Abstract:
An automatic shutoff mechanism for a pneumatic tool having an air motor, a pressurized airflow path to the motor and a torquing mechanism driven by the motor, includes a valve member moveable between open and closed positions relative to the airflow path and biased to the open position, and a trip apparatus responsive to application of a predetermined torque for moving the valve member to a trip position disposed in the airflow path sufficiently to expose the valve member to the pressurized airflow for driving the valve member from the trip position to the closed position. Mechanical and electromechanical embodiments are disclosed.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of the filing date of copending U.S. Provisional Application No. 60/379,071, filed May 9, 2002. 
     
    
     
       BACKGROUND  
         [0002]    This application relates to pneumatic tools and, in particular, to control mechanisms therefore. The application relates specifically to shutoff mechanisms for disconnecting a pneumatic motor from a supply of pressurized pneumatic fluid.  
           [0003]    Pneumatically operated tools of varying types are known, including a wide variety of pneumatically-operated hand tools. Many such tools are designed for torque application to a workpiece and may include devices such as screw or nut driving tools, impact wrenches and the like. Such tools are typically provided with a trigger valve mechanism to manually control the flow of pressurized pneumatic fluid, typically air, to an air motor. Some pneumatic tools are also provided with automatic shutoff mechanisms, responsive to a particular event or condition, such as the application of a predetermined torque level. Such prior shutoff arrangements have typically been rather complex, bulky, expensive, relative slow acting and/or difficult to adjust.  
         SUMMARY  
         [0004]    There is disclosed herein an improved technique for automatic shutoff of a pneumatic tool.  
           [0005]    The technique includes use of a valve member biased to a normal open position and a trip apparatus responsive to application of a predetermined torque by the tool for moving the valve member into the pressurized airflow path a distance sufficient that the airflow itself will then drive the valve member to a closed position, shutting off airflow to the motor.  
           [0006]    In a mechanical embodiment of the shutoff mechanism, the trip assembly includes an inertia member coaxial with the motor rotor shaft and a helical coupling between the inertia member and the rotor shaft such that they rotate together at constant velocity, but that upon rapid deceleration of the rotor shaft the inertia member moves rotatably and axially relative to the rotor shaft to a position spaced from the valve member a distance inversely proportional to the torque applied by the tool, the trip assembly moving the inertia member into engagement with the valve member upon application of the predetermined torque.  
           [0007]    Where the pneumatic tool is an impact tool, in one mechanical embodiment of the shutoff mechanism the inertia member is biased to a home position spaced a maximum distance from the valve member and, in response to each impact, moves toward the valve member a distance proportional to the torque applied and then back to the home position.  
           [0008]    In another embodiment, the trip assembly includes a clutch mechanism responsive to movement of the inertia member from its home position for preventing its return to the home position until the valve has been tripped, and preventing premature tripping upon transition from free run down of a fastener to initial torque resistance.  
           [0009]    In another embodiment, the shutoff mechanism is electromechanically operated, the valve member being a solenoid actuated in response to a torque sensing device.  
           [0010]    There is also a disclosed method for automatically shutting off a pneumatic torque-applying tool when a predetermined torque is reached by disposing a valve member adjacent to the pressured airflow path upstream of the motor and, when the predetermined torque is reached, moving the valve member from its open position to a trip position disposed in the airflow path and spaced from the open position a distance such that the valve member is exposed to a pressured air load which drives it to the closed position. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For the purpose of facilitating an understanding of the subject matter sought to be protected, there are illustrated in the accompanying drawings embodiments thereof, from an inspection of which, when considered in connection with the following description, the subject matter sought to be protected, its construction and operation, and many of its advantages should be readily understood and appreciated.  
         [0012]    [0012]FIG. 1 is a vertical sectional view of a pneumatic impact tool having an automatic shutoff mechanism in accordance with a first embodiment;  
         [0013]    [0013]FIG. 2 is an enlarged, fragmentary view of a rear portion of FIG. 1 showing the automatic shutoff mechanism;  
         [0014]    [0014]FIG. 3 is a fragmentary front perspective view in partial section of the automatic shutoff mechanism of FIG. 2;  
         [0015]    [0015]FIG. 4 is a further enlarged, exploded, front perspective view of the trip assembly of the automatic shutoff mechanism of FIG. 2;  
         [0016]    [0016]FIG. 5 is a side elevational view of the valve plate of the shutoff mechanism of FIG. 3;  
         [0017]    [0017]FIG. 6 is a rear elevational view of the valve plate of FIG. 5;  
         [0018]    [0018]FIG. 7 is a front elevational view of the valve plate of FIG. 5;  
         [0019]    [0019]FIG. 8 is a front perspective view of the valve seat of the shutoff mechanism of FIG. 3;  
         [0020]    [0020]FIG. 9 is a side elevational view of the valve seat of FIG. 8;  
         [0021]    [0021]FIG. 10 is a front perspective view of the end plate of the shutoff mechanism of FIG. 3;  
         [0022]    [0022]FIG. 11 is a view similar to FIG. 2 of another embodiment of automatic shutoff mechanism;  
         [0023]    [0023]FIG. 12 is an enlarged view similar to FIG. 3 of the automatic shutoff mechanism of FIG. 11;  
         [0024]    [0024]FIG. 13 is an enlarged, exploded, front perspective view of the trip assembly of the automatic shutoff mechanism of FIG. 11;  
         [0025]    [0025]FIG. 14 is a view similar to FIG. 12 of another embodiment of automatic shutoff mechanism.  
         [0026]    [0026]FIG. 15 is a rear perspective view of a modified end plate for use with the automatic shutoff mechanism of FIG. 14; and  
         [0027]    [0027]FIG. 16 is a view similar to FIG. 14 of another embodiment of automatic shutoff mechanism. 
     
    
     DETAILED DESCRIPTION  
       [0028]    Referring to FIG. 1, there is illustrated a pneumatic torquing tool  10  in the nature of an impact tool, having a housing  11  with an elongated, generally cylindrical barrel portion  12  and a depending handle portion  13  cooperating to define a known pistol grip configuration. The distal end of the handle portion  13  is adapted to be coupled to a source of pressurized pneumatic fluid, such as air, in a known manner, the airflow to the motor being controlled by a known trigger valve assembly  14 . Mounted in the barrel portion  12  of the housing  11  is a known air motor  15  having a cylinder  16  surrounding a rotor  17  provided with a plurality of circumferentially spaced and radially extending vanes, the front and rear ends of the motor  15  being respectively closed by front and rear end plates  18  and  19 , again all in a known manner. Coupled to the forward or output end of the rotor  17  is an output mechanism  20 , which, in the illustrated embodiment, includes a torquing mechanism in the nature of a known impact mechanism  21 , which may be of the double dog type. The output mechanism  20  also includes an output member  22  which is connected to the impact mechanism  21  and is adapted for coupling to a suitable drive tool, such as a socket, for coupling to an associated fastener or other work piece to which torque is to be applied, again all in a known manner. The rear end of the rotor  17  defines a stub shaft  23  journaled in a bearing  24 . The tool  10  may also be provided with a reversing valve assembly  25 , again of a known construction, for cooperation with the trigger valve assembly  14  to control the direction of rotation of the air motor  15 . In operation, the valve assemblies  14  and  25  channel the input pressurized airflow through passageways to the rear of the housing  11 , where the airflow enters the air motor  15 , exiting at the forward end thereof. The passages permit the pressured air stream to enter the rear of the air motor  15  at different locations, depending upon the condition of the reversing valve assembly  25 , as will be explained more fully below.  
         [0029]    Referring now also to FIGS.  2 - 9 , the pneumatic tool  10  is provided at its rear end with an automatic shutoff mechanism, generally designated by the numeral  30  (FIGS. 1 and 2) for automatically shutting off the air motor  15  upon the occurrence of a predetermined event, such as the development of a predetermined reactive force on the tool which, in the embodiment of FIGS.  1 - 9 , corresponds to the application of a predetermined torque to the associated work piece. The shutoff mechanism  30  includes a valve plate  31  having a front face  32  and a rear face  33  (see FIGS.  5 - 7 ), with an inlet port  34  extending therethrough between the two faces. Formed in the rear face  33  and communicating with the port  34  is a generally Y-shaped groove  35 , the arms of which partially encircle a central cylindrical bore  36  formed through the plate  31  and provided at the rear face  33  with a first relatively deep counterbore  37  and a larger-diameter shallow counter bore  37   a.  The bore  36  is also provided with a counterbore  37   b  in the front face  32  (FIG. 7). Formed through the plate  31  adjacent to its upper end is an arcuate port  38 . Formed in the front face  32  of the valve plate  31  is a generally question mark-shaped groove  39 , which partially encircles the central bore  36  for reversing the direction of the air motor  15 .  
         [0030]    The shutoff mechanism  30  also includes a valve seat  40  (FIG. 8) which is in the nature of a relatively thin plate having a front face  41  which is disposed in use against the rear face  33  of the valve plate  31  substantially congruent therewith (see FIGS. 2 and 3). Formed through the valve seat  40  are a pair of diametrically opposed, concentric arcuate apertures  43 . Formed in the front face  41 , respectively radially inwardly and outwardly of the arcuate apertures  43 , are concentric circular grooves for receiving O-ring seals  44 . Also formed in the front face  41  is a generally Y-shaped groove  45  which is disposed so as to be matingly congruent with the Y-shaped groove  35  in the valve plate  31  for cooperation therewith to define a channel providing communication between the inlet port  34  and the arcuate apertures  43 . Formed through the valve seat  40  is a circular central bore  46  concentrically inside the inner O-ring seal  44 . Formed through the valve seat  40  adjacent to the upper end thereof is an arcuate aperture  48  disposed for registry with the port  38  in the valve plate  31 .  
         [0031]    The shutoff mechanism  30  also includes an end plate  50  which has a front face  51  (see FIG. 9) disposed in use against the rear face of the valve seat  40  substantially congruent therewith (see FIGS. 2 and 3). Formed in the front face  51  is an arcuate groove  53  which forms a nearly complete circle and terminates in radially outwardly extending legs  54 . The groove  53  is positioned for registry in use with the arcuate apertures  43  in the valve seat  40 , with the ends of the legs  54  being in registry with the arcuate aperture  48  in the valve seat  40 . A central bore  56  is formed through the end plate  50  inside the arcuate groove  53  coaxially therewith for registry with the central bore  46  of the valve seat  40 , the bore  56  being provided in the front face  51  with a shallow counterbore  57 . An end cap  58  is disposed in use against the rear face of the end plate  50  substantially congruent therewith, and has a central bore  59  formed therethrough in registry with the central bore  56  of the end plate  50 . In use, the front face  32  of the valve plate  31  is disposed against the rear end plate  19  of the air motor  15 , being preferably spaced therefrom by a suitable gasket  58   a  (FIGS. 2 and 3). The valve plate  31 , valve seat  40 , end plate  50  and end cap  58  are secured together and to the motor  15  by suitable fasteners  59   a  (one shown in FIG. 1).  
         [0032]    Referring in particular to FIGS.  1 - 4 , the shutoff mechanism  30  also includes a hollow cylindrical plug insert  60  having an internally threaded bore  61  therethrough provided at the forward end thereof with a counterbore  62 . Projecting radially outwardly from the front end of the plug insert  60  is an annular flange  63 . In use, the plug insert  60  is received through the central bores  56  and  59  in the end plate  50  and in the end cap  58 , with the flange  63  seated in the counterbore  57 . A slotted adjusting screw  65  is threadedly engaged in the plug insert  60  and is provided with a radially outwardly projecting annular flange  66  having a circumferential groove  67  therein for receiving an O-ring seal  68  circumferentially sealing the forward end of the adjusting screw  65  against the counterbore  62  of the plug insert  60 . Alternatively, the screw could have a lever that seats in circumferentially spaced detent recesses to facilitate manual adjustment and ensure repeatability of settings.  
         [0033]    The shutoff mechanism  30  also includes a shaft extension  70  having a coupling end  71  with flats formed thereon and mateably receivable in the stub shaft  23  of the motor rotor  17  for rotation therein. Just rearwardly of coupling end  71  is a radially outwardly projecting annular flange  73  which is disposed in the central bore  36  of the valve plate  31  and is encircled by a lip seal  74 . Formed in the outer surface of the shaft extension  70  rearwardly of the flange  73  are a plurality of circumferentially spaced helical grooves  75 , which may be three in number, in each of which is seated a corresponding ball  76 . The shaft extension  70  is coaxially encircled by an annular actuation member in the form of an inertia ring  80 , which has plural helical grooves  81  formed in the inner surface thereof, respectively cooperating with the grooves  75  in the shaft extension  70  for forming helical tracks for the balls  76  and confining the balls therein. Provided on the rear face of the inertia ring  80  is an annular thrust bearing  82 , which is engaged with an annular end flange  83  of a cylindrical thrust washer  84 . The rear end of the cylindrical thrust washer  84  is counterbored to define an annular shoulder  85 , against which is seated one end of a helical compression adjustment spring  86 , the other end of which is seated against the flange  66  of the adjustment screw  65  (see FIG. 2).  
         [0034]    The cylindrical thrust washer  84  extends through the center of an annular valve member in the nature of a disc valve  87 , which seats in the counterbore  37   a  of the valve plate  31 . The disc valve  87  has a annular counterbore  88  formed in the rear face thereof, in which is seated one end of a helical compression reset spring  89 , the rear end of which is seated against the flange  63  of the plug insert  60  (see FIG. 2). It will be appreciated that the disc valve  87  is resiliently retained by the reset spring  89  in a normal open position seated in the valve plate counterbore  37   a.  This spring force also retains the plug insert  60  seated in the end plate counterbore  57 . Also, the thrust washer  84  and the inertia ring  80  are biased forwardly to a normal rest or home position, shown in the drawings, by the adjustment spring  86  with a force which can be varied by the adjustment screw  65 .  
         [0035]    In operation of the air motor  15  in a forward or fastener-tightening direction, when the trigger valve assembly  14  is actuated, pressurized airflow will pass upwardly through the handle portion  13  of the housing, through the open trigger valve assembly  14 , and then rearwardly through the inlet port  34  of the valve plate  31  to the rear face thereof, and then upwardly through the channel formed by the Y-shaped grooves  35  and  45 , as indicated by the arrows in FIG. 2, then rearwardly through the arcuate apertures  43  in the valve seat  40  to the arcuate groove  53  and the end plate  50 , to the ends of the legs  54 , and then back forwardly through the arcuate aperture  48  in the valve seat  40  and the port  38  in the valve plate  31  to the rotor  17  of the air motor. Thus, it can be seen that this pressurized airflow path passes rearwardly of the disc valve  87 , which is seated in its normally open position. The air pressure may serve to assist the reset spring  89  in urging the disc valve  87  to its seated open position in the counterbore  88 .  
         [0036]    As is well known, when a fastener is being run in, there will initially be negligible torque and the motor rotor  17 , shaft extension  70  and inertia ring  80  will all rotate together. As torque builds up, the impact mechanism  21  will begin imparting impulses or impacts to the work piece. With each such impact, the rotor  17  and shaft extension  70  will momentarily stop. However, the inertia ring  80 , which is not fixed to the shaft extension  70 , will try to continue rotating. The continued rotation of the inertia ring  80  relative to the shaft extension  70  will cause the inertia ring  80  to move axially rearwardly by operation of the helical ball-and-groove coupling to the shaft extension  70 , thereby driving the thrust washer  84  axially rearwardly against the urging of the adjustment spring  86 . The extent of the axial movement will be proportional to the amount of torque applied. Immediately after the rotor  17  and the shaft extension  70  resume rotation, the thrust washer  84  and inertia ring  80  will be returned forwardly to their home positions under the urging of the adjustment spring  86 .  
         [0037]    Typically, each successive impact will exert a slightly higher torque than the preceding one. Thus, with each impact of the impact mechanism  21 , the inertia ring  80  will move axially a slightly greater distance rearwardly, returning each time to its home position between impacts. Eventually, when a predetermined torque level is reached, corresponding with the adjustment setting of the adjustment screw  65 , the inertia ring  80  will move rearwardly a sufficient distance that the end flange  84  of the thrust washer  84  will engage the front face of the disc valve  87 , unseating it and pushing it rearwardly from its normal open position a slight distance into the pressurized airflow. This will expose the front face of the disc valve  87  to the pressurized airflow, the pressure of which will then slam the disc valve  87  rearwardly the rest of the way to a closed position, sealed against the O-rings  44  of the valve seat  40 , thereby shutting off airflow through the arcuate apertures  48  in the valve seat  40 , blocking airflow to the air motor  15  and shutting it off. It will be appreciated that the O-rings  44  could be located on the disc valve  87  instead of on the valve seat  40 . As soon as the operator releases the trigger valve assembly  14 , the pressurized airflow from the source will be shut off, relieving the air pressure on the disc valve  87 , and permitting it to return to its normal open position under the urging of the reset spring  89 .  
         [0038]    Thus, automatic shutoff of the tool  10  is accomplished at a predetermined torque level preventing over torquing of the work piece. It is significant that the disc valve  87  need be moved only a very small distance from its normal open position, typically in the range of from about 0.01 inch to about 0.02 inch, to permit the pressurized airflow to take over and drive the disc valve  87  to its closed position, thereby using the pressurized airflow to perform most of the work in overcoming the force exerted by the reset spring  89  and effecting a very rapid shutoff. The shutoff mechanism is easily adjusted to vary the shutoff torque, is very compact, with all parts located at the rear of the air motor, and is relatively inexpensive.  
         [0039]    If the reversing valve assembly  25  is actuated to operate the air motor  15  in a reverse or fastener-loosening direction, the pressurized airflow path will be different, bypassing the shutoff mechanism  30 , which is not needed, since there will be no torque limit to be concerned with. Thus, in this case, the airflow will be directed so that, at the front face  32  of the valve plate  31 , it will not enter the inlet port  34 , but will rather enter the reverse groove  39 , which channels it directly to a reverse-direction inlet port in the motor rear end plate  19  without going past the disc valve  87 .  
         [0040]    Referring now also to FIGS.  11 - 13 , there is illustrated another embodiment of automatic shutoff mechanism, generally designated by the numeral  90 , which utilizes substantially the same valve plate  31 , valve seat  40 , end plate  50  and end cap  58  described above in connection with the automatic shutoff mechanism  30  of FIGS.  1 - 10 , and creates the same airflow paths. The same plug insert  60  and adjusting screw  65  are also used. The shutoff mechanism  90  utilizes a global shaft extension  91  which differs somewhat from the shaft extension  70 , described above. The shaft extension  91  has plural helical grooves  92  formed in the outer surface thereof for respectively receiving balls  93 . However, in this case, each of the helical grooves  92  has a sloping base or root  94 , which is inclined so that the forwardmost end of the groove is further from the rotational axis than the rearwardmost end thereof, as can best be seen in FIG. 11. The shaft extension  91  has a reduced-diameter rearward end  95 , provided at its distal end with a plurality of radially outwardly projecting spokes  96 , which may be three in number, and cooperate to define a slotted annular ring provided with a circumferential groove  97  in its outer surface, in which are seated a washer  98  and retaining ring  99 .  
         [0041]    The shutoff mechanism  90  includes an inertia ring  100  which coaxially encircles the shaft extension  91  and has plural helical grooves  101  formed on the inner surface thereof for cooperation with the grooves  92  in the shaft extension  91  to form helical tracks for the balls  93 . Mounted at the rear end of the inertia ring  100  is a thrust bearing  102  which engages the forward end of a thrust washer  103 , which has at its rearward end a reduced-diameter cylindrical portion which is axially slotted to define a plurality of equiangularly spaced fingers  104 , the inner surfaces of which are counterbored to define a part-annular shoulder  105 .  
         [0042]    The forward end of the adjustment spring  86  seats against the shoulder  105  on the fingers  104  of the inertia ring  100 . The shutoff mechanism  90  also includes a disc valve  106 , which is similar to the disc valve  87  described above and again seats in a normal open position in the counterbore  37   a  of the valve plate  41 . However, the disc valve  106  is provided with a counterbore  107  and with a plurality of equiangularly spaced arcuate apertures  108  therethrough, shaped and dimensioned for respectively receiving therethrough the fingers  104  of the inertia ring  100 . The disc valve  106  is retained in its open position by the reset spring  89  in the same manner as was described above with respect to the disc valve  87 .  
         [0043]    Disposed coaxially within the inertia ring  100  is a cylindrical reset sleeve  110  which has a main body  111  disposed in use coaxially between the helically grooved portions of the shaft extensions  91  and the inertia ring  100 , the main body  111  having plural circumferentially extending slots  112  therein for respectively receiving the balls  93  therethrough. The main body  111  is integral at its rearward end with a radially inwardly extending annular shoulder  113 , which is in turn integral at its radially inner end with a rearwardly projecting, reduced-diameter end portion  114  which has a plurality of equiangularly spaced axial slots  115  formed therein defining fingers  116 , the outer surfaces of which are grooved adjacent to their distal ends for receiving therein a washer  117  and a retaining ring  118 . When assembled, the radial spokes  96  of the shaft extension  91  will respectively project radially outwardly through the slots  115  of the reset sleeve  110 , but remain inside the fingers  104  of the inertia ring  100 , as can best be seen in FIG. 11. A helical compression reset spring  119  encircles the reset sleeve fingers  116 , having one end thereof seated against the washer  117  and the other end thereof seated against the shoulder  113 , for resiliently urging the reset sleeve  110  forwardly against the shoulder  95   a  of the shaft extension  91 .  
         [0044]    The operation of the shutoff mechanism  90  is similar to that of the shutoff mechanism  30 , described above. However, in this case, with each impact of the impact mechanism  21 , when the inertia ring  100  moves axially rearwardly relative to the shaft extension  91 , it will not return to its normal home position before the next impact. Rather, the reset sleeve  110  cooperates with the sloping helical grooves  92  in the shaft extension  91  to operate as a clutch to prevent return of the inertia ring  100  between impacts. More specifically, it can be seen that the reset spring  119  continuously urges the reset sleeve  100  and, thereby, the balls  93 , forwardly, continuously tending to wedge the balls  93  between the radially converging helical grooves  92  and  101 . Thus, in response to an impact, the inertia ring  100  is permitted to move rearwardly through the helical groove-and-ball coupling action described above, but is prevented from returning forwardly to its home position by its wedging action of the balls. Thus, there is a step-wise or additive movement of the inertia ring  100  rearwardly until, when the predetermined torque is reached, the thrust washer  103  engages and unseats the disc valve  106 , which is slammed to its closed position by the pressurized airflow stream in the manner described above. As the disc valve  106  moves to its closed position, it engages the washer  117  on the reset sleeve fingers  116 , pulling the reset sleeve  110  and, thereby, the balls  93 , rearwardly, releasing the clutch wedging action and permitting the inertia ring  100  to return to its home position under the urging of the adjustment spring  86 . The disc valve  106  will be reset after release of the trigger valve assembly  14 , in the same manner as described above.  
         [0045]    Referring now to FIGS. 14 and 15, there is illustrated another embodiment of automatic shutoff mechanism, generally designated by the numeral  120 . Many of the parts of the shutoff mechanism  120  are the same as were used in the shutoff mechanisms  30  and  90 , described above, and common parts in those several embodiments bear the same reference numerals. The shutoff mechanism  120  utilizes a modified end plate  121 , which is similar to the end plate  50 , described above, except that it has a rear face  122  in which is formed a rectangular circuit board recess  123  and an aperture  124  through the end plate  121  for circuit leads. The rear face  122  of the end plate  121  is covered, in use, by an end cap  125  (FIG. 14), which has therein a display window  126  for viewing a display which may form a part of a circuit board mounted in the recess  123 .  
         [0046]    The shutoff mechanism  120  includes a trip assembly  129 , which includes a shaft extension  70 A which is substantially the same as the shaft extension  70 , described above, except that its helical grooves  75 A are disposed adjacent to its distal end rather than adjacent to the flange  73 . An inertia ring  130  encircles the shaft extension  70 A and has helical grooves  131  on its inner surface which cooperate with the grooves  75 A on the shaft extension  70 A to perform helical tracks for balls  76 A, in the manner described above, except that the helices are curved in the opposite direction. The inertia ring  130  has a radially inwardly extending annular end flange  132  at its forward end and has formed axially in the front surface thereof an annular groove  133 . Encircling the shaft extension  70 A adjacent to the flange  73  is an annular thrust washer  134  which is channel-shaped in transverse section and is secured to the valve plate  31  as by screws  135  (one shown). The thrust washer  134  seats a thrust bearing  136 . A helical reset spring  137  has one end thereof seated against the thrust washer  134  and the other end thereof seated in the groove  133  of the inertia ring  130  for resiliently urging the inertia ring  130  rearwardly. A suitable magnetic sensor  138  is seated in a radial cavity  139  in the valve plate  31  immediately above the inertia ring  130 .  
         [0047]    A disc valve  140  is seated in the counterbore  37   a  of the valve plate  31  so that it is spaced a slight distance rearwardly of the inertia ring  130  in its normal home position illustrated in the drawings. Formed in the rear face of the disc valve  140  is an annular spring groove  141  in which is seated one end of a helical reset spring  142 , the rear end of which is seated in a counterbore  143  in the end plate  121  for resiliently urging the disc valve  140  to its normal open position. Disposed in the central bore of the end plate  121  is a solenoid  145 , which has a forwardly extending plunger or shaft  146  which extends through a central opening in the disc valve  140  and is connected to a suitable retainer on the front side of the disc valve  140 . A circuit board  147  is seated in the circuit board recess  123  of the end plate  121  and is electrically connected to the solenoid  145  and to the sensor  138  by suitable leads (not shown). It will be appreciated that the circuit board  147  may include a suitable display which is visible through the display window  126  in the end cap  125 , and may also be provided with suitable input devices, such as a push buttons or the like, which may extend through suitable apertures (not shown) in the end cap  125 .  
         [0048]    In operation, the inertia ring  130  will move axially back and forth in response to impacts delivered by the impact mechanism  21 , in much the same way as was described above in connection with the shutoff mechanism  30 , except that in this case the inertia ring  130  will move forwardly when the rotor extension  70 A stops and will return rearwardly to its home position. These movements will be sensed by the sensor  138 , which will output an electrical signal having a value proportional to the axial extent of the movement, which signal will be compared by a microprocessor or other suitable circuitry on the circuit board  147 , with a preset signal level corresponding to a predetermined torque value, which may be input by the user through the input means described above. When the predetermined torque level is reached, the circuit board  147  will output a signal to the solenoid  145 , which will actuate to pull the disc valve  140  a slight distance rearwardly into the air stream, causing it to slam to a closed position in the manner described above.  
         [0049]    Referring now to FIG. 16, there is illustrated a trip assembly, generally designated by the numeral  150 , which may be substituted for the trip assembly  129  in the shutoff mechanism  120  of FIG. 14. The trip assembly  150  has a modified shaft extension  151  provided at its end with a an axial bore  152  which receives the shaft  146  of the solenoid  145  and its associated coupler. Integral with the shaft extension  151  at its rear end is a radially outwardly extending annular end wall  153  which terminates at its radially outer edge in a forwardly projecting cylindrical flange  154 . Encircling the shaft extension  151  is an annular bobbin sensor  155 , which is a field sensor, which may be a magnetoelastic sensor of the type sold by Magna-Lastic Devices, Inc., or other contactless stress measuring device. The sensor  155  has an annular, radially outwardly extending flange at its forward end which is secured, as by fasteners  156 , to the valve plate  31 . The forward end of the cylindrical flange  154  of the shaft extension  151  may slightly overlap the bobbin sensor  155 . The region of shaft extension  151  within the bobbin sensor  155  is specifically magnetized so that it can generate an electromagnetic field signal which can be sensed by the sensor  155  in a non-contact manner. The sensor  155  detects changes of torque through the magnetization and outputs a signal which is interpreted by the electronics on the circuit board  147  for measuring the amount of force reflected from the impact mechanism  21 , which results in torsional stresses in the shaft extension  155  proportional to the torque applied and sensed by the sensor  155 . The signal generated by the sensor  155  is proportional to the torque applied and is compared by the electronics on the circuit  147  to a predetermined reference torque level and, when they match, the solenoid  145  is actuated in the manner described above. If desired, the achieved torque value could then be displayed on the display of the circuit board  147  and the solenoid  145  is then deactivated, permitting the disc valve  140  to be returned to its normally opened position by the spring  142  when the trigger valve assembly  14  is released. A method of producing a circular magnetized, non-contact torque sensor of the type just described is disclosed in U.S. Pat. No. 5,887,335.  
         [0050]    While, in the illustrated embodiments, the pneumatic tool  10  is a hand tool, it will be appreciated that the automatic shutoff principles disclosed herein would be applicable to other types of pneumatic devices. Also, while the illustrated embodiments are utilized in a torque-applying tool, it will be appreciated that the automatic shutoff principles disclosed herein, particularly those in the electromagnetic embodiments of FIGS.  14 - 16 , could be used in pneumatic tools delivering other types of forces to a work piece, such as pneumatic hammers, chisels and the like. Also, while the illustrated embodiments have been shown as utilized in a torquing tool of the impact type, it will be appreciated that certain of the automatic shutoff principles herein could be utilized with other types of non-impact torquing tools.  
         [0051]    From the foregoing, it can be seen that there has been provided an improved automatic shutoff mechanism for a pneumatic tool which is relatively simple, compact, inexpensive, fast-acting and easy to adjust.  
         [0052]    The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. While particular embodiments have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made without departing from the broader aspects of applicants&#39; contribution. The actual scope of the protection sought is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.