US3596542A

US3596542A - Pneumatic push-start, torque shut-off screw driver

Info

Publication number: US3596542A
Application number: US820749A
Authority: US
Inventors: William K Wallace
Original assignee: Chicago Pneumatic Tool Co LLC
Current assignee: Chicago Pneumatic Tool Co LLC
Priority date: 1969-05-01
Filing date: 1969-05-01
Publication date: 1971-08-03
Anticipated expiration: 1988-08-03
Also published as: DE7010334U; FR2046453A5; DE2013257A1

Abstract

A pneumatic screw driver, the operation of which is automatically started when it is pressed against the work and automatically interrupted when the work has been driven to a predetermined degree of tightness, and the mechanism of which is automatically restored to normal condition when the tool is removed from the work. A clutch assembly which is slidable relative to the tool housing when the tool is pressed against the work, actuates a control valve mechanism by push rod means to cause live air flow to start operation of the tool. During the time the tool is pressed against the work, a torque releasable overriding cam clutch is adapted, upon delivery of a predetermined maximum torque to the work, to disengage and cause the push rod means to actuate the control valve mechanism to interrupt operating air flow to the motor. Whenever the tool is removed from the work, the clutch assembly and valve mechanism automatically return to normal condition. The invention is adapted to be incorporated in a linear type housing as well as in a piston grip type housing.

Description

United States Patent Primary Examiner-Theron E. Condon Assistant Exammer-Horace M Culver www W.N KM, m" um wm m n .m n .n

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ABSTRACT: A pneumatic screw driver, the o which is automatically started when it is work and automatically interrupted when driven to a predetermined degree of tightness, and the mechanism of which is automatically restored to normal condition when the tool is removed from the work. A clutch as sembly which is slidable relative to the tool housin tool is pressed against the work mechanism by push rod means to cause live air flow to start operation of the tool. During the time the tool is against the work adapted, upon delivery of a predetermined maximum tor to the work, to disengage and cause the push rod means to actuate the control valve mechanism to interru 4b.is SR2 5243 W HB7 i U 8 wm y w 8M m. m 1 m E u u u l Q N w R m m E T 0 m m T c Tv m m MA v.. m m m T man a w A 911 r H ma m m mT ou ...,S .mlsY wkn mmm mm y u maumw Pmm mmh. n sMACN ma... www N HD1 U 0 d Mwsw W mm. UEM mmm Pwi ER .M Pa NCC Stk AFPA PS9 UMF HiZisliJ.. M, UHO.. N rZtHU U rS.. U

moved from the the clutch assembly and valve mechanism automatically return to normal condition. The invention is adapted to be i corporated in a linear type housing as well as in a type housing.

715 12/1966 DeGroffetal...............

flow to the motor. Whenever the tool is re work,

3/1963 lWilmerding et al. 754 11/1967 Amtsberg et al..

piston grip aus" .o mo k AS 29 65 99 ll 55 35 27 60 46 38 08 32 vivi PMENTED AUG 3 ISYI SHEU l UF 2 PMENTEU AUG 31971 SHEET a uf 2 BACKGROUND OF TI-IE INVENTION This invention is directed to a pneumatically powered screw driver of a push-start, torque shut-oft' type.

A tool of this general nature is of advantage in that its mode of operation reduces the time interval` the tool operates to drive the fastener home." This results in a saving of operating air, in a reduction in the cycling frequency ot' the tool and consequent wear, and in a reduction of the usual attendant noise. Tools of this general nature are known which require detent mechanism associated with the clutch system, or require multiple clutches.

The tool of the present invention is relatively more durable than such known tools in that it does not have the problem of wear or malfunbtioning associated with the detent mechanism and multiple clutches of such tools.

A feature ot the present invention is a torque releasable clutch in association with operating air control valve mechanism. The clutch functions to disconnect the driving motor from the work following maximum torque delivery and cooperates with the valve mechanism to shutoff operating air tlow to the motor without repeated ratcheting of the clutch members. This resultsl in a more accurate control of the delivered torque to the work since residual energy stored in the motor following maximum torque delivery is not delivered to the work. It also serves to avoid undesirable reactionary impulses being transmitted to the operator as might otherwise occur if the operating air were shut-off without disconnection ofthe motor from the work.

Another feature of the present invention is in the nature of its clutch which is subject to disengagement following maximum torque delivery when operating in a positive direction during tightening of a fastener but is not disengageable when operating under load in a negative direction to loosen a fastener.

BRIEF DESCRIPTION F THE DRAWINGS In the accompanying drawings:

FIG. I is a longitudinal section through a rotary screw driving tool embodying the invention;

FIG. 3 is a section taken on line 3-3 of FIG. ll;

FIG. 3 is a plan view of the clutch face of the driving clutch member;

FIG. I is an enlarged detail section through one of the clutch hall pockets of the driving clutch member;

FIG. 5 is a plan view ofthe clutch face of the driven clutch member;

FIG. ti is a fragmentary development view of the cam surface and associated troughs of the driven clutch member, showing the cycle of movement of the driving clutch member, indicated by the ball, from engaged condition through disengaged condition back to engaged condition;

FIGS. '7--Ill are detail views of the valve assembly, in which:

FIG. 7 shows the condition ofthe valve assembly when the tool is at rest and not pressed against the work, as in FIG. ll;

FIG. d shows the condition of the valve assembly when the tool is driving the work, in which condition the main valve is held in open condition by the pressed relation of the tool against the work;

FIG. 3 shows the condition the valve assembly obtains when the set torque has been delivered, in which condition the main valve has been raised further upwardly by the disengaging action ofthe clutch to carry the spool valve to closed condition over the air inlet port; and

FIG. Ill shows the condition the valve assembly obtains upon reengagement of the clutch and prior to removal of the tool from the work, in which condition the main valve has dropped to the FIG. d position but the spool valve remains in closed condition over the air inlet port.

FIG. II shows the valve assembly incorporated in a tool of the pistol grip housing type.

DESCRIPTION OF A PREFERRED EMBODIMENT The screw driving tool shown in the drawings includes a general housing I@ in which is contained a reversible pneumatic rotary motor Il of a conventional radially slidable vane type. An inlet I3 in the rear extremity ofthe housing connects an external source (not shown) off live air with an inlet chamber I3. A valve assembly ld controls air flow from the chamber to passage means I3 leading to the motor chamber 1lb. A conventional manually settable reversing valve I7 interposed in the passage means I3 is effective in a normal condition (FIG. 3) to direct air from the passage means I5 to passages Ib connected with a positive area of the motor chamber so as to drive the motor in a forward direction; and the reversing valve is etective in a reverse condition to direct air from passage means I3 to passages I9 connected with a negative area of the motor chamber so as to drive the motor in a reverse direction. A set screw 3l is engageable with the reversing valve so as to lock it in its selected condition.

The motor drive shaft 33 is drivingly splined at 33 to a reduction gearing assembly 3d, the output spindle 35 of which is iournaled in a bearing 3b having a fixed position in the housing. A driving shank 37 of a rotary clutch assembly 33 has a sleeve portion 39 fixed in its end, which portion has a straight splined slidable connection 31 with the spindle 35; whereby the clutch assembly has limited axial movement relative to the spindle and to an internal shoulder 33 of the housing and is adapted to be rotated by the spindle. A shoulder 33 defined at the upper extremity of the clutch shank is cooperable with the inner race ot' the bearing 3o to limit the extent of rearward movement of the clutch assembly away from the shoulder 33.

Mounted upon the clutch shank 37 is a torque releasable overriding cam clutch 3d comprising a driving member 35 held under the bias of a clutch spring 36 in clutched engagement with a driven member 37. The driven member has a circular head 33 which is keyed to the clutch shank by means of a ring of ball bearings 39 whereby the driven member is rotatable relative to the shank but is restrained against relative axial movement. The clutch head 3@ normally rests upon the housing shoulder 33 in the normal inoperative condition of the tool, as in FIG. I. A cylindrical stem il depends axially from the clutch head 3h with a slide tit through an opening d3 centrally of the housing shoulder 33. The stern cooperates with the wall of the opening I3 to provide a bearing support for the lower end of the clutch assembly. The stem has a polygonal socket d3 in which a complementary head ad of a screw driver bit d3 is slidably received. A ball detent I6 carried in the stem is cooperable with the head of the bit to retain the latter against endwise escape from the socket.

A work finder d3 slidably sleeves the cylindrical shank portion ofthe bit. The nder has a counter recess t9 at its front end adapted to engage over the head ofthe work, represented by bit screw 5I, so as to facilitate engagement of the bit with the screw slot and to retain the bit in such relation. The finder is slidable axially into the bottom end of the housing against the resistance of a'spring 33. A key 53,. carried by the housing and engaged under the pressure of an annular holding spring 3d in a longitudinal keyway 55 of the finder, is cooperable with the ends 3d of the keyway to limit the extent of axial movement of the finder relative to the bit and to the housing. The key 53 also serves to restrain the finder to the housing against being frictionally rotated with the bit. The nder has an enlarged rear recess 37 which enables it to be slid back over the stem Ill of the driven clutch member. This permits axial movement of the finder with the bit as the latter is pressed against the work and moves the clutch assembly 3h rearwardly from the housing shoulder 33.

The driving clutch member 33 has a group of ball spline and longitudinal groove connections 38 with the clutch shank 27 whereby it is rotatable as a unit with the shank. The connections 58 also permit limited axial movement of the driving clutch member relative to the shank out of clutched engagement from the driven clutch member. The clutch spring 36 is positioned between an annular shoulder 59 of the driving clutch member and an overhead flange of a nut 6l screwed upon a threaded portion 62 of the clutch shank. The nut can be adjustably threaded along the shank to increase or relax the tension of the clutch spring.

Clutched engagement of the driving and driven clutch members is provided by means of a group of balls 63 carried in pockets 64 of the driving member. Each ball has a protruding portion normally seated in a separate relatively shallow trough 65 of the driven member (FIGS. l, 3 6). The pockets 64, here three in number, are formed in a flat annular end face 66 of the driving member and are spaced circumferentially equally apart. The troughs 65 are formed in an opposed flat annular end face 67 of the driven member and are similarly spaced equally apart. Each trough terminates in an inside radius 68 at its forward end, and in a similar radius 69 at its opposite end. Extending angularly over the end face 67 from each trough to the next is a cam lobe 70. In the engaged condition of the clutch members, the cam lobes are slidably received in grooves 81 formed in the end face of the driving member. Each groove extends angularly fromone ball pocket 64 to the next. Each lobe 70 has a forwardly inclined short slope 72 which rises from a forward edge M of one trough to a peak 73 from which a long back slope 74 declines toward the next succeeding trough to a point 75. Point 75 is elevated above the rear edge of the next succeeding trough by means of a short end wall 76 of the lobe. The wall 76 has a radius in con tinuation of the rear inside radius 69 of the trough. The structural relation of the clutch members is such that, during operation of the tool in a positive direction to tighten a fastener, the driving clutch member 35 upon experiencing torque overload will override in an axially disengaging action the cam lobes of the driven member 3,7 from one trough to the next against the resistance of the clutch spring 36; but when the tool is operated in a reverse directionto loosen a fastener, the steep slope defined by the combined inside radius 69 and the lobe end wall 76 of each trough will curb the driving clutch member from overriding the driven member. Accordingly, the

driving clutch member is subject to overriding clutch disengaging action in a Apositive direction, but not in a reverse direction.

The limited axial movement of the clutch assembly 28 is utilized to control starting of the operation of the tool and final restoration of the tool to normal condition; and the limited axial movement of the driving clutch member 35 as it overrides the driven member 37 is utilized to cause interruption of the operation of the motor upon the work being driven to a predetermined degree of tightness. Toward these ends, the air feed control valve assembly 14 is operatively associated with the clutch assembly 28 and with the driving clutch member 35 by means of an extended pushrod 80. The pushrod extends axially and slidably through the tool from the valve assembly into the clutch shank wherein it abuts a cross pin 77. Pin 77 extends slidably through a relatively vertically enlarged slot 78 of the clutch shank and is fixed at its ends in the body of the driving clutch member 35.

The valve assembly 14 (FIGS. l and 7) includes a cylindrical valve case 79 having an open bottom end retained by means of an overhead spring 82 in the inlet chamber 13 upon the top end of a bushing 91.*A port 85 in the sidewall of the valve case communicates the inlet chamber 13 with the interior of the case. An -ring 83 seals against leakage of inlet air around a cylindrical neck portion 88 of a spool valve 86 to an opening 84 provided by an internal shoulder 71 of the bushing. The spool valve is slidable relative to the inner wall of the valve case from a normal open position, asin FIGS. l and 7, in which inlet air can flow freely into the case to a closed condition as in FIG. 9 in which it blocks inlet air flow into the case. In its normal position, the spool valve rests upon the shoulder 7l. A main control valve 93 located inside the spool valve is mounted axially atop the pushrod 80, and it has a stem portion 101 extending with a slide fit through the top wall of the valve case.

The main valve 93 is normally held closed over the opening 84 upon the shoulder or seat 71, as in FIGS. l and 7, by means of an overhead closing spring 95 whereby inlet air flow to the motor is blocked. The main valve is caused to be raised from its seat, as in FIG. y8, against the resistance of the closing spring by means of the pushrod during axial rearward movement of the clutch assembly 28 from the housing shoulder 32, as occurs when the bit 45 is pressed against the work. The inner race of the bearing 26 cooperates with the end shoulder 33 of the clutch shank 27 to limit the extent to which the clutch assembly may be moved rearwardly so that in this movement of the clutch assembly the main valve 93 will be carried by the pushrod into abutment with an overhead shoulder 97 of the spool valve 86. When the main valve is so raised, as appears in FIG. 8, inlet air entering port of the valve case 79 is caused to flow about the peripheral ribs 96 of the main valve to the motor chamber to operate the motor.

As the motor operates, its torque is transmitted through the reduction gearing 24 and clutch assembly 28 to the work 51. As the work attains a predetermined degree of tightness, the driving clutch member 35 advances angularly and moves axially as it is forced away from the driven member 37 by the increasing torque resistance of the work. In this action, the driving clutch member 35 carries the clutch balls 63 out of the troughs 65 of the driven clutch member onto the initial short slopes 72 of the cam lobes. Maximum torque is delivered to the work as the driving clutch member advances the balls out of the troughs over the points M onto the cam slopes 72 thus disengaging the motor from the work. The axial disengaging movement of the driving clutch member 35 in this action relative to the driven member is transmitted through the pushrod 80 to carry the main valve 93 further rearwardly and to cause the main valve to slide the spool valve 86 into a closed position blocking the inlet port 85 of the valve case, as appears in FIG. 9. As the spool valve is moving to this blocking position, inlet air `bleeding through the diametric clearance between the spool valve 86 and the valve case 79 enters and builds up in the annular space 98 below the spool valve sufficiently to hold the spool valve raised independently of the main valve with the Oring 90 of the spool valve in sealing contactwith the inside wall of the valve case. l v

Closing of the inlet port 85 effectively cuts off flow of operating air to the motor. But, to insure that the clutch balls will not stop on the peaks 73 of the cam lobes, flats 102 are provided on the stem 101 of the main valve. The flats allow a limited volume of inlet air to the motor so as to continue motor operation until the clutch balls have advanced up the short slopes and over the peaks 73 of the cam lobes. The flats 102 are caused to protrude out of the valve case, as in FIG. 9, at the same time as the main valve carries the spool valve to its blocking position over the inlet port 85. When the clutch balls 63 have been advanced over the cam peaks 73 onto the long declining back slopes 74 of the lobes, as indicated by the intermediate broken line ball in FIG. 6, the clutch spring 36 reexpands and forces the driving clutch member 35 axially and angularly to carry the clutch balls down the back slopes 74 into reengagement with the next succeeding troughs of the driven member, as indicated by the second broken line ball in FIG. 6. In this movement of the driving clutch member down the back slopes into reengagement, the main valve 93 is forced downwardly by its return spring and brings the stem flats 102 back inside the valve case. A smooth and final stopping of the motor is effected as the clutch balls reengage the troughs of the driven clutch member, the valve assembly having at this time the condition indicated in FIG. 10. At this time, since the work end of the tool is still pressed against the work 5l, the main valve 93 will still be held short of its seat by the elevated pushrod 80 so that a small amount of air bleeding through the diametric clearance between the main valve stem 101 and the top wall of the valve case 79 is permitted to vent ineffectively through the motor.

Since the spool valve do continues, as tip-pears in lFlG. lill, after reengagement of the clutch members to be held closed over the inlet port 8% by the live air trapped in the space lill at its underside, the motor remains stopped until the tool is lifted by the operator from the worlrl When the latter occurs, the main valve @El drops to closed condition upon its seat 7l under the bias ot its return spring dll; and the clutch assembly 2li returns to the housing shoulder 32. 'llhe air bleeding around the stem ltlll of the main valve then rapidly builds up in the valve case 7@ above the closed main valve sufficiently to allow the spring llflllll to return the spool valve liti to normal position and thereby uncover the inlet port dll to open condition, thus returning the tool to its normal inoperative PltGS. ll and '7 con dition.

The tool shown in lFlG. l is of the linear housing type 'lhe valve assembly ld ol lFlG. l may also be incorporated in a tool of the pistol grip housing type as indicated in lFllG. lill. To

avoid unduly elongating the housing ot the liltl. llll tool, theV valve assembly ld is located in the upper area of the pistol grip portion llll'l of the housing. in this pivoted in the housing translates movement of the pushrod 80a through a pin Mill slidable in a bushing llllll and through an abbreviated pushrod hub to unseat the main valve hlt.

lclaim:

ll. In a pneumatic screw drive including a rotary air motor and a live air supply chamber, a valve assembly controlling live air flow from the chamber to the motor comprising a valve case having a side inlet port connecting the chamber with the interior of the case for allowing air ll'low in large volume into the case, an open-ended slide valve interiorly ofthe case having a normal condition clear of the port and being movable to a condition closing the port, the slide valve having an internal abutment and having a valve seat opening communicating the interior of the case with the motor, a main valve interiorly of the slide valve having a normal condition closed over the valve seat opening, the main valve being movable relative to the slide valve an initial distance to open condition against the abutment and being movable for a further distance beyond the initial distance in engagement with the abutment to carry the slide valve to its closed condition over the port, and a stem carried by the main valve having a normal clearance condition in an opening of the valve case allowing bleed air from the chamber into the case, the stern having a portion formed with flats movable with the main valve to protrude from said openn ing as the main valve moves beyond said initial distance so as to allow air llow from the chamber in limited volume into the case.

2. ln a pneumatic screw driver as in claim ll, wherein closing spring means biases the main valve to its normal condition.

3. ln a pneumatic screw driver as in claim l, wherein means is provided which is elilective in response to movement of the slide valve to closed condition to cause the slide valve to be pneumatically retained in said condition.

4. ln a pneumatic screw driver as in claim l, including first means for moving the main valve against the bias of the closing spring means to the extent of said initial distance and second means for moving the main valve against the bias of the closing spring means over said further distance.

5. ln a pneumatic screw driver as in claim il, wherein the arrangement, a lever llllld ti first means for moving the main valve is a slidable clutch assembly having a pushrod engaged with the main valve.

d. in a pneumatic screw driver as in claim d, wherein the second means for moving the main valve over the further distance is a torque responsive cam disengageable overriding driving clutch member having an abutment engaged with the pushrod, and the closing spring means is responsive to the overriding action of the clutch member to return the main valve to its initially moved condition.

'i'. in a push-start, torque shut-ofi screw driving tool including a casing having an internal shoulder, a rotary pneumatic motor, a valve seat, and a main valve to control operating air flow to the motor having a normal condition closed upon the seat, a clutch shaft having rotation with the motor and having relative axial movement, a drivin torque releasable cam clutch member carried by the sh t for unitary rotation but having relative axial movement, a driven cam clutch member keyedto the shaft for relative rotation and for axial movement unitary with the shaft, a clutch spring biasing the driving clutch member into clutched engagement with the driven clutch member, the driving clutch member adapted upon delivery of a preset maximum torque to the driven clutch member to disengage axially from the driven clutch member against the bias of the clutch spring while at the same time overriding the driven clutch member for a predetermined degree and then returning under the bias of the clutch spring to reengaged condition, the driven. clutch member having a normal condition seated upon the internal shoulder of the casing limiting the extent ol axial movement of the clutch shaft away from the motor, bit means carried by the driven clutch member engageable with the work and adapted when pressed upon the work to force the clutch shaft together with the driving and driven clutch members away from the internal shoulder toward the motor, an abutment carried by the driving clutch member, a slide rod extending axially ofthe tool resting at its bottom end upon the abutment adapted upon axial movement of the clutch shaft together with the driving and driven clutch members to unseat the main valve so as to allow operating air flow to the motor, means responsive to the disengaging and subsequent reengaging action of the driving clutch member to substantially shut off operating air flow to the main valve upon delivery of said maximum torque and then upon said reengagement to finally shut ofi said air flow to the main valve; and means responsive to removal of the bit means from the worlr to axially return the clutch shaft together with the reengaged clutch members to normal so as to reseat the driven clutch member upon the internal shoulder' and to return the main valve closed upon its seat.

d. ln a push-start, torque shutoff screw driving tool according to claim 7, wherein the motor is reversible, reversing valve means is provided between the main valve and the motor for directing operating air selectively to the motor so as to drive the motor in either a positive or negative direction.

il. ln a push-start, torque shut-off screw driver according to claim ll, wherein the driven clutch member has a cam engagement with the driving clutch member permitting said overriding disengaging action ofthe driving clutch member in a positive direction, but blocking it against. such action in a reverse direction.

Claims

1. In a pneumatic screw drive including a rotary air motor and a live air supply chamber, a valve assembly controlling live air flow from the chamber to the motor comprising a valve case having a side inlet port connecting the chamber with the interior of the case for allowing air flow in large volume into thE case, an open-ended slide valve interiorly of the case having a normal condition clear of the port and being movable to a condition closing the port, the slide valve having an internal abutment and having a valve seat opening communicating the interior of the case with the motor, a main valve interiorly of the slide valve having a normal condition closed over the valve seat opening, the main valve being movable relative to the slide valve an initial distance to open condition against the abutment and being movable for a further distance beyond the initial distance in engagement with the abutment to carry the slide valve to its closed condition over the port, and a stem carried by the main valve having a normal clearance condition in an opening of the valve case allowing bleed air from the chamber into the case, the stem having a portion formed with flats movable with the main valve to protrude from said opening as the main valve moves beyond said initial distance so as to allow air flow from the chamber in limited volume into the case.

2. In a pneumatic screw driver as in claim 1, wherein closing spring means biases the main valve to its normal condition.

3. In a pneumatic screw driver as in claim 2, wherein means is provided which is effective in response to movement of the slide valve to closed condition to cause the slide valve to be pneumatically retained in said condition.

4. In a pneumatic screw driver as in claim 3, including first means for moving the main valve against the bias of the closing spring means to the extent of said initial distance and second means for moving the main valve against the bias of the closing spring means over said further distance.

5. In a pneumatic screw driver as in claim 4, wherein the first means for moving the main valve is a slidable clutch assembly having a pushrod engaged with the main valve.

6. In a pneumatic screw driver as in claim 5, wherein the second means for moving the main valve over the further distance is a torque responsive cam disengageable overriding driving clutch member having an abutment engaged with the pushrod, and the closing spring means is responsive to the overriding action of the clutch member to return the main valve to its initially moved condition.

7. In a push-start, torque shut-off screw driving tool including a casing having an internal shoulder, a rotary pneumatic motor, a valve seat, and a main valve to control operating air flow to the motor having a normal condition closed upon the seat, a clutch shaft having rotation with the motor and having relative axial movement, a driving torque releasable cam clutch member carried by the shaft for unitary rotation but having relative axial movement, a driven cam clutch member keyed to the shaft for relative rotation and for axial movement unitary with the shaft, a clutch spring biasing the driving clutch member into clutched engagement with the driven clutch member, the driving clutch member adapted upon delivery of a preset maximum torque to the driven clutch member to disengage axially from the driven clutch member against the bias of the clutch spring while at the same time overriding the driven clutch member for a predetermined degree and then returning under the bias of the clutch spring to reengaged condition, the driven clutch member having a normal condition seated upon the internal shoulder of the casing limiting the extent of axial movement of the clutch shaft away from the motor, bit means carried by the driven clutch member engageable with the work and adapted when pressed upon the work to force the clutch shaft together with the driving and driven clutch members away from the internal shoulder toward the motor, an abutment carried by the driving clutch member, a slide rod extending axially of the tool resting at its bottom end upon the abutment adapted upon axial movement of the clutch shaft together with the driving and driven clutch members to unseat the main valve so as to allow operating air flow to the motor, means responsive to the disengaging and subsequent reengaging action of the driving clutch member to substantially shut off operating air flow to the main valve upon delivery of said maximum torque and then upon said reengagement to finally shut off said air flow to the main valve; and means responsive to removal of the bit means from the work to axially return the clutch shaft together with the reengaged clutch members to normal so as to reseat the driven clutch member upon the internal shoulder and to return the main valve closed upon its seat.

8. In a push-start, torque shut-off screw driving tool according to claim 7, wherein the motor is reversible, reversing valve means is provided between the main valve and the motor for directing operating air selectively to the motor so as to drive the motor in either a positive or negative direction.

9. In a push-start, torque shut-off screw driver according to claim 8, wherein the driven clutch member has a cam engagement with the driving clutch member permitting said overriding disengaging action of the driving clutch member in a positive direction, but blocking it against such action in a reverse direction.