Abstract:
A power driver may include a motor that rotates upon receiving an input current. A tool chuck may have chuck jaws to hold an accessory. The tool chuck may be coupled to the motor. A power take off mechanism may be connected between the motor and the tool chuck. The power take off mechanism may be adjustable into a CHUCK MODE to one of open and close the chuck jaws while the motor rotates. An electronic clutch may interrupt the input current to the motor if the power take off mechanism is in the CHUCK MODE and if the input current exceeds a trip value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This U.S. non-provisional application claims priority under 35 USC §119 to U.S. Provisional Application No. 60/672,504 filed Apr. 19, 2005, the content of which is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     Example, non-limiting embodiments of the present invention relate in general to an electronic clutch for a power driver, and more particularly to an electronic clutch that may be operational when a tool chuck is actuated (i.e., to open or close the chuck jaws) via power from the power driver&#39;s transmission.  
         [0004]     2. Description of Related Art  
         [0005]     A power driver may have a multi-speed transmission for rotationally driving the tool chuck at various operating speeds. A user may select the operating speed of the power driver via a speed selector.  
         [0006]     The power driver may also include an electronic clutch that may interrupt the power supply to the motor of the driver when the torque applied to the motor armature shaft exceeds a trip torque. Here, the current passing through the motor coils may be sensed and used as an indicator of torque performance. Such electronic clutches are conventionally known in this art. For example, a representative electronic clutch is described in U.S. Pat. No. 4,503,370, the entire contents of which are incorporated herein by reference. The electronic clutch described in U.S. Pat. No. 4,503,370 may have a plurality of operating levels. The operating levels may respectively correspond to the operating speeds of the power driver. That is, the trip torque of the electronic clutch may be set depending on the user selected operating speed of the power driver.  
         [0007]     The power driver may also include a tool chuck with chuck jaws that may be actuated (i.e., opened and closed) via power from the transmission. Such chuck actuation may be referred to as a power take off (“PTO”) feature. The PTO feature (when closing the chuck jaws) may result in the chuck jaws applying a more than desirable clamping force. Also, the motor of the power driver may be wound to give greater performance (e.g., more torque) when driven in a forward direction than in a reverse direction. This may cause the tool chuck to be tightened to a more than desirable level.  
         [0008]     Tool chucks with various PTO features are described in commonly-assigned, Copending Provisional Application entitled “TOOL CHUCK WITH POWER TAKE OFF AND DEAD SPINDLE FEATURES,” filed Apr. 19, 2005, Attorney Docket No. 0275L-001056/US/PS1 (the “Copending Provisional Application”). The disclosed example embodiments of the Copending Provisional Application are set forth below in sections I-V.  
         [0009]     I. Example Embodiment Depicted in FIGS.  1 - 4 :  
         [0010]      FIG. 1  schematically shows an example, non-limiting embodiment of a tool chuck  50  that may be provided on a power driver (e.g., a drill) for holding an accessory (e.g., a drill bit). It will be appreciated, however, that the tool chuck  50  may be suitably implemented on a variety of power drivers (other than drills) for holding a variety of accessories (other than drill bits).  
         [0011]     The tool chuck  50  may be connected to the transmission  70  of a power driver via a power take off (“PTO”) mechanism  10 . The transmission  70  may be coupled to an electric motor  90 . The transmission  70  may use gearing to effect a change in the ratio between an input rpm (from the electric motor  90 ) and an output rpm (delivered to the tool chuck  50 ).  
         [0012]     In this example embodiment, the transmission  70  may include three planetary reduction systems. It will be appreciated, however, that the invention is not limited in this regard. For example, more or less than three planetary reduction systems may be implemented. Further, transmissions other than planetary reduction system transmissions (e.g., conventional parallel axis transmissions) may be suitably implemented. Planetary reduction transmissions are well known in this art, and therefore a detailed discussion of the same is omitted. The PTO mechanism  10  may be provided at the output of the transmission  70 .  
         [0013]     A. The Structure:  
         [0014]      FIG. 2  is an exploded perspective view of the PTO mechanism  10 . In this example embodiment, the PTO mechanism  10  may include a shift ring  12 , an output coupling  20  and a PTO drive disk  30 .  
         [0015]     The shift ring  12  may have a radial inward facing surface provided with splines  13  (for selectively engaging with the output coupling  20 , the PTO drive disk  30  and a disk  74  of the third stage carrier  72 ). The shift ring  12  may have a radial outward facing surface provided with forwardly extended splines  15  and rearwardly extended splines  16  (for selective engaging with a housing of the driver, not shown) and a continuous circumferential groove  17  (for accommodating a wire  18 ).  
         [0016]     The wire  18 , which may be slidable through the circumferential groove  17 , may have free ends that extend in a radial direction and out of the circumferential groove  17 . The fee ends of the wire  18  (serving as cam followers) may be received in a slot of a shift collar (not shown in  FIG. 2 ) rotatably mounted on the driver housing. Upon rotating the shift collar, the slot may influence the cam followers (and thus the shift ring  12 ) to the desired axial positions, as will be discussed in more detail below.  
         [0017]     The output coupling  20  may include a central aperture  22  having a shape that corresponds to the shape of an input shaft  60  (not shown in  FIG. 2 ), discussed in more detail below. The output coupling  20  may have a radial outward facing surface provided with splines  24  that selectively cooperate with the radial inward facing splines  13  of the shift ring  12 .  
         [0018]     The PTO drive disk  30  may include a central aperture  32  having a shape that corresponds to the shape of a PTO actuator shaft (not shown in  FIG. 2 ), discussed in more detail below. The PTO drive disk  30  may have a radial outward facing surface provided with splines  34  that selectively cooperate with the radial inward facing splines  13  of the shift ring  12 . The PTO drive disk  30  may have an axial rearward facing surface provided with clutch features  36 . In this example embodiment, the clutch features  36  may be in the form of elongated projections that extend in a radial fashion across the axial rearward facing surface of the PTO drive disk  30 .  
         [0019]     The disk  74  of the third stage carrier  72  may include a central aperture  76  that extends axially through the third stage carrier  72 . The disk  74  may have a radial outward facing surface provided with splines  78  that selectively cooperate with the radial inward facing splines  13  of the shift ring  12 . The disk  74  may also include an axial forward facing surface provided with clutch features  79 . In this example embodiment, the clutch features  79  may be in the form of elongated projections that extend in a radial fashion across the axial forward facing surface of the disk  74 . The clutch features  79  of the disk  74  may cooperate with the clutch features  36  of the PTO drive disk  30 . As is well known in this art, the third stage carrier  72  may include shafts  80  that rotatably support planetary gears (not shown).  
         [0020]      FIG. 3  is a sectional perspective view of the PTO mechanism  10  assembled together with the tool chuck  50 . Here, the shift ring  12  is shown in phantom for clarity.  
         [0021]     The tool chuck  50  may include an input shaft  60 . A forward end of the input shaft  60  may support a nose portion (not shown) that may include passageways through which chuck jaws (not shown) are respectively slidable. The passageways of the nose portion may rotationally fix the input shaft  60  to the chuck jaws. The input shaft  60  may have a rear end that extends through the central aperture  22  of the output coupling  20 . The rear end of the input shaft  60  may have a radial outward facing surface provided with features that cooperate with corresponding features provided on the radial inward facing surface defining the central aperture  22  so that the input shaft  60  may be rotationally locked to the output coupling  20 . Such features are well known in this art. By way of example only, the input shaft  60  may be provided with flats against which flats of the central aperture  22  may abut to rotationally lock together the input shaft  60  and the output coupling  20 . The input shaft  60  may include a through bore  62 . The through bore  62  may rotatably support a chuck actuating shaft  64 .  
         [0022]     The chuck actuating shaft  64  may include a through bore  66 . The through bore  66  may have a rear end receiving a PTO actuator shaft  40 . The rear end of the through bore  66  and the PTO actuator shaft  40  may have corresponding shapes to rotationally fix the chuck actuating shaft  64  to the PTO actuator shaft  40 . The forward end of the through bore  66  may be provided with radial inward facing threads  68  that may interact with radial outward facing threads  58  of a chuck actuating screw  55 . That is, the chuck actuating shaft  64  may be screw coupled to the chuck actuating screw  55 .  
         [0023]     The chuck actuating screw  55  may include radial passageways  56  through which the chuck jaws are respectively slidable. The radial passageways  56  may rotationally fix the chuck actuating screw  55  to the chuck jaws. The interaction between the threads  58  and  68  may cause the chuck actuating screw  55  to advance and retract in the axial direction relative to the input shaft  60 . It will be appreciated that the chuck actuating screw  55  and input shaft  60  may be rotationally locked together via the chuck jaws.  
         [0024]     The PTO actuator shaft  40  extends through the through bore  66  of the chuck actuating shaft  64 , the central aperture  32  of the PTO drive disk  30  and the central aperture  76  of the disk  74 . A keeper  41  (in the form of a snap ring, for example) may be mounted on the PTO actuator shaft  40 . A spring  44  may be mounted on the PTO actuator shaft  40  and compressed between the third stage carrier  72  and the keeper  41 . The PTO actuator shaft  40  may support another keeper (not shown for clarity) via a slot located axially forward of the PTO drive disk  30 . As noted above, the PTO actuator shaft  40  may have a shape that corresponds to the shape of the central aperture  32  of the PTO drive disk  30 . In this way, the PTO actuator shaft  40  may be rotationally fixed to the PTO drive disk  30 .  
         [0025]     As shown in  FIG. 3 , the output coupling  20 , the PTO drive disk  30  and the disk  74  of the third stage carrier  72  may be assembled together in a coaxial fashion. Here, the clutch features  36  of the PTO drive disk  30  may face (and engage with) the clutch features  79  of the disk  74 . Also, the shift ring  12  (shown in phantom) may be mounted for axial movement so that the radial inward facing splines  13  of the shift ring  12  may selectively engage with the radial outward facing splines  24  of the output coupling  20 , the radial outward facing splines  34  of the PTO drive disk  30  and the radial outward facing splines  78  of the disk  74 .  
         [0026]     B. The Operation:  
         [0027]     The tool chuck  50  may operate differently depending on the axial position of shift ring  12 , which may assume three different operating positions inclusive of a MANUAL OVERRIDE MODE, a DRILL/DRIVE MODE and a CHUCK MODE.  
         [0028]      FIG. 3  illustrates the shift ring  12  in the MANUAL OVERRIDE MODE, in which the shift ring  12  may be located at an axial rearward position. Here, the radial outward facing splines  16  of the shift ring  12  may engage with corresponding features provided on the driver housing (not shown). Thus, the shift ring  12  may be rotationally fixed (or grounded) to the driver housing. The radial inward facing splines  13  of the shift ring  12  may engage with the radial outward facing splines  34  of the PTO drive disk  30  and the radial outward facing splines  78  of the disk  74 . Thus, the shift ring  12 , the PTO drive disk  30  (and therefore the PTO actuator shaft  40 ) and the disk  74  (and therefore the third stage carrier  72 ) may be rotationally grounded to the driver housing. In this condition, the output coupling  20  and the input shaft  60  may remain rotatable relative to the driver housing.  
         [0029]     A user may grasp and manually rotate the input shaft  60  (together with the chuck jaws and the chuck actuating screw  55 ) relative to the driver housing. The chuck actuating screw  55  may rotate relative to the chuck actuating shaft  64 , which may be rotationally fixed to the PTO actuator shaft  40  (and therefore may be rotationally grounded to the driver housing). This relative rotation may cause the chuck actuating screw  55  to advance or retract in the axial direction (depending on the rotation direction of the input shaft  60 ) by virtue of the interaction between the radially inward facing threads  68  and the radially outward facing threads  58 . The translational movement of the chuck actuating screw  55  may push or pull on the chuck jaws to open or close the same.  
         [0030]     For example, during a closing operation, the chuck actuating screw  55  (together with the chuck jaws) may be advanced in the axial direction. During this time, the passageways of the nose portion of the input shaft  60  may influence the chuck jaws  2  in a radial inward direction through the radial passageways  56  of the chuck actuating screw  55 . This pusher type jaw action is well known in the pertinent art.  
         [0031]     The DRILL/DRIVE MODE may be achieved by sliding the shift ring  12  forward (from its position in the MANUAL OVERRIDE MODE) to an intermediate axial position. Here, the shift ring  12  may be disengaged from (and rotatable relative to) the driver housing. The radial inward facing splines  13  of the shift ring  12  may engage with the radial outward facing splines  24  of the output coupling  20 , the radial outward facing splines  34  of the PTO drive disk  30  and the radial outward facing splines  78  of the disk  74 . Thus, the shift ring  12 , the output coupling  20  (and therefore the input shaft  60 ), the PTO drive disk  30  and the disk  74  (and therefore the third stage carrier  72 ) may be rotationally fixed together and rotatable as a unit. Since the PTO drive disk  30  (and therefore the PTO actuator shaft  40  and the chuck actuating shaft  64 ) and the output coupling  20  (and therefore the input shaft  60  and the chuck actuating screw  55 ) may be rotationally locked together, the tool chuck  50  may not loosen during operation. A user may then power up the driver to rotationally drive the tool chuck  50 .  
         [0032]     The CHUCK MODE may be achieved by sliding the shift ring  12  (from its position in the DRILL/DRIVE MODE) to a forward axial position. Here, the radial outward facing splines  15  of the shift ring  12  may engage with corresponding features provided on the driver housing. Thus, the shift ring  12  may be rotationally grounded to the driver housing. The radial inward facing splines  13  of the shift ring  12  may engage with the radial outward facing splines  24  of the output coupling  20 . Thus, the shift ring  12  and the output coupling  20  (and therefore the input shaft  60  and the chuck actuating screw  55 ) may be rotationally grounded to the driver housing. Here, the PTO drive disk  30  (and therefore the PTO actuator shaft  40  and the chuck actuating shaft  64 ) and the disk  74  (and therefore the third stage carrier  72 ) may remain rotatable relative to the driver housing.  
         [0033]     A user may then power up the driver to actuate the tool chuck  50 . At this time, the third stage carrier  72  may rotationally drive the PTO drive disk  30  via the cooperating clutch features  79  and  36  respectively provided on the confronting surfaces of the disk  74  and the PTO drive disk  30 . The PTO drive disk  30  may rotationally drive the PTO actuator shaft  40 , which in turn may rotationally drive the chuck actuating shaft  64 . The chuck actuating shaft  64  may rotate relative to the chuck actuating screw  55 , which may remain rotationally grounded to the driver housing (via the chuck jaws, the input shaft  60 , the output coupling  20  and the shift ring  12 ). This relative rotation may cause the chuck actuating screw  55  to advance or retract in the axial direction (depending on the rotation direction of the chuck actuating shaft  64 ) by virtue of the interaction between the radial inward facing threads  68  and the radial outward facing threads  58 . The translational movement of the chuck actuating screw  55  may push or pull on the chuck jaws to open or close the same.  
         [0034]     During chuck actuation, the input shaft  60 , the chuck jaws and the chuck actuating screw  55  may remain rotationally grounded to the driver housing, while the chuck actuating screw  55  may move axially (via the rotational movements of the chuck actuating shaft  64 ) relative to the input shaft  60  to open and close the chuck jaws. This may be referred to as a dead spindle feature since the user may not be exposed to (or observe) any rotating parts.  
         [0035]     Once the tool chuck  50  is tight (i.e., when the chuck jaws clamp the accessory) or fully opened, the cooperating clutch features  79  and  36  respectively provided on the confronting surfaces of the disk  74  and the PTO drive disk  30  may give way and slip relative to each other. At this time, the disk  74  (together with the third stage carrier  72 ) may move in an axial rearward direction against the influence of the spring  44 . When the cooperating clutch features  79  and  36  slip, they may produce an audible indication that the chuck actuation process is complete.  
         [0036]     The cooperating clutch features  79  and  36  may give way or slip at a predetermined torque threshold. The predetermined torque threshold may be suitably adjusted by selecting an appropriate spring  44  and/or by suitably designing the geometries of the cooperating clutch features  79  and  36 . Further, the predetermined torque threshold for tightening the tool chuck  50  may be less than the predetermined torque threshold for loosening the tool chuck  50 . This feature may be obtained by suitably designing the geometries of the cooperating clutch features  79  and  36 . Numerous and varied clutch surface geometries are well known in this art, and therefore a detailed discussion of the same is omitted.  
         [0037]     C. The Shift Collar/Mode Ring:  
         [0038]      FIG. 4  shows an example, non-limiting embodiment of a mode ring  45  and a shift collar  42  that may be implemented to axially position the shift ring  12  depicted in  FIGS. 2 and 3  to achieve the various operational modes. In  FIG. 4 , the portion of the drawing above the axis  43  depicts the DRILL/DRIVE MODE (where the shift ring  12  may be located at the intermediate axial position), and the portion of the drawing below the axis  43  depicts the CHUCK MODE (where the shift ring  12  may be located at the forward axial position).  
         [0039]     The mode ring  45  and the shift collar  42  may be mounted for rotation on the driver housing  95 . The mode ring  45  and the shift collar  42  may be rotationally fixed together via a radial extension  46 . Thus, the mode ring  45  and the shift collar  42  may be rotatable together relative to the driver housing  95 .  
         [0040]     The shift collar  42  may include a slot that extends in a circumferential direction around the shift collar  42 . In this example embodiment, the shift collar  42  may include two circumferential slots. The driver housing  95  may include longitudinal slots  96 . The longitudinal slots  96  may extend across (and underneath) the circumferential slots of the shift collar  42 . The ends of the wire  18  may extend in a radial outward direction from the shift ring  12 , through the longitudinal slots  96  of the driver housing  95  and into the slots of the shift collar  42 .  
         [0041]     A user may rotate the mode ring  45  (and thus the shift collar  42 ) relative to the housing  95 . At this time, the wire  18  may remain rotationally fixed to the housing  95  via the longitudinal slots  96 . During this relative rotation, the ends of the wire  18  may slide through the circumferential slots of the shift collar  42 . The shapes of the circumferential slots of the shift collar  42  may influence the wire  18  (and thus the shift ring  12 ) to the desired axial position. In this regard, the ends of the wire  18  may serve as cam followers and the corresponding circumferential slots may serve as cams. It will be appreciated that the circumferential slots of the shift collar  42  may extend in axial directions to thereby axially displace the shift ring  12 .  
         [0042]     II. Example Embodiment Depicted in FIGS.  5 - 9 :  
         [0043]      FIGS. 5-9  show another example, non-limiting embodiment of a PTO mechanism  110  that may support a tool chuck  150 . This example embodiment is similar to the one noted in section I above to the extent that it provides a dead spindle feature when operated in a CHUCK MODE, but there are some notable differences.  
         [0044]     A. The Structure:  
         [0045]     With reference to  FIGS. 5 and 6 , the PTO mechanism  110  may include an output coupling  120 . The output coupling  120  may have a radial outward facing surface provided with forwardly extended splines  124  (for selectively engaging with a driver housing, not shown), and a continuous circumferential groove  117  (for accommodating a wire  118 ).  
         [0046]     The wire  118 , which may be slidable through the circumferential groove  117 , may have free ends that extend in a radial direction and out of the circumferential groove  117 . The free ends of the wire  118  (serving as cam followers) may be received in a slot a shift collar rotatably mounted on the driver housing. Upon rotating the shift collar (via a mode ring, which may be similar to the one discussed above in section I), the slot may influence the cam followers (and thus the output coupling  120 ) to the desired axial positions. In  FIG. 5 , the output coupling  120  may be located in axial rearward position (to achieve a DRILL/DRIVE MODE), and in  FIG. 6 , the output coupling  120  may be located in an axial forward position (to achieve a CHUCK MODE).  
         [0047]     The output coupling  120  may include a central aperture  122  for receiving an end of an input shaft  160 . The central aperture  122  may have a shape corresponding to the shape of the input shaft  160 . By way of example only, a wall defining the central aperture  122  may include flats that abut against flats  167  provided on the input shaft  160 . In this way, the output coupling  120  may be rotationally fixed to (and axial moveable relative to) the input shaft  160 .  
         [0048]     The output coupling  120  may rotatably support pawls  127  (see  FIGS. 7 and 8 ). A spring  126 , mounted on the radial outward facing surface of the output coupling  120 , may bias the pawls  127  in a radial inward direction. The pawls  127  may selectively engage with ratchet features  175  provided on a radial outward facing surface of a disk  174 .  
         [0049]      FIGS. 7 and 8  illustrate the cooperation between the pawls  127  of the output coupling  120  and the ratchet features  175  of the disk  174 . As shown, the output coupling  120  may include radial openings into which shafts  128  may extend. The pawls  127 , which may be respectively provided in the radial openings, may be rotatably mounted on the shafts  128 . In  FIG. 8 , only the shafts  128  and the pawls  127  of the output coupling  120  are illustrated for clarity.  
         [0050]     As in the previous embodiment, a transmission may include a planetary reduction system. The disk  174  may be fixed to a third stage carrier  172 . The third stage carrier  172  may include a central aperture  176  having a shape that corresponds to a shape of a chuck actuating shaft (not shown). In this way, the third stage carrier  172  (and thus the disk  174 ) may be rotationally fixed to the chuck actuating shaft.  
         [0051]     Turning to  FIG. 9 , which does not illustrate the output coupling  120  for clarity, the tool chuck  150  may be somewhat similar to the one described in section I above. For example, the tool chuck  150  may include the input shaft  160 . As shown, the input shaft  160  may include two component parts inclusive of a nose portion  165  and a main body portion  163  that may be press fitted together, for example. The nose portion  165  may include passageways  161  through which chuck jaws (not shown) are respectively slidable. The input shaft  160  may include a through bore that rotatably supports a chuck actuating shaft  164 .  
         [0052]     In this example embodiment, the rear end of the chuck actuating shaft  164  extends from the input shaft  160  and into the central aperture  176  of the third stage carrier  172 . As noted above, the chuck actuating shaft is rotationally fixed to the third stage carrier  172 . The chuck actuating shaft  164  may include a through bore, the forward end of which may be provided with radial inward facing threads that may interact with radial outward facing threads of a chuck actuating screw (not shown). That is, the chuck actuating shaft may be screw coupled to the chuck actuating screw.  
         [0053]     As described above in section I, the chuck actuating screw may include radial passageways through which the chuck jaws are respectively slidable. The radial passageways may rotationally fix the chuck actuating screw to the chuck jaws. The chuck actuating screw and the input shaft  160  may be rotationally locked together via the chuck jaws.  
         [0054]     B. The Operation:  
         [0055]     The tool chuck  150  may operate differently depending on the axial position of the output coupling  120 , which may assume two different operating positions inclusive of a DRILL/DRIVE MODE (as shown in  FIG. 5 ) and a CHUCK MODE (as shown in  FIG. 6 ).  
         [0056]     As shown in  FIG. 5 , the output coupling  120  may located at an axial rearward position to achieve the DRILL/DRIVE MODE. Here, the output coupling  120  may be disengaged from (and rotatable relative to) the driver housing. The pawls  127  of the output coupling  120  may engage with the radial outward facing ratchet features  175  of the disk  174  (as shown in  FIG. 8 ). Thus, the output coupling  120  (and therefore the input shaft  160 , the chuck jaws and the chuck actuating screw) and the disk  174  (and therefore the third stage carrier  172 ) may be rotatable together as a unit, relative to the driver housing. A user may then power up the driver to rotationally drive the tool chuck  150 .  
         [0057]     The pawls  127  of the output coupling  120  may interact with the ratchet features  175  of the disk  174  so that the tool chuck  150  may tighten when driven in a forward direction as application torque increases, and may not loosen when driven in a reverse direction.  
         [0058]     With respect to the tightening feature, the third stage carrier  172  (together with the disk  174 ) may rotationally drive the output coupling  120  (together with the input shaft  160 , the chuck jaws and the chuck actuating screw). As the application torque increases, a rotational force applied by the ratchet features  175  of the disk  174  to the pawls  127  of the output coupling  120  may increase. This rotational force may increase to a threshold at which the ratchet features  175  may drive the pawls  127  in a radial outward direction and against the influence of the spring  126 , thereby causing the pawls  127  to rotate about the shafts  128 . In  FIG. 8 , the pawls  127  would rotate in a clockwise direction about the shafts  128 . At this time, the third stage carrier  172  (and thus the disk  174  and the chuck actuating shaft  164 ) may rotate relative to the output coupling  120  (and thus the input shaft  160 , the chuck jaws and the chuck actuating screw). The relative rotation between the chuck actuating shaft  164  and the chuck actuating screw (which may be screw coupled together) may cause the chuck jaws to tighten on the accessory.  
         [0059]     With respect to loosening, the driver may be operated in a reverse direction. Here, the ratchet features  175  of the disk  174  may apply a rotational force to the pawls  127  of the output coupling  120 . In this case, however, and with reference to  FIG. 8 , the pawls  127  may not rotate in a counter clockwise direction about the shafts  128 . This is due to the elongated shape of the pawls  128  and because of the radial location of the shafts  128 . Thus, the chuck actuating shaft  164  and the chuck actuating screw may remain rotationally locked together when the power driver is operated in the reverse direction.  
         [0060]     As shown in  FIG. 6 , the output coupling  120  may located at an axial forward position to achieve the CHUCK MODE. Here, the radial outward facing splines  124  of the output coupling  120  may engage with corresponding features provided on the driver housing. Thus, the output coupling  120  (and therefore the input shaft  160  and the chuck actuating screw) may be rotationally grounded to the driver housing. The pawls  127  of the output coupling  120  may be disengaged from ratchet features  175  of the disk  174  so that the disk  174  is rotatable relative to the output coupling  120 .  
         [0061]     A user may then power up the driver to actuate the tool chuck  150 . At this time, the third stage carrier  172  may rotationally drive the disk  174  and the chuck actuating shaft  164 . The chuck actuating shaft  164  may rotate relative to the chuck actuating screw, which may remain rotationally grounded to the driver housing (via the chuck jaws, the input shaft  160  and the output coupling  120 ). This relative rotation may cause the chuck actuating screw to advance or retract in the axial direction (depending on the rotation direction of the chuck actuating shaft  164 ). The translational movement of the chuck actuating screw may push or pull on the chuck jaws to open or close the same.  
         [0062]     As in the embodiment discussed above in section I, this embodiment also provides a dead spindle feature. For example, during chuck actuation, the input shaft  160 , the chuck jaws and the chuck actuating screw may remain rotationally grounded to the driver housing, while the chuck actuating screw may move axially (via the rotational movements of the chuck actuating shaft  164 ) relative to the input shaft  160  to open and close the chuck jaws.  
         [0063]     C. Example Modification for Shift Ring—FIGS.  10 - 13 :  
         [0064]      FIGS. 10-13  illustrate an example modification of the PTO mechanism depicted shown in  FIGS. 5-9 . Here, the PTO mechanism  110 ′ may additionally include an axially moveable shift ring  112 , and the output coupling  120 ′ may remain axially fixed to the input shaft  160 ′.  
         [0065]     With reference to  FIGS. 10 and 11 , the shift ring  112  may have a radial outward facing surface provided with splines  116  (for selectively engaging with a driver housing, not shown), and a continuous circumferential groove (not shown) for accommodating a wire. The free ends of the wire may be received in a slot of a shift collar rotatably mounted on the driver housing. Upon rotating the shift collar (via a mode ring, for example), the slot may influence the cam followers (and thus the shift ring  112 ) to the desired axial positions. In  FIG. 10 , the shift ring  112  may be located in an axial forward position (to achieve a DRILL/DRIVE MODE), and in  FIG. 11 , the shift ring  112  may be located in an axial rearward position (to achieve a CHUCK MODE).  
         [0066]     Turning to  FIGS. 12 and 13 , the output coupling  120 ′ may include a central aperture  122 ′ having a shape corresponding to the shape of the input shaft  160 ′ so that the output coupling  120 ′ may be rotationally fixed to the input shaft  160 ′. In this example modification, the output coupling  120 ′ may also be axially fixed to the input shaft  160 ′ by features that are well known in this art.  
         [0067]     The output coupling  120 ′ may rotatably support pawls  127 ′. And a spring  126 ′, mounted on the radial outward facing surface of the output coupling  120 ′, may bias the pawls  127 ′ in a radial inward direction. The pawls  127 ′ may engage with ratchet features  175 ′ provided on a radial outward facing surface of a disk  174 ′. Since the output coupling  120 ′ may be axially fixed, the pawls  127 ′ may remain engaged with the ratchet features  175 ′. As described in more detail below, the pivot action of the pawls  127 ′ about the shafts  128 ′ may be selectively enabled/disabled via the axial location of the shift ring  112 .  
         [0068]     The disk  174 ′ may be fixed to a third stage carrier  172 ′. The third stage carrier  172 ′ may include a central aperture  176 ′ having a shape that corresponds to a shape of a chuck actuating shaft  164 ′ so that the third stage carrier  172 ′ (and thus the disk  174 ′) may be rotationally fixed to the chuck actuating shaft  164 ′.  
         [0069]     The tool chuck  150  may be similar to the one described in section II A above.  
         [0070]     The example modification may operate differently depending on the axial position of the shift ring  112 , which may assume two different operating positions inclusive of a DRILL/DRIVE MODE (as shown in  FIG. 10 ) and a CHUCK MODE (as shown in  FIG. 11 ).  
         [0071]     As shown in  FIG. 10 , the shift ring  112  may be located at an axial forward position to achieve the DRILL/DRIVE MODE. Here, the shift ring  112  may be disengaged from (and rotatable relative to) the driver housing. In this condition, a radial inward facing surface of the shift ring  112  may not abut against and prevent the pawls  127 ′ from disengaging from the ratchet features  175 ′. In this respect, the pawls  127 ′ may be considered as being “enabled” to the extent that they may (given an appropriate application torque) rotate about the shafts  128 ′. A user may then power up the driver to rotationally drive the tool chuck  150 ′.  
         [0072]     The pawls  127 ′ of the output coupling  120 ′ may interact with the ratchet features  175 ′ of the disk  174 ′ so that the tool chuck  150 ′ may tighten when driven in a forward direction as application torque increases, and may not loosen when driven in a reverse direction. In this regard, the functional aspects of the example modification are similar to those discussed above in section II B.  
         [0073]     As shown in  FIG. 11 , the shift ring  112  may located at an axial rearward position to achieve the CHUCK MODE. Here, the radial outward facing splines  116  of the shift ring  112  may engage with corresponding features provided on the driver housing. Thus, the shift ring  112  (and therefore the input shaft  160 ′ and the chuck actuating screw) may be rotationally grounded to the driver housing.  
         [0074]     The shift ring  112  may cover the outer circumference of the output coupling  120 ′. In this condition, a radial inward facing surface of the shift ring  112  may contact a tail end of the pawls  127 ′, causing the pawls  127 ′ to rotate (clockwise when viewed from the front of the driver) about the shafts  128 ′ and completely out of engagement from the ratchet feature  175 ′. This may “disable” the ratcheting and anti-reverse characteristics of the pawl mechanism so that the tool chuck  150 ′ may be loosened freely with a counter-clockwise rotation of the third stage carrier  172 ′ (and thus the disk  174 ′).  
         [0075]     A user may then power up the driver to actuate the tool chuck  150 ′. At this time, the third stage carrier  172 ′ may rotationally drive the disk  174 ′ and the chuck actuating shaft  164 ′. The chuck actuating shaft  164 ′ may rotate relative to the chuck actuating screw, which may remain rotationally grounded to the driver housing (via the chuck jaws, the input shaft  160 ′, the output coupling  120 ′ and the shift ring  112 ). This relative rotation may cause the chuck actuating screw to advance or retract in the axial direction (depending on the rotation direction of the chuck actuating shaft  164 ′) to open or close the chuck jaws.  
         [0076]     This example modification may also provide a dead spindle feature. For example, during chuck actuation, the input shaft  160 ′, the chuck jaws and the chuck actuating screw may remain rotationally grounded to the driver housing, while the chuck actuating screw may move axially (via the rotational movements of the chuck actuating shaft  164 ′) relative to the input shaft  160 ′ to open and close the chuck jaws.  
         [0077]     III. Example Embodiment Depicted in FIGS.  14 - 16 :  
         [0078]      FIGS. 14-16  show another example, non-limiting embodiment of a PTO mechanism  210  that may support a tool chuck. As in the previous embodiments, the PTO mechanism  210  may be provided at the output end of the transmission  270  and have elements that may be positioned to operate the tool chuck in various modes. However, there are some notable differences.  
         [0079]     A. The Structure:  
         [0080]     With reference to  FIGS. 14 and 15 , the PTO mechanism  210  may include an output coupling  220 , a shift spider  212 , and a shift coupling  230 .  
         [0081]     The output coupling  220  may include a central aperture  222  having a shape that corresponds to the shape of an input shaft (not shown) of the tool chuck. In this way, the output coupling  220  may be rotationally fixed to the input shaft. The output coupling  220  may include lugs  224 .  
         [0082]     The shift spider  212  may have a radial outward facing surface provided with a continuous circumferential groove  217  for accommodating a wire (not shown). The free ends of the wire may be received in a slot of a shift collar rotatably mounted on the driver housing. Upon rotating the shift collar (via a mode ring, for example), the slot may influence the cam followers (and thus the shift spider  212 ) to the desired axial positions, as will be discussed in more detail below.  
         [0083]     The shift spider  212  may include lug openings  214  through which the lugs  224  of the output coupling  220  extend. The lug openings  214  may be separated from each other via radial extending tabs  216 . The radial inner ends of the tabs  216  may support a drive ring  218 .  
         [0084]     The shift coupling  230  may include a central aperture  232  having a shape that corresponds to the shape of a chuck actuating shaft  264  (which is only partially shown in  FIGS. 14-16 ) of the tool chuck. In this way, the shift coupling  230  may be rotationally fixed to (and axially moveable with respect to) the chuck actuating shaft  264 . The shift coupling  230  may have a radial outward facing surface supporting a flange  234 . The axial forward facing surface of the flange  234  may cooperate with the drive ring  218  of the shift spider  212 , as will be discussed in more detail below. The axial rearward facing surface of the flange  234  may support a spring (not shown) that may influence the shift coupling  230  in an axial forward direction. The shift coupling  230  may have an axial rearward facing surface that support lugs  236 .  
         [0085]     The transmission  270  may include three planetary reduction systems. The third stage carrier  272  may have a central aperture  274  into which the lugs  236  of the shift coupling  230  may extend. The third stage carrier  272  may have an axial forward facing surface that supports drive lugs  276 . The drive lugs  276  may extend through the lug openings  214  of the shift spider  212  and engage with the lugs  224  of the output coupling  220 . In this way, the third stage carrier  272  may be rotationally fixed to the output coupling  220 .  
         [0086]     The third stage sun gear  280  may be mounted on the second stage carrier  290 . The third stage sun gear  280  may have an axial forward facing surface supporting drive lugs  282 . The drive lugs  282  may extend into the central aperture  274  of the third stage carrier  272  and selectively engage with the lugs  236  of the shift coupling  230 .  
         [0087]     In this example embodiment, the tool chuck may be similar to those described with respect to the previous embodiments. Here, however, the rear end of the chuck actuating shaft  264  may extend from the input shaft and into the central aperture  232  of the shift coupling  230 . As noted above, the shift coupling  230  may be rotationally fixed to (and axially moveable relative to) the chuck actuating shaft  264 .  
         [0088]     B. The Operation:  
         [0089]     The tool chuck may operate differently depending on the axial position of the shift spider  212 , which may assume two different operating positions inclusive of a DRILL/DRIVE MODE and a CHUCK MODE. The axial movements of the shift spider  212  will be appreciated with reference to  FIG. 16 .  
         [0090]     The shift spider  212  may be located at an axial forward position to achieve the DRILL/DRIVE MODE. Here, the spring (not shown) abutting against the axial rearward facing surface of the flange  234  of the shift coupling  230  may influence the shift coupling  230  to move axially along the chuck actuating shaft  264  to an axial forward position. The drive ring  218  of the shift spider  212  (abutting against the axial forward facing surface of the flange  234 ) may limit the axial forward travel of the shift coupling  230 . In this condition, the lugs  236  of the shift coupling  230  may be disengaged from the drive lugs  282  of the third stage sun gear  280 .  
         [0091]     A user may then power up the driver. At this time, the third stage sun gear  280  may rotationally drive the third stage carrier  272  (via third stage planetary gears  278 ), which in turn may rotationally drive the output coupling  220  (via the interacting lugs  276  and  224 ). The output coupling  220  may rotationally drive the tool chuck (by virtue of being rotationally fixed to the input shaft). The chuck actuating shaft  264  (together with the shift coupling  230 ) may rotate relative to the third stage sun gear  280 .  
         [0092]     The shift spider  212  may be located at an axial rearward position to achieve the CHUCK MODE. Here, the drive ring  218  (of the shift spider  212 ) abutting against the axial forward facing surface of the flange  234  may drive the shift coupling  230  axially along the chuck actuating shaft  264  (and against the influence of the spring) to an axial rearward position. In this condition, the lugs  236  of the shift coupling  230  may engage with the drive lugs  282  of the third stage sun gear  280  so that the shift coupling  230  may be rotationally fixed to the third stage sun gear  280 .  
         [0093]     A user may then power up the driver. At this time, the third stage sun gear  280  may rotationally drive the third stage carrier  272  (via third stage planetary gears  278 , only one of which is depicted for clarity), which in turn may rotationally drive the output coupling  220  (via the interacting lugs  276  and  224 ). The output coupling  220  may rotationally drive the input shaft (and thus the chuck jaws and the chuck actuating screw). At the same time, the third stage sun gear  280  may rotationally drive the shift coupling  230  (via the interacting lugs  282  and  236 ), which in turn may rotationally drive chuck actuating shaft  264 .  
         [0094]     As is well known in this art, one rotation of the third stage sun gear  280  may cause (via the third stage planetary gears  278 ) only a fractional rotation of the third stage carrier  272 . In other words, relative to the driver housing, the third stage sun gear  280  (and thus the shift coupling  230  and the chuck actuating shaft  264 ) may rotate faster than the third stage carrier  272  (and thus the output coupling  220 , the chuck input shaft, the chuck jaws and the chuck actuating screw). The speed differential between the rotationally driven chuck actuating shaft  264  and the rotationally driven chuck actuating screw may result in a relative rotation between these two component parts. This relative rotation may advance or retract the chuck actuating screw in the axial direction (depending on the rotation direction of the transmission  270  output) to open or close the chuck jaws.  
         [0095]     When the tool chuck tightens on an accessory, the rotational movements of the input shaft, the chuck jaws, the chuck actuating screw and the chuck actuating shaft  264  stops. This stop when tight feature may be referred to as a semi-dead spindle feature.  
         [0096]     C. Example Modification for Dead Spindle Feature—FIGS.  17 - 19 :  
         [0097]      FIGS. 17-19  depict an example modification of the PTO mechanism shown in  FIGS. 14-16 . Here, the PTO mechanism  210 ′ may provide a dead spindle feature when operated in the CHUCK MODE.  
         [0098]     With reference to  FIGS. 17 and 18 , the PTO mechanism  210 ′ may include an output coupling  220 ′, a PTO actuator shaft  240 ′, a shift spider  212 ′ and a shift coupling  230 ′.  
         [0099]     The output coupling  220 ′ may include a central aperture  222 ′ having a shape that corresponds to the shape of the input shaft  260 ′ of the tool chuck  250 ′ so that the output coupling  220 ′ may be rotationally fixed to the input shaft  260 ′. The output coupling  220 ′ may include lugs  224 ′.  
         [0100]     The PTO actuator shaft  240 ′ may have a shape that corresponds to the shape of a chuck actuating shaft (not shown). In this way, the PTO actuator shaft  240 ′ may be rotationally fixed to (and axially moveable relative to) the chuck actuating shaft. A rear end of the PTO actuator shaft  240 ′ may be provided with a clutch feature  265 ′. A spring (not shown), may be captured between the output coupling  220 ′ and the clutch feature  265 ′ to influence the PTO actuator shaft  240 ′ in an axial rearward direction. The axial rearward travel of the PTO actuator shaft  240 ′ (relative to the chuck actuating shaft) may be limited by dogs  277 ′ of the third stage carrier  272 ′.  
         [0101]     The shift spider  212 ′ may have a radial outward facing surface provided with forwardly extended splines (not shown) for selectively engaging with the driver housing (not shown). The radial outward facing surface may also be provided with a continuous circumferential groove  217 ′ for accommodating a wire (not shown). The free ends of the wire may be received in a slot of a shift collar rotatably mounted on the driver housing. Upon rotating the shift collar (via a mode ring, for example), the slot may influence the cam followers (and thus the shift spider  212 ′) to the desired axial positions, as will be discussed in more detail below.  
         [0102]     The shift spider  212 ′ may include lug openings  214 ′ through which the lugs  224 ′ of the output coupling  220 ′ extend. The lug openings  214 ′ may be separated from each other via radial extending tabs  216 ′. The radial inner ends of the tabs  216 ′ may support a drive ring  218 ′.  
         [0103]     The shift coupling  230 ′ may include a radial outward facing surface provided with a continuous circumferential groove  234 ′ for accommodating the drive ring  218 ′ of the shift spider  212 ′. A retaining ring (not shown) may be provided on the shift coupling  230 ′ to axially capture the drive ring  218 ′. In this way, the shift coupling  230 ′ may be axially fixed to the shift spider  212 ′. An axial forward facing surface of the shift coupling  230 ′ may be provided with drive lugs  237 ′ for selectively engaging with the clutch feature  265 ′ provided on the PTO actuator shaft  240 ′. An axial rearward facing surface of the shift coupling  230 ′ may support lugs  236 ′ for selectively engaging with drive lugs  282 ′ of a third stage sun gear  280 ′. The shift coupling  230 ′ may support a shaft  238 ′ having a shape that corresponds to the shape of a central aperture  292 ′ of a second stage carrier  290 ′. In this way, the shift coupling  230 ′ may be rotationally fixed to (and axially moveable relative to) the second stage carrier  290 ′.  
         [0104]     The transmission  270 ′ may include three planetary reduction systems. The third stage carrier  272 ′ may have a central aperture  274 ′ through which the shaft  238 ′ of the shift coupling  230 ′ may extend. The third stage carrier  272 ′ may have an axial forward facing surface that supports drive lugs  276 ′. The drive lugs  276 ′ may extend through the lug openings  214 ′ of the shift spider  212 ′ and engage with the lugs  224 ′ of the output coupling  220 ′. In this way, the third stage carrier  272 ′ may be rotationally fixed to the output coupling  220 ′. The axial forward distal ends of the drive lugs  276 ′ may support dogs  277 ′. The dogs  277 ′ may selectively engage with the clutch feature  265 ′ of the PTO actuator shaft  240 ′. As shown, the dogs  277 ′ of the third stage carrier  272 ′ may be located on the radial outside of the drive lugs  237 ′ of the shift coupling  230 ′.  
         [0105]     In this example modification, the third stage sun gear  280 ′ may be mounted for rotation on the second stage carrier  290 ′. The third stage sun gear  280 ′ may have an axial forward facing surface supporting drive lugs  282 ′. The drive lugs  282 ′ may extend into the central aperture  274 ′ of the third stage carrier  272 ′ and selectively engage with the lugs  236 ′ of the shift coupling  230 ′.  
         [0106]     In this example embodiment, the tool chuck  250 ′ may be similar to those described with respect to the previous embodiments. Here, however, the rear end of the chuck actuating shaft (not shown) may receive the axial forward end of the PTO actuator shaft  240 ′.  
         [0107]     The example modification may operate differently depending on the axial position of the shift spider  212 ′, which may assume two different operating positions inclusive of a DRILL/DRIVE MODE and a CHUCK MODE.  
         [0108]      FIG. 19  shows the shift spider  212 ′ located at an axial rearward position to achieve the DRILL/DRIVE MODE. Here, the shift coupling  230 ′ (axially fixed to the shift spider  212 ′) may assume an axial rearward position in which the lugs  236 ′ may engage with the drive lugs  282 ′ of the third stage sun gear  280 ′. In this condition, the drive lugs  237 ′ of the shift coupling  230 ′ may be located in the axial direction further rearward than the dogs  277 ′ of the third stage carrier  272 ′. Thus, the clutch feature  265 ′ (under the influence of the spring) may engage with the dogs  277 ′ of the third stage carrier  272 ′, and not the drive lugs  237 ′ of the shift coupling  230 ′.  
         [0109]     A user may then power up the driver to rotationally drive the tool chuck  250 ′. At this time, the second stage carrier  290 ′ may rotationally drive the shift coupling  230 ′ (via the shaft  238 ′), which in turn may rotationally drive the third stage sun gear  280 ′ (via the interacting lugs  236 ′ and  282 ′). In this way, the second stage carrier  290 ′, the shift coupling  230 ′ and the third stage sun gear  280 ′ may rotate together as a unit. The third stage sun gear  280 ′ may rotationally drive the third stage carrier  272 ′ (via the third stage planetary gears  278 ′), which in turn may rotationally drive the output coupling  220 ′ (and therefore the input shaft  260 ′, the chuck jaws and the chuck actuating screw). Since the clutch feature  265 ′ (and therefore the PTO actuator shaft  240 ′ and the chuck actuating shaft) and the third stage carrier  272 ′ (and therefore the input shaft  260 ′ and the chuck actuating screw) may be rotationally fixed together, the tool chuck  250 ′ may not loosen during operation.  
         [0110]     The shift spider  212 ′ may be located at an axial forward position to achieve the CHUCK MODE. Here, the forwardly extended splines (not shown) of the shift spider  212 ′ may engage with corresponding features provided on the driver housing. Thus, the shift spider  212 ′, the third stage carrier  272 ′ and the output coupling  220 ′ (and therefore the input shaft  260 ′, the chuck jaws and the chuck actuating screw) may remain rotationally grounded to the driver housing.  
         [0111]     The drive ring  218 ′ (of the shift spider  212 ) may drive the shift coupling  230 ′ to an axial forward position. As the shift coupling  230  advances forward, the drive lugs  237 ′ may engage with the clutch feature  265 ′ and push the clutch feature  265 ′ (together with the PTO actuator shaft  240 ′) in an axial forward direction against the influence of the spring (not shown). In this condition, the clutch feature  265 ′ may disengage from the dogs  277 ′ of the third stage carrier  272 ′. At the same time, the lugs  236 ′ of the shift coupling  230 ′ may disengage from the lugs  282 ′ of the third stage sun gear  280 ′, while the shaft  238 ′ of the shift coupling  230 ′ may remain inserted into the central aperture  292 ′ of the second stage carrier  290 ′.  
         [0112]     A user may then power up the driver to actuate the tool chuck  250 ′. At this time, the second stage carrier  290 ′ may rotationally drive the shift coupling  230 ′ (via the shaft  238 ′), which in turn may rotationally drive the PTO actuator shaft  240 ′ (via the interacting drive lugs  237 ′ and the clutch feature  265 ′). The second stage carrier  290 ′ may not, however, rotationally drive the third stage sun gear  280 ′ since the lugs  236 ′ (of the shift coupling  230 ′) may be disengaged from the lugs  282 ′ (of the third stage sun gear  280 ′). In this way, the second stage carrier  290 ′ (and therefore the shift coupling  230 ′, the PTO actuator shaft  240 ′ and the chuck actuating shaft) may rotate relative to the third stage sun gear  280 ′ (and therefore the third stage carrier  272 ′, the output coupling  220 ′, the input shaft  260 ′ and the chuck actuating screw), which may remain rotationally grounded to the driver housing (via the shift spider  212 ′). The rotational movements of the chuck actuating shaft relative to the input shaft  260 ′ may cause the chuck actuating screw to advance or retract in the axial direction (depending on the rotation direction of the chuck actuating shaft  264 ′) to open or close the chuck jaws.  
         [0113]     This example modification may also provide a dead spindle feature. For example, during chuck actuation, the input shaft  260 ′, the chuck jaws and the chuck actuating screw may remain rotationally grounded to the driver housing, while the chuck actuating screw may move axially (via the rotational movements of the PTO actuator shaft  240 ′ and the chuck actuating shaft) relative to the input shaft  260 ′ to open and close the chuck jaws.  
         [0114]     IV. Example Embodiment Depicted in  FIGS. 20 and 21 :  
         [0115]      FIGS. 20 and 21  show another example, non-limiting embodiment of a PTO mechanism  310  that may support a tool chuck  350 . As in the previous embodiments, the PTO mechanism  310  may be provided at the output end of the transmission  370 . This example embodiment, however, may implement a spindle lock.  
         [0116]     A. The Structure:  
         [0117]     With reference to  FIGS. 20 and 21 , the PTO mechanism  310  may include a shift coupling  330  and a spindle lock  340 .  
         [0118]     The shift coupling  330  may have an axial shaft  338 . The shaft  338  may be inserted into a through bore  366  of the chuck actuating shaft  364 . The shaft  338  may have a shape corresponding to the shape of the through bore  366  so that the shift coupling  330  may be rotationally fixed to (an axially moveable relative to) the chuck actuating shaft  364 . From the chuck actuating shaft  364 , the shaft  338  of the shift coupling  330  may extend in an axial rearward direction through the main body portion  363  of the input shaft  360  and through the spindle lock  340 . The shaft  338  may have an axial rear end that may support a clutch feature  337 . The clutch feature  337  may operatively engage with a clutch feature  377  provided on the third stage carrier  372 .  
         [0119]     The spindle lock  340  may be mounted between the main body portion  363  of the input shaft  360  and the driver housing  395 . It will be appreciated that spindle locks are conventionally known in this art. For example, a representative automatic spindle lock is described in U.S. Pat. No. 6,311,787, the entire contents of which is incorporated herein by reference. The spindle lock described in U.S. Pat. No. 6,311,787 could be suitably implemented in the example embodiment with only slight modifications that may be readily apparent to those skilled in the art. In any event, the spindle lock  340  may provide the following functionality.  
         [0120]     On the one hand, the spindle lock  340  may allow the input shaft  360  to rotate (relative to the housing  395 ) when the third stage carrier  372  is in an axial forward position (as shown in  FIG. 20 ). Here, drive lugs  376  on the front surface of the third stage carrier  372  may interact with a cage  342  of the spindle lock  340 . When the driver is powered up, the third stage carrier  372  (via the drive lugs  376 ) may rotationally drive the cage  342 , which in turn may rotationally drive the input shaft  360  relative to the housing  395 .  
         [0121]     On the other hand, the spindle lock  340  may prevent the input shaft  360  from rotating (relative to the housing  395 ) when the third stage carrier  372  is in the axial rearward position (as shown in  FIG. 21 ). Here, the drive lugs  376  may disengage the spindle lock  340 . Thus, when the driver is powered up, the third stage carrier  372  may not rotationally drive the cage  342  so that the input shaft  360  may be rotationally locked to the housing  395 .  
         [0122]     The transmission  370  may include three planetary reduction systems. The third stage carrier  372  may include a central aperture  374  through which the shaft  338  of the shift coupling  330  may extend. The third stage carrier  372  may have an axial rearward facing surface that supports a clutch feature  377 . As noted above, the clutch feature  377  may operatively engage with the clutch feature  337  of the shift coupling  330 .  
         [0123]     In this example embodiment, the second stage carrier  390  and the third stage sun gear  380  may be fixed together (e.g., the two component parts may of a unitary, one-piece construction). The third stage sun gear  380  may support a spring  391 . The spring  391  may influence the shift coupling  330  in an axial forward direction, thereby loading the operative engagement between the clutch feature  337  (of the shift coupling  330 ) and the clutch feature  377  (of the third stage carrier  372 ). The second stage carrier  390  may support a spring  392 . The spring  392  may (in conjunction with a plate  393 ) influence the third stage planetary gears in an axial forward direction to ensure that the third stage planetary gears may remain in the desired position on the third stage carrier  372 .  
         [0124]     In this example embodiment, the tool chuck  350  may be similar to those described with respect to the previous embodiments. Here, however, the rear end of the chuck actuating shaft  364  receives the shaft  338  of the shift coupling  330  so that the shift coupling  330  is rotationally fixed to (and axially moveable relative to) the chuck actuating shaft  364 .  
         [0125]     B. The Operation:  
         [0126]     The tool chuck  350  may operate differently depending on the axial position of the third stage carrier  372 , which may assume two different operating positions inclusive of a DRILL/DRIVE MODE and a CHUCK MODE. The third stage carrier  372  may be moved to the desired axial position via (for example) a wire cooperating with a shift collar, as will be readily apparent to those skilled in the art.  
         [0127]      FIG. 20  illustrates the third stage carrier  372  at an axial forward position to achieve the DRILL/DRIVE MODE. Here, the drive lugs  376  may engage with the cage  342  of the spindle lock  340 . Also, the shift coupling  330  (under the influence of the spring  391 ) may be located at an axial forward position so that the clutch feature  337  (of the shift coupling  330 ) may be engaged with the clutch feature  377  (of the third stage carrier  372 ).  
         [0128]     A user may then power up the driver. At this time, the third stage carrier  272  may rotationally drive the spindle lock  340  (via the interacting drive lugs  376  and the cage  342 ) and the shift coupling  330  (via the interacting clutch features  377  and  337 ). The spindle lock  340  may rotationally drive the input shaft  360 , which may rotate together with the chuck jaws, and the chuck actuating screw  355 . At the same time, the shift coupling  330  may rotationally drive the chuck actuating shaft  364 . In this way, the components of the tool chuck  350  may rotate together as a unit and relative to the driver housing  395 .  
         [0129]      FIG. 21  illustrates the third stage carrier  372  at an axial rearward position to achieve the CHUCK MODE. Here, the drive lugs  376  may be disengaged from the cage  342  of the spindle lock  340 . The third stage carrier  372  (when moved to the axial rearward position), drives the shift coupling  330  (against the influence of the spring  291 ) to an axial rearward position.  
         [0130]     A user may then power up the driver to actuate the tool chuck  350 . At this time, the third stage carrier  372  may rotationally drive the shift coupling  330  (via the interacting clutch features  377  and  337 ), which in turn may rotationally drive the chuck actuating shaft  364 . The input shaft  360  (and therefore the chuck jaws and the chuck actuating screw  355 ) may be rotationally grounded to the driver housing  395  via the spindle lock  340 . The rotational movements of the chuck actuating shaft  364  relative to the chuck actuating screw  355  may cause the chuck actuating screw  355  to advance or retract in the axial direction (depending on the rotation direction of the chuck actuating shaft  364  to open or close the chuck jaws.  
         [0131]     This example embodiment may also provide a dead spindle feature during chuck actuation.  
         [0132]     Once the tool chuck  350  is tight (i.e., when the chuck jaws clamp the accessory) or fully opened, the cooperating clutch features  377  and  337  (respectively provided on the third stage carrier  372  and the shift coupling  330 ) may give way and slip relative to each other. At this time, the shift coupling  330  may move in an axial rearward direction against the influence of the spring  391 . When the cooperating clutch features  377  and  337  slip, they may produce an audible indication that the chuck actuation process is complete. Further, the cooperating clutch features  377  and  337  may give way or slip at a predetermined torque threshold, which may be less during a tightening operation than during a loosening operation.  
         [0133]     C. First Example Modification— FIGS. 22 and 23 :  
         [0134]      FIGS. 22 and 23  depict a first example modification of the PTO mechanism shown in  FIGS. 20 and 21 . Here, the coil spring  391  (provided between the shift coupling  330  and the third stage sun gear  380 ) may be dispensed with in favor of a hub  331 ′ and a spring  391 ′. The hub  331 ′ may be press fit and fixed to the shaft  338 ′ of the shift coupling  330 ′. The spring  391 ′ (e.g., a belville spring) may be captured between the hub  331 ′ and the third stage carrier  372 ′. The spring  391 ′ may load the operative engagement between the clutch features  337 ′ and  377 ′ respectively provided on the shift coupling  330 ′ and the third stage carrier  372 ′. Numerous and alternative clutch features  337 ′ and  377 ′ may be suitably implemented. For example, the clutch features may be in the form of corresponding profiled surfaces and/or friction surfaces, which will be readily appreciated by those skilled in the art.  
         [0135]     In this example modification, the spring  391 ′ may not provide an axial load on the transmission  370 ′. Further, this example modification may provide for a preset clutch/third stage carrier/spring subassembly that may reduce (or possibly altogether prevent) transmission stack up effects on clutch loading. Otherwise, this example modification is structurally and functionally similar to the example embodiment depicted in  FIGS. 20 and 21 .  
         [0136]      FIG. 22  illustrates the third stage carrier  372 ′ at an axial forward position to achieve the DRILL/DRIVE MODE. And  FIG. 23  illustrates the third stage carrier  372 ′ at an axial rearward position to achieve the CHUCK MODE.  
         [0137]     D. Second Example Modification— FIGS. 24 and 25 :  
         [0138]      FIGS. 24 and 25  depict a second example modification of the PTO mechanism shown in  FIGS. 20 and 21 , in which the clutch feature of the shift coupling may be dispensed with. Here, the shift coupling  330 ″ may include a shaft  338 ″ having an axial rear end that may be fixed to the third stage carrier  372 ″. The shaft  338 ″ may be in the form of a flexible torsion rod.  
         [0139]      FIG. 24 , illustrates the third stage carrier  372 ″ at an axial rearward position to achieve the CHUCK MODE. When the driver is powered up, the power path may run from the third stage carrier  372 ″ and into the shaft  338 ″ of the PTO mechanism  310 ″ to actuate the tool chuck  350 ″. When tool chuck  350 ″ becomes tight, a transmission clutch may give way. Transmission clutches are well known in this art, and therefore a detailed discussion of the same is omitted.  
         [0140]      FIG. 25  illustrates the third stage carrier  372 ′ at an axial forward position to achieve the DRILL/DRIVE MODE. When shifting to the DRILL/DRIVE MODE, the shaft  338 ″ (by virtue of being a flexible torsion rod) may remain loaded with torque as the third stage carrier  372 ″ engages the cage  342 ″ of the spindle lock  340 ″. In this way, a prevailing load may be applied to the tool chuck  250 ″ to keep it tight. Otherwise, this example modification is structurally and functionally similar to the example embodiment depicted in  FIGS. 20 and 21 .  
         [0141]     E. Third Example Modification— FIGS. 26 and 27 :  
         [0142]      FIGS. 26 and 27  depict a third example modification of the PTO mechanism shown in  FIGS. 20 and 21 . Here, the spindle lock  340 ′″ may be moveable to an axial forward position (as shown in  FIG. 26 ) to achieve the CHUCK MODE and an axial rearward position (as shown in  FIG. 27 ) to achieve a DRILL/DRIVE MODE. In this example modification, the CHUCK MODE and the DRILL/DRIVE MODE may be achieved while maintaining the third stage carrier  372 ′″ and the shift coupling  330 ′″ at fixed axial positions.  
         [0143]     A mode ring  345 ′″ may be mounted for rotation on the driver housing  395 ′″. The mode ring  345 ′″ may have cam surfaces  346 ′″ that interact with pins  341 ′″ extended axial forward from the spindle lock  340 ′″. The spindle lock  340 ′″ may be influenced by a spring  343 ′″ (captured between the spindle lock  340 ′″ and the third stage carrier  372 ′″) in an axial forward direction. A user may turn the mode ring  345 ′″ (relative to the driver housing  395 ′″) so that the cam surfaces  346 ′″ may interact with the pins  341 ′″ to move the spindle lock  340 ′″ to the desired axial position.  
         [0144]     This example modification may employ a hub  331 ′″ and a spring  391 ′″, similar to that described above in section IV C, to load the operative engagement between the clutch features respectively provided on the shift coupling  330 ′″ and the third stage carrier  372 ′″.  
         [0145]      FIG. 26 , illustrates the spindle lock  340 ′″ at an axial forward position to achieve the CHUCK MODE. And  FIG. 25  illustrates the spindle lock  340 ′″ at an axial rearward position to achieve the DRILL/DRIVE MODE. Otherwise, this example modification is structurally and functionally similar to the example embodiment depicted in  FIGS. 20 and 21 .  
         [0146]     V. Example Embodiment Depicted in  FIG. 28 :  
         [0147]      FIG. 28  shows another example, non-limiting embodiment of a PTO mechanism  410 . Here, the PTO mechanism  410  may be connected to a conventional parallel axis transmission  470 .  
         [0148]     A. The Structure:  
         [0149]     With reference to  FIG. 28 , the PTO mechanism  410  may include a chuck actuating hammer  482  that may be mounted for rotation on the chuck actuating shaft  464  of the tool chuck  450 . The chuck actuating hammer  482  may also be axially moveable along the chuck actuating shaft  464 . The chuck actuating hammer  482  may include hammer lugs  468 , which may cooperate with corresponding hammer lugs  466  provided on a hammer anvil  465  of the chuck actuating shaft  464  of the tool chuck  450 . A spring  498  (captured between the chuck actuating hammer  482  and a keeper mounted on the chuck actuating shaft  464 ) may influence the chuck actuating hammer  482  in an axial forward direction. The chuck actuating hammer  482  may have a radial outward facing surface provided with gear teeth that may engage with a chuck actuating drive gear  492 .  
         [0150]     The conventional parallel axis transmission  470  may couple an electric motor to a tool chuck  450 . The electric motor may have a rotary shaft that supports an output gear  499 . The output gear  499  may engage with and rotationally drive an intermediate shaft  490 .  
         [0151]     The intermediate shaft  490  may be mounted for rotation in the housing  495  of the driver. The intermediate shaft  490  may support an input gear  491 , the chuck actuating drive gear  492 , and an input shaft drive gear  493 . The input gear  491  (rotationally fixed to the intermediate shaft  490 ) may engage with the output gear  499 , the chuck actuating drive gear  492  (rotatable relative to the intermediate shaft  490 ) may engage with the chuck actuating hammer  482  of the PTO mechanism  410 , and the input shaft drive gear  493  (rotationally fixed to the intermediate shaft  490 ) may engage with an input shaft driven gear  483  fixed to the input shaft  460  of the tool chuck  450 .  
         [0152]     The intermediate shaft  490  may also support a shift plate  494 . The shift plate  494  may be rotationally fixed to (and axially moveable relative to) the intermediate shaft  490 . The shift plate  494  may include drive lugs  497  for selectively engaging with corresponding features provided on the chuck actuating drive gear  492 . A selector  445  may be mounted on the driver housing  495  for axial movement. The selector  445  may be coupled to the shift plate  494 . A user may manipulate the selector  445  to drive the shift plate  494  to the desired axial position.  
         [0153]     Power may be taken off the transmission  470  via the input shaft drive gear  493  in both a DRILL/DRIVE MODE and a CHUCK MODE. Power may also be taken off the transmission  470  via the chuck actuating drive gear  492  in the CHUCK MODE.  
         [0154]     In this example embodiment, the tool chuck may be similar to those described with respect to the previous embodiments. Here, however, the input shaft  460  may be fixed to the input shaft driven gear  483 , and the chuck actuating shaft  464  may be fixed to the hammer anvil  465 .  
         [0155]     B. The Operation:  
         [0156]     The tool chuck  450  may operate differently depending on the axial position of the shift plate  494 , which may assume two different operating positions inclusive of the DRILL/DRIVE MODE and a CHUCK MODE.  
         [0157]      FIG. 28  depicts the shift plate  494  at an axial rearward position to achieve the DRILL/DRIVE MODE. Here, the shift plate  494  may be disengaged from the chuck actuating drive gear  492 .  
         [0158]     When the driver is powered up, the input shaft drive gear  493  may rotationally drive the input shaft driven gear  483  together with the input shaft  460 . The input shaft  460  may rotate together with the chuck jaws, the chuck actuating screw  455 , the chuck actuating shaft  464  and the chuck actuating hammer  482 . In this condition, the chuck actuating drive gear  492  may rotate relative to the intermediate shaft  490 . In the DRILL/DRIVE mode, the power from the transmission  470  is delivered to the input shaft driven gear  483  to rotationally drive the tool chuck  450 .  
         [0159]     The shift plate  494  may be located at an axial forward position to achieve the CHUCK MODE. Here, the shift plate  494  (via the drive lugs  497 ) may be rotationally fixed to the chuck actuating drive gear  492 .  
         [0160]     When the driver is powered up, the input shaft drive gear  493  may rotationally drive the input shaft driven gear  483  together with the input shaft  460 . The input shaft  460  may rotate together with the chuck jaws and the chuck actuating screw  455 . At the same time, the chuck actuating drive gear  492  may rotationally drive the chuck actuating hammer  482 , which in turn may rotationally drive the chuck actuating shaft  464  (via the interacting hammer lugs  466  and  468 ).  
         [0161]     The chuck actuating drive gear  492  may be larger than the input shaft drive gear  493 , and thus the chuck actuating hammer  482  may be driven at a faster rotational speed than the input shaft driven gear  483 . As a result, relative to the housing  495 , the chuck actuating shaft  464  may rotate faster than the chuck actuating screw  455 . This speed differential may result in a relative rotation between the chuck actuating shaft  464  and the chuck actuating screw  455 , thereby causing the chuck actuating screw  455  to advance or retract in the axial direction to open or close the chuck jaws.  
         [0162]     Once the tool chuck  450  is tight (e.g., when the chuck jaws clamp the accessory) or fully opened, the hammer lugs  468  on the chuck actuating hammer  482  may give way and slip relative to the hammer lugs  466  on the hammer anvil  465 . At this time, the chuck actuating hammer  482  may move in an axial rearward direction against the influence of the spring  498 . A gap between the chuck actuating hammer  482  and the chuck actuating drive gear  492  may accommodate the rearward axial movement of the chuck actuating hammer  482 . As the chuck actuating hammer  482  and the hammer anvil  465  slip, the hammer lugs  466  and  468  may impact with each other to further tighten or loosen the tool chuck  450 .  
         [0163]     C. Example Modification— FIG. 29 :  
         [0164]      FIG. 29  depicts an example modification of the PTO mechanism shown in  FIG. 28 . Here, the shift plate  494 ′ may be mounted on the chuck actuating shaft  464 ′ (instead of the intermediate shaft of the transmission).  
         [0165]     The shift plate  494 ′ may be mounted for rotation on the chuck actuating shaft  464 ′. The shift plate  494 ′ may be axially moveable relative to the chuck actuating shaft  464 ′. The shift plate  494 ′ may be driven to the desired axial position by a variety of mechanisms (e.g., a selector, not shown). The shift plate  494 ′ may include a radial outward facing surface provided with gear teeth  496 ′ that may engage with the chuck actuating drive gear  492 ′. The shift plate  494 ′ may include drive lugs  497 ′ for selectively engaging with corresponding features provided on the chuck actuating hammer  482 ′.  
         [0166]     The transmission  470 ′ is similar to the one described above. Here, however, the chuck actuating drive gear  492 ′ may be fixed to the intermediate shaft  490 ′.  
         [0167]     The tool chuck  450 ′ may operate differently depending on the axial position of the shift plate  494 ′, which may assume two different operating positions inclusive of the DRILL/DRIVE MODE and a CHUCK MODE.  
         [0168]      FIG. 29  depicts the shift plate  494 ′ at an axial rearward position to achieve the DRILL/DRIVE MODE. Here, the shift plate  494 ′ may be disengaged from the chuck actuating hammer  482 ′. When the driver is powered up, the input shaft drive gear  493 ′ may rotationally drive the input shaft driven gear  483 ′ together with the input shaft  460 ′. The input shaft  460 ′ may rotate together with the chuck jaws, the chuck actuating screw  455 ′, the chuck actuating shaft  464 ′ and the chuck actuating hammer  482 ′. In this condition, the shift plate  494 ′, rotationally driven by the chuck actuating drive gear  492 ′, may free wheel about the chuck actuating shaft  464 ′. In the DRILL/DRIVE mode, the power from the transmission  470 ′ is delivered to the input shaft driven gear  483 ′ to rotationally drive the tool chuck  450 ′.  
         [0169]     The shift plate  494 ′ may be located at an axial forward position to achieve the CHUCK MODE. Here, the shift plate  494 ′ (via the drive lugs  497 ′) may be rotationally fixed to the chuck actuating hammer  482 ′. When the driver is powered up, the input shaft drive gear  493 ′ may rotationally drive the input shaft driven gear  483 ′ together with the input shaft  460 ′. The input shaft  460 ′ may rotate together with the chuck jaws and the chuck actuating screw  455 ′. At the same time, the chuck actuating drive gear  492 ′ may rotationally drive the shift plate  494 ′. The shift plate  494 ′ may (via the drive lugs  497 ′) rotationally drive the chuck actuating hammer  482 ′. The chuck actuating hammer  482 ′ may rotationally drive the chuck actuating shaft  464 ′ (via the interacting hammer lugs  466 ′ and  468 ′).  
         [0170]     Relative to the housing  495 ′, the chuck actuating shaft  464 ′ may rotate faster than the chuck actuating screw  455 ′. This speed differential may result in a relative rotation between the chuck actuating shaft  464 ′ and the chuck actuating screw  455 ′, thereby causing the chuck actuating screw  455 ′ to advance or retract in the axial direction to open or close the chuck jaws.  
         [0171]     Once the tool chuck  450 ′ is tight (e.g., when the chuck jaws clamp the accessory) or fully opened, the hammer lugs  468 ′ on the chuck actuating hammer  482 ′ may give way and slip relative to the hammer lugs  466 ′ on the hammer anvil  465 ′. At this time, the chuck actuating hammer  482 ′ (together with the shift plate  494 ′) may move in an axial rearward direction against the influence of the spring  498 ′. As the chuck actuating hammer  482 ′ and the hammer anvil  465 ′ slip, the hammer lugs  466 ′ and  468 ′ may impact with each other to further tighten or loosen the tool chuck  450 ′.  
       SUMMARY  
       [0172]     According to an example, non-limiting embodiment, a power driver may include a motor that rotates upon receiving an input current. A tool chuck may have chuck jaws to hold an accessory. The tool chuck may be coupled to the motor. A power take off mechanism may be connected between the motor and the tool chuck. The power take off mechanism may be adjustable into a CHUCK MODE to one of open and close the chuck jaws while the motor rotates. An electronic clutch may be provided to interrupt the input current to the motor if the power take off mechanism is in the CHUCK MODE and if the input current exceeds a trip value.  
         [0173]     According to another example, non-limiting embodiment, a power driver may include a motor that receives an input current to drive a tool chuck supporting chuck jaws. A control circuit may be provided to selectively interrupt the input current to the motor. The control circuit may receive a current value input indicating the input current to the motor. The control circuit may also receive a mode input indicating whether the power driver is set to operate in CHUCK MODE in which the motor rotates to one of open and close the chuck jaws. The control circuit may interrupt the input current to the motor if the mode input indicates that the power driver is set to operate in the CHUCK MODE and if the current value input exceeds a trip value.  
         [0174]     According to another example, non-limiting embodiments, an electronic clutch may be provided for a power driver having a motor that receives an input current to drive a tool chuck supporting chuck jaws. The electronic clutch may have a control circuit to receive a first input indicative of a clamping force applied by the chuck jaws, and a second input to indicate whether the power driver is set to operate in a CHUCK MODE in which the motor rotates to one of open and close the chuck jaws. The control circuit may interrupt the input current to the motor if the second input indicates that the power driver is set to operate in the CHUCK MODE and if the first input exceeds a trip value.  
         [0175]     According to another example, non-limiting embodiment, an electronic clutch may be provided for a power driver having a motor to drive a tool chuck supporting chuck jaws. The electronic clutch may include a control circuit to receive a mode input to indicate whether the power driver is set to operate in a CHUCK MODE in which the motor rotates to one of open and close the chuck jaws. The control circuit may limit a torque output of the motor, if the mode input indicates that the power driver is set to operate in the CHUCK MODE.  
         [0176]     The above and other features of the invention including various and novel details of construction and combinations of parts will now be more particularly described with reference to the accompanying drawings. It will be understood that the details of the exemplary embodiments are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0177]     Example, non-limiting embodiments of the present invention will become more fully understood from the detailed description below and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limiting of the present invention.  
         [0178]      FIG. 1  is a schematic illustration of a tool chuck with a power take off mechanism according to an example, non-limiting embodiment disclosed in the Copending Provisional Application.  
         [0179]      FIG. 2  is an exploded perspective view of the power take off mechanism of  FIG. 1 .  
         [0180]      FIG. 3  is a sectional perspective view of the tool chuck mounted on the power take off mechanism of  FIG. 1 .  
         [0181]      FIG. 4  is a sectional view of a mode ring and shift collar that may be suitably implemented to change the operational modes of the tool chuck.  
         [0182]      FIGS. 5-9  are schematic views of a tool chuck with a power take off mechanism according to another example, non-limiting embodiment disclosed in the Copending Provisional Application.  
         [0183]      FIGS. 10-13  are schematic views of an example modification of the power take off mechanism illustrated in  FIGS. 5-9 .  
         [0184]      FIGS. 14-16  are schematic views of a tool chuck with a power take off mechanism according to an another example, non-limiting embodiment disclosed in the Copending Provisional Application.  
         [0185]      FIGS. 17-19  are schematic views of an example modification of the power take off mechanism illustrated in  FIGS. 14-16 .  
         [0186]      FIGS. 20 and 21  are schematic views of a tool chuck with a power take off mechanism according to an another example, non-limiting embodiment disclosed in the Copending Provisional Application.  
         [0187]      FIGS. 22 and 23  are schematic views of a first example modification of the power take off mechanism illustrated in  FIGS. 20 and 21 .  
         [0188]      FIGS. 24 and 25  are schematic views of a second example modification of the power take off mechanism illustrated in  FIGS. 20 and 21 .  
         [0189]      FIGS. 26 and 27  are schematic views of a third example modification of the power take off mechanism illustrated in  FIGS. 20 and 21 .  
         [0190]      FIG. 28  is a schematic view of a tool chuck with a power take off mechanism according to an another example, non-limiting embodiment disclosed in the Copending Provisional Application.  
         [0191]      FIG. 29  is a schematic view of an example modification of the power take off mechanism illustrated in  FIG. 28 .  
         [0192]      FIG. 30  is a schematic view of a power driver having an electronic clutch according to an example, non-limiting embodiment of the present invention.  
         [0193]      FIG. 31  is a schematic view of an example modification of the electronic clutch illustrated in  FIG. 30 . 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0194]     I. Example Embodiment Depicted in  FIG. 30 :  
         [0195]      FIG. 30  schematically illustrates an example, non-limiting embodiment of a power driver  500  (e.g., a drill) having an electronic clutch in form of a control circuit  540 . The power driver  500  may have a tool chuck  550  for holding an accessory (e.g., a drill bit). It will be appreciated, however, that the control circuit  540  may be suitably implemented on a variety of power drivers (other than drills) for driving a variety of accessories (other than drill bits).  
         [0196]     A. The Structure:  
         [0197]     As shown in  FIG. 30 , the tool chuck  550  may be connected to a PTO mechanism  510 . The tool chuck  550  and the PTO mechanism  510  may have a structure like any of those illustrated in  FIGS. 1-29 .  
         [0198]     The PTO mechanism  510  may include a mode selector  512  (e.g., a mode ring or a selector button). A user may manipulate the mode selector  512 , thereby moving the component parts of the PTO mechanism  510 , to achieve various operating modes of the power driver  500 . The operating modes of the power driver  500  may include (for example) a MANUAL OVERRIDE MODE (where the user may grasp and manually rotate an input shaft of the tool chuck  550  to open and close the chuck jaws of the tool chuck  550 ), a DRILL/DRIVE MODE (where the user may power up the driver  500  to rotationally drive the tool chuck  550 ) and a CHUCK MODE (where the user may power up the driver to actuate the tool chuck  550  to open or close the chuck jaws). The PTO mechanism  510  may be coupled to a multi-speed transmission  570 .  
         [0199]     The multi-speed transmission  570  may include a speed selector  572  (e.g., a shift collar or a shift button). A user may manipulate the speed selector  572 , thereby moving the component parts of the multi-speed transmission  570 , to achieve the desired operating speed of the power driver  500 . By way of example only, the multi-speed transmission  570  may have three gear reductions (and thus three operating speeds), but the invention is not limited in this regard. A variety of multi-speed transmissions (inclusive of planetary reduction transmissions and conventional parallel axis transmissions) are well known in this art, and therefore a detailed discussion of the same is omitted. The multi-speed transmission  570  may be coupled to a motor  590 .  
         [0200]     The motor  590  may be electrically coupled to (and draw current from) a battery pack  520 . In alternative embodiments, however, the same control principles may be implemented on a corded power driver with an AC main line as a power source. A switch  542  may be provided on the power line extending between the motor  590  and the battery pack  520 . The switch  542  may be opened and closed via the control circuit  540 .  
         [0201]     The control circuit  540  may receive three inputs inclusive of a current value input  543 , a mode input  544  and a speed input  545 . The current value input  543  may indicate the current of the battery pack  520  that is drawn by the motor  590  (and thus may provide an indication of the torque applied to the armature shaft of the motor  590 ). The mode input  544  may be supplied by a mode selector position sensor  514  that may be operatively coupled to the mode selector  512 . The mode input  544  may indicate the position of the mode selector  512  (and thus the user selected operating mode of the power driver  500 ). The speed input  545  may be supplied by a speed selector position sensor  574  that may be operatively coupled to the speed selector  572 . The speed input  545  may indicate the position of the speed selector  572  (and thus the user selected operating speed of the power drive  500 ).  
         [0202]     The control circuit  540  may provide an output signal  546  to selectively open and close the switch  542 .  
         [0203]     B. The Operation:  
         [0204]     The electronic clutch (or control circuit  540 ) may operate differently depending on the three inputs  543 ,  544  and  545 . In this example embodiment, the control circuit  540  may be active when the mode input  544  indicates that the power driver  500  is in the CHUCK MODE, and it may be inactive otherwise. When the control circuit  540  is inactive, the switch  542  may default to a normal operating mode (e.g., where the switch  542  may open and close to provide a Pulse Width Modulated (“PWM”) control and/or a variable speed control, as is well known in this art) so that the motor  590  may draw current from the battery pack  520  (assuming the user powers up the driver  500 ).  
         [0205]     When the control circuit  540  is active (i.e., when the mode input  544  indicates that the power drive is in the CHUCK MODE), it may open the switch  542  (via the output signal  546 ) based on the current value input  543 . As noted above, the current value input  543  may indicate the current drawn by the motor  590  (and thus the torque applied to the armature shaft of the motor  590 ). The current value input  543  may increase with an increase in torque applied to the armature shaft of the motor  590 . In this way, the control circuit  540  may operate to open the switch  542  (to interrupt the power supply to the motor  590 ) when the torque applied to the armature shaft of the motor  590  exceeds a trip value.  
         [0206]     Consider the following example. In the CHUCK MODE, a user may power up the driver  500  in a forward direction to tighten the tool chuck  550  onto an accessory. Initially, the chuck jaws may be space apart from the accessory so that the torque applied to the armature shaft of the motor  590  may remain below the trip value (and thus the switch  542  may remain closed). Once the tool chuck is tight (i.e., when the chuck jaws clamp the accessory), the torque applied to the armature shaft of the motor  590  (and thus the current value input  543 ) may increase to a level above the trip value. Once the trip value is exceeded, the control circuit  540  may generate the output signal  546  to open the switch  542 , thereby interrupting current supply to the motor  590 . In this way, the control circuit  540  may limit the clamping force that the chuck jaws apply to the accessory.  
         [0207]     The relationship between the current drawn by the motor  590  (and thus the current value input  543 ) and the output torque of the transmission  570  may vary depending on the user selected operating speed. For example, assume the user operates the power driver  500  in the CHUCK MODE to open or close the chuck jaws. If the power drive  500  is operated in high speed, then the output torque of the transmission  570  may be less than if the power driver  500  had been operated in low speed. Thus, the chuck jaws may apply a clamping force on the accessory that varies from one user selected operating speed to the next.  
         [0208]     To compensate for the variability in the output torque of the transmission  570 , the trip value of the current value input  543  may be proportionalized so that the output torque of the transmission  570  (and thus the clamping force applied to the accessory) may remain the same, regardless of the user selected operating speed. In this way, the tightening force of the chuck  550  may remain the same, regardless of the user selected operating speed. By way of example only, consider the following scenario.  
         [0209]     Assume the power drive has three operating speeds inclusive of speed  1  (where the tool chuck  550  may be rotationally driven at 1000 rmp), speed  2  (where the tool chuck  550  may be rotationally driven at 2000 rpm) and speed  3  (where the tool chuck  550  may be rotationally driven at 3000 rmp). Further assume that the power drive  500  is operated in the CHUCK MODE. If the power driver  500  is operated in speed  3  (as selected by the user), the control circuit  540  may generate the output signal  546  (to open the switch  542 ) when the current value input  543  exceeds a trip value I 3 . If the power driver  500  is operated in speed  2  (as selected by the user), the control circuit  540  may generate the output signal  546  when the current value input  543  exceeds a trip value (⅔)I 3 . And if the power driver  500  is operated in speed  1  (as selected by the user), the control circuit  540  may generate the output signal  546  when the current value input  543  exceeds a trip value (⅓)I 3 . In this way, the control circuit  540  may operate to open the switch  542  at a trip value (as indicated by the current value input  543 ) corresponding to the user selected operated speed, so that the tool chuck  550  (when tightened) may apply a consistent clamping force on the accessory.  
         [0210]     II. First Example Modification— FIG. 31 :  
         [0211]      FIG. 31  shows an example modification of a control circuit  540  that is somewhat similar to the one depicted in  FIG. 30 . Here, however, the control circuit  540  may receive an input from a direction sensor  530 .  
         [0212]     The direction sensor  530  may be operatively coupled to a direction selector (not shown in  FIG. 31 ) of the power driver. A user may manipulate the direction selector so that the power driver may be operated in a forward or a reverse direction. The direction sensor  530  may generate a direction input  547 . The direction input  547  may indicate the position of the direction selector (and thus the user selected operating direction of the power driver).  
         [0213]     The trip value (as indicated by the current value input  543 ) of the control circuit  540  may depend on the direction input  547  (in addition to the speed input  545 ). It may be desirable, for example, for the control circuit  540  to operate at a relatively low trip value when the power drive is operated in the forward direction, and a relatively high trip value when the power driver is operated in the reverse direction. Consider the following scenario.  
         [0214]     Assume the power driver is operated in the CHUCK MODE (as indicated by the mode input  544 ) and at operating speed  3  (as indicated by the speed input  545 ). If the power drive is operated in the forward direction (as indicated by the direction input  547 ) to close the chuck jaws, then the control circuit  540  may generate the output signal  546  (to open the switch  542 ) when the current value input  543  exceeds a trip value I 3F . And if the power driver is operated in the reverse direction (as indicated by the direction input  547 ) to open the chuck jaws, then the control circuit  540  may generate the output signal  546  (to open the switch  542 ) when the current value input  543  exceeds a trip value I 3R . Here, I 3F  may be less than I 3R . In this way, the control circuit  540  may operate to open the switch  542  at a trip value (or current value input  543 ) corresponding to the user selected operating direction to ensure that the tool chuck may be operated in the CHUCK MODE to open the chuck jaws.  
         [0215]     It will be readily apparent that a trip value may be appropriately set in the forward and the reverse directions for each available operating speed.  
         [0216]     III. Second Example Modification:  
         [0217]     Some power drivers may include a mechanical clutch mechanism. The user may select the torque threshold of the mechanical clutch mechanism to limit the output torque of the multi-speed transmission. Such mechanical clutch mechanisms are well known in this art, and therefore a detailed discussion of the same is omitted.  
         [0218]     If the user selects a relatively low torque threshold for the mechanical clutch mechanism, and the selected low torque threshold is less than the trip torque (corresponding to the trip value) of the control circuit  540 , then (assuming the power driver is operated in the CHUCK MODE) the chuck jaws may not apply the desired amount of clamping pressure on the accessory. That is, the mechanical clutch mechanism may give way and prevent the current value input  543  from reaching a level that would exceed the trip value of the control circuit  540 .  
         [0219]     Accordingly, the mode selector  512  may be suitably designed so that when the user selects the CHUCK MODE, the mode selector  512  may lock out (or disable) the mechanical clutch mechanism. Such lock out features are well known in this art, and therefore a detailed discussion of the same is omitted. In this way, when the user manipulate the mode selector  512  to achieve the CHUCK MODE, the tool chuck may be actuated so that the chuck jaws apply the desired clamping pressure on the accessory.  
         [0220]     IV. Third Example Modification:  
         [0221]     In this example modification, the control circuit  540  may be active in the CHUCK MODE (as in the previous embodiments) and in the DRILL/DRIVE MODE. Here, the power driver may or may not include a mechanical clutch mechanism.  
         [0222]     When the power driver  500  is operated in the DRILL/DRIVE mode (as indicated by the mode input  544 ), the control circuit  540  may monitor the current value input  543  and may operate (at a trip value) to limit the torque of the tool chuck  550  as it is being rotationally driven. Further, the trip value of the control circuit  540  may vary depending on the user selected operating speed (as indicated by the speed input  545 ).  
         [0223]     When the power driver  500  is operated in the CHUCK mode (as indicated by the mode input  544 ), the control circuit  540  may monitor the current value input  543  and may operate (at a trip value) to limit the clamping force that the chuck jaws apply to the accessory. Further, the trip value of the control circuit  540  may vary depending on the user selected operating speed (as indicated by the speed input  545 ).  
         [0224]     As discussed above, the trip values of the control circuit  540  may depend on the mode input  544  and the speed input  545 . It will be appreciated that the trip values established for the DRILL/DRIVE mode may be different than the trip values established for the CHUCK mode. Further, the trip values of the control circuit  540  may depend on the direction input  547 .  
         [0225]     V. Fourth Example Modification:  
         [0226]     Conventionally, power drivers may include a trigger switch that is actuatable to power up the driver. As is well known in this art, such trigger switches may offer a variable speed feature in which the user may vary the power supplied to the driver based on the actuation level of the trigger switch. For example, if the user fully actuates the trigger switch, then the power driver may operate at a relatively fast speed, and if the user actuates the trigger switch (but not fully), then the power driver may operate a relatively slow speed. When the power driver is operated at full speed (i.e., the trigger switch is fully actuated), a relatively small amount of current may be drawn by the motor for a given level of torque. And when the same power driver is operated at a lower speed (i.e., the trigger switch is not fully actuated), a relatively high amount of current may be drawn by the motor to supply the same level of torque.  
         [0227]     The control circuit  540  may compensate for the variability of the torque that may be applied by the motor armature shaft as a function of the trigger switch&#39;s variable speed feature. To this end, when the power driver is operated in the CHUCK MODE (as indicated by the mode input  544 ), the control circuit  540  may override the variable speed feature of the trigger switch and operate the power driver at a set speed. In this way, the control circuit  540  may ensure that the trip level may achieve the appropriate tightening torque at the tool chuck  550 .  
         [0228]     VI. Fifth Example Modification:  
         [0229]     When the tool chuck  550  tightens on an accessory, the tightening force may come from two sources: the power of the motor  590 ; and kinetic energy stored in the components of the motor (e.g., the armature shaft) and the gear train (e.g., the multi-speed transmission  570 ). The kinetic energy may vary as a function of the user selected operating speed. For example, operating at a high speed may result in an amount of kinetic energy that is greater than an amount of kinetic energy that may result from operating at a low operating speed. Also, as the gear reduction increases through the use of more reductions in the transmission  570 , more of the gear train may be rotating, which will also store more kinetic energy.  
         [0230]     To compensate for the various amounts of kinetic energy, the control circuit  540  may implement Pulse Width Modulated (“PWM”) speed adjustments that depend on the user selected operating speed (as indicated by the speed input  545 ). The PWM compensation technique may not be directly proportional to the user selected speed.  
         [0231]     The above example electronic clutches (and modifications) are schematically illustrated in that they do not show the individual circuit components in detail. However, a designer in this art will appreciate numerous and alternative circuit components that may be suitably implemented to achieve the described functionality.  
         [0232]     For example, in the disclosed embodiments, the control circuit  540  may open the switch  542  to interrupt the power supply to the motor  590 , thereby limiting the clamping force of the chuck jaws. In alternative embodiments, the control circuit  540  may open and close the switch  542  to pulse the power supply to the motor  590 , thereby limiting the clamping force of the chuck jaws. In alternative embodiments, the control circuit  540  may electronically limit the power supply to the motor  590  for a period of time, and then open the switch  542 .