Abstract:
A variably amplified load indicating fastener includes a fastener ( 100 ) detachably secured to a load indicator via a base member. The fastener ( 100 ) has an abutment which is moved by a change in length under an applied load. The load indicator includes a dial plate ( 16 ), a needle ( 24 ) pivotally supported by the plate ( 16 ), a compensating slot ( 36 ), a driving slot, an elongated U-shaped element ( 50 ) disposed in the direction in which the load is applied when the fastener ( 100 ) is in use, and a first reference mark ( 40   a ). One side of the U is rigidly attached to the fastener. The closed end of the U is proximate to and movable by the abutment. The other side of the U extends through the compensating slot ( 36 ) and the driving slot ( 26 ). The needle ( 24 ) is operatively coupled to the outer end of said second side. A sleeve with a second reference mark fits over the outer circumference of both the load indicator and the base member.

Description:
PRIORITY 
   The present application claims priority to provisional application No. 60/191,217 filed on Mar. 22, 2000. 

   BACKGROUND 
   The present invention relates generally to devices and methods of measuring and indicating applied bolt loads, and, more particularly, to a mechanically actuated bolt load indicator having variable, adjustable indicator amplification which provides easy calibration independent of a fastener and more accurate bolt load measurement and indication thereof. 
   Mechanical bolt load indicating devices are used where it is necessary to continuously monitor the tensile strain that a bolt or other fastener is subjected to while in use. Such load indicating apparatuses are typically installed into a recessed portion of a bolt head such that a mechanical load indicator element is caused to move relative to the amount the bolt stretches or elongates as a load is applied thereto. A dial plate having a reference scale is used to indicate to the user the amount of load to which the bolt is being subjected when it is initially tightened and thereafter as the load changes over time. 
   Prior art examples of mechanical bolt load indicating apparatuses include Maxbolt™ and Betabolt™ load-indicators. The structure and operation of the Maxbolt™ and Betabolt™ load indicators are described in detail, respectively, in U.S. Pat. Nos. 5,668,323 and 5,584,627 the disclosures of which are incorporated herein by reference in their entirety. 
     FIGS. 1 and 2  show a prior art mechanically actuated bolt load indicator and fastener  70  composed of a body  72  having a shank  74  and an abutment  60 A formed at the end of a gauge pin  60 , which is moved by the changing length of the body  72  under an applied load, and a load sensing member  11 . The body is drilled with a blind bore  76 . The fastener may be used, for example, as shown to fasten two flat members  80  together using the nut  78 . The load sensing member  11  is composed of an element  51  disposed in the direction in which the load is applied when the fastener is in use. The element  51  is of elongated U-shape, a first limb  51  C of the U rigidly attached to the body  72 , a closed end  51  A of the U bearing on the abutment  60 A so as to move with the abutment  60 A, and a second limb  51  B of the U having registering the applied load  5  on a scale  17  (FIG.  2 ).  FIG. 18  describes a process of making this fastener. 
   Indicator needles in conventional mechanical load indicators, such as that of  FIGS. 1 and 2 , provide a fixed and limited amount of amplified movement relative to the amount that the bolt stretches when subjected to a load. Therefore, the sweep distance of the indicator needle from a zero reference to a maximum reference is relatively small thereby decreasing the overall precision with which the device can be read because the dial plate is relatively small with few well defined bolt load references. For example, a typical bolt load that causes a 0.004″ elongation of a bolt produces a mere 0.125″ total sweep distance of the indicator needle from a zero reference to a maximum reference. 
   Another problem that exists with conventional bolt load indicators is that their relatively small dial plates may be difficult to see and read correctly due to the parallax effect. Adverse environmental conditions such as poor lighting, moisture, debris or the location of the bolt in which the load indicator is installed may make it even more difficult to read the indicator correctly. 
   A still further problem with conventional bolt load indicators is that conventional indicator needles travel or move in a generally linear direction across the dial plate. Therefore, the maximum needle moment amplification that is obtainable is limited to the diameter of the dial plate which in turn is limited by the size of the bolt head itself 
   A still further problem that exists with conventional bolt load indicators is that they can not be universally calibrated during the assembly process. 
   SUMMARY 
   Accordingly, embodiments of the present invention provide a mechanical bolt load indicator apparatus and method having a radially adjustable and dynamically variable moment arm that amplifies the indicator needle&#39;s non-linear circumferencial travel. The present invention also provides a dial plate with greater surface area to increase overall precision. The mechanical bolt load indicator apparatus is simply calibrated and set in the factory during manufacture and may be used universally with any fastener in the field adapted to receive such a load indicator, including small diameter bolts with small bolt heads. The load indicator may be made from a material having the same coefficient of thermal expansion as the fastener. 
   One aspect of the invention is a variable, adjustable amplification load indicator for fasteners. This embodiment includes a main body having a central bore and a U-shaped spring wire having an attachment end secured to the main body, a pivot point, and a drive end extending through the central bore of the main body. The spring wire pivot point is mechanically responsive to the extension of the fastener bolt in proportion to the amount of load applied to the fastener bolt to cause correlative movement of the drive end of the spring wire. This embodiment also includes a plate having a compensating slot, and a needle pivotally mounted to a dial plate and having a drive slot. The drive end of the spring wire extends through the compensating slot and the drive slot in order to angularly drive the needle about its pivot pin. The distance between the pivot pin and the spring wire drive end is adjustable to amplify the needle movement for a given fastener extension and spring wire drive end movement. 
   In this aspect of the invention the adjustable distance is effected by the movement of the dial plate carrying the needle pivot pin. The distance between the pivot pin and the spring wire drive end may be increased to reduce the amount of angular needle rotation for a given fastener extension. The distance between the pivot pin and the spring wire drive end is shortened to increase the amount of angular needle rotation for a given fastener extension. 
   Another embodiment of this invention includes a base member having a central bore mounted in a recessed opening in the fastener, the main body of the load indicator removably secured in the base member, and a reference sleeve surrounding the base member and main body. The reference sleeve and main body have alignable reference marks, one fixed relative to the base member, the other fixed relative to the main body. In this way the main body may be removed from a fastener under load and re-installed or replaced and be repositioned at its position at the time of removal and continue to indicate the correct applied load without further calibration. 
   The advantages of the present invention include:
         1. Greater needle movement for the same amount of spring wire movement enables a mechanically actuated load indicator to fit into small diameter fastener bolts.   2. Increased accuracy in indicating applied loads.   3. Simplicity of calibration.   4. Factory calibration during manufacture that fits any unit in the field.   5. Removable in the field from a fastener under load for maintenance, repair, or replacement, and reinstallable to continue indicating applied load accurately without further calibration.   6. Easily readable and visible.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevation view in partial section of a prior art mechanically actuated bolt load indicator and fastener. 
       FIG. 2  is a top plan view of FIG.  1 . 
       FIG. 3  is a perspective view of the mechanically actuated bolt load indicator assembly of the present invention. 
       FIG. 4  is a perspective view of the reference sleeve  54  of the present invention. 
       FIG. 5  is a perspective view of the base member  90  of the present invention. 
       FIG. 6  is a perspective view of the gauge pin  60  of the present invention. 
       FIG. 7  is an exploded perspective view of the mechanically actuated bolt load indicator assembly of the present invention. 
       FIG. 8  is a front elevation view in partial section of FIG.  3 . 
       FIG. 9  is an enlarged top plan view of FIG.  3 . 
       FIG. 10  is a top plan view of the main body  40  of FIG.  9 . 
       FIG. 11  is a top plan view of the compensating plate  34  of FIG.  9 . 
       FIG. 12  is a top plan view of the spacer plate  30  of FIG.  9 . 
       FIG. 13  is a top plan view of the indicator needle  24  of FIG.  9 . 
       FIG. 14  is a top plan view of the dial plate  16  of FIG.  9 . 
       FIG. 15  is a top plan view of FIG.  4 . 
       FIG. 16  is a flow chart of the process of assembling the mechanical load indicator apparatus of  FIGS. 3-17  of the present invention. 
       FIG. 17  is a partially exploded front elevational view in partial section of the improved Betabolt™ mechanical load indicator apparatus of  FIGS. 3-17  installed in a fastener including a pre-drilled fastener head  72  and fastener shank  74  of the type seen in FIG.  1 . 
       FIG. 18  is a flow chart of the prior art process of making a bolt for use with a load indicator apparatus of the present invention. 
       FIG. 19  is a flow chart of the process of installing the load indicator assembly  10  of reference sleeve of the present invention into a fastener of the type described in  FIG. 18  to form a fastener assembly of the present invention. 
       FIG. 20  is a flow chart of the process of removing the load indicator assembly  10  of the present invention from the fastener assembly of the present invention described in FIG.  19 . 
       FIG. 21  is a flow chart of the process of re-installing the load indicator assembly  10  of the present invention into the fastener assembly of the present invention described in FIG.  19 . 
       FIG. 22  is a front elevation view in partial section of the mechanical load indicator apparatus of  FIGS. 3-17  installed in a fastener including a pre-drilled fastener head  72  and fastener shank  74  of the type seen in  FIG. 1  to form a fastener assembly of the present invention. 
       FIG. 23  is a top plan view of FIG.  22 . 
       FIG. 24  is a view similar to  FIG. 22  showing a 0% load applied to the fastener of the present invention. 
       FIG. 24A  is a top plan view of FIG.  24 . 
       FIG. 25  is a view similar to  FIG. 22  showing a 50% load applied to the fastener of the present invention. 
       FIG. 25A  is a top plan view of FIG.  25 . 
       FIG. 26  is a view similar to  FIG. 22  showing a 100% load applied to the fastener of the present invention. 
       FIG. 26A  is a top plan view of FIG.  26 . 
       FIG. 27  is a flow chart of the process of calibrating the load indicator assembly  10  of the present invention in a fastener assembly of the present invention. 
       FIG. 28  is a top plan view similar to  FIG. 9  showing the dial plate  16  in its fully raised (top) position providing the smallest distance E between the wire drive end  50 B and pivot pin  29 , the position of maximum amplification of needle  24  movement. 
       FIG. 29  is a top plan view similar to  FIG. 9  showing the dial plate  16  in its middle position providing the median distance E between the wire drive end  50 B and pivot pin  29 , the position of median amplification of needle  24  movement. 
       FIG. 30  is a top plan view similar to  FIG. 9  showing the dial plate  16  in its lowered (bottom) position providing the maximum distance E between the wire drive end  50 B and pivot pin  29 , the position of smallest amplification of needle  24  movement. 
       FIG. 31  is a table showing the angular rotation of the needle in its top, middle and bottom position as shown schematically in  FIGS. 28-30 . 
   

   DETAILED DESCRIPTION 
   a. Method of Making the Fastener Bolt 
   As shown in  FIG. 17  the load indicating fastener assembly  110  of the present invention includes a fastener  100 . The fastener  100  includes a bolt  70  having a forged metal, for example steel, body which includes a hexagonal head  72  and a shank  74  which has an externally screw-threaded portion  73  spaced from the head  72  by a plain cylindrical portion  75 . Drilled into the head  72  and shank  74  coaxially with its rotational axis, is a blind bore  76  of predetermined diameter and depth for receiving a clearance gauge pin  60 . The blind bore  76  extends from the top of the head  72 , through the head  72  and into the shank  74  for approximately half the length of the plain cylindrical portion  75 . The outer end of the bore  76  is counter bored to a predetermined diameter and depth to form an enlarged recessed opening  71  which is threaded to receive a base member  90  which is adapted to receive a load indicator assembly  10  of the present invention. Blind bore  76  is further drilled to a smaller predetermined diameter and depth to provide a bore  76 A for receiving by press fit therein gauge pin  60 . Base member  90  is press fit into counter bore  71  in the fastener head  72  and has a threaded bore for receiving therein securing threads  46  of the main body  40  of the load indicator assembly  10  of the present invention, as more fully described below. The main body  40  may be made out of metal or a non-metal. 
   A reference sleeve  54  slips over the outer circumference of both base member  90  and main body  40  and is secured in position resting atop the fastener head  72  by a screw  58 A which contacts main body  40  and by a screw  58 B which contacts base member  90 , as more fully described below. Reference sleeve  54  carries a reference mark  54 A ( FIG. 4 ) on its outer top rim as more fully described below. 
   b. The Process of Assembling the Variable Amplification Load Indicator Apparatus 
   As seen in  FIGS. 3-15  and described in  FIG. 16 , the variable amplification bolt load indicator of the present invention has the following features and is assembled according to the following process: 
   A spring wire  50  is secured to a spring wire support member  48  by crimping tabs  48 A or other suitable securing means around attachment end  50 C of spring wire  50 . Support member  48  is then secured to the bottom end of main body  40  by inserting the driving end  50 B of spring wire  50  into and through the central bore  42  of main body  40  and then securing support member  48  to main body  40  using screws  52  or other suitable securing means. (FIG.  8 ). If main body  40  and support member  48  are both metal, they may also be secured together, for example by spot welding, crimping or the like. Components may also be made out of plastic or other material to suit the environment. 
   A compensating plate  34  is then positioned into the upper recess  41  of the main body  40  with the driving end SOB of spring wire  50  received in and extending through a compensating slot  36  with mounting holes  38  aligned with threaded holes  44  in upper recess  41 . Compensating slot  36  is a cammed slot which is programmed to give a needle indicator  24 , which is coupled to driving end SOB in a manner described below, a linear travel when converting the wire  50  straight line motion to needle indicator  24  rotary motion. 
   A spacer plate  30  is then positioned atop compensating plate  34  with its mounting holes  32  aligned with holes  38  and  44 . Meanwhile indicator needle  24  is sub-assembled by being pivotally mounted to the underside of a dial plate  16 , having a window  22  therethrough, on a pivot pin  29 . Pivot pin  29  is held in an opening  20  through dial plate, and extends into and is held in an opening  28  through indicator needle  24 . Then dial plate  16  with needle  24  pivotally mounted thereon is positioned atop spacer plate  30  with the driving end SOB of spring wire  50  received in and extending through a driving slot  26  of needle  24  and with its adjustment slots  18  aligned with holes  32 , 38 , and  44 . The above components are then secured to the main body  40  by screws  14  extending through slots  18  and holes  32  and  38  and screwing into threaded holes  44 . The assembly is completed by press fitting a lens  12  into upper recess  41  of main body  40  after the calibration process is completed as more fully described below. 
   c. The Process of Installing the Load Indicator Apparatus in the Fastener Bolt 
   Referring to  FIG. 19 , the process of installing the pre-calibrated variable amplification load indicator assembly  10  into a fastener bolt  70  of a fastener  100  includes the following steps:
         1. The pre-calibrated load indicator  10  is threaded into base member  90  of fastener bolt  70  until the indicator needle indicates zero on the dial plate  16 .   2. Reference sleeve  54  is then placed over main body  40  and base member  90  in a rest position atop fastener head  72 . The reference sleeve  54  is then rotated until a sleeve reference mark  54 A is aligned with a main body reference mark  40 A ( FIG. 3 ) indicating the reference position for zero load on the load indicator dial.   3. The reference sleeve  54  is then locked in this position by applying adhesive to the screw  58 B and threading it into threaded hole  56 B ( FIG. 4 ) until the screw tightly and securely holds the reference sleeve  54  in contact with the exposed surface portion of base member  90 .   4. The load indicator  10  is then locked in its referenced position by threading screw  58 A (without adhesive) into threaded hole  56 A of reference sleeve  54  until the screw tightly and securely holds the reference sleeve  54  in contact with the exposed surface portion of load indicator  10 . A small portion of the top end circumferential wall of the main body  40  extends beyond the top end of reference sleeve  54  when the main body  40  is fully assembled inside of reference sleeve  54 . (FIG.  22 ). This extended lip functions as a gripping surface for a wrench or pliers for installing and removing the load indicator  10 .       

   As more fully described below, the above structure allows the variable amplification load indicator  10  to be removed for routine maintenance and repair and then re-installed in its former position. When the reference mark  40 A on the load indicator  10  is aligned with the reference sleeve mark  54 A the device can immediately accurately indicate the actual load on the fastener without recalibration. 
   d. The Process of Removing the Load Indicator From a Load Bearing Fastener 
   Referring to  FIG. 20 , the process of removing the load indicator from a load bearing fastener bolt  100  of a fastener  110  is accomplished by the following steps:
         1. Unscrew upper set screw  58 A from holding contact with main body  90 .   2. Use an insertion/extraction tool to engage gripping slots  40 B or other suitable gripping portion located at or near the end portion of the main body  40  proximate to the top of sleeve  54 .       

   Unscrew the load indicator  10  from the base member  90  and remove it from reference sleeve  54 . 
   e. The Process of Re-installing the Load Indicator into a Load Bearing Fastener 
   Referring to  FIG. 21 , the process of re-installing the load indicator  10  into a load bearing fastener bolt  70  of a fastener  100  is accomplished by the following steps:
         1. Insert load indicator  10  through the reference sleeve  54  and threadingly secure main body  40  into the base member  90  until the load indicator needle  24  begins to show movement towards zero on the dial plate  16 . This movement indicates the spring wire pivot point  50 A ( FIG. 3 ) has contacted the top  60 A of gauge pin  60 .   2. Continue threadingly securing main body  40  into base member  90  until main body reference mark  40 A is aligned with sleeve reference mark  54 A indicating the main body is aligned with the position of zero % load from whence it started. The main body will correctly and accurately indicate the load that is on the fastener  110 .   3. Screw upper set screw  58 A into holding contact with main body  90  to lock the main body in position.
 
f. The Process of Calibrating the Variable Amplification Load Indicator
       

   As seen in  FIGS. 22-26A , and  28 - 30 , and as described in  FIG. 27 , the process of calibrating the variable amplification load indicator will now be described.  FIGS. 24-26A  show a series of cross-sections of the variable amplification load indicator of the present invention showing the range of wire movement for 0 to 100% rotation of the needle. In  FIGS. 24 and 24A  the wire position when the load indicator is inserted into the fastener is “zeroed-out” indicating no load. Note that in this position, spring wire pivot point  50 A is pushed upwardly by the raised position of the gauge pin  60  forcing the spring wire indicator driving end SOB to deflect to the left. 
   In  FIGS. 25 and 25A  the wire position when the load indicator is inserted into the fastener indicates 50% load. Note that in this position, spring wire pivot point  50 A experiences less upward push by the now partially extended (lowered) position of the gauge pin  60  forcing the spring wire indicator driving end  50 B to deflect to the right of its no load position. 
   In FIGS.  26  and  26 A_the wire position when the load indicator is inserted into the fastener indicates 100% load. Note that in this position, spring wire pivot point  50 A experiences much less upward push by the now fully extended (lowered) position of the gauge pin  60  allowing the spring wire indicator driving end  50 B to deflect to its extreme right position relative to its no load position. Thus, elongation of the fastener results in the relaxing of the wire and the driving of the needle through its captured position in compensating slot  36  and indicator needle driving slot  26  to indicate a percentage of maximum load. 
   The variable amplification load indicator of the present invention is designed so that a predetermined elongation of the fastener drives the wire a predetermined distance thereby also rotating the needle through a predetermined angle. In the preferred embodiment, 0.004″ fastener bolt elongation causes 0.125″ wire drive end movement. 
   However, the angular rotation of the needle caused by the spring wire drive end wire movement is adjustable by vertically moving the dial plate  16  to which the needle  24  is pivotally attached. As the dial plate  16  is moved the distance between the needle pivot pin  29  and the drive end  50 B changes shortening or lengthening the moment arm E (FIGS.  28 - 30 ). The change in the moment arm E causes a corresponding increase or decrease of angular travel of the needle  24 . As seen in  FIGS. 28-30 , the following values are displayed for a 0.004″ fastener bolt elongation and a 0.125″ wire end movement:
         A−B=the distance the wire travels.   C−D=the range of needle position.   E=the distance between the wire and the pivot pin.   F=the distance the face plate is raised.   G=the distance the face plate is lowered.   H=the reference line.   I=the needle span in degrees is 74.6° (raised dial), 64.0° (middle dial), 55.5° (lowered dial).       

   As described in  FIG. 27 , the process of calibrating the variable amplification load indicator  10  of the present invention is accomplished by the following steps:
         1. Load indicator  10  is threadingly inserted into base member  90  of fastener  110 . Spring wire  50  and pivot point  50 A are received in bore  76  of fastener bolt  70  to contact gauge pin  60 . As the load indicator  10  is threadingly inserted into the base member  90  the spring wire drive end  50 B deflects and rotates needle  24  until the needle  24  indicates zero on the dial plate  16 .   2. Apply a known load (i.e., 50%, 75%, or 100%) to the fastener bolt  70  and observe the reading indicated by the needle  24 .   3. If the needle indicates within ±6% of the known load being applied to the fastener  70 , the load indicator is properly calibrated and the process ends.   4. If the needle indicates more than 6% greater than the known load being applied to the fastener  70 , apply a correction to reduce the needle travel by lowering the dial plate  16  thereby lengthening the moment arm E and the process ends.   5. If the needle indicates more than 6% less than the known load being applied to the fastener  70 , apply a correction to increase the needle travel by raising the dial plate  16  shortening the moment arm E and the process ends.       

   This new calibration adjustment feature of the present invention is required to compensate for uncontrollable variations inherent in the manufacture of the indicator components. 
   g. Operation of the Spring Needle to Indicate Actual Load Applied to the Fastener Bolt 
   The spring wire  50 , gauge pin  60 , and needle  24  cooperate to indicate applied load as follows: 
   The principle described above in which the spring wire drive end  50 B drives the needle  24  in the drive slot  26  resulting in angular rotation of the needle  24  and an amplification of angular rotation at the end of needle  24 , represents a significant improvement over the prior art. This amplification of lateral motion enables a smaller hole to be drilled in the fastener head which allows the load indicator assembly  10  to be applied to smaller size fasteners. 
   Spring wire  50  responds to loads up to the proof load applied to the shank  74  when the fastener is in use. For this to be achieved the spring wire  50  is set in the bore  76  such that the pivot point  50 A bears on the top of gauge pin  60 . The drive end  50 B is under a compressive load which, in the unloaded condition of the fastener, causes that spring wire attachment end to bow. The bowing of the spring wire causes the spring wire drive end  50 B to deflect towards the attachment end by a distance corresponding to the extension of the shank under the proof load all according to Young&#39;s Modulus. 
   At no load then with the bowing of the attachment end  50 C and the deflecting of the drive end  50 B toward the attachment end  50 C, the spring wire drive end  50 B starts out at the zero end of the scale. As load is applied to the fastener bolt  70  in use and the shank extends under the load, the force of the gauge pin against the spring wire pivot point  50 A is gradually reduced. Thus the attachment end  50 C progressively returns towards its normal straight form and the drive end  50 B moves away from the attachment end  50 C. When the proof load is reached and the bearing force of the gauge pin is zero. At this point the attachment end  50 C end has fully straightened, the drive end  50 B has moved its full extent away from the attachment end limb and the indicator is at or near the opposite end of the compensation slot from which it began. For a more detailed description, the reader is referred to U.S. Pat. No. 5,584,627. 
   The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in the light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.