Abstract:
A method of making a direct tension indicating washer including: forming a first surface having a discrete protuberance formed thereon; forming a second surface having a discrete indentation formed opposite the protuberance; and positioning an indicating material in the indentation, wherein the indicating material has a cured skin on an outside of the indicating material and a liquid core in an interior of the indicating material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/875,224 filed Oct. 19, 2007, the entire contents of which are incorporated herein by reference, which claims the benefit of U.S. provisional patent application Ser. No. 60/874,457 filed Dec. 12, 2006, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The invention relates in general to direct tension indicating and maintaining washers and in particular to direct tension indicating and maintaining washers that maintain bolt tension and/or indicate when the proper bolt tension has been achieved. 
     High strength metal bolts, along with their corresponding nuts, when used in metal connections, are almost always intended to be installed so that they are tensioned. The tensioning is usually accomplished by turning the nut relative to the bolt a specified amount of rotation or until a certain torque resistance has been reached. The tension in the bolt stretches the bolt and compresses the connection plates between the bolt head and nut so the connection plates do not slip relative to each other. It is this slip resistance which holds structures such as bridges and buildings together. 
       FIG. 1  is a top view of a conventional direct tension indicating washer  10 , commonly referred to as a DTI. The direct tension indicating washer  10  includes a series of protuberances  12  formed on a first surface  14 . As shown in  FIG. 2 , corresponding indentations  16  are formed in a second surface  18  of the direct tension indicating washer  10 . As shown in  FIGS. 3 and 4 , the direct tension indicating washer  10  is manufactured through a stamping operation in which the blank metal washer  10  is placed between a tool  20  and a die  22 . Pressure is applied to the tool  20  (e.g. by a press) and a protrusion  24  on the tool  20  creates the protuberance  12 , and the corresponding indentation  16 , in the direct tension indicating washer  10  as shown in  FIG. 4 . 
     The DTI is manufactured so that when the tightening of the bolt forces the protrusions  12  to compress, or reduce height, sufficiently, the bolt is said to have sufficient tension to meet code requirements. Determination of sufficient protrusion compression is accomplished in the field by the attempt to insert a feeler gage of specified thickness (e.g. 0.015″) into the residual gap between the underside of the bolt head and the first surface  14  of the DTI.  FIG. 1A  shows a feeler gage being placed on a first surface of a direct tension indicating to test bolt tension (the bolt is not shown for clarity). If the attempt to insert the feeler gage is unsuccessful, that is, the inspector cannot insert the feeler gage into the gap half way radially around the DTI circumference between the DTI protuberances, the residual DTI gap (the space between the first surface of the direct tension indicating washer and the bottom of the bolt head) is said to be, on average, of dimension less than the thickness of the feeler gage, and is therefore judged acceptable. Conventional direct tension indicating washers are manufactured so that surfaces  14  and  18  are parallel and perpendicular to the longitudinal axis of the bolt which they are made to fit on, that is when the bolt is inserted through the inner diameter. 
     Conventional direct tension indicating washers, though well suited for their intended purposes, have several drawbacks. First, the process of attempting to insert the feeler gage into the DTI gap is time consuming and must be done after the bolt installer has stopped the tightening process. Second, over time, bolt tension declines as the bolt/nut threads seat into the mating steel surfaces and as external loads shake the connections and try to pry the bolt/nut apart. This problem is especially prominent in connections that are subject to vibration such as bridges, structures subject to earthquakes, connections supporting moving cranes and other equipment, etc. As the bolt tension declines, so does the clamping force and the capacity to carry external loads by resistance to slip. The conventional direct tension indicating washer  10  is flat and has the disadvantage that, after bolt installation, the protrusions  12  are essentially flattened inelastically and will not spring back to their original positions if the bolt begins to loose tension. 
     SUMMARY 
     Embodiments of the invention include a method of making a direct tension indicating washer including: forming a first surface having a discrete protuberance formed thereon; forming a second surface having a discrete indentation formed opposite the protuberance; and positioning an indicating material in the indentation, wherein the indicating material has a cured skin on an outside of the indicating material and a liquid core in an interior of the indicating material. 
    
    
     
       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 and maintaining washer; 
         FIG. 1A  is a perspective view of the use of a feeler gage to test bolt tension; 
         FIG. 2  is a cross-sectional view of the direct tension indicating and maintaining washer shown in  FIG. 1  taken along line  2 - 2 ; 
         FIGS. 3 and 4  illustrate a process for creating the protuberances/indentations in the conventional direct tension indicating and maintaining washer; 
         FIG. 5  is a cross-sectional view of a direct tension indicating and maintaining washer in accordance with a first embodiment of the invention; 
         FIG. 6  illustrates a process for forming the direct tension indicating and maintaining washer shown in  FIG. 5 ; 
         FIGS. 7-9  illustrate the state of the direct tension indicating and maintaining washer during the installation process; 
         FIG. 10  illustrates the state of the direct tension indicating and maintaining washer after a decrease in bolt elongation; 
         FIG. 11  is a graph of bolt tension versus bolt elongation when using the conventional direct tension indicating and maintaining washer shown in  FIGS. 1-2 ; 
         FIG. 12  is a graph of bolt tension versus bolt elongation when using the direct tension indicating and maintaining washer shown in  FIG. 5 ; 
         FIG. 13  is a top view of direct tension indicating washer in an alternative embodiment of the invention; 
         FIG. 14  is a cross-sectional view taken along line  14 - 14  of  FIG. 13 ; 
         FIG. 15  is an enlarged view of a portion of the direct tension indicating washer of  FIG. 14 ; 
         FIGS. 16-21  illustrate the state of the direct tension indicating washer of  FIG. 13  during the installation process; 
         FIG. 22  is a graph of bolt tension versus bolt elongation when using the conventional direct tension indicating washer shown in  FIGS. 1-2 ; 
         FIG. 23  is a graph bolt tension versus bolt elongation when using the direct tension indicating washer shown in  FIG. 13 ; 
         FIG. 24  is a cross-sectional view of a direct tension indicating and maintaining washer in yet another embodiment of the invention; 
         FIGS. 25A-25E  illustrate emission of indicating material with increased bolt tension; and 
         FIGS. 26A-26E  illustrate emission of indicating material with increased bolt tension in an alternate embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  is a cross-sectional view of the direct tension indicating and maintaining washer of the present invention shown generally at  30 . The direct tension indicating and maintaining washer is similar to the conventional washer shown in  FIG. 1  in that a first surface  14  includes protuberances  12  and the second surface  18  includes indentations  16 . The direct tension indicating and maintaining washer  30  has an opening formed therein having an inner diameter wall  31 . The inner diameter wall  31  meets the second surface  18  at an inner diameter edge  32 . The direct tension indicating and maintaining washer  30  has an outer diameter having an outer diameter wall  33 . The outer diameter wall  33  meets the second surface  18  at outer diameter edge  34 . The inner diameter edge  32  lies in a plane different from the plane of the outer diameter edge  34 . In the exemplary embodiment shown in  FIG. 5 , the direct tension indicating and maintaining washer  30  has a conical cross section. It is understood that other geometries may be used. 
       FIG. 6  illustrates a step in the process of making the direct tension indicating and maintaining washer  30 . First, the protuberances  12  and indentations  16  are formed as described above with respect to  FIGS. 3 and 4 . Second, the direct tension indicating and maintaining washer  30  is placed between a second tool  40  and a second die  42 . The second die includes recess  44  for receiving the protuberances  12  formed on the first surface of the direct tension indicating and maintaining washer  30 . Pressure is applied to the tool  40  and the direct tension indicating and maintaining washer is formed into the conical shape shown in  FIG. 5 . 
     Although  FIG. 6  illustrates a metal stamping manufacturing process, there are other processes, such as metal machining or metal casting, may be used to form the direct tension indicating and maintaining washer  30 . In some cases, the metal product will be heat treated by quenching and tempering after forming to produce the required spring-like load/deformation properties. In an exemplary embodiment, the direct tension indicating and maintaining washer  30  is made from carbon steel, but stainless steel and other alloy products and non ferrous metals may also be used. 
       FIGS. 7-9  illustrate the state of the direct tension indicating and maintaining washer  30  during the installation process.  FIG. 7  illustrates the placement of the direct tension indicating and maintaining washer  30  adjacent to a bolt head  50  which is threaded to a nut  52  (shown in  FIG. 10 ). The conditions shown in  FIG. 7  are identified as stage  1  where the bolt is at rest. As the bolt  50  is tightened by turning its nut  52 , the direct tension indicating and maintaining washer  30  is flattened as shown in  FIG. 8 . At this state, identified as stage  2 , the protuberances  12  on the first surface  14  of the direct tension indicating and maintaining washer  30  have not been fully compressed. The DTI gap, the distance between the first surface  14  and the bottom of the bolt head, is designated as x. Although the direct tension indicating and maintaining washer  30  has been flattened so that the inner diameter edge and the outer diameter edge lie in the same plane, the direct tension indicating and maintaining washer  30  maintains its resiliency and will tend to return to its original conical shape when the bolt tension is released. As the bolt  50  is further tightened, the protuberances  12  are compressed and tightening is discontinued when the DTI gap is at or below a predetermined distance y. At this point, the bolt installer knows that the bolt tension is equal to or greater than the required minimum. This state is identified as stage  3  and is shown in  FIG. 9 . The tension may now be tested with a feeler gage as shown in  FIG. 1A . 
       FIG. 10  illustrates the state of the direct tension indicating and maintaining washer  30  as external loads, metal to metal deformation, or seating in of the bolt, vibration, temperature and other influences cause the bolt to tend to lose elongation. As shown in  FIG. 10 , the direct tension indicating and maintaining washer  30 , due to its resilient nature, pushes against the bottom of the bolt  50  and maintains tension on the bolt. Accordingly, the direct tension indicating and maintaining washer  30  not only indicates when a predetermined bolt tension has been achieved upon bolt installation but maintains bolt tension even as the bolt tension would tend to decline during the life of the bolt/nut assembly. 
       FIG. 11  is a graph of bolt tension versus bolt elongation for a bolt/nut assembly using the conventional direct tension indicating washer  10  shown in  FIG. 1 . As the bolt is tightened, the bolt tension and the bolt elongation increase as shown in  FIG. 11  until the bolt installer is satisfied that the proper tension is achieved at point A. Bolt elongation refers to the increase in bolt length due to tension on the bolt. As shown in  FIG. 11 , any reduction in the bolt elongation (due to vibration, temperature, etc.) will cause the bolt tension to tend to decline until ultimately, the bolt tension is zero or close to zero (point B). 
       FIG. 12  is a graph of bolt tension versus bolt elongation for a bolt/nut assembly using the direct tension indicating washer  30  shown in  FIG. 5 . As the bolt is tightened, the bolt tension and the bolt elongation increase as shown in  FIG. 12  until the bolt installer is satisfied that the proper tension is achieved at point A. The numerals  1 - 4  in  FIG. 12  correspond to states  1 - 4  illustrated in  FIGS. 7-10 . Subsequently, any decrease in bolt elongation will cause a reduction in bolt tension (from A to a minimum level). However, the bolt tension will be maintained at a minimum level  4  by the force applied by virtue of the resilient nature of the direct tension indicating washer  30 . 
       FIG. 13  is a top view of a direct tension indicating washer  60  in an alternative embodiment of the invention. The direct tension indicating washer  60  is similar to the direct tension indicating washer  10  shown in  FIG. 1  in that protuberances  12  are formed on a first surface  14  and corresponding indentations  16  are formed on a second surface  18 . 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  as shown in detail in  FIG. 15 . The direct tension indicating washer  60  is manufactured in a process similar to the direct tension indicating washer  30 . A tool and die are used to stamp the protuberances  12 , indentations  16  and channels  62  into a blank washer. Other processes, such as metal machining or metal casting may be used to form the direct tension indicating washer  60 . In some cases, the metal product will be heat treated by quenching and tempering after forming to produce the required spring-like load/deformation properties. In an exemplary embodiment, the 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 or other materials such a butadiene urethane. 
       FIGS. 16-21  illustrate the state of the direct tension indicating washer  60  as a bolt  50  is tightened on the direct tension indicating washer  60 . As shown in  FIG. 16 , the bolt is placed through an opening in the center of the direct tension indicating washer  60 . This state is identified as stage  1  in  FIG. 16 . As the bolt  50  is tightened, the bottom of the bolt head contacts the protuberances  12  and begins to compress the protuberances towards the first surface  14  as shown in  FIG. 17 .  FIG. 18  is an enlarged view of one of the indentations  16  illustrating that the force exerted by the protuberance  12  on the indicating material  64  forces the indicating material into channel  62 . This state is identified in  FIG. 17  as stage  2 . 
       FIG. 19  illustrates the state of the direct tension indicating washer  60  when the bolt  50  has been tightened to a predetermined tension. When the desired bolt tension is achieved, the protuberances  12  have been compressed into the indentations  16  by a distance sufficient to force the indicating material  64  completely through the channel  62  so that the indicating material  64  appears at the outer diameter of the direct tension indicating washer  60 .  FIG. 20  shows the change in volume of the protuberances  12  from stage  1  to stage  3 . The change in volume of the indentations  16  is calibrated to the volume of channel  62 .  FIG. 21  is an enlarged view of one of the protuberances  12  and the channel  62  when the predetermined bolt tension has been achieved. As shown in  FIG. 21 , the indicating material  64  has been forced through the channel  62  and exits the channel at the outer diameter of the direct tension indicating washer  60 . 
       FIG. 22  is a graph of bolt tension versus bolt elongation when using a convention direct tension indicating washer such as that shown in  FIG. 1 . The desired bolt tension is shown at point B. The bolt installer, however, has no indication that the desired bolt tension has been reached and continues to tighten the bolt past the desired bolt tension B.  FIG. 23  is a graph of bolt tension versus bolt elongation when using the direct tension indicating washer  60  shown in  FIGS. 13-14 . The numerals  1 - 3  on the graph of  FIG. 23  correspond to stages  1 - 3  shown in  FIGS. 16-21 . When the desired bolt tension is achieved at point B, the indicating material  64  will appear at the outer diameter of the direct tension indicating washer  60  and the bolt installer knows to discontinue tightening of the bolt. If the extruded indicating material has worn away over time, and confirmation of bolt tension is needed, a feeler gage may be used to confirm adequate bolt tension as shown in  FIG. 1A . 
       FIG. 24  is a cross sectional view of another embodiment of the invention. The direct tension indicating washer  70  in  FIG. 24  combines the features of direct tension indicating washer  30  and direct tension indicating washer  60  described above. Accordingly, the direct tension indicating washer  70  both maintains tension by using a conical washer body but also includes the channel  62  and indicating material  64  for immediate detection of the proper bolt tension. The inner diameter edge  32  lies in a plane different than the plane of the outer diameter edge  34 . 
       FIGS. 25A-25E  illustrates emission of the indicating material with increased bolt tension. As shown in  FIG. 25 , as bolt tension, P, increases from zero to Ps, the intended calibration tension, the indicating material exits the channel. In  FIG. 25A , the indicating material is silicone, which typically cures in the indentation prior to use. When cured, although flexible, the silicone is not compressible. Consequently, as the volume of the indentations becomes smaller as the protuberance is compressed, the silicone begins appearing on the outside at around 70% of the intended bolt pretension, shown as P=0.7 Ps in  FIG. 25C . This sometimes confuses bolt installers, and they stop tightening at that point. To simply fill the depression with less silicone, does result in a later (higher bolt preload) appearance of silicone, however this practice is risky because of inherent variables in the metal strength the bolt pretension sometimes climbs too high before the silicone is clearly visible. Additional silicone exits the channel as the bolt tension reaches the desired level as shown in  FIGS. 25D and 25E . 
     Accordingly, in alternate embodiments of the invention, a different indicating material is used in the indentations.  FIGS. 26A-26E  illustrate emission of indicating material with increased bolt tension in an alternate embodiment. In the embodiment of  FIG. 26 , the indicating material has compressibility and modified polymerization to control the release of indicating material through the channel. 
     The indicating material in  FIGS. 26A-26E  is made more compressible than typical silicone. The compressibility is achieved by formulating and then energetically mixing the silicone under an inert blanket of nitrogen in such a way as to deposit micro bubbles of nitrogen into the silicone. In lieu of nitrogen, ultra-dry air may be used. These micro-bubbles have the property of being compressible within the liquid silicone, while not affecting the curing properties of the silicone. 
     Modified polymerization generates a cured skin over the compressible silicone in the indentation. Most moisture/heat cured silicones have the property of curing from the outside surface inward without stopping, thereby resulting, after a complete cure, in uniform properties all the way through the cured and solid elastomeric silicone. Embodiments of the invention use silicone with an additive material mixed into the liquid silicone which has, when triggered by heat or infrared radiation, the effect of terminating the polymerization process. This process results in the polymerization of about 1 millimeter of thickness, leaving the remaining silicone core encapsulated in liquid form, ready to compress and then explode out in liquid form. 
     In one embodiment, the silicone is cured by the addition of a platinum based catalyst containing an amine or sulphur based molecular sieve which has the capability of poisoning the curing reaction when activated by heat. After the liquid silicone is deposited into the metal DTI, a short interval of time will allow the surface curing to begin, and then an application of heat will terminate the curing process, leaving the liquid compressible core. 
     In an alternate embodiment, the compressible silicone is deposited in the indentation. The compressible silicone is then capped right away with a second fast curing skin which can be any of a large number of materials, sprayed of otherwise deposited on top of the compressible silicone underneath. The indentation is then flash cured by heat or Ultraviolet Light (UV) so that the compressible silicone underneath stops curing. 
     When this combination of skinned over and liquid core of compressible silicone is used, the onset of silicone appearance at the periphery of the DTI is delayed as shown in  FIGS. 26A-26E . As shown in  FIGS. 26A-26D , as the bolt tension increases from 0 to 0.9 Ps, the silicone indicating material is not emitted through the channel right to the end. This is due to the compressibility of the silicone and the cured skin on the outside of the indicating material. At a particular bolt pretension about 90-95% of the intended bolt calibration load, the silicone appearance on the outside is climactic and dramatic as shown in  FIG. 26E , effectively alerting the installer that the correct bolt pretension has been achieved. 
     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. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.