GB2328635A

GB2328635A - Impact screwdriver

Info

Publication number: GB2328635A
Application number: GB9818222A
Authority: GB
Inventors: Helmut Achtzehnter
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1997-09-01
Filing date: 1998-08-20
Publication date: 1999-03-03
Anticipated expiration: 2018-08-20
Also published as: JP4309498B2; DE19738094C1; GB9818222D0; GB2328635B; JPH11123664A

Abstract

The invention relates to an electric tool, in particular to an impact screwdriver, with a rotary drive motor, a tool and means of superimposing on the rotary motion a hammering motion. An impact hammer 16 acts upon the tool holder and executes an attemating axial motion. Means of detecting an actual strike number per unit of time comprise a switching means (22) operable directly by the impact hammer (16), an operating means (24) of the switching means (22) being disposed inside the path of axial motion (20) of the impact hammer (16). Upon a pre-set count being achieved a switching device disconnects the electric tool.

Description

2328635 1 Electric tool The invention relates to an electric tool, in

particular to an impact screwdriver, having the features indicated in the preamble of claim 1.

Background art

Electric tools of the type described are known. They are used, for example, as socalled impact screwdrivers or hammer drills. In said tools, a tool holder, which may be equipped with a selectable tool, is set in rotation by a drive motor. A hammering motion, during which an impact hammer acting upon the tool holder executes an 10 alternating axial motion, is superimposed on the rotary motion of the tool holder.

When such an electric tool is used, for example, for defined production of a screwed connection, it is desirable upon attainment of the end position of the screwed connection to disconnect the electric tool because, otherwise, the screwed connection isis mechanically loaded by the impact hammer. From EP 0 552 990 B 1 an electric tool is known, in which a strike number of the impact hammer is detected by means of a microphone. In said tool, the striking noises of the impact hammer are picked up by the microphone, converted into an electrical signal and supplied to a target device, which counts the received pulses and compares their number with a setpoint 20 number of pulses. When the actual strike number reaches the setpoint strike number, the electric tool is disconnected. The drawback of said known arrangement is that detection of the strike number entails indirect measurement in that the impact is converted (inevitably) into a noise and said noise is picked up by a diaphragm of a microphone and then converted into an electrical signal. As a result, inaccuracies are possible during detection of the actual strike number.

2 Advantages of the invention The electric tool according to the invention having the features indicated in claim 1 offers the advantage of enabling accurate measurement of an actual strike number of the electric tool in a simple manner. By virtue of the fact that the strike counting is effected by a switching means operable directly by the impact hammer, an operating means of the switching means being disposed inside the path of axial motion of the impact hammer, it is possible without intermediate conversion of the striking motion into a physical quantity to effect direct and hence very accurate measurement of the strike number. With each axial motion of the impact hammer the switching means is operated, making it possible for the number of strikes to be inferred immediately from the number of operations of the switching means. By means of simple, known per se, evaluation circuits said received pulses may be processed, e.g. compared with a preselectable setpoint strike number, with disconnection of the electric tool advantageously being effected upon the actual strike nuniber reaching a setpoint strike number. Since the number of strikes is in a known manner torque-dependent, the measured actual strike number may easily be used to realize a torque adjustment of the electric tool.

In a preferred refinement of the invention, it is provided that the switching means comprises a magnet, which is displaceable along a Hall sensor and, depending on the position of the impact hammer, is moved into and out of the effective range of the Hall sensor. In dependence upon the position of the magnet there is generated in the Hall sensor a Hall voltage of a specific frequency, which corresponds to the strike number of the impact hammer. Said Hall voltage may, as an electrical signal, be evaluated by means of an evaluation circuit. Such a switching means may be manufactured with a very small overall size so that it may easily be integrated into 3 electric tools without either substantially increasing their weight or requiring the availability of a large installation space.

Further advantageous refmements of the invention arise from the remaining features 5 indicated in the sub-claims.

Drawings There follows a detailed description of an embodiment of the invention with 10 reference to the accompanying drawings. The drawings show:

Figure 1 a diagrammatic sectional view of an impact screwdriver and is Figure 2 a sectional view of a switching means for detecting the strike number of the impact screwdriver.

Description of the embodiment

Figure 1 shows a diagrammatic sectional view of an impact screwdriver 10. The 20 latter has a tool holder 12, which is disposed on the drive shaft of an electric motor (not shown) and may be equipped with various tools (not shown), e.g. crosspoint screwbits, plainslot screwbits, bolt screwbits etc. An anvil 14 carried in a nonrotatable manner by the tool holder 12 is radially strikable by an impact hammer 16. Provided for said purpose is a mechanism, which is not shown in detail and which in dependence upon a rotational speed of the tool holder generates an axial sg motion of the impact hammer 16. The axial motion is achieved by means of an oblique guide groove, the energy of the motor, which continues to rotate relative to the stationary tool, therefore being stored in the compression spring and, after 4 sufficient "wmding-up" m axial direction, bemig released as a rotary impact upon the anvil. During said process the impact hammer 16 is preloaded counter to the force of a resilient spring element e.g. a spiral spring 18, and upon attainment of a specific angular position of the tool holder 12 is knocked by the force of the spring element 18 against the anvil 14. As a result, the striking motion of the ninpact hammer 16 is superimposed on the rotary motion of the tool holder 14 so that, for example, a screw or the like may be driven in faster by means of the hammer dn 1110.

In a known manner, it is frequently desirable to enable the screwed connection to be produced with a specific, selectable torque. This means that the hammer drill 10 disconnects upon attainment of the desired selectable torque.

The impact hammer 16 during striking executes an alternating axial motion indicated by the double arrow 20, the impact hammer either being preloaded counter to the force of the spring element 18 or being moved in the opposite direction by the force of the spring element 18. Disposed within the range of axial motion, which is predetermined by the design of the hammer drill 10, is a switching means denoted as a whole by 22. Said switchmig means 22 comprises an operating means 24, which is loadable with an operating force by an end face 26 of the impact hammer 16. The moment the impact hammer 16 is preloaded counter to the force of the spring element 18, the end face 26 comes into contact with the operating means 24 and pushes the latter into the switching means 22 counter to the force of a spring element 28. The operating means 24 carries a permanent magnet 30 which, upon movement of the operating means 24, is likewise axially displaced. For assembly of the operating means 24 and the permanent magnet 30, the latter are disposed in a housing 32 forming a bore 34, inside which the operating means 24 and the permanent magnet are displaceable. A Hall sensor 36 is disposed on the outerperiphery of the housing 32. The Hall sensor 36 is positioned in such a way that the permanent magnet 30 in the initial position of the switching means 22 is not situated in the effective range of the Hall sensor 33. The Hall sensor 36 is connected by connecting leads 38, which are merely indicated here, to an evaluation circuit 40, the parameters of which are adjustable"by a setting means 42 which may be formed, for example, by 5 a manually operable setting wheel.

Figure 2 shows an enlarged view of a possible concrete embodiment of the switching means 22. Parts identical to those in Figure 1 are provided with the same reference characters. The housing 32 may be formed, for example, by a cube-shaped basic body 44, inside which the bore 34 takes the form of a stepped bore. An annular step 46 of the bore 34 serves as a stop for the operating means 24, which is formed by a bolt 48 which springs from a guide portion 50. The guide portion 50 has an outside diameter substantially corresponding to the inside diameter of the bore 34 so that said guide portion is axially displaceable without clearance inside the bore 34. The end face of the bolt 48 has a crowning 52, which comes into contact with the end face 46 of the impact hammer 16. The crowning 52 ensures a substantially point- like contact with the end face 26.

The guide portion 50 carries the permanent magnet 30, it being possible for there to 20 be a frictional connection between the permanent magnet 30 and the guide portion 50, e.g. as a result of an adhesive join. According to a further embodiment it is conceivable for the permanent magnet 30 to rest loosely against the guide portion 50. Acting against the permanent magnet 30 is the spring element 28 which may be forTned, for example, by a spiral spring. One limb 54 of the spring element 28 exerts a spring force upon the permanent magnet 30 so that the latter presses against the guide portion 50 and hence holds the operating means 24 in its initial position. The operating means 24 occupies the - viewed in the drawing plane in Fig. 2 - right initial position when the guide portion 50 rests against the annular step 46. The spring 6 element 28 is guided on a pin 56, a further limb 58 of the spring element being supported against the housing 332. The housing 32 has an appropriate recess 60 for receiving the spring element 28.

In Figure 2, an initial position of the permanent magnet 330 'm relation to a longitudinal axis 62 of the bore 34 is projected into the paper plane and denoted by a. In relation to said initial position a, the Hall sensor 36 (Figure 1) is arranged so as to be disposed in an effective range w, which is situated between the initial position a and an end position a,.

The hammer drill 10 operates as follows:

During correct use of the hammer drill 10 the tool holder 12 is set in rotation by the drive motor (not shown). At the same time the impact hammer 16 executes an alternating axial motion 20 in dependence upon the rotation of the tool holder 12. During said process the operating means 24 of the switching means 22 is, depending on the instantaneous position of the impact hammer 16, either displaced into the effective range w counter to the force of the spring element 28 or pushed out of the effective range w by the spring element 28. The moment the permanent magnet 30 passes into the effective range w of the Hall sensor 3 6, a Hall voltage is in a known manner generated in said Hall sensor. In accordance with the sg frequency of the impact hammer 16 the permanent magnet 30 is moved with a corresponding frequency into and out of the effective range w, with the result that a corresponding signal characteristic may be picked off at the Hall sensor 36. The signal characteristic of the Hall sensor 36 is evaluated by means of the evaluation circuit 40, the evaluation process for example consisting of counting the number of maxima and/or the number of minima of the signal characteristic. The number determined is compared with a number filed in a programmable memory. The number of maxima 7 and/or minima acquired by means of the Hall sensor 36 corresponds to an actual strike number of the impact hammer 16, while the number filed in the programmable memory corresponds to a setpoint strike number of the impact hammer 16. The setting means 42 may be used to vary the setpoint strike number filed in the programmable memory. For said purpose the setting means 42 is coupled, for example, to a suitable register which calls the programmable memory.

When during correct use of the hammer drill 10 a specific tightening position of the screwed connection to be produced is reached, the torque acting upon the tool holder 12 increases. Said applied torque has a braking effect upon the rotating tool holder 12 and reduces its speed. This in turn leads to a reduction in the striking number of the impact hammer 16, the striking number being proportionally dependent upon the speed. There is a corresponding change in the characteristic of the Hall voltage of the Hall sensor 36 detected by the switching means 22, with the result that the actual is strike number and hence the speed proportional to the actual strike number and hence the torque are detectable by means of the electronic circuit 40. Thus, the setting means 42, by means of which the setpoint strike number and hence the setpoint torque are preselectable, enables very precise adjustment of the impact screwdriver 10. In particular, even low torques may be adjusted very accurately with reproducible results. By virtue of the axial displacement of the permanent magnet 30 along the longitudinal axis 62 of the bore 34, which displacement is proportional to the axial motion 20 of the impact hammer 16, the motion of the impact hammer 16 may be mapped and evaluated very accurately and directly. In particular, because the operating means 24 is made of a non-magnetic material and a negligible magnetic influence is exerted by the spring element 28 upon the permanent magnet 30, the switching means 22 may be realized with a very small overall size and correspondingly low switching tolerances. The separation between mechanical detection of the axial motion 20 and electronic evaluation by means of the Hall 8 sensor 36 and the electronic circuit 40 provides a maximum degree of operational reliability, which is essential in view of the rugged operating conditions of hammer drills 10.

9

Claims

1. Electric tool, in particular an impact screwdriver, having a tool holder which may be set in rotation by a drive motor and equipped with a tool, and having means of superimposIng on the rotary motion a hammering motion, during which an impact hammer acting upon the tool holder executes an alternating axial motion, and having means of detecting an actual strike number per unit of time of the impact hammer, wherein upon the actual strike number reaching a setpoint strike number a switching device for disconnecting the electric tool activates, characterized in that the strike counting is effected by a switching means (22) operable directly by the impact hammer (16), an operating means (24) of the switching means (22) being disposed inside the path of axial motion (20) of the impact hammer (16).

2. Electric tool according to claim 1, characterized in that the switching means (22) comprises a magnet (30), which is movable along a Hall sensor (36).

3. Electric tool according to one of the preceding claims, characterized in that the magnet (30) is movable counter to the force of a spring element (28) into the effective range of the Hall sensor (36) by the operating means (24).

4.

Electric tool according to one of the preceding claims, characterized in that the magnet (30) is supported in an axially displaceable manner in an axial bore (34) of a housing (32), a guide portion (50) of the operating means (24) engaging into 2 5 the axial bore (34) and the Hall sensor (36) being disposed on the outer periphery of the housing (32) in the region of the axial bore (34).

5. Electric tool according to one of the preceding claims, characterized in that the operating means (24) is made of a non-magnetic material.

6. Electric tool according to one of the preceding claims, characterized in that the magnet (30) occupies an initial position (a), which does not lie in an effective range (w) of the Hall sensor (36).

7. Electric tool according to one of the preceding claims, characterized in that a generated Hall voltage of the Hall sensor (36) may be supplied to an evaluation circuit (40), which compares the actual signal characteristic with a setpoint signal characteristic, and upon the actual signal characteristic reaching the setpoint signal characteristic a signal for disconnection of the electric tool (10) is supplied.

8. Electric tool according to claim 7, characterized in that the setpoint signal characten stic and hence the setpoint strike number are van ably preselectable by a setting means (42).

9. An electric tool substantially as heremi described with reference to the accompanying drawings.