JP2649837B2 - Fastener - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a load indicating fastener.

In many operations, it is desirable to measure the amount of longitudinal load that results when a member is subjected to longitudinal stress.
Knowing such a load is particularly useful when the member is a fastener, as it determines whether or not an appropriate joint can be obtained according to the magnitude of the longitudinal stress.

Conventionally, many techniques have been developed for indicating the magnitude of the longitudinal stress generated in a fastener having a load indicating characteristic. This is typically done by connecting one end of an elongated member, such as a pin, to a portion of the fastener prior to installation.

FIG. 1 shows an example of this type of load indicating member. The elongated member 10 extends parallel to the fastener 12, and is not affected by plastic deformation of the fastener due to longitudinal stress. Therefore, the free end 14 of the elongated member 10 functions as an index indicating the extension of the fastener 12 due to the longitudinal stress. Typically, elongate member 10 is a pin received within longitudinal slot 16 of fastener 12 and extends from head 18 of fastener 12 into a portion of shank 20 of the fastener. One end 22 of pin 10
Is connected to the shaft 20 of the fastener 12 at the bottom of the hole 16 by, for example, an adhesive, a screw, or an interference fit. In the past, various types of load indicating members and fasteners of this type were very different in construction and method and apparatus and were used to indicate the elongation of the load indicating member or fastener. Examples of such fasteners are described in U.S. Pat. No. 3,812,758 (published May 28, 1974), and U.S. Pat. No. 3,823,639.
No. (announced July 16, 1974), No. 2,503,141 (April 1950
No. 3,943,819 (March 16, 1976), No. 2,600,029 (June 10, 1952), No. 3,90
No. 8,508 (announced September 30, 1975), No. 3,987,668 (1976
No. 4,144,428 (announced October 26, 1978)
(Published on Nov. 9), as well as U.S. Pat. No. 4,676,109, filed on Nov. 13, 1984, which is to be published soon.

Although the various pin-type load indicating members and load measuring devices described above have their respective advantages in terms of accuracy, manufacturability, or readability, there are many correction and pin members at the center of the load indicating member. It is necessary to add the above, so that the manufacturing cost is high. As a result, the above-described load indicating member is selectively used only when a special immediate diagnosis is required or when there is a danger of a recognized safety. It would be too expensive to use the components and devices described above for normal assembly operations where such monitoring or benefit would be obtained.

Another way to measure the elongation of a member or fastener is to use an ultrasonic measuring device. Typically, such a measurement scheme is described in US Pat. No. 3,759,090, No. 1.
The ultrasonic transducer 58 is detachably connected to one end of a member to be measured, typically the head 106 of the fastener 98, as shown in FIG. 2 attached to the present specification corresponding to FIG. 3 and FIG. Performed by To get a reliable display,
The bolt head needs to be ground extremely flat and a reliable ultrasonic transmission medium needs to be attached to the bolt head. The transducer must be correctly positioned on the bolt and fixed during the measurement. Examples of techniques and devices using this method are described in U.S. Pat. No. 3,306,100 (February 28, 1967).
No. 3,307,393 (announced March 7, 1967), No.
No. 3,308,476 (announced on March 7, 1967), No. 3,759,090 (announced on September 18, 1973), and No. 3,812,709 (May 2, 1974)
8th announcement), No. 3,822,587 (announced July 9, 1974),
4,014,208 (announced March 29, 1977), 4,062,227 (announced December 13, 1977), 4,117,731 (announced October 3, 1978), 4,363,242 (announced December 14, 1982) ,
Nos. 4,402,222 (announced September 6, 1983), 4,413,518 (announced November 8, 1983), and 4,471,657 (1984)
(Announced September 18, 2008).

The aforementioned U.S. Patent combines a measuring device with a tightening tool and uses the information obtained from measuring the elongation of the bolt, thereby measuring the time to stop the tightening tool, or monitoring the tightening process. To determine whether a proper joint has been obtained. Examples of such fastening tools are described in U.S. Pat. No. 3,969,960 (July 2, 1976).
0 date announcement) and 3,969,810 (announced July 20, 1976)
Is disclosed.

Although the methods and apparatus of the aforementioned U.S. patents provide reliable information about fasteners and joints, they are very limited in their use. The main reason is that the bolts must be manufactured with care and the components must be easily manipulable. Thus, ultrasonic tension measurement is understood as a high-precision laboratory clamping method for calibration application tests and purely critical clamping joints. The method is to replace the strain gauge bolt with several uses of calibration and critical property control. However, the practical difficulties in measuring ultrasonic tension described above hinder its application as a general assembly tightening strategy.

Some attempts have been made to combine the advantages of the pin-type load indicating member described above with the ultrasonic elongation measuring device described above by attaching a piezoelectric or other ultrasonic sensor to the member itself. Such members are described, for example, in U.S. Pat.
No. 788 (announced November 28, 1978) and No. 4,294,122 (19
Published on October 13, 1981). These patents teach a load-bearing fastener that has been modified to have stress indicating properties. However, like the pin-type fasteners described above, these equipped fasteners are heavily modified to mount complex and large ultrasonic detectors. Thus, such fasteners are prohibitively expensive for widespread use.

Other prior U.S. patents teaching or claiming the ultrasonic piezoelectric method or another are as follows.

Only a very small number of achievements have been achieved from the above research and development, and its use has proven to be difficult to maintain a reliable connection during tightening, the cost and complexity of the required equipment, and the fastener materials and The demands on tight control over properties have largely restricted laboratory research and expensive and rigorous equipment.

Therefore, what is needed is a permanent installation of an inexpensive ultrasonic transducer on a fastener in an inexpensive manner to provide accurate tightening information in a mass production format. Such an ultrasonic load indicating member can be easily connected to a measuring device or an assembly tool,
It would also solve the problems encountered when conventional ultrasonic measurement devices attempt to create reliable acoustic coupling.

(Summary of the Invention)

While the present invention provides a load indicating fastener, the present invention provides a number of advantages, whereas various load indicating members or fasteners have conventionally been utilized individually, i.e., only a single member or fastener. It is a combination.

Further, the present invention provides additional features and advantages not previously utilized in load indicating members, ie, load indicating fasteners.

The load indicating member of the present invention includes a shaft portion that is plastically deformed when subjected to a longitudinal stress, and a substantially flat first surface and a second surface that are respectively close to both longitudinal end portions of the shaft portion. The first surface and the second surface are parallel to and separated from each other by a substantially predetermined distance when the shaft is unstressed.
A piezoelectric film permanently, mechanically and electrically connected to the first and second electrodes is disposed on the first surface of the shaft. The first electrode is permanently, mechanically, electrically and acoustically connected to the first surface. The first electrode may be made of, for example, an adhesive or a metal film, or may itself be made of an electrode member.

In a preferred embodiment, the piezoelectric film is a thin flexible piezoelectric film disk, and the first and second electrodes are metal layers on both sides of the disk. Also, in this preferred embodiment, the load indicating member is a load indicating fastener having an enlarged head, and a flat first surface is formed on the head of the fastener.

The fastener according to the present invention has a flat surface at one end in the vertical direction, and a first electrode and a second electrode are arranged on both surfaces of the flexible piezoelectric film. The first electrode is permanently, mechanically, electrically, and acoustically arranged such that the flat surface of the fastener and the second electrode are insulated from the flat surface.

The load measuring device according to the present invention includes a first contact that electrically engages with the first electrode, and a second contact that engages with the second electrode.
A contact, an electronic measuring device that responds to an electronic differential signal that can measure a tensile load of the fastener, that is, the load indicating member when stress is applied between the first electrode and the second electrode in a longitudinal direction; Is provided.

In a preferred embodiment, the piezoelectric film comprises an excitation device for generating an ultrasonic signal such as for generating an electronic differential signal. Further, in a preferred embodiment, the load indicating member is conductive, and the first contact is electrically connected to the first electrode by the first contact indirectly engaging with the load indicating member. Combine.

The tightening tool used in the present invention comprises a first contact and a second contact which can be electrically engaged with the first electrode and the second electrode, respectively.
A contact, a load inducing device for applying a tensile load in the load carrying member, a load device for applying a tensile load in the load carrying member, and a response to an electric differential signal capable of accurately measuring the tensile load. Load measuring device.

A preferred example of the tightening tool used in the present invention is a conductive fastener engaging device capable of engaging with the load indicating fastener,
A contact member capable of engaging with the second electrode of the load indicating fastener, a driving device for applying torque to the fastener engaging device so as to rotationally drive the load indicating fastener, and a tightening operation receiving from the socket and the contact member And a load measuring device responsive to an electric differential signal capable of accurately measuring a tensile load on the shaft portion of the fastener when subjected to a longitudinal stress as a result.

The output of the load measuring device can be used to continuously read the instantaneous tensile load of the fastener, or to measure the end point of the tightening operation or to indicate the load in the pre-fastened fastener. Can be used for When the load indicating member is a fastener, the load measuring device can be used simultaneously with the fastener tightening tool or can be directly attached to the fastener tightening tool. When the fastener tightening tool including the load measuring device is of an automatic tightening type, the indication of the tensile load in the load measuring device is combined with other parameters, and the angle and torque are monitored by the fastener tightening tool. Also, it is possible to measure the end point of the tightening and detect the abnormality of the joint.

When measuring the load of the fastener of the present invention, a load measuring device is connected to the fastener, and the tensile load of the shaft portion of the fastener is calculated from the ultrasonic measurement value.

When tightening the load indicating fastener of the present invention, a load measuring device is connected to the head of the load indicating fastener of the present invention, and the load indicating fastener is tightened and the load measuring device is continuously monitored. Is measured, and the tightening of the load indicating fastener is stopped at the time of arrival as displayed by the load measuring device.

A primary object of the present invention is to provide an inexpensive load indicating fastener that can be easily installed with conventional tools.

Yet another object of the present invention is to provide a load indicating fastener that can be tightened by a conventional tightening tool, and in particular, to provide a load indicating fastener that can be monitored by a load measuring device during the tightening operation.

That is, according to the present invention, a fastener adapted to measure its own longitudinal strain, having a longitudinal axis and a predetermined longitudinal length and receiving longitudinal strain along said longitudinal axis. A head formed at one end of the shaft and having a substantially flat surface perpendicular to the longitudinal axis; and a permanent, mechanical, electrical and acoustical head. A first electrode interconnected with the first electrode; a piezoelectric film member permanently, mechanically, and electrically interconnected with the first electrode; a permanent, mechanical, and electrically interconnected with the piezoelectric film member. A fastener is provided that is connected and electrically insulated from the head and the first electrode, the second electrode forming an ultrasonic transducer in cooperation with the first electrode and the piezoelectric film member.

Further in accordance with the present invention, a fastener adapted to measure its own longitudinal strain has a longitudinal axis and a predetermined longitudinal length and is subject to longitudinal strain along said longitudinal axis. A head formed at one end of the shaft and having a substantially flat surface perpendicular to the longitudinal axis; and a permanent, mechanical, and electrical connection to the head by an adhesive. And a first electrode acoustically connected; a thin piezoelectric film disk permanently, mechanically and electrically connected to the first electrode; a permanent, mechanical and electrical connection to the piezoelectric film disk. And a second electrode electrically connected to the head and the first electrode to form an ultrasonic transducer in cooperation with the first electrode and the piezoelectric film disk. Fastener Beauty second electrode including a metal film on both surfaces of the piezoelectric film disk is given.

The foregoing objects, features, and advantages of the present invention, as well as other objects, features, and advantages, will be apparent to those skilled in the art from the following detailed description of the drawings.

〔Example〕

In the drawings, particularly FIGS. 3 and 4, a load indicating portion, in particular, a load indicating fastener 110 is illustrated. The load indicating fastener 110 is formed from a conventional bolt modified to indicate the tensile load, stress, elongation, and other properties of the bolt during the tightening operation and at various times during the joint life. This bolt has a thread 11 at one end
4 and a shaft 112 having a head 116 at the other end.
A shoulder 118 is formed between the head 116 and the shaft 112. The head 116 has a substantially flat upper surface 120 perpendicular to the longitudinal axis 122 of the shaft 112. A lower surface 124 is formed at the other end of the shaft 112 perpendicular to the longitudinal axis 122 of the shaft. Further, the head has an engaging surface of a tool such as a hexagonal wrench on the periphery thereof.
It has 126.

In another embodiment, as shown in FIG.
A flat surface 120a may be formed in the recess 121 of the 116a. The recess 121 may be a tool engagement socket or a discard hole, or may be a shallow groove formed to protect the piezoelectric film sensor 128 from environmental danger.

The piezoelectric film sensor 128 is located on the upper surface 120 or 120 of the head 116.
It is permanently or semi-permanently mounted on a. As shown, the piezoelectric film sensor 128 preferably comprises a disk 130 of piezoelectric film material, a first electrode 132 mounted on a flat surface on each side of the disk, and a second electrode 134.

The piezoelectric film sensor is formed from a thin layer of a flexible and inexpensive piezoelectric film material, such as a polymer material, and has a thickness of approximately 5 to 110 microns. In a preferred embodiment, polyvinylidene fluoride and its derivatives are chosen because of their strong corrosion resistance. However, other materials, i.e., copolymers, may be present to provide satisfactory properties, and separate bi- and multi-molar forms of the stack are also contemplated. The electrodes may be formed from a piezoelectric disk, a conductive ink or paint, or a metal layer vacuum treated on an adhesive. Alternatively, one or both electrodes may be a conductive foil permanently bonded to the disc. The first electrode is electrically and acoustically coupled to the head 116, while the piezoelectric disk 130 and the second electrode 134 are electrically isolated from the head 116. In some embodiments, head 116 performs the function of first electrode 132.

In some embodiments, the first electrode 132 comprises a conductive or non-conductive adhesive, which is non-conductive and provides a capacitive coupling for electrical communication between the piezoelectric film sensor 128 and the head 116. Is preferred.

In experimental work, the echo signal level of a thin film polymer piezoelectric transducer adhesively bonded to the component was compared to a conventional thick ceramic transducer removably mechanically coupled to the component. The film converter not only has a very high signal level, but also a very high signal-to-noise ratio (SN ratio), which proves that echo detection is very easy and reliable.

The high noise level of ceramic transducers is due to echoes from internally reflected ultrasound waves within the transducer and its housing. The reflected sound waves in the film transducer will attenuate quickly because the low acoustic impedance of polymeric piezoelectric films, such as polyvinylidene fluoride and its derivatives, effectively transfers energy to air or conductive rubber contacts.

Therefore, it should be noted that the piezoelectric film transducer used herein has significantly improved transducer performance over the prior art.

In FIG. 5, one of the load indicating fasteners 110 described above is shown.
An example is shown, wherein a fastener fastening tool 140 is engaged with the fastener. The tool 140 includes a normal power tool 142,
The power tool is only shown in its rotary drive part. The conventional power tool 142 has a rotary output driver 144 that engages with the socket member 146. The fastener tightening tool 140 and the rotary output driver 144 rotate together. Socket member 146 electrically and mechanically engages with head 116 of fastener 110.

The contact pin 148 is reciprocally mounted on the fastener tightening tool 140 and reciprocates with respect to the socket member 146 to reciprocate the second electrode 134 of the piezoelectric film sensor 128 and the fastener 110.
With the head 116. The contact pins 148 preferably have a conductive rubber tip 148a to provide a low acoustic impedance surface and not damage the transducer.

5 is electrically connected to the contact pin 148 and the socket member 146 by wires 152, 154, respectively, in the form of slip ring wipers 156, 158, as is well known. Alternatively, the signal can be transmitted by non-contact means such as capacitive coupling or other known techniques. The electronic control unit 150 supplies and measures an electronic differential signal between the first electrode 132 and the second electrode 134 of the piezoelectric film sensor 128 to measure and exceed the tensile load, stress, or elongation of the shaft 112 of the fastener 110. Measure the sound wave.

One skilled in the art will appreciate that the fastener tightening tool 140 can include a display (not shown) to display ultrasonic measurements of tensile load, stress, or elongation obtained during fastener operation. Alternatively, the fastener tightening tool 140 may use information continuously provided by the electronic controller 150 to determine when a predetermined amount of tensile load or elongation has occurred, and thus when to stop the tightening operation. be able to.

Those skilled in the art will also appreciate that the selected power tool can monitor, in a known manner, the characteristics of the formed joint, such as the torque and instantaneous angle of the load indicating fastener. An example of such a power tool is disclosed in U.S. Pat. No. 4,344,216 issued Aug. 17, 1982. Such information obtained from the power tool is combined with the tensile load, stress or elongation information provided by the electronic control unit 150 so that various measured parameters can be used to control the continuous operation of the tightening and consequently , A precisely controlled tightening operation such as monitoring the For example, the socket member 146 can be used with a power tool using a method conventionally known as a "nut tightening method"
The stretch information is used to determine whether the joint formed by the tightening operation satisfies certain instructions.

In operation, the load indicating fastener 110 of the present invention is progressively penetrated through appropriate holes in the panels 136, 138 and the panel is secured by tightening the nut behind the panel. A fastener tightening tool 140 and, if necessary, a standard tightening tool is engaged with the tool engaging surface 126 of the load indicating fastener 110 and rotated to tighten the joint. When the panels 136 and 138 are engaged with the shoulder 118 and the nut 139, respectively, the shaft 112 of the load indicating fastener 110 receives vertical stress and is elastically deformed in the vertical direction. Tensile load of the shaft 112,
The amount of stress, or elongation, is measured by fastener fastening tool 140.

Although FIG. 5 shows a fastener tightening tool 140 including a conventional power tool 142 and an electronic control unit 150, those skilled in the art will be able to form a fastener measuring tool including all the components of the fastener tightening tool 140 except for the power tool 142. Let's understand. Such a device can be used to measure the elongation of the bolt independently of the tightening tool.

It should also be noted that the shape of the load indicating fastener 110 described above allows for a quick change of existing bolts. Apart from the substantially flat upper surface 120, no special surface treatment is required. In fact, in the conventional method, 63
A surface finish of 5 microcentimeters (250 microinches) is specified as acceptable for acoustic coupling purposes, but a surface finish of about 318 microcentimeters (125 microinches) is desirable Was known. However, a surface finish of 41 to 510 microcentimeters (16 to 200 microinches) has been found to be preferred in the present invention because of the use of adhesive bonding. Piezoelectric film sensor 128 may be formed independent of bolts by attaching a metal material film to each of the two sides of piezoelectric disc 130. A suitable adhesive, which acoustically and mechanically couples the piezoelectric film sensor 128 to the top surface 120 of the head 116, is not shown to store the piezoelectric film sensor until it is attached to the fastener 110, although not shown. As is well known, the sensor is previously attached to the sensor together with a release sheet of an inert material.

According to the load measuring device, the traveling time of ultrasonic waves along the axial length of the bolt can be directly measured. This travel time varies according to the length of the bolt and the stress acting on the bolt.

For the measurement of the transit time, many different electronic technologies are conventionally known as a result of advances in ultrasound in the field of non-destructive testing. Although the vast majority of electronics can provide the required resolution and accuracy, a few show advantages in terms of pulse count, circuit complexity, and power consumption for accurate measurements.

A salient factor is that the effect of axial stress on the shear wave velocity is much less than on the longitudinal wave velocity. Therefore, the measurement of the transit time of both waves can be used to measure the axial stress ignoring the bolt length. Thus, the axial load in the pre-installed fastener can be measured. A simple comparison of the information obtained by using shear waves and longitudinal waves is shown below.

All currently available ultrasonic devices are used to measure the transit time of longitudinal ultrasonic waves generated by a piezoelectric transducer mounted on one end (usually a head) of a bolt. Ultrasound runs towards the reflecting end, reflects off that end and is detected by the same transducer. Numerous attempts to time this run are briefly described below, but all measure the change in time from zero tension and calculate the tension by that measurement.

It is common practice to grind the ends of the fastener parallel and to a surface finish better than 250 microinches (635 microcentimeters). Good surface finish is required for proper acoustic coupling to the bolt.

The ultrasonic wave used for this travel time measurement is a longitudinal wave.
The particle motion in longitudinal waves is in the direction of propagation forming a region of compression and tension motion. The transit time of the longitudinal wave depends on the length of the bolt and the speed of the ultrasonic wave. The change in bolt length
Thermal expansion, elongation due to axial load based on tightening,
And plastic deformation caused by tightening. The speed of the ultrasonic waves depends on the nature of the material, i.e., composition, heat treatment, and temperature, as well as the axial stress in the fastener caused by the tightening. Parameters relating to material properties and property changes with temperature are determined empirically and are usually entered into an ultrasonic tension measurement device along with ambient temperature measurements.

When the fastener is tightened, the bolt is stretched by the axial load and the propagation speed of the ultrasonic wave is reduced by the axial stress. This corresponds to about 2/3 of the measured increase in travel time.

When measuring the tension with this technique, it is necessary to add the grip length, that is, an appropriate correction stress factor selected in advance as a parameter. The reason for this is that the gripping length affects not only the elongation of the bolt due to the load, but also the average stress and thus the average velocity propagating through the length of the bolt.

Thus, there is some limiting feature that uses the transit time of the longitudinal wave over the entire length of the fastener. This measurement depends on the grip length that needs to be added as a control parameter. In addition, variations in the local stress distribution in the region of the nut affect the accuracy. All bolts are typically ± 2.54x
Measure the tension in the fastener after it has been installed, without having to grind it to an exact length of 10 cm (± 1 x 10 inches) or measure this exact length and put it in the measurement system as a parameter It is not practical to do so. Usually, the tension measurement is based on an increase in the transit time for the zero load measurement made before the start of the tightening.

Longitudinal waves are sound waves that are well known in that they are waves used to transmit the vibrations of an acoustic energy source from the air to the ear. In contrast, the shear wave particles do not oscillate in the direction of propagation but oscillate perpendicular to that direction. This shear wave is often referred to as a shear wave because adjacent particles are subjected to shear forces. Particularly viscous fluids need to be acoustically coupled to transducers temporarily attached to bolts by known devices, since gases and liquids generally cannot transmit shear waves.

Longitudinal waves run through steel at almost twice as fast as transverse waves, and transit time is affected to varying degrees by axial stress. The longitudinal wave velocity change due to stress is about 1.5 times faster than the shear wave velocity change.
Twice as large.

As described in the aforementioned U.S. Pat. No. 4,602,511, a longitudinal wave and a transverse wave are applied to one end of the fastener by a piezoelectric crystal that generates a wave having a longitudinal component and a transverse component. The running time of each of the longitudinal wave and the horizontal wave is measured. From these two measurements, the tensile force can be measured without knowing in advance the exact length of the bolt.

The use of both longitudinal and shear waves theoretically offers two significant advantages over using only longitudinal waves. First, the tensile force can be measured without first measuring zero load, that is, the load can be measured on an already installed fastener. Second, the effect on the various bolts, such as due to material properties, heat treatment, etc., is reduced since they affect both longitudinal and transverse waves.

The load measuring device may be used with one or both of longitudinal waves and shear waves in accordance with the intent of the invention.

As briefly described below, the load measuring device is capable of direct time measurement, indirect time measurement, clock interpolation, double pulse transmission, resonance frequency detection, acoustic impedance detection, harmonic frequency detection,
Any of a variety of timing schemes can be used, including phase detection.

In the direct timekeeping method, as shown in FIG. 6, a time interval from the transmission of a driving pulse to the reception of an echo signal is measured by a gate-controlled oscillator and a counter. For example, one device sold by Raymond Engineering uses a 100 MHz clock and an average of 160 measurements to achieve the required analysis. When using gallium arsenide, a clock rate as high as 2 gigahertz could be considered.

The indirect timing method relates to a time interval from the first echo to the second echo as shown in FIG. By measuring the time interval between the first echo and the second echo, errors due to fluctuations in the trigger point due to various echo waveforms and transmission delays of circuits and wiring are eliminated. Bolt end finishing is important because the second echo attenuates each of these three effects.

In clock interpolation, the analysis is improved by using analog methods to measure a fraction of the clock cycle in addition to clock counting. One method, shown in FIG. 8, is to use two synchronized phase clocks at the end of the gate interval, each integrated over the same short period of time.
Use clocks.

In the double pulse transmission method, two pulses A and B are transmitted as shown in FIG. Pulse A is the first
Echo A1 and second echo A2 are generated, and pulse B is
An echo B1 and a second echo B2 are generated. Pulse A,
The time interval between B is adjusted so that the second echo A2 from pulse A coincides with the first echo B1 from pulse B.
This result is corrected and integrated to generate a frequency control voltage for a voltage controlled oscillator that adjusts the pulse timing. The frequency of this voltage controlled oscillator is used to calculate the tension.

In the fundamental frequency detection scheme, the bolt is maintained so that longitudinal waves resonate at its fundamental frequency, and the difference before and after tightening is used for tension measurement.

In the acoustic impedance detection scheme, the volt is driven near the fundamental frequency and changes in mechanical or acoustic impedance are used for tension measurement.

The harmonic resonance frequency detection method is a modification of the resonance method,
The harmonic resonance of the longitudinal wave is maintained at a harmonic frequency in the range of, for example, 5 to 10 MHz. The bolt tension is calculated from the frequency shift during tightening.

Phase detection method is pulse phase lock loop (PLL)
Use method. The radio frequency output of the voltage controlled oscillator is periodically gated to the converter. The received signal generated by the reflected wave is mixed with the output of the voltage controlled oscillator as shown in FIG. If the transmitter signal and the received signal are in phase, the sum signal will be maximum, and if the phases are 180 ° out of phase, the two signals will cancel and the sum signal will be minimum. Thus, this sum signal indicates the phase difference and is used to control the frequency of the voltage controlled oscillator such that the phase remains constant, ie, the sum signal is minimum or maximum. Thus, the frequency of the oscillator is adjusted to maintain the transit time at the correct number of cycles. The running time is determined by the frequency and the number of cycles calculated from the start of transmission to the start of reception.

It will be appreciated that the methods described above, as well as the two types of waves, can be used in various combinations to measure one or more parameters to the desired accuracy.

It will be understood by those skilled in the art that the load indicating fastener 110 of the present invention can be easily connected to a conventional tightening tool or a tightening tool having a structure such as the tightening tool 140 described above.

The fastener 110 and fastener fastening tool 140 can quickly form a reliable joint having repeatable and predictable characteristics. Also, defects in the joint during the actual mounting process can be detected, thus reducing the risk of catastrophic joint failure. In addition, the quality of the joint can be monitored at a later date.

It should be noted that the fastener of the present invention can monitor and inspect the condition of the load indicating member. If the approximate length of the member is known, the approximate transit time of the ultrasonic pulse is also known. If the measurement is inconsistent or the signal is other than the expected signal, there will be defects such as cracks in the part. It should also be noted that the sensor 128 can be used on a bolted end or a threaded bar, and the tool can be used in engagement with another head or nut engaging device.

[Brief description of the drawings]

FIG. 1 is a partially cutaway side view of a prior art load indicating member (fastener), FIG. 2 is a partially cutaway side view of a conventional ultrasonic load measuring device, and FIG. 3 is a perspective view of an example of the fastener of the present invention. FIG. 4 is an enlarged partial cross-sectional view of the fastener of FIG. 3, FIG.
FIGS. 4b and 4c are partial sectional views similar to FIG. 4 of another embodiment of the fastener of the present invention, and FIG. 5 is a load indicating fastener of the present invention and a load measurement engaged with the fastener. Partial cut-away schematic side view of the fastener tightening tool including the device, FIG.
Figures 10 to 10 show graphs of the acoustic operation of the present invention in various embodiments.

Claims (30)

(57) [Claims] 1. A fastener adapted to measure its own longitudinal strain, having a longitudinal axis and a predetermined longitudinal length, and receiving longitudinal strain along said longitudinal axis. A head formed at one end of the shaft and having a substantially flat surface perpendicular to the longitudinal axis; and a permanent, mechanical, electrical and acoustically interconnected head. A first electrode connected thereto; a piezoelectric film member permanently, mechanically and electrically interconnected with the first electrode; a permanent, mechanical and electrical interconnected with the piezoelectric film member And a second electrode electrically insulated from the head and the first electrode to form an ultrasonic transducer in cooperation with the first electrode and the piezoelectric film member. 2. The fastener according to claim 1, wherein said piezoelectric film member comprises a thin layer of piezoelectric film material. 3. The fastener according to claim 2, wherein said piezoelectric film material is polyvinylidene fluoride and its derivatives. 4. The fastener of claim 2, wherein said piezoelectric film material has a thickness of about 5 to 110 microns. 5. The fastener of claim 3, wherein said first and second electrodes have a thickness of less than 30 microns. 6. The fastener according to claim 3, wherein said piezoelectric film member has a disk shape and is located at an axially aligned position with said shaft portion. 7. The fastener of claim 6, wherein said disk-shaped piezoelectric film member has a diameter between about a diameter of said shaft and half of said shaft. 8. The fastener according to claim 6, wherein at least one of said first and second electrodes has a disk shape and has a diameter substantially the same as that of said piezoelectric film member. 9. The fastener of claim 1, wherein at least one of said first and second electrodes comprises a thin layer of metal on a surface of said piezoelectric film member. 10. The fastener according to claim 9, wherein at least one of said first and second electrodes is formed by vapor deposition. 11. The fastener according to claim 1, wherein said piezoelectric film member comprises polyvinylidene fluoride and a derivative thereof. 12. The fastener according to claim 1, wherein said first electrode is connected to said face of said head by an adhesive for mechanical and acoustic connection. 13. The fastener of claim 1 including a bolt having a thread located on said shaft spaced from said head. 14. The fastener according to claim 1, wherein said head is made of a conductive material, and said first electrode is electrically or capacitively connected to said head. 15. The fastener according to claim 1, wherein at least one of the first and second electrodes is formed by depositing, electrodepositing, or coating a thin metal layer on the surface of the piezoelectric film member. 16. The fastener according to claim 1, wherein said piezoelectric film member is a polymer material. 17. The fastener of claim 1, wherein said piezoelectric film member comprises a thin layer of a polymeric film material. 18. The fastener according to claim 1, wherein said first electrode includes a non-conductive adhesive for capacitively coupling said piezoelectric film member to said shaft. 19. The fastener of claim 2, wherein said piezoelectric film material has a thickness between 20 and 60 microns. 20. The fastener of claim 1, wherein said substantially planar first surface has a finished surface of about 41 to 510 microcentimeters (16 to 200 microinches). 21. A fastener adapted to measure its own longitudinal strain, having a longitudinal axis and a predetermined longitudinal length, and receiving longitudinal strain along said longitudinal axis. A shaft formed at one end of the shaft and having a substantially flat surface perpendicular to the longitudinal axis; and a permanent, mechanical, electrical,
A first electrode electrically and acoustically connected; a thin piezoelectric film disk permanently, mechanically and electrically connected to the first electrode; and a permanent, mechanically and electrically connected to the piezoelectric film disk. A second electrode connected and electrically insulated from the head and the first electrode to form an ultrasonic transducer in cooperation with the first electrode and the piezoelectric film disk; A fastener wherein the electrodes comprise metal films on both sides of the piezoelectric film disk. 22. The fastener according to claim 21, wherein said piezoelectric film disk is polyvinylidene fluoride. 23. The piezoelectric film disk according to claim 20, wherein
22. The fastener of claim 21 having a thickness of 60 microns. 24. The fastener of claim 21, wherein said piezoelectric film disk has a diameter approximately between said shaft diameter and half of said shaft diameter. 25. The film according to claim 21, wherein at least one of the first and second electrodes is formed by vapor deposition, electrodeposition, or metal coating. 26. The film according to claim 21, wherein said first electrode comprises an adhesive. 27. The fastener according to claim 21, wherein said piezoelectric film disk is formed in a disk shape and is axially aligned with said shaft portion. 28. The fastener of claim 21, wherein said piezoelectric film disk is a polymeric material. 29. The method according to claim 29, wherein the piezoelectric film disk is 5 to 110.
22. The fastener of claim 21 having a thickness of microns. 30. The fastener of claim 21, wherein said substantially planar first surface has a finished surface of about 41 to 510 microcentimeters (16 to 200 microinches).