Abstract:
End mounts are used to secure a helical tension spring to end fixtures with various shapes and sizes. These end mounts contain an inner hole to encase the inner spring end mount and secure the end mount making it like a cap. There is also a keyhole created in the top surface that goes through the end mount allowing it to fit over the fixtures but not over the inner end mount, holding it in place. Grooves are machined in a helical pattern on the cylindrical side wall of the end mount. The spring is wound onto the grooves of the end mount.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to mounts for springs and, more specifically, to end-mounting tension springs. 
       BACKGROUND OF THE INVENTION 
       [0002]    Nested springs are used for various applications. For example, they can be used in the landing gear of airplanes, or on any type of device for securing fixtures to a relatively moving component. They have been used where reliability is paramount or in some situations to reduce the space envelope required where the spring is mounted. 
         [0003]    Often, nested springs are used to avoid single-point failures by providing a redundant system. They replace a single-spring system. In a single-spring system however, when the spring fails, the whole system fails. Having a dual spring system increases the factor of safety. If one spring fails, the other can still properly function for a time until the broken spring is replaced. 
         [0004]    Some other examples include throttle return springs, brake return springs, and valve springs. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention relates to mounts for springs. More specifically, the present invention comprises an end-mount system for tension springs to attach to fixtures in various applications using grooves and spring tension to keep the spring and fixtures secured together. The system comprises an inner spring and end mount along with an outer spring, encasing the inner spring, and outer end mount resting on the inner end mount. These are attached to fixtures on either end, which can differ depending on the application. 
         [0006]    In accordance with further aspects of the invention, the end mounts are cylindrical in shape to allow for the helical tension spring to be secured to the end mount by way of being wound onto grooves on the end mount. The inner diameter of the grooves is slightly larger than the coil diameter of the spring. As the spring is tensioned the increased inward forces of the spring hold the spring on the end mount in a fashion similar to a Chinese finger trap. If the spring were free of the end mount these forces would tend to reduce the spring diameter all along the spring. 
         [0007]    In accordance with other aspects of the invention, the end mounts have keyholes to fit over end fixtures. The keyholes are shaped so that the end mount fits over the fixture but not over the inner spring and inner end mount. The keyhole can be any shape as long as it fills this purpose. 
         [0008]    In accordance with still further aspects of the invention, the fixtures are machined or attached to the inner spring end mount. These fixtures can be a variety of shapes and sizes depending on the application at hand. 
         [0009]    In accordance with yet other aspects of the invention, a dual spring system, with an outer spring encasing an inner spring, allows each spring to be reduced in mass and the strength of the system increases greatly. In addition, a measure of safety is added with another spring introduced to the system. The resonate frequency of the combined spring system is also dramatically changed in comparison to a single spring system for the same application. This change in frequency can also be used to increase the safety of the overall system by avoiding dangerous frequency coupling. 
         [0010]    As may be appreciated from the foregoing summary, the invention provides a more secure and clean attachment that adds a factor of safety, is reliable in spring attachments without fasteners, and saves weight. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0012]      FIG. 1  is a side view of a prior-art nested tension spring arrangement, 
           [0013]      FIG. 2  is a side view of the spring assembly, 
           [0014]      FIG. 3  is a side view of the inner portion spring assembly, 
           [0015]      FIG. 4  is a side view of the outer spring assembly, 
           [0016]      FIG. 5   a  is an isometric view of the outer spring end mount, 
           [0017]      FIG. 5   b  is an end view of the outer spring end mount having a wider key-hole opening, 
           [0018]      FIG. 5   c  is a side view of the outer spring end mount, and 
           [0019]      FIG. 6  is a side view of the outer portion spring assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    As illustrated in  FIG. 1 , prior-art nested tension springs  4 ,  6  use opposing hooks to restrain the spring from releasing from their mount locations. The outer hook  10  opposes the inner hook  8 . It is to be noted that the previous tension springs  4 ,  6  typically failed at the hook most particularly at the location where the large bending stress on the hook met the large torsion stress coming from the coil under a tensile load. The relationship of the hooks  8 ,  10  to the springs  4 ,  6  will be better understood after the following description. The operation of the springs  4 ,  6  may be understood upon reference to  FIG. 1 . The embodiment consists of two tensions springs  4 ,  6 , an inner spring  4  and an outer spring  6 . The above springs  4 ,  6  connect at hooks  8 ,  10  on either end. These springs  4 ,  6  are assembled together so that the coiled spirals are wound in opposing directions. The hooks  8 ,  10  however, are connected to fixtures in such a manner that upon application of repeated tensile forces the hooks may fail. These hooks  8 ,  10  do not have a clean or secure attachment. Stress concentrations arise at the fixture or element to which the hooks secure and at the bend in the hooks prior to the windings. Also with the prior art, when one spring fails, the hooks move, often causing the entire system to fail. 
         [0021]    To resolve the difficultly noted above, embodiments of the present invention include an inner spring  12  and an outer spring  16  that are connected to fixtures with end mounts  14 ,  18  in such a manner that upon application of tensile forces there is much less chance for failure and a more secure and clean attachment. These end mounts  14 ,  18  may be machined from various materials such as metals, plastics, or composites. The relationship of the end mounts  14 ,  18  to the embodiment will be better understood upon reference to  FIG. 2 . 
         [0022]    As shown in  FIG. 2 , an inner spring  12  is attached to fixtures  20 ,  21  with end mounts  14  on either end. Surrounding the inner spring  12  is the outer spring  16  wound onto end mounts  18 . Because there is more than one spring in the system, each individual spring can have a lower mass as strength from multiple springs is combined. As a result of the lower mass of each individual spring in the system, the dual-spring embodiment has a higher intrinsic natural frequency. This higher natural frequency adds safety as the frequency is out of the range of frequencies experienced in use. When the apparatus experiences frequencies similar to its natural frequency it can more easily fail. This has been seen in airplane landing gear. Because the natural frequency of the system is higher a measure of safety is added. 
         [0023]    Returning to the end mounts  18 , the inner diameter of the grooves  28  in end mounts  18  is slightly larger than the resting coil diameter of the outer spring  16 . For this reason, coupled with the fact that the mentioned springs are tension springs, as the spring is introduced to tension the spring diameter naturally tends to decrease, however, the end mount  18  forces the coil diameter to remain the same on the end mounts  18 . This makes the outer spring  16  compress around the end mount  18 , holding it on securely without slippage. 
         [0024]    Another advantage these end mounts  18  provide for the embodiment is that if one spring fails the end mounts  14 ,  18  hold the other spring in the same alignment. With the prior art the hooks  8 ,  10  move when one spring fails. Such movement can be a problem, especially with a tight space envelope or if the springs are near a fuel tank or other combustible where a spark may be an issue. Furthermore, since conventional fasteners are eliminated, failures are further reduced. There are fewer parts to fail. The bending stresses are minimized and the torsional stresses are spread over multiple coils in the end-mounting region. 
         [0025]    Turning now to the end mount  18  more specifically as seen in  FIG. 5 , there is a keyhole  24  created. This keyhole  24  extends through wall  26  of end mount  18  in order that the end mount  18  may fit over the end fixtures  20  and  21  attached to the outer spring. The keyhole  24  has the shape as shown on the wall  26  of the end mount. The hole shape is configured to provide clearance for fitting over one end of the fixture while being small enough to engage the inner spring mount or other stop as discussed below. The wall  26  of the end mount is preferably a flat surface on which a snap ring  22  rests to hold the end mount  18  in place as shown in  FIG. 2 . 
         [0026]    The end mount  18  also preferably has a recess such that it can nest over the inner end mount  14  and the inner spring  12 . An inner surface  36  of wall  26  in the end mount  18  rests on the inner end mount  14  or the flange  15 , if a flange is used. Alternately to a flange, a separate stop, such as a washer may be used. The shape of the keyhole  24  may be specific to the fixture or can be a fixed shape such as a circle (as in  FIG. 5   a ). The keyhole  24  is created to allow the end mount  18  to fit over the end fixtures  20  and  21  attached to the outer spring  16  so that the spring  16  is able to be wound onto the end mounts  18 . Fixtures  20  and  21  attached to the outer spring  16  may vary in shape, size and length depending on need from the desired application. 
         [0027]    Also as noted in  FIG. 5 , the end mount  18  has a taper  32  which then turns into the grooves  28  that the outer spring  16  is wound onto. The grooves  28  are constantly spaced and have a constant diameter. As the spring is wound onto the mount, the force to turn it on tends to enlarge the spring diameter. However, once mounted, the tensile forces during use tend to keep the spring securely mounted. 
         [0028]    The cylindrical side wall  30  of the end mount  18  is preferably generally constant in diameter along its length, except of course for the variation introduced by the grooves in the outer portion of the wall. The inner hole  34  has a constant diameter that is able to fit over inner end mount  14  and shoulder  15  or  17  (as seen in  FIG. 3 ). However, the inner hole  34  diameter is preferably tight enough around the inner end mount  14  and shoulder  15  or  17  that it does not slide around and come out of place. The inner hole surface  36  is preferably flat and perpendicular to the side wall  30  of the end mount  18 . This allows the end mount  18  to rest on the shoulder  15  or the inner end mount  14  securely. 
         [0029]    As shown in  FIG. 2 , the outer spring end mount  18  rests on the inner spring end mount  14 . The inner spring end mount  14  has a machined shoulder  17  or a flange  15  with a wall  19 , which serves the purpose of holding the outer spring  16  in place and provides a wall  19  for the outer spring end mount  18  to be fixed. As a consequence of having two springs  12  and  16  in the assembly each spring can weigh less because of the added strength of including an additional spring. It is to be noted that the springs  12  and  16  coil in opposite directions. For this reason coupled with the fact that there are two tension springs  12  and  16 , great strength is added to the apparatus. 
         [0030]    As tension is added to the springs  12 ,  16  they pull in opposing directions because they are wound oppositely. Incidentally, it will be appreciated that a greater factor of safety is added in the use of two springs  12 ,  16 . As shown in  FIG. 3  the apparatus is capable of functioning with the use of only one spring. The fixtures  20  and  21  are still securely connected and the embodiment can still function. Also, as shown in  FIGS. 4 and 6 , the outer spring  16  can still function if the inner spring  12  fails. If one spring fails, the other one is still present and can function without the other. This added measure of safety is especially important as this device is often used in applications with high stress and multi-directional, vibrating, and/or high speed motion. 
         [0031]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, a safety rod could be added for an even higher measure of safety. Thus if both springs  12  and  16  were to fail, the rod would hold the fixtures together. The rod could be attached using keyholes from the rod into the fixtures. 
         [0032]    Another example used in the application of securing the outer end mounts  18  could be the use of flanges. These flanges could take the place of the shoulder of the inner end mount. Flanges could be machined directly onto the fixtures  20  and  21  or arm of the fixture  20  and  21  depending on the application. These flanges would then hold the outer end mount  18  in place. 
         [0033]    Also, depending on the fixture  20  and  21 , differing sizes of outer end mounts  18  and keyholes may be used. In relation to the springs  12  and  16 , differing sizes and materials may be used depending on the strength need and the function of the device. Free length, pitch, diameter, coil diameter, and the number of coils may all vary. Similarly the diameter of the outer end mounts  18  may vary depending on the outer spring  16  coil diameter and the size of the fixtures  20  and  21  attached. 
         [0034]    The dual-spring system can alternatively be more than two springs, even as many as four or five springs. These do not necessarily have to be nested. They may be secured one next to the other, however, this does not necessarily reduce the space envelope. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.