Patent Publication Number: US-2023143261-A1

Title: Clutch mechanism and power tool having same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 63/263,712, filed on Nov. 8, 202, entitled “CLUTCH MECHANISM AND POWER TOOL HAVING SAME,” the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     This document relates, generally, to a clutch mechanism, and in particular to a clutch mechanism for a power tool. 
     BACKGROUND 
     A power-driven tool may output a force generated by a motor of the tool to perform an operation on a workpiece. Power-driven tools may operate at multiple different settings (speed settings, torque settings and the like) to accomplish different types of tasks. Power-driven tools may employ a transmission mechanism and a clutch mechanism, allowing for power from the motor to be output by an output device of the tool at different torques and/or speeds to accommodate a variety of different tasks with the same tool. A substantially linear operation profile of the clutch mechanism may produce inconsistencies in output torque settings, particularly at upper and lower end portions of the operation profile. A clutch mechanism having a variable operation profile may enhance utility and functionality of this type of power-driven tool, thus enhancing utility to the operator. 
     SUMMARY 
     In one general aspect, a power-driven tool includes a motor; an output shaft; a transmission configured to transmit a torque generated by the motor to the output shaft; and a clutch configured to selectively disengage torque transfer from the transmission to the output shaft when an output torque exceeds a threshold torque value. The clutch may include a clutch selector actuatable to select the threshold torque value; a retaining ring moveably coupled to the selector to move relative to the transmission in response to selection of the threshold torque value by actuation of the selector; a clutch engagement member selectively engageable with a component of the transmission to interrupt torque transfer from the transmission to the output shaft; and a biasing mechanism coupled between the retaining ring and the clutch engagement member, the biasing mechanism including at least one spring member having a variable spring rate. A biasing force applied to the clutch engagement member by the biasing mechanism corresponds to the selected threshold torque value and can be varied in a non-linear manner in accordance with movement of the retaining ring that adjusts the biasing force in accordance with the variable spring rate. 
     In some implementations, the clutch engagement member is a ball or a pin that engages a ramped surface on the transmission. The clutch engagement member may also include a clutch plate disposed between the ball or pin and the biasing member. The selector may include a clutch collar that is rotatable relative to the housing. A clutch nut may be disposed between the clutch collar and the retaining ring. The clutch may also include a clutch housing with a threaded front end portion, the clutch nut threadably engaged with the threaded front end portion to be axially movable relative to the transmission. 
     In some implementations, the at least one spring member includes a dual coil spring, including a first coil portion having a first length and a first diameter; and a second coil portion having a second length that is different than the first length, and a second diameter that is different than the first diameter. The first coil portion may be positioned within the second coil portion and may be concentrically arranged with the second coil portion; the first length of the first coil portion is greater than the second length of the second coil portion; and the first diameter of the first coil portion is less than the second diameter of the second coil portion. The retaining ring may be moveable axially relative to the clutch engagement member such that at a first axial position of the retaining ring relative to the clutch engagement member, the first coil portion of the dual coil spring contacts the clutch engagement member and is compressed to exert a first biasing force on the clutch engagement member and the second coil portion of the dual coil spring is not compressed; and at a second axial position of the retaining ring relative to the clutch plate, both the first coil portion and the second coil portion of the dual coil spring contact the clutch engagement member and are compressed to exert a second biasing force on the clutch engagement member that is greater than the first biasing force. The first axial position may correspond to a first clutch setting corresponding to a first threshold value torque setting for the power-driven tool, and the second axial position may correspond to a second clutch setting corresponding to a second threshold value torque setting that is greater than the first output torque setting. In some implementations, the dual coil spring includes a first coil spring defining the first coil portion, and a second coil spring defining the second coil portion. In some implementations, the first coil portion follows a first helical pattern, and the second coil portion follows a second helical pattern that is opposite the first helical pattern of the first coil portion. 
     In some implementations, the at least one spring member includes a dual rate spring, including a first coil portion having a first spring rate; and a second coil portion coupled to the first coil portion and having a second spring rate. At a first axial position of the retaining ring relative to the clutch plate, the first coil portion of the dual rate spring may contact the clutch engagement member and be compressed, and the second coil portion is not compressed, such that the dual coil spring exerts a first biasing force corresponding to the first spring rate on clutch engagement member. At a second axial position of the retaining ring relative to the clutch engagement member, both the first coil portion and the second coil portion of the dual rate spring may be compressed, and the dual rate spring exerts a second biasing force corresponding to the second spring rate on the clutch engagement member, the second biasing force being greater than the first biasing force. The first axial position may correspond to a first clutch setting corresponding to a first output torque setting for the power-driven tool, and the second axial position may correspond to a second clutch setting corresponding to a second output torque setting that is greater than the first output torque setting. A first end of the first coil portion may be configured to selectively contact the clutch engagement member based on an axial position of the retaining ring relative to the clutch plate; a second end of the first coil portion may be coupled to a first end of the second coil portion such that the second coil portion extends from the first end of the first coil portion of the dual rate spring; and a second end of the second coil portion may be retained by a corresponding pin and recess defined in the retaining ring. 
     In some implementations, the at least one spring member includes a plurality of spring members, each of the plurality of spring members having a first end portion thereof configured to selectively contact the clutch engagement member based on an axial position of the retaining ring relative to the clutch plate, and a second end thereof retained by a corresponding pin and recess defined in the retaining ring. In some implementations, the at least one spring member includes a single spring member, the single spring member having a first end portion thereof configured to selectively contact the clutch engagement member based on an axial position of the retaining ring relative to the clutch plate, and a second end thereof retained by a corresponding pin and recess defined in the retaining ring. 
     In some implementations, the retaining ring is configured to move in a first axial direction in response to rotation of the clutch collar in a first rotational direction; and the retaining ring is configured to move in a second axial direction in response to rotation of the clutch collar in a second rotational direction. The at least one spring member may be configured to be compressed in response to movement of the retaining ring in the first axial direction to exert a biasing force on clutch engagement member; and compression of the at least one spring member may be configured to be released in response to movement of the retaining ring in the second axial direction to release the biasing force exerted on the clutch engagement member. 
     In some implementations, thee at least one spring member includes a plurality of springs, including at least one first spring having a first length; and at least one second spring having a second length, the second length being different from the first length. The at least one first spring may be configured to exert a biasing force on the clutch engagement member at a first axial position of the retaining ring relative to the clutch engagement member; and the at least one second spring may be configured to exert a biasing force on the clutch engagement member at a second axial position of the retaining ring relative to the clutch engagement member. The at least one first spring may be configured to exert a biasing force on the clutch engagement member at a first axial position of the retaining ring relative to the clutch engagement member; and the at least one first spring and the at least one second spring may be configured to exert a biasing force on the clutch engagement member at a second axial position of the retaining ring relative to the clutch engagement member. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic illustration of an example power-driven tool. 
         FIG.  2 A  is a side view of an example power-driven tool, in accordance with implementations described herein. 
         FIG.  2 B  is an internal view of the example tool shown in  FIG.  2 A , with a portion of a housing removed, so that internal components are visible. 
         FIG.  3    is a close-in view of an example clutch mechanism including a biasing member. 
         FIG.  4    is a graph of output torque as a function of clutch setting for various example biasing members. 
         FIGS.  5 A- 5 C  are assembled perspective views of an example clutch mechanism coupled to an example transmission mechanism of a power-driven tool, such as the example power-driven tool  100  described above with respect to  FIGS.  2 A- 3   , in accordance with an implementation described herein. 
         FIG.  5 D  is a partially exploded view of the example clutch mechanism shown in  FIGS.  5 A- 5 C . 
         FIG.  5 E  is a fully exploded view of the example clutch mechanism shown in  FIGS.  5 A- 5 C . 
         FIG.  5 F  is a perspective view of an example retaining ring of the example clutch mechanism shown in  FIGS.  5 A- 5 E , in accordance with implementations described herein. 
         FIG.  5 G  is a perspective view illustrating interaction of an example clutch selector, an example clutch nut, and an example retaining ring of the example clutch mechanism shown in  FIGS.  5 A- 5 E , in accordance with implementations described herein. 
         FIGS.  6 A and  6 B  are side views of an example arrangement of biasing members of an example biasing mechanism for a clutch mechanism of a power-driven tool, in accordance with implementations described herein. 
         FIG.  6 C  is a cross-sectional view taken alone line A-A of  FIG.  6 A . 
         FIGS.  7 A- 7 C  are side views of an example arrangement of biasing members of an example biasing mechanism for a clutch mechanism of a power-driven tool, in accordance with implementations described herein. 
         FIG.  7 D  is an exploded, partial perspective view of the example arrangement of biasing members shown in  FIGS.  7 A- 7 C . 
         FIGS.  7 E and  7 F  are side views of example biasing members of the example biasing mechanism shown in  FIGS.  7 A- 7 C . 
         FIGS.  8 A- 8 D  are side views of an example arrangement of biasing members of an example biasing mechanism for a clutch mechanism of a power-driven tool, in accordance with implementations described herein. 
         FIG.  8 E  is a side view of an example biasing member of the example biasing mechanism shown in  FIGS.  8 A- 8 D . 
     
    
    
     DETAILED DESCRIPTION 
     Power-driven tools such as, for example, drills, drivers, impact drills/drivers and other such power-driven tools, may apply torque to a workpiece to accomplish a task, with different tasks sometimes requiring different levels of output torque. In some examples, the power-driven tool may be configured to output variable or adjustable torque levels such that a torque level, or an amount of torque, to be output by the tool and/or applied to the workpiece to accomplish a particular task may be selected by the operator. In some examples, the power-driven tool may include a torque control device, or a torque limiting device, that may selectively engage the clutch mechanism and/or the transmission mechanism based on, for example, a torque level selected by an operator of the tool. In some examples, such a torque control device, or torque limiting device, may control a maximum amount of torque that is transmitted from a driving mechanism (i.e., a motor) to an output mechanism of the power-driven tool. Such a torque control device, or torque limiting device may cause the clutch mechanism to engage and disengage the output mechanism and/or the transmission mechanism based on the selected output torque level, or selected trip torque, for a particular task. This may allow the operator to apply a desired level of torque to a workpiece, a uniform level of torque to multiple portions of a workpiece (such as, for example, the tightening of multiple fasteners at a uniform torque level with respect to the workpiece), avoid over-torquing, and the like, thus enhancing functionality and utility of the power-driven tool. In a power-driven tool including a clutch mechanism that provides torque limiting functionality, in accordance with implementations described herein, a level or amount of torque output by the power-driven tool may be accurately and reliably controlled by a biasing mechanism of the clutch mechanism that provides for variable levels of biasing, based on a selected output torque level. 
     A schematic view of an example power-driven tool  10  is shown in  FIG.  1   . The example tool  10  includes a driving mechanism  11  generating a driving force, for example, a rotational driving force. In the example shown in  FIG.  1   , a transmission mechanism  12  is coupled to the driving mechanism  11 , to transfer force, for example, rotational force, from the driving mechanism  11  to an output mechanism  13 . A clutch mechanism  14  may be coupled, for example, between the transmission mechanism  12  and the output mechanism  13  and/or between the driving mechanism  11  and the transmission mechanism  12 . The driving mechanism  11 , the transmission mechanism  12 , the output mechanism  13  and the clutch mechanism  14  may be received in a housing  19 . A selection mechanism  18  may be coupled to the clutch mechanism  14  and/or the transmission mechanism  12  and/or the driving mechanism  11 . The selection mechanism  18  may provide for user selection of an operation mode of the tool  10 , an operation speed to be output by the tool  10 , a torque level to be output by the tool  10 , and the like. In some implementations, the driving mechanism  11  may be an electric motor that receives power from, for example, a power storage device (such as, for example, a battery), an external electrical power source, and the like. In some implementations, the driving mechanism  11  may be an air driven, or pneumatic motor, that is powered by compressed air introduced into the housing  19  from an external compressed air source. Other types of driving mechanisms, and other sources of power, may provide for power driven operation of the tool  10 . 
       FIGS.  2 A and  2 B  are side views of an example power-driven tool  100 , in accordance with implementations described herein.  FIG.  2 B  provides an internal view of the example tool  100 , with a portion of a housing  190  shown in  FIG.  2 A  partially removed, so that internal components of the example tool  100  are visible. The example tool  100  shown in  FIGS.  2 A and  2 B  includes a housing  190 , with a chuck assembly  170  at an end portion of the housing  190 , for example, at an end portion of the housing  190  corresponding to a working end of the tool  100 . A trigger  125  for triggering operation of the tool  100  may be provided at a handle portion  196  of the housing  190 . The example power-driven tool  100  shown in  FIGS.  2 A and  2 B  includes multiple selection mechanisms  180  provided on the housing  190  for user control of the tool  100 . For example, a first selection mechanism  180 A may provide for user selection of a torque setting, for example, a maximum torque setting, or a maximum torque level. The first selection mechanism  180 A may be operably coupled to a clutch  140  received in the housing  190 , to control a maximum torque level to be output by the tool  100 . A second selection mechanism  180 B may provide for user selection of an operating mode of the tool  100  such as, for example, an operating speed, an operating direction and the like. 
     The example power-driven tool  100  illustrated in  FIGS.  2 A and  2 B  is a driving tool, or a drill, simply for purposes of discussion and illustration. The principles to be described herein may be applied to other types of power-driven tools, including more, or fewer, components and/or features. 
     As shown in  FIG.  2 B , the example tool  100  may include a motor  110  received in the housing  190 . The motor  110  may output a force, for example, a rotational force, via an output shaft  112 , to a transmission  120 . The transmission  120  may, in turn, transmit the rotary force from the motor  110  to an output mechanism  130 , for example, an output shaft  130 . An output tool (for example, a bit, a blade, and the like) may be coupled to the output shaft  130 , and may be positioned outside of the housing  190  to perform an operation on a workpiece. The output tool may be coupled to the power-driven tool  100  by, for example, the chuck assembly  170 , and driven by a force transmitted thereto by the output shaft  130 . 
     Referring also to  FIG.  2 B , in some examples, an output torque level may be set by the operator, for example through operator manipulation of the first selection mechanism  180 A. Operator manipulation of the selection mechanism  180 A may adjust an axial position of a retaining ring  146 , which in turn may adjust compression of a biasing device  144  against a clutch plate  142 . In the example shown in  FIG.  3   , the biasing device  144  is a single compression spring  144  retained between the clutch plate  142  and the retaining ring  146 . An amount of axial pressure applied to the clutch plate  142  and corresponding axial movement of the clutch plate  142 /interaction with the transmission  120  (in response to the operator manipulation of the selection mechanism  180 A) may provide for engagement/disengagement of the clutch  140  and the transmission  120  in accordance with the selected output torque level. 
     In some situations, the biasing device  144  in the form of the single compression spring  144 , which provides a single spring rate, or a substantially uniform stiffness, may produce an output torque clutch setting that is too low for a high clutch setting, resulting in unintended disengagement at a lower output torque level than selected. In some cases, this may be addressed by using a compression spring having a greater stiffness. However, this may result in an output torque clutch setting that is too high for a low clutch setting, resulting in possible over-torquing. This is graphically illustrated in  FIG.  4   . As shown in  FIG.  4   , a biasing device having a variable spring rate, or a dual spring rate, may provide for output torque levels that are consistent with the selected clutch setting. In particular, as shown in  FIG.  4   , a biasing device having a non-linear spring rate, or a dual spring rate, or a variable spring rate, has a lower slope/lower spring constant/lower stiffness at lower clutch settings, and has a greater slope/greater spring constant/greater stiffness at higher clutch settings, allowing the device to achieve output torque along a relatively wide range of output torque levels. 
       FIGS.  5 A- 5 C  are assembled perspective views of an example clutch coupled to an example transmission of a power-driven tool, such as the example power-driven tool  100  described above with respect to  FIGS.  2 A- 3   , in accordance with implementations described herein. In the views shown in  FIGS.  5 A- 5 C , a housing portion of the power-driven tool has been removed, so that components of the example clutch are visible.  FIG.  5 D  is a partially exploded view, and  FIG.  5 E  is a fully exploded view, of the example clutch shown in  FIGS.  5 A- 5 C .  FIG.  5 F  is a perspective view of an example retaining ring  560  of the example clutch shown in  FIGS.  5 A- 5 E , in accordance with implementations described herein.  FIG.  5 G  is a perspective view illustrating the operation of a clutch selector  540  including a rotatable clutch collar of the example clutch shown in  FIGS.  5 A- 5 E , in accordance with implementations described herein. The clutch selector  540  is actuatable by the operator to select a threshold torque value, or output torque value, or trip torque level, at which the clutch will disengage the transmission. 
     The example clutch  500  shown in  FIGS.  5 A- 5 E  includes a biasing mechanism  570  positioned between a retaining ring  560  and a clutch engagement member  585  including a clutch plate  580  that selectively interacts with a plurality of balls  505  received in an output stage ring gear  520  of the transmission. In the example arrangement shown in  FIGS.  5 A- 5 E , the clutch plate  580  has a substantially annular shape and is positioned at a nose portion of a clutch housing  590 . The clutch plate  580  applies a force (i.e., an axial force) to the balls  505 , which are received in recessed portions  522  between ramped surfaces defined in a front face of the output stage ring gear  520  in response to a biasing force applied thereto by the biasing mechanism  570 , to selectively retain the balls  505  in the recessed portions  522 . The example biasing mechanism  570  shown in  FIGS.  5 A- 5 D  includes a plurality of biasing members  571 , or spring members  571 . A first end portion of each spring member  571  may be retained by a corresponding pin  561  and spring recess  563  defined in a mating surface  564  of the retaining ring  560 . The example arrangement of pins  561  and spring recesses  563  defined in the mating surface  564  of the retaining ring  560  shown in  FIG.  5 F  is just one example of how the pins  561  and spring recesses  563  may be arranged. The retaining ring  560  may incorporate other arrangements of pins  561  and spring recesses  563  to accommodate the retention of other arrangements, sizes, combinations and the like of biasing members of a biasing mechanism in accordance with implementations described herein. 
     A clutch nut  550  is engaged between the retaining ring  560  and a clutch selector  540  (see  FIGS.  5 E and  5 F ). The clutch selector  540  is accessible from the outside of the tool, to provide for operator manipulation of the clutch selector  540 . A threshold torque value, or output torque level, or trip torque level, may be selected through operator actuation, or manipulation, for example, rotation, of the clutch selector  540 . In some examples, protrusions  554 , or splines  554 , or lugs  554  defined on an outer peripheral surface of the clutch nut  550  (see  FIGS.  5 C and  5 D ) engage with corresponding protrusions, or splines (not shown) formed on an inner peripheral surface of the clutch selector  540  to couple the clutch selector  540  and the clutch nut  550 , and the retaining ring  560  coupled thereto. In some examples, the clutch nut  550  is fixed to the clutch selector  540 , so that the clutch selector  540  and the clutch nut  550  rotate together for the setting of an output torque level, or trip torque level, or clutch setting via manipulation of the clutch selector  540 . In some examples, a detent plate  515  (see  FIG.  5 G ) is fixed to the housing of the power-driven tool. As the clutch selector  540  and clutch nut  550  are rotated, a protrusion  519 , or detent  519 , formed at an outer peripheral portion of the detent plate  515  moves along an inner peripheral surface of the clutch selector  540  and is received in one of a plurality of detent recesses  559  formed in the inner peripheral surface of the clutch selector  540 . The plurality of detent recesses  559  formed in the inner peripheral surface of the clutch selector  540  may correspond to a plurality of clutch settings selectable via rotation of the clutch selector  540 . In some examples, a stop  558  formed on the inner peripheral surface of the clutch selector  540  interacts with a tab  518  formed on the outer peripheral portion of the detent plate  515  to prevent over rotation of the clutch selector  540  in either the direction R 1  or the direction R 2 . 
     A threaded interior portion  552  of the clutch nut  550  may be engaged with threaded portions  592  on a front protruding portion  593  of the clutch housing  590 . As the clutch nut  550  rotates in response to rotation of the clutch selector  540 , the threaded engagement of the clutch nut  550  with the clutch housing  590  translates the rotational movement into axial movement. 
     Rotation of the clutch selector  540  in a first rotational direction R 1  causes corresponding rotation of the clutch nut  550  and the retaining ring  560 , and axial movement of the clutch nut  550  and retaining ring  560  in a first axial direction Al. The axial movement of the clutch nut  550  and retaining ring  560  in turn causes compression of the biasing mechanism  570 . The compression of the biasing mechanism  570  causes a first force to be exerted on the clutch plate  580 . The first force exerted on the clutch plate  580  may position the clutch plate  580  so as to exert a force on the plurality of balls  505  received in the channels  522  of the output stage ring gear  520 , to retain the balls  505  in the channels  522 . Rotation of the clutch plate  580  may be restricted by the positioning of one or more protrusions  584  of the clutch plate  580  in a corresponding one or more recesses  594  formed in the clutch housing  590 . 
     During operation of the tool, the clutch  500  may selectively provide for engagement between the transmission and the output shaft  530  (to in turn drive an output tool secured in the chuck  510 ). That is, an amount of compression of the biasing mechanism  570  (and corresponding magnitude of the biasing force exerted on the clutch plate  580 ) may correspond to a set output torque level, or trip torque, selected via manipulation (i.e., rotation) of the clutch selector  540 . As the clutch nut  550  and retaining ring  560  move further in the first axial direction Al, the amount of compression of the biasing mechanism  570  (and corresponding force exerted on the clutch plate  580 ) increases. 
     As the force exerted on the clutch plate  580  increases (corresponding to an increased output torque level), an amount of torque required to cause the balls  505  to travel in the channels  522  and over ramped portions  524  of the output stage ring gear  520 , to cause disengagement of the clutch  500 , also increases. Once disengaged, force is no longer transmitted from the transmission to the output shaft  530 . That is, when a level of torque output by the transmission is greater than the selected torque level, the biasing force retaining the balls  505  in the channels  522  of the output stage ring gear  520  is overcome, and balls  505  travel over the ramped portions  524 , allowing the output stage ring gear  520  to spin freely. This disengages the output of the transmission from the output shaft  530 , so that torque is no longer transmitted from the transmission to the output shaft  530 . 
     In a similar manner, rotation of the clutch selector  540  in a second rotational direction R 2  (opposite the first rotational direction R 1 ) may cause axial movement of the clutch nut  550  and the retaining ring  560  in a second axial direction A 2  (opposite the first axial direction A 1 ). As the clutch nut  550  and retaining ring  560  move further in the second axial direction A 2 , the amount of compression of the biasing mechanism  570  (and corresponding force exerted on the clutch plate  580 ) decreases. The decreased force exerted on the clutch plate  580  in turn decreases an amount of torque that will cause the balls  505  to travel in the channels  522  and over the ramped portions  524  of the output stage ring gear  520 , causing disengagement of the clutch  500  such that force is no longer transmitted from the transmission to the output shaft  530 . 
     As described above with respect to  FIG.  4   , a biasing mechanism having a single spring rate, or a substantially uniform stiffness (such as the example biasing device  144  in the form of the single spring  144  shown in  FIG.  3   ) may produce an output torque clutch setting that is too low for a high clutch setting, resulting in unintended disengagement at a lower output torque level than selected. Increasing stiffness of the biasing mechanism may result in an output torque clutch setting that is too high for a low clutch setting, resulting in greater output torque levels than desired, or over-torquing. A biasing mechanism, in accordance with implementations described herein, may employ a variable overall spring rate, to provide for output torque levels that are consistent with the selected clutch setting. 
     In the example arrangement shown in  FIGS.  5 A- 5 D , the biasing mechanism  570  includes a plurality of coil spring members having a first end thereof retained by the retaining ring  560 . A second end of one or more of the coil spring members selectively contacts the clutch plate  580 , depending on an axial position of the retaining ring  560  and the clutch nut  550  (based on the selected output torque corresponding to the rotational position of the clutch selector  540 ). 
       FIGS.  6 A and  6 B  are side views of an example arrangement of biasing members of an example biasing mechanism  600  for a clutch of a power-driven tool, such as the clutch  500  described above with respect to  FIGS.  5 A- 5 D .  FIG.  6 C  is a cross-sectional view taken alone line A-A of  FIG.  6 A . The example biasing mechanism  600 , in accordance with implementations described herein, provides a variable, or non-linear, or dual spring rate, allowing the device to achieve a desired output torque along a relatively wide range of output torque levels. 
     As shown in  FIGS.  6 A- 6 C , in some implementations, the biasing mechanism  600  includes a plurality of first biasing members  671 , or first coil spring members  671 , and a plurality of second biasing members  672 , or second coil spring members  672 . In the example arrangement shown in  FIGS.  6 A- 6 C , the first coils spring members  671  and the second coil spring members  672  are arranged circumferentially about an axis defined by the output shaft  530 . In the example arrangement shown in  FIGS.  6 A- 6 C , a first coil spring member  671  is positioned between two adjacent pairs of second coil spring members  672 , simply for purposes of discussion and illustration. The biasing mechanism  600  can include more, or fewer first coil spring members  671 , and/or more, or fewer second coil spring members  672  than shown in  FIGS.  6 A- 6 C , and/or different combinations and/or arrangements of first coil spring members  671  and second coil spring members  672 . 
     As shown in  FIG.  6 A , a length of the first coil spring members  671  is greater than a length of the second coil spring members  672 .  FIG.  6 A  illustrates the first coil spring members  671  and the second coil spring members  672  relative to the clutch nut  550 , the retaining ring  560  and the clutch plate  580  at a first clutch setting, for example, a low clutch setting corresponding to a low output torque level. At the first clutch setting, the first (longer) coil spring members  671  contact the clutch plate  580 , thus imparting a first biasing force on the clutch plate  580 , while the second (shorter) coil spring members  672  do not contact the clutch plate  580 , and thus do not exert any biasing force on the clutch plate  580 . 
       FIG.  6 B  illustrates the first coil spring members  671  and the second coil spring members  672  relative to the clutch nut  550 , the retaining ring  560  and the clutch plate  580  at a second clutch setting, for example a clutch setting that is higher than the first clutch setting, corresponding to a higher output torque level than shown in  FIG.  6 A . At the second clutch setting, the clutch nut  550 /retaining ring  560  has moved axially closer to the clutch plate  580 , so that the first (longer) coil spring members  671  and the second (shorter) coil spring members  672  together impart a second biasing force on the clutch plate  580  that is greater than the first biasing force described with respect to  FIG.  6 A . 
     The example biasing mechanism  600  includes the first coil spring members  671  and second coil spring members  672 , simply for purposes of discussion and illustration. In some implementations, the biasing mechanism  600  can include additional coil spring members having different lengths than the first coil spring members  671  and the second coil spring members  672 . This may allow for additional variation in the spring rate provided by the biasing mechanism  600 . 
     The combination of the first (longer) coil spring members  671  and the second (shorter) coil spring members  672  in the example biasing mechanism  600  provides a variable, or non-linear, or dual spring rate. Coupled with the varying degrees of biasing force generated by the first coil spring members  671  and the second coil spring members  672  depending on the relative position of the clutch nut  550 /retaining ring  560  and the clutch plate  580  and the corresponding amount of compression of the first and second coil spring members  671 ,  672 , this allows the tool to output a desired output torque along a relatively wide range of output torque levels. 
       FIGS.  7 A- 7 C  are side views of an example arrangement of biasing members of an example biasing mechanism  700  for a clutch of a power-driven tool, such as the clutch  500  described above with respect to  FIGS.  5 A- 5 D .  FIG.  7 D  is an exploded perspective view of the example arrangement of biasing members of the example biasing mechanism  700  shown in  FIGS.  7 A- 7 C .  FIGS.  7 D and  7 E  are side views of example spring members  770 ′ and  770 ′ that can be used in the example biasing mechanism  700  shown in  FIGS.  7 A- 7 D . The example biasing mechanism  700 , in accordance with implementations described herein, provides a variable, or non-linear, or dual spring rate, allowing the device to achieve a desired output torque along a relatively wide range of output torque levels. 
       FIG.  7 E  illustrates an example spring member  770 , which can be incorporated into the example biasing mechanism  700  shown in  FIGS.  7 A- 7 D . In some implementations, the example spring member  770  can replace some or all of the coil spring members  671 ,  672  of the example biasing mechanism  600  shown in  FIGS.  6 A- 6 C . In some implementations, the example spring member  770  shown in  FIG.  7 E  can replace some of the coil spring members  671 ,  672  of the example biasing mechanism  600  shown in  FIGS.  6 A- 6 C  such that the biasing mechanism includes a combination of spring members, including one or more of the coil spring members  671 ,  672  and one or more of the spring members  770 . 
     The example spring member  770  shown in  FIG.  7 E  is a double coil spring member  770  including a first coil portion  771  and a second coil portion  772 . In the example shown in  FIG.  7 E , a diameter, of the first coil portion  771  is less than a diameter of the second coil portion  772 . In the example shown in  FIG.  7 D , the first coil portion  771  and the second coil portion  772  are substantially concentric, with the second coil portion  772  positioned outside of the first coil portion  771 . In the example shown in  FIG.  7 E , a helix pattern of the first coil portion  771  is different, for example, opposite a helix pattern of the second coil portion  772 . For example, a first coil portion  771  having a right-hand pattern and a second coil portion  772  having an opposite, left-hand pattern may allow the example double coil spring member  770  to operate without the coils of the first coil portion  771  interfering with the coils of the second coil portion  772  as the first and second coil portions  771 ,  772  are independently compressed and released. 
     As shown in  FIG.  7 F , in some examples, a double coil spring member  770 ′ may include a first spring defining the first coil portion  771 ′ and a separate second spring defining the second coil portion  772 ′, the first and second springs having different lengths and different diameters consistent with the description provided above with respect to the first and second coil portions  771 ,  772  of the double coil spring member  770  shown in  FIG.  7 E . 
     Hereinafter, simply for purposes of discussion and illustration, the example biasing mechanism  700  will be described with respect to the double coil spring member  770 . The principles to be described can be similarly applied to a biasing mechanism  700  including the double coil spring member  770 ′. 
     The example biasing mechanism  700  includes spring members  770  each including the first coil portion  771  and the second coil portion  772 , simply for purposes of discussion and illustration. In some implementations, the biasing mechanism  700  can include additional coil portions, for example additional coil portions having different diameters and/or lengths that the first coil portion  771  and the second coil portion  772 . This may allow for additional variation in the spring rate provided by the biasing mechansim  700 . 
     In  FIGS.  7 E and  7 F , the example spring members  770 ,  770 ′ are in an at rest state, corresponding to a disengaged state of the clutch shown in  FIG.  7 A . In the at rest state of the example spring member  770 / 770 ′, a length L 1  of the first coil portion  771 / 771 ′ is greater than a length L 2  of the second coil portion  772 / 772 ′. In the at rest state of the example spring member  770 / 770 ′ corresponding to the disengaged state of the clutch mechanism shown in  FIG.  7 A , the clutch nut  550 /retaining ring  560  and clutch plate  580  are positioned such that neither the first coil portion  771 / 771 ′ nor the second coil portion  772 / 772 ′ is contacting, or engaged with the clutch plate  580 . 
       FIG.  7 B  illustrates the double coil spring members  770  (or  770 ′) relative to the clutch nut  550 , the retaining ring  560  and the clutch plate  580  at a first clutch setting, for example, a low clutch setting corresponding to a low output torque level. At the first clutch setting, the clutch nut  550 /retaining ring  560  has moved axially closer to the clutch plate  580 . At the first clutch setting, the first (longer) coil portion  771 / 771 ′ of each of the double coil spring members  770 / 770 ′ contacts the clutch plate  580 , thus imparting a first biasing force on the clutch plate  580 . At the first clutch setting shown in  FIG.  7 B , the second (shorter) coil portions  772 / 772 ′ of each of the double coil spring members  770 / 770 ′ do not contact the clutch plate  580 , and thus do not exert any biasing force on the clutch plate  580 . Thus, at the first clutch setting shown in  FIG.  7 B , only the first coil portion  771 / 771 ′ of each of the double coil spring members  770 / 770 ′ imparts any force on the clutch plate  580 . 
       FIG.  7 C  illustrates the double coil spring members  770 / 770 ′ relative to the clutch nut  550 , the retaining ring  560  and the clutch plate  580  at a second clutch setting, for example a clutch setting that is higher than the first clutch setting, corresponding to a higher output torque level, or trip torque, than shown in  FIG.  7 B . At the second clutch setting, the clutch nut  550 /retaining ring  560  has moved axially closer to the clutch plate  580 , so that the first (longer) coil portion  771 / 771 ′ and the second (shorter) coil portion  772 / 772 ′ together impart a second biasing force on the clutch plate  580  that is greater than the first biasing force described with respect to  FIG.  7 B . At the second clutch setting, the first coil portion  771 / 771 ′ is more compressed than at the first clutch setting shown in  FIG.  7 B , thus exerting a greater biasing force at the second clutch setting than at the first clutch setting, with the compression of the second coil portion  772 / 772 ′ of the double coil spring members  770 / 770 ′ now also contributing to the biasing force exerted on the clutch plate  580 . 
     The example biasing mechanism  700  shown in  FIGS.  7 A- 7 F  includes a plurality of double coil spring members  770  and/or  770 ′, simply for ease of discussion and illustration. As noted above, the biasing mechanism  700  can include a combination of different types and/or arrangements and/or numbers of biasing members including the double coil spring members  770  and/or  770 ′. In some examples, a single, larger double coil spring members  770 / 770 ′ having the first coil portion  771 / 771 ′ and the second coil portion  772 / 772 ′ as described above may replace the single biasing device  144  shown in  FIG.  3   . The example biasing mechanism  700  has been described with respect to the use of the dual coil spring members  770 / 770 ′, simply for ease of discussion and illustration. The principles described with respect to the biasing mechanism  700  may be applied to spring members having multiple spring rates other than the dual spring rate as described (for example, triple spring rates, quadruple spring rates, and the like), non-linear spring rates, and the like. 
       FIGS.  8 A- 8 C  are side views of an example arrangement of biasing members of an example biasing mechanism  800  for a clutch of a power-driven tool, such as the clutch  500  described above with respect to  FIGS.  5 A- 5 D .  FIG.  8 D  is an exploded perspective view of the example arrangement of biasing members of the example biasing mechanism  800  shown in  FIGS.  8 A- 8 C .  FIG.  8 E  is a side view of one of the example biasing members  870  of the example biasing mechanism  800  shown in  FIGS.  8 A- 8 D . The example biasing mechanism  800 , in accordance with implementations described herein, provides a variable, or non-linear, or dual spring rate, allowing the device to achieve a desired output torque along a relatively wide range of output torque levels. 
       FIG.  8 E  illustrates an example spring member  870 , and in particular an example dual rate spring member  870 , which can be incorporated into the example biasing mechanism  800  shown in  FIGS.  8 A- 8 D . In some implementations, the example spring member  870  can replace some or all of the coil spring members  671 ,  672  of the example biasing mechanism  600  shown in  FIGS.  6 A- 6 C  and/or some or all of the example double coil spring members  770  shown in  FIGS.  7 A- 7 F . In some implementations, the example spring member  870  shown in  FIG.  8 D  can replace some of the coil spring members  671 ,  672  of the example biasing mechanism  600  shown in  FIGS.  6 A- 6 C  and/or some or all of the example double coil spring members  770 / 770 ′ shown in  FIGS.  7 A- 7 F  such that the biasing mechanism includes a combination of springs, including one or more of the coil spring members  671 ,  672  and/or one or more of the double coil spring members  770 / 770 ′ and/or one or more of the dual rate spring members  870 . The example biasing mechanism  800  includes the spring members  870  each including the first coil portion  871  and the second coil portion  872 , simply for purposes of discussion and illustration. In some implementations, the spring members  870  having additional coil portions incorporated into the spring member. This may allow for additional variation in the spring rate provided by the biasing mechanism  800 . 
     The example spring member  870  shown in  FIG.  8 E  is a dual rate spring member  870  including a first coil portion  871  and a second coil portion  872 . In the example shown in  FIG.  8 E , the coils of the first coil portion  871  are arranged at a first pitch, and the coils of the second coil portion  872  are arranged at a second pitch that is greater than the first pitch, and a spring rate, or a stiffness of the first coil portion  871  is less than a spring rate, or a stiffness of the second coil portion  872 . In the example shown in  FIG.  8 E , the first coil portion  871  and the second coil portion  872  are substantially concentric, with the first coil portion and the second coil portion  872  arranged end to end and aligned along substantially the same central axis as the first coil portion  871 . 
     In  FIG.  8 E , the example spring member  870  is in an at rest state, corresponding to a disengaged state of the clutch shown in  FIG.  8 A . In the at rest state of the example spring member  870  corresponding to the disengaged state of the clutch shown in  FIG.  8 A , the clutch nut  550 /retaining ring  560  and clutch plate  580  are positioned such that neither the first coil portion  771  nor the second coil portion  772  are compressed against the clutch plate  580 . 
       FIG.  8 B  illustrates the dual rate spring members  870  relative to the clutch nut  550 , the retaining ring  560  and the clutch plate  580  at a first clutch setting, for example, a low clutch setting corresponding to a low output torque level. At the first clutch setting, the clutch nut  550 /retaining ring  560  has moved axially closer to the clutch plate  580 . At the first clutch setting, the first coil portion  871  (having the lower stiffness) contacts the clutch plate  580 , thus imparting a first biasing force on the clutch plate  580 . At the first clutch setting shown in  FIG.  8 B , the second coil portions  872  of each of the dual rate spring members  870  are not compressed and do not exert any biasing force on the clutch plate  580 . Thus, at the first clutch setting shown in  FIG.  8 B , only the first coil portion  871  of each of the dual rate spring members  870  imparts any force on the clutch plate  580 . 
     As the clutch nut  550 /retaining ring  560  moves axially closer to the clutch plate  580 , the first coil portion  871  (having the lower stiffness) of each of the dual rate spring members  870  continues to be compressed and the pitch between adjacent coils of the first coil portion  871  continues to decrease. At the point at which the first coil portion  871  is substantially fully compressed, the second coil portion  872  (having the greater stiffness) will be compressed in response to continued axial movement of the clutch nut  550 /retaining ring  560  toward the clutch plate  580 . 
       FIG.  8 C  illustrates the dual rate spring members  870  relative to the clutch nut  550 , the retaining ring  560  and the clutch plate  580  at a second clutch setting, for example a clutch setting that is higher than the first clutch setting, corresponding to a higher output torque level, or trip torque, than shown in  FIG.  8 B . At the second clutch setting, the clutch nut  550 /retaining ring  560  has moved axially closer to the clutch plate  580 , so that the first coil portion  871  (having the lower stiffness) and the second coil portion  872  (having the greater stiffness) together impart a second biasing force on the clutch plate  580  that is greater than the first biasing force described with respect to  FIG.  8 B . At the second clutch setting, both the first coil portion  871  and the second coil portion  872  are compressed and contribute to the biasing force exerted on the clutch plate  580 . 
     The example biasing mechanism  800  shown in  FIGS.  8 A- 8 E  includes a plurality of dual rate spring members  870 , simply for ease of discussion and illustration. As noted above, the biasing mechanism  800  can include a combination of different types and/or arrangements and/or numbers of biasing members including the dual rate spring members  870 . In some examples, a single, larger dual rate spring member  870  having the first coil portion  871  and the second coil portion  872  as described above may replace the single biasing device  144  shown in  FIG.  3   . The example biasing mechanism  800  has been described with respect to the use of the dual spring members  870  including the first coil portion  871  and the second coil portion  872 , simply for ease of discussion and illustration. The principles described with respect to the biasing mechanism  800  may be applied to spring members having multiple coil portions other than the two coil portions as described (for example, three or more coil portions). 
     In a power-driven tool including a clutch that provides torque limiting functionality, in accordance with implementations described herein, a level or amount of torque output by the power-driven tool may be accurately and stably controlled by a biasing mechanism of the clutch that provides for variable levels of biasing, based on a selected output torque level. 
     In the following, some examples are described. 
     Example 1: A power-driven tool, including a motor; an output shaft; a transmission configured to transmit a torque generated by the motor to the output shaft; and a clutch configured to selectively disengage torque transfer from the transmission to the output shaft when an output torque exceeds a threshold torque value, the clutch including a clutch selector actuatable to select the threshold torque value; a retaining ring moveably coupled to the selector to move relative to the transmission in response to selection of the threshold torque value by actuation of the selector; a clutch engagement member selectively engageable with a component of the transmission to interrupt torque transfer from the transmission to the output shaft; and a biasing mechanism coupled between the retaining ring and the clutch engagement member, the biasing mechanism including at least one spring member having a variable spring rate, wherein a biasing force applied to the clutch engagement member by the biasing mechanism corresponds to the selected threshold torque value and can be varied in a non-linear manner in accordance with movement of the retaining ring that adjusts the biasing force in accordance with the variable spring rate. 
     Example 2: The power-driven tool of example 1, wherein the clutch engagement member comprises a ball or a pin that engages a ramped surface on the transmission. 
     Example 3: The power-driven tool of example 2, wherein the clutch engagement member further comprises a clutch plate disposed between the ball or pin and the biasing member. 
     Example 4: The power-driven tool of example 1, wherein the selector comprises a clutch collar that is rotatable relative to the housing. 
     Example 5: The power-driven tool of example 4, further comprising a clutch nut disposed between the clutch collar and the retaining ring. 
     Example 6: The power-driven tool of example 5, wherein the clutch further includes a clutch housing with a threaded front end portion, the clutch nut threadably engaged with the threaded front end portion to be axially movable relative to the transmission. 
     Example 7: The power-driven tool of example 1, wherein the at least one spring member includes a dual coil spring, including a first coil portion having a first length and a first diameter; and a second coil portion having a second length that is different than the first length, and a second diameter that is different than the first diameter. 
     Example 8: The power-driven tool of example 7, wherein the first coil portion is positioned within the second coil portion and is concentrically arranged with the second coil portion; the first length of the first coil portion is greater than the second length of the second coil portion; and the first diameter of the first coil portion is less than the second diameter of the second coil portion. 
     Example 9: The power-driven tool of example 8, wherein the retaining ring is moveable axially relative to the clutch engagement member such that at a first axial position of the retaining ring relative to the clutch engagement member, the first coil portion of the dual coil spring contacts the clutch engagement member and is compressed to exert a first biasing force on the clutch engagement member and the second coil portion of the dual coil spring is not compressed; and at a second axial position of the retaining ring relative to the clutch plate, both the first coil portion and the second coil portion of the dual coil spring contact the clutch engagement member and are compressed to exert a second biasing force on the clutch engagement member that is greater than the first biasing force. 
     Example 10: The power-driven tool of example 9, wherein the first axial position corresponds to a first clutch setting corresponding to a first threshold value torque setting for the power-driven tool, and the second axial position corresponds to a second clutch setting corresponding to a second threshold value torque setting that is greater than the first output torque setting. 
     Example 11: The power-driven tool of example 7, wherein the dual coil spring includes a first coil spring defining the first coil portion, and a second coil spring defining the second coil portion. 
     Example 12: The power-driven tool of example 7, wherein the first coil portion follows a first helical pattern, and the second coil portion follows a second helical pattern that is opposite the first helical pattern of the first coil portion. 
     Example 13: The power-driven tool of example 1, wherein the at least one spring member includes a dual rate spring, including a first coil portion having a first spring rate; and a second coil portion coupled to the first coil portion and having a second spring rate. 
     Example 14: The power-driven tool of example 13, wherein at a first axial position of the retaining ring relative to the clutch plate, the first coil portion of the dual rate spring contacts the clutch engagement member and is compressed, and the second coil portion is not compressed, such that the dual coil spring exerts a first biasing force corresponding to the first spring rate on clutch engagement member; and at a second axial position of the retaining ring relative to the clutch engagement member, both the first coil portion and the second coil portion of the dual rate spring are compressed, and the dual rate spring exerts a second biasing force corresponding to the second spring rate on the clutch engagement member, the second biasing force being greater than the first biasing force. 
     Example 15: The power-driven tool of example 14, wherein the first axial position corresponds to a first clutch setting corresponding to a first output torque setting for the power-driven tool, and the second axial position corresponds to a second clutch setting corresponding to a second output torque setting that is greater than the first output torque setting. 
     Example 16: The power-driven tool of example 13, wherein a first end of the first coil portion is configured to selectively contact the clutch engagement member based on an axial position of the retaining ring relative to the clutch plate; a second end of the first coil portion is coupled to a first end of the second coil portion such that the second coil portion extends from the first end of the first coil portion of the dual rate spring; and a second end of the second coil portion is retained by a corresponding pin and recess defined in the retaining ring. 
     Example 17: The power-driven tool of example 1, wherein the at least one spring member comprises a plurality of spring members, each of the plurality of spring members having a first end portion thereof configured to selectively contact the clutch engagement member based on an axial position of the retaining ring relative to the clutch plate, and a second end thereof retained by a corresponding pin and recess defined in the retaining ring. 
     Example 18: The power-driven tool of example 1, wherein the at least one spring member comprises a single spring member, the single spring member having a first end portion thereof configured to selectively contact the clutch engagement member based on an axial position of the retaining ring relative to the clutch plate, and a second end thereof retained by a corresponding pin and recess defined in the retaining ring. 
     Example 19: The power-driven tool of example 4, wherein the retaining ring is configured to move in a first axial direction in response to rotation of the clutch collar in a first rotational direction; and the retaining ring is configured to move in a second axial direction in response to rotation of the clutch collar in a second rotational direction. 
     Example 20: The power-driven tool of example 19, wherein the at least one spring member is configured to be compressed in response to movement of the retaining ring in the first axial direction to exert a biasing force on clutch engagement member; and compression of the at least one spring member is configured to be released in response to movement of the retaining ring in the second axial direction to release the biasing force exerted on the clutch engagement member. 
     Example 21: The power-driven tool of example 1, wherein the at least one spring member includes a plurality of springs, including at least one first spring having a first length; and at least one second spring having a second length, the second length being different from the first length. 
     Example 22: The power-driven tool of example 21, wherein the at least one first spring is configured to exert a biasing force on the clutch engagement member at a first axial position of the retaining ring relative to the clutch engagement member; and the at least one second spring is configured to exert a biasing force on the clutch engagement member at a second axial position of the retaining ring relative to the clutch engagement member. 
     Example 23: The power-driven tool of example 21, wherein the at least one first spring is configured to exert a biasing force on the clutch engagement member at a first axial position of the retaining ring relative to the clutch engagement member; and the at least one first spring and the at least one second spring are configured to exert a biasing force on the clutch engagement member at a second axial position of the retaining ring relative to the clutch engagement member. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Terms of degree such as “generally,” “substantially,” “approximately,” and “about” may be used herein when describing the relative positions, sizes, dimensions, or values of various elements, components, regions, layers and/or sections. These terms mean that such relative positions, sizes, dimensions, or values are within the defined range or comparison (e.g., equal or close to equal) with sufficient precision as would be understood by one of ordinary skill in the art in the context of the various elements, components, regions, layers and/or sections being described. 
     While certain features of the described implementations have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the scope of the implementations. It should be understood that they have been presented by way of example only, not limitation, and various changes in form and details may be made. Any portion of the apparatus and/or methods described herein may be combined in any combination, except mutually exclusive combinations. The implementations described herein can include various combinations and/or sub-combinations of the functions, components and/or features of the different implementations described.