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RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of co-pending patent application Ser. No. 15/093,727, which claims the priority of 62/144,898 filed Apr. 8, 2015. 
         [0002]    This application also claims priority of U.S. Provisional application No. 62/165,177 filed May 21, 2015. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    In general, the present invention relates to counterbalance systems for windows that prevent open window sashes from moving under the force of their own weight. More particularly, the present invention system relates to the structure of both the brake shoe and the spring mount components of the counterbalance system that interconnect to inhibit the unintentional movement, known as drift, of a window sash that has been fully open. 
         [0005]    2. Description of the Prior Art 
         [0006]    There are many types and styles of windows. One of the most common types of window is the double-hung window. Double-hung windows are the window of choice for most home construction applications. A double-hung window consists of an upper window sash and a lower window sash. Either the upper window sash or the lower window sash can be selectively opened and closed by a person sliding the sash up and down within the window frame. 
         [0007]    The sash of a double-hung window has a weight that depends upon the materials used to make the window sash and the size of the window sash. Since the sashes of a double-hung window are free to move up and down within the frame of a window, some counterbalancing system must be used to prevent the window sashes from constantly moving to the bottom of the window frame under the force of their own weight. 
         [0008]    A popular variation of the double-hung window is the tilt-in double-hung window. Tilt-in double-hung windows have sashes that can be selectively moved up and down. Additionally, the sashes can be selectively tilted into the home so that the exterior of the sashes can be cleaned from within the home. 
         [0009]    Modern tilt-in double-hung windows are primarily manufactured in one of two ways. There are vinyl frame windows and wooden frame windows. In the window manufacturing industry, different types of counterbalance systems are traditionally used for vinyl frame windows and for wooden frame windows. The present invention is mainly concerned with the structure of vinyl frame windows. As such, the prior art concerning vinyl frame windows is herein addressed. 
         [0010]    Vinyl frame, tilt-in, double-hung windows are typically manufactured with guide tracks along the inside of the window frame. Brake shoe assemblies, commonly known as “shoes” in the window industry, are placed in the guide tracks and ride up and down within the guide tracks. Each sash of the window has two tilt pins or tilt posts that extend into the shoes and cause the shoes to ride up and down in the guide tracks as the window sashes are opened or closed. 
         [0011]    The shoes contain a brake mechanism that is activated by the tilt post of the window sash when the window sash is tilted inwardly away from the window frame. The shoe therefore locks the tilt post in place and prevents the base of the sash from moving up or down in the window frame once the sash is tilted open. Furthermore, the brake shoes are attached to coil springs inside the guide tracks of the window assembly. Coil springs are constant force springs, made from wound lengths of metal ribbon. The coil springs supply the counterbalance force needed to suspend the weight of the window sash. 
         [0012]    Small tilt-in windows have small, relatively light window sashes. Such small sashes may only require a single coil spring on either side of the window sash to generate the required counterbalance forces. However, due to the space restrictions present in modern tilt-in window assemblies, larger springs cannot be used for heavier window sashes. Rather, multiple smaller coil springs are ganged together to provide the needed counterbalance force. A large tilt-in window sash may have up to eight coil springs to provide the needed counterbalance force. 
         [0013]    The counterbalance force created by the coil spring is fairly constant. However, there is a slight decrease in the counterbalance force that occurs when the window sash is open to the fullest degree. In this position, the coil springs are nearly fully wound. As such, very little of the coil spring is stressed and is opposing the weight of the window sash. It is for this reason that when a window sash is fully open, it may close slightly without being touched. Gravity can cause a window sash to drift closed. Furthermore, when an upper window sash is open, it creates friction against the lower window sash that it passes. If the lower window sash is open, then the friction can cause the upper window sash to drift open. 
         [0014]    A need therefore exists for a system that prevents a window from drifting away from its fully open position. This need is met by the present invention as described and claimed below. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention is a system and method for inhibiting inadvertent movement of a window sash out of a fully open position. The window sash is set in guide tracks that run along the sides of the overall window assembly. The window sash is a tilt-in window with pivot posts that engage brake shoes. The brake shoes travel up and down in the guide tracks as the window sash is moved between a fully open position and a fully closed position. 
         [0016]    A stop is mounted within the guide tracks. The brake shoe and the stop have a connector that joins the brake shoe to the stop when the window sash is moved to its fully open position. The connection made the connector is tenuous. The brake shoe is separable from the stop when a closing force is manually applied to the window sash that acts to move the window sash away from its fully open position. The force applied must exceed a threshold level. In this manner, the window sash will remain in its fully open position and will not inadvertently drift closed due to gravity, vibrations, or friction with another window sash. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a better understanding of the present invention, reference is made to the following description of exemplary embodiments thereof, considered in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  is an end view showing an exemplary embodiment of a counterbalance system in a guide track of a tilt-in window; 
           [0019]      FIG. 2  is an exploded perspective view of the primary components of the counterbalance system; 
           [0020]      FIG. 3  shows the embodiment of  FIG. 2  with the brake shoe and stop mount engaged and without the coiled ribbon springs; 
           [0021]      FIG. 4  is a cross-sectional view of the embodiment shown in  FIG. 3 ; 
           [0022]      FIG. 5  shows an alternate embodiment for the connector between the brake shoe and the stop mount; 
           [0023]      FIG. 6  shows an alternate embodiment for the connector between the brake shoe and the stop mount; 
           [0024]      FIG. 7  shoes an alternate embodiment for the connector between the brake shoe and the stop mount; 
           [0025]      FIG. 8  shoes an alternate embodiment for the connector between the brake shoe and the stop mount. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The features of the present invention counterbalance system can be incorporated into many window designs. However, the illustrations provided only show a few exemplary embodiments of the counterbalance system for the purpose of description. The embodiments illustrated are selected in order to set forth some of the best modes contemplated for the invention. The illustrated embodiments, however, are merely exemplary and should not be considered limitations when interpreting the scope of the claims. 
         [0027]    Referring to  FIG. 1 , in conjunction with  FIG. 2 ,  FIG. 3  and  FIG. 4 , there is shown a first exemplary embodiment of a counterbalance system  10  that is used to counterbalance the sashes contained within a window assembly. The counterbalance system  10  utilizes a brake shoe  12  that supports one or more coiled ribbon springs  14 . The counterbalance system  10  also includes a stop mount  16  that attaches to the frame of the window in the window guide track  18 . It will be understood that each window sash typically utilizes two counterbalance systems on opposite sides of a window sash  20 . However, for the sake of simplicity and clarity, only one counterbalance system  10  is being illustrated. 
         [0028]    The brake shoe  12  rides up and down in the guide track  18  of the window assembly. The brake shoe  12  and the window sash  20  it supports are pulled upwardly within the guide track  18  by the coiled ribbon springs  14 . The coiled ribbon springs  14  rotate around spool posts  21  that extend from the brake shoe  12 . 
         [0029]    A tilt arm  22  extends into the brake shoe  12 . The tilt arm  22  clips into the window sash  20 , therein interconnecting the window sash  20  with the brake shoe  12 . When the window sash  20  is tilted, the tilt arm  22  turns and causes the brake shoe  12  to lock in place in the guide track  18 . 
         [0030]    When the window sash  20  is opened to its fullest extent, the coiled ribbon springs  14  are nearly fully wound on the spool posts  21 . In this fully open position, the brake shoe  12  contacts the stop mount  16 . The presence of the stop mounts  16  prevents any further movement of the brake shoe  12  beyond the fully open position. However, the mere presence of the stop mount  16  does not prevent the brake shoe  12  from drifting away from the stop mount  16 , therein causing the window sash  20  to move away from its fully open position. 
         [0031]    To prevent the brake shoe  12 , and the window sash  20  it supports, from drifting away from the stop mount  16 , an interconnection is created between the stop mount  16  and the brake shoe  12  that physically prevents the brake shoe  12  from inadvertently drifting away from the stop mount  16 . In  FIG. 1  through  FIG. 4 , the interconnection between the brake shoe  12  and the stop mount  16  is created by a mechanical connector  24 . 
         [0032]    The mechanical connector  24  includes a flexible locking finger  26  that protrudes from the first end of the brake shoe  12 . The locking finger  26  has a flexible neck  29  and a shaped head  30  that extends from the flexible neck  29 . The locking finger  26  and the flexible neck  29  form the male section of the mechanical connector  24 . The interconnecting female section of the mechanical connector  24  is formed on the stop mount  16 . 
         [0033]    The stop mount  16  is anchored to the rear wall of the guide track  18 . When the window sash  20  is opened, the brake shoe  12  moves up the guide track  18  until it contacts the stop mount  16 . The stop mount  16  is set in a fixed location on the guide track  18  with screws. The stop mount  16  has anchor elements  32  that interconnect with the free ends of the coiled ribbon springs  14 . In this manner, the stop mount  16  anchors the free ends of the coiled ribbon springs  14  and the coiled ribbon springs  14  act to bias the brake shoe  12  toward the stop mount  16 . This bias counterbalances the force of gravity, which acts to move the window sash  20  away from the stop mount  16 . 
         [0034]    The female section of the mechanical connector  24  is configured as a depression  34  that is formed into the stop mount  16 . The depression  34  presents an inclined surface  36  at the bottom edge of the stop mount  16 . The inclined surface  36  extends into the depression  34  and terminates with a ledge  38 . When the window sash  20  is fully open, the locking finger  26  from the brake shoe  12  comes into contact with the stop mount  16 . The locking finger  26  advances over the inclined surface  36 , wherein the shaped head  30  moves over the inclined surface  36  and passes over the ledge  38 . The presence of the shaped head  30  latched over the ledge  38  completes the mechanical connector  24  that interconnects the brake shoe  12  and the stop mount  16 . The interconnection provided by the mechanical connector  24  is sufficient to prevent the window sash  20  from drifting, due to gravity or contact with another sash. However, the interconnection is tenuous. The mechanical connector  24  can be separated by manually applying a separation force in excess of a designed threshold. The preferred threshold separation force corresponds to a downward force of between one and ten pounds being applied to the window sash  20 . Such a downward force will separate the mechanical connector  24  and pull the brake shoe  12  free of the stop mount  16 , wherein the window now functions in the traditional manner. 
         [0035]    It will therefore be understood that the window sash  20  will operate in a completely ordinary manner until the window sash  20  is opened to its fullest extent. At that point, the brake shoe  12  interconnects with the stop mount  16 . This interconnection prevents the window sash  20  from drifting closed. To close the window sash  20 , a threshold force must be applied to the window sash  20  in a manner that acts to separate the interconnection. Once the threshold force is reached, the brake shoe  12  will separate from the stop mount  16  and the window sash  20  is free to close. 
         [0036]    In the embodiment illustrated in  FIG. 1  through  FIG. 4 , the male section of the mechanical connector  24  is positioned on the brake shoe  12  and the female section is positioned on the stop mount  16 . It will be understood that such positions are arbitrary and can be reversed. What is of importance is that the mechanical connector  24  exists between the brake shoe  12  and the stop mount  16  so that the brake shoe  12  and the stop mount  16  automatically interconnect when brought into contact. 
         [0037]    Referring to  FIG. 5 , an alternate embodiment of the mechanical connector  50  between the stop mount  16  and the brake shoe  12  is shown. In this embodiment, a female receptacle  52  is formed at the first end of the brake shoe  12 . The female receptacle  52  is shaped and positioned to receive a locking finger  54  that extends from the stop mount  16 . When the brake shoe  12  advances to the stop mount  16 , the locking finger  54  enters the receptacle  52  and a mechanical interconnection is created. The mechanical connector  50  is designed to separate should a separating threshold force be applied. 
         [0038]    Referring to  FIG. 6 , another embodiment of the mechanical connector  60  between the stop mount  16  and the brake shoe  12  is shown. In this embodiment, the mechanical connector  60  is configured as a clip having two locking fingers  64  that extend into a female receptacle  66 . The female receptacle  66  is formed at the first end of the brake shoe  12 . The female receptacle  66  is shaped and positioned to receive the locking fingers  64  that extend from the stop mount  16 . When the brake shoe  12  advances to the stop mount  16 , the locking fingers  64  enter the receptacle  66  and a mechanical interconnection is created. The mechanical connector  60  is designed to separate should an appropriate separating threshold force be applied. 
         [0039]    Referring to  FIG. 7 , another embodiment of the mechanical connector  70  is shown between the stop mount  16  and the brake shoe  12 . In this embodiment, a female receptacle  72  is formed at the first end of the brake shoe  12 . A male connector  74  is configured as a tab  75  that contains one or more spring-loaded ball bearings  76 . The female receptacle  72  is shaped and positioned to receive a tab  75  and engage the ball bearings  76  with detents  78  that accept the ball bearings  76 . When the brake shoe  12  advances to the stop mount  16 , the tab  75  and ball bearings  76  enter the receptacle  72  and engage the ball detents  78 . This creates a mechanical interconnection. The mechanical connector  70  is designed to separate should an appropriate separating threshold force be applied. 
         [0040]    Referring to  FIG. 8 , an embodiment is shown that uses a magnetic connector  80  to create an interconnection between the stop mount  16  and the brake shoe  12 . In this embodiment, a first magnet  82  is mounted to the first end of the brake shoe  12 . A second magnet  84  is mounted to the stop mount  16 . When the brake shoe  12  advances to the stop mount  16 , the magnets  82 ,  84  interconnect. This creates a physical interconnection between the brake shoe  12  and the stop mount  16 . The magnets  82 ,  84  are of a size and power that they separate should an appropriate separating threshold force be applied. 
         [0041]    It will be understood that the embodiments of the present invention counterbalance system that are described and illustrated herein are merely exemplary and a person skilled in the art can make many variations to the embodiments shown without departing from the scope of the present invention. All such variations, modifications, and alternate embodiments are intended to be included within the scope of the present invention as defined by the claims.

Summary:
A system and method for inhibiting inadvertent movement of a window sash out of a fully open position. The window sash is set in guide tracks. The window sash engages brake shoes that travel up and down in the guide tracks. A stop is mounted within the guide tracks. The brake shoe and the stop have a connector that joins the brake shoe to the stop when the window sash is moved to its fully open position. The brake shoe is separable from the stop when a closing force is manually applied to the window sash. The force applied must exceed a threshold level. In this manner, the window sash will remain in its fully open position and will not inadvertently drift closed due to gravity, vibrations, or friction with another window sash.