Abstract:
This invention installs multiple miniature strain gauges inside a regular dimensioned bolt washer to accurately measure bolt preload. To enhance the strain gauge sensitivity, an alloy with low elastic modulus and high yield strength is selected to fabricate the metal washer. In addition, multiple gauges are connected in series to multiply the effective gauge length and enhance measurement sensitivity. Further, the stain gauges are encapsulated in the middle of the washer as opposed to on the external surface which offers much improved sensitivity and physical protection of the strain gauges.

Description:
BACKGROUND 
     Field 
     This invention relates generally to a strain-gauged washer and, more particularly, to a conventional-thickness washer for measuring bolt preload including one or more strain gauges placed in a slot in the washer and oriented through a thickness of the washer to measure compressive strain in the washer, where the slot is located between an inner diameter and an outer diameter of the washer in order for the strain gauge to detect a maximum strain field, and more than one strain gauge may be connected in series in order to increase effective strain gauge length. 
     Discussion 
     Bolts are commonly used to fasten components together in assemblies of all types—ranging from simple, inexpensive household items to multi-billion dollar aircraft and space vehicles. Most bolted joints include a washer under the bolt head—where the washer serves to provide uniform contact and prevent damage to the underlying component. 
     In many bolted joint applications, it is important to achieve a prescribed preload in the bolt. Proper bolt preloading is effective in minimizing joint fatigue due to cyclic loading, and is also effective in preventing bolt loosening or back-out. Bolt preloading requirements are especially important in applications where the article of manufacture is large (requiring significant disassembly in order to access and replace/tighten a bolt), expensive (costly downtime for bolt replacement/tightening) and/or remotely deployed (impossible to replace/tighten a bolt on a satellite in space). 
     Many techniques for determining bolt preload have been developed over the years. One of the most basic forms of bolt preload estimation is through simple torque measurement during bolt tightening. However, surface friction and thread friction variations make torque-based bolt preload estimation inherently imprecise—with accuracies often no better than +/−30%. Other bolt preload techniques involve instrumentation or inspection of the bolt itself. These techniques also have disadvantages, however, including the cost and complexity of fitting sensors inside of the bolt, and the time and labor involved in performing ultrasound or other inspections on every bolt after it is installed. Still other bolt preload techniques involve the use of a thick collar in place of a standard washer under the bolt head, where the collar is fitted with instruments for measuring or estimating the load applied by the bolt head. However, these thick instrumented collars change the geometry of the bolted joint, necessitating a different bolt length to be used and/or dimensional changes to the fastened components. 
     As discussed above, all of the traditional techniques for bolt preload estimation or measurement suffer from significant drawbacks. Therefore, a need remains for a bolt preload measurement technique which is simple, inexpensive, reliable, accurate, and does not require any changes to the bolts or fastened components which are used in a bolted assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional illustration of a bolt and washer in a bolted-joint assembly; 
         FIG. 2  is an isometric view illustration of a washer configured to include strain gauges oriented through the thickness of the washer to directly measure compressive strain; 
         FIG. 3  is a cross-sectional illustration of a first configuration of strain-gauged washer according to an embodiment of the invention; 
         FIG. 4  is a cross-sectional illustration of a second configuration of strain-gauged washer according to an embodiment of the invention; and 
         FIG. 5  is a cross-sectional illustration of a strain-gauged washer including a plurality of strain gauges in series within each slot in the washer, according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a strain-gauged washer for measuring bolt preload is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the embodiments discussed below are described in the context of a flat washer employed between a flat bolt head and a flat fastened component. However, the disclosed invention is equally suitable for use in other shapes and configurations of washers and joint geometries. 
       FIG. 1  is a cross-sectional illustration of a bolted-joint assembly including a bolt  10 , a nut  20 , a first fastened component  30  and a second fastened component  40 . To join the assembly, the bolt  10  is inserted through holes in the components  30  and  40 , the nut  20  is threaded onto the extended end of the bolt  10 , and the bolt  10  and the nut  20  are tightened. Alternatively, in lieu of the nut  20 , the fastened component  40  may include a threaded portion into which the bolt  10  is threaded. A washer  50  is also included in the assembly, between the head of the bolt  10  and the adjacent surface of the first fastened component  30 , as is commonly done and would be understood by anyone familiar with mechanical assemblies. Another washer (not shown) may also be used between the second fastened component  40  and the nut  20 , where this additional washer may be a plain flat washer or lock washer, or it may be a strain-gauged washer according to the embodiments discussed below. 
     The bolt  10  has an axial direction  12 , as shown in  FIG. 1 . The axial direction  12  is aligned along the length of the bolt  10 ; for example the axial direction  12  may be defined by the centerline of the bolt  10 , where the bolt  10  is in tension in the axial direction  12  when the nut  20  is tightened to compress the assembly together. 
     Bolted joints are widely used in spacecraft construction and other long durability crafts and structures. To prevent joints from gapping and slipping, an adequate knowledge of the bolt preload is usually required in these applications. Often, such knowledge is gained by extensive structural testing of bolted joints and specification control of the bolt torque using a calibrated torque wrench. Also, in spacecraft and launch vehicle construction, mechanical separation devices rely on correct bolt preload for firing the separation device. In some cases, for example, separation bolts are made of shape memory alloys (SMA), and the separation load is linearly added to the bolt preload. Therefore, accurately measuring and controlling bolt preload is critically important in operation of SMA and similar separation devices. 
     Controlling bolt preload by specifying bolt tightening torque, preload estimation can be managed only to a precision of approximately 30%, due to variations in friction. On the other hand, direct measurement of bolt preload can allow this precision to be within a single digit. However, prior art techniques for measuring or estimating bolt preload have proven unsatisfactory for a variety of reasons. For example, measuring bolt elongation using ultrasonic waves offers improved preload precision over torque-based methods, but is labor intensive and not possible in some applications. Measuring bolt elongation using a strain gauge can also offer improved preload precision, but requires modification of the expensive and complex-shaped bolt. Installing thick collar-type measurement devices under the bolt head allows compressive load measurement, but changes the geometry of the bolted joint, necessitating use of a different bolt and/or modification of the fastened components. 
     The current invention embeds strain gauge sensors inside a standard-size washer used as part of the bolted joint. The compressive load inside the washer directly reflects the preload in the bolt. This device can be used with regular bolts without modification of either the bolt or the structural components which are being fastened by the bolt. 
     In the following discussions, a standard-sized washer is used as an example for illustration of the inventive concept. Specifically, a NAS1587 washer is illustrated—in both flat and countersunk varieties. NAS1587 is a family of washers designed to accommodate bolts ranging in size from ¼″ diameter to 1¼″ diameter, where all of the washers in the family have a thickness of 0.062″ (or about 1.6 mm thickness). In one embodiment illustrated in the figures, a NAS1587-6C washer is shown, which is designed for a ⅜″ bolt, has an inside diameter of about 0.38″ and an outside diameter of about 0.69″ and a conically-shaped inside diameter to accommodate a countersunk bolt head. For those skilled in the art, the concept explained below can be readily applied to other washers. 
     Embedding a strain sensor inside a washer rather than inside a bolt has not been done in the past due to the challenges involved. Specifically, the challenge of fitting a strain gauge with suitable sensitivity (accuracy) within the thickness of a standard-size washer. The gauge factor, or sensitivity, of a strain gauge is linearly proportional to the strain gauge length. These challenges are overcome by several aspects of this invention discussed below. 
     Firstly, modern strain gauge technology has advanced greatly to make small strain gauges possible. For example, strain gauges are now commercially available with gauge length as small as 0.2 mm, which is about an order of magnitude smaller than the thickness of the NAS1587 washer family discussed above. 
     Secondly, to enhance the strain gauge sensitivity for a short gauge length sensor, embodiments of the disclosed invention provide multiple strain gauges sensors embedded into the washer and connected in series, thus providing a longer effective gauge length. As such, the measured stain gauge resistance change is multiplied by the number of embedded strain sensors. 
     Thirdly, to further enhance the measurement sensitivity, materials are selected for fabrication of the washer which maximize the strain induced in the washer relative to bolt preload. By maximizing the compressive strain in the washer as a function of bolt preload (while of course ensuring that the washer material is strong enough to withstand the load), strain gauge sensitivity is improved. 
     Finally, modern finite element stress/strain analysis allows the strain field distribution inside a washer under a bolt load to be studied. This analysis provides the optimal locations for embedding strain gauge sensors inside a washer, further increasing the sensitivity and effectiveness of the strain gauges. 
       FIG. 2  is an isometric view illustration of a washer  100  configured to directly measure compressive strain and therefore bolt preload in a bolted-joint assembly, according to a preferred embodiment of the invention. The washer  100  could be used as the washer  50  in the assembly of  FIG. 1 , as the washer  100  is a standard-sized flat washer such as, for example, one of the NAS1587 washers having a thickness of 1.6 mm discussed previously. 
     The washer  100  is configured with one or more small vertical slots  110 . In the embodiment shown in  FIG. 2 , two of the slots  110  are provided, on opposite sides from each other surrounding the center bolt hole. The slots  110  are located about half-way between the inner diameter (ID) of the washer and the outer diameter (OD) as shown. The term “vertical” is used to describe the slots  110 , where “vertical” means aligned with the axial direction  12  of the bolt  10 , discussed previously. In a preferred embodiment, the slots  110  are round holes drilled or otherwise formed through the entire thickness of the washer  100 . The washer  100  also includes a channel  120  extending from each of the slots  110  to the outer diameter of the washer  100 . The channel(s)  120  allow for signal wires to be routed from the slot  110  to the outer diameter of the washer  100 .  FIG. 2  shows just the configuration of the washer  100  itself; additional details of the invention are discussed below. 
       FIG. 3  is a cross-sectional illustration of the washer  100  of  FIG. 2 .  FIG. 3  (along with  FIGS. 4-5  discussed below) shows just one half of the cross-section of the washer  100 —specifically, the right-hand half—as indicated by centerline  102  visible at the left. The opposite side of the washer  100  may be similarly configured—that is, a mirror image of what is shown in  FIG. 3 . Furthermore, the cross-section shown in  FIG. 3  may be repeated multiple times around the circumferential direction of the washer  100 —such as four of the slots  110  at equally-spaced 90° intervals. The same is true of  FIGS. 4-5 . 
     The half cross-section of the washer  100  has a width  104  and a thickness  106 , which depend on exactly which washer size is used in a particular application. The width  104  is of course equal to one-half of the difference between the OD and the ID. The shape of the washer  100  shown in  FIGS. 2 and 3  corresponds with one of the larger-diameter washers from the NAS1587 family, such as a 2″ outer diameter washer, having the standard thickness (1.6 mm). 
     The slot  110  and the channel  120 , shown in  FIG. 2  and discussed above, are seen more clearly in the cross-sectional view of  FIG. 3 . The slot  110  may be a round hole drilled all the way through the thickness of the washer  100 , as discussed above. The channel  120  may have any suitable cross-sectional shape (for example, U-shaped with rounded or square corners), and extends from the slot  110  to the outer peripheral edge of the washer  100 . 
     A strain gauge  130  is attached to an inner surface of the slot  110 . As discussed above, strain gauges are commercially available which are small enough to easily fit within the thickness  106  of the washer  100 . The strain gauge  130  is oriented “vertically” in the slot  110 —that is, parallel with the axial direction  12  of the bolt  10 , so that the strain gauge  130  measures compressive strain in the washer  100 . The strain gauge  130  may be attached to the inner surface of the slot  110  in any typical fashion, such as by bonding. The backing material on the strain gauge  130  may be trimmed in order to provide a proper fit within the slot  110 . 
     If two of the slots  110  are provided in the washer  100 , as shown in  FIG. 2 , then one of the strain gauges  130  is provided in each of the slots  110  and the two gauges  130  are wired in series and form gauge elements of a Wheatstone quarter bridge. The serial connection of the two gauges  130  multiplies as well as balances the strain gauge measurement as compared to a single strain gauge configuration. 
     An insulated wire  132  connects the strain gauge  130  to a connector  134 , where the wire  132  passes though the channel  120  and extends to the exterior periphery of the washer  100 . The connector  134  is preferably a miniature connector, for example, a micro USB connector, suitable for attachment to a data collection/display instrument (not shown). The wire  132  carries a data signal (a voltage) from the strain gauge  130  to the data collection instrument. The data collection instrument may be used to calibrate strain gauge readings to actual bolt preload in a controlled laboratory-type setting, and may be used to monitor strain gauge readings and display bolt preload in real time during actual production assembly of the bolted-joint. 
     The wires  132  may be encapsulated inside the channels  120  using a polymeric compound, for example, a polyurethane compound, or an epoxy. The encapsulation material is applied so as to have a height which does not protrude beyond the surface of the washer  100 . 
     The washer  100  may be constructed using a standard stainless steel material. However, as an enhancement to the preferred embodiment, a titanium alloy, such as Ti-6Al-4V heat treated to STA condition, is used to make the washer  100 . Titanium alloys have a very high yield strength, yet have an elastic modulus (stiffness) which is about 40% less than the modulus (stiffness) of the stainless steel which is typically used to make NAS washers. Thus, a titanium alloy washer can produce over one and a half times the elastic strain under the same load as compared to stainless steel. The increased strain magnitude of the washer  100  when composed of titanium further enhances the strain gauge sensitivity to bolt preload. 
       FIG. 4  is a cross-sectional illustration of a washer  200  which is a second configuration of strain-gauged washer according to an embodiment of the invention. The washer  200  of  FIG. 4  has a shape which approximately corresponds to a NAS1587-6C washer, which has a width  204  of 0.155″ (3.9 mm) and a thickness  206  of 0.062″ (1.6 mm), and is shaped to accommodate a countersunk-style bolt head. The washer  200  includes (in the portion shown in  FIG. 4 ; others may be included elsewhere in the washer  200 ) a slot  210 , a channel  220  and a strain gauge  230 , which are comparable to the corresponding items in  FIG. 3 . The signal wire leading from the strain gauge  230  through the channel  220  to an outer periphery of the washer  200  is not shown, for the sake of simplicity and clarity. 
     As mentioned earlier, finite element analysis (FEA) can be performed on a washer compressed under a bolt head to determine the strain field in the washer. Such a finite element analysis was performed on the washer  200 , simulating a NAS1587-6C washer compressed from above by a flat-head bolt which extends to the outer diameter of the washer  200 . (The washer  200 , although designed to accommodate a countersunk-style bolt head, may also be used with flat-head bolts, for example where the bolt has a large-radius fillet blending the bolt shank to the bolt head, and the conical relief of the washer ID allows room for the fillet.) 
     The FEA of the washer  200  resulted in a strain field which was greatest in a region  208  depicted in  FIG. 4 . In order to optimize the sensitivity of the strain gauge  230 , the slot  210  and the strain gauge  230  may be placed within or very near to the maximum strain region  208 . Such a configuration results in maximum compression of the strain gauge  230  for a given bolt preload, thereby delivering optimum strain gauge sensitivity and bolt preload accuracy. The slot  210  in  FIG. 2  is shown slightly displaced from the region  208  simply for clarity. If more than one of the slots  210  and the strain gauges  230  are used in the washer  200 , they would preferably all be located at the same diameter, at spaced-apart positions around a circle. FEA of any bolt head and washer combination can readily be performed, where the results of the FEA will prescribe the location of maximum washer strain and hence the optimal location for slot and strain gauge placement. 
       FIG. 5  is a cross-sectional illustration of the strain-gauged washer  100  including a plurality of strain gauges  130  A-D within each of the slots  110  in the washer  100 , according to another embodiment of the invention. In this embodiment, four of the strain gauges  130  A-D are arranged in the slot  110  and connected in series, thus providing an increased effective strain gauge length and increasing the sensitivity of the output signal. In this way, for a given bolt preload, the strain gauge output signal will be four times greater than it would be for a single strain gauge in the slot  110 . More or fewer than four of the strain gauges  130  can be used in a single slot  110 , with the main constraint simply being physical space within the slot  110 . Multiple slots  110  can be employed in the washer  100 , as shown in  FIG. 2 , with multiple strain gauges within each of the slots  110 , as shown in  FIG. 5 , to provide a magnified and balanced signal proportional to bolt preload. 
     To those skilled the art, other implementations of the strain-gauged washer for bolt preload measurement can be readily realized in multiple ways. One example is to make the washer using other materials with low elastic modulus but high strength, in order to maximize the actual strain magnitude in the washer and thereby improve output signal strength. Several candidate alloys can be made of magnesium, zirconium, erbium, aluminum, hafnium, gold, silver, niobium, zinc, titanium, palladium, vanadium, copper or a combination thereof. 
     Another example is to place strain gauges in four equally-spaced slots around the face of the washer, thus making the system of strain gauges quadruple multiplied and balanced. Referring back to  FIG. 2 , four (or more) of the slots  110  can be included so that multiple strain gauges can be installed to further enhance the measurement sensitivity and balance out any off-axial moment of the bolt. Yet another embodiment includes multiple strain gauge sensors and temperature sensors into a single washer so that a full Wheatstone bridge and fully temperature compensated strain gauge washer is realized. 
     Still another example is to use strain gauges fabricated using microelectromechanical systems (MEMS) technologies in lieu of regular metal foil gauges. These MEMS devices can further miniaturize the sensors and permit installation on smaller washers, or the use of more strain gauges within a single slot. 
     The strain-gauged washer for measuring bolt preload described above provides numerous advantages over previous systems. These advantages include greatly improved strain gauge sensitivity compared to other systems—due to placement of the strain gauges within the interior of the washer to directly measure compressive strain in the maximum strain region of the washer, the use of multiple strain gauges connected in series to amplify the output signal, and the selection of a washer material which maximizes absolute strain while still elastically withstanding the compressive stress. Advantages of the disclosed invention also include the extremely low cost and small size of the strain-gauged washer, where the strain-gauged washer can be used in place of any standard-size washer in a bolted joint. This combination of features facilitates bolt preload measurement which is accurate, repeatable, inexpensive, and does not require changes to the bolt specifications or the design of the fastened assembly—thereby enabling bolt preload measurement to be reliably employed in any assembly where bolt preload criteria are important. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.