Abstract: A load indicating fastener composed of a body (2) having an abutment (9) which is moved by the changing length of the body (2) under an applied load, and a load sensing member (10) composed of an element (11) disposed in the direction in which a load is applied to the body (2) when the fastener is in the user characterized in that the element (11) is of elongated U-shape, a first limb (13) of the U providing an anchorage part (14) which is rigidly anchored to the body (2), the closed end of the U providing a bearing part (15) which bears on the abutment (9) so as to move with the abutment relative to the anchorage part (14) in response to a load applied to the body, and a second limb (16) of the U having an indicator (17) at its outer end which responds movement of the bearing part (15) and registers the applied load related to that movement on a scale (19).

Abstract: An annular fluid type pusher for tightening a part on a shaft comprises, a body mounted on the shaft, a pushing piece for pushing the part with a fluid pressure, a sealed fluid path formed annular in the body for sealingly containing a fluid and slidably holding said pushing piece in part, a fluid pressure adjusting section for adjusting the fluid pressure, an indicator cylinder chamber communicated with the sealed fluid path, an indicator piston slidably provided in the cylinder chamber, a spring to urge the piston against the fluid pressure, and a thrust indicating bar. Thrust graduation lines have been marked on the thrust indicating bar. And one end portion of the thrust indicating bar is connected to the piston, and the other end portion is provided in such a manner as to be protruded in association with the movement of the piston which is slidably moved in proportion of the fluid pressure.

Abstract: A smart fastener having corrosion detection features. Within the fastener is a corrosion detecting array that receives an electrolyte that corrodes sacrificial materials formed on electrodes of the corrosion detecting array. The materials are the same as those materials which are secured, fastened, or attached by the fastener. There is parallel corrosion of the sacrificial materials and the attached materials since the electrolyte which is channeled via capillary tubes into the corrosion detecting array is the same electrolyte that is in the environment of the fastened materials. Other electrodes sense various properties of the electrolyte. Information sensed by the corrosion sensing electrodes of the array is in the form of very small electrical signals which are amplified and stored in an analog storage memory. A clock and addressing device time stamps and address the signals.

Abstract: A device is disclosed for adjusting and aligning an axle of a wheeled vehicle wherein a pair of such devices are disposed on either side of a vehicle such as a motorcycle. The devices then provide a scale on both sides of the machine which are visible through sights. A person attempting to adjust and align the axle of the machine must then simply match the scales on either side of the machine to ensure that the axle is aligned.

Abstract: A method and apparatus for graphically indicating the dimensions of fasteners on the fastener itself. According to a preferred embodiment, graphical indica are disposed on the head of a nail to visually indicate the length and/or gauge of the nail. Such indicia may include numerals and other shapes or objects. Preferably, one or more hatch marks are disposed about the face of the nail in the form of a clock face and representing such dimensions.

Abstract: A bolt includes a movable pin in the center thereof. This pin can freely move up and down in the bolt. If the pin protrudes out of bolt a predetermined distance after the bolt is screwed into a blind bolt hole, then an inspector knows that the bolt is fully engaged. If, on the other hand, the bolt is not fully engaged, the pin will not protrude the predetermined distance out of the bolt. A second embodiment of the similar invention is a stud having an indicator pin. In third and fourth embodiments, the bolts and studs have indicator pins including grease zerks. These grease zerks allow the holes to be filled with grease to prevent damage to the hole due to corrosion.

Abstract: A force sensor for measuring tightening tension applied to an assembly component having first and second faces and an axis of symmetry includes a ceramic support layer having a mounting side to be secured to the first face and an exposed side carrying a piezoresistive layer having a resistance that varies in response to its deformation. The force sensor may comprise a central metallic layer and an annular metallic layer joined by the piezoresistive layer or at least two separate sensors having angularly disposed longitudinal axes. A machine for tightening the assembly component includes a motor driven socket for torquing the component in accordance with a sensed tightening tension.

Abstract: A compression system for joining structural components under tension utilizing a bolt with a shank, an end and a head, one or more flat washers and one or more compression springs. One of the structural components, the bolt end and the nut carry indicia means that form a linear relationship with each other when the system is in an initial tensioned and starting condition and form a selectably angular relationship when the nut is tightened on the bolt a predetermined amount. The system enables the consistent uniform tightening of the system elements to combine structural components and provide consistent spring loading throughout.

Abstract: A rock bolt load sensor assembly for use to determine the load on a nut screwed onto a bolt embedded in a rock face, the assembly comprising a plurality of spring washers arranged to be mounted on the bolt between the nut and a roof plate, and a sensor comprising an induction coil adapted to be, in use, placed around the washers, the sensor including an indicator coupled to the induction coil whereby in use, the spring washers define an air space which varies in dependence on the load on the bolt and the induction coil provides an electrical indication of variation of the air space and the indicator provides an indication of variation of the load on the bolt.

Abstract: The present invention apparatus is a novel and unique combination of a damping member adapted to an indexed scale fastener for use with an archery bow. The indexed scale fastener and the damping member can be used together or separately. The damping member can be used at locations on the archery bow which are remote from the indexed scale fastener. The concept is to have the indexed scale fastener adapted to the conventional threaded openings where the prior art hex screws are located. A user can screw the indexed scale fastener to the archery bow and measure the poundage applied to the archery bow by simply reading the scale provided on the indexed scale fastener. The damping member can be threadedly mounted to the indexed scale fastener or to the archery bow, where the damping member absorbs the vibrations and shock in both the longitudinal and lateral directions caused by releasing of the arrow from the archery bow.

Abstract: A pre-load indicating washer in which a plurality of protuberances, struck and partially sheared from the annular body of the washer, are pushed under load back into a corresponding plurality of indentations in the annular body which are formed when the protuberances are formed. The mass of material defined by each protuberance/indentation pair is concentrated along the lengthwise central region of the protuberance/indentation pair, thereby stiffening the protuberance, so that when load is applied to the washer, the protuberance will first undergo elastic deformation prior to collapsing back into the indentation from which it was formed.

Abstract: The present invention relates to a fastening system comprising a nut with a locking threadform and a nut driving system which visually marks the locking nut when the proper torque is reached. The locking nut with a locking threadform has been designed so that the major diameter thread of a typical bolt will engage a wedge ramp portion of the nut threadform thereby resisting unintentional loosening. The nut driving system of the present invention comprises a wrench socket wherein the socket contains a ball set; when the socket is used to tighten the locking nut, it visually marks the nut head when the proper torque is reached.

Abstract: A load indicating fastener such as a bolt (10) or stud, has a shank (12) which is subject to tensile loading in use. The shank (12) has an axially extending bore (22) which receives a first gauge pin (30) anchored at its inner end (32) to the shank (12) and protruding outwards beyond an end face (16) of the fastener. An indicating member (40) is rotatable on the protruding portion of the gauge pin (30) between the end face (16) of the fastener and the head (34) of the pin (30), so that the indicating member (40) is free to rotate when the shank (12) of the fastener is unstressed but when the shank (12) is subject to a predetermined tensile loading is held against rotation by contact with the end face (16) of the fastener. A second gauge pin (42) is received within an axially extending bore (36) in the first gauge pin (30) and is anchored at is inner end and protrudes outwards beyond the outer end of the first gauge pin (30).

Abstract: A load-indicating fastener, such as a bolt (10), has a bore (18) which extends from an end face (16) of the bolt and which can receive a gauge pin (30). The gauge pin (30) is free to rotate so that it can be withdrawn completely from the bolt. In use of the bolt, the gauge pin (30) is reinserted into the bore (18) and screwed down until the end of the pin engages the inner end of the bore. As the bolt is tightened, causing elongation of the shank (12) of the bolt (10) and therefore of the bore (18), the user can check the loading on the bolt by periodically rotating the gauge pin (30), to maintain contact between the end of the pin and the inner end of the bore, until the pointer (40) reaches the mark (28) on the end face (16) of the bolt, indicating that the predetermined loading has been reached.

Abstract: An anti-corrosion plug for closing a threaded condensate drain opening in a compressor air tank. The plug is formed from a material having a lower potential in the electrochemical series than said air tank to prevent electrolytic corrosion of the tank. The plug has a central cavity open to receive any condensate in the tank. An acid neutralizing insert, for example, of limestone, is located in the cavity to neutralize any acidity in the tank condensate.

Abstract: A bolt tension indicator having an annular washer part of concave cross section for fitment beneath the head of a bolt or beneath a nut, the indicator being made of spring steel and having at least one projecting tab element continuous with the annular washer part so that the lying flat of the tab element against a clamping face indicates a tension in the bolt.

Abstract: A fastener which allows a user to easily utilize the Torque Angle Method to provide a predictable clamp load to a clamped element. The fastener includes an externally threaded shank portion, a torque receiving portion integrally connected on one end of the shank portion and a radially extending flange located between the shank portion and the torque receiving portion. The flange includes a pair of cut-out notches located on the peripheral edge of the flange which are formed prior to initial tightening of the fastener and are spaced apart at a predetermined angular distance to allow a user to utilize the Torque Angle method to provide predictable clamp load across a joint connection.

Abstract: A piston rod, having a threaded end, is threadedly received in a split collar which is (a) internally threaded and (b) externally tapered. The collar is received in a tapered bore formed in a cross-head. The crosshead has a plurality of threaded holes formed therein to receive studs which penetrate a pressure plate. Nuts secured to ends of the studs draw the plate against the collar to cause its threads to fully engage the threads of the rod end, and secure the collar in place. The rod, of course, can be turned in the collar, threadedly to adjust its axial positioning in the collar.

Abstract: A device for setting an anchor rod (3) in a borehole includes a housing (4) and a socket (5). The socket (5) is made up of a sleeve (6) and a liner (7). The sleeve (6) is rotatable relative to the housing (4) and the liner (7). The liner (7) is in threaded engagement with the anchor rod (3) by a standard thread and with the sleeve (6) by a fine pitch thread (6c). Due to the fine pitch thread engagement of the sleeve (6) and the liner (7), by rotating the sleeve (6), which remains axially stationary, the liner (7) and the anchor rod (3) are moved axially with the anchor and also moving axially relative to an expansion sleeve (2) so that the expansion sleeve (2) is expanded into an undercut in the borehole.

Abstract: A fluid cooled thermal skin is bolted to a sufficient back structure. In-line coolant flow paths are arranged to pass the cooling fluid through the bolt head. The in-line paths are recessed adjacent the bolt head to mate with flow openings in the head. In one embodiment the paths through the bolt head are parallel paths, requiring alignment of the bolt head. In another embodiment alignment of the bolt head is not required.

Abstract: A warning device for indicating undesired rotation of a threaded nut on a non-rotatable threaded shaft includes a tight fitting shell for covering the nut and a manually rotatable highly visible indicator on the end of the shell to be adjusted to a predetermined initial alignment. Any torque change caused by nut rotation will therefore rotate the indicator from its predetermined alignment.

Abstract: A composite nut is shown which permits a fastener to be inserted or removed from either side with an indicator of fastener engagement. The nut has a plurality of segments, preferably at least three segments, which are internally threaded, spring-loaded apart by an internal spring, and has detents on opposite sides which force the nut segments into operative engagement with a threaded member when pushed in and release the segments for quick insertion or removal of the nut when moved out. When the nut is installed, end pressure on one of the detents presses the nut segments into operative engagement with a threaded member where continued rotation locks the structure together with the detents depressed to indicate positive locking engagement of the nut. On removal, counterclockwise rotation of the nut relieves the endwise pressure on the detents permitting internal springs to force the detents outward and allowing the nut segments to move outward and separate to permit quick removal of the fastener.

Abstract: As used with a nut, a device for indicating tensile strength comprises an annular groove approximately 1/8 inch in depth on the bottom of the nut adjacent to a washer. The same groove may be fitted in a bolt if the device is used with a bolt. A vertical shaft of the same diameter as the groove is also located within the nut or bolt engaging member and connects at one end to the groove. The groove and vertical shaft contain a "filler" material. A washer having a boss or protrusion is utilized with the engaging member wherein the boss or protrusion fits into the said groove. Once the washer encounters friction, when the combination nut and washer is tightened about a threaded bolt, with the connecting elements, the protrusion inside the groove pushes the filler up the vertical shaft as the nut rotates over the washer.

Abstract: A bolt for attaching a first member to a second with a desired compressive force. The bolt has a cylindrical body with a threaded section on a first end and a head on a second end. The cylindrical body had a longitudinal bore which extends to a cross bore in the head. A pin located in the cross bore has a first flat surface separated from as second flat surface by a shoulder. A rod located in the longitudinal bore has a first end fixed to the cylindrical body and a second end that is located on the first flat surface. A spring acts on the pin to hold the shoulder against the rod. An input torque applied to the head causes the threads to move into threads on the second body. When the head engages the first body, further torque results in expansion or elongation of the cylindrical body. The rod freely moves in the longitudinal bore and when a predetermined elongation occurs corresponding to the depth of the shoulder on the pin, the spring moves the pin to align the second flat surface over the rod.

Abstract: A composite fastener which provides up to 70 KSI in shear strength in a 1/4 inch shank diameter and includes several layers having long parallel carbon fibers embedded in resin. The layers are pressed together and heated to form a panel. The panel is cut into bars which are then machined into rods, used to form threaded or unthreaded composite shear pins. The fibers in adjacent layers are angled with respect to one another to provide maximum shear strength in a particular direction. The head of the shear pin is preferably marked to indicate the orientation of maximum shear strength.

Abstract: A failure indicating device wherein a fluid is encased within a bore in a fastener. In another embodiment of the invention, the fluid is encased within a thin walled capsule which in turn is encapsulated within a bore in the fastener. Upon failure of the fastener, the encased fluid will leak out into the external surface and its detection will indicate that the fastener has failed. The stress which causes the fastener to fail will also assist the fluid to leak out onto its external surface. To facilitate the leakage of the fluid, one or more shallow and narrow groove(s) may be machined onto the external surface of the fastener so that the leaked fluid will travel along this groove(s) to the preferred external end of the fastener. This is desirable in cases where only one side of the joint is accessible.

Abstract: A indicating-nut threaded coupling with load-indicating device (1) adapted to be arranged in particular on grouted anchors for mining or the like and in order to obtain a simple solution which is advantageous in use a free space is formed by at least one groove (4) equipped with a fold bottom (5), the groove being arranged in the outer wall (3) of a bushing (2) which extends spaced from the outer surface of the anchor.

Abstract: A self locking panel fastener comprising a receptacle connected to one of the two panels being secured together, the receptacle having an internally threaded nut portion, an externally threaded stud having a head portion extendible through the apertures of the panels, a shaped end portion and a bore formed therein; a first locking member within said receptacle having a shaped aperture and sized to receive the shaped end portion of the stud to drivingly connect the first locking member and the stud; a second locking member mounted within the receptacle for generally axial movement relative to the nut portion; a spring member for forcing the second locking member into engagement with the first locking member to form a locking connection between the stud and the second locking member; and a member for driving the second locking member generally axially relative to the first locking member against the force of the spring to break the locking connection and to allow counterrotation of the stud relative to the nut

Abstract: Apparatus for indicating incomplete and complete fastening of a screw includes a washer and a ring of a generally flat synthetic resin sheet. The washer includes a pair of washer portions of a generally flat, round shape and a joint portion interconnecting the washer portions to face them in opposite directions to enable the washer portions to receive a shank of a screw in the openings thereof. The lengthwise section of the joint portion forms generally a U shape. The ring is made of a generally round, deformable material and is inserted between the pair of washer portions thus facing each other. The deformable material is expanded radially outward, when compressed by the washer portions receiving the shank of the screw when fastened. Upon the compressive force reaching a predetermined value, the ring will be broken due to the pair of washer portions overlaying one another.

Abstract: For anchoring heavy machinery to foundations, previously tensioned stress-level indicating bolt connectors are provided. In a top axial cavity of the connector, threads are provided to mount an angularly adjustable stress-level indicating pin. In use, when the design tension stress level is reached and the connector thereby extended, angular freedom or restraint of the gaging pin indicates that the pre-set stress has been reached or exceeded. Indicia angularly arranged on the connector end facilitate adjustment of the pin to provide upper or lower stress levels.

Abstract: An apparatus for testing the integrity of a fastener under load. The apparatus includes an actuating assembly and a tensile loading structure mounted in serial relationship. The actuating assembly includes a piston and cylinder assembly. The cylinder heads are supported by a plurality of rods which are secured to a plurality of compressions rods of the tensile loading structure. The piston rod extends through an upper cylinder head and is connected to a movable shackle. A second shackle is secured to a cap or top member of the tensile loading structure. A bolt to be tested is secured in removable inserts carried in the shackles. A gage reads the amount of force, in pounds, applied to the bolt. An enclosure is provided for enclosing the tensile loading structure for protecting people and materials in the testing area, and wheels are disposed on a cabinet which supports the actuating mechanism and the tensile loading structure to provide portability to the apparatus.

Abstract: A connector element is described which is suitable for connecting together two components (2a, 2b) in a system which is likely to be subject to vibration. The connector element has an indicating mechanism which provides an easily visible effect to show whether or not the two components are mated. In one embodiment the edges (10a) of a spring (10) protrude from a recessed portion (18a) in a nut (18) at the end of the bolt to indicate the unmated condition. In another form the end of an indicating lever (110) protrudes from a slot (112) in an adjusting nut (180).

Abstract: Torque verification apparatus and methods for verifying that an amount of torque applied to the nut of a fastener of the type comprising a threaded nut and a bolt having a threaded body portion and a head portion is equal to or greater than a predetermined torque value are provided. The apparatus and methods are particularly useful for verifying that the amount of torque applied to the lug nuts used to mount a wheel to a wheel hub of a vehicle is equal to or greater than a predetermined torque value.

Abstract: A fastener and tightening device for exerting force and equalization of force upon the nut or head of a bolt or threaded stud. The device is comprised of a shank fitting through a support member within an abutment head adapted to be drawn up tight against the support member. A plurality of jack bolts are threadedly engageable with holes in the head and are adapted to be tightened to exert pressure against the support member. Separate ball members are positioned between the jack bolts and a wear plate on the support member to minimize friction and provide for force equalization to counteract any off center alignment.

Abstract: Disclosed is a fastener assembly fixable to a structural member, the assembly having means to indicate the torque and clamping force of a bolt which forms part of the fastener assembly.

Abstract: Attainment of a predetermined stress in a bolt (10) which is being tightened is indicated by interference with the freedom of a pin (15) to rotate relative to the bolt. The pin extends along a bore (14) from an end of the bolt and the bore lies in that part of the bolt which presents the minimum diameter of the bolt. The invention is especially useful for detecting when the stress to which the bolt is subjected exceeds the elastic limit of the bolt, because the radial contraction of the bore which then occurs is large, relative to the contraction which occurs during elastic deformation of the bolt.

Abstract: Apparatus for affixing a coin drop mechanism to the front panel of a vending machine in situations where internal tactile and visual access to the rear surface of the machine panel is limited. An external plate assembly includes guide rods extending from threaded studs which are used to align an internal plate assembly supporting the end drive mechanism. Internally threaded tubular locking fixtures applied over the guide rods to engage the studs for clamping the first and second plate assemblies around the panel. When proper assembly has been achieved the tip to the guide rods extends to the locking members to provide a tactical indication that proper installation has been achieved.

Abstract: In an indicator for visually displaying extremely small displacements between two points within a structure, a change of color visible through an indicator window occurs during a preselected portion of a predetermined displacement within the structure. The color change occurs due to an increase in thickness of a light-absorbing fluid in the central area of a clear, flexible indicator means separating a colored portion from the indicator window. Substantially no light-absorbing fluid is permitted to enter the central area until a preselected displacement has occurred since the colored portion is firmly, resiliently biased against the indicator window until that amount of displacement has occurred. Thereafter, light-absorbing fluid is permitted to enter the central area. This device concentrates visual indications of displacements into limited segments of an overall displacement.

Abstract: A scaled releasable fastener for temporarily joining a plurality of sheets through apertures located in the sheets has a body with a work engaging surface and an opposite surface. The body includes a longitudinal bore extending through the body from the work engaging surface to the opposite surface. A pair of laterally expandable work engaging pins are positioned in the bore. Each of the pins has a work engaging shoulder. A spreader member is fixedly carried by the body and is positioned between the work engaging pins for laterally spreading the pins apart upon retraction of the pins into the bore. An elongated element is positioned in the bore in association with the pins for moving the pins in the bore. A portion of the elongated element extends out of the body from the opposite surface of the body with the remaining portion of the elongated element located in the bore and connected to the pins. Indicia marks are located on the elongated element to be readable against the opposite surface of the body.

Abstract: Deformable washer, particularly for predetermining the force needed for tightening a nut mounted on a screw or a screw cooperating with a tapped hole. The washer is provided on one of its faces, preferably the face which is meant to rest against one of the parts to be tightened, with a circular spot face, concentric to the central hole, the external diameter of which spot face is notably greater than the diameter of the central hole, while its depth is preferably near to the width of the spot faced ring. The washer further comprises, on its edge, a visible groove of which the cylindrical base is also concentric to the central hole and has a diameter at the most equal to the external diameter of the circular spot face and preferably just under said external diameter, the thickness of said groove being also as near as possible to the depth of the spot face.

Abstract: A coupling for making electrical connection of an avionics box and for retaining it in a rack has a rotatable jackscrew, by which cooperating parts of an electrical connector on the box and the rack can be drawn together. The jackscrew has a knob at one end with an indicator button which is pulled into the knob by a mechanical linkage when the two parts of the connector are fully mated. The linkage includes a sleeve which embraces the jackscrew and which is pushed towards the knob against a spring when the connector is mated. A ratchet mechansim limits the torque applied by the jackscrew. Ball catches engage asymmetric grooves on the sleeve to make it more difficult to unfasten the jackscrew than to fasten it.

Abstract: The element is constituted by an elongated tube whose height is equal to or exceeds 0.8 times its diameter, and which has a bearing surface on one face of the members to be assembled and a bearing surface on one face of a nut, the two bearing surfaces being interrupted by a slot completely passing through the tube in a generally longitudinal directional section, a displacement being provided on each of the bearing surfaces and between the zones of the bearing surfaces respectively located on either side of the slot, the two displacements being in opposite directions in the longitudinal direction of the element.

Abstract: A stud/bolt load gauge assembly. A gauge screw threadably engaged in a stud or bolt has a top, belleville, overload, and bottom washer mounted between the head of the bolt and the head of the gauge screw. All of the washers are movable until axial stress on the bolt equal to the minimum desired load causes the underload gap to be taken up by the gauge screw. Then only the overload washer is movable. The overload washer becomes immovable between the top and bottom washers when the bolt load is increased beyond the desired maximum load, causing compression of the belleville spring washer to a substantially flat state.

Abstract: A new fitting indicating mechanism in screw type connector housings is provided. Said mechanism comprises a male connector housing having an engaging portion; a female connector housing having cavity means for receiving said engaging portion therewithin; a tubular body having a bore contiguous to said cavity means and a plurality of flexible support arms extending into said bore toward a direction opposite to said female connection housing; and an indicator rod held to extend within said cavity means and said bore, said indicator rod being biased toward the male connector housing such that said flexible support arms covers the indicator rod when fitting engagement of both housing connectors is not completed but pushes open said flexible support arms upon completion thereof such that said indicator rod is exposed out the flexible arms.

Abstract: A cable fixing device which is constructed by superposing between an outer housing (1) and an inner housing (7) both of which can be fitted with each other by screwing, a hanging bell-shaped packing (14) having an inner hole, a guiding ring (19) having a guiding hole, and a tightening ring (20) having a plurality of flexible fins (21) reclining in the inward direction, and which is adapted for clamping an inserted cable with the flexible fins (21) of the tightening ring (20) by resiliently contacting the hanging bell-shaped packing (14) with the outer periphery of said cable and screwing the inner housing (7) into the outer housing (1). By the water protecting effect of the hanging bell-shaped packing (14) and the fixing effect of the tightening ring, it can prevent the penetration of water from the outside and simultaneously can securely fix cables of various diameters without changing the parts.

Abstract: A load indicating flange is provided either separately or on a nut or a bolt head. This load indicating flange has an axially directed face which has a wavy surface on which a plurality of crests are disposed for initial engagement with a work surface. A plurality of troughs are provided on the wavy surface one each between two of the crests. A gauge groove is provided at each trough and when the bolt and nut combination is properly tightened on a workpiece, the troughs are flattened toward the work surface to such an extent that a relatively thick gauge cannot enter the gauge groove. If the bolt and nut are improperly tightened, then the gauge will enter the gauge groove. A narrow band peripheral bearing surface is provided on the outer periphery of the face to prevent defacing the work surface.

Abstract: A bolt for transmitting axial load having a head provided with a weakening slot for promoting elastic deformation of the head in response to the axial load magnitude. On the head there is a pair of measure points located at each side of the slot in or adjacent the free end surface of the head at a distance between them that decreases at increasing axial load on the bolt. The bolt head also comprises a pair of reference points located either in the same axial plane as the measure points but in a direction perpendicular to the latters or in a plane axially spaced from that of the measure points adjacent the bottom of the weakening slot. The location of the reference points is such that the relative distance between them remains unaffected by the deformation of the bolt head and thereby the axial load transmitted by the bolt.