This application relates to machines for binding together concrete reinforcing bars using wire ties.

Examples of such machines are described in WO 2004/083559 and WO 2007/042785. They typically comprise mechanisms for feeding a length of wire from a spool such that it passes round the bars to be tied, clamping the end of the wire, retracting the wire back towards the spool to pull it tight, cutting off a length of wire and then twisting the wire to form a tie. The present invention is intended to be applicable to such machines although it is not necessarily limited thereto.

In WO 2007/042785 an arrangement is disclosed for cutting off the length of wire prior to twisting, whereby the initial torque of the twisting head is used to cut the wire against the static body of the machine where it passes from the body into the twisting head. At first sight this is an attractive solution since it obviates the need for a separate cutting mechanism. However the Applicant has appreciated that it carries disadvantages. In particular the requirements for cutting the wire: a very high torque at low speed; are incompatible with those for twisting the ends of the wire together once it has been cut: this requires a high speed at low torque. Accordingly previous proposals have employed a motor and corresponding gearing and battery to provide sufficient torque to cut the wire, at the expense of tying speed and also with an unnecessarily high torque for the twisting operation.

This problem could be addressed by providing an appropriate selective gearbox; or a separate motor or other wire cutting mechanism. However these are both unattractive as they introduce additional cost and complexity.

The present invention provides a machine for tying a wire around a pair of bars comprising means for passing a first end of said wire in a loop around the bars and a twisting head adapted to cut said wire to form a second end and thereafter to twist the ends of said loop together, said twisting head being driven by an impact drive system. Thus the machine of the invention uses an impact drive motor both to cut the wire and twist the ends to form a tie. An impact drive motor is one in which a rotating 'hammer', driven by a motor, strikes an 'anvil' coupled to the output shaft. If the output shaft is free to rotate, the hammer remains in contact with the anvil to rotate the output shaft. However if there is a strong enough resistance to rotation by the output shaft, the hammer does not rotate the output shaft but instead the rotation of the driving motor is converted into an axial and/or radial motion, typically against a spring bias, to move the hammer away from the anvil. Eventually the hammer can rotate again without striking the anvil. When the hammer is once again able to rotate, its kinetic energy is enhanced by the potential energy stored in the spring and the now free running motor which has until the next hammer/anvil strike to impart kinetic energy into the hammer, and it is allowed to revert to its previous axial and/or angular position. This means that it will be aligned so as to strike the anvil, or more typically another similar anvil spaced circumferentially from the first, when it reaches the appropriate angular position. The subsequent impact of the hammer transfers its enhanced kinetic energy to the anvil as a high-torque pulse. The torque transmitted by the impact is dependent on the resistance torque at the anvil.

The Applicant has realised that this is a very elegant solution which reconciles the apparently conflicting requirements described above since an impact drive motor can deliver a high instantaneous-torque pulse when it encounters a high static resistance, making it suitable for initially cutting the wire; however when it encounters a low resistance it can simply transmit the rotation of the driving motor direct and uninterrupted to the anvil and hence the output shaft. The torque of the driving motor need only be high enough to turn the twisting mechanism after breaking; the impact drive mechanism provides the torque amplification to cut the wire.

Accordingly the requirement for a very high initial torque is met by the initial impact or series of impacts without the need for that high torque to be provided directly by the motor. Thereafter the twisting head can be driven at high speed by the motor whilst having a low power requirement commensurate with the low torque required to twist the wire. As a result a much smaller lighter motor and lighter and simpler gearbox can be used. The impact drive system preferably comprises a driving motor, an output shaft coupled to the twisting head and an impact drive mechanism between the driving motor and the output shaft. The impact drive mechanism preferably comprises an anvil member coupled to the output shaft. The impact drive mechanism preferably comprises a hammer member mounted to allow rotation relative to the anvil member. The hammer member is preferably resiliently mounted on a carrier member, said carrier member being coupled to the driving motor, possibly through a gearbox. Preferably the hammer member is coupled to the carrier member such that relative rotation between them disengages the hammer member from the anvil member. In a set of preferred embodiments the hammer member is mounted to the carrier member on one or more helical ramps to provide an axial disengagement. In other embodiments the hammer member could be disengaged radially, or a combination of the two.

The hammer member could have other forms, but preferably comprises one or more discrete protrusions. Preferably the anvil member comprises one or more corresponding protrusions for engagement with the hammer protrusion(s). The hammer and anvil members could have different numbers of protrusions but preferably have the same. In a set of preferred embodiments the hammer and anvil members each have two diametrically opposed protrusions. In a set of preferred embodiments the protrusions protrude axially from the body of the respective member. In one set of embodiments they are substantially rectangular, although the faces thereof are conveniently arcuate rather than planar.

The machine could be relatively large and operated robotically, either manually or automatically, e.g. to tie a large number of ties simultaneously. Preferably however the machine comprises a cordless hand-held tool. The motor could be powered pneumatically or otherwise, but preferably is electrically driven by a battery integral to the tool.

It will be seen therefore that not only can a very simple mechanism be employed, which makes manufacturing less costly and makes the tool more reliable, but in preferred embodiments which incorporate an electric motor and battery, the lower power requirement translates into a lower current drain requirement which means that a smaller battery can be used and/or it will last longer between charges than is the case with previous proposals. There is also a positive knock-on effect on the control electronics which can be operated at a lower voltage meaning that smaller components can be used, that they have a longer mean life, and that less heat is generated.

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1a is a perspective view of an apparatus embodying the invention above a pair of crossed bars prior to a tying operation being initiated;

Fig. 1 b is a view similar to Fig. 1a with the main mounting bracket removed;

Fig. 2 sectional view through the apparatus shown in Fig. 1 ;

Fig. 3 is a view of the apparatus from beneath;

Fig. 4 is a sectional view similar to Fig. 2 showing the apparatus part-way through a tying operation;

Fig. 5a is another sectional view showing the wire tensioned prior to twisting;

Fig. 5b is an enlargement of the circled part of Fig. 5a;

Fig. 6 is an exploded view of the parts of the impact drive mechanism; and

Figs. 7a to 7d are views of the impact drive mechanism at various points during its operation.

Referring first to Figs. 1a, 1 b and 2 there are shown two perspective views and a sectional view of part of an apparatus in accordance with the invention with certain parts such as the housing, handle, battery, controls, lower shroud and wire spool removed for clarity. The apparatus is shown situated over a junction where two steel bars 2 cross over each other at right angles. The steel bars 2 are intended to form a rectangular grid to be embedded in a concrete structure in order to reinforce it.

Sitting in use above the uppermost bar 2 is the rotary head of the apparatus 4. This includes a horizontal circular base plate 6 extending up from which is a channel 8 which is approximately semi-circular in vertical section and of approximately constant width in the orthogonal direction. The underneath of the base plate 6 is shown in Fig. 3 from which it will be seen that on one side there is a narrow slot 10 corresponding to one end of the semi-circular channel and on the other side of the plate 6 corresponding to the other end of the channel is a funnel region 12.

Returning to Figs. 1a, 1 b and 2, attached to the semi-circular channel 8 is the upper cylindrical portion of the head 14 which is rotatably mounted in the cylindrical portion 16a of a bracket member mounted to the housing (not shown) by a flange portion 16b (omitted from Fig. 1b). The upper head portion is supported by two rotary bearings 18. A toothed gear wheel, 20 is provided fixed at the top of the head to allow it to be driven by a motor 22 via a worm gear. However the head may instead be driven directly by the motor. The motor is an impact drive motor and will be described in greater detail below with reference to Figs. 6 and 7a to 7d.

Extending through the gear wheel 20 into the open upper end of the head 4 is a solenoid assembly comprising a cylindrical outer tube 26 housing the coil and an inner plunger 28 which is able to slide vertically relative to the coil 26. At the bottom end of the plunger 28 is an actuating disc 30, the purpose of which will be explained later.

The internal construction of the head 4 will now be described. On the left hand side as seen from Fig. 2, there may be seen a pivotally mounted angled clamping member 32. The member comprises a longer, upper arm and a shorter, lower arm. A pair of compression springs 36 act on the upper arm so as to bias the member in an anti-clockwise direction in which the lower arm is held in contact with the wire. Of course any number of springs might be used or the springs could be omitted.

The Figures show the clamping members 32 only schematically, and so do not allow a deduction to be made as to angles. However in one example the angle between the central axis of the lower arm of the clamping member 32 and the normal to the wire (i.e. the direction perpendicular to the length of the wire) is between 12 and 15 degrees.

To the right of the clamping member 32 are a series of roller wheels 38a, 38b, 38c the purpose of which will be explained below. A second clamping member is provided displaced approximately 180 degrees around the head. This is not therefore visible in the sectional view.

To the left of the upper head portion 14, connected to the main bracket flange portion 16b, is a wire feed inlet guide 40 which receives the free end of wire 46 which has been unwound from the spool (not shown).

Fig. 6 shows schematically most of the parts of the impact drive motor. At the right hand side of the Figure is a carrier member 50. This comprises an input shaft 52 connected to the driving motor or gearbox (not shown) and an annular wall 54 which gives it a generally cup-shaped form. On the inside of the annular wall is a series of helically-extending ribs 56. Next along the axis is a hammer member 58 which is generally disc-shaped with an annular wall 60. The annular wall 60 has a slightly smaller diameter than the wall 54 of the carrier member and it is formed with a series of helically-extending slots 61 which correspond to the ribs on the carrier member 50 so that, when assembled, hammer member 58 can telescope in and out of the carrier member 50 as they rotate relative to one another. In practice a compression coil spring (omitted for clarity) is provided between the carrier member 50 and the hammer member to provide a resilient bias towards the 'telescoped out' position.

Protruding from the annular wall 60 of the hammer member, in a direction away from the carrier member 50, are a pair of hammer protrusions 62. The hammer protrusions are generally rectangular and arranged at diametrically opposed points of the annular wall 60.

Finally there is an anvil member 64, which has a pair of anvil protrusions 66 corresponding to the hammer protrusions 62. The anvil member also has an output shaft 68 which is coupled to the twisting head either directly or through the worm drive or gearbox arrangement shown in the previous Figures.

Operation of the apparatus will now be described. The apparatus is first placed above the uppermost of a pair of steel reinforcing bars 2 which are crossed at right angles. The operator may then commence the tying operation. The first part of this operation is to energise the solenoid coil 26 which pushes the plunger member 28 downwardly. This causes the actuating member 30 at the end of the plunger to be pressed downwardly onto the upper arms of the clamping members 32 to press them down against the respective compression springs 36 and therefore raise the shorter, lower arms. This is the position which is shown in Figure 2.

A motor (not shown) is operated to drive a wire feed roller (also not shown) that acts on the wire 46 to feed it from the spool through the wire inlet guide 40 and into the aligned channel in the upper head portion 14. The wire is fed in horizontally past the end 32a of the first clamping member 32 which is held away from the wire by the actuating disc 30 acting on the long arm of the clamping member. The wire encounters the first of the passive rollers 38a. The first roller 38a causes the wire to bend downwardly slightly so that it passes between the second and third rollers 38b, 38c. The relative positions of the three passive rollers 38a, 38b, 38c is such that when the wire 46 emerges from them it is bent so as to have an arcuate set. As the wire 46 continues to be driven by the wire feed roller, it encounters and is guided by the inner surface of the semi-circular channel 8.

When the wire 46 emerges from the channel 8, its arcuate set causes it to continue to describe an approximately circular arc, now unguided in free space, around the two reinforcing bars. This is shown in Figure 4. As the wire 46 continues to be driven, the free end will eventually strike the mouth of the funnel region 12 in the bottom of the base plate 6 and therefore be guided back into the semi-circular channel 8. However it is not guided back precisely diametrically opposite where it was issued from but rather slightly laterally offset therefrom. This allows the second clamping member (not shown) to be located next to the first clamping member 32 which enables the apparatus to be kept relatively compact.

As the free end of the wire re-enters the semi-circular channel 8, it passes beneath the second clamping member, also held away from the wire end by the actuating disc 30 acting on the long arm of the clamping member.

Once the free end of the wire 46 is detected by a suitable detector, the motor feeding the wire is stopped and therefore the wire does not advance any further. At this point the solenoid coil 26 is then de-energised which causes the plunger 28 to be retracted by a spring (not shown) which releases the two clamping members 32 so that the respective compression springs 36 act to return the respective ends 32a into contact with the two ends of the wire loop and therefore hold the wire 46 in place.

The wire feed motor is driven in reverse in order to apply tension to the wire loop which draws the wire in around the reinforcing bars 2. This may be seen in Figure 5a. Figure 5b shows detail of the clamping member 32 on the feed side clamping the end of the wire 46. A similar arrangement clamps the other end of the wire as explained above. As the tension in the wire increases, the ends of the clamping members 32a roll over the wire slightly. The curvature of these ends of the clamping members 32a causes them to increase the clamping force as the tension in the wire increases to firmly clamp the ends. When the wire 46 is fully tensioned it will be seen from Fig. 5a that the two ends of the loop are pulled up almost vertically from their initial circular profile.

Next the impact drive motor 22 is energised. The motor itself (not shown) begins to spin which rotates the carrier member 50. This in turn rotates the hammer member 58 which is at its axially outermost position relative to the carrier member. Within half a rotation the hammer members 62 will strike the anvil members 66 as shown in Fig. 7a. Initially, because the wire 46 crosses from the inlet guide 40 to the upper head portion 14, the head 4 is prevented from rotating which therefore prevents the anvil member 64 from rotating. However rather than stalling the motor, continued rotation of the carrier member 50 causes the hammer member 58 to begin to telescope into it, compressing the coil spring.

As the hammer member 58 telescopes into the carrier member 50, the hammer protrusions 62 are withdrawn axially along the anvil protrusions 66, reducing the amount they overlap as may be seen in Fig. 7b. When the hammer member 58 has telescoped fully into the carrier member 50, as shown in Fig. 7c, the hammer protrusions 62 are drawn axially fully free of the anvil protrusions 66 and thus the hammer member 58 can once again be rotated by the carrier member 50 relative to the anvil member. The relative rotational speed between the hammer member 58 and the anvil member 64 is enhanced by the rotation of the hammer member 58 with respect to the carrier member 50 as it telescopes out again, guided by the helical ribs and slots 56, 61 , under the action of the coil spring. As well as giving the hammer member extra rotational speed, this also brings the hammer protrusions 62 back into alignment with the anvil protrusions 66.

The result is that after another half-revolution, the hammer protrusions 62 strike the anvil protrusions 66 with significant force, thereby imparting a high impulse of torque, before the process is repeated. Only a few such impacts are necessary to shear the wire 46. With the wire thus broken, when the hammer protrusions 62 next strike the anvil protrusions 66, instead of the hammer member 58 telescoping into the carrier member 50, as there is no longer significant torsional resistance, the hammer member 58 simply drives the anvil member 64 and output shaft 68 as shown in Fig. 7d, thereby rotating the head 4 and beginning to twist the sides of the loop together above the reinforcing bars 2

As the twisting proceeds, the tension in the wire 46 continues to increase. The shape of the rounded ends 32a of the clamping members causes them to roll over the end of the wire and bite down harder on the wire to increase the clamping force on the wire so that a very tight tie can be formed. For example the clamping member might pivot between 0.5 and 1 degree as the tension increases. The maximum clamping force applied is for example between 2000 and 3000 Newtons. When the tension in the wire reaches a maximum value, e.g. in the range 250-350 Newtons, the maximum clamping force applied by the clamping members can no longer hold the ends of the wire and the wire then slips past the clamping members 32 until it is released. This maximum wire tension gives rise to a much lower torsional resistance to the head 4 twisting than is encountered before the wire is initially broken. Thus while the hammer member 58 of the impact drive mechanism might telescope a little way into the carrier member 50, it will not be sufficient for the hammer protrusions 62 to bypass the anvil protrusions 66.

The continued twisting of the head 4 causes the ends of the wire to be neatly wrapped at a low tension as the ends of the wire are pulled completely out of the head. This reduces the risk of sharp ends being left protruding which would be a snagging hazard. The machine can sense when the ends of the wire have come out as there will be a sudden reduction on the torque on the motor driving the impact drive mechanism. This can be sensed by a corresponding reduction in the electrical current drawn by the motor. The motor can be stopped when this is sensed or a short time thereafter to allow final twisting of the emerging ends of the wire.

It will be appreciated from the above, that the use of an impact drive mechanism means that a high peak torque can be delivered to the wire to shear it initially even with a relatively modest motor. To give an example of the benefit that can be achieved in accordance with one embodiment of the invention, using a Bosch 14.4V GDR impact driver mechanism available from Robert Bosch GmbH weighing 1.0 kg, a 135 Nm initial torque can be delivered at a running speed of 2800 rpm. This gives sufficient torque to break a standard 16 gauge wire used for reinforcement bar binding and a high speed tying operation. By contrast a typical conventional motor used for such a tying machine (Bosch GSR 14.4 VE2) is unable to provide sufficient torque even at 1400 rpm to cut the wire; the speed must be reduced to below 400 rpm before it can deliver even 70 Nm. Furthermore this previously used motor weighs an additional 0.6 kg which is clearly significant in the context of a hand tool which is typically lifted and used hundreds of times a day.