Abstract:
Low cost transducer bonding tooling and techniques are provided which can be used to duplicate the quality of factory manufactured load indicating members, and to allow transducers to be attached to fasteners irrespective of their orientation. The bonding tooling and techniques can also be used to bond other small objects requiring precise placement and reliable bonding such as, for example, the bonding of identification labels to manufactured components or structures.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a Section 371 national phase application claiming the benefit of International Patent Application No. PCT/US15/19883 filed Mar. 11, 2015 and claims the benefit of U.S. Provisional Patent Application No. 61/951,376 filed Mar. 11, 2014, both titled, “System for Applying an Ultrasonic Transducer to a Fastener.” 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to the bonding of a small object, such as a transducer or an identification label, precisely and reliably to a component for the purpose of measurement or identification, and more specifically, to the bonding of an ultrasonic transducer to a fastener to produce a load indicating fastener using a process which is both easy and reliable for a technician in a test lab or at an assembly or construction site. 
         [0003]    For some time, ultrasonics has been used to accurately measure the load in bolts. Initially, removable ultrasonic devices were the most commonly used. More recently, low-cost permanent ultrasonic transducers, which can be permanently attached to one end of the fastener, have come to be used. Permanent fasteners of this type are described, for example, in U.S. Pat. No. 4,846,001 (Kibblewhite), issued Jul. 11, 1989; U.S. Pat. No. 5,131,276 (Kibblewhite), issued Jul. 21, 1992; U.S. Pat. No. 6,990,866 (Kibblewhite), issued Jan. 31, 2006; and U.S. Pat. No. 8,028,585 (Kibblewhite) issued Oct. 4, 2011, the subject matter of each of which is incorporated by reference as if fully set forth herein. 
         [0004]    Load measurements in fasteners using ultrasonics use a pulse-echo technique to determine the time-of-flight or acoustic length of the fastener. With permanently attached transducers that are bonded to an end of a fastener with an adhesive, such measurements are very dependent on the position of the transducer and the characteristics of the bond. In addition to providing the mechanical attachment, the adhesive acts as an acoustic couplant and an electrical capacitive coupling to the bolt surface. In order for the ultrasonic transducer to transmit and receive the acoustic waves effectively, the adhesive layer or “bond line” must be thin and free of air bubbles. In order to measure load accurately, the transducer must be precisely centered relative to the bolt shank on the end surface to minimize the effect of variations in stress across the diameter of the shank. Since this thin, repeatable bond line is difficult to achieve, the most accurate permanent ultrasonic transducer load indicating fasteners, such as i-Bolt® fasteners manufactured by Load Control Technologies of King of Prussia, Pa., are bonded in a manufacturing facility with precision automated robotic equipment. 
         [0005]    There is a need for producing load indicating fasteners quickly for urgent test and development programs and, consequently, ultrasonic load measurement equipment manufacturers provide ultrasonic transducers for bonding to fasteners by the user to make load indicating fasteners. 
         [0006]    An example of such an ultrasonic transducer and attachment procedure is described in the operating manual for the MC900 Transient Recorder manufactured by MicroControl, Inc. of Troy, Mich., the subject matter of which is incorporated by reference as if fully set forth herein. However, these manual manufacturing processes lack an effective way to precisely position the transducer and the operation is difficult and time consuming. Furthermore, poor bond line repeatability can cause measurement unreliability and inaccuracies. 
         [0007]    It therefore remains desirable to provide a system which allows a user to make load indicating fasteners easily and quickly while achieving the precise placement and bond integrity of factory produced load indicating fasteners, and to provide a practical technique for applying a transducer to an already installed fastener, or in other situations in which the fastener cannot be oriented with its bonding surface facing up. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Ultrasonic transducer attachment kits currently available for end users to attach transducers to fasteners are supplied without tooling for positioning and bonding the ultrasonic transducer. Transducer placement by hand requires a skilled technician, is very slow and tedious and results in a load indicating member which lacks the reliability accuracy of load indicating members produced in a manufacturing facility. 
         [0009]    In accordance with the present invention, low cost transducer bonding tooling and techniques are provided which can be used to duplicate the quality of factory manufactured load indicating members. The present invention further allows transducers to be attached to fasteners irrespective of their orientation. In addition, the bonding tooling and techniques of the present invention can be used to bond other small objects requiring precise placement and reliable bonding such as, for example, the bonding of identification labels to manufactured components or structures. 
         [0010]    For further detail regarding preferred embodiments for implementing the improvements of the present invention, reference is made to the description which is provided below, together with the following illustrations. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]      FIG. 1  shows an example of a typical load indicating fastener with an ultrasonic transducer permanently attached to the head. 
           [0012]      FIG. 2  shows a transducer with additional layers applied to facilitate precise bonding to a fastener to form a load indicating member. 
           [0013]      FIG. 3  shows a preferred embodiment of the tooling of the present invention, for use in attaching an ultrasonic transducer to an end of a fastener to form a load indicating member. 
           [0014]      FIG. 4  is a cross-sectional view of part of the tooling of  FIG. 3 , with like numbers referring to corresponding elements. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  shows a typical load indicating fastener produced in accordance with the present invention. In this illustrative example, the load indicating fastener generally includes a fastener  1  and a permanent transducer  2  (e.g., of the type disclosed in the above-referenced U.S. Pat. No. 6,990,866, issued to Kibblewhite) attached to one end. The fastener  1  includes a head  3 , which can be suitably engaged by an appropriate tool (not shown) for applying torque to the fastener  1 , and a threaded or thread-forming body portion  4 . As is disclosed in the above referenced U.S. Pat. No. 6,990,866, the transducer  2  can further include a bar code  6  applied to the top electrode  5  of the transducer  2 , for purposes of facilitating the subsequent steps taken to obtain an indication of tensile load, stress, elongation or other characteristic of the fastener  1  during a tightening operation, or at various other times during the service life of the fastener  1 . 
         [0016]    A load indicating member of the type shown in  FIG. 1  can be produced with the tooling shown in  FIG. 3  using transducer and transducer attachment materials in accordance with the present invention. 
         [0017]    In a preferred embodiment of the present invention, the transducer is supplied with additional layers temporarily attached to the top electrode, as illustrated in the cross-sectional view in  FIG. 2 , to facilitate bonding of the transducer to the bolt.  FIG. 2  shows a transducer  2 , which is preferably circular in shape, and which includes a laminate of polyvinylidene fluoride  11  and a metallic foil top electrode  12  with an etched bar code  13 . Temporarily attached to top electrode  12  is an adhesive backed tape  14  including plastic film  16  and pressure adhesive layer  15 , an example of which is commercially available Scotch® brand tape available from the 3M Company of St. Paul, Minn. The purpose of adhesive backed tape  14  is to protect the top surface of electrode  12  from adhesive during the bonding of the transducer. A low tack adhesive layer  17 , of a type similar to the adhesives used for the commercially available Post-it® brand products also available from the 3M Company of St. Paul, Minn., is selectively applied to the top surface of adhesive backed tape  14 , preferably using a mask with a hole diameter smaller than that of adhesive backed tape  14  to leave an area around the outer diameter of adhesive backed tape  14  top surface without adhesive. A first function of the low tack adhesive layer  17  is to temporarily attach transducer  10  with adhesive backed tape  14  to carrier strip  18  for delivery to the user. A second function of the low tack adhesive layer  17  is to precisely position the transducer during bonding of the transducer to the fastener with the transducer bonding tooling shown in  FIG. 3 . 
         [0018]    The tooling  30  of  FIG. 3  includes a transducer placement tooling  32  mounted above a self-centering vise  31  used to clamp fastener  1  during the transducer bonding operation. Self-centering vise  31  has two clamping “V” jaws  34  and  35 , actuated during clamping such that each jaw moves equally in distance and opposite in direction, always locating the fastener axis in the same position irrespective of the diameter of the fastener  1 . Rotation of adjustment nut  36  allows clamping of different sizes of fasteners, for example, from 6 mm diameter to 40 mm diameter, with over-center toggle lever  33 . A hole  37  in bottom plate  38  below the center clamping axis of self-centering vise  31  enables clamping of very long fasteners, such as studs, when mounted with hole  37  over the edge of a work bench, for example. 
         [0019]    The operation of tooling  30  is herein described with reference to  FIG. 4 , showing a cross-sectional view of part of tooling  30  of  FIG. 3 . Fastener  1 , with its top end prepared for transducer bonding by providing a clean flat bonding surface, is clamped in jaws  34  and  35  of the self-centering vise so the axis of fastener  1  is aligned with the pressure pin  40 . Pressure pin  40  is provided with pressure tip  41  and pressure controlling spring  42  and is screwed into pressure pin rod  43  which is free to move axially in alignment bar  44 . 
         [0020]    During operation, alignment bar  44 , with pressure rod  43  and pressure pin  40  is lowered so that pressure pin tip  41  rests on the fastener  1  in the position as shown in  FIG. 4 . As alignment bar  44  is lowered further with pressure rod  43  and pressure pin  40  resting against fastener  1 , pressure pin spring  42  is compressed with the amount of compression determining the force of pressure pin tip  41  against fastener  1 . The axial distance in lowering alignment bar  44  is limited by adjustment thumb nut  45  and, since the distance of alignment bar  44  from fastener  1  determines the amount of compression of pressure pin spring  42 , thumb nut  45  can be used to control the force exerted by pressure tip  41  on fastener  1 . The protrusion of the top end  46  of pressure rod  43  from the alignment bar  44  provides an indication of the amount of compression of pressure pin spring  42  and, consequently, an indication of the force applied by pressure tip  41  on fastener  1 . Thumb screw  47  is used to lock alignment bar  44  in its support block  48  in any required position. Alignment bar spring  49  returns alignment bar  44  to its up position when alignment bar  44  is released with thumb screw  47 . 
         [0021]    Pressure pin tip  41  is made of a soft elastic material, such as rubber or polyurethane, to provide even pressure on the transducer and eliminate any minor alignment variations during adhesive curing. Preferably, a thin adhesive backed replaceable protective plastic cap  50  of slightly larger diameter than pressure tip  41  is adhered to the bottom surface to extend beyond the outer edge of pressure tip  41  to protect pressure tip  41  from excess adhesive during bonding. In the preferred embodiment, protective plastic cap  50  preferably takes the form of a boot drawn over pressure tip  41  and retained in position through an interference fit between pressure tip  41  and protective cap  50 , and is preferably formed as a cylindrical part having a flat bottom and made of a thin Teflon® material commercially available from E. I. duPont de Nemours and Company Corporation of Wilmington, Del. As an alternative, protective cap  50  can be made from adhesive backed Teflon® brand tape, also commercially available from E. I. du Pont de Nemours and Company Corporation of Wilmington, Del. 
         [0022]    An example of a method for attaching a transducer  2  supplied on a carrier strip  18 , as illustrated in  FIG. 2 , to a fastener to make a load indicating member using the tooling illustrated in  FIG. 4  is as follows. With alignment bar  44  in the up position, a fastener with one end surface prepared flat and clean for bonding is placed in the self-centering vise and clamped between jaws  34  and  35 , with the bonding surface up. The transducer is removed from the carrier strip using tweezers or similar tool and placed centered on the end of fastener  1 . If the fastener has been prepared with the bonding surface in a recess centered on the fastener axis, the transducer is positioned in the center of the recess. Alternatively, if the entire fastener end surface is flat, the transducer is centered by placing it on the bonding surface directly below pressure pin tip  41 . The transducer can be positioned easily by moving it with the tweezers or other pointed tool by contacting the area of protective tape  14  top surface without adhesive. 
         [0023]    Alignment bar  44  is lowered fully at which time the pressure pin  41  protective cap  50  is firmly in contact with the transducer positioned on the top of fastener  1 . The alignment bar  44  is then raised with the transducer adhered to protective cap  50  by the low tack adhesive layer  17 . A very small drop of transducer bonding adhesive is placed in the center of the bonding surface using a fine adhesive dispensing needle. Alignment bar  44  is then fully lowered to the position preset with thumb nut  45  and alignment bar  44  is locked in its lowered position using thumb screw  47 . 
         [0024]    The above-described operation has placed the transducer back on fastener  1  in precisely the same position that it was originally placed and holds it securely with a preset pressure for a specified time sufficient to allow the adhesive to cure. After the specified cure time has elapsed, alignment bar  44  is raised after releasing thumb screw  47  and fastener  1  is removed from the self-centering vise with its bonded transducer. Excess adhesive is then removed, for example, using Q-Tips® brand swabs commercially available from Chesebrough-Pond&#39;s Inc. of Wilmington, Del., protective tape layer  14  is removed from top electrode  12  using a plastic or wooden pointed tool, such as the handle end of a Q-Tips® brand swab, and the entire transducer is cleaned with a solvent, such as isopropyl alcohol, once again, by conveniently using Q-Tips® brand swabs. The load indicating fastener is then ready for use. 
         [0025]    In the preferred embodiment of the present invention, a cyanoacrylate adhesive with a cure time of 20 to 60 seconds is used to bond the transducer to the fastener, enabling a high production rate. Alternative adhesives, such as anaerobic or epoxy adhesives can also be used if required to meet specific application environmental conditions. 
         [0026]    In an alternative embodiment, carrier strip  18  is thin and flexible and transducer  2  remains on carrier strip  18  during bonding. After the drop of adhesive is applied to the bonding surface, the transducer on the carrier strip is positioned just above the bonding surface and then alignment bar  44  is lowered, pressing the transducer  2  on the bonding surface through carrier strip  18 . This embodiment eliminates the need for pressure tip cap  50  since the carrier strip protects pressure pin tip  41  from the bonding adhesive. This embodiment is most suited for bonding to a flat top surface rather than a bonding surface in a recess. 
         [0027]    In yet another embodiment of the present invention pressure cap  50  has a reusable low tack adhesive applied to its bottom surface and the transducer is provided without low tack adhesive  17  on the adhesive backed tape layer  14 . In this embodiment the reusable low tack adhesive on the bottom surface of protective cap  50  temporarily attaches the transducer to the pressure pin in the above described bonding operation. 
         [0028]    It will be appreciated by those skilled in the art that protective cap  50  is only provided to prevent excess adhesive from adhering to pressure pin tip  41 . If pressure pin tip  41  is made of a material that does not adhere to the adhesive used for bonding, protective cap  50  is not required. The above described embodiments use ultrasonic transducers including a laminate of polyvinylidene fluoride and metallic foil. Alternative types of ultrasonic transducers known in the art and described in the above referenced patents and documents, such as piezoelectric ceramic elements and thin film transducers deposited on metal foil, can also be attached to fasteners to form load indicating members using the improvements of the present invention. 
         [0029]    An alternative embodiment of the present invention can be used to apply or reapply transducers on already installed fasteners. This may be necessary in highly corrosive environments, very high temperature environments, for very large fasteners or if the transducer fails and has to be replaced while in service. In this embodiment, transducer placement tooling similar to that described in the preferred embodiment is alternatively mounted directly to the fastener or joint, but in such a way that pressure tip  41  is aligned with and able to move along the axis of the fastener. 
         [0030]    It will be appreciated by one skilled in the art that the above described invention can be used to precisely and reliably bond other small objects, such as other transducers, identification labels and RFID tags, to fasteners and to parts other than fasteners, such as manufactured parts, machines, vehicles and structures on site. 
         [0031]    It will also be understood that while the present invention has been described based on specific embodiments incorporating specified parts, the present invention further encompasses all enabled equivalents of the parts described, and that various changes in the details, materials and arrangement of parts which have been herein described and illustrated in order to explain the nature of this invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the following claims.