Abstract:
A power-driven saw comprising a housing; a blade support; a motor coupled to a drive shaft; a pair of saw blades slideably supported by the blade support; and a drive mechanism for generating reciprocating rectilinear motion of the saw blades in response to rotation of the drive shaft. The drive mechanism comprises a crank assembly with a pair of diametrically-opposed crank pins rotatable about a crank axis in response to rotation of the drive shaft; and a pair of connecting-rods. The drive shaft extends from the motor, between rotational sweep of the crank pins and reciprocating sweep of the drive couplings and to where a bearing fixed in relation to the motor supports the drive shaft for rotation. The connecting rods are shaped to avoid contact with the drive shaft during rotation thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to British Patent Application Number 1205272.6, filed Mar. 26, 2012, which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a portable power-driven saw of the type using reciprocating rectilinear motion to perform cutting operations. 
       BACKGROUND OF THE INVENTION 
       [0003]    A portable power-driven saw is disclosed by U.S. patent publication No. 4,031,622. The power-driven saw comprises a blade support; a motor coupled to a drive shaft rotatable by the motor; a saw blade slideably supported by the blade support for reciprocating rectilinear motion; and a drive mechanism for generating reciprocating rectilinear motion of the saw blade in response to rotation of the drive shaft. The drive mechanism operates like a crank shaft. A link member connects a rotating crank pin to the saw blade. The link member has a pair of arms defining an aperture through which the drive shaft extends. In use, vibrations may propagate from the reciprocating saw blade to the motor and to the power-driven saw&#39;s housing when held by a user. These vibrations may be tiresome to the user over time. The greater the amplitude of the vibrations the less time a portable power-driven saw may be held by the user. An embodiment of the portable power-driven saw of U.S. Pat. No. 4,031,622 has a flexible drive chain to transmit rotation of the drive gear to the driven gear. According to U.S. Pat. No. 4,031,622, the flexible drive chain minimises transmission of vibrations from the saw blade to the motor. 
         [0004]    Another portable power-driven saw is disclosed by U.S. patent publication No. 5,031,324. The power-driven saw comprises a blade support; a motor coupled to a drive shaft rotatable by the motor; a pair of saw blades slideably supported by the blade support for rectilinear motion; and a drive mechanism for generating counter-reciprocating rectilinear motion of the saw blades in response to rotation of the drive shaft. The drive mechanism operates like a scotch yoke. A pair of sliding yokes, each shaped like a cross with a longitudinal slot traversed by a lateral slot, couples the rotating crank pins to a respective saw blade. The drive shaft extends through the longitudinal slots. Each crank pin is mounted in a sliding piece which slides in a respective lateral slot. According to U.S. Pat. No. 5,031,324, the length of a sliding piece corresponds to at least twice the width of a longitudinal slot plus the diameter of an associated crank pin. This is to ensure that the sliding pieces are reliably guided and do not tilt in the longitudinal direction in the transition region where the lateral and longitudinal slots intersect. Otherwise, the crank pins would not reliably traverse the transition region of the slots alone. The sliding pieces introduce additional components. The sliding pieces and lateral slots are made to tight tolerances to ensure smooth operation. This may increase manufacturing cost of the power-driven saw. The sliding pieces and lateral slots are subject to rapid wear by sliding friction and high contact pressures. This may reduce the service life of the power-driven saw, especially if the power-driven saw is used in a dusty environment as is commonly the case with saws. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a portable power-driven saw comprising: a housing; a blade support fixed in relation to the housing; a motor coupled to a drive shaft rotatable by the motor; a pair of saw blades slideably supported by the blade support for reciprocating rectilinear motion relative thereto wherein each saw blade has a respective drive coupling disposed near or at one end of the saw blade; and a drive mechanism for generating reciprocating rectilinear motion of the saw blades in response to rotation of the drive shaft, wherein the drive mechanism comprises: a crank assembly rotatable about a crank axis wherein the crank assembly has a pair of axially-spaced substantially diametrically-opposed crank pins rotatable about the crank axis in response to rotation of the drive shaft; and a pair of connecting-rods each connecting a respective crank pin to the drive coupling of a respective saw blade to impart reciprocating motion to each saw blade in response to rotation of the respective crank pin about the crank axis, wherein the drive shaft extends from the motor on one side of the drive mechanism, through a region between rotational sweep of the crank pins and reciprocating sweep of the drive couplings and to a second opposite side of the drive mechanism where a bearing fixed in relation to the motor supports the drive shaft for rotation and wherein the connecting rods are shaped to avoid contact with the drive shaft during rotation thereof. 
         [0006]    The present invention provides a compact and manoeuvrable design of portable power-driven saw by locating the bulk of the motor, and its drive shaft, in a central region of the saw&#39;s housing while its counter-reciprocating rectilinear cutting motion may provide approximately double the output of the power-driven saw disclosed by U.S. Pat. No. 4,031,622 with only a marginal increase in the overall height of the drive mechanism. Also, the drive mechanism of the present invention does not feature tight tolerances between moving parts such as the sliding pieces and lateral slots of U.S. Pat. No. 5,031,324. This may reduce wear caused by sliding friction with high contact pressures which may prolong the service life of the portable power-driven saw of the present invention. Vibration caused by each of the counter-reciprocating saw blades of the present invention tends to counter-act each other. This reduces vibration without dedicated anti-vibration features in the drive mechanism such as the flexible drive chain of U.S. Pat. No. 4,031,622. This may simplify the design and assembly of the portable power-driven saw according to the present invention. 
         [0007]    Preferably, the drive shaft has at least one drive gear rotatable with the drive shaft, wherein the crank assembly comprises a first driven gear and a second driven gear, wherein each driven gear independently rotatable about the crank axis on a respective driven gear axle supported by a respective bearing, wherein each driven gear has a respective one of the crank pins connected to a first non-toothed side of the driven gear and a respective driven gear axle connected to a second opposite non-toothed side of the driven gear, wherein the at least one drive gear meshes with the first and second driven gears to transmit rotation of the drive shaft to the crank pins. The nature of the power-driven saw&#39;s operation is such that there are sharp counter-reciprocating forces acting along the saw blades which are transmitted to the drive mechanism. Independent driven gears, each having its own crank pin and driven gear axle, divides the crank axis and diminishes counter-rotational torque which may otherwise be transmitted along, and cause fatigue in, an integral crank axis such as a crank shaft. Also, the separate driven gears facilitate manufacture and repair of the power-driven saw by constructing the drive mechanism from readily assembled parts. 
         [0008]    Preferably, the first non-toothed side of each driven gear face each other. As a result, the crank pins are sandwiched between the driven gears. This facilitates construction of the drive mechanism by making it easier to position the crank pins relative each other because they can be observed at the same time. 
         [0009]    Preferably, each connecting-rod comprises a pair of arms spaced apart to define a central aperture therebetween, wherein each arm extends between a respective crank pin at a first end thereof and a respective drive coupling at a second end thereof and wherein the drive shaft extends through the central apertures. Having two arms adds rigidity to the connecting-rods. Forces acting along the connecting-rod are divided and balanced substantially equally between the arms, thereby reducing the risk of fatigue. 
         [0010]    Preferably, the blade support, the crank axis and an axis of the drive shaft are substantially located in a central plane of the power-driven saw with one of the drive couplings disposed on each side of the central plane, wherein one of the arms of each connecting-rod is shorter than the other of the arms and wherein the shorter arm of each connecting-rod is disposed on the same side of the central plane as the drive coupling connected to said connecting-rod when the crank pins are substantially located in the central plane. The longer of the arms of each connecting-rod arches around the drive shaft and traverses the central plane to where it connects to the drive coupling of a respective saw blade. The shorter of the arms arches around the drive shaft but it does not traverse the central plane. The connecting-rods are opposite and asymmetric about the central plane. In use, each shorter arm&#39;s lateral sweep is closer to the central plane than it would be if the arms of each connecting-rod were a mirror image of each other. This reduces the width of the drive mechanism which may diminish the lateral profile of the power-driven saw&#39;s housing. 
         [0011]    Preferably, the central plane is substantially equidistant between the outermost edges of the arms of each connecting-rod when the crank pins occupy the central plane. The lateral sweep of each connecting-rod is substantially equal on both sides of the central plane. This minimizes the space occupied by the drive mechanism within the housing of the power-driven saw. 
         [0012]    Preferably, the arms of the connecting-rods are arched in a smooth curve about the drive shaft. The smooth curve of the arms may be any curved profile without notches or corners to act as stress concentration points. 
         [0013]    A drive coupling may be any feature capable of coupling one end of a saw blade to a respective connecting-rod. A drive coupling may be integral with the saw blade, for example. Preferably, each drive coupling comprises a shaft coupled to a respective connecting-rod at a first end thereof and coupled to a respective saw blade at a second end thereof. The shafts provide an interface between the connecting-rods inside the housing and the saw blades outside the housings. The drive mechanism and motor may be a self-contained unit shielded within the housing from dust and debris which may impair functioning of the drive mechanism over time. 
         [0014]    Preferably, each shaft is supported for sliding rectilinear motion by a bearing fixed in relation to the housing and wherein the first end of each shaft is pivotally coupled to a respective connecting-rod. The counter-reciprocating rectilinear motion delivered by the drive mechanism may provoke vigorous movement, especially when the saw blades are cutting. The bearing provides additional structural support to the drive mechanism. 
         [0015]    Preferably, the second end of each shaft comprises a blade clamp detachably connected to a respective saw blade. The blade clamp facilitates renewal of the saw blades when damaged or worn. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A specific embodiment of the present invention will now be described with reference to the accompanying drawings of which: 
           [0017]      FIG. 1  shows a side elevation view of a power-driven saw with counter-reciprocating rectilinear cutting motion; 
           [0018]      FIG. 2  shows a side elevation view of a pair of saw blades each with a convex array of cutting teeth, a blade support and a drive mechanism of the power-driven saw of  FIG. 1 ; 
           [0019]      FIG. 3  shows a perspective view of the electric motor and the drive mechanism of  FIG. 2 ; 
           [0020]      FIG. 4  shows a side elevation view of the electric motor and the drive mechanism of  FIG. 2 ; 
           [0021]      FIG. 5  shows a cross-section V-V of the drive mechanism of  FIG. 4 ; 
           [0022]      FIG. 6  shows a cross-section VI-VI of the drive mechanism of  FIG. 4 ; 
           [0023]      FIG. 7  shows a cross-section VII-VII of the one of the saw blades and the blade support of  FIG. 1 ; 
           [0024]      FIG. 8  shows a cross-section VIII-VIII of one saw blade and one side of the blade support of  FIG. 7 ; and 
           [0025]      FIG. 9  shows a side elevation view of a saw blade with a concave array of cutting teeth. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring to  FIG. 1 , a power-driven saw  2  comprises a housing  4 , an elongate saw blade support  6  and a pair of saw blades  8   a,    8   b  for counter-reciprocating rectilinear cutting motion in the direction of double-headed arrow A. The housing comprises a front handle  10  and a rear handle  12  each to be grasped by a user of the power-driven saw. The housing encloses a drive mechanism  14 , a mains electrical power supply cable  16  depending from the rear of the housing, an on/off trigger switch  18  protruding from inside the rear handle and an electric motor  20  fixed to the interior of the housing. The blade support is fixed to the housing at a location below the front handle. The saw blades are supported for sliding motion under the blade support. The saw blades are coupled to the drive mechanism. 
         [0027]    In the present embodiment, the electric motor  20  provides the power-saw  2  with a drive device, although such a power tool may alternatively have a combustion engine or an electric motor powered by batteries, the choice of which depends on the environment in which the power tool is designed to be used. Referring to  FIGS. 2 to 4 , the electric motor  20  is coupled to a drive shaft  22  with a longitudinal central axis  23 . Two gears are attached to the drive shaft: an upper drive gear  24   a  spaced axially apart from a lower drive gear  24   b.  A free end  26  of the drive shaft  22  is supported for rotation by a bearing  28  connected to the housing. This is to prevent deflection of the drive shaft  22  when under load. 
         [0028]    The drive mechanism  14  comprises a pair of independent driven gears: an upper driven gear  30   a  spaced axially apart from a lower driven gear  30   b.  The upper driven gear has a toothed circumference  32   a  meshing with the upper drive gear  24   a.  The lower driven gear has a toothed circumference  32   b  meshing with the lower drive gear  24   b.    
         [0029]    The upper driven gear  30   a  rotates about a central upper driven gear axle  34   a.  The upper driven gear  30   a  has an upper crank pin  36   a  disposed eccentricly with regard to the upper driven gear axle  34   a.  The upper crank pin  36   a  protrudes from a first non-toothed side of the upper driven gear  30   a  facing towards the lower driven gear  30   b  and the upper driven gear axle  34   a  protrudes from a second opposite non-toothed side of the upper driven gear  30   a.  The upper driven gear axle  34   a  is supported for rotation by a bearing  38   a  connected to the housing  4 . A first end of an upper connecting-rod  40   a  is pivotally coupled to the upper crank pin  36   a.  A second opposite end of the upper connecting-rod  40   a  is pivotally coupled to an upper pivot  41   a  at a first end of an upper drive coupling in the form of an upper shaft  42   a  which is supported for reciprocating rectilinear motion by a bearing  44  connected to the housing  4 . A second opposite end of the upper shaft  42   a  comprises a left blade clamp, as is described in more detail below. 
         [0030]    The lower driven gear  30   b  rotates about a central lower driven gear axle  34   b  which is concentric with the upper driven gear axle  34   a.  The driven gear axles  34   a,    34   b  each form independent parts of a crank axis  45 . The lower driven gear  30   b  has a lower crank pin  36   b  disposed eccentricly with regard to the lower driven gear axle  34   b.  The lower crank pin  36   b  protrudes from a first non-toothed side of the lower driven gear  30   b  facing towards the upper driven gear  30   a  and the lower driven gear axle  34   b  protrudes from a second opposite non-toothed side of the lower driven gear. The lower driven gear axle  34   b  is supported for rotation by a bearing  38   b  connected to the housing  4 . A first end of a lower connecting-rod  40   b  is pivotally coupled to the lower crank pin. A second opposite end of the lower connecting-rod  40   b  is pivotally coupled to a lower pivot  41   b  at a first end of a drive coupling in the form of a lower shaft  42   b  which is supported for reciprocating rectilinear motion by the bearing  44 . A second opposite end of the lower shaft  42   b  comprises a right blade clamp, as is described in more detail below. 
         [0031]    The left and right blade clamps may be any feature which is capable of releasably fixing one end of the saw blades  8   a,    8   b  to the upper and lower shafts  42   a,    42   b,  respectively, for saw blade renewal. In the present embodiment, the left blade clamp comprises a vertical slit  46   a  in the second end of the upper shaft  42   a.  The slit  46   a  is traversed by a hole  47   a  which is threaded on one side of the slit  46   a  to engage a threaded clamp fastener  48   a.  The clamp fastener  48   a  passes through an eyelet (not shown) in one end of the left saw blade  8   a  when located in the slit  46   a.  Tightening rotation of the clamp fastener  48   a  clamps the left saw blade  8   a  to the upper shaft  42   a.  The right blade clamp comprises a slit  46   b  in the second end of the lower shaft  42   b.  The slit  46   b  is traversed by a hole  47   b  which is threaded on one side of the slit  46   b  to engage a threaded clamp fastener  48   b.  The clamp fastener  48   b  passes through an eyelet (not shown) in one end of the right saw blade  8   b  when located in the slit  46   b.  Tightening rotation of the clamp fastener  48   b  clamps the right saw blade  8   b  to the lower shaft  42   b.    
         [0032]    Referring to  FIGS. 5 and 6 , the power-driven saw  2  has a longitudinal central plane  43  which is coplanar with the saw blade support  6  and which bisects the housing  4 . The drive shaft central axis  23  is located in the central plane  43 . The upper connecting-rod  40   a  comprises a pair of arms  50   a,    52   a  spaced, or bowed, apart to define a central aperture  54   a  through which passes the drive shaft  22  without touching either of the arms  50   a,    52   a.  The arms have different lengths such that the upper connecting-rod is asymmetrical. When viewed from above the central plane, as shown in  FIG. 5 , the shorter arm  50   a  is disposed on the same side as the upper shaft  42   a  when the upper crank pin  36   a  is located in the central plane  43 . 
         [0033]    The lower connecting-rod  40   b  comprises a pair of arms  50   b,    52   b  spaced, or bowed, apart to define a central aperture  54   b  through which passes the drive shaft  22  without touching either of the arms  50   b,    52   b.  The arms have different lengths such that the lower connecting-rod is asymmetrical. When viewed from above the central plane, as shown in FIG.  5 , the shorter arm  50   b  is disposed on the same side as the lower shaft  42   b  when the lower crank pin  36   b  is located in the central plane  43 . 
         [0034]    In use, the trigger switch  18  is depressed and the electric motor  20  energized by electricity fed by the mains electrical power supply cable  16 . The motor rotates the drive gears  24   a,    24   b  which rotate the driven gears  30   a,    30   b  in the same direction as each other. Rotation of the driven gears is translated, via the connecting-rods  40   a,    40   b,  into reciprocating rectilinear motion of the shafts  42   a,    42   b.  The upper crank pin  36   a  is located on the upper driven gear  30   a  at a location diametrically-opposed to where the lower crank pin  36   b  is located on the lower driven gear  30   b.  Movement of the upper connecting-rod  40   a  and upper shaft  42   a  are 180 degrees out of phase with the lower connecting-rod  40   b  and lower shaft  42   b  and, as a result, the rectilinear motion of the shafts  42   a,    42   b,  and the saw blades  8   a,    8   b  connected thereto, is counter-reciprocating. The driven gears, the crank pins and the connecting-rods operate as a crank assembly. The drive shaft  22  extends through a region between the rotational sweep of the crank pins  36   a,    36   b  and the reciprocating sweep of the shafts  42   a,    42   b.  The arms  50   a,    50   b,    52   a,    52   b  are shaped to avoid contact with the drive shaft  22  all the while the connecting-rods  40   a,    40   b  are rotating and reciprocating back and forth. 
         [0035]    Referring to  FIGS. 7 and 8 , the blade support  6  comprises an elongate planar blade support body  56  coplanar with the central plane  43 . A lower edge of the blade support body  56  is bifurcated by a pair of shoulders  58   a,    58   b.  The blade support  6  comprises a pair of parallel mutually spaced rims  60   a,    60   b  arranged along the lower edge of the blade support body  56 . As viewed in  FIG. 7 , the left shoulder  58   a  is clad with the left rim  60   a  and the right shoulder  58   b  is clad with the right rim  60   b.  The left rim and shoulder, on one side, and the right rim and shoulder, on the other side, define a slot  62  along the lower edge of the blade support body  56 . 
         [0036]    Each saw blade  8   a,    8   b  comprises an elongate body  64   a,    64   b  with an array of cutting teeth  66  arranged along a respective convex lower edge  68   a,    68   b.  In use, an upper edge  70   a,    70   b  of each respective saw blade  8   a,    8   b  is located in the slot  62  of the blade support  6 . The first end of each saw blade  8   a,    8   b  is coupled to the drive mechanism  14  by a respective blade clamp in the manner already described above. A second opposite end of each saw blade  8   a,    8   b  has a respective mouth  72  facing away from the drive mechanism  14 . Each mouth  72  surrounds a lateral pin  74  spanning the slot  62  in the blade support. The lateral pin is orientated perpendicularly to the central plane  43 . Each mouth  72  is long enough to accommodate the lateral pin  74  through the full sweep of reciprocating rectilinear motion of the saw blades. As a result, the upper edges  70   a,    70   b  remain in the slot  62  while the saw blades  8   a,    8   b  are coupled to their respective blade clamps. 
         [0037]    For the purpose of saw blade  8   a,    8   b  renewal, the upper edges  70   a,    70   b  of the saw blades  8   a,    8   b  may be removed from the slot  62 . Using the left saw blade  8   a  as an example, initially, the left blade clamp is released by loosening rotation of the clamp fastener  48   a  and removing it from the hole  47   a  so that the saw blade  8   a  is free to pivot about the lateral pin  74 , out from the slot  46   a  and away from the blade support  6 . Next, the left saw blade  8   a  is slid towards the drive mechanism  14 , the lateral pin  74  leaves the mouth  72  of the left saw blade  8   a,  and the left saw blade  8   a  is free. A corresponding process is followed to release the right saw blade  8   b.    
         [0038]    Referring in particular to  FIG. 8 , and again using the left saw blade  8   a  as an example, the upper edge  70   a  of the left saw blade  8   a  is shaped as a smooth undulating wave, the peaks and troughs of which match the correspondingly shaped left shoulder  58   a  of the blade support body  56 . In use, the upper edge  70   a  slides against the shoulder  58   a.  Contact between the upper edge  70   a  and shoulder  58   a  varies between peak-to-peak contact, and peak-to-trough contact, as the left saw blade  8   a  performs reciprocating rectilinear motion. The left saw blade  8   a  and the blade support body  56  are closest at peak-to-tough contact and, conversely, they are furthest apart at peak-to-peak contact. The relative movement between left saw blade  8   a  and blade support body  56  superimposes a percussive motion upon the left saw blade  8   a  in a direction approximately perpendicular to the reciprocating rectilinear motion and indicated by a double-headed arrow B. The upper edge  70   b  of the right saw blade  8   b  is also shaped as a smooth undulating wave, the peaks and troughs of which match the correspondingly shaped right shoulder  58   b  of the blade support body  56 . Consequently, a percussive motion, also in the direction of double-headed arrow B, is superimposed upon the right saw blade  8   b.  The percussive motion increases downward cutting force of the saw blades and helps dislodge and clear debris between the cutting teeth  66  thereby improving the cutting speed of the power driven saw  2 . 
         [0039]    While the upper edges  70   a,    70   b  and shoulders  62   a,    62   b  have the same waveform, the peaks of the left shoulder  58   a  are adjacent the troughs of the right shoulder  58   b,  and vice versa. The direction of the percussive force on the left blade  8   a  is 180 degrees out of phase with the percussive force on the right blade  8   b.  One blade moves downward while the other moves upward, and vice versa. The opposing percussive motion of one saw blade in relation to the other saw blade helps balance and cancels-out forces acting in the direction of arrow B and, in doing so, reduces vibration in the power-driven saw  2 . 
         [0040]    Referring to  FIG. 9 , the power-driven saw may be modified with a different pair of saw blades  108 . Each saw blade  108  comprises an elongate body  164  with an array of cutting teeth  166  arranged along a concave lower edge  168 . Otherwise, the concave saw blade  108  is the same as the convex saw blades  8   a,    8   b  described above and the same feature numbers are used to identify the same features. The arched lower edges  68 ,  168  of the convex saw blades  8   a,    8   b  and the concave saw blades  108  introduce a downward component to the reciprocating rectilinear motion at the contact point between the cutting teeth  66 ,  166  and a work-piece. This increases downward cutting force of the saw blades thereby improving the cutting speed of the power driven saw  2 .