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FIELD OF THE INVENTION 
       [0001]    This invention relates generally to overhead doors and, more particularly, to a method and apparatus for preventing a door lift cable from disengaging from its mounting pulley. 
         [0002]    Overhead doors are used to occlude openings in structures such as warehouses, factories, and other commercial establishments. Typically, such a door is comprised of a series of panels hinged together and moveable between a doorway blocking position to an overhead storage position. Accordingly, the doors can be relatively heavy for an operator to move especially when moving the door against gravity, i.e. to the upward position. 
         [0003]    One approach to solving the weight problem is that of using one or more torsion springs, which are usually located at the top of the doorway for biasing the door to an upward position. Thus, when the door is moving upwardly, the spring is being unwound, and when the door is being moved to a downward position, the spring is being wound, or the operator is working against the tension of the springs. 
         [0004]    Another device that is helpful in offsetting the effect of gravity is that of using a counterweight such that the door is attached to one end of a pulley mounted cable and a counterweight is attached to the other end thereof so as to substantially balance the weight of the door and allow easy up and down movement by the operator. 
         [0005]    Another approach is a fully mechanized system wherein an electric drive unit is selectively actuated to rotate the cable shaft to either wind up the cable on the pulley to raise the door or to unwind the cable on the pulley to lower the door. Such a drive unit is normally programmed to automatically turn off when the door reaches its fully downward position. However, if an object of any substantial size is located under the door, as occasionally happens, then the drive unit will continue to unwind the cable, with the result being that the cable slackens and often comes off the pulley. The system is then useless until the cable is returned to its proper position on the pulley, a process which can be relatively difficult and time consuming since the pulley is located at a rather high position above the doorway. 
         [0006]    The problem of the cable slackening is also true in manually operated doors that simply have the torsion springs or the counterweight as discussed hereinabove. That is, a sudden stopping of the door by an object such as discussed hereinabove when the door is in its downward movement, or possibly even a sudden stopping of the door when it reaches the floor, may result in the cable continuing to move such that it is slackened and may allow one or more windings to come off of the pulley. 
         [0007]    What is needed is a method and apparatus for preventing such occurrences. 
       SUMMARY OF THE INVENTION 
       [0008]    Briefly, in accordance with one aspect of the invention, provision is made for sensing when the cable tends to slacken and to responsively take up the slack as it occurs. 
         [0009]    In accordance with another aspect of the invention, a biasing mechanism moves to take up the slack in the cable. 
         [0010]    In accordance with another aspect of the invention, a cable engagement apparatus is installed on each side of the doorway adjacent the downwardly extending cable, with each having a biasing mechanism to take out the slack in its associated cable. 
         [0011]    In accordance with another aspect of the invention, when the door includes an automated driving system, at least one of the cable engagement apparatuses includes a switch which, when a slack absorbing device has been moved to a predetermined position, is activated to turn off the driving system. A reset switch can then be actuated to continue operation after the object has been removed from its position under the door. 
         [0012]    In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a perspective view of a loading dock door with the present invention incorporated therein. 
           [0014]      FIG. 2  is a partial view thereof showing the cable engagement apparatus of the present invention as installed. 
           [0015]      FIG. 3  is a front view of the cable engagement apparatus. 
           [0016]      FIG. 4  is a right side view thereof. 
           [0017]      FIG. 5  is a perspective view of the switch portion thereof. 
           [0018]      FIG. 6  is a perspective view of a left cable engagement apparatus in its normal operating position. 
           [0019]      FIG. 7  is a perspective view of a left side cable engagement apparatus as shown in a position just prior to tripping the switch. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    Referring now to  FIG. 1 , the invention is generally shown at  10  as applied to an overhead door  11  which is mounted in a vertically orientated, closed position to close an opening  12  in a building side wall  13 . The door  11  is comprised of a plurality of panels  14  which are hingedly connected at their edges such that the door  11  is flexible to permit its movement around a curved track as it is moved to an open and stored horizontal position. The door  111  has upper and lower edges  16  and  17  and side edges  18  and  19 . 
         [0021]    Mounted to the wall  13  on either side of the door  11  are vertical support members  21  and  22 . The door  11  is supportably mounted within the vertical support members  21  and  22  in a manner in which permits the door to be slidably moved in the grooves of the support members  21  and  22  so as to open and close the door. 
         [0022]    Considering now the manner and structure for moving the door  11  to an open position, a lift bracket (not shown) is attached to each door side edge  18  and  19  at a point near the bottom of the door  11  and a cable  28  is secured to the lift bracket. The cable  28  passes upwardly to be wound around a pulley  29  mounted on a shaft  31 . A torsion spring  32  mounted to the shaft  31  biases the rotation of the shaft  31  such that the door  11  is biased to move in the upward direction. 
         [0023]    It should be recognized that other mechanisms for opening and closing the door can be used while remaining within the scope of the invention. For example, rather than the pulley  29  and spring  32 , the cable may be secured to a counterweight to provide the biasing effect, and the door  11  may be opened and closed manually by an attendant. 
         [0024]    It has now become common to mechanize the opening of the door by providing an electrically powered actuator operator box  30  mechanically attached to one end of the shaft  31  to selectively rotate the shaft in the proper direction to open or close the door  11 . 
         [0025]    It should be mentioned that the mounting of warehouse doors is generally accomplished in a manner which allows the doors to be stored in the horizontal, oblique, or upright open positions. As shown in  FIG. 1 , a horizontal disposition is shown wherein a pair of horizontal rails or tracks  33  and  34  are provided to guide the movement of the door  11  to an open/storage position. Also, in order to bridge the movement of the door  11  between the vertical primary track members  23  and the tracks  33  and  34 , arcuate track members  36  and  37  are provided as shown. In operation, the doors  11  are usually in a fully closed position as shown but are moved to the fully opened position when loading or unloading freight. Thus, for the door to be in an intermediate position is somewhat out of the ordinary. One situation that arises is when the door is being moved to a closed position and there is an object under the door which prevents it from being fully closed. If this occurs, the pulleys  29  may continue to rotate such that the cable is slackened and may be disengaged from the sheath(s) of the pulley. This can occur in a manually operated door, but is even more likely to occur when the automated system is installed. That is, the operator  30  is designed to move the door to a fully opened or fully closed position. When the door is stopped from its movement to a fully closed position, the operator  30  tends to continue to operate for a time period which allows the shaft  31  to continue to rotate and the cable to become slack and to slip off its pulley. The present invention is intended to address this problem. 
         [0026]    Mounted on the wall  13  between the vertical support members  21  and  22  and their respective pulleys  29  are mounting brackets  41  and  42  which are L-shaped in cross section and provide a means for mounting the cable engagement apparatus  43  and  44  on either side of the door as shown. The purpose of these is to automatically take up the slack in the cable when that occurs so as to thereby prevent the undesirable occurrence of the cable coming off the pulleys  29 . 
         [0027]    Referring now to  FIG. 2 , the right mounting rail  41  is shown with its cable engagement apparatus  43  attached thereto. The cable engagement apparatus  43  includes an extender arm  46  which is attached to the mounting rail  41  at its one end by a plurality of bolts  47 . Integrally attached to the other end of the extender arm  46  is a box-like housing  48  having a front wall  49 , a top wall  51 , and an outer wall  52 . 
         [0028]    Secured to the outer wall  52  and extending toward the mounting rail  41  is a mounting shaft  53 , on which is coaxially mounted a torsion spring  54  and a lever arm  56 . As will be seen, the torsion spring has its one end  57  secured to the outer wall  52  and its other end  58  secured to one end of the lever end  56 . In this way, the torsion spring  54  acts to bias the lever arm  56  to rotate in the counterclockwise direction as seen in  FIG. 2 . 
         [0029]    Also attached to the outer wall  52  and extending toward the mounting rail  41  is a stop bolt  59  which extends outwardly from the front wall  52  to a greater extent than the lever arm  56  as shown in  FIG. 3 . The purpose of the stop bolt  59  is to eventually engage the one side of the lever arm  56  near its bottom end  61  such that the freedom of movement in the counterclockwise direction is limited. This feature will be more fully described hereinafter. 
         [0030]    Attached to the lever arm  56  near its top end  62 , and extending outwardly toward the mounting rail  41  is a slide shaft  63  with a relatively small pulley  64  rotatable and slidably, mounted thereon. The pulley  64  is intended to not only rotate on the slide shaft  63  so as to allow rotating engagement with the cable  28 , but also to slide between the two ends of the slide shaft  63  to accommodate the various positions of the cable  28  on the drum of the pulley  29 . Accordingly, the pulley  64  preferably includes a bearing  65  which facilitates this movement. 
         [0031]    The manner in which the cable engagement apparatus  43  is connected to and operates with a conventional cable and pulley system is shown in  FIG. 2 . The cable  28  which normally comes off the larger sheath end of the pulley  29  and passes downwardly to the door is flexed forwardly to fit over the forward edge of the pulley  64  as shown. The cable  28  will normally be taut so as to overcome the counterclockwise biased movement of the lever arm  56  such that the lever arm remains in the position as shown in  FIG. 2 . However, in the event that the cable  28  tends to slacken in situations as described hereinabove, the biasing movement of the lever arm  56  will automatically take up the slack in the cable  28  as the lever arm  56  is biased to the counterclockwise position. In this way, the cable  28  is prevented from becoming so slack as to allow it to disengage from the drum of the pulley  29 . 
         [0032]    It should be recognized that the cable engagement apparatus as described hereinabove, and as shown in  FIGS. 2-4 , is applicable to doors which are opened and closed by manual operation or by an automated drive system. In the later case, however, it is desirable to provide a further function, i.e. that of automatically turning off the drive mechanism when the cable engagement apparatus has been activated and the lever arm  56  has been rotated to a certain position. For that purpose, a mechanically actuated switch  66  is provided as shown in  FIGS. 2-5 . 
         [0033]    As will be seen in  FIG. 5 , the switch apparatus  66  includes a base member  67  to which is attached a housing  68 . Within the housing  68  is contained an electrical switch which is mechanically triggered to send a signal along a lead  69  to the operator (see  FIG. 1 ) to turn off the mechanism within the operator  30  such that the shaft  31  discontinues its rotation. 
         [0034]    At one end of the housing  68  is a probe  71 , which is fabricated of a flexible material such as plastic or the like. A coil spring  72  is disposed over the probe  71  to provide a resiliency thereto. 
         [0035]    As shown in  FIGS. 2-4 , the switch apparatus  66  is mounted inside the housing  48  such that its probe  71  extends outwardly through an opening  73 . The switch apparatus  66  is then so positioned with respect to the lever arm  56  as to be activated by the biased rotation of the lever arm  56  when a slack occurs in the cable  28  and the lever arm  56  is moved to a predetermined position. This movement can be seen in  FIGS. 6 and 7 . 
         [0036]    In  FIGS. 6 and 7 , the left side cable engagement apparatus  44  is shown in its normal position in  FIG. 6  and in its switch actuating position in  FIG. 7 . In this regard, it should be recognized that the left side cable engagement apparatus  44  is a mirror image of that of the right side cable engagement apparatus  43 . The structure and method of operation is therefore identical to that of the cable engagement apparatus  43  except that in the right side cable engagement apparatus, the lever arm is biased to rotate in a clockwise rather than a counterclockwise direction. 
         [0037]    As shown in  FIG. 6 , the cable  74  engages the pulley  64  to hold the lever arm  56  in a “loaded” position against the bias of the torsion spring  54 . So long as the cable  74  is taut, the lever arm  56  will be held in that position. However, when the cable  74  tends to slacken, such as when the door descends to rest against an object as described hereinabove, then the cable will tend to slacken, and the biasing action of the torsion spring  54  will cause the lever arm  56  to rotate to take up that slack. This action, by itself, will prevent the disengagement of the cable  74  from the pulley drum as might otherwise occur. Both manually operated doors and mechanized driven doors will benefit from this feature. 
         [0038]    In the case of mechanized doors, the switch apparatus  66  will further come into play to shut down the operation of the operator  30 . This will be seen in  FIG. 7  wherein the lever arm  56  has been rotated to overcome the slack in the cable and when it reaches the position as shown, it engages the probe  71  to activate the switch  66  so as to thereby shut down the operator  30 . The lever arm  56  and its biasing torsion spring  54 , however, will continue to operate to take up any slack in the cable  74  by further rotation of the lever arm  56 . When the lever arm  56  reaches the position of the stop bolt  59 , the lever arm will be prevented from further rotation even though some tension may remain in the torsion spring  54 . A reset switch can then be actuated to continue operation, after the object has been removed from its position under the door. 
         [0039]    While the invention has be described in terms of use for overcoming the problem of cable disengagement when the door meets an object to prevent its being fully closed, it should be recognized that the invention is intended, and will operate, to take up cable slack and prevent pulley disengagement at anytime that this may tend to occur. For example, even though overhead door systems are generally designed such that all movement stops when the door comes to the fully closed position, because of the inertia of the rapidly descending door, there may still be some tendency for the cable to continue to move and thereby become slackened to the point where it could be disengaged from its pulley. The present invention is intended to correct this problem. 
         [0040]    It should also be recognized that, although the present invention has been described in terms of use with a warehouse door, it may also be useful in non-industrial settings such as with an overhead door in a residential garage, for example. 
         [0041]    While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and various modifications can be made thereto without departing from the scope of the invention as defined in the following claims.

Summary:
A cable-and-pulley mounted overhead door includes a mechanism for sensing when the cable is tending to slacken and to automatically take-up the slack so as to thereby prevent the cable from disengaging from its pulley. When the overhead door includes an automated drive system, provision is further made for sensing when the take up of slack has progressed to a determined degree and to responsively turn off the automated drive system.