WO2000019179A1

WO2000019179A1 - Strain gauge and strain-indicating devices

Info

Publication number: WO2000019179A1
Application number: PCT/US1999/022155
Authority: WO
Inventors: Malcolm H. Hodge
Original assignee: Structural Integrity Monitoring Systems, Inc.
Priority date: 1998-09-25
Filing date: 1999-09-24
Publication date: 2000-04-06
Also published as: AU6262199A

Abstract

A fastener strain sensing device (218) and system for providing discrete or continuous sensing of fastener tension in situ suitable for use, for example, with bolt-type fasteners having a bolt head (212) with an exposed surface (216) suitable for receiving the fastener tension sensing device (218). The fastener tension sensing device (18) comprises a pair of rigid opposed sensing arms (222a, 222b) and a pointer arm (232). The rigid opposed sensing arms (222a, 222b) reversibly move toward or away from one another in response to the natural distortion occurring on the bolt head (212) upon application or release of tension. Movement of the rigid opposed sensing arms (222a, 222b) causes corresponding reversible movement of the pointer arm (230). A method for sensing and indicating fastener tension using the fastener tension sensing device is also provided.

Description

STRAIN GAUGE AND STRAIN-INDICATING DEVICES

CROSS REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. provisional application Serial Number 60/101,889, filed September 25, 1998.

BACKGROUND OF THE INVENTION

Field of the Invention The present invention relates generally to a strain-indicating device which responds to small dimensional changes in a structure to which the device is fastened, to provide an indication of the extent of such dimensional changes. More specifically, the present invention is concerned with strain-indicating devices which employ leverlike mechanical advantage to magnify the movement of a pointer component of the device so that very small changes in dimension of the structure may be rendered more readily discernible both to the eye and to any of a variety of sensing devices, such as potentiometers, strain gauges, or optical and electronic devices generally. The structures to which the strain-indicating devices of the present invention may be attached are any structures which are subject to dimensional changes, whether caused by ther- mal expansion and contraction, imposed stress or load, mechanical impact or the like. Such structures include structural components of bridges, buildings and the like, the rails of railroad tracks, furnace, boiler or engine components, vehicle components, aircraft components and tension fastener devices, such as nut and bolt fasteners, rivets and the like. In the case of tension fastener devices, the strain-indicating devices of the present invention provide an indication of the tension to which such fastener is subjected and, therefore, to the "tightness" of a nut and bolt fastener. The rails of railroad tracks, the components of bridges and other structures, such as beams, girders and cables, are subjected to thermal expansion and contraction with changes in ambient temperature. The size of these structures is such that the resulting dimensional changes become large enough to affect connections between structural members and the ability to retain essential dimensions and spacings. The same applies to boiler, furnace and engine components, which are subject to wide temperature swings and consequent thermal expansion. In the case of structures subject to being heated, such as the components of a boiler, furnace or engine, the degree of strain resulting from thermal expansion and contraction which is indicated by the devices of the invention also provides an indication of the temperature to which the part is being subjected. In one embodiment, the present invention relates to a strain-indicating device which exploits the natural elastic "dishing" distortion of a member such as a bolt or rivet head, nut or washer, which is sustained when the shaft of the device is placed under tension, such as when a bolt head or nut is tightened on, or a rivet is placed through, a sub- strate. For purposes of explanation, bolt and nut fasteners are discussed below and illustrated, but it will be appreciated that this aspect of the invention applies to tension fasteners generally, such as rivets and the like, as well as to nut and bolt fasteners, and other aspects of the invention apply to any structural members, nails, parts or members for which it is desired to measure even very small amounts of dimensional change. Dimensional change measurable by the devices of the present invention can be as small as about two microns, e.g., about 2 to 20 microns. Obviously, much larger dimensional changes can also be measured by the devices of the invention by appropriately sizing them.

Nut and bolt fasteners and the like ("bolt-type fasteners") are common compo- nents of fastening systems used for securing two or more substrates together, for securing tension cables to an anchor or base member, or for fastening any of a wide variety of substrates or members that must be secured reliably together under a predetermined load. Because of load stresses, vibration, temperature changes and other causes, the desired tension imposed by such a fastener may change, e.g., the fastener may loosen. On the other hand, a bolt-type fastener may be over-torqued when applied or maintained, resulting in imposing a tension higher than that desired.

Related Art

It is known to carry out tension monitoring of, for example, bolt-type fasteners by utilizing sophisticated monitoring means such as bolt load analyzers using computers, or by employing workers who periodically "torque" a nut, sometimes with a tor- quing wrench, to insure that it is exerting the proper clamp load. It is also known to fasten a tension-indicating device to a bolt-type fastener.

For example, commonly assigned U.S. Patent 5,722,807, issued March 3, 1998, entitled "CLAMP LOAD INDICATOR", Serial Number 08/729,203, filed Oc- tober 15, 1996, discloses a mechanical indicator comprising a washer having at least one arm extending therefrom to monitor the clamp load of a fastener by exploiting the washer's natural tendency to dish or cup when subjected to pressure to monitor the clamp load of a clamped substrate. The washer portion is interposed between the fastener and the structure to which the fastener is attached and functions by measuring the dishing of the washer by the pressure imposed upon it by the fastener. Commonly assigned parent application entitled "FASTENER TENSION INDICATOR", Serial No. 08/771,087, filed December 20, 1996, discloses a structure mounted on the ten- sioning device, such as a bolt head, which utilizes differential substrate distortion between a portion of the substrate which is distorted by being clamped by the fastener and a less distorted or undistorted portion of the substrate adjacent to the distorted portion of the substrate in order to indicate the fastener tension.

SUMMARY OF THE INVENTION The present invention provides a strain gauge for indicating strain in a sub- strate, the gauge comprising (a) a sensing member having a mounting portion and a moving portion and (b) a pointer arm having at least one free end and a fulcrum portion. The gauge is configured and positioned so that the free end of the pointer arm moves with magnifying effect in response to relative movement between the moving portion of the sensing member and the fulcrum portion of the pointer arm. According to one aspect of the invention, the strain gauge may comprise two pointer arms, each having a free end and a fulcrum portion, and the strain gauge being configured to magnify at the free ends of both pointer arms movement of the moving portion of the sensing member relative to the fulcrum portions of the pointer arms. According to another aspect of the invention, the strain gauge may comprise two sensing members, each sensing member comprising a moving end connected to the pointer arm on opposite sides thereof and at different points along the length of the pointer arm and wherein each sensing member comprises a mounting end for attachment to a stressed member. Optionally, the pointer arm may have two free ends and a fulcrum portion therebetween. Also optionally, each sensing member may comprise a sensing arm and a mounting portion, and the mounting portion of each sensing mem- ber may comprise a peripheral member to which the moving portions of the sensing arms are connected. There may be indicia markings on the peripheral member near at least one free end of the pointer arm.

According to yet another aspect of the invention, the sensing member may comprise a sensing arm comprising the mounting end, and the sensing member mounting portion may comprise two sensing webs, each sensing web having two ends and being secured at one end to the sensing arm and having an anchor end for connection to a member, wherein the moving end of the sensing arm is connected to a pointer arm having a free end and a fulcrum portion, the strain gauge being configured so that relative motion between the anchor ends of the webs causes the free end of the pointer arm to move with magnifying effect.

This invention also provides a strain gauge comprising a torsion pivot bar, two mounting feet extending from the pivot bar for connection to the substrate and a connector member extending from the pivot bar for connection to the substrate.

This invention further provides a strain gauge comprising two sensing mem- bers, each comprising a sensing arm comprising a mounting portion and a moving portion, wherein each mounting portion comprises two webs each having two ends, the webs being secured at one end to the sensing arm and comprising a free anchor end. The gauge further comprises two pointer arms each having a free end and a fulcrum portion, the fulcrum portions being joined together and being connected to the moving portion of a sensing arm. Optionally, the pointer arms may be configured to move relative to each other with mutual magnifying effect.

A particular embodiment of this invention provides a fastener comprising a shaft, a fastener head on the shaft, and a strain gauge as described herein mounted on the fastener head for indicating strain in the fastener head. Optionally, the strain gauge may comprise at least one sensing member having a stationary portion securely mounted to the fastener head and having a movable por- tion that can move relative to the fastener head to indicate strain in the fastener, and further comprising at least one pointer arm mounted to the fastener head, the pointer arm having a free end. The strain gauge is configured so that strain in the fastener head causes a sensing member to bear upon the pointer arm and to move the free end with a magnifying effect relative to the motion of the sensing member. There may be two pointer arms, each having a free end, and the strain gauge may be configured so that strain in the fastener head causes a sensing member to bear upon each pointer arm and to move the free end thereof with a magnifying effect relative to the motion of the sensing member. Optionally, one sensing member may be configured to bear upon two pointer arms, which may optionally be pivotably secured to the fastener head.

The pointer arms may be resiliently connected to each other whereby movement of the pointer arms caused by strain in the fastener head is reversed upon removal of the strain. Alternatively, the two sensing members may bear on opposite sides of a pointer arm having two free ends. Two pointer arms may be mounted on the free end of a second sensing member having a stationary portion securely mounted to the fastener head and having a movable portion that can move relative to the fastener head in response to strain in the fastener head.

In any of the foregoing embodiments, the fastener head comprises a recess and the strain gauge may be disposed within the recess. The fastener may optionally comprise a clear disc affixed to the fastener head over the strain gauge.

This invention also provides a fastener comprising a shaft, a fastener head on the shaft and a strain gauge mounted on the fastener head for indicating strain in the fastener head, the strain gauge comprising a torsion pivot bar suspended across the top of the fastener head and a connector joining a central portion of the pivot bar to the fastener head.

Alternatively, the strain gauge may comprise a sensing arm connected to the fastener head by two webs, each web having two ends and being secured at one end to the fastener head and to the sensing arm at the other, the webs and sensing arm being configured to move the sensing arm when strain is imposed on the fastener head, and wherein the sensing arm is connected to a pointer arm having a free end, the strain gauge being configured so that movement of the sensing arm causes the free end of the pointer arm to move with a magnifying effect relative to the motion of the sensing arm. There may be two sensor arms and two pointer arms, and the webs and the sensing arm may be configured so that the sensing arm moves with a magnifying effect relative to the webs.

Alternatively, there may be two sensing arms and two pointer arms and the pointer arms may be hinged to each other and the strain gauge may be configured so that the relative motion between the free ends of the pointer arms further magnifies the movement of the sensing arms.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view in elevation of a bolt-type fastener comprising a bolt head, a shaft and a nut, the fastener securing together two elements of a substrate and having on the bolt head strain-indicating means, and comprising one em- bodiment of the present invention;

Figure 2 is a perspective view of a typical bolt head showing the peripheral and interior areas thereof;

Figure 3 is a top plan view of the bolt head of the fastener of Figure 1, with the strain-indicating means omitted to show the holes in which the strain-indicating means are mounted;

Figure 3 A is a top plan view of the bolt-type fastener of Figure 1 in an unstressed and undistorted state;

Figure 3B is a view corresponding to Figure 3 A but showing the fastener in a stressed, distorted shape; Figures 3C and 3D are views similar to Figures 3 A and 3B of a fastener according to an alternative embodiment of the invention;

Figures 4A and 4B provide schematic elevation views of the operation of a second embodiment of the present invention;

Figure 5 is a schematic illustration showing plan and elevation views of a bolt head in a distorted state having thereon a device in accordance with another embodiment of the invention; Figure 5A is an illustration corresponding to Figure 5, but showing the bolt head in a relaxed state;

Figure 6 is a schematic plan view of another embodiment of the device of the invention shown in a distorted state; Figure 6A is a view corresponding to Figure 6, but showing the device in a relaxed state;

Figure 7 is a schematic illustration shown in plan and elevation views of a bolt head in a distorted state having thereon yet another embodiment of the invention;

Figure 7A is an illustration corresponding to Figure 7, but showing the device in a relaxed state;

Figure 7B is a side view of the device of Figure 7 in its distorted state; Figure 8 is a plan view of yet another embodiment of the present invention shown in a distorted state;

Figure 8A is a perspective view of a bolt head of a tension fastener with the device of Figure 8 mounted thereon, the bolt head being in its distorted state;

Figure 8B is a view corresponding to Figure 8 showing the device in a relaxed state;

Figure 8C is a view corresponding to Figure 8A but showing the bolt head in its relaxed state; Figure 9 is a perspective view of the bolt head of a tension fastener having a recess fashioned thereon;

Figure 9A is a view corresponding to Figure 9 showing a strain-indicating device in accordance with yet another embodiment of the present invention received within the recess of the bolt head; and Figure 9B is a plan view slightly enlarged relative to Figure 9A, showing the strain-indicating device of Figure 9 A.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF The devices of the present invention exploit expansion and contraction to which structural members and other articles are inherently subject in use. For exam- ple, the rails of railroad tracks, structural members of bridges and buildings, pressurized boilers and other containers and other structures or substrates are subject to expansion and/or contraction upon being subjected to temperature changes, stresses imposed by live and dead loads, etc. The present invention provides strain gauges that can be applied to any such structure to indicate the strain produced by such loads, temperature changes, etc. Devices according to the present invention rely on the fact that such strain causes some parts of the structure to move relative to others. They indicate strain by signaling movement of one part of a structure relative to another. In addition, there are various embodiments of the invention in which the relative move- ment of two points on a structure is magmfied by mechanical means, to facilitate detection of the strain. For example, in the case of tension fasteners, the natural elasticity and reversible deformation characteristics inherent in standard bolt heads is taken advantage of by certain embodiments of the present invention to allow reversibility and reproducibility in fastener tension measurement. Thus, in some embodiments, devices of the present invention exploit the elasticity in the bolt head to provide a continuous monitoring of the fastener tension over time and will not interfere with the application of standard tools (such as wrenches) to its bolt heads.

The devices of the present invention provide an indication of elongation and shrinkage of any structural member or component substrate, and thus directly indicate strain. In the case of fasteners, the devices of the present invention provide a direct (rather than indirect, such as with the torquing method of measuring tension) indication of fastener tension and they do so without having to contact the substrate to which the fastener is attached. Prior art tension indicators typically require that a lever-like indicator arm be mounted on the fastener for contact with the substrate. Such devices may be referred to as stress indicators since they respond to force applied between two different structures. The motion of a prior art stress indicator in response to a given degree of stress may vary according to the toughness or rigidity of the substrate because the motion of the indicator is based in part on the configuration of the stressed substrate. The strain gauges of the present invention, in contrast, are optionally, but preferably, situated on the fastener so that they do not contact the substrate. As a result, the deformation manifested by the gauge more reliably corre- sponds to imposed stress because it is independent of the characteristics of the substrate to which the fastener is secured. Devices of the present invention can be used with conventional, commercially-available bolt-type fasteners, and require minimal or no alteration to the commercially-available fasteners. Therefore, the present invention provides an inexpensive alternative to currently available tension indicators. Various embodiments of the present invention provide a mechanical multiplying effect, and preferably a series of such mechanical multiplying effects, to provide a cascade of movements resulting in magnifying the small movement of one part of the device (usually referred to herein as a "sensor") into a larger movement of another compo- nent of the device (usually referred to herein as a "pointer") to facilitate visual or instrumental discernment of the indication of strain in the member to which the device of the invention is attached.

Referring now to Figure 1, there is shown a typical bolt-type fastener suitable for use in connection with the present invention. The bolt-type fastener 10 comprises a bolt head 12 and a shaft 14. A washer 15 is interposed between bolt head 12 and substrate S,. Bolt-type fastener 10 is used to secure substrate S, to substrate S₂ by compressing the two substrates between bolt head 12 and a nut 17 secured at or near the distal end 14b of shaft 14. A bore 19a is formed in substrate S, in alignment with a bore 19b formed in substrate S₂, to permit passage of shaft 14 therethrough. Bolt head 12 has a major exposed top surface 16 (seen in perspective view in Figure 2, and in plan view in Figure 3). The surface 16 of the bolt head 12 is provided with surface apertures 20a, 20b located at opposite extremities of the peripheral area of bolt head 12, and a central aperture 20c. These apertures provide a means for attachment of a fastener strain sensing device (also referred to herein as a "strain gauge") 18 to the bolt head 12, as described below.

Figures 3 A and 3B show the fastener tension sensing device 18 disposed upon the surface 16 of the bolt head 12. The device comprises a rigid sensing arm 22a, and a pair of pointer arms 32a, 32b. Sensing arm 22a has a proximal mounting end where there is an aperture 24a for alignment with bolt head surface aperture and an opposite, distal mounting end. Pointer arms 32a, 32b have free ends formed in the shape of arrowheads and fulcrum portions that merge their apex and which have a common ap- erture 24b therein. The apex formed by pointer arms 32a, 32b, at which they merge to share mounting end 22b, acts as a fulcrum or pivot point for the pointer arms, as will be described below. The mounting end of sensing arm 22a and the fulcrum portions of pointer arms 32a, 32b may be attached to the surface 16 by any suitable means, such as pins 26a, 26b (Figure 3A). The paired apertures 20a, 24a and 20b, 24b and the pins 26a, 26b which, respectively, pass therethrough, may be replaced by other suitable fastener means. For example, they may be laser spot-welded to the surface 16 at the locations shown for pins 26a, 26b. The moving end of arm 22a is free to move so that when strain is imposed on the fastener head, the mounting end moves with the head and sensing arm 22a translates that movement to the moving end, which moves along the fastener head surface. Sensing arm 22a and pointer arms 32a, 32b are mounted and positioned on the fastener head so that strain in the fastener head will cause the moving end of sensing arm 22a to move towards pointer arms 32a, 32b. A gap 28 may be left between the moving end of sensing arm 22a and the pointer arms 32a, 32b.

It is known in the art that all components in the clamping train (e.g., the fastener, the nut, the washer, the substrate) of a bolt-type fastener undergo substantial distortion (i.e., strain) when force is applied to a fastener, such as when tightening a nut and bolt assembly. The upper lateral surface extremities of the bolt head of a bolt-type fastener undergo the most distortion (often referred to in the art as "dishing" or "cupping") and the adjacent peripheral area of the top surface 16 undergoes nearly as much distortion. The tightening of the fastener causes the upper surface of the bolt head to cup or dish so that the outer edges approach one another, as seen by comparing Figures 3 A and 3B (the relaxed configuration being indicated in Figure 3B by dotted outline; see also Figures 5 and 5A, discussed above). As noted above, the rigid sensing arm 22a is fastened at one end to the top surface 16 of the bolt head 12 at the peripheral area thereof. The mounted end of arm 22a and pointer arms 32a, 32b approach one another in response to the dishing of the bolt head upon application of tension to the shaft 14 (Figure 1) thereof and consequent dishing of bolt head 12. Ac- cordingly, arm 22a will bear against at least one pointer arm 32a or 32b. The distal or moving end of sensing arm 22a is configured to bear upon both pointer arms 32a, 32b at a point near their apex, and preferably has a notch or indentation formed therein as shown in Figures 3 A and 3B.

The pointer arms 32a, 32b are displaceable in an amount corresponding to the degree to which the moving portion of sensing arm 22a and pointer arms 32a, 32b approach each other when the bolt head dishes, as shown in Figure 3B. As tension is increased and the bolt head distorts by dishing, pointer arms 32a, 32b are driven by sensing arm 22a to the position indicated in Figure 3B (their initial position in the unstressed state being shown in phantom outline in Figure 3B), thus providing a visual indication of the amount of dishing of bolt head 12, and thereby of the amount of tension imposed on shaft 14 and the tightness of the assembly of bolt type fastener 10. The movement of sensing arm 22a is magnified at the free end of the pointer arms 32a, 32b because the apex of the pointer arms acts as a fulcrum and sensing arm 22a bears upon the pointer arms at a point much closer to the fulcrum than to the free end. Pointer arms 32a and 32b are preferably resihently displaceable, so that when tension on the shaft 14 is relieved somewhat and sensing arm 22a and pointer arms 32a, 32b move away from each other, pointer arms 32a, 32b return to the position indicated in Figure 3A. Pointer arms 32a, 32b can thus provide a dynamic, reversible indication of the amount of tension present in the fastener at any given time. A scale or indicia may be applied to the surface 16 of bolt head 12 so that the amount of distortion, and thereby the tightness of the bolt-type fastener 10, may be indicated by the position of the arrowheads on first and second pointer arms 32a, 32b relative to the indicia or scale. The strain gauge 18 may be covered by a clear plastic member to seal strain gauge 18 from the elements, thereby preventing dirt or other foreign objects entering therein and interfering with movement of first and second pointer arms 32a, 32b.

The strain gauge 18 of the present invention may be permanently attached to the fastener used in connection with the sensing device or may be removably attached allowing a worker to use it to monitor a series of bolt fasteners or of bolt fasteners lo- cated at various sites. Figures 3C and 3D show a similar strain gauge 18' disposed on the surface 16 of bolt head 12. Strain gauge 18' comprises the same sensing arm 22a as device 18 of Figures 3 A and 3B. In strain gauge 18', however, pointer arms 32a and 32b are secured to surface 16 at point 26c by means of a pin, spot weld, etc., and are displace- able about point 26c in response to movement of sensing arm 22a. Pointer arms 32a, 32b and sensing arm 22a are positioned so that when bolt head 12 manifests strain, the . free end of sensing arm 22a bears against pointer arms 32a and 32b, displacing them from the position shown in Figure 3C to indicate the strain, as seen in Figure 3D.

Those skilled in the art will recognize that, although the foregoing description refers to the "dishing" action of distorting a bolt head, the invention will work even when strain is manifested by expansion and contraction of a substrate without a dishing action. For example, when a bolt head undergoing dishing distortion is viewed in plan view, there is relative movement as discerned in a plane taken parallel to the un- distorted top surface of the bolt head, i.e., the plane of the paper in which the plan view is depicted. For example, see the dash and solid outlines of bolt head 12 in Figure 3B. This provides lateral movement between two fixed peripheral points on the structure. Planar expansion and contraction of a structure without a dishing effect, as may occur due to changes in temperature, ambient pressure, etc., will likewise effectuate movement between two spaced-apart fixed points on the structure. It is impor- tant, therefore, to realize that the dishing action described above is not essential to practices of the present invention, which operates equally well to measure linear expansion and contraction.

Figure 4 A shows a member 112 having connected to the underside thereof a member 114, member 112 being supported by members 115 on a substrate S₃. It will be appreciated that the schematic illustration of Figure 4A could be considered to be a partial longitudinal cross section taken through bolt-type fastener 10 of Figure 1. Alternatively, Figure 4 A could be an end view of a structure in which members 112 and 115 are plates, member 114 is a rod or other such structure secured as by welding to the underside of plate 112, or formed integrally therewith, the structure being carried upon a substrate S₃ which has a bore (unnumbered) formed therein through which member 114 passes. Member 114 may be subjected to tension as by being pulled in the direction indicated by the unnumbered downward-pointing arrow in Figure 4A, resulting in distortion of members 112, 115 and substrate S₃, as shown in Figure 4B. In this embodiment, first and second pointer arms 132a, 132b are fixed to member 112 and, in the unstressed condition shown in Figure 4A, are seen to be pointed vertically upwardly. When tension is imposed upon member 114, causing member 112 to be forced into the dished position shown in Figure 4B, first and second pointer arms 132a, 132b will be tilted towards each other as shown by the curved arrows (unnumbered) in Figure 4B, thereby providing an indication of the degree of distortion by the amount of tipping imposed upon first and second pointer arms 132a, 132b. It will be appreciated that the distal ends 132c, 132d of pointer arms 132a, 132b will pivot a greater distance than will the opposite proximal ends thereof (unnumbered) fastened to member 112, because of the mechanical advantage provided by the lever-type action involved. A suitably supported indicia means (not shown) may be positioned in proximity to the distal ends 132c, 132d to enable the use of first and second pointer arms 132a, 132b as gauge needles or pointers to enable reading the scale to provide a quantitative indication of the degree of distortion being sustained.

Although a device of the present invention can be affixed to a member (sometimes referred to herein as a "stressed member") while the member is in its relaxed state to indicate strain induced therein, it has been found, especially in the case of stressed members comprising bolt heads, that it is advantageous to affix the device of the invention to the top surface of the bolt head while the bolt head is in the desired stressed state, i.e., while the bolt-type fastener has been tightened onto a fixture to the torque level desired for tightening in use. The bolt-type fastener may then be released from the fixture on which it has been tightened, which will result in movement of the pointer arm or arms to a position indicating release of tension on the bolt-type fastener. The bolt-type fastener may be shipped in that condition and when it is applied to the structure on which it is to be used, the pointer arm or arms will then move, and the tightening is continued until the pointer arm or arms are in the original position, indicating that the desired degree of tightness has been attained. Thereafter, should the bolt-type fastener loosen due to vibration, weathering, or any other reasons, the pointer arm or arms will move away from the desired position, alerting an inspector that the bolt needs re-tightening.

The following embodiments of the invention illustrate structures, including unitary devices in accordance with the present invention, which may be cut, as by a laser, from a thin, flat coupon, which coupon is then laser spot-welded to the structure whose dimensional changes are to be measured. The coupon may be cut from a thin stainless steel sheet, e.g., one of from about 0.001 inch to 0.030 inch thickness, or from any other suitable material.

Figure 5 A shows a bolt head 212 having a major exposed top surface 216 on which is affixed a cruciform-shaped strain gauge or fastener strain sensing device 218 comprised of first and second sensing arms 222a, 222b and a pointer arm 232 having two freely-moving pointer arm ends 232a, 232b and a fulcrum portion (unnumbered) therebetween. The outer ends of first and second sensing arms 222a, 222b comprise mounting portions thereof which are affixed, e.g., laser spot-welded, at 34a, 34b to top surface 216. The opposite ends of arms 222a, 222b comprise the moving portions thereof which bear on the fulcrum portion of pointer arm 232 through line hinge connections thereto at points offset from each other along the length of pointer arm 232. As shown in Figure 5 A, the spot- welding is carried out while bolt head 212 is in a deformed, dished condition, its normal outer periphery being indicated by the dotted line in the plan view portion of Figure 5 and its dished position being shown in solid outline. When the bolt of which bolt head 212 is a part is released from the tightening fixture, it resumes its relaxed condition shown in Figure 5 A, which causes the spot welds 34a, 34b to slightly move away from each other. The live hinge arrangement provided by the notches (unnumbered) at the intersection of the sensing arms 222a, 222b and the fulcrum portion of pointer arm 232 impose force moments on pointer arm 232 to shift it to the position shown in Figure 5 A, effecting rotation of pointer arm 232 about a fulcrum point or center of rotation near the moving ends of sensing arms 222a, 222b. Since the moving ends of sensing arms 222a, 222b bear on pointer arm 232 at points closer to the fulcrum point than to the free ends thereof, the free ends move with magnifying effect, i.e., to a greater degree than the moving ends of sensing arms 222a, 222b. Top surface 216 is provided with indicia scales 36a, 36b which are positioned so that pointer arm ends 232a, 232b are disposed over, respectively, indicia scales 36a and 36b. In this manner, a visual reading of the dishing distortion of bolt head 212, and consequently of the tension imposed on the bolt-type fastener of which it is a part, is readily obtained. When the bolt-type fastener 210 is placed in use, it is tightened until first and second pointer arm ends 232a, 232b are restored to the position shown in Figure 5, which indicates to the operator that the required degree of tension has been imposed on bolt-type fastener 210.

Figures 6 and 6A show, respectively, a variation of the tension fastener indicator of Figures 5 and 5 A in which the spot welds 134a, 134b connect, respectively, the sensing arms 222a' and 222b' to a peripheral ring 38, which comprises part of the mounting portion thereof and which in turn may be secured to the periphery of a bolt head (not shown in Figures 6 and 6 A) or the like. In this case, an indicia scale 136a, 136b may be formed on peripheral ring 38 itself. This device will function in the same manner as the device illustrated in Figures 5 and 5 A, and is of otherwise identi- cal construction. Therefore, the remainder of the device and its function are not further described.

For a M-8 bolt, which is a commonly used bolt, the bolt head will deflect a total of about 5 micrometers under an overall strain of about 2 percent at the preferred maximum tension. The corresponding movement of the sensing arms will cause an angular displacement in the pointer arm of about 5 degrees. The length of the pointer arm from the fulcrum portion to ends 232a, 232b provides a mechanical advantage which may magnify the movement by, e.g., a factor of 80. The magnified movement is indicated at the indicia scales 136a, 136b.

Figures 7, 7 A and 7B illustrate another embodiment of a strain gauge accord- ing to the present invention, wherein a pivot bar 39 has laser-welded thereto a connector 40, the other end of which is laser spot- welded to the center of the top surface 316 of bolt 312. The center of the top surface of a bolt head has been found to undergo the greatest amount of movement during stress-imposed dishing distortion and recovery to the unstressed, undistorted state. The opposite ends of pivot bar 39 are connected by feet 39a, 39b to the opposite peripheral edges of the top surface 316 bolt head 312. Tension imposed upon shaft 314 by tightening bolt-type fastener 310 causes bolt head 312 to be deformed, the center of top surface 316 dishing downwardly, whereby connector 40 rotates pivot bar 39 to the position shown in the plan view portion of Figure 7. Figure 7B shows a schematic partial side view of pivot bar 39 in the distorted position of Figure 7. When tension is released as shown in Figure 7 A, the center portion of bolt head 312 rises to its unstressed, undeformed level, thereby returning pivot bar 39 to the perpendicular position as shown in the plan view portion of Figure 7A.

Figure 8 is a plan view of another embodiment of a strain gauge or tension indicator 418 of the invention which may be laser cut from a sheet of stainless steel or other suitable material to provide first and second pointer arms 432a and 432b, which are connected at the fulcrum portions thereof by a live hinge 42, and then by connecting webs or tensor hinges (unnumbered) to sensing members comprising moving portions and mounting portions. The moving portions comprise first and second sensing arms 422a and 422b, and the mounting portions comprise webs or tensor hinges 424a, 424b, and 426a, 426b, each of which has two ends, one being connected to the associated sensing arm and the other comprising anchor ends 44a, 44b and 46a, 46b, which may be affixed, e.g., spot welded, to the member on which indicator 418 will be mounted, or can join a common base bar 44 which will be affixed to the strained member. For example, the resultant unitary coupon cutout may be laser spot- welded at 44a, 44b and 46a, 46b to a major exposed top surface 416 (Figures 8 A and 8C) of bolt head 412.

Figure 8 A shows the tension indicator 418 of Figure 8 mounted on the bolt head 412 which surmounts a calibration fixture 415 with the bolt head 412 tightened precisely to a desired tension against fixture 415. The bolt head 412 and the device 418 are shown in the stressed, dishing-deformed condition. The mounting portion of tension indicator 418, i.e., anchor ends 44a, 44b, etc., is spot-welded or otherwise secured to the bolt head surface 416 while the bolt head 412 is stressed and while the tension indicator is in its relaxed state as shown in Figure 8 and is positioned so that the relaxation of stress-induced dishing causes the anchor ends to move away from each other. The bolt is then released from the stress and it relaxes, causing the pointer arms 432a, 432b to move away from each other, as shown in Figure 8C, with the movement of the free ends of the pointer arms magnifying the movement of the anchor points relative to each other and/or to the fulcrum portions of the pointer arms. This occurs because as the strain in the bolt head recedes, the spot welds 44a and 46a move away from each other, and spot welds 44b, 46b do the same, and force moments are imposed upon sensing arms 422a, 422b via webs 424a, 424b, 426a, 426b, and they in turn exert a force moment on first and second pointer arms 432a, 432b, in the direction of the force arrows (unnumbered) shown in Figure 8B. This brings the V- shaped distal tips, of pointer arms 432a, 432b apart, as illustrated in Figure 8C.

When the bolt-type fastener is tightened onto a substrate, bolt head 412 again becomes stressed so that surface 416 begins to return to its strained, dished configuration of Figure 8 A. Consequently, spot welds 44a and 46a move towards each other, as do spot welds 44b, 46b. This imposes, through webs 424a, 424b, 426a and 426b, forces opposite to those shown by the unnumbered arrows in Figure 8B, and thereby moves the V-shaped distal ends of pointer arms 432a, 432b towards the position shown in Figures 8 and 8 A. When the position shown in Figures 8 and 8 A is attained, the requisite degree of tightening has been imposed.

It will be noted from Figures 8A and 8C that the surface 416 is contained within a recess at the top of the bolt head 412. This protects the tension indicator means 418, which may be covered by a clear plastic disk affixed at the periphery of the indentation in order to protect tension indicator 418 from the elements, dirt, etc. It will be appreciated by those of ordinary skill in the art, based on the teachings herein, that tension indicator 418 is bilaterally symmetric and that it could be cut in half along axis line X in Figure 8B to produce two fully functional tension indicators. In such an embodiment, spot welds 44a, 46a would be located on the bolt head as described above and the cleaved end of hinge 42 would also be spot welded to the bolt head to provide the necessary fulcrum effect for pointer arm 432a. Alternatively, a pivot pin could be inserted in pointer arm 432a (or a pivot point could be established by other means) at a point offset from the connection between sensing arm 422a and pointer arm 432a but closer thereto than to the free end of the pointer arm. Figures 9, 9 A and 9B show yet another embodiment of the invention in which the bolt head 512 of a bolt-type fastener (the shaft corresponding to shaft 14 of Figure 1 being omitted from these figures and from Figures 8 A and 8B for simplicity of illustration) has formed therein a circular recess to define the major exposed surface 516 within which a tension indicator device 518 (Figures 9A and 9C) is disposed and spot- welded at laser spot welds 134a, 134b. A calibration washer or extension collar 515 (Figures 9 and 9A) is at the underside of bolt head 512. As in the case of the embodiment of Figure 8 A and 8C, a clear plastic disk may be fixed over tension indicator . device 518 to protect it from the elements. The disk-like coupon of tension indicator 518 has cutouts formed therein to define a pointer arm 532 having a pointed free end 532a and a fulcrum portion 532b. Tension indicator 518 also comprises first and sec- ond sensing arms 522a, 522b each having a moving end connected to the fulcrum portion of pointer arm 532. Sensing arms 522a, 522b comprise mounting portions that include webs or tensor hinges (unnumbered) that connect the arms to peripheral structure 538. Peripheral structure 538 is spot-welded or otherwise secured to bolt head 512 at spots 134a, 134b. The sensing arms 522a, 522b which will react when laser spot welds 134a, 134b move towards or away from each other to bear upon the fulcrum portion of pointer arm 532 and impart a pivoting motion thereto, which will indicate the degree of distortion of bolt 512 in response to tension imposed upon the bolt-type fastener of which bolt head 512 is a part.

It will be appreciated that the description of the above embodiments shows a method of applying the tension indicator which comprises stressing the bolt head or other structure involved into its distorted condition, affixing the tension indicator to the bolt head or other structure while in its distorted condition, as by laser spot- welding or any other suitable means, and then allowing the bolt head or other structure to relax to its undistorted condition. The tension indicator device is so configured that when the bolt head or other structure to which it is affixed is in the distorted condition, the pointer arms are in a position which indicates the degree of tension which will be desired when the fastener is placed into use. The fastener is then removed from the fixture in which it is tightened and placed into inventory or shipped for use. While the invention has been described in detail with reference to particular embodiments thereof, it will be apparent that upon a reading and understanding of the foregoing, numerous alterations to the described embodiments will occur to those skilled in the art.

Claims

THE CLAIMSWhat is claimed is:

1. A strain gauge for indicating strain in a substrate, the gauge comprising (a) a sensing member having a mounting portion and a moving portion and (b) a pointer arm having at least one free end and a fulcrum portion, the gauge being configured so that the free end of the pointer arm moves with magnifying effect in response to relative movement between the moving portion of the sensing member and the fulcrum portion of the pointer arm.

2. The strain gauge of claim 1 comprising two pointer arms each having a free end and a fulcrum portion, the strain gauge being configured to magnify at the free ends of both pointer arms movement of the moving portion of the sensing member relative to the fulcrum portions of the pointer arms.

3. The strain gauge of claim 1 comprising two sensing members, each sensing member comprising a moving end connected to the pointer arm on opposite sides thereof and at different points along the length of the pointer arm and wherein each sensing member comprises a mounting end for attachment to a stressed member.

4. The strain gauge of claim 3 wherein the pointer arm has two free ends and a fulcrum portion therebetween.

5. The strain gauge of claim 3 or claim 4 wherein each sensing member comprises a sensing arm and a mounting portion and wherein the mounting portion of each sensing member comprises a peripheral member to which the moving portions of the sensing members are connected.

6. The strain gauge of claim 5 further comprising indicia markings on the peripheral member near at least one free end of the pointer arm.

7. The strain gauge of claim 1 wherein the sensing member comprises a sensing arm comprising the mounting end, and wherein the sensing member mounting portion comprises two sensing webs, each sensing web having two ends and being secured at one end to the sensing arm and having an anchor end for connection to a member, wherein the moving end of the sensing arm is connected to a pointer arm having a free end and a fulcrum portion, the strain gauge being configured so that relative motion between the anchor ends of the webs causes the free end of the pointer arm to move with magnifying effect.

8. A strain gauge for indicating strain in a substrate, the gauge comprising a torsion pivot bar, two mounting feet extending from the pivot bar for connection to the substrate and a connector member extending from the pivot bar for connection to the substrate.

9. A strain gauge comprising two sensing members, each comprising a sensing arm comprising a mounting portion and a moving portion, each mounting portion comprising two webs each having two ends, the webs being secured at one end to the sensing arm and comprising a free anchor end, and comprising two pointer arms each having a free end and a fulcrum portion, the fulcrum portions being joined together and being connected to the moving portion of a sensing arm.

10. The strain gauge of claim 9 wherein the pointer arms are configured to move relative to each other with mutual magnifying effect.

11. A fastener comprising: a shaft; a fastener head on the shaft; and a strain gauge mounted on the fastener head for indicating strain in the fastener head.

12. The fastener of claim 11 wherein the strain gauge comprises at least one sensing member having a stationary portion securely mounted to the fastener head and having a movable portion that can move relative to the fastener head to indicate strain in the fastener, and further comprising at least one pointer arm mounted to the fastener head, the pointer arm having a free end, the strain gauge being configured so that strain in the fastener head causes a sensing member to bear upon the pointer arm and to move the free end with a magnifying effect relative to the motion of the sensing member.

13. The fastener of claim 12 comprising two pointer arms, each having a free end, wherein the strain gauge is configured so that strain in the fastener head causes a sensing member to bear upon each pointer arm and to move the free end thereof with a magnifying effect relative to the motion of the sensing member.

14. The fastener of claim 13 wherein one sensing member is configured to bear upon two pointer arms.

15. The fastener of claim 13 or claim 14 wherein the pointer arms are resil- iently connected to each other whereby movement of the pointer arms caused by strain in the fastener head is reversed upon removal of the strain.

16. The fastener of claim 13 or claim 14 comprising the two sensing members that bear on opposite sides of a pointer arm having two free ends.

17. The fastener of claim 13 wherein the pointer arms are pivotably secured to the fastener head.

18. The fastener of claim 13 wherein the two pointer arms are mounted on the free end of a second sensing member having a stationary portion securely mounted to the fastener head and having a movable portion that can move relative to the fastener head in response to strain in the fastener head.

19. The fastener of claim 11 or claim 12 wherein the fastener head comprises a recess and wherein the strain gauge is disposed within the recess.

20. The fastener of claim 11 or claim 12 comprising a clear disc affixed to the fastener head over the strain gauge.

21. A fastener comprising: a shaft; a fastener head on the shaft; and a strain gauge mounted on the fastener head for indicating strain in the fastener head, the strain gauge comprising a torsion pivot bar suspended across the top of the fastener head and a connector joining a central portion of the pivot bar to the fastener head.

22. A fastener comprising: a shaft; a fastener head on the shaft; and a strain gauge mounted on the fastener head for indicating strain in the fastener head, the strain gauge comprising a sensing arm connected to the fastener head by two webs, each web having two ends and being secured at one end to the fastener head and to the sensing arm at the other, the webs and sensing arm being configured to move the sensing arm when strain is imposed on the fastener head, and wherein the sensing arm is connected to a pointer arm having a free end, the strain gauge being configured so that movement of the sensing arm causes the free end of the pointer arm to move with a magnifying effect relative to the motion of the sensing arm.

23. The fastener of claim 22 comprising two sensing arms and two pointer arms wherein the webs and the sensing arm are configured so that the sensing arm moves with a magnifying effect relative to the webs.

24. The fastener of claim 13 or claim 23 comprising two sensing arms and two pointer arms, the pointer arms being hinged to each other and wherein the strain gauge is configured so that the relative motion between the free ends of the pointer arms further magnifies the movement of the sensing arms.

25. A fastener comprising: a shaft; a fastener head on the shaft; and a strain gauge mounted on the fastener, the gauge comprising (a) a sensing member having a mounting portion secured to the fastener head and a moving portion and (b) a pointer arm having at least one free end and a fulcrum portion, the gauge being configured and positioned so that the free end of the pointer arm moves with magnifying effect in response to relative movement between the moving portion of the sensing memeber and the fulcrum portion of the pointer arm.

26. The fastener of claim 25 wherein the strain gauge comprises two pointer arms each having a free end and a fulcrum portion, the strain gauge being configured to magnify at the free ends of both pointer arms movement of the moving portion of the sensing member relative to the fulcrum portions of the pointer arms.

27. The fastener of claim 25 wherein the strain gauge comprises two sensing members, each sensing member comprising a moving end connected to the pointer arm on opposite sides thereof and at different points along the length of the pointer arm and wherein each sensing member comprises a mounting end for attachment to a stressed member.

28. The fastener of claim 27 wherein the pointer member has two free ends.

29. The fastener of claim 27 or claim 28 wherein each sensing member comprises a sensing arm and a mounting portion and wherein the mounting portion of each sensing member comprises a peripheral member to which the moving portions of the sensing members are connected.

30. The fastener of claim 29 further comprising indicia markings on the peripheral member near at least one free end of the cross bar.

31. The fastener of claim 25 wherein the sensing member comprises a sensing arm comprising the mounting end, and wherein the sensing member mounting portion comprises two sensing webs, each sensing web having two ends and being secured at one end to the sensing arm and having an anchor end connected to the fastener head, wherein the moving end of the sensing arm is connected to a pointer arm having a free end and a fulcrum portion, the strain gauge being configured so that relative motion between the anchor ends of the webs causes the free end of the pointer arm to move with magnifying effect.

32. A fastener comprising: a shaft; a fastener head on the shaft; and a strain gauge on the fastener, the strain gauge comprising a torsion pivot bar, two mounting feet extending from the pivot bar for connection to the substrate and a connector member extending from the pivot bar for connection to the substrate.

33. A fastener comprising: a shaft; a fastener head on the shaft; and a strain gauge on the fastener comprising two sensing members, each sensing member comprising a sensing arm comprising a mounting portion and a moving portion, each mounting portion comprising two webs each having two ends, the webs being secured at one end to the sensing arm and comprising a free anchor end, and comprising two pointer arms each having a free end and a fulcrum portion, the fulcrum portions being joined together and being connected to the moving portion of a sensing arm.

34. The fastener of claim 33 wherein the pointer arms are configured to move relative to each other with mutual magnifying effect.