TECHNICAL FIELD

This invention generally relates to load indicating, and more particularly, to precise positioning or "indexing" of a load indicator removably coupled to a fastener.

BACKGROUND

Fasteners are used in a wide variety of applications, where achieving proper fastener loading (tension) and maintaining this loading in service can be problematic. Monitoring the load on a fastener during installation and service can be equally problematic.

As a fastener is continually tightened, the load increases until the fastener ultimately yields, breaks, or its integrity is otherwise compromised. Thus, overtightening a fastener can lead to catastrophic failures. Conversely, fasteners typically experience some loss of tension in service due to, for example, a variety of in-service occurrences including: relaxation (thread embedment), vibration loosening, compressive deformation in the joint or flange, temperature expansion or contraction, etc. Loss of tension from these occurrences can cause misalignment or premature wear in a bolted assembly, leakage (in applications where the fastener is used for sealing), or catastrophic joint failure due to excessively high loads on other members of the assembly.

In some applications, knowledge of a fastener load, upon installation and over time, is desirable for avoiding the potentially dangerous consequences of a compromised or loosened fastener, such as slippage, wear, leakage and/or possible failure. In other applications, for example when working with a group of bolts around a flange of a sealed assembly, it is important to evenly tighten the group of bolts. By uniformly tightening a group of bolts or studs to an appropriate load, and maintaining this load over time, potential failures are less likely to be experienced.

Determining the tensile load of a conventional fastener often entails cumbersome methods to check the tightness of each bolt, such as loosening and re-tightening all of the fasteners regardless of whether such re-tightening is needed. The retorquing (i.e. tightening) of a fastener, however, induces wear and strain in the fastener system. Additionally, corrosion, friction, variations in nut condition, and the like can cause variations in torque values and introduce error into tensile load measurements.

One earlier proposed load indicating fastener, which is the subject of U.S. Pat. No. 5,668,323 issued Sept. 16, 1997 to Cory S. Waxman includes a single pivot lever positioned within a bore in the fastener head with the actuator end of the pivot lever in contact with a reference post or datum rod seated or formed in the end of the bore and an indicator end of the pivot lever being visible at the head of the fastener.

Other proposed methods often require costly tools or readers or special training. For example, electronic or ultrasonic methods for determining tensile loads require experienced operators, expensive equipment, clean surfaces and records of pre- installation test values for each bolt or stud. In addition, such systems may be adversely affected by shock and other extreme conditions.

Accordingly, improvements are sought in the monitoring of loading in fasteners.

DISCLOSURE OF INVENTION

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

It has been discovered that a load indicator may be removably coupled to a fastener with a displacement gauge precisely indexed relative to the fastener head via a plurality of indexing projections on the load indicator that engage a coupler on the fastener head. The precise positioning or "indexing" of the displacement gauge relative to a reference surface defined on the male coupler of the fastener head allows for precise measurement of fastener elongation based on movement of a datum rod disposed in a bore in the fastener.

In some embodiments, a fastener load indicating system includes a fastener defining a shank bore extending to a surface of a fastener head and a datum rod anchored at a first end within the central bore such that elongation of the fastener causes displacement of a second free end of the datum rod relative to the fastener head. A male coupler is defined on the fastener head.

A load indicator is removably couplable to the fastener via the male coupler and includes a gauge housing defining a female coupler having an interior dimension sized to receive at least a portion of the male coupler defined on the fastener head. A displacement gauge is disposed within the housing and includes a probe disposed within the female coupler and response to movement of the second free end of the datum rod when the load indicator is coupled to the fastener. A plurality of indexing projections are spaced about an interior of the female coupler and moveable to engage the male coupler of the fastener head to thereby index the probe relative to the fastener proximate to the second free end of the datum rod.

A lower end of the datum rod is anchored within a bore in the fastener while the upper free end of the datum rod moves relative to a top surface of the fastener head during fastener elongation. As the fastener is tightened, the fastener is elongated causing the free end of the datum rod to be drawn or displaced further into the internal fore of the fastener. The probe measures displacement of the datum rod free end.

In some implementations, the gauge housing further includes a moveable locking ring concentric to the indexing projections. The locking ring is moveable between a first locked or coupled position maintaining the indexing projections in engagement with the male coupler and a second released position permitting disengagement of the indexing projections from the male coupler and decoupling of the load indicator from the fastener.

In some cases, a locking ring is biased towards the first locked position to thereby provide a snap-fit locking of the load indicator in place on the fastener upon indexing of the probe.

In some cases, the indexing projections are retractable away from the male coupler on the fastener head via movement of a release mechanism on the indicator. In some implementations, the male coupler defines a circumferential groove engageable by the indexing projections independent of rotational positioning of the load indicator on the fastener head.

In some implementations, the female coupler further defines a seat engageable with an end face of the male coupler.

Precise positioning of the indexing projections relative to the probe while the load indicator is coupled to the fastener facilitates precise measurement by the probe of displacement of the datum rod within the bore.

In some implementations, the probe is moveable in response to displacement of the datum rod. In some implementations, the probe is configured to measure a gap between the probe and the datum rod. Thus, the probe can be indexed proximate to or in contact with the datum rod.

In some implementations, a load indicator is removably couplable to a fastener for measuring fastener loading. The load indicator includes a gauge housing including a female coupler having an interior dimension sized to receive at least a portion of a male coupler defined on the fastener. A displacement gauge is disposed within the housing and includes a probe disposed within the female coupler. A plurality of indexing projections are spaced about the interior of the female coupler and positioned a predetermined distance from the probe when the load indicator is coupled to the fastener to thereby index the probe relative to the fastener.

In some implementations, the female coupler includes a piston bearing the probe and biased towards an unlocked position. A sleeve is positioned concentric to the piston and bearing the indexing projections. A locking ring is positioned concentric to the sleeve and biased towards a locked position. The piston is retractable within the sleeve via insertion of the male coupler and the indexing projections are inwardly extendable in response to movement of the locking ring into the locked position to thereby engage the male coupler and securely index the probe relative to the fastener. In some implementations, the indexing projections engage a first portion of the male coupler and the piston contacts a second portion of the male coupler to index the probe relative to the fastener.

In some cases, the locking ring is concentric to the indexing projections and is biased to maintain the indexing projections in a locked position and is moveable to allow retraction of the indexing projections during insertion and removal of the male coupler into or from the female coupler.

In some implementations, the load indicator further includes the datum rod anchored within a bore in the fastener and engageable by the probe to measure displacement of the datum rod relative to the male coupler when the probe is indexed relative to the fastener via engagement of the indexing projections with the male coupler.

In some cases, the load indicator includes a visual scale on which an indication of loading is presented in response to displacement of the probe.

In has been discovered that a load indicator may be indexed to a fastener via a plurality of indexing projections on the load indicator for engaging a coupler on the fastener. In some applications, the fastener includes a shank defining a central bore, a datum rod anchored within the central bore, and a fastener head defining a portion of the bore therein and a male coupler thereon.

In some applications an indexing method includes positioning a load indicator on the fastener head, the load indicator including a probe responsive to displacement of the datum rod relative to the fastener head and a load gauge responsive to measurements by the probe. The load indicator further includes a female coupler including a plurality of indexing projections engageable with the male coupler on the fastener head. The method further includes sliding the female coupler over the male coupler and indexing the probe relative to the fastener head via engagement of the indexing projections with the male coupler to provide a fixed reference for measurement by the probe of displacement of the datum rod relative to the fastener head. The loading of the fastener may then be determined using the probe based on displacement of the datum rod relative to the fastener head. In some applications, the method includes retracting the indexing projections from engagement with the male coupler and decoupling the load indicator from the fastener.

In some applications, the method includes sliding the female coupler over the male coupler to bring a seat portion of the female coupler into contact with an end face portion of the male coupler, and wherein sliding thereafter of the female coupler over the male coupler advances the indexing projections into locked indexing engagement with the male coupler. In some cases, retracting the indexing projections comprises sliding a locking ring disposed concentric to the indexing projections.

In some applications, measuring with the probe includes measuring displacement of the datum rod via probe contact with the datum rod. In some cases, measuring with the probe includes non-contact measurement by the probe of displacement of the datum rod.

While various implementations are described herein in the context of a dial gauge indexed via engagement of a plurality of indexing projections with a male coupler, other implementations and variations are included within the scope of the claims. In some implementations, the displacement gauge is a single or multi-lever gauge. In some implementations, the displacement gauge employs non-contact measurement techniques, e.g., by measurement of an air gap between the gauge and a reference surface. In some cases the indicator includes a sensor for sensing displacement of the probe and translating the displacement into an electronic signal. In some cases, the sensor is one of a displacement transducer and a pressure transducer. In some cases the sensor provides a capacitance measurement of an air gap between the probe and the datum rod. In some implementations, the load indicator can produce at least one of an electrical signal, an audible signal and a light signal. The signal produced by the load indicator can be useful for auto-shut off of a tightening tool or for remote monitoring.

In some implementations, the indicator is coupled to the fastener via a snap fit. In some implementations, the probe acts on one or more levers to move a pointer along a visual scale. In some cases, the probe is a first end of a first lever and the pointer is a second end of a second lever, the first and second levers moveable about pivots with a second end of the first lever cooperating with a first end of the second lever. In some cases the probe and pointer are opposite ends of a single lever.

In some implementations, the fastener or bolt includes a head, a shank and a central bore defined in the head and shank. A datum rod is anchored within the central bore in the fastener. The fastener head defines a reference surface on a load indicator coupler, e.g., on an end face or lateral surface of a male coupler. A free end of the datum rod moves relative to a reference surface on the fastener head upon loading and elongation of the fastener. The respective couplers on the load indicator and the fastener head provide for indexing and mounting of the load indicator atop the fastener.

As the load indicator is pressed onto the bolt head, the probe of the load indicator is progressively pushed into the load indicator housing. A gap defined between the free end of the datum rod and the reference surface is translated into a measurement of fastener loading via the corresponding probe measurement, e.g., movement of the probe and associated dial gauge or levers and pointer. Thus, the load indicator displays an indication of the loading in proportion to the displacement of the datum rod as a result of fastener loading. As the load indicator is mounted on the fastener, in some cases, the probe contacts the datum rod and moves the pointer down the visual scale to an appropriate percentile load indicator.

In some implementations, the probe includes a ball bearing displaceable within a journal.

In some cases, movement of the probe is translated electronically into a visual indication of fastener loading, e.g., via an LCD display. In some cases, the gap between the datum rod and reference surface can be additionally or alternatively measured using sonic, optical, capacitance or resistance measurements or other non-contact gap measuring techniques. In some implementations, the respective couplers of the fastener and load indicator include cooperative releasably engageable features. In some cases, the couplers together include a snap-fit indexing coupling to align and removably secure the load indicator on the fastener.

For protection from corrosion, in some cases grease is provided around the datum rod within the central bore and a removable cap is provided over the male coupler and central bore when the load indicator is not attached. For example, the cap can be retained on the fastener head via the male coupler.

In some implementations, the load indicator can maintain a loading indication for reading after removal of the load indicator from the fastener. For example, in some cases, a second moveable indicator accompanies the pointer along the visual scale in a first direction, but remains subsequently fixed along the scale to provide an indication of the reading obtained prior to separation of the load indicator. The second indicator can then be reset adjacent the pointer for a subsequent reading.

In some implementations, the load indicator can be coupled to the fastener independent of a particular rotational orientation, i.e., the couplers together provide for a wide range of orientations. Alternatively, in some implementations, more precise positioning of the load indicator on the fastener head may be provided, for example, by a series of complementary projections and detents may allow for only a finite number of discrete orientations.

In some implementations, the load indicator is configured to be further connected to a power tightening tool to facilitate automatic shut-off of the tool upon reaching a certain loading, as indicated by the load indicator. For example, the load indicator can provide an electrical signal corresponding to the movement of the probe and the tool can be configured to shut-off upon detection of a predetermined electrical signal value.

In some cases, the load indicator is positioned within a driver on the tool such that the load indicator and driver are simultaneously pressed onto the fastener. Thus, a fastener may be retained by the load indicator for ease of installation and the load indicator may be retained on the fastener for ease in reading. In some implementations, the indexing projections may be used with multiple different sized fasteners and/or male couplers.

In some implementations, the load indicator includes an audible signal or light generator. The audible signal or light generator can be set to activate when a detected loading is within a predetermined range. Alternatively, it can activate when a detected loading falls outside an acceptable range, for example, when the fastener is overloaded or insufficiently loaded.

In some implementations, the load indicator can be semi-permanently coupled to a fastener to provide ongoing loading measurements. For example, the load indicator can be removably snap-fit onto a fastener head for periodic readings in some cases or can be left snap-fitted onto the fastener head for extended periods to provide ongoing measurements. Thus, the load indicator can be removably, semipermanently, or effectively permanently coupled to the fastener to provide a desired frequency of readings.

Multiple levers within the load indicator may cooperate to indicate the load on the fastener by amplification of the elongation experienced by the fastener. A multi- lever design may provide a visible range of pointer movement even with a significantly reduced indicator diameter and a reduced indicator depth.

BRIEF DESCRIPTION OF DRAWINGS

Like reference symbols in the various drawings indicate like elements.

FIG. 1 a is a prior art fastener without an applied load.

FIG. 1 b is a prior art fastener with an applied load.

FIG. 2 is a schematic diagram of a cut-away view of a bore of a fastener housing a single-lever in-fastener prior art load indicating assembly.

FIG. 3 is a schematic diagram of a cut-away view of a bore of a loaded fastener housing a dual-lever in-fastener load indicating assembly according to one embodiment. FIG. 4 is a schematic diagram of a cut-away view of the fastener bore and load indicating assembly of FIG. 3 without a load.

FIG. 5 is a top view of a load indicator visual scale and housing.

FIG. 6A is a schematic diagram of a cut-away view of a fastener providing in- fastener recessed cartridge mounting in accordance with one embodiment.

FIG. 6B is a schematic diagram of a cut-away view of an in-fastener flush-mounted cartridge in accordance with another embodiment.

FIG. 7 is a schematic elevation view of a fastener configured for use with a load indicator according to one implementation.

FIG. 8 is a perspective view of a load indicator for use with the fastener of FIG. 7 according to one implementation.

FIG. 9 is a sectional view of the load indicator of FIG. 8 along line A-A.

FIG. 10 is a side view of the load indicator of FIG. 8 coupled to the fastener of FIG. 7.

FIG. 11 is a front view of a load indicator coupled to a fastener head.

FIG. 12 is a sectional front view of a load indicator coupled to a fastener head with a probe indexed relative to the fastener head.

DETAILED DESCRIPTION

The following description is of example implementations of the invention only, and is not intended to limit the scope, applicability or configuration of the invention. As will become apparent, various changes may be made in the function and arrangement of the elements described in these implementations without departing from the scope of the invention as set forth herein.

While described here in the example context of a bolt, it should be appreciated that a load indicator may be used with any fastener, for example, studs, pins, dowels, jack bolts, thread stock, and/or the like may incorporate aspects of the present invention. Thus, a fastener, as used herein, means any securing device or structure capable of elongation in response to an applied force.

The term "lever" as used herein, refers to any rigid or substantially rigid member having a portion configured to be moveable relative to a reference point or "pivot" in response to movement of another portion of the lever. Thus, suitable levers may be of any shape or size and may be configured and associated with a pivot in any manner to achieve a desired lever movement.

"Pivot" as used herein, generally may be construed to mean any fulcrum or reference by which actuation of a first portion of a lever causes movement of an opposing second portion of the lever. For example, the pivot may be a pin positioned to obtain a specific amplification ratio of the movement of the second lever portion in response to movement of the first lever portion. Similarly, the lever lengths and pivot points of the levers may be selected to achieve a combined amplification ratio.

In some implementations, elongation of a fastener causes multiple interoperable levers to display an indication of the tensile load on a visual scale. The indicator may be used to establish the proper initial loading of a fastener or to monitor the subsequent loading of the fastener. That being said, implementations are described herein in the context of torquing or tightening a fastener to the proper loading.

In accordance with various embodiments, one or more of the levers may be biased to return one or more of the levers to a default position. For example, in various embodiments, the pointer end of the second lever is biased toward the maximum load portion of the visual scale.

In the context of a removable load indicator, the load indicator housing/cartridge is temporarily mounted to the head of the fastener by aligning and engaging indexing projections on the load indicator with a coupler feature on the fastener. As the indicator is mounted to the fastener, a probe extending from the load indicator contacts a datum rod anchored within a bore in the fastener causing the pointer at the end of the second lever to move proportionately along a visual scale. The indicator may be snap-fitted or otherwise accurately positioned on the fastener to ensure accurate readings.

With reference now to FIGS. 1a and 1 b, conventional fasteners 5 are shown in an unloaded state and in a loaded state. As appreciated by those skilled in the art, a force F applied to fastener 2 causes fastener elongation that is proportional to the force (F) applied. Elongation of fastener 2 adheres to Hooke's law, which dictates that elongation is directly proportional to the load applied to fastener 2. For example, in FIG. 1 a, an initial distance Io is defined between Point A adjacent the bolt head and Point B located a predetermined distance from Point A on the shank. With reference to FIG. 1 b, as a force F is applied, fastener 2 is elongated such that the distance between Points A and B (as shown in FIG. 1 b) is greater than I ₀ , namely, as depicted, the distance I between Points A and B. As those skilled in the art will appreciate, 1-I ₀ =Δ.I and Δ.l is proportional to F, where F is the force which is applied to fastener 2.

As will be appreciated, Δ.l will vary depending upon the specific section of fastener 2 that is analyzed. For example, the elongation in the upper portion of the fastener will tend to be different than that in the lower i.e. threaded portion of the fastener. However, within a given region, the percent elongation is substantially constant over that region. As will be described in greater detail herein below, the percent elongation over the upper region of the fastener, such as shown in FIGS. 1 a and 1 b, is utilized in determining the load that is applied to the fastener. The indicator provides a visual indication of clamp load status during tightening and throughout the life of fastener 2.

With reference to FIG. 2, a schematic diagram shows a cut-away view of a prior art in-fastener single lever load indicator 1 within an elongated internal bore 4 of a fastener, including bore sections 4a, 4b, and 4c extending from the head of the fastener to a lower internal point. Bore section 4a extends into the shank or yieldable portion of the fastener and is configured to receive a reference datum rod 6 configured to act upon a single elongated lever 8 disposed in bore section 4b. Lever 8 is moveable within bore 4 about a pivot 10 in response to movement of datum rod 6 during elongation of the fastener. The dashed lines indicate the resting position 5 of lever 8 prior to elongation of the fastener and the corresponding solid lines indicate the elongation position 7 of lever 8. As a first end 12 of lever 8 moves in contact with datum rod 6, a second end 14 of lever 8 produces an amplified response, moving second end 14 between position 5 and position 7. Section 4c is configured to accommodate the amplified range of movement of second end 14. Thus, a smaller, less visible movement of datum rod 6 within bore 4 is converted into a larger, more visible movement at second end 14 of lever 8.

With the single lever configuration shown in FIG. 2, the depth and diameter of bore section 4c is determined by the desired amplified range of movement of second end 14 and the length of lever 8. For example, for the range of movement of second end 14 to travel a full half-inch visual scale, the diameter of section 4c must be substantially equal to one-half inch. The depth of section 4c, in turn, must be sufficient to provide clearance for movement of the remaining length of lever 8. Single lever indicators have proven very reliable and effective, but have typically been limited to use in fasteners having larger shanks or thicker heads due to the required depth of bore section 4c. Thus, use of multiple levers provides for a significantly shallower bore section 4c to enable use of load indicator 1 in fasteners having smaller diameter shanks and smaller heads.

With reference to FIG. 3, a cut-away view is shown of a load indicator 1 having a first lever 8 within bore section 4b and a second lever 16 disposed within bore section 4c. First end 12 of lever 8 engages datum rod 6 disposed in bore section 4a causing lever 8 to respond to movement of datum rod 6 during elongation of the fastener. Datum rod 6 may be integral to the fastener or may be a separate component inserted into bore section 4.

A datum rod 6 comprises a post 7 configured to extend a predetermined distance into section 4a in the shank of the fastener. Different lengths of post 7 may be used to enable use of a standard size of load indicator 1 with various lengths of fasteners having various lengths of bore 4. Datum rod 6 further includes an annular rim 9 about the top of post 7. Pivot 10 and any portion of lever 8 may extend into the opening in the center of annular rim 9. Annular rim 9 serves to contact first end 12 of lever 8 independent of the orientation of load indicator 1 within bore 4. It is understood that datum rod 6 may include any other suitable feature for engaging lever 8 and may be configured for a particular orientation of load indicator 1 within bore 4. Datum rod 6 may anchored at its lower end in section 4a and substantially free along the length of post 7 to convey elongation of bore 4 to load indicator 1.

With continued reference to FIG. 3, a second lever 16 is disposed within bore 4 and is moveable about second pivot 18. A first end 20 of second lever 16 is responsive to movement of second end 14 of lever 8 causing corresponding movement of a second end 22 of second lever 16. First end 20 may be configured to engage lever 8 in both directions. Alternatively, second lever 16 may be biased in one direction and moveable in a second direction in response to movement of lever 8.

Second lever 16 may be sized and configured to provide any desired degree of response to movement of lever 1. For example, second pivot 18 may be positioned at the midpoint of second lever 16 or may be shifted towards one end to further amplify the response of second lever 16. For example, both levers 8 and 16 may produce amplified responses, with the sum response being significantly greater than that provided by a single lever of comparable length. Second lever 16 is depicted with second pivot 18 positioned adjacent first end 20 of second lever 16 to provide an amplified response at second end 22 of second lever 16. Bore section 4c is sized to accommodate the amplified response movement of second lever 16.

Comparison of FIGS. 2 and 3 demonstrates advantage provided by aspects of the present invention with in-fastener mounting regarding the depth of bore section 4c. By limiting the range of movement of lever 8 to the smaller diameter of bore section 4b and shifting the broader amplified response to second lever 16, the degree of clearance required for lever 8 is significantly reduced and the depth of section 4c is likewise significantly reduced. Additional advantages afforded by aspects of the present invention include increased sensitivity of load indicator 1 , a reduction in the overall length of load indicator 1 and in the depth of bore 4. Further advantages include increased manufacturing efficiency, less loss of fastener material to bore 4, and compatibility with smaller or more conventional fasteners. A multi-lever configuration provides similar advantages in separable indicators. For example, the indicator housing or cartridge may be made more compact for use in tighter areas or with smaller fasteners while allowing for use of wrenches or other tools to tighten the fastener. With reference to FIG. 4, a schematic diagram of a cut-away view of the load indicator of FIG. 3 is shown with levers 8 and 16 in a second position corresponding to maximum elongation of the fastener as well as to the default position of levers 8 and 16 prior to insertion into bore 4. During installation and calibration of load indicator 1 in bore 4 of a fastener, datum rod 6 is first installed in bore section 4a and load indicator 1 is then press-fitted or threaded into bore 4 or is otherwise secured to the fastener. Additional methods of attaching load indicator 1 to a fastener are described with reference to FIGS. 5-6 and 10-12.

Load indicator 1 is advanced within bore 4 until first end 12 of lever 8 contacts datum rod 6. Load indicator 1 is further advanced until the second end 22 of second lever 16 indicates zero load on a visual scale associated with second lever 16. It is understood that any number of load indicator components may be suitably independently or jointly installed and adjusted to achieve calibration of load indicator 1. As datum rod 6 moves due to elongation of the fastener, second end 22 of second lever 16 travels along the visual scale to indicate the corresponding elongation or loading conditions.

Calibration of load indicator 1 may be performed by loading an associated fastener in a hydraulic testing fixture to apply a predetermined load while monitoring the response of load indicator 1. Levers 8 and 16 may be replaced or modified as needed to obtain proper calibration of load indicator 1. For example, lever 8 may be replaced with a lever having a slightly different amplification ratio to achieve a desired amplified response. Once load indicator 1 exhibits the desired amplified response, it may be locked within bore 4 to maintain proper calibration.

With reference to FIG. 5A, second end 22 of second lever 16 is moveable along a visual scale 26 within a bezel 28. Visual scale 26 may be viewed from top of the fastener through a protective transparent lens secured within bezel 28. Visual scale 26 includes percentage markings indicating a range of proof loads. Any suitable indication may be used with visual scale 26, including percentages, fractions, letters, numbers, colors, and the like. A "0%" marking indicates some minimum load. The location of the "100%" designation corresponds to a predetermined maximum acceptable load. Load indicator 1 may be calibrated to about "0%," indicating that no clamp load forces are present when installed in an unloaded fastener. Calibration may be suitably accomplished by varying the position of load indicator 1 within the fastener or the configuration of any of the components within load indicator 1. Incremental markings on visual scale 26 suitably allow an indication of intermediate fastener loads. Alternatively, a separable load indicator 1 may be calibrated during temporary mounting on a test fastener.

As previously discussed, upon tightening of a fastener 2 associated with load indicator 1 , fastener 2 experiences tensile loading and elongates according to Hooke's Law. Elongation of fastener 2 results in movement of datum rod 6 causing the amplified response of levers 8 and 16 thereby moving second end 22 towards the "100%" designation on visual scale 26. Upon loosening of fastener 2, fastener 2 experiences a reduction in tensile loading and shortens in length. The shortening of fastener 2 increases the pressure against lever 8 moving second end 22 towards the "0%" designation on visual scale 26.

In an alternative embodiment, load indicator 1 may include or be attached to an electronic circuit or other mechanism for initiating auto shut-off of a pneumatic tightening tool or other powered tool to prevent over-tightening. Load indicator 1 can also be attached electronically to a remote reader for automated monitoring of the clamp load status of a large number of fasteners 2.

Bezel 28 can further include a series of fastener openings 32 and 34. Openings 32 and 34 may be threaded to retain fasteners therein or may simply permit installation of a fastener to be threaded into the head of fastener 2 or the end of an indicator housing. For example, openings 32 permit installation of fasteners configured to secure bezel 28 to fastener 2 using threaded holes in the head of fastener 2. Openings 34 are threaded and retain set screws or locking fasteners for locking bezel 28 into position with respect to fastener 2 or other indicator housing. For example, fasteners in openings 32 may be tightened until load indicator 1 provides a zero load reading and may then be locked in this calibrated position by tightening fasteners in openings 34, thereby locking bezel 28 in the calibrated position. With reference now to FIGS. 6A-6B, a schematic diagram of a cut-away view of the load indicator 1 of FIG. 5 is shown installed within fastener 2. In one implementation, load indicator 1 includes cartridge 36 configured for insertion into bore 4. Cartridge 36 retains pivots 10 and 18 securing levers 8 and 16 within bore 4. Cartridge 36 may further retain a spring for biasing one of levers 8 or 16 in a default position. Association of levers 8 and 16 and pivots 10 and 18 with cartridge 36 facilitates easy installation and removal of load indicator 1 from bore 4. Cartridge 36 is connected at its upper end to bezel 28.

Bezel 28 may be mounted in a recess in the head of fastener 2 as shown in FIG. 6A or on top of the head of fastener 2 as shown in FIG. 6B. In certain applications involving limited space requirements or high-impact conditions, it may be advantageous for bezel 28 to be located flush with or below the top surface of the head of fastener 2. Alternatively, as described below, bezel 28 may be attached to an indicator housing to secure a bi-directional viewing window.

Load indicator 1 may further include a housing seal 30 and wave washer 44 between bezel 28 and fastener 2. Housing seal 30 serves to exclude water, dirt, and other contaminants from bore 4 and load indicator 1. Housing seal 30 may be omitted, for example, where load indicator is sealingly press-fitted into bore 4 or otherwise sealed within an indicator housing. Wave washer 44 serves to maintain resistance against the fasteners connecting bezel 28 to fastener 2. Bezel 28 may further retain a transparent lens 40 and lens seal 42. Lens 40 need not be flat, but may be concave, convex, curved, angled or otherwise shaped to provide viewing of visual scale 26. In various alternative embodiments, bezel 28, cartridge 36, or other components of load indicator 1 may serves to attach load indicator 1 to fastener 2. For example, bezel 28 or cartridge 36 may be press fitted into bore 4. Alternatively, cartridge 36 may be an enclosed housing as described below that is temporarily attached to fastener 2 to obtain a load measurement.

With reference now to FIG. 7, a fastener 2 is configured for use with a separable tensile load indicator shown in FIGS. 8-9. Fastener 2 defines a central bore 4 extending from the head 3 of fastener 2 into the shank of the fastener. Datum rod 6 is anchored at a lower end 6a within bore 4. Head 3 of fastener 2 further defines a male coupler 5 for use as a reference surface and/or for use in indexing and temporarily mounting a load indicator on fastener 2. Top reference surface 5a of male coupler 5 and top reference surface 6b of datum rod 6 are coplanar when fastener 2 is in a non-loaded state. As fastener 2 is tightened, bore 4 is elongated drawing datum rod top surface 6b downward away from top surface 5a of male coupler 5. In other implementations the datum rod and reference surface are not necessarily coplanar and another predetermined unloaded relative positioning of the free end of the datum rod and the reference surface can be used.

With reference now to FIG. 8, a separable load indicator 80 is configured to be detachably mountable to fastener 2 of FIG. 7. Load indicator 80 includes a sealed housing 86. Visual scale 26 is visible through lens 40 secured by bezel 28 atop housing 86. Load indicator 80 is mounted atop fastener 2 by a female coupler 82 configured to connect to male coupler 5 of fastener 2.

Female coupler 82 may be configured as a socket to couple with male coupler 5. Female coupler 82 is shown in FIG. 9 as an annular rim constructed and arranged to engage male coupler 5 on fastener 2. It should be appreciated that male coupler 5 and female coupler 82 may be constructed and arranged in any manner suitable to properly index indicator 80 with fastener 2 for accurate readings.

For example, fastener 2 may include any number of alignment or attachment features to ensure proper alignment and attachment during use. For example, a key and slot combination feature may ensure proper alignment, while a snap fit indexing feature may ensure full indexing engagement and attachment prior to reading of load indicator 80.

FIGS. 9-10 show sectional views of the tensile load indicator 80 of FIG. 8 taken along line A-A while FIG. 10 shows female coupler 82 receiving male coupler 5 to position probe 12 in abutting engagement with second free end 6b of datum rod 6.

Housing 86 houses first lever 16 moveably mounted at pivot 8 and responsive to movement of lever 8 attached at pivot 10. In this regard, housing 86 is similar to cartridge 36 described earlier. A spring 84 biases lever 8 or lever 16 towards one end of visual scale 26. For example, second end 22 of lever 16, which extends over visual scale 26 as a moveable pointer, may be biased towards the 100% load end of visual scale 26.

First end 12 of lever 8 serves as a probe to contact top surface 6b of datum rod 6, causing lever 8 and interoperable lever 16 to move second end 22 of lever 16 down visual scale 26 to the appropriate corresponding load indication. Visual scale 26 may be positioned on an incline below a multi-faceted lens 40 so as to be visible both from the top and the side of housing 86. Lens 40 may be configured to provide magnification of visual scale 26.

Alternatively, first end 12 of lever 8 may rest in a default retracted position until extended into a reading position by a load indicator operator. Similarly, first end 12 of lever 8 may be associated with any number of additional probe elements, such as a ball bearing interposed in a journal between first end 12 and upper surface 6b during reading.

Thus, load indicator 80 may be used during installation or routine maintenance of fasteners to measure the tensile load or clamp force of a fastener. Use of a dual lever embodiment provides for a reduced indicator diameter and height for use in tight areas, or for ease of use, transportation, storage and manufacture. The angled lens 40 and inclined visual scale 26 enable observation of indicator readings from various angles.

With reference to FIG. 11 , a load indicator 100 is shown coupled to a fastener 102. Load indicator 100 includes a gauge housing 104 and a displacement gauge 106 disposed within housing 104. Gauge housing 104 includes a female coupler 108 at one end for coupling to a complementary male coupler formed on the head of fastener 102.

With reference to FIG. 12, load indicator 100 is indexed and removably coupled to fastener 102 via a male coupler 110 defined on fastener 102. Displacement gauge 106 is disposed within housing 104 and includes a probe 114 disposed within female coupler 108. A plurality of indexing projections 122 are spaced about an interior of female coupler 108 and are moveable to engage male coupler 110 to index probe 114 relative to fastener 102. A datum rod 112 is anchored at a first end within a central bore 113 defined in fastener 102 such that elongation of fastener 102 causes displacement of a second free end of datum rod 112 relative to male coupler 110. Displacement of datum rod 112 is measured by probe 114 to provide an indication of fastener loading.

Female coupler 108 includes a piston 118 bearing probe 114. Piston 118 is biased towards an unlocked or decoupled position via spring 128. Spring 128 may be a coil spring concentric to piston 118 and acting between housing 104 and a flange or other feature formed on piston 118.

A sleeve 120 is positioned concentric to piston 118 and bears a plurality of indexing projections 122. Indexing projections are shown as ball bearings retained in a journal formed radially through sleeve 120. A locking ring 124 is positioned concentric to sleeve 120 and is biased by coil spring 126 towards a locked position in which locking ring 124 maintains indexing projections 122 in engagement with indexing contour 116 on male coupler 110. Piston 118 is retractable within sleeve 120 via insertion of male coupler 110. Indexing projections 122 are inwardly extendable in response to movement of locking ring 124 into the locked position to thereby engage indexing contour 116 on male coupler 110 and securely index probe 114 relative to fastener 102.

Spring loading of piston 118 and locking ring 124 provide for snap-fit indexing and coupling of loading indicator 100 to fastener 102 by sliding female coupler 108 over male coupler 110. For example, female coupler 108 is pressed onto male coupler 110 until an end face of piston 118, which forms a seat portion of female coupler 108, contacts and end face of male coupler 110. Continued movement of female coupler 108 over male coupler 110 compresses spring 128 as male coupler 110 displaces piston 118 within sleeve 120. As the indexing contour 116 on male coupler 110 aligns with indexing projections 122, indexing projections 122 are urged into engagement with indexing contour 116 by spring loaded locking ring 124.

Indexing projections 122 are shown as ball locks, e.g., ball bearings formed of stainless steel. In alternative implementations, indexing projections may include non-spherical elements such as spring tabs depending from sleeve 120 or other feature suitable to engage an indexing contour 116 formed on male coupler 110. Indexing contour 116 is shown as a circumferential groove or annular recess defined on a lateral surface of male coupler 110. Indexing contour 116 allows indexing projections 122 to engage male coupler 110 at any rotational orientation of load indicator 100 atop fastener 102. The semi-circular profile of and circumferential continuity of indexing contour 116 provides a self-centering alignment aspect of indexing of load indicator 100 atop fastener 102.

"Indexing" as used herein refers to precise positioning of probe 114 relative to a fixed reference on fastener 102, e.g., an end face of male coupler 110. This precise positioning can include both axial and radial alignment and positioning components. For example, probe 112 can be indexed to fastener 102 via indexing projections 122 to both position the probe 112 axially proximate or in abutment with datum rod 112 and radially centered over datum rod 112. Accordingly, indexing contour 116 on male coupler 110 and indexing projections 122 on load indicator 100 cooperate to index probe 114 relative to a fixed reference surface of fastener 102, such as an end face of male coupler 110. Displacement of a free end of datum rod 112 relative to the end face of male coupler 110 or other fixed reference surface can provide an indication of fastener elongation and thereby loading in fastener 102.

In some implementations, the end face of male coupler 110 or other fixed reference surface can be used in combination with indexing projections 122 and indexing contour 116 to securely index probe 114. Thus, in some cases, indexing projections 122 may be the primary or sole means of indexing probe 114, while in some cases, additional coupler features may additionally contribute to indexing of probe 114.

In some implementations, displacement gauge 106 is moveable within housing 104 with movement of piston 118. Movement of probe 114 relative to piston 118 and the connected displacement gauge 106 provides an indication of displacement of datum rod 112 and elongation of fastener 102. In some cases, probe 114 includes at its tip a ball bearing to provide a point contact with the free end of datum rod 112. The ball bearing is axially movable within an axial journal to act upon a push rod causing a dial of a dial gauge to traverse a dial gauge scale visible through an opening defined in housing 104. It will be appreciated that a common dial gauge or any other suitable mechanical, electro-mechanical, or electrical gauge or probe may be used in conjunction with the described indexing features to obtain a suitably precise measurement of elongation and lending.

Housing 104, piston 118, sleeve 120 and locking ring 124 may be machined, molded or otherwise formed of aluminum, steel, brass, or other suitably durable and rigid material, e.g., plastic or composite material. Locking ring 124 can include a taper, knurling, or other feature to facilitate retraction of locking ring 124 to decouple load indicator 100 from fastener 102. Retraction of locking ring 124 moves a locking portion of locking ring 124 out of locking engagement with indexing projections 122. Upon retraction of locking ring 124, indexing projections may be retracted from engagement with indexing contour 116 of male coupler 110 and load indicator 100 may be decoupled from fastener 102.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, spring action of the locking ring may be replaced by manual action. For example, both a visual indication and an electrical signal can be provided in response to probe measurement. Accordingly, other embodiments are within the scope of the following claims.