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BACKGROUND OF THE INVENTION 
     The present invention relates to the general field of pedestrian barriers, and more particularly to the field of barriers used to control and direct groups of people in public places. 
     Queue barriers are commonly used to guide and control crowds of people at public events and exhibits. Typical freestanding queue barriers comprise a draped rope or retractable belt stretched between upright tubular stanchions, each mounted on a weighted circular base. For aesthetic reasons, it is often desirable to minimize the diameter of the stanchions and the bulk of the base. The preference for a sleek, unobtrusive look, particularly at artistic exhibits, can dictate the use of slender cords rather than belts between the stanchions. 
     While spring-loaded spool mechanisms are suitable for use with retractable belt barriers, a spool for the equivalent length of cord would need to be much wider—requiring an unsightly larger stanchion diameter. For retractable cord barriers, proper cord tension is a critical element, since a sagging cord is a visual distraction, while an excessively taut, unyielding cord can pose a tripping or safety hazard. 
     The present invention addresses these requirements by providing a retraction mechanism in which the cord is helically wound around one or more pairs of opposing pulleys. When the cord is extended, one set of pulleys in each pair remains fixed, while the other slides toward it against the resistance of a constant-force spring. In order to achieve the proper balance of cord and spring tension, the optimal stretch factor of the cord is less than 50%, as compared to 100% stretch cord commonly used in other applications. The optimal stretch factor of the cord is selected to achieve the correct balance between the retraction force of the spring, which is constant, and the extension force of the cord, which increases as the cord stretches. The excessive stiffness of 100% stretch cord translates into a large force that must be exerted to extend the cord. That large extension force must be balanced by an equally large refraction force of the spring, thereby requiring a large spring. But the refraction force of a large spring will cause a stanchion to tip over unless its base is heavily weighted. High spring tension will also cause an extended cord to snap back forcefully and hazardously when released. On the other hand, a cord with minimal or no stretch will be unyielding when taut and can become slack and develop an unsightly sag when extended between stanchions. 
     There are several U.S. patents directed to spring-biased retraction mechanisms. The systems described in the U.S. patents of Carlson (U.S. Pat. No. 5,117,859), Schwendinger (U.S. Pat. No. 6,338,450) and Bertagna et al. (U.S. Pat. No. 5,421,530) do not employ constant force springs, because there is no need in these applications to maintain a constant tension on the extended hose/cable/cord. Moreover, since the stretch factor of the hose/cable/cord in these applications is negligible, these mechanisms do not need to balance the opposing forces of a spring and a stretched cord, as does the present invention. 
     While the phone cord rewinder described in the U.S. patent of Ditzig (U.S. Pat. No. 5,507,446) does utilize a constant-force coiled metal spring as the biasing mechanism between the pulleys, it lacks any means of maintaining a constant taut tension on the extended phone cord, which must have a certain amount of slack to be usable. 
     The U.S. patent of Knapp et al. (U.S. Pat. No. 6,143,985) discloses a cable retracting system for modular components, using a pulley system biased by constant-force coiled metal springs. Unlike the Ditzig mechanism, this apparatus is designed to maintain a low constant force on the extended cable sufficient to prevent dangling and entanglement. But the Knapp system is incapable of providing the “straight line” tension required in a queue barrier and cannot be adapted to handle a stretchable cord. 
     In short, none of the spring-biased pulley retraction mechanisms disclosed in the prior art address the problem of achieving a constant taut, but yielding, tension in a stretchable cord. Nor can the features of the prior art mechanisms be combined in an obvious way to achieve this functionality of the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a queue barrier specifically suited for applications, such as museums, which demand an aesthetically pleasing, unobtrusive appearance. In addition to directing the flow of patrons entering an exhibit, these barriers are often used to keep patrons at a safe distance from sensitive art objects. For that reason, barriers that deploy retractable belt or tape restraints between the stanchions are not desirable, because the breadth of the belt or tape interferes with the patrons&#39; view of the protected object. For the same reason, the stanchion itself should have the minimal diameter consistent with its function. 
     Although a retractable cord has much less visual impact than a belt or tape, it has a greater bulk when wrapped around a spool than does a belt or tape. Since spring-loaded spools are the standard retraction mechanisms in existing queue barriers, the objective of combining a retractable cord with a slender stanchion is the central technical problem which the present invention addresses. 
     The present invention addresses this technical problem by providing, instead of the standard spring-loaded spool retraction mechanism, a spring-biased pulley retraction mechanism acting on a stretchable cord. A constant-force coiled metal spring is used, such that the retraction force on the cord does not increase as the cord is extended—as it would for a helical spring governed by Hooke&#39;s Law. The use of a constant-force spring avoids abrupt snap-back of the extended cord when released, as well as the need for excessive pulling force on the cord as it approaches full extension, which tends to cause the stanchion to tip over. 
     The present invention achieves a dynamic balance between the constant retraction force of the spring-biased pulley system and the opposing contraction force of the stretched cord as it extends. The elastic cord most commonly used in other applications has a stretch factor of 100%—i.e., it will expand to twice its unstretched length. The contraction force exerted by 100% stretch cord will increase proportionally to its stretch until it reaches full extension. While it&#39;s possible to maintain a balance between this contraction force and the retraction force of the spring if the latter force also proportionally increases in accordance with Hooke&#39;s Law, the barrier stanchion would tend to tip over at full extension unless its base were heavily weighted to anchor the spring. In combination with a constant-force spring, on the other hand, a balance between the proportionally increasing contraction force of 100% stretch cord and the constant refraction force of the spring cannot be maintained over the entire extension of the cord. Either the spring must be over-sized, in which case the extended cord will be excessively taut, creating a tripping/safety hazard, or the spring must be under-sized, in which case the extended cord will be slack and unsightly and will not retract properly. 
     By utilizing a cord with a stretch factor of less then 50%, the present invention achieves a dynamic balance between the contraction force of the cord and the constant retraction force of the spring-biased pulley system. As the cord is extended, it initially stretches until it becomes taut, yet yielding if engaged by a patron. As the cord is further extended, its contraction force and the retraction force of spring-biased pulley system remain in balance, allowing the taut but yielding tension of the cord to be maintained without exerting an excessive tipping force on the stanchion. 
     The foregoing summarizes the general design features of the present invention. In the following sections, specific embodiments of the present invention will be described in some detail. These specific embodiments are intended to demonstrate the feasibility of implementing the present invention in accordance with the general design features discussed above. Therefore, the detailed descriptions of these embodiments are offered for illustrative and exemplary purposes only, and they are not intended to limit the scope either of the foregoing summary description or of the claims which follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of an exemplary queue barrier comprising three (3) interconnected stanchions; 
         FIG. 2A  is a perspective view of a retraction mechanism, comprising two pairs of spring-biased opposing pulleys, according to the preferred embodiment of the present invention; 
         FIG. 2B  is a front view of a refraction mechanism, comprising two pairs of spring-biased opposing pulleys, according to the preferred embodiment of the present invention; 
         FIG. 2C  is a rear view of a retraction mechanism, comprising two pairs of spring-biased opposing pulleys, according to the preferred embodiment of the present invention; 
         FIG. 3  is an exploded view of a retraction mechanism, comprising two pairs of spring-biased opposing pulleys, according to the preferred embodiment of the present invention; 
         FIG. 4  is a front view of a retraction mechanism, comprising two pairs of spring-biased opposing pulleys, with an elastic cord helically winding around each pair of opposing pulleys, according to the preferred embodiment of the present invention; 
         FIG. 5A  is a detail view of a spring-loaded cord connector in the closed position; 
         FIG. 5B  is a detail view of a spring-loaded cord connector in the unlocked open position; 
         FIG. 5C  is a detail view of a spring-loaded cord connector in the locked open position; 
         FIG. 6A  is a detail view of the closed position of the spring mechanism of the spring-loaded cord connector as depicted in  FIG. 5A ; 
         FIG. 6B  is a detail view of the unlocked open position of the spring mechanism of the spring-loaded cord connector as depicted in  FIG. 5B ; 
         FIG. 6C  is a detail view of the locked open position of the spring mechanism of the spring-loaded cord connector as depicted in  FIG. 5C ; and 
         FIGS. 7A-7D  are views of an exemplary floor socket for the support of one of the stanchions of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , an exemplary queue barrier system  10  according to the present invention comprises three (3) tubular stanchions  11 , each supported by a weighted base  12 . Alternately, each of the stanchions can be anchored in a floor socket  13 , of which  FIGS. 7A-7D  depict an illustrative example. 
     In  FIG. 1 , a first stanchion  14  is releasably connected to a second stanchion  15  by two retractable elastic cords  17 , which extend from two cord apertures  18  in the first stanchion  14 . A first upper cord  19  extends from a first upper cord aperture  20  of the first stanchion  14  and releasably attaches to a second upper cord connector  26  of the second stanchion  15 . A first lower cord  22  extends from a first lower cord aperture  23  of the first stanchion  14  and releasably attaches to a second lower cord connector  28  of the second stanchion  15 . 
     The reason for having both upper and lower cords  17  interconnecting the stanchions  11  is compliance with ADA requirements, with the lower cords serving as an indicator for visually-impaired persons. The upper cords are set at approximate hip-to-waist level for a standing person, while the lower cords are at approximate knee level. 
     Referring again to  FIG. 1 , the first stanchion  14  is releasably connected to a third stanchion  16  by two retractable elastic cords  17 , which extend from two cord apertures  18  in the third stanchion  16 . A third upper cord  29  extends from a third upper cord aperture  30  of the third stanchion  16  and releasably attaches to a first upper cord connector  21  of the first stanchion  14 . A third lower cord  32  extends from a third lower cord aperture  33  of the third stanchion  16  and releasably attached to a first lower cord connector  24  of the first stanchion  14 . 
     It is understood that this illustrative three-stanchion barrier system can be further extended. For example, the second stanchion  15  can be further connected to a fourth stanchion (not shown) by extending upper and lower elastic cords (not shown) from a second upper cord aperture  25  and a second lower cord aperture  27  to corresponding upper and lower cord connectors of the fourth stanchions (not shown). Similarly, the third stanchion  16  can be connected to a fifth stanchion (not shown) by extending upper and lower elastic cords (not shown) from the fifth stanchion to the third upper cord connector  31  and the third lower cord connector  34 , respectively. In this manner, the queue barrier can be indefinitely extended in either direction according to the desired area to be enclosed. 
     Although, in the exemplary barrier system  10  depicted in  FIG. 1 , the stanchions  11  are arranged in a straight line, it is understood that angular connections between the stanchions  11  are also feasible, and that multiple cord connectors can be located on the stanchions  11  at various angles with respect to the cord apertures  18 . 
       FIGS. 2A-2C  and  FIG. 3  depict an exemplary mechanism  35  within each stanchion  11  which controls the extension and retraction of the elastic cords  17 . The depicted embodiment  35  comprises two pairs of opposing spring-biased pulleys  36 , which are mounted on a pulley frame  37  consisting of two parallel frame rods  38  anchored to the stanchion  11 . An upper pair of pulleys  39  comprises an upper fixed pulley  40 , which is fixedly attached to the upper end of the pulley frame  37 , and an upper movable pulley  41 , which is slidably attached to the midsection of the pulley frame  37 . A constant-force upper coil spring  42  is anchored to the pulley frame  37  immediately below the upper movable pulley  41 , with the free end of the coil  42  attached to the upper movable pulley  41  and restraining its movement toward the upper fixed pulley  40 . 
     Similarly, a lower pair of pulleys  43  comprises a lower fixed pulley  44 , which is fixedly attached to the midsection of the pulley frame  37  below the upper coil spring  42 , and a lower movable pulley  45 , which is slidably attached to the lower end of the pulley frame  37 . Optionally, the upper coil spring  42  can be anchored to the pulley frame  37  by the same structure that attaches to the lower fixed pulley  44  to the midsection of the pulley frame  37 . A constant-force lower coil spring  46  is anchored to the pulley frame  37  immediately below the lower movable pulley  45 , with the free end of the coil  46  attached to the lower movable pulley  45  and restraining its movement toward the lower fixed pulley  41 . 
     Referring now to  FIG. 4 , the upper cord  19  helically winds around the upper pair of pulleys  39 , with its proximal end  47  anchored in the upper fixed pulley  40 , and its distal end  48  extending outward from the upper fixed pulley  40  through the upper cord aperture  20  of the stanchion  11 . When the distal end  48  of the upper cord  19  is pulled away from the stanchion  11  to interconnect it with an adjoining stanchion (as shown in  FIG. 1 ), the shortening of the length of the upper cord  19  helically winding around the upper pair of pulleys  39  draws the upper movable pulley  41  toward the upper fixed pulley  40  against the constant retractive force of the upper coil spring  42 . 
     As the elastic upper cord  19  is extended, it stretches to its maximum length, which is preferably about 20% greater than its unstretched length. The 20% stretch factor allows the upper coil spring  42  to be moderately sized, so that its retraction force is not so great as to tip the stanchion  11  to which it&#39;s anchored or to cause the upper cord to snap back forcefully when released. The size of the upper coil spring  42  is selected so that its constant retractive force balances the contractive force of the upper cord  19  when fully stretched. 
     Referring again to  FIG. 4 , the lower cord  22  helically winds around the lower pair of pulleys  43 , with its proximal end  49  anchored in the lower fixed pulley  44 , and its distal end  50  extending outward from the lower fixed pulley  44  through the lower cord aperture  23  of the stanchion  11 . When the distal end  50  of the lower cord  22  is pulley away from the stanchion  11  to interconnect it with an adjoining stanchion (as shown in  FIG. 1 ), the shortcoming of the length of the lower cord  22  helically winding around the lower pair of pulleys  43  draws the lower movable pulley  45  toward the lower fixed pulley  44  against the constant retractive force of the lower coil spring  46 . 
     As the elastic lower cord  22  is extended and stretched to its maximum length, its contractive tension balances the retractive force of the lower coil spring  46  in the same way as described above with reference to the dynamic balance between upper cord  19  and upper coil spring  42 . 
       FIGS. 5A-5C  and  FIGS. 6A-6C  depict an optional configuration for accessing the upper cord connector  21  of the stanchions  11 . The top of the stanchion  11  is configured with a spring-loaded liftable access cap  51 , through which the upper cord connector  21  can be accessed with a connecting cord from an adjoining stanchion. As shown in  FIGS. 5A and 6A , the access cap  51  is retained in the closed position by a spring mechanism  52 —in this example a helical spring. As the cap  51  is lifted into the open position, depicted in  FIG. 5B , the spring  52  is compressed, as shown in  FIG. 6B . When the cap  51  is swiveled outward, as shown in  FIG. 5C , it locks in the open position against the restoring force of the spring  52 , as depicted in  FIG. 6C . With the access cap  51  locked in the open position, the upper cord connector  21  is accessible to a connecting cord extending from another stanchion, as shown in  FIG. 5C . Once the connecting cord is in place, the access cap  51  is swiveled inward again, as shown in  FIG. 5B , and the spring  52  is able to retract ( FIG. 6B ) and restore the cap  51  to the closed position depicted in  FIGS. 5A and 6A . 
     Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible, without departing from the scope and spirit of the present invention as defined by the accompanying claims.

Summary:
A retractable cord queue barrier system uses a spring-biased pulley refraction mechanism acting on a stretchable cord. A constant-force coiled metal spring is used, such that the retraction force on the cord does not increase as the cord is extended—as it would for a helical spring governed by Hooke&#39;s Law. The use of a constant-force spring avoids abrupt snap-back of the extended cord when released, as well as the need for excessive pulling force on the cord as it approaches full extension, which tends to cause the stanchion to tip over. Dynamic balance between the contractive force of the stretchable cord and the retractive force of the constant-force spring achieves a taut but not unyielding tension in the interconnecting cords between stanchions.