Patent Publication Number: US-8123099-B2

Title: Cam and clutch configuration for a power tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional patent application Ser. No. 60/559,344 filed Apr. 2, 2004 entitled “Fastening Tool”. 
    
    
     INTRODUCTION 
     The present invention generally relates to a driving tool, such as a nailer, and more particularly to a driving tool having a cam and a clutch that cooperate to move and hold an activation arm in an actuated position. 
     Fastening tools, such as power nailers and staplers, are relatively common place in the construction trades. Often times, however, the fastening tools that are available may not provide the user with a desired degree of flexibility and freedom due to the presence of hoses and such that couple the fastening tool to a source of pneumatic power. 
     Recently, several types of cordless nailers have been introduced to the market in an effort to satisfy the demands of modern consumers. Some of these nailers, however, are relatively large in size and/or weight, which renders them relatively cumbersome to work with. Others require relatively expensive fuel cartridges that are not re-fillable by the user so that when the supply of fuel cartridges has been exhausted, the user must leave the work site to purchase additional fuel cartridges. Yet other cordless nailers are relatively complex in their design and operation so that they are relatively expensive to manufacture and do not operate in a robust manner that reliably sets fasteners into a workpiece in a consistent manner. 
     Accordingly, there remains a need in the art for an improved fastening tool. 
     SUMMARY 
     In one form, the present teachings provide a power tool with a flywheel, an activation arm, a driver, a cam and a clutch. The activation arm can have a first arm, a second arm, which can be pivotally mounted to the first arm, and a roller that can be coupled to the second arm. The driver can be disposed between the roller and the flywheel. The cam can be moveable in a predetermined direction for causing the activation arm to rotate about a pivot to drive the roller against the driver so that the driver frictionally engages the flywheel. The clutch can cooperate with the cam to inhibit the cam from moving in a direction opposite the predetermined direction in response to a reaction force that is applied by the driver onto the activation arm. 
     In another form, the present teachings provide a power tool having a structural backbone, a flywheel mounted on the structural backbone, an activation arm, a pivot, a driver, a cam, an actuator and a ground plate that can be coupled to the structural backbone. The activation arm can have a first arm, which can be coupled to a follower, a second arm that can be pivotally mounted to the first arm, and a roller that can be coupled to the second arm. The pivot can pivotally couple the first arm to the structural backbone. The driver can be disposed between the roller and the flywheel and movable along a translation axis between a retracted position and an extended position. The cam can have a cam surface that can be engaged to the follower. The actuator can have a body, which can be coupled to the structural backbone, and a member that is movable relative to the body along an actuation axis. The member can be coupled to the cam such that the cam is movable along the actuation axis in a first direction and a second direction that is opposite the first direction. Actuation of the power tool is at least partially accomplished through movement of the member in the first direction such that the cam surface contacts the follower and pivots the first arm about the pivot pin and the roller engages the driver against the flywheel to thereby transfer energy from the flywheel to the driver. A reaction force applied to the activation arm drives the cam into locking frictional engagement with the cam to the ground plate to thereby resist movement of the cam in the second direction. 
     In yet another form, the present teachings provide a tool with a housing that defines an internal cavity, a backbone that is attached to the housing and disposed within the internal cavity and a drive motor assembly. The drive motor assembly can have a driver and a power source that is mounted on the backbone and configured to translate the driver along an axis. 
     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 right side elevation view of a fastening tool constructed in accordance with the teachings of the present invention; 
         FIG. 2  is a left side view of a portion of the fastening tool of  FIG. 1  illustrating the backbone, the drive motor assembly and the control unit in greater detail; 
         FIG. 3  is a right side view of a portion of the fastening tool of  FIG. 1  illustrating the backbone, depth adjustment mechanism and contact trip mechanism in greater detail; 
         FIG. 4  is a rear view of the a portion of the fastening tool of  FIG. 1  illustrating the backbone, the drive motor assembly and the control unit in greater detail; 
         FIG. 5  is a top plan view of a portion of the backbone illustrating the motor mount in greater detail; 
         FIG. 5A  is a view similar to that of  FIG. 5  but illustrating an optional isolator member as installed to the motor mount; 
         FIG. 6  is another top plan view of the motor mount with a motor strap attached thereto; 
         FIG. 7  is a perspective view of the motor strap; 
         FIG. 8  is a top plan view of the motor mount with the motor operatively attached thereto; 
         FIG. 9  is a view similar to that of  FIG. 4  but illustrating the cam in operative association with the clutch; 
         FIG. 10  is a right side view of a portion of the fastening tool of  FIG. 1  illustrating the motor mount and the actuator mount and the return mechanism in greater detail; 
         FIG. 11  is a partial longitudinal sectional view of the backbone illustrating the nosepiece mount in operative association with the nosepiece assembly; 
         FIG. 12  is a side view of the belt tensioning mechanism; 
         FIG. 13  is a longitudinal section view of the flywheel assembly; 
         FIG. 14  is a side view of a flywheel constructed in accordance with the teachings of the present invention; 
         FIG. 15  is a side view of another flywheel constructed in accordance with the teachings of the present invention; 
         FIG. 16  is a sectional view taken through a portion of the flywheel and the driver; 
         FIG. 17  is a sectional view of yet another flywheel constructed in accordance with the teachings of the present invention; 
         FIG. 18  is a side view of still another flywheel constructed in accordance with the teachings of the present invention; 
         FIG. 19  is a sectional view taken along the line  19 - 19  of  FIG. 18 ; 
         FIG. 20  is a sectional view of an alternately constructed outer rim; 
         FIG. 21  is a sectional view of another alternately constructed outer rim; 
         FIG. 22  is a perspective view in partial section of a portion of the flywheel assembly wherein the flywheel pulley is molded directly onto the flywheel shaft; 
         FIG. 23  is a front view of a driver constructed in accordance with the teachings of the present invention, the keeper being shown exploded from the remainder of the driver; 
         FIG. 24  is a sectional view taken along the line  24 - 24  of  FIG. 23 ; 
         FIG. 25  is a right side view of the driver of  FIG. 23 ; 
         FIG. 26  is a longitudinal section view of a portion of an alternately constructed driver; 
         FIG. 27  is a top view of a portion of the driver of  FIG. 23 ; 
         FIG. 28  is a bottom view of an alternately constructed driver having a driver blade that is angled to match a feed direction of fasteners from a magazine assembly that is angled relative to the axis about which the drive motor assembly is oriented; 
         FIG. 29  is a sectional view of an alternately constructed nosepiece assembly wherein the nosepiece is configured to receive fasteners from a magazine assembly that is rotated relative to a plane that extends through the longitudinal center of the fastening tool; 
         FIG. 30  is a front view of a portion of the fastening tool of  FIG. 1  illustrating the backbone, the flywheel, the skid plate, the skid roller, the upper bumper and the lower bumper in greater detail; 
         FIG. 31  is a front view of a portion of the drive motor assembly illustrating the follower assembly in greater detail; 
         FIG. 32  is a sectional view taken along the line  32 - 32  of  FIG. 31 ; 
         FIG. 33  is a sectional view taken along the line  33 - 33  of  FIG. 32 ; 
         FIG. 34  is a sectional view taken along the line  34 - 34  of  FIG. 31 ; 
         FIG. 35  is a sectional view taken along the line  35 - 35  of  FIG. 31 ; 
         FIG. 36  is a right side view of a portion of the follower assembly illustrating the activation arm in greater detail; 
         FIG. 37  is a front view of the activation arm; 
         FIG. 38  is a plan view of a key for coupling the arm members of the activation arm to one another during the manufacture of the activation arm; 
         FIG. 39  is a right side view of a portion of the follower assembly illustrating the roller cage in greater detail; 
         FIG. 40  is an exploded view of a portion of the roller assembly; 
         FIG. 41  is a side elevation view of a portion of the drive motor assembly illustrating the actuator and the cam in greater detail; 
         FIG. 42  is a right side view of a portion of the roller assembly; 
         FIG. 43  is a front view of a portion of the drive motor assembly illustrating the return mechanism in greater detail; 
         FIG. 44  is a sectional view taken along the line  44 - 44  of  FIG. 43 ; 
         FIG. 45  is a partial longitudinal section view of a portion of the return mechanism illustrating the keeper in greater detail; 
         FIG. 46  is a sectional view taken along the line  46 - 46  of  FIG. 43 ; 
         FIG. 47  is a right side view of a portion of the fastening tool of  FIG. 1 ; 
         FIG. 48  is an exploded perspective view of the upper bumper; 
         FIG. 49  is a perspective view of the driver and the beatpiece; 
         FIG. 50  is a longitudinal section view of a portion of the fastening tool of  FIG. 1  illustrating the upper bumper, the driver and portions of the backbone and the flywheel; 
         FIG. 51  is a perspective view of the backbone illustrating the cavity into which the upper bumper is disposed; 
         FIG. 52  is a front view of a portion of the fastening tool of  FIG. 1  illustrating the driver in conjunction with the lower bumper and the backbone; 
         FIG. 53  is a sectional view taken along the line  53 - 53  of  FIG. 52 ; 
         FIG. 54  is a view similar to  FIG. 52  but illustrating an alternately constructed lower bumper; 
         FIG. 55  is a sectional view taken along the line  55 - 55  of  FIG. 54 ; 
         FIG. 56  is a sectional view taken along the line  56 - 56  of  FIG. 54 ; 
         FIG. 57  is a sectional view taken along the line  57 - 57  of  FIG. 54 ; 
         FIG. 58  is a schematic illustration of a portion of the fastening tool of  FIG. 1 , illustrating the control unit in greater detail; 
         FIG. 59  is a front view of a portion of the fastening tool of  FIG. 1 ; 
         FIG. 60  is a right side view of a portion of the fastening tool of  FIG. 1  illustrating the backbone and the drive motor assembly as received into a left housing shell; 
         FIG. 61  is a left side view of a portion of the fastening tool of  FIG. 1  illustrating the backbone, the drive motor assembly, the control unit and the trigger as received into a right housing shell; 
         FIG. 61A  is an enlarged partially broken away portion of  FIG. 61 ; 
         FIG. 62  is a front view of the housing; 
         FIG. 63  is a view of a portion of the housing with the trigger installed thereto; 
         FIG. 64  is a sectional view of the trigger; 
         FIG. 65  is a view of the cavity side of the backbone cover; 
         FIG. 66  is a partial section view taken along the line  66 - 66  of  FIG. 65 ; 
         FIG. 67  is a right side view of a portion of the drive motor assembly illustrating the clutch, the cam and the actuator in greater detail; 
         FIG. 68  is a rear view of the clutch and the cam; 
         FIG. 69  is a view similar to that of  FIG. 67  but including a spacer that is configured to resist lock-up of the cam to the clutch when the driver is moving toward a returned position; 
         FIG. 70  is a perspective view of the spacer; 
         FIG. 71  is a back view of a portion of the fastening tool of  FIG. 1  illustrating the actuator in greater detail; 
         FIG. 72  is a side view of an exemplary tool for adjusting a position of the solenoid relative to the backbone; 
         FIG. 73  is an end view of the tool of  FIG. 72 ; 
         FIG. 74  is a plot that illustrates the relationship between electrical current and the amount of time constants that are required to bring a given motor to a given speed; 
         FIG. 75  is a schematic of an electrical circuit that is analogous to a mechanical motor-driven system having a given inertia; 
         FIG. 76  is a plot that illustrate the relationships of a motor (ke) value to energy losses and the amount of time needed to bring the motor to a given speed; 
         FIG. 77  is an exploded perspective view of a portion of the fastening tool of  FIG. 1  illustrating a belt hook constructed in accordance with the teachings of the present invention; 
         FIG. 78  is a sectional view of the belt hook of  FIG. 77 ; 
         FIG. 79  is an exploded perspective view of a portion of a fastening tool similar to that of  FIG. 1  but illustrating a second belt hook constructed in accordance with the teachings of the present invention; 
         FIG. 80  is a sectional view of the fastening tool of  FIG. 79  illustrating the second belt hook in greater detail; 
         FIG. 81  is a sectional view of a portion of the belt hook of  FIG. 79  illustrating the leg member as engaged to the fastener; 
         FIG. 82  is an exploded perspective view of a portion of another fastening tool similar to that of  FIG. 1  but illustrating a third belt hook constructed in accordance with the teachings of the present invention; and 
         FIG. 83  is a sectional view of a portion of the fastening tool of  FIG. 82  illustrating the third belt hook in greater detail. 
     
    
    
     DETAILED DESCRIPTION OF THE VARIOUS EMBODIMENTS 
     With reference to  FIG. 1  of the drawings, a fastening tool constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . The fastening tool  10  may include a housing assembly  12 , a backbone  14 , a backbone cover  16 , an drive motor assembly  18 , a control unit  20 , a nosepiece assembly  22 , a magazine assembly  24  and a battery pack  26 . While the fastening tool  10  is illustrated as being electrically powered by a suitable power source, such as the battery pack  26 , those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently and that aspects of the present invention may have applicability to pneumatically powered fastening tools. Furthermore, while aspects of the present invention are described herein and illustrated in the accompanying drawings in the context of a nailer, those of ordinary skill in the art will appreciate that the invention, in its broadest aspects, has further applicability. For example, the drive motor assembly  18  may also be employed in various other mechanisms that utilize reciprocating motion, including rotary hammers, hole forming tools, such as punches, and riveting tools, such as those that install deformation rivets. 
     Aspects of the control unit  20 , the magazine assembly  24  and the nosepiece assembly  22  of the particular fastening tool illustrated are described in further detail in copending U.S. patent application Ser. No. 11/095,723 filed Mar. 31, 2005, entitled “Method For Controlling A Power Driver”, U.S. patent application Ser. No. 11/068,344 filed Feb. 28, 2005, entitled “Contact Trip Mechanism For Nailer”, and U.S. patent application Ser. No. 11/050,280filed Feb. 3, 2005, entitled “Magazine Assembly For Nailer”, all of which being incorporated by reference in their entirety as if fully set forth herein. The battery pack  26  may be of any desired type and may be rechargeable, removable and/or disposable. In the particular example provided, the battery pack  26  is rechargeable and removable and may be a battery pack that is commercially available and marketed by the DeWalt Industrial Tool Company of Baltimore, Md. 
     With additional reference to  FIGS. 2 and 3 , the backbone  14  may be a structural element upon which the drive motor assembly  18 , the control unit  20 , the nosepiece assembly  22 , and/or the magazine assembly  24  may be fully or partially mounted. The drive motor assembly  18  may be of any desired configuration, but in the example provided, includes a power source  30 , a driver  32 , a follower assembly  34 , and a return mechanism  36 . In the particular example provided, the power source  30  includes a motor  40 , a flywheel  42 , and an actuator  44 . 
     In operation, fasteners F are stored in the magazine assembly  24 , which sequentially feeds the fasteners F into the nosepiece assembly  22 . The drive motor assembly  18  may be actuated by the control unit  20  to cause the driver  32  to translate and impact a fastener F in the nosepiece assembly  22  so that the fastener F may be driven into a workpiece (not shown). Actuation of the power source may utilize electrical energy from the battery pack  26  to operate the motor  40  and the actuator  44 . The motor  40  is employed to drive the flywheel  42 , while the actuator  44  is employed to move a follower  50  that is associated with the follower assembly  34 , which squeezes the driver  32  into engagement with the flywheel  42  so that energy may be transferred from the flywheel  42  to the driver  32  to cause the driver  32  to translate. The nosepiece assembly  22  guides the fastener F as it is being driven into the workpiece. The return mechanism  36  biases the driver  32  into a returned position. 
     Backbone 
     With reference to  FIGS. 3 and 4 , the backbone  14  may include first and second backbone portions  14   a  and  14   b , respectively, that may be die cast from a suitable structural material, such as magnesium or aluminum. The first and second backbone portions  14   a  and  14   b  may cooperate to define a motor mount  60 , an actuator mount  62 , a clutch mount  64 , a flywheel mount  66 , a follower pivot  68  and a nosepiece mount  70 . 
     With reference to  FIGS. 4 through 6 , the motor mount  60  may include an arcuate surface  80  having features, such as a plurality of tabs  82 , that abut the motor  40 . In the particular example provided, the tabs  82  support the opposite longitudinal ends of the motor  40  and serve to space a flux ring that is disposed about the middle of the motor  40  apart from the motor mount  60 . In another example, the motor mount  60  may be configured such that a continuous full sweeping arc of material is disposed at both ends of the motor  40  for support, while the flux ring is elevated above the motor mount  60 . As motion of motor  40  against the backbone  14  may cause wear, rotational constraint of the motor  40  relative to the backbone  14  may be obtained through the abutment of the transmission plate  256  against a feature on the backbone  14 . Additionally, an optional isolator member IM ( FIG. 5A ) may be disposed between the motor  40  and the backbone  14 . The motor mount  60  may also include first and second engagements  88  and  90 , respectively, that cooperate with another structural element to secure the motor  40  in the motor mount  60  against the arcuate surface  80 . In the particular example provided, the other structural element is a motor strap  92  which is illustrated in detail in  FIGS. 6 and 7 . The motor strap  92  may include a hook portion  100 , an attachment portion  102  and an intermediate portion  104  that interconnects the hook portion  100  and the attachment portion  102 . The hook portion  100  may be pivotally coupled to the first engagement  88  so that the motor strap  92  may pivot relative to the backbone  14  between a first position, which permits the motor  40  to be installed to the motor mount  60 , and a second position in which the attachment portion  102  may be abutted against the second engagement  90 , which is a flange that is formed on the backbone  14  in the example provided. A threaded fastener  106  ( FIG. 8 ) may be employed to secure the attachment portion  102  to the second engagement  90 . 
     With reference to  FIGS. 4 and 6  through  8 , the motor strap  92  may be configured to apply a force against the body  108  of the motor  40  that tends to seat the motor  40  against the tabs  82  of the motor mount  60 . Accordingly, the intermediate portion  104  may be appropriately shaped so as to apply a load to one or more desired areas on the body  108  of the motor  40 , for example to counteract a force, which is applied by the belt  280 , that tends to pivot the motor  40  out of the motor mount  60  when the flywheel  42  stalls. In the example provided, the intermediate portion  104  is configured with a gooseneck  110  and a sloped section  112  that cooperate to apply a force to the motor  40  over a relatively small circular segment of the body  108  that may be in-line with the rotational axis  114  of the motor  40  and the rotational axis  116  of the flywheel  42  and which is generally perpendicular to an axis  118  about which the driver  32  is translated. 
     In the particular example illustrated, the first engagement  88  includes a pair of bosses  120  that are formed onto the backbone  14 . Those of ordinary skill in the art will appreciate in light of this disclosure that the motor mount  60  and/or the motor strap  92  may be otherwise configured. For example, a pin, a threaded fastener, or a shoulder screw may be substituted for the bosses  120 , and/or the hook portion  100  may be formed as a yoke, or that another attachment portion, which is similar to the attachment portion  102 , may be substituted for the hook portion  100 . In this latter case, the first engagements  88  may be configured in a manner that is similar to that of the second engagements  90 , or may include a slotted aperture into which or pair of rails between which the attachment portion may be received. 
     With reference to  FIGS. 9 and 10 , the actuator mount  62  may include a bore  150 , a pair of channels  152  and a pair of slotted apertures  154 . The bore  150  may be formed through the backbone  14  about an axis  158  that is generally perpendicular to the rotational axis  116  of the flywheel  42 . A plurality of stand-offs  160  may be formed about the bore  150  which cooperate to shroud the actuator  44  ( FIG. 2 ) so to protect it from deleterious contact with other components (e.g., the housing assembly  12 ) if the fastening tool  10  should be dropped or otherwise roughly handled. The channels  152  may be formed in the first and second backbone portions  14   a  and  14   b  so as to extend in a direction that is generally parallel the axis  158 . The slotted apertures  154  are disposed generally perpendicular to the channels  152  and extend therethrough. 
     The clutch mount  64  is configured to receive a wear or ground plate  170 , which is described in greater detail, below. The clutch mount  64  may be formed in the backbone  14  so as to intersect the bore  150 . In the example provided, the clutch mount  64  includes retaining features  172  that capture the opposite ends of the ground plate  170  to inhibit translation of the ground plate  170  along a direction that is generally parallel to the axis  158 , as well as to limit movement of the ground plate  170  toward the bore  150 . Threaded fasteners, such as cone point set screws  174 , may be driven against side of the ground plate  170  to fix the ground plate  170  to the backbone  14  in a substantially stationary position. The ground plate  170  may include outwardly projecting end walls  178 , which when contacted by the set screws  174 , distribute the clamp force that is generated by the set screws  174  such that the ground plate  170  is both pinched between the two set screws  174  and driven in a predetermined direction, such as toward the bore  150 . 
     The flywheel mount  66  includes a pair of trunnions  190  that cooperate to define a flywheel cavity  192  and a flywheel bore  194 . The flywheel cavity  192  is configured to receive the flywheel  42  therein, while the flywheel bore  194  is configured to receive a flywheel shaft  200  ( FIG. 13 ) to which the flywheel  42  is coupled for rotation. 
     With reference to  FIG. 3 , the follower pivot  68  may be formed in a pair of arms  204  that extend from the first and second backbone portions  14   a  and  14   b . In the example provided, the follower pivot  68  is disposed above the flywheel cavity  192  and includes a pair of bushings  206  that are received into the arms  204 . The bushings  206  define an axis  210  that is generally perpendicular to the axis  118  and generally parallel to the axis  116  as shown in  FIG. 4 . 
     With reference to  FIGS. 4 and 11 , the nosepiece mount  70  may include a pair of flanges  220  and a pair of projections  222 . The flanges  220  may extend outwardly from the backbone  14  along a direction that is generally parallel to the axis  118  about which the driver  32  ( FIG. 2 ) translates, whereas the projections  222  may be angled relative to an associated one of the flanges  220  to define a V-shaped pocket  226  therebetween. The nosepiece assembly  22  may be inserted into the V-shaped pocket  226  such that the nosepiece assembly  22  is abutted against the flanges  220  on a first side and wedged against the projections  222  on a second side. Threaded fasteners  228  may be employed to fixedly but removably couple the nosepiece assembly  22  to the flanges  220 . 
     Drive Motor Assembly 
     With reference to  FIG. 2 , the drive motor assembly  18  may include the power source  30 , the driver  32 , the follower assembly  34 , and the return mechanism  36 . The power source  30  is operable for propelling the driver  32  in a first direction along the axis  118  and may include the motor  40  and a flywheel assembly  250  that includes the flywheel  42  and is driven by the motor  40 . 
     Drive Motor Assembly: Power Source: Motor &amp; Transmission 
     In the particular example provided, the motor  40  may be a conventional electric motor having an output shaft (not specifically shown) with a pulley  254  coupled thereto for driving the flywheel assembly  250 . The motor  40  may be part of a motor assembly that may include a transmission plate  256  and a belt-tensioning device  258 . 
     With additional reference to  FIG. 4 , the transmission plate  256  may be removably coupled to an end of the body  108  of the motor  40  via conventional threaded fasteners and may include a structure for mounting the belt-tensioning device  258 . In the example provided, the transmission plate includes a pivot hub  260 , a foot slot  262  and a reaction arm  264 . The pivot hub  260  may extend upwardly from the main portion of transmission plate  256  and may include a hole that is formed therethrough. The foot slot  262  is a slot that may be formed about a portion of the pivot hub  260  concentrically with the hole. The reaction arm  264  also extends upwardly from the main portion of the transmission plate  256  and is spaced apart from the pivot hub  260 . 
     With additional reference to  FIG. 12 , the belt-tensioning device  258  has a configuration that is similar to that of a conventional automotive automatically-adjusting belt tensioner. In the example provided, the belt-tensioning device  258  includes an idler wheel  270  that is rotatably mounted to an idler arm  272 . The idler arm  272  includes a post  274  that is received into the hole in the pivot hub  260  so that the idler arm  272  (and the idler wheel  270 ) may pivot about the pivot hub  260 . A foot  276  that is formed on the idler arm  272  extends through the foot slot  262 ; contact between the foot  276  and the opposite ends of the foot slot  262  serves to limit the amount by which the idler arm  272  may be rotated about the pivot hub  260 . A torsion spring  278  may be fitted about the pivot hub  260  and engaged to the foot  276  and the reaction arm  264  to thereby bias the idler arm  272  in a desired rotational direction, such as counterclockwise toward the pulley  254 . 
     Drive Motor Assembly: Power Source: Flywheel Assembly 
     With reference to  FIG. 13 , the flywheel assembly  250  may include the flywheel  42 , the flywheel shaft  200 , a flywheel pulley  300 , a first support bearing  302  and a second support bearing  304 . The flywheel  42  is employed as a kinetic energy storage device and may be configured in any manner that is desired. For example, the flywheel  42  may be unitarily formed in any suitable process and may be cast, forged or formed from a powdered metal material. Alternatively, the flywheel  42  may be formed from two or more components that are fixedly coupled to one another. 
     With reference to  FIG. 14 , the flywheel  42  may include a hub  320 , an outer rim  322  and means for coupling the hub  320  and the outer rim  322  to one another. The coupling means may comprise a plurality of blades  326  that may be employed to generate a flow of air when the flywheel  42  rotates; the flow of air may be employed to cool various components of the fastening tool  10  ( FIG. 1 ), such as the motor  40  ( FIG. 2 ), the control unit  20  ( FIG. 2 ) and the flywheel  42  itself. The blades  326  may have any appropriate configuration (e.g., straight, helical). Alternatively, the coupling means may comprise a plurality of spokes  328  ( FIG. 15 ) or any other structure that may be employed to couple the hub  320  and the outer rim  322  to one another. 
     Returning to  FIGS. 13 and 14 , the hub  320  may be formed from a hardened material such that the ends of the hub  320  may form wear-resistant thrust surfaces. The hub  320  includes a through-hole  330  that is sized to engage the flywheel shaft  200 . In the example illustrated, the through-hole  330  includes a threaded portion and a counterbored portion that is somewhat larger in diameter than the threaded portion. 
     The outer rim  322  of the flywheel  42  may be configured in any appropriate manner to distribute energy to the driver  32  in a manner that is both efficient and which promotes resistance to wear. In the particular example provided, the outer rim  322  of the flywheel  42  is formed from a hardened steel and includes an exterior surface  350  that is configured with a plurality of circumferentially-extending V-shaped teeth  360  that cooperate to form a plurality of peaks  362  and valleys  364  as shown in  FIG. 16 . The valleys  364  in the exterior surface  350  of the outer rim  322  may terminate at a slot  366  having spaced apart wall members  368  rather than at a sharp corner. The slot  366  that is formed in the valleys  364  will be discussed in greater detail, below. 
     Examples of flywheels  42  having a configuration with two or more components are shown in  FIGS. 17 through 19 , wherein the outer rim  322  has a relatively high mass and is coupled to the remainder of the flywheel  42 , the remainder having a relatively low mass. In the example of  FIG. 17 , the outer rim  322  is threadably engaged to the hub  320  using threads  370  having a “hand” (i.e., right-handed or left-handed) that is opposite the direction with which the flywheel  42  rotates so as to self-tighten when the fastening tool  10  is utilized. 
     In the example of  FIGS. 18 and 19 , the hub  320  and the outer rim  322  are discrete components, and the coupling means  374  is a material, such as a thermoplastic, that is cast or molded to the hub  320  and the outer rim  322 . The hub  320  may have a flat or contoured outer surface  376 , while the outer rim  322  is formed with an interior flange  378 . The interior flange  378  may extend about the interior of the outer rim  322  in an intermittent manner (i.e., with portions  378   a  that are circumferentially-spaced apart as shown) and includes a pair of abutting surfaces  380  that are configured to be engaged by the coupling means  374 . The coupling means  374  may be molded or cast between the hub  320  and the outer rim  322 . 
     Hoop stresses that are generated when the coupling means  374  cools and shrinks are typically sufficient to secure the coupling means  374  and the hub  320  to one another. Shrinkage of the coupling means  374 , however, tends to pull the coupling means  374  away from the outer rim  322 , which is why insert molding has not been employed to mold to the interior surface of a part. In this example, however, shrinkage of the coupling means  374  applies a force (i.e., a shrink force) to the abutting surfaces  380  on the interior flange  378 , which fixedly couples the coupling means  374  to the outer rim  322 . 
     To eliminate or control a cupping effect that may occur when one side of the interior flange  378  is subjected to a higher load than the other side, the abutting surfaces  380  may be configured to divide the shrink force in a predetermined manner. In the example provided, it was desirable that the cupping effect be eliminated and as such, the abutting surfaces  380  were formed as mirror images of one another. Other examples of suitably configured abutting surfaces  380  may include the configurations that are illustrated in  FIGS. 20 and 21 . Those of ordinary skill in the art will appreciate from this disclosure that although the interior-insert molding technique has been illustrated and described in conjunction with a flywheel for a nailer, the invention in its broadest aspects are not so limited. 
     Returning to  FIGS. 13 and 16 , an optional wear-resistant coating  390  may be applied to the outer rim  322  to improve the longevity of the flywheel  42 . The wear-resistant coating  390  may comprise any coating having a relatively high hardness, a thickness greater than about 0.001 inch, and a coefficient of friction against steel or iron of about 0.1 or greater. For example, if the outer rim  322  of the flywheel  42  were made of SAE 4140 steel that has been through-hardened to a hardness of about 35 R C  to about 40 R C , or of SAE 8620 steel that has been case-hardened to a hardness of about 35 R C  to about 40 R C , the wear-resistant coating  390  may be formed of a) tungsten carbide and applied via a high-velocity oxy-fuel process, b) tantalum tungsten carbide and applied via an electro-spark alloying process, c) electroless nickel and applied via a chemical bath, or d) industrial hard chrome and applied via electroplating. 
     Returning to  FIG. 13 , the flywheel shaft  200  includes a central portion  400 , a first end portion  402  and a second end portion  404 . The central portion  400  is relatively smaller in diameter than the first end portion  402  but relatively larger in diameter than the second end portion  404 . The first end portion  402  may be generally cylindrically shaped and may be sized to engage the flywheel pulley  300  in a press fit or shrink fit manner. The central portion  400  is sized to receive thereon the first support bearing  302  in a slip fit manner. The second end portion  404  includes a threaded portion  410  and a necked-down portion  412  that is adjacent the threaded portion  410  on a side opposite the central portion  400 . The threaded portion  410  is sized to threadably engage the flywheel  42 , while the necked-down portion  412  is sized to engage the second support bearing  304  in a slip-fit manner. 
     With additional reference to  FIGS. 9 and 14 , the first and second support bearings  302  and  304  may be pressed into, adhesively coupled to or otherwise installed to the first and second backbone portions  14   a  and  14   b , respectively in the flywheel bore  194 . The flywheel  42  may be placed into the flywheel cavity  192  in the backbone  14  such that the through-hole  330  in the hub  320  is aligned to the flywheel bore  194 . The flywheel shaft  200 , with the flywheel pulley  300  coupled thereto as described above, is inserted into the flywheel bore  194  and installed to the flywheel  42  such that the threaded portion  410  is threadably engaged to the threaded portion of the through-hole  330  in the hub  320  of the flywheel  42 , the central portion  400  is supported by the first support bearing  302 , the portion of the central portion  400  between the first support bearing  302  and the threaded portion  410  of the flywheel shaft  200  is received into the counterbored portion of the hub  320  of the flywheel  42 , and the necked-down portion  412  is supported by the second support bearing  304 . As noted above, the first and second support bearings  302  and  304  engage the flywheel shaft  200  in a slip fit manner, which permits the flywheel shaft  200  to be slidably inserted into the flywheel bore  194 . 
     The flywheel shaft  200  may be rotated relative to the flywheel  42  to draw the flywheel  42  into abutment with the first support bearing  302  such that the inner race  302   a  of the first support bearing  302  is clamped between the flywheel  42  and a shoulder  420  between the first end portion  402  and the central portion  400 . To aid the tightening of the flywheel  42  against the first support bearing  302 , an assembly feature  422 , such as a non-circular hole (e.g., hex, square, Torx® shaped) or a slot may be formed in or a protrusion may extend from either the flywheel pulley  300  or the first end portion  402 . The assembly feature  422  is configured to be engaged by a tool, such as an Allen wrench, an open end wrench or a socket wrench, to permit the flywheel shaft  200  to be rotated relative to the flywheel  42 . 
     Returning to  FIGS. 2 and 13 , a belt  280 , which may have a poly-V configuration that matches that of the pulley  254  and the flywheel pulley  300 , may be disposed about the pulley  254  and the flywheel pulley  300  and engaged by the idler wheel  270  of the belt-tensioning device  258  to tension the belt  280 . The load that is applied by the belt  280  to the flywheel assembly  250  places a load onto the flywheel shaft  200  that is sufficient to force the necked-down portion  412  against the inner bearing race  304   a  of the second support bearing  304  to thereby inhibit relative rotation therebetween. In the particular example provided, the motor  40 , belt  280 , flywheel pulley  300  and flywheel  42  may be configured so that the surface speed of the exterior surface  350  of the flywheel  42  may attain a velocity of about 86 ft/sec to 92 ft/sec. 
     While the flywheel pulley  300  has been described as being a discrete component, those skilled in the art will appreciate that it may be otherwise formed. For example, the flywheel shaft  200  may be formed such that the first end portion  402  includes a plurality of retaining features  450 , such as teeth or splines, that may be formed in a knurling process, for example, as is shown in  FIG. 22 . The flywheel pulley  300  may be insert molded to the flywheel shaft  200 . In this regard, the tooling that is employed to form the flywheel pulley  300  may be configured to locate on the outer diameters of the central portion  400  or the second end portion  404 , which may be ground concentrically about the rotational axis of the flywheel shaft  200 . Accordingly, the flywheel pulley  300  may be inexpensively attached to the flywheel shaft  200  in a permanent manner without introducing significant runout or other tolerance stack-up. 
     Drive Motor Assembly: Driver 
     With reference to  FIGS. 23 and 24 , the driver  32  may include an upper driver member  500 , a driver blade  502  and a retainer  504 . The upper driver member  500  may be unitarily formed in an appropriate process, such as investment casting, from a suitable material. In the particular example provided, the upper driver member  500  was formed of titanium. Titanium typically exhibits relatively poor wear characteristics and as such, those of ordinary skill in the art would likely consider the use of titanium as being unsuitable and hence, unconventional. We realized, however, that as titanium is relatively lightweight, has a relatively high strength-to-weight ratio and has excellent bending and fatigue properties, an upper driver member  500  formed from titanium might provide a relatively lower mass driver  32  that provides improved system efficiency (i.e., the capacity to set more fasteners). In the particular example provided, the use of titanium for the upper driver member  500  provided an approximately 20% increase in capacity as compared with upper driver members  500  that were formed from conventional materials, such as steel. The upper driver member  500  may include a body  510  and a pair of projections  512  that extend from the opposite lateral sides of the body  510 . The body  510  may include a driver profile  520 , a cam profile  522 , an abutment  524 , a blade recess  526 , a blade aperture  528 , and a retainer aperture  530 . 
     With additional reference to  FIG. 16 , the driver profile  520  is configured in a manner that is complementary to the exterior surface  350  of the outer rim  322  of the flywheel  42 . In the particular example provided, the driver profile  520  includes a plurality of longitudinally extending V-shaped teeth  534  that cooperate to form a plurality of valleys  536  and peaks  538 . The valleys  536  may terminate at a slot  540  having spaced apart wall members  542  rather than at a sharp corner. The slots  366  and  540  in the outer rim  322  and the body  510 , respectively, provide a space into which the V-shaped teeth  534  and  360 , respectively, may extend as the exterior surface  350  and/or the driver profile  520  wear to thereby ensure contact between the exterior surface  350  and the driver profile  520  along a substantial portion of the V-shaped teeth  360  and  534 , rather than point contact at one or more locations where the peaks  362  and  538  contact the valleys  536  and  364 , respectively. 
     To further control wear, a coating  550  may be applied to the body  510  at one or more locations, such as over the driver profile  520  and the cam profile  522 . The coating may be a type of carbide and may be applied via a plasma spray, for example. 
     In  FIG. 23  through  FIG. 25 , the cam profile  522  may be formed on a side of the body  510  opposite the driver profile  520  and may include a first cam portion  560  and a second cam portion  562  and a pair of rails  564  that may extend between the first and second cam portions  560  and  562 . The abutment  524  may be formed on the body  510  on a side opposite the side from which the driver blade  502  extends and may include an arcuate end surface  570  that slopes away from the driver profile  520 . The cam profile  522  and the abutment  524  are discussed in greater detail, below. 
     The blade recess  526  may be a longitudinally extending cavity that may be disposed between the rails  564  of the cam profile  522 . The blade recess  526  may define an engagement structure  590  for engaging the driver blade  502  and first and second platforms  592  and  594 , that may be located on opposite sides of the engagement structure  590 . In the example provided, the engagement structure  590  includes a plurality of teeth  600  that cooperate to define a serpentine-shaped channel  602 , having a flat bottom  606  that may be co-planar with the first platform  592 . The first platform  592  may begin at a point that is within the blade recess  526  proximate the blade aperture  528  and may extend to the lower surface  612  of the body  510 , while the second platform  594  is positioned proximate the retainer aperture  530 . 
     The blade aperture  528  is a hole that extends longitudinally through a portion of the body  510  of the driver  32  and intersects the blade recess  526 . The blade aperture  528  may include fillet radii  610  ( FIG. 26 ) so that a sharp corner is not formed at the point where the blade aperture  528  meets the exterior lower surface  612  of the body  510 . 
     The retainer aperture  530  may extend through the body  510  of the driver  32  in a direction that may be generally perpendicular to the longitudinal axis of the driver  32 . In the example provided, the retainer aperture  530  is a slot having an abutting edge  620  that is generally parallel to the rails  564 . 
     The projections  512  may be employed both as return anchors  630 , i.e., points at which the driver  32  is coupled to the return mechanism  36  ( FIG. 2 ), and as bumper tabs  632  that are used to stop downward movement of the driver  32  after a fastener has been installed to a workpiece. Each return anchor  630  may be formed into portions of an associated projection  512  that extends generally parallel to the longitudinal axis of the driver  32 . The return anchor  630  may include a top flange  650 , a rear wall  652 , a pair of opposite side walls  654  and a front flange  656 . The top flange  650  may extend between the side walls  654  and defines a cord opening  660 . The rear wall  652 , which may intersect the top flange  650 , cooperates with the top flange  650 , the side walls  654  and the front flange  656  to define an anchor cavity  662 . In the particular example provided, the rear wall  652  is generally parallel to the longitudinal axis of the driver  32  at a location that is across from the front flange  656  and is arcuately shaped at a location below the front flange  656 . The side walls  654  may be coupled to the rear wall  652  and the front flange  656  and may include an anchor recess  664 , which may extend completely through the side wall  654 . 
     The bumper tabs  632  define a contact surfaces  670  that may be cylindrically shaped and which may be arranged about axes that are generally perpendicular to the longitudinal axis of the driver  32  and generally parallel one another and disposed on opposite lateral sides of the driver profile  520 . 
     The driver blade  502  may include a retaining portion  690  and a blade portion  692 . The retaining portion  690  may include a corresponding engagement structure  700  that is configured to engage the engagement structure  590  in the body  510 . In the particular example provided, the corresponding engagement structure  700  includes a plurality of teeth  702  that are received into the serpentine-shaped channel  602  and into engagement with the teeth  600  of the engagement structure  590 . Engagement of the teeth  600  and  702  substantially inhibits motion between the driver blade  502  and the body  510 . The retaining portion  690  may further include an engagement tab  710  that is configured to be engaged by both the second platform  594  and the retainer  504  as shown in  FIG. 24 . The engagement tab  710  may have any desired configuration but in the example provided tapers between its opposite lateral sides. 
     Returning to  FIG. 23 , the blade portion  692  extends downwardly from the retaining portion  690  and through the blade aperture  528  in the body  510 . The opposite end of the driver blade  502  may include an end portion  720  that is tapered in a conventional manner (e.g., on the side against which the fasteners in the magazine assembly  24  are fed) and on its laterally opposite sides. 
     With additional reference to  FIGS. 24 and 25 , the retainer  504  may be configured to drive the retaining portion  690  of the driver blade  502  against the second platform  594  and to inhibit movement of the driver blade  502  relative to the body  510  in a direction that is generally transverse to the longitudinal axis of the driver  32 . In the example provided, the retainer  504  includes a pair of feet  730 , an engagement member  732  and a tab  734 . The engagement member  732  is inwardly sloped relative to the feet  730  and disposed on a side of the retainer  504  opposite the tab  734 . 
     To assemble the driver  32 , the driver blade  502  is positioned into the blade aperture  528  and slid therethrough so that a substantial portion of the driver blade  502  extends through the blade aperture  528 . The corresponding engagement structure  700  is lowered into the engagement structure  590  such that the teeth  702  are engaged to the teeth  600  and the engagement tab  710  is disposed over the second platform  594 . The retainer  504  is inserted into the retainer aperture  530  such that the feet  730  are disposed against the abutting edge  620 , the engagement tab  710  is in contact with both the engagement member  732  and the second platform  594 , and the tab  734  extends out the retainer aperture  530  on an opposite side of the body  510 . The sloped surface of the engagement member  732  of the retainer  504  is abutted against the matching sloped surface of the engagement tab  710 , which serves to wedge the engagement tab  710  against the second platform  594 . The tab  734  may be deformed (e.g., bent over and into contact with the body  510  or twisted) so as to inhibit the retainer  504  from withdrawing from the retainer aperture  530 . 
     Engagement of the teeth  600  and  702  permits axially directed loads to be efficiently transmitted between the driver blade  502  and the driver body  510 , while the retainer  504  aids in the transmission of off-axis loads as well as maintains the driver blade  502  and the driver body  510  in a condition where teeth  600  and  702  are engaged to one another. 
     Optionally, a structural gap filling material  740 , such as a metal, a plastic or an epoxy, may be applied to the engagement structure  590  and the corresponding engagement structure  700  to inhibit micro-motion therebetween. In the example provided, the structural gap filling material  740  comprises an epoxy that is disposed between the teeth  600  and  702 . Examples of suitable metals for the structural gap filling material  740  include zinc and brass. 
     In the example provided, the magazine assembly  24  slopes upwardly with increasing distance from the nosepiece assembly  22 , but is maintained in a plane that includes the axis  118  as shown in  FIG. 1  as well as the centerline of the housing assembly  12 . In some situations, however, the slope of the magazine assembly  24  may bring it into contact with another portion of the fastening tool  10 , such as the handle of the housing assembly  12 . In such situations, it is desirable that the driver blade  502  ( FIG. 23 ) be arranged generally perpendicular to the axis along which fasteners F are fed from the magazine assembly  24 . One solution may be to rotate the orientation of drive motor assembly  18  and nosepiece assembly  22  so as to conform to the axis along which fasteners F are fed from the magazine assembly  24 . This solution, however, may not be implementable, as it may not be practical to rotate the drive motor assembly  18  and/or the appearance of the fastening tool  10  may not be desirable when its nosepiece assembly  22  has been rotated into a position that is different from that which is illustrated. 
     The two-piece configuration of the driver  32  ( FIG. 23 ) permits the driver blade  502  ( FIG. 23 ) to be rotated about the axis  118  and the centerline of the housing assembly  12  so as to orient the driver blade  502  ( FIG. 23 ) in a desired manner. Accordingly, the driver  32  may be configured as shown in  FIG. 28 , which permits the drive motor assembly  18  to be maintained in the orientation that is shown in  FIGS. 2 and 4 . 
     Alternatively, the nosepiece  22   a  of the nosepiece assembly  22  may be coupled to the housing assembly  12  and backbone  14  ( FIG. 2 ) as described herein, but may be configured to receive fasteners F from the magazine assembly  24  along the axis along which the fasteners F are fed. This arrangement is schematically illustrated in  FIG. 29 . The drive motor assembly  18  ( FIG. 1 ), however, may be rotated about the axis  118  ( FIG. 1 ) and the centerline of the housing assembly  12  to align the driver blade  502  to the nosepiece  22   a.    
     Drive Motor Assembly: Skid Plate &amp; Skid Roller 
     With reference to  FIG. 30 , the backbone  14  may optionally carry a skid plate  750  and/or a skid roller  752 . In the example provided, the skid plate  750  is coupled to the backbone  14  on a side of the flywheel assembly  250  opposite the skid roller  752 . The skid plate  750  may be formed of a wear resistant material, such as carbide, and is configured to protect the backbone  14  against injurious contact with the body  510  ( FIG. 23 ) of the driver  32  ( FIG. 23 ) at a location between the flywheel  42  and the nosepiece assembly  22  ( FIG. 1 ). 
     As the interface between the exterior surface  350  of the flywheel  42  and the driver profile  520  ( FIG. 23 ) of the driver  32  ( FIG. 23 ) are not directly in-line with the center of gravity of the driver, the driver may tend to porpoise or undulate as the flywheel  42  accelerates the driver. The skid roller  752  is configured to support the driver  32  ( FIG. 23 ) in a location upwardly of the flywheel  42  so as to inhibit porpoising or undulation of the driver  32  ( FIG. 23 ). The skid roller  752  may have any desired configuration that is compatible with the driver  32 , but in the example provided, the skid roller  752  comprises two rollers  754 , which are formed from carbide and which have sloped surfaces  756  that are configured to engage the V-shaped teeth  534  ( FIG. 23 ) of the driver profile  520  ( FIG. 23 ). In some situations, an upper skid plate (not shown) may be substituted for the skid roller  752 . In the example provided, however, the rollers  754  of the skid roller  752  engage a relatively large surface area of the driver profile  520  ( FIG. 23 ) with relatively lower friction than an upper skid plate. 
     Drive Motor Assembly: Follower Assembly 
     With reference to  FIGS. 2 and 9 , the follower assembly  34  may include the actuator  44 , the ground plate  170 , a clutch  800 , and an activation arm assembly  804  with an activation arm  806  and a roller assembly  808 . 
     Drive Motor Assembly: Follower Assembly: Actuator, Clutch &amp; Cam 
     The actuator  44  may be any appropriate type of actuator and may be configured to selectively provide linear and/or rotary motion. In the example provided, the actuator  44  is a linear actuator and may be a solenoid  810  as shown in  FIG. 41 . With additional reference to  FIG. 4 , the solenoid  810  may be housed in the bore  150  of the actuator mount  62  in the backbone  14 . The solenoid  810  may include a pair of arms  812  that are received into the channels  152  that are formed in the actuator mount  62 . Threaded fasteners  814  may be received through the slotted apertures  816  ( FIG. 3 ) in the actuator mount  62  and threadably engaged to the arms  812  to thereby fixedly but removably and adjustably couple the solenoid  810  to the backbone  14 . The solenoid  810  may include a plunger  820  that is biased by a spring  822  into an extended position. The plunger  820  may have a shoulder  824 , a neck  826  and a head  828 . 
     In  FIG. 4 , the ground plate  170  may be disposed in the clutch mount  64  and fixedly coupled to the backbone  14  as described above. The ground plate  170  may include a set of ways  830 , which may extend generally parallel to the axis  158  of the bore  150 , and a plurality of inwardly tapered engagement surfaces  836  that may be disposed on the opposite sides of the ways  830  and which extend generally parallel to the ways  830 . 
     The clutch  800  may be employed to cooperate with the activation arm  806  ( FIG. 2 ) to convert the motion of the actuator  44  into another type of motion. With reference to  FIGS. 9 and 36 , the clutch  800  may include a way slot  840 , a yoke  842 , a cam surface  844  and a pair of engagement surfaces  846 . The way slot  840  is configured to receive therein the ways  830  so that the ways  830  may guide the clutch  800  thereon for movement in a direction that is generally parallel to the axis  158  of the bore  150 . The yoke  842  is configured to slide around the neck  826  of the plunger  820  between the shoulder  824  and the head  828 . 
     Drive Motor Assembly: Follower Assembly: Activation Arm Assembly 
     With reference to  FIGS. 31 and 32 , the activation arm  806  may include an arm structure  850 , a cam follower  852 , an arm pivot pin  854 , a follower pivot pin  856  and a spring  858 . With reference to  FIGS. 36 and 37 , the arm structure  850  may include a pair of arm members  870  that are spaced apart by a pair of laterally extending central members  872  that is disposed between the arm members  870 . Each arm member  870  may be generally L-shaped, having a base  880  and a leg  882  that may be disposed generally perpendicular to the base  880 . Each base  880  may define a pivot aperture  890 , which is configured to receive the arm pivot pin  854  therethrough, a coupling aperture  892 , which is configured to receive the follower pivot pin  856  therethrough, a rotational stop  894 , which limits an amount by which the roller assembly  808  may rotate relative to the activation arm  806  in a given rotational direction, while each leg  882  may define a follower aperture  898  that is configured to receive the cam follower  852  therein. 
     With reference to  FIGS. 31 and 33 , the cam follower  852  may be a pin or roller that is rotatably supported by the legs  882 . In the example provided, the cam follower  852  is a roller with ends that are disposed in the follower apertures  898  in a slip-fit manner. In  FIGS. 2 ,  31  and  36 , the arm pivot pin  854  may be disposed through the follower pivot  68  and the pivot apertures  890  in the bases  880  to pivotably couple the activation arm  806  to the backbone  14 . In the example provided, the activation arm  806  is disposed between the arms  204  that form the follower pivot  68  and the arm pivot pin  854  is inserted through the bushings  206  and the pivot apertures  890 . 
     The follower pivot pin  856  may extend through the coupling apertures  892  and pivotably couple the roller assembly  808  to the activation arm  806 . The spring  858  may bias the roller assembly  808  in a predetermined rotational direction. In the example provided, the spring  858  includes a pair of leaf springs, whose ends are abutted against the laterally extending central members  872 , which may include features, such as a pair of spaced apart legs  900 , that are employed to maintain the leaf springs in a desired position. The leaf springs may be configured in any desired manner, but are approximately diamond-shaped in the example provided so that stress levels within the leaf springs are fairly uniform over their entire length. 
     The arm structure  850  may be a unitarily formed stamping which may be made in a progressive die, a multislide or a fourslide, for example, and may thereafter heat treated. As the sheet material from which the arm structure  850  may be formed may be relatively thin, residual stresses as well as the heat treating process may distort the configuration of the arm members  870 , which would necessitate post-heat treatment secondary processes (e.g., straightening, grinding). To avoid such post-heat treatment secondary processes, one or more slots  910  may be formed in the arm members  870  as shown in  FIG. 36  to receive a key  912  (which is shown in  FIG. 38 ) therethrough prior to the heat treatment operation. One or more sets of grooves  916  may be formed in the key  912  so as to permit the key  912  to engage the arm members  870  as is schematically illustrated in  FIG. 37 . In the example provided, two sets of grooves  916  are employed wherein the grooves  916  are spaced apart on the key  912  by a distance that corresponds to a desired distance between the arm members  870 . Rotation of the key  912  in the slots  910  after the grooves  916  have been aligned to the arm members  870  locks the key  912  between the arm members  870 . The key  912  thus becomes a structural member that resists deformation of the arm members  870 . Accordingly, one or more keys  912  may be installed to the arm members  870  prior to the heat treatment of the activation arm  806  to thereby inhibit deformation of the arm members  870  relative to one another prior to and during the heat treatment of the activation arm  806 . Moreover, the keys  912  may be easily removed from the activation arm  806  after heat treatment by rotation of the key  912  in the slot  910  and re-used or discarded as appropriate. Advantageously, the key  912  or keys  912  may be formed by the same tooling that is employed to form the arm structure  850 . More specifically, the key  912  or keys  912  may be formed in areas inside or around the blank from which the arm structure  850  is formed that would otherwise be designated as scrap. 
     With reference to  FIGS. 31 and 35 , the roller assembly  808  may include a roller cage  920 , a pair of eccentrics  922 , an axle  924 , a follower  50 , and a biasing mechanism  928  for biasing the eccentrics  922  in a predetermined direction. With reference to  FIGS. 31 and 39 , the roller cage  920  may include a pair of auxiliary arms  930  and a reaction arm  932  that is disposed between the auxiliary arms  930  and which may be configured with an cylindrically-shaped contact surface  934  that is employed to contact the spring  858 . Each auxiliary arm  930  may include an axle aperture  940 , a range limit slot  942 , which is concentric with the axle aperture  940 , a pin aperture  944 , an assembly notch  946 , and a stop aperture  948 , which is configured to receive the rotational stops  894  that are formed on the arm members  870 . Like the arm structure  850 , the roller cage may be unitarily formed stamping which may be made in a progressive die, a multislide or a fourslide, for example, and may thereafter heat treated. Accordingly, one or more slots  952 , which are similar to the slots  910  ( FIG. 36 ) that are formed in the arm structure  850 , and keys, which that are similar to the keys  912  ( FIG. 38 ) that are described above, may be employed to prevent or resist warping, bending or other deformation of the auxiliary arms  930  relative to one another prior to and during heat treatment of the roller cage  920 . 
     With reference to  FIGS. 32 ,  35  and  40 , each of the eccentrics  922  may be a plate-like structure that includes first and second bosses  970  and  972 , which extend from a first side, and an axle stub  974  and a stop member  976  that are disposed on a side opposite the first and second bosses  970  and  972 . The axle stub  974  is configured to extend through the axle aperture  940  ( FIG. 39 ) in a corresponding one of the auxiliary arms  930  and the stop member  976  is configured to extend into the range limit slot  942  to limit an amount by which the eccentric  922  may be rotated about the axle stub  974 . 
     An axle aperture  980  may be formed into the first boss  970  and configured to receive the axle  924  therein. In some situations, it may not be desirable to permit the axle  924  to rotate within the axle aperture  980 . In the example provided, a pair of flats  982  are formed on the axle  924 , which gives the ends of the axle  924  a cross-section that is somewhat D-shaped. The axle aperture  980  in this example is formed with a corresponding shape (i.e., the axle aperture  980  is also D-shaped), which permits the axle  924  to be slidingly inserted into the axle aperture  980  but which inhibits rotation of the axle  924  within the axle aperture  980 . The second boss  972  may be spaced apart from the first boss  970  and may include a pin portion  986 . Alternatively, the pin portion  986  may be a discrete member that is fixedly coupled (e.g., press fit) to the eccentric  922 . The follower  50 , which is a roller in the example provided, is rotatably disposed on the axle  924 . In the particular example provided, bearings, such as roller bearings, may be employed to rotatably support the follower  50  on the axle  924 . 
     With reference to  FIGS. 31 ,  32  and  35 , the biasing mechanism  928  may include a yoke  1000 , a spacer  1002  and a spring  1004 . The yoke  1000  may include a generally hollow cross-bar portion  1010  and a transverse member  1012  upon which the spring  1004  is mounted. The cross-bar portion  1010  may have an aperture  1016  formed therein for receiving the pin portions  986  of the second boss  972  of each eccentric  922 . 
     With additional reference to  FIG. 42 , the spacer  1002  may include a body  1020  having a pair of flange members  1022  and  1024 , a coupling yoke  1026 , a cantilevered engagement member  1028 . A counterbore  1030  may be formed into the body  1020  for receiving the spring and the transverse member  1012  of the yoke  1000 . The flange members  1022  and  1024  extend outwardly from the opposite lateral sides of the body  1020  over the auxiliary arms  930  that abut the body  1020 . Accordingly, the flange members  1022  and  1024  cooperate to guide the spacer  1002  on the opposite surfaces of the auxiliary arms  930  when the spacer  1002  is installed to the auxiliary arms  930 , as well as inhibit rotation of the spacer  1002  relative to the roller cage  920  about the follower pivot pin  856 . The engagement member  1028  may be engaged to the assembly notches  946  ( FIG. 39 ) that are formed in the auxiliary arms  930 . The coupling yoke  1026  includes an aperture  1036  formed therethrough which is configured to receive the follower pivot pin  856  to thereby pivotably couple the roller assembly  808  to the activation arm  806  as well as inhibit translation of the spacer  1002  relative to the roller cage  920 . With the spacer  1002  in a fixed position relative to the roller cage  920 , the spring  1004  exterts a force to the yoke  1000  that is transmitted to the eccentrics  922  via the pin portions  986 , causing the eccentrics  922  to rotate in a rotational direction toward such that the stop members  976  are disposed at the upper end of the range limit slots  942 . Engagement of the cantilevered engagement member  1028  to the assembly notches  946  ( FIG. 39 ) inhibits the spacer  1002  from moving outwardly from the auxiliary arms  930  during the assembly of the roller assembly  808  in response to the force that is applied by the spring  1004 , as well as aligns the aperture  1036  in the coupling yoke  1026  to the pin aperture  944  ( FIG. 39 ) in the auxiliary arms  930 . 
     In view of the above discussion and with reference to  FIGS. 31 through 40 , those of ordinary skill in the art will appreciate from this disclosure that the roller assembly  808  may be assembled as follows: a) the follower  50  is installed over the axle  924 ; b) a first one of the eccentrics  922  is installed to the axle  924  such that the axle  924  is disposed in the axle aperture  980 ; c) the yoke  1000  is installed to the pin portion  986  of the first one of the eccentrics  922 ; d) the other one of the eccentrics  922  is installed to the axle  924  and the yoke  1000 ; e) the subassembly (i.e., eccentrics  922 , axle  924 , follower  50  and yoke  1000 ) is installed to the roller cage  920  such that the axle stubs  974  are located in the axle apertures  940  and the stop members  976  are disposed in the range limit slots  942 ; f) the spring  1004  may be fitted over the transverse member  1012 ; g) the spacer  1002  may be aligned between the auxiliary arms  930  such that the flange members  1022  and  1024  extend over the opposite sides of the auxiliary arms  930  and the transverse member  1012  and spring  1004  are introduced into the counterbore  1030 ; h) the spacer  1002  may be urged between the auxiliary arms  930  such that the flange members  1022  and  1024  cooperate with the opposite sides of the auxiliary arms to guide the spacer  1002  as the spring  1004  is compressed; i) sliding movement of the spacer  1002  may be stopped when the cantilevered engagement member  1028  engages the assembly notches that are formed in the auxiliary arms  930 ; j) the roller assembly  808  may be positioned between the arm members  870  of the arm structure  850  and pivotably coupled thereto via the follower pivot pin  856 , which extends through the coupling apertures  892 , the pin apertures  944  and the aperture  1036  in the coupling yoke  1026 ; k) optionally, one or both of the ends of the follower pivot pin  856  may be deformed (e.g., peened over) to inhibit the follower pivot pin  856  from being withdrawn; l) the spring  858  may be installed to the arm structure  850 ; and m) the roller assembly  808  may be rotated about the follower pivot pin  856  to position the rotational stops  894  on the arm members  870  within the stop apertures  948  that are formed on the auxiliary arms  930  and thereby pre-stress the spring  858 . In this latter step, the reaction arm  932  of the roller cage  920  engages and loads the leaf springs so as to bias the roller assembly  808  outwardly from the activation arm  806 . 
     Drive Motor Assembly: Return Mechanism 
     With reference to  FIGS. 2 ,  43  and  44 , the return mechanism  36  may include a housing  1050  and one or more return cords  1052 . The housing  1050  may include a pair of housing shells  1050   a  and  1050   b  that cooperate to define a pair of spring cavities  1056  that are generally parallel one another. The housing shell  1050   a  may include a set of attachment features  1058  that permit the housing shell  1050   a  to be fixedly coupled to the backbone  14 . In the example provided, the set of attachment features  1058  include a pair of legs  1060  and a pair of bayonets  1062 . The legs  1060  are coupled to a first end of the housing shell  1050   a  and extend outwardly therefrom in a direction that is generally parallel to the spring cavities  1056 . The bayonets  1062  are coupled to an end of the housing shell  1050   a  opposite the legs  1060  and extend therefrom in a direction that is generally perpendicular to the legs  1060 . 
     With additional reference to  FIG. 10 , the legs  1060  and bayonets  1062  are configured to be received under laterally extending tabs  1066  and  1068 , respectively, that are formed on the backbone  14 . More specifically, the legs  1060  may be installed to the backbone  14  under the laterally extending tabs  1066  and thereafter the housing  1050  may be rotated to urge the bayonets  1062  into engagement with the laterally extending tabs  1068 . Those of ordinary skill in the art will appreciate from this disclosure that as the laterally extending tabs  1068  may include an arcuately shaped surface  1070 , which may cooperate with the bayonets  1062  to cause the bayonets  1062  to resiliently deflect toward the legs  1060  as the housing  1050  is being rotated toward the backbone  14 . 
     Returning to  FIGS. 43 and 44 , each return cord  1052  may include a cord portion  1080 , a spring  1082  and a keeper  1084 . The cord portion  1080  may be a resilient cord that may be formed of a suitable rubber or thermoplastic elastomer and may include a first retaining member  1090 , which may be configured to releasably engage the return anchors  630 , a second retaining member  1092 , which may be configured to be engaged by the keeper  1084 , and a cord member  1094  that is disposed between the first and second retaining members  1090  and  1092 . The second retaining member  1092  may include a conical face  2000  and a spherical end  2002 . 
     The first retaining member  1090  may include a body  2006  and a pair of tab members  2008  that extend from the opposite sides of the body  2006 . The first retaining member  1090  may be configured to couple the cord portion  1080  to the driver  32  ( FIG. 23 ). In the particular example provided, the body  2006  may be received into the anchor cavity  662  ( FIG. 25 ) such that the tab members  2008  extend into the anchor recesses  664  ( FIG. 23 ) and the cord member  1094  extends outwardly of the cord opening  660  ( FIG. 27 ) in the top flange  650  ( FIG. 27 ). In the example provided, the arcuate portion of the rear wall  652  ( FIG. 25 ) is configured to guide the first retaining member  1090  into the anchor cavity  662  ( FIG. 25 ) and the tab members  2008  extend through the side walls  654  ( FIG. 23 ) when the first retaining member  1090  is engaged to the return anchor  630  ( FIG. 23 ). 
     The cord member  1094  may have a substantially uniform cross-sectional area over its entire length. In the example provided, the cord member  1094  tapers outwardly (i.e., is bigger in diameter) at its opposite ends where it is coupled to the first and second retaining members  1090  and  1092 . Fillet radii  2012  are also employed at the locations at which the cord member  1094  is coupled to the first and second retaining members  1090  and  1092 . 
     The spring  1082  may be a conventional compression spring and may include a plurality of dead coils (not specifically shown) on each of its ends. With additional reference to  FIG. 45 , the keeper  1084  is employed to transmit loads between the cord member  1094  and the spring  1082  and as such, may include first and second contact surfaces  2016  and  2018 , respectively, for engaging the second retaining member  1092  and the spring  1082 , respectively. In the particular example provided, the keeper  1084  is a sleeve having a first portion  2020 , a smaller diameter second portion  2022  and a longitudinally extending slot  2024  into which the cord member  1094  may be received. The first contact surface  2016  may be formed onto the first portion  2020  and may have a conically-shaped surface that is configured to matingly engage the conical face  2000  of the second retaining member  1092 . The second portion  2022  may be formed such that its interior surface  2024  tapers outwardly toward it lower end. A shoulder that is formed at the intersection of the first portion  2020  and the second portion  2022  may define the second contact surface  2018 , which is abutted against an end of the spring  1082 . 
     With the spring  1082  disposed over the cord member  1094  and the keeper  1084  positioned between the spring  1082  and the second retaining member  1092 , the return cord  1052  is installed to the spring cavity  1056  in the housing  1050 . More specifically, the lower end of the spring  1082  is abutted against the housing  1050 , while the spherical end  2002  of the second retaining member  1092  abuts an opposite end of the housing  1050 . Configuration of the second retaining member  1092  in this manner (i.e., in abutment with the housing  1050 ) permits the second retaining member  1092  to provide shock resistance so that shock loads that are transmitted to the keeper  1084  and the spring  1082  may be minimized or eliminated. The two-component configuration of the return cord  1052  is highly advantageous in that the strengths of each component offset the weakness of the other. For example, the deceleration that is associated with the downstroke of the driver  32  (i.e., from abut 65 f.p.s. to about 0 f.p.s. in the example provided) can be detrimental to the fatigue life of a coil spring, whereas the relatively long overall length of travel of the driver could be detrimental to the life of a rubber or rubber-like cord. Incorporation of a coil spring  1082  into the return cord  1052  prevents the cord member  1094  from overstretching, whereas the cord member  1094  prevents the coil spring  1082  from being overshocked. Moreover, the return mechanism  36  is relatively small and may be readily packaged into the fastening tool  10 . 
     Drive Motor Assembly: Anti-Hammer Mechanism 
     Optionally, the fastening tool  10  may further include an stop mechanism  2050  to inhibit the activation arm  806  from engaging the driver  32  to the flywheel  42  as shown in  FIG. 2 . With reference to  FIGS. 10 ,  43 ,  44  and  46 , the stop mechanism  2050  may include a rack  2052 , a spring  2054  and an actuating arm  2056 . The rack  2052  may be mounted to the housing shell  1050   b  for translation thereon in a generally vertical direction that may be parallel to the axis  118 . The rack  2052  may include one or more rack engagements  2060 , a generally H-shaped body  2062  and an arm  2064 . The rack engagements  2060  may be coupled to the body  2062  and may have a sloped engagement surface  2070  with teeth  2072  formed thereon. The body  2062  may define one or more guides  2074  and a crossbar  2076 , which may be disposed between the guides  2074 . The guides  2074  may be received into corresponding structures, such as a guide tab  2080  and a spring cavity  2082 , that are formed on the housing shell  1050   b . The structures on the housing shell  1050   b  and the guides  2074  cooperate so that the rack  2052  may be translated in a predetermined direction between an extended position and a retracted position. Placement of the rack  2052  in the extended position permits the teeth  2072  of the sloped engagement surface  2070  to engage an upper one of the laterally extending central members  872  ( FIG. 47 ) of the arm structure  850  ( FIG. 47 ), while placement of the rack  2052  in the retracted position locates the teeth  2072  of the sloped engagement surface  2070  in a position that does not inhibit movement of the arm structure  850  ( FIG. 47 ) about the pivot arm pin  854 . 
     The spring  2054  may be a conventional compression spring that may be received into a spring cavity  2082  that is formed into the housing shell  1050   b . In the example provided, the spring  2054  is disposed between the housing shell  1050   b  and one of the guides  2074  and biases the rack  2052  toward the extended position. 
     A feature, such as a bayonet  2080 , may be incorporated into the housing shell  1050   b  to engage the rack  2052  when the rack  2052  is in the extended position so as to inhibit the rack  2052  from disengaging the housing shell  1050   b . In the example provided, the bayonet  2080  engages the lower end of the crossbar  2076  when the rack  2052  is in the extended position. 
     The actuating arm  2056  is configured to engage the arm  2064  on the rack  2052  and selectively urge the rack  2052  into the disengaged position. In the example provided, the actuating arm  2056  is mechanically coupled to the mechanical linkage of a contact trip mechanism  2090  ( FIG. 1 ) that is associated with the nosepiece assembly  22  ( FIG. 1 ). A detailed discussion of the contact trip mechanism  2090  is beyond the scope of this disclosure and moreover is not necessary as such mechanisms are well known in the art. In a discussion that is both brief and “general” in nature, contact trip mechanisms are typically employed to identify those situations where the nosepiece of a tool has been brought into a desired proximity with a workpiece. Contact trip mechanisms typically employ a mechanical linkage that interacts with (e.g., pushes, rotates) a trigger, or a valve or, in the example provided, an electrical switch, to permit the fastening tool to be operated. 
     In the example provided, the actuating arm  2056  is coupled to the mechanical linkage and as the contact trip mechanism  2090  ( FIG. 1 ) biases the mechanical linkage downwardly (so that the contact trip is position in an extended position), the actuating arm  2056  is likewise positioned in a downward position that permits the rack  2052  to be moved into the extended position. Placement of the contact trip mechanism  2090  ( FIG. 1 ) against a workpiece pushes the mechanical linkage upwardly by a sufficient distance, which closes an air gap between the actuating arm  2056  and the arm  2064 , to thereby cause the actuating arm  2056  to urge the rack  2052  upwardly into the disengaged position. 
     Drive Motor Assembly: Upper &amp; Lower Bumpers 
     With reference to  FIG. 30 , the backbone  14  may carry an upper bumper  2100  and a lower bumper  2102 . With additional reference to  FIG. 48 , the upper bumper  2100  may be coupled to the backbone  14  in any desired manner and may include a beatpiece  2110  and a damper  2112 . Formation of the upper bumper  2100  from two pieces permits the materials to be tailored to specific tasks. For example, the beatpiece  2110  may be formed from a relatively tough material, such as glass-filled nylon, while the damper  2112  may be formed from a material that is relatively more resilient than that of the beatpiece  2110 , such as chlorobutyl rubber. Accordingly, those of ordinary skill in the art will appreciate from this disclosure that the combination of the beatpiece  2110  and the damper  2112  permit the upper bumper  2100  to be formed with highly effective impact absorbing characteristics and a highly impact resistant interface where the driver  32  ( FIG. 49 ) contacts the upper bumper  2100 . 
     With additional reference to  FIGS. 49 and 50 , the beatpiece  2110  may be trapezoidal in shape, having a sloped lower surface  2116 , and may include a cavity  2118  having a ramp  2120  that conforms to the arcuate end surface  570  of the abutment  524  that is formed on the upper end of the driver  32 . The arcuate end surface  570  of the abutment  524  and the ramp  2120  of the beatpiece  2110  may be shaped so that contact between the arcuate end surface  570  and the ramp  2120  urges the driver  32  horizontally outward away from the flywheel assembly  250  to thereby ensure that the driver  32  does not contact the flywheel assembly  250  when the driver  32  is being returned or when the driver  32  is at rest. The arcuate end surface  570  and the ramp  2120  may also be shaped so that contact between the arcuate end surface  570  and the ramp  2120  causes the driver to deflect laterally, rather than vertically or toward the fasteners F, so that side-to-side movement (i.e., in the direction of arrow  2126 ) of the driver  32  within the cavity  2118  is initiated when the driver  32  impacts the upper bumper  2100  and the driver  32  is less apt to travel vertically downwardly toward the flywheel  42 . 
     The damper  2112  may be configured to be fully or partially received into the beatpiece  2110  to render the upper bumper  2100  relatively easier to install to the backbone  14 . In the particular example provided, the beatpiece  2110  includes an upper cavity  2130  having an arcuate upper surface  2132  that is generally parallel to the ramp  2120 , while the damper  2112  includes a lower surface  2134  that conforms to the arcuate upper surface  2132  when the damper  2112  is installed to the beatpiece  2110 . 
     With reference to  FIGS. 50 and 51 , the upper bumper  2100  may be inserted into an upper bumper pocket  2150  that is formed in the backbone  14 . The upper bumper pocket  2150  may include a pair of side walls  2152 , an upper wall  2154  and a pair of lower ribs  2156 , each of which being formed on an associated one of the side walls  2152 . The side walls  2152  may be generally orthogonally to the upper wall  2154  and the ribs  2156  may be angled to match the sloped lower surface  2116  of the beatpiece  2110 . As the material from which the damper  2112  is formed may have a relatively high coefficient of friction, the angled ribs  2156  facilitate installation of the upper bumper  2100  to the backbone  14 , since the narrow end of the upper bumper  2100  is readily received into the upper bumper pocket  2150  and the angled ribs  2156  permit the upper bumper  2100  to be slid both into the upper bumper pocket  2150  and upwardly against the upper wall  2154 . A feature  2160  ( FIG. 65 ) that is formed onto the backbone cover  16  ( FIG. 65 ) may contact or otherwise restrain the upper bumper  2100  so as to maintain the upper bumper  2100  within the upper bumper pocket  2150 . 
     In  FIGS. 30 and 52 , the lower bumper  2102  may be coupled to the backbone  14  in any desired manner and may be configured to contact a portion of the driver  32 , such as the contact surfaces  670  of the bumper tabs  632 , to prevent the driver  32  from directly contacting the backbone  14  at the end of the stroke of the driver  32 . The lower bumper  2102  may be configured of any suitable material and may have any desired configuration, but in the example provide a pair of lower bumper members  2200  that are disposed in-line with a respective one of the bumper tabs  632  on the driver  32 . In the particular example provided, the bumper members  2200  are interconnected by a pair of ribs  2202  and include locking tabs  2204  that extend from a side opposite the other bumper member  2200 . The lower bumper  2102  may be configured to be slidably engaged to the backbone  14  such that the locking tabs  2204  and one of the ribs  2202  are disposed in a mating recess  2210  that is formed in the backbone  14  and the bumper members  2102  abut a flange  2212  that extends generally perpendicular to the axis  118 . With brief additional reference to  FIGS. 65 and 66 , the backbone cover  16  may be configured with one or more mating tabs  2216  that cooperate with the backbone  14  to capture the other rib  2202  to thereby immobilize the lower bumper  2102 . 
     Returning to  FIGS. 52 and 53 , the lower bumper members  2200  may have a cylindrical upper surface  2230  that may be aligned about an axis  2232 , which may be generally perpendicular to both the axis  118  and the axes  2234  about which the contact surfaces  670  may be formed. Configuration in this manner permits the lower bumper members  2200  to loaded in a consistent manner without the need to precisely guide the driver  32  onto the lower bumper members  2200  and without transmitting a significant shear load to the lower bumper members  2200 . 
     As another example, each lower bumper member  2200  may be formed with a channel  2270  that extends about the lower bumper member  2200  inwardly of the perimeter of the lower bumper member  2200  as shown in  FIGS. 54 through 57 . The channel  2270  may be formed in a lower surface of the lower bumper member  2200  so as to be open at the bottom of the lower bumper member  2200  (as shown), or may be a closed cavity that is disposed within the lower bumper member  2200  (not shown). While the lower bumper member  2200  and the channel  2270  are illustrated to have a generally rectangular shape, those of ordinary skill in the art should appreciate from this disclosure that the lower bumper member  2200  and the channel  2270  may be otherwise formed. For example, the lower bumper member  2200  may be generally cylindrically shaped, and/or the channel  2270  may be annular in shape. The area at which the driver  32  contacts the lower bumper members  2200  is subject to relatively high stresses that are mitigated to a large degree by the channels  2270 . 
     Control Unit 
     With reference to  FIG. 58 , the control unit  20  may include various sensors (e.g., a trigger switch  2300  and contact trip switch  2302 ) for sensing the state of various components, e.g., the trigger  2304  ( FIG. 1 ) and the contact trip mechanism  2090  ( FIG. 1 ), respectively, and generating signals in response thereto. The control unit  20  may further include a controller  2310  for receiving the various sensor signals and controlling the fastening tool  10  ( FIG. 1 ) in response thereto. The control unit  20  may further include a DC/DC converter  2312  with a switching power supply  2314  for pulse-modulating the electrical power that is provided by the battery pack  26  and supplied to the motor  40 . More specifically, the switching power supply  2314  switches (i.e., turns on and off) to control its output to the motor  40  to thereby apply power of a desired voltage to the motor  40 . Consequently, electrical power of a substantially constant overall voltage may be provided to the motor  40  regardless of the voltage of the battery pack  26  by adjusting the length of time at which the switching power supply  2314  has been turned off and/or on. 
     With additional reference to  FIG. 2 , the control unit  20  may include one or more circuit boards  2320  onto which the electrical components and circuitry, including the switches, may be mounted. A wire harness  2322  may extend from the circuit board  2320  and may include terminals for electrically coupling the circuit board  2320  to the battery pack  26  and the motor  40 . 
     Housing Assembly, Backbone Cover &amp; Trigger 
     With reference to  FIGS. 1 ,  59  and  60 , the housing assembly  12  may include discrete housing shells  2400   a  and  2400   b  that may be formed from a thermoplastic material and which cooperate to define a body portion  2402  and a handle portion  2404 . The body portion  2402  may define a housing cavity  2410  that is sized to receive the backbone  14 , the drive motor assembly  18  and the control unit  20  therein. The handle portion  2404  may extend from the body portion  2402  and may be configured in a manner that permits an operator to manipulate the fastening tool  10  in a convenient manner. Optionally, the handle portion  2404  may include a mount  2418  to which the battery pack  26  may be releasably received, and/or a wire harness guard  2420  that confines the wire harness  2322  to a predetermined area within the handle portion  2404 . The mount  2418  may include a recess  2422  that is configured to be engaged by a latch  2424  on the battery pack  26  so that the battery pack  26  may be fixedly but removably coupled to the handle portion  2404 . The wire harness guard  2420  may include a plate member  2430  that extends inwardly from the housing shell  2400   a  and a plurality of ribs  2432  that cooperate to form a cavity into which a tool terminal block  2436  may be received. The tool terminal block  2436  includes electrical terminals that engage corresponding terminals that are formed on the battery pack  26 . 
     Optionally, portions of the housing assembly  12  may be overmolded to create areas on the exterior of and/or within the housing assembly  12  that enhance the capability of the housing assembly  12  to be gripped by an operator, provide vibration damping, and/or form one or more seals. Such techniques are described in more detail in commonly assigned U.S. Pat. No. 6,431,289 entitled “Multispeed Power Tool Transmission” and copending U.S. patent application Ser. No. 09/963,905 entitled “Housing With Functional Overmold”, both of which are hereby incorporated by reference as if fully set forth herein. 
     With reference to  FIGS. 60 through 62 , the housing shells  2400   a  and  2400   b  may employ a plurality of locating features to locate the housing shells  2400   a  and  2400   b  to one another as well as to the backbone  14 . In the example provided, the housing shells  2400   a  and  2400   b  are located to one another with several sets of bosses and a rib-and-groove feature. Each set of bosses includes a first boss  2450  and a second boss  2542  into which the first boss  2450  is received. The set of bosses may be configured to receive a threaded fastener  2456  therein to secure the housing shells  2400   a  and  2400   b  to one another. The rib-and-groove feature may include a rib member  2460 , which extends from a first one of the housing shells, e.g., housing shell  2400   a , about selected portions of the surface  2462  that abuts the other housing shell, and a mating groove  2468  that is formed in the other housing shell, e.g., housing shell  2400   b.    
     The housing assembly  12  may also include a trigger mount  2470  and a belt clip mount, which is discussed in greater detail below. The trigger mount  2470  may be configured in an appropriate manner to as to accept a desired trigger, including a rotary actuated trigger or a linearly actuated trigger. In the example provided, the trigger  2304  has characteristics of both a rotational actuated trigger and a linearly actuated trigger and as such, the trigger mount may include a backplate  2480 , a trigger opening  2482 , a pair of first trigger retainers  2484 , and a pair of second trigger retainers  2486 . The backplate  2480  may be formed on one or both of the housing shells  2400   a  and/or  2400   b  and includes an abutting surface  2490  that extends generally perpendicular to the trigger opening  2482 . Each of the first and second trigger retainers  2484  and  2486  may be defined by one or more wall members  2492  that extends from an associated housing shell (e.g., housing shell  2400   a ) and defines first and second cams  2500  and  2502 , respectively. In the particular example provided, the handle angle is positive and as such, the first cam  2500  is aligned about a first axis  2506 , while the second cam  2502  is aligned about a second axis  2508  that is skewed (i.e., angled) to the first axis  2506  such that the angle therebetween is obtuse. In instances where the handle angle is negative, the angle between the first and second axes  2506  and  2508  may be 90 degrees or less. Those of ordinary skill in the art will appreciate in view of this disclosure that the cams  2500  and  2502  may have any configuration, provided that they define the axes  2506  and  2508 , respectively, along which corresponding portions of the trigger  2304  travel. In this regard, each end of the first and second trigger retainers  2484  and  2486  may be open or closed and as such, need not limit the travel of the trigger  2304  along a respective axis. 
     With reference to  FIGS. 63 and 64 , a trigger assembly  2510  may include the trigger  2304  and a trigger spring  2512 , which may be a conventional compression spring. Except as noted below, the trigger  2304  may be substantially symmetrical about its longitudinal centerline and may include a spring mount  2520 , a first pair of pins  2522  and a second set of pins  2524 . The spring mount  2520  may be configured to receive the trigger spring  2512  thereon and may serve as a guide for the trigger spring  2512  when it is compressed. The first and second sets of pins  2522  and  2524  extend from the opposite lateral sides of the trigger  2304  and are configured to be disposed in the first and second cams  2500  and  2502 , respectively, that are formed in the housing assembly  12 . 
     The wall members  2492  of the first and second trigger retainers  2484  and  2486  operatively restrict the movement of the first and second sets of pins  2522  and  2524 , respectively, to thereby dictate the manner in which the trigger  2304  may be moved within the trigger mount  2470 . More specifically, when the trigger  2304  is urged into a retracted position by the finger of an operator, the wall members  2492  of the first trigger retainers  2484  guide the first pins  2522  along the first axis  2506  so that they move along a vector having two directional components—one that is toward the centerline of the handle portion  2404  (i.e., toward a side of the handle portion  2404  opposite the trigger  2304 ) and another that is parallel the centerline of the handle portion  2404  (i.e., toward the battery pack  26  ( FIG. 1 )). Simultaneously, the wall members  2492  of the second trigger retainers  2486  guide the second pins  2524  along the second axis  2508 . As thus constructed, the trigger  2304  has a “feel” that is similar to a linearly actuated trigger, but is relatively robust in design like a rotationally actuated trigger. 
     From the foregoing, those of ordinary skill in the art will appreciate that force is transmitted through the trigger  2304  at a location that is off-center to the trigger  2304  and its linkage. If a purely linear trigger were to be loaded in this manner, wracking would result as such triggers and linkages always act more smoothly when the loads are applied in a direction that is in-line with bearing surfaces. If a purely rotational trigger were to be loaded in this manner, it would function smoothly as they are generally tolerant of off-axis loads, but would be relatively less comfortable for a user to operate. 
     Those of ordinary skill in the art will also appreciate from this disclosure that the shape and angle of the cams  2500  and  2502  are a function of the path over which the user&#39;s finger travels. In other words, the cam  2502  may be generally parallel to or in-line with the center of the handle portion  2404 . To determine the shape of the cam  2500 , the trigger  2304  may be translated from an initial position (i.e., an unactuated position) into the handle portion  2404  to an end position (i.e., an actuated position). Movement of the trigger  2304  from the initial position to the end position is controlled at a first point by the cam  2502  (i.e., the trigger  2304  moves along the cam  2502 ). Movement of the trigger  2304  at a second point is controlled by a finger contact point (i.e., the point at which the user&#39;s finger contacts the trigger  2304 ). The finger contact point on the trigger  2304  is translated in a direction that is generally perpendicular to the handle portion  2404  when the trigger  2304  is moved between the initial position and the end position. The cam  2500  is constructed to confine the movement of the second point of the trigger  2304  along the perpendicular line along which the finger contact point translates. 
     Returning to  FIGS. 61 and 61A , the trigger  2304  may further include a switch arm  2550  that is configured to engage an actuator  2552  of a trigger switch  2300  that is employed in part to actuate the fastening tool  10 . In the example provided, the trigger switch  2300  is a microswitch and the actuator  2552  is a spring-biased plunger that is slidably mounted to the backbone  14 . The switch arm  2550  is configured to contact and move the actuator  2552  when the trigger  2304  is depressed so as to change the state of the microswitch. 
     To prevent the trigger switch  2300  from being damaged as a result of over-traveling the actuator  2552 , the trigger switch  2300  is configured such that the actuator  2552  is biased into contact with the microswitch and the trigger  2304  is employed to push the actuator  2552  away from the microswitch. Accordingly, the only force that is applied to the microswitch is the force of the spring  2558  that biases the actuator  2552  into contact with the trigger switch  2300 ; no forces are applied to the microswitch when the trigger  2304  is depressed, regardless of how far the actuator  2552  is over-traveled. 
     With reference to  FIG. 1 , the backbone cover  16  may be employed to cover the top of the backbone  14  and may attach to both the housing assembly  12  and the backbone  14 . In this regard, the housing assembly  12  and the backbone cover  16  may employ a rib-and-groove feature, which is similar to that which is described above, to locate the backbone cover  16  relative to the housing assembly  12 . In the example provided and with additional reference to  FIGS. 62 and 65 , the housing assembly  12  includes a rib member  2600  that extends from selected portions of the surface  2602  that abuts the backbone cover  16 , and a mating groove  2602  that is formed in the backbone cover  16 . Bosses  2604  may be formed into the backbone cover  16  to receive threaded fasteners (not shown) therethrough to permit the backbone cover  16  to be fixedly but removably secured to the backbone  14 . Configuration of the fastening tool  10  in this manner provides a means by which an operator may readily gain access to the drive motor assembly  18  to inspect and/or service components, such as the flywheel  42  ( FIG. 2 ), the driver  32  ( FIG. 2 ) and the return mechanism  36  ( FIG. 2 ), as well as provides a structural element that is relatively strong and durable and which may extend over the upper end and/or lower end of the housing assembly  12 . Alternatively, the housing assembly  12  may be configured to cover the top of the backbone  14 . 
     Tool Operation 
     In the particular example provided and with reference to  FIG. 58 , the control unit  20  may activate the motor  40  upon the occurrence of a predetermined condition, such as a change in the state of the contact trip switch  2302  that indicates that the contact trip mechanism  2090  has been abutted against a workpiece, and thereafter activate the actuator  44  upon the occurrence of a second predetermined condition, such as a change in the state of the trigger switch  2300  that indicates that the trigger  2304  has been depressed by the operator. As there is typically a short delay between the activation of the contact trip switch  2302  and the trigger switch  2300 , configuration in this manner permits the flywheel  42  ( FIG. 2 ) to be rotated prior to the time at which the operator has called for the fastening tool  10  to install a fastener F ( FIG. 1 ) (e.g., the time at which the operator depressed the trigger  2304  in the example provided). Accordingly, the overall time between the point at which the operator has called for the fastening tool  10  to install a fastener F ( FIG. 1 ) and the point at which the fastening tool  10  installs the fastener F ( FIG. 1 ) may thereby be shortened relative to the activation times of other known cordless nailers. 
     With reference to  FIGS. 1 ,  2  and  4 , when the fastening tool  10  is actuated, the control unit  20  cooperates to activate the drive motor assembly  18  to cause the motor  40  to drive the flywheel  42  and thereafter to cause the actuator  44  to move the follower  50  so that the follower  50  contacts the driver  32  such that the driver profile  520  ( FIG. 16 ) of the driver  32  is engaged to the exterior surface  350  ( FIG. 16 ) of the flywheel  42  ( FIG. 16 ) with sufficient clamping force so as to permit the flywheel  42  ( FIG. 16 ) to accelerate the driver  32  to a speed that is within a desired speed range. In the particular example provided and with additional reference to  FIGS. 67 and 68 , activation of the actuator  44  causes the plunger  820  of the solenoid  810  to travel away from the driver  32 . As the plunger  820  and the clutch  800  are coupled to one another, movement of the plunger  820  causes corresponding translation of the clutch  800  along the ways  830 . The follower  852 , which is engaged to the cam surface  844 , follows the cam surface  844  as the clutch  800  translates, which causes the activation arm assembly  804  to pivot relative to the backbone  14  about the arm pivot pin  854 , which in turn rotates the follower  50  about the arm pivot pin  854  into engagement with the first cam portion  560  ( FIG. 23 ) of the cam profile  522  ( FIG. 23 ). Engagement of the follower  50  to the first cam portion  560  ( FIG. 23 ) translates the driver  32  into contact with the rotating flywheel  42  so that the flywheel  42  may transmit kinetic energy to the driver  32  to accelerate the driver  32  along the axis  118 . The spring  858  of the activation arm  806  provides a degree of compliance between the activation arm  806  and the roller assembly  808  that permits the follower  50  to pivot away from the driver  32  to thereby inhibit the activation arm assembly  804  from overloading the driver  32  and/or the flywheel assembly  250 . 
     The first cam portion  560  ( FIG. 23 ) of the cam profile  522  ( FIG. 23 ) may be configured such that the clamping force that is exerted by the follower  50  onto the driver  32  is ramped up quickly, but not so quickly as to concentrate wear at a single location on the cam profile  522  ( FIG. 23 ). Rather, the ramp-up in clamping force may be distributed over a predetermined length of the cam profile  522  ( FIG. 23 ) to thereby distribute corresponding wear over an appropriately sized area so as to increase the longevity of the driver  32 . Note, too, that the ramp-up in clamping force cannot be distributed over too long a length of the cam profile  522  ( FIG. 23 ), as this may result in the transfer of an insufficient amount of energy from the flywheel  42  to the driver  32 . In the example provided, the first cam portion  560  ( FIG. 23 ) of the cam profile  522  ( FIG. 23 ) may have an angle of about 4 degrees to about 5 degrees relative to the rails  564  ( FIG. 23 ) of the cam profile  522  ( FIG. 23 ). 
     While the solenoid  810 , clutch  800  and activation arm assembly  804  cooperate to apply a force to the driver  32  that initiates the transfer of energy from the flywheel  42  to the driver  32 , it should be appreciated that this force, in and of itself, may be insufficient (e.g., due to considerations for the size and weight of the actuator  44 ) to clamp the driver  32  to the flywheel  42  so that a sufficient amount of energy may be transferred to the driver  32  to drive a fastener F into a workpiece. In such situations, the reaction force that is applied to the follower  50  will tend to pivot the activation arm assembly  804  about the arm pivot pin  854  so that the cam follower  852  is urged against the sloped cam surface  844 , which tends to urges the clutch  800  in a direction away from the solenoid  810 , as well as toward the ground plate  170  such that the engagement surfaces  846  engage the engagement surfaces  836  and lock the clutch  800  to the ground plate  170 . In this regard, the ground plate  170  operates as a one-way clutch to inhibit the translation of the clutch  800  along the ways  830  in a direction away from the solenoid  810 . Accordingly, the clamping force that is exerted by the follower  50  onto the cam profile  522  ( FIG. 23 ) of the driver  32  increases to a maximum level wherein the follower  50  is disposed on the rails  564  ( FIG. 23 ) of the cam profile  522  ( FIG. 23 ). The maximum level of clamping force is highly dependent upon numerous factors, including the type of fastener that is to be driven, the configuration of the interface between the driver  32  and the flywheel  42 , etc. In the particular example provided, the clamping force may range from about 150 lbf. to about 210 lbf. 
     Those of ordinary skill in the art will appreciate from this disclosure that the consistency of the interface between the ground plate  170  and the clutch  800  is an important factor in the operation of the fastening tool  10  and that variances in this consistency may prevent the clutch  800  from properly engaging or disengaging the ground plate  170 . As such, the ground plate  170  and the clutch  800  may be shrouded by one or more components from other components, such as the flywheel  42  that tend to generate dust and debris due to wear. In the particular example provided, the clutch  800  and the ground plate  170  are disposed within cavities in the backbone  14  so that a portion of the backbone  14  extends between the flywheel  42  and the interface between the clutch  800  and the ground plate  170  as is best shown in  FIG. 4 . Alternatively, a discrete component may be coupled to the backbone  14  upwardly of the flywheel  42  to shroud the interface in an appropriate manner. 
     The energy that is transferred from the flywheel  42  to the driver  32  may be of a magnitude that is sufficient to drive a fastener F of a predetermined maximum length into a workpiece that is formed of a relatively hard material, such as oak. In such conditions, the driving of the fastener F may consume substantially all of the energy that has been stored in the flywheel  34  and the armature of the motor  40 . In situations where the fastener F has a length that is smaller than the maximum length and/or is driven into a workpiece that is formed of a relatively softer material, such as pine, the flywheel  34  et al. may have a significant amount of energy after the fastener F has been driven into the workpiece. In this latter case, the residual energy may cause the driver  32  to bounce upwardly away from the nosepiece assembly  22 , as the lower bumper  2102  ( FIG. 30 ) may tend to reflect rather than absorb the energy of the impact with the driver  32 . This residual energy may tend to drive the driver  32  into the follower  50 , which may in turn apply a force to the activation arm assembly  804  that pivots it about the arm pivot pin  854  in a direction that would tend to cause the clutch  800  to lock against the ground plate  170 . 
     With brief additional reference to  FIGS. 32 and 35 , the magnitude of the force with which the driver  32  may impact the follower  50  may be reduced in such situations through the pivoting of the eccentrics  922  about the axle stubs  974  such that the stop members  976  travel toward or are disposed in an end of the range limit slots  942  opposite the end into which they are normally biased. Rotation of the eccentrics  922  pivots the follower  50  away from the driver  32  when the driver  32  bounces off the lower bumper  2102 . To accelerate the process by which the follower  50  is pivoted away from the driver  32 , the second cam portion  562  ( FIG. 23 ) is provided on the cam profile  522  ( FIG. 23 ) of the driver  32 . The second cam portion  562  ( FIG. 23 ) is configured to permit the spring  858  to unload to thereby permit the clutch  800  to disengage and permit the activation arm assembly  804  to return to it&#39;s “home” position when the driver  32  is starting to stall (i.e., is proximate the lowest point in its stroke), which permits the eccentrics  922  to pivot about the axle stubs  974  and rotate the follower  50  upwardly and away from the cam profile  522  ( FIG. 23 ) such that the clamp force exerted by the follower  50  actually decreases. In the particular example provided, the follower  50  does not disengage the cam profile  522  ( FIG. 23 ) of the driver  32 . 
     A spring  2700  ( FIG. 59 ) may be employed to apply a force to the activation arm assembly  804  that causes it to rotate about the arm pivot pin  854  away from the flywheel  42  to thereby ensure that the stop mechanism  2050  will engage the activation arm assembly  804 . Alternatively, as is shown in  FIGS. 69 and 70 , a spacer  2800  may be disposed between the cam follower  852  and the yoke  842  that is formed on the clutch  800 . The spacer  2800  may include a sloped counter cam surface  2802  that may be generally parallel to the cam surface  844  when the spacer  2800  is operatively installed. In the particular example provided, the spacer  2800  is a sheet metal fabrication (e.g., clip) that engages the neck  826  ( FIG. 41 ) of the plunger  820 . 
     When the solenoid  810  is de-energized, a spring  2810  may be employed to urge the plunger  820  away from the body  810   a  of the solenoid  810  (i.e., extend the plunger  820  in the example provided). As the plunger  820  is coupled to the clutch  800  (via the yoke  842 ), the clutch  800  may likewise be urged away from the body  810   a  of the solenoid  810 . The residual energy in the driver  32  ( FIG. 2 ) may cause the driver  32  ( FIG. 2 ) to bounce into contact with the follower  50  ( FIG. 2 ), which may thereby urge the activation arm assembly  804  to rotate about the arm pivot pin  854  ( FIG. 2 ), which may initiate contact between the cam follower  852  and the sloped cam surface  844  that tends to lock the clutch  800  to the ground plate  170 . To guard against this condition, the second cam portion  562  ( FIG. 23 ) of the cam profile  522  ( FIG. 23 ) on the driver  32  ( FIG. 2 ) may be configured such that the activation arm assembly  804  pivots about the arm pivot pin  854  ( FIG. 2 ) in a direction that brings the cam follower  852  into contact with the counter cam surface  2802  on the spacer  2800  when the driver  32  ( FIG. 2 ) is proximate the bottom of its stroke. Contact between the cam follower  852  and the counter cam surface  2802  permits force to be transmitted along a vector FN that is generally normal to the counter cam surface  2802 ; this vector FN, however, includes a component FC that is generally normal to the path of the clutch  800 . When FC is transmitted to the clutch  800 , the clutch  800  separates from the ground plate  170  such that the engagement surfaces  846  are disengaged from the engagement surfaces  836  on the ground plate  170  to thereby inhibit lock-up of the clutch  800  to the ground plate  170 . The remaining force vector FR will cause the clutch  800  to translate to thereby rotate the activation arm assembly  804 . 
     With reference to  FIGS. 1 ,  2  and  62 , the configuration of the drive motor assembly  18  that is illustrated is advantageous in that the center of gravity CG of the fastening tool  10  is laterally centered to the handle portion  2404 , as well as vertically positioned so as to lie in an area of the handle portion  2404  proximate the trigger  2304  to thereby provide the fastening tool  10  with a balanced feeling that is relatively comfortable for an operator. Furthermore, the positioning of the various components of the fastening tool  10 , such that the relatively large sized components including the motor  40 , the solenoid  810  and the flywheel  42 , are in locations toward the upper end of the fastening tool  10  permits the fastening tool  10  to be configured with a shape that corresponds to an upwardly extending wedge, as is shown in  FIG. 62 , wherein a lower end of the housing assembly  12  is relatively smaller than an upper end of the housing assembly  12 . The wedge shape of the fastening tool  10  improves the ability with which the operator may view the placement of the nosepiece assembly  22  as well as improves the capability of the fastening tool  10  to be used in relatively tight workspace areas (so that the nosepiece assembly  22  may reach an area on a workpiece prior to a point where another portion of the fastening tool  10 , such as the housing assembly  12 , contacts the workpiece). 
     Drive Motor Assembly: Solenoid Adjustment 
     From the foregoing, those of ordinary skill in the art will appreciate that the drive motor assembly  18  include some means for adjusting the amount of clearance between the follower  50  and the cam profile  522  ( FIG. 23 ) so as to compensate for issues such as normal manufacturing variation of the various components and wear. Provided that the clearance between the follower  50  and the cam profile  522  is sufficient to permit the activation arm assembly  804  to return to the “home” position, the ability of the fastening tool  10  to tolerate wear (i.e., the capability of the fastening tool  10  to fire with full energy) improves as the clearance between the follower  50  and the cam profile  522  decreases. In this regard, the capability of the activation arm assembly  804  to apply full pinch force to the driver  32  is lost when the various components of the fastening tool  10  (e.g., flywheel  42 , driver  32 ) have worn to the point where the plunger  820  of the solenoid  810  is out of stroke before the follower  50  contacts the driver  32 . With reference to  FIGS. 2 ,  4 ,  41  and  71 , this adjustability may be provided, for example, by moving the solenoid  810  to change the position of the activation arm assembly  804  about the arm pivot pin  854 . In this regard, the arms  812  of the solenoid  810  may be telescopically received into the channels  152  that are formed in the actuator mount  62  in the backbone  14 . 
     The position of the solenoid  810  within the bore  150  may be adjusted by positioning the follower  50  onto a predetermined portion of the cam profile  522  ( FIG. 23 ), e.g., on the rails  564  ( FIG. 23 ), pulling the solenoid  810  in the bore  150  in a direction away from the cam follower  852  ( FIG. 32 ) until the occurrence of a first condition, pushing the solenoid  810  in the bore  150  in an opposite direction, i.e., toward the cam follower  852  ( FIG. 32 ), until the occurrence of a second condition, and securing the solenoid  810  to the backbone  14 , as by tightening the fasteners  814 . The first condition may be position-based (e.g., where each pair of elements contacts one another: the cam profile  522  ( FIG. 23 ) and the exterior surface  350  of the flywheel  42 , the cam follower  852  ( FIG. 32 ) and the cam surface  844 , the engagement surfaces  836  and  846  ( FIG. 16 ), and the yoke  842  and the head  828  of the plunger  820 ) or may be based on an amount of force that is applied to the body  810   a  of the solenoid  810  to push the solenoid  810  in the first direction. The second condition may be a displacement of the body  810   a  of the solenoid  810  in the second direction from a given reference point, such as the location where the first condition is satisfied. 
     In the particular example provided and with additional reference to  FIGS. 72 and 73 , the body  810   a  of the solenoid  810  includes a key-hole shaped aperture  2900  that is configured to be engaged by a correspondingly shaped tool  2910 . The tool  2910  is inserted into the key-hole shaped aperture  2900  and rotated such that the tool  2910  may not be withdrawn from the body  810   a  of the solenoid  810 . The tool  2910  is pulled in the first direction, carrying with it the body  810   a  of the solenoid  810 , until a force of a predetermined magnitude has been applied to the body  810   a  of the solenoid  810 . The body  810   a  of the solenoid  810  is thereafter translated in the second direction by a predetermined distance and the fasteners  814  are tightened against the backbone  14  to fix the solenoid  810  to the backbone  14  in this desired position. The tool  2910  is thereafter rotated into alignment with the key-hole shaped aperture  2900  and withdrawn from the body  810   a  of the solenoid  810 . As one of ordinary skill in the art will appreciate from this disclosure, this process may be automated through the use of a piece of equipment that employs force and displacement transducers. 
     Alternatively, a shim or spacer may be employed to set the location of the solenoid  810  relative to the backbone  14 . For example, with the stop mechanism  2050  in a disengaged condition, a shim or spacer of a predetermined thickness may be inserted between the cam profile  522  ( FIG. 23 ) on the driver  32  and the follower  50  when the driver  32  is in a predetermined condition, e.g., in the fully returned position so that the shim or spacer is abutted against the first cam portion  560  ( FIG. 23 ) of the cam profile  522  ( FIG. 23 ), the solenoid  810  is pulled in the first direction (as described in the immediately preceding paragraphs) so that no “slop” or clearance is present between the follower  50  and the shim or spacer, between the shim or spacer and the driver  32 , and between the driver  32  and the flywheel  42 . 
     Motor Sizing 
       FIG. 74  is a plot that illustrates a typical relationship between current and time is illustrated for a given arrangement having a predefined motor, inertia and battery arrangement where power is applied to the motor at time=0 and the motor is initially at rest. The mechanical inertia and motor combination, together with the battery/source may be simplified with reference to  FIG. 75 . The power source be a battery B with a no-load voltage (V), while the total resistance (R) is equal to the sum of the battery/source resistance and the motor resistance. The capacitor (C) represents the mechanical inertia of the combined motor and system inertia, together with the energy conversion process from electrical to mechanical energy, which is typically quantified as a back-emf value in the electrical circuit. The value of (C) relates to a given DC motor with a back emf constant (ke) and the system inertia (J) as follows: C=J÷(ke) 2  and the time constant of the electrical analogy is equal to R×C. 
     As the mechanical inertia and the required speed of the inertia are predefined for a given application, the energy stored may also be considered to be known or predefined. For a mechanical system, the energy stored is equal to 0.5×J×ω 2 , where ω is the angular speed of the inertia. For the above electrical analogy, the mechanical/electrical stored energy is 0.5×C×v 2 , where v is the instantaneous voltage across the capacitor (C). By definition, these two relationships must be equal (i.e., 0.5×J×ω 2 =0.5×C×v 2 ) and thus ke=v÷ω. Assuming that the total resistance (R) and the voltage of the power source (V) are constant, the only way to reduce the time to attain a given speed (or voltage across the capacitor) is to modify the value of ke and/or J. 
     If ke is reduced, the value of C increases and as such, the magnitude of each time constant increases as well. However, to attain a given speed, and thus a given speed/mechanical stored energy, the number of time constants is actually less as is shown in the plot of  FIG. 76 . The plot illustrates energy loss as a function of the value of ke, which is depicted by the line  4000 , and time to attain a desired speed as a function of the value of ke, which is depicted by the line  4020 . As is shown in the particular example provided, energy losses associated with bringing the mechanical inertia to the required rotational speed are minimized by utilizing a motor with a value of ke that approaches 1.0. However, the time that is needed to bring the mechanical inertia to the required rotational speed is relatively long. In contrast, if motor has a value of ke that is about 0.85 to about 0.55, and preferably about 0.80 to about 0.65 and more preferably about 0.75 to about 0.70, the amount of time that is needed to bring the mechanical inertia to the required rotational speed is minimized. Sizing of the motor  40  ( FIG. 2 ) in this manner is advantageous in that it can significantly reduce the amount of time that an operator of the fastening tool  10  ( FIG. 1 ) will need to wait after actuating a trigger  2304  ( FIG. 1 ) and/or the contact trip mechanism  2090  ( FIG. 1 ) to installing a fastener into a workpiece. 
     Belt Hook With reference to  FIGS. 77 and 78 , the belt hook  5000  may include a clip structure  5002  that may be keyed to the housing assembly  12 . The clip structure  5002  may be generally L-shaped, having a base  5004  and an arm  5006 . The base  5004  may include a boss  5010  for receiving a fastener  5012 , and a keying feature  5020  that is coupled to the boss  5010 . The arm  5006  may include a portion that extends in a direction that is generally transverse to the base  5004  and may include an arcuate end portion  5022  at its distal end. 
     The housing assembly  12  may be configured with an aperture  5030  that is configured to receive the boss  5010  and the keying feature  5020  therein and a second aperture  5032  that is configured to receive the fastener  5012 . Preferably, the aperture  5030  and the second aperture  5032  are mirror images of one another so that the clip structure  5002  may be selectively positioned on one or the other side of the fastening tool  10 . In the example provided, the fastener  5012  is inserted into the second aperture  5032  and threadably engaged to the boss  5010  to thereby fixedly but removably couple the clip structure  5002  to the housing assembly  12 . 
     With reference to  FIGS. 79 through 81 , a belt hook constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  5050 . The belt hook  5050  may have a body  5052 , one or more legs  5054 , and one or more fasteners  5056  that are employed to secure the legs  5054  to the housing assembly  12 . The body  5052  may extend downwardly along a side of the housing assembly  12  and may terminate in a shape which may be rounded to an appropriate degree. 
     The legs  5054  may extend outwardly from the body  5052  and may include features  5060  that are configured to engage the fasteners  5056 . In the example provided, the features  5060  include at least one non-uniformity, such as axially spaced apart recesses  5062  that are configured to be engaged by annular protrusions  5064  that are formed on the fasteners  5056 . In the example illustrated, the body  5052  and the legs  5054  are unitarily formed from a suitable heavy-gauge wire, but those of ordinary skill in the art will appreciate that the body  5052  and legs  5054  may be formed otherwise. 
     The fasteners  5056  may be disposed within the housing assembly  12 , as for example between the housing shells  2400   a  and  2400   b . More specifically, the housing shells  2400   a  and  2400   b  may include leg bosses  5070  that may be configured to receive the legs  5054  therethrough. The inward end  5072  of each leg boss  5070  is configured to abut an associated end of one of the fasteners  5056 . In the example provided, a counterbore is formed in each end of the fasteners  5056 , with the counterbore being sized to receive the inward end of a leg boss  5070 . Threaded fasteners  5056  may be employed to secure the housing shells  2400   a  and  2400   b  to one another to thereby secure the fasteners  5056  within the housing assembly  12 . In the particular example provided, the legs  5054  are forcibly inserted to the fasteners  5056  to align the recesses  5062  with the protrusions  5064 . Engagement of the recesses  5062  and the protrusions  5064  inhibits movement of the legs  5054  relative to the fasteners  5056  to thereby secure the belt hook  5050  to the housing assembly  12 . 
     The example of  FIGS. 82 and 83  is generally similar to the example of  FIGS. 79 through 81  described above, except for the configuration of the legs  5054 , the fasteners  5056  and the leg bosses  5070 . In this example, the features  5060  on the legs  5054  include male threads, whereas the fasteners  5056  are sleeve-like elements having an internal threadform, which is configured to threadably engage the male threads on the legs  5054 , and a driving end  5080 . The leg bosses  5070  may abut an opposite leg boss  5070  at their inward end and may include a counterbored section  5084  that is configured to receive an associated one of the fasteners  5056 . To secure the belt hook  5050  to the housing assembly  12 , the legs  5054  are inserted into the leg bosses  5070  and the fasteners  5056  are threadably engaged to the male threads on the legs  5054 . The driving end  5080 , if included, may be employed to rotate the fastener  5056  so that it does not extend above the outer surface of the housing assembly  12 . In the particular example provided, the driving end  5080  includes a slot, which may be engaged by a conventional slotted-tip screwdriver. Those of ordinary skill in the art will appreciate, however, that the driving end  5080  may be configured differently and may have a configuration, for example, that permits the user to rotate the fastener  5056  with a Phillips screwdriver, an Allen wrench, a Torx® driver, etc. 
     While the invention has been described in the specification and illustrated in the drawings with reference to various embodiments, 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. Furthermore, the mixing and matching of features, elements and/or functions between various embodiments is expressly contemplated herein so that one of ordinary skill in the art would appreciate from this disclosure that features, elements and/or functions of one embodiment may be incorporated into another embodiment as appropriate, unless described otherwise, above. Moreover, 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 and the appended claims.