WO1996000139A1

WO1996000139A1 - Torque impulse drive unit for power tool

Info

Publication number: WO1996000139A1
Application number: PCT/GB1995/001484
Authority: WO
Inventors: Michael George Allen; Melvyn Cassell; Christopher John Dudden
Original assignee: Desoutter Limited
Priority date: 1994-06-27
Filing date: 1995-06-23
Publication date: 1996-01-04
Also published as: GB9412865D0; AU2748095A

Abstract

A driven input body (24) is coupled to an output body (28) by dogs (30, 30a) capable of riding over one another when rotation of the output body (28) is checked. A further pair of dogs (31, 31a) is provided at a different distance from the rotation axis. The input body (24) has a cylinder (42) and piston (44) defining an internal hydraulic space which communicates with an external hydraulic space (45, 46) via a bleed valve (43) controlling the rate of axial movement of the piston (44) as the dogs ride up one another. A one-way valve (39) allows hydraulic fluid to flow back into the internal hydraulic space as the piston (44) moves downwards after the dogs have ridden over one another. The pressure in the internal space may be relieved just before the dogs ride over one another, to reduce frictional wear of the dogs.

Description

TORQUE IMPULSE DRIVE UNIT FOR POWER TOOL

This invention relates to torque impulse drive units for power tools used for driving a screw-threaded member such as a bolt, a wood screw, or a self-tapping screw, or for running a nut on to a screw thread.

Such tools are generally elongate and have a metal or plastics casing. It is known to provide a power tool with a switch arm which, when the tool is grasped initiates the drive and, when it is released, stops the drive, or operatively disconnects the driving energy source if the drive has already been stopped by a cut-out arrangement.

Typically such tools have a rearward portion for control of the source of driving energy. This energy may be an electrical supply, but is more typically a compressed air supply. In this latter case the rearward portion will include valving means and ducting to allow a supply of compressed air into the tool, to discontinue such supply, and to vent the supply to atmosphere as needed in different stages of operation-

Known tools also possess a central rotary motor portion, i.e. a pneumatic (or electric) motor, and a forward impulse-drive portion rotatably connected thereto. The forward impulse-drive portion is itself in two parts, an input body and an output body, separate but adjacent and sharing a common axis of rotation. The part nearest the forward end of the tool is the output body, which carries a suitable exchangeable bit (or socket) for engagement with the member to be driven. With no encountered resistance the output body rotates with the input body. When resistance is encountered, torque impulses are intermittently transmitted from the input body to the output body.

It is also known (GB-A-2 240 632) to provide, at the rear of the impulse drive portion, a two-part inertial switch or control around the rotary axis. In normal operation the two parts are resiliently separated. As the progressive impacts become stronger the separation will eventually be overcome, thus actuating a transverse slide in one of the two parts to open an axial gate. This in turn permits forward movement of an axial actuating rod operatively connected at its rearward end to compressed air valving, to discontinue the supply of compressed air to the motor.

In one known power tool inclined dog surfaces located between the input body and the output body slide on one another to force the input and output bodies apart against the action of a spring when the output body encounter resistance to rotation. Eventually the output body is brought to a standstill and the dogs ride over one another so that the input body can continue to rotate, being accelerated until the next impact of the dog surfaces, thereby transmitting a torque impulse from the input body to the output body. A disadvantage of this arrangement is that the necessarily high spring force causes severe wear of the dogs as they ride over one another.

In another known power tool the input and output bodies are connected by a hydraulic device comprising a rotor mounted in a casing and defining hydraulic spaces between which a pressure differential builds up when the output body encounters resistance to rotation. The rotor and casing rotate together until the resistance brings the output body to a standstill, whereupon the rotor and casing rotate relative to each other, temporarily interconnecting the hydraulic spaces until a position is reached at which the rotor and casing cooperate to close off the hydraulic spaces, thereby transmitting a torque impulse from the input body to the output body.

The hydraulic spaces thus vary size as the input unit rotates. Alternatively stated, a transverse section through the hydraulic spaces (i.e. across the tool) will vary in dimension at different times. This is disadvantageous in terms of complexity of structure (and consequent cost) and non-symmetry of operation leading to increased wear and maintenance requirements.

Another disadvantage is that the impulse drive comes into effect at any but the most minor resistance, which slows down the initial stage of driving wood screws, self-tapping screws, or anti-vibration nuts, for example.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome or mitigate these problems.

The present invention provides a torque impulse drive unit for a power tool, comprising: an input body which is to be driven in rotation about an axis, and an output body rotatable about the said axis relative to the input body, the input body having a cylinder part and a piston part which are coaxial with the said axis and which define an internal hydraulic space, the input body also defining an external hydraulic space, one of the said parts being axially movable relative to the output body; a rotation coupling between the input body and the output body, the coupling comprising a first dog which rotates with the output body and a second dog which rotates with the input body and moves axially with the said movable part, the dogs being capable of riding over one another when rotation of the output body is checked while the input body is driven; a bleed valve which communicates between the said hydraulic spaces so as to permit restricted flow and thereby to control the rate of axial movement of the said movable part as the dogs ride up one another; and a one-way valve which communicates between the said hydraulic spaces so as to permit free flow, in the opposite sense to the flow through the bleed valve, after the dogs have ridden over one another.

The torque resistance at which impulse drive comes into effect may be selected by adjustment of the bleed valve.

In a preferred embodiment, there is a duct for communicating between the said hydraulic spaces, the duct being open when the said axially movable part is in the axial position at which the dogs ride over one another, the said part closing the duct until just before the dogs ride over one another. This minimises wear of the dogs. The invention also provides a torque impulse drive unit for a power tool, comprising an input body which is to be driven in rotation about an axis, an output body rotatable about the said axis relative to the input body, and a rotation coupling between the input body and the output body, the coupling comprising two pairs of first and second dogs in which the first dog rotates with the output body and the second dog rotates with the input body, one dog of each pair being axially movable relative to the other and riding over the other when rotation of the output body is checked while the input body is driven, wherein the two pairs of dogs are at different distances from the said axis.

The invention further provides a torque impulse drive unit for a power tool, comprising an input body which is to be driven in rotation about an axis, an output body rotatable about the said axis relative to the input body, an impulse clutch which transmits rotation from the input body to the output body until a sufficiently high reaction torque is applied to the output body, whereupon the clutch intermittently transmits torque impulses from the input body to the output body, the clutch including hydraulic spaces between which a pressure differential builds up as the torque transmitted rises, an axial control rod for cutting off a motor driving the input body, an inertial control device on the input body, the rod extending into the, inertial control device, the said device including a latch which has a first position in which it engages the control rod to prevent cutting-off and a second position in which it allows the control rod to be axially urged to a cut-off position, the latch moving from the first position to the second position when a given torque impulse level is reached, and a valve which communicates between the said hydraulic spaces and which is opened by movement of the control rod to the cut-off position so as to equalise the pressure in the said hydraulic spaces.

In one aspect the invention provides an impulse-drive power tool comprising an elongate casing enclosing a rearward control portion for a driving energy source, an intermediate drive portion, and a forward impulse portion rotary therewith, the impulse portion comprising an input body coaxial with and adjacent to an output body, the bodies being rotatably coupled by dogs capable of riding over one another, the input body comprising a cylinder and piston arrangement coaxial with the axis of rotation and connected to one of the facing coupling dogs so that when the dogs ride over one another as the output body rotation is checked, an internal hydraulic space defined within the said arrangement is placed under pressure, the said internal space communicating with a bleed valve, which restricts the rate of decrease in volume of the space and thus restricts the rate of relative sliding movement of the dogs, and a one-way valve to permit hydraulic fluid to enter as the dogs disengage and the space again increases in volume.

Preferably, the internal space with the piston/cylinder arrangement is defined between a cylinder end and a hollow open-ended piston.

Preferably, the said internal space includes a compression spring biassing the piston to an outer position. The use of an internal biassing compression spring of this nature is valuable for running a nut, or driving a screw, against prevailing torque. Examples of this are antivibration nuts or driven wood screws. Typically, (and in contrast to a simple nut-running requirement) both of these exert some intermediate resistance to driving long before the "impulse drive" characteristics are actually needed. With the spring bias simple non-impulse drive, i. e. without disengagement of the dogs, takes place until a substantial resistance, adequate to start to compress the spring, is present, at which time the tool effectively changes to impulse drive.

The one-way valve to the internal piston-cylinder space most preferably comprises a ball located on a seat around an opening at the base of the piston.

Alternatively or additionally it may comprise a seated ball valve located over an axial opening at the end of the cylinder; this is especially the case when the tool comprises the pressure-venting arrangement described below.

The dogs are preferably such as to permit relative rotation of the tool drive unit and holder for nearly 360" between impacts. To preserve the improved rotary balance of the tool we have established that a preferred arrangement is to provide two pairs of impacting dogs at different distances from the axis of rotation. This is also applicable to the known type of impulse-drive tool.

In the case in which the one-way valve to the piston-cylinder space comprises a ball located on a seat around an axial opening at the base of the cylinder, the tool preferably comprises a two-part inertial control extending around the rotary axis, one part carrying a slidable gate at the said axis to control passage of a longer axial rod extending forward from the rearward control portion as a cut-out actuator for the driving energy source, and the two parts being mounted to come together to cause the said gate to slide open at a predetermined level of torque impulse and thus permit the longer axial rod to slide to a drive cut-out position, the tool being further characterized in that a separate shorter axial actuator rod is slidably located with a rearward end adjacent and forward of the gate and a forward end located near the ball of the valve in the cylinder base, whereby movement of the longer axial drive cut-out rod past the gate pushes the separate shorter actuator rod to dislodge the ball and thereby releases the pressure in the internal hydraulic space.

This aspect of the invention thus provides an impulse- drive tool which (as is known) cuts out the drive at a predetermined impulse level and which also simultaneously vents the hydraulic chamber so that the tool is ready for further use.

Preferably, the internal space also comprises a pressure-sensitive relief valve to vent the space at a predetermined pressure level. Most preferably, the pressure-sensitive valve is also operatively connected to the rearward control portion for the driving energy source so as to discontinue the drive when the predetermined pressure opens the relief valve.

Thus, the tool preferably comprises, as the one-way valve, a ball located on a seat extending around an opening in the base of the piston and, as the relief valve, a slide member with a conical end seating in an axial opening in the base of the cylinder.

As the operative connection there is preferably provided an axial rod extending forward from the rearward control portion as a cut-out actuator for the driving energy source, and with its forward end received in an axial bore in the said slide member at either an outer position or an inner position separated by a latching means releasable so that the end of the axial rod can move from its outer to its inner position in relation to the slide member axial bore, thereby actuating the drive energy cut-off, when the slide member is displaced from its seat by the predetermined pressure.

The slide member may itself slide in a bore of a surrounding ducted block against a resetting spring. The latching means may then comprise a plurality of members, e. g. balls, each loosely held in a respective opening, extending between the slide member axial bore and the slide member outer wall, so as to project into the said axial bore when the slide member openings are located within the block bore, thereby to prevent the end of the axial rod from moving to its inner position, but capable of moving from that inner position when the slide member openings lie outside of the block bore, thereby to allow the axial rod to move to its inner position and actuate the driving energy cut-off. For example, the block bore may have a chamfered rear edge surface, and the balls may be held in recesses in the slide bore walls (a) through which they may project internally but not pass into the slide bore, and (b) from which they may fall on to the chamfered rear edge surface.

This arrangement thus permits automatic pressure release, and thus cessation of drive, at a predetermined pressure (rather than at a predetermined impact force) with simultaneous cut-out of the drive energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: -

Figure 1 shows a hand-held impulse-drive tool from the side;

Figure 2 is a longitudinal medial section of the tool shown in Figure 1;

Figure 3 shows on a larger scale the rotary impulse portion of Figure 2, also in longitudinal medial section; Figures 3a to c are diagrammatic longitudinal medial sectional views of part of the rotary impulse portion, showing successive stages in its operation;

Figure 3d is an enlarged fragmentary exploded perspective view of part of the rotary impulse portion;

Figure 4 is a transverse section through Figure 3 along the line IV-IV; and

Figure 5 shows diagrammatically a variant of selectively operable relief valve and exhaust port which can be incorporated into the rotary impulse portion as shown in Figure 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The power tool illustrated is used for impulse driving of a bit or socket. It has an external plastics casing 1 shaped at regions 2, 3 and 3a to facilitate forceful manual grasping, and has at a rearward end a socket 5 for a compressed air line and at a forward end a holder 4 to accommodate interchangeably a driving bit or socket. The tool further comprises a switch arm 6 pivoted at 7. When the tool is grasped, this arm 6 pivots inwards to lie along the casing (see also Figure 2). A further external switch 8 can be selectively operated to reverse the drive, for example to unscrew a nut or bolt, or to remove a wood screw or the like.

The tool, shown in Figure 2 in longitudinal medial section, comprises internally a rearward structural zone 9, generally controlling the air supply as described in more detail below, a central pneumatic motor 10 (having a vaned rotor), and a forward rotary impulse structure 11. In Figure 2 the arm 6 is also shown in cross-section, and for purposes of description is shown in both its outer position and its inner position. This arm 6 is further shown as provided with an outer extension sleeve 12, capable of being selectively positioned to provide an effective and adjustable extension to the arm.

The rearward structure 9 comprises (as known per se) a compressed air inlet duct 13, and a ball valve consisting of a ball 14 and a seat 15. The ball 14 is shown in two positions, i. e. with the valve both shut and open. Transversely extending across the structure 9 is an operating rod 16 for the ball 14, extending between the surface of the ball and an inner face of the external operating arm 6.

Downstream of the ball valve 14, 15 is ball valve opening 18 shown closed by a movable closure member 17 with a spring 17a serving to re-set the closure member 17 in this closed position against the outlet of the ball valve opening 18. This closure member is surrounded by chamber 19 which vents via a duct 20 to atmosphere. The closure member also carries one end of a long rod 21 extending through the motor 10 to the rotary impulse structure 11.

The motor 10 is also of conventional nature, and will not be described in detail. Basically, the air entering through the opened ball valve 14, 15 forces the closure member 17 on its rod 20 downwards (to an extent governed by a trap member discussed below) and places in communication the air supply line and aerodynamically shaped portions of the motor 10 so that the rotor of the motor rotates upon suitable bearings as conventional in the art.

The rotary impulse structure 11 is shown in more detail in Figure 3. It comprises an outer casing 24 having an end cap 25 attached thereto. Within the end cap are ball races 26, 27 carrying an internally hollow shaft 28 connecting to the holder 4. The shaft 28 and the holder 4 are thereby capable of rotating relative to the overall structure 11. To provide intermittent or impulse characteristics to this relative rotation interacting sets of dogs are provided, as shown in more detail in Figure 3d, one set 30,31 being provided on a flanged end face of the hollow shaft 28, the other set 30a, 31a being provided similarly at the end of a relatively longitudinally movable piston structure 34.

The piston structure 34 comprises a base portion 35 with a central bore 36 which communicates with a larger bore 37 via a divergent throat 38. The throat 38 constitutes a seat for an internal ball 39.

A thin wall 40 of the piston structure 34 surrounds a compression spring 40a extending from the base of the piston hollow 41 (defined within the wall 40) to bear upon an end wall of a cylinder structure 42 surrounding the piston. The space within the cylinder structure 42 and the piston hollow 41 communicates via a needle valve 43 (with setting screw 44) and a duct 45, with a space 46 at the forward end of the piston and cylinder arrangement 34. The duct 45 is provided with a ball valve 47, seating at 47a when permitted by a short operating rod 48 extending to a gate structure 49.

The nature of the gate structure 49 can be seen from Figure 4, which is a transverse section taken through the forward rotary structure 11 at the level of such structure. The gate itself is a slidable member 50, spring biassed outwardly at 51 to present between the forward end of the long rod 21 and rearward end of the short operating rod 48 an impassable barrier when biassed outwardly as shown. The gate 50 and spring 51 are mounted in a generally circular inertial structure, one component 53 of which houses the gate 50 and its biassing spring 51, and the other, generally L-shaped, component 54 being located in its neutral position as shown to hold the gate 50 to protrude against the spring 51 to act as the barrier between the rods 21 and 48.

As described in more detail below, if the L-shaped component 54 should pivot about its pivot 55 then it will force the gate 50 inwards against the action of the spring 51. Conveniently, adjustment members 56 and 58 can be positioned so as to control this movement precisely. If such inward movement does take place, however, the rods 21 and 48 then register axially with the hole 50a in the gate 50 with consequences as discussed below.

The device operates as follows.

The tool is connected to a compressed air line at 5. It is picked up and grasped naturally around the configured surface at 3, with the thumb resting against the projection 2 (to force the tool longitudinally to the extent that may be required) and the remainder of the hand resting on the smooth portion 3a and, in particular, over the end of operating arm 6. The arm 6 is therefore forced in towards the body of the tool.

As can be seen from Fig. 2, the inward position of the arm 6 forces the operating rod 16 across the structure 9. This has the effect of displacing the ball 14 from the seat 15. The compressed air from the rearward connection 5 can thus pass downstream of the ball 14, and in particular to the duct or opening 18. It forces back the closure member 17 against spring 17a to the extent permitted by the rod 21 abutting on the gate 50, so that air passes around the closure member 17 and onwards to the pneumatic motor 10, to impart to the motor a rotation in one or other sense as dependent upon the setting of switch 8. The motor is rotationally connected to the structure 11.

If the forward holder 4 end of structure 11 is thus pressed against a nut to be run onto a thread, or to a suitable bolt or screw (depending upon the structure) then the nut, bolt or screw will rotate. If the nut, bolt or screw encounters resistance, however, the forward shaft 28 will try to slow down or stop. Since the rearward part of the device is rotating rapidly, and has an inertial resistance to stopping, the opposed inclined faces of respective dogs 30, 31 and 30a, 31a on the end of the shaft 28 and on the opposed base 35 of the piston 34 will contact, and will start to rise up and force the piston 34 and shaft 28 apart. Movement of the piston wall 40 within the cylinder structure 42 will decrease the available internal space at a restricted rate dependent on the presetting of needle valve 43, creating a high pressure H (Fig. 3a), since the hydraulic medium within the piston wall 40 cannot escape through the ball valve 38/39, or through the ball valve 47/47a, to the low pressure side of the arrangement. As the hydraulic medium bleeds through the needle valve 43 and along the duct 45 to the forward space 46 the piston wall 40 eventually rises to a point (Fig. 3b) at which a duct 40b for communicating between the internal space and the forward space 46 is open just before the position (Fig. 3c) where the interengaged dogs 30 and 30a, 31 and 31a become eventually able to ride over one another so that the base of the piston and the end face of hollow shaft 28 may rotate relatively for another revolution. This reduces to a low value L the pressure applied to the piston, so that its internal spring 40a forces the piston structure to its initial relative position, hydraulic fluid entering over ball valve 47/47a to permit such movement.

Fig. 3d shows that one interacting pair of dogs 30, 30a, located as between the end of shaft 28 and the bottom of the piston structure, is radially inward of the other pair 31,31a whereby a balanced structure but a single impact once per revolution is jointly obtained. The upshot of this is that the forward bit or holder 4 is given a rotary movement consisting of a sequence of rapidly applied impulses, one per revolution of the piston/cylinder arrangement.

In practice, as the screw or nut is tightened each successive impulse becomes stronger. Eventually the impulse will be strong enough to rock the L-shaped component 54 about its pivot 55 so that the slide 50 is pressed inwards against the action of its biassing spring 51. When this is done, the hole 50a registers with the bore containing the respective rods 21 and 48. The effect of this is that the rod 21 is forced through hole 50, by the pressure of the drive air upon the closing member 17. This rod 21, as it passes through the hole 50a and moves the rod 48 to displace the ball 47 against its own biassing spring from seat 47a. This opens the port from the interior of the piston/cylinder arrangement, so that the high pressure therein is rapidly vented through the duct 45 to the low pressure side of the hydraulic arrangement. Thus, at a predetermined impulse force (a) the air supply is cut off from the pneumatic motor (because of the downward movement of the closure member 17 carrying the rod 21) and (b) the high hydraulic pressure interior of the rotary unit is vented through the duct 45.

It will be appreciated by those skilled in the art that this particular arrangement, involving a biassing spring 40a is of considerable advantage in both manufacture and use. It is in its major structure symmetrical about the axis of the tool, and in particular involves as a pressure chamber the space within an open ended piston within a cylinder, both being essentially of uniform shape and symmetrically arranged about the rotary axis of the tool. Advantage is also achieved by the use of spring 40a in that as a biassing spring it permits operation against a prevailing torque. Whereas in the assembly of a simple nut or bolt there is no effective torque exerted until engagement against the abutment surfaces, in other circumstances such as driving a wood screw or setting an antivibration assembly, there is torque at all stages. There is a disadvantage if the machine produces impulse drive at any such low prevailing torque, since slow, intermittent, drive will be given at all stages. Use of a spring 40a resisting the displacement of the dogs enables a wood screw, by way of example, to be driven in smoothly for a considerable distance by an exerted torque before the pressure is such as to dislodge the dogs 30, 30a or 31,31a and convert the tool to an impulse drive. Fig. 5 shows an alternative structure to be located in place of the ball 47 seated at 47a to seal the internal space of the cylinders. As described above, with reference to the embodiment shown in Fig. 3, there is a level of resistance to further turning which increases the impulse of the drive enough to swing the L-shaped member 54 around its pivot 55 to an extent as adjusted members 56 and 58 and thereby operate the trap member and consequently ball valve 47/47a. Using the construction shown in Fig. 5 it is possible to achieve a similar objective which is responsive to the pressure in the cylinder.

Fig. 5 shows the rearward end of a cylinder arrangement. At this end there is provided a major bore 60, and an external annular collector space 61 communicating with the low pressure side of the system by a duct 62. Within bore 60 there is provided a block 63, with suitable seals 64. The block itself possesses a longitudinal bore, having a small diameter entrance 66 nearest the cylinder end, a larger general diameter 68 throughout most of the length of the block, and an angled or chamfered surrounding edge 70 at the rear of the block 64. Within this bore there is located a movable slide valving member 72, of generally cylindrical shape but pointed at one end 73 to seat at the narrow bore 66. This valving member 72 is itself provided with a longitudinal bore 74 and with for example three or four, recesses 76 drilled inward from the outside but being larger at their outer openings 78 than their inner openings 80. Each such recess contains a ball 82, and in the position shown the balls are constrained by the walls of the bore block 63 to project inwardly of the longitudinal bore 74 in the valving member 72. Thus, the end of the rod 21 attached as before to the air inlet closure member 17 cannot in the position shown move past the balls 82.

When the pressure in the piston/cylinder spaces reaches a predetermined level it will push the valving member 72 backwards up the bore 68 so that the pressure can be vented through the transverse duct 84 in block 63 duct into the collector space 61 and thence via duct 62 to the low pressure side of the piston/cylinder. Upon such movement of valving member 72, however, the balls 82 in their recesses 76 are brought level with the chamfered or inclined edge 70 of the main bore 68 and thus fall outwards. This disencumbers the internal bore 74 of the valving member 72, whereby rod 21 is allowed to move downwards and, as described above, cut off the air supply to the pneumatic motor.

Thus, the same objectives of releasing the hydraulic pressure in the cylinder and at the same time cutting off the pneumatic drive are achieved. In this instance they are achieved in response to the pressure developed in the cylinder rather than to the force necessary at the driving end of the bit or socket.

It will be appreciated that to reset the device there is provided a spring 86 behind valving member 72. Thus, when the predetermined pressure is reached (a) the cylinder pressure is equalised; (b) when arm 6 of the tool is released to cut off the air supply the resetting spring 17a pushes the closure member 17 and rod 21 towards the rear of the tool; (c) this then allows the balls 82 to roll back into their recesses, whereby (d) spring 86 resets valving member 72 on its seat.

Claims

Claims :

1. A torque impulse drive unit for a power tool, comprising: an input body which is to be driven in rotation about an axis, and an output body rotatable about the said axis relative to the input body, the input body having a cylinder part and a piston part which are coaxial with the said axis and which define an internal hydraulic space, the input body also defining an external hydraulic space, one of the said parts being axially movable relative to the output body; a rotation coupling between the input body and the output body, the coupling comprising a first dog which rotates with the output body and a second dog which rotates with the input body and moves axially with the said movable part, the dogs being capable of riding over one another when rotation of the output body is checked while the input body is driven; a bleed valve which communicates between the said hydraulic spaces so as to permit restricted flow and thereby to control the rate of axial movement of the said movable part as the dogs ride up one another; and a one-way valve which communicates between the said hydraulic spaces so as to permit free flow, in the opposite sense to the flow through the bleed valve, after the dogs have ridden over one another.

2. A torque impulse drive unit as claimed in claim 1, including a duct for communicating between the said hydraulic spaces, the duct being open when the said axially movable part is in the axial position at which the dogs ride over one another, the said part closing the duct until just before the dogs ride over one another.

3. A torque impulse drive unit as claimed in claim 2, in which the duct is defined between the exterior of the piston part and the interior of the cylinder part.

4. A torque impulse drive unit as claimed in claim 1, including urging means acting on the said movable part to urge the second dog into an axial position in which it engages with the first dog.

5. A torque impulse drive unit as claimed in claim 4, in which the urging means comprises a coil spring coaxial with the said axis.

6. A torque impulse drive unit as claimed in claim 1, in which the piston part constitutes a hollow open-ended piston.

7. A torque impulse drive unit as claimed in claim 1, in which the said movable part is the piston part.

8. A torque impulse drive unit as claimed in claim 1, in which the axial movement of the said movable part as the dogs ride up one another causes compression of the said internal hydraulic space.

9. A torque impulse drive unit as claimed in claim 1, including two pairs of said first and second dogs, the two pairs of dogs being at different distances from the said axis.

10. A torque impulse drive unit as claimed in claim 1, including an axial control rod for cutting off a motor driving the input body, an inertial control device on the input body, the rod extending into the inertial control device, the said device including a latch which has a first position in which it engages the control rod to prevent cutting-off and a second position in which it allows the control rod to be axially urged to a cut-off position, the latch moving from the first position to the second position when a given torque impulse level is reached, and a pressure-equalization valve which communicates between the said hydraulic spaces and which is opened by movement of the control rod to the cut-off position so as to equalise the pressure in the said hydraulic spaces.

11. A torque impulse drive unit as claimed in claim 10, in which the said pressure-equalization valve comprises a seat around an axial opening communicating between the said hydraulic spaces, a ball located on the seat, and an axial actuating rod which is pushed by the control rod to dislodge the ball from the seat when the control rod moves to its cut-off position.

12. A torque impulse drive unit as claimed in claim 10, including a pressure-sensitive valve which opens communication between the said hydraulic spaces when a predetermined pressure difference is reached.

13. A torque impulse drive unit as claimed in claim 12, including means associated with the pressure-sensitive valve for cutting off a driving motor when the valve opens.

14. A torque impulse drive unit as claimed in claim 12, in which the pressure-sensitive valve comprises an axial slide member with a conical end seating in an axial opening communicating between the said two hydraulic spaces.

15. A torque impulse drive unit as claimed in claim 14, in which the axial control rod has a forward end received in an axial bore in the said slide member at one of an outer position and an inner position separated by the said latch, which is releasable so that the end of the axial rod can move from its outer to its inner position in relation to the slide member axial bore, thereby allowing the rod to move to a cut-off position, when the slide member is displaced by the predetermined pressure difference.

16. A torque impulse drive unit as claimed in claim 15, in which the slide member slides in an axial bore in a ducted block, against the action of a resetting spring.

17. A torque impulse drive unit as claimed in claim 16, in which the latching means comprise a plurality of members, each loosely held in a respective opening, extending between the slide member axial bore and the slide member outer wall, so as to project into the said axial bore when the slide member openings are located within the block bore, thereby to prevent the end of the axial rod from moving to its inner position, but capable of moving from that inner position when the slide member openings lie outside the block bore, thereby to allow the axial rod to move to its inner cut-off position.

18. A torque impulse drive unit for a power tool, comprising an input body which is to be driven in rotation about an axis, an output body rotatable about the said axis relative to the input body, and a rotation coupling between the input body and the output body, the coupling comprising two pairs of first and second dogs in which the first dog rotates with the output body and the second dog rotates with the input body, one dog of each pair being axially movable relative to the other and riding over the other when rotation of the output body is checked while the input body is driven, wherein the two pairs of dogs are at different distances from the said axis.

19. A torque impulse drive unit for a power tool, comprising an input body which is to be driven in rotation about an axis, an output body rotatable about the said axis relative to the input body, an impulse clutch which transmits rotation from the input body to the output body until a sufficiently high reaction torque is applied to the output body, whereupon the clutch intermittently transmits torque impulses from the input body to the output body, the clutch including hydraulic spaces between which a pressure differential builds up as the torque transmitted rises, an axial control rod for cutting off a motor driving the input body, an inertial control device on the input body, the rod extending into the inertial control device, the said device including a latch which has a first position in which it engages the control rod to prevent cutting-off and a second position in which it allows the control rod to be axially urged to a cut-off position, the latch moving from the first position to the second position when a given torque impulse level is reached, and a valve which communicates between the said hydraulic spaces and which is opened by movement of the control rod to the cut-off position so as to equalise the pressure in the said hydraulic spaces.