Abstract:
An adjustment device that compensates for changes in location of the connection point of an aerial cable to a main cable. The device deflects under varying tension in the cable to maintain the cable at a desired ground clearance.

Description:
PRIORITY APPLICATION 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/407,731, filed on Oct. 28, 2010, the content of which is relied upon and incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to methods and apparatus for adjusting the ground clearance for aerial cables. 
       BACKGROUND 
       [0003]    In order to provide cable access to individual residences, businesses, etc. cables such as electrically conductive, fiber optic, and other types, are connected to an exterior point on a structure. The cables connected to the structure, such as drop cables, are often connected to a larger, “main” cable that is in turn mounted to support poles. Because a structure may be remote from support poles, the drop cable is often connected to a point of the main cable located between the support poles. Connection at a location between support poles is often referred to as “mid-span” attachment, although the term does not require connection at the actual midpoint of the main cable between the poles. 
         [0004]      FIG. 1  illustrates a conventional mid-span attachment. In the illustrated arrangement, an aerial drop cable  10  is connected at one end to a main cable  20  at a connection point  30 , and to a structure  40  at its other end. Cables supported in this manner hang with a catenary shape. When the main cable  20  is subject wind loading, it may sway back and forth. When the main cable  20  sways toward the structure  40  a distance generally indicated as D (although the cable&#39;s motion will be generally arcuate), the elevation of a lowest point of the drop cable  10  drops a distance indicated as H so that the lowest point on the drop cable  10  has a ground clearance C. Minimum ground clearances for aerial cables are regulated to ensure that cables do not strike people or objects beneath the cable, so vertical translation of drop cables must be accounted for when planning aerial installations. In extreme weather, both the main cable  20  and the cable  10  may move vertically and horizontally to such a degree that the aerial cable  10  can strike the ground. 
         [0005]    A common solution to ensure minimum ground clearance is to use increased tension when connecting the aerial cable to the structure and to the main cable. Increased tension, however, increases the difficulty of installation and also increases the strain on the aerial cable. The drop cable must therefore be constructed to more stringent specifications, which increases the cost of the cable. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]    The present embodiments are explained in more detail below with reference to figures which show the exemplary embodiments. 
           [0007]      FIG. 1  illustrates a conventional mid-span attachment. 
           [0008]      FIG. 2  illustrates a mid-span aerial cable installation according to a first embodiment of the present invention. 
           [0009]      FIGS. 3A and 3B  illustrate operation of an adjustment device according to the first embodiment. 
           [0010]      FIGS. 4A and 4B  illustrate operation of an adjustment device according to a second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 2  illustrates a mid-span attachment of an aerial cable  50  according to a first embodiment of the present invention. In  FIG. 1 , the aerial cable  50  is connected at one end to a main cable  60  at a connection point  64 , and at a second end to a structure  40 . The main cable  60  is supported by two poles  74 ,  78  in the illustration, although in practice the main cable may extend for long distances to either side of the poles  74 ,  78 . The aerial cable  50 , can be, for example, an aerial drop cable having fiber optic waveguides capable of conveying optical communication signals. The main cable  60  can also be an aerial fiber optic cable, such as a cable sold under the FlexNAP® trademark available from Corning Cable Systems of Hickory N.C. 
         [0012]    According to one aspect of the invention, an adjustment device  80  regulates the elevation of the aerial cable  50 , as well as reducing lateral translations of the cable, to and compensate for swaying and other motion of the main cable  60 . The adjustment device  80  maintains a portion  82  of the cable closest to the structure  40  at a raised elevation, and compensates for movement of the midspan connection point  64  by laterally and/or vertically translating the portion  82  of the cable  50 . The adjustment device  80  can be, for example, an elongate flexible rod rigidly secured to the structure  40  at its base  86 , and secured to the cable  50  at a connection point  88  at or near its distal end  90 . The attachment of the device  80  to the drop cable  50  is along a medial part of the span of the cable  50 , and is secure enough so that that relatively high tensions in the drop cable  50  are borne by the device  80 . The cable  50  extends past the connection point  88  and can be terminated at the structure  40  to provide optical and/or electrical connectivity from the main cable  60  to the structure  40 . Because the device  80  can bear the majority of the tension in the cable  50 , the tension in the cable  50  between the cable connection point  88  and the connectivity point(s) at the structure  40  can be minimal. 
         [0013]      FIGS. 3A and 3B  are schematic illustrations of the operation of the adjustment device  80 . In  FIGS. 3A and 3B , the device  80  is illustrated as an elongate flexible rod  94  with the aerial cable  50  connected to the distal end  90  at the connection point  88 . The aerial cable  50  further extends from the connection point  88  down the structure (not shown). 
         [0014]    Referring to  FIG. 3A , the rod  94  and aerial cable  50  are shown when the main cable  60  ( FIG. 2 ) is under no wind load. Under no wind load, which can be described as “static” conditions, the aerial cable  50  is designed to hang with sufficient ground clearance. When the flexible rod  94  supports the cable  50  under static conditions, static tension in the cable  50  causes the connection point  88  to be deflected laterally away from the structure and toward the main cable  60 , and also downwardly. The cable  50  is therefore initially connected to the device  80  so that the rod  94  has a static strain that provides static vertical and lateral deflection of the connection point  88 . 
         [0015]      FIG. 3B  shows the orientation of the rod  94  and cable  50  when the main cable  60  has been subjected to wind loads and is swaying toward the structure. As the connection point  64  with the main cable  60  ( FIG. 2 ) moves toward the structure  40 , the entire cable  50  also translates. In conventional arrangements, this would result in the lowest point in the cable  50  having a reduced ground clearance, or unwanted horizontal swaying of the aerial cable, or both. According to the present embodiment, the movement in the cable  50  temporarily lessens the tension in the aerial cable  50 , and the rod  94 , which was deflected under stress in its static state, deflects upwardly and away from the main cable  60  as the cable tension lessens. The cable  50  is therefore pulled upwardly and translated laterally away from the main cable  60  by an adjustment translation distance AT, and an adjustment height AH. The translations AT, AH are shown is idealized vertical and horizontal values, although the rod  94  may also sway from side to side. 
         [0016]    When the midspan connection point  64  translates laterally away from the structure  40 , the tension in the cable  50  increases. The rod  94  is sufficiently flexible to deflect under increased tension so that the cable  50  can translate towards the main cable  60  to avoid excessively high tension in the cable  50 . The rod  94  can have an undeflected, zero strain length L from base  86  to connection point  88 . 
         [0017]    The device  80  can have a length L in the range of 0.5-4.0 meters, and can be constructed of materials such as graphite, fiberglass, and composites thereof. The device  80  should be sufficiently flexible to undergo substantial static deflections, yet have a high enough elastic modulus to withstand the stresses induced by wind loading. In a typical installation as shown in  FIGS. 2 ,  3 A, and  3 B, the adjustment translation distance AT may fall in the range of 0.25-2.0 meters. The adjustment height AH may fall in the range of 0.25-2.0 meters. Lateral, or side to side motion of the connection point  88  may fall in the range of 0.25-2.0 meters. 
         [0018]      FIGS. 4A and 4B  are schematic illustrations of an adjustment device  130  according to another embodiment. The adjustment device  130  has a weight  140  that exerts tension on the cable  50  to regulate the elevation of the cable  50  and thereby compensate for swaying in the main cable  60  ( FIG. 2 ). The weight  140  is connected to a tension cable  150 , which is in turn connected to the aerial cable  50  at a connection  160 . The tension cable  150  hangs over a pulley  170  that can be supported at the structure  40  on a pin  174 . The weight  140  can travel vertically along a vertically extending guide  180 , such as, for example, a tube. The weight  140  exerts a tension force T on the tension cable  150 , which in turn exerts the tension T on the cable  50 . A portion  52  of the cable  50  extends from the connection  160  down to the structure  40  where it can provide various services to the structure. The portion  52  can be relatively tension-free. 
         [0019]    In  FIG. 4A , the cable aerial cable  50  is shown in its static state, with the weight at a first height H 1  in the guide  180 .  FIG. 4B  illustrates the operation of the adjustment device  130  as the cable  50  sags, such as would happen when the main cable  60  sways as shown in  FIG. 2 . When the cable  50  sags, the weight  140  falls to a height H 2 , which is below H 1 . The tension cable  150  then pulls the cable  50  to compensate for the sag in the cable  50 , as illustrated by the arrow below the connection  160 . 
         [0020]    In practice, under shifting winds, the main cable  60  would sway back and forth, and have other irregular motions, so that the aerial cable  50  alternately sag and then be pulled taught in irregular motions. The weight  140  exerts a relatively constant tension on the cable  50  so that the clearance height of the cable  50  can be relatively constant. Lateral motion and other movements of the aerial cable  50  are also inhibited by the adjustment device  130 . 
         [0021]    According to an alternative embodiment, the adjustment devices  80 ,  130  could be mounted to the main cable to compensate for sag and other variables at the mid-span attachment. 
         [0022]    Many modifications and other embodiments of the present invention, within the scope of the claims will be apparent to those skilled in the art. For instance, the concepts of the present invention can be used with any suitable fiber optic cable design and/or method of manufacture. For instance, the embodiments shown can include other suitable cable components such as an armor layer, coupling elements, different cross-sectional shapes, or the like. Thus, it is intended that this invention covers these modifications and embodiments as well those also apparent to those skilled in the art.