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FIELD OF THE INVENTION 
     The present invention relates to a system in which outer lifting cords are eliminated from blinds or shades. More specifically, the present invention relates to window covering systems which employ one or springs to balance the weight of window covering material and to accumulate the lifting cord within the head rail and/or bottom rail as the blind or shade is raised or lowered. 
     BACKGROUND OF THE INVENTION 
     Venetian blinds have known for many years and typically include a plurality of slats made from metal, plastic, wood or other materials and supported by ladders. FIG. 1 shows a conventional venetian blind system  10  that includes a plurality of slats  12  located between a head rail  14  and a bottom rail  16 . Prior art blind system  10  typically include a tilt mechanism  18  so that slats  12  can be moved from a horizontal position to a nearly vertical position to control the amount of light passing therethrough. As also conventional, blind system  10  includes lifting cords  20  and  22  which are coupled to the bottom rail, pass upwardly through the slats and into mechanisms within the head rail  14 , and terminate in an exposed cord loop  24  outside the blind or shade. The lifting cord is so exposed to facilitate pulling of the outer pull cord  24  by hand, which in turn raises or lowers the bottom rail and any accumulated slats. Because of the natural tendency of the bottom rail and accumulated slats to free fall, locking mechanisms  25  are also commonly employed with such prior art blind systems. 
     Similar lift cord systems are used in a variety of the “soft” window products which are currently popular, including window coverings having pleated fabric between the head rail and the bottom rail, window coverings which have cellular fabric material between the head rail and the bottom rail, light control products which include cells having opaque portions arranged between the bottom rail and the head rail for light control and the like. 
     Systems are also known wherein the lift cords do not exit the head rail at all. Such systems are shown in Kuhar U.S. Pat. No. 6,234,236, issued May 22, 2001, U.S. Pat. No. 6,079,471, issued Jun. 27, 2000, U.S. Pat. No. 5,531,257, issued Jul. 2, 1996, and U.S. Pat. No. 5,482,100, issued Jan. 9, 1996. These systems use spring motors to balance the weight of the bottom rail and accumulating window covering material as the window covering is raised or lowered by simply grasping the bottom rail and urging it upwardly or downwardly. 
     Other patents show various spring devices used with venetian blinds. For example, in Cohn&#39;s U.S. Pat. No. 2,390,826, issued Dec. 11, 1945 for “Cordless Venetian Blinds,” two coil springs are used to provide even force, with a centrifugal pawl stop. The blind is raised by freeing the pawl to allow the spring to provide a lift assist. Other more conventional systems employing springs and ratchet and pawl mechanisms include those shown in Etten&#39;s U.S. Pat. No. 2,824,608, issued Feb. 25, 1958 for “Venetian Blind”; U.S. Pat. No. 2,266,160, issued Dec. 16, 1941 to Burns for “Spring Actuated Blind”; and U.S. Pat. No. 2,276,716, issued Mar. 17, 1942 to Cardona for “Venetian Blind.” 
     It would be desirable to provide a cordless window covering system with an inexpensive and simple cordless mechanism. 
     SUMMARY OF THE INVENTION 
     The present invention features a cordless blind system which employs one or more linearly shaped springs (i.e., an extension or compression spring) to balance the weight of window covering material and to accumulate the lifting cord within the head rail and/or bottom rail. The present invention further features a system which is easy to adapt to a wide variety of blind designs and sizes and has the capability of applying spring forces in a variety of ways and combinations. 
     According to a first aspect of the present invention, a window covering system comprises a plurality of slats located between a head rail and a bottom rail. The bottom rail is connected to the head rail by at least one lifting cord. At least one first biasing devices is located in one of the head rail and the bottom rail. The at least one first biasing devices has a fixed end and a free end that is free to move in a direction along an axis of the head rail or bottom rail. The at least one lifting cord is operatively connected to the free end of the at least one of the first biasing device so that movement of the bottom rail causes a corresponding movement in the free end of the first biasing device in the direction of the axis of the head rail or bottom rail. 
     According to another aspect of the present invention, a window covering system comprises a plurality of slats located between a head rail and a bottom rail. The bottom rail is connected to the head rail by at least two lifting cords extending through the slats. A pair of first linear springs is located in one of the head rail and the bottom rail. The first linear springs has first ends anchored to an inner surface of the head rail or the bottom rail and second ends that are free to move within the head rail or the bottom rail. At least one of the lifting cords is operatively connected to the free end of at least one of the linear springs so that movement of the bottom rail causes a corresponding movement in the second end of the linear spring. 
     According to another aspect of the present invention, a window covering system comprises a plurality of slats located between a head rail and a bottom rail. The bottom rail is connected to the head rail by at least two lifting cords extending through the slats. A first spring motor and storage device is located in one of the head rail and the bottom rail. The first spring motor and storage device includes a linear spring having one end that is fixedly secured in the head rail or bottom rail and a second end that is free to move within the head rail or bottom rail. At least one of the lifting cords is operatively connected to the free end of at least one of the coil springs so that movement of the bottom rail causes a corresponding movement in the second end of the coil spring. 
     According to a further aspect of the present invention, a method for balancing a window covering system includes operatively connecting a fixed end of a linearly shaped spring to a non-movable anchor in a hear rail or bottom rail so that the fixed end remains stationary, an opposite free end of the linearly shaped spring being free to move toward and away from the fixed end. The method further includes operatively connecting the at least one lifting cord to the free end of the linear shaped spring so that movement of the bottom rail in a vertical direction causes a corresponding movement in the free end of the linearly shaped spring in a direction along an axis of the head rail or bottom rail. 
     These and other benefits and features of the invention will be apparent upon consideration of the following detailed description of preferred embodiments thereof, presented in connection with the following drawings in which like reference numerals are used to identify like elements throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a conventional venetian blind in accordance with the prior art. 
     FIG. 2 is a front elevation schematic representation of a venetian blind and slat lifting mechanism in accordance a first embodiment of the present invention, with the blind shown in a closed position. 
     FIG. 3 is a front elevation schematic representation of the venetian blind and slat lifting mechanism of FIG. 2 with the blind shown in an open position. 
     FIG. 4 is a front elevation schematic representation of a venetian blind and slat lifting mechanism in accordance a second embodiment of the present invention. 
     FIG. 5 is a top plan schematic representation of the Venetian blind and lifting mechanism shown in FIG.  4 . 
     FIG. 6 is a top plan schematic representation of a Venetian blind and slat lifting mechanism in accordance a third embodiment of the present invention. 
     FIG. 7 is a front elevation schematic representation of a venetian blind and slat lifting mechanism in accordance a fourth embodiment of the present invention. 
     FIG. 8 is a top plan schematic representation of the venetian blind and lifting mechanism shown in FIG. 7 taken along the line  8 — 8 . 
     FIG. 9 is a top plan schematic representation of the venetian blind and lifting mechanism shown in FIG. 7 taken along the line  9 — 9 . 
     FIG. 10 is a front elevation schematic representation of a venetian blind and slat lifting mechanism in accordance a fifth embodiment of the present invention. 
     FIG. 11 is a top plan schematic representation of the venetian blind and lifting mechanism shown in FIG. 10 taken along the line  11 — 11 . 
     FIG. 12 is a top plan schematic representation of the venetian blind and lifting mechanism shown in FIG. 10 taken along the line  12 — 12 . 
     FIG. 13 is a front elevation schematic representation of a bottom rail and slat lifting mechanism in accordance a sixth embodiment of the present invention. 
     FIG. 14 is an enlarged, horizontal sectional view of a cord brake shown in FIG. 13 taken along the line  14 — 14 , the cord brake shown in the engaged position. 
     FIG. 15 is a similar view as FIG. 14 but with the cord brake shown in the disengaged position. 
    
    
     Before explaining at least one preferred embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIGS. 2 and 3, a first embodiment of a blind system  110  in accordance with the present invention is shown in a fully lowered (closed) position (see FIG. 2) and a fully raised (open) position (see FIG.  3 ). For convenience, elements of blind system  110  that are substantially similar to corresponding elements of blind system  10  will be indicated by the same reference numerals but preceded by a “1”. 
     Blind system  110  includes a plurality of slats  112  located between a head rail  114  and a bottom rail  116 . When bottom rail  116  is in its fully lowered position (see FIG.  2 ), all the slats  112  are individually suspended from ladders (not shown) attached to head rail  114  and rotatable to different angles by a tilt mechanism (not shown) for selectively restricting the amount of light passing therethrough. The ladders and tilt mechanism are not illustrated in the FIGURES but are conventional and, in and of themselves, do not form part of the present invention. 
     Blind system  110  includes a pair of lifting cords  120  and  122  for raising and lowering bottom rail  116  and any accumulated slats  112 . Cords  120  and  122  extend upwardly from bottom rail  116  through apertures formed in slats  112  and into head rail  114  via associated openings  124  and  126 , respectively, formed in a bottom wall  128  of head rail  114 . In head rail  114 , cords  120  and  122  extend generally inwardly past each other as they proceed to a spring motor and storage unit  130 . 
     Spring motor and storage unit  130  comprises a pair of elongated biasing devices  132  and  134  mounted in head rail  114 . Each biasing device  132 ,  134  comprises a linearly shaped extension (or tension) spring  136 ,  138  having an elongated central portion  137 ,  139  terminated by a fixed (immovable) end  140 ,  142  and a free (movable) end  144 ,  146 . Springs  136  and  138  are oriented with their central portions  137  and  139  generally in alignment with (i.e., parallel to) the central axes of head rail  114  and bottom rail  116 . In addition, springs  136 ,  138  are oriented with their fixed ends  140  and  142  facing away from each other and their free ends  144  and  146  facing toward each other. The fixed ends  140  and  142  of springs  136  and  138  are connected to associated anchors  148  and  150 , respectively, adjacent opposite end walls  152  and  154  of head rail  114  or at any other suitable location within head rail  114 . The free ends  144  and  146  of springs  136  and  138  are slidably engaged with lift cords  122  and  120 , respectively. When bottom rail  116  is fully lowered (see FIG.  2 ), blind system  110  will be at its maximum height H MAX  and each spring  136 ,  138  will be at its maximum length L MAX . 
     To open blind system  110 , bottom rail  116  is manually urged toward head rail  114 . When this occurs, slats  112  will begin to accumulate on bottom rail  16  and any resulting slack created in lifting cords  120  and  122  will be immediately taken up by spring motor and storage unit  130  as a result of the free ends  144  and  146  of springs  136  and  138  moving away from each other. When bottom rail  116  is fully raised (see FIG.  3 ), blind system  110  will be at its minimum height H MIN  and each spring  136 ,  138  will be at its minimum length L MIN . From FIGS. 2 and 3, it can be seen that the height of blind system  110  will always vary in a predetermined manner in relation to the length of each spring  136 ,  138 . 
     In the embodiment of FIGS. 2 and 3, each cord  120 ,  122  is looped one time in spring motor and storage unit  130 . In particular, cord  120  is looped once about free end  146  and cord  122  is looped once about free end  144 . Cords  120  and  122  may be two portions of a single cord having its ends operatively coupled to bottom rail  116  or, alternatively, cords  120  and  122  may be separate cords connected together at a point between free ends  144  and  146  or secured to a fixed anchor in head rail  114  between free ends  144  and  146 . In either case, any change in the height of blind system  110  resulting from bottom rail  116  being vertically urged from a first position to a second position will cause a corresponding change in the length of each spring  136 ,  138 . In particular, this relationship can be described by the following equation: 
     
       
           H   1   −H   2 =2×( L   1   −L   2 ),  (1)  
       
     
     where L 1  is the spring length when bottom rail  116  is in the first position, L 2  is the spring length when bottom rail  116  is in the second position, H 1  is the blind height when bottom rail  116  is in the first position, and H 2  is the blind height when bottom rail  116  is in the second position. Thus, the length of each extension spring  136 ,  138  will change about ½ the amount of any change in the height of blind system  110 . 
     Extension springs  136  and  138  should be selected to provide sufficient tension forces over their entire working range (i.e., between their expected maximum and minimum lengths) to support the weight of bottom rail  116  and any accumulated slats  112 , taking into account any frictional forces in the system, so that bottom rail  116  does not free fall when released. However, extension springs  136  and  138  should not be selected to provide a tension force that is so strong that bottom rail  116  moves upwardly on its own accord when released. By selecting springs of the appropriate strengths and/or manipulating the frictional forces in blind system  110 , the blind system can be properly balanced so that bottom rail  116  reliably remains in the position to which it is urged. 
     According to a well known equation known as Hooke&#39;s law, the force that an extension spring exerts on a mass is directly proportional to its extension and always acts to reduce this extension: 
     
       
         
           f=−k×Δ,  
         
       
     
     where f is the spring force, k is a positive quantity called the force constant of the spring, and Δ is the change in length (or extension) of the spring. Hence, it will be noted that the spring force f provided by extension springs  136  and  138  increases as bottom rail  116  is lowered because lowering bottom rail  116  results in further extension of springs  136  and  138 . As persons skilled in the art will recognize, this provides a force curve that is precisely opposite what would be ideal because springs  136  and  138  are required to do less work as bottom rail  116  is lowered as a result of less slats being accumulated thereon. 
     Accordingly, to properly balance blind system  110  it may be desirable or necessary to employ various well known devices or techniques for increasing or decreasing the amount of frictional forces. For example, the components of blind system  110  can be made from certain materials having known high or low (as appropriate) frictional coefficients, or lubricants can be used to alter the natural frictional coefficients of the materials. In addition, blind system  110  may be provided with features that are specifically designed for increasing or decreasing the amount of friction in blind system  110 . For example, friction can be reduced by positioning a pair of guides  156  and  158  within head rail  114  adjacent openings  124  and  126 , respectively, to assist the sliding movement of each cord  120 ,  122  as it transitions from its generally vertical orientation below head rail  114  to its generally horizontal orientation within head rail  114 . Guides  156  and  158  may take the form of simple rods, small rollers or any other appropriate form. 
     Referring now to FIGS. 4 and 5, a second embodiment of a blind system  210  is shown. For brevity, the description of blind system  210  will be generally limited to its differences relative to blind system  110 . For convenience, elements of blind system  210  that are substantially similar to corresponding elements of blind system  110  will be identified by the same reference numerals but preceded by a “2” instead of a “1”. 
     Blind system  210  includes a plurality of slats extending between a head rail  214  and a bottom rail  216 . A pair of lifting cords  220  and  222  extend upwardly from bottom rail  216  through the slats and into head rail  214  via a pair of openings  224  and  226 , respectively, to a spring motor and storage unit  230 . 
     Blind system  210  differs from blind system  110  primarily that each cord  220 ,  222  is looped multiple times in spring motor and storage unit  230 . As explained in detail below, each loop of cord  220 ,  222  in spring motor and storage unit  230  will act as a reducer, that is, any change in the height of blind system  210  will produce a correspondingly smaller change in the length of each spring  236 ,  238  due to the multiple cord loops. This can be particularly advantageous in blind systems that have relatively narrow widths in comparison to the height or length of the blind. 
     Blind system  210  also differs from blind system  110  in that the free end  244 ,  246  of each spring  236 ,  238  includes a block and tackle (or pulley)  260 ,  262  for reducing the friction in blind system  210 . As seen in FIG. 5, each block and tackle  260 ,  262  includes one or more rollers  264 ,  266  mounted for rotation about an axle  268 ,  270  formed in a generally flat plate  272 ,  274 . Each axle  268 ,  270  preferably extends generally transversely to the central axes of the head rail and bottom rails. Each roller  264 ,  266  may include one or more grooves so that the multiple cord loops remain separated from each other during movement of bottom rail  216 . This not only helps prevent cord entanglement but also reduces the friction in blind system  210  because the cords do not have to slide over one another. Cords  220  and  222  may be connected to one another in head rail  214  or tied to a post or anchor  280  secured to an inner surface of head rail  214 . 
     In the embodiment of FIGS. 4 and 5, each cord  220 ,  222  is looped a total of three times in spring motor and storage unit  230 . Specifically, cord  220  is looped twice about free end  246  and once about free end  244 , and cord  222  is looped twice about free end  244  and once about free end  246 . Hence, any change in the height of blind system  210  resulting from vertical movement of bottom rail  216  will cause about a corresponding change in the length of each spring  236 ,  238 . In particular, this relationship can be described by the following equation: 
     
       
           H   1   −H   2 =2× N ×( L   1   −L   2 ),  (2)  
       
     
     where N is the total number of times that each cord  220 ,  222  is looped over the free ends  244  and  246  in spring motor and storage unit  230 . Thus, the length of each extension spring  136 ,  138  will change about ½n times the amount of any change in the height of blind system  110 . 
     Referring now to FIG. 6, a third embodiment of a blind system  310  is shown. For brevity, the description of blind system  310  will be generally limited to its differences relative to blind system  210 . For convenience, elements of blind system  310  that are substantially similar to corresponding elements of blind system  210  will be identified by the same reference numerals but preceded by a “3” instead of a “2”. 
     Blind system  310  includes a plurality of slats extending between a head rail  314  and a bottom rail. A pair of lifting cords  320  and  322  extend upwardly from the bottom rail through the slats and into head rail  314  via a pair of openings  324  and  326 . 
     Blind system  310  differs from blind system  210  primarily in that cords  320  and  322  are looped around separate rollers  364 A,  366 A and  364 B,  366 B, respectively, rather than shared rollers. In addition, each cord  320 ,  322  is tied to itself in a knot  321 ,  323 , respectively, rather than tied to the opposite cord. As shown by the solid lines in FIG. 6, each roller  364 A,  366 A,  364 B,  366 B may be individually mounted in head rail  414  by a separate extension spring  336 A,  338 A,  336 B,  338 B, respectively. Alternatively, rollers  364 A,  366 A and  364 B,  366 B may be mounted in head rail  414  by only two extension springs  336 ′ and  338 ′, respectively (see the phantom lines in FIG.  6 ). 
     In either case, cords  320  and  322  each loop around their respective rollers  364 B,  366 B and  364 A,  366 A a total of six times. Thus, the height of blind system  310  will change about six times as much as the length of each extension spring  336 A,  338 A,  336 B,  338 B (or  336 ′,  338 ′ in the alternative arrangement) when the bottom rail is moved vertically from one position to another. Once again, this relationship can be described by equation (2) described above. 
     Referring now to FIGS. 7-9, a fourth embodiment of a blind system  410  is shown. For brevity, the description of blind system  410  will be generally limited to its differences relative to blind system  210 . For convenience, elements of blind system  410  that are substantially similar to corresponding elements of blind system  210  will be identified by the same reference numerals but preceded by a “4” instead of a “2”. 
     Blind system  410  includes a plurality of slats extending between a head rail  414  and a bottom rail  416 . A pair of lifting cords  420  and  422  extend upwardly from bottom rail  416  through the slats and into head rail  414  via a pair of openings  424  and  426  to a spring motor and storage unit  430 . 
     Blind system  410  differs from blind system  210  primarily in that it includes an additional (lower) spring motor and storage unit  430 ′ in bottom rail  416 . In addition, each cord  420 ,  422  is not simply tied to bottom rail  416  but instead extends to lower spring motor and storage unit  430 ′ via a pair of openings  424 ′ and  426 ′. 
     In the embodiment of FIGS. 7-9, each cord  420 ,  422  makes a total of three loops in upper spring motor and storage unit  430  (see FIG. 8) and three loops in lower spring motor and storage unit  430 ′ (see FIG.  9 ). Thus, each cord  420 ,  422  makes a combined total of six loops in upper and lower spring motor and storage units  430  and  430 ′. Accordingly, the height of blind system  410  will change about twelve times as much as the length of each spring  436 ,  438  and  436 ′,  438 ′ when bottom rail  416  is moved vertically from one position to another. Once again, this relationship can be described by equation (2) described above. 
     Referring now to FIGS. 10-12, a fourth embodiment of a blind system  510  is shown. For brevity, the description of blind system  510  will be generally limited to its differences relative to blind system  410 . For convenience, elements of blind system  510  that are substantially similar to corresponding elements of blind system  410  will be identified by the same reference numerals but preceded by a “5” instead of a “4”. 
     Similar to all the previous embodiments, bind system  510  includes a plurality of slats extending between a head rail  514  and a bottom rail  516 . Blind system  510  differs from the previous embodiments, however, in that it includes a pair of lifting cords that extend in opposite directions to each other. Specifically, one lifting cord  520  extends upwardly from bottom rail  516  through the slats and into head rail  514  via an opening  524  to an upper spring motor and storage unit  530 . The other lifting cord  522  extends downwardly from upper rail  514  through the slats and into bottom rail  516  via an opening  526 ′ to a lower spring motor and storage unit  530 ′. 
     In the embodiment of FIGS. 10-12, cord  520  makes a total of six loops in upper spring motor and storage unit  530  (see FIG.  11 ), and cord  522  makes a total of six loops in lower spring motor and storage unit  530 ′ (see FIG.  12 ). Accordingly, the height of blind system  510  will change about twelve times as much as the length of each spring  536 ,  536 ′, and  538 ,  538 ′ when bottom rail  516  is moved vertically from one position to another. Once again, this relationship can be described by equation (2) described above. 
     As explained above, persons skilled in the art may find it desirable or necessary to employ devices for altering the amount of friction in a blind system constructed in accordance with the present invention. One such device for substantially increasing the amount of friction is shown in the embodiment of FIGS. 13-15. In FIG. 13, a bottom rail  616  of a blind system  610  is shown with a lower spring motor and storage unit  630 ′. Lower spring motor and storage unit  630 ′ receives a pair of lift cords  620 ,  622 . 
     Blind system  610  differs from all the above-described blind systems in that it further includes a braking device  682  associated with cord  620 . As shown in FIG. 14, braking device  682  has a case  684  that is provided with a pair of cord holes  686  and  688  aligned with each other on opposite sides of case  684 . Case  684  is also provided with a bore  690  configured to receive a compression spring  692  and a retaining member  694 . Spring  692  and retaining member  694  are situated in bore  690  such that spring  692  naturally biases retaining member  694  out of bore  690 . Lift cord  620  passes through cord holes  686  and  688  of case  684  and also through a cord hole  696  formed in retaining member  694 . As shown in FIG. 14, when retaining member  694  is naturally urged by spring  692 , cord hole  696  of retaining member  694  and cord holes  686  and  688  of case  684  are located alternately to bring about the clamping effect that acts on lift cord  620 . By means of the clamping force and the resulting frictional resistance of braking device  682 , the rewinding force of spring motor and storage means  630 ′ is overcome. As a result, bottom rail  616  can be located at any desired position without inadvertent rewinding. 
     Now referring to FIG. 15, when retaining member  694  is pushed deeper into bore  690  by an external force, cord hole  696  of retaining member  694  moves substantially into alignment with cord holes  686  and  688  of case  684 . As a result, the frictional forces acting on cord  620  are substantially reduced, whereby bottom rail  616  can be readily moved to a new position. 
     It is important to note that the above-described preferred embodiments of the blind system are illustrative only. Although the invention has been described in conjunction with specific embodiments thereof, those skilled in the art will appreciate that numerous modifications are possible without materially departing from the novel teachings and advantages of the subject matter described herein. For example, although the blind system is described above with each spring motor and storage unit including a pair of extension springs, the spring motor and storage unit could employ as few as one extension spring or more than two extension springs. In addition, although the linear springs of each spring motor and storage unit are described as extension (or tension) springs, those skilled in the art would understand that the extension springs could be replaced with compression springs by making relatively simple modifications to the existing structures. For example, the inner ends of the compression springs could be secured to fixed anchors in the head rail or bottom rail and the outer ends of the compression springs could be allowed to move freely toward and away from the fixed ends as the bottom rail is moved vertically. Thus, the term “linear” spring is intended to encompass both compression springs and extension springs. Accordingly, these and all other such modifications are intended to be included within the scope of the present invention. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the preferred and other exemplary embodiments without departing from the spirit of the present invention.

Summary:
A window covering system comprises a plurality of slats located between a head rail and a bottom rail. The bottom rail is connected to the head rail by a pair of lifting cords extending through the slats. A first spring motor and storage device is located in one of the head rail and the bottom rail. The first spring motor and storage device includes at least one extension spring having a first end that is fixedly secured in the head rail or bottom rail and a second end that is free to move within the head rail or bottom rail. At least one of the lifting cords is looped around the free end of at least one of the extension springs so that movement of the bottom rail in a vertical direction causes a corresponding movement in the second end of the extension spring in a direction along the longitudinal axis of the head rail or bottom rail. A method for balancing a window covering system using a pair of extension springs is also disclosed.