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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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 the brake shoe component of counterbalance systems for tilt-in windows and the manner in which springs connects to the brake shoe. 
     2. Description of the Prior Art 
     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. 
     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. 
     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. 
     For many years, counterbalance weights were hung next to the window frames in weight wells. The weights were attached to window sashes using a string or chain that passed over a pulley at the top of the window frame. The weights counterbalanced the weight of the window sashes. As such, when the sashes were moved in the window frame, they had a neutral weight and friction would hold them in place. 
     The use of weight wells, however, prevents insulation from being packed tightly around a window frame. Furthermore, the use of counterbalance weights on chains or strings cannot be adapted well to tilt-in double-hung windows. Accordingly, as tilt-in windows were being developed, alternative counterbalance systems were developed that were contained within the confines of the window frame and did not interfere with the tilt action of the tilt-in windows. 
     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. 
     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. 
     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 curl springs inside the guide tracks of the window assembly. Curl springs are constant force coil springs, made from wound length of metal ribbon, that supply the counterbalance force needed to suspend the weight of the window sash. 
     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. Counterbalance systems that use ganged assemblies of coil springs are exemplified by U.S. Pat. No. 5,232,208 to Braid, entitled Springs For Sash Frame Tensioning Arrangements. 
     The metal ribbons of coil springs in a window counterbalance system usually experience tension as they support the weight of the window sash. However, this is not always the case. When a window sash is rapidly opened, the upward speed of the window sash may exceed the recoil speed of the counterbalance springs. In such a situation, the metal ribbons of the coil springs may experience a brief period of compression. The ribbons of coil springs are typically uniform in width, except for the free ends of the spring ribbon. The free ends of the spring ribbon are often stamped and shaped so that the end of the spring can engage the structure of the brake shoe. Since the areas near the ends of the spring ribbons are reduced in width, the repeating tension and compression stresses tend to concentrate in these reduced areas. The cycles of tension forces and compressive forces cause the metal ribbon of the coil spring to fatigue. Eventually, the fatigue forces can cause the coil spring to break, thereby disconnecting the coil spring from the brake shoe. This causes the overall counterbalance system to fail. 
     A need therefore exists in the field of vinyl, tilt-in, double-hung windows, for a counterbalance system with a brake shoe that can attach to a coil spring in such a way that the structure of the brake shoe prevents fatigue stresses from compromising the coil spring. This need is met by the present invention as described and claimed below. 
     SUMMARY OF THE INVENTION 
     The present invention is an assembly of components that are use in a counterbalance system for a tilt-in window. A coil spring of wound ribbon is provided that has a free end that terminates with a shaped head. A brake shoe housing is provided that connects to the coil spring in such a manner that fatigue stresses are reduced in the coil spring as the tilt-in window is repeatedly opened and closed. 
     The brake shoe housing has a receptacle slot formed into one of its side surfaces. The receptacle slot is formed low on the side of the brake shoe housing. An open relief is formed immediately above the receptacle slot. The open relief abuts against and supports the ribbon of the coil spring just behind the shaped head. By engaging the shaped head of the coil spring and supporting the coil spring adjacent to the shaped head, stresses experienced by the shaped head are greatly reduced. The result is a coil spring that has a much longer service life. Furthermore, the connection between the coil spring and the housing also assist in preventing excessive cocking of the brake shoe housing. This prevents wear of the brake shoe housing and increases its operational life. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the following description of an exemplary embodiment thereof, considered in conjunction with the accompanying drawings, in which: 
         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; 
         FIG. 2  is a cross section of the embodiment of the counterbalance system shown in  FIG. 1 , viewed along line  2 - 2 ; 
         FIG. 3  is an exploded perspective view of the brake shoe housing and cam element of the counterbalance system; 
         FIG. 4  is a front view of the brake shoe housing and cam element shown with the cam element holding a tilt post of a vertically oriented window sash; 
         FIG. 5  is a front view of the brake shoe housing and cam element shown with the cam element holding a tilt post of a tilted window sash; 
         FIG. 6  is a perspective view of the brake shoe assembly and the free end of the coil spring to show interconnection features; and 
         FIG. 7  is a cross-sectional view of the subassembly of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The claimed features of the present invention brake shoe can be incorporated into many window counterbalance designs. However, the embodiment illustrated shows only one exemplary embodiment of the counterbalance system for the purpose of disclosure. The embodiment illustrated is selected in order to set forth one of the best modes contemplated for the invention. The illustrated embodiment, however, is merely exemplary and should not be considered a limitation when interpreting the scope of the appended claims. 
     Referring to  FIG. 1 , in conjunction with  FIG. 2 , there is shown an exemplary embodiment of a counterbalance system  10  that is used to counterbalance the sashes  12  contained within a window assembly  14 . The counterbalance system  10  utilizes a brake shoe housing  16 , a cam element  18 , and at least one coil spring  20  on either side of each window sash  12 . The brake shoe housing  16  engages a tilt post  22  that extends from the bottom of the window sash  12 . As the window sash  12  is opened and closed, the brake shoe housing  16  travels up and down in vertical guide tracks  24 . It will be understood that each window sash  12  typically utilizes two counterbalance systems on opposite sides of the sash  12 . However, for the sake of simplicity and clarity, only one counterbalance system  10  is illustrated. 
     The brake shoe housing  16  receives the cam element  18  to form a brake shoe assembly  19 . The brake shoe assembly  19  rides up and down in its guide track  24 . The brake shoe assembly  19  is biased upwardly within the guide track  24  by at least one coil spring  20 . The guide track  24  has a rear wall  26  and two side walls  27 ,  28 . The brake shoe assembly  19  is sized to be just narrow enough to fit between the side walls  27 ,  28  of the guide track  24  without causing excessive contact with the guide track  24  as the brake shoe assembly  19  moves up and down with the window sash  12 . 
     Referring to  FIG. 3  in conjunction with  FIG. 1  and  FIG. 2 , it can be seen that the brake shoe housing  16  is a unistructurally molded unit that requires no assembly. The brake shoe housing  16  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  20  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. 
     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 into the cam opening  36 . 
     A catch finger  40  protrudes from the first sloped surface  39  of the first arm element  30 . The catch finger  40  extends into the gap space  38  between the first arm element  30  and the second arm element  32 . The catch finger  40  is integrally molded as part of the first arm element  30  and the overall brake shoe housing  16 . The catch finger  40  has a first section  42  that extends away from the first sloped surface  39  at an acute angle. This causes the catch finger  40  to extend in a downward direction. The catch finger  40  then curves into a nearly vertical orientation proximate its free end  44 . The free end  44  is molded to be slightly bulbous in order to prevent the catch finger  40  from hanging up on the tilt post  22 , as will later be explained. 
     The cam opening  36 , although generally circular, is not round. Rather, the cam opening  36  has a rounded bottom section  46 . On the first arm element  30 , the rounded bottom section  46  transitions into a first curved section  48  that has a larger radius of curvature than the rounded bottom section  46 . On the opposite second arm element  32 , there is a second curved section  49  with the same general radius of curvature as the first curved  48  section. However, the second curved section  49  does not transition directly into the rounded bottom section  46 . Rather, the second curved section  49  is offset from the rounded bottom section  46  with a flat ridge  50 . The flat ridge  50  acts as a stop for the cam element  18 , as will later be explained. 
     The brake shoe housing  16  has a face surface  52  and a rear surface  54 . The cam opening  36  extends from the face surface  52  back to the rear surface  54 . The dimensions of the cam opening  36  decrease just behind the face surface  52  and the rear surface  54  of the brake shoe housing  16 . The decreases in dimensions create ledges  56  in the cam opening  36  just behind the face surface  52  and the rear surface  54 . The ledges  56  are used to help retain the cam element  18 , which will be later described in more detail. 
     A key projection  58  protrudes into the cam opening  36  from the second curved section  49 . The key projection  58  is positioned approximately midway between the face surface  52  and the rear surface  54 . Again, the key projection  58  is used to help retain the cam element  18 , which will be later described in more detail. 
     The cam element  18  is generally cylindrical in shape. The cam element  18 , however, does not have a circular cross-sectional profile. Rather, the cross-sectional profile of the cam element  18  is oblong, being mildly elliptical in its general shape. The cam element  18  has a midsection  60  positioned between a front flange  62  and a back flange  64 . The midsection  60  of the cam element  18  has a long axis  61  and a short axis  63  when viewed in cross-section from either end. The front flange  62  and the back flange  64  are slightly larger than the midsection  60 , therein providing the cam element  18  with a slight spool configuration. 
     A tilt post receiving slot  66  is formed in the cam element  18 . The receiving slot  66  extends from the front flange  62  to the back flange  64 . However, the receiving slot  66  is not symmetrically positioned. Rather, the receiving slot  66  is eccentrically positioned, so that the receiving slot  66  is closer to one side of the cam element  18  than to the other. For the purposes of this description, the side of the cam element  18  that contains most of the receiving slot  66  shall be referred to as the narrow side  68  of the cam element  18 . Conversely, the side of the cam element  18  that does not retain much of the receiving slot  66  is referred to as the wide side  69  of the cam element  18 . 
     A groove  70  is formed in the exterior of the midsection  60  of the cam element  18  in the wide side  69  of the cam element  18 . The groove  70  is sized to receive the key projection  58  formed into the cam opening. 
     Referring to  FIG. 4 , in conjunction with  FIG. 1  and  FIG. 3 , it can be seen that the cam opening  36  receives and retains the cam element  18 . During manufacture in the factory, the cam element  18  is inserted into the cam opening  36  by forcing the cam element  18  into the gap space  38  between the first arm element  30  and the second arm element  32  of the brake shoe housing  16 . When pressed into the gap space  38 , the cam element  18  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  16 , which acts as a living hinge. The cam element  18  also elastically deforms the catch finger  40  down until the cam element  18  passes. Once the cam element  18  is inside the cam opening  36 , the first arm element  30  and the second arm element  32  rebound to their original positions. Likewise, the catch finger  40  returns to its original orientation. The presence of the catch finger  40  helps hinder the removal of the cam element  18  from the cam opening  36 . 
     Once the cam element  18  is displaced into the cam opening  36  of the brake shoe housing  16 , the front flange  62  and the back flange  64  of the cam element  18  engage the ledges  56  inside the cam opening  36  and prevent the cam element  18  from exiting the cam opening  36  either through the face surface  52  of the brake shoe housing  16  or the rear surface  54  of the brake shoe housing  16 . Furthermore, the key projection  58  in the cam opening  36  engages the groove  70  of the cam element  18 . This interconnection helps retain the cam element  18  in place, while still enabling the cam element  18  to rotate within the cam opening  36 . The length of the groove  70  and the presence of the flat ridge  50  within the cam opening  36  limit the range of rotation achievable by the cam element  18  in the cam opening  36 . In this manner, the over-rotation of the cam element  18  can be prevented. 
     The narrow side  68  of the cam element  18  is positioned toward the bottom of the brake shoe housing  16 . This causes the tilt post receiving slot  66  to lie close to the thin bottom section  34  of the brake shoe housing  16 . The tilt post receiving slot  66  receives the tilt post  22 . Consequently, the tilt post  22  of the window sash  12  is held close to the thin bottom section  34  of the brake shoe housing  16 . The result is that the window sash  12  can move to a lower position in the window frame than prior art brake shoe assemblies that support tilt posts in a cam near the center of the brake shoe housing. 
     Referring to  FIG. 5  in conjunction with  FIGS. 1-4 , it can be seen that when the window sash  12  is tilted inwardly, the tilt posts  22  of the window sash  12  causes the cam element  18  to turn. Prior, the long axis  61  of the cam element  18  had been vertically oriented. When the window sash  12  is tilted, that orientation changes toward the horizontal. The cam element  18  is oblong in shape since it has a long axis  61  and short axis  63 . Consequently, when the cam element  18  turns, the cam element  18  spreads the first arm element  30  from the second arm element  32  of the brake shoe housing  16 . As the cam element  18  spreads the brake shoe housing  16 , the brake shoe housing  16  flexes in its bottom section  34 . The first arm element  30  and the second arm element  32  engage the side walls  27 ,  28  of the track  24 . The result is that the brake shoe assembly  19  becomes locked in position within the guide track  24 . 
     As the cam element  18  spreads open the brake shoe housing  16 , the gap space  38  between the first arm element  30  and the second arm element  32  increases. The tilt post  22  can therefore be removed from the cam element  18  through the widened gap space  38 . Removal of the cam element  18  in such a manner is hindered by the presence of the catch finger  40 . The catch finger  40  extends into the gap space  38  and provides a physical barrier that prevents the tilt post  22  from exiting the cam element  18 . In this manner, the catch finger  40  prevents a user from inadvertently pulling the tilt post  22  out of the cam element  18  while tilting the window sash  12  inwardly. 
     It will be understood that if the window sash  12  is broken or otherwise is intended to be removed from the window assembly, such a removal is possible. A person intending to remove the window sash  12  can simply depress the catch finger  40  while pulling up on the window sash  12 . If the catch finger  40  is depressed, it will not block the gap space  38  above the tilt post  22  and the tilt post  22  can be freely removed. 
     Alternately, since the receiving slot  66  that retains the tilt post  22  is eccentrically positioned toward the narrow side  68  of the cam element  18 , it will be understood that the catch finger  40  will not align directly above the tilt post  22 . Rather, as is shown in  FIG. 5 , the enlarged free end  44  of the catch finger  40  aligns above one side of the tilt post  22 . This enables the catch finger  40  to prevent most accidental removals of the tilt post  22 . However, if the window sash  12  is pulled upwardly with a sufficient and determined force, the tilt post  22  will contact the catch finger  40  at an angle. Provided the upward force exceeds a predetermined threshold force of at least five pounds, for example, the catch finger  40  will then elastically yield to the tilt post  22  and the window sash  12  can be removed. Once the window sash  12  is removed, the temporarily displaced catch finger  40  will return to its original position. In this manner, a serviceman or homeowner can intentionally pull the window sash  12  out of the window assembly without any tools or manual brake shoe manipulations. The requirement of sufficient and sustained force required for the removal eliminates most all inadvertent removals of the window sash  12 . 
       FIGS. 2 and 4  show the brake shoe housing  16 , cam element  18  and tilt post  22  when the window sash  12  is vertical and in its regular operating position.  FIG. 5  shows the brake shoe housing  16 , cam element  18  and tilt post  22  when the window sash  12  is tilted and the brake shoe housing  16  is locked in the guide track  24 . The shape of the cam opening  36  varies between the regular operating position of  FIG. 4  and the locked position of  FIG. 5 . As can be seen from  FIG. 4  and  FIG. 5 , the shape of the cam element  18  is designed to more precisely fit the cam opening  36  when the cam opening  36  is in its locked position. The result is fewer gaps  75  where no contact exists. In this manner, the cam opening  36  better engages the brake shoe housing  16  and is more resistant to accidental replacement while the window sash  12  is being tilted in. This helps prevent the cam element  18  from being advertently pulled, pushed or otherwise displaced from the brake shoe housing  16 . 
     In the shown embodiment, the coil spring  20  attaches to the first arm element  30  of the brake shoe housing  16 . This causes the brake shoe housing  16  to have a rotational bias in the clockwise direction as it travels up and down the guide track  24 . To prevent the brake shoe housing  16  from cocking in the guide track  24 , the second arm element  32  is provided with an extension  72 . The extension  72  elongates the second arm element  32  and provides more surface contact with the side walls  27 ,  28  of the window guide track  24 . This extended contact prevents the brake shoe assembly  19  from cocking to the bias of the coil spring  20  and binding in the guide track  24 . 
     Referring to  FIG. 6  and  FIG. 7 , it can be seen that the coil spring  20  is made of a wound ribbon  81  of steel. The free end of the ribbon  81  is shaped into a T-shaped head  80  that is more narrow than the ribbon  81 . The T-shaped head has a length L 1 . The T-shaped head  80  interconnects with the first arm element  30  of the brake shoe housing  16 . The first arm element  30  of the brake shoe housing  16  is specially designed to receive both the T-shaped head  80  of the coil spring  20  and a length of the ribbon  81  proximate the T-shaped head  80  so as to reduce fatigue stresses in the coil spring  20 . 
     A receptacle slot  82  is formed in a side wall  83  of the first arm element  30 . The receptacle slot  82  is sized to receive and retain the T-shaped head  80  of the coil spring  20 . A relief area  84  is formed in the side wall  83  of the first arm element  30  just above the receptacle slot  82 . The receptacle slot  82  has a transition section  86  that smoothly leads the receptacle slot  82  into the relief area  84 . When the coil spring  20  is engaged with the brake shoe housing  16 , the T-shaped head  80  of the coil spring  20  enters the receptacle slot  82 , therein mechanically interconnecting the coil spring  20  with the brake shoe housing  16 . Once in this position, a length of the ribbon  81  proximate the T-shaped head  80  lays flush in the relief area  84 . The length of the ribbon  81  supported by the relief area  84  is preferably at least as long as the length L 1  of the T-shaped head  80 . As a consequence, the receptacle slot  82  and the relief area  84  combine to form an anchor structure  85  that engages both the T-shaped head  80  of the coil spring  20  and the length of ribbon  81  behind the T-shaped head  80 . 
     The T-shaped head  80  of the coil spring  20  is much narrower than the remaining ribbon  81  of the coil spring  20 . As such, as a window sash  12  ( FIG. 1 ) is opened and closed, changing tension forces and even some compression forces can be experienced by the coil spring  20 . These changing forces create stresses that tend to concentrate in the thin T-shaped head  80  of the coil spring  20 . The stresses fatigue the metal of the coil spring  20  and can eventually cause the T-shaped head  80  to break. By supporting both the T-shaped head and the segment of ribbon  81  behind the T-shaped head  80 , the stress forces are prevented from concentrating in the T-shaped head  80 . The result is that the coil spring  88  does experiences far less fatigue forces and therefore has a much longer operating life. 
     In order to accommodate both the receptacle slot  82  and the relief area  84 , the receptacle slot  82  must be positioned low on the side wall  83  of the first arm element  30 . The brake shoe housing  16  has a bottom surface  87  at the bottom of the bottom section  34 . The cam opening  36  in the brake shoe housing  16  has a center point CP a predetermined distance D 1  above the bottom surface  87 . The receptacle slot  82  is positioned on the first arm element  30  at a height above the bottom surface  87  that is no higher than that of the center point CP of the cam opening  36 . 
     Attaching the coil spring  20  to the brake shoe housing  16  at this low point of attachment has secondary advantages. The T-shaped head  80  of the coil spring  20  is generally horizontally aligned with the center of the cam element  18 . Since the brake shoe housing  16  can rotate relative the cam element  18 , this horizontal alignment minimizes the rotational torque experienced by the brake shoe housing  16 . As a result, the cocking forces on the brake shoe housing  16  are minimized. 
     It will be understood that the embodiment of the present invention counterbalance system that is described and illustrated herein is merely exemplary and a person skilled in the art can make many variations to the embodiment 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 appended claims.

Summary:
An assembly of components that are use in a counterbalance system for a tilt-in window. A coil spring of wound ribbon is provided that has a shaped head. A brake shoe housing is provided that connects to the coil spring in such a manner that fatigue stresses are reduced in the coil spring as the tilt-in window is operated. The brake shoe housing has a receptacle slot formed into one of its side surfaces. An open relief is formed immediately above the receptacle slot. The open relief abuts against and supports the ribbon of the coil spring just behind the shaped head. By engaging the shaped head of the coil spring and supporting the coil spring adjacent to the shaped head, stresses experienced by the shaped head are greatly reduced. The result is a coil spring that has a much longer service life.