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RELATED APPLICATIONS 
       [0001]    The application claims the benefit of U.S. Provisional Patent Application No. 62/144,898, filed Apr. 8, 2015. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    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 window track stops that help inhibit the unintentional movement, known in the industry as drift, of a window sash during use. 
         [0004]    2. Description of the Prior Art 
         [0005]    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. 
         [0006]    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. 
         [0007]    The sash of a double-hung window can be very heavy. The weight of the window sash depends upon both 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]    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 than 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. 
         [0009]    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. 
         [0010]    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 a coiled length of metal ribbon. The coil springs supply the counterbalance force needed to suspend the weight of the window sash. 
         [0011]    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 small 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. 
         [0012]    The coil springs used to counterbalance the weight of a window sash typically only approximate the weight of the window sash. Often, the upward force of the coil springs is slightly less than the downward force of gravity. A window manufacturer, therefore, relies on friction to retain the window in an open position. However, as windows wear, surfaces become smooth and friction can be significantly reduced. Accordingly, when a window sash is fully open, it may begin to drift closed without being touched. Furthermore, when an upper window sash is being opened, it creates friction against the lower window sash that it passes. If the lower window sash is opened, then the movement can cause the open upper window sash to drift closed. 
         [0013]    A need therefore exists for a system and method that can prevent a window sash from drifting under the force of its own weight as friction forces vary over time. This need is met by the present invention as described and claimed below. 
       SUMMARY OF THE INVENTION 
       [0014]    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. 
         [0015]    A stop is mounted within the guide tracks. The brake shoe and the stop contact and interconnect 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 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 contact with another window sash. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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: 
           [0017]      FIG. 1  is an exploded perspective view of a section of a tilt-in window assembly containing a counterbalance system in accordance with the present invention; 
           [0018]      FIG. 2  is an end view of the embodiment of the system shown in  FIG. 1 , shown in an unengaged condition; 
           [0019]      FIG. 3  is an end view of the embodiment of the system shown in  FIG. 2 , shown in an engaged condition; and 
           [0020]      FIG. 4  is an end view of an alternate embodiment of the system, shown in an engaged condition; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The features of the present invention system can be incorporated into many window counterbalance designs. However, the embodiments illustrated show only two exemplary embodiments of the system for the purpose of description. The exemplary embodiments illustrated are selected in order to set forth two of the best modes contemplated for the invention. The illustrated embodiments, however, are merely exemplary and should not be considered a limitation when interpreting the scope of the appended claims. 
         [0022]    Referring to  FIG. 1 , in conjunction with  FIG. 2 , there is shown a first exemplary embodiment of a counterbalance system  10  that is used to counterbalance a window sash  12  contained within a window assembly  14 . The window sash  12  is a tilt-in window sash and therefore has pivot posts  16  that extend laterally from the bottom of the window sash  12 . Although only one pivot post  16  is illustrated, it will be understood that the window sash  12  is symmetrical and that a pivot post  16  extends from both sides of the window sash  12 . 
         [0023]    Each pivot post  16  extends into a brake shoe assembly  20 . The brake show assembly  20  moves up and down in a guide track  18  on either side of the window sash  12 . The brake shoe assembly  20  serves multiple functions. First, the brake shoe assembly  20  is designed to move smoothly within the guide trade  18 . As such, the window sash  12  can open and close smoothly without binding or rattling. Second, the brake shoe assembly  20  engages the guide track  18  and holds the pivot post  16  of the window sash  12  in a fixed position, when the window sash  12  is tilted inwardly for cleaning or removal. Lastly, the brake shoe assembly  20  attaches to one or more coil springs  22  that are used to counterbalance the weight of the window sash  12 . The coil springs  22  are mounted in the guide track  18  at some elevation above the brake shoe assembly  20 . As such, the brake shoe assembly  20  and the window sash  12  it supports are biased upwardly by the coil springs  22 . 
         [0024]    Each brake shoe assembly  20  includes a brake shoe housing  24  and a cam element  26 . The brake shoe housing  24  retains the cam element  26 . The cam element  26  receives the pivot post  16  extending from the window sash  12 . The brake shoe assembly  20  rides up and down in its guide track  18 . Each guide track  18  has a rear wall  29  and two side walls  27 ,  28 . The brake shoe assembly  20  is sized to be just narrow enough to fit between the side walls  27 ,  28  of the guide track  18  without causing excessive contact with the guide track  18  as the brake shoe assembly  20  moves up and down with the window sash  12 . 
         [0025]    The brake shoe housing  24  is plastic and is preferably unistructurally molded as a single unit that requires no assembly. The brake shoe housing  24  is generally U-shaped, having a first arm element  30  and a second arm element  32  that are interconnected by a thin bottom section  34 . In the shown embodiment, the coil spring  22  attaches to the first arm element  30 . In the preferred embodiment, the second arm element  32  has a length that is at least twenty-five percent longer than that of the first arm element  30 . This prevents the coil spring  22  from being able to twist or cock within the brake shoe housing  24  in the guide track  18 . 
         [0026]    A generally circular cam opening  36  is formed between the first arm element  30 , the second arm element  32  and the bottom section  34 . Above the cam opening  36 , the first arm element  30  and the second arm element  32  are separated by a gap space  38 . The first arm element  30  has a first sloped surface  39  that faces the gap space  38 . Likewise, the second arm element  32  has a second sloped surface  41  that faces the gap space  38 . Taken together, the first sloped surface  39  and the second sloped surface  41  diverge away from each other as they ascend above the cam opening  36 . The result is that the gap space  38  has tapered sides that lead toward the cam opening  36 . 
         [0027]    The cam element  26  is inserted into the cam opening  36 . The cam element  26  receives the pivot post  16  from the window sash  12 . The cam opening  36  and the cam element  26  are configured so that the cam element  26  will cause the cam opening  36  to enlarge as the cam element  26  rotates within the cam opening  36 . The cam element  26  can be rotated by the pivot post  16  when the window sash  12  is tilted inwardly. When rotated, the cam element  26  spreads the first arm element  30  and the second arm element  32  apart. This is achieved by the elastic flexing of the thin bottom section  34  of the brake shoe housing  24 , which acts as a living hinge. The first arm element  30  and the second arm element  32  engage the sides of the guide track  18  and lock the brake shoe assembly  20  in place within the guide track  18 . 
         [0028]    A locking projection  44  is formed on one of the two sloped surfaces  39 ,  41 . In the shown embodiment, the locking projection  44  is formed on the second sloped surface  41 . However, this position is arbitrary and its position can be reversed to be formed on the first sloped surface  39 . 
         [0029]    A catch stop  50  is provided. The catch stop  50  is a small shaped body that is mounted to the rear wall  29  of the guide track  18 . The catch stop  50  is positioned in the guide track  18  so that the brake shoe assembly  20  contacts and interconnects with the catch stop  50  when the window sash  12  is in its fully open position. 
         [0030]    The catch stop  50  has a curved head  52  and a catch relief  54  along one side adjacent to the curved head  52 . The curved head  52  of the catch stop  50  is sized to pass into the gap space  38  between the first sloped surface  39  and the second sloped surface  41  as the window sash  12  is fully opened. As the window sash  12  is opened, the brake shoe assembly  20  moves up the guide track  18  until it contacts the catch stop  50 . As the window sash  12  reaches its fully open position, the catch stop  50  advances between the sloped surfaces  39 ,  41  of the first arm element  30  and the second arm element  32 . 
         [0031]    Referring to  FIG. 3  in conjunction with  FIG. 2  and  FIG. 1 , it can be understood that the initial contact between the locking projection  44  and the curved head  52  of the catch stop  50  causes the gap space  38  to spread. This makes it a little harder to move the window sash  12 , since the brake shoe assembly  20  is engaging the guide track  18  with increased force. This provides a tactile indication to a person that the window sash  12  is almost at its fully open position. Upon a slight further opening, the locking projection  44  advances into the catch relief  54 . 
         [0032]    The presence of the locking projection  44  in the catch relief  54  creates a mechanical interconnection between the brake shoe assembly  20  and the catch stop  50 . The mechanical interconnection is sufficient to prevent the window sash  12  from drifting, due to gravity or contact with another window sash. However, the mechanical connection is tenuous. The mechanical connection can easily be overcome by manually applying a downward force of a few pounds to the window sash  12 . The preferred force is between one and five pounds. Such a downward force will pull the brake shoe assembly  20  free of the catch stop  50 , wherein the window functions in the traditional manner. 
         [0033]    The locking projection  44  and/or the catch relief  54  can have angled surfaces that facilitate the separation of the locking projection  44  from the catch relief  54  as a downward force is applied to the window sash  12 . The angled surfaces prevent binding and excessive wear between the locking projection  44  and the catch relief  54 . 
         [0034]    Referring to all figures, it will be understood that to utilize the present invention, the catch stops  50  are mounted into the guide tracks  18  of the window assembly  14 . Likewise, the brake shoe assemblies  20  of the present invention are used in the counterbalance system. As the window sash  12  is opened, the brake shoe assemblies  20  move toward the catch stops  50 . As the window sash  12  reaches its fully open position, the brake shoes assemblies  20  contact and interconnect with the catch stops  50 . The locking projections  44  on the brake shoes assemblies  20  enter the catch reliefs  54  in the catch stops  50 , therein creating a mechanical interconnection between the brake shoe assemblies  20  and the catch stops  50 . The mechanical interconnection is strong enough to prevent the window sash  12  from drifting closed. However, once a downward force of a few pounds is applied to the window sash  12 , the mechanical interconnection releases and the window sash  12  is free to close in the normal manner. 
         [0035]    The connection between the catch stop  50  and the brake shoe assembly  20  can be made in many ways. There are many alternative mechanical connections, such as pawls or detents, that can create a similar temporary interconnection. Alternatively, a magnetic connection can also be used in place of the mechanical interconnection. Referring to  FIG. 4 , such an alternate embodiment is shown. In  FIG. 4 , magnets  60  are coupled to the brake shoe assembly  62 . Magnets  64  of the opposite polarity or ferromagnetic plates are coupled to a catch stop  50 . As the brake shoe assemblies  62  contact the catch stop  50 , there is a magnetic interconnection created between the magnets  60 ,  64 . The strength of the interconnection can be engineered by controlling the strength, size and location of the magnets  60 ,  64 . Preferably the magnetic connection is engineered to create an attraction force of between one and five pounds. In this manner, the magnetic connection can be undone by simply applying a slight downward force to the window sash and separating the magnets  60 ,  64 . 
         [0036]    It will be understood that the embodiments of the present invention 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. Brake shoes travel in the guide tracks as the window sash is moved between a fully open position and a fully closed position. A stop is mounted within the guide tracks. The brake shoe and the stop contact and interconnect 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 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 contact with another window sash.