Method of and device for tightening a screw connection.

p0001The invention relates to a method according to the preamble of claim 1 and an apparatus for implementing the method.

p0002In automated production processes, especially in engine and automotive screw are manufactured by machines. Such machines have electric screwdriver which z. B. Tighten the nuts by hand cranked automatically. The problem here is to tighten the screw in exactly reproducible manner so that z. B. the cylinder head sits with a predetermined contact pressure on the engine block.

p0003It is well known that one - can then cancel the screwing if the applied torque that the contact pressure is proportional even substantially, has reached a certain amount - similar to the assembly by hand.

p0004To save costs, to choose the strength of the screw connection, eg. As the diameter of the stud, only so large that just the desired holding force can be taken up. When tightening the screw connection is now loaded the material to above the Hooke's area so that an elongation occurs.

p0005From DE-OS 17 03 681 is known a screw in which the tightening of the screw is then stopped when the slope of the torque has reached a predetermined value after the rotation angle. Here, the knowledge is used that during tightening of a screw is initially present, a linear region (elastic deformation) of the torque according to the rotational angle and decreases the slope of the curve after reaching the yield point. Here, it is assumed that the course of the fastening operation is known and predictable. It is here therefore always necessary to first create a sample-threaded in order to measure the desired curve slope and store in the screwing can.

p0006From DE-PS 23 36 896 a device is known, in which first the slope of the curve is measured in the linear range and stored, and then, as soon as changes the slope of the curve, or flattens, the relationship between this (maximum) and slope determines the current slope of the curve and with a (minimum) value is compared. Once this minimum value is not reached, stops the screwdriver. The disadvantage of this device or method but that the moment curve is not ideally smooth or continuous runs, but wavy due Materialrauhigkeiten. This results in that the first derivative or the result of which has such a fluctuating course that the threshold is exceeded even at excessively high curvature means. Man tries thereby manage that one stops the power wrench only after a certain number of undershooting of the threshold value. If this number is too large, the screwdriver can no longer be stopped, because after reaching the desired curvature value occurs no more falling below threshold more.

p0007Starting from on above-mentioned prior art, the object of the present invention to further develop a method of the aforementioned kind such that a correct shutdown of the screwdriver in reproducible Längungswerten achievable.

p0008This object is achieved by the features specified in the characterizing part of patent claim 1 features an apparatus for performing the method described in claim 4th

p0009Surprisingly, it has been found that on the one hand, the second derivative of the analyzed function, the material behavior in the flow region reproduces much better and more reproducible than the slope (first derivative) of the function, on the other hand, the sharp peaks in from the relatively slow variations in the first derivative result, are filtered out by simple means, so that a limit is reached is more accurate than previously defined.

p0010Preferred embodiments of the invention result from the dependent claims and the following embodiments which are defined in detail with reference to figures. In the drawing:<ul><li>Fig. 1 is a diagram illustrating the torque curve over the rotational angle and</li><li>Fig. 2 is a schematic Blockchaltbild of the apparatus for performing the method.</li></ul>

p0011First, the inventive method is illustrated with reference to FIG. 1. In the figure, the upward force applied to the screwing torque M, to the right of the rotational angle β, or at a constant rotational speed, applied time.

p0012When one puts on a nut on a bolt with an electric screwdriver, so initially increases the torque applied to the landing moment M_AL and then decreases when tightening the screws almost up to zero. Once the nut face contact, has increased - provided contact the parts to be joined - the torque to the threshold torque M_S steeply and then goes into a linear region. This linear range means that the studs or any other elastically deformable parts, is tensioned. The resulting curve is with M_I designated. If the material of the stud is stretched over the Hooke's range, then the curve starts M_I flatten, thus differs from the linear course. If a certain degree of elongation of the bolt is reached the electric screwdriver has to be turned off or stopped.

p0013M'with the first derivative of the curve is M_I designated by M"_I is the second derivative of the curve M_I β on the angle (and after the time), respectively. According to a first preferred embodiment of the inventive method, the electric screw driver is then turned off when the second derivative M_I"A preset value K falls below or exceeds the amount stipulated. But occur upon initiation of screwing, z. B. in the area where the plant torque is reached, extreme fluctuations in the torque curve, the (still) are not to lead to a shutdown of the wrench, the torque curve is in addition to exceeding the threshold torque M_S monitored. Exceeding the Schwellmomentes M_S is an output signal A_S generated. Only with the simultaneous presence of the output signal A_S and falls below the threshold K by the second derivative of the torque representation 1 I will be given a shutdown signal A shown in FIG., the electric screwdriver is stopped.

p0014The cornering in the accompanying FIG. 1 is drawn idialisiert. In practice, the torque curve M passes_I more jagged, resulting in the curve M_I'To slow, in the curve M_I"Leads to very rapid fluctuations. These rapid fluctuations are smoothed by filtering (moving average or the like.). Since the fluctuations of the curve M_I"Extend substantially faster, ie higher than the frequent change of the slope of the curve M_I, The lower limit frequency of the "LPF" can be set relatively low, without running the risk that the screwdriver can not be stopped in time.

p0015In a further preferred embodiment of the invention, the switch-off signal A is issued immediately after falling below the threshold K, but only when the screw β yet an additional angular amount_n solid was pulled. This embodiment is shown in the curve II. This ensures that even in the event despite filtering premature fall below the threshold K nevertheless a sufficient elongation is obtained.

p0016Preferably to faulty bolted, resulting in fundamentally different torque curves as with M_II and M_III are designated, cover aufzu, in addition to the above-described method further decision criteria (priority) used to generate a Abschaltsignales A.

p0017In the case of M_IIIn which the second derivative of the torque according to the rotational angle never drops below the threshold K, then a turn-off signal A is added (curve III), when a preset maximum angle β_Max is achieved at this value but a minimum torque M_min has not been reached. Simultaneously, a warning is given so that an operator can detect the error and correct it if necessary.

p0018When faulty torque curve M_III the switch-off signal A is then added when the torque has a maximum value M_Max exceeds, a minimum rotation angle value β_min but has not yet been reached. Also in this case, a warning signal is given (curve IV).

p0019In the following the basic structure of the apparatus for performing the method according to the invention will be described. As is apparent from Fig. 2, the electric screwdriver includes an electric motor 10 to which a planetary gear 13 is connected downstream. The output shaft of the planetary gear 13 passes over a rotary encoder 11 and a torque transmitter 12, a spindle bearing 14 which carries a tool holder 15.

p0020The electric motor 10 is preferably a three-phase motor, which is driven via a servo amplifier 17th The servo amplifier 17 includes a four-quadrant control which, in conjunction with the three-phase motor, a particularly exact control (rapid shutdown). The servo amplifier 17 is powered by a power supply 16 with power.

p0021The output signals of the rotary encoder 11 and the torque sensor 12 are fed via a measuring interface 19 a (commercial) control electronics 18th The control electronics 18 is connected via a data bus to a processing unit 20 in connection, comprising an input keyboard 21 and an output screen 22nd

p0022The measurement interface 19 further comprises an output interface via which it is connected to the servo amplifier 17th

p0023The computing unit 20 is preferably constructed as a digital computer, which performs the calculation and comparison operations already described above, and the stop signal A of the control electronics supplying the eighteenth About the input keyboard 21 of the computer unit 20, the fastening parameters, as well as the limits described above are input. Further finds in the computing unit 20 and the (low-pass) filtering the torque curve in a conventional manner instead.

p0024For better monitoring the screwing, it is advantageous when the torque curve can be plotted against the angle of rotation of the screen 22nd In particular, it is advantageous if the display takes place when a fault occurs (curves M_II or M_III) Is present. The torque curve can then give the operator informs about the present fault. Furthermore, it is advantageous if the entry of the screw parameters carried in the interactive operation on the screen 22, so that the operator is prompted for storing data required for operations.

p0025An additional security monitoring results in an advantageous manner if a shutdown is (in addition to the above criteria) output when the electric motor 10 supplied current exceeds a maximum value. This ensures that even if an incorrect number of maxi times expended torque no motor overload can occur 10th The maximum current setting can also be set to a value of the maximum torque above M_Max equivalent. thereby achieved a double protection against destruction of the screw, even if the torque transducer has failed.

p0026Monitoring of the rotation angle Gebes 11 is thereby possible that if, despite power to the electric motor 10, no change in the rotation angle β is carried out, switches the system off, and outputs an alarm signal. This ensures that the torque curve as described above M_IIIn which never reach the maximum torque and the second derivative of the torque limits, and do occur breakage of the screw could be safely avoided.

p0027Selbsvertständlich extends the invention also on the possible combinations of the features described above.