Power screwdriver
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a power screwdriver having a torque or rotating angle sensing element associated with the output shaft. 
     2. Background 
     Power screwdrivers are known such as described in German Patent Specifications DE-AS 24 23 300 and DE-OS 31 27 753, which exhibit right-angle gear units as an output or downline spur gear units including transducers which serve to measure the torque and/or the rotation angle during driving. As soon as the required rotation angle and/or the required torque is obtained, the measuring/control electronics generates a signal, usually a cutoff signal, with the result that the driving operation is terminated. 
     In the known power screwdrivers, the transducers and the transmitter are provided in the area of the drive shaft. Consequently, for example, a torque which is applied to a screw can be measured only indirectly. This is because the right-angle gear unit of the spur gear unit is situated between the bit, which is in engagement with the screw, and the drive shaft. Such speed-transforming gear units and above all the occurring friction and the associated inefficiency of the gear unit adversely affect the measuring accuracy. In the case of safety-type screw connections, the screws are tightened to their yield point. However, when the yield point is reached, the power screwdriver must reliably switch off so that the screw is not inadmissibly strained. With the known power screwdrivers in which the transducers are disposed in the area of the drive or on the drive shaft, this requirement cannot be met. 
     The object of the invention is to provide a power screwdriver of the initially mentioned kind such that the transducers for the torque and/or rotation angle can be disposed without increasing the overall dimensions of the power screwdriver housing in such a way that the influence of the gear unit and the friction are eliminated during measuring. 
     SUMMARY OF THE INVENTION 
     The transducer and the transmitter of the power screwdriver according to the invention are disposed inside the housing of the power screwdriver in the area of the output shaft. The transducer is therefore situated in the area of the output part, with the result that the influence of the gear unit and of the friction occurring in the measurements during driving have no influence on the measuring accuracy. With a power screwdriver according to the invention, the parts which are to be screw-connected can be reliably tightened to the yield point without there being the danger of their being overstrained. As a result of the design according to the invention, the measured values are established directly by bypassing the speed-transforming gear unit and are tranmitted to the control electronics. The transmission element, which transmits the measured values from the transducer to the measuring/control electronics, is rotationally rigidly connected to the output shaft and rotates with the latter. Its slipring is contacted by the sensor with which the measured signals are relayed to the outside to the measuring/control electronics. The transmitter is situated in an easily accessible place inside the gear unit housing. 
     Further features of the invention will become apparent from the claims, the detailed description and the drawings. 
    
    
     The invention is explained in greater detail with reference to some embodiments shown in the drawings. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows in a diagrammatic representation a stationary power screwdriver according to the invention with flanged-on spur gear unit; 
     FIG. 2 shows in an enlarged representation and in a partial section the spur gear unit of the power screwdriver according to the invention in FIG. 1; 
     FIG. 3 shows a section taken on line III--III in FIG. 2; 
     FIG. 4 shows a section taken on line IV--IV in FIG. 2; 
     FIG. 5 shows in a diagrammatic representation a power screwdriver according to the invention with a right-angle gear unit; 
     FIG. 6 shows in an enlarged representation and in a partial section the right-angle gear unit of the power screwdriver according to the invention in FIG. 5 with integrated torque transducer and rotation angle pickup; and FIG. 7 shows in a partial section a further embodiment of a right-angle gear unit with a pressure transducer for indirect torque measurement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The power screwdriver shown in FIGS. 1 to 4 has a straight motor part 1 with a motor and a planetary gear unit. Disposed in known manner on the motor part 1 is the housing of a spur gear unit 2 which has a drive pinion and an output gearwheel 3 which is seated on an output shaft 3a. The output shaft 3a is supported in the gear unit housing 4 by two spaced-apart bearings 5 and 6. In the area between the output gearwheel 3 and the bearing 6 the output shaft 3a is provided with a torsion part 7 which is formed by a reduced-diameter section of the output shaft. Strain gauges 8, acting as a transducer, are bonded on the torsion part 7 in known manner as a signal generator for the torque applied to a screw. 
     The signal generated by the strain gauges 8 when a torque is applied to a screw is supplied via connecting lines 9 to a transmitter 10. The connecting lines 9 extend through a bore 11 in the output shaft 3a to the transmitter 10. The bore 11 may be provided centrally or eccentrically in the output shaft 3a. The transmitter 10 is disposed at the upper end of the output shaft 3a and has a pin-shaped transmission element 12 which is rotationally rigid in the bore 11. 
     The connecting lines 9 are connected to soldering lugs 13 of the transmission element 12 which is provided at its end projecting beyond the output shaft 3a with a number of sliprings 14 corresponding to the number of connecting lines 9. The sliprings 14 are contacted by sensors 15 of a brushgear unit 16 from which the measured signals are relayed in a multi-core cable 17 to the outside to the measuring/control electronics (not shown). 
     During screwdriving, the torsion part 7 is subjected to torsion by the reactive force of the screw. The magnitude of the torsion is a measure of the torque applied and is converted by the strain gauges 8 into corresponding signals which are evaluated by the measuring/control electronics. 
     In similar manner to the torque signals, the revolutions of the output spindle are also monitored, for example, by a rotation angle pickup 18 which is disposed on the output shaft 3a. The rotation angle pickup 18, which may be, for example, a magneto-resistive sensor, is likewise bonded on the output shaft 3a. In the specimen embodiment, the rotation angle pickup 18 is situated in the area outside the torsion part 7. In the area of the rotation angle pickup 18 the output shaft 3a is surrounded by a disk-shaped magnet core 19 which is attached on the inside of the gear unit housing 4 and whose active zone 20 (FIG. 4) facing the rotation angle pickup 18 is in the form of a spiral. Consequently, as the output shaft 3a turns, the distance between the rotation angle pickup 18 and the active zone 20 changes. The output voltage of the rotation angle pickup 18 changes according to this distance, as a result of which the precise measurement of the instantaneously covered rotation angle is made possible. The electrical lines 35 bringing the rotation angle signal from the rotation angle pickup 18 are likewise routed through the bore 11 in the output shaft 3a. 
     The two bearings 5, 6 are provided for guiding and supporting the output shaft 3a. The bearing 5 is fixed in the axial direction. On its side facing the bearing 6, the bearing 5 is in contact with the inward-projecting shoulder 21 of the gear unit housing 4. On the other side, the bearing 5 is axially locked by a nut 22 which is screwed into a threaded hole 23 in the gear unit housing 4 and with which the bearing 5 is pressed against the shoulder 21. The nut 22 is in the form of a cap nut within which there is a further nut 24 which is screwed onto a threaded, axial stud 25 of the output shaft 3a and which is supported on the inner race 26 of the bearing 5. Consequently, the output shaft 3a is axially fixed. The brushgear unit 16 is mounted on the nut 22. For the cable 17 a strain relief device 27 is provided which is likewise releasably mounted on the nut 22. The cable 17 is brought out of the gear unit housing 4 to the outside in known manner. 
     In this embodiment, the strain gauges 8 and the rotation angle pickup 18 are disposed on the output shaft 3a. Since the appropriate bit 28 (FIG. 1) is attached to the output shaft 3a, the corresponding torques and rotation angles can be measured directly during screwdriving and can be transmitted to the control electronics. Between the bit 28 and the strain gauges 8 as well as the rotation angle pickup 18 there is no interposed gear unit which might adversely affect the measuring accuracy. The transmitter 10 is easily accessible since it is mounted on the end face of the output shaft 3a directly under a removable cover 29 of the gear unit housing 4. 
     FIG. 5 shows a further embodiment of a power screwdriver. A right-angle gear unit 30 is provided on the straight motor part 1&#39; as a step-down gear unit. Provided as the right-angle gear unit is a bevel gear unit, of which one bevel gear 31 is shown in FIG. 6. The bevel gear 31 is likewise seated on the output shaft 3a&#39; which is rotatably supported in the gear unit housing 4&#39; by radial bearings 32 and 33 as well as by a thrust bearing 34. The thrust bearing 34 is situated in the area below the bevel gear 31 which is supported on the thrust bearing. The output shaft 3a&#39; has the reduced-diameter torsion part 7&#39; on which the strain gauges 8&#39; are mounted. The connecting lines 9&#39; are, as in the previous embodiment, routed through the bore 11&#39; in the output shaft 3a&#39; to the transmitter 10&#39; which is of identical design to the transmitter used in the previously described embodiment. Provided directly adjacent to the radial bearing 33 on the side facing the torsion part 7&#39; is the rotation angle pickup 18&#39; which is surrounded by the magnet core 19&#39; which is attached on the inside of the housing 4&#39;. Lines 35&#39;, which relay the rotation angle signal from the rotation angle pickup 18&#39;, are routed through the bore 11&#39;. 
     The transmitter 10&#39; is accommodated in a recess 36 of the gear unit housing 4&#39;, said recess 36 being closable by the cover 29&#39;. The brushgear unit 16&#39; is seated on an inward-projecting shoulder 37 of the housing 4&#39;. The transmission element 12&#39; with the sliprings 14&#39; as well as the sensors 15&#39; are of identical design to those in the previously described embodiment. The strain relief device 27&#39; is attached on the cover 29&#39;. The transmission element 12&#39;, which is mounted in the bore 11&#39;, rotates together with the output shaft 3a&#39; and the bevel gear 31 while the brushgear unit 16&#39; with the cable 17&#39; is stationary. The bearings 32 to 34 are axially locked inside the housing 4&#39;. At its end facing away from the transmitter 10&#39;, the output shaft 3a&#39; is provided with a connecting part 38 for the respective bit. 
     In this embodiment, which shows a predominantly hand-held power screwdriver, the measurements of the torque and of the rotation angle can once again be performed without impairment by the right-angle gear unit, with the result that the parts which are to be screw-connected can be brought reliably, for example, to the yield point. 
     FIG. 7 shows an embodiment of a power screwdriver in which the torque is measured by a force transducer 39 whose measured signals are relayed via the connecting lines 9&#34; in the described manner through the bore 11&#34; in the output shaft 3a&#34;. In this embodiment, the torque at the output is measured indirectly. This embodiment is similar in design in some respects to the embodiments shown in FIGS. 5 and 6. The step-down gear unit is once again a bevel gear unit, of which merely the bevel gear 31&#34; is shown. The output shaft 3a&#34; is rotatably supported inside the gear unit housing 4&#34; by two abutting radial bearings 32&#34;, 33&#34; and a thrust bearing 34&#34; near the connecting piece 38&#34;. The bevel gear 31&#34; is seated on a shoulder 40 of the output shaft 3a&#34; and is axially locked by a retaining ring 41 which is situated between the bevel gear and a further shoulder 42 on the inside of the gear unit housing 4&#34;. Furthermore, the retaining ring 41 is locked by a spring lock washer 43 which engages an annular groove 44 in the output shaft 3a&#34;. The transmitter 10&#34; is of identical design to the one used in the embodiment in FIG. 6. 
     The transmission of torque to the output bevel gear 31&#34; results in axial force components of the peripheral force. These force components are in a certain ratio to the transmitted torque and can, therefore, also be used to measure the torque. The axial force component of the bevel gear 31&#34; is transmitted via the shoulder 40 to the output shaft 3a&#34;. The latter, in turn, is supported via a further shoulder 45 on the force transducer 39 which, in turn, rests on the thrust bearing 34&#34;. The magnitude of the axial force component and thus the transmitted torque are measured by the force transducer and are relayed through the connecting lines 9&#34; to the transmitter 10&#34;, from where the signals are supplied via the cable 17&#34; to the measuring/control electronics. 
     The transducers 39 which is in the form of a force transducer is of annular design and is seated on the output shaft 3a&#34;. 
     In the embodiments in FIGS. 1 to 6, the torque is measured by means of strain gauges acting as transducers. Instead of such strain gauges, it is also possible to employ piezo measuring elements. Also possible are inductive transducers or permeability measurements which are performed at the torsion-exposed outer cylindrical surface of the output shaft. 
     In all embodiments, the sliprings 14, 14&#39;, 14&#34; are provided at their points of contact with the sensors 15, 15&#39;, 15&#34; with grooves, fillets or similar configurations in order to guarantee undisturbed transmission of the measured values also in the case of axial impacts or slight shifts of the output shaft 3a, 3a&#39;, 3a&#34;.