You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to tensioning devices and more specifically to an apparatus for tensioning devices for live axle doors.  
       BACKGROUND OF THE INVENTION  
       [0002]     A live axle roll up door is a door made of a flexible material, or having hinged segments, that raises and lowers by means of a rotating axle located above the door frame. The door is typically attached at an end to a set of drum wheels that rotate with the axle. As the axle rotates, the door rolls up onto the drum wheels. If the direction of rotation of the axle is reversed, the door rolls off of the drum wheel and travels downward to close. In warehouses and mini-storage facilities it is common to place a live axle roll up door at the opening to the building. These doors are usually relatively lightweight and designed to be easily and quickly retracted either manually or automatically.  
         [0003]     In order to allow for manual operation of the door, a coil spring is usually installed at the top of the door to counterbalance the weight of the door. The coil spring is usually disposed around a shaft and fixed to the shaft at one end, such that the spring rotates with the shaft, and fixed to a stationary structure at the other end. The torsional forces created in the spring by the rotation of the shaft provide a variable torque which counteracts the weight of the door. As the door is lowered, the torsional forces developed in the spring pull in the opposite direction of the travel of the door. The amount of tension resulting from the torsional forces generated in the coil spring will determine the performance characteristics of the door.  
         [0004]     If there is too little tension, the weight of the door may cause the door to drift down from the open position to the closed position. If the tension is too great, the door may be hard to pull down and it may not stay closed. Also, a door with too much tension in the spring will fly up upon exertion of an upward force to open the door. If the amount of tension is set correctly, the door can be lowered gently and a balance will be struck between the weight of the door and the force exerted by the spring. At certain positions, the weight of the door may balance with the force of the spring and the door can be left partially open. In the closed position, the weight of the door will overcome the force of the spring and the end of the door will rest on the ground. Also, if the door is adjusted properly, a small amount of upward force will release the potential energy of the spring and the door will easily travel in the vertical direction.  
         [0005]     The live axle is free to rotate within and is supported by brackets on either side of the opening of the door. On one end of the axle, a tension bracket connects to one end of the spring. The other end of the spring is attached to a drum wheel that rolls the door. The drum wheel rotates with the axle and the end of the door is attached to the drum wheel. As the axle turns, the door, which is constructed of a sectional material that is flexible enough to roll up, rolls onto the drum wheels.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is a spring tensioning mechanism incorporating a frame having a substantially-planar main panel and a bore passing therethrough, and an axle disposed orthogonally to the substantially-planar main panel and passing through the bore. A spring is disposed around the axle. An outboard plate, disposed about the axle, is secured to the frame. The outboard plate incorporates a clocking feature. An inboard plate, having a clocking feature mated to the clocking feature of the outboard plate, is connected to the end of the spring. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:  
         [0008]      FIG. 1  is an isometric view of an upwardly-acting rolling door;  
         [0009]      FIG. 2  is an exploded isometric view of a spring tensioning mechanism in accordance with certain embodiments of the present invention;  
         [0010]      FIG. 3  is an isometric view of the spring tensioning mechanism depicted in  FIG. 2 ;  
         [0011]      FIG. 4  is a section view of the spring tensioning mechanism depicted in  FIG. 3 ;  
         [0012]      FIG. 5  is an isometric view of an opening mechanism for a rolling door having a tensioning mechanism installed at one end thereof; and  
         [0013]      FIG. 6  is an isometric view of an upwardly-acting rolling door having tensioning mechanisms installed at both ends thereof. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.  
         [0015]     Door assembly  100  includes a first vertical guide  102  at a first end thereof and a second vertical guide  104  at a second end thereof. Vertical guides  102  and  104  are secured to the door jambs by a set of mounting brackets  106 , although other methods of securement may be employed in alternate embodiments. A first support bracket  108  is disposed at the upper end of vertical guide  102 . A second support bracket  110  is disposed at the upper end of vertical guide  104 . Together, support brackets  108  and  110  support an axle  112  in an orientation generally parallel to both the ground and the principal plane of the door assembly  100 .  
         [0016]     A first drum  114  is mounted on axle  112  at a first position and a second drum  116  is mounted on axle  112  at a second position. Drums  114  and  116  are secured to the top edge of the door  118  at a point on the perimeter of each. Accordingly, the door  118  will wrap around the perimeter of drums  114  and  116  if the axle  112  is rotated in one direction, and will unwrap from the perimeter of drums  114  and  116  if the axle is rotated in the opposite direction. Because of the effect of gravity on the door  118 , the weight of door  118  will tend to draw the door  118  downward and unwrap the door  118  from drums  114  and  116 .  
         [0017]     In order to counteract the effect of gravity on the door  118 , a first coil spring  120  is disposed at a first end of axle  112  and a second coil spring  122  is disposed at a second end of axle  112 . As door  118  unwraps from drums  114  and  116 , the portion of the door  118  hanging free, and therefore imparting a moment on drums  114  and  116 , increases. At the same time, coil springs  120  and  122  wind up as axle  112  rotates, thereby increasing the energy stored in coil springs  120  and  122  and the torque exerted on axle  112  by coil springs  120  and  122 . If the torsional spring rate of coil springs  120  and  122  is selected appropriately, the door assembly  100  can be designed so that the torque of coil springs  120  and  122  substantially counteracts the weight of the door  118  at every point through the range of motion of door  118 .  
         [0018]     Although door assembly  100  is an effective design, it suffers from certain drawbacks. In certain applications it may be desirable to modify a door assembly such as door assembly  100  in order to increase its functionality. The present invention involves a method and apparatus for improving a door assembly such as door assembly  100  while retaining many of the same components of the assembly, thereby minimizing the expense of the upgrade.  
         [0019]     As seen in  FIGS. 2-6 , the components added to the door assembly  100  in order to construct an improved door assembly  200  include outboard plates  210 , axle bearings  230  and inboard plates  240 . The improved door assembly  200  retains most of the original components of door assembly  100 , including support brackets  108  and  110 , axle  112  and coil springs  120  and  122 .  
         [0020]     In order to modify door assembly  100  to incorporate the advantages of door assembly  200 , the tension in springs  120  and  122  must first be relieved, in a manner known to those of skill in the art. Once the tension is relieved, axle  112  can be supported in place and one or more of support brackets  108  and  110  can be removed from the ends of axle  112 , so as to facilitate addition of upgrade components, as described in more detail below.  
         [0021]     Support brackets  108  and  110 , which may be retained from the original door assembly  100 , have inboard surfaces  202 , axle bores  206  and arrays of fastener bores  204 . Disposed adjacent to the inboard surface  202  of each support bracket  108  or  110  is the outboard surface  212  of an outboard plate  210 . Extending from the inboard surface  214  of the outboard plate  210  is a hollow cylindrical protrusion  220  having an outer surface  222  and an inner surface  226  defining an axle bore  224 . Protrusion  220  is surrounded by a set of fastener bores  216 , one or more of which has a position aligned to one or more of the fastener bores  204  in the support bracket  108  or  110 , so as to facilitate securement of the outboard plate  210  to the support bracket  108  or  110 . Outboard plate  210  may also incorporate one or more pin bores  218  sized and shaped to receive a retaining pin  228 .  
         [0022]     Disposed within the hollow cylindrical protrusion  220  of each outboard plate  210  is a bearing  230  having an outer surface  232  and an inner surface  234 . The outer surface  232  is shaped and sized to mate with the inner surface  226  of the axle bore  224 , while the inner surface  234  is shaped and sized to mate with the outer surface  236  of the axle  112 . Axle  112  rides in the bearings  230  within the outboard plates  210 , thereby facilitating smooth, low-friction operation of door assembly  200 .  
         [0023]     Disposed adjacent to the inboard surface  214  of each outboard plate  210  is the outboard surface  242  of an inboard plate  240 , which is not fastened to either of outboard plate  210  or support bracket  108 . Each inboard plate  240  includes an inboard surface  244 , an axle bore  246 , and one or more pin bores  248  positioned to align to at least one pin bore  218  in the outboard plate  210  and inboard plate  240  further includes a set of receivers  260  disposed about the axle bore  246 , each having an aperture  262  therein shaped and sized to receive a tensioning lever (not shown). As assembled, each inboard plate  240  is secured to the outboard end of a coil spring  120  or  122 . Although only one end of the improved door assembly  200  is shown in  FIGS. 2-4 , those of skill in the art will appreciate that the opposite end of door assembly  200  is the mirror image of the end shown in  FIGS. 2-4 . The full door assembly  200  is shown in  FIGS. 5 and 6 .  
         [0024]     Once the improved door assembly  200  is fully assembled, the tension in coil springs  120  and  122  can be adjusted by disposing tensioning levers in the receivers  260  of the inboard plates  240  and unloading the retaining pins  228  so that they can be removed. Once the retaining pins  228  are removed from the improved door assembly  200 , the inboard plates  240  are free to rotate with respect to the outboard plates  210 . The tension in coil springs  120  and  122  can be increased by rotating the inboard plates  240  in one direction, or decreased by rotating the inboard plates  240  in the opposite direction.  
         [0025]     Once the proper tension is imparted to coil springs  120  and  122 , respective pin bores  218  and  248  in the outboard plates  210  and inboard plates  240  are aligned and the retaining pins  228  are re-inserted therein, thereby fixing the angular position of the inboard plates  240  with respect to the outboard plates  210 .  
         [0026]     Although preferred embodiments of the invention have been described in detail, it will be understood by those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.

Summary:
A spring tensioning mechanism incorporating a frame having a substantially-planar main panel and a bore passing therethrough, and an axle disposed orthogonally to the substantially-planar main panel and passing through the bore. A spring is disposed around the axle. An outboard plate, disposed about the axle, is secured to the frame. The outboard plate incorporates a clocking feature. An inboard plate, having a clocking feature mated to the clocking feature of the outboard plate, is connected to the end of the spring.