Abstract:
A direct tension indicating washer includes a body having a first surface having a protuberance formed thereon; the body having a second surface having an indentation formed opposite the protuberance; and an insert positioned internal to the body, the body and the insert being made from materials of different hardness.

Description:
BACKGROUND 
     The invention relates in general to direct tension indicating washers and in particular to direct tension indicating washers that limit creep. 
     Direct tension indicating (DTI) washers are used in the art to indicate when proper bolt tension has been reached. U.S. Pat. No. 5,931,618, the entire contents of which are incorporated herein by reference, discloses an exemplary DTI.  FIG. 1  is a top view of the DTI from U.S. Pat. No. 5,931,618. A direct tension indicating washer  60  includes protuberances  12  formed on a first surface  14  and corresponding indentations  16  formed on a second surface  18  opposite the protuberances  12 . The direct tension indicating washer  60  also includes channels  62  that lead from each indentation  16  to the outer diameter of the direct tension indicating washer  60 . The indentation  16  is filled with an indicating material  64 . The direct tension indicating washer  60  is manufactured using a tool and die to stamp the protuberances  12 , indentations  16  and channels  62  into a blank washer. Other processes, such as metal machining, electronic printing, or metal casting may be used to form the direct tension indicating washer  60 . Direct tension indicating washer  60  is made from carbon steel, but stainless steel, nonferrous metals, and other alloy products may also be used. The indicating material  64  is an extrudable, elastomeric solid material such as colored silicone.  FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1 . 
     While the direct tension indicating washer of  FIGS. 1-2  is well suited for its intended purposes, a tension indicating device for compressive loads required by many non-structural industries (e.g., mining, gasketed pipe flange joints) is significantly lower than what the typical steel material for a structural DTI, (ASTM 959) is manufactured with. As such, the amount of plastic deformation required to provide the appropriate user feedback and other performance requirements of a tension indicating device are difficult to achieve with steel as a sole material of manufacture. Thermoplastics or other intrinsically softer materials are available to provide the required amount of deformation for the intended performance criteria, but such materials have long term compressive creep concerns that would exclude their suitability. 
     SUMMARY 
     Embodiments of the invention include a direct tension indicating washer having a body having a first surface having a protuberance formed thereon; the body having a second surface having an indentation formed opposite the protuberance; and an insert positioned internal to the body, the body and the insert being made from materials of different hardness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  is a top view of a conventional direct tension indicating washer; 
         FIG. 2  is a cross-sectional view of the direct tension indicating washer shown in  FIG. 1  taken along line  2 - 2 ; 
         FIG. 3  is a top view of a direct tension indicating washer in exemplary embodiments; 
         FIG. 4  depicts a cross-section of the direct tension indicating washer of  FIG. 3  with a bolt; and 
         FIG. 5  depicts a cross-section of the direct tension indicating washer of  FIG. 3  with a bolt in an alternate embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention related to a direct tension indicating washer that indicates proper bolt tension at loads lower than those used with conventional direct tension indicating washers. In order to allow the direct tension indicating washer to indicate lower bolt tension, the direct tension indicating washer is made from a plastic or thermoplastic. To prevent the long term creep of thermoplastics or other soft materials, an insert is included within the formation of the direct tension indicating washer of a non-deformable structure to prevent the long term creep while under load. Such inserts could be in the form of one or more pellets or rings placed within the direct tension indicating washer to provide a limit to the amount of deformation possible to the softer direct tension indicating washer material between the nut, bolt head or washer and the face of the materials being fastened. 
       FIG. 3  is a top view of a direct tension indicating washer  100  in exemplary embodiments. The direct tension indicating washer  100  is similar in construction to the direct tension indicating washer  60  of  FIG. 1 . Direct tension indicating washer  100  includes a body  102  having protuberances  12  formed on a first surface  14  and corresponding indentations  16  formed on a second surface  18  opposite the protuberances  12 . The direct tension indicating washer  100  also includes channels  62  that lead from each indentation  16  to the outer diameter of the direct tension indicating washer  100 . The indentation  16  is filled with an indicating material  64 . The indicating material  64  is an extrudable, elastomeric solid material such as colored silicone. 
     Direct tension indicating washer  100  is made from a softer, deformable material than that used to construct direct tension indicating washer  60  of  FIG. 1 . Exemplary materials that may be used to form direct tension indicating washer  100  include plastics and thermoplastics such as ABS, acetal, PTFE, polyester, nylon and polypropylene. By using a softer material, the direct tension indicating washer  100  may be used to indicate lower bolt tensions than the direct tension indicating washer  60  of  FIG. 1 . Direct tension indicating washer  100  operates in a similar manner as direct tension indicating washer  60  of  FIG. 1 . That is, as a nut is tightened on a bolt passing through direct tension indicating washer  100 , pressure on protuberances  12  causes the protuberances  12  to be pressed towards surface  14 . This causes the indicating material  64  to be forced through channel  62 . The direct tension indicating washer  100  is calibrated so that the indicating material  64  appears at the outer periphery of the direct tension indicating washer  100  when the proper bolt tension is achieved. Direct tension indicating washer  100  may include features from U.S. Pat. No. 6,425,718, U.S. Patent Application Publication 20080138167, U.S. Pat. No. 7,857,562 and U.S. Patent Application Publication 20110123288, the entire contents of all these publications are incorporated herein by reference. 
     As direct tension indicating washer  100  is made from a deformable material, there is a need to prevent washer creep after installation to maintain bolt tension. To address creep, direct tension indicating washer  100  includes one or more inserts  110 . The inserts  110  may be a plurality of inserts positioned within the body of direct tension indicating washer  100 , at equally spaced radial angles. In the example in  FIG. 3 , six inserts are used, each spaced at 60-degree intervals. The thickness of the inserts  110  is approximately equal to the thickness between first surface  14  and second surface  18 . Inserts  110  may be flush with first surface  14  and/or second surface  18 , or may be recessed with respect to first surface  14  and/or second surface  18 . In alternate embodiments, the insert is a single ring insert positioned in the direct tension indicating washer  100  and is positioned between the ID and OD of the direct tension indicating washer  100 . 
     Inserts  110  are made from a material that is harder than the material used for the body of the direct tension indicating washer  100 . In exemplary embodiments, inserts  110  are made from metal (e.g., steel or stainless steel). To generalize, the body  102  of direct tension indicating washer  100  is made from a material having a hardness that is lower than the hardness of the inserts  110 . Using metal for inserts  110  also provides for conduction of electricity between the nut  132  and structure  140  ( FIG. 4 ) which is necessary in some applications. 
     Washer  100  may be formed using rapid prototyping methods, such as stereolithography, selective laser sintering, fused deposition modeling, etc. The washer  100  may be formed with pockets to accept inserts  110  by press fitting the inserts  110  into washer  100 . The use of rapid prototyping techniques allows manufacturers to quickly meet customer demand. 
       FIG. 4  depicts a cross-section of the direct tension indicating washer of  FIG. 3  with a bolt  120  inserted through the ID of the direct tension indicating washer  100 . A washer  130  is shown positioned adjacent protuberances  12 . As nut  132  is tightened, direct tension indicating washer  100  is compressed between washer  130  and structure  140 . When the calibrated bolt tension is reached, indicating material  64  forced through channels  62  appears at the outer periphery of direct tension indicating washer  100 . By this point, or shortly thereafter, washer  130  is also in contact with inserts  110 . As such, creep of the direct tension indicating washer  100  will not result in a decrease in bolt tension. Further, compression of the direct tension indicating washer  100  causes the inner diameter of the direct tension indicating washer  100  to decrease due to the Poisson effect. This reduction in the inner diameter of the direct tension indicating washer  100  can cause the direct tension indicating washer  100  to seal around the bolt shank, and prevent corrosion. 
       FIG. 5  depicts a cross-section of the direct tension indicating washer of  FIG. 3  with a bolt in an alternate embodiment. The embodiment of  FIG. 5  is similar to that in  FIG. 4 , and similar reference numerals are used for similar elements. The insert  200  in  FIG. 5  is an active insert that generates a shut off signal upon a certain condition. Insert  200  may include a micro-emitter, such as a micro-RF emitter. In one embodiment, insert  200  generates an RF shut off signal when each end of the insert  200  is in electrical contact with metal. For example, in  FIG. 5 , tool  220  is used to tighten nut  132 . When nut  32  is tightened sufficiently, washer  130  contacts one end of insert  200  and structure  140  contacts the other end of insert  200 . This causes the insert  200  to emit a shut off signal that is received by a controller  210  associated with tool  220 . Controller  210  may be integrated with tool  220  or be a separate device from tool  220 . When the insert  200  emits the shut off signal, controller  210  ceases operation of the tool  220 . This prevents operators from over tightening nut  132 . 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. For example, the inserts may be used with conventional, non-squirting, direct tension indicating washers. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.