Abstract:
An ultrasonic load measurement transducer ( 14 ) is mated with a thread-forming fastener ( 12 ) to provide a load indicating thread-forming fastener ( 10 ) that can be used for precise and reliable assembly of critical bolted joints, such as those in the automobile and aerospace industries, among others. Steps can then be taken to accurately measure and control the load in the thread-forming fastener ( 10 ) during tightening, and to inspect the load in the thread-forming fastener ( 10 ) after assembly. A similar result can be achieved for a thread-locking fastener by mating an ultrasonic transducer with the thread-locking fastener assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/412,125, filed Sep. 19, 2002. This application is also a continuation-in-part of U.S. patent application Ser. No. 10/470,372, which has an assigned filing date of Jul. 25, 2003, which was the National Stage of International Application No. PCT/US02/03920, filed Jan. 28, 2002, and which claims the benefit of U.S. Provisional Application No. 60/264,877, filed Jan. 29, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to load indicating fasteners that are “thread-forming” (also referred to as “thread-rolling” or “self-tapping” fasteners), methods for making load indicating thread-forming fasteners, and methods for measuring the load in thread-forming fasteners. 
     Thread-forming fasteners are well known in many industries, such as in high-volume automotive assembly. Examples of such fasteners are described in U.S. Pat. No. 5,242,253 (Fulmer), issued Sep. 7, 1993, for example. Such fasteners are also marketed commercially, for example, by Reminc, Research Engineering and Manufacturing Inc., Middletown, R.I., USA, under the trademark “Taptite” and “Taptite 2000”, and a description of such fasteners can be found in their product literature, entitled “Taptite 2000 Thread Rolling Fasteners”. 
     The major advantage of thread-forming fasteners is that they can be installed directly into a drilled hole, eliminating the cost of tapping the hole. Additionally, the thread formed by a thread-forming fastener has very tight tolerance since it is formed by the fastener itself and therefore forms a better nut. 
     Although thread-forming fasteners have been used in numerous applications in the automotive and aerospace industries to reduce cost, such fasteners are generally restricted to non-critical or less-critical applications. The difficulty in controlling the tightening process prevents their use in critical applications. 
     The primary reason for this is that the thread-forming process requires torque, in addition to the tightening torque, and this thread-forming torque varies significantly with hole tolerance, material, friction conditions, etc. As a result, the precise tightening of a thread-forming fastener to a specified torque into a blind hole, where the thread is still being formed as the bolt is being tightened, will result in a 3 sigma load scatter of typically +/−50%, which is unacceptable in critical applications. 
     SUMMARY OF THE INVENTION 
     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. Provisional Patent Application No. 60/264,877 (Kibblewhite), filed Jan. 29, 2001, and International Application No. PCT/US02/03920 (Kibblewhite), filed May 17, 2002, the subject matter of which is incorporated by reference herein. 
     In accordance with the present invention, it has been determined that such ultrasonics can be mated with an otherwise conventional thread-forming fastener to provide a load indicating thread-forming fastener that can be used for precise and reliable assembly of critical bolted joints, such as those in automobile engines (e.g., cylinder heads, connecting rods, main bearings, etc.), drive trains, steering, brakes, suspensions, and a variety of other applications, including aerospace applications. 
     Steps can then be taken, using equipment and methods that are otherwise known and conventional, to accurately measure and control the load in the thread-forming fastener during tightening, and to inspect the load in the thread-forming fastener after assembly. 
     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 DRAWINGS 
         FIG. 1  shows an example of a typical load indicating thread-forming fastener which is produced in accordance with the present invention. 
         FIGS. 2 and 3  are graphs showing typical load and torque characteristics plotted against the angle of rotation of the load indicating thread-forming fastener of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a typical embodiment of a load indicating thread-forming fastener which is produced in accordance with the present invention. In this illustrative example, the load indicating thread-forming fastener has been implemented in conjunction with an otherwise conventional “Taptite” fastener, which is commercially available from Reminc, Research Engineering and Manufacturing Inc., Middletown, R.I., USA. It is to be understood, however, that this embodiment is shown only for purposes of illustration, and that the load indicating thread-forming fastener of the present invention can also be implemented using any of a variety of known and available load indicating devices, coupled or combined with any of a variety of known and available thread-forming fasteners. 
     In the illustrative embodiment of  FIG. 1 , the load indicating thread-forming fastener  10  generally includes a fastener  12  (e.g., the above-mentioned “Taptite” fastener) and a permanent piezoelectric polymer film transducer  14  (e.g., of the type disclosed in the above-mentioned U.S. Pat. No. 4,846,001, issued to Kibblewhite) attached to one end. The fastener  12  includes a head  16 , which can be suitably engaged by a tool (not shown) for applying torque to the fastener  12 , and a thread-forming body portion  18 . As disclosed in U.S. Provisional Patent Application No. 60/264,877 (Kibblewhite) and International Application No. PCT/US02/03920 (Kibblewhite), the transducer  14  can further include a two-dimensional high-density bar code (not shown) applied to the top electrode  20  of the transducer  14 , for purposes of facilitating the subsequent steps taken to obtain an indication of tensile load, stress, elongation or other characteristic of the fastener  12  during a tightening operation, or at various other times during the service life of the fastener  12 , as will be discussed more fully below. 
     As an example, the transducer  14  can be implemented using a thin piezoelectric polymer sensor (e.g., a 9 micron thick, polyvinylidene fluoride copolymer film, of the type manufactured by Measurement Specialties Inc., Valley Forge, Pa.) permanently, mechanically and acoustically attached to an end surface  22  of the fastener  12 . The top electrode of the transducer  14  can be implemented as a thin metallic foil (e.g., an approximately 50 micron thick, type 316, full-hard, dull or matte finished stainless steel) which has been treated to provide a black oxide finish, which is then preferably provided with a black oxide treatment to provide an extremely thin, durable, corrosion resistant and electrically conductive, black coating. A high-resolution bar code can then be marked on this surface by removing selected areas of the coating (e.g., by conventional laser ablation techniques) to provide a high contrast mark easily read with conventional, commercially available optical readers. 
     It is again to be understood that such implementations are described only for purposes of illustration, and that any of a variety of transducer configurations can be used to implement the transducer  14  applied to the fastener  12 , as desired. For example, the ultrasonic transducer  14  can be implemented as an oriented piezoelectric thin film, vapor deposited directly on the head of the fastener  12 , as is described in U.S. Pat. No. 5,131,276 (Kibblewhite), issued Jul. 21, 1992. As a further alternative, the ultrasonic transducer  14  can be implemented as a piezoelectric polymer film, chemically grafted on the head of the fastener  12 , as is described in U.S. Provisional Patent Application No. 60/264,877 (Kibblewhite), filed Jan. 29, 2001, and International Application No. PCT/US02/03920 (Kibblewhite), filed May 17, 2002. It will be readily understood that other alternative implementations are also possible. 
     In the embodiment illustrated in  FIG. 1 , the ultrasonic transducer  14  is permanently attached to the head  16  of the fastener  12 , as described in the above-referenced patents issued to Kibblewhite. An essentially flat, or spherically radiused surface  24  is provided on at least a portion of the threaded end of the fastener to provide an acoustically reflective surface to reflect the ultrasonic wave transmitted by the transducer back to the transducer. Load is then measured using standard, pulse-echo ultrasonic techniques, which are themselves known in the art and described, for example, in the above-referenced patents issued to Kibblewhite. Load control accuracies of +/−3% have been achieved when tightening thread-forming fasteners into blind holes during both the first and subsequent tightenings. 
     In an alternative embodiment, an essentially flat surface is provided on the head  16  of the thread-forming fastener  12  and a removable ultrasonic transducer is temporarily attached to the fastener for the purpose of making load measurements. The threaded end of the fastener  12  is identical to the previous embodiment with the permanent ultrasonic transducer. 
     In practice, heat is generated as a result of the thread-forming work that takes place during the thread-forming run-down stage of the installation of a thread-forming fastener. This results in a slight increase in temperature in both the fastener (the bolt) and the resulting joint. This increase in temperature can cause errors in the ultrasonic load measurements to be taken because of thermal expansion effects. For this reason, when using ultrasonics for inspecting the load in a fastener, it is usual to measure the temperature of the fastener or the joint in order to compensate for the effects of thermal expansion. 
     However, in conjunction with a thread-forming fastener, the average temperature increase due to the heat generated during thread-formation can not be measured directly during the installation process and is subject to variations in material, friction, and heat conduction properties of the joint components. Without compensation, this thermal effect can result in inaccuracies of load measurement on the order of 5% to 20%, depending on the bolt, the joint and the assembly process being used. 
       FIGS. 2 and 3  show typical load and torque characteristics plotted against the angle of rotation of a typical bolt.  FIG. 2  shows the tightening curves for a typical through-hole application, in which the torque reduces after the thread is formed through the entire hole.  FIG. 3  shows the tightening curves for a typical blind hole application, in which the thread is still being formed as the bolt is tightened. 
     Further in accordance with the present invention, more accurate load measurements in the thread-forming load indicating fasteners are provided by eliminating the effects of fastener heating resulting from the thread-forming process. This is achieved by measuring the load (or acoustic time-of-flight) value immediately prior to the load-inducing stage of the assembly process, and by using this measured value as the zero-load reading. 
     The load-inducing stage of the assembly process can be detected by any one of a variety of methods. For example, an increase in load above a predetermined threshold, a change in the increase in load with time, angle of rotation of the fastener or torque, an increase in torque above a predetermined threshold, or a change in the increase in torque with time, angle or load can be detected. Irrespective of the method used to detect the load-inducing stage of the assembly process, a new zero-load base measurement is taken as a value just prior to the load-inducing assembly stage by selecting or calculating a load measurement prior to the load-inducing stage. This can be achieved by selecting a load measurement corresponding to a fixed time or angle prior to the detection of the commencement of the load-inducing stage, for example. Alternatively, for through-hole applications, the end of the thread-forming phase can be detected by a reduction in torque. It is again to be understood that such methods are only illustrative, and that there are numerous other methods for determining the new zero-load base measurement prior to tightening, from load, time, torque and angle of rotation measurements recorded during assembly operations with hand and powered assembly tools. 
     The thermal effect of thread forming causes an apparent positive load value at zero load just prior to tightening. An alternative to zeroing the load (or time-of-flight measurement) is to add this load offset, measured prior to the load-inducing stage of the assembly process, to the target load (or target time-of-flight). The result is the same since the increase in measured load is the same. 
     Yet another alternative is to experimentally determine an average value of load error due to the thread forming and adjust the zero-load measurement or target tightening parameter to compensate for this effect using one of the above-described methods. This approach, however, does not compensate for variations with individual fasteners or joint components and is therefore presently considered less desirable. 
     The result is that, for the first time, ultrasonic load measurement technology can be used with thread-forming fasteners. Errors in load measurement resulting from the thermal effects of thread-forming can be compensated. This then results in accurate load measurement and tightening control of the thread-forming fasteners. 
     The above-described method of eliminating the effects of fastener heating resulting from the thread-forming process can also be used with other fastener assembly processes that generate heat prior to the load-inducing tightening stage. Thread-locking bolts and nuts, for example, are manufactured with a prevailing “locking” torque to prevent the fastener from loosening during service. Most often, the thread of either the bolt or nut has an irregular profile causing the threads to elastically deform slightly upon mating. Alternatively, the bolt or nut has an insert or patch of a soft material to provide the prevailing torque or resistance to loosening. The prevailing torque provided by these thread-locking features produces heating of the fastener during rundown in the same manner as the tapping torque does with a thread-forming fastener. Consequently, the above-described method for compensating for thermal-related errors in accordance with the present invention can be used with prevailing torque-locking fasteners to improve the accuracy of ultrasonic load measurement during assembly. 
     It will be understood 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.