Abstract:
A threaded engagement element comprising a body having a longitudinal axis and a thread adapted to be received by a corresponding thread of a second element. The threads are characterized by a thread parameter. The body has first and second portions that each has a different value for the thread parameter. The body may have more than one thread count or more than one geometry. This configuration creates a threaded engagement element with self-locking threads, which prevents unintentional loosening.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to cooperative threaded engagement elements with self-locking threads. More particularly, it relates to a pair of threaded engagement elements, one of which has a portion with differing thread parameters, which prevents one element of the pair from loosening relative to the other after tightening. 
       BACKGROUND OF THE INVENTION 
       [0002]    Bolted joints are used to affix two or more parts together and typically have two engaging elements (a bolt and nut or equivalent structure) that are characterized by cooperative threads. The threads are most often spiral and have a given pitch and thread count. Relative longitudinal movement between the nut and bolt along a common axis is produced when one or the other element is rotated. At some longitudinal position, the two elements engage the parts that are to be fastened, and fix the same in place when sufficient torque is applied to one element relative to the other. The parts remain fixed due to the frictional force that is generated at the interfaces between threads, the parts and the surfaces of the engaging elements (nut and bolt) that contact the parts. This type of fastened joint suffers from the problem of unintentional loosening due to operational conditions. Tightened bolts or nuts rotate loose when relative motion between the cooperating threads occurs. 
         [0003]    Several factors can cause this relative motion. First, parts can bend which results in forces being induced at the friction surface. Second, changes in temperature can cause the bolt and nut to alternatively constrict and expand, causing slight relative movement that reduces those frictional forces. Finally, applied forces on the joint components can lead to shifting of the joint surfaces. 
         [0004]    Many attempts have been made to prevent a bolt from being loosened unintentionally. Some attempts involve the increase of friction between the male and female threads. Other attempts try to increase the frictional force between the bottom of the bolt head and the work piece by the use of extra components such as washers interposed between an engaging element and the part that it contacts. These can be split, star or spring washers. Others have added a chemical adhesive to the threads to bond the nut to the bolt at the threads. 
         [0005]    However, these attempts have several disadvantages. First, these attempts still suffer from relative loosening when different forces are applied. Second, an increase in friction often takes more torque to install than a standard nut and bolt. Third, the extra parts or chemicals significantly increase assembly cost and time. The present invention overcomes these disadvantages by providing a bolt that is as easy to install as a standard assembly but does not suffer from loosening over time. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to provide a threaded element with threads that self-lock when engaged with a cooperatively configured second threaded element. 
         [0007]    Another object of the present invention is to prevent relative loosening between cooperative threads of first and second fastening elements. 
         [0008]    Still another object of the present invention is to shorten the cost and assembly time of a bolted joint. 
         [0009]    According to one aspect of the present invention, a threaded engagement element includes a body having a longitudinal axis and a thread adapted to be received by a corresponding thread of a second element. The threads are characterized by a thread parameter. The element body has first and second portions each with a different value of the thread parameter. 
         [0010]    According to another aspect of the present invention, a cooperative element pair for use in affixing a first part to a second part includes a first element having a longitudinal axis and a thread formed on a surface thereof. A second element has a thread formed in a surface thereof. The thread of the second element is adapted to be received by the first element thread. Each of the first and second element threads are characterized by a thread parameter. The first element further has first and second portions, each with a different value of the thread parameter. 
         [0011]    According to still another aspect of the present invention, a bolt includes a generally cylindrical body having a longitudinal axis and an external thread. The bolt also has a head continuous with the body. In addition, the thread on the body may have more than one thread geometry. Preferably, the thread geometry may be steeper as it becomes closer to the head of the bolt. More preferably, the thread geometry may be steeper for the last four threads closest to the head of the bolt. 
         [0012]    The body may also have more than one thread count. Preferably, the thread count may be higher closer to the head of the bolt. More preferably, the thread count may be higher for the last four threads closest to the head of the bolt. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a simplified schematic side view of a bolt with self-locking threads provided in accordance with the present invention. 
           [0014]      FIG. 2  is a simplified schematic end view of the bolt of  FIG. 1 . 
           [0015]      FIG. 3  is a simplified schematic illustration of the bolt of  FIG. 1 . 
           [0016]      FIG. 4  is a simplified schematic illustration of a second element configured to be received by the bolt of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    The present invention is directed to fastening elements that are configured to prevent unintentional loosening.  FIG. 1  shows one embodiment of the present invention in a simplified schematic form. In  FIG. 1 , a bolt  10  has a generally cylindrical body  12  having a longitudinal axis  14  and an external thread  16 . The bolt  10  also has a head  18  continuous with the body  12 . As seen in  FIGS. 2 and 3 , the head  18  has a series of flat peripheral surfaces configured to receive cooperative surfaces of a tool (not shown) that is used to rotate the bolt about the longitudinal axis  14 . The head  18  extends radially beyond the body  12  by a pre-selected amount. 
         [0018]    The body  12  has an external thread  16  all the way to the head  18 , or the body  12  may have a shank  20  disposed between the threaded body and the head  18 . The shank  20  does not have any threads in the preferred embodiment. The longitudinal dimension of the shank is important as detailed hereinafter. 
         [0019]    The threads in both the first and second elements are characterized by certain thread parameters such as pitch, thread count, and thread geometry. “Thread geometry” includes the thread angle, which is the angle of an individual thread as measured from a thread trough  21  to an outer thread surface  23  when compared to a perpendicular  25  extending from longitudinal axis  14 . Thread count is the number of threads per unit of longitudinal measure (e.g. “threads per inch”). “Pitch” is the distance from one thread to the next as measured along the length of the axis. Other thread parameters may also be configured as thread count, pitch, and thread geometry are described herein alone or in combination and are therefore contemplated by the present invention. 
         [0020]    A cooperative threaded engagement element pair includes first and second elements such as a bolt and a nut shown schematically in  FIG. 4 . In the prior art, these elements are characterized by uniform thread pitch and thread count allowing for smooth advancement of one element relative to the other until the parts that are to be joined are in fixed contact with one another. As noted, this arrangement often requires additional measures to preclude loosening of the joined parts by way of slight rotational movement. 
         [0021]    With the present invention, the external thread  16  may have more than one thread geometry or more than one thread count. There are at least two regions of the threaded body. The first threaded body portion  28  has a pre-selected thread geometry and count. The second threaded body portion  30  is located adjacent the shank  20  and has a second pre-selected thread count or thread geometry, one (or both) of which is (are) different from that of the first threaded body portion. 
         [0022]    Preferably, the thread geometry becomes steeper closer to the head  18  of the bolt  10 . More preferably, the thread geometry is steeper for the last four threads closest to the shank  20  of the bolt  10 . By “steeper” it is meant that the thread angle increases in magnitude. A change in thread angle of about 10 per cent is preferable. 
         [0023]    In certain applications, it is preferable for the thread count of threaded body portion  30  to become higher closer to the shank  20 . More preferably, the thread count is higher for the last four threads closest to the head  18  of the bolt  10 . In the embodiment of the present invention shown in  FIG. 1 , the thread count is increased in threaded body region  30  by about 10% as compared to that of body region  28 . 
         [0024]    In some applications it may be desirable for the threaded body portion  30  to have more than one thread count and/or more than one thread geometry, with a multitude of combinations of the same all contemplated by the present invention. The thread may have both a steeper thread angle and a higher thread count closer to the head  18  of the bolt  10 . In the preferred embodiment, the thread of threaded body portion  30  has a longitudinal dimension that extends to encompass at least the last four threads closest to the head  18  of the bolt  10 . Those skilled in the art will note that the longitudinal dimension of threaded body portion  30  is a function of the particular application. 
         [0025]    In the preferred embodiment, the thread geometry and count of the corresponding thread  42  of the second element  40  that receives that element of the complimentary pair having threaded body portion  30  is the same as that of threaded body portion  28 . However, those skilled in the art will note that the present invention contemplates embodiments wherein the thread geometry and thread count of the corresponding thread  42  of the second element  40  may contain a portion in which either thread geometry or count is varied as well as that in first element threaded body portion  30 . 
         [0026]    In the embodiment of  FIG. 1 , the bolt  10  may be used by fitting it into a threaded nut, bore, or hole, any of which comprises the second element  40  of the complimentary element pair. The bolt  10  is then twisted until the corresponding threads  42  of the nut, bore, or hole engage the differing thread geometry or thread count of the external thread  16 . Preferably, the bolt  10  is twisted until the threads  42  of the nut, bore, or hole engage the external thread  16  at a point where the thread angle is steeper or the thread count is higher. More preferably, the bolt  10  is twisted until the threads  42  of the nut, bore, or hole engage the external thread  16  at the last four threads closest to the head  18  of the bolt  10 , where the thread angle is steeper or the thread count is higher. As stated above, the external thread  16  of bolt  10  may have both a steeper thread angle and a higher thread count. 
         [0027]    Those skilled in the art will note that the location of threaded body portion  30  is preferably adjacent the shank whose longitudinal dimension is a function of the particular application. In that sense, the thickness of the parts to be joined are taken into consideration so that the segment of threaded body portion  30  that is engaged with the second element  40  is sufficient to ensure the two elements remain tightly affixed to one another. Furthermore, the present invention also contemplates embodiments where the thread count or geometry of the threaded body portion  30  can decrease as compared to threaded portion  28 . 
         [0028]    Those skilled in the art will note that the cooperative threaded engagement element pair provided in accordance with the present invention is configured to be reusable in that the elements thereof can be repeatedly engaged and disengaged with the benefits of the present invention and without performance degradation. As such it is seen that the present invention overcomes the limitation of the prior art in requiring an additional component such as a split washer or a bonding agent to be applied each and every time the element pair are used to fasten parts together. 
         [0029]    Moreover, the prior art teaches away from the present invention in that it was believed that differences in thread geometry and/or thread count between threaded fastening elements was to be avoided as the threads were inevitably damaged when engaged, thereby precluding proper fastening torque and subsequent reuse. However, the present invention overcomes these drawbacks. 
         [0030]    While the invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various obvious changes may be made, and equivalents may be substituted for elements thereof, without departing from the essential scope of the present invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention includes all embodiments falling within the scope of the appended claims.