Patent Publication Number: US-2021170564-A1

Title: Power tool with multispeed transmission

Description:
RELATED APPLICATIONS 
     This application claims priority, under 35 U.S.C. § 119(e), to U.S. Provisional Application No. 63/198,123, filed Sep. 30, 2020 and U.S. Provisional Application No. 62/941,043, filed Nov. 27, 2019, each of which is incorporated by reference. 
    
    
     FIELD 
     This application relates, generally, to a multi-speed 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. Multi-speed power tools may operate at multiple different speeds to, for example, accommodate different types of tasks, and/or allow different tasks to be performed at an appropriate operational speed for the particular task. Such multi-speed power tools may employ a transmission mechanism and a clutching mechanism, allowing for power from the motor to be output by an output device of the tool at different speeds to accommodate a variety of different tasks with the same tool. The arrangement of these components within a housing of the tool may render the tool relatively large, and relatively difficult to operate in confined spaces. More compact internal components may enhance utility and functionality of this type of multi-speed tool, thus enhancing utility to the operator. 
     SUMMARY 
     In one general aspect, a multi-speed power tool may include a housing, a motor received in the housing, and a multi-speed planetary transmission rotationally driven by the motor and defining a transmission axis, wherein the multi-speed transmission includes a plurality of subsections, each of the plurality of subsections being configured to produce a corresponding speed reduction, the plurality of subsections including a first subsection having a first set of planet gears and a first ring gear meshed with the first set of planet gears, and a second subsection having a second set of planet gears and a second ring gear meshed with the second set of planet gears. The transmission may include a carrier assembly, including a first carrier segment, a second carrier segment, a third carrier segment, a first set of supports or pins connecting the first carrier segment to one of the second carrier segment and the third carrier segment and carrying the first set of planet gears, and a second set of supports or pins connecting the third carrier segment to one of the first carrier segment and the third carrier segment and carrying the second set of planet gears, such that the first, second, and third carrier segments rotate in unison about the transmission axis, wherein the first and second sets of supports or pins are arranged such that each of the first set of planet gears is radially offset from each of the second set of planet gears. The tool may also include an output shaft rotationally driven by the transmission, and a speed selector moveable among a plurality of positions where the speed selector engages one or more of the plurality of subsections so as to cause the transmission to rotate the output shaft at a plurality of different output speeds. 
     In some implementations, each of the first set of planet gears may include a front face facing the second set of planet gears and aligned in a first plane, and each of the second set of planet gears may include a rear face facing the first set of planet gears and aligned in a second plane, wherein the first plane and the second plane are substantially coplanar. 
     In some implementations, the transmission may also include a first sun gear meshed with the first set of planet gears and a second sun gear meshed with the second set of planet gears. The first and second sun gears may be axially aligned, with at least the first sun gear being rotatably driven by the motor. 
     In some implementations, the speed selector may be configured to ground the first ring gear to the housing in a first position to cause the transmission to rotate the output shaft a first speed, and to ground the second ring gear to the housing in a second position to cause the transmission to rotate the output shaft at a second speed that is different than the first speed. 
     In some implementations, the transmission may include a third subsection having a third set of planet gears and a third ring gear meshed with the third set of planet gears. The third set of planet gears may be carried by a third set of supports or pins connected to one of the first carrier segment, the second carrier segment or the third carrier segment. The speed selector may be configured to ground the first ring gear to the housing in a first position to cause the transmission to rotate the output shaft a first speed, to ground the second ring gear to the housing in a second position to cause the transmission to rotate the output shaft at a second speed that is different from the first speed, and to ground the third ring gear to the housing in a third position to cause the transmission to rotate the output shaft at a third speed that is different from the first speed and the second speed. 
     In some implementations, the third set of supports or pins may be arranged such that the third set of planet gears is radially offset from each of the first set of planet gears and second set of planet gears. In some implementations, the carrier assembly further may include a fourth carrier segment, wherein the third set of supports or pins connect the fourth carrier segment to one of the second carrier segment or the third carrier segment. 
     In some implementations, the transmission may include a plurality of sun gears that includes a first sun gear, a second sun gear and a third sun gear. The first, second and third sun gears may be axially aligned. The first set of planet gears may include a first stage thereof engaged with the first sun gear and a second stage thereof engaged with the second sun gear. The second set of planet gears may include a first stage thereof engaged with the second sun gear and a second stage thereof engaged with the third sun gear. The third set of planet gears may be engaged with the third sun gear. 
     In some implementations, the speed selector may include a shift member having an at least partial ring shape configured to move axially with respect to the transmission, and a plurality of engagement members coupled to the shift member and configured to selectively engage one of the ring gears based on an axial position of the shift member with respect to the transmission. The plurality of engagement members may include a plurality of rods arranged on the shift member and extending in parallel to the transmission axis. At a first axial position of the shift member, a first portion of the plurality of rods may be configured to engage a respective plurality of lugs on an outer circumferential portion of the first ring gear, so as to ground the first ring gear, while the second ring gear remains freely rotatable. At a second axial position of the shift member, a plurality of engagement lugs formed on an inner circumferential portion of the shift member may be configured to engage a respective lug of a plurality of lugs on an outer circumferential portion of the second ring gear, while the first ring gear remains freely rotatable. 
     In some implementations, the plurality of engagement members may include a plurality of rods arranged circumferentially along the shift member and extending parallel to the transmission axis, the rods including a first protrusion formed at an end portion of the rod and a second protrusion formed at an intermediate portion of the rod between the first protrusion and the shift member. At a first axial position of the shift member, the first protrusion may be configured to engage a lug on an outer circumferential portion of the first ring gear, so as to ground the first ring gear, while the second ring gear remains freely rotatable. At a second axial position of the shift member, the second protrusion may be configured to engage a lug on an outer circumferential portion of the second ring gear, while the first ring gear remains freely rotatable. 
     In some implementations, the speed selector further may include a switch that is accessible from an outside of the housing, and that is movable within an opening formed in the housing, and at least one shift rail having a first end portion fixed to the switch and a second end portion fixed to the shift member. The switch may include a base portion having a first side portion coupled to the at least one shift rail, a shutter slidably coupled on a second side portion of the base portion, opposite the first side portion thereof, and a grasping member (e.g., a button or lever) extending from the base portion through an opening formed in the shutter. The grasping member (e.g., a button or lever) may be movable to a plurality of different positions between a first end of the opening in the housing and a second end of the opening in the housing, the plurality of different positions corresponding to a plurality of different axial positions of the shift member with respect to the transmission. In some implementations, a distance from a first end of the base portion to a second end of the base portion may be less than a distance from the first end of the opening in the housing to the second end of the opening in the housing, and the shutter may be configured to slide with respect to the base portion as the selection device moves in the opening, such that the shutter and the base portion close the opening in the housing at the plurality of positions of the grasping member (e.g., a button or lever) between the first end and the second end of the opening. 
     In another general aspect, a multi-speed power tool may include a housing, a motor received in the housing, a multi-speed planetary transmission rotationally driven by the motor and defining a transmission axis. The multi-speed transmission may include a plurality of subsections, each of the plurality of subsections being configured to produce a corresponding speed reduction. The plurality of subsections may include a first subsection having a first set of planet gears and a first ring gear meshed with the first set of planet gears, and a second subsection having a second set of planet gears and a second ring gear meshed with the second set of planet gears. The transmission may include a carrier assembly, including a first carrier segment, a second carrier segment, a third carrier segment, a first set of supports or pins connecting the first carrier segment to one of the second carrier segment or the third carrier segment, and a second set of supports or pins connecting the third carrier segment to one of the first carrier segment or the third carrier segment. The first set of supports or pins carrying the first set of planet gears and the second set of supports or pins may carry the second set of planet gears such that the first, second, and third carrier segments rotate in unison about the transmission axis. Each of the first set of planet gears may include a front face facing the second set of planet gears and aligned in a first plane, and each of the second set of planet gears may include a rear face facing the first set of planet gears and aligned in a second plane, wherein the first plane and the second plane are substantially coplanar. The tool may also include an output shaft rotationally driven by the transmission, and a speed selector moveable among a plurality of positions where the speed selector engages one or more of the plurality of subsections so as to cause the transmission to rotate the output shaft at a plurality of different output speeds. 
     In some implementations, the transmission may include a third subsection having a third set of planet gears and a third ring gear meshed with the third set of planet gears. The third set of planet gears may be carried by a third set of supports or pins connecting one of the first carrier segment, the second carrier segment or the third carrier segment to a fourth carrier segment. The first set of supports or pins, the second set of supports or pins and the third set of supports or pins may be arranged such that the first set of planet gears, the second set of planet gears, and the third set of planet gears are radially offset from each other. In some implementations, the speed selector may be configured to ground the first ring gear to the housing in a first position to cause the transmission to rotate the output shaft a first speed, to ground the second ring gear to the housing in a second position to cause the transmission to rotate the output shaft at a second speed that is different from the first speed, and to ground the third ring gear to the housing in a third position to cause the transmission to rotate the output shaft at a third speed that is different from the first speed and the second speed. 
     In some implementations, the speed selector may include a shift member having an at least partial ring shape configured to move axially with respect to the transmission, and a plurality of engagement members arranged circumferentially along the shift member, and extending in parallel to the transmission axis. At a first axial position of the shift member, a first engagement portion defined on the plurality of rods may be configured to engage a respective plurality of lugs on an outer circumferential portion of the first ring gear, so as to ground the first ring gear, while the second ring gear and the third ring gear remain freely rotatable. At a second axial position of the shift member, a second engagement portion defined by engagement lugs formed on an inner circumferential portion of the shift member may be configured to engage a respective lug of a plurality of lugs on an outer circumferential portion of the second ring gear, while the first ring gear and the third ring gear remain freely rotatable. At a third axial position of the shift member, a third engagement defined on the plurality of rods may be configured to engage a respective plurality of lugs on an outer circumferential portion of the third ring gear, so as to ground the third ring gear, while the first ring gear and the second ring gear remain freely rotatable. 
     In some implementations, the speed selector may include a switch that is accessible from an outside of the housing, and that is movable within an opening formed in the housing, and at least one shift rail having a first end portion fixed to the switch and a second end portion fixed to the shift member. The switch may include a base portion having a first side portion coupled to the at least one shift rail, a shutter slidably coupled on a second side portion of the base portion, opposite the first side portion thereof, and a grasping member (e.g., a button or lever) extending from the base portion through an opening formed in the shutter. The grasping member (e.g., a button or lever) may be movable to a plurality of different positions between a first end of the opening in the housing and a second end of the opening in the housing, the plurality of different positions corresponding to a plurality of different axial positions of the shift member with respect to the transmission. 
     In another general aspect, a multi-speed power tool may include a housing, a motor received in the housing, a multi-speed planetary transmission rotationally driven by the motor and defining a transmission axis, wherein the multi-speed transmission may include a plurality of subsections, each of the plurality of subsections being configured to produce a corresponding speed reduction, an output shaft rotationally driven by the transmission, and a speed selector moveable among a plurality of positions in which the speed selector engages one or more of the plurality of subsections of the transmission so as to cause the transmission to rotate the output shaft at a plurality of different output speeds. The speed selector may include a switch that is accessible from an outside of the housing, and that is movable within an opening formed in the housing, and at least one shift rail having a first end portion fixed to the switch and a second end portion fixed to a shift member having an at least partial ring shape that selectively engages one of the plurality of subsections of the transmission. The switch may include a base portion having a first side portion coupled to the at least one shift rail, a shutter slidably coupled on a second side portion of the base portion, opposite the first side portion thereof, and a grasping member (e.g., a button or lever) extending from the base portion through an opening formed in the shutter. The grasping member (e.g., a button or lever) may be movable to a plurality of different positions between a first end of the opening in the housing and a second end of the opening in the housing, the plurality of different positions corresponding to a plurality of different axial positions of the shift member with respect to the transmission. 
     In some implementations, the tool may include a hammer mechanism coupled to the output shaft. The hammer mechanism may include a first ratchet fixed to the output shaft so as to rotate with the output shaft, a second ratchet axially aligned with the first ratchet, and a cam mechanism surrounding the second ratchet. The second ratchet may be non-rotatable, and may be axially movable with respect to the first ratchet. 
     In some implementations, the cam mechanism may include a cam ring, a plurality of lugs extending radially inward from the cam ring, and a plurality of ramp surfaces defined on the plurality of lugs, and the second ratchet may include a hub, a plurality of lugs extending outward from the hub, and a plurality of teeth formed on a surface of the hub facing the first ratchet. In a first mode of operation, the cam ring may be in a first axial position and a first radial position, each lug of the plurality of lugs of the second ratchet may be positioned between a pair of lugs of the cam mechanism, and the first ratchet and the second ratchet may be axially separated and in a disengaged state. In a second mode of operation, the cam ring may be in a second axial position and a second radial position, the ramp surface of each lug of the plurality of lugs of the cam mechanism may be positioned against a corresponding lug of the plurality of lugs of the second ratchet, and the hub of the second ratchet may be positioned against a mating surface of the first ratchet, such that the teeth defined on the hub of the second ratchet are engaged with teeth on the mating surface of the first ratchet, and an axial percussive force is imparted on the output shaft. 
     In another general aspect, a multi-speed power tool includes a housing, a motor received in the housing, and a multi-speed planetary transmission rotationally driven by the motor and defining a transmission axis. The multi-speed transmission includes a plurality of sub-sections each of which is configured to provide an intermediate speed reduction when that subsection is active. An output shaft is rotationally driven by the transmission at an output rotational speed defined by intermediate speed reductions of any of the active subsections. A speed selector includes a shift member inside the housing that is moveable among a first plurality of positions where the shift member engages a one or more of the subsections of the transmission to activate one or more of the subjections so as to cause the transmission to rotate the output shaft at a plurality of different output rotational speeds. A switch is accessible from outside the housing, coupled to the shift member, and moveable among a second plurality of positions corresponding to selected output speeds of the transmission to cause movement of the shift member among the first plurality of positions. A plurality of springs is disposed in parallel between the shift member and the switch without any of the plurality of springs arranged serially, the plurality of springs exerting a bi-directional biasing force on the shift member in both an axial forward and an axial rearward direction relative to the switch. If the switch is moved from a first switch position to a second switch position of the second plurality of positions and the shift member is blocked from moving from the a first speed position to a second speed position of the first plurality of positions, the shift member remains stationary and when the shift member is later unblocked from moving from the first speed position to the second speed position, a force provided by the plurality of springs urge the shift member to move to the second speed position while the switch remains stationary. 
     In some implementations, each subsection may include a plurality of planet gears and a ring gear engaged with the planet gears. The shift member may include an at least partially ring shaped shift ring configured to engage a ring gear in at least one of the subsections of the transmission, wherein the subsection is active when the shift ring engages the ring gear. The shifter may be fixedly coupled to a first end of an axial pin that and is moveable with the axial pin parallel to the transmission axis. The speed selector may include a carriage fixedly coupled to a second end of the axial pin and moveable with the axial pin parallel to the transmission axis. The switch may include a base portion moveably coupled to the carriage with the plurality of springs disposed between the base portion and the carriage. The switch may include a shutter slidably coupled on a second side portion of the base portion, opposite the first side portion thereof, and a grasping portion extending from the base portion through an opening formed in the shutter, wherein the grasping is movable to a plurality of different positions between a first end of the opening in the housing and a second end of the opening in the housing, the plurality of different positions corresponding to a plurality of different axial positions of the shift member with respect to the transmission. A distance from a first end of the base portion to a second end of the base portion may be less than a distance from the first end of the opening in the housing to the second end of the opening in the housing, and the shutter may be configured to slide with respect to the base portion as the selection device moves in the opening, such that the shutter and the base portion close the opening in the housing at the plurality of positions of the button between the first end and the second end of the openings. 
     In some implementations, the plurality of springs comprises two parallel compression springs. The compression springs may be disposed on a pair of axial pins fixedly coupled to the shifting member. The plurality of sub-sections may include a first subsection having a first set of planet gears and a first ring gear meshed with the first set of planet gears, and a second subsection having a second set of planet gears and a second ring gear meshed with the second set of planet gears. The shifting member may be configured to selectively ground the first ring gear relative to the housing in a first shift position of the shifting member for a first output speed of the output shaft and the second ring gear relative to the housing in a second shift position of the shifting member for a second output speed of the output shaft. The plurality of sub-sections may further include a third subsection having a third set of planet gears and a third ring gear meshed with the third set of planet gears. The shifting member may be configured to selectively ground the third ring gear relative to the housing in a third shift position of the shifting member for a third output speed of the output shaft. 
     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 multi-speed power-driven tool. 
         FIG. 2  is a side view of an example multi-speed power-driven tool, in accordance with implementations described herein. 
         FIG. 3  is an internal view of the example tool shown in  FIG. 2 , with a portion of a housing removed, so that internal components are visible. 
         FIG. 4  is a side view of an example transmission mechanism of an example multi-speed power-driven tool, in accordance with implementations described herein. 
         FIG. 5  is a side view of a plurality of sun gears of the example transmission mechanism shown in  FIG. 4 , in accordance with implementations described herein. 
         FIG. 6  is a perspective view of a plurality of example planet gears of a plurality of example planet gear assemblies of the example transmission mechanism shown in  FIG. 4 , in accordance with implementations described herein. 
         FIG. 7  is a perspective view of a plurality of example ring gears of the example transmission mechanism shown in  FIG. 4 , in accordance with implementations described herein. 
         FIG. 8A  is a side view of the example transmission mechanism shown in  FIG. 4 , with the plurality of example ring gears shown in  FIG. 7  removed, in accordance with implementations described herein. 
         FIG. 8B  is a perspective view of an example first ring gear and an example first planet gear assembly of the example transmission mechanism shown in  FIG. 4 , in accordance with implementations described herein. 
         FIG. 8C ( 1 ) is a first side perspective view and  FIG. 8C ( 2 ) is a second side perspective view of an example second ring gear, an example first planet gear assembly and an example second planet gear assembly of the example transmission mechanism shown in  FIG. 4 , in accordance with implementations described herein. 
         FIG. 8D  is a perspective view of an example third ring gear, portions of an example first planet gear assembly, portions of an example second planet gear assembly, and portions of an example third planet gear assembly of the example transmission mechanism shown in  FIG. 4 , in accordance with implementations described herein. 
         FIG. 9  is a perspective view of an example planet carrier assembly of the example transmission mechanism shown in  FIG. 4 , in accordance with implementations described herein. 
         FIG. 10  is a perspective view of the example planet carrier assembly shown in  FIG. 9 , with a plurality of separator plates removed, in accordance with implementations described herein. 
         FIG. 11A  is a schematic illustration of the example transmission mechanism in a first output speed mode of operation,  FIG. 11B  is a schematic illustration of the example transmission mechanism in a second, intermediate output speed mode of operation, and  FIG. 11C  is a schematic illustration of the example transmission mechanism in a third output speed mode of operation, in accordance with implementations described herein. 
         FIG. 11D  is a schematic illustration of the example transmission mechanism in the third output speed mode of operation, in accordance with another implementation described herein. 
         FIGS. 12A-12C  are side views of the example transmission system and an example speed selection mechanism of the example power-driven tool, in a first output speed mode of operation, in a second, intermediate output speed mode of operation, and in a third output speed mode of operation, respectively, in accordance with implementations described herein. 
         FIG. 13A  is a perspective view of an example speed selection mechanism of the example multi-speed power-driven tool, in accordance with implementations described herein. 
         FIG. 13B  is a cross-sectional view taken along line A-A of  FIG. 13A . 
         FIG. 14A  is a side view, and  FIG. 14B  is an axial end view, of the example speed selection mechanism engaged with the example transmission mechanism in a first mode of operation, in accordance with implementations described herein. 
         FIG. 14C  is a cross sectional view taken along line B-B of  FIG. 14A . 
         FIG. 15A  is a side view, and  FIG. 15B  is an axial end view, of the example speed selection mechanism engaged with the example transmission mechanism in a second mode of operation, in accordance with implementations described herein. 
         FIG. 15C  is a cross sectional view taken along line C-C of  FIG. 15A . 
         FIG. 16A  is a side view, and  FIG. 16B  is an axial end view, of the example speed selection mechanism engaged with the example transmission mechanism in a third mode of operation, in accordance with implementations described herein. 
         FIG. 16C  is a cross sectional view taken along line D-D of  FIG. 16A . 
         FIG. 17  is a perspective view of an example shift member having an at least partial ring shape of an example speed selection mechanism, in accordance with implementations described herein. 
         FIG. 18  is a side view of the example shift member having an at least partial ring shape engaged with an example transmission mechanism, in accordance with implementations described herein. 
         FIG. 19  is a perspective view of an example switching device of an example speed selection mechanism, in accordance with implementations described herein. 
         FIGS. 20A-20D  are partial cross-sectional views of the example switching device shown in  FIG. 19 , installed in an example housing of a multi-speed power-driven tool, in accordance with implementations described herein. 
         FIG. 21  is a partial perspective view of a working end portion of an example power-driven tool, in accordance with implementations described herein. 
         FIG. 22A  is a perspective view of an implementation of a cam ring in the power tool of  FIG. 21 , and  FIG. 22B  is a perspective view of an implementation of a fixed ratchet configured to be engaged by the cam ring of  FIG. 22A . 
         FIG. 23A  is a cross-sectional view taken along line G-G of  FIG. 21 , and  FIG. 23B  is a cross-sectional view taken along line H-H, with the example power-driven tool in a second mode of operation, in accordance with implementations described herein. 
         FIG. 24A  is a perspective view of an example ratchet mechanism of an impacting mechanism of the example power-driven tool shown in  FIGS. 22A-23B , in accordance with implementations described herein. 
         FIG. 24B  is a perspective view of an example cam mechanism of an impacting mechanism of the example power-driven tool shown in  FIGS. 22A-23B , in accordance with implementations described herein. 
         FIG. 25A  is a side perspective view, and  FIG. 25B  is a bottom perspective view, of an example speed selection mechanism  1400 , in accordance with implementations described herein. 
         FIG. 25C  is a cross-sectional view, taken along line L-L of  FIG. 25B . 
         FIG. 25D  is a close in perspective view of an example compliant mechanism of the speed selection mechanism  1400 , in accordance with implementations described herein. 
         FIGS. 25E and 25F  are cross-sectional views of the example speed selection mechanism  1400  installed in a housing of the example multi-speed power-driven tool. 
         FIGS. 26A-26D  are perspective views sequentially illustrating an assembly of the example speed selection mechanism shown in  FIGS. 25A-26B , in accordance with implementations described herein. 
         FIGS. 27A and 27B  are partial perspective views of the example speed selection mechanism shown in  FIGS. 25A-25C , illustrating operation of the example compliant mechanism shown in  FIG. 25D , in accordance with implementations described herein. 
         FIGS. 27C-27E  are partial cross-sectional views of an interaction between the example speed selection mechanism, the example compliant mechanism, and the example transmission mechanism, in accordance with implementations described herein. 
         FIG. 28A  is a top perspective view of an example speed selection mechanism, in accordance with implementations described herein. 
         FIG. 28B  is a bottom perspective view of a base of a switching device of the example speed selection mechanism shown in  FIG. 28A . 
         FIGS. 29A-29D  are perspective views sequentially illustrating an assembly of the example speed selection mechanism shown in  FIG. 27 , in accordance with implementations described herein. 
         FIGS. 30A and 30B  are perspective views first and second fork bushings and first and second pins of the example speed selection mechanism shown in  FIG. 27 , in accordance with implementations described herein. 
         FIG. 30C  is a cross sectional view taken along line M-M of  FIG. 29D   
         FIGS. 31A-31C  illustrate a positioning of components of the example speed selection mechanism during an example shifting movement, in accordance with implementations described herein. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic view of an example multi-speed power-driven tool  10  is shown in  FIG. 1 . The example tool  10  may include a driving mechanism  11  generating a driving force, for example, a rotational driving force. A transmission mechanism  12  may be coupled to the driving mechanism  11 , to transfer force, for example, rotational force, from the driving mechanism  11  to an output mechanism  13 . A speed selection mechanism  14  may be coupled to the transmission mechanism  12 . The speed selection mechanism  14  may provide for user selection of an operation speed to be output by the tool  10 . In response to such a user selection, the speed selection mechanism may interface with the transmission mechanism  12  to control a rotational speed transmitted from the driving mechanism  11  to the output mechanism  13  accordingly. Some multi-speed power-driven tools may include a hammer or impact mechanism  15 , which may be coupled to the output mechanism  13  to selectively impart a repeated impacting force on a workpiece. The driving mechanism  11 , the transmission mechanism  12 , the output mechanism  13 , the speed selection mechanism  14  and the hammer or impact mechanism  15  (when included) may be received in and/or coupled to a housing  19 . 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 and 3  are side views of an example power-driven tool  100 , in accordance with implementations described herein. In particular,  FIG. 3  provides an internal view of the example tool  100 , with a housing  190  shown in  FIG. 2  partially removed, so that internal components of the example tool  100  are visible. As shown in  FIGS. 2 and 3 , the example power tool  100  may include a housing  190 , with a chuck assembly  180  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  120  for triggering operation of the tool  100  may be provided at a handle portion  196  of the housing  190 . A speed selection mechanism  400 , may provide for user selection of an operational speed of the tool  100  through user manipulation of the speed selection mechanism  400 . 
     The example power tool  100  illustrated in  FIGS. 2 and 3  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 multi-speed power-driven tools. As noted above, in some implementations, power-driven tools such as, for example, the example tool  100  shown in  FIGS. 2 and 3 , may also optionally include a hammer or impact mechanism  500 , which may selectively impart a hammering or impacting force on a fastener/workpiece. The principles to be described herein may applied to other implementations of multi-speed power tools, with or without a hammer/impact mechanism. 
     The example tool  100  shown in  FIG. 3  may include a motor  110  housed in the housing  190  that may output a force, for example, a rotational force, via an output shaft  112 , to a transmission mechanism  200 . The transmission mechanism  200  may, in turn, transmit the rotary force from the motor  110  to an output mechanism  130 , for example, an output spindle  130 . As noted above, in some implementations, the tool  100  may include a hammer mechanism  500 , or impact mechanism  500 , positioned between the transmission mechanism  200  and the chuck assembly  180 , to selectively output a hammering/impacting force via the output spindle  130 . 
       FIG. 4  is a side perspective view of the example transmission mechanism  200  shown in  FIG. 3 . As shown in  FIG. 4 , in some implementations, the transmission mechanism  200  may be a multi-speed compound planetary transmission mechanism  200 . In particular, the example transmission mechanism  200  shown in  FIG. 4  is a 3-speed compound planetary transmission mechanism  200 , having three sets of planet gears, mounted in a common planet carrier assembly  300 . The arrangement of the individual components of the compound planetary transmission mechanism  200  in the common planet carrier assembly  300 , in accordance with implementations described herein, may provide for an axially compact arrangement of the component planet gear assembly. The example arrangement of the components of the compound planetary transmission mechanism  200  in the common planet carrier assembly  300 , may result in a reduced axial length of the transmission mechanism compared to, for example, a radially aligned arrangement of transmission mechanism components. The example arrangement to be described below may reduce bending stresses in the components of the transmission mechanism  200 , while allowing the transmission mechanism to achieve a relatively great speed reduction in a relative compact profile. 
     The example transmission mechanism  200  shown in  FIG. 4  may include example subsections  200 A,  200 B,  200 C,  200 D,  200 E, and  200 F. In some implementations, the example transmission mechanism  200  may include a first sun gear  210 , a second sun gear  220 , and a third sun gear  230 . The example first, second and third sun gears  210 ,  220 ,  230  are not visible in  FIG. 4 , but are separately illustrated in  FIG. 5 . The first sun gear  210  may be driven by rotational force of the output shaft  112 . In some implementations, the first, second and third sun gears  210 ,  220 ,  230  may mesh with first, second, and third stepped compound planet gear assemblies  240 ,  250 ,  260 , such that the second sun gear  220  rotates in response to rotation of the first compound planet gear assembly  240  and causes rotation of the second compound planet gear assembly  250  and the third sun gear  230  rotates in response to rotation of the second compound planet gear assembly  250  and causes rotation of the of third compound planet gear assembly  260 . The example first, second and third stepped compound planet gear assemblies  240 ,  250 ,  260  are partially visible in  FIG. 4 , and are separately illustrated in  FIG. 6 . The three stepped compound planet gear assemblies  240 ,  250 ,  260  may be pinned to and carried by a common planet carrier assembly  300 , so as to rotate together as a unit about an axis of the output shaft  112 . In some implementations, the three planet gear assemblies  240 ,  250 ,  260 , may be respectively engaged by three ring gears  270 ,  280 ,  290 . The example first, second and third ring gears  270 ,  280 ,  290  are separately illustrated in  FIG. 7 . The three ring gears  270 ,  280 ,  290  may respectively mesh with a corresponding portion of each of the three compound planet gear assemblies  240 ,  250 ,  260 . A shift member  410  having an at least partial ring shape  410  (not shown in  FIG. 3 ) of the speed selection mechanism  400  may move axially with respect to the transmission mechanism  200 , to selectively ground one of the three ring gears  270 ,  280 ,  290  to the housing  190  of the tool  100 , to set a selected speed reduction ratio to be output by the transmission mechanism  200 . The shift member or ring  410   410  may be a complete ring (e.g., a circular shape) or a partial ring (e.g., a C-shape). 
       FIG. 8A  is a side view of the example transmission mechanism  200  shown in  FIG. 4 , with the ring gears  270 ,  280 ,  290  removed, so that the arrangement of the planet gear assemblies  240 ,  250 ,  260  with respect to the components of the planet carrier assembly  300  are more easily visible.  FIG. 8B  is a perspective view of an arrangement of the first planet gear assembly  240  and the first ring gear  270 .  FIGS. 8C ( 1 ) and  8 C( 2 ) are opposite side views of an arrangement of the first planet gear assembly  240 , the second planet gear assembly  250  and the second ring gear  280 .  FIG. 8D  is a perspective view of the first, second and third planet gear assemblies  240 ,  250 ,  260  and the third ring gear  290 , with some of the planet gears removed so that the engagement of the planet gear assemblies  240 ,  250 ,  260  is visible.  FIG. 9  is a side view of the example planet carrier assembly  300 , i.e., a side view without the ring gears  270 ,  280 ,  290 , the planet gear assemblies  240 ,  250 ,  260  and the sun gears  210 ,  220 ,  230  of the example transmission mechanism  200  installed therein, so that the arrangement of the components of the planet carrier assembly  300  are more easily visible.  FIG. 10  is a side view of the example planet carrier assembly  300 , with intermediary plates, or separating plates, or wear plates, removed, so that the arrangement of carrier segments of the planet carrier assembly  300 , and of supports or pins connecting the carrier segments of the planet carrier assembly  300 , are more easily visible. 
     As noted above, the first sun gear  210  may be driven in response to a rotational force transmitted from the motor  110 , via the output shaft  112 , to the transmission mechanism  200 . In this example implementation, the first sun gear  210  is engaged with, for example, in meshed engagement with, the first planet gear assembly  240 . For example, each stepped planet gear of the first planet gear assembly  240  may include a first stage gear  242  and a second stage gear  244 , as shown in  FIG. 6 . A dimension such as, for example, a diameter, of the second stage gear  244  may be different from, a corresponding dimension, or diameter, of the first stage gear  242  of the stepped planet gear(s) of the first planet gear assembly  240 . In this example, the first sun gear  210  is a single stage (i.e., not stepped) gear, in meshed engagement with the first stage gear  242  of the first planet gear assembly  240 . 
     In this example implementation, the second sun gear  220  may be a compound sun gear including a first stage gear  222  and a second stage gear  224 . The second sun gear  220  may span subsections  200 B and  200 C of the transmission mechanism  200 . That is, the first stage gear  222  of the second sun gear  220  may be in meshed engagement with the second stage gear  244  of the first stepped planet gear assembly  240 , and the second stage gear  224  of the second sun gear  220  may be in meshed engagement with a first stage gear  252  of the second planet gear assembly  250 . In this example implementation, the third sun gear  230  may be a single stage (i.e., not stepped) gear that may span subsections  200 D and  200 E of the transmission mechanism  200 . That is, the third sun gear  230  may include a first portion  232  and a second portion  234 . The first portion  232  of the third sun gear  230  may be in meshed engagement with a second stage gear  254  of the second stepped planet gear assembly  250 , and the second portion  234  of the third sun gear  230  may be in meshed engagement with a first stage gear  262  of the third planet gear assembly  260 . 
     In this example implementation, each compound stepped planet gear of the first, second and third compound planet gear assemblies  240 ,  250 ,  260  may include a concentrically formed larger diameter portion and smaller diameter portion, each larger diameter portion having a greater number of teeth than the respective smaller diameter portion. In some implementations, the larger diameter portion and the smaller diameter portion may be integrally formed. Each compound stepped planet gear of the first, second and third compound planet gear assemblies  240 ,  250 ,  260  may rotate about its own respective axis at a substantially constant speed, so that each compound stepped planet gear orbits its respective sun gear at substantially the same rate. 
     As described above, the first ring gear  270  may be engaged with the first stepped planet gear assembly  240 . Similarly, the second ring gear  280  may be engaged with the second stepped planet gear assembly  250 , and the third ring gear  280  may be engaged with the third stepped planet gear assembly  260 . For example, in some implementations, the first ring gear  270  may be in meshed engagement with the second stage gear  244  of the first planet gear assembly  240 ; the second ring gear  280  may be in meshed engagement with the second stage gear  254  of the second planet gear assembly  250 ; and the third ring gear  290  may be in meshed engagement with the second stage gear  264  of the third planet gear assembly  260 . In some implementations, for example, an implementation in which an orientation of the third planet gear assembly  260  is reversed from the orientation shown in  FIGS. 4, 6 and 8 , the third ring gear  290  may be in meshed engagement with the first stage gear  262  of the third planet gear assembly  260 . 
     One of the first ring gear  270 , the second ring gear  280  or the third ring gear  290  may be selectively grounded to the housing  190  in response to manipulation of the speed selection mechanism  400  and corresponding axial movement of the shift ring  410  to set a gear reduction ratio of the transmission mechanism  200 , and an output speed of the tool  100 . When one of the ring gears  270 ,  280 ,  290  is grounded to the housing  190 , rotation of the grounded ring gear is restricted, i.e., the grounded ring gear cannot rotate. The remaining ring gears (i.e., the ring gears that are not grounded to the housing  190 ) can freely rotate. For example, in this example implementation, grounding of the first ring gear  270  may produce a first (for example, low) speed reduction ratio through the transmission mechanism  200 , and a corresponding first (for example, high) output speed for the tool  100 . Grounding of the second ring gear  280  may produce a second, intermediate speed reduction ratio through the transmission mechanism  200 , and a corresponding second, intermediate output speed for the tool  100 . Grounding of the third ring gear  290  may produce a third (for example, high) speed reduction ratio through the transmission mechanism  200 , and a corresponding third (for example, low) output speed for the tool  100 . 
     As shown in  FIGS. 8A, 9 and 10 , in some implementations, the planet carrier assembly  300  may include a first carrier segment  310  (or set of carrier segments), a second carrier segment  320  (or set of carrier segments), a third carrier segment  330  (or set of carrier segments), and a fourth carrier segment  340  (or set of carrier segments). A set of first supports or pins  315  may connect the first carrier segment  310  and the second carrier segment  320 . The gear assemblies of the first stepped planet gear assembly  240  may be carried on the first set of supports or pins  315 . A set of second supports or pins  325  may connect the first carrier segment  310  and the third carrier segment  330 . The gear assemblies of the second stepped planet gear assembly  250  may be carried on the second set of supports or pins  325 . A set of third supports or pins  335  may connect the second carrier segment  320  and the fourth carrier segment  340 . The gear assemblies of the third stepped planet gear assembly  260  may be carried on the third set of supports or pins  335 . The connection of the first, second, third and fourth carrier segments  310 ,  320 ,  330  and  340  by the first, second and third sets of supports or pins  315 ,  325 ,  335  in this manner may allow the carrier segments  310 ,  320 ,  330  and  340  to rotate together, as a single unit. The first supports or pins  315 , the second supports or pins  325  and the third supports or pins  335  may be radially offset from each other. The radial offset of the first, second and third supports or pins  315 ,  325 ,  335  may, in turn, provide for a radially offset positioning of the first, second and third planet gear assemblies  240 ,  250 ,  260  carried thereon. 
     This radially offset positioning of the first, second and third planet gear assemblies  240 ,  250 ,  260  may allow the first, second and third planet gear assemblies  240 ,  250 ,  260  to be axially closer to one another, compared to, for example, an otherwise radially aligned arrangement of the planet gear assemblies along an axial direction of the transmission mechanism. For example, this radially offset positioning of the first, second and third planet gear assemblies  240 ,  250 ,  260  may allow a rear plane of one planet gear assembly to be essentially in line with a front plane of the next adjacent planet gear assembly. This relatively compact arrangement of the first, second and third planet gear assemblies  240 ,  250 ,  260  may provide for a more axially compact transmission mechanism  200 , and a more compact overall tool profile. 
     The radially offset positioning of the first, second and third planet gear assemblies  240 ,  250 ,  260  may allow for the use of multiple, shorter supports or pins  315 ,  325 ,  335 , compared to, for example, longer supports or pins on which radially aligned gear assemblies might otherwise be axially arranged. The shorter supports or pins  315 ,  325 ,  335  may experience reduced bending stress (compared to, for example, longer supports or pins), thus enhancing durability and reliability. 
     In some implementations, the planet carrier assembly  300  may include a plurality of plates axially positioned between the first carrier segment  310  and the fourth carrier segment  340  of the carrier assembly  300 . For example, a first plate  350  may separate subsection  200 B from subsection  200 C; a second plate  360  may separate subsection  200 C from subsection  200 D; a third plate  370  may separate subsection  200 D from subsection  200 E; and a fourth plate  380  may separate subsection  200 E from subsection  200 F. In some implementations, the plates  350 ,  360 ,  370 ,  380  may be carried on the respective pins  315 ,  325 ,  335  and/or on the respective carrier segments  310 ,  320 ,  330 ,  340 . Each of the plates  350 ,  360 ,  370 ,  380  may be positioned, for example, axially positioned, between axially adjacent planet gear assemblies  240 ,  250 ,  260 , to prevent interference between the adjacent planet gear assemblies during operation. In some implementations, the plates  350 ,  360 ,  370 ,  380  may prevent contact between adjacent planet gear assemblies which may impede rotation of the planet gear assemblies and/or cause undue wear of the planet gear assemblies. 
     In some implementations, one or more of the plates  350 ,  360 ,  370 ,  380  may include cut outs, or cored out areas, to accommodate respective planet gears of the planet gear assemblies  240 ,  250 ,  260  therethrough. For example, in the example implementation illustrated in  FIG. 9 , the second plate  360  includes a cored out area  365  to accommodate the second planet gear assembly  250 . Similarly, in the example implementation illustrated in  FIG. 9 , the fourth plate  380  includes a cored out area  385  to accommodate the third planet gear assembly  260 . In some implementations, one or more of the plates  350 ,  360 ,  370 ,  380  may be made of, for example, a steel material. In some implementations, a material for one or more of the plates  350 ,  360 ,  370 ,  380  may be selected based on, for example, a material of a corresponding one or more of the planet gear assemblies  240 ,  250 ,  260  and related wear properties, weight considerations, lubrication properties, and other such factors. 
       FIGS. 11A-11C  are schematic illustrations of the engagement of the sun gears  210 ,  220 ,  230 , the planet gear assemblies  240 ,  250 ,  260 , the ring gears  270 ,  280 ,  290 , and the shift ring  410  of the speed selection mechanism  400 , at a first (for example, high) output speed setting, a second, intermediate output speed setting, and a third (for example, low) output speed setting, respectively.  FIG. 11D  is a schematic illustration of the engagement of the sun gears  210 ,  220 ,  230 , the planet gear assemblies  240 ,  250 ,  260 , the ring gears  270 ,  280 ,  290 , and the shift ring  410  of the speed selection mechanism  400 , at the third (for example, low) output speed setting, in which an orientation of the third planet gear assembly  260  is reversed from the orientation shown in  FIGS. 11A-11C .  FIGS. 12A-12C  are corresponding side views of the transmission mechanism  200  and the shift selection mechanism  400  at the first (for example, high) output speed setting, the second, intermediate output speed setting, and the third (for example, low) output speed setting, respectively. 
     As described above, in some implementations, the first sun gear  210  (a single stage, i.e., not stepped, gear in this example) may be engaged with, for example, in meshed engagement with, the first stepped planet gear assembly  240 . For example, the first sun gear  210  may be in meshed engagement with the first stage gear  242  of the first stepped planet gear assembly  240 . The second sun gear  220  (a stepped, compound sun gear in this example) may span subsections  200 B and  200 C of the transmission mechanism  200 , and may be in meshed engagement with both the first planet gear assembly  240  and the second planet gear assembly  250 . That is, the first stage gear  222  of the second sun gear  220  may be in meshed engagement with the second stage gear  244  of the first stepped planet gear assembly  240 , and the second stage gear  224  of the second sun gear  220  may be in meshed engagement with the first stage gear  252  of the second planet gear assembly  250 . The third sun gear  230  (a single stage i.e., not stepped, gear in this example) may span subsections  200 D and  200 E of the transmission mechanism  200 . That is, the first portion  232  of the third sun gear  230  may be in meshed engagement with the second stage gear  254  of the second planet gear assembly  250 , and the second portion  234  of the third sun gear  230  may be in meshed engagement with the first stage gear  262  of the third planet gear assembly  260 . 
     In the first (for example, low) speed reduction/first (for example, high) output speed mode shown in  FIGS. 11A and 12A , the shift ring  410  of the speed selection mechanism  400  is engaged with one or more of the lugs  272  of the first ring gear  270 . This engagement of the shift ring  410  and the first ring gear  270  may ground the first ring gear  270  to the housing  190  of the tool  100 , or may fix a position of the first ring gear  270  relative to the housing  190  of the tool  100 . The grounding of the first ring gear  270  relative to the housing  190  in this first (for example, high) output speed mode of operation restricts rotation of the first ring gear  270 , while the second ring gear  280  and the third ring gear  290  may rotate freely. In this example arrangement, the rotational force generated by the motor  110  goes through a first (for example, low) speed reduction in the transmission mechanism  200 , resulting in a first (for example, high) speed output by the tool  100 , as shown by the arrow F 1  in  FIG. 11A . 
     In the intermediate speed reduction ratio/intermediate output speed mode shown in  FIGS. 11B and 12B , the shift ring  410  of the speed selection mechanism  400  is engaged with a lug  282  of the second ring gear  280 . This engagement of the shift ring  410  and the second ring gear  280  may ground the second ring gear  280  to the housing  190  of the tool  100 , or fix a position of the second ring gear  280  relative to the housing  190  of the tool  100 . The grounding of the second ring gear  280  relative to the housing  190  in this intermediate output speed mode of operation restricts rotation of the second ring gear  280 , while the first ring gear  270  and the third ring gear  290  may rotate freely. In this example arrangement, the rotational force generated by the motor  110  goes through an intermediate speed reduction in the transmission mechanism  200 , resulting in an intermediate speed output by the tool  100 , as shown by the arrow F 2  in  FIG. 11B . 
     In the third speed reduction ratio/third output speed mode shown in  FIGS. 11C and 12C , the shift ring  410  of the speed selection mechanism  400  is engaged with a lug  292  of the third ring gear  290 . This engagement of the shift ring  410  and the third ring gear  290  may ground the third ring gear  290  to the housing  190  of the tool  100 , or fix a position of the third ring gear  290  relative to the housing  190  of the tool  100 . The grounding of the third ring gear  290  relative to the housing  190  in this third output speed mode of operation restricts rotation of the third ring gear  290 , while the first ring gear  270  and the second ring gear  280  may rotate freely. In this example arrangement, the rotational force generated by the motor  110  goes through a third speed reduction in the transmission mechanism  200 , resulting in a third speed output by the tool  100 , as shown by the arrow F 3  in  FIG. 11C . 
     As noted above, and as shown in  FIG. 11D , in some implementations, an orientation of the first (larger diameter) stage gear  262  and the second (smaller diameter) stage gear  264  of the third planet gear assembly  260  may be reversed from the orientation shown in  FIGS. 11A-11C . When oriented, or arranged, in this manner, the third ring gear  290  may be in meshed engagement with the second (smaller diameter) stage gear  264  of the third planet gear assembly  260 , as illustrated schematically in  FIG. 11D . Engagement of the shift ring  410  and the third ring gear  290  may ground the third ring gear  290  to the housing  190  of the tool  100 . The grounding of the third ring gear  290  relative to the housing  190  in this third output speed mode of operation restricts rotation of the third ring gear  290  (now meshed to the second, smaller diameter stage gear  264  of the third planet gear assembly  260  in this example), while the first ring gear  270  and the second ring gear  280  may rotate freely. In this example arrangement, the rotational force generated by the motor  110  goes through a third speed reduction in the transmission mechanism  200 , resulting in a third speed output by the tool  100 , as shown by the arrow F 4  in  FIG. 11D . 
       FIG. 13A  is a perspective view of an example speed selection mechanism of the example multi-speed power-driven tool, in accordance with implementations described herein.  FIG. 13B  is a cross-sectional view taken along line A-A of  FIG. 13A . 
     As described above, the speed selection mechanism  400 , in accordance with implementations described herein, may provide for multi-level shifting amongst a plurality of different speed reduction ratios, and amongst a plurality of corresponding operational output speeds, of the multi-speed power-driven tool. In the example implementation described above, the example multi-speed power-driven tool  100  included three modes of operation, or three output speeds (i.e., a first speed operation mode shown in  FIGS. 11A and 12A , a second, intermediate speed operation mode shown in  FIGS. 11B and 12B , and a third speed operation mode shown in  FIGS. 11C, 11D and 12C ). Accordingly, the example speed selection mechanism  400  will be described with respect to shifting amongst three operation modes, or three output speeds, simply for purposes of discussion and illustration. 
     As shown in  FIG. 13A , in some implementations, the speed selection mechanism  400  may include a switching device  490 , or shifting device  490 . The switching device  490  may be mounted, for example, movably mounted, on one or more shift rails  480 . In some implementations, the switching device  490  may extend at least partially out of the housing  190  of the tool  100 , and/or may be accessible from an exterior of the tool  100 , for manipulation by an operator of the tool  100 . In some implementations, index marks (not shown) may be provided on the housing  190  of the tool  100 , to provide the operator with a visual indication of alignment of a position of the switching device  490  with an index mark corresponding to a desired mode of operation of the tool  100 , or output speed of the tool  100 . The shift ring  410  may be coupled at an opposite end of the shift rail(s)  480 , such that the shift rail(s)  480  couple the switching device  490  and the shift ring  410 . In some implementations, one or more guide lugs  412  formed at an outer circumferential portion of the shift ring  410  may be received in corresponding channels formed in an interior of the housing  190  (not shown) to guide the axial movement of the shift ring  410  relative to the transmission mechanism  200 . 
     A plurality of shift pins  420  may be coupled to, or provided on, the shift ring  410 . In some implementations, the plurality of shift pins  420  may be positioned circumferentially on the shift ring  410 , at circumferential positions on the shift ring  410  corresponding to the lugs  272 ,  292  of the first and third ring gears  270 ,  290 , respectively. One or more engagement lugs  430  may be defined on an inner circumferential portion of the shift ring  410 , at circumferential positions corresponding to the lugs  282  of the second ring gear  280 . The circumferential positions of the shift pins  420  and the engagement lugs  430  may be generally aligned or may be offset from each other. A tip end portion  422  of each of the shift pins  420  may define a level one stop  440 , or a first grounding device  440 , or a first stopping device  440 , or a first engagement device  440 , of the speed selection mechanism  400 . The engagement lugs  430  may define a level two stop  450 , or a second grounding device  450 , or a second stopping device  450 , or a second engagement device  450 , of the speed selection mechanism  400 . A head end portion  424  of each of the shift pins  420  may define a level three stop  460 , or a third grounding device  460 , or a third stopping device  460 , or a third engagement device  460 , of the speed selection mechanism  400 . 
       FIG. 14A  is a side view of the example speed selection mechanism  400  engaged with the example transmission mechanism  200  in the first output speed mode of operation.  FIG. 14B  is an axial end view of the engagement between the example speed selection mechanism  400  and the example transmission mechanism  200  illustrated in  FIG. 14A .  FIG. 14C  is a cross-sectional view taken along line B-B of  FIG. 14A . As shown in  FIG. 14A , in the first output speed mode of operation, a manipulation of the switching device  490  may position the shift ring  410  such that the level one stop  440 , or first grounding device  440 , defined by the tip end portion  422  of the shift pins  420  is positioned against the lugs  272  of the first ring gear  270 , as shown in  FIG. 14B . In this arrangement, level two stop  450 , or second grounding device  450 , defined by the engagement lugs  430  of the shift ring  410  remain clear of/disengaged from the lugs  282  of the second ring gear  280 , and the level three stop  460 , or third grounding device  460 , defined by the head end portion  424  of the shift pins  420  each remain clear of/disengaged from the lugs  292  of the third ring gear  290 , allowing the second and third ring gears  280 ,  290  to rotate freely. The positioning of tip end portion  422  of the shift pins  420  against the lugs  272  of the first ring gear  270  grounds the first ring gear  270  relative to the housing  190  of the tool  100 , thus restricting rotation of the first ring gear  270 . Restriction of rotation of the first ring gear  270  in this manner (while the second ring gear  280  and the third ring gear  290  rotate freely) produces a first speed reduction through the transmission mechanism  200 , resulting in a first speed output by the tool  100 , as described above in detail. 
       FIG. 15A  is a side view of the example speed selection mechanism  400  engaged with the example transmission mechanism  200  in the intermediate output speed mode of operation.  FIG. 15B  is an axial end view of the engagement between the example speed selection mechanism  400  and the example transmission mechanism  200  illustrated in  FIG. 15A .  FIG. 15C  is a cross-sectional view taken along line C-C of  FIG. 15A . As shown in  FIG. 15A , in the second, intermediate output speed mode of operation, a shifting of the switching device  490  along the shift rail(s)  480 , for example, in the direction of the arrow L, from the position shown in  FIG. 14A  to the position shown in  FIG. 15A , may position the shift ring  410  such that the engagement lugs  430  defining the level two stop  450 , or second grounding device  450 , are positioned against the lugs  282  of the second ring gear  280 , as shown in  FIG. 15B . In this arrangement, the level one stop  440 , or first grounding device  440 , defined by the tip end portion  422  of the shift pins  420  remains clear of/disengaged from the lugs  272  of the first ring gear  270 , and the level three stop  460 , or third grounding device  460 , defined by the head end portion  424  of the shift pins  420  each remain clear of/disengaged from the lugs  292  of the third ring gear  290 , allowing the first and third ring gears  270 ,  290  to rotate freely. The positioning of engagement lugs  430  of the shift ring  410  against the lugs  282  of the second ring gear  280  grounds the second ring gear  280  relative to the housing  190  of the tool  100 , thus restricting rotation of the second ring gear  280 . Restriction of the rotation of the second ring gear  280  in this manner (while the first ring gear  270  and the third ring gear  290  rotate freely) produces a second, intermediate speed reduction through the transmission mechanism  200 , resulting in a second, intermediate speed output by the tool  100 , as described above in detail. 
       FIG. 16A  is a side view of the example speed selection mechanism  400  engaged with the example transmission mechanism  200  in the third output speed mode of operation.  FIG. 16B  is an axial view of the engagement between the example speed selection mechanism  400  and the example transmission mechanism  200  illustrated in  FIG. 16A .  FIG. 16C  is a cross-sectional view taken along line D-D of  FIG. 16A . As shown in  FIG. 16A , in the third output speed mode of operation, a shifting of the switching device  490  along the shift rail(s)  480 , for example, in the direction of the arrow L, from the position shown in  FIG. 15A  to the position shown in  FIG. 16A , may position the shift ring  410  such that the head end portion  424  of the shift pins  420  defining the level three stop  460 , or third grounding device  460 , are positioned against the lugs  292  of the third ring gear  290 , as shown in  FIG. 16B . In this arrangement, the level one stop  440 , or first grounding device  440 , defined by the tip end portion  422  of the shift pins  420  remain clear of/disengaged from the lugs  272  of the first ring gear  270 , and the level two stop  450 , or second grounding device  450 , defined by the lugs  430  of the shift ring  410  remain clear of/disengaged from the lugs  282  of the second ring gear  280 , allowing the first and second ring gears  270 ,  280  to rotate freely. The positioning of head end portion  424  of the shift pins  420  against the lugs  292  of the third ring gear  290  grounds the third ring gear  290  relative to the housing  190  of the tool  100 , thus restricting rotation of the third ring gear  290 . Restriction of the rotation of the third ring gear  290  in this manner (while the first ring gear  270  and the second ring gear  280  rotate freely) produces a third (for example, high) speed reduction through the transmission mechanism  200 , resulting in a third speed output by the tool  100 , as described above in detail. 
       FIG. 17  is a perspective view of a shift ring  410 A including a plurality of shift pins  420 A, for use with a speed selection mechanism for a multi-speed power driven tool, in accordance with implementations described herein.  FIG. 18  is a side view of the shift ring  410 A shown in  FIG. 17 , engaged with the example transmission mechanism  200 , in accordance with implementations described herein. The shift ring  410 A may be a complete ring (e.g., a circular shape) or a partial ring (e.g., a C-shape). 
     As shown in  FIGS. 17 and 18 , the shift ring  410 A may include a plurality of shift pins  420 A, circumferentially arranged on the shift ring  410 A, at positions corresponding to the lugs  272 ,  282 ,  292  of the first, second and third ring gears  270 ,  280 ,  290 . Each of the shift pins  420 A may include a first protrusion  425 A defining a level one stop  440 , or first grounding device  440 , or first stopping device  440 , or first engagement device  440 . Each of the shift pins  420 A may include a second protrusion  427 A defining a level two stop  450 , or second grounding device  450 , or second stopping device  450 , or second engagement device  450 . Each of the shift pins  420 A may include a third protrusion  429 A defining a level three stop  460 , or third grounding device  460 , or third stopping device  460 , or third engagement device  460 . 
     Axial movement of the shift ring  410 A in the manner described above (for example, with respect to  FIGS. 14A through 16C ) may engagement between one of the protrusions  425 A,  427 A,  429 A with corresponding lugs  272 ,  282 ,  292  of a selected one of the ring gears  270 ,  280 ,  290 , to ground the selected ring gear  270 ,  280 ,  290  as described above. For example, as shown in  FIG. 18 , at an intermediate axial position of the shift ring  410 A relative to the transmission mechanism  200 , the second protrusion  427 A of each of the shift pins  420 A may be positioned against the corresponding lugs  282  of the second ring gear  280 , thus restricting rotation of the second ring gear  280  (while rotation of the first ring gear  270  and the third ring gear  290  remain unrestricted, and rotate freely). Restriction of the rotation of the second ring gear  280  in this manner (while the first ring gear  270  and the third ring gear  290  rotate freely) produces an intermediate speed reduction through the transmission mechanism  200 , resulting in an intermediate speed output by the tool  100 , as described above in detail. 
     A speed selection mechanism  400  including the switching device  490 , shift rails  480  and shift ring(s)  410 ,  410 A that incorporate multi-level shift pins, in accordance with implementations described herein, may provide for a relatively axially compact mechanism for grounding one of a plurality of ring gears of the example transmission mechanism, while allowing the remaining ring gears to rotate freely. The use of multi-level shift pins  420 ,  420 A as described above, providing for multiple levels of speed selection along the same set of multi-level shift pins  420 ,  420 A may provide for speed selection with relatively minimal axial travel of the shift ring  410 ,  410 A and shift pins  420 ,  420 A relative to the transmission mechanism  200 . 
     A reduction in an overall profile of the example tool  100 , and in particular, a reduction in an axial length of the tool  100 , as provided by the features described above, may enhance functionality and utility of the tool  100  to the user. However, a reduction in the axial length of the tool  100 , and a corresponding adjustment in the dimension(s) of the housing  190 , may, in turn, dictate a reduction in the overall profile of the shift selection mechanism  400 , such as, for example, a reduction in the axial travel distance of the switching device  490  to effect the shifting, or switching, or speed selection as described above (i.e., shifting between operation at the first output speed, the second output speed and the third output speed, as described above). 
     As shown in  FIG. 19 , in some implementations, the switching device  490 , or shifting device  490 , or selection device  490 , may include a grasping member (e.g., a button or lever)  492  and a base portion  494 . In some implementations, the grasping member (e.g., a button or lever)  492  and the base portion  494  may be integrally formed. A shutter  496  may be positioned on the base portion  494 , with the grasping member (e.g., a button or lever)  492  extending out through an opening  495  in the shutter  496 . The opening  495  may be formed between a first end  496 A and a second end  496 B of the shutter  495 . The shutter  496  may be freely slidable relative to the base portion  494  such that the grasping member (e.g., a button or lever)  492  can move between a first end  495 A of the opening  495  and a second end  495 B of the opening  495  in the shutter  496 . 
     The switching device  490  may be positioned within an opening  195 , or a slot  195 , formed in the housing  190 , so that the grasping member (e.g., a button or lever)  492  may be accessible for manipulation by the user through the opening  195 , for selection of one of the plurality of modes of operation of the tool  100  (i.e., selection of one of the plurality of output speeds of the tool  100 ). In some implementations, the sliding interaction of the shutter  496  relative to the base portion  494  may provide for enclosure of the opening  195  in the housing  190 . The combination of the shutter  496  and the base portion  494  may provide for enclosure of the opening  195  with a relatively shorter overall length compared to, for example, a single piece base for the switching device  490 . In some implementations, the shutter  496  may be freely slidable relative to the base portion  494 , so that a length covered by the shutter  496  and the base portion  494  is variable. The variable length provided by the slidable interaction of the shutter  496  and the base portion  494  may occupy a shorter overall axial length (i.e., a shorter maximum axial length) to extend across the opening  195  than would otherwise be required by a single piece base, without the variable length afforded by the slidable interaction of the shutter  496  and the base portion  494 . This will be described in more detail with respect to  FIGS. 20A-20D . 
       FIGS. 20A-20C  are partial cross-sectional views of the switching device  490  of the speed selection mechanism  400 , installed in the housing  190  of the tool  100 . In particular,  FIG. 20A  is a cross-sectional view of the switching device  490  shown in  FIG. 19 , at a first position, corresponding to a first (for example, high) output speed mode of operation of the tool  100  (for example, as shown in  FIG. 12A ).  FIGS. 20B ( 1 ) and  20 B( 2 ) are cross-sectional views of the switching device  490  shown in  FIG. 19 , at a second position, corresponding to a second, intermediate output speed mode of operation of the tool  100  (for example, as shown in  FIG. 12B ).  FIG. 20C  is a cross-sectional view of the switching device  490  shown in  FIG. 19 , at a third position, corresponding to a third (for example, low) output speed mode of operation of the tool  100  (for example, as shown in  FIG. 12C ).  FIG. 20D  is a cross-sectional view of the base portion  494  and the grasping member (e.g., a button or lever)  492  of the switching device  490 , without the shutter  496 , installed in the housing  190  of the tool  100 . 
     As shown in  FIG. 20A , in the first position of the switching device  490 , corresponding to the first (for example, high) output speed mode of operation of the tool  100 , the grasping member (e.g., a button or lever)  492  is positioned at a first end portion  195 A of the opening  195  in the housing  190 , at, essentially, a first extreme axial position of the switching device  490 . In the position shown in  FIG. 20A , a portion of the shutter  496 , and a portion of the base portion  494  (exposed through the opening  495  in the shutter  496 ) extend across the opening  195  in the housing  190 . In this manner, the housing  190  can remain enclosed (by the grasping member (e.g., a button or lever)  492 , the base portion  494  and the shutter  496 ) in this first position of the switching device  490 . 
     As shown in  FIG. 20B , in the second position of the switching device  490 , corresponding to the second, intermediate output speed mode of operation of the tool  100 , the grasping member (e.g., a button or lever)  492  is positioned at an intermediate portion  195 B of the opening  195  in the housing  190 . In  FIG. 20B ( 1 ), the shutter  496  is positioned at a first extreme end of travel of the shutter  496  relative to the base portion  494 . In  FIG. 20B ( 2 ), the shutter  496  is positioned at a second extreme end of travel of the shutter  496  relative to the base portion  494 . In either of these positions, the opening  195  in the housing  190  is enclosed by the base portion  494  and the shutter  496 . In this manner, the housing  190  can remain enclosed (by the grasping member (e.g., a button or lever)  492 , the base portion  494  and the shutter  496 ) in this second, intermediate position of the switching device  490 , regardless of the free sliding movement of the shutter  496  relative to the base portion  494 . 
     As shown in  FIG. 20C , in the third position of the switching device  490 , corresponding to the third (for example, low) output speed mode of operation of the tool  100 , the grasping member (e.g., a button or lever)  492  is positioned at a second end portion  195 C of the opening  195  in the housing  190 , at, essentially, a second extreme axial position of the switching device  490 . In the position shown in  FIG. 20C , a portion of the shutter  496 , and a portion of the base portion  494  (exposed through the opening  495  in the shutter  496 ) extend across the opening  195  in the housing  190 . In this manner, the housing  190  can remain enclosed (by the grasping member (e.g., a button or lever)  492 , the base portion  494  and the shutter  496 ) in this third position of the switching device  490 . As shown in  FIG. 20C , in some implementations, the switching device  490  may include a stopping mechanism  498  to, for example, limit the axial movement of the shutter  496  in the housing  190 . In the example implementation shown in  FIG. 20C , the stopping mechanism  498  may include a first protrusion  498 A extending upward at the first end  496 A of the shutter  496 , that is configured to engage a second protrusion  498 B extending downward from a corresponding peripheral portion of the opening  195  in the housing  190 . 
     In  FIG. 20D , the switching device  490  is installed in the housing  190 , at the same position as shown in  FIG. 20C , but without the shutter in place on the base portion  494 . As shown in  FIG. 20D , without the variable length of the enclosure area provided by the sliding shutter  496  relative to the base portion  494  as described above, the base portion  494  of the switching device  490  would have to be elongated, for example, considerably elongated, to extend across an open area  0 , and completely enclose the opening  195  in the housing  190 , particularly with the grasping member (e.g., a button or lever)  492  in the first position shown in  FIG. 20C , and in the third position shown in  FIGS. 20C and 20D . Elongating the base portion in this manner would, in turn, occupy additional axial space within the housing  190 , thus requiring an elongated axial space in the housing to accommodate the movement of the switching device. 
     Thus, a switching device  490  including a slidable shutter  496  as described above may allow for a larger opening in the tool housing  190  to access the switching device  490  while still preventing dust and debris from entering the tool housing  190 . Similarly, a switching device  490  including a slidable shutter  496  as described above may allow for an increased axial travel distance of the switching device for a given size of housing  190 . 
     As noted above, in some implementations, the tool  100  may include a hammer mechanism  500  or an impact mechanism (not shown), which may selectively impart a repeated axial hammering force or a repeated rotational impacting force to a bit coupled to the power tool, respectively. In some implementations, an example rotational impact mechanism may have a design similar to one or more of the impact mechanisms disclosed in U.S. Pat. No. 5,016,501 and U.S. Patent Application Pub. Nos. 2007/0267207 and 2010/0071923, which are incorporated herein by reference. Other example impact mechanisms can be found in a model IDS600 Impact Screwdriver and a model FDS600 Impact Screwdriver commercially available from Black &amp; Decker and in a model DCF895 Impact Driver, DCF896 Impact Driver, and DCF880 Impact Wrench available commercially from DeWalt Industrial Tool Co. 
       FIG. 21  is a perspective view of a working end of the tool  100 , in which a hammer mechanism  500  may be housed, within the housing  190  of the tool  100 . The example power-driven tool  100  including the hammer mechanism  500 , in accordance with implementations described herein, may include a rotating cam mechanism  510 , as shown in  FIG. 22A , that interacts with a non-rotating ratchet  550 , as shown in  FIG. 22B . The rotating cam mechanism  510  may include cam lugs  520  having ramp surfaces  525 , the cam lugs  520  extending radially inward from a cam ring  530 . The non-rotating ratchet  550  may include lugs  560  having ramp surfaces  565 , the lugs  560  extending radially outward from a hub  570 . The cam mechanism  510  may surround the non-rotating ratchet  550 , such that that the non-rotating ratchet  550  is nested within the cam mechanism  510 . This arrangement may reduce an axial length of the hammer mechanism  500  (compared to, for example, an impacting mechanism in which a cam mechanism and a ratcheting mechanism are axially arranged), resulting in a more axially compact impacting mechanism  500 , thus further enhancing utility and functionality of the tool  100 . Operation of the hammer mechanism  500  will be described in more detail with respect to  FIGS. 23A-23B . 
       FIGS. 23A and 23B  are cross-sectional views taken along line taken along line G-G of  FIG. 22 .  FIGS. 24B and 25B  are cross-sectional views taken along line H-H of  FIG. 22 . In the cross-sectional views illustrated in  FIGS. 23A and 23B , the example tool  100  is in a first mode of operation. In the cross-sectional views illustrated in  FIGS. 24A and 24B , the example tool  100  is in a second mode of operation. In some implementations, in a power-driven tool including a hammer mechanism, in accordance with implementations described herein, the first mode of operation may be a driving mode, or a drilling mode of operation, and the second mode of operation may be a hammer mode of operation. 
     As shown in the cross-sectional views illustrated in  FIGS. 23B and 24B , the non-rotating ratchet  550  may be non-rotatably fixed and axially moveable relative to the housing  190  of the tool  100 . A rotating ratchet  590  may be non-rotatably fixed and axially movable relative to the output spindle  130 . The fixed ratchet  590  and the rotating ratchet  550  may be axially aligned, such that teeth  575  formed on the hub  570  of the non-rotating ratchet  550  face teeth  585  on the rotating ratchet  590 . In the first (driving, or drill) mode of operation, shown in  FIGS. 23A and 23B , a collar  135  provided on an exterior of the housing  190  and coupled to the cam ring  530  is in a first selection position, for operation in the first mode of operation. In this first (drill) mode, the cam lugs  520 /ramp surfaces  525  of the cam mechanism  510  and the lugs  560 /ramp surfaces  565  of the non-rotating ratchet  550  are radially offset, and not engaged, as shown in  FIG. 23A . In this first (drill) mode of operation, the non-rotating ratchet  550  and the rotating ratchet  590  are axially spaced apart, or separated, as shown in  FIG. 24B , such that the teeth  575  of the non-rotating ratchet  550  and the teeth  595  of the rotating ratchet  590  are not engaged, and no axial percussion is applied to the output spindle  130 . In the first (drill) mode, force (for example, rotational force) may be output from the output spindle  130  to, for example, an output accessory held in the chuck assembly  180 . 
     Rotation of the collar  135  from the first position shown in  FIGS. 23A and 23B , to a second position shown in  FIGS. 24A and 24B , may enable the second (hammer) mode of operation. In some implementations, rotation of the collar  135  (coupled to the cam ring  530  of the cam mechanism  510 ) to the second position may rotate the cam mechanism  510  by a set angular distance, as shown in  FIG. 24A . This rotation of the cam mechanism  510  may cause the radial cam lugs  520 /ramp surfaces  525  of the cam mechanism  510  to be aligned with the lugs  560 /ramp surfaces  565  of the non-rotating ratchet  550 . As shown in  FIG. 24A , in this second position (corresponding to the second mode of operation), the cam lugs  520  are positioned behind the lugs  560  of the non-rotating ratchet  550 . This movement of the cam mechanism  510  and alignment of the cam lugs  520  with the lugs  560  of the non-rotating ratchet  550  may push the non-rotating ratchet  550  axially forward, toward the rotating ratchet  590 , so that that teeth  575  of the non-rotating ratchet  550  and the teeth  595  of the rotating ratchet  590  are engaged. Engagement of the teeth  575  of the non-rotating ratchet  550  and the teeth  595  of the rotating ratchet  590  may generate an axial percussion that is imparted on the output spindle  130  as the output spindle  130  rotates. This arrangement of components of the hammer mechanism  500  may provide for an axially compact hammer mechanism  500 , contributing to a reduced overall tool profile, and further enhancing utility and functionality of the tool. 
       FIG. 25A  is a side perspective view, and  FIG. 25B  is a bottom perspective view, of an example speed selection mechanism  1400 , for use with the example multi-speed power-driven tool, in accordance with implementations described herein.  FIG. 25C  is a cross-sectional view, taken along line L-L of  FIG. 25B .  FIG. 25D  is a close in perspective view of a compliant mechanism of the speed selection mechanism  1400 , in accordance with implementations described herein.  FIGS. 25E and 25F  are cross-sectional views of the example speed mechanism  1400  installed in a housing of the example multi-speed power-driven tool. 
     The speed selection mechanism  1400 , in accordance with implementations described herein, may provide for compliant shifting, for example, compliant multi-level shifting amongst a plurality of different speed reduction ratios, and amongst a plurality of corresponding operational output speeds, of the multi-speed power-driven tool. In the example implementation described above, the example multi-speed power-driven tool  100  included three modes of operation, or three output speeds (i.e., a first speed operation mode shown in  FIGS. 11A and 12A , a second, intermediate speed operation mode shown in  FIGS. 11B and 12B , and a third speed operation mode shown in  FIGS. 11C, 11D and 12C ). The example speed selection mechanism  1400  will be described with respect to shifting amongst three operation modes, or three output speeds, simply for purposes of discussion and illustration. The principles to be described herein with respect to the example speed selection mechanism may be adapted for use with a power driven tool that is operable in more, or fewer, operation modes and/or at more, or fewer, output speeds. 
     In some implementations, the speed selection mechanism  1400  may include a switching device  1490 , or shifting device  1490 , or speed selector  1490 . The switching device  1490  may be coupled to a shift ring  1410  that is moved axially, in response to movement of the switching device  1490 , to shift the output speed of the transmission mechanism  200  into the first, second or third mode of operation as described above, based on a position of the switching device  1490 . The shift ring  1410  may be a complete ring (e.g., a circular shape) or a partial ring (e.g., a C-shape). In some implementations, the shift ring  1410  may include one or more guide lugs  1412  formed at an outer circumferential portion of the shift ring  1410 . The guide lug(s)  1412  may be received in corresponding channels  198  formed in an interior of the housing  190 , as shown in  FIG. 25E , to guide the axial movement of the shift ring  1410 . In some implementations, the shift ring  1410  may include one or more engagement lugs  1415  formed at an inner circumferential portion of the shift ring  1410 . The engagement lugs  1415  may selectively engage features ring gears  270 ,  280 ,  290 , for example, the lugs  272 ,  282 ,  292  of the first, second and third ring gears  270 ,  280 ,  290 , respectively, to ground the selected ring gear  270 ,  280 ,  290  to the housing  190  for the selected output speed, as described above. A pair of axial pins  1471  (i.e.,  1471 A,  1471 B) may each have a first end thereof coupled, for example, fixedly coupled, to the shift ring  1410 . A first fork bushing  1441  may include a first fitting  1441 A fixed to the first axial pin  1471 A, and a second fitting  1441 B fixed to the second axial pin  1471 B. A second fork bushing  1442  may include a first fitting  1442 A fixed to the first axial pin  1471 A, and a second fitting  1442 B fixed to the second axial pin  1471 B. Thus, the first and second fork bushings  1441 ,  1442  may be fixed to, and may extend between, the pair of axial pins  1471 , and the pair of axial pins  1471 , and the first and second fork bushings  1441 ,  1442  may move axially, together with the shift ring  1410 . 
     The switching device  1490  may be positioned over the axial pins  1471  and the fork bushings  1441 ,  1442 . The switching device  1490  may include a finger grip  1492  that extends at least partially out of the housing  190  of the tool  100 , so that the switching device  1490  is accessible from an exterior of the tool  100 , for manipulation by an operator of the tool  100 . The switching device  1490  may include a shutter  1496  positioned on a base  1494 , with the finger grip  1492  extending up from the base  1494  and out through an opening  1495  in the shutter  1496 , so that the finger grip  1492  is movable within the opening  1495  to a plurality of selection positions, allowing for operator selection of different output modes, or speeds, of the tool  100 . In some implementations, a plurality of detents  1421  may be formed on opposite lateral sides of the base  1494 . The detents  1421  may selectively engage leaf springs  197  provided in the housing  190  (see  FIG. 25F ) to retain an axial position of the speed selection mechanism  1400  relative to the transmission mechanism  200 . 
     A pair of compression springs  1451  may be respectively positioned on the pair of axial pins  1471 . For example, a first compression spring  1451 A may positioned on the first axial pin  1471 A, between the first fitting  1441 A of the first fork bushing  1441  and the first fitting  1442 A of the second fork bushing  1442 . A second compression spring  1451 B may be positioned on the second axial pin  1471 B, between the second fitting  1441 B of the first fork bushing  1441  and the second fitting  1442 B of the second fork bushing  1442 . In some implementations, bushings  1431  may be installed at the second ends of the axial pins  1471 . 
       FIGS. 26A-26D  are perspective views sequentially illustrating the assembly of the speed selection mechanism  1400 , in accordance with implementations described herein, to illustrate the interaction of the components of the speed selection mechanism  1400 . As shown in  FIG. 26A , the first end portion of the first and second axial pins  1471  may be fit, for example, press fit, into corresponding receiving openings in the shift ring  1410 , to fix the coupling of the pins  1471  to the shift ring  1140 . As shown in  FIG. 26B , the first fork bushing  1441  may be installed onto the pins  1471 . For example, the first axial pin  1471 A may be fit, for example, press fit, in the first fitting  1441 A of the first fork bushing  1441 , and the second axial pin  1471 B may be fit, for example, press fit, in the second fitting  1441 B of the first fork bushing  1441 . The springs  1451  may be positioned within axial support arms  1530  of the base  1494  of the switching device  1490 , and the base  1494  may be slidably coupled onto the pins  1471 , as shown in  FIG. 26C . The second fork bushing  1442  may then be installed by fitting, for example, press fitting the first and second axial pins  1471  into the first and second fittings  1442 A,  1442 B, respectively, and then fitting the bushings  1431  on the ends of the axial pins  1471 , as shown in  FIG. 26D . 
     In some implementations, a compliant mechanism  1500  may provide the speed selection mechanism  1400  with compliance in shifting between the various output speeds of the power-driven tool  100 . For example, in some implementations, the pair of compression springs  1451  and corresponding portions of the pair of axial pins  1471  on which the springs  1451  are positioned, may be respectively received in a pair of support arms  1530  formed at a bottom portion of the base  1494 . For example, the first pin  1471 A and first spring  1451 A may be received in a first support arm  1530 A formed on a corresponding bottom surface portion of the base  1494 . The second pin  1471 B and the second spring  1451 B may be received in a second support arm  1530 B formed on a corresponding bottom surface portion of the base  1494 . Interaction of the guide pins  1471  and compression springs  1451  with the first and second fork bushings  1441 ,  1442 , and with the support arms  1530 , in providing for compliance in shifting, will be described in more detail below. 
     In some implementations, protrusions  1443  may extend from the fittings of the first fork bushing  1441  toward the respective spring  1451 , and protrusions  1444  may extend from the fittings of the second fork bushing  1442  toward the respective spring  1451 . For example, as shown in  FIG. 25B , a protrusion  1443 A may extend from the first fitting  1441 A of the first fork bushing  1441  toward the first spring  1451 A on the first axial pin  1471 A, and a protrusion  1443 B may extend from the second fitting  1441 B of the first fork bushing  1441  toward the second spring  1451 B on the second axial pin  1471 B. Similarly, a protrusion  1444 A may extend from the first fitting  1442 A of the second fork bushing  1442  toward the first spring  1451 A on the first axial pin  1471 A, and a protrusion  1444 B may extend from the second fitting  1442 B of the second fork bushing  1442  toward the second spring  1451 B on the second axial pin  1471 B. Each of the support arms  1530  may include a first pair of tabs  1510  at a first axial end of the support arm  1530 , and a second pair of tabs  1520  at a second axial end of the support arm  1530 .  FIG. 25D  provides a close in view of the arrangement of these components of the compliant mechanism  1500 , shown from a first side of the speed selection mechanism  1400 . It is understood these components will be similarly arranged on the second side of the speed selection mechanism  1400 . 
       FIGS. 27A and 27B  are partial perspective views of the speed selection mechanism  1400 , illustrating operation of the compliance mechanism  1500  for engagement of the speed selection mechanism  1400  and the transmission mechanism  200  (not shown in  FIGS. 27A and 27B ), in accordance with implementations described herein. 
     In the position shown in  FIG. 27A , the switching device  1490  has been moved to a position at a first end of the opening  1495  in the shutter  1496 , to select the first output speed. This axial selection position may be retained by interaction of the leaf spring  197  attached to the interior of the housing  190  with corresponding detent(s)  1421  formed in the lateral side(s) of the base  1494  (see  FIG. 25F ). In this position, the springs  1451  are compressed in a forward position on the axial pins  1471  (in the orientation shown in  FIG. 27A ), with the tabs  1520  at the axially rearward end portion of the support arms  1530  pressing the springs  1451  against the protrusions  1443  of the first fork bushing  1441 . In this arrangement, the first fork bushing  1441 , the axial pins  1471 , and the shift ring  1410  are axially stationary. Due to, for example, the relative arrangement of the ring gears  270 ,  280 ,  290 , and in particular the circumferential positioning of the lugs  272 ,  282 ,  292  of the ring gears  270 ,  280 ,  290 , with respect to the engagement lugs  1415  of the shift ring  1410 , it may be that the shift ring  1410  is temporarily blocked, or restricted, from moving axially to engage the desired ring gear  270 ,  280 ,  290 . In this case, the shift ring  1410  may move to an intermediate position between the previously engaged ring gear and the desired ring gear. 
     For example,  FIG. 27C  provides a cross-sectional view illustrating a position of the shift ring  1410  relative to the ring gears  270 ,  280 ,  290  at the first output speed. In this position, the shift ring  1410  is engaged with the first ring gear  270 , so as to ground the first ring gear  270  relative to the housing  190  and select operation of the tool  100  at the first output speed. An operator may move or slide the grasping member (e.g., a button or lever)  1492 , from the position shown in  FIG. 27C  to the left in the orientation shown in  FIGS. 27C-27E , to select the second output speed. As shown in  FIG. 27D , the relative arrangement of the ring gears  270 ,  280 ,  290  and the corresponding circumferential positioning of the lugs  272 ,  282 ,  292  with respect to the engagement lugs  1415  of the shift ring  1410  has temporarily blocked, or restricted axial movement of the shift ring  1410 , and the shift ring  1410  is positioned between the previously engaged ring gear (the first ring gear  270  in this example) and the desired ring gear (the second ring gear  280  in this example). In response to application of power, one or more of the ring gears  270 ,  280 ,  290  may rotate so that the axial movement of the shift ring  1410  is no longer obstructed. The force of the springs  1451  pushes against the protrusion  1443  of the first fork bushing  1441 , allowing the first and second fork bushing  1441 ,  1442  and the shift ring  1410  to move axially into engagement with the desired ring gear (the second ring gear  280  in this example), as shown in  FIG. 27E . 
     As noted above, the axial selection position may be retained by interaction of the leaf spring  197  attached to the interior of the housing  190  with corresponding detent(s)  1421  formed in the lateral side(s) of the base  1494 . 
     In this position, the springs  1451  are compressed in a rearward position on the axial pins  1471  (in the orientation shown in  FIG. 27B ), with the tabs  1510  at the axially forward end portion of the support arms  1530  pressing the springs  1451  against the protrusions  1444  of the second fork bushing  1442 . As described above, due to, for example, the relative arrangement of the ring gears  270 ,  280 ,  290 , and in particular the circumferential positioning of the lugs  272 ,  282 ,  292  of the ring gears  270 ,  280 ,  290 , with respect to the engagement lugs  1415  of the shift ring  1410 , it may be that the shift ring  1410  is temporarily blocked, or restricted, from moving axially to engage the desired ring gear  270 ,  280 ,  290 . In response to application of power, one or more of the ring gears  270 ,  280 ,  290  may spin, the axial movement of the shift ring  1410  may no longer be obstructed, and the force of the springs  1451  will push against the protrusion  1444  of the second fork bushing  1442 , allowing the first and second fork bushing  1441 ,  1442  and the shift ring  1410  to move axially into engagement with the desired ring gear  270 ,  280 ,  290 . 
     The first and second axial springs  1451  are arranged substantially in parallel on the first and second axial pins  1471 , which move axially together with the shift ring  1410 . Axial movement of the axial springs  1451  on the axial pins  1471  exert a biasing force on the shift ring  1410  in both the forward and the rearward directions relative to switching device  1490 . The bi-directional biasing provided by the two axial springs  1451  arranged in parallel provides for shifting compliance in both movement directions of the speed selection mechanism  1400 , and may reduce imbalance experienced during shifting, particularly when compared to an arrangement in which the springs are positioned serially. 
       FIG. 28A  is a top perspective view of another example speed selection mechanism  2400 , for use with the example multi-speed power-driven tool, in accordance with implementations described herein.  FIG. 28B  is a bottom perspective view of portion of the example speed selection mechanism  2400  shown in  FIG. 28A . 
     The speed selection mechanism  2400 , in accordance with implementations described herein, may provide for compliant shifting, for example, compliant multi-level shifting amongst a plurality of different speed reduction ratios, and amongst a plurality of corresponding operational output speeds, of the multi-speed power-driven tool. In the example implementations described above, the example multi-speed power-driven tool  100  included three modes of operation, or three output speeds (i.e., a first speed operation mode shown in  FIGS. 11A and 12A , a second, intermediate speed operation mode shown in  FIGS. 11B and 12B , and a third speed operation mode shown in  FIGS. 11C, 11D and 12C ). The example speed selection mechanism  2400  will be described with respect to shifting amongst three operation modes, or three output speeds, simply for purposes of discussion and illustration. The principles to be described herein with respect to the example speed selection mechanism may be adapted for use with a power driven tool that is operable in more, or fewer, operation modes and/or at more, or fewer, output speeds. 
     As shown in  FIG. 28A , in some implementations, the speed selection mechanism  2400  may include a switching device  2490 , or shifting device  2490 , or speed selector  2490 . The switching device  2490  may be coupled to a shift ring  2410  that is moved axially, in response to movement of the switching device  2490 , to shift the output speed of the transmission mechanism  200  into the first, second or third mode of operation as described above, based on a position of the switching device  2490 . The shift ring  2410  may be a complete ring (e.g., a circular shape) or a partial ring (e.g., a C-shape). In some implementations, the shift ring  2410  may include one or more guide lugs  2412  formed at an outer circumferential portion of the shift ring  2410 . The guide lug(s)  21412  may be received in corresponding channels formed in an interior of the housing  190  (not shown), to guide the axial movement of the shift ring  2410 . In some implementations, the shift ring  2410  may include one or more engagement lugs  2415  formed at an inner circumferential portion of the shift ring  2410 . The engagement lugs  2415  may selectively engage features ring gears  270 ,  280 ,  290 , for example, the lugs  272 ,  282 ,  292  of the first, second and third ring gears  270 ,  280 ,  290 , respectively, to ground the selected ring gear  270 ,  280 ,  290  to the housing  190  for the selected output speed, as described above. A pair of axial pins  2471  (i.e., a first axial pin  2471 A and a second axial pin  2471 B) may each have a first end thereof coupled, for example, fixedly coupled, to the shift ring  2410 . A first fork bushing  2441  may include a first fitting  2441 A fixed to the first axial pin  2471 A, and a second fitting  2441 B fixed to the second axial pin  2471 B. A second fork bushing  2442  may include a first fitting  2442 A fixed to the first axial pin  2471 A, and a second fitting  2442 B fixed to the second axial pin  2471 B. Thus, the first and second fork bushings  2441 ,  2442  may be fixed to, and may extend between, the pair of axial pins  2471 , and the first and second fork bushings  2441 ,  2442  may move axially, together with the shift ring  2410 . 
     The switching device  2490  may be positioned over the axial pins  2471  and the fork bushings  2441 ,  2442 . The switching device  2490  may include a finger grip  1492  that extends at least partially out of the housing  190  of the tool  100 , so that the switching device  2490  is accessible from an exterior of the tool  100 , for manipulation by an operator of the tool  100 . The switching device  2490  may include a shutter  2496  positioned on a base  2494 , with the finger grip  2492  extending up from the base  2494  and out through an opening  2495  in the shutter  2496 , so that the finger grip  2492  is movable within the opening  2495  to a plurality of selection positions, allowing for operator selection of different output modes, or speeds, of the tool  100 . 
     As illustrated in the bottom perspective view of the base  2494  of the switching device  2490  shown in  FIG. 28A , the first spring  2451 A may be positioned in a first pocket  2481 A defined in a bottom surface of the base  2494 . The second spring  2451 B may be positioned in a second pocket  2481 B defined in the bottom surface of the base  2494 . The first pocket  2481 A may be at least partially defined by a first rail  2475 A, together with a first side portion of a central post  2485  and a first channel  2461 A extending along the length of the bottom portion of the base  2494 . The second pocket  2481 B may be at least partially defined by a second rail  2475 B, together with a second side portion of the central post  2485  and a second channel  2461 B extending along the length of the bottom portion of the base  2494 . Opposite ends of the first spring  2451 A may be retained against a first block  2455 A of the first fork bushing  2441  and a first block  2457 A of the second fork bushing  2442 . Similarly, opposite ends of the second spring  2452  may be retained against a second block  2455 B of the first fork bushing  2441  and a second block  2457 B of the second fork bushing  2442 . 
     A first set of detents  2421  (for example, three detents  2421 A,  2421 B and  2421 C in the example arrangement shown) may be defined in a first side surface portion of the base  2494 . A second set of detents  2422  (for example, three detents  2422 A,  2422 B and  2422 C in the example arrangement shown) may be defined in a second side surface portion of the base  2494 , at positions corresponding to the first set of detents  2421 . 
       FIGS. 29A through 29D  are perspective views sequentially illustrating the assembly of the speed selection mechanism  2400 , in accordance with implementations described herein, to illustrate the interaction of the components of the speed selection mechanism  2400 . As shown in  FIG. 29A , the first end portion of the first and second axial pins  2471  may be fit, for example, press fit, into corresponding receiving openings in the shift ring  2410 , to fix the coupling of the pins  2471  to the shift ring  2140 . As shown in  FIG. 29B , the first fork bushing  2441  may be installed onto the pins  2471 . For example, the first axial pin  1471 A may be fit, for example, press fit, in the first fitting  2441 A of the first fork bushing  2441 , and the second axial pin  1471 B may be fit, for example, press fit, in the second fitting  2441 B of the first fork bushing  2441 . With the springs  2451  compressed into the respective pockets  2481  of the base  2494  of the switching device  2490 , the base  2494  may be slidably coupled onto the first fork bushing  2441 , as shown in  FIG. 29C . In some implementations, the first rail  2475 A formed on the bottom portion of the base  2494  may be slidably received in a corresponding first recess  2445 A formed in the first fork bushing  2441 , and the second rail  2475 B of the base  2494  may be slidably received in a corresponding second recess  2445 B formed in the first fork bushing  2441 , to couple the base  2494  of the switching device  2490  to the first fork bushing  2441 . The second fork bushing  2442  may then be installed by fitting, for example, press fitting the first and second axial pins  1471  into the first and second fittings  2442 A,  2442 B, respectively. This may also include slidably coupling the first and second rails  2475 A,  2475 B into corresponding first and second recesses  2447 A and  2447 B in the second fork bushing  2442 . 
       FIG. 30A  is a perspective view of the arrangement of the first and second fork bushings  2441 ,  2442  on the first and second pins  2471 , with the base  2494  removed, so that the alignment of the first and second recesses  2475 A,  2475 B of the first fork bushing  2441  with the first and second recesses  2477 A,  2477 B of the second fork bushing  1442  is more clearly visible.  FIG. 30B  is a perspective view of the arrangement of the first and second fork bushings  2441 ,  2442  on the first and second rails  2475 , with the base  2494  illustrated in shadow, so that the relationship of the first and second recesses  2475 A,  2475 B of the first fork bushing  2441 , the first and second recesses  2477 A,  2477 B of the second fork bushing  2442 , the first and second channels  2461 A,  2461 B of the base  2494 , and the pockets  2481 A,  2481 B in which the springs  2451 A,  2451 B are respectively received, is more easily visible.  FIG. 30C  is a cross sectional view taken along line M-M of  FIG. 29D . 
       FIGS. 31A-31C  illustrate movement of some of the components of the example speed selection mechanism  2400  during an example shifting movement, in accordance with implementations described herein. In particular,  FIG. 31A  illustrates a positioning of components at a first output speed setting,  FIG. 31C  illustrates a positioning of components at a second output speed setting, and  FIG. 31B  illustrates in interim arrangement of the components, as the mechanism  2400  shifts between the first output speed and the second output speed. 
     In the arrangement shown in  FIG. 31A , the selection device  2490  is positioned so as to select a first output speed. At the first output speed, the end of base  2494  is positioned at the line L 1 , and the first and second fork bushings  2441 ,  2441  aligned at the position L 3 , with a position of the selection device  2490  retained by, for example, the engagement of one of the set of detents  2421 / 2422  engaged with a leaf spring fixed in the housing  190  (now shown). In  FIG. 31B , the selection device  2490  has been moved from the position L 2  to the position L 2 . At this interim position, the axial pins  1471  and the first and second fork bushings  2441 ,  2442  have remained stationary, aligned at the position L 3 . In this interim position, corner portions of the pockets  2481 A,  2481 B defined in the base  2494  compress the springs  2451 A,  2451 B against the blocks  2455 A,  2455 B of the first fork bushing  2441 . At this interim position, due to, for example, the relative arrangement of the ring gears  270 ,  280 ,  290 , and in particular the circumferential positioning of the lugs  272 ,  282 ,  292  of the ring gears  270 ,  280 ,  290 , with respect to the engagement lugs  2415  of the shift ring  2410 , it may be that the shift ring  2410  is temporarily blocked, or restricted, from moving axially to engage the desired ring gear  270 ,  280 ,  290 . In response to application of power, one or more of the ring gears  270 ,  280 ,  290  may spin, the axial movement of the shift ring  2410  may no longer be obstructed, and the force of the springs  2451 A,  2451 B will push against the blocks  2455 A,  2455 B of the first fork bushing  2441 , moving the first and second fork bushings  2441 ,  2442  and the axial pins  1471  from the position L 3  to the position L 4 , and allowing the shift ring  2410  to move axially into engagement with the desired ring gear  270 ,  280 ,  290 . 
     Example embodiments have been provided so that this disclosure will be thorough, and to fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     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. 
     Numerous modifications may be made to the exemplary implementations described above. These and other implementations are within the scope of this application.