Abstract:
A self-centering clamp having three jaws, for the industrial automation field is provided and is adapted to equip robotic arms. One of the jaws is controlled by a cursor directly activated by the clamp actuator, which can be electric, pneumatic, oleopneumatic, etc. The other two jaws are controlled by corresponding transfer levers kinematically coupled to the cursor. The levers rotate in a lying plane parallel to the handling plane of the jaws and the cursor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a clamp having three jaws, in particular to a self-centering type of clamp, adapted to be used as gripping element of industrial manipulators. 
     STATE OF THE ART 
     In the field of industrial automation is known the use of robotized manipulators to which a gripping clamp for the objects to be manipulated is normally associated. 
     The clamps are provided with a body housing the jaws, or clamps, and the corresponding activating device. The jaws are movable between a first not-operating position or releasing position, where they don&#39;t apply any pressure on the piece to be manipulated and an operating position, or gripping position, where they apply a pressure on the piece to be manipulated adequate to provide the workpiece not becoming accidentally free during its displacement. The device for activating the jaws can be of an electric, pneumatic, oleo-pneumatic, etc type. 
     According to the number of the jaws and their movement, they can be clamps having two jaws, which can be parallel, radial or angular, clamps having three jaws, etc. 
     In case of three jaws these are arranged inside the clamp body, translatable in respective seats extending along radial directions intersecting each other at the longitudinal axis of the clamp (that is in turn orthogonal to such radial directions), intercepting three 120° central angles. 
     A kind of clamp has the gripping position corresponding to the jaws being proximal to the clamp longitudinal axis, for the external gripping of pieces that are inserted between the jaws themselves; the releasing position corresponds to the jaws being distal from the clamp longitudinal axis. 
     In another kind of clamp, the gripping position corresponds to the jaws being distal from the clamp longitudinal axis, for the inner gripping of pieces surrounding the jaws themselves; the releasing position corresponds to the jaws being proximal to the clamp longitudinal axis. 
     An example of a clamp having three translatable jaws is described in the U.S. Pat. No. 6,193,292, in the name of the Applicant. 
     Two technical requirements are usually needed for a clamp having three jaws for the industrial automation field. 
     Firstly, the three jaws have to move simultaneously to cover same travels in response to the respective activation. This requirement arises because of the need of preventing a jaw from applying a pressure before the other jaws to the piece to be manipulated, causing its undesirable and potentially dangerous misalignment with respect to the clamp longitudinal axis. Further, the three jaws coming in the gripping position at the same time makes the clamp to be self-centering. 
     Secondly, always more frequently is required a minimization of the clamp size and weight. A small sized clamp, and particularly vertically small sized along the longitudinal axis, is versatile in use, since it allows to manipulate even complicated shape workpieces or workpieces that are initially arranged very close to each other. The clamp lightness is important for the dynamic performances of the related manipulator, for example a robotic arm; actually, the clamp constitutes a weight applied to the end of the robotic arm, and it is evidently advantageous to reduce such a weight as much as possible. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a three jaw clamp being light and having minimum size. 
     In particular, it is an object of the present invention to provide a self-centering clamp of the previously mentioned type, characterized by the three jaws moving simultaneously and having identical travels, regardless of the type of the respective activating device. 
     Therefore, the present invention relates to a clamp having three jaws according to claim  1 . 
     Particularly, the clamp comprises a body provided with a longitudinal axis and three coplanar seats for housing the jaws. The jaws are each alternately translatable into the respective seat in the two ways along a direction orthogonal to the longitudinal axis, between a proximal position and a distal position with respect to the longitudinal axis itself. 
     The distal and proximal positions correspond to the gripping and releasing positions, respectively, or vice versa, according to the requirements. 
     The clamp comprises as well a device for activating the jaws, that is in its turn provided with an actuator and means to drive the motion of the actuator to the jaws. The drive means comprise a cursor translatable in the clamp body, integrally with a first jaw, in response to the start of the actuator. Suitable transfer means impart the movement of the cursor to the second jaw and the third jaw too. 
     The second and the third jaws translate along directions inclined with respect to the translation direction of the first jaw, preferably according to center angles of 120°. So, the transfer means are configured to take into account this aspect. 
     Particularly, the transfer means comprise two controlling levers, both rotatable on a respective rotation axis parallel to the aforesaid longitudinal axis, in response to the translatory movement of the cursor. Both levers are constrained to a corresponding jaw of the second and the third jaw to impart it a travel equivalent to the travel imparted by the cursor to the first jaw. 
     The term “jaw” is referred to any movable element intended to interact with the surface of the workpiece to be manipulated, regardless of its shape. 
     The clamp according to the present invention provides multiple advantages with respect to the known art. 
     The transfer levers rotate in a plane orthogonal to the longitudinal axis of the clamp body, that is a plane parallel to the sliding plane of the jaw; this arrangement allows for minimizing the bulk of the clamp along its longitudinal axis, or rather its vertical size. In other words, the clamp can have a particularly flat shape with respect to known clamps, leading to obvious positive effects on reduction and distribution of the overall weight of the clamp itself. 
     The use of transfer levers movable in a lying plane parallel to the sliding plane of the jaws, and that can rotate at the same time, allows to move the second and the third jaws in synchronism with the first jaw and to have three jaws with identical travels, de facto making the clamp self-centering. This prevents one of the jaws from interacting with the workpiece to be manipulated before the others, which could change the spatial position or cause breakages or damages. 
     Furthermore, a single cursor operates all jaws. 
     Preferably, the transfer levers are substantially coplanar, for example flat or nearly flat, so as not to affect the bulk of the clamp height, and they are interposed between the cursor and the jaws. 
     In a preferred embodiment of the invention, the second and the third jaws are each provided with a pin, or a similar connecting element, extending parallel to the longitudinal axis through a corresponding eyelet formed in the clamp body. Each of the levers comprises a guide, slidingly housing the pin, according to a coupling that can be assimilate to a cam and follower type. The guide of each transfer lever is arranged as an inner cam wherein the pin is driven arranged as a follower constrained to follow the path imparted by the guide by virtue of its shape. 
     In such an embodiment, the guide of each transfer lever extends preferably along an arc of a circumference whose center of curvature does not coincide with the rotation axis of the respective lever. According to this feature, when the transfer levers rotate, a corresponding translation of the respective pins along the moving direction of the connected jaws is provided. Actually, the pin moves with respect to the guide of the transfer lever and, at the same time, with respect to the clamp body. 
     More particularly, in a first position of the cursor all jaws are in the respective distal position and the pins of the second jaw and the third jaw are substantially at a first end of the respective guide of the corresponding transfer lever, that is the maximum distance from the longitudinal axis. In a second position of the cursor all jaws are in the respective proximal position and the pins of the second and third jaws are substantially at the second end of the respective guide of the corresponding transfer lever, that is the minimum distance from the longitudinal axis. 
     Preferably, the transfer levers insert at least partially in each other, together defining an eyelet extending along an arc of a circumference whose center of curvature is in the median plane of the first jaw, or else along the respective translation axis, at the opposite side with respect to the longitudinal axis. For example, the two transfer levers both comprise a perimetrical recess forming a half of the previously mentioned eyelet. The cursor comprises a dragging pin, or a similar coupling element, slidingly engaging the eyelet. The pin extends preferably parallel to the longitudinal axis from the cursor and inserts into the eyelet. 
     In more detail, in a first position of the cursor, each of the jaws are in the respective distal position and the levers are rotated in opposite directions with respect to the aforesaid median plane, each lever on the respective rotation axis. In a second position of the cursor each of the jaws are in the respective proximal position and the levers are rotated one towards each other and preferably are inserted at least partially one in another, towards the above mentioned median plane. Therefore, the eyelet defined by the levers extends and shortens as a consequence of the rotation of the levers themselves, respectively increasing or reducing the distance between each other. 
     Preferably another pin or a similar connecting element couples the cursor to the first jaw to make these components translationally integral. 
     Generally the clamp actuator can be any kind of actuator, as long as it is arranged for controlling the alternate movements of the cursor in the two ways of the respective translation direction. For example, the actuator is electric, or pneumatic or oleodynamic. 
     In the preferred embodiment the actuator is an electric motor and the drive means comprise a tow slide translatable in parallel with the cursor, at least one elastic element, and a thrust screw of the tow slide. The thrust screw is non-reversible, meaning that the respective thread prevents the unwanted rotation of the screw. The screw is oriented in parallel with the translation direction of the cursor. The tow slide is threaded too and it meshes the thrust screw, the latter operating the translation thereof in the two ways in response to the clockwise/anticlockwise rotation in its turn imparted by the shaft of the electric motor. 
     The cursor is supported by the tow slide and between these elements the elastic element, preferably a preloaded coil spring, is interposed. The preload is fixed by the manufacturer based on the use requirements of the clamp. The cursor and the tow slide can translate both integrally, till the preload of the elastic element does not change, and independently from one another, in response to a preload change of the elastic element produced by a resistance exerted against the translation movement of the jaws. 
     Actually, the slide-elastic element-cursor assembly acts as a compensator of the travel of the jaws that is able to adjust beforehand the force exerted by the jaws onto the pieces to be manipulated, even if the diameter thereof changes. This assembly also performs the function of releasing the electric motor from the task of pushing and keeping the jaws exactly in the respective end positions they can take, the distal and the proximal one. Preferably, the electric motor and the thrust screw are aligned one above the other to allow to minimize the side sizes of the clamp. 
    
    
     
       LIST OF THE FIGURES 
       Further characteristics and advantages of the present invention will be more evident from a review of the following specification of a preferred, but not exclusive, embodiment, shown for illustration purposes only and without limitation, with the aid of the attached drawings, in which: 
         FIG. 1  is a perspective view of a clamp having three jaws according to the present invention; 
         FIG. 2  is an exploded view of the clamp shown in  FIG. 1 ; 
         FIG. 3  is a side elevation view of the clamp shown in  FIG. 1 ; 
         FIG. 4  is an first vertical section view of the clamp shown in  FIG. 1 ; 
         FIG. 5  is a second vertical section view of the clamp shown in  FIG. 1 ; 
         FIG. 6  is a first cross section view of the clamp shown in  FIG. 1 ; 
         FIG. 7  is a second cross section view of the clamp shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The enclosed  FIGS. 1-6  show the preferred embodiment of the clamp  1  according to the present invention, comprising a body  2  housing the respective components. 
     The body  2  of the clamp  1  is made up of two portions, an upper portion  2 A and a lower portion  2 B. The upper portion  2 A is substantially circular and extends around the longitudinal axis Z-Z and has a center recess  3 , which is circular too. 
     The portion  2 A of the body  2  houses the jaws G 1 , G 2  and G 3 , that are slidable within respective seats, or tracks,  21 - 23  radially extending with respect to the axis Z-Z, along directions forming angles of 120° at the intersection of each other, and in the same lying plane. In  FIG. 1  the jaws G 1 -G 3  are fully retracted, i.e. they are in the position of minimum distance from the longitudinal axis Z-Z with respect to the travel each jaw can run, corresponding to the releasing position of the workpiece to be manipulated. The jaws G 1 -G 3  can be translationally pushed in the respective seats  21 - 23  to be partially ejected beyond the perimeter of the portion  2 A of the clamp body  2 , taking the gripping position of the workpiece to be handled, which obviously is at least partially hollow, allowing the clamp  1  to be inserted therein. 
     In another embodiment, not shown in figures, the jaws G 1 -G 3  can move towards the longitudinal axis Z-Z, beyond the position shown in  FIG. 1 , to close against a piece to be manipulate having a diameter small enough to be inserted in the jaws G 1 -G 3  themselves. 
     By way of example only, the travel of each jaw G 1 , G 2  or G 3  is 3 mm, 6 mm or 9 mm, according to the size of the clamp  1 . 
     The lower portion  2 B of the body  2  houses most of the clamp components intended to activate the jaws G 1 -G 3 .  FIG. 2  shows the elements in detail. 
     The portion  2 B of the body  2  houses an actuator  4 , for example an electric motor as illustrated in figures, aligned with the seat  21  of the first jaw G 1 . A thrust screw  5 , comprising a threaded shank  51  having a non-reversible thread and a gear wheel  52  is kinematically connected to the shaft of the electric motor  4  by gears  6  and  7 . In particular, the gear wheel  52  is cascade connected to the gear  7 , the gear  6  and the shaft of the electric motor  4 . 
     The threaded shank  51  of the thrust screw  5  engages a tow slide  8 , threaded at its underside, to operate the movements thereof along the respective activating direction, parallel to the screw  5 , and, accordingly, parallel to the activating direction of the first jaw G 1 . 
     In practice, the electric motor  4  controls the rotation of the corresponding shaft and the connected gear  6 , inducing corresponding rotations of the screw  5  and translations of the tow slide  8 . 
     The tow slide  8  supports a cursor  11 , the latter being translatable too in the portion  2 B of the body  2  of the clamp  1 . Between the slide  8  and the cursor  11  two coil springs  9  and  10  are interposed, not only in a material way, but above all in an operational way. The coil spring  9  and  10  are compressively pre-loaded during the assembling step, directly by the manufacturer. If needed, during the following life of the clamp  1 , the springs  9  and  10  can be replaced by springs having a different preload. 
     The assembly comprising the slide  8 , the springs  9  and  10  the cursor  11 , forms a device elastically compensating the travels of the jaws G 1 -G 3 , the device being equivalent to the one described in the Italian Patent Application BS2010A000074 of the 12 Apr. 2010. 
     The assembly comprises the afore said compensating device with the addiction of the actuator  4 , the gears  6  and  7  and the thrust screw  5 , the device A activating the clamp  1 . 
     The pins  12  are integral with the cursor  11  and jut towards the tow slide  8  such that they act as plungers of the springs  9  and  10  when the cursor translates with respect to the tow slide  8 . The pins  12  can be inserted between the shoulders  81  of the slide  8 , that represent the extreme limit position of the springs  9  and  10  towards the respective side of the tow slide  8 . 
     Substantially the tow slide  8 , translated by the screw  5 , always run a fixed travel, whereas the cursor  11  can also translate with respect to the tow slide  8  so as to run travels being variable within certain limits according to the resistance to movement the jaws G 1 -G 3  meet in picking up a work piece. The springs  9  and  10  are compressed for absorb exceeding force that the jaws G 1 -G 3  would otherwise apply on the piece to be manipulated. 
     The pin  13  and the dragging pin  14  extend vertically, that is parallel to the axis Z-Z, from the upper part of the cursor  11 , with a different extension. The pin  13  directly engages the first jaw G 1 , passing through two aligned openings  15  and  16  obtained respectively through the lower portion  2 B and the upper portion  2 A of the body  2 . The resulting connection makes the cursor  11  and the first jaw G 1  translationally integral with respect to the body  2  of the clamp  1 . 
     The cinematic chain formed by the elements  6 ,  7 ,  8 - 11 ,  13  and  14  accomplishes the motion transmission from the electric motor to the jaws G 1 -G 3 . 
     The dragging pin  14 , pushed by the cursor  11 , actually drives the movement of the remaining jaws G 2  and G 3 , not directly but by interposing proper transfer means that will be described hereinafter. 
     The transfer means comprise two horizontally actuated levers  17  and  18  and respective pins  21  and  22  connecting them to the jaw G 2  and the jaw G 3 , respectively. The levers  17  and  18  are intended to rotate around pivots that are referred to with the numerals  171  and  181 . The pivots  171  and  181  are parallel to the longitudinal axis Z-Z and insert in corresponding seats  19  and  20  obtained on the upper surface of the portion  2 B of the body  2 . 
     As it could be appreciated in  FIG. 2 , the horizontal transfer levers  17  and  18  are shaped so as to partially insert one in another when the angle between the levers themselves decreases as a consequence of their rotation. At the bottom of the upper portion  2 A of the body  2  a special recess is provided for housing the levers  17  and  18 . 
     The transfer levers  17  and  18  are respectively provided with guides  172  and  182  shaped as an inner cam, slidingly housing the pins  24  and  25  constrained to the jaws G 2  and G 3 . The levers  17  and  18  together define an eyelet  26  wherein the pin  14  from the cursor  11  is inserted. 
       FIG. 3  shows the clamp  1  in elevation view; as it could be appreciated, the arrangement of the drive elements and the actuator in the lower portion  2 B of the body  2 , allows to limit the side sizes of the portion  2 B itself to a dimension far smaller than the sizes of the upper portion  2 A. 
     The operation of the clamp  1  will be now described, referring to  FIGS. 4 and 5  which respectively show two orthogonal longitudinal section, the first one taken along the median plane of the seat  21  of the jaw G 1 , and a cross section taken along a plane parallel to the lying plane of the seats  21 - 23  flush with the transfer levers  17  and  18 . 
     In  FIG. 4 , the jaw G 1  is shown fully retracted in the respective seat  21 , that is the position proximal to the longitudinal axis Z-Z. The jaws G 2 -G 3  are also in the proximal position, previously referred to as no-working position or releasing position of the piece. 
     The rotation of the screw  5  is controlled by activating the motor  4  and giving the gears  6 ,  7  and  52  a rotation. This causes the tow slide  8  and the cursor  11  to be translated (to the right in  FIG. 4 ) in the direction leading the jaw G 1  to project beyond the perimeter of the portion  2 A of the body  2 , for example by about 3 mm. The transfer levers  17  and  18  push the jaws G 2  and G 3  into the respective seats  22  and  23  with the same travels of the jaw G 1 . 
     The assembly comprising the springs  9  and  10 , the tow slide  8  and the cursor  11 , moves rigidly as a whole provided that the jaws G 1 -G 3  do not meet resistance. If the jaws G 1 -G 3  meet resistance, for example coming in abutment against the inner surface of the piece to be manipulated, the springs  9  and  10  compress to absorb the thrust corresponding to the remaining travel of the jaws G 1 -G 3 . Under this circumstances, the cursor  11  stops moving integrally with the tow slide  8  and translates with respect to the latter; for example, the slide  8  completes its travel and the cursor  11  stays still because of the jaws G 1 -G 3  are in abutment against the piece to be manipulated. 
       FIG. 5  shows a section orthogonal to the springs  9  and  10 . 
       FIG. 6  shows a cross section of the clamp  1  wherein the springs  9  and  10  appear compressed no more than the initial preload, that is to say in the releasing position of the jaws G 1 -G 3 . 
     Referring in particular to  FIG. 7 , there is shown a top view of the transfer lever  17  and  18 , partially inserted in one another, and ready to be rotated about the respective pivots  171 ,  181  in the sheet plane. The pins  13  and  14  alternately translate in the two ways of the axis X-X integrally with the cursor  11 , according to the direction of rotation of the shaft of the motor  4 . The pins  24 ,  25  are at a first end of the respective guide  172 ,  182 . 
     The dotted lines show the distal position of the jaws G 1 -G 3 , i.e. the maximum radial position projecting from the perimeter of the portion  2 A, to grip the piece to be manipulated. The pins  24 ,  25  are at the second end of the respective guide  172 ,  182 . 
     To drive the jaws G 1 -G 3  from the releasing position to the gripping position, the cursor moves to the left in  FIG. 6 . The pin  13  drives the jaw G 1  directly; the pin  14  applies a thrust onto the walls of the eyelet  26  causing the levers  17  and  18  to rotate simultaneously in opposite directions. Under these circumstances, referring to  FIG. 6 , the lever  18  rotates clockwise and the lever  17  rotates counterclockwise; the eyelet  26 , extending along an arc of circumference transverse to axis X-X, tends to open that is to extend. In particular, as it could be appreciated, the center of curvature of the eyelet  26  is on the median plane of the seat  21  of the jaw G 1 , or rather the vertical plane containing the axis X-X. 
     The guides  172  and  182  slide with respect to the pins  24  and  25  that are, at the same time, translationally pushed along the directions X′ and X″, actually causing the jaws G 2  and G 3  to translate in the respective seats  22  and  23 . The directions X-X, X′ and X″ form angles of 120°. 
     Obviously a translation of the cursor  11  in the opposite way returns the jaws G 1  and G 3  into the initial releasing position. 
     This dynamics is possible due to the shape of the guides  172  and  182 , each extending along an arc of circumference having its rotation center offset with respect to the pivots  171  and  181 . 
     The described configuration allows the synchronous movement of all jaws G 1 -G 3  with identical travels, resulting in the self-centering effect with respect to the longitudinal axis Z-Z as previously described. 
     Preferably the jaws G 1 -G 3  transfer from the releasing position to the gripping position, and vice versa, in less than a tenth of a second. 
     According to the sizing of electric motor  4 , or the pneumatic/oleo pneumatic actuator used as an alternative, each jaw G 1 -G 3  can preferably apply a force of 100 N to 400 N to the piece to be manipulated.