Measuring and monitoring a preload force in a screw connection

Abstract

The invention relates to a device for measuring a preload force in a screw connection, wherein the screw connection has a screw having an ultrasonic transducer for introducing ultrasound into the screw, having a securing device, a temperature sensor, an excitation component, which is designed to excite the ultrasonic transducer to transmit monofrequency ultrasonic waves when a start signal is received, a measuring component, which is designed to determine the time of flight from transmission of the ultrasonic waves to receipt of the echo by the ultrasonic transducer using an evaluation algorithm, which is free of pulse compression and cross-correlation, a signal-transmission element, which is connected to the excitation component and to the measuring component, and which is also connected to the ultrasonic transducer when the device is secured to the screw, a microprocessor, which is designed to receive the start signal from an input module and to forward same to the excitation component and to forward the received temperature signal, the received ultrasonic time of flight signal and an identifier to an output module. The invention also relates to a system for monitoring a preload force in a screw connection with a device of this type.

Description

[0001] Measuring and monitoring a preload force in a bolted joint

[0002] The invention relates to a device for measuring a preload force in a bolted joint, wherein the bolted joint comprises a screw with an ultrasonic transducer attached to a surface of the screw head or to a surface at the end of the screw opposite the screw head for introducing ultrasound into the screw. The invention also relates to a system for monitoring a preload force in a bolted joint.

[0003] For the purposes of this application, the term "screws" also includes bolts that do not have a screw head.

[0004] From US patent 2017 / 0105058 A1, a device of the type mentioned above is known, comprising a fastening device for attaching the device to the screw, a temperature sensor for measuring the temperature of the screw; an excitation component configured to excite the ultrasonic transducer to emit ultrasonic waves by means of an excitation signal upon receipt of a start signal; a measuring component configured to determine the transit time from the transmission of the ultrasonic waves until the reception of the echo by the ultrasonic transducer and to transmit a signal corresponding to the ultrasonic transit time; and a signal transmission element connected to the excitation component and to the measuring component, and also connected to the ultrasonic transducer when the device is attached to the screw.a microprocessor that is connected to the temperature sensor, the excitation component and the measuring component and is connectable to an input module and an output module and is configured to receive the start signal from the input module and forward it to the excitation component and to forward a temperature signal, an ultrasonic transit-time signal and an identifier to the output module.

[0005] For the precise determination of preload in bolted connections, an ultrasound-based method developed by INTELLIFAST, Speyer, Germany, is also known. This method is also based on the principle of transit-time difference measurement, i.e., the transit time from the transmission of the ultrasound signal to the reception of its echo is measured. This ultrasound transit time increases due to the thermal expansion of the screw or bolt and due to material stress (acoustoelastic effect). The method complies with DIN / EN 4859. A highly accurate measuring device was developed to perform the method, enabling the determination of the preload even in small, high-strength screws, such as an M4 titanium screw. This measuring device uses an evaluation method that employs several algorithms to determine the corresponding echo and transit time from the chirp signal using pulse compression and cross-correlation.This evaluation method is based on a measurement card (Data Acquisition Unit [DAU]) developed by INTELLIFAST, which generates so-called DAUgrams. These DAUgrams are then converted into the corresponding forces by the INTELLIFAST software "LoadLab". This method operates at a high frequency (it performs up to 1000 measurements per second). It is primarily used in research and development by, among others, aircraft manufacturers and suppliers, automotive manufacturers, and mechanical engineering companies to check and optimize bolted connections.

[0006] In recent years, this method and the corresponding measuring device have been increasingly used for large bolted connections (>M24), particularly in wind turbines. The primary interest in this application lies in measuring and documenting the preload forces in the bolted connections – for example, in wind farms – after installation and in monitoring them repeatedly over extended periods. This is done either manually or using a wired monitoring system.

[0007] For use with large bolted connections, especially in applications like the one described above, this method and the corresponding measuring device are too expensive.

[0008] Other well-known systems for measuring preload forces in large bolted joints are based on mechanical principles of length measurement, which is relatively inaccurate. Alternatively, they measure preload forces using applied strain gauges, which must be mechanically attached to the bolt. This requires drilling, milling, and the use of organic adhesives, which in turn alter the bolt's properties and make it more susceptible to corrosion. Furthermore, the organic components of the adhesive age over time.

[0009] Therefore, the objective of the invention is to propose a device and a system with which the preload forces in large bolted connections can be measured and monitored more cost-effectively.

[0010] According to the invention, this problem is solved by a device according to claim 1 and by a system according to claim 13. The measures according to the invention allow the preload forces of large bolted connections with a bolt length of 200 mm or more to be measured and monitored extremely economically, so that the number of monitored bolted connections in a flange connection – for example, in a wind turbine – can be considerably increased, thus ensuring the highest level of structural safety over long periods.

[0011] The invention is based on the finding that with large screws, especially those larger than M24, the ultrasonic transit times between the transmission of the ultrasonic waves and the reception of the echo are significantly longer than with, for example, an M4 titanium screw, which typically has a comparatively short length, due to the screw length also increasing with the screw diameter. This makes it possible to use monofrequency ultrasonic waves, i.e., single-frequency sound waves.

[0012] It has been found that monofrequency ultrasonic waves can reliably determine the ultrasonic transit time differences between an unloaded and a loaded state of a screw, even with a screw length as short as 200 mm, thus enabling the determination of the corresponding screw preload forces. From a screw length of 200 mm, these ultrasonic transit time differences are readily measurable even without high demands on resolution and accuracy, meaning they are no longer as critical as with shorter screws.

[0013] Preferably the screw length is at least 250 mm and particularly preferably at least 300 mm.

[0014] The use of monofrequency ultrasound waves enables ultrasound generation and echo detection without the need for high-frequency analog-to-digital converters or digital-to-analog converters. Consequently, data processing is simplified, as the evaluation algorithms do not require pulse compression or cross-correlation, which are necessary with multifrequency ultrasound waves. Therefore, data processing, including the temperature signal from the temperature sensor, in the microprocessor takes significantly less time, which in turn considerably reduces energy consumption.Due to the simpler ultrasound generation and echo detection / evaluation, fewer components are required, which also significantly reduces energy consumption. These components can be cost-effective standard parts, with a relatively large selection of standard parts available, allowing for the choice of energy-optimized components. Furthermore, the small number of components allows for compact external dimensions of the device, enabling easy attachment to screws. Additionally, the low component count results in a lighter device, preventing significant additional stress on the structure when the device is mounted on screw connections.

[0015] In an advantageous embodiment of the invention, the excitation component includes a pulse generator, and the excitation signal is an excitation pulse and the trigger signal is a trigger pulse. The microprocessor is configured to cause the pulse generator to send an excitation pulse to the ultrasonic transducer and to send an associated trigger pulse to the measuring component to start a time measurement. Pulse generators are standard components and readily available commercially. A pulse generator is pre-programmed and therefore contains pre-defined pulse patterns. As a result, it has low energy consumption and is cost-effective, in contrast to a digital-to-analog converter required in the prior art.

[0016] Preferably, the measuring component includes a converter configured to generate a sequence of square-wave pulses from the analog ultrasonic echo signal for determining the ultrasonic transit time. Since such a converter only generates a sequence of square-wave pulses, it is a relatively simple and inexpensive component, readily available as a standard part on the market. Furthermore, it consumes little energy because the ultrasonic echo signals are not sampled and therefore no corresponding data sets are generated.

[0017] In a favorable embodiment of the invention, the measuring component includes a timing device configured to receive the sequence of rectangular pulses and featuring an interface for data exchange with the microprocessor. This device is configured to determine the difference between the reception of a trigger signal and the reception of the sequence of rectangular pulses associated with that trigger signal, and to transmit this difference to the microprocessor as the ultrasound transit time. Such a timing device is also a relatively simple, standard component, since determining the ultrasound transit time from the sequence of rectangular pulses and the associated trigger pulse does not require any complicated, extensive calculations; only the differences need to be determined. This significantly reduces the energy consumption and manufacturing costs of a device according to the invention.

[0018] Advantageously, the fastening device is designed to detachably secure the fixture to the screw connection. This allows the screw connection or the fixture attached to it to be easily replaced if necessary. The fastening device preferably includes at least one magnet by means of which the fixture can be detachably attached to the screw connection. This design enables a very cost-effective fastening device.

[0019] If the ultrasonic transducer mounted on the screw includes a piezoelectric element for generating ultrasound and receiving echo signals, the signal transmission element, in a favorable embodiment of the invention, has a signal transmission pin that is connected to the excitation component and the measuring component, and is also connected to the piezoelectric element when the device is mounted on the screw. With such a signal transmission pin, the ultrasonic transducer can be easily excited by the excitation component, and the echo signals can be transmitted to the measuring component.

[0020] Preferably, the microprocessor includes a data memory for storing temperature signals and ultrasonic transit-time signals. This allows measurements to be performed autonomously by the device and the measurement data to be forwarded to an output module at a later time.

[0021] In an advantageous further development of the invention, all electronic and electrical components are configured to automatically switch to a standby mode after measurement and data transmission. This measure minimizes energy consumption.

[0022] In a favorable embodiment of the invention, the device includes a radio module that is interposed between the microprocessor on the one hand and an input module and an output module on the other for data transmission. This measure also enables the aforementioned remote control.

[0023] Advantageously, the device has a battery that is connected to the temperature sensor, the excitation component, the measuring component, the microprocessor and the radio module for their power supply.

[0024] For the purposes of this application, the term "batteries" also includes accumulators.

[0025] If the device does not require an external power supply, remote control is possible. This means the device can be "woken up" from sleep mode by remote control using a start signal, whereupon the battery power supply is activated and the measurement is performed. The measurement data is then forwarded to an output module. Afterwards, the device returns to sleep mode. Preferably, the battery capacity is matched to the temperature sensor, excitation component, measuring component, microprocessor, and radio module such that the battery life is at least 10 years, and preferably at least 15 years. This results in long maintenance intervals and thus significantly reduces maintenance costs.

[0026] A system according to the invention comprises a device according to the invention with a battery and a radio module, and further a base station with an input module that can be connected to the microprocessor of the device and via which a user can input data, in particular the start signal for the ultrasonic transducer, further with an output module that can be connected to the microprocessor of the device and via which the microprocessor can output data, in particular the temperature signal, the ultrasonic transit-time signal and an identifier, to a user, and with a second radio module that is interposed between the input module and the output module on the one hand and the microprocessor of the device on the other hand for data transmission.

[0027] With such a system, the preload force of bolted connections can be reliably and precisely measured and monitored without requiring personnel on site. The data transmitted by the device is evaluated by the base station using the calibration data of the corresponding bolt.

[0028] Preferably, the system includes an evaluation module that communicates with the output module and is configured to calculate the preload force from the data output to the output module and store it in a database with corresponding timestamps. The user can then graphically display or output the relevant data for the screw, in particular the preload force and its development over time.

[0029] The database is advantageously located in the cloud. This allows users to access the data and analysis from any location via the internet.

[0030] In a preferred embodiment of the invention, the device includes a gateway that is connected to the base station and is configured to query at least two devices sequentially. A gateway allows the measurement and monitoring of preload forces in bolted connections to be further automated and thus made more economical.

[0031] The invention is explained in more detail below with reference to the drawings by way of example. Figure 1 shows a vertical view of a bolted connection in a flange connection shown in section, wherein an embodiment of a device according to the invention is attached to the end face of one end of a bolt;

[0032] Figure 2 shows a schematic representation of an embodiment of a system according to the invention, with an exploded view in which the device from Figure 1 is shown above the screw bolt end provided with an ultrasonic transducer;

[0033] Figure 3 shows a partially cut-away side view of the device from Figure 2;

[0034] Figure 4 in perspective view shows a bottom view of the device from Figure 2;

[0035] Figure 5 in perspective view shows a top view of the device from Figure 2;

[0036] Figure 6 shows a block diagram of the system from Figure 2.

[0037] Figure 1 shows, as an embodiment, a screw connection 1 with a bolt 2 in a flange connection 3. At one end of the bolt 2, an embodiment of a device 4 according to the invention for measuring a preload force in a screw connection 1 is attached to the end face 5 of the bolt 2.

[0038] Figure 2 shows that an ultrasonic transducer 7 is attached to the center of this end face 5 (center line 6 in Figure 1). An exploded view shows that the device 4 is arranged centrally above the ultrasonic transducer 7.

[0039] The ultrasound transducer 7 has a piezoelectric element 7a (Figure 6) for generating ultrasound and receiving echo signals.

[0040] The device 4 for measuring a preload force in a screw connection 1 is shown in more detail in Figures 3 to 5.

[0041] The device 4 has a circular cylindrical section 8, on the side of which facing away from the screw connection 1 a hexagonal section 9 is attached.

[0042] Figure 3 schematically illustrates the components arranged in the device 4. These are a temperature sensor 10 for measuring the temperature of the screw or bolt 2, an excitation component 11 which, upon receiving a start signal, excites the ultrasonic transducer 7 to emit monofrequency ultrasonic waves, a measuring component 12 which measures the transit time from the transmission of the ultrasonic waves until the reception of the echo by the ultrasonic transducer 7, a microprocessor 13 which is connected to the temperature sensor 10, the excitation component 11, and the measuring component 12, and a battery 14 which is connected to the temperature sensor 10, the excitation component 11, the measuring component 12, the microprocessor 13, and any radio module 19 (Figure 9) for their power supply.

[0043] The excitation component 11 has a pulse generator for sending an excitation pulse to the ultrasonic transducer 7 and for sending an associated trigger pulse to the measuring component 12 to start a time measurement.

[0044] The measuring component 12 has a converter which is set up to generate a sequence of rectangular pulses from the analog ultrasonic echo signal for determining the ultrasonic transit time.

[0045] Furthermore, the measuring component 12 has a timing device which is set up to receive the sequence of rectangular pulses and has an interface for data exchange with the microprocessor 13 and is set up to determine the difference between the reception of a trigger signal and the reception of the sequence of rectangular pulses assigned to this trigger signal and to transmit it to the microprocessor 13 as the ultrasound transit time.

[0046] Optionally, the device 4 can also have a data storage device (not shown) to be able to temporarily store temperature signals and ultrasonic wave transit time signals and only forward them later to an output module 25 (Figure 6).

[0047] Furthermore, the device 4 can include a radio module 19 (Figure 6) which is configured to send / receive data to / from a base station 23 (Figures 2 and 5). The radio module 19 is interposed between a microprocessor 13 of the device 4 on the one hand and an input module 24 and an output module 25 of the base station 23 on the other hand for data transmission.

[0048] In this way, the microprocessor 13 can receive the start signal from the input module 24 via radio and forward it to the excitation component 11, and forward the temperature signal from the temperature sensor 10 and the ultrasonic transit-time signal received from the measuring component 12, as well as an identifier, via radio to the output module 25. A signal transmission element 16 in the form of a signal transmission pin (see Figures 3 and 4) protrudes from the underside 15 of the circular cylindrical section 8 facing the screw connection 1. This pin is connected to the excitation component 11 and the measuring component 12 and, when the device 4 is attached to the end face 5 of the screw bolt 2 and thus positioned above the ultrasonic transducer 7, is also connected to the piezoelectric element 7a.

[0049] The fastening device 17 is also formed in the underside 15 of the circular cylindrical section 8 (Figures 3 and 4). In the illustrated embodiment, it has three recesses in the underside 15, which are arranged on a circular ring at intervals of 120° to each other around the signal transmission pin 16. The recesses are designed to receive magnets (not shown) with which the device can be detachably fastened to the end face 5 of the screw bolt 2.

[0050] In the upper surface 18 of the hexagonal section 9, facing away from the screw connection 1, a recess 20 is formed, which is rectangular in the illustrated embodiment. An identifier for the screw connection 1 can, for example, be placed in this recess 20 for identification purposes.

[0051] Furthermore, an LED display 20a is arranged on this top surface 18, which indicates the status of the device 4.

[0052] In the illustrated embodiment, a hexagonal logo 21 of the manufacturer is also arranged on the upper surface 18.

[0053] The battery supplies power to all electronic and electrical components of the device 4. The capacity of the battery 14 is matched to these components—in particular to the temperature sensor 10, the excitation component 11, the measuring component 12, the microprocessor 13, the radio module 19, and any data storage device that may be present—such that the operating time of the battery 14 is at least 10 years and preferably at least 15 years.

[0054] The data flow for determining the ultrasound transit time in a screw is as follows:

[0055] The base station 23 sends a command to the microprocessor 13 via radio or the wireless module 19 to perform an ultrasonic measurement. The microprocessor 13 then "wakes up" all components of the device 4 that are in standby or sleep mode.

[0056] The pulse generator receives a command from the microprocessor 13 to generate an excitation pulse. This pulse is then forwarded to the ultrasonic transducer 7. Simultaneously, the pulse generator sends a trigger pulse to the timing device.

[0057] The excitation pulse first reaches the signal transmission element 16, which is connected to a piezoelectric element 7a of the ultrasonic transducer 7 and converts the electrical excitation pulse into ultrasonic vibrations. The ultrasonic vibrations are coupled into the screw, pass through it, and are reflected at the shaft end. The reflected ultrasonic vibrations are referred to as an echo signal.

[0058] The echo signal travels back through the screw to the piezoelectric element 7a, which converts the echo signal – i.e., the reflected ultrasonic vibrations – into an electrical signal.

[0059] The echo signal reaches the converter and is transformed into a sequence of rectangular pulses.

[0060] This sequence of rectangular pulses reaches the timing device. The timing device's measurement was started by the pulse generator with the trigger pulse. The time values ​​determined by the timing device are stored in its internal memory.

[0061] The microprocessor 13 retrieves the time values ​​via the interface and calculates the transit time of the ultrasound signal in the screw from these values.

[0062] The microprocessor 13 uses the temperature sensor 10 to determine the temperature in the screw and corrects the calculated ultrasonic transit time using the determined temperature value.

[0063] The temperature-compensated ultrasound transit time is communicated by the microprocessor 13 to the base station 23.

[0064] For measuring the ultrasound wave transit time, the measuring component 12 thus has an evaluation unit with an evaluation algorithm that is free of pulse compression and cross-correlation.

[0065] Afterwards, all unnecessary electronic and electrical components of device 4 are returned to sleep mode.

[0066] The value of the preload force of the monitored screw is calculated in the base station 23. The embodiment of a system 22 according to the invention for monitoring preload forces in bolted connections 1, shown in Figure 2, comprises the device 4 described above for measuring preload forces in bolted connections 1, including a battery 14 and a radio module 19, as well as a base station 23.

[0067] A block diagram of the system is shown in Figure 6. In this illustration, the device 4 is attached to the top of a screw head 2a.

[0068] The base station 23 has an input module 24 and an output module 25 and a second radio module 26, which is interposed between input module 24 and output module 25 on the one hand and microprocessor 13 of the device 4 on the other hand for data transmission and can be brought into radio communication with the radio module 19 of the device 4.

[0069] A user can input data, in particular the start signal for the ultrasonic transducer 7, via the input module 24, which can be connected to the microprocessor 13 of the device 4 via the radio modules 19, 26.

[0070] By means of the output module 25, which can also be brought into data communication with the microprocessor 13 of the device 4 via the radio modules 19, 26, the microprocessor 13 can output data, in particular the temperature signal, the ultrasonic transit time signal and an identifier, to a user.

[0071] Base station 23 also includes an evaluation module 27, which is in data communication with output module 25. Evaluation module 27 calculates the preload force from the data output to output module 25. Evaluation module 27 then stores the preload force, along with corresponding timestamps, in a database 28.

[0072] In the illustrated example, database 28 is located in a cloud. However, database 28 can also be stored locally.

[0073] To monitor multiple bolted connections 1, for example in a flange connection 3, a gateway 29 can be connected to the base station 23. This gateway has switching logic, meaning that different channels can be selected and thus different bolted connections 1 can be selected sequentially for measuring and monitoring their preload forces. (Reference numeral list)

[0074] 1 screw connection

[0075] 2 screw bolts

[0076] 2a Screw head

[0077] 3 Flange connection

[0078] 4 Device for measuring a preload force in a bolted joint

[0079] 5. End face of the screw bolt

[0080] 6 Center line

[0081] 7 ultrasound transducers

[0082] 7a Piezoelectric element

[0083] 8 circular cylindrical section of the device

[0084] 9 hexagonal section of the device

[0085] 10 Temperature sensor

[0086] 11 Excitation component

[0087] 12 Measuring component

[0088] 13 Microprocessor

[0089] 14 batteries

[0090] 15 Underside of the circular cylindrical section

[0091] 16 Signal transmission element

[0092] 17 Fastening device

[0093] 18 Top side of the hexagonal section

[0094] 19 Radio module of the device

[0095] 20 In-depth study

[0096] 20a LED display

[0097] 21 Logo

[0098] 22 System for monitoring preload forces in bolted connections

[0099] 23 Base station

[0100] 24 Input module

[0101] 25 Output module

[0102] 26 Radio module of the base station

[0103] 27 Evaluation module of the base station

[0104] 28 database

[0105] 29 Gateway

Claims

Patent claims 1. Device for measuring a preload force in a screw connection (1), wherein the screw connection (1) comprises a screw (2) having a minimum length of 200 mm and comprising an ultrasonic transducer (7) attached to a surface of the screw head of the screw (2) or to a surface at the end of the screw (2) opposite the screw head for introducing ultrasound into the screw (2), with a fastening device (17) for attaching the device (4) to the screw (2), a temperature sensor (10) for measuring the temperature of the screw (2); an excitation component (11) configured to excite the ultrasonic transducer (7) upon receiving a start signal by means of an excitation signal to emit monofrequency ultrasonic waves and simultaneously to send an associated trigger signal to a measuring component (12) to start time measurements, each of which is associated with this trigger signal;wherein the measuring component (12) is configured to determine the transit time from the transmission of the ultrasonic waves to the reception of the echo by the ultrasonic transducer (7), and has for the ultrasonic waves an evaluation unit with an evaluation algorithm that is free of pulse compression and cross-correlation and is configured to determine the ultrasonic transit time and forward a corresponding signal; a signal transmission element (16) which is connected to the excitation component (11) and to the measuring component (12) and, in the state of the device (4) being attached to the screw (2), is also connected to the ultrasonic transducer (7);a microprocessor (13) which is connected to the temperature sensor (10), the excitation component (11) and the measuring component (12) and which can be connected to an input module (24) and to an output module (25) and is configured to receive the start signal from the input module (24) and forward it to the excitation component (11) and to forward a temperature signal, an ultrasonic transit-time signal and an identifier to the output module (25).

2. Device according to claim 1, characterized in that the excitation component (11) includes a pulse generator and the excitation signal is an excitation pulse and the trigger signal is a trigger pulse, wherein the microprocessor (13) is configured to cause the pulse generator to send an excitation pulse to the ultrasonic transducer (7) and to send an associated trigger pulse to the measuring component (12) to start a time measurement.

3. Device according to one of the preceding claims, characterized in that the measuring component (12) has a converter which is configured to generate a sequence of rectangular pulses from the analog ultrasonic echo signal for determining the ultrasonic transit time.

4. Device according to claim 3, characterized in that the measuring component (12) has a timing device which is configured to receive the sequence of rectangular pulses and has an interface for data exchange with the microprocessor (13) and is configured to determine the difference between the reception of a trigger signal and the reception of the sequence of rectangular pulses associated with this trigger signal and to transmit it to the microprocessor (13) as the ultrasound transit time.

5. Device according to one of the preceding claims, characterized in that the fastening device (17) is configured to detachably fasten the device (4) to the screw connection (1).

6. Device according to claim 5, characterized in that the fastening device (17) has at least one magnet by means of which the device (4) can be detachably fastened to the screw connection (1).

7. Device according to one of the preceding claims, wherein the ultrasound transducer (7) comprises a piezoelectric element (7a) for generating ultrasound and receiving echo signals, characterized in that the signal transmission element (16) has a signal transmission pin which is connected to the excitation component (11) and the measuring component (12) and, in the state of the device (4) being attached to the screw (2), is also connected to the piezo element (7a).

8. Device according to one of the preceding claims, characterized in that the microprocessor (13) has a data storage device for storing temperature signals and ultrasonic transit-time signals.

9. Device according to one of the preceding claims, characterized in that all electronic and electrical components are configured to automatically enter a sleep mode after measurement and data transmission.

10. Device according to one of the preceding claims, characterized by a radio module (19) which is interposed between microprocessor (13) on the one hand and input module (24) and output module (25) on the other hand for data transmission.

11. Device according to one of the preceding claims, characterized by a battery (14) which is connected to the temperature sensor (10), the excitation component (11), the measuring component (12), the microprocessor (13) and the radio module (19) for their power supply.

12. Device according to claim 11, characterized in that the capacity of the battery (14) is matched to the temperature sensor (10), the excitation component (11), the measuring component (12), the microprocessor (13) and the radio module (19) such that the operating time of the battery (14) is at least 10 years and preferably at least 15 years.

13. System for monitoring a preload force in a bolted joint (1), wherein the bolted joint (1) comprises a screw (2) having a minimum length of 200 mm and having an ultrasonic transducer (7) attached to a surface of the screw head of the screw (2) or to a surface at the end of the screw (2) opposite the screw head for introducing ultrasound into the screw (2), with a device (4) according to claim 10 and according to claims 11 or 12, and with a base station (23) comprising an input module (24) which can be connected to the microprocessor (13) of the device (4) and via which a user can input data, in particular the start signal for the ultrasonic transducer (7), an output module (25) which can be connected to the microprocessor (13) of the device (4) and via which the microprocessor (13) can output data, in particular the temperature signal, the ultrasonic transit-time signal and an identifier, to a user, a second radio module (26) which is interposed between the input module (24) and the output module (25) on the one hand and the microprocessor (13) via the first radio module (19) of the device (4) on the other hand for data transmission.

14. System according to claim 13, characterized by an evaluation module (27) which is in data communication with the output module (25) and is configured to calculate the preload force from the data output to the output module (25) and to store it with corresponding timestamps in a database (28).

15. System according to claim 13 or 14, characterized by a gateway (29) which is connected to the base station (23) and is configured to query at least two devices (4) successively.

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