, now U.S. Pat. No. 6,748,829, which is a continuation of U.S. application Ser. No. 09/594,191, filed Jun. 14, 2000, now U.S. Pat. No. 6,279,431, which is a continuation-in-part of U.S. application Ser. No. 09/334,055, filed Jun. 15,1999, now U.S. Pat. No. 6,212,978. The disclosure of each of the above-identified applications is incorporated herein by reference. 

   FIELD OF INVENTION 
   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 
   The traditional version of a pliers includes two elongated members jointed 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. 
   U.S. Pat. No. 4,651,598, for example, provides a 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. In addition, the clamping force applied by these pliers depends upon the size of the workpiece being grasped. 
   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. However, overcenter locking pliers are conventional, and are described, for example, in a series of patents such as U.S. Pat. No. 4,541,312. Such 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. 
   Other types of locking pliers such as the AutoLock™ pliers combine the self-adjusting feature with an overcenter locking mechanism. These 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. 
   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. The present invention fulfills this need. 
   SUMMARY OF THE INVENTION 
   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 operable with one hand. 
   In accordance with the invention, a self-adjusting pliers is operable to grasp a workpiece between an upper jaw and a lower jaw. The pliers includes an upper arm having a first end and a second end. The upper jaw is 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 lower jaw is 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 low 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. 
   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. 
   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. 
   The engagement mechanism releasably engages the jaw arm to the upper arm. There also may 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, but does not interfere with rotation of the control arm about the upper control-arm pivot pin in the releasable engagement-and-lock embodiment. 
   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. 
   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. 
   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 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. 
   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. 
   The pliers described above preferably includes an overcenter lock switch mechanism in the lower handle which is movable between a first locking position and a second non-locking position. In the first position the overcenter lock switch mechanism allows the pivoting movement of the lower arm relative to the control arm prior to reaching an overcenter lock position, while in the second non-locking position the overcenter lock switch mechanism prevents the control arm from reaching an overcenter lock position. The movement of the locking switch mechanism to the second switch position thus prevents the pivoting movement of the lower arm and the control arm from reaching an overcenter locking position, and thereby prevents the overcenter locking function. 
   In one form of the invention, the lock switch mechanism is a slidable lock which is movable to interfere with the operation of a release arm. The release arm is mounted on the lower handle and when the switch is in the lock position the release arm is pivoted to a release position by the control arm when the control arm moves to its overcenter locking position. The release arm may then be operated to move the control arm out of the overcenter locking position to unlock the plier. To prevent locking, the switch is moved to a no-lock position where it prevents the release arm from pivoting to the release position, thereby preventing the control arm from moving to its overcenter locking position. 
   In another form of the invention, the lock switch mechanism is a slidable lock selector, or locking engagement controller, which includes a pin on the release arm which is slidable in a track on the lower handle to allow the release arm to be moved between lock and no-lock positions. When the selector is in the no-lock position, a blocking pad on the release arm engages a lobe on the control arm to prevent the control arm from moving to an overcenter locking position. To permit locking of the pliers, the lock selector is moved to its lock position which moves the blocking pad out of the path of the control arm lobe, allowing the control arm to move to its overcenter locking position. The release arm also includes a release pad which engages the lobe in the lock position to enable release of the control arm. 
   In a related form of the invention, the overcenter lock switch mechanism includes an overcenter limiting arm affixed to the lower arm of the pliers, as by a slider pin in a slot. The limiting arm includes a contact surface which can be moved toward or away from a control arm lobe to prevent or to allow the overcenter locking operation. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of preferred embodiments, 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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevational view of a pliers; 
       FIG. 2  is a schematic perspective view of the pliers of  FIG. 1 , with portions of the external structure removed; 
       FIG. 3  is a schematic perspective view of the pliers of  FIG. 1 , with additional portions of the external structure removed; 
       FIG. 4  is a schematic perspective view of the pliers of  FIG. 1 , with further portions of the external structure removed; 
       FIG. 5  is a detail perspective view near the second end of the upper arm of the pliers of  FIG. 1 ; 
       FIG. 6  is a detail perspective view in the region of the shifter of the pliers of  FIG. 1 ; 
       FIG. 7  is a partial elevational view of a pliers incorporating a modified overcenter lock switch mechanism; 
       FIGS. 8–11  are a series of partial schematic elevational views of a modified switchable overcenter locking mechanism in the lock mode; and 
       FIGS. 12–15  are a series of partial schematic elevational views of the switchable overcenter locking mechanism of  FIGS. 8–11  in the non-locking mode. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1–6  illustrate a self-adjusting pliers  20  according to the 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  illustrate details. “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. 
   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. 
   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. 
   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. The support engagement slot  48  desirably includes small support engagement teeth  50  along a side  51  of the slot  48  nearest the jaw  24  and  26 . 
   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 . 
   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 . 
   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 . 
   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 . 
   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  ( FIG. 5 ) in the jaw arm  34 . 
   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 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 . 
   An engagement mechanism  86  ( FIG. 6 ) 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. 
   The engagement mechanism  86  includes a pivotably supported pawl  88  ( FIG. 3 ). 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 . 
   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 . 
   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 pin  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  FIG. 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. 
   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 element 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 workpiece  22 , the anti-squat 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 . 
   After the jaws  24  and  26  have contacted the workpiece  22  and have 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 anti-squat 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 . 
   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 form the contact face  98  of the shifter  92  to the pawl pivot pin  93 . The contact face  98  rides on the included 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. 
   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  form the engagement teeth  50 , allows the lower jaw  26  to move downwardly, and disengages the jaws  24  and  26  from the workpiece  22 . 
   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. 
   The overcenter locking mechanism  150  incorporates an unlocking lobe  106  on the lower side of the control arm  58 . A release arm  108  is pivotably connected to the end of the lower arm  52  remote from shifter  92 , and is accessible to the hand of the user of the pliers  20 . A release pad  110  is located on the upper side of the release arm  108  ( FIG. 2 ) and is disposed to contact the unlocking lobe  106  when the release arm  108  is rotated. In operation, when the lower arm  52  is moved upwardly to the limit of its travel established by the operation of the engagement mechanism  86 , 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 . 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 , to thereby force the lower arm  52  downwardly and out of the overcenter relationship. This type of overcenter locking capability may be provided instead of or in addition to the engaging-but-non-locking embodiment described previously. 
   The embodiment of  FIGS. 1–4  includes a locking engagement control  112  that allows the pliers  20  to be selectively shifted between a non-locking version and a locking/release version. The locking engagement control  112  includes an overcenter lock selector  114  that enables the release arm  108  to move along a track  124  between a locking position and a non-locking position. In the non-locking position, an overcenter blocking pad  111  on the release arm  108  is positioned to contact 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. In the locking position, 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. This allows the force on the arms  28  and  52  to cause the jaws  24  and  26  first to grasp and, then with continued force, allows overcenter motion of control arm  58  to allow the jaws 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, to unlock the jaws the release arm  108  is pivoted to cause release pad  110  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 thereby released, and the disengagement of the jaws  24  and  26  from the workpiece proceeds. This ability to readily switch between non-locking and locking pliers is an important advantage of the invention. 
   As illustrated the overcenter lock selector  114  may comprise a generally rectangular pin secured to, and movable with, release arm  108 . The pin is located in and is slidable along the slot  124 , which extends generally longitudinally along the lower arm  52 , to permit longitudinal motion of the release arm  108 . This allows arm  108  to be shifted lengthwise along arm  52  to switch the pliers between the non-locking mode of operation in the forward position (illustrated in  FIG. 4 ) and the locking/release mode of operation in the rearward position. The pin  114  and the forward and rearward ends of the slot are angled slightly with respect to the generally longitudinal direction of the slot  124  to provide latching regions which receive the pin to secure the release arm in either its forward or rearward positions. The release arm is pivoted clockwise to align the pin  114  with the slot  124  to allow the arm to move back and forth along the slot to switch the pliers between the locking and non-locking modes. 
   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 through 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 of the user of the pliers  20 . 
   A modification of the selective overcenter locking and release mechanism  150  described above is illustrated in  FIG. 7 , and is described in detail in the aforesaid parent application Ser. No. 09/334,055, now U.S. Pat. No. 6,212,978. In this embodiment, an unlocking lobe  160  is located on a control arm  162  of the pliers  164 . A release arm  166  is pivotably connected to the lower arm  168 , at a location between the first end (not shown) and the second end  170  and accessible to the hand of the user of the pliers at the second end  170 . A release pad  172  on the upper side of the release arm is disposed to contact the unlocking lobe  160 . In operation the lower control arm pivot pin  174  moves to an overcenter position relative to an upper control arm pivot pin when the lower arm  168  is moved upwardly to the limit of its travel. Stated alternatively, when the lower arm is fully open (moved to its downward limit of travel), the lower control arm pivot pin  174  lies below a straight line drawn between the upper control arm pivot pin and the lower arm pivot pin. As the lower handle is moved upwardly, the pivot pins move closer to a straight-line relationship, and eventually pin  174  crosses over that straight line to the overcenter lock position. To release the pliers from this overcenter lock position, the release arm  166  is operated to rotate the release pad  172  upwardly against the unlocking lobe  160 , and thereby forces the lower arm  168  downwardly and out of the overcenter relationship. 
   The embodiment of  FIG. 7  allows the pliers to be selectively shifted between the non-locking version and the locking/release version. A lock switch  180  is provided to selectively prevent the pivoting movement of the release arm  166 . That is, when the movement of the pliers control arm  162  passes into the overcenter relationship, the release arm  166  is forced to pivot in the direction (counter-clockwise in the embodiment of  FIG. 7 ) opposite to the pivoting movement of the release arm during unlocking (clockwise in  FIG. 7 ). The locking function may be prevented by preventing this movement of the release arm  166  as the movement reaches the overcenter position as the jaws are closed, so that the stationary release arm  166  prevents the movement of the control arm  162  from passing to the overcenter position. The lock switch  180  prevents the movement of the release arm and the control arm by physically contacting and interfering with the movement of the release arm. Thus, in the embodiment of  FIG. 7 , the lock switch  180  slides into an interfering position relative to the release arm when slid to the right, so that the overcenter locking is not permitted. The pliers then serves as an ordinary non-locking pliers. When the lock switch is slid to the left in the view of  FIG. 7 , it does not interfere with the rotation of the release arm, and the release arm does not prevent the movement of the lobe  160  and thence the control arm  162  as it passes to the overcenter position. The pliers is a locking pliers in this configuration. 
   Another modification of the selective overcenter lock mechanism is illustrated in  FIGS. 8–13 , and is further described in parent application Ser. No. 09/594,191, now U.S. Pat. No. 6,279,431 and in parent application Ser. No. 09/942,095, now U.S. Pat. No. 6,748,829. These figures are partial views to illustrate a lock mechanism  190  which is similar to those described above for pliers  20  and  164 . In this embodiment, the pliers  192  incorporates a lower arm  194  having an end  196  and a control arm  198  having a lobe  200  similar to those described above, with the control arm being pivotally connected to the lower arm  194  at pivot pin  202 . The release arm  108  and its associated structure described above is replaced by a shaped overcenter lock switch mechanism  240 , which has some of the same functionality as the release arm  108 . The overcenter lock switch mechanism  240  includes a contact surface  242  at the end of an overcenter-limiting arm  244 . The overcenter-limiting arm  244  is affixed to the lower arm  194  at a location adjacent to the second end  196  thereof. The overcenter-limiting arm  244  is affixed to the lower arm  194  by any operable approach, such as an illustrated slider pin  246  in a slot  248 . Other affixing approaches include, for example, a hinge mechanism and a slotted receiver such as discussed above and often used at the jaw end of a conventional pliers. The movement of the overcenter-limiting arm  244  on the slider pin  246  or other affixing approach allows the overcenter-limiting arm  244 , and thence the contact surface  242 , to be positioned relative to the lobe  200  to allow an overcenter locking function or to prevent an overcenter locking function, depending upon the positioning. A leaf spring  250  extends between the overcenter-limiting arm  244  and the lower arm  194  to bias the overcenter-limiting arm  244  in the straight extended position. 
     FIGS. 8–11  sequentially illustrate the operation of the pliers when the overcenter-limiting arm  244  is moved to its rearward position on the slider pin  246 . In  FIG. 8 , the lower jaw of the plier is separated from the workpiece (not shown) so that no force is applied to the pliers arms. In  FIG. 9 , force is applied through the handles so that the lower handle  194  is moved counterclockwise and the lower jaw of the pliers (not shown) just contacts the workpiece. The contact surface  242  has not contacted the lobe  200 . In  FIG. 10 , the handles are squeezed together, so that a gripping load is applied to the workpiece and the lower arm  192  and pivot point  202  have moved almost, but not quite, to the overcenter position. The contact surface  242  has not contacted the lobe  200 , so that in  FIG. 11  the lower arm  192  can move further to the overcenter position. At this point, there is contact between the contact surface  242  and the lobe  200 , so that the lower arm  192  can not move further. To unlock the overcenter position, the overcenter-limiting arm  244  is rotated against the force of the leaf spring  250 , clockwise in the view of  FIG. 11 , to push the lower arm  192  back through the overcenter position. 
     FIGS. 12–15  illustrate substantially the same sequence as  FIGS. 8–11 , except that the overcenter-limiting arm  244  is moved to its forward position on the slider pin  246 . Closing the lower handle produces a progression from the fully open position of  FIG. 12 , to the contacting of the lower jaw of the pliers to the workpiece in  FIG. 13 , to the near-contact of the contact surface  242  to the lobe  200  in  FIG. 14 , to the contacting of the contact surface  242  to the lobe  200  in  FIG. 15 . The contact of the contact surface  242  to the lobe  200  in  FIG. 15 , before the lower control arm pivot pin  202  reaches the overcenter position, prevents movement to the overcenter position and thereby prevents the engagement of an overcenter lock. 
   The ability to readily switch between a pliers configuration that permits an overcenter lock, as in  FIGS. 8–11 , and a pliers configuration that does not permit an overcenter lock, as in  FIGS. 12–15 , is an important advantage. Some pliers uses, such as the initial tightening of a fitting, are best accomplished without an overcenter lock to permit the user to move the pliers quickly. Then, when the fitting is nearly tightened, the user may switch to the overcenter lock configuration to allow the final tightening to be most easily accomplished. 
   Although a particular embodiment of the invention has been described in detail for purpose 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.