BACKGROUND It is important that threaded fasteners such as nuts and bolts should be correctly tightened. If they are not tightened sufficiently, they may not correctly transfer loads between the components they are connecting, and/or they may work loose in service. If they are tightened too much, they or the components they are connecting may be damaged. Thus there is a requirement for a means for indicating when a threaded fastener is correctly tightened.

SUMMARY OF THE INVENTION According to the invention, we provide a load indicating device for a threaded fastener, the device being adapted to be placed in relation to a fastener to be deformed as the fastener is tightened and including means arranged to change an electrical parameter in accordance with the deformation, the electrical parameter being measurable to provide an indication of the amount of said deformation.

Preferably the device comprises electrical contact means arranged to change its state after a predetermined amount of deformation.

Preferably the electrical contact means comprises electrical contact elements supported in spaced relation to one another, and arranged to make electrical contact with one another after said predetermined deformation.

Preferably they are spaced in the direction of the length of a fastener with which the load indicating device is to be used.

The device may comprise an annular body through which the fastener may extend, and which may define an internal cavity wherein said electrical contact elements are disposed.

The body may include first and second radially inwardly extending annular flange portions whose central apertures are of a size to fit closely on the fastener with which the load indicating means is to be used, and between which flange portions the internal cavity is defined.

Preferably the contact elements are carried by the flange portions.

One of said contact elements may be insulated from the body and have electrical connection means associated with it whereby an electrical connection may be made with the contact element from the exterior of the body.

The body may carry one part of a releasable electrical connector, eg. a socket, by which a removable electrical

connection may be established with the contact element.

The other contact element may be supported by the body in electrical connection therewith, so that when the contact elements establish electrical contact with one another an electrical circuit may be established between the electrical connector on the body and"earth" as constituted by the body. In this arrangement, the body is, of course, required to be at least partially of an electrically conductive material which usually will be a metal, eg. steel.

At least one of the contact elements may be adjustably supported by the body, to enable adjustment of the amount of deformation of the body required to cause the change of state of contact of the elements.

At least one of the contact elements may incorporate yieldable means so that damage is avoided in case deformation of the body takes place beyond the point at which the contact elements engage one another.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a broken-away perspective view of a first embodiment of the present invention ; Figure 2 is a section through a further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring firstly to Figure 1 of the drawings, a load indicating device for a fastener such as a bolt

comprises a body 10 which overall is of annular configuration. The body may be of steel or other metal appropriate to the intended environment and use of the device. It comprises a cylindrical wall portion 11 and two spaced flange portions 12,13 which extend radially (having regard to an axis 14 which is the longitudinal axis of a fastener with which the load indicating device is to. be used) inwardly at opposite ends of the wall portion 11. The flange portions 12,13 which are somewhat frusto-conical in configuration, diverging axially from one another as they extend radially inwardly, have central apertures 15, 16 respectively whose diameter is sufficient for the fastener to pass therethrough. Adjacent the aperture 15 the flange portion 12 has a planar annular abutment surface 17 to be engaged by the head of a fastener such as a bolt, or a nut, or a washer, or a component which the fastener is securing, and the flange portion 13 has a similar abutment surface 18. Between the flange portions 12, 13, an annular internal cavity 19 is defined.

The flange portion 13 carries a contact element in the form of a conductive rod 20 extending axially of the device towards the opposite flange portion 12. The rod 20 is a press fit in an insulating sleeve 21 in an aperture 22 in the flange portion 13. The rod 20 is electrically connected by a wire 23 to a connection socket 24 supported in the wall portion 11 of the device. An electrical connection, insulated from the

remainder of the device, can thus be established with the rod 20 by a connector plug inserted in the socket 24.

The flange portion 12 of the device carries a further contact element in the form of a grub screw 25 received in a screw-threaded opening 26 in the flange portion 12. The fit between the threads of the grub screw 25 in aperture 26 is such that the grub screw will retain a set position in the aperture, and not be moved if it should be subject to vibration or the like. The grub screw 25 is hollow and carries at its end facing the end of the rod 20 a captive ball 27 biased towards the rod 20 by a spring 28.

When the device is provided on a fastener and the fastener tightened, the load thus applied to the device in the direction of the axis 14 deforms and compresses the device axially and moves the contact element afforded by grub screw 25 towards the contact element afforded by the rod 20. Assuming the grub screw 25 has been set at an appropriate position relative to the flange portion 12 of the device, after a certain amount of such deformation of the device the rod will contact the ball 27 at the tip of the grub screw. Thus an electrical connection between the rod 20 and"earth"as constituted by the body of the device will be established. If the fastener is tightened further, the ball 27 will retract into the interior of the grub screw 25 against the action of spring 28, thus avoiding

damage.

Figure 2 of the drawings shows in section a device whose principle of construction and operation is the same as that of the device in Figure 1, but whose physical configuration is different. Parts corresponding to those parts in Figure 1 are identified by the same reference numerals as used in relation to Figure 1 : in particular the rod 20, insulating sleeve 21, grub screw 25, ball 27, and spring 28 are clearly visible. In this embodiment, however, the flange portions 12,13 are of frusto-conical configuration in the interior of the device and generally planar, but with raised annular bosses as indicated at 17a, 18a, at the outer ends of the device.

A device such as one of the embodiments illustrated may be used, for example, on each fastener in an item of equipment, plant or machinery whose correct degree of tightening is important to be established. When the fastener is being tightened, an instrument such as a test meter may be connected between the electrical connection socket of the device and earth, such meter indicating when the resistance therebetween falls substantially to zero to indicate when the contact elements of the device have made contact with one another as the fastener is tightened. Thereafter the meter. may be removed. Alternatively, some form of electrical testing device or circuit could remain permanently connected to the device if it is required to

monitor on a permanent basis whether the fastener remains tight.

Although it is convenient in the embodiments illustrated above for one of the contact elements of the device to be earthed to the body thereof, it would be possible for both contact elements to be insulated from the body of the device and to be arranged to have electrical connection made to them, so that the establishment of an electrical circuit to indicate correct fastener tightening is not made through the body of the device. This would render it possible for several such indicating devices provided on a number of fasteners to be connected in an electrical circuit with one another, eg. in series with one another, to provide an indication if any one of such fasteners should not be tightened adequately or become loosened in service.

This might be required to be done, for example, on a number of bolts provided at a flanged joint in a pipeline.

It would be possible for a device according to the invention to be provided with more than one set of contact elements, which may be arranged to change their state of contact at different amounts of deformation of the body of the device. Thus, signals may be provided to represent different fastener loads, eg. maximum and minimum loads.

It would be within the broadest scope of the present invention for the device to incorporate, for

example, a variable resistance, inductance of capacitor whose value is varied in accordance with deformation of the body of the device. For example, in the case of an inductance, a coil may be supported by one flange portion of the device and a core by the other flange portion, relative movement therebetween changing the value of the inductance. This would enable actual fastener tightness to be measured on a continuous basis, rather than a satisfactory/unsatisfactory indication to be given as by the use of contact elements.