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FIELD OF THE INVENTION  
         [0001]    The invention relates generally to a drive system for shifting a movable barrier and, more particularly, to a drive system for shifting a garage door using a flexible actuator.  
         BACKGROUND OF THE INVENTION  
         [0002]    Garage door systems, such as shown in U.S. Pat. Nos. 5,803,149 and 6,326,751, include a garage door that is normally shifted between a substantially vertical orientation, where the door is in a closed position, and a substantially horizontal position, where the door is in an open position. Jackshaft operators as disclosed in the &#39;149 patent are available that employ a spring-loaded drive shaft to assist in controlled shifting of the heavy weight of the door as it is moved between its horizontal open and vertical closed positions along a guide track as by application of a counterbalancing force thereto. For lifting the door open, a pull cable connected near the bottom of the door is spooled on a drum mounted to the rotating shaft.  
           [0003]    Garage door systems have been developed that also use an upper cable operatively connected adjacent the top of the door to pull the garage door from the open position to the closed position. The upper cable is tensioned with an extension spring, such as disclosed in the aforementioned patents. The &#39;751 patent also shows a torsion spring that exerts a torsional or rotational force on links that are pivotally connected in order to tension the cable. Such a torsion spring and link arrangement introduces undesirable complexities and pivot points that can quickly wear and fail with repeated cycling and especially over prolonged periods of garage door operation.  
           [0004]    During winding and unwinding of the cables from the drum or drums, the cables are more likely to spool onto the drums improperly or actually fall off of the drums, also known as cable throw, unless properly tensioned. In particular, the cable not bearing the majority of the load tends to come off of its drum unless properly tensioned. For example, when the door is nearly to its closed position, the majority of the door&#39;s weight is supported by the lower cable, thus reducing the tension in the upper cable which, unless proper tension is applied, results in cable throw. Cable throw causes the improper winding and/or unwinding of the cable from the drum, resulting in the malfunction of the garage door system in terms of properly opening and closing as is desired.  
           [0005]    The use of extension or coil springs to tension upper cables of garage door systems is problematic from a security standpoint. More specifically, extension springs are attached between the upper cable and the door. Generally, there is a pivotal bracket arm attached adjacent the upper end of the door at one end and to a roller at its other end with the spring operatively attached between the arm and cable. Accordingly, with the door closed, the spring allows an intruder to exert an upward lifting force on the door to push the roller in the guide track with the spring deflecting or stretching, thus raising the door despite lack of rotation of the drive shaft and drum on which the upper cable is spooled. In other words, the intruder can lift the door by way of spring deflection, even though the length of the upper cable between the drum and spring does not increase. The intruder usually will be able to lift the door by deflection of the spring by a vertical amount sufficient so that they can gain access to the interior of the garage by fitting under the door, e.g., by lifting the door by a height off the ground large enough for the intruder to pass through. Further, if the yield strength of the spring is exceeded, the overflexed spring may not be able to exert the same tensioning force on the cable and generally will see its usable spring life cycles reduced. In some instances an intruder may stretch the spring so that the spring breaks, thereby allowing the garage door to be lifted completely up.  
           [0006]    A further complication in designing drive systems comes from the use of multi-panel doors that travel curved paths as these doors move between open and closed positions. As the panels pivot relative to adjacent panels during travel along the curved path, the respective distances traveled by between the top end and the bottom end of the door are not the same for a given elevation of the door. Since the upper and lower cables are attached to these ends of the garage door, the length of travel required of the upper cable also varies relative to the length of travel required of the lower cable as the door is raised and lowered. The variance in the travel distance of the cables can cause fluctuations in the tension in the cables, which can result in the build up of slack and thus cable throw.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with the invention, a drive system for a moveable barrier, e.g., garage door, is provided that limits unauthorized shifting thereof. In particular, the drive system includes a biasing mechanism having a biasing member, such as a compression spring, associated with a flexible actuator, e.g., cable or chain, operably connected between a drive shaft and the door such as toward the upper end thereof for keeping the cable actuator tensioned. The biasing mechanism also includes a stop assembly which provides a well-defined, generally precise limit to the amount of deflection or flexing the compression spring can undergo. In this way, the present biasing mechanism incorporating the stop assembly only allows the garage door to be lifted from the closed position without operation of the drive shaft by a predetermined small, vertical distance that is insufficient in terms of allowing unauthorized access to the garage. At the same time, the stop assembly does not allow the spring to be overflexed even when the stop assembly is operable to stop unauthorized door shifting thus maintaining spring performance for actuator tensioning and maximizing the life thereof.  
           [0008]    It is preferred that the biasing member exert a linearly directed biasing force with the stop assembly being connected to the mechanism for similarly flexing the member in the linear direction, preferably in line with the cable actuator. In this way, operation of the biasing mechanism and stop assembly thereof do not require pivot members for transmission of the tensioning force to the cable and the wear and reliability problems these pose.  
           [0009]    As is apparent, this linearly directed biasing force is akin to that provided by prior extension springs which, however, lack the stop assembly of the present invention. In this manner, the present biasing mechanism can be implemented in much the same manner as prior extension springs in terms of the surrounding hardware necessary for attaching it between the cable and the door. For instance, the normal arm having a roller riding in the guide track for the door and being pivotally mounted to the upper end of the door at one end with the other having a bracket for pivotally attaching to the present biasing mechanism can generally still be employed with only relatively minor modifications thereto. Accordingly, the present drive system can more easily be substituted for prior systems employing extension springs with a minimum of added expense and effort for installation and retrofitting thereof.  
           [0010]    In the preferred and illustrated form, the biasing mechanism and connected stop assembly are a commercially available extension spring assembly that include pull devices. The pull devices include a pair of elongate U-shaped loops that each pass through the barrel of the coils in opposite directions to each other and hook around the opposite end coils of the spring so that when a tension force is applied to the loops, they pull toward each other compressing the spring coils together. Once the coils are completely compressed, there is a hard, physical limit to the deflection of the spring regardless of loading so that the garage door cannot be lifted further once this point is reached. In addition, this prevents the spring from being overflexed or overstretched which otherwise can adversely effect the bias force applied by the spring to keep the cable tensioned and can reduce spring life.  
           [0011]    It should be noted that the construction of the present spring assembly is interchangeably called an extension or a compression spring as it includes physical characteristics of both. Common characteristics include loops that in operation are pulled away from each other similar to expansion springs. The loops are connected to hooks of the pull devices that are operable to pull the opposing end coils toward each other to compress the coils together like operation of a compression spring when the loops are pulled as described. Nevertheless, the present spring assembly is constructed to provide additional advantages over simple extension or compression springs, as described herein.  
           [0012]    More specifically and in a preferred form, the present drive system is employed with a jackshaft garage door operator including a drive shaft that is driven to raise the garage door from the closed position via a lower cable tat is taken up to pull the door toward the open position while the upper cable pays out. Conversely, when the drive shaft is driven to lower the garage door from the open position, the upper cable is taken up to pull the door toward the closed position while the lower cable pays out. Once the upper cable begins to urge the garage door toward its closed position, the lower cable assists in supporting the weight of the door as it is being lowered.  
           [0013]    As mentioned, the biasing mechanism is provided between the cable and the garage door in order to provide tension to the upper cable. The biasing mechanism includes a spring, as discussed above, to provide sufficient tension to the cable to prevent the cable from being thrown off of the drum or otherwise hindering movement of the door. The spring of the biasing mechanism is configured to apply tension to the flexible actuator within a range before the spring is completely compressed to a predetermined maximum limit, i.e., about two inches. When the predetermined maximum limit is reached, the stop assembly does not allow further resilient flexing of the spring and movement of the garage door beyond the predetermined limited amount when the drive shaft is not rotated.  
           [0014]    Many garage doors include a plurality of pivotally connected panels with connected rollers positioned within the guide track. The track has a generally vertical portion for supporting the garage door in the closed position and a generally horizontal portion for supporting the door in the open position. Connecting the vertical and horizontal track portions is an arcuate portion.  
           [0015]    As the rigid panels are pivoted for articulating to travel along the arcuate track portion, the upper and lower cables will travel by different distances with respect to each other for a given position of the garage door between the closed and open positions. As one is being paid out and the other is being taken up by the rotating drum(s) to which they are secured, as previously discussed. It has been found that the travel differences between the cables vary and oscillate in a fairly predictable range that can be measured. At different positions of the door between its open and closed positions, there is a travel differential amount, i.e., the difference the upper cable has traveled relative to the lower cable. The travel differential amount varies depending upon the position of the garage door. Throughout the travel of the door there is a largest measured difference, which is termed the maximum travel differential amount. As is apparent, since the cable drum is mounted on the rotating drive shaft that is fixed in position relative to the door, the lack of a constant one-to-one correspondence between the cable travel distances creates slack in the cables, and most typically the upper cable, during garage door operations.  
           [0016]    While prior extension springs would generally allow a sufficient amount of deflection to take-up the maximum travel differential amount so as to keep the cables tensioned during garage door operations, these springs are typically oversized in that they have almost no practical limit on the maximum deflections, thereby allowing far greater deflection that the maximum differential travel amount. In other words, there has been no consideration given to the travel differential, and certainly these prior drive systems have not identified the maximum travel differential as being of importance.  
           [0017]    Accordingly, in another form of the invention, a drive system is provided that has a pair of flexible actuators, i.e., cables, connected to shift the movable barrier. A resilient take-up device that provides one of the actuators with a biasing force by resilient deflection or flexing minimizes slack in the actuator due to the travel differential. The take-up device is provided with a limit assembly which defines a predetermined maximum limit of deflection of the take-up device. In particular, the limit assembly allows the maximum deflection limit to be preselected to generally correspond to the maximum travel differential. In this way, the present take-up device can be carefully tailored to provide the deflection or flexing and bias force to the flexible actuator that is need to avoid slack due to travel differential, while avoiding the oversizing thereof as occurred with prior extension springs that were not selected based on an identification of the maximum travel differential amount similar to the take-up device incorporating the limit assembly herein. At the same time, the limit assembly avoids overflexing of the take-up device such as could occur if an intruder is attempting to push the door up, which could deflect and stretch the prior extension springs of the upper cables until they can gain access by fitting under the door to the garage.  
           [0018]    As previously discussed, the resilient take-up device is preferably in the form of a compression coil spring and the limiting stop assembly preferably includes a pair of opposing drawbars having the compression spring positioned therebetween. The drawbars and spring are configured and arranged to apply tension to the cable when the drawbars are drawn toward each other due to the biasing force of the spring. When the spring coils are fully compressed between the drawbars, the maximum limit of applied tension to the flexible actuator is reached. The engagement of the drawbars against the fully compressed coils of the spring prevents further extension of the flexible actuator, thereby allowing the upper cable to become taunt. If this point has been reached without rotation of the drive shaft, i.e., by an intruder lifting the door, further unauthorized shifting of the garage door is prevented.  
           [0019]    Over time, the cable may stretch and deform so that it is longer than its initial length. If the cable increases in length, then the biasing mechanism is required to take up the slack in the cable so that tension in the cable stays relatively constant. The compression spring needs to deflect or expand axially taking up the preload initially set therein as described hereinbelow thus requiring an increase the length between opposite end coils to pull the two opposing drawbars closer together, and particularly the loop connection points thereof. However, as mentioned above, the distance between the two opposing drawbars and the preloaded, partially compressed axial length of the spring are carefully selected to permit deflection of the spring generally corresponding only to the maximum travel differential amount. The change in the distances in the drawbar spring assembly, such as by taking up slack in an elongated cable, reduces the ability of the spring assembly to compensate for the predetermined maximum travel differential amount. In other words, if the coil spring becomes axially longer than it is in its preloaded, partially compressed state, the drawbars will no longer fully compress the cables when the maximum travel differential amount is reached.  
           [0020]    In order to maintain a generally constant maximum differential travel amount, even when the upper cable lengthens over time, herein a tensioner is provided between the arm pivotally attached to the door at one end and to the spring assembly at its other end. The distance between the connection point of the tensioner relative to the arm is made to be adjustable. The tensioner includes an adjustment device so that the connection point can be controllably shifted relative to the arm in order to change the distance between the connection point and the drive shaft prior to garage operations. In this manner, the preload tensioner allows a user to more precisely set the tension in the upper cable during system set-up procedures, such as with the door in its closed position. Shifting the connection point further away from the shaft via the preload tensioner allows for the take up of slack in an elongated upper cable to maintain the spring at its preload, partially compressed axial length which accommodates the maximum travel differential amount.  
           [0021]    The tensioner may include a supplemental adjustment mechanism that causes the connection point to automatically shift away from the shaft, such as in predetermined increments, to take up slack in the upper cable. In this manner, the tensioner is adapted to allow the drawbar and compression spring assembly to maintain a generally constant range of tension on the cable, even as the cable is stretched and lengthens over time, so that the drawbar and spring assembly stays tailored to address only the necessary amount of the travel differential between the upper and lower cable actuators, namely the maximum travel differential amount as described hereinabove. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a perspective view of a garage door in a closed position thereof and a drive system therefore including a drive shaft and upper and lower flexible cable actuators operatively attached to the door in accordance with an embodiment of the invention;  
         [0023]    [0023]FIG. 2 is an enlarged perspective view of the drive system showing a spring assembly attached between the upper cable and an arm pivotally attached adjacent the upper end of the door with spring assembly coils that are compressed to apply a tension force to the cable as the door is being shifted;  
         [0024]    [0024]FIG. 3 is a view similar to FIG. 2 showing the door lowered closer to its closed position with the coils of the spring assembly expanded for decreasing the applied tension force to the cable;  
         [0025]    [0025]FIG. 4 is perspective view of the spring assembly showing a compression spring and a pair of drawbars extending therethrough with each drawbar including a connection loop and a hook end;  
         [0026]    [0026]FIG. 5 is a perspective view of a preload tensioner for the drawbar spring assembly showing a turnbuckle including hook screws threaded thereto connected to a bracket attached to the arm pivotally connected to the upper end of the door at one end and to one of the drawbar loops at the other end for keeping the preload in the spring substantially constant during garage door operation;  
         [0027]    [0027]FIG. 6 is a perspective view of another preload tensioner for the drawbar spring assembly showing a hook screw threaded into a block attached to the arm pivotally connected to the upper end of the door and having one of the drawbar loops connected at the hook end for keeping the preload in the spring substantially constant during garage door operation;  
         [0028]    [0028]FIG. 7 is a perspective view of a self-adjusting preload tensioner for the drawbar spring assembly showing a hook screw inserted through a block attached to the arm pivotally connected to the upper end of the door and threaded into a split nut and having a spring biasing the screw from the block and having one of the drawbar loops connected at the hook end on the other side of the block for keeping the preload in the spring substantially constant during garage door operation;  
         [0029]    [0029]FIG. 8 is a perspective view of the self-adjusting preload tensioner of FIG. 7 with the spring removed showing the split nut and a cap on the threaded end of the hook screw against which the spring of FIG. 7 biases the screw from the block; and  
         [0030]    [0030]FIG. 9 is a chart comparing the differences between travel of the upper flexible cable actuator and the lower flexible cable actuator of the system of FIG. 1 to the elevation of the garage door as it travels from its closed position to its open position. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]    In FIGS.  1 - 3 , a garage door  20  and its drive system  10  are shown for shifting the door  20  between a closed position (FIG. 1) and an open position in accordance with the present invention. More particularly, the drive system  10  includes a lower cable  44  that exerts a lifting force on the vertical door  20  as it is shifted to the open position, which as shown will be with the door  20  in a generally horizontal orientation due to the configuration of its guide track  60 . Most residential garage door systems will have a vertical portion or run  66  that guides the door to its closed position and a horizontal portion or run  62  adjacent and below the ceiling of the garage  5  so that the door  20  is lifted open to a horizontal position. A curved or arcuate track portion  64  interconnects the vertical and horizontal track runs  66  and  62 , as is known. For shifting the door  20  closed, the present drive system  10  includes an upper cable  42  that is operable to exert a closing force on the door  20 .  
         [0032]    With the drive shaft  30  being a component of the typical jackshaft operator  32  and disposed over the garage door opening  7  as shown in FIG. 1, and having drums  36  on which the cables  42  and  44  are spooled, the lower cable  44  is operatively connected toward the lower end of the door  20 , and the upper cable  42  is operatively connected toward the upper end of the door  20 . In this regard, an extension arm  122  is pivotally attached to the door  20  via a bracket  124  and pivot pin  126  at one end of the arm  122 . As best seen in FIG. 2, a biasing mechanism or resilient take-up device  50  is shown pivotally attached between the other end of the arm  122  via a bracket  128  secured thereto. The biasing mechanism  50  keeps tension in the cable  42  so that it does not develop slack during garage door operations.  
         [0033]    The biasing mechanism  50  is also provided with a stop or limit assembly  70  that provides a hard stop to the maximum deflection the biasing member in the form of a coil spring  52  can undergo. In this manner, unlike prior extension springs, the present biasing mechanism  50  provides a precise, known limit to how much shifting the door  20  can undergo without operation of the rotating drive shaft  30 . Accordingly, with the door  20  closed an intruder attempting to gain access to the interior space of the garage  5  will only be able to lift the closed garage door  20  off from the ground by a predetermined limited amount which is defined by the arrangement of the coil spring  52  and the stop assembly  70 . On the other hand, the present biasing mechanism  50  employs the coil spring  52  advantageously as it applies a linear bias force for tensioning the cable  42  with the force in line or coaxial with the cable  42  so as to keep the number of pivoting parts in the present biasing mechanism  50  to a minimum. In addition, by utilizing a coil spring  52  similar to prior extension coils springs but having a stop assembly  70  incorporated therewith, the present biasing mechanism  50  can be more readily installed in current garage door drive systems that employ an upper cable with an extension spring for keeping tension thereon without requiring significant modifications thereto. In the preferred form, the present biasing mechanism  50  can be a commercially available drawbar spring assembly such as provided by McMaster-Carr of Chicago, Ill. These spring assemblies  50  have a size or form similar to prior extension springs so they can be easily substituted therefor. Furthermore, this allows the drive system  10  incorporating the biasing mechanism  50  as described herein to be implemented with a minimum of expense as custom made parts therefor are avoided.  
         [0034]    Referring to FIG. 4, the drawbar assembly  70  includes a pair of drawbars  72  and  172  that extend through the barrel of the spring coil  52  in opposite directions. The drawbars  72  and  172  each include a loop  76  or  176  at one end and hooks  74  or  174  at the other end. Accordingly, there is a loop  76  of one drawbar  72  that projects beyond one end of the coil spring  52  while the hooks  174  of the other drawbar  172  are engaged about the coils thereat. The loop  76  is connected to the end of the upper cable  42  while the other loop  176  is connected to the bracket  128  of the arm  122 , as best seen in FIGS. 2 and 3. Thus, the coil spring  52  is loaded by axial compression such as during system set-up for preloading thereof as will be described hereafter, and during garage door operations either by the arm  122  pushing on the loop  176  causing the hooks  174  to pull on the end coil for compressing the coils during door opening operations, or by take-up of the cable  42  on the drum pulling on drawbar loop  76  causing hook end  74  to pull on the end coil for compressing the coils  52  during door closing operations. Accordingly, unlike prior extension springs, there is an axial shortening of the coil spring  52  that is effective to load the biasing mechanism  50  for keeping tension on the upper cable  42 .  
         [0035]    In each instance when the door  20  shifts as by drive shaft rotation, the above-described arrangement of the drawbars  72  and  172  allows the assembly  50  to exert a linear compressive force on the coil spring  52  aligned with the force applied by the spring assembly  50  to the upper cable  42 . As is apparent, the drawbars  72  and  172  can only pull the coils together until they all are engaged with adjacent coils. At this point, the coil spring  52  can not be deflected further, thereby providing a well-defined limit to its maximum deflection which cannot be exceeded. In this manner, the present spring assembly  50  cannot be overflexed as possible with prior extension springs. Importantly, the hard limit provided to the spring deflection is effective in stopping unauthorized entry into the garage door space  5  as no longer will an intruder be able to continually stretch and deflect the spring  52  of the upper cable  42  until they can fit under the door  20 . Again, this overflexing is avoided with the present drawbar spring assembly  50  along with the potential for plastic deformation thereof, and even complete failure of the coil spring  52 . More specifically, when an intruder attempts to open the fully closed garage door  20  without the drive shaft  30  being driven for rotation by the operator motor  34 , the garage door  20  will initially move along the track  60  toward its open position with the lower end of the door  20  raised off from the ground. While the garage door  20  is being lifted upwardly, the distance between the drawbar  176  and arm  122  connection and the drum  36  increases from its nominal distance, with the upper cable  42  tensioned and coils of the compression spring  52  shifting axially toward each other. When the coils have shifted linearly along their axis by the maximum deflection amount due to the lifting force, they are fully axially compressed between the hooks  74  and  174  of the opposing drawbars  72  and  172  so that with the upper cable  42  fully taunt the door  20  cannot undergo any further upward movement as might allow an intruder access to the garage interior space  5 .  
         [0036]    As the drawbar spring assembly  50  is commercially available in different sizes, it can be selected so that the amount of shifting or lifting of the door  20  absent drive shaft rotation and motor operation will be known in advance, with allowance taken in to account for preloading of the spring assembly  52 , as will be described herein. The limited amount of shifting that is allowed can be selected to be, for example, approximately two inches with the coil spring  52  preloaded as by axially compressing the coils by approximately two inches with the door  20  lifted off of the ground by this short vertical distance, e.g. two inches, at which point further raising of the door  20  cannot occur substantially irrespective of the manual lifting force applied by an intruder, and they will be unable to fit under to door  20  to effectively keep them out of the garage interior space  5 .  
         [0037]    Many garage doors  20  are of a multi-panel construction including several panels  26  that are hinged together to allow them to pivot relative to each other. As seen best in FIGS.  1 - 3 , the panels  26  have a hinge  28  adjacent each lateral side thereof and in the mid-section thereof. The hinges  28  each include an upper hinge portion  132  attached to the lower end of the upper adjacent panel  26  and a lower hinge portion  134  attached to the upper end of the lower adjacent panel  26 . Connecting the two hinge portions  132  and  134  is a pivot pin  136  that allow the hinge portions  132  and  134 , and thus the adjacent door panels  26 , to pivot relative to each other.  
         [0038]    Rollers  24  are positioned to extend past the lateral edges of the door  20  for traveling in the track portions  62 ,  64 , and  66 . The rollers  24  are mounted in several locations. Some of the rollers  24  are mounted to the hinges  28  adjacent the lateral edges of the panels  26  via pins  27  with rollers  24  on the ends thereof rotatable mounted thereto. As best seen in FIG. 1, the roller pins  27  can be mounted to the lower hinge portions  134 . The roller pin  27  and the pivot pin  136  may also be combined. That is, the same pin that pivotally connects the upper and lower hinge portions  132  and  134  may also extend past the lateral edge of the door panel  26  and have a roller  24  mounted thereto for travel in the track  60 . Other rollers  24  may have their roller pins  27  mounted to the garage door  20  via brackets  29  and  124  independent of the hinges  28 . For example, rollers  24  may be mounted to pins  27  attached to brackets  29  and  124  fixed adjacent to lateral edges of the door  20  at the top end of the uppermost panel  26  and the bottom end of the lower most panel  26  for guiding the top and bottom of the door  20 . Rollers  24  are also mounted relative to both ends of the arm  122  to guide the arm  122  along the track  60 . These rollers  24  have pins  27  that extend through holes in the end of the arm  122  pivotally attached to the door  20  with a hinge bracket  124  and the end opposite the door  20 .  
         [0039]    The positions of the rollers  24  relative to the panels  26  and the arm  122  are carefully selected to allow the door panels  26  and arm  122  to travel through the arcuate portion  64  of the track  60 . For instance, the rollers  24  are positioned near the top and bottom ends of the panels  26  and arm  122 , as opposed to in the midsections thereof, to allow the panels  26  and arm  122  to move through the arcuate track portion  64  as the panels  26  and arm  122  transition between horizontal and vertical orientations. As illustrated in FIG. 1, for a garage door  20  having four panel sections  26  five rollers  24  are positioned along each lateral side thereof for travel in the track  60 , along with one roller  24  at the end of the arm  122  opposite the connection of the arm  122  to the uppermost panel  26  of the door  20 . Rollers  24  are mounted to brackets  29  attached toward the bottom end of the bottom most panel  26 . A pair of rollers  24  are also connected to a combined pivot pin and roller pin  126  joining the upper and lower hinge portions  132  and  134  of the hinge  28  connecting the lowermost panel  26  to the panel  26  adjacent thereto. The hinges  28  joining the two intermediate panels  26  and the uppermost panel  26  and its adjacent panel  26  each have a roller  24  connected to a roller pin  27  connected to the lower hinge portion  134 . At each side of the top end of the uppermost panel  26  a bracket  124  is provided having a roller pin  27  with a roller  24  on the end thereof. For the side of the panel  26  having the arm  122  connected thereto, the combined roller pin  126  also pivotally connects the arm  122  to the bracket  124 .  
         [0040]    As the door  20  is shifting through its curved path adjacent panels  26  pivot relative to each other which is believed to be at least one reason for the travel differential between the upper and lower cables  42  and  44 , as previously described. The present drive system  10  via the resilient take-up device  50  and limit assembly  70  is very well adapted to keep proper tension on the cables  42  and  44  despite the travel differential therebetween during garage door operations. In this regard, the resilient take-up device  50  including the limit assembly  70  is sized with precision to deflect the coil spring  52  by no more than is needed to accommodate the maximum amount of travel differential between the cables  42  and  44 . In this way, the size of the take-up device  50  in terms of how much resilient deflection it needs to be able to undergo is kept to a minimum.  
         [0041]    Where the resilient take-up device  50  and limit assembly  70  are as shown in their preferred form, i.e., the drawbar spring assembly  50  as shown in FIG. 4, another advantage is that by minimizing the maximum resilient deflection that is selected, the predetermined limited amount of unauthorized garage door  20  shifting allowed by the device is also kept to a minimum. In other words, the maximum resilient deflection is the linear distance that the coils can be shifted or compressed along their axis before they are engaged together or fully compressed by the pulling force on the drawbars  72  and  172 . As such, this maximum resilient deflection level also defines the limited amount of door  20  shifting that can occur absent drive shaft rotation. Accordingly, identifying the maximum travel differential between the cables  42  and  44  as done herein allows the drawbar spring assembly  50  to be selected in a way that also affords optimized advantages as the limited amount of allowed door  20  shifting can be kept to a minimum.  
         [0042]    As discussed above, the biasing mechanism  50  is preferably preloaded such that the spring  52  is in a partially compressed state when the garage door  20  is in its closed position to tension the upper cable  42 . The length of the upper cable  42  when the garage door  20  is in the closed position and/or the size of the spring and drawbar assembly  50  are selected so that the spring  52  is partially compressed to the preselected amount that allows for the spring  52  to be compressed an amount corresponding to the maximum differential travel amount. A supplemental tensioner  80 ,  89 , or  90  is provided to allow for adjustment of the axial distance the spring  52  can compress from its partially compressed state, i.e., when the garage door  20  is in its closed position, to its fully compressed state, to achieve only the amount of garage door  20  travel necessary to compensate for the maximum travel differential amount before further travel is prevented by the stop assembly  70 .  
         [0043]    Adjustments may be needed when installing a drive system  10  in accordance with the invention, and when retrofitting an existing system with the biasing mechanism  50 . In particular, the supplemental tensions  80 ,  89 , and  90  allow for the fine-tuning of the biasing mechanism  50 . Adjustments may also be needed periodically over time during use of the garage door drive system  10  due to stretching, and thus an increase in length, of the cables  42  and  44 . For example, if the upper cable  42  increases in length, the spring  52  of the biasing mechanism  50  must increase in axial length from its preselected preload length to take up the slack therein due to the increased length thereof. As discussed above, an increased preload spring  52  axial length will allow the garage door  20  to travel from its closed position a greater distance before further travel is prevented by the stop assembly  70  fully compressing the spring  52 .  
         [0044]    The supplemental tension  80 , as shown in FIG. 5, includes a turnbuckle  82  having hooks screws  84  and  184  with threaded ends  88  and  188  threaded thereinto. The hooked end  86  of the hook screw  84  is connected to the loop end  176  of the drawbar  172  of the spring and drawbar assembly  50 . The other hook screw  184  has its hooked end  186  connected to the bracket  128  mounted to the end of the arm  122  opposite the end of the arm  122  attached to the door  20  with the bracket  124 . The threads of the threaded ends  88  and  188  of the hooks screws  84  and  184  allow for the distance between the opposing hooked ends  86  and  186  thereof to be increased or decreased, which causes the distance between the bracket  129  and the spring and drawbar assembly  50  to increase or decrease. When the distance is decreased, the hooked end  174  of the drawbar  172  can be set to apply a greater preload to the spring, compressing the spring  52  to the preselected amount necessary allow the spring  52  to be fully compressed once the maximum predetermined travel differential has been reached. Conversely, increasing the distance using the tensioner  80  allows the spring  52  to increase in axial length, increasing the amount of travel of the door  20  before the limit assembly  70  fully compresses the spring  52  to prevent further travel of the door  20 .  
         [0045]    [0045]FIG. 6 shows a supplemental tensioner  89 , different from the tensioner  80  discussed above, that allows for the change in distance between the end of the arm  122  and the spring and drawbar assembly  50 . The supplemental tensioner  89  includes a hook screw  104  having a threaded end  102  passing through a bore in a mounting block  130  fixed to the bracket  128  on the end of the arm  122 . The threaded end  102  threads into a nut  106  that prevents the hook screw  104  from passing back through the bore of the block  130 . The hook end  108  of the screw  104  is connected to the loop end  176  of the drawbar  172  of the spring and drawbar assembly  50 . Adjustment of the nut  106  either increases or decreases the distance between the end of the arm  122  and the connection of the hook end  108  to the spring and drawbar assembly  50 . When the distance is increased, the preload on the spring  52  is decreased which increases the axial travel of the spring  52  prior to full compression of the coils thereof, allowing for greater travel of the door  20  from its closed position before the spring  52  is fully compressed and the stop assembly  70  and upper cable  42  prevent further raising of the door  20 . To reduce the travel of the door  20  from its closed position before further travel is prevented by the stop assembly  70  and taunt upper cable  42 , the distance between the end of the arm  122  and the spring and drawbar assembly  50  is decreased, causing the hooked ends  174  of the drawbar  172  to compress the spring  52  to have a smaller initial axial length, i.e., the axial length of the spring  52  when the door  20  is fully closed.  
         [0046]    Another supplemental tensioner  90  is shown in FIGS. 7 and 8 for adjusting the preload in the spring  52  of the spring and drawbar assembly  50 . The loop end  176  of the spring and drawbar assembly  50  is connected relative to the arm  122  via a hook screw  93 . The hook screw  93  has a hook end  92  for connecting to the loop end  176  of the drawbar  172  and a threaded end  95  that passes through a bore in a block  94  mounted to the bracket  128  attached to the arm  122 . A split-nut  98  generally prevents, as will be described in more detail below, the screw  93  from passing back out the bore of the block  94  when the screw  93  is pulled upon by the spring and drawbar assembly  50 . The rotation of the split-nut  98  in the clockwise direction draws the hook end  92  of the screw  93  toward the end of the arm  122 , thereby decreasing the distance between the end of the arm  122  and the connection between the hook end  92  of the screw  93  and the spring and drawbar assembly  50  to increase the precompression of the spring  52  which decreases the distance the opposing drawbars  72  and  172  travel to fully compress the spring  52  therebetween, such as to prevent further travel of the door  20  from the closed position absent rotation of the drive shaft  30 . To increase the axial length of the preloaded spring  52 , causing the drawbars  72  and  172  to travel a greater distance before the spring  52  becomes fully compressed therebetween, the split-nut  98  is turned counter-clockwise, thereby increasing the distance between the end of the arm  122  and the connection between the hook end  92  of the screw  93  and the spring and drawbar assembly  50 .  
         [0047]    In addition to being moved by rotation along the threaded portion  95  of the hook screw  93 , the split-nut  98  also moves along the threaded portion  95  when the threaded portion  95  is pulled either away from or toward the mounting block  94  when a predetermined force is exceeded. The split-nut  98  functions similar to a ratchet, allowing the screw  93  to move relative to the block  94  when the predetermined force is exceeded before reengaging the threaded portion  95  thereof and preventing further movement until the predetermined force is again exceeded. A cap  99  is attached to the end of the threaded portion  95  of the screw  93  and a spring  96  is disposed between the block  94  and the cap  99  to bias the cap  99  and thus the screw  93  away from the block  94 .  
         [0048]    The biasing force of the spring  96  is selected to balance the biasing force of the spring and drawbar assembly  50  attached at the hooked end  92  of the screw  93  on the opposite side of the block  94  from the spring  96  to maintain the distance between the block  94 , fixed relative to the end of the arm  122 , and the connection between the hook end  92  of the screw  93  and the loop end  176  of the drawbar  172  of the spring and drawbar assembly  50  to correspond to the preloaded, precompressed axial length of the spring  52  selected to allow the spring  52  to fully compress once the maximum differential travel amount has been reached. If the spring  52  becomes axially longer than its preselected length, the biasing force of the spring  96  will be greater than the biasing force of the spring  52 , and thus the spring  96  will bias the cap  99  and thus the threaded end  95  of the screw  93  from the block  94  to decrease the distance between the block  94  and the hook end  92  of the screw  93  before the spring forces are balanced and the split-nut  98  prevents further movement, thereby causing the hooks  174  of the drawbar  172  to preload and compress the spring  52  until its preselected axial length is returned. Oppositely, if the biasing force of spring  52  becomes larger than that of spring  95 , such as when the spring  52  is precompressed beyond its desired preload axial length, the split-nut  98  allows the threaded portion  95  of the screw  93  to move toward the block  94  until the spring forces are balanced  96  and  52  to increase the distance between the block  94  and the hooked end  92  of the screw  93  and thus the end of the arm  122  and the connection to the spring and drawbar assembly  50 , thereby allowing the spring  52  to expand back to its preselected axial length.  
         [0049]    Turning to more of the details, the upper and lower cables  42  and  44  may wrap around the same drum  36 , as illustrated in FIG. 2, or may each have separate drums  36 . The drums  36  include lips  38  projecting upward on both sides thereof for assisting in preventing cable throw as the cables  42  and  44  are taken up thereby or payed out therefrom. As illustrated in FIG. 1, the upper cable  42  may be attached only on one side of the door  20 . During door  20  travel, the upper cable  42  is used primarily for urging the door  20  from the open position to the closed position, and particularly the initial movement of the door  20  from its fully open position. Thus, the upper cable  42 , unlike the weight bearing lower cable  44 , is only necessary to be on one side of the door  20 .  
         [0050]    To assist in raising the door  20  from its closed position, the jackshaft operator  32  includes a large torsion spring  38 , as illustrated in FIG. 1, that is configured to bias the door  20  from the closed position, thus reducing the amount of pulling the lower cables  44  need to do as they are taken up on the drums  36  to pull the door  20  open. When lowering the door  20 , the spring  38  assists in counteracting the heavy weight of the door  20  in order to ensure a smooth, controlled descent thereof. A motor  34  is operatively connected to the jackshaft operator  32  to prevent the shaft  30  from rotating unless caused by the motor  34 . When the motor  34  causes the shaft  30  to rotate in a first direction and the door  20  is in its closed position, the torsion spring  38  and the taking up of the lower cables  44  on the drums  36  causes the lifting of the door. Conversely, to move the door  20  from its fully open position, the motor  34  causes rotation of the shaft  30  in a direction opposite the first direction, taking up the upper cable  42  on the drum  36  to pull the arm  122  and thus the door  20  from the open position until the weight of the door  20  against the biasing force of the torsion spring  38  allows the controlled descent of the door  20 .  
         [0051]    The differential travel amount and the maximum differential travel amount between upper and lower cables  42  and  44  during travel of the garage door  20  between open and closed positions, discussed above, depends, at least in part, on the dimensions and geometry of the track  60  and the garage door  20 . In particular, the length of the arm  122 , the height of the panel sections  26 , and the radius of the arcuate portion  64  of the track  60  contribute to the differential travel amounts and the maximum differential travel amount. For example, analysis has shown that an arcuate portion  64  having a fifteen inch radius and an eighteen inch arm  122  will have a larger maximum differential travel amount as compared to a twenty inch arm  122 . Similarly, a different maximum differential travel differential amount will result for an arcuate portion  64  having a twelve inch radius when used with an eighteen inch arm  122  as compared to an arcuate portion  64  with a fifteen inch radius used with an eighteen inch arm  122 . These particular configurations are discussed in greater detail the examples and analysis below.  
       EXAMPLE 1  
       [0052]    The follow example illustrates the difference in the travel between the lower and upper cables  44  and  42  as the garage door  20  is moved from a closed position to an open position. The garage door  20  comprises four panel sections  26  hinged together with hinges  28 , with each panel  26  being approximately twenty-one inches in height, for a total door height of approximately eighty-four inches. An arm  122  about twenty inches in length is pivotably connected with a bracket  124  to an upper panel  26  of the door  20  approximately six inches below its upper edge. Rollers  24  are attached to either hinges  28  or brackets  29  and  128  and extend from the lateral edges of the panels  26  and the arm  122  at positions similar to those illustrated in FIG. 1 for travel within tracks  60  having an arcuate portion  64  with a fifteen inch radius.  
         [0053]    As the garage door  20  was move from its closed position to its open position, the length and relative travel of both the lower and upper cables  44  and  42  was measured for every twelve inches that the garage door  20  was raised from its closed position, as set forth in the table below.  
                                                                                   15″ Door Track Radius with 20″ Arm                Lower   Lower   Upper   Upper   Travel       Door   Cable   Cable   Cable   Cable   Difference       Height   Length   Travel   Length   Travel   (Upper − Lower)                    0   96.127   0.000   12.311   0.000   0.000       12   84.122   12.005   25.072   12.761   0.756       24   72.117   24.010   36.753   24.442   0.432       36   60.110   36.017   49.207   36.896   0.879       48   48.099   48.028   60.981   48.670   0.642       60   36.078   60.049   73.789   61.478   1.429       72   24.043   72.084   85.477   73.166   1.082       84   12.167   83.960   96.506   84.195   0.235                  
 
         [0054]    As illustrated in the chart of FIG. 9, plotting the differential travel amount between the upper and lower cables  42  and  44  in the above example relative to the height of the garage door  20  illustrates an oscillating pattern of the differential travel amount. The three peaks of the differential travel amount illustrated in FIG. 9 correspond to travel of the three sets of rollers  24  proximate the hinge connections  28  between the adjacent four panels  26  of the garage door  20  traveling through the arcuate portion  64  of the track  60 . Further, as the garage door  20  is raised further, the magnitude of the differential travel amount increases due to the decrease in the distance between the lower end of the garage door  20  and the shaft  30 .  
         [0055]    The maximum difference between the upper cable travel and the lower cable travel, i.e, the maximum differential travel amount, is 1.429 inches. Thus, a tensioner  50  could be placed at an end of the upper cable  42  and adjusted to have a maximum limit of extension of 1.429 inches before further extension is prevented by the stop assembly  70 , just enough extension to allow for the upper cable  42  to accommodate the variation between its travel and the travel of the lower cable  42 . If desired, the limit of extension can be increased, such as to 1.50 inches, to accommodate for variations in reproducing the above results.  
       EXAMPLE 2  
       [0056]    The following example is similar to EXAMPLE 1, however instead of an arm  122  twenty inches in length, an arm  122  eighteen inches in length is used. As the garage door  20  moves from its closed position to its open position, the corresponding length and differential travel between both the lower and upper cables  44  and  42  was measured for every inch the garage door  20  was raised, as set forth in the table below.  
                                                                                   15″ Door Track Radius with 18″ Arm                Lower   Lower   Upper   Upper   Travel       Door   Cable   Cable   Cable   Cable   Difference       Height   Length   Travel   Length   Travel   (Upper − Lower)                    0   96.127   0.000   9.886   0.000   0.000       1   95.126   1.001   10.917   1.031   0.030       2   94.126   2.001   12.013   2.127   0.126       3   93.126   3.001   13.147   3.261   0.260       4   92.125   4.002   14.281   4.395   0.393       5   91.125   5.002   15.401   5.515   0.513       6   90.125   6.002   16.513   6.627   0.625       7   89.124   7.003   17.617   7.731   0.728       8   88.124   8.003   18.712   8.826   0.823       9   87.124   9.003   19.799   9.913   0.910       10   86.123   10.004   20.876   10.990   0.986       11   85.123   11.004   21.940   12.054   1.050       12   84.122   12.005   22.990   13.104   1.099       13   83.122   13.005   24.200   14.314   1.309       14   82.122   14.005   25.025   15.139   1.134       15   81.121   15.006   25.989   16.103   1.097       16   80.121   16.006   26.903   17.017   1.011       17   79.121   17.006   27.820   17.934   0.928       18   78.120   18.007   28.788   18.902   0.895       19   77.120   19.007   29.785   19.899   0.892       20   76.120   20.007   30.781   20.895   0.888       21   75.119   21.008   31.776   21.890   0.882       22   74.119   22.008   32.768   22.882   0.874       23   73.118   23.009   33.758   23.872   0.863       24   72.118   24.009   34.750   24.864   0.855       25   71.117   25.010   35.746   25.860   0.850       26   70.117   26.010   36.751   26.865   0.855       27   69.116   27.011   37.767   27.881   0.870       28   68.116   28.011   38.795   28.909   0.898       29   67.115   29.012   39.838   29.952   0.940       30   66.115   30.012   40.892   31.006   0.994       31   65.114   31.013   41.954   32.068   1.055       32   64.114   32.013   43.020   33.134   1.121       33   63.113   33.014   44.088   34.202   1.188       34   62.113   34.014   45.154   35.268   1.254       35   61.112   35.015   46.202   36.316   1.301       36   60.111   36.016   47.204   37.318   1.302       37   59.111   37.016   48.161   38.275   1.259       38   58.110   38.017   49.129   39.243   1.226       39   57.110   39.017   50.145   40.259   1.242       40   56.109   40.018   51.161   41.275   1.257       41   55.109   41.018   52.143   42.257   1.239       42   54.108   42.019   53.096   43.210   1.191       43   53.107   43.020   54.041   44.155   1.135       44   52.106   44.021   54.996   45.110   1.089       45   51.104   45.023   55.966   46.080   1.057       46   50.102   46.025   56.952   47.066   1.041       47   49.101   47.026   57.956   48.070   1.044       48   48.099   48.028   58.980   49.094   1.066       49   47.098   49.029   60.022   50.136   1.107       50   46.097   50.030   61.085   51.199   1.169       51   45.096   51.031   62.162   52.276   1.245       52   44.095   52.032   63.251   53.365   1.333       53   43.094   53.033   64.346   54.460   1.427       54   42.091   54.036   65.445   55.559   1.523       55   41.090   55.037   66.531   56.645   1.608       56   40.088   56.039   67.602   57.716   1.677       57   39.086   57.041   68.615   58.729   1.688       58   38.084   58.043   69.637   59.751   1.708       59   37.081   59.046   70.713   60.827   1.781       60   36.078   60.049   71.788   61.902   1.853       61   35.075   61.052   72.817   62.931   1.879       62   34.072   62.055   73.804   63.918   1.863       63   33.069   63.058   74.768   64.882   1.824       64   32.066   64.061   75.727   65.841   1.780       65   31.063   65.064   76.687   66.801   1.737       66   30.060   66.067   77.650   67.764   1.697       67   29.057   67.070   78.614   68.728   1.658       68   28.054   68.073   79.582   69.696   1.623       69   27.051   69.076   80.555   70.669   1.593       70   26.048   70.079   81.530   71.644   1.565       71   25.045   71.082   82.505   72.619   1.537       72   24.043   72.084   83.480   73.594   1.510       73   23.038   73.089   84.443   74.557   1.468       74   22.051   74.076   85.401   75.515   1.439       75   21.073   75.054   86.346   76.460   1.406       76   20.089   76.038   87.264   77.378   1.340       77   19.103   77.024   88.138   78.252   1.228       78   18.114   78.013   88.995   79.109   1.096       79   17.126   79.001   89.897   80.011   1.010       80   16.138   79.989   90.843   80.957   0.968       81   15.140   80.987   91.792   81.906   0.919       82   14.147   81.980   92.710   82.824   0.844       83   13.153   82.974   93.608   83.722   0.748       84   12.167   83.960   94.506   84.620   0.660                  
 
         [0057]    When the differential travel amount between the upper and lower cables  42  and  44  is plotted against the elevation of the bottom end of the garage door  20 , as illustrated in FIG. 9, an oscillation pattern similar to that of EXAMPLE 1 is apparent. However, by shortening the arm length compared to that of EXAMPLE 1, the maximum variation between the cable travels is increased to 1.879 inches. Accordingly, the biasing mechanism  50  could be placed at an end of the upper cable  42  and have the stop assembly  70  configured to provide a maximum extension limit of 1.879 inches, corresponding to the maximum travel differential amount between the cables  42  and  44 .  
       EXAMPLE 3  
       [0058]    The following example is similar to EXAMPLES 1 and 2, however an arm  122  eighteen inches in length and a track  60  having an arcuate portion  64  with a radius of twelve inches are used. As the garage door  20  was move from its closed position to its open position, the corresponding length and travel of both the lower and upper cables  44  and  42  was measured for every twelve inches the door  20  was raised, as set forth in the table below.  
                                                                                   12″ Door Track Radius with 18″ Arm                Lower   Lower   Upper   Upper   Travel       Door   Cable   Cable   Cable   Cable   Difference       Height   Length   Travel   Length   Travel   (Upper − Lower)                    0   96.127   0.000   12.391   0.000   0.000       12   84.122   12.005   25.166   12.775   0.770       24   72.117   24.010   36.326   23.935   −0.075       36   60.110   36.017   49.906   37.515   1.498       48   48.099   48.028   60.771   48.380   0.352       60   36.078   60.049   73.938   61.547   1.498       72   24.043   72.084   85.563   73.172   1.088       84   12.167   83.960   95.962   83.571   −0.389                  
 
         [0059]    When the differential travel amount for the upper and lower cables  42  and  44  of EXAMPLE 3 is plotted against the garage door elevation, an oscillation pattern similar to that of EXAMPLES 1 and 2 is apparent. However, the change in the radius of the arcuate portion  64  of the track  60 , as compared to EXAMPLES 1 and 2, and the arm length, as compared to EXAMPLE 1, combine to result in a maximum travel difference of 1.498 inches. Thus, a biasing mechanism  50  having a stop assembly  70  configured to allow for a maximum of 1.498 inches of movement, corresponding to the maximum travel difference, can be placed the upper cable  42  and the top end of the garage door  20 .  
         [0060]    While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.  
         [0061]    The invention is defined more particularly by the following claims:

Summary:
A drive system is provided for a moveable barrier, such as a garage door, that limits unauthorized shifting thereof. The drive system includes a cable actuator for lowering the door. The cable actuator is tensioned with a biasing mechanism to minimize cable throw, and a stop assembly of the biasing mechanism limits travel of the garage door from the closed position by a predetermined amount that is sufficiently small so as to keep intruders out of the garage.