Patent Publication Number: US-11034006-B2

Title: Pneumatic linear fastener driving tool

Description:
BACKGROUND 
     When working with a material such as wood or concrete, there is a frequent need to attach items to the material for structural, mechanical, plumbing, and electrical installations. Using a linear fastener driving tool makes efficient work when attaching or connecting items for these applications. Linear fastener driving tools are portable, hand-held tools that drive staples, nails or other linearly driven fasteners into a workpiece. 
     Some conventional linear fastener driving tools use a gas spring as the motive force that drives the fastener into a workpiece. In a gas spring driving tool, a cylinder filled with compressed gas is used quickly force a piston through a driving stroke, while a driver that is mechanically connected to the piston drives the fastener into the workpiece. The cylinder discharge, piston stroke and impact of the driver with the fastener are collectively referred to as a driving operation. The piston, and thus also the driver, may be returned to the starting, or “ready” position via a reset mechanism before another driving stroke can be made. During the reset operation, the piston compresses the gas within the cylinder, thereby preparing the linear fastener driving tool for another driving operation. 
     Linear fastener driving tools employ various mechanisms to achieve tool reset, including rack-and-pinion gear systems, secondary pneumatic systems, or cam-driven rotary lifting mechanisms. Such systems can be complex and thus difficult and/or expensive to manufacture while adding significant weight to a portable hand tool. Thus, it is desirable to provide a reset mechanism for a linear fastener driving tool that is relatively simple and more mechanically efficient when compared to known reset mechanisms. 
     SUMMARY 
     In some aspects, a fastener driving tool includes a hollow cylinder having a cylinder longitudinal axis, and a piston disposed in the cylinder in such a way that a) a centerline of the piston is coaxial with the cylinder longitudinal axis, and b) the piston is movable along the cylinder longitudinal axis between a ready position and a driven position. The piston includes a peripheral seal that forms a fluid seal with an inner surface of the cylinder and segregates the cylinder into a first chamber that is configured to contain a pressurized fluid and a second chamber that is open to the atmosphere. The fastener driving tool includes a blade that is at least partially disposed in the second chamber. The blade has a blade first end that is connected to the piston, and a blade second end that is opposed to the first end and configured to contact a fastener during a fastener driving operation. In addition, the fastener driving tool includes a reset mechanism that is configured to translate the piston along the cylinder longitudinal axis. The reset mechanism includes a hollow screw having a screw external thread. An inner surface of the screw defines a passageway that extends between a screw first end and a screw second end that is opposed to the screw first end. The screw has a screw longitudinal axis that extends between the screw first end and the screw second end and is parallel the cylinder longitudinal axis. The reset mechanism includes a nut having a nut internal thread that is engaged with the screw external thread. The nut is configured to engage the piston for certain positions of the nut relative to the screw. The reset mechanism includes a gear that is fixed to the hollow screw in such a way that rotation of the gear results in rotation of the screw about the screw longitudinal axis, and rotation of the screw about the screw longitudinal axis results in translation of the nut relative to the screw. The reset mechanism also includes an actuator that is configured to drive the gear. When the gear is driven by the actuator, the nut engages with and drives the piston toward the ready position via a force that is concentric with the centerline of the piston. 
     In some embodiments, the nut is engaged with the piston via a sleeve that surrounds, and is secured to, an outer surface of the nut. 
     In some embodiments, the sleeve includes a sleeve first end that surrounds, and is secured to, the outer surface of the nut, and a sleeve second end that protrudes outward from the nut and toward the piston. The sleeve second end is configured to directly contact the piston for certain positions of the nut relative to the screw. 
     In some embodiments, the sleeve second end directly contacts the piston along a circle that is centered on the cylinder longitudinal axis. 
     In some embodiments, the fastener driving tool includes a sensor that is configured to determine a position of the sleeve relative to the cylinder. In some embodiments, the sensor is a hall effect sensor that is configured to detect a magnetic element, and the magnetic element is fixed to the sleeve. 
     In some embodiments, the blade extends through the passageway. 
     In some embodiments, the blade has a circular cross-sectional shape. 
     In some embodiments, the blade is concentric with the screw longitudinal axis and freely movable relative to the screw. 
     In some embodiments, the screw external thread and the nut internal thread are directly engaged to provide a lead screw mechanism. 
     In some embodiments, the reset mechanism comprises ball bearings, the screw external threads and the nut internal threads are indirectly engaged via the ball bearings, and the screw, the nut and the ball bearings cooperate to provide a ball screw mechanism. 
     In some embodiments, the nut includes an internal passageway configured to allow recirculation of the ball bearings through the ball screw mechanism. 
     In some embodiments, the nut includes an external passageway configured to allow recirculation of the ball bearings through the ball screw mechanism. 
     In some embodiments, the reset mechanism includes a sear that is supported on the tool. The sear is moveable between an advanced position and a retracted position. In the advanced position, an engaging portion of the sear is engaged with a notch provided in the blade whereby the blade is retained in the ready position. In the retracted position, the engaging portion is disengaged from the notch whereby the blade can be driven to the driven position. The notch is one of multiple notches provided in the blade, each notch providing a unique blade firing position, and each notch corresponds to a unique power output applied to the blade by the driver. 
     In some embodiments, the sear rotates relative to the cylinder about a rotational axis between the advanced position and the retracted position. 
     In some embodiments, the sear is biased toward the advanced position via an elastic member. 
     In some embodiments, the nut is engaged with the piston via a sleeve that surrounds, and is secured to, an outer surface of the nut. The sleeve includes a sleeve first end that surrounds, and is secured to, the outer surface of the nut, and a sleeve second end that protrudes outward from the nut and toward the piston. The sleeve second end is configured to directly contact the piston for certain positions of the nut relative to the screw. In addition, the sleeve has a slot that extends in a direction parallel to the screw longitudinal axis, and a portion of the sear protrudes through the slot. 
     In some aspects, a fastener driving tool includes a hollow cylinder having a cylinder longitudinal axis, and a piston disposed in the cylinder in such a way that a) a centerline of the piston is coaxial with the cylinder longitudinal axis, and b) the piston is movable along the cylinder longitudinal axis between a ready position and a driven position. The piston includes a peripheral seal that forms a fluid seal with an inner surface of the cylinder and segregates the cylinder into a first chamber that is configured to contain a pressurized fluid and a second chamber that is open to the atmosphere. The fastener driving tool includes a blade that is at least partially disposed in the second chamber. The blade has a blade first end that is connected to the piston, and a blade second end that is opposed to the first end and configured to contact a fastener during a fastener driving operation. The fastener driving tool also includes a reset mechanism that is configured to translate the piston along the cylinder longitudinal axis. The reset mechanism includes a hollow screw having a screw external thread. An inner surface of the screw defines a passageway that extends between a screw first end and a screw second end that is opposed to the screw first end. The screw has a screw longitudinal axis that extends between the screw first end and the screw second end and is parallel the cylinder longitudinal axis. The reset mechanism includes a nut having a nut internal thread that is engaged with the screw external thread. The nut is configured to engage the piston for certain positions of the nut relative to the screw. The reset mechanism includes a gear that is fixed to the hollow screw in such a way that rotation of the gear results in rotation of the screw about the screw longitudinal axis, and rotation of the screw about the screw longitudinal axis results in translation of the nut relative to the screw. In addition, the reset mechanism includes an actuator that is configured to drive the gear. The blade extends through the passageway and is freely movable relative to the screw. 
     The pneumatic linear fastener driving tool includes a reset mechanism that resets the tool to the ready-to-fire configuration following a fastener driving operation. More particularly, the reset mechanism translates the piston from a low energy state that is associated with an advanced position of the piston within the cylinder following completion of the driving operation, to a high energy state that is associated with a retracted position of the piston within the cylinder providing stored energy that allows the tool to be driven. 
     The reset mechanism provides several advantages relative to that of some conventional pneumatic linear fastener driving tools. For example, in some embodiments, the reset mechanism uses a hollow ball screw to achieve translation of the piston within the cylinder to a position of maximum energy storage. In addition, the driver blade, which strikes the nail, extends through the hollow ball screw and thus is substantially co-axial with the ball screw axis. This placement allows the driver blade to translate, while the ball screw rotates during the time interval that the piston is being moved to the firing position (high energy position). 
     The hollow ball screw has the distinct advantage of applying a force to the piston that is effectively on the piston centerline, eliminating any side loads, which reduce the drive energy to translate the piston. This also minimizes any piston side loads that could lead to loss of the gas charge above the piston, due to unbalanced lateral seal loading. Advantageously, this configuration also eliminates any cylinder scoring/scratching issues due to undesirable piston to cylinder side loads. 
     The reset mechanism employing a hollow ball screw that applies a force to the piston that is effectively on the piston centerline has advantages when compared to some conventional linear fastener driving tools that accomplish tool reset using eccentric drives or rack and pinions. Such mechanisms have frictional losses, since the force applied to the piston to achieve reset is not purely axial. In addition, some conventional reset mechanisms may have sliding contact between elements in the drive mechanism instead of rolling contact, which contributes to additional frictional losses. The mechanical efficiency of a rolling contact ball screw is high, so frictional losses in the area of highest mechanical loads are minimized. In addition, high mechanical efficiency has the benefit to store more energy in the gas piston, in a shorter reset time. 
     Using a hollow ball screw in the reset mechanism has the advantage of creating a larger pitch diameter, which then allows a reduced thread pitch to be specified. A lower pitch value, effectively becomes a gear reduction and reduces the number of gear stages that are needed between the motor and ball screw. 
     Using a hollow ball screw in the reset mechanism has further advantages. The ball screw is a separate component and is not part of the driver blade. This can be compared to some conventional linear fastener driving tools that are forced to integrate the drive geometry into the driver blade, resulting in an expensive driver blade and adding substantial mass/inertia to driver blade. Also, for the end user, the maintenance cost of such conventional linear fastener driving tools is very high, since the driver blade is a complicated and expensive wear part. By providing a linear fastener driving tool that employs a hollow ball screw, the metallurgical properties of the driver blade can be optimized for impact, while the ball screw can be optimized for cyclic durability. In addition, the proposed driver blade design can follow a conventional manufacturing approach, which has already been optimized in pneumatic linear fastener driving tools. 
     The hollow ball screw includes a bore that provides the longitudinal passageway through which the blade extends. The bore has a circular shape. Since the circular shape of the driver blade can be fitted to the circular passageway in the ball screw, the pathway for concrete dust and other jobsite debris is significant restricted to the piston and cylinder. This reduces dust exposure at the piston and cylinder interface, prolonging the life of the tool. Thus, using a hollow ball screw having a circular bore provides improved durability relative to some conventional linear fastener driving tools that have a substantial pathway for contaminants by including gear teeth or cogs as part of the driver blade. 
     In addition, use of a ball screw in the reset mechanism creates opportunities to improve the safety of the linear fastener driving tool. Since the ball screw motion is independent of the position of the driver blade, the tool control system can control the translating portion of the ball screw to position it in close proximity or in contact with the piston, preventing the fastener firing sequence. This can be beneficial if the linear fastener driving tool is unattended for a period of time or an accelerometer or similar device detects an accidental drop and commands the ball screw to a position that prevents firing. 
     The reset mechanism can also move the piston to intermediate firing positions, creating variable power settings. This feature is desirable to the end user, as it accommodates the variability of concrete hardness, or the ability to drive nails of differing length. Fastening operations in wood and other substrates can also benefit from a power setting adjustment. This can be compared to some conventional linear fastener driving tools that accomplish tool reset using eccentric drives or rack and pinions, and thus are forced to reach a singular firing position and do not have the advantage of intermediate power outputs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view in partial cross section of a pneumatic linear fastener driving tool. 
         FIG. 2  is a cross sectional view of the fastener driving tool as seen along line  2 - 2  of  FIG. 1 , illustrating the fastener driver mechanism and the fastener driver reset mechanism. 
         FIG. 3  is an enlarged cross-sectional view of the reset mechanism of  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the blade. 
         FIG. 5  is a cross-sectional view of an alternative embodiment blade. 
         FIG. 6  is a perspective view of a sleeve of the reset mechanism of  FIG. 3 . 
         FIG. 7  is a schematic illustration of a ball screw device having an internal ball bearing recirculation path. 
         FIG. 8  is a schematic illustration of a ball screw device having an external ball bearing recirculation path. 
         FIG. 9  is a schematic illustration of a lead screw device. 
         FIG. 10  is a cross-sectional view of the blade as seen along line  10 - 10  of  FIG. 4 . 
         FIG. 11  is a cross-sectional view of an alternative embodiment blade. 
         FIG. 12  is a cross-sectional view of another alternative embodiment blade. 
         FIG. 13  is a cross-sectional view of still another alternative embodiment blade. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a linear fastener driving tool  2  is designed to linearly drive fasteners such as nails and staples. The tool  2  includes a handle  4  that forms an upper mid portion of the tool  2 , a fastener driver mechanism  100  that is positioned forward of the handle to provide the front of the tool  2 , and a fastener driver reset mechanism  200  that is disposed below the fastener driver mechanism  100  along the front of the tool  2 . The tool  2  includes a fastener exit portion  10  and a guide body  11  that are disposed below the fastener driver reset mechanism  200 . A battery pack is mounted to a rear side of the handle  4 , and a fastener magazine  6  is disposed below the handle  4  and battery pack  12  so as to communicate with the guide body  11 . An actuator  244  that is used to drive the fastener driver reset mechanism  200  is disposed between the handle  4  and the fastener magazine  6 . The directional nomenclature recited herein such as above, below, front (see reference number  3 ), rear (see reference number  5 ), forward, rearward, upper, lower, etc., is used with respect to orientation of the tool  2  illustrated in  FIG. 1 , and is not intended to be limiting since the tool  2  can be used in other orientations in space without departing from the principles of the present invention. 
     The handle  4  is hollow, and a printed circuit board  14  is disposed in the interior space of the handle  4 . The printed circuit board  14  supports a controller  16 . The handle  4  includes a trigger switch  18  that is activated by a trigger  20 . As can been seen in  FIG. 1 , the handle  4  is designed for gripping by a human hand, and the trigger  20  is designed for actuation by a user&#39;s finger while gripping the handle  4 . The trigger switch  18  provides an input to the controller  16 . There are also other input devices for the controller  16  (not shown). The controller  16  may include a microprocessor or a microcomputer device that acts as a processing circuit. At least one memory circuit will may also be part of the controller  16 , including Random Access Memory (RAM) and Read Only Memory (ROM) devices. To store user-inputted information (if applicable for a particular tool model), a non-volatile memory device may be included, such as EEPROM, NVRAM, or a Flash memory device. 
     The fastener magazine  6  includes a magazine housing  22 , and a fastener track  24  is disposed in the magazine housing  22 . The individual fasteners (for example a nail  32 ,  FIG. 2 ) run along the fastener track  24  while they remain within the magazine  6 . A feeder carriage  26  is disposed in the magazine housing  22 , and is used to feed an individual fastener from the magazine  6  into the drive mechanism area, and a back plate  28  is used to carry an individual fastener while it is being driven. In the illustrated embodiment, the feeder carriage  26  positions a fastener in a location within the guide body  11  that is coincident with the path of a driver member (e.g., a blade  150 , discussed below with respect to  FIG. 2 ) of the fastener driver mechanism  100 , so that when the blade  150  moves through a driving stroke, its driving end will intercept the fastener and carry that fastener to the fastener exit portion  10 , essentially at the bottom portion of the tool&#39;s exit area. 
     The actuator  244  acts as a prime mover for the tool  2 , and has an output that drives a gear set  246 . An output shaft  248  of the gear set  246  drives the fastener driver reset mechanism  200 , as discussed further below. The actuator  244  may be, for example, an electric brushless DC motor. 
     A solenoid  30  is disposed in the vicinity of the output shaft  248  of the gear set  246  that is powered by the battery pack  12  and controlled by the controller  16 . Further details of the operation of the solenoid  30  are discussed below with respect to  FIG. 3 . 
     The battery pack  12  is to the rear of the handle  4 , and provides electrical power for the controller  16 , the actuator  244  and the solenoid  30 . The battery pack  12  is rechargeable. To this end, the battery pack  12  may be selectively removable from the handle  4  to allow recharging within a dedicated charging device. 
     Referring now to  FIG. 2 , the fastener driver mechanism  100  includes a cylinder  102  that provides a portion of a housing of the fastener driver mechanism  100 , a piston  130  that is disposed in the cylinder  102 , and a blade  150  that is fixed to the piston  130 . The elements of the fastener driver mechanism  100  will now be described in detail. 
     The cylinder  102  has a closed cylinder first end  104 , a cylinder second end  106  that is opposed to the cylinder first end  104  and is open to the atmosphere. The cylinder  102  includes a cylinder longitudinal axis  108  that extends along a centerline of the cylinder  102  and through the first and second ends  104 ,  106 . 
     The piston  130  is disposed in the cylinder  102  so as to translate along the cylinder longitudinal axis  108 . The piston  130  is prevented from exiting the cylinder second end  106  via an annular, stationary stop  116  disposed adjacent the cylinder second end  106 . The piston  130  is generally disk shaped, and has opposed piston first and second surfaces  132 ,  134  that are oriented perpendicular to the cylinder longitudinal axis  108 . The piston peripheral edge  136  includes a groove  138  that extends about the circumference of the piston  130 , and an annular, elastic seal  140  is disposed in the groove  138 . The piston  130 , including the seal  140 , is shaped and dimensioned to form a fluid tight seal with an inner surface  110  of the cylinder  102 . As a result, the piston  130  segregates the interior space of the cylinder  102  into a first fluid chamber  112  that is disposed between the cylinder first end  104  and the piston  130 , and a second fluid chamber  114  that is disposed between the piston  130  and the cylinder second end  106 . The first fluid chamber  112  is fluid-tight, while the second fluid chamber  114  is open to the atmosphere. 
     The piston  130  is moveable within the cylinder  102  along the cylinder longitudinal axis  108  between a first, retracted position (shown in  FIG. 2  in broken lines and identified by reference number “ 130 ( 1 )”) and a second, advanced position (shown in  FIG. 2  in broken lines and identified by reference number “ 130 ( 2 )”). In the first position  130 ( 1 ), the piston  130  is disposed between the cylinder first end  104  and a mid point  105  of the cylinder  102 . In this position, the fluid, for example a gas such as air, nitrogen or other appropriate compressible fluid, is compressed between the piston  130  and the cylinder first end  104 , providing a gas spring that is at maximum energy. The first position  130 ( 1 ) is also referred to as the “ready” position. In the second position  130 ( 2 ), the piston  130  is disposed between the mid point  105  of the cylinder  102  and the cylinder second end  106 . In particular, the piston  130  abuts the stop  116 , and the gas spring is at a minimum energy. Since the piston  130  is movable along the cylinder longitudinal axis  108 , the first and second fluid chambers  112 ,  114  do not have a fixed volume. Rather, the volumes of the first and second fluid chambers  112 ,  114  vary as the piston  130  moves longitudinally. In addition, although the pressure of the fluid within the second fluid chamber  114  is at atmospheric pressure for all positions of the piston  130 , the pressure of the fluid in the first fluid chamber  112  increases as the piston  130  moves toward the first position  130 ( 1 ), and is a maximum when the piston  130  is in the first position  130 ( 1 ). 
     Referring to  FIGS. 3 and 4 , the blade  150  is fixed to the piston second surface  134  (e.g., the surface that faces the cylinder second end  106 ), and serves as the portion of the fastener driver mechanism  100  that contacts the fastener  32  and drives the fastener  32  into a workpiece  34 . The blade  150  is an elongate, solid cylindrical rod having a blade first end  152  that is joined to the piston  130  via, for example, a threaded connection, and a blade second end  154  that is opposed to the blade first end  152 . The blade  150  includes a blade longitudinal axis  156  that extends between the blade first and second ends  152 ,  154 , and is co-linear with the cylinder longitudinal axis  108 . 
     The blade first end  152  includes an external thread  152   a  that engages with a corresponding internal thread  142   a  provided in a central blind hole  142  provided in the piston second surface  134 . The external thread  152   a  terminates at an integrally-formed annular protrusion  152   b  that abuts the piston second surface  134  when the blade  150  is fully engaged with, and secured to, the piston  130 . 
     The blade second end  154  terminates in a blunt tip  158  that is perpendicular to the blade longitudinal axis  156  and provides a fastener contact surface during a driving operation of the tool  2 . 
     The blade  150  has a circular cross-section and a diameter that varies along the blade longitudinal axis  156 . In particular, the blade  150  includes a blade first portion  153  that adjoins the blade first end  152  and has a blade first diameter d 1 , and a blade second portion  155  that adjoins the blade second end  154  and has a blade second diameter d 2  that is smaller than the blade first diameter d 1 . A blade shoulder  159  is provided at the transition between the blade first and second diameters d 1 , d 2 . 
     Referring to  FIGS. 4 and 5 , the blade first portion  153  includes a circumferential notch  160 . The notch  160  is shaped and dimensioned to engage with a portion of a latch mechanism  300 . The latch mechanism  300  is used to retain the piston  130  in the retracted first position  130 ( 1 ) once the piston  130  has been positioned in the first position  130 ( 1 ), for example in readiness for a driving operation of the tool  2 . The latch mechanism  300  is described in detail below. Although the blade  150  illustrated in  FIGS. 3 and 4  includes a single notch  160 , it is understood that the blade  150  may include a greater number of notches  160 . For example,  FIG. 5  illustrates an alternative embodiment blade  450  that includes three notches  160 ,  162 ,  164 . When the tool  2  employs the alternative embodiment blade  450  that includes multiple notches  160 ,  162 ,  164 , the piston  130  is capable of being positioned in a maximum energy position (e.g., the first position  130 ( 1 )), or in one of two intermediate positions provided between the minimum energy position (e.g., the second position  130 ( 2 )) and the maximum energy position, creating variable power settings for the tool  2 . This feature is desirable to the end user, as it accommodates the variability of concrete hardness, or the ability to drive nails of differing length. Fastening operations in wood and other substrates can also benefit from a power setting adjustment. 
     The blade second portion  155  has a circular cross sectional, shape and is of uniform outer diameter. 
     The blade  150  is configured, for example via conventional forming and treating processes, to accommodate the frequent, high-load impacts associated with driving fasteners into substrates (such as wood, concrete, etc.) having a range of hardnesses. 
     Referring to  FIGS. 3 and 6-8 , the fastener driver reset mechanism  200  is configured to translate the piston  130  along the cylinder longitudinal axis  108  from the second position  130 ( 2 ) to the ready position. In the embodiment illustrated in  FIG. 3 , the ready position corresponds to the first position  130 ( 1 ). In other embodiments, the ready position may correspond to the first position  130 ( 1 ), or to an intermediate position associated with the one of the intermediate notches  162 ,  164  as selected by the user. 
     The fastener driver reset mechanism  200  includes a ball screw device  202 , and a driven gear  216  that is fixed to a screw  204  of the ball screw device  202 , and is mechanically connected to the actuator  244  via the gear set  246 . In addition, the fastener driver reset mechanism  200  includes a sleeve  260  that is disposed on a nut  220  of the ball screw device  202  and protrudes outward from the nut  220  toward the piston  130 . The elements of the fastener driver reset mechanism  200  will now be described in detail. 
     The ball screw device  202  includes the screw  204 , the nut  220  that is driven by the screw  204 , and ball bearings  230  that provide a mechanical interface between exterior threads of the screw  204  and interior threads of the nut  220 . The screw  204  is mounted via a bearing  219  to a housing  40  of the tool  2  for rotation relative to the cylinder  102 . 
     The screw  204  is an elongate, hollow element that includes an open screw first end  206  and an open screw second end  208 , where the screw second end  208  is opposed to the screw first end  206 . In addition, the screw  204  has a screw longitudinal axis  214  that extends between the opposed first and second ends  206 ,  208 . The screw longitudinal axis  214  is parallel to, and co-linear with, both the cylinder longitudinal axis  108  and the blade longitudinal axis  156 . 
     The screw  204  has a screw external thread  210  that extends from the screw first end  206  to a location that is closely spaced to the screw second end  208 . Between the screw external thread  210  and the screw second end  208 , the screw outer surface is thread-free. A protrusion  209  extends around at least a portion of the circumference of the screw  204  in the thread-free region. The protrusion  209  serves as a key that retains the driven gear  216  on the screw second end  208 . 
     The screw  204  has an inner surface  212  that provides a cylindrical passageway  213  that extends between the screw first end  206  and the screw second end  208 . The passageway  213  has a passageway first portion  205  that adjoins the screw first end  206  and has a first diameter p 1 , and a passageway second portion  207  that adjoins the screw second end  208  and has a second diameter p 2 . The passageway second diameter p 2  is less than the passageway first diameter p 1 , and a passageway shoulder  215  is provided at the transition between the passageway first and second diameters p 1 , p 2 . A mid-portion of the blade  150  is disposed in the passageway  213  in such a way that the blade  150  translates freely along the screw longitudinal axis  214 . In some embodiments, when the piston  130  is in the second position  130 ( 2 ), the blade shoulder  159  abuts the passageway shoulder  215 , or is closely spaced relative to the passageway shoulder  215 . 
     The driven gear  216  is fixed to the screw second end  208  in such a way that rotation of the driven gear  216  results in rotation of the screw  204  about the screw longitudinal axis  214 . For example, in the illustrated embodiment, the protrusion  209  is embedded in the driven gear  216  whereby the driven gear  216  is secured to the screw  204 . The driven gear  216  has external teeth  218  that are engaged with a drive gear  249  of the gear set  246 , whereby the driven gear  216  is actuated by the actuator  244 . Along with the screw  204 , the driven gear  216  is supported for rotation relative to the housing  40  of the tool  2  by the bearings  219 , which are disposed along an inner circumference of the driven gear  216 . 
     Although the screw  204  is rotatable about the screw longitudinal axis  214  that is co-linear with the cylinder longitudinal axis  108 , the screw  204  does not translate within the tool  2 . In addition, the blade  150  translates in a reciprocating motion within the passageway  213  in accordance with alternating driving and resetting operations of the tool  2 , as discussed in detail below. 
     The nut  220  is an elongate, hollow element having a nut internal thread  226  that is engaged with the screw external thread  210  via the ball bearings  230 . In some embodiments, the ball bearings  230  are recirculated to the nut  220  internally, for example via internal passages  232  provided within the nut  220 ′ ( FIG. 7 ). In other embodiments, the ball bearings  230  are recirculated to the nut  220  externally, for example via external passages  234  overlying an outer surface of the nut  220 ″ ( FIG. 8 ). 
     The nut  220  has a longitudinal dimension that is much smaller than that of the screw  204 , and rotation of the screw  204  about the screw longitudinal axis  214  results in translation of the nut  220  relative to the screw  204  along the screw longitudinal axis  214 . 
     The sleeve  260  is a rigid, hollow cylindrical element that is supported on the nut  220 . The sleeve  260  has a first end  262 , and a second end  264  that is opposed to the first end  262 . The sleeve  260  has a longitudinal dimension that is greater than the longitudinal dimension of the nut  220 , whereby the sleeve first end  262  protrudes outward from the nut  220  toward the piston  130 . 
     The sleeve  260  has a generally uniform wall thickness (e.g., a uniform radial dimension) and a diameter that varies longitudinally. In particular, the sleeve  260  includes a sleeve first portion  288  that adjoins the sleeve first end  262  and has a sleeve first diameter s 1 , and a sleeve second portion  290  that adjoins the sleeve second end  264  and has a sleeve second diameter s 2  that is greater than the sleeve first diameter s 1 . A sleeve shoulder  292  is provided at the transition between the sleeve first and second diameters s 1 , s 2 . 
     The sleeve second portion  290  surrounds, and is fixed relative to, the nut  220 . To this end, the sleeve second diameter s 2  is set so that the sleeve second portion  290  receives the nut  220  therein in a closely-fit manner. In some embodiments, the nut  220  is press fit within the sleeve second portion  290 . In other embodiments, sleeve  260  is molded-in-place on the nut  220  in an injecting molding process. In the illustrated embodiment, the entirety of the nut  220  is disposed within the sleeve second portion  290 , but the sleeve  260  is not limited to this configuration. For example, in some embodiments (not shown), the sleeve second portion  290  may enclose only the nut first end  222 . 
     The sleeve first portion  288  protrudes from the sleeve second portion  290  toward the piston  130 . The sleeve first diameter s 1  is less than an outer diameter of the nut  220  and greater than an inner diameter of the nut  220 . The sleeve first diameter s 1  is set so that there is a gap between the sleeve inner surface  266  and the screw  204  whereby the sleeve  260  can move freely relative to the screw  204 . In addition, the sleeve first diameter s 1  is set so that the sleeve first end  262  can pass through the opening defined by the stop  116  provided at the cylinder second end  106 . 
     The sleeve first portion  288  has a slot  280  that extends in a direction parallel to the screw longitudinal axis  214 , and opens at the sleeve first end  262 . In the illustrated embodiment, the slot  280  extends longitudinally to the sleeve shoulder  292 , and circumferentially along an arc having an arc length in a range of about 60 degrees to 90 degrees. The slot  280  allows a sear  302  of the latch mechanism  300  to extend into an interior space of the sleeve  260  and engage with the blade  150 , as discussed further below. To this end, the sleeve  260  is oriented on the nut  220  so that the slot  280  faces the latch mechanism  300 . 
     As previously discussed, rotation of the screw  204  about the screw longitudinal axis  214  results in translation of the nut  220  relative to the screw  204  along the screw longitudinal axis  214 . Because the sleeve  260  is fixed to the nut  220 , the sleeve  260  also translates along the screw longitudinal axis  214  upon rotation of the screw  204 . In certain positions of the nut  220  relative to the screw  204 , and the outer end face  294  of the sleeve first portion  288  abuts the piston second surface  134 . In particular, the outer end face  294  directly contacts the piston  130  along a circle that is centered on the cylinder longitudinal axis  108 . Once the sleeve  260  has engaged the piston  130 , further rotation of the screw  204  causes the sleeve  260  to drive the piston  130  along the cylinder longitudinal axis  108  toward the piston first position  130 ( 1 ). Because the cylinder  102 , the blade  150 , the screw  204 , the nut  220  and the sleeve  260  are all concentric, when the driven gear  216  is driven by the actuator  244 , the nut  220  engages with and drives the piston  130  (via the sleeve  260 ) toward the ready position via a force that is concentric with the centerline of the piston  130 . 
     In some embodiments, a sensor  282  is provided in the vicinity of an outer surface of the sleeve  260 . The sensor  282  is configured to determine a position of the sleeve  260  relative to the cylinder  102 . The sensor  282  may be any type of sensor that may be configured to determine a position of the sleeve relative to the cylinder, such as a mechanical contact sensor, an optical sensor, etc. In the illustrated embodiment, the sensor  282  is a Hall effect sensor that is configured to detect a magnetic element  284 , and the magnetic element  284  is fixed to an outer surface of the sleeve  260 . Sensor output is directed to the controller  16 . In some embodiments, the controller  16  will stop the actuator  244  when the piston  130  has reached the desired ready position. It is understood that the controller  16  may also receive the output of other sensors in addition to, or as an alternative to, the position sensor  282  described here. For example, in other embodiments, the controller  16  may be configured to stop the actuator  244  upon detection that the first chamber  112  has reached a pre-determined pressure as detected by a pressure sensor (not shown) within the first fluid chamber  112 . 
     Once the piston  130  has been moved to the ready position by the fastener driver reset mechanism  200 , the latch mechanism  300  is employed to retain the piston  130  in the desired ready position until the tool  2  is fired by the user. The latch mechanism  300  includes a sear  302  that is mounted to the tool housing  40  (or an adjacent ancillary element of the tool  2 ) via a pivot pin  312 . The sear  302  is a rigid, generally “L” shaped structure that is configured to selectively engage with, and be disengaged from, the notches  160 ,  162 ,  164  provided in the blade  150 ,  450 . 
     The sear  302  includes an engaging portion  304  that constitutes one “leg” of the “L” shaped structure, and a pivot arm  306  that constitutes the other “leg” of the “L” shaped structure. A terminal end  308  of the engaging portion  304  is shaped and dimensioned to engage with the notches  160 ,  162 ,  164 . For example, in the illustrated embodiment, the terminal end  308  is beveled to conform to the contour of the notches  160 ,  162 ,  164 . The pivot arm  306  is angled relative to the engaging portion  304 . An end of the pivot arm  306  that is distant from the engaging portion  304  has an opening that receives the pivot pin  312 . 
     The sear  302  is rotatable about the pivot pin  312  between an advanced position ( FIG. 3 ) and a retracted position (not shown). When the sear  302  is in the advanced position, the engaging portion  304  extends through the slot  280  and the terminal end  308  of the engaging portion  304  is engaged with the notch  160 , whereby the blade  150  is retained in the ready position. When the sear  302  is in the retracted position, the terminal end  308  of the engaging portion  304  is disengaged from the notch  160  whereby the blade  150  is free to move longitudinally, and the piston  130  can be driven by the fastener driver mechanism  100  to the second position  130 ( 2 ). 
     The latch mechanism  300  includes an elastic member  314  such as a spring that extends between the pivot arm  306  and the tool housing  40 . The sear  302  is biased toward the advanced position via the spring force of the elastic member  314 . 
     The latch mechanism  300  is mechanically connected to the solenoid  30  via a link arm  310 , and the position of the sear  320  relative to the blade  150  is controlled by the controller  16  via the solenoid  30 , as discussed further below. 
     The fastener driver mechanism  100  is used to perform a driving operation of the tool  2 . In use, two independent actions are performed by the user to actuate the fastener driver mechanism  100 . In some embodiments of the invention, the two independent actions can occur in either order. In other embodiments, there is also an optional “restrictive mode” of operation, in which the two independent actions occur in a specific order. The two independent actions are 1) pressing the nose  13  of the guide body  11  against a solid surface (e.g., the workpiece  34 ), and 2) depressing the trigger  20 . The trigger  20  will cause the trigger switch  52  to change state, which is one condition that will allow current to be sent to the actuator  244 . As the nose  13  is pushed against the workpiece  34 , this condition is detected by another sensor, for example a limit switch (not shown). When both the pressing and depressing conditions occur simultaneously, the controller will energize the solenoid  30 , which will rotate the sear  302  about the pivot pin  312  a small angular distance clockwise to the retracted position, whereby the sear first end  304  disengages from the notch  160 , where the term “clockwise” is used with respect to the orientation of  FIG. 3 . Immediately upon withdrawal of the sear first end  304 , from the notch  160 , the piston  130  is driven from the first position  130 ( 1 ) to the second position  130 ( 2 ) via the energy stored in the first fluid chamber  112 . As the piston  130  is moved from the first position  130 ( 1 ) to the second position  130 ( 2 ), the blade  150  is quickly driven through the guide body  11  toward the fastener exit portion  10 . As the blade  150  moves through the guide body  11 , the tip  158  intercepts the fastener  32  and carries the fastener  32  to the fastener exit portion  10 , where it exits the tool  2  and is propelled into the workpiece  34 . 
     Following the driving operation, the fastener driver reset mechanism  200  is used to return the piston  130  from the advanced, low-energy second position  130 ( 2 ) to the retracted, high energy first position  130 ( 1 ) so that the tool is ready for the next firing (driving) stroke. In particular, the sear  302  remains in the retracted position while the actuator  244  drives the driven gear  216  via the gear set  246 , which results in rotation of the screw  204  about the screw longitudinal axis  214 , and translation of the nut  220  toward the piston  130 . Upon sufficient rotation of the screw  204 , the sleeve  260  engages the piston  130  and pushes it into the first position  130 ( 1 ). When the piston  130  has been returned to the first position  130 ( 1 ), the controller de-energizes the solenoid  30 , allowing the sear  302  to move to the advanced position where it is engaged with the notch  160 . In some embodiments, the actuator  244  is operated in a reverse direction to return the nut  220  and sleeve  260  to a position outside the cylinder  102  in readiness for the next driving operation. 
     Referring to  FIG. 9 , although the fastener driver reset mechanism  200  disclosed herein employs a ball screw device  202  to drive the piston  130  from the second position  130 ( 2 ) to the first position  130 ( 1 ), the fastener driver reset mechanism  200  is not limited to using the ball screw device. For example, in some embodiments, the fastener driver reset mechanism  200  employs a lead screw device  502 . The term ‘lead screw device’ as used herein refers to a device that is similar to a ball screw, and in which the nut  220 ″′ directly engages the screw  204 , and ball bearings are omitted. Substituting a lead screw device for the ball screw device provides a mechanism that is smaller, lower cost, as well as quieter and smoother than some comparable ball screw devices, but which has lower efficiency due to frictional losses and supports relatively lighter loads. 
     Referring to  FIGS. 10-13 . in the illustrated embodiment, the blade  150  is described herein as being a solid cylindrical rod, and the blade second portion  155  has a circular cross-sectional shape ( FIG. 10 ). However, the blade second portion  155  is not limited to having a circular cross-sectional shape, and can have other cross-sectional shapes to accommodate specific types of fasteners. For example, an alternative embodiment blade  250  includes a blade second portion  255  that has a rectangular cross-section ( FIG. 11 ), which may be advantageous when the fastener being driven is a staple. Another alternative embodiment blade  350  includes a blade second portion  355  that has a crescent shaped cross-section ( FIG. 12 ), which may be advantageous when the fastener being driven is a nail having a clipped-head. Yet another alternative embodiment blade  450  includes a blade second portion  455  having a “tee” cross-sectional shape ( FIG. 13 ), which may be advantageous when the fastener being driven is a framing nail. 
     In the illustrated embodiments, the sleeve  260  is formed separately from the nut  220 , and then is assembled therewith. However, in other embodiments, the sleeve  260  and the nut  220  may be formed integrally so as to constitute a single element. In still other embodiments, the sleeve  260  may be omitted, and the nut  220  includes an annular projection that protrudes from the piston-facing end of the nut  220 . In this embodiment, the annular projection serves to directly contact the piston during the reset operation. 
     In the illustrated embodiment, the cylinder  102  is a hollow right cylinder of uniform diameter. However, the cylinder  102  is not limited to this configuration. For example, in some embodiments, working portions of the cylinder  102  that are below the upper limit of piston travel (e.g., portions of the cylinder  102  that are below the first position  130 ( 1 ) and correspond to portions of the cylinder through which the piston  130  travels) have a uniform diameter, while portions of the cylinder  102  that are above the upper limit of piston travel, and provide a stored volume of gas, can be contained in chamber of any shape. In some embodiments, the cylinder may have a concentric auxiliary chamber that surrounds, and is in fluid communication with, the working portions of the cylinder  102 . In other embodiments, the cylinder may include an auxiliary chamber of irregular shape that is attached to, and in fluid communication with, the working portions of the cylinder  102 . In still other embodiments, the cylinder  102  may include one or more fixed volume storage chambers that are offset from the working portions of the cylinder  102 . Alternatively, the storage chamber(s) may be connected to the working portions of the cylinder  102  with a hose or tube, as long as the hose or tube has sufficient cross-section to allow for rapid gas flow. The location and sizing of the storage chamber may be optimized for the optimum tool ergonomics and balance. 
     Although the latch mechanism  300  is described herein as being actuated by the solenoid  30 , the latch mechanism  300  is not limited to this configuration. For example, in some embodiments, the latch mechanism  300  may be mechanically actuated, and a small solenoid used as a safety mechanism to allow the sear  302  to be actuated. In other embodiments in which the tool  2  relatively small, a magnetic latch could be used instead of the solenoid  30 . 
     Selective illustrative embodiments of the pneumatic linear fastener driving tool and fastener driver reset mechanism are described above in some detail. It should be understood that only structures considered necessary for clarifying the tool and reset mechanism have been described herein. Other conventional structures, and those of ancillary and auxiliary components of the tool and reset mechanism, are assumed to be known and understood by those skilled in the art. Moreover, while working examples of the tool and reset mechanism have been described above, the tool and reset mechanism are not limited to the working examples described above, but various design alterations may be carried out without departing from the tool and reset mechanism as set forth in the claims.