Abstract:
A hand-held power tool having a multi-speed transmission and a clutch. The multi-speed transmission and the clutch are coupled to one another via a set of interconnecting tabs that are slidingly engaged to one another and secured with pins to inhibit the withdrawal of the tabs from one another. The clutch may include a clutch member, a unitarily formed clutch plate and a plurality of engagement members. The clutch plate includes an annular plate member and a plurality of leg members that extend generally perpendicularly from the annular plate member and which bias the engagement members into engagement with the clutch member. The clutch member may be coupled to an element of the multi-speed transmission, such as to the ring gear of a planetary gear set, so as to reduce the overall size of the power tool.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/352,045, filed Jan. 25, 2002. 

   FIELD OF THE INVENTION 
   The present invention generally relates to power tools such as rotatable drills, power screwdrivers, and rotatable cutting devices. More specifically, the present invention relates to improvements in power tools and more particularly to the construction of a clutch mechanism and its coupling to a transmission. 
   BACKGROUND OF THE INVENTION 
   Modernly, manufacturers of power tools desire to reduce the cost of producing power tools by providing designs that provide the product with a high level of robustness while reducing complexity at the assembly level and minimizing components that do not add value to the product. Manufacturers are further challenged by the demand of modern consumers for tools that are relatively smaller in size, lighter in weight and more powerful. 
   Accordingly, it is highly desirable to eliminate threaded fasteners from the power tool, such as those that are typically employed to couple the transmission assembly to the clutch mechanism. The use of threaded fasteners in these situations necessitates the incorporation of bosses to the transmission assembly and the clutch mechanism that tend to enlarge the size of the tool and which add a degree of weight to the power tool. The fastening process itself tends to be relatively slow and errors in the process, such as over tightening, which can lead to the stripping of threads or cracking of the components, or under tightening, which can create an interference that prevents the components from operating properly, are possible. The use of torque controlled fastening equipment is known to alleviate such processing errors, but this equipment can be relatively expensive to purchase and operate. 
   It is also desirable to better integrate the clutch mechanism with the transmission assembly. Many of the known power tool designs employ a modular design that is based on a power tool having no torque controlled clutch. In cases where precise torque control was needed, a clutch module could be coupled to the output end of the base tool. While configuration in this manner effectively accommodated consumer demands for both the base and torque controlled models in an economical manner, the modular configuration tended to add considerable length and weight to the power tool. 
   SUMMARY OF THE INVENTION 
   In one preferred form, the present invention provides a coupling mechanism for coupling the components of a power tool, such as a transmission assembly and a clutch mechanism. The coupling mechanism includes a first structure, such as a gear case, having at least one fastening tab that defines a coupling recess. The coupling mechanism also includes a second structure, such as a clutch sleeve, having an aperture for receiving a part of the first structure, and at least one outboard tab for receiving the fastening tab or tabs. The outboard tab(s) include a pin aperture that is aligned to the coupling recess when the first and second structures are fitted together. A pin is placed into each pin aperture and an associated coupling recess and operates to lock the fastening tab within the outboard tab to thereby inhibit relative movement between the first and second structures. 
   In another preferred form, the present invention provides a power tool having an improved clutch mechanism for limiting the torsional output of a transmission assembly. The clutch mechanism includes a clutch member that is fixedly coupled to a portion of the transmission assembly, such as the ring gear of a planetary-type reduction gear set. The clutch member includes a contoured clutch face against which a rotation-inhibiting element is biased. The torsional output of the transmission assembly is limited by the force that is exerted by the rotation-inhibiting element onto the clutch face. When the torque that is exerted on the portion of the transmission assembly exceeds a predetermined magnitude, the clutch member, along with the portion of the transmission assembly, rotates relative to the rotation inhibiting element to thereby limit the torsional output of the power tool. 
   In yet another preferred form, the present invention provides an improved clutch mechanism. The clutch mechanism includes a unitarily formed clutch plate having an annular plate member and a plurality of legs that extend outwardly from the annular plate member toward a clutch member. The opposite end of the legs may be contoured to receive force-transmitting elements, such as bearing balls, which are employed to transmit a biasing force to the clutch member to bias the clutch member in a stationary condition. Alternatively, the opposite ends of the legs may be contoured to act as force transmitting elements. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a side view of a power tool constructed in accordance with the teaching of the present invention; 
       FIG. 2  is an exploded perspective view of a portion of the power tool of  FIG. 1 ; 
       FIG. 3  is an enlarged view of a portion of  FIG. 2  illustrating the transmission assembly and the clutch mechanism in greater detail; 
       FIG. 4  is an exploded perspective view of a portion of the power tool of  FIG. 1  illustrating the construction of the gear case and the clutch sleeve; 
       FIG. 5  is a sectional view of a portion of the power tool of  FIG. 1  taken along the longitudinal axis of the power tool and illustrating the construction of the transmission assembly; 
       FIG. 6  is a sectional view of a portion of the transmission assembly illustrating the second planetary gear set in the active position; 
       FIG. 7  is a perspective view of a portion of the transmission assembly illustrating the contour of the top and rear surfaces of the second reduction carrier; 
       FIG. 8  is a perspective view of a portion of the transmission assembly illustrating the third ring gear in greater detail; 
       FIG. 9  is a sectional view taken along the line  9 — 9  of  FIG. 3 ; 
       FIG. 10  is a partial bottom view of a portion of the transmission assembly illustrating the speed selector mechanism in greater detail; 
       FIG. 11  is a sectional view of a portion of the power tool of  FIG. 1  taken through the gear case and clutch sleeve and illustrating the method by which the transmission assembly and the clutch mechanism are coupled; 
       FIG. 12  is a side view of a the clutch plate; 
       FIG. 13  is an exploded side view in partial section illustrating the clutch plate and the balls; 
       FIG. 14  is a sectional view similar to that of  FIG. 13  but illustrating an alternate embodiment of the clutch plate; and 
       FIG. 15  is a sectional view of a portion of the power tool of  FIG. 1  taken along the longitudinal axis and illustrating the clutch mechanism in greater detail. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIGS. 1 and 2  of the drawings, a power tool constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . As those skilled in the art will appreciate, the preferred embodiment of the present invention may be either a cord or cordless (battery operated) device, such as a portable screwdriver or drill. In the particular embodiment illustrated, the power tool  10  is a cordless drill having a housing  12 , a motor assembly  14 , a multi-speed transmission assembly  16 , a clutch mechanism  18 , an output spindle assembly  20 , a chuck  22 , a trigger assembly  24  and a battery pack  26 . Those skilled in the art will understand that several of the components of the power tool  10 , such as the chuck  22 , the trigger assembly  24  and the battery pack  26 , are conventional in nature and therefore need not be discussed in significant detail in the present application. Reference may be made to a variety of publications for a more complete understanding of the conventional features of the power tool  10 . One example of such publications is U.S. Pat. No. 5,897,454 issued Apr. 27, 1999, the disclosure of which is hereby incorporated by reference as if fully set forth herein. 
   The housing  12  includes a pair of mating handle shells  34  that cooperate to define a handle portion  36  and a drive train or body portion  38 . The trigger assembly  24  and the battery pack  26  are mechanically coupled to the handle portion  36  and electrically coupled to the motor assembly  14  in a conventional manner that is not specifically shown but which is readily the capabilities of one having an ordinary level of skill in the art. The body portion  38  includes a motor cavity  40  and a transmission cavity  42 . The motor assembly  14  is housed in the motor cavity  40  and includes a rotatable output shaft  44 , which extends into the transmission cavity  42 . A motor pinion  46  having a plurality of gear teeth  48  is coupled for rotation with the output shaft  44 . The trigger assembly and battery pack  26  cooperate to selectively provide electric power to the motor assembly  14  in a manner that is generally well known in the art so as to permit the user of the power tool  10  to control the speed and direction with which the output shaft  44  rotates. 
   The transmission assembly  16  is housed in the transmission cavity  42  and includes a speed selector mechanism  60 . The transmission assembly  16  receives a rotary input from the motor pinion  46  and converts that input to a relatively lower speed, higher torque output that is transmitted to the shaft  62  of the output spindle assembly  20 . The transmission assembly  16  includes a plurality of reduction elements that are selectively engaged by the speed selector mechanism  60  to provide a plurality of speed ratios. Each of the speed ratios multiplies the speed and torque of the drive input in a predetermined manner, permitting the output speed and torque of the transmission assembly  16  to be varied in a desired manner between a relatively low speed, high torque output and a relatively high speed, low torque output. Rotary power output from the transmission assembly  16  is transmitted to the output spindle assembly, to which the chuck  22  is coupled for rotation, to permit torque to be transmitted to a tool bit (not shown). The clutch mechanism  18  is coupled to the transmission assembly and is operable for limiting the magnitude of the torque associated with the output of the transmission assembly  16  to a predetermined, selectable torque limit. 
   Transmission Assembly 
   With additional reference to  FIG. 3 , the transmission assembly  16  is illustrated to further include a gear case  100  that houses a three-stage, two-speed gear train  102 . With additional reference to  FIG. 4 , the gear case  100  is shaped in a generally hollow cylindrical manner and includes a fastening tab  104  and a clip aperture  106  on each of its lateral sides, a pair of guide rails  108  and a guide tab  110  that is located on its top surface, and a central cavity  112  that extends longitudinally through the gear case  100 . Each fastening tab  104  terminates at its outward face at a coupling recess  114  that extends in a direction that is generally transverse to the central cavity  112 . The coupling recess  114  is preferably arcuately shaped, and in the particular embodiment illustrated, is illustrated to be generally U-shaped. Each clip aperture  106  extends through the wall  116  of the gear case  100  along the longitudinal axis  118  of the central cavity  112  and intersects the central cavity  112 . The guide rails  108  positioned rearwardly of the guide tab  110  and are spaced laterally apart from one another. The guide rails  108  and the guide tab  110  will be discussed in further detail, below. 
   The gear train  102  is illustrated to be a planetary type gear train, having a first planetary gear set  120 , a second planetary gear set  122  and a third planetary gear set  124 . In the example provided, each of the first, second and third gear sets  120 ,  122  and  124  are operable in an active mode, wherein the gear set performs a speed reduction and torque multiplication operation, while the second planetary gear set  122  is also operable in an inactive mode, wherein it provides a rotary output having a speed and torque that is about equal to that which is input to it. 
   The first planetary gear set  120  includes first ring gear  130 , a first set of planet gears  132  and a first reduction carrier  134 . The first ring gear  130  is an annular structure, having a plurality of gear teeth  130   a  that are formed about its interior diameter and a plurality of gear case engagement teeth  130   b  that are formed onto its outer perimeter. With additional reference to  FIG. 5 , the first ring gear  130  is disposed within the central cavity  112  of the gear case  100  such that the gear case engagement teeth  130   b  engage mating teeth  130   c  formed on the inner surface of the gear case  100  to inhibit relative rotation between the first ring gear  130  and the gear case  100 . As the gear case engagement teeth  130   b  terminate prior to the rear face  130   d  of the first ring gear  130 , forward movement of the first ring gear  130  is halted by interference between the mating teeth  130   c  that are formed on the inner surface of the gear case  100  and the portion of the first ring gear  130  that is disposed rearwardly of the gear case engagement teeth  130   b.    
   The first reduction carrier  134  includes a body  134   a , which is formed in the shape of a flat cylinder and a plurality of cylindrical pins  134   b  that extend from the rearward face of the body  134   a , and a plurality of  134   c    
   A plurality of gear teeth  134   c  are formed into the outer perimeter of the body  134   a  and are sized to engage the gear teeth  152   a  of the second ring gear  152 . With reference to  FIG. 7 , the profile of the gear teeth  134   c  of the body  134   a  is illustrated in greater detail. As shown, each tooth  134   c  terminates at a gradual radius  190  at the forward face of the body  134   a  but terminates abruptly at the rearward face of the body  134   a . A radius  192  is also formed on the valleys  194  between the gear teeth  134   c . The first set of planet gears  132  includes a plurality of planet gears  132   a , each of which being generally cylindrical in shape and having a plurality of gear teeth  132   b  formed onto its outer perimeter and a pin aperture (not specifically shown) formed through its center. Each planet gear  132   a  is rotatably supported on an associated one of the pins  132   b  of the first reduction carrier  134  and is positioned to be in meshing engagement with the gear teeth of the first ring gear  130 . A first annular thrust washer  140  is fitted to the end of the gear case  100  proximate the motor assembly  14  and prevents the planet gears  132  from moving rearwardly and disengaging the pins  134   b  of the first reduction carrier  134 . A raised portion  142  is formed onto the front and rear faces of each planet gear  132  to inhibit the gear teeth  132   b  of the planet gears  132  from rubbing on the first reduction carrier  134  and the first thrust washer  140 . The teeth  46   a  of the motor pinion  46  are also meshingly engaged with the teeth  132   b  of the planet gears  132  and as such, the motor pinion  46  serves as the first sun gear for the first planetary gear set  120 . 
   The second planetary gear set  122  is disposed within the central cavity  112  forward of the first planetary gear set  120  and includes a second sun gear  150 , a second ring gear  152 , a second reduction carrier  154  and a second set of planet gears  156 . The second sun gear  150  is fixed for rotation with the first reduction carrier  134  and includes a plurality of gear teeth  150   a  that extend forwardly from the flat, cylindrical portion of the first reduction carrier  134 . 
   The second ring gear  152  is an annular structure having a plurality of gear teeth  152   a  formed about its interior diameter, an annular clip groove  158  formed into its outer perimeter and a plurality of gear case engagement teeth  160  that are formed onto its outer perimeter. The gear teeth  152   a  may be heavily chamfered at the rear face  152   b  of the second ring gear  152  but terminate abruptly its front face. More preferably, a heavy radius  170  is formed onto the rear face  152   b  and the sides of each of the gear teeth  152   a  as illustrated in  FIG. 6 , with the heavy radius  170  being employed rather than the heavy chamfer as the heavy radius  170  on the gear teeth  152   a  provides for better engagement between the second ring gear  152  and the second reduction carrier  154 , as will be described in more detail, below. In the example illustrated, the clip groove  158  is a rectangular slot having a pair of sidewalls  174 . The clip groove  158  will be discussed in further detail, below. 
   The second ring gear  152  is movably disposed within the central cavity  112  of the gear case  100  between a first position as shown in  FIG. 6 , wherein the gear case engagement teeth  160  engage mating teeth  180  formed on the inner surface of the gear case  100  to inhibit relative rotation between the second ring gear  152  and the gear case  100 , and a second position as shown in  FIG. 5 , wherein the gear case engagement teeth  160  are axially spaced apart from the mating teeth  180  to thereby permit relative rotation between the second ring gear  152  and the gear case  100 . 
   The second reduction carrier  154  includes a body  154   a , which is formed in the shape of a flat cylinder, and plurality of pins  154   b  that extend from the rearward face of the body  154   a.    
   Referring back to  FIGS. 3 and 5 , the second set of planet gears  156  is shown to include a plurality of planet gears  156   a , each of which being generally cylindrical in shape and having a plurality of gear teeth  156   b  and a pin aperture (not specifically shown) in its center. Each planet gear  156   a  is supported for rotation on an associated one of the pins  154   b  of the second reduction carrier  154  and is positioned such that the gear teeth  156   b  are in meshing engagement with gear teeth  152   a  of the second ring gear  152 . 
   The third planetary gear set  124  is disposed on the side of the second planetary gear set  122  opposite the first planetary gear set  120 . Like the second planetary gear set  122 , the third planetary gear set  124  includes a third sun gear  200 , a third ring gear  202 , a third reduction carrier  204  and a third set of planet gears  206 . The third sun gear  200  is fixed for rotation with the body  154   a  of the second reduction carrier  154  and includes a plurality of gear teeth  200   a  that extend forwardly from the body  154   a . An annular second thrust washer  210  is disposed between the second ring gear  152  and the third ring gear  202  and operates to limit the forward movement of the second ring gear  152  and the rearward movement of the third ring gear  202  and the third set of planet gears  206 . The second thrust washer  210 , which includes an aperture  212  through which the third sun gear  200  extends, engages the inner surface of the gear case  100 . 
   The third ring gear  202  is an annular structure having a plurality of gear teeth  202   a  formed about its interior diameter and an outer radial flange  220  that forms its outer perimeter. A clutch face  222  is formed into the forward surface of the outer radial flange  220 . In the particular embodiment illustrated, the clutch face  222  is shown to have an arcuate cross-sectional profile and is further defined by a plurality of peaks  224  and valleys  226  that are arranged relative to one another to form a series of ramps that are defined by an angle of about 18°. Those skilled in the art will understand, however, that clutch faces of other configurations, such as those having a sinusoidal shape, may also be employed. Those skilled in the art will also understand that while the clutch face  222  is shown to be unitarily formed with the third ring gear  202 , multi-component configurations may also be employed. Such multi-component configurations include, for example, an annular clutch face ring (not shown) having a rearward facing first side for engaging the third ring gear  202  and a forward facing second side that forms the clutch face  222 . Configuration in this latter manner may be advantageous, for example, when it is necessary for the clutch face  222  to have properties or characteristics (e.g., lubricity, hardness, toughness, surface finish) that are different from the properties or characteristics of the third ring gear  202 . 
   The third reduction carrier  204  includes a body  204   a , which is formed in the shape of a flat cylinder, and a plurality of cylindrical pins  204   b , which extend from the rearward face of the body  204   a , and a coupling portion  204   c  that extends from the forward face of the body  204   a . Rotary power transmitted to the third reduction carrier  204  is transmitted through the coupling portion  204   c  to a coupling member  230  that engages the shaft  62 of the output spindle assembly  20 . Those skilled in the art will understand that various other coupling devices and methods may be utilized to couple the third reduction carrier  204  to the output spindle assembly  20 , such as a direct coupling of the shaft  62  of the output spindle assembly  20  to the body  204   a  of the third reduction carrier  204 . 
   The third set of planet gears  206  includes a plurality of planet gears  206   a , each of which being generally cylindrical in shape and having a plurality of gear teeth  206   b  formed onto its outer perimeter and a pin aperture (not specifically shown) formed through its center. Each planet gear  206   a  is rotatably supported on an associated one of the pins  204   b  of the third reduction carrier  204  and is positioned to be in meshing engagement with the gear teeth  202   a  of the third ring gear  202 . 
   The speed selector mechanism  60  is illustrated to include a slider body  240  and a clip structure  242 . The slider body  240  is an elongated structure that is configured to be housed between the handle shells  34  and selectively slid along the top of the gear case  100 . The slider body  240  includes an attachment groove  246 , which permits the clip structure  242  to be attached to the slider body  240 , and a selector tab  248 , which is configured to receive an input from the user of the power tool  10  to switch the second planetary gear set  122  between the active and inactive modes. With additional reference to  FIGS. 9 and 10 , a slot  250  is formed into the underside of the slider body  240  and is sized to engage the guide tab  110  that extends from the top surface of the gear case  100 . The guide rails  108  are spaced laterally apart to receive the slider body  240 . The guide tab  110  and the guide rails  108  cooperate with the sides of the slot  250  and the sides of the attachment groove  246 , respectively, to guide the slider body  240  as the slider body  240  is moved in an axial direction along the top surface of the gear case  100 . 
   Returning to  FIG. 3 , the clip structure  242  is a wire that is formed to include a circular body portion  256  and a pair of end tabs  258  that extend inwardly from the body portion  256 . The body portion  256  is fixedly coupled to an attachment tab  260 , which is illustrated to be a pair of trunnions that extend downwardly from the slider body  240 . The body portion  256  is sized to fit over the outer circumference of the gear case  100  and preferably includes a rotation-inhibiting element  262  to inhibit the clip structure  242  from rotating relative to the attachment tab  260 . In the embodiment provided, the rotation-inhibiting element  262  is illustrated to include a plurality of bends, such as M-, N-, S-, or Z-shaped bends, that are formed into the wire and which are molded into or abut the underside of the slider body  240 . Each of the end tabs  258  extends through an associated one of the clip apertures  106  in the sides of the gear case  100  and engages the annular clip groove  158  that is formed into the perimeter of the second ring gear  152 . The wire that forms the clip structure  242  is somewhat smaller in diameter than the width of the clip groove  158 . 
   Alternatively, the rotation-inhibiting element  262  may include a plurality of tabs that are formed from bends in the body portion  256  of the wire, wherein each tab is defined by a circumferentially extending segment that is offset radially outwardly from the remainder of the body portion  256 . Each of the tabs is configured to be received in a corresponding aperture formed into the slider body  240  such that the front and rear faces of each tab engage the sides of the apertures in the slider body  240 . The tabs, being confined within an associated aperture in the slider body  240 , inhibit relative movement between the slider body  240  and the body portion  256  of the clip structure  242 . 
   Sliding movement of the slider body  240  relative to the gear case  100  is operable for transmitting a force through the end tabs  258  of the clip structure  242  and to the second ring gear  152  which may be used to move the second ring gear  152  between the first and second positions. When the second ring gear  152  is positioned in the first position as illustrated in  FIG. 6 , the engagement teeth  160  of the second ring gear  152  are engaged to the mating engagement teeth  180  of the gear case  100  and the gear teeth  152   a  of the second ring gear  152  are engaged to only the gear teeth  156   b  of the planet gears  156   a  of the second planet gear set  156 , thereby permitting the second planetary gear set  122  to operate in the active mode. When the second ring gear  152  is positioned in the second position as illustrated in  FIG. 5 , the engagement teeth  160  of the second ring gear  152  are not engaged to the mating engagement teeth  180  of the gear case  100  and the gear teeth  152   a  of the second ring gear  152  are engaged to both the gear teeth  156   b  of the planet gears  156   a  of the second planet gear set  156  and the gear teeth  134   c  of the first reduction carrier  134 , thereby permitting the second planetary gear set  122  to operate in the inactive mode. 
   Clutch Mechanism 
   In  FIG. 3 , the clutch mechanism  18  is illustrated to include a clutch sleeve  300 , a clutch member  302 , a plurality of balls  304 , a clutch plate  306 , a spring  308 , an adjustment collar  310 , a detent mechanism  312  and a clutch cover  314 . With additional reference to  FIG. 4 , the clutch sleeve  300  is illustrated to include a wall member  320 , which defines a hollow cavity or bore  322  that extends along the longitudinal axis of the clutch sleeve  300 , a base portion  324  and a nose portion  326  that extends forwardly from the base portion  324 . The rearward end of the bore  322  is sized to receive a forward portion of the gear case  100 , the third ring gear  202  and the third reduction carrier  204 , while the forward portion of the bore  322  is sized somewhat smaller so as to receive the coupling member  230  and the shaft  62  of the output spindle assembly  20 . The nose portion  326 , which is somewhat smaller in diameter than the base portion  324 , is generally cylindrical, having a helical thread form  330  that wraps around its perimeter. 
   The base portion  324  includes a pair of outboard tabs  334 , which are formed on the lateral sides of the base portion  324 , a plurality of leg apertures  336 , which extend generally perpendicular to the longitudinal axis of the bore  322 , and a detent aperture  338  for receiving the detent mechanism  312 . Each outboard tab  334  is configured to receive an associated one of the fastening tabs  104  and includes a pin aperture  340 . In the particular embodiment illustrated, each outboard tab  334  is defined by an outer lateral wall  342 , a lower wall  344 , and an upper wall  346 , through which the pin aperture  340  extends. With additional reference to  FIG. 11 , a cylindrical locking pin  350  is fitted through the pin aperture  340  in each outboard tab  334  and the coupling recess  114  in the associated fastening tab  104  and thereby fixedly but removably couples the clutch sleeve  300  to the gear case  100 . The locking pins  350  are highly advantageous in that they eliminate the need for threaded fasteners, fastening tools and the use of bosses in the gear case  100  and the clutch sleeve  300  that are configured for receiving a conventional threaded fastener. The leg apertures  336  are circumferentially spaced about the nose portion  326  and extend through the base portion  324  and intersect the rearward portion of the bore  322 . The detent aperture  338  extends through the base portion  324  between the clutch cover  314  and the gear case  100  and is sized to receive a portion Of the detent mechanism  312 . 
   In  FIGS. 3 ,  12  and  13 , the clutch plate  306  is illustrated to be a unitarily formed structure that includes a washer-like annular plate member  360  and a plurality of leg members  362  that are coupled to and circumferentially spaced about the annular plate member  360 . The leg members  362  have a generally circular cross-section and extend generally perpendicularly from the plate member  360 . The end of the each leg member  362  opposite the plate member  360  terminates in a spherical recess  364  that is configured to receive one of the balls  304 , which are illustrated to be hardened bearing balls. The clutch plate  306  is disposed over the nose portion  326  of the clutch sleeve  300  and moved axially rearward to push the leg members  362  through the leg apertures  336  in the base portion  324 , as well as to bring each of the balls  304  into contact with the clutch face  222  and an associated one of the spherical recesses  364 . 
   In an alternate embodiment illustrated in  FIG. 14 , the clutch plate  306 ′ is illustrated to be similar to the clutch plate  306 , except that the ends of the leg members  362 ′ opposite the annular plate member  360  terminate at a spherical protrusion  370 , rather than a spherical recess. Configuration in this manner is advantageous in that it eliminates the balls  304  from the clutch mechanism  18 . 
   Returning to  FIG. 3  and with additional reference to  FIG. 15 , the spring  308  is illustrated to be a conventional compression spring having ground ends. The spring  308  is disposed over the nose portion  326  of the clutch sleeve  300  between the plate member  360  of the clutch plate  306  and the adjustment collar  310 . The adjustment collar  310  is an annular structure that is illustrated to include an internal annular flange  380 , a threaded portion  382  and an engagement portion  384 . The internal annular flange  380  extends around the inner circumference of the adjustment collar  310  and sized somewhat smaller in diameter than the spring  308  but larger than the nose portion  326  of the clutch sleeve  300 . The threaded portion  382  intersects the internal annular flange  380  and is sized to threadably engage the thread form  330  that is formed on the outer diameter of the nose portion  326 . The engagement portion  384  is configured to permit the adjustment collar  310  to be rotatably coupled to the clutch cover  314  and well as to move axially within the clutch cover  314 . In the example provided, the engagement portion  384  includes a plurality of engagement teeth  384   a  that are formed about the outer perimeter of the adjustment collar  310 . The engagement teeth  384   a  will be described in further detail, below. 
   A wire clip  400  is coupled to the nose portion  326  to inhibit the removal of the adjustment collar  310  from the thread form  330 . The wire clip  400  is formed in U-shape, having a base  402  that is disposed between a pair of spaced apart legs  404 . Each of the legs  404  extends in a generally perpendicular direction away from the base  402 . With the clutch plate  306  and spring  308  fitted over the nose portion  326  and the adjustment collar  310  engaged to the thread form  330 , the wire clip  400  is fitted over the nose portion  326  generally perpendicular to the longitudinal axis of the clutch sleeve  300  such that legs  404  are engaged to leg apertures  408  in the clutch sleeve  300  and the base  402  is disposed in a shallow U-shaped recess  410  that is situated on the top surface of the nose portion  326  as best shown in  FIG. 4 . Engagement of the wire clip  400  into the leg apertures  408  and recess  410  operatively locks the wire clip  400  to the nose portion  326  and thereby creates a positive stop that is configured to prevent the adjustment collar  310  from being threaded out of engagement with the thread form  330  that is formed onto the nose portion  326 . 
   The clutch cover  314  is constructed in the form of a hollow sleeve that shrouds the clutch plate  306 , the spring  308 , the nose portion  326  and the wire clip  400 . The clutch cover  314  extends forwardly of the base portion  324  and includes a gripping surface  420  that is formed on its outer perimeter. The gripping surface  420  is contoured to permit the user of the power tool  10  to rotate the clutch cover  314  about the longitudinal axis of the power tool  10  to adjust the setting of the clutch mechanism  18  as will be discussed in greater detail, below. 
   A plurality of mating engagement teeth  422  are formed onto the inner diameter of the clutch cover  314  which are sized to engage the engagement teeth  384   a  of the adjustment collar  310 . The mating engagement teeth  422  are relatively longer than the engagement teeth  384   a  and as such, permit the engagement teeth  384   a  to axially slide along the mating engagement teeth  422  along the longitudinal axis of the power tool  10  when the clutch cover  314  is rotated. 
   In the example provided, the detent mechanism  312  is illustrated to include a detent spring  430 , a plunger  432  and a detent ring  434 . The detent spring  430  and plunger  432  are housed in the detent aperture  338  that is formed through the base portion  324  of the clutch sleeve  300 . The detent spring  430 , which is illustrated to be a conventional compression spring, abuts the gear case  100  on a first side and a flattened end of the plunger  432  on the opposite side, thereby biasing the plunger  432  in a direction outwardly from the base portion  324 . The plunger  432  includes a contact end  440 , which is defined by a spherical radius in the example illustrated, and which is biased forwardly by the detent spring  430  into contact with the detent ring  434 . In the particular embodiment provided, the detent ring  434  is integrally formed with the clutch cover  314  and includes a plurality of circumferentially spaced recesses or detents  442  that are sized to engage the contact end  440  of the plunger  432 . Each of the detents  442  is illustrated to be defined by a spherical radius that conforms to the contact end  440 . A setting indicator  450  ( FIG. 2 ) may be employed to indicate the position of the adjustment collar  310  relative to the clutch sleeve  300 . In the example provided, the setting indicator  450  includes an arrow  452  that is formed into the handle shells  34  and a scale  454  that is marked into the circumference of the clutch cover  314 . 
   Rotation of the clutch cover  314  relative to the clutch sleeve  300  causes the adjustment collar  310  to rotate in an equivalent manner to thereby alter the amount by which the spring  308  is compressed. Interaction between the contact end  440  of the plunger  432  and the detents  442  in the detent ring  434  provide the user of the power tool  10  with feedback as to the setting of the clutch mechanism  18 , as well as inhibit the clutch cover  314  from inadvertently rotating out of the position to which it has been set. The spring  308  exerts a compression force onto the annular flange  380  of the adjustment collar  310  and the plate member  360  of the clutch plate  306 , driving the leg members  362  of the clutch plate  306  rearwardly and biasing the balls  304  into engagement with the clutch face  222 . The balls  304  exert a counter torque onto the clutch face  222  that tends to inhibit rotation of the third ring gear  202  relative to the clutch sleeve  300 . 
   When the power tool  10  is operated and the torque that is exerted through the gear teeth  202   a  of the third ring gear  202  exceeds the counter torque, the peaks  224  of the clutch face  222  ride over the balls  304  to enable the third ring gear  202  to rotate relative to the third reduction carrier  204  and greatly reduce the torque that is applied to the output spindle assembly  20 . 
   While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description.