Abstract:
A reusable washer assembly apparatus includes a generally ring-shaped flexible inner member, a generally ring-shaped outer member, a distal washer member, and a proximate washer member. The outer member is disposed around the inner member and is adapted for at least partial rotation about the inner member. The flexible inner member and outer member define a first gap between each other. The distal washer member communicates with a first surface of the inner member and the proximate washer member communicates with a second surface of the inner member. An applied force to at least one of the proximate washer member and the distal washer member causes the flexible inner member to deform and at least partially fill the first gap thereby preventing rotation of the outer member when the applied force reaches a threshold value.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     This is a continuation of prior application Ser. No. 10/991,639 filed Nov. 17, 2004, by inventors Jeffery A. Wilkerson and John R. Porter, entitled “Reusable Preload Indicating Washer Assembly,” now U.S. Pat. No. 7,096,740, which is incorporated by reference in its entirety. 
     
    
     FIELD OF INVENTION  
       [0002]     This invention relates to fastening devices, and more particularly, to preload indicating washer assemblies that may be used in connection with bolts and nuts, with externally threaded studs, or with other fasteners, shafts, axles or rods.  
       BACKGROUND  
       [0003]     Preload indicting washer assemblies are used with fastening devices such as, for example, nuts and bolts, for critical applications where it is important to know a predetermined compression value applied by the fastening devices or an axial tension in a bolt, rod, etc. Examples of important applications for such assemblies may include the fastening of certain critical joints of aircraft, etc.  
         [0004]     The use of torque wrenches do not always provide consistent indications of the compression or load actually applied by the fasteners. That is, even when the same torque (as read by a torque wrench) is applied to a plurality of fasteners of the same fastener type and size, differing preloads are actually experienced on the fasteners due to tolerance variations in the manufacture of the fasteners, including small variations in fastener dimensions, materials and coatings, as well as variations in any lubrication that may be applied. It is believed that under some circumstances, the application of the same torque as read by a torque wrench to a plurality of bolts supplied under the same part number, can result in a variation of 25% or more in the load actually applied by the bolts. Thus preload indicating washer assemblies can be used with application-critical fasteners to provide a more accurate indication of the actual compression applied by the fasteners on their associated joints. Additionally, these assemblies can be used to provide an indication of compression in joint locations that are not accessible by torque wrenches.  
         [0005]     Known preload indicating washer assemblies function by a plastic deformation of an inner ring member to “indicate” when the proper bolt load is reached. That is, the inner ring member is concentrically mated with an outer ring member having a shorter axial height than the inner ring member. The inner and outer ring members are sandwiched between two, standard washers. This assembly is mated with a bolt or other fastener so that the shorter, outer ring member can be made to manually rotate around both the taller inner ring member as well as the fastener when no compression, or when a compression below the rated load, is applied.  
         [0006]     In operation, the inner ring member is subjected to compression applied by the fastener, such as a bolt, as two or more components of a structure are being assembled. When the desired, predetermined compression is reached, the inner ring member is plastically deformed so that the two standard washers abut each end of the outer ring member with sufficient force that the outer ring member is no longer able to rotate around the inner member or the fastener. Thus by manually attempting to rotate the outer ring member from time to time as increasing compression is applied, the user can know that the desired, predetermined compression has been reached when the outer ring member can no longer be manually rotated.  
         [0007]     Because the deformation of the inner rings of known, preload indicating washer assemblies is plastic and permanent, these assemblies cannot be reused to indicate proper bolt or fastener loading. Another washer assembly, or at least another inner ring component, must be used. Additionally, if the joint or bolt loses tension due to relaxation etc. of the assembled structure, the act of simply retightening the nut (and thereby re-applying increased compression to the previously-deformed inner ring) will not ensure an adequate joint load.  
         [0008]     Thus it is desirable to provide an improved preload indicating washer assembly that is repeatable, i.e. that can be reused and/or re-tightened while retaining the ability to accurately indicate the design or rated compression experienced by it.  
       SUMMARY OF THE INVENTION  
       [0009]     Washer assemblies are provided that indicate proper fastener loading. In addition, these assemblies are reusable. For instance, after an applied force is removed, the force can later be reapplied (i.e., a fastener can be re-tightened) and the assembly will still indicate when proper fastener loading is reached.  
         [0010]     In many of these embodiments, a reusable washer assembly apparatus includes a generally ring-shaped flexible inner member, a generally ring-shaped outer member, a distal washer member, and a proximate washer member. The outer member is disposed around the inner member and is adapted for at least partial rotation about the inner member. The flexible inner member and outer member define a first gap between each other.  
         [0011]     The distal washer member communicates with a first surface of the inner member and the proximate washer member communicates with a second surface of the inner member. An applied force to at least one of the proximate washer member and the distal washer member causes the flexible inner member to deform and at least partially fill the first gap thereby preventing rotation of the outer member when the applied force reaches a threshold value.  
         [0012]     In other examples, the outer member and the proximate washer member define a second gap. In this case, the applied force causes the flexible inner member to deform and at least partially fill the second gap, thereby preventing rotation of the outer member when the applied force reaches the threshold value.  
         [0013]     The applied force can be applied via a fastener. In one example, the fastener is a bolt. Other types of fasteners may also be used.  
         [0014]     Additionally, the inner member may be constructed from various types of materials. For instance, the inner member may be constructed of nitinol.  
         [0015]     In others of these embodiments, the outer member has an outer surface and a lug extending from an outer surface of the outer member. In other examples, the apparatus is used with an actuator and the outer member has an outer surface defining a bore adapted to receive the actuator.  
         [0016]     Thus, various types of washer assemblies are provided that indicate when a certain force level has been applied to the assembly. The washer assemblies can be reused so that forces can be applied, removed, and reapplied and still accurately indicate whether a force threshold has been reached.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is an exploded, perspective view of a preload indicating washer assembly according to one embodiment of the present inventions.  
         [0018]      FIG. 2  is a top plan view of the washer assembly of  FIG. 1 .  
         [0019]      FIG. 3  is a cross-sectional view of the washer assembly of  FIG. 2  without any compression force being applied, as viewed along lines  3 - 3  of  FIG. 2 .  
         [0020]      FIG. 4  is a cross-sectional view of the washer assembly of  FIG. 2  with a compression force applied.  
         [0021]      FIG. 5  is a stress-strain diagram of two materials. 
     
    
     DETAILED DESCRIPTION  
       [0022]     In the following description, reference is made to the accompanying drawings which form a part hereof and which illustrate several embodiments of the present invention. It is understood that other embodiments may be utilized and structural and operational changes may be made without departing from the scope of the present invention.  
         [0023]     One embodiment of the present invention provides an improved preload indicating washer assembly designed to operate in the elastic region for the rated, predetermined compression that is desired. Because it operates in the elastic region, the preload indicating washer assembly resumes its original shape when the compression is decreased or removed. The washer assembly elastically deforms again to the same extent as its original deformation when the predetermined compression is restored. Thus the washer assembly will indicate the proper fastener preload when used for multiple loading cycles or in multiple locations.  
         [0024]     Referring to  FIGS. 1-4 , there is shown a preload indicating washer assembly  100  comprising an inner ring  102 , an outer ring  104 , a distal washer  106 , and a proximate washer  108 . The outer ring  104  has a cylindrically-shaped inner surface  110 , a cylindrically-shaped outer surface  112  and an axial height  114 . The inner diameter of the outer ring  104  is adapted to mate with a threaded extension of a bolt or with another elongated member (not shown) extending from a fastening apparatus or from any other surface. The outer ring  104  further has a proximate side  116  and a distal side  118  each of which connects the inner surface  110  and outer surface  112 .  
         [0025]     The inner ring  102  has a cylindrically-shaped outer surface  120 , a cylindrically-shaped inner surface  122  and an axial height  124  that is greater than the axial height  114  of the outer ring  104 . The inner ring  102  further has a proximate side  126  and a distal side  128 , each of which connects the outer surface  120  and inner surface  122 . The inner diameter of the inner ring  102  is also adapted to mate with the same bolt extension or other elongated member (not shown) as is used with the outer ring  104 .  
         [0026]     As best seen in  FIGS. 2 and 3 , the outer diameter of the inner ring  102  is less than the inner diameter of the outer ring  104  so that the inner and outer rings may be concentrically mated such that the outer ring inner surface  110  surrounds a portion of the inner ring outer surface  120 . When the inner and outer rings  102 ,  104  are concentrically mated, the proximate washer  108  is placed so that it abuts the proximate side  126  of the inner ring  102 . Similarly, the distal washer  106  is placed so that it abuts the distal side  128  of the inner ring  102 . As so arranged, the assembly  100  is placed on the threaded extension of a bolt or on another elongated member. As best seen in  FIG. 3 , a first gap  130  is thereby formed between the inner surface  110  of the outer ring  104  and the outer surface  120  of the inner ring  102 . Also, a second gap  138  is formed between the proximate washer  108  and the proximate side  116  of the outer ring  104 . While the embodiment of  FIGS. 1-4  shows the use of inner and outer rings, it will be appreciated that inner and outer members having other shapes and geometries may be used as well.  
         [0027]     The outer ring  104  has a plurality of bores  132  extending from the outer surface  112  to the inner surface  110 . These bores  132  are adapted to receive a peg or other actuator (not shown). When the assembly  100  is placed on a bolt or other elongated member, the user can insert the peg into one of the bores  132  so as to provide a manual grip for assistance in manually rotating the outer ring  104  around the inner ring  102  and the bolt. As best seen in  FIG. 4 , when a compression force (represented by the arrows designated by the letter “F”) of a predetermined value corresponding to the rated load is placed on the inner ring  102 , the inner ring deforms elastically such that any further rotation of the outer ring  104  is prevented or inhibited. This prevention/inhibition of the rotation is caused by the frictional force of the proximate and distal washers  108 ,  106  acting on the proximate and distal sides  116 ,  118  of the outer ring as is evident by the disappearance of the second gap  138  ( FIG. 3 ).  
         [0028]     In alternative embodiments, however, the inhibition/prevention of the rotation is caused by the deformation of the inner ring outer surface  120  such that it abuts the outer ring inner surface  110  with sufficient frictional force to inhibit or prevent any further rotation of the outer ring  104 . Thus in such alternative embodiments, the first gap  130  ( FIG. 3 ) would disappear when the predetermined compression force is applied.  
         [0029]     While the embodiment of  FIGS. 1-4  shows bores  132  that are used with pegs, it will be appreciated that other designs may be employed to facilitate the rotation of the outer ring  104 . For example, the outer ring  104  can include one or more lugs extending from the outer surface  112  for use in gripping the outer ring  104 . Alternatively, the outer surface  112  can have a plurality of knurls to assist in gripping the outer ring  104 .  
         [0030]     The inner ring  102  is constructed of a material that is adapted to deform elastically in the design operating range of the rated, predetermined compression. According to one embodiment of the invention, the inner ring  102  is constructed of nitinol, Le. a nickel titanium alloy. Exemplary nitinol can be obtained from Nitinol Technologies, Inc., of Auburn, Wash. under the designation “60 Nitinol.” However, other embodiments of the invention may be constructed of other materials that deform elastically for the desired, rated compression load.  
         [0031]      FIG. 5  is a stress-strain diagram showing force deflection curves for two materials used as inner ring components of preload indicating washer assemblies. The material represented by the dotted line  134  shows the behavior of a material used for known inner ring components. The design or rated compression for the known device is in the plastic region of this curve  134 . Thus as can be seen, any removal and subsequent restoration of compression on such a device would not result in a reliable and repeatable deformation of the material for purposes of indicating the actual compression at a joint.  
         [0032]     The solid line  136  shows the behavior of a material used according to one embodiment of the invention. In contrast to known preload indicating devices, this material operates in the elastic region for the rated, predetermined compression. Thus any removal and subsequent restoration of compression on a device made of this material results in a reliable change in the deformation for purposes of indicating the amount of applied compression. This therefore provides the desired “repeatability” of a preload indicating device.  
         [0033]     It will be appreciated that preload indicating washer assemblies can be designed for different rated or predetermined compression loads by the selection of different materials according to their stress-strain characteristics. Additionally, variations in design loads can be achieved by varying the height or thickness of the inner ring component, or in alternative embodiments of the invention, by varying the width of the gap between the outer surface of the inner ring and the inner surface of the outer ring.  
         [0034]     In operation according to some embodiments, the distal washer  106  is placed around a threaded extension of a bolt or other elongated member (not shown). Next, the inner ring  102  and the outer ring  104  are placed around the bolt extension and are aligned concentrically with one another so that the outer ring inner surface  110  surrounds a portion of the inner ring outer surface  120 . Then, the proximate washer  108  is placed around the threaded extension so that it abuts the outer ring proximate side  116 .  
         [0035]     By exerting a torque on a nut engaged with the threaded extension, a first axial compression is applied to the assembly  100 , including the inner ring  102 . This first compression is less than a predetermined value corresponding to the design or rated compression of the inner ring  102 . Using an actuator engaged in one of the bores  132  of the outer ring  104 , the outer ring  104  is at least partially rotated around both the inner ring  102  and the threaded extension thus confirming that the design compression has not been reached. Next, a second compression that is less than the rated load is applied to the assembly, including the inner ring  102 . The outer ring  104  is again rotated thus again confirming that the design compression has not yet been achieved.  
         [0036]     This process is continued until the compression experienced by the inner ring  102  reaches the predetermined, rated compression value. At this point, the inner ring  102  will elastically deform such that the proximate washer  108  abuts the proximate side  116  of the outer ring  104 , and the distal washer  106  abuts the distal side  118  of the outer ring  104  with sufficient force to inhibit rotation of the outer ring  104 . (Or in alternative embodiments, the inner ring  102  will elastically deform such that the inner ring outer surface  120  abuts the outer ring inner surface  110  with sufficient force to inhibit the rotation of the outer ring  104 .) At this point, when the actuator is used in an attempt to rotate the outer ring  104 , it will be observed that the outer ring  104  cannot rotate thus indicating that the rated compression as experienced by the inner ring  102 , the fastener and the joint has been reached.  
         [0037]     A some later time, the compression on the assembly  100  is reduced from the predetermined, rated load to a lower, second value, either intentionally or due to relaxation of the joint. The outer ring  104  is rotated thus confirming that the rated load is no longer present at the bolt. Next, a compression in the amount of the rated load is again applied to the assembly  100  and the inner ring  102 . Once again the inner ring  102  elastically deforms as previously described so as to inhibit the rotation of the outer ring  104 .  
         [0038]     At yet another time, the compression on the assembly  100  is removed entirely, the nut is disengaged from the bolt and the assembly  100  is removed. This same assembly  100  is then reassembled on the original members or placed on another bolt or other type of fastener, and the above process is repeated.  
         [0039]     Thus there is disclosed an improved preload indicating washer assembly that operates in the elastic region for the rated, predetermined compression that is desired. Because it operates in the elastic region, the preload indicating washer assembly resumes its original shape when the compression is decreased or removed. The washer assembly elastically deforms again to the same extent as its original deformation when the predetermined compression is restored. Thus the washer assembly will indicate the proper fastener preload when used for multiple loading cycles or in multiple locations.  
         [0040]     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.