PRIORITY CLAIM 
     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 12/710,899 filed Feb. 23, 2010, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to mounts for springs and, more specifically, to end-mounting tension springs and associated friction-resistant coatings. 
     BACKGROUND OF THE INVENTION 
     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. Examples of applications of nested springs are throttle return springs, brake return springs, and valve springs. 
     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. In some cases, the springs wear, twist, or bend excessively where portions of the spring contact one another. The effect is worsened where the contact is acute, such as with a small contact surface area under high load. Also, some materials are abrasive, such as Titanium, and wear more quickly than other materials. 
     SUMMARY OF THE INVENTION 
     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. 
     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 grooves of the end mount in one embodiment have a greater pitch than the pitch of the tension spring before being secured to the mount. In a preferred embodiment, the pitch of the mounting portion of the spring has a greater pitch than the remainder of the spring, to match the pitch of the mount grooves. 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. 
     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. 
     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. 
     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 resonant 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. 
     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 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  is a side view of a prior-art nested tension spring arrangement, 
         FIG. 2  is a side view of the spring assembly, 
         FIG. 3  is a side view of the inner portion spring assembly, 
         FIG. 4  is a side view of the outer spring assembly, 
         FIG. 5 a    is an isometric view of the outer spring end mount, 
         FIG. 5 b    is an end view of the outer spring end mount having a wider key-hole opening, and 
         FIG. 5 c    is a side view of the outer spring end mount. 
         FIG. 6  is a side view of the outer portion spring assembly. 
         FIG. 7  is an isometric view of an end mount with a machined channel and a friction-resistant coating in the channel. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     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 . 
       FIG. 2  shows a spring assembly  11  including an inner spring  12  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 . Any component of the spring assembly  11  can have a coating  40  applied to the outer surface to ease friction, prevent bending, twisting, and other wear that may occur between contacting portions of the spring assembly  11 . The springs  12 ,  16 , the end fittings  14 ,  18 , and any other component of the assembly  11  can have the coating  40 . The coating  40  can be between any two contacting surfaces. The coating  40  can be applied to either one of the contacting surfaces or to both contacting surfaces. For example, the coating  40  can be applied to the grooves of the end fittings  14 ,  18 , or to the springs  12 ,  16 , or to both the end fittings  14 ,  18  and the springs  12 ,  16 . The coating  40  can be a Nylon material, such as Nylon 11 or another suitable coating material. The coating  40  can be any suitable thickness. In some embodiments the coating  40  is approximately 0.011 inches thick. The coating  40  can be applied through a powder coating application method to the external surface of the springs  12 ,  16 . In some embodiments the springs  12 ,  16  are made of a material such as 15-5 ph stainless steel or titanium. Some materials wear more quickly than others. In particular, titanium is abrasive and may wear through other contacting materials if subjected to sufficient stress. The coating  40  minimizes wear from twisting, bending, and other deformation as well as from friction. 
     The assembly  11  includes more than one spring, so 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. 
     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. The coating  40  mitigates wear between the spring  16  and the end mount  18 , which is especially beneficial given the pressure created between these two members when the spring  16  is in tension and therefore constricts around the end mount  18 . Slippage in the axial direction is prevented by the end mount  18 , but the coating  40  can allow the spring  16  to move slightly relative to the end mount  18  in a circumferential direction as the spring  16  is tensioned. During tensioning of the spring a slight twist is induced in the spring. Slight movement of the spring away from the end mount occurs, especially where the spring exits the end mount. The coating  40  applied to the groove of the end mount where the spring exits provides a smooth, durable interface to resist wear on the mount and the spring. 
     Also notable to mention, in some embodiments the grooves  28  in which the outer spring rests in the end mounts  18  may be spaced further apart than the natural pitch of the outer spring  16 . This creates an associated tension on the grooves  28  of the end mount  18  as the outer spring  16  tries to recoil to its free length. In the preferred embodiment, the ends of the springs are manufactured with a greater pitch to more closely match the pitch of grooves  28 . Such springs are easier to secure to the mounts. However, slippage is still avoided due to the tighter grip of the spring upon tensioning, as discussed above. 
     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. The coating  40  further prevents friction that may cause sparks, further mitigating this risk. 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. 
     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 . The coating  40  can be applied to any suitable region of the end mount  18  or to the entire external surface. In some embodiments the external, grooved region that engages the spring  16  is coated, but the remainder is uncoated. Preferably, only the final portion of the groove is coated, adjacent the final winding of the spring on the end mount near the exit of the spring from the end mount. 
     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 inner surface  36  can be coated as described above. 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. Any portion of the fixtures  20 ,  21  can have a coating applied similar to what is described in detail above. 
     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. As previously noted, these grooves  28  may be spaced further apart than the natural resting pitch of the outer spring  16 . 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. The grooves  28  can have a coating  40  applied to help installation by reducing the friction between the end mount  18  and the spring  16 . The coating  40  on the tapered portion can be thicker and stiffer than on other portions of the assembly  11  to further mitigate wear and to further reduce twisting and bending. The angle of taper can be increased for an application calling for a thicker coating to create more space between the fitting and the spring in which the thicker coating will more easily fit. 
     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. The shoulder  15  can also be coated to mitigate wear and improve contact qualities similar to what is described above. 
     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. Any surface of the end mount  14  can be coated with a coating similar to what is described above and for similar reasons. 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. In some embodiments the inner spring  12  and outer spring  16  may contact one another. With the coating  40  applied to the external surface of the springs  12 ,  16 , such contact does not adversely affect performance of the assembly  11 . 
     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  FIG. 4 , 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. 
     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. 
     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. 
     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. 
       FIG. 7  illustrates an end mount  50  according to embodiments of the present disclosure in which a coating  40  has been applied to certain surfaces of the end mount  50 . The end mount  50  is generally similar to other end mounts disclosed herein. The grooves of the end mount  50  come to an end  52 . The final portion  54  of the groove is machined to have a slightly larger radius than the remainder of the coils to accommodate the thickness of the coating  40 . The coating  40  can be applied to this final portion  54 . When the spring is threaded onto this end mount  50 , the coils exit the mount at end  52 . Thus, this final portion  54  encounters most of the spring twisting and rubbing in use. The coating  40  helps to lessen the wear and early failure of the springs and fittings. The dimensions of the final portion  54  can be varied for a given application depending on how much the spring moves along the final portion  54  during use. The wear caused by the contact of the spring ends and the end mount are mitigated. 
     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.
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. A friction-resistant coating is applied between components of the assembly to mitigate wear and to prevent bending and twisting.