Abstract:
A method of making a wear and fatigue resistant component of a power tool (e.g., a clutch) includes providing a quantity of base steel and a quantity of alloying elements to be added to the base steel to form a desired alloyed grade of steel. The base steel and alloying elements are combined and melted to produce a molten alloyed steel (e.g., SAE 9310 or AISI M2). The molten alloyed steel is cast using a near-net-shape investment casting process to form a component of a power tool. An edge of the component is pre-radiused. and the component is case hardened after the edge of the component has been pre-radiused. In one implementation, the component is a clutch that has a lifespan of at least twice a lifespan of a second clutch that has not been pre-radiused prior to case hardening.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/404,877, filed Apr. 17, 2006, which is a divisional of U.S. patent application Ser. No. 10/361,609, filed Feb. 11, 2003 (now U.S. Pat. No. 7,047,848), which is a continuation-in-part of U.S. patent application Ser. No. 09/923,434 filed Aug. 8, 2001 (now U.S. Pat. No. 6,665,923), and claims priority from provisional U.S. Patent Application Ser. No. 60/354,943 filed Feb. 11, 2002 and provisional U.S. Patent Application Ser. No. 60/301,450, filed Jun. 29, 2001. Each of the foregoing applications is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to wear and fatigue resistant clutches for screw guns which automatically disengage when a screw is driven to a selected depth and which disengage positively to prevent clashing, and also materials and processes for manufacturing wear and fatigue resistant alloy steel components for screw gun clutches, and for power tools generally, especially hand-held power tools. 
       INTRODUCTION 
       [0003]    The prior art includes several examples of positive disengagement clutches, or “quiet” clutches, for screw guns. Generally, these clutches include a mechanism for moving the clutch surfaces away from one another when they disengage to prevent the clutch surfaces from clashing. This positive disengagement leads to an extended life of the clutch and results in other desirable operating characteristics such as reduced noise and vibration. 
         [0004]    U.S. Pat. Nos. 4,655,103, 4,809,572, and 4,947,714 disclose exemplary quiet clutches for screw guns. The &#39;714 patent discloses a clutch with three clutching elements—a drive element, an intermediate element and an output element. To drive a screw, all three clutch elements are initially engaged with one another (see FIG. 2 of the &#39;714 patent). At some point while driving the screw, torque causes the intermediate element and the output element to separate from each other due to the cam surfaces 30, 15 (see FIG. 3 of the &#39;714 patent). When the screw is almost driven to its intended depth, the intermediate element and the output element slide axially forward away from the input element, following the screw into the work piece (see FIG. 4 of the &#39;714 patent). When the screw reaches its intended depth, the input element and the intermediate element slip. Once the slip occurs, the torque is released and the separation between the output element and the intermediate element caused by the torque and the cam surfaces 30 and 15 is no longer present. A spring positioned between the input element and the intermediate element returns the intermediate element back together with the output element. This return creates a gap “s” between the input element and the intermediate element (see FIG. 5 of the &#39;714 patent). The gap prevents clashing of the clutch surfaces during disengagement. 
         [0005]    While the clutch described in the &#39;714 patent prevents clashing of the clutch surfaces during disengagement, the contact area between the intermediate element and the output element in the &#39;714 patent decreases as the intermediate element moves away from the output element. This decrease in the contact area leads to an additional increase in the stress which arises on the intermediate element and the output element from the transfer of torque from one to the other. The increased stress could lead to a decrease in the fatigue life of each part. 
         [0006]    Clutch components such as the drive element, intermediate element and output element described in the &#39;714 patent have been made from steel such as SAE 8620 steel. While this steel has been selected because of its desirable wear and fatigue properties, clutches in screw guns nevertheless remain one of the shortest-lived wear components in these tools. Breakdown of screw guns due to clutch failure causes the user to spend considerable time and money repairing the clutch or replacing the tool in its entirety. 
         [0007]    Screw gun clutches, as well as other high-wear components in power tools, would benefit from greater wear and fatigue resistance properties. Many high-wear components in hand-held power tools are especially sensitive to wear and fatigue because in hand-held power tools the weight and size of these components is typically minimized. Use of higher alloy steels can improve the wear and fatigue resistance of these components but does so typically at the expense of the cost competitiveness of the tool. Higher alloy steels can lead to greater initial manufacturing and inventory costs and increased difficulty in final machining. 
         [0008]    The wear and fatigue resistance of screw gun clutches is improved herein through both improvements in the design of these clutches and improvements in the materials and manufacturing methods for producing them. These improvements are achieved with very little increase in the cost of the clutches. 
         [0009]    Thus, one aspect of the invention is a clutch design which prevents clashing of the clutch surfaces during disengagement and is also more wear resistant than previous designs. This clutch design is also simple and inexpensive. 
         [0010]    Another aspect of the invention is materials and methods for manufacturing the components of the clutch. The components can be cast to near-net shape with a higher alloy steel, and can later be carburized or nitrided and subjected to other treating processes. In addition to screw gun clutches, these materials and methods are also applicable to other high-wear components in power tools generally, such as clutch components for drill/drivers and rotary ratchets for hammerdrills and hammers. 
         [0011]    These and other advantages and features will be apparent from the description, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a side view of a clutch according to one embodiment of the invention. 
           [0013]      FIG. 2  is a sectional view of the clutch taken along line A-A of  FIG. 1 . 
           [0014]      FIG. 3  is an isometric view of the input spindle for the clutch in  FIG. 1 . 
           [0015]      FIG. 4  is an isometric view of the driving clutch for the clutch in  FIG. 1 . 
           [0016]      FIG. 5  is a side view of the clutch in  FIG. 1  together with a sectional portion of a nosepiece for use with the clutch. 
           [0017]      FIGS. 6-9  are side views of the clutch in  FIG. 5  in various stages of engagement. 
           [0018]      FIGS. 10A-10B  are views of an embodiment of the clutch including a first alternative to the retaining ring in  FIG. 1 . 
           [0019]      FIGS. 11A-11B  are views of an embodiment of the clutch including a second alternative to the retaining ring in  FIG. 1 . 
           [0020]      FIG. 12  is a schematic illustration of another embodiment of the input spindle and the diving clutch. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The invention provides a clutch which prevents clashing of the clutch surfaces during disengagement. This clutch design is more resistant to wear and fatigue than some previous designs. The invention also provides materials and methods for manufacturing the components of the clutch. The principles of the materials and methods for manufacturing the components of the clutch are equally applicable, in a similar manner, to other high-wear components in hand-held power tools generally, and especially to high-wear components such as clutch components for drill/drivers and rotary ratchets for hammerdrills and hammers. The principles of this aspect of the invention will be explained as applied to the manufacture of clutch components for a screw gun. Those of skill in the art will recognize their application to other power tools generally. 
         [0022]    With reference to  FIGS. 1 and 2 , the clutch includes a driving clutch  100  and an output clutch  200 . The driving clutch  100  and output clutch  200  are configured with mating surfaces, such as lugs  101  and  201 , that, when engaged, are capable of transmitting torque from one to the other. 
         [0023]    An input spindle  300  is driven by a driving means housed in the tool, such as an electric motor or the like. Bearing  301  supports input spindle  300  so that it can rotate relative to the tool housing. Input spindle  300  and driving clutch  100  engage one another through a helical spline assembly, as will be described in detail below. 
         [0024]    Output spindle  400  includes an end  420  into which a bit holder  430  is detachably mounted. A bit  440  adapted for driving a screw or some other fastener is in turn detachably mounted into bit holder  430 . Another end  410  of the output spindle  400  is journaled with a sliding fit inside of a bore  330  formed in the input spindle  300 . The sliding fit allows output spindle  400  to rotate and freely slide axially in bore  330 . The output clutch  200  is fixed to and rotates with output spindle  400 . While output clutch  200  preferably does not slide axially any appreciable amount relative to the output spindle  400 , it may slide axially together with output spindle  400 . 
         [0025]    A compression spring  500  biases the output clutch  200  and the output spindle  400  apart from the driving clutch  100 . The respective ends of compression spring  500  push against the driving clutch  100 , or an optional thrust washer  322 , and the output clutch  200 . 
         [0026]    With reference to  FIGS. 3 and 4 , the input spindle  300  and the driving clutch  100  are illustrated, respectively. The input spindle  300  and the driving clutch  100  engage one another through a helical spline assembly. The driving clutch  100  has helical splines  110  formed on its interior bore  130 . The input spindle  300  has corresponding helical splines  310  formed on an exterior cylindrical surface  320 . The helical splines  110 ,  310  are sized to mesh with one another in a manner similar to the meshing of threads on a nut and bolt. When the driver is operated in the forward direction to drive a fastener, torque is transferred from the input spindle  300  to the driving clutch  100 . The force at the helical splines  110 ,  310  caused by the torque creates an axial reaction force tending to push the driving clutch  100  axially away from the input spindle  300 . When the driving clutch  100  moves axially away from the input spindle  300 , the driving clutch  100  also rotates a few degrees relative to the input spindle  300 . The axial movement of the driving clutch  100  relative to the input spindle  300  can be limited by a retaining ring  340  ( FIG. 2 ) or other structure. Thrust washer  322  is optionally provided in one embodiment to ride against an annular surface  120  ( FIG. 4 ) of the driving clutch  100  to distribute the load from the driving clutch  100  to the retaining ring  340 . 
         [0027]    Besides the illustrated helical spline assembly, the engagement between the input spindle  300  and the driving clutch  100  may be through any arrangement including complementary engaging surfaces formed on the input spindle  300  and the driving clutch  100  where the transfer of torque from one to the other causes an axial reaction force between the input spindle  300  and the driving clutch  100 , and where the complementary engaging surfaces are formed on the exterior cylindrical surface  320  of the input spindle and on the interior bore  130  of the driving clutch. 
         [0028]    For example,  FIG. 12  illustrates an alternative preferred embodiment where the input spindle  300   a  includes a square-sectioned, helically twisted engaging surface  310   a  formed on its exterior cylindrical surface  320   a . The surface  310   a  is complementary with and engages a similarly square-sectioned, helically twisted engaging surface  110   a  formed on the interior bore  130   a  of driving clutch  100   a . As with the input spindle  300  and the driving clutch  100 , when the input spindle  300   a  transfers torque in a forward direction to the driving clutch  10   a , an axial reaction force is created tending to push the driving clutch  100   a  away from the input spindle  300   a.    
         [0029]    One advantage of such an arrangement is that the contact area between the input spindle  300  and the driving clutch  100  remains substantially constant. Even though the driving clutch  100  moves away from the input spindle  300 , the contact area does not decrease. Thus, the pressure on the contact area which arises during torque transfer does not increase due to a decrease in the contact area. 
         [0030]    The helical splines  110  and  310  on the driving clutch  100  and the input spindle  300  can be machined. Alternatively, they may be formed in a metal injection molding, powder metal forming, or investment casting operation. The helix angle of each of the splines (measured from a plane parallel to the axis of the input spindle) is preferably between about 45.degree. and 75.degree., and more preferably between about 55.degree. and 65.degree., and most preferably about 60.degree. 
         [0031]    Typically, the components of the clutch in a screw gun are the most prone to early failure requiring repair of the clutch or replacement of the tool in its entirety. Even with the positive disengagement of the clutch described herein—which helps greatly reduce wear and fatigue—the driving clutch  100  and the output clutch  200 , and to a lesser extent the input spindle  300 , remain sensitive to failures caused by wear and fatigue. It would be highly advantageous if the clutches  100  and  200  were less prone to wear and fatigue so that they would not as often require repair or replacement of the tool. 
         [0032]    It has been found that forming the clutches  100  and  200  from a steel containing relatively large amounts of chromium and relatively large amounts of nickel as alloying elements increases the life of the clutches  100  and  200 . Preferably, the steel contains more than 1% nickel by weight; more preferably the steel contains more than 1% nickel by weight and more than 0.4% chromium by weight; more preferably the steel contains more than 2% nickel by weight and more than 0.8% chromium by weight; and most preferably the steel contains more than 3% nickel by weight and more than 1% chromium by weight. For example, testing has indicated that when the clutches  100  and  200  are formed of SAE 9310 steel, which has 1-1.4% chromium and 3-3.5% nickel by weight, the clutches enjoy a longer life than past clutches. 
         [0033]    Alternatively, it has also been found that forming the clutches  100  and  200  from a steel containing a relatively large total amount of molybdenum and tungsten as alloying elements increases the life of the clutches  100  and  200 . Preferably, the total amount of molybdenum and tungsten in the steel is more than 5% by weight; more preferably the total amount of molybdenum and tungsten in the steel is more than 7% by weight; and most preferably the total amount of molybdenum and tungsten in the steel is more than 9% by weight. Preferably, this steel also contains relatively high amounts of chromium and vanadium as alloying elements in addition to the molybdenum and tungsten. For example, at least approximately 0.5% by weight vanadium and at least approximately 3% by weight chromium are preferable. For example, testing has shown that when the clutches  100  and  200  are formed of AISI M2 steel, which has a total amount of molybdenum and tungsten of approximately 11% by weight, the clutches enjoy a longer life than past clutches. 
         [0034]    Also, it has been found that case hardening of the clutches  100  and  200  further increases their lifespan. The case hardening can be accomplished through carburizing techniques or nitriding techniques, as appropriate or desirable for the given steel. 
         [0035]    In order to ensure a sufficient case depth on the edges of the lugs  101  and  201 , the edges can be “pre-radiused” before case hardening. The edges between the top and sides of lugs  101  and  201  wear against one another during operation and are the primary wear surfaces of the lugs. In the design of the clutches  100  and  200  and the lugs  101  and  201  depicted in the figures, the clutches begin to operate roughly when the edges of the lugs have been rounded to a radius of approximately 0.065 to 0.075 inches. If the case hardening is performed while the edges of the lugs  101  and  201  are sharp, then the hardened case on the edges may still be quickly worn away as the sharp corners are rounded by wear. To avoid having the hardened case worn away too quickly at the critical edges of the lugs  101  and  201 , those edges may be pre-radiused up to an approximate radius of about 0.030 to about 0.035 inches. The case hardening is then performed after this pre-radiusing so that the case depth extends a uniform depth below the pre-radiused surface, and further into the clutches than it does if the case hardening is performed while the edges are sharp. This technique is very effective in maintaining a case hardened surface on the edges of the lugs for a greater portion of the clutch&#39;s life. 
         [0036]    In the case of using nitriding as a case hardening technique, the pre-radiusing may be necessary to ensure that the edges of the lugs  101  and  201  do not become too brittle and chip during use of the screw gun. 
         [0037]    In addition to case hardening, the clutches can be treated with heat treatment processes, including cryogenic treatments. 
       EXAMPLES 
       [0038]    Tests were conducted to compare screw gun clutches made from a steel with relatively large amounts of chromium and nickel to screw gun clutches marketed by a competitor. Each of the screw guns in this test operated at a speed of approximately 5,000 r.p.m. and was used to drive 15/8 inch long screws into a test material. The competitor&#39;s screw gun utilized a clutch with lugs similar to those depicted on clutches  100  and  200  herein. The clutches in the competitor&#39;s screw gun are made from SAE 8620 steel. The competitor&#39;s screw gun drove approximately 6,000 screws when it was judged to be operating roughly because the clutch was chattering. 
         [0039]    A screw gun built by the assignee of this invention was also used at approximately 5,000 r.p.m. to drive 15/8 inch long screws into the test material. This screw gun utilized a clutch with the helical spline design depicted in the drawings herein and which was made from a steel with relatively large amounts of chromium and nickel—in this case SAE 9310 steel. The clutch was also case hardened through a carburizing technique with an effective case depth of 0.040 to 0.050 inches to a hardness of 62-65 Rockwell C, and was subjected to a cryogenic treatment after the carburizing. This screw gun drove 15,000 screws before it was judged to be operating roughly because of clutch chatter. 
         [0040]    In order to further enhance this performance, a test was made with an identical screw gun except in this case the lugs on the clutches were pre-radiused to a radius of approximately 0.035 inches before the carburizing treatment. With the pre-radiused clutch, the screw gun drove 30,000 screws before it was judged to be operating roughly because of clutch chatter. Thus, it can be seen that the pre-radiusing of the clutch before carburizing resulted in a much longer life for the clutch. 
         [0041]    A further test was conducted with a screw gun built by the assignee of his invention and operated at approximately 5,000 r.p.m. to drive 15/8 inch long screws into the test material. This screw gun utilized a clutch with the helical spline design depicted in the drawings herein and which was made from a steel with a relatively large total amount of molybdenum and nickel—in this case AISI M2 steel. This screw gun drove 30,000 screws and was still judged to be running smoothly before the test was stopped. An identical screw gun, except that the edges on the lugs  101  and  201  were pre-radiused to a radius of about 0.030 to 0.035 inches and the clutches were nitrided in a salt bath, was put through the same test. This screw gun drove 35,000 screws and was still judged to be running smoothly before the test was stopped. 
         [0042]    A summary of these test results is displayed in the chart below. 
         [0000]    
       
         
               
             
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Competitor&#39;s Screw Gun 
               
             
          
           
               
                   
                 Steel 
                 No. of Cycles 
               
               
                   
                   
               
               
                   
                 SAE 8620 
                 6,000 
               
               
                   
                   
               
             
          
           
               
                 Test Screw Guns 
               
             
          
           
               
                   
                 Steel 
                 Pre-Radiused 
                 Case Hardened 
                 No. of Cycles 
               
               
                   
                   
               
               
                   
                 SAE 9310 
                 No 
                 Yes 
                 15,000 
               
               
                   
                 SAE 9310 
                 Yes 
                 Yes 
                 30,000 
               
               
                   
                 AISI M2 
                 No 
                 No 
                 &gt;30,000 
               
               
                   
                 AISI M2 
                 Yes 
                 Yes 
                 &gt;35,000 
               
               
                   
                   
               
             
          
         
       
     
         [0043]    Several drawbacks can arise from using a steel with large amounts of alloying elements to form components in a screw gun clutch. The presence of alloying elements in the steel can render it very difficult to machine. A decrease in machinability will contribute to an overall increase in the cost of the part. 
         [0044]    Also, steel with large amounts of alloying elements can typically only be ordered from a mill at a competitive price in relatively large quantities. The large quantity is necessary because these steels are often not widely used and therefore not routinely made by steel mills. Such a large quantity order of steel will result in large inventories of the steel to be held by the manufacturer. Such a large inventory can be particularly burdensome when only very small parts are to be formed, such as clutch components for a screw gun. The cost of purchasing so far in advance and keeping such a large inventory of a raw material is a major drawback. 
         [0045]    Normally these drawbacks of decreased machinability and large inventories would necessitate against the use of a specialized steel for small parts such as screw gun clutch components where costs need to be tightly controlled. 
         [0046]    However, it has been found that these and other drawbacks of using a specialized steel for small, high-wear components can be overcome by using a near-net-shape investment casting process. The investment casting process can use a more widely available base steel as a raw material. The base steel can be melted and combined with additional alloying elements with the final chemistry of the steel being controlled by the investment caster. This eliminates the drawback of large inventories of a specialized steel. For example, SAE 1010 steel, a widely available and relatively inexpensive steel, can be used as the base steel. Alloying elements can be combined with the molten SAE 1010 steel to result in molten steel with the same chemistry as SAE 9310 steel. 
         [0047]    Also, because of the relatively low tooling costs, parts can be investment cast in short runs of low quantities because there is little cost advantage to large runs. The small quantities of the short runs result in small inventories of the finished part, further reducing the inventory costs. 
         [0048]    The investment casting operation can also be selected to produce components with acceptable final dimensions and tolerances so that further machining is unnecessary. This type of investment casting is referred to as near-net-shape. While near-net-shape investment casting is relatively expensive, it has been found that cost savings gained from eliminating most final machining steps help to keep the cost of this process competitive. In fact, it has been found that the driving clutch  100  and the output clutch  200  can each be formed of SAE 9310 steel in a near-net-shape investment casting operation for about 91 cents each, which compares very well to the cost of about 89 cents for producing the same parts with SAE 8620 steel. Thus, for a very small increase in cost, the clutches  100  and  200  can be made to heavily outperform clutches that have been used in the past 
       Operation 
       [0049]    Two modes of operation of the clutch depicted in  FIGS. 1-9  will now be described. 
         [0050]    In the first mode of operation,  FIG. 5  depicts the clutch in a disengaged state, and it will be assumed that the drive means of the tool is not yet activated. 
         [0051]    In  FIG. 6 , the user pushes the bit  440  and a screw or other fastener against a work piece W. This pushing force is transferred from the screw gun to the input spindle  300 , to the driving clutch  100 , to the thrust washer  322  and spring  500 . This pushing force is also transferred from the spring  500  to the output clutch  200 , output spindle  400 , bit holder  430 , bit  440  and finally to the screw. The pushing force compresses spring  500 , causing output spindle  400  and output clutch  200  to slide axially closer to the driving clutch  100  and input spindle  300 . Eventually, when the pushing force is great enough, the driving clutch  100  and the output clutch  200  begin to engage. When the driving clutch  100  and output clutch  200  begin to engage, they are ready to transfer torque. When the driving clutch  100  and the output clutch  200  are fully engaged, the pushing force is directly transferred between them. 
         [0052]    When the driving means is activated, torque is applied to the input spindle  300  and in turn is transferred to the driving clutch  100 , the output clutch  200 , the output spindle  400 , the bit holder  430 , the bit  440  and finally to the screw, rotating each of these components together. The screw is thereby driven into the work piece W. 
         [0053]    In  FIG. 7 , at some point while driving the screw, the torque transferred through the input spindle  300  and driving clutch  100  becomes great enough that the reaction force resulting from the torque on the helical splines  310 ,  110  causes driving clutch  100  to rotate slightly relative to the input spindle  300  and move axially away from the input spindle  300  by a small distance D, until the driving clutch  100  and the thrust washer  322  ( FIG. 2 ) abut retaining ring  340  ( FIG. 2 ). This movement of the driving clutch  100  also causes the output clutch  200 , output spindle  400 , bit holder  430  and bit  440  to slide axially forward a small amount. 
         [0054]    Eventually the screw is driven to a predetermined depth and an end of the nosepiece  600  abuts the work piece W, as depicted in  FIGS. 7 and 8 . The user&#39;s pushing force against the screw gun is then transferred through the nosepiece  600  to the work piece W, and no longer through the clutch. With the user&#39;s pushing force no longer fully compressing spring  500 , the output spindle  400  and the output clutch  200  are pushed axially away from the driving clutch  100  and input spindle  300  as the screw is driven further into the work piece W. In one embodiment, the output spindle  400  and the output clutch  200  are pushed axially away from the driving clutch  100  and input spindle  300  by a reaction force created at the surface of sloped mating teeth formed on the mating surfaces of each of the output clutch  200  and driving clutch  100 . As output clutch  200  moves axially away from the driving clutch  100 , it begins to disengage from driving clutch  100 , as depicted in  FIG. 8 . 
         [0055]    Eventually, the mating surfaces of output clutch  200  move just beyond the mating surfaces of driving clutch  100  and they slip and disengage. At the instant they slip, the torque transmitted from the input spindle  300  to the driving clutch  100  is released. When the torque is released, the reaction force from the helical splines  110 ,  310  which pushes the driving clutch  100  axially away from the input spindle  300  is also released. Once this reaction force is released, spring  500  pushes the thrust washer  322  and the driving clutch  100  back into their original positions relative to input spindle  300  so that there is no longer a distance D separating them. This motion also moves driving clutch  100  axially away from the output clutch  200 , creating a new clearance distance D between the output clutch  200  and the driving clutch  100 , as depicted in  FIG. 9 . This clearance distance D ensures a clean disengagement of the clutch and reduces clashing. 
         [0056]    In the second mode of operation,  FIG. 5  depicts the clutch in a disengaged state, and it will be assumed that the drive means of the tool is already activated. This mode of operation is commonly employed by tradesmen who lock the screw gun in its “ON” position during continuous use for driving one fastener after another in rapid succession. 
         [0057]    When the user pushes a screw against a work piece W with the screw gun, as in  FIG. 6 , output clutch  200  moves toward engagement with driving clutch  100 , as in the first mode. Since the drive means is already activated, input spindle  300  and driving clutch  100  are already rotating. As soon as driving clutch  100  and output clutch  200  begin to engage, output clutch  200  and output spindle  400  immediately begin to rotate along with driving clutch  100  and input spindle  300  and the screw is driven into work piece W. 
         [0058]    As in the first mode, at some point while driving the screw, the torque transferred through the input spindle  300  and driving clutch  100  becomes great enough that the reaction force resulting from the torque on the helical splines  310 ,  110  causes driving clutch  100  to move away from input spindle  300  by a small distance D, as depicted in  FIG. 7 . From this point forward, the clutch will continue to operate as described in the first mode until the clutch disengages. 
         [0059]    Besides simplicity, this design includes other advantages not present in the prior art. For example, the retaining ring  340 , shown in  FIG. 2 , is advantageously placed on the input spindle  300  so that at the time when the driving clutch  100  and thrust washer  322  contact retaining ring  340 , the retaining ring  340  rotates at the same speed as driving clutch  100 . In the design disclosed in U.S. Pat. No. 5,538,089, an annular projecting shoulder  88  (see FIG. 1 of the &#39;089 patent) is formed in the clutch housing and bears against an annular shoulder  56  on the intermediate clutch. The projecting shoulder  88  stops the forward axial movement of the intermediate clutch, but also causes friction as the clutch housing does not rotate and the intermediate clutch rotates at a high speed. The disclosure suggests that a journal bearing  89  can be placed between the shoulders  88  and  56 , but heat and wear are nevertheless created by the friction resulting from the contact. Retaining ring  340  disclosed herein produces substantially no heat or wear. 
         [0060]    Retaining ring  340  must be able to withstand the axial force placed upon it by the driving clutch  100 . To this end, two alternative embodiments are proposed in  FIGS. 10-11  which may provide an increased fatigue life for the clutch. 
         [0061]      FIGS. 10A-10B  illustrate a preferred embodiment which includes a first alternative to retaining ring  340 : a nut  340   b . Nut  340   b  is threaded on its inside diameter. The outside diameter of the input spindle  300  has complementary threads in this embodiment so that nut  340   b  is held onto input spindle  300  with a threaded connection. The threaded connection provides additional strength to keep nut  340   b  securely positioned on the input spindle  300  during the life of the tool. 
         [0062]      FIGS. 11A-11B  illustrate a preferred embodiment which includes a second alternative to retaining ring  340 : a flange  340   c  on a stud  341 . One end of stud  341  is sized to fit inside of bore  330  of input spindle  300 . Bore  330  has threads formed therein and one end of stud  341  has complementary threads which engage the threads of bore  330  to form a threaded connection between input spindle  300  and stud  341 . Flange  340   c  is formed as an annular shoulder on the other end of stud  341  opposite the threaded end. In this embodiment, output spindle  400  would be formed with a bore in one end sized so that stud  341  can be journaled in the bore and the output spindle  400  can be rotationally supported by stud  341 . 
         [0063]    Although the invention has been described in relation to certain preferred embodiments, the invention is not limited to these embodiments. Many possible variations of the clutch may be realized without departing from the scope of the invention. For example, in lieu of the helical spline arrangement between the input spindle  300  and the driving clutch  100 , the output clutch  200  may be connected to the output spindle  400  through a helical spline. Such an arrangement would be essentially a reversal of the arrangement illustrated in  FIG. 2 . Other alternatives are within the scope of the preferred embodiments and shall be regarded as equivalents. Likewise, the material and process for manufacturing the clutch components, and for manufacturing high-wear components for power tools generally, have been described in relation to a specific application. This aspect of the invention should not be limited to this single application. The principles of the material and process for producing high-wear parts in power tools generally encompass the various alternatives which will be apparent to those of skill in the art.