WO2021121520A1

WO2021121520A1 - Bolt tag system

Info

Publication number: WO2021121520A1
Application number: PCT/DK2020/050390
Authority: WO
Inventors: Flemming Selmer Nielsen
Original assignee: R&D Engineering A/S
Priority date: 2019-12-18
Filing date: 2020-12-18
Publication date: 2021-06-24
Also published as: DK180817B1; DK201970786A1

Abstract

Ensuring proper maintenance efficiency and quality for bolted joints is important. A method and system are described, the tag system to be used when evaluating preload in a bolted joint (10), and comprising: - a bolt tag (100) comprising an encoded identifier being readable while the bolt tag (100) is attached to said bolted joint (10), - a joint database (150) storing a joint profile (151) associated with said encoded identifier, said joint profile (151) relating at least to bolt material data and bolt length data of the bolt of said bolted joint (10) and an ultrasonic waveform (174) describing the ultrasonic signal intensity as a function of time-of-flight through the bolt (1) of said bolted joint (10), and - a portable interrogating device having interrogation means to interrogate said bolt tag (100) and receive the encoded identifier here from, and further to retrieve said joint profile (151) from said joint database (150), and ultrasonic measuring means for measuring the length of a bolt in the bolted joint ultrasonically.

Description

BOLT TAG SYSTEM

FIELD OF THE INVENTION

The invention relates to a system and method for tracking over time the preload force of bolted joints, and a tag therefore.

BACKGROUND OF THE INVENTION

Ensuring that a bolt joint has an adequate preload is instrumental when servicing industrial machinery, such as in the tower of wind turbines. Routine servicing and control include assessing and maintaining bolt joint preloads, where bolt joint preload is calculated using a reference torque. A given joint may be specified for a torque of 100 Nm, and a torque wrench is used to apply the desired torque. Various issues exist with this method, such as lubrication friction in the joint depending on lubricant age and temperature, whereby a given torque may produce an unsatisfactory preload.

A newer way of ensuring proper bolt preload is measuring bolt elongation in response to the tightening of the bolt joint. To calculate the elongation, it is necessary to know the unloaded bolt length however, which requires unfastening the bolted joint.

Mechanical and optical techniques are used in the art to measure bolt length. Micrometre callipers and laser measurement devices are useful, while ultrasound has been used in some cases as well. Irrespective of the measurement technique used, however, it is necessary to release the bolted joint to measure unloaded bolt length which is a time- consuming process and potentially leads to technician errors. Furthermore, even if no joints have crept out of desired preload range, unloading, measuring and reloading the joints are necessary to ensure precise preload.

The object of the invention is to help avoid unloading joints while still allowing precise joint analysis and adjustment.

SUMMARY OF THE INVENTION

It is the aim of the current invention to alleviate at least some of the above-mentioned problems. This is achieved by a tag system which is to be used when evaluating preload in at least one bolted joint, where the tag system comprises:

- a bolt tag comprising an encoded identifier being readable by an interrogating device, while the bolt tag is attached to said bolted joint, - a joint database storing a joint profile associated with said encoded identifier, said joint profile relating at least to bolt material data and bolt length data of the bolt of said bolted joint and an ultrasonic waveform describing the ultrasonic signal intensity as a function of time-of-flight through the bolt of said bolted joint, and.

- a portable interrogating device having interrogation means to interrogate said bolt tag and receive the encoded identifier here from, and further to retrieve said joint profile from said joint database, and ultrasonic measuring means for measuring the length of a bolt in the bolted joint ultrasonically.

Thereby, important bolt information is retained that allows data analysis and precise bolt inspection and tightening. Further, it allows saving time and manual labour by only requiring bringing the nut tightening equipment if the bolt has loosened / slipped.

Bolt length data refers to unloaded bolt length, loaded mechanical bolt length and loaded ultrasonic bolt length, or any of these.

Genreally, bolted joints denote any assemblies, where a tightened mechanical fastener ensures the proper clamping force of the assembly, i.e. maintains the retaining force in the joint. Examples thereof, which are not limiting to the scope of the invention, are a bolt and a nut, a threaded rod with two nuts, a bolt screwed into a hole, a threaded rod screwed into an undersized or threaded hole with a nut attached at the other end, or any other such joint.

By storing and comparing waveform length data, subsequent inspections can compare and identify the specific bolt to assure that it is the correct bolt, as bolts have unique ultrasonic waveforms. Further, amplitude readings on the loaded bolt can take unloaded bolt amplitudes into account. This allows avoiding/mitigating false negatives during noise removal steps of ultrasonic measurements of the elongated bolt.

Bolt material data refers to bolt material constant / elasticity modulus and may further include bolt dimensions and expected environment/weathering at the installation site that affects the unloading or slippage of the bolted joint.

The portable interrogating device can be such as an electronic device with interrogating means, such as an RFID interrogating device or another type of interrogating means, such as a barcode interrogator QR-code interrogator or data matrix interrogator.

The joint database may be stored on an electronic device controlled by the technician user, such as a portable interrogating device, or preferably remotely and accessed through a network. In an embodiment, the joint database is stored on a portable interrogating device. In an embodiment, the joint database is stored remotely. For embodiments where the joint database is stored remotely, an electronic device controlled by the technician user and a remote database can preferably exchange parts of the data in the database prior to making inspections.

By the bolt tag being attached to the bolted joint is meant that it can be attached on the bolt or nut or adjacent to the bolted joint. It is not necessary that it is attached specifically to the nut or bolt or other part of the bolted joint itself, as long as the technician can associate the bolted joint and the bolt tag clearly.

By interrogation is meant the conventional process of transmitting, over RFID, a radio frequency signal and receiving information in return. In the current disclosure, such information is the encoded identifier, such as a number. This interrogation process can be a two-channel process, or the RFI D antenna can react with a readably modified impedance, or be interrogated by other conventional means such as any technical means of interrogating the bolt tag to be provided an encoded identifier, such as interrogating/reading a 2-dimensional barcode, for example a QR-code, data matrix or Aztec code, or even a string of alphanumeric characters.

In the majority of the current disclosure, example is taken in RFID, but the encoded identifier may be any convenient method, such as a QR-code.

In an embodiment, the system further comprises a bolted joint, and where the bolt tag is magnetically attached to the bolted joint via magnetic attraction by a magnetic part. Thereby, the bolt tag is associated specifically with the given bolt.

In an embodiment, bolt length data comprises an ultrasonic waveform describing the ultrasonic signal intensity as a function of time-of-flight through the bolt of said bolted joint. This may be a partial cut of the waveform around the part of the waveform informing on the length of the bolt, or it may be a longer waveform comprising the part of the waveform informing on the length of the bolt.

Thereby, subsequent inspections can compare and identify the specific bolt to assure that it is the correct bolt, as bolts have unique ultrasonic waveforms. Further, amplitude readings on the loaded bolt can take unloaded bolt amplitudes into account. This allows avoiding/mitigating false negatives during noise removal steps of ultrasonic measurements of the elongated bolt.

In an embodiment, the bolt length data comprise a reference waveform being an ultrasonic waveform that describes an unloaded bolt. This achieves even further precision. In an embodiment, the bolt length data comprise a preload waveform being an ultrasonic waveform that describes a bolt elongated to a preload clamping force. This achieves even further precision.

In an embodiment, the joint profile data comprise at least an unloaded bolt length and a preload bolt length reflecting the bolt length that provides the necessary bolt preload.

Thereby, the joint profile comprises data that is specific to the field of bolt elongation.

In an embodiment, the bolt tag of said tag system further comprises an attachment part for attaching said bolt tag to said bolted joint. The attachment part is preferably one of: a magnetic part, an adhesive, an outer surface integrally formed with a bolt, a weld-suitable outer surface and a mechanical fastening part. Thereby, the bolt tag can be fastened securely to the bolted joint.

It should be noted that although magnetic attachment using a magnetic part is the embodiment described most throughout the disclosure, this is merely one effective embodiment of the invention. Welding, adhesive, a mechanical fastener, and forming the bolt tag integrally with the bolt or nut are further alternatives.

In an embodiment, the bolt tag has a magnetic part. Thereby, it is user-friendly and easy to use. In an embodiment, the attachment part is an adhesive disposed on an outer surface of the bolt tag. Thereby, the position of the bolt tag can be retained between inspection jobs.

In an embodiment, the attachment part is a mechanical fastening part, for example openings, to allow a screw or bolt to pass through and into the bolt or nut for threaded engagement, or the bolt may have an outer surface adapted to fit snugly and/or frictionally into a hole drilled for the purpose into the bolt or nut.

In an embodiment, the attachment means is a weld-suitable outer surface that is adapted to be welded onto the bolt or nut. Thereby, an effective bonding is achieved.

In an embodiment, the bolt tag is at least partially integrally formed with the bolt or nut at production. In an aspect of the invention, it relates to a nut or bolt with a bolt tag integrally formed therein. Integrally forming is for example etching of the encoded identifier into the bolt, such as any type of barcode or an alphanumeric string. Thereby, the bolt tag is further securely placed.

In an embodiment, the tag system comprises a plurality of bolt tags, and where the ultrasonic measuring means of the portable interrogating device is adapted to measure the length a plurality of bolted joints corresponding to said plurality of bolt tag. In an embodiment, the bolt tag has a first outer surface and a second, oppositely arranged outer surface, where the magnetic part is disposed adjacent the first outer surface and an RFID transceiver is disposed between the magnetic part and the second outer surface. Thereby, the magnetic element has been found to further shield an RFID interrogation process from eddy currents of the ferrous and, thus, magnetisable bolt and/or nut.

In an embodiment, the RFID transceiver of the bolt tag is an active tag, and the bolt tag further has a battery for keeping the tag powered. Thereby, it may be easier to interrogate the bolt tag.

In an embodiment, the RFID transceiver is a passive tag and is powered by the interrogating signal of the interrogating device. Thereby, the bolt tag has a longer life span and/or service life.

In an embodiment, the interrogating device is an RFID interrogating device adapted to interrogate an RFID transceiver and receive the encoded identifier here from, and further to retrieve joint profile data from the joint database, the RFID interrogating device further optionally having a screen for displaying length data. Thereby, the technician can manage the bolt tags and bolt data in a user-friendly manner.

In an embodiment, the portable interrogating device further has measuring means for measuring the length of a bolt in a bolted joint. Thereby, the technician only has to carry a single, user-friendly device.

Naturally, embodiments, which are mentioned throughout the specification as useful with the RFID interrogating device, are also generally useful with the portable interrogating device and vice versa.

In an embodiment, the portable interrogating device has mechanical measuring means, such as a micrometre calliper. Thereby, the technician only has to carry a single, user- friendly device.

In an embodiment, the portable interrogating device has an ultrasonic measuring means and a mechanical measuring means such as a micrometre calliper. Thereby, the technician only has to carry a single, user-friendly device, and the further advantages of dual measuring are attained as described, such as not needing to unload bolts to ascertain their elongation and thus load.

In an aspect of the invention, it relates to a bolt tag comprising a magnetic part and an encoded identifier, encoded identifier being readable by a portable interrogating device while the bolt tag is attached to said bolted joint, where reading said encoded identifier directs said portable interrogating device to a joint database storing a joint profile associated with said encoded identifier, said joint profile relating at least to bolt material data and bolt length data, as well as an ultrasonic waveform for the bolted joint. Thereby, a specialised identifying unit is provided that is easy for the technician to work with. Optionally, the bolt tag uses RFID communication to communicate the encoded identifier and magnetic attachment to attach to the bolted joint.

In an aspect of the invention, it relates to a method to be used when evaluating preload in a bolted joint with a bolt and a nut, the method comprising the steps:

- interrogating a bolt tag using a portable interrogating device, thereby receiving an encoded identifier comprised in the bolt tag, while said bolt tag is attached in a manner associated to said bolted joint,

- accessing a joint profile in a joint database using said encoded identifier and deriving for the bolt a preload bolt length from said joint profile, the preload bolt length being a length that achieves a predetermined preload clamping force in the joint and being determined from bolt material data and bolt length data, wherein said bolt length data comprises an ultrasonic waveform describing the ultrasonic signal intensity as a function of time-of-flight through the bolt of said bolted joint, and

- measuring length of said bolt in said bolted joint, where said measuring length of said bolt is performed at least ultrasonically, and

- comparing said measured bolt length of the bolt with the preload bolt length derived from said joint profile, where comparing said measured length comprises comparing a measured waveform with said waveform derived from said joint database.

Thereby, the easy inspection of the invention is achieved, such as a very user-friendly method to determine whether a specific bolt needs to be further tightened or not, and if so, by how much. The further benefits of the wavefunction, which are achieved as well, are ensuring that the bolted joint is indeed the correct one, which ensures measurement integrity and thus proper maintenance of the bolted structure such as a wind turbine.

In an embodiment, a signal is transmitted indicative of the result of comparing said measured bolt length of the bolt with the preload bolt length derived from said joint profile. The signal may for example be an acceptance signal indicating that the bolted joint is sufficiently tightened. The signal may for example be a retightening signal indicating that the bolted joint needs retightening. Thereby, the technician can use the signal to tighten the bolts as needed. In an embodiment, tThe signal may also be an error signal indicating that the measured waveform for the currently measured bolted joint does not correspond to the bolt tag that has been interrogated. Thereby, even further ease is achieved in ensuring proper maintenance.

In an embodiment, the method further comprises transmitting the measured length to the joint database. Thereby, the bolt length data is retained between inspection/maintenance cycles, allowing even better bolt elongation analysis, and can be used to ensure that all bolts of a maintenance site are inspected during a specific inspection job.

In an embodiment, bolt length data comprises an ultrasonic waveform describing the ultrasonic signal intensity as a function of time-of-flight through the bolt of said bolted joint, where said measuring length of said bolt is performed at least ultrasonically, and where comparing said measured length comprises comparing a measured waveform with said reference waveform derived from said joint database.

Preferably, the ultrasonic waveform is a reference waveform being a waveform for the bolt in an unloaded state. In an embodiment, the bolt length data comprise a preload waveform being the ultrasonic waveform of a bolt when it is loaded to preload clamping force.

In an embodiment, the bolt tag is provided on an unreliably loaded bolted joint.

This allows, given sufficient bolt material data and bolt length data, the technician to achieve a new level of localised bolt data for previously installed joints.

In an embodiment, the method further comprises reference measuring the lengths of at least one, or preferably at least three unreliably loaded bolts of a batch of bolts, reference measuring comprising unloading them and measuring their lengths, then finding the average between their unloaded lengths then using that average as a reference unloaded length for the rest of a given batch of bolts maintenance site of bolts.

Thereby, the rest of the bolted joints do not need to be unloaded, but can instead rely on the produced reference measurement.

In an embodiment, if measured bolt length is shorter than preload bolt length, the bolt is retightened to at least said preload bolt length, preferably to a calibration bolt length being longer than said preload bolt length. Thereby, the joint is tightened. Preferably, the bolt may slip or stretch in the joint while still remaining within safe preload levels and a technician may even be required to service the joint less. This achieves hitherto unseen levels of ease by not requiring untightening bolts while still ascertaining and using precise calibration lengths.

In an embodiment, length measurement is performed concurrently with retightening the bolt, where retightening is informed by said length measurement, and where it is stopped once a desired length is achieved, and where retightening and length measurement are controlled by a processor. Thereby, an easy process is achieved where preload clamping force is achieved accurately and fast.

In an embodiment, bolt length data is displayed on a screen for the technician to inspect.

In an embodiment, the displayed data is at least a preload bolt length being the calculated bolt length of the bolted joint achieving the necessary bolt preload. Thereby, the technician can see the specific length that the bolted joint is supposed to achieve, which may serve as a guarantee ensuring that the technician remains in control during the process and can still use his/her intuition about the bolt elongation.

In an embodiment, the concurrent length measurement comprises ultrasonic measurement to produce a waveform of the concurrent measurement, and further the waveform retrieved from the joint profile and the waveform of the concurrent measurement are displayed on a screen for a technician to inspect. Thereby, the tensioning is easily reviewable and controllable for a technician.

In an embodiment, measuring said bolt length is performed by both mechanical measurement and ultrasonic measurement. This utilises a relationship between the mechanical (true) length of a tightened bolt and the ultrasonically measured length of the same bolt. Ultrasonic measurements characteristically overshoot length of materials as a function of tension in the material. The relationship between the mechanical length and the ultrasonic length depends on the elasticity of the material and is thus derived from or is an aspect of the material constant. The material constant and/or unloaded mechanical length are/is sufficiently homogenous between bolts of a single product batch that a single calibration measurement will be accurate for the whole batch. Furthermore, the ultrasonic measurement also provides a unique profile in the form of a waveform that is further kept and later stored for future comparison.

Thereby, increased precision is achieved.

SHORT LIST OF THE DRAWINGS

In the following, example embodiments are described according to the invention, where: Fig. 1 is a cross-sectional side view of a bolted joint with a bolt tag according to the invention,

Fig. 2 is a zoomed in view of the bolt tag shown in Fig. 1 ,

Fig. 3 is a perspective partial view of two joined tower pieces with bolted joints having bolt tags according to the invention,

Fig. 4 is a cross-sectional top view of a sandwiched bolt tag according to the invention,

Fig. 5 is a cross-sectional side view of the bolt tag of Fig. 4,

Fig. 6 is a cross-sectional top view of a doughnut bolt tag according to the invention,

Fig. 7 is a cross-sectional side view of the bolt tag of Fig. 6,

Fig. 8 is a cross-sectional top view of ring bolt tag according to the invention,

Fig. 9 is a cross-sectional side view of the bolt tag of Fig. 8,

Fig. 10A and 10B illustrate ultrasonic waveforms stored as data according to the invention, Fig. 11 illustrates an embodiment of the bolt tag system according to the invention, and Figs. 12-14 illustrate example uses of the method according to the invention.

DETAILED DESCRIPTION OF DRAWINGS

In the following the invention is described in detail through embodiments hereof that should not be thought of as limiting to the scope of the invention.

Fig. 1 is a cross-sectional side view of a bolted joint 10 with a joint tag 100 according to the invention. The bolted joint 10 comprises a nut 2 and a bolt 1 and two jointed elements 3, 4 and the bolt tag 100. The bolt tag 100 can be seen in more detail in Fig. 2 which shows a closer view conceptual cross section hereof.

Over time, vibrations, heat and other environmental factors may reduce the clamping force of the bolted joint, and so it is important to routinely ensure that joints provide a sufficient clamping force. At joint installation, the nut can be threaded onto the bolt to form the joint 10. The bolt tag 100 is attached to the bolt 1 or nut 2, and may be attached during joint installation, in this case preferably before the joint is tightened to preload. Alternatively, the bolt tag 100 may be attached to the joint later, such as during an inspection.

As is seen from Fig. 2, the bolt tag 100 is attached by magnetic attachment, using a magnetic element 120 of the bolt tag 100. The bolt tag 100 further has an RFID transceiver 110 located in a way that makes it interrogatable while the bolt tag 100 is installed on the bolted joint 10. This RFID transceiver 110 is adapted to be interrogated by an RFID interrogator and provides its unique identifier to the interrogator hereby. As is seen, the magnetic element is located between the bolted joint and the RFID transceiver. This not only provides easy attachment means for the bolt tag, but further has been found to surprisingly prevent the ferrous bolt and nut from interfering with the RFID interrogation process.

Fig. 3 is a perspective partial view of two joined pieces 3, 4, here specifically tower pieces. From the slight curvature of the rear wall of the upper tower piece 3, it may be envisioned that several hundreds of bolted joints 10 are required along the periphery of the two joined tower pieces. The assembly as illustrated is near to the ground and a technician can walk around the periphery. Other situations may have the joints being internal to the tower or other structure and/or may require crawling up ladders to perform the required operations.

A technician 20 interrogates a bolt tag 100’ to retrieve an encoded unique identifier stored on the bolt tag 100’, using an electronic device 130 having a screen 131. With access to a joint database (not shown), the electronic device 130 now uses the unique identifier to access a location within the database specific to the unique identifier and the bolt tag 100’ and, thus, the bolted joint 10’.

For the shown example, a technician 20 can walk to a bolted joint 10, interrogate the bolt tag 100’ and measure the bolt length. By interrogating the bolt tag 100’, the technician 20 accesses a database where data of the bolt 1 is stored, that may otherwise have been lost. For example, the specific material properties of the batch of bolts may be necessary for precise preload calculations and may be impossible to remember for every bolt manually. Further, even if such information is written by each joint, the bolts may be swapped out or the information may be exposed to degrading weathering or be written illegibly for the next technician. By these two simple steps, the technician can ascertain the bolted joint preload. If the bolt 1 is still of a length indicating a sufficient clamping force in the bolted joint 10, the technician 20 may then simply proceed to the next bolted joint 10 without further action. If not, the technician 20 may then tighten it before moving on. In either case, the new bolt length is preferably transmitted to the joint database 150.

When moving inside other crammed spaces or remote parts, such as in or near the nacelle / head / control hub of a wind turbine, there may be fewer bolted joints 10 to measure than around a periphery of the tower, but if all bolted joints are sufficiently tightened, it is not necessary to move heavy equipment to the top of the tower to tighten the bolted joints 10, saving precious time. The bolt tag 100 may be attached to the bolt as desired. This can be achieved through magnetic attachment as discussed, or it can be achieved using adhesive or a mechanical fastening part. Bolt tag 100’ is attached with a mechanical fastening part. Bolt tag 100” is attached through an adhesive. Bolt tag 100’” is attached using a magnetic part.

Once installed, it may be useful to retain it at a given location, as it may affect ultrasonic measurements. It is advantageous to place the bolt tag 100 to the bolted joint 10 away from the nut hexagonal side faces to avoid obstructing tightening operation of the bolted joint.

A reference bolt length measurement is conducted while the joint is still not tightened to preload, but is instead preferably installed, or even before installation, and this reference length is stored in the database at the bolt-specific location. Bolt material data is also stored in the database.

A given bolt elongation in a specific bolted joint exposed to specific environmental conditions corresponds to a specific bolt preload. Taking bolt material properties, bolt dimensions and perhaps environmental conditions into consideration then, it is possible to calculate a desired bolt elongation to achieve a certain clamping force. The material and bolt type and possibly environmental parameters are stored in the joint database and associated with the unique identifier for the given bolt tag. Therefore, for the technician in the field, it is simply a matter of measuring the length of the bolt, and comparing it to a desired reference bolt length being the length that achieves the desired preload. If the actual bolt length is shorter than the desired reference length, the technician 20 further tightens the bolt or marks it for tightening later.

When returning to the bolted joint in the future to assess joint preload, both the bolt reference length and bolt material is now readily accessible for the technician, as well as any previous elongation measurements. For example, a given bolt may have had a reference length of 475 mm and then an original preloaded length of 476 mm. Both are available in the joint database at a location associated with the encoded identifier. When returning now to the joint, the technician measures the current bolt length at 475,8 mm and, interrogating the bolt tag, compares this to previous measurements to find that this is within desired length to retain a sufficient preload. S/he then does not need to bring heavy and cumbersome tools to the bolted joint.

Alternatively, should the bolted joint have slipped further (or should 475,8 be considered to be too loose), the technician can tighten it to the right preload, both without first removing the bolt and further without the tightening being corrupted by torque-affecting factors such as degraded oil/grease. The new measurement, whether it reflects a slight slippage within acceptable conditions or a retightening, can then be added to the joint database to attain two further advantages. Firstly, it provides a history of bolt elongation which may give a prediction of future bolt slippage. Secondly, if a given maintenance system comprises a specific number of bolted joints each with a bolt tag 100, it can be assured that every bolt has been measured with every maintenance cycle, instead of the technician needing to for example mark and unmark the joints or even forget a joint or redo a joint to be sure. For example, there may be one hundred joints in a given maintenance cycle. When the technician begins measuring bolt lengths, each access to the joint database and adding of an entry marks the specific tag as completed. When the technician is finished, it can be assured that s/he has indeed measured a hundred joints, or if he had perhaps forgotten two or three, it is noticed. This can be useful both for the specific technician and for insurance and due diligence.

Any time the bolted joint is to be measured, the process then merely has the steps of interrogating the bolt tag to get a desired elongation and measuring the bolt length for the actual elongation, then comparing the two. The bolt tag has the encoded identifier that specifies the specific bolt, and the encoded identifier is generally any readable unique code that can be read by an interrogating electronic device. Some examples thereof may be a barcode such as a 2-dimensional barcode, for example a QR-code or a data matrix, an NFC element, an RFID transceiver, or any other convenient means.

The database may be stored on the electronic measuring device and then be synchronised with an external central database as it is being measured or at a later convenient time.

Figs. 4 and 5 show a sandwiched bolt tag 200 being an embodiment of the invention. Fig. 4 illustrates a top view within the sandwiched bolt tag 200 with an outer shell 101, and shows an RFID transceiver 110 with an antenna 111 being a circuit of a tuned length to catch specific radio frequency waves, and a chip 112 with an encoded unique identifier to provide it for communicating to an interrogator, such as through electric, magnetic or electromagnetic signalling through the antenna 111.

Fig 5 is a cross-sectional view as taken along axis l-l of Fig. 4. As illustrated in Fig. 5, the bolt tag 200 has a magnetic element 120 adjacent a first outer surface 114 and the RFID transceiver 110 adjacent an opposite second outer surface 113. This achieves two purposes. It ensures that the antenna 111 of the RFID transceiver faces away from the bolted joint and is thus readily accessible to the technician and the interrogation device. Further, it has been found that surprisingly, the magnetic element 120 may function as an RFID shield between the antenna and the ferrous bolt and nut, preventing the eddy currents that the interrogator otherwise may produce here to corrupt the transceiver-interrogator communication.

Figs. 6 and 7 show a ring bolt tag 300 being an embodiment bolt tag 300 of the invention. Fig 7 is a cross-sectional view as taken along axis ll-ll of Fig. 6. The ring bolt tag 300 has the magnetic element 120 arranged in a ring along the periphery of the ring bolt tag 300. As can be seen, six smaller magnets form the magnetic element 120. The magnetic element 120 could also be formed as another number of magnets, such as three, or as a single ring element or a partial ring while still being compatible with this embodiment. In the centre of the magnetic element 120, such as between the ring of magnets of the magnetic element 120, the RFID transceiver 110 is located, having an antenna 111 and chip 112.

This embodiment achieves a flat profile while allowing the RFID transceiver 110 to be interrogated while the bolt tag is mounted on the bolted joint. In other words, the magnetic element does not obstruct the RFID communication / interrogation.

Figs. 8 and 9 shows a doughnut bolt tag 400 being an embodiment in which the RFID transceiver 110 is located around a central magnetic element 120. Fig 9 is a cross-sectional view as taken along axis Ill-Ill of Fig. 8. This achieves a flat profile while allowing the RFID transceiver 110 to be interrogated while the bolt tag is mounted on the bolted joint. In other words, the magnetic element does not obstruct the RFID communication / interrogation.

Fig. 10 shows a method of using the bolt tag 100 according to the invention to quickly and precisely inspect the preload in a bolted joint as previously described.

A loaded bolt length is measured and presented on the RFID interrogating device. The measurement can be performed by ultrasound or another method such as calliper measurement. Preferably, the interrogating device has the measurement capability and the length is automatically converted to a digital signal. Even further, it may be useful to present the loaded bolt length on the RFID interrogating device directly.

A bolt tag magnetically fastened to the bolted joint is interrogated by the RFID interrogating device using RFID communication, and the encoded identifier is retrieved therefrom.

Using the encoded identifier, a preload bolt length is retrieved from a joint profile in the joint database. The preload bolt length may have been determined at joint installation or at a later time and will be further described below. In any case, the preload bolt length and/or elongation are/is preferably presented along with the measured length on the RFID interrogating device screen. Further yet measurement history may be retrieved as well to provide a context for any slippage or lack thereof, that may help inform the technician and help him/her to do an even better job. Any bolt material data may also be retrieved if desired.

The measured length and the previously determined preload bolt length are compared, and any slippage that has occurred is identified as a discrepancy. If the bolt has come loose / slipped, it will be physically shorter than the length it needs, to have the desired preload.

If the bolt has slipped, it is tightened to achieve the preload bolt length. It may be tightened to within a tolerance of useful lengths, whereby it may be tightened more than the lower sufficient limit to achieve the preload length. This may provide a longer service span.

Further, there may be two different calculated bolt lengths, where a first is the preload bolt length being the aforementioned length above which no tightening needs to take place (a lower threshold), while a second calculated bolt length is a calibration length being longer than the preload bolt length and is a length to which the bolt can be safely tightened. The calibration length may simply fall within a safe range before the bolt is overtightened, or it may be an upper threshold of safe bolt elongation. By evaluating bolt length by compliance with the preload bolt length and tightening it to the calibration length, a bolt may be tightened to a relatively high preload to later be inspected with longer periods between inspections, while it decreases within an acceptable range. Then, when it finally does fall below the lower threshold, it is tightened back up, such as to near an upper threshold. This method may be especially advantageous where it is difficult to bring the tightening equipment, such as the nacelle of a wind turbine.

Such a method further takes especial advantage of the invention by using the accrued bolt data made available to avoid costly and unnecessary processes such as bringing tightening tools to hard-to-reach places.

In an embodiment, the bolt length data can simply be numerical values. In a preferred embodiment, however, the bolt length data further comprises ultrasonic waveform plots / functions. Figs. 10A and 10B illustrate such part of a preferred embodiment of measurement data according to the invention.

Fig 10A shows a reference waveform 174 of an unloaded bolt. The diagram shows ultrasonic amplitude 161 as a function of time, as an ultrasonic signal is emitted. This produces time-of-flight as the signal travels through the bolt and back, which time can be converted to bolt length 162. Finding the correct bolt length is then a matter of reading the waveform and selecting a point along the x-axis, denoting bolt length 162. A given reference length is thereby calculated based on the preload waveform 174. The length of the bolt can be calculated based on the waveform by any conventional method. The reference length 171 is for example calculated between ‘cycles’. It may for example be calculated as the point between the two first significant cycles, as shown in Fig. 10A.

The first significant cycle consists of a high peak 163A and a low peak 163B, both of which are lower than their counterparts of the following high peak 164A and low peak 164B of the second cycle, being a ‘high cycle’.

Preferably, the reference waveform 174 is stored in the joint profile for each joint.

Fig. 10B illustrates the reference waveform 174 as shown in Fig. 10A, and further preload waveform 175 of the bolt. The preload waveform 175 describes the ultrasonic measurement of a bolt tightened to an ultrasonic preload elongation 172. The ultrasonic bolt preload length 173 is calculated by the same analysis as achieves the reference length, and may even preferably be informed thereby.

Because the bolt is tensioned when the preload waveform 175 is produced, the amplitudes are smaller. As this may reduce an amplitude to below significance threshold for noise reduction of the measurement device, errors are often produced in tensioned materials. This can take the shape of disregarding a cycle entirely or mismatching the amplitude pairs. This can then result in the conventional, uninformed method placing the elongated bolt length in the middle of a cycle or by moving the measurement farther along the time-of- flight.

By comparing the preload waveform 175 with the corresponding reference waveform 174, the false negatives of the noise reduction software are mitigated and precise calculations can be ensured even for tensioned materials because un-tensioned data can be referenced.

Although an ultrasonic measurement overshoots the length of the material due to the tension in the bolt, this can be corrected by using the material constant of the bolt, preferably combined with mechanical measurements.

The preload waveform 175 can be stored in the joint profile along with the reference waveform 174. When ultrasonic measurements are made, a measured waveform can then be displayed to the technician, overlapping the reference waveform 174 and/or the preload waveform 175 on a graph screen.

Further than false negatives mitigation, the waveforms achieves another benefit for the technician. When a tag is read and a reference waveform 174 is retrieved from the joint profile, the technician then measures the ultrasonic length of the bolt. Because waveforms rely significantly on bolted joint geometry, each bolt has a unique waveform. If the bolt tag has been placed by an incorrect bolted joint, or if the technician interrogates an erroneous tag before ultrasonic measurement, an error message can be produced, informing the technician that the measured bolt and the interrogated bolt tag do not match.

On the other hand, if the waveform read matches the reference waveform 174, the technician knows s/he is proceeding correctly.

The two waveforms have a mathematical relationship that can be calculated, where the preload waveform 175 can be thought of as an elastically stretched version of the reference waveform.

In a preferred embodiment, the ultrasonic measurement is continued during calibration, while the bolt is tightened. This produces a transformation of the waveform that may ensure increased precision.

Fig. 11 shows an embodiment of the bolt tag system of the invention. In the illustrated embodiment, the system comprises a bolt tag 100, an interrogating device 130 and a joint database 150. The bolt tag comprises a magnetic part 120 and an RFID transceiver 110. An interrogating device 130 interrogates the bolt tag 100 with interrogation means 134, through the RFID transceiver 110 and receives an encoded identifier stored in the RFID transceiver. The interrogating device 130 then uses the encoded identifier to communicate with a joint database 150 to find a joint profile 151 specific to the encoded identifier and, thus, to the bolt tag 100 and the joint it is installed on.

The interrogating device 130 has a device screen 131 for communicating to a technician and, preferably, it has input means as well, such as a keyboard or a touch screen. It has a processor 132 to execute the necessary communications and to perform the calculations needed. The interrogating device may further comprise length measuring means 135 such as a micrometre calliper instrument, as well as ultrasonic measuring means 136.

The database has joint profiles 151. A joint profile has an identifier matching it with an encoded identifier of only one bolt tag 100. The joint profile 151 further has bolt length data being length measurement data and bolt material data being data relating to the material constant, elasticity and dimensions of a particular bolt or a batch of bolts. The joint profile may also comprise weathering data relating to expected weathering of the installation site and nut data and/or other data relating to the joint. The joint database 150 is preferably stored at a central location, and relevant parts of it is retrieved to the memory 133 of the interrogating device 130 prior to an inspection job. Such memory 133 can be volatile memory or preferably a local non-volatile database. After an inspection job, the retrieved aspects of the joint database 150 are updated from the interrogating device to the joint database 150.

The interrogating device may further have tightening means 137 to apply a tightening force to the nut of the bolted joint and, thus, perform the necessary preload tightening. This significantly eases the process of the inspection job. An interrogating device may simply be installed onto the joint, where it performs interrogating, ultrasonic measuring, mechanical measuring, length calculations, then tightening the joint to specified preload.

Using the mentioned waveshapes, this becomes even more precise.

EXAMPLES

The invention can be used in a variety of ways, a couple of which will be described in the following.

EXAMPLE A - INSTALLATION AT ASSEMBLY

One way to use the system and method of the invention is to install a bolt tag at joint installation. When a wind turbine is erected and tower pieces installed and before operation commences, the bolt tags can advantageously be installed. Such an installation process can be conducted generally as described with Figs. 1-3 and is reiterated here. This example describes a way to install the bolt tag at joint assembly, and is shown in Fig. 12.

In step a), the bolt tag is mounted on the bolt or nut of the assembled and unloaded joint.

In step b), the unloaded bolt length is measured by any conventional means such as a calliper measurement or using ultrasound measurement. It is preferable to use both mechanical measurement and ultrasonic measurement, as the mechanical measurement is more precise while the ultrasonic measurement provides a waveform that has other advantages, and further, the combination of the two provides further advantages.

In step c), bolt material data is derived by consulting reference data available for the bolt production batch. If no such data is available, one or more reference bolts may be reference measured as discussed through example B.

In step d) based on bolt length data and bolt material data, a precise preload bolt length is calculated. The preload bolt length is provided to the technician, being a bolt length that ensures sufficient bolted joint preload. In step e), the technician then tightens the bolt to achieve the preload bolt length.

In step f), the bolt tag is interrogated by the RFID interrogating device to retrieve an encoded identifier specific to the bolt tag and, thus, the bolted joint. The RFID interrogating device may be the same device as performs measurement of step b). Further, steps f) and g) may of course take place after, concurrently with, or before any of steps b) through e).

In step g), the encoded identifier then used to identify a location in the joint database dedicated to the specific bolted joint. When the joint profile is accessed, step h) can be initiated.

In step h), the RFID interrogating device, or another device, is then used to transmit bolt length data and bolt material data or any derivative data hereof to the joint profile in the bolt database connected to the unique identifier of the tag attached to the joint.

Thereby, the bolt is precisely preloaded and the requirements for further and later preloading is retained on site that are specific to the given bolt, while using conventional tools with the addition of the bolt tag of the invention.

EXAMPLE B - INSTALLATION IN TENSIONED JOINTS

For systems where bolted joints are already installed, it may be unfeasible or at least difficult to measure unloaded bolt length of all bolts, which is normally necessary for precise bolt preload length calculations. Further, bolt parameters, such as importantly the material constant of the bolts, may not be documented or may have been lost. For such situations, the bolt tag may be used by following the method of example B.

This example describes a way to upgrade a preloaded bolted joint system in situ, to use bolt tags for precise maintenance and calibration, and is shown in Fig. 13.

In step a), the bolt tag is mounted on the bolt or nut of an unreliably loaded bolted joint. This will typically be on a non-work surface of the nut or the end of the bolt, or any other convenient place to reach for the technician.

In step b), the loaded length is measured by any mechanical length means such as a calliper measurement and is further additionally measured ultrasonically. This step utilises a relationship between the mechanical (true) length of a tightened bolt and the ultrasonically measured length of the same bolt. Ultrasonic measurements characteristically overshoot length of materials as a function of tension in the material. The relationship between the mechanical length and the ultrasonic length depends on the elasticity of the material, and is thus derived from or is an aspect of the material constant. The material constant and/or unloaded mechanical length is sufficiently homogenous between bolts of a single product batch that a single calibration measurement will be accurate for the whole batch. Furthermore, the ultrasonic measurement also provides a unique profile in the form of a waveform that is further kept and stored for future comparison.

In step c), bolt material data is derived by: i. either consulting predetermined reference data if available for the bolt material and preferably even for the bolt production batch, or ii. in-situ measuring three reference bolts produced from the same bolt production batch as the unreliably loaded bolt. At least three lengths are measured of: unloaded mechanical bolt length, unloaded ultrasonic bolt length, loaded mechanical bolt length and loaded ultrasonic bolt length; after which the material constant and/or lengths of the reference bolts can be used as predetermined reference data.

The first bolted joints of a maintenance site will likely be calibrated by using step c)ii, while the remainder will be calibrated using step c)i, relying on the reference data produced by the first calibration. Alternatively, if bolt batch material characteristics are available, step c)ii is unnecessary. On the other hand, only one or two bolted joints may be used, if desired. Alternatively, more than three bolted joints may be used if desired for improved precision. Further, if a maintenance site has bolts of more than one production batch, reference data should be created for each batch that has no such reference data.

In step d), based on bolt length data and bolt material data, a precise bolt preload length is calculated, being a bolt length that ensures sufficient bolted joint preload.

In step e), measured mechanical length of the unreliably loaded bolt is compared with preload length of the unreliably loaded bolt.

In step f), the bolt tag is interrogated by the RFID interrogating device to retrieve an encoded identifier specific to the bolt tag and, thus, the bolted joint. The RFID interrogating device may be the same device as performs either or both measurements of step b). Interrogating the bolt tag provides an encoded identifier and can be performed at any time before step g), which in turn can be performed at any time prior to the commencement of step h).

In step g), the encoded identifier is then used to identify a location in the joint database dedicated to the specific bolted joint. When the joint profile is accessed, step h) can be initiated. In step h), the RFID interrogating device or another convenient device is then used to transmit measurement and material data to the joint database, which is used to store the derived lengths. This preferably includes the ultrasonic waveform. Even where unloaded ultrasonic measurements are not performed, the loaded ultrasonic measurement provides a useful alternative waveform that may be transformed mathematically to substitute for, or provide, the reference waveform.

A step i) is then preferably performed where the technician is notified of any discrepancy between the mechanical length of the bolt and the preload bolt length that has been calculated, such that s/he can tighten the bolt to remedy this. If so, the new length is preferably input into the joint database and provides a historic development view of a bolt.

EXAMPLE C - ROUTINE INSPECTION

Either of examples A and B can be used to provide the basis for easy quality preload inspection and serves as examples of onboarding a bolted joint at different points during the bolted joint life cycle. No matter how the bolted joint first is calibrated, however, the inspection of a bolted joint can then be performed as follows, also shown in Fig. 14: a) the loaded bolt length is measured by ultrasonic measurement or mechanical measurement, b) the bolt tag is interrogated by the RFID interrogating device to retrieve an encoded identifier, c) bolt length data and bolt material data are retrieved from the joint profile by using the encoded identifier, providing preload bolt length to the technician, being a bolt length that ensures sufficient bolted joint preload, d) measured length is compared to preload bolt length, e) if measured length is less than preload length, a signal indicative hereof is transmitted to the technician.

Where a waveform, such as a reference waveform, is stored in the joint profile, and where step a) comprises ultrasonic measurement, the waveform retrieved from the joint profile is during step c) or d) compared to the waveform measured during step a). This ensures that the correct bolted joint is being measured and improves the ultrasonic measurement.

Two further steps are then preferably performed: f) the technician then tightens the bolt to achieve the preload bolt length if necessary, g) the new bolt length is input through the interrogating device to the joint profile in the bolt database. Throughout the specification, the encoded identifier is mostly described as being provided by an RFID transceiver. Obviously, other unique identifiers can be used instead to achieve many of the same benefits. For example, barcodes, 2-dimensional barcodes, QR-codes, data matrixes, alphanumeric codes and so on, could naturally also be used.

Claims

1. A tag system to be used when evaluating preload in a bolted joint (10), the tag system comprising:

- a bolt tag (100) comprising an encoded identifier being readable while the bolt tag (100) is attached to said bolted joint (10),

- a joint database (150) storing a joint profile (151) associated with said encoded identifier, said joint profile (151) relating at least to bolt material data and bolt length data of the bolt of said bolted joint (10) and an ultrasonic waveform (174) describing the ultrasonic signal intensity as a function of time-of-flight through the bolt (1) of said bolted joint (10), and

- a portable interrogating device having interrogation means to interrogate said bolt tag (100) and receive the encoded identifier here from and further to retrieve said joint profile (151) from said joint database (150), and ultrasonic measuring means for measuring the length of a bolt in the bolted joint ultrasonically.

2. A tag system according to claim 1 , where said bolt tag further comprises an attachment part for attaching said bolt tag (100) to said bolted joint, where the attachment part is preferably one of: a magnetic part, an adhesive, an outer surface at least partially integrally formed with a bolt, a weld-suitable outer surface and a mechanical fastening part.

3. A tag system according to claim 2, wherein said bolt tag (100) has a first outer surface (114) and a second, oppositely arranged outer surface (113), where the magnetic part (120) is disposed adjacent the first outer surface (114) and an RFID transceiver (110) having the encoded identifier is disposed between said magnetic part (120) and said second outer surface (113).

4. A tag system according to any of claims 1-3 further for evaluating preload in a plurality of bolted joints (10), the tag system further comprising a plurality of bolt tags (100) and where the ultrasonic measuring means of the portable interrogating device is adapted to measure the length a plurality of bolted joints (10) corresponding to said plurality of bolt tag (100).

5. A method to be used when evaluating preload in a bolted joint (10) with a bolt (1) and a nut (2), the method comprising the steps:

- interrogating a bolt tag (100) using a portable interrogating device, thereby receiving an encoded identifier comprised in the bolt tag (100), while said bolt tag (100) is attached in a manner associated to said bolted joint (10), - accessing a joint profile (151) in a joint database (150) using said encoded identifier, and deriving for the bolt (1) a preload bolt length from said joint profile, the preload bolt length being a length that achieves a predetermined preload clamping force in the joint, and being determined from bolt material data and bolt length data, wherein said bolt length data comprises an ultrasonic waveform (174) describing the ultrasonic signal intensity as a function of time-of-flight through the bolt (1) of said bolted joint (10), and

- measuring length of said bolt (1) in said bolted joint (10), where said measuring length of said bolt is performed at least ultrasonically, and

- comparing said measured bolt length of the bolt with the preload bolt length derived from said joint profile (151) where comparing said measured length comprises comparing a measured waveform with said waveform (174) derived from said joint database.

6. A method according to claim 5, where a signal is transmitted being indicative of the result of comparing said measured bolt length of the bolt with the preload bolt length derived from said joint profile (151).

7. A method according to any of claims 5-6, where said bolt tag further comprises an attachment part for attaching said bolt tag (100) to said bolted joint (10), where the attachment part is preferably one of: a magnetic part, an adhesive, an outer surface at least partially integrally formed with a bolt, a weld-suitable outer surface and a mechanical fastening part.

8. A method according to any of claims 5-7, wherein if measured bolt length is shorter than preload bolt length, the bolt is retightened to at least said preload bolt length, preferably to a calibration bolt length being longer than said preload bolt length.

9. A method according to claim 8, wherein the length measurement is performed concurrently with retightening the bolt, where retightening is informed by said length measurement, and where it is stopped once a desired length is achieved, and where retightening and length measurement are controlled by a processor (132).

10. A method according to claim 9, where the concurrent length measurement comprises ultrasonic measurement to produce a waveform of the concurrent measurement, and further the waveform (174) retrieved from the joint profile and the waveform of the concurrent measurement are displayed on a screen for a technician to inspect.

11. A method according to any of claims5-10, wherein said measuring said bolt length is performed by both mechanical measurement and ultrasonic measurement.