Method of monitoring and controlling a screwing process

Details Method of monitoring and controlling a screwing process Method of monitoring and controlling a screwing process

2001-08-29

2003-06-24

Kwok, Helen (Department: 2856)

Measuring and testing

Specimen stress or strain, or testing by stress or strain...

Threaded fastener stress

C073S597000, C073S598000

Reexamination Certificate

active

06581472

ABSTRACT:

TECHNICAL FIELD OF APPLICATION
The present invention relates to a method of monitoring and, if needed be, of controlling a screwing process in the creation of a screw connection using a screw, wherein ultrasonic pulses are repeatedly coupled into the screw starting from the head of the screw using a pulse-echo method and the time (propagation time) it takes the ultrasonic pulses to traverse the length of the screw is measured.
The present method is particularly suited for automated screwing processes in industrial production in order to continually determine the predetermined tension force of the screw during the screwing process and to be able to control the screwing process accordingly.
STATE OF THE ART
Reliable achievement of a minimum predetermined tension force in a screw connection in serial assembly plays an essential role in the manufacture of many products. Poor predetermined tension force may result in the failure of heavy-duty screw connections with serious consequences. Due to additional influencing factors, such as friction losses, traditional methods with torque control and torque angle control for producing a screw connection, however, do not achieve the precision required for reliable setting of the predetermined tension force. For several years, ultrasonic methods of determining the predetermined tension force have been employed in addition to these indirect means of determining the predetermined tension force.
For example, Krautkrämer GmbH & Co. sells ultrasonic control devices which repeatedly couple ultrasonic pulses into the screw starting from the head of the screw using a pulse-echo method before and during the tightening process and measure the time it takes the ultrasonic pulses to traverse the length of the screw. Taking into account the acousto-elastic effect, screw elongation can be calculated from the time it takes the ultrasonic pulse to traverse the length of the screw and therefrom the, if desired, tension or the predetermined tension force. The systems known under the names Stress Mike® and Boltmike® store in a microcomputer system the calibration constants for determining the predetermined tension from the measured propagation times. The calibration has to be determined for the respective screw materials before conducting the screwing process using concrete measurements and then those have to programmed into the computer. In one embodiment of the mentioned systems, upper and lower predetermined tension force limit values, with an alarm signal sounding off when they are achieved, can also be entered.
In both systems, a separate measuring probe is mounted on the head of the screw via which the ultrasonic pulse can be coupled in and the reflected signals can be received.
In order to improve sound propagation and reflection relationships, the surfaces of the screw head and screw end are designed by the screw manufacturer according to various optimization concepts. Thus, for example, it is known to apply a piezoelectric layer on each screw head during screw production. The ultrasonic pulses can then be generated in a simple manner via this piezoelectric layer simply by means of suited electric activation and the reflected signals can be received again. This facilitates, in particular, application of known ultrasonic methods in rotating screw tools, as is realized in Ultrafast®'s control systems.
Despite many attempts to improve the methods of determining the predetermined tension force of screws, hitherto such type systems have proven unsatisfactory in regard to measuring precision and reliability in serial production, particularly in automobile production. Especially the required precision and reliability have hitherto not fulfilled expectations.
DE 42 31 429 C1 describes a screwing process in which monitoring the tightening process is conducted using an ultrasonic measuring process. The ultrasonic pulses are repeatedly coupled into the screw starting from the head of the screw using a pulse-echo method before and during the tightening process and the time it takes the ultrasonic pulses to traverse the length of the screw are measured (propagation time). In order to begin the screwing process, a first propagation time measurement is conducted on the load-free screw and the measured value is stored. During the screwing-in process respectively the tightening process, further propagation time measurements are conducted with a high rate of repetition. The detected changes in the propagation time are compared with reference values the propagation time change, which were previously recorded in a table. These table reference values were previously determined empirically. When a change in the propagation time corresponds to a desired predetermined tension in the table is achieved, tightening is discontinued.
Based on this state of the art, the object of the present invention is to provide a further method of monitoring and, if need be, of controlling a screwing process in the creation of a screw connection using a screw which provides great reliability and precision and, in particular, permits detection of any significant deviation from the prescribed specifications of the used screw.
DESCRIPTION OF THE INVENTION
In the present method ultrasonic pulses are repeatedly coupled into the screw starting from the head of the screw using a pulse-echo method before and during the tightening process and the time it takes the ultrasonic pulses to traverse the length of the screw is measured. It is fundamentally sufficient to measure the value of the sound propagation time of the load-free screw once before starting to tighten, for example already during screwing in of the screw. However, repeated measuring increases measuring precision.
Furthermore, prior to starting to tighten the screw, the change in sound propagation time is calculated from the material-specific characteristic values of the screw and from the once or repeatedly measured values of the sound propagation time in the load-free screws before tightening the screw as a function of the tension in the screw respectively the predetermined tension of the screw is calculated and stored. This calculation is based on the known material-specific constants of the type of screw employed. From the sound propagation time values repeatedly measured during tightening of the screw, at least one value is determined for the relationship between the change in the propagation time and the change in a screw parameter which influences the predetermined tension of the screw. This screw parameter may be, for example, the torque, the torque angle or the screwing-in time with a constant torque. Finally during tightening of the screw, the value or values for the relationship between the stored dependency of the changes in propagation time and the change in predetermined tension determined from the propagation time measurements are compared.
This comparison of the rise of a calculated theoretic curve of the type of screw used with the values measured during tightening of the screw permits drawing a conclusion as to whether the actually employed screw meets the specifications. Furthermore, this comparison permits drawing conclusions as to whether the selected screw parameters have been correctly selected for carrying out the current screw process. Thus, the comparison can, in particular, be utilized to influence the respective screw parameter in the event of a deviation from the values determined from measuring the propagation time from the stored values in order to minimize the deviation. A deviation, lying significantly above or below the prescribed value, of the value determined from the propagation time measurements from a stored value is preferably indicated so that, if need be, the screwing process is discontinued manually or automatically. Such a significant deviation is a sign that the specifications of the used screw do not meet the prescribed specifications. Furthermore, in such an event there may be unanticipated secondary conditions for screwing which may negatively influence the screwing result. Keeping in mind

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