Load indicating

A hollow bolt (10) contains first and second electrical contact elements (13, 16) which are moved apart when the bolt is subjected to a predetermined tensile lead. A carrier (14) for a first of the contact elements contains an inductor (18) connected electrically between the contact elements, the condition of the contact elements is ascertained by inductive coupling of test apparatus with the inductor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the indication of a load to which a member 
is subjected. The invention has been devised primarily for use in 
fasteners or other members which are to be subjected to a predetermined 
load and the invention is useful during initial loading of the member and 
subsequently for checking that the required lead has been maintained. 
2. Description of the Related Art 
In GB 2212284 A., there is disclosed a hollow bolt containing first and 
second carriers, each carrying a respective electrically conductive 
element. When the bolt is subjected to a first load, these elements are in 
mutual contact. When the bolt is subjected to a second load, the 
conductive elements are out of mutual contact. This change in the 
condition of the device is detected electrically. To facilitate this, the 
first electrical contact extends through the first carrier to a face 
thereof exposed at an end of tile bolt. The second carrier provides an 
electrically conductive path between the second element and the bolt. To 
ascertain whether the elements are in mutual contact, electrically 
conductive probes are applied to an end of the bolt and an exposed end of 
the first element, these probes being connected in an electrical circuit 
which includes a source of electrical power and a lamp or other signalling 
device. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a more reliable indication of 
load than can be achieved by the known arrangement under certain 
conditions. 
According to the present invention, there is provided a device comprising a 
hollow member, an inductor and first and second electrically conductive 
elements which are anchored to or freed with respect to respective 
portions of the member, which portions undergo relative movement when the 
member is stressed, wherein, when the member is subjected to a first load, 
the elements are in mutual electrically conductive contact at a position 
inside the hollow member and, when the member is subjected to a second 
load, the elements are out of mutual electrically conductive contact and 
wherein the inductor is connected electrically between said elements so 
that a closed circuit including the inductor exists when the elements are 
in mutual contact. 
The presence or absence of the closed circuit can be checked, using 
inductive coupling between testing apparatus and the inductor of the 
device embodying the invention. It is therefore unnecessary to establish 
an electrically conductive relation between test apparatus and the device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The particular fastener illustrated in the accompanying drawing is a bolt 
10 having a male screw thread 11 and a head 12. The bolt is hollow. In the 
particular example illustrated, a blind bore extends into the bolt from an 
end face thereof presented by fie head 12. The bore is preferably coaxial 
with the thread 11 and extends substantially beyond the head 12. 
There is disposed inside the bolt a first electrically conductive element 
13 carried by a carrier 14. The carrier also is disposed at least partly 
inside the bolt. However, at least one face 15 of the carrier is exposed 
at the outside of the bolt. The carrier 14 is fixed with respect to the 
bolt 10. In the example illustrated, the carrier and fie bolt have 
respective cooperating screw threads and fie carrier is restrained against 
turning relative to the bolt, for example by means of an adhesive on the 
threads. Also in the example illustrated, the electrically conductive 
element 13 is fled with respect to the carrier 14. 
A second electrically conductive element 16 is also disposed inside the 
bolt. This element occupies an inner portion of the bore formed in fie 
bolt and is urged by a spring 17 onto a seat at the blind end of the bore. 
The spring 17 acts between the element 16 and the carrier 14. The element 
16 is resiliently compressible and may comprise a plastics composition 
loaded with an electrically conductive material. The bolt 10 is formed of 
an electrically conductive material and the element 16 is in electrically 
conductive relation with the bolt. 
The carrier 14 is formed of an electrically insulating material, 
conveniently a plastics material There is encapsulated in the carrier an 
inductor 18. The inductor is connected electrically between the element 13 
and the element 16. In the example illustrated, the inductor is a coil 
having one end connected by a lead 19 with the element 13 and opposite end 
of the coil is connected by a lead 20 with a metal sleeve 2 on the outside 
of the carrier 14. A part of the screw thread of the carder may be formed 
on this sleeve. The sleeve is in electrical contact with the bolt 10, 
which provides an electrically conductive path to the element 16. 
The device is prepared for use by placing the element 16 in the bore of the 
bolt, then placing the spring 17 in the bore and then screwing the carrier 
14 into the bore until the element 13 just contacts the element 16. This 
condition may be detected electrically. The carrier is then screwed a 
further, predetermined distance into the bore. This further movement of 
the carrier and element 13 is accommodated by compression of the element 
16. The carrier is then fixed in the position to which it has been set. 
This may be achieved by curing of an adhesive on the internal thread of 
the bolt. Other ways of fixing the carrier 14 wig respect to the bolt may 
be employed. For example, the carrier may be a force-fit inside the bolt. 
When the bolt is in use and is subjected to tension, the shank of the bolt 
stretches and the carrier 14 moves away from the seat at the blind end of 
the bore. When the bolt has been subjected to the required load, the 
element 14 moves out of electrical contact with the element 16. This is 
detected electrically. 
For detecting whether the elements 13 and 16 are in electrical contact with 
each other, there is used a test apparatus (not shown) which is capable of 
inductive coupling with the inductor 18 of the circuit in the bolt. The 
test apparatus may be arranged to provide an indication of whether there 
is a closed circuit containing the inductor 18 in the bolt. Additionally, 
the test apparatus may be arranged to provide an indication which vanes in 
a manner dependent upon the capacitance between the elements 13 and 16. It 
will be appreciated that this capacitance will change as the separation 
between these elements changes. 
The bolt 10 may be used in place of bolts disclosed in GB 2212284A. 
Furthermore, the device illustrated in the accompanying drawing may be 
modified by the incorporation of features disclosed in fie aforesaid 
specification. For example, there may be substituted for the element 16 a 
second carrier carrying an electrically conductive element and containing 
a spring which acts between the second carrier and the second conductive 
element. 
The inductor 18 lies close to the face 15 of the carrier 14 so that an 
inductor of test apparatus can be brought into close proximity with the 
inductor 18. 
The carrier 14 preferably closes the bore of the bolt to prevent ingress of 
moisture and of air. 
The features disclosed in the foregoing description, or the following 
claims, or the accompanying drawings, expressed in their specific forms or 
in terms of a means for performing the disclosed function, or a method or 
process for attaining the disclosed result, as appropriate, may, 
separately or in any combination of such features, be utilised for 
realising the invention in diverse forms thereof.