DK180269B1

DK180269B1 - Bolt tension measuring device and method

Info

Publication number: DK180269B1
Application number: DKPA201870822A
Authority: DK
Inventors: Selmer Nielsen Flemming
Original assignee: R&D Eng A/S
Priority date: 2018-12-18
Filing date: 2018-12-18
Publication date: 2020-09-22
Also published as: DK201870822A1; WO2020108721A1

Abstract

The invention relates to a device and method for determining the elongation (E) of a preloaded bolt (1) in a bolted joint. The method comprises the steps of: ultrasonically measuring (110) the time-of-flight of an ultrasonic wave through the pre-loaded bolt (1) to determine an ultrasonic bolt length (C); reference measuring (120) a mechanical length (B) of said preloaded bolt (1) using a technique that is different from ultrasonic measurement; determining a difference between said lengths obtained by said two measurements, and determining the elongation (E) of said loaded bolt (1) compared to a corresponding unloaded bolt based on said difference in length and on a predetermined material constant (Km) of the bolt. Thereby, it is possible to calculate the elongation and tension of a preloaded bolt (1) without unloading it.

Description

DK 180269 B1 1

BOLT TENSION MEASURING DEVICE AND METHOD

FIELD OF THE INVENTION The invention relates to an apparatus and method for measuring the length of bolts.

BACKGROUND OF THE INVENTION Ensuring that a bolt joint has an adequate preload is instrumental to proper bolt fastening in industrial machinery, such as in the tower of windmills. Routine ser- vicing and control include assessing and maintaining bolt joint preloads, where bolt joint preload is calculated using bolt elongation as response to the tightening of the bolt joint. To calculate the elongation, it is necessary to know the unloaded bolt length, which then 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.

In ultrasonic measurements, an ultrasonic transducer may be brought into align- ment at one end of a bolt to be measured. An ultrasonic wave is emitted through the bolt and the duration from emission to reception of the reflected wave is meas- ured.

The unloaded bolt length is then used in combination with other variables and constants such as bolt dimensions and material and the distance between bolt head and nut in loaded state to calculate the required bolt elongation for correct bolt joint.

Irrespective of the measurement technique used, however, it is necessary to re- lease the bolted joint to measure unloaded bolt length which is a time-consuming process and potentially leads to technician errors.

Furthermore, the ultrasonic devices produce erroneous results in strained mate- rials, and so only work poorly for bolt joints.

DK 180269 B1 2 The object of the invention is to help determine the elongation or preload force affecting a bolt, even when it is already mounted. US 2018/0346012 A1 describes an adjustment rod inserted in a width direction through displacement-side through holes and one pair of vehicle body-side through holes. One pair of pressing parts are disposed at positions where both end parts of the adjustment rod protrude from respective outer surfaces of one pair of support plate parts. An adjustment lever is secured to the adjustment rod. With the distance between the one pair of pressing parts being reduced by turning the adjustment lever in a predetermined direction, the amount of tightening of an adjustment nut serving as one of the pressing members is adjusted while check- ing an axial force applied to the adjustment nut. The tightening amount adjust- ment of the adjustment nut is completed with the checked axial force remaining in a predetermined range.

SUMMARY OF THE INVENTION It is the aim of the present invention to alleviate at least some of the above-men- tioned problems. This is achieved by providing a bolt elongation measuring method and device. The method comprising the steps of ultrasonically measuring the time-of-flight of an ultrasonic wave through said preloaded bolt to determine an ultrasonic bolt length C and reference measuring the mechanical length B of said preloaded bolt using a technique that is different from ultrasonic measurement. The difference between said lengths B and C can then be obtained. The elongation E of said loaded bolt compared to a corresponding unloaded bolt is calculated based on the difference in lengths B, C and on a predetermined material constant Km of the bolt. Ultrasonic measurements of the bolt length are affected not only by the elonga- tion of the bolt but also by the strain in the bolt. Measuring a preloaded bolt using ultrasonic measuring produces an erroneous result as the sound waves propa- gate slower through the stretched material. The ultrasonic measurement C will overshoot the mechanical length B of any strained bolt. How much an ultrasonic

DK 180269 B1 3 measurement overshoots depends on the elasticity modulus A of the material of the bolt as well as an empirically determined factor Kr. Let Km be the product of the elasticity module A of the material and the empirically determined factor Kr.

Elasticity will vary between bolts of different materials, treatments and geome- tries, affecting the relationship between the preload force F applied to the bolt and the produced bolt elongation E. This means that the two measurements are not enough in themselves to calculate the strain in a preloaded bolt without re- moving it. The method requires knowing the material constant Km.

The empirically determined factor Kr is found through measurements on unloaded bolts and increasing the load on them to produce two functions for the lengths of the preloaded bolt for a given material. This produces a reference function for the reference measurements which plots the mechanical length B; and an ultrasonic function for the ultrasonic measurements which plots out the ultrasonic length C.

Bolts can be made from a long list of different materials, with certain materials being more common than others. For example, bolts can be made of such mate- rials as iron, brass, various grades of steel, carbon steel, steel alloys, silicon bronze, aluminium and chrome. Surface treated materials are also possible, such as zinc plated steel, galvanized steel and chrome plated steel. Determining the material constant Km for the relevant materials is performed ahead of time for the relevant materials, and stored in an easily retrievable location.

From this predetermined material constant Km and the lengths B, C measured by the technician on site, identifying the elongation E of the bolt becomes: C-B C-B E = — DD ———>—%>9%»—%8£ Having the ultrasonic bolt length measurement, a normal bolt length measure- mentand the material constant Km, a technician is enabled to calculate unloaded bolt length and elongation without unloading the bolt first.

By providing the material constant Km, the relationship between the two meas- urement functions is known. This allows finding both the unloaded bolt length D

DK 180269 B1 4 and the bolt elongation E without removing the bolt preload, decreasing time used and the needed skill, while allowing a higher standard of service and even docu- mentation for bolt preload. Typical bolt lengths are between 150 — 500 mm, although the invention works on bolts of any length. In an embodiment, the method further comprises supplying a preloaded bolt. In a preferred embodiment, a batch calibration is further used to calculate the elongation E of said loaded bolt. Thereby, the precision of the measurement is improved.

The batch calibration informs on the relationship between ultrasonic length C and mechanical length B for the bolts of the production batch of the preloaded bolt. More precisely, it informs where the intersection of the two functions will be and/or the value of the empirically determined factor Kr. The intersection represents the unloaded bolt length, and the fine-tuning of the relationship between the functions for the two then improves precision.

Even though bolt producers aim to keep bolt material, treatment and geometry constant between production batches, differences creep in. These may simply be within the normal tolerances in material quality, treatment time, and bolt length and so on. For example, one batch may have slightly different material composi- tion or a slightly longer treatment time. Different batches of bolts have different elasticities and/or lengths.

Material, treatment and/or lengths have been found to be sufficiently homoge- nous within individual batches however. For that reason, calibration can be per- formed on a subset of bolts within a batch, whose results apply for the entire batch. In an embodiment, all bolts may be part of the batch calibration. In an embodiment, each individual bolt is individually calibrated.

In an embodiment, the method comprises unloading at least two bolts and then measuring them in the unloaded state to determine the batch calibration.

DK 180269 B1

Thereby, a batch of bolts can have their material batch calibration determined on site by a technician, and the technician only has to unload a few bolts instead of all the bolts of a type.

In an embodiment, at least three bolts are unloaded.

In an embodiment, at least four bolts are unloaded.

In an embodiment, at least five 5 bolts are unloaded.

In an embodiment, batch calibration is performed by adjusting the preload in a bolt and measuring the mechanical length B and the ultrasonic length C for vari- ous preloads.

This allows determining the batch calibration without unloading the bolt.

In an embodiment, continual measurement is performed on the bolt during adjusting the preload.

In an embodiment, batch calibration is performed by ad- justing the preload of at least two bolts.

In an embodiment, batch calibration is performed by adjusting the preload of at least three bolts.

In an embodiment, batch calibration is performed by adjusting the preload of at least four bolts.

In an embodiment, batch calibration is performed by adjusting the preload of at least five bolts.

The invention is made for bolts in all types of apparatuses, machines, buildings and so on.

The examples given are of a bolt with a head and a tightening nut, though a thread with two nuts can be used, or other tensioned bolted joint type of fasteners.

The invention is applicable to objects other than bolts too, as long as the measured objects are elastic objects under elastic deformation and the length can be measured referentially as well as ultrasonically.

In an embodiment, the method further comprises a step of determining the pre- load force F of said loaded bolt based on said lengths C, D.

Thereby, technicians do not need to remove bolts to gauge their tension.

In an embodiment, reference measuring is mechanical measuring such as a mi- crometre calliper measuring.

Thereby, a precise and convenient method is used.

In an embodiment, reference measuring is optical length measuring such as a laser distance measuring.

Thereby, a precise and convenient method is used.

DK 180269 B1 6 In an embodiment, the method comprises partially unloading a preload to perform measurements of the bolt on at least two different preloads, by which the material constant Km can be calculated. Thereby, even if there is no predetermined Km for a certain bolt, complete unloading is unnecessary.

In an aspect of the invention it relates to an elongation measuring device for de- termining the elongation E of a preloaded bolt in a loaded joint. The device com- prises: an ultrasonic sensor adapted to measure the time-of-flight of an ultrasonic wave through said preloaded bolt; a reference sensor for measuring a mechani- cal length B of said loaded bolt, where said reference sensor is not an ultrasonic sensor; a control unit; and a user interface for the input from the user and for displaying the determined elongation E of the bolt to the user. The control unit is coupled to the reference sensor and the ultrasonic sensor and has a data processor. The control unit is adapted to receive data from the refer- ence sensor regarding a mechanical length B of said loaded bolt and data from said ultrasonic sensor regarding an ultrasonic time-of-flight through said pre- loaded bolt and to determine an ultrasonic length C of the loaded bolt. The control unit is adapted to determine the difference between said lengths obtained by said two measurements, and from that to determine the elongation E of said preloaded bolt compared to a corresponding unloaded bolt based on said difference in length and on a predetermined material constant Km of the bolt. By providing a device according to the invention, it is easy and user friendly to achieve the ends of the invention. A technician can mount the device to the bolt, measure it, gauge whether it requires fine-tuning, and move on very quickly. This decreases time used and the needed skill, while allowing a higher standard of service and even documentation for bolt preload. In an embodiment, the device further uses a batch calibration to calculate the elongation E of said loaded bolt. Thereby, the precision of the measurement is improved.

DK 180269 B1 7 In an embodiment, the device is further adapted to determine the preload force F of said loaded bolt based on said lengths C, D. Thereby, technicians do not need to remove bolts to gauge their tension.

In an embodiment, the reference sensor is a mechanical measuring unit such as a micrometre calliper. Thereby, a precise and convenient method is used.

In an embodiment, the reference sensor is optical length measuring such as a laser distance sensor. Thereby, a precise and convenient method is used.

The ultrasonic sensor of the invention is conveniently a transceiver adapted to emit and receive an ultrasonic wave. In another embodiment, a separate receiver and emitter are provided.

LIST OF THE DRAWINGS In the following, example embodiments are described according to the invention, where: Fig. 1 illustrates a measuring device for assessing clamp load according to the invention, Fig. 2 illustrates measurement data from a preloaded bolt according to the inven- tion, Fig. 3 illustrates a processing unit and device according to the invention, Fig. 4 illustrates a method for assessing clamp load according to the invention, Fig. 5 illustrates a method for assessing clamp load according to the invention, and Fig. 6 illustrates a further method for assessing clamp load according to the in- vention.

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.

DK 180269 B1 8 Fig. 1 illustrates a bolt elongation measuring device 200 in the process of meas- uring a bolt elongation in a preloaded bolt 1. The preloaded bolt 1 is located in a bolt joint, tightening some parts of an apparatus 9, e.g. a machine, using a nut 5 and a washer 7.

The bolt elongation measuring device 200 has a top calliper part 201 that is brought into alignment with a top head 3 of the loaded bolt 1. A lower calliper jaw 203 slides by translating along a rail 205 in the upper calliper part 201 to bring it into alignment with the opposite end of the preloaded bolt 1 relative to the upper calliper part 201.

The lower calliper arm 203 has an ultrasonic sensor 205 being a transceiver for bringing into close proximity to, or into contact with, the end surface of the pre- loaded bolt 1.

After the measuring device 200 has been brought into contact with both bolt ends, a reference sensor 209 produces a measurement of the mechanical length B of the preloaded bolt by measuring the distance between the inner surfaces of the top calliper arm 201 and the bottom calliper arm 203. In other embodiments, other conventional sensor types can be used to measure the mechanical length of the bolt.

After the device has been brought into contact with both bolt ends, an ultrasonic sensor 211 transmits an ultrasonic wave through the preloaded bolt. The time it takes for the wave to reflect and propagate back to the sensor is measured (time- of-flight) and an ultrasonic length C of the preloaded bolt is determined. In other embodiments, there may be an ultrasonic emitter at one end and a receiver at the other end of the bolt 1. In one embodiment, the ultrasonic sensor 211 and the reference sensor 209 are provided individually of each other.

These two measurements are stored at a bolt database 131.

Fig. 2 is a graph of the relationship between a preload force F in a bolt in a bolted joint and the length L of that preloaded bolt. The reference function 11 is deter- mined by reference sensor 209 that allows determining a mechanical length B of

DK 180269 B1 9 the bolt irrespective of its internal strains, such as a millimetre.

The mechanical length B is the actual length of the bolt before and after strain.

An ultrasonic function 13 describes the length of the preloaded bolt measured ultrasonically at various levels of preload force F.

For each individual preload force F to be measured, an ultrasonic length C of the bolt is measured.

As can be seen, the length C measured by the ultrasonic method is not identical to the mechanical length B measured mechanically.

The discrepancy between the mechanical length B and the ultrasonic length C is called the ultrasonic overshot G.

Since ultrasonic measurements are accurate in unstrained materials, the two functions 11, 13 has an intersection 10 at zero strain, denoting the unloaded bolt length D.

In other words, the ultrasonic over- shot is zero G for unloaded bolts.

Since the functions 11, 13 are known ahead of time, it is possible to equate the functions to find the bolt length without having to dismount the bolted joint.

Further, the ultrasonic function 13 co-varies with the reference function 11. The example bolt characterised in the figure is affected by clamping force F', re- sulting in a mechanical length B that is the sum of the unloaded bolt length D and an elongation E.

Fig. 3 is a schematic view of a bolt elongation measuring device 200. The device comprises an ultrasonic sensor 211 having a transceiver adapted to transmit and receive ultrasonic waves.

When an ultrasonic wave is emitted, it reflects off the far end of the bolt and returns to the sensor 211. The device 200 further has a reference sensor 209 for measuring a mechanical length B.

When the measure- ments are completed, they are written to a measurement database 131 where they are stored for future use.

The measurement database 131 can be a local storage medium such as a hard drive or random access memory.

A user can control the device 200 through a user interface 204 having a display 206 as well as input means 208. The input means can be a keyboard and mouse, a touch functionality in the display 206 or any other convenient input means 208 for a user to control the device 200.

DK 180269 B1 10 The device 200 has a material database 111 adapted to store data on the bolt material, such as the material constant Km. The material database 111 has been prepared with material characteristics ahead of time, at least the material con- stant Km. In an embodiment, the specific production batch calibration is also pro- vided ahead of time. In one embodiment, the bolt elongation measuring device 200 is adapted to provide data for the material database 111 by performing ini- tialization measurements 110 and having write access to the material database

111. Thereby, the batch calibration can be performed on site. The device 200 is adapted to perform calculations on the measured lengths B, C as well as material data from the material database 111 using a processing means 212. The calculations and/or the results are then displayed to the techni- cian user on the display 206, and the technician can then perform the relevant adjustments to the bolted joint. In one embodiment, the bolt elongation measuring device 200 further is adapted to inform the technician whether the preload is sufficient, and otherwise provide an alert. This allows better compliance as number of bolts accepted can be con- trolled against number of total bolts as well as having the bolt elongation meas- uring device 200 perform evaluations based on actual bolt strain. Although the device 200 has been described as a discrete whole, certain parts of it may be separated physically while retaining necessary functionality. For exam- ple, the sensors 209, 211 may be one unit or even two individual sensors, while their measurements are then signalled to a personal computer. For example, an individual ultrasonic sensor 211 may be placed against the end of the bolt 1, and the reference sensor 209 then placed over it, whereby the length of the bolt 1 and the ultrasonic sensor 211 is measured. This can be practical because it allows the technician to move the ultrasonic sensor 211 around to get the best response through the bolt. For the reference measurement, the length of the ultrasonic sen- sor 211 is then subtracted. The personal computer can be a laptop, a tablet or such device having storage means 131, 111, processing means 212 and a user interface 204.

DK 180269 B1 11 In one embodiment, the databases 111, 131 are not local or not only local, but are accessed wirelessly by the device 200. The data may then be stored or backed up off-site such as at a central location, or in another manner that is con- venient to set up and access.

Fig. 4 is a diagram of a method according to the invention.

The method performed using e.g. a device as described above is as follows to determine the elongation E / preload force F on a bolt: - calibrating 110 the system to ensure that the ultrasonic measurements 209 and the reference measurements 211 measure the same length 10 in a situation where the bolt is at rest, i.e. is not elongated.

This will probably only be necessary from bolt batch to bolt batch; - measuring 130 the ultrasonic length C of the tensioned bolt 1 and the me- chanical length B of the tensioned bolt 1; - calculating 150 the elongation E, where the difference between the two measurements are determined and reducing with a material constant Km being specific to the bolt material; - optionally calculating the preload force F based on the calculated mechan- ical elongation E.

The method will be further described in the following.

First, a calibration phase 110 is initialised.

In this phase, the batch characteristics are determined.

This is done by measuring a series of bolts both ultrasonically and referentially.

Even when bolts are made of the same material, every batch of bolts have differ- ent treatments, exact material composition and/or lengths.

Therefore, a batch calibration is determined.

Several bolts from a batch can be measured or every bolt can be measured.

This batch calibration 110 comprises measuring unloaded bolt length D, which informs on ultrasonic length function 13 and/or the mechan- ical length function 11. The calibration database 111 holds the data used to compare with the measured data to calculate bolt elongation and/or tension.

The material constant Km is stored in it ahead of time.

After the calibration phase, the batch calibration is

DK 180269 B1 12 stored in the calibration database 111 too. Ultrasonic length function 13 as well as mechanical length function 11 can conveniently be stored in the calibration database 111, if needed. In one embodiment, a reference tension table is also stored in the calibration database 111.

In the measurement phase 130, a technician measures a preloaded bolt using the ultrasonic sensor of the device. Ultrasonic measurement 133 is performed by projecting ultrasonic waves through the bolt. Conveniently, a transceiver is used to measure the time-of-flight of a reflected wave through the bolt to obtain the ultrasonic length of the bolt. Further, a reference measurement 135 obtains the mechanical length B of the bolt. A technician may bring a millimetre calliper into contact with the bolt to measure its mechanical length B, or another convenient manner of measuring length may be used.

This provides measurement data on the mechanical length B and ultrasonic length C which is then stored in a measurement database 131.

In the calculation phase 150, the measured data is retrieved from the measure- ment database 131 while calibration data is retrieved from the calibration data- base 111.

Bolt elongation E can then be determined by dividing the ultrasonic overshot G by the material constant Km. The ultrasonic overshot G is calculated by subtract- ing the mechanical length B from the ultrasonic length C.

E — C-B G&G — Km Km The bolt elongation E could be stored to the calculation database 151. It could further be shown to the technician through the display 206. The method uniquely allows a technician to only need to measure mechanical length B and ultrasonic length C of a preloaded bolt joint on-site to calculate its elongation E. Loosening preloaded bolts to measure their lengths is unnecessary when material constant Km and preferably the batch calibration are known.

DK 180269 B1 13 In the case of servicing a series of old bolted joints, such as on a single machine or a machine floor, these may have no pre-established batch calibration. In these cases, the method can be calibrated by measuring a few unloaded bolts. If a machine has fifty bolted joints of one type, unscrewing two or three of the bolts can be sufficient to establish the batch calibration for all fifty. The technician can calculate the elongation E and, thereby, the preload force F in the remainder of the bolted joints. Furthermore, the data gathered can be stored and used for the next time the bolts needs servicing. In that way, no further bolts need to be un- fastened to ensure a proper preload.

Fig. 5 illustrates an embodiment of the invention where batch calibration is pro- vided ahead of time or not at all, and a technician only needs to measure and calculate.

The material constant Km is provided. The technician measures 130 the mechan- ical bolt length B, as well as the ultrasonic bolt length C, which are stored in a measurement database 131. Calculations are performed 150 on the measure- ments in combination with predetermined material constant Km and any batch calibration provided, retrieved from a calibration database 111. These calcula- tions informs the technician about the elongation of the bolt.

Fig. 6 illustrates an embodiment of the calculation phase 150 according to the invention. In this embodiment, the elongation calculations are followed by strain calculations 153, in which the preload force F' is identified. The preload force is calculated to inform the technician directly whether the bolted joint is sufficiently fastened. This requires first finding the unloaded bolt length D. The unloaded bolt length D is obtained by subtracting the bolt elongation E from the mechanical bolt length B.

D=B— c =B-E Km Then, taking material parameters and bolt geometry into account, the tension is calculated and stored in a strain database 151. The technician can now easily compare the tension in the bolt stored in the strain database 151 with a reference

DK 180269 B1 14 tension.

The reference tension is the proper range of tensions suitable for the particular material and bolt geometry.

In a preferable embodiment, a reference tension table is stored in the calibration database 111. After strain calculations 153, the calculated strain is compared 155 with reference strains for the given material and bolt geometry.

Reference strains are retrieved from the reference tension table in the calibration database 111 to compare with the calculated bolt strain to evaluate whether or not the bolt is suf- ficiently tightened.

Preferably, the device then directly informs the technician if the bolt needs either tightening or slackening in a feedback step 160. In one embodiment, the feedback step includes continual sensing of the bolt while the technician tightens or slack- ens it, informing them when the bolt is sufficiently tensioned.

In an embodiment, the device is adapted to be attached to the tightening part of the bolted joint and to manipulate it until it has the proper tension.

Claims

DK 180269 B1 1 Patentkrav

A method of determining the extension (E) of a preloaded bolt (1) in a bolted joint, the method comprising the steps of: - reference measurement (120) of the mechanical length (B) of the preloaded bolt (1) using a technique other than ultrasonic measurement, - characterized by ultrasonic measurement (110) of the flight time of an ultrasonic wave through the preloaded bolt (1) to determine an ultrasonic bolt length (C), - determination of the difference between the lengths obtained by the two measurements, and - determination of the loaded bolt (1) elongation (A) compared to a corresponding unloaded bolt based on the length difference and the predetermined material constant (Km) of the bolt.

A method according to claim 1, wherein a predetermined batch calibration is used to calculate the extension (E) of the loaded bolt (1).

A method according to any one of claims 1-2, further comprising a step of determining the preload force (F) of the loaded bolt based on said lengths (C, D).

A method according to any one of claims 1 to 3, wherein the reference measurement (120) is a mechanical measurement, such as a micrometer caliper learning measurement.

A method according to any one of claims 1 to 4, wherein the reference measurement (120) is an optical length measurement, such as a laser distance measurement.

Bolt extension measuring device (200) for determining the extension (E) of a preloaded bolt (1) in a loaded connection, which device (200) comprises: - a reference sensor (209) for measuring a mechanical length (B) of the load. loaded bolt (1), where the reference sensor (209) is not an ultrasonic sensor,

180269 B1 2 - a control unit comprising a data processor (212), the control unit being coupled to the reference sensor (209) and arranged to receive data from the reference sensor (209) regarding a mechanical length (B) of the loaded bolt ( And - a user interface unit for receiving input from the user, - which bolt elongation measuring apparatus is characterized by further comprising: - an ultrasonic sensor (211), which is arranged to measure the flight time of an ultrasonic wave (1) through the preloaded bolt (1). The control unit is connected to the ultrasonic sensor (211) and is further arranged to: - receive data from the ultrasonic sensor (211) regarding an ultrasonic flight time through the preloaded bolt (1) and determine an ultrasonic length (C) of the preloaded bolt (1 ), - determine the difference between the lengths obtained by means of the two measurements, and - determine the elongation (E) of the preloaded bolt compared to a corresponding unloaded bolt based on the difference in length and a front determined material constant (Km) in connection with the bolt, which apparatus further comprises that - the user interface unit is arranged to show the determined extension (E) of the loaded bolt (1).

The apparatus (200) of claim 6, further using a predetermined batch calibration to determine the extension (E) of the preloaded bolt (1).

Apparatus (200) according to any one of claims 6 to 7, further comprising a step of determining the preload force (F) of the loaded bolt based on the lengths (C, D).

An apparatus (200) according to any one of claims 6 to 8, wherein the reference sensor (209) is a mechanical measuring unit, such as a micrometer caliper.

DK 180269 B1 3

An apparatus (200) according to any one of claims 6 to 9, wherein the reference measurement (120) is an optical length measurement, such as a laser distance sensor.