Patent Publication Number: US-2004055164-A1

Title: Portable motor-powered shears

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to a portable motor-driven scissor applicable to constitute a tool for pruning in agricultural works, provided with two exchangeable jaws, both provided with motion in opposite senses from a open position to a closed position and vice versa, which allows the scissor has a very lightweight and compact design.  
       PRIOR ART  
       [0002] Different devices of portable scissor are known in the prior art, driven by an battery-powered electric motor either integrated in the scissor itself or, more generally, carried by the operator thereof, the scissor being provided with a handle which allows that it can be used with a single hand.  
       [0003] In this line, EP 0 291 431 discloses a portable electric tool, such as a plate shears or scissor for pruning comprising a moving working part, such as a bit, arranged at a front end of an empty body, a part thereof forms a handle of the said tool. Within the said empty body are arranged an electric motor and a speed reducer the outlet of which is associated to the said moving working part by means of a transmission system comprising a moving driving part. The tool comprises a trigger accessible from outside the said body and an electrical control device driven by the said trigger. As well the trigger as the electrical control device are controlled by the moving driving part of the moving working part the displacements of which they follow and they may adopt different relative positions. In this cutting electrical tool, only one of the jaws is provided with motion, while the other is fixed on the supporting body. This has, on one hand, the drawback that it provides an asymmetric cut which is little neat, and that in the case of pruning the fixed jaw or dolly tends to crush the branch damaging it. On the other hand, the moving jaw needs to move between the open and closed positions and therefore its driving requires a relatively big and heavy mechanism which is very bulky very close to the cutting area, which is also a drawback. Driving, in addition, is carried out by means of a worm device having a low mechanical efficiency, so part of the motor power is missed which means a relatively high energy consumption which is adverse in the case that the tool is fed by batteries having a limited load and carried by the user.  
       [0004] DE 34 46 802 discloses another of the said motor-driven tools specially dedicated to act as a shear, in which only one of the two jaws is provided with motion while the other is fixed. This model shows approximately same drawbacks as those above discussed.  
       [0005] DE 35 24 443 discloses a scissor provided with two jaws of which only one is provided with motion while the other is fixed. In this model, the moving jaw is hinged with respect to an axis and shows a lever arm on the side opposite to the cutting area with respect to the said hinge axis, the said lever arm being ended by a sector of gear wheel which interlocks with a pinion of a gear reducer drive connected to an electric motor. Gear between the gear wheel and the pinion is a cylindrical gear having parallel axis. This makes that the diameter of the pinion is added to the radius of the circular sector lengthwise the scissor, bringing the motor far apart the cutting area and providing a significantly long tool. In addition, the said gear reducer drive includes other cylindrical and conical steps, which are even more bulky and add- even more weigh to the area between the motor and the jaws, which constitutes a serious drawback. Also in this case, the fact it has a moving sheet which is applied against another fixed sheet as a dolly, provides an asymmetric cut which, when the scissor is used, for example, for pruning trees, it has to chop off the cutting area of the part of the branch which is joined to the tree, which causes a serious prejudice.  
       [0006] Utility model ES 1006938 discloses a motor-driven and self-powered scissor in which both jaws are provided with a symmetric motion. The said jaws are associated to a device having bent levers forming a hinged quadrilateral with a vertex at the hinge point of the jaws, fixed to the frame, and a movable opposite vertex driven by an endless screw mechanism. This device has a very important length and is enclosed within an elastic shell such as a corrugated rubber sleeve, which is deformed by gradually be widened as the jaws are approaching to their closed position by the effect of the bent levers deployment until reaching a level of protrusion hindering the access to parts of a tree in the case of pruning, which in addition could even be dangerous for the operator in narrow enclosures. In addition this mechanism does not provide an even closing speed but the speed is reduced as the jaws are closing.  
       EXPLANATION OF THE INVENTION  
       [0007] This invention seeks to provide a motor-driven scissor which overcomes above discussed drawbacks by providing two moving jaws, with symmetric displacements, driven by a small-sized lightweight mechanism which has a high mechanic efficiency in order to optimize the energy consumed and with it extend the operational time of batteries load in the event they are used.  
       [0008] For this, this invention provides a portable motor-driven scissor of the kind which comprises a support associated to a power takeoff from a motor, and a cutting mouth comprising two jaws, at least one of which is a working cutting jaw rotatably mounted on the said support and kinetically connected to the output of the said power takeoff by means of a gear drive to carry out a closing-opening motion with respect to the other jaw. The said support includes or has fastened a handle to which are associated means for controlling the tool operation. The said working cutting jaw, which is at least one, is fastened to a part for power drive which may rotate about an axis mounted on a support, the said power drive part includes a lever arm at the end of which is arranged a sector of crown wheel which interlocks with a pinion joined to an axis of the said power takeoff constituting the said gear drive.  
       [0009] An essential and differentiating characteristic of the scissor of this invention is that the said spin axis of the power drive part forms a predetermined angle with respect to the said pinion spin axis. Preferably, the spin axis of the power drive part joins the pinion spin axis and the said predetermined angle is a straight angle. According to an example of embodiment most preferred, the gear between the pinion and the said gear wheel sector is an external conical gear of axis crossing themselves at straight angle.  
       [0010] According to an example of embodiment, only one jaw is a working jaw while the other is fixed on the support. This single working jaw is driven by the conical gear above disclosed. According to another example of embodiment, both jaws are working jaws and each of them is fixed to a respective power drive part hinged with respect to the said axis, which is common for both. Each of the power drive parts includes one of the said lever arms with a respective sector of crown wheel which interlocks with areas diametrally opposite to the said pinion, so that, when rotating the said pinion, both power drive parts and the respective jaws rotate a same angle in opposite directions with respect to the said axis, each running half the travel of the said closing-opening motion.  
       [0011] This arrangement has the advantage that the diameter of the pinion is not added to the radius of the sector of the gear wheel in the scissor lengthwise direction, a more compact design being achieved. In addition, by means of the example of embodiment in which both jaws are working jaws, a more even and smooth cutting action is achieved as both jaws are closing symmetrically on the object to be cut.  
       [0012] According to an example of embodiment of a double working jaw, the portable motor-driven scissor comprises a supporting body and a pair of jaws fixed to respective power drive parts. Both power drive parts with their respective jaws incorporated, are hinged and, may rotate about a common axis with respect to the said supporting body. From the said power drive parts lever arms are deriving, located on sides opposite to the cutting mouth with, respect to the said spin axis. The said lever arms carry the respective sectors of gear wheel which interlocks with areas diametrally opposite to a same pinion constituting a gear drive arranged at the output of a speed reducer connected to a driving electric motor.  
       [0013] Advantageously, the gear between the pinion and the said gear wheel sectors is an external conical gear, so that the diameter of the pinion is not added tot he radius of the gear wheel sectors of the jaws in the scissor lengthwise direction, but the pinion remains interposed between the said lever arms carrying the sectors of gear wheel. In addition, as the angular displacements of the lever arms are only half the wideness between the positions open and closed of the jaws, this drive can be substantially included in a cylinder having a diameter comparable to the diameter of the motor group with a planetary reducer coupled to it. On the other hand, the conical gear jointly with the planetary reducer has a very high mechanical efficiency which optimizes to the maximum the motor power and the energy consumption, which allows more operational time with a same battery load. The scissor can incorporate jaws having edges shaped so that it allows to optimize the motor power.  
       [0014] In the example of embodiment provided with single working jaw, the support of the portable scissor is of a single part and comprises a portion for coupling to the power takeoff and a side arm forwardly extended on which are mounted two jaws, a working jaw and a fixed jaw, forming the cutting mouth. On the support, a recess is foreseen for the pinion or the conical gear for the working jaw drive.  
       [0015] The motor is coupled to a reducer the output of which constitutes the said power takeoff, and are provided with means for fastening the motor and reducer group to the said support coupling portion. On one of the internal faces of the said supporting side arm, the fixed jaw is locked and adjacent to this is mounted the working jaw. In the said coupling portion, there exists a passageway through which a conical pinion fixed to the power takeoff axis passes up to an area of the arm adjacent to the coupling portion where is located a recess which provides, at least partly, a space for the conical pinion where this later interlocks with the sector of crown wheel arranged in the power drive part associated to the working jaw.  
       [0016] The example of embodiment of a single working jaw takes profit of the advantages from using a conical gear having simpler mechanisms which means cutting down the production cost and the fact that it is mounted on a support provided with a side arm makes that the said mechanisms are more accessible, providing a greater easiness for mounting, dismounting and servicing.  
       [0017] As well in the example of embodiment with double working jaw as for single working jaw, the rotation of the pinion is guided and supported by a radial bearer which is externally seated on a cylindrical area of the support, and the rotation of each working jaw is guided and supported by a radial bearer and by an axial bearer. In the event of the double moving jaw, a single axial bearer is interposed between both jaws. The radial bearers are preferably simple ball bearers and the axial bearer is preferably a needle bearer and to prevent that dirtiness enters between both jaws up to the axial bearer area, the tool includes an elastic annular sealing element, such as a O-ring, trapped between the fixed and working jaws and surrounding the said axial bearer.  
       [0018] Optionally, arranged on the bottom of an offset housing the axial bearer there is several adjusting washers, of metallic or synthetic material, of one or several thicknesses which together with a given pre-load. of axis tightening, adjust the closing-opening motion of the working jaw.  
       [0019] The tool of this invention comprises a housing fixed to the support which incorporates a handle and controlling means associated to it. The controlling means are located so that they may be driven with the fingers of the same handle which grasps the handle and include at least a driving push-button of the said motor for controlling a closing-opening motion of the working jaw with respect to the fixed jaw and at least a control associated to a safety device.  
       [0020] The tool optionally includes an overload electronic control which when detecting an intensity of current higher than a preestablished threshold, stops the closing-opening cycle of the working jaw and brings it back to an open position,  
       [0021] When both jaws are working jaws, they can be two bits or one bit and a dolly. When only one jaw is a working jaw, it is preferably a bit and the fixed jaw is a dolly, although it could be the contrary or that the working jaw and the fixed jaw are both bits. 
     
    
    
     SHORT EXPLANATION OF THE DRAWINGS  
     [0022] These and other advantages will be apparent from following detailed description of examples of embodiment with reference to the drawings appended in which:  
     [0023]FIG. 1 is a view in perspective, exploded, of a head of a motor-driven scissor according to this invention;  
     [0024]FIG. 2 is a view in perspective of the head of FIG. 1, assembled;  
     [0025]FIGS. 3 and 4 are side elevation views and plan view, respectively, of the head of FIG. 2;  
     [0026]FIG. 5 is a central lengthwise sectional view of the head of FIG. 3 with an enlarged detail;  
     [0027]FIGS. 6 a  y  6   b  are plan view of a variation of the head of FIG. 2 with a model of jaws in open and closed positions, respectively;  
     [0028]FIGS. 7 a  and  7   b  are plan views of same variation of FIGS. 6 a  and  6   b  with another model of jaws in open and closed positions, respectively;  
     [0029]FIG. 8 is a right side elevation view of the portable motor driven scissor in which the stroke lines indicate the profile of the housing configuring the handle:  
     [0030]FIG. 9 is a left side elevation view with the stroke lines indicating the profile of the said housing configuring the handle;  
     [0031]FIG. 10 is a top view taken in the direction of arrow X of FIG. 9 in which the stroke lines indicate the profile of the housing;  
     [0032]FIG. 11 is an enlarged view in lengthwise section taken by a medium plane perpendicular to the cutting plane;  
     [0033]FIG. 12 is an enlarged cross sectional view taken by plane XII-XII of FIG. 8; and  
     [0034]FIG. 13 is a view in perspective of the support of FIGS.  1  to  5 . 
    
    
     DETAILED DESCRIPTION OF EXAMPLES OF PREFERRED EMBODIMENTS  
     [0035] Referring initially to FIGS.  1 - 5 , the portable motor-driven scissor of this invention comprises, according to an example of embodiment, a head essentially formed by a support  1  which supports a pair of moving jaws, which incorporate jaws  3   a ,  3   b . Although the said jaws could be integral with the said jaws, it is preferred that the said jaws are formed, as it is best shown in FIG. 1 by power drive parts  6   a ,  6   b  on which are fixed jaws  3   a ,  3   b , which are independent and can adopt different configurations, anchoring means being provided for locking jaws  3   a ,  3   b  on respective power drive parts  6   a ,  6   b . Both jaws, formed by the power drive parts  6   a ,  6   b  with jaws  3   a ,  3   b  incorporated, are hinged and may rotate about an axis  6  common with respect to support  1  and the power drive parts derive, on opposite sides of the cutting area respect to the said axis  7 , respective lever arms  8   a ,  8   b  carrying the gear wheel sectors  9   a ,  9   b  which interlock with diametrally opposite areas of a same pinion  10  pertaining to a gear drive at the output of a speed reducer  84  connected to an electric motor  73  (not shown). Preferably, the spin axis of pinion  10  orthogonally cuts the spin axis  7  of the jaws and respective jaws  3   a ,  3   b  and levers  8   a ,  9   a ;  8   b ,  9   b . With this arrangement, when rotating the said pinion  10 , both jaws and the respective jaws  3   a ,  3   b  rotate with respect to the said axis  7  a same angle in opposite directions, each making half the travel between an open position and a closed position and vice versa. The scissor includes controlling means of the said motor  73  for displacing the hinged jaws between an open position and a closed position and vice versa, the assembly being housed in a housing which incorporates a handle provided with a controlling trigger (not shown).  
     [0036] Advantageously, the said speed reducer  84  is a reducer having planetary gears with their entering shaft coaxial to its exiting shaft and to the spin axis  10  and the gear between the pinion  10  and the said gear wheel sector  9   a ,  9   b  is an external conic gear, so that the diameter of the pinion  10  is not added to the-radius of the sectors of the gear wheel  9   a ,  9   b , of the jaws lengthwise the scissor. This, together with the fact that each jaw only makes half the total travel, allows that the design is extremely compact and lightweight as well in width as in length, of the head of the portable motor-driven scissor of this invention.  
     [0037] For mounting them, as well the power drive parts.  6   a ,  6   b  as the jaws  3   a ,  3   b  comprise respective passing holes  1 ,  2  coaxial to each other, for axis  7  and mentioned means for anchoring jaws  3   a ,  3   b  to the power drive parts  6   a ,  6   b , comprise at least a fastening point in addition to the said passing holes  1 , 2  for the axis  7 . Preferably, the scissor of the invention includes at least two and more preferably three of the said anchoring points angularly distributed about the passing holes  1 ,  2  for axis  7 . As it can be best see from details on FIG. 5, each of the said anchoring points comprises an externally cylindric centring socket  24 , passed through respective holes  36 ,  37 , coaxial to each other, of the power drive part  6   a ,  6   b  and the jaw  3   a ,  3   b , the said centring socket  24  being provided with a head  25  which is housed in an external offset of jaws,  3   a ,  3   b  and a central threaded hole  26  in which a screw  21  provided with a head  27  is screwed, which remains housed in an external offset of the power drive part  6   a ,  6   b  without protruding from the external surface thereof. Although the presence of the centring socket  24  provides a very accurate centring of the jaws  3   a ,  3   b  with respect to the power drive parts  6   a ,  6   b  at same time it protects the screw thread  21  in the event both parts slide, a simpler mounting would also be possible in which the screws  21  will be directly screwed in threaded holes of the power drive part.  
     [0038] The pinion  10  is mounted on a shaft  11  supported by a bearing  31  housed in an offset  30  of the said support  1  from a end of which frontally protrudes a fork  12  supporting spin axis  7  of the power drive parts  6   a ,  6   b  and respective jaws  3   a ,  3   b , the said fork  12  comprising two arms located on sides diametrally opposite of the pinion  10 , including it. On the other end of the support  1 , motor  73 -reducer  84  are mounted (not shown). Between both arms of the fork  12  are trapped two power drive parts  6   a ,  6   b  with respective jaws  3   a ,  3   b  fastened to them, both jaws  3   a ,  3   b  being leaning against each other, trapping an axial bearing  13  half housed in respective offsets  32  of the jaws  3   a ,  3   b  and axially pressing an O-ring  33  closing the perimeters of the said offsets. Preferably, the said axial bearing  13  is a needle bearing using as thread track the bottoms of the said offsets  32 . However, on the bottom of at least one of the said offsets  32  at least one adjusting washer  34  can be housed to offset possible height differences, in which case it is necessary to have additionally available a supporting washer  13   a  for the bearing  13 . Obviously, the said bearing  13  could be suppressed in a simplified embodiment.  
     [0039] Although it is not strictly necessary, the scissor of this invention comprises respective bearings  14  housed in offsets  38  of the external faces of the power drive parts  6   a ,  6   b , the said bearing being protected by O-rings  35  axially pressed down by covers or enlarged ends of the fork  12  arms at which ends are located holes  20  for the axis  7  which crosses throughout the power drive parts  6   a ,  6   b  and respective jaws  3   a ,  3   b . The figures illustrate the bearings  14  as radial ball bearings, although they could also be another kind of bearings or friction socket.  
     [0040] As it is best shown in FIG. 5 to make its mounting and good operation easier, the axis  7  comprises a socket  15  provided with a head  16  and a cylindric end surface  11  on which are internally adjusted the said holes  20  of the fork  12  arms and the bearings  14  and a threaded central hole  17 , the head of which  16  in operating position, is stopped by the external face of one of the fork  12  arms, while a head  18  of a screw  19  threaded in the said central hole  17  is stopped against the external face of the other fork  12  arm.  
     [0041] Referring now to FIGS. 8, 9 and  10 , the portable motor-driven scissor of this invention comprises, according to another example of embodiment, a support  5  associated to a power takeoff  42  from a motor  43  and a cutting mouth  44  comprising two cutting jaws  3   a ,  3   b , one of which is a working cutting jaw  3   a , mounted so that it rotates on the said support  5  and connected to the output of the said power takeoff  42  by means of a conical gear  47  for performing a closing-opening motion with respect to the other jaw  3   b  which is fixed. The said support  5  includes or has fixed a housing  46  (shown by means of stroke line in the figures) which defines a handle  48  to which are associated means  49 ,  50  controlling the tool. The said motor  43  is coupled to a reducer  54  provided with one output cylindric end from which is protruding an axis  11 , constituting the power takeoff  42 .  
     [0042] Although the example of embodiment with double working jaw illustrated in FIGS.  1  to  7   b  are shown without motor and handle, the arrangement of motor  43 , reducer  54 , housing  54 , handle  48  etc. could be applied to it, which is disclosed referring to the example of embodiment of a single working jaw illustrated in FIGS.  8  to  13 .  
     [0043] The said support  5  which is shown separately in FIG. 13 is of a single piece and comprises a coupling portion  57  to the power takeoff  42  and a side arm  60  forwardly extended on which is mounted the cutting mouth  44  and the mechanism for controlling it.  
     [0044] The said coupling portion  57  is substantially cylindric and comprises a glass-shaped end hollow area  65  within which is arranged a first fastening configuration having the shape of an internal thread  58  which is fastened in a second fastening configuration having the shape of an external thread  62 , interconnected, arranged at the said cylindric output end of the reducer  54 .  
     [0045] On an internal face  64  of the said arm  60  is locked the fixed jaw  3   b , and adjacent to this is mounted the working jaw  3   a . Through the coupling portion  57  there exists a passageway  66  for passing conical pinion  10  fixed to the said axis  11  of the power takeoff  42  until a space located in front of an area were the arm  60  starts, where the said conical pinion  10  interlocks with a sector of crown wheel  9  interconnected integral with the working jaw  5 . The said pinion  10  and crown  9  constitute a conical gear  47 , preferably having a straight dentate and axis orthogonally crossing. As it is shown in FIG. 11, in an internal area of the passageway  66  of the coupling portion  57  an external seat is defined for a radial bearing  30  which is internally seated on a cylindric area of a conical pinion hub  10 . Thanks to it, the stresses radially the axis  11  are transmitted by the said bearing  30  to the support  5  preventing to damage the reducer  54  output internal bearing. The radial bearing  30  is preferably a simple ball bearing.  
     [0046] As it is best shown in FIG. 13, the internal face  64  of the arm  60  has a flat end area, having a circular outline, in which centre there is a hole  67  for mounting an axis which, as disclosed in details below, passes through the fixed jaw  3   b  and acts as a guide for the rotating motion of the working jaw  3   a . At a medium area of the internal face  64  are provided protrusions  69  for fastening on the sides the fixed jaw  3   b , the said protrusions  69  have a height substantially equal to the thickness of the fixed jaw  3   b . In the example illustrated, in an area located between the said protrusions  69 , there exists two holes for screws  70  to fasten the fixed jaw  3   b , although at least one would be sufficient. In an area of the arm  60  adjacent to the coupling portion  57  there is arranged a recess  61  which provides at leat part of the said space for the conical pinion  10 , where it interlocks with the said crown wheel sector  9   a.    
     [0047] To transmit the gear  47  force to the working jaw  3   a , the tool comprises a power drive part  6  provided with a portion substantially circular leaning against the external face of the working jaw  3   a  and crossed by its central area by the said axis  7 . From the said substantially circular portion protrudes a lever arm  8   a  at which end the dentate of the said crown wheel  9   a . At this force drive part  6  a hole  73  is provided (see FIG. 12) which remains facing a corresponding hole of the working jaw  3   a  for housing, between them, a driving pin  74 . However, and depending on the requirements of design, two or more driving pins could be housed in corresponding pairs of holes facing each other located on the drive part  6   a  and the working jaw  3   a.    
     [0048] The axis  7  consecutively crosses the power drive part  6   a , the working jaw  3   a , the fixed jaw  3   b  and the arm  60  of the support  5  acting as hinge axis of the assembly of force drive part  6  and working jaw  3   a , as positioning pin of the fixed jaw  3   b  and as packing and fastening element of the assembly of arm  60  of the support  5 . For this and as it is best shown in FIG. 12, the axis  7  comprises: at one end a head, wide and flat, trapping a radial bearing  14  housed in a recess of an external face of the force drive part  6 ; in a medium area of a cylindric section  83  on which the assembly of the force drive part  6   a  and working jaw  3   a  rotates and which acts as positioning pin for the fixed jaw  3   b ; and on the opposite end, a threaded area  76  which is fixed to a nut  77  which remains embedded in an external face of the arm  60  of the support  5 . Between the working and fixed jaws  3   a  and  3   b , an axial bearing  13  is arranged which remains housed in a recess of the fixed jaw  3   b . The said axial bearing  13  is protected from dirtiness by an annular sealing element  33 , such as an elastic O-ring, trapped between the working and fixed jaws  3   a  and  3   b  surrounding the said axial bearing  13 .  
     [0049] In the example of embodiment illustrated, the said radial bearing  14  is a simple ball bearing and the axial bearing  13  is a needle bearing. The adjustment of the closing and opening motion of the working jaw  3   a  with respect to the fixed jaw  3   b  is carried out by means of one or several adjusting washers  34  arranged on the bottom of the said recess of the fixed jaw  3   b  which houses the axial bearing  13  in combination with a given preload of the axis  7  tightening by means of its threaded area  76 . The said adjusting washers  34  can be of metallic or synthetic material and when there is more than one, they can all be of same thickness or of different thicknesses.  
     [0050] The housing  46 , shown by stroke lines in FIGS.  8  to  10  and to which it has been referred to at the beginning of this description is preferably fixed to the support  5 , although they could form an integral part thereof. The housing  46  incorporates the said handle  48  and the said controlling means  49 ,  50 , which typically include a push-button  49  for driving the said motor  43  and a control  50  associated to a safety device. Each time the said push-button  49  is pushed, it starts a sequence of motions of the motor  43  which results in quick closing-opening motion of the working jaw  3   a  with respect to the fixed jaw  3   b . The portable scissor of this invention also optionally includes an overload electronic control which, when detecting an intensity of the current higher than a preestablished threshold, stops the closing-opening cycle of the working jaw  3   a  and takes it back to an open position.  
     [0051] Although in the example of embodiment illustrated in the figures, the said working jaw  3   a  is actually a jaw and the said fixed jaw  3   b  is actually a dolly, the reversed arrangement could be possible or an arrangement in which the working jaw  3   a  and the said fixed jaw  3   b  are bits, depending on the application wished.  
     [0052] The characteristics of operation of the motor-driven scissor of this invention, common for all the examples of embodiment, are shown with reference to FIGS. 6 a ,  6   b ; and  7   a ,  7   b , in which the head illustrated is analogous in everything to that shown in FIGS.  1 - 5  except in a different distribution of the points anchoring the jaws  3   a ,  3   b  to the power drive parts  6   a ,  6   b  around axis  7 .  
     [0053] Referring now concretely to FIGS. 6 a ,  6   b , the head shows a variation for the two jaws  3   a ,  3   b  which have respective concave edge  28   a ,  28   b , the said concavities, in operation arrangement, remain facing each other and during the cutting operation they tends to trap the object to be cut  4  in a position as close as possible to the axis  7 , optimizing thus the scissor lever arms and with it the consumption of energy for each cut, which contributes to extend the operational time for a same battery load. With this model of jaws, it is essential to have available a sensor system to control the motor  73  operation, for example the intensity of the current consumed by the motor  73 . When the object to be cut  4  is too hard, that it does not allow the normal travel of the jaws  3   a ,  3   b  and the cut progression, the consumption of current by the motor  73  increases beyond a preestablished threshold, the consumption being detected by the sensor system which orders a current switch-off or reversing the rotation senses of the motor  73 , which does not damage it nor the gear drives.  
     [0054] In FIGS. 7 a ,  7   b , the head of the scissor of the invention  9  is showing another variation for the two jaws  3   a ,  3   b , where one of them has a concave edge  29   a  while the other has a convex edge  29   b , so that, in an operating arrangement, the first shows its concavity facing the convexity of the second. This kind of edges makes that when the object to be cut  4 , located on the bottom of the opening between the jaws  3   a ,  3   b  is too hard, it tends to outwardly slide until even being expelled, protecting the mechanism, although the referred system protecting by means of intensity sensors can still be used.  
     [0055] Another, characteristic seeking to optimize the energy consumption consists on end of stroke sensors, typically sensors having Hall effect, which according to this invention carries out motor  73  switch-off during a predetermined period of time before the power drive parts  6   a ,  6   b  and respective jaws  3   a ,  3   b  reached their respective mechanical ends of stroke as a precaution against the displacements which are produced afterwards by inertia during the transitory stopping displacements. Thus, when reaching the end of stroke of closing the device, it does not reverses the motor  73  rotation sense when it is still rotating by inertia en opposite sense, which consumes a peak of energy, but that during the time the motor  73  rotates by inertia, the motor  73  is disconnected and is only connected in opposite sense when it is already stopped.  
     [0056] Last, it must be pointed out that the examples of embodiment disclosed and shown in the figures are for simple illustrative purpose which does not limit the scope of this invention which is defined in appended claims.