Socket for use in measuring clamp load DESCRIPTION Inventive Field

The invention relates to the measurement of clamp load in bolted joints.

Background of the Invention

In the assembly of bolted joints, it is important that the bolt is tightened until a correct predetermined clamp load is applied to the joint. If the optimum clamp load is achieved on assembly, most subsequent joint failures, due for instance to fastener fatigue, vibration loosening or joint leakage, can be prevented.

At present there is no satisfactory method for measuring directly and economically the clamp load in bolted joints. Instead, readings are taken of the torque applied to the bolt, sometimes in combination with a measurement of its angle of rotation. Unfortunately such torque or torque and angle measurements do not give a very accurate indication of clamp load. For the torque only measurement the accuracy is affected by variations in friction in the underhead and screw thread mating surfaces, and for the torque and angle measurement, the accuracy is affected by joint compression. Discrepancies in clamp load measurements of plus or minus 30% for torque only, and plus or minus 15% for torque and angle, are to be expected. This means that parts must be overdesigned, and larger or more numerous bolts used than strictly necessary, to produce the desired reliability. This results in increased manufacturing times and costs.

One method that has been used in the past for measuring directly the clamp load in bolted joints involves the use of ultrasonic waves. The ends of a bolt to be tested are ground down and a piezo-electnc transducer is mounted on

one end of the bolt, coupled to the bolt with an acoustic coupling gel. When an electrical signal is applied to the transducer it transmits an ultrasonic wave down the length of the bolt. The wave is deflected from the opposite end of the bolt and detected by the same transducer. The time taken for this echo depends on the length of the bolt and the speed of the wave, both of which are dependent on the tension in the bolt. Consequently, comparisons of the time of flight of the ultrasonic wave when the bolt is unstressed with the time of flight under load can be used to calculate the tensile load in the bolt.

This is a viable method of measuring clamp load, but extreme care is needed in setting up the equipment, and ensuring that the acoustic coupling is sound, and that there is no movement of the transducer relative to the bolt. The method is therefore only used in operations where cost is not of prime importance, such as in nuclear power stations. It is not suitable for giving the cheap, fast and reliable measure of clamp loads needed, for instance in vehicle production lines.

A recent advance in ultrasonic technology involves the use of so called "intelligent fasteners", bolts with piezo-electπc thin films deposited on their heads by sputtering, a vacuum deposition process. This avoids coupling problems and improves accuracy when used for the conventional method discussed above. More importantly, such bolts enable the use of a new ultrasonic technique which simultaneously exploits two different ultrasonic waves. By manipulating the orientation of the crystals in the piezo-electnc film on the surface of the bolt, the piezo-electnc can be made to send one ultrasonic wave directly down the centre of the bolt and one wave at an angle so that it bounces off the sides of the bolt. This second, angled "transverse" wave could not be used

with previous techniques because such waves are not transmitted through fluids like coupling gels.

By using both longitudinal and transverse waves, the clamp load can be measured directly without knowing the unstressed length of the bolt, and therefore without having to untighten the fastener. Measurements are highly accurate in comparison with all previous techniques (around +3%) .

The major problem with the above technique is that of providing an electrical signal to the bolt head. It has been proposed to use a torque wrench with a probe, such as a retractable spring-pin, at the socket for establishing electrical contact with the piezo-electnc film on the bolt head. However, in order that electrical signals may pass to and from the probe, it has been thought necessary to provide a path for these signals right through the socket, the assembly tool drive head, and out of the handle of the tool. This has a number of disadvantages.

Firstly, such electrical wiring makes it impossible to use any kind of ratchet wrench due to twisting of the wiring. Secondly, it requires a hole to be drilled right through the socket and through the drive head of the wrench. This is highly impractical and indeed impossible with mechanical wrenches which contain gearing mechanisms ust where the drilled hole would be needed. Therefore, the cost of providing new tooling compatible with this ultrasonic technique would be so high as to make the technique impractical for most applications.

The important concept which has now been appreciated by the Applicants is that it might be possible to transmit the electric signal to and from the probe via the socket. Such an "intelligent" socket would allow the use of

ultrasomcs techniques without the necessity to make any alterations to the wrench at all.

The Applicants' published Patent Specification WO 94/07119 describes a novel torque transducer in which the conventional brushes for picking the electrical signals from the commutator sliprings are replaced by flexible strips of metal, passing partially around carbon sliprings and tensioned against the sliprings. The flexible strips are in brushing contact with the sliprings and the long arc of contact removes substantially all signal noise.

Furthermore by using carbon sliprings and metal flexible strips, wear and tear on the carbon is kept to a minimum, and any such wear that does occur does not result in disruption of the signal.

It has been appreciated that aspects of the brush technology of the above torque transducer might be useful in providing a particularly advantageous embodiment of the "intelligent" socket discussed above.

The Invention

The invention provides a socket for use in measuring the clamp load of a bolted joint, the socket being adapted at one end for receiving torque from a wrench, and adapted at the other end for transmitting torque to a bolt, and including: an electrically conductive pin mounted within the socket for contacting the head of the bolt and allowing the transmission of an electrical signal thereto or therefro ; slipring means mounted for conjoint rotation with the socket, and in electrical connection with the pin for allowing the transmission of the signal to or from the pin; and

-5- brush means passing partially around and tensioned against the slipring means in electrical brushing contact therewith for transmitting the signal to or from an electrical circuit.

Preferably electrical connection from the pin to the slipring means is provided by an annular or substantially annular printed circuit board.

By using such a socket, electrical signals can be transmitted to and from a piezo-electnc coated bolt head via the socket only. No changes have to be made to the wrench or impulse tool being used to tighten the bolt, and the only change to the equipment that is necessary is the replacement of the standard socket with a socket according to the invention, which makes substantially the same space demands.

Different sockets would be needed for each different size of nut. Preferably sockets would initially be made in a limited range of sizes and then their hexagonal sockets finished to precisely the desired size.

Preferably the slipring means comprises one or more carbon discs and the brush means comprises one or more flexible metallic strips. In this case the socket has all the advantages discussed in relation to the torque wrench of International Application WO 94/07119 in respect of low signal noise and reliability. This s particularly important for the applications in which ultrasonic measuring of clamp load can be of particular use, for example vehicle production lines, where sockets are expected to withstand hundreds of millions of revolutions, and to cope with the extra stresses involved with use in conjunction with impulse tools.

The socket of the invention is particularly suited for

use with impulse tools, because the time between impulses can be used to check the clamp load of the bolt using ultrasonic techniques. Depending on the measured value of clamp load a decision can be made whether or not to continue tightening the bolt.

If desired, a socket according to the invention can be equipped also with the torque measuring apparatus of WO 94/07119. A further four sliprings and brushes would be necessary to conduct the torque responsive signals to the controller.

Drawings

Figure 1 is a side view of a socket according to the invention;

Figure 2 is an end view of the socket of Figure 1;

Figure 3 is a section on the line A-A of the socket of

Figure 2 ;

Figure 4 is a section of the line B-B of the socket of

Figure 1 ; and

Figure 5 is a plan view of a printed circuit board suitable for use with the invention.

Description with reference to the drawings

Referring to Figure 3, a socket according to the invention includes a shaft 1 having at one end a square socket 5 for receiving the square end of, for instance, an impact or impulse tool for tightening a bolt. At the other end of the shaft 1 is a hexagonal socket 10 for receiving the head of the bolt to be tightened. Protruding into this hexagonal socket is the end of an electrically conductive spring loaded pin 3, used for transmitting electrical signals to and from the head of the bolt. Because the pin is spring loaded, it always presses against the head of the bolt, maintaining a good contact.

The spring loaded pin 3 is electrically connected to a printed circuit board 4, of the shape shown in Figure 5. This circuit board has an insulated backing material on one side, and a continuous layer of conductive material on the other side, to provide electrical contact between the spring loaded pin, and a carbon slipring 2. A second carbon slipring 2 is used as an earth.

Referring to Figure 4, a flexible metallic strip 6 is connected at its ends between an insulating bar 8 and a printed circuit board 9. The strip is held in tension around and against slipring 2 by a tensioning spring 7. Thus, a good electrical brushing contact is maintained between the slipring and the metallic strip for the transmission of electrical signals therebetween. The electrical signals are transmitted to and from the printed circuit board 9 by a controller which s not illustrated.

An example of the use of the socket is in the tightening of a joint to a predetermined clamp load, using an impulse tool. A bolt having its head coated with piezo- -electπc material is used, as previously discussed. First, the impulse tool applies a torque to the bolt via the socket. There is then a delay before the next pulse of torque is applied. During this delay, an electrical signal is transmitted from the controller via the printed circuit board 9, to the metallic strip 6. From there it passes via the slipring 2 and the printed circuit board 4 to the pin 3 thereby to stimulate the piezo-electnc material on the head of the bolt.

This piezo-electnc material produces a longitudinal ultrasonic wave, which is transmitted down the centre of the bolt, and a "transverse" ultrasonic wave which bounces off the sides of the bolt. Both of the waves are reflected from the end of the bolt back towards the

piezo-electπc crystal. The reflected waves excite the piezo-electnc material, and the resultant electrical signal passes back via the pin 3, the printed circuit board 4, the slipring 2, the metallic strip 6 and the printed circuit board 9 to the controller. Analysis of the signals by the controller gives a measure of the clamp load in the joint. The controller therefore compares this measured clamp load with a predetermined desired clamp load to determine whether or not the impulse tool should apply more torque.