Abstract:
An article having a first structure and a second structure. The first structure includes a structural portion and an overmold portion, which is formed from a resilient material and molded onto the structural portion. The overmold portion is configured to perform an auxiliary function, such as creating a seal portion that is configured to sealing engage the second structure, an isolator portion that is configured to contact the second structure and dampen vibrations that are transmitted thereto and/or an auxiliary gripping surface.

Description:
PRIORITY &amp; CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/263,379, filed Jan. 23, 2001. Other features of the present invention are discussed and claimed in commonly assigned copending U.S. application Ser. No. 09/964,226 entitled Multi-Speed Power Tool Transmission; U.S. application Ser. No. 09/964,078 entitled First Stage Clutch; and U.S. application Ser. No. 09/965,108 entitled 360 Degree Clutch Collar. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to housings for devises such as power tools, including rotatable drills, power screwdrivers and cutting tools. More particularly, the present invention relates to a housing having an overmold portion in which the overmold portion performs an auxiliary function. 
     2. Discussion 
     Modern manufactures of power tools typically seek to provide powerful and robust tools that are both ergonomically configured and which offer exceptionally high value at a relatively inexpensive price. Often times, however, the goal of a robust, ergonomic configuration seems to be at odds with the offering of the tool at a relatively inexpensive price as additional processes, such as overmolding, or a multiplicity of parts that do not appear to add significant value, such as vibration isolators and seals, are required. 
     The overmolding of the grip of a hand tool is known in the art for purposes of reducing the transmission of vibration to the user&#39;s hand. In these situations, the outer surface of the grip of the tool is overmolded with a resilient material; the overmolded portion tends to damp the vibrations that are transmitted between the housing of the tool and the user&#39;s hand. The overmolded portion does not effect the operation of the tool per se, and as such, the use of overmolding tends to be limited to a relatively small area on the exterior of the tool so as to minimize the cost of the tool. More specifically, the overmolding operation is typically employed in a non-functional manner which adds cost to the article of manufacture without adding a commensurate amount of value. 
     While parts, such as isolators and even seals, may be necessary for the proper operation of the power tool, their discrete nature nonetheless increases the cost of the power tool, both in terms of materials and in assembly labor. Furthermore, the proliferation of component parts is known to have a detrimental on defect rates and warranty costs. Accordingly, there remains a need in the art for a functional overmold which adds value to the article of manufacture in an amount commensurate with its cost. 
     SUMMARY OF THE INVENTION 
     In one preferred form, the present invention provides an article having a first structure and a second structure. The first structure includes a structural portion and an overmold portion, which is formed from a resilient material and molded onto the structural portion. The overmold portion is configured to perform an auxiliary function, such as creating a seal portion that is configured to sealing engage the second structure, an isolator portion that is configured to contact the second structure and dampen vibrations that are transmitted thereto and/or an auxiliary gripping surface. 
    
    
     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 a perspective view of a portion of the housing of the power tool of FIG. 1 illustrating the rear of the end cap assembly; 
     FIG. 4 is a front view of the end cap assembly; 
     FIG. 5 is a section view taken along the line  5 — 5  of FIG. 4; 
     FIG. 6 is a rear view of a portion of the power tool of FIG. 1 with the end cap assembly removed; 
     FIG. 7 is a side view of a portion of the power tool of FIG. 1 with the end cap assembly removed; 
     FIG. 8 is a view similar to that of FIG. 4, but illustrating the end cap shell prior to the overmolding operation; 
     FIG. 9 is a view similar to that of FIG. 5, but illustrating the end cap shell prior to the overmolding operation; 
     FIG. 10 is a view similar to that of FIG. 4, but illustrating an alternate construction of the overmold member; 
     FIG. 11 is a partial sectional view of a portion of a power tool that employs an end cap assembly having an overmold member constructed in the manner illustrated in FIG. 10; 
     FIG. 12 is an exploded perspective view of a portion of the power tool of FIG. 1, illustrating the transmission assembly in greater detail; 
     FIG. 13 is an exploded perspective view of a portion of the power tool of FIG. 1, illustrating the reduction gearset assembly, the transmission sleeve, a portion of the housing and a portion of the clutch mechanism in greater detail; 
     FIG. 13 a  is a sectional view taken along a longitudinal axis of the second ring gear; 
     FIG. 13 b  is a sectional view taken along a longitudinal axis of the third ring gear; 
     FIG. 14 is a side view of the transmission sleeve; 
     FIG. 15 is a rear view of the transmission sleeve; 
     FIG. 16 is a sectional view taken along the line  16 — 16  of FIG. 15; 
     FIG. 17 is a sectional view taken along the line  17 — 17  of FIG. 15; 
     FIG. 18 is an exploded view of the reduction gearset assembly; 
     FIG. 19 is a sectional view taken along a longitudinal axis of the power tool of FIG. 1 illustrating a portion of the reduction gearset assembly in greater detail; 
     FIG. 20 is a front view of a portion of the first reduction carrier; 
     FIG. 21 is a sectional view taken along a longitudinal axis of the power tool of FIG. 1 illustrating a portion of the reduction gearset assembly in greater detail; 
     FIG. 22 is a rear view of a portion of the third reduction carrier; 
     FIG. 23 is an sectional view taken along the longitudinal axis of the power tool of FIG.  1  and illustrating the transmission assembly as positioned in the first speed ratio; 
     FIG. 24 is a sectional view similar to that of FIG. 23 but illustrating the transmission assembly as positioned in the second speed ratio; 
     FIG. 25 is a sectional view similar to that of FIG. 23 but illustrating the transmission assembly as positioned in the third speed ratio; 
     FIG. 26 is a top view of a portion of the power tool of FIG. 1 illustrating the speed selector mechanism in greater detail; 
     FIG. 27 a  is a side view of the rotary selector cam; 
     FIG. 27 b  is a top view of the rotary selector cam; 
     FIG. 27 c  is a sectional view taken through along the central axis of the speed selector mechanism; 
     FIG. 28 is a rear view of the output spindle assembly; 
     FIG. 29 is an exploded perspective view of the clutch mechanism; 
     FIG. 29 a  is a perspective view of a portion of the clutch mechanism illustrating another configuration of the clutch member; 
     FIG. 29 b  is an exploded perspective view illustrating a multi-piece construction for the first ring gear and clutch member; 
     FIG. 30 is a schematic illustration of the adjustment structure in an “unwrapped” state; 
     FIG. 31 is a schematic illustration similar to that of FIG. 30 but showing an alternate construction of the adjustment profile; and 
     FIG. 32 is a schematic illustration similar to that of FIG. 30 but showing a portion of another alternate construction of the adjustment profile; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Overview 
     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, 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 power tool  10 , such as the chuck  22 , the trigger assembly  24  and the battery pack  26 , are conventional in nature and need not be described in significant detail in this application. Reference may be made to a variety of publications for a more complete understanding of the operation of the conventional features of power tool  10 . One example of such publications is commonly assigned 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. 
     Housing  12  includes an end cap assembly  30  and a handle shell assembly  32  that includes a pair of mating handle shells  34 . Handle shell assembly  32  includes a handle portion  36  and a drive train or body portion  38 . Trigger assembly  24  and battery pack  26  are mechanically coupled to handle portion  36  and electrically coupled to motor assembly  14 . Body portion  38  includes a motor cavity  40  and a transmission cavity  42 . Motor assembly  14  is housed in motor cavity  40  and includes a rotatable output shaft  44 , which extends into transmission cavity  42 . A motor pinion  46  having a plurality of gear teeth  48  is coupled for rotation with output shaft  44 . Trigger assembly  24  and battery pack  26  cooperate to selectively provide electric power to motor assembly  14  in a manner that is generally well known in the art so as to control the speed and direction with which output shaft  44  rotates. 
     Transmission assembly  16  is housed in transmission cavity  42  and includes a speed selector mechanism  60 . Motor pinion  46  couples transmission assembly  16  to output shaft  44 , transmitting a relatively high speed, low torque drive input to transmission assembly  16 . Transmission assembly  16  includes a plurality of reduction elements that are selectively engaged by 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. The transmission output is delivered the output spindle assembly  20 , 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 transmission assembly  16  and is operable for limiting the magnitude of the torque associated with the drive input to a predetermined, selectable torque limit. 
     Functional Overmold 
     With specific reference to FIGS. 2 through 9, end cap assembly  30  is shown to include an end cap shell  100  and an overmold member  102 . In the example provided, the end cap shell  100  is injection molded from a plastic material, such as ABS. The end cap shell  100  defines an end cap cavity  104  that is sized to receive the portion of the motor assembly  14  that extends rearwardly of the handle shell assembly  32 . A plurality of first and second radial tab apertures  108  and  110  and the abutting face  128  are formed into the forward face  114  of the end cap shell  100  and a plurality of screw bosses  116  are formed into the perimeter of the end cap shell  100 . Each of the first and second radial tab apertures  108  and  110  is sized to receive one of the first radial tabs  120  and second radial tabs  122 , respectively, that are formed into the rearward face  124  of the handle shells  34 . The first and second radial tab apertures  108  and  110  cooperate with the first and second radial tabs  122  to properly align the end cap shell  100  to the handle shell assembly  32 , as well as to inhibit relative rotation therebetween. An arcuate portion  128  of the forward face  114  of the end cap shell  100  is angled to match the abutting face  132  of the rearward face  124  of the handle shells  34 . The screw bosses  116  permit the end cap shell  100  to be fixedly coupled to the motor cover  136  via a plurality of screws  138 . The geometry of the motor cover  136  is such that it is constrained to the handle shells  34 . As such, fastening of the end cap shell  100  to the motor cover  136  operates to fixedly retain the end cap shell  100  against the rearward face  124  of the handle shell assembly  32 , as well as to close off the rear handle aperture  139  in the handle shell assembly  32 . 
     A plurality of side apertures  140  are formed into the sides of the end cap shell  100  to permit air to flow through the handle shell assembly  32  and cool the motor assembly  14  in a manner that is well known in the art. A plurality of rear apertures  144  are formed into the rear of the end cap shell  100 , with each of the rear apertures  144  including a recessed portion  146  that extends only partially into the outer surface  148  of the end cap shell  100  and a through-portion  150  that extends completely through the end cap shell  100 . A pair of retaining tabs  152  are formed to extend from the interior surface  154  of the end cap shell  100  inwardly into the end cap cavity  104 . A channel  156  is formed into the interior surface  154  of the end cap shell  100  and intersects each of the rear apertures  144  and the retaining tabs  152 . 
     The overmold member  102  is formed from a resilient material, such as thermoplastic elastomer (e.g., HYTREL® manufactured by E.I. du Pont de Nemours and Company) and is simultaneously formed and coupled to the end cap shell  100  in an injection molding operation. In the particular example provided, the overmold member  102  includes a plurality of bumper members  170 , a pair of isolators  172  and a linking member  174 . Each of the bumper members  170  extends from a point roughly coincident with the interior surface  154  of the end cap shell  100  to a point rearwardly of the outer surface  148  of the end cap shell  100  by about 0.5 mm to about 1.5 mm and preferably about 0.75 mm. Construction in this manner permits the bumper members  170  to provide a degree of shock absorption which reduces the likelihood of damaging the end cap shell  100  in the event that the tool  10  is dropped. Furthermore, it is sometimes necessary for an operator to apply a relatively high force to the tool  10 , as when employing a hole saw to drill large diameter holes. In such situations, the operator is inclined to press onto the rear of the tool  10  to apply a force that is in-line with the axis of the chuck  22 . In such situations, the bumper members  170  provide the operator with a relatively soft and comfortable surface which tends to resist slipping as well as attenuate the vibrations that are transmitted to the operator. 
     The isolators  172  are formed about the retaining tabs  152  on the interior surface  154  of the end cap shell  100 . In the example provided, each of the isolators  172  includes an annular member  180  that extends forwardly of the interior surface  154  of the end cap shell  100 . Construction in this manner permits the end cap shell  100  to engage the isolators  172  to the outer diameter  14   a  and the rear surface  14   b  of the motor housing  14   c  to fixedly retain the motor  14   d  within the motor cover  136 . This prevents the components of the motor assembly  14  from moving along the longitudinal axis of the tool  10 , as well as dampens vibrations that are created during the operation of the motor assembly  14 . The linking member  174  is fixedly coupled to each of the bumper members  170  and the isolators  172 . The linking member  174  provides a flow path through which the resilient material flows during the formation of the bumper members  170  and the isolators  172 . The linking member  174  also interconnects the bumper members  170  and the isolators  172 , thereby rendering their removal from the end cap shell  100  more difficult. 
     Those skilled in the art will appreciate that this aspect of the present invention may be incorporated into various other positions within the handle assembly  32  for sealing between two or more components, dampening vibrations or positioning one component relative to another. One such example is illustrated in FIGS. 10 and 11 where the isolators  172  are modified to extend around the perimeter of a portion of the end cap cavity  104  and sealingly contact the rear surface  14   b  of the motor  14   d . The isolators  172  seal the interface between the end cap shell  100  and the motor assembly  14 , while the bumper members  170  seal the rear apertures  144  in the end cap shell  100 . The space  188  defined by the isolators  172  is then filled with grease or another suitable lubricant, which lubricates a motor armature bearing  190 . 
     Transmission Assembly 
     With reference to FIG. 12, the transmission assembly  16  is shown to be a three-stage, three-speed transmission that includes a transmission sleeve  200 , a reduction gearset assembly  202  and the speed selector mechanism  60 . With additional reference to FIGS. 13 through 17, the transmission sleeve  200  includes a wall member  210  that defines a generally transmission bore or hollow cavity  212  into which the reduction gearset assembly  202  is disposed. The transmission sleeve  200  includes a body  214  and a base  216 . The body  214  of the transmission sleeve  200  is fairly uniform in diameter and generally smaller in diameter than the base  216 . The inside diameter of the base  216  is sized to receive the cylindrical nose portion  220  of the motor cover  136 . 
     A plurality of raised lands  226  are formed into the base  216 . The raised lands  226  define a plurality of first grooves  228  in the outer surface  230  of the base  216  and a plurality of second grooves  232  in the inner surface  234  of the base  216 . The first grooves  228  are configured to receive the alignment ribs  238  that are formed into the inner surface  242  of the handle shells  34  to align the transmission sleeve  200  to the handle shells  34  and inhibit relative rotation between the transmission sleeve  200  and the housing  12 . Preferably, the first grooves  228  and alignment ribs  238  are configured in a manner that the transmission sleeve  200  can only be assembled to the handle shells  34  in one orientation (i.e., the configuration of the first grooves  228  and alignment ribs  238  prevents the transmission sleeve  200  from being rotated 180° out of position relative to the handle shells  34 ). The second grooves  232  will be discussed in greater detail, below. 
     The body  214  of the transmission sleeve  200  is shown to include a cylindrical body portion  246  and a pin housing portion  248 . In the particular embodiment illustrated, the cylindrical body portion  246  includes a selector cam guide  250 , a plurality of lubricant grooves  252  and first and second sets of ring engagement teeth  254  and  256 , respectively. The selector cam guide  250  is generally rectangular in cross section, extending outwardly from the top of the outer surface  258  of the body portion  246 . The lubricant grooves  252  are formed concentrically around the upper half of the perimeter of the body portion  246 . The lubricant grooves  252  have a depth of about 0.01 inch to about 0.030 inch to hold a lubricant, such as grease, on the upper half of the perimeter of the body portion  246 . The operation of the selector cam guide  250  and the lubricant grooves  252  will be discussed in detail, below. 
     A raised bead  264  segregates the interior of the body portion  246  into first and second housing portions  260  and  262 , respectively. The first set of ring engagement teeth  254  are formed onto the inner surface  266  of the body portion  246  and extend rearwardly from the raised bead  264  toward the base  216 . The second set of ring engagement teeth  256  are also formed into the inner surface of the body portion  246  but extend forwardly from the raised bead  264 . The teeth  268  of the first and second sets of ring engagement teeth  254  and  256  are uniformly spaced around the inner surface  266  of the body portion  246 . The configuration of each tooth  268  in the first and second sets of ring engagement teeth  254  and  256  is similar in that each tooth extends from the raised bead  264 , has a pair of parallel engagement surfaces  270  and terminates at a tip portion  272 . The tip portion  272  of each tooth  268  is both rounded and tapered to enhance the ability with which it will mesh with a portion of the reduction gearset assembly  202  as will be described in detail, below. 
     The pin housing portion  248  extends downwardly from the body portion  246  over a significant portion of the length of the body portion  246 . An actuator aperture  274  is formed into the pin housing portion  248  and extends rearwardly through the base  216  of the transmission sleeve  200 . In the particular embodiment illustrated, the actuator aperture  274  is stepped, having a first portion  276  with a first diameter at the rear of the transmission sleeve  200  and a second portion  278  with a smaller second diameter at the front of the transmission sleeve  200 . In the example shown, the first portion  276  of the actuator aperture  274  breaks through the wall of the first housing portion  260  and forms a groove  280  into the inner surface  234  of the base  216 . The pin housing portion  248  will be discussed in further detail, below. 
     A pair of first clip slots  284  and a pair of second clip slots  286  are formed into the transmission sleeve  200 , extending along the sides of the transmission sleeve  200  in a manner that is parallel the longitudinal axis of the transmission sleeve  200 . The first pair of clip slots  284  is formed through the sides of the body portion  246  rearwardly of the raised bead  264  and extends rearwardly toward the base  216 . The depth of the first pair of clip slots  284  is such that they do not extend through the portion of the wall member  210  that defines the base  216 . The second pair of clip slots  286  are also formed through the sides of the body portion  246  beginning forwardly of the raised bead  264  and extending through the front face  288  of the transmission sleeve  200 . 
     With reference to FIGS. 12,  13 ,  18  and  23 , the reduction gearset assembly  202  includes a first reduction gear set  302 , a second reduction gear set  304  and a third reduction gear set  306 . The first, second and third reduction gear sets  302 ,  304  and  306  are operable in an active mode and an inactive mode. Operation in the active mode causes the reduction gear set to perform a speed reduction and torque multiplication operation, while operation of the reduction gear set in an inactive mode for causes the reduction gear set to provide an output having a speed and torque that is about equal to the speed and torque of the rotary input provided to that reduction gear set. In the particular embodiment illustrated, each of the first, second and third reduction gear sets  302 ,  304  and  306  are planetary gear sets. Those skilled in the art will understand, however, that various other types of reduction gear sets that are well known in the art may be substituted for one or more of the reduction gear sets forming the reduction gearset assembly  202 . 
     As shown, the first reduction gear set  302  includes a first reduction element or ring gear  310 , a first set of planet gears  312  and a first reduction carrier  314 . The first ring gear  310  is an annular structure, having a plurality of gear teeth  310   a  formed along its interior diameter. A clutch face  316  is formed into the outer perimeter of the front face  318  of the first ring gear  310  and will be discussed in greater detail, below. The first ring gear  310  is disposed within the portion of the hollow cavity  212  defined by the base  216 ; the front face  318  of the first ring gear  310  contacts a step  320  formed into the transmission sleeve  200 , thereby limiting the ability of the first ring gear  310  to move forwardly into the hollow cavity  212 . 
     The first reduction carrier  314  is formed in the shape of a flat cylinder, having plurality of pins  322  that extend from its rearward face  324 . A plurality of gear teeth  314   a  are formed into almost the entire outer perimeter of the first reduction carrier  314 , with a valley  314   b  being formed between each pair of adjacent gear teeth  314   a . Due to the spacing of the gear teeth  314   a , one of the valleys (i.e., valley  314   b ′) is relatively larger than the remaining valleys  314   b  due to the omission of a tooth  314   a  in the outer perimeter of the first reduction carrier  314 . In the particular embodiment illustrated, the gear teeth  314   a  of the first reduction carrier  314  are configured so as not to be meshingly engagable with the gear teeth  310   a  of the first ring gear  310 . 
     With specific reference to FIGS. 19 and 20, the profile of the gear teeth  314   a  is illustrated in greater detail. As shown, each gear tooth  314   a  terminates at a gradual radius  326  at the forward face  328  of the first reduction carrier  314  but terminates abruptly at the rearward face  324  of the first reduction carrier  314 . A radius  330  is also formed on the valleys  314   b  between the gear teeth  314   a.    
     Returning to FIGS. 12,  13 ,  15 ,  18  and  23 , a first thrust washer  332  having a first annular portion  334 , a second annular portion  336  and a plurality of retaining tabs  338  is positioned rearwardly of the first reduction gear set  302 . The retaining tabs  338  engage the second grooves  232  in the base  216  of the transmission sleeve  200  and as such, relative rotation between the first thrust washer  332  and the transmission sleeve  200  is inhibited. The inside diameter of the base  216  is sized to receive the motor cover  136  and as such, the front face  340  of the motor cover  136  inhibits the axial movement of the first thrust washer  332 . The first annular portion  334  contacts the rear face  342  of the first ring gear  310 , providing a wear surface and controlling the amount by which the first ring gear  310  is able to move in an axial direction. The second annular portion  336  is spaced axially apart from the first annular portion  334 , extending forwardly of the first annular portion  334  to provide a wear surface for the first set of planet gears  312  that also controls the amount by which they can move in an axial direction. 
     The first set of planet gears  312  includes a plurality of planet gears  344 , each of which being generally cylindrical in shape, having a plurality of gear teeth  344   a  formed into its outer perimeter and a pin aperture  346  formed its their center. Each planet gear  344  is rotatably supported on an associated one of the pins  322  and the first reduction carrier  314  and is positioned such that its teeth  344   a  meshingly engage the teeth  314   a  of the first ring gear  310 . A raised portion  348  is formed into the front and rear face  350  and  352  of each planet gear  344  that inhibits the teeth  344   a  from rubbing on the first reduction carrier  314  and the first thrust washer  332  and creating dust or chips that would impair the performance of the transmission assembly  16  and reduce its operating life. As the teeth  46   a  of the motor pinion  46  on the output shaft  44  are also meshingly engaged with the teeth  344   a  of the planet gears  344 , the motor pinion  46  serves as a sun gear for the first reduction gear set  302 . 
     The second reduction gear set  304  is disposed within the portion of the hollow cavity  212  defined by the first housing portion  260  and includes a second sun gear  358 , a second reduction element or ring gear  360 , a second set of planet gears  362  and a second reduction carrier  364 . The second sun gear  358  is fixed for rotation with the first reduction carrier  314 . The second sun gear  358  includes a plurality of gear teeth  358   a  that extend forwardly of the forward face  328  of the first reduction carrier  314 . 
     The second ring gear  360  is an annular structure, having a plurality of gear teeth  360   a  formed along its interior diameter. The gear teeth  360   a  may be heavily chamfered at the rear face  366  of the second ring gear  360  but terminate abruptly at the front face  368 . More preferably, a heavy radius  369  is formed onto the rear face  366  and the sides of each of the gear teeth  360   a , with the heavy radius  369  being employed rather than the heavy chamfer as the heavy radius  369  on the gear teeth  360   a  provides for better engagement between the second ring gear  360  and the first reduction carrier  314 . 
     A plurality of sleeve engagement teeth  370  are formed into the outer perimeter of the second ring gear  360 ; the sleeve engagement teeth  370  extend forwardly toward the front face  368  of the second ring gear  360  and terminate at a tip portion  372  that is rounded and tapers forwardly and inwardly. An annular clip groove  374  is also formed into the outer perimeter of the second ring gear  360 . In the example illustrated, the clip groove  374  is a rectangular slot having a pair of sidewalls  376 . The clip groove  374  will be discussed in greater detail, below. 
     The second reduction carrier  364  is formed in the shape of a flat cylinder, having plurality of pins  378  that extend from its rearward face  380 . The second set of planet gears  362  is shown to include a plurality of planet gears  382 . Each planet gear  382  is generally cylindrical in shape, having a plurality of gear teeth  382   a  formed into its outer perimeter and a pin aperture  384  formed its center. Each planet gear  382  is rotatably supported on an associated one of the pins  378  and the second reduction carrier  364  is positioned such that the gear teeth  382   a  of the planet gears  382  meshingly engage the gear teeth  360   a  of the second ring gear  360 . The gear teeth  358   a  of the second sun gear  358  are also meshingly engaged with the gear teeth  382   a  of the planet gears  382 . 
     The third reduction gear set  306  is disposed within the portion of the hollow cavity  212  defined by the second housing portion  262  and includes a third sun gear  398 , a third reduction element or ring gear  400 , a third set of planet gears  402  and a third reduction carrier  404 . The third sun gear  398  is fixed for rotation with the second reduction carrier  364 . The third sun gear  398  includes a plurality of gear teeth  398   a  that extend forwardly of the front face  406  of the second reduction carrier  364 . 
     The third ring gear  400  is an annular structure, having a plurality of gear teeth  400   a  formed along its interior diameter. The gear teeth  400   a  may be heavily chamfered at the front face  412  of the third ring gear  400 , but terminate abruptly at the rear face  414 . More preferably, a heavy radius  407  is formed onto the front face  412  and the sides of each of the gear teeth  400   a , with the heavy radius  407  being employed rather than the heavy chamfer as the heavy radius  407  on the gear teeth  400   a  provides for better engagement between the third ring gear  400  and the third reduction carrier  404 . A plurality of sleeve engagement teeth  418  are formed into the outer perimeter of the third ring gear  400 ; the sleeve engagement teeth  418  extend rearward toward the rear face  414  of the third ring gear  400  and terminate at a tip portion  420  that is rounded and tapers rearwardly and inwardly. An annular clip groove  422  is also formed into the outer perimeter of the third ring gear  400 . In the example illustrated, the clip groove  422  is a rectangular slot having a pair of sidewalls  424 . The clip groove  422  will be discussed in greater detail, below. 
     The third reduction carrier  404  is formed in the shape of a flat cylinder, having plurality of pins  428  that extend from its rearward face  430 . A plurality of gear teeth  404   a  are formed into almost the entire outer perimeter of the third reduction carrier  404 , with a valley  404   b  being formed between each pair of adjacent teeth  404   a . Due to the spacing of the teeth  404   a , one of the valleys  404   b  (i.e., valley  404   b ′) is relatively larger than the remaining valleys  404   b  due to the omission of a tooth  404   a  in the outer perimeter of the third reduction carrier  404 . In the particular embodiment illustrated, the gear teeth  404   a  of the third reduction carrier  404  are configured so as not to be meshingly engagable with the gear teeth  382   a  of the second planet gears  382 . 
     With brief additional reference to FIGS. 21 and 22, the profile of the gear teeth  404   a  is illustrated in greater detail. As shown, the rear face  430  of the third reduction carrier  404  is chamfered and a heavy radius  434  is formed into each of sides of the teeth  404   a  and valleys  404   b . Each gear tooth  404   a  terminates abruptly at the forward face  436  of the third reduction carrier  404 . 
     Returning back to FIGS. 12,  13 ,  15 ,  18  and  23 , the third set of planet gears  402  is shown to include a plurality of planet gears  438 . Each planet gear  438  is generally cylindrical in shape, having a plurality of gear teeth  438   a  formed into its outer perimeter and a pin aperture  440  formed through its center. Each planet gear  438  is rotatably supported on an associated one of the pins  428  and the third reduction carrier  404  is positioned such that the gear teeth  438   a  of the planet gears  438  meshingly engage the gear teeth  400   a  of the third ring gear  400 . A raised portion  442  is formed into each of the front and rear faces of the planet gears  438  which inhibits the gear teeth  438   a  from rubbing on the third reduction carrier  404  and creating dust or chips that would impair the performance of the transmission assembly  12  and reduce its operating life. A second thrust washer  450  is disposed around the third sun gear  398  and the teeth  398   a  of the third sun gear  398  are meshingly engaged with the gear teeth  438   a  of the planet gears  438 . The second thrust washer  450  includes a plurality of retaining tabs  452  that are configured to engage corresponding tab grooves  454  (FIG. 13) that are formed in the inner surface  266  of body portion  246  of the transmission sleeve  200 . The retaining tabs  452  and the tab grooves  454  cooperate to inhibit relative rotation between the second thrust washer  450  and the transmission sleeve  200 . 
     The output spindle assembly  20  includes a transmitting means  458  for coupling a spindle  460  for rotation with the third reduction carrier  404  so as to transmit drive torque from the reduction gearset assembly  202  to the chuck  22 . Such transmitting means  458  are well known in the art and easily adapted to the transmission assembly of the present invention. Accordingly, a detailed discussion of the transmitting means  458  need not be included herein. 
     With reference to FIGS. 13,  13   a ,  13   b ,  16 ,  17 ,  18  and  23  through  28 , the speed selector mechanism  60  is movable between a first position  500 , a second position  502  and a third position  504  and includes a switch portion  510  for receiving a speed change input and an actuator portion  512  for manipulating the reduction gearset assembly  202  in accordance with the speed change input. The actuator portion  512  is operatively coupled to the reduction gearset assembly  202  and moves the second and third reduction gear sets  304  and  306  between the active and inactive modes in response to movement of the switch portion  510  between the first, second and third positions  500 ,  502  and  504 . In the particular embodiment illustrated, the actuator portion  512  includes a rotary selector cam  520 , a plurality of wire clips  522  and a spring member  523 . Each of the wire clips  522  is formed from a round wire which is bent in the shape of a semi-circle  524  with a pair of tabs  526  extending outwardly from the semi-circle  524  and positioned on about the centerline of the semi-circle  524 . The semi-circle  524  is sized to fit within the clip grooves  374  and  422  in the second and third ring gears  360  and  400 , respectively. In this regard, the semi-circle  524  neither extends radially outwardly of an associated one of the ring gears ( 360 ,  400 ), nor binds against the sidewalls ( 376 ,  424 ) of the clip grooves ( 374 ,  422 ). In the example provided, the sidewalls ( 376 ,  424 ) of the clip grooves ( 374 ,  422 ) are spaced apart about 0.05 inch and the diameter of the wire forming the wire clips  522  is about 0.04 inch. 
     The tabs  526  of the wire clips  522  extend outwardly of the hollow cavity  212  into an associated one of the clip slots ( 284 ,  286 ) that is formed into the transmission sleeve  200 . The tabs  526  are long enough so that they extend outwardly of the outer surface  258  of the body  214  of the transmission sleeve  200 , but not so far as to extend radially outwardly of the portion of the first clip slots  284  in the base  216  of the transmission sleeve  200 . Configuration of the wire clips  522  in this manner facilitates the assembly of the transmission assembly  16 , permitting the wire clips  522  to be installed to the second and third ring gears  360  and  400 , after which these assemblies are inserted into the hollow cavity  212  along the longitudinal axis of the transmission sleeve  200 . 
     With specific reference to FIGS. 13 and 27 a  through  27   c , the rotary selector cam  520  is illustrated to include an arcuate selector body  530 , a switch tab  532  and a plurality of spacing members  534 . A pair of first cam slots  540   a  and  540   b , a pair of second cam slots  544   a  and  544   b , a spring aperture  546  and a guide aperture  548  are formed through the selector body  530 . The selector body  530  is sized to engage the outside diameter of the body portion  246  of the transmission sleeve  200  in a slip-fit manner. The guide aperture  548  is generally rectangular in shape and sized to engage the front and rear surfaces of the selector cam guide  250 . The guide aperture  548  is considerably wider than the width of the selector cam guide  250 , being sized in this manner to permit the rotary selector cam  520  to be rotated on the transmission sleeve  200  between a first rotational position, a second rotational position and a third rotational position. The selector cam guide  250  and cooperates with the guide aperture  548  to limit the amount by which the rotary selector cam  520  can be rotated on the transmission sleeve  200 , with a first lateral side of the selector cam guide  250  contacting a first lateral side of the guide aperture  548  when the rotary selector cam  520  is positioned in the first rotational position, and a second lateral side of the selector cam guide  250  contacting a second lateral side of the guide aperture  548  when the rotary selector cam  520  is positioned in the third rotational position. 
     Each of the first cam slots  540   a  and  540   b  is sized to receive one of the tabs  526  of the wire clip  522  that is engaged to the second ring gear  360 . In the particular embodiment illustrated, first cam slot  540   a  includes a first segment  550 , a second segment  552  and an intermediate segment  554 . The first segment  550  is located a first predetermined distance away from a reference plane  558  that is perpendicular to the longitudinal axis of the rotary selector cam  520  and the second segment  552  is located a second distance away from the reference plane  558 . The intermediate segment  554  couples the first and second segments  550  and  552  to one another. The configuration of first cam slot  540   b  is identical to that of first cam slot  540   a , except that it is rotated relative to the rotary selector cam  520  such that each of the first, second and intermediate segments  550 ,  552  and  554  in the first cam slot  540   b  are located 180° apart from the first, second and intermediate segments  550 ,  552  and  554  in the first cam slot  540   a.    
     Each of the second cam slots  544   a  and  544   b  is sized to receive one of the tabs  526  of a corresponding one of the wire clips  522 . In the particular embodiment illustrated, second cam slot  544   a  includes a first segment  560 , a second segment  562 , a third segment  564  and a pair of intermediate segments  566  and  568 . The first and third segments  560  and  564  are located a third predetermined distance away from the reference plane and the second segment  562  is located a fourth distance away from the reference plane  558 . The intermediate segment  566   a  couples the first and second segments  560  and  562  to one another and the intermediate segment  568  couples the second and third segments  562  and  566  together. The configuration of second cam slot  544   b  is identical to that of second cam slot  544   a , except that it is rotated relative to the rotary selector cam  520  such that each of the first, second, third and intermediate segments  560 ,  562 ,  564  and  566  and  568  in the second cam slot  544   b  are located 180° apart from the first, second, third and intermediate segments  560 ,  562 ,  564  and  566  and  568  in the second cam slot  544   a.    
     With the tabs  526  of the wire clips  522  engaged to the first cam slots  540   a  and  540   b  and the second cam slots  544   a  and  544   b , the rotary selector cam  520  may be rotated on the transmission sleeve  200  between the first, second and third positions  500 ,  502  and  504  to selectively engage and disengage the second and third ring gears  360  and  400  from the first and third reduction carriers  314  and  404 , respectively. During the rotation of the rotary selector cam  520 , the first cam slots  540   a  and  540   b  and the second cam slots  544   a  and  544   b  confine the wire tabs  526  of their associated wire clip  522  and cause the wire tabs  526  to travel along the longitudinal axis of the transmission sleeve  200  in an associated one of the first and second clip slots  284  and  286 . Accordingly, the rotary selector cam  520  is operative for converting a rotational input to an axial output that causes the wire clips  522  to move axially in a predetermined manner. A lubricant (not specifically shown) is applied to the lubricant grooves  252  formed into body portion  246  of the transmission sleeve  200  is employed to lubricate the interface between the transmission sleeve  200  and the rotary selector cam  520 . 
     Positioning the rotary selector cam  520  in the first rotational position  500  causes the tabs  526  of the wire clip  522  that is engaged to the second ring gear  360  to be positioned in the first segment  550  of the first cam slots  540   a  and  540   b  and the tabs  526  of the wire clip  522  that is engaged to the third ring gear  400  to be positioned in the first segment  560  of the second cam slots  544   a  and  544   b . Accordingly, positioning of the rotary selector cam  520  in the first rotational position causes the second and third ring gears  360  and  400  to be positioned in meshing engagement with the second and third planet gears  362  and  402 , respectively. Simultaneously with the meshing engagement of the second and third ring gears  360  and  400  with the second and third planet gears  362  and  402 , the sleeve engagement teeth  370  and  418  of the second and third ring gears  360  and  400 , respectively, are positioned in meshing engagement with the first and second sets of ring engagement teeth  254  and  256 , respectively, to inhibit relative rotation between the second and third ring gears  360  and  400  and the transmission sleeve  200  to thereby providing the transmission assembly  16  with a first overall gear reduction or speed ratio  570  as shown in FIG.  23 . Those skilled in the art will understand that the tip portion  272  of the teeth  268  of the first and second sets of ring engagement teeth  254  and  256  and the tip portions  372  and  420  of the sleeve engagement teeth  370  and  418 , respectively, are rounded and tapered so as to improve their capability for meshing engagement in response to axial repositioning along a longitudinal axis of the transmission assembly  16 . 
     Positioning the rotary selector cam  520  in the second rotational position  502  causes the tabs  526  of the wire clip  522  that is engaged to the second ring gear  360  to be positioned in the first segment  550  of the first cam slots  540   a  and  540   b  and the tabs  526  of the wire clip  522  that is engaged to the third ring gear  400  to be positioned in the second segment  562  of the second cam slots  544   a  and  544   b . Accordingly, positioning of the rotary selector cam  520  in second rotational position causes the second ring gear  360  to be in meshing engagement with the second planet gears  362  and the third ring gear  400  in meshing engagement with both the third planet gears  402  and the third reduction carrier  404 . Positioning of the rotary selector cam  520  in the second rotational position  502  also positions the sleeve engagement teeth  370  of the second ring gear  360  in meshing engagement with the first set of ring engagement teeth  254  while the sleeve engagement teeth  418  of the third ring gear  400  are not meshingly engaged with the second set of ring engagement teeth  256 . As such, relative rotation between the second ring gear  360  and the transmission sleeve  200  is inhibited, while relative rotation between the third ring gear  400  and the transmission sleeve  200  is permitted to thereby provide the transmission assembly  16  with a second overall gear reduction or speed ratio  572  as illustrated in FIG.  24 . 
     Positioning the rotary selector cam  520  in the third rotational position  504  causes the tabs  526  of the wire clip  522  that is engaged to the second ring gear  360  to be positioned in the second segment  552  of the first cam slots  540   a  and  540   b  and the tabs  526  of the wire clip  522  that is engaged to the third ring gear  400  to be positioned in the third segment  564  of the second cam slots  544   a  and  544   b . Accordingly, positioning of the rotary selector cam  520  in the third rotational position causes the second ring gear  360  to be in meshing engagement with both the second planet gears  362  and the first reduction carrier  314  while the third ring gear  400  in meshing engagement with only the third planet gears  402 . Positioning the rotary selector cam  520  in the third rotation position  504  also positions the sleeve engagement teeth  370  on the second ring gear  360  out of meshing engagement with the first set of ring engagement teeth  254  and the sleeve engagement teeth  418  on the third ring gear  400  in meshing engagement with the second sets of ring engagement teeth  256  to inhibit relative rotation between the second ring gear  360  and the transmission sleeve  200  and permit relative rotation between the third ring gear  400  and the transmission sleeve  200  to provide the transmission assembly  16  with a third overall gear reduction or speed ratio  574 . 
     In the example shown in FIGS. 13,  27   b  and  28 , the spring member  523  is formed from a flat rectangular piece of spring steel and includes a flattened Z-shaped portion  580  and a raised portion  584 . The flattened Z-shaped portion  580  is configured to wrap around two reinforcement bars  586  that extend into the spring aperture  546 , thereby permitting the raised portion  584  to be maintained at a predetermined position and also to transmit a spring force between the rotary selector cam  520  and the spring member  523 . With additional reference to FIG. 28, the raised portion  584  of the spring member  523  is sized to engage internal notches  590  formed in the housing  592  of the output spindle assembly  20 . Lands  594  that are circumferentially spaced from the rotary selector cam  520  are formed between the notches  590 . When the output spindle assembly  20  is positioned over the transmission assembly  16  and the speed selector mechanism  60  is positioned in one of the first, second and third rotational positions  500 ,  502  and  504 , the raised portion  584  of the spring member  523  engages an associated one of the notches  590 . The force that is generated by the spring member  523  when the raised portion  584  is moved downwardly toward the rotary selector cam  520  in response to contact between the raised portion  584  and the land  594  acts to inhibit unintended rotation of the speed selector mechanism  60 . Furthermore, placement of the raised portion  584  in a notch  590  provides the user with a tactile indication of the positioning of the rotary selector cam  520 . 
     In the particular embodiment illustrated in FIGS. 13 and 27 c , switch portion  510  is shown to include an arcuate band  600  having a raised hollow and rectangular selector button  602  formed therein. The arcuate band  600  is formed from a plastic material and is configured to conform to the outer diameter of the rotary selector cam  520 . The open end of the selector button  602  is configured to receive the switch tab  532 , thereby permitting the switch portion  510  and the rotary selector cam  520  to be coupled to one another in a fastenerless manner. The plurality of spacing members  534  are raised portions formed into the rotary selector cam  520  that are concentric to and extend radially outwardly from the selector body  530 . The spacing members  534  elevate the arcuate band  600  to prevent the arcuate band from contacting the wire tabs  526  in the first cam slots  540   a  and  540   b . The spacing members  534  may also be employed to selectively strengthen areas of the rotary selector cam  520 , such as in the areas adjacent the first cam slots  540   a  and  540   b.    
     Those skilled in the art will understand that the rotary selector cam  520  (i.e., the first cam slots  540   a  and  540   b  and the second cam slots  544   a  and  544   b ) could be configured somewhat differently so as to cause the second ring gear  360  meshingly engages both the second planet gears  362  and the first reduction carrier  314  while the third ring gear  400  meshingly engages both the third planet gears  402  and the third reduction carrier  404  to thereby providing the transmission assembly  16  with a fourth overall gear reduction or speed ratio. 
     Those skilled in the art will also understand that selector mechanisms of other configurations may be substituted for the selector mechanism  60  illustrated herein. These selector mechanisms may include actuators that are actuated via rotary or sliding motion and may include linkages, cams or other devices that are well known in the art to slide the second and third ring gears  360  and  400  relative to the transmission sleeve  200 . Those skilled in the art will also understand that as the second and third ring gears  360  and  400  are independently movable between the active and inactive modes (i.e., the placement of one of the second and third ring gears  360  and  400  does not dictate the positioning of the other one of the second and third ring gears  360  and  400 ), the switch mechanism  60  could also be configured to position the second and third ring gears  360  and  400  independently of one another. 
     Clutch Mechanism 
     In FIGS. 23,  26  and  28  through  30 , the clutch mechanism  18  is shown to include a clutch member  700 , an engagement assembly  702  and an adjustment mechanism  704 . The clutch member  700  is shown to be an annular structure that is fixed to the outer diameter of the first ring gear  310  and which extends radially outwardly therefrom. The clutch member  700  includes an arcuate clutch face  316  that is formed into the front face  318  of the first ring gear  310 . The outer diameter of the clutch member  700  is sized to rotate within the portion of the hollow cavity  212  that is defined by the base  216  of the transmission sleeve  200 . With specific brief reference to FIG. 29, the clutch face  316  of the example illustrated is shown to be defined by a plurality of peaks  710  and valleys  712  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 other clutch face configurations may also be employed, such as a sinusoidally shaped clutch face  316 ′ (FIG. 29 a ). 
     While the first ring gear  310  and the clutch member  700  have been illustrated as a one piece (i.e., unitarily formed) construction, those skilled in the art will understand that they may be constructed otherwise. One such embodiment is illustrated in FIG. 29 b  wherein the first ring gear  310 ′ is shown to include an annular collar  1000  and a plurality of tab apertures  1002 . The annular collar  1000  is illustrated to include a plurality of ramps  1004  that have dual sloping sides, but is otherwise flat. The first ring gear  310 ′ is otherwise identical to the first ring gear  310 . An annular damper  1008  abuts the annular collar  1000  and includes a plurality of tab members  1010  that engage the tab apertures  1002  in the first ring gear  310 ′ to prevent the damper  1008  from rotating relative to the first ring gear  310 ′. The damper  1008  includes a body portion  1012  that is configured to match the contour of the annular collar  1000  and as such, includes a plurality of mating ramped portions  1014  that are configured to engage each of the ramps  1004 . The damper  1008  is formed from a suitable impact dampening material, such as acetyl. The clutch member  700 ′, which is an annular member that is formed from a wear resistant material, such as hardened  8620  steel, is disposed over the damper  1008 . Like the damper  1008 , the clutch member  700 ′ includes a plurality of tab members  1020 , which lock into the tab apertures  1002  to prevent rotation relative to the first ring gear  310 ′, and a plurality of mating ramped portions  1022 . The mating ramped portions  1022  of the clutch member  700 ′, however, matingly engage the mating ramped portions  1014  of the damper  1008 . While the construction in this manner is more expensive relative to the previously described embodiment, it is more tolerant of high impact forces that are associated with the operation of the clutch mechanism  18 . 
     In the particular embodiment illustrated, the engagement assembly  702  includes a pin member  720 , a follower spring  722  and a follower  724 . The pin member  720  includes a cylindrical body portion  730  having an outer diameter that is sized to slip-fit within the second portion  278  of the actuator aperture  274  that is formed into the pin housing portion  248  of the transmission sleeve  200 . The pin member  720  also includes a tip portion  732  and a head portion  734 . The tip portion  732  is configured to engage the adjustment mechanism  704  and in the example shown, is formed into the end of the body portion  730  of the pin member  720  and defined by a spherical radius. The head portion  734  is coupled to the end of the body portion  730  opposite the tip portion  732  and is shaped in the form of a flat cylinder or barrel that is sized to slip fit within the first portion  276  of the actuator aperture  274 . Accordingly, the head portion  734  prevents the pin member  720  from being urged forwardly out of the actuator aperture  274 . 
     The follower spring  722  is a compression spring whose outside diameter is sized to slip fit within the first portion  276  of the actuator aperture  274 . The forward end of the follower spring  722  contacts the head portion  734  of the pin member  720 , while the opposite end of the follower spring  722  contacts the follower  724 . The end portion  740  of the follower  724  is cylindrical in shape and sized to slip fit within the inside diameter of the follower spring  722 . In this regard, the end portion  740  of the follower acts as a spring follower to prevent the follower spring  722  from bending over when it is compressed. The follower  724  also includes a follower portion  744  having a cylindrically shaped body portion  746 , a tip portion  748  and a flange portion  750 . The body portion  746  is sized to slip fit within the first portion  276  of the actuator aperture  274 . The tip portion  748  is configured to engage the clutch face  316  and in the example shown, is formed into the end of the body portion  746  of the follower  724  and defined by a spherical radius. The flange portion  750  is formed at the intersection between the body portion  746  and the end portion  740 . The flange portion  750  is generally flat and configured to receive a biasing force that is exerted by the follower spring  722 . 
     The adjustment mechanism  704  is also shown to include an adjustment structure  760  and a setting collar  762 . The adjustment structure  760  is shaped in the form of a generally hollow cylinder that is sized to fit a housing portion  766  of the output spindle assembly  20 . The adjustment structure  760  includes an annular face  768  into which an adjustment profile  770  is formed. The adjustment profile  770  includes a first adjustment segment  772 , a last adjustment segment  774 , a plurality of intermediate adjustment segments  776  and a ramp section  778  between the first and last adjustment segments  772  and  774 . In the embodiment illustrated, a second ramp section  779  is included between the last intermediate adjustment segment  776   z  and the last adjustment segment  774 . Also in the particular embodiment illustrated, the portion of the adjustment profile  770  from the first adjustment segment  772  through the last one of the intermediate adjustment segments  776   z  is formed as a ramp having a constant slope. Accordingly, a follower  780  that is coupled to the housing portion  766  of the output spindle assembly  20  is biased radially outwardly toward the inside diameter of the adjustment structure  760  where it acts against the plurality of detents  782  that are formed into the adjustment mechanism  704  (e.g., in the setting collar  762 ). The follower  724  and plurality of detents  782  cooperate to provide the user of tool  10  with a tactile indication of the position of the adjustment profile  770  as well as inhibit the free rotation of the adjustment structure  760  so as to maintain the position of the adjustment profile  770  at a desired one of the adjustment segments  772 ,  774  and  776 . 
     The setting collar  762  is coupled to the exterior of the adjustment structure  760  and includes a plurality of raised gripping surfaces  790  that permit the user of the tool  10  to comfortably rotate both the setting collar  762  and the adjustment structure  760  to set the adjustment profile  770  at a desired one of the adjustment segments  772 ,  774  and  776 . A setting indicator  792  is employed to indicate the position of the adjustment profile  770  relative to the housing portion  766  of the output spindle assembly  20 . In the example provided, the setting indicator  792  includes an arrow  794  formed into the housing portion  766  of the output spindle assembly  20  and a scale  796  that is marked into the circumference of the setting collar  762 . 
     During the operation of the tool  10 , an initial drive torque is transmitted by the motor pinion  46  from the motor assembly  14  to the first set of planet gears  312  causing the first set of planet gears  312  to rotate. In response to the rotation of the first set of planet gears  312 , a first intermediate torque is applied against the first ring gear  310 . Resisting this torque is a clutch torque that is applied by the clutch mechanism  18 . The clutch torque inhibits the free rotation of the first ring gear  310 , causing the first intermediate torque to be applied to the first reduction carrier  314  and the remainder of the reduction gearset assembly  202  so as to multiply the first intermediate torque in a predetermined manner according to the setting of the switch mechanism  60 . In this regard, the clutch mechanism  18  biases the first reduction gearset  302  in the active mode. 
     The magnitude of the clutch torque is dictated by the adjustment mechanism  704 , and more specifically, the relative height of the adjustment segment  772 ,  774  or  776  that is in contact with the tip portion  732  of the pin member  720 . Positioning of the adjustment mechanism  704  at a predetermined one of the adjustment segments  772 ,  774  or  776  pushes the pin member  720  rearwardly in the actuator aperture  274 , thereby compressing the follower spring  722  and producing the a clutch force. The clutch force is transmitted to the flange portion  750  of the follower  724 , causing the tip portion  748  of the follower  724  to engage the clutch face  316  and generating the clutch torque. Positioning of the tip portion  748  of the follower  724  in one of the valleys  712  in the clutch face  316  operates to inhibit rotation of the first ring gear  310  relative to the transmission sleeve  200  when the magnitude of the clutch torque exceeds the first intermediate torque. When the first intermediate torque exceeds the clutch torque, however, the first ring gear  310  is permitted to rotate relative to the transmission sleeve  200 . Depending upon the configuration of the clutch face  316 , rotation of the first ring gear  310  may cause the clutch force to increase a sufficient amount to resist further rotation. In such situations, the first ring gear  310  will rotate in an opposite direction when the magnitude of the first intermediate torque diminishes, permitting the tip portion  748  of the follower  724  to align in one of the valleys  712  in the clutch face  316 . If rotation of the first ring gear  310  does not cause the clutch force to increase sufficiently so as to fully resist rotation of the first ring gear  310 , the first reduction gearset  302  will be placed in the inactive mode wherein the first ring gear  310  will rotate so as to inhibit the transmission of the first intermediate torque to the first reduction carrier  314 . In such situations, no torque will be transmitted through the portions of the transmission assembly  16  that are located forwardly of the first set of planet gears  312  (e.g., the first reduction carrier  314 , the second sun gear  358 , the second set of planet gears  362 ). 
     Configuration of the clutch mechanism  18  in this manner is highly advantageous in that the clutch torque is sized to resist the first intermediate torque, as opposed to the output torque of the tool  10  that is generated by the multi-reduction transmission assembly  16  and transmitted through the chuck  22 . In this regard, the clutch mechanism  18  may be sized in a relatively small manner, thereby improving the ability with which it can be incorporated or packaged into the tool  10 . Furthermore, as the speed or gear ratios are changed after or down stream of the first ring gear  310 , the clutch mechanism  18  is operable over a relatively large span of output torques. In comparison with conventional clutch mechanisms that operate to limit the output torque of a transmission, these devices are typically operable over a relatively narrow torque band, necessitating a change in their clutch spring if a considerable shift in the magnitude of the output torque is desired. In contrast, the clutch mechanism  18  of the present invention can accommodate a considerable shift in the magnitude of the output torque of the tool  10  by simply operating the transmission assembly  16  in a different (i.e., lower or higher) gear ratio. 
     In the operation of rotary power tools such as tool  10 , it is frequently desirable to change between two clutch settings, as when the tool  10  is used to both drill a hole and thereafter install a screw in that hole. Accordingly, the adjustment mechanism  704  may be rotated relative to the output spindle assembly  20  to position the adjustment mechanism  704  at a desired one of the adjustment segments  772 ,  774  and  776  to perform the first operation and thereafter rotated to a second one of the adjustment segments  772 ,  774  and  776  to perform the second operation. In contrast to the known clutch arrangements, the adjustment mechanism  704  of the present invention is configured such that the adjustment structure  760  and the setting collar  762  are rotatable through an angle of 360°. Assuming the adjustment structure  760  to be positioned at an intermediate adjustment segment  776   x , rotation of the adjustment mechanism  704  through an angle of 360° would rotate the adjustment structure  760  past the other intermediate adjustment segments  776 , as well as the first and last adjustment segments  772  and  774  and the ramp section  778  such that the adjustment structure  760  would again be positioned at the intermediate adjustment segment  776   x . The feature is especially convenient when it is necessary to change the clutch setting between a relatively high clutch setting and a relatively low clutch setting. In this regard, the ramp section  778  permits the setting collar  762  (and adjustment structure  760 ) to be rotated from highest clutch setting, corresponding to the last adjustment segment, to the lowest clutch setting, corresponding to the first clutch setting, without positioning the clutch mechanism  18  in one of the intermediate clutch settings. Accordingly, the user of the tool  10  is able to vary the clutch setting from its maximum setting to its minimum setting (and vice versa) by rotating the setting collar  762  a relatively small amount. 
     While the adjustment profile  770  has been described thus far as having a constant slope, those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. For example, the adjustment profile  770 ′ may be formed such that each of the first, last and intermediate adjustment segments  772 ′,  774 ′ and  776 ′ is detented as illustrated in FIG.  31 . In this arrangement, the detents  782  in the adjustment structure  760  and the follower  780  in the housing portion  766  of the output spindle assembly  20  are unnecessary as the adjustment segments  772 ′,  774 ′ and  776 ′ will cooperate with the engagement  702  to provide the user of the tool  10  with a tactile indication of the position of the adjustment profile  770 ′, as well as inhibit the free rotation of the adjustment structure  760 . 
     Another example is illustrated in FIG. 32 wherein the adjustment profile  770 ″ is generally similar to the adjustment profile  770  except that the ramp section  779  has been omitted so that the last intermediate adjustment segment  776   z  is immediately adjacent the last adjustment segment  774 . 
     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 description of the appended claims.