Patent Publication Number: US-2023140382-A1

Title: Cord retractor and window covering having same

Description:
BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure generally relates to a cord retractor of a window covering, and more particularly relates to the cord retractor providing different resistance in the cord retracting direction and the cord releasing direction. 
     Description of the Prior Art 
     Window coverings are commonly applied to the openings of buildings, e.g., window or door, for adjusting light or improving privacy. The window coverings, which have vertically adjustable shading areas, can be roughly divided into two types by way of operation. One is controlled by using the cords as another is not, which is so-called cordless window covering. A conventional cordless window covering includes an upper rail, one or more coverings, a bottom rail and a driving mechanism which assists in extending or retracting the covering. Some cordless window coverings may further include a middle rail. The driving mechanism may include a lift cord and a driving unit, wherein the driving unit is usually a spring box disposed in the upper rail and may include a reeling wheel, a driving wheel, a spring wheel and a spring. The spring is coupled with the driving wheel and the spring wheel. The reeling wheel and the driving wheel are intermeshed for driving each other to rotate. One end of the lift cord is coupled with the reeling wheel as another end is extending out from the upper rail, penetrating through the coverings, and extending to and coupled with the bottom rail. When the bottom rail is pulled down for extending the covering, the part of the lift cord wrapped around the reeling wheel is released from the reeling wheel, and the reeling wheel is driven to rotate. The rotation of the reeling wheel drives the driving wheel to rotate correspondingly, by which the spring is wound on the driving wheel and stores a spring force. When the bottom rail is raised for retracting the covering, the spring force is released and the driving wheel is driven to rotate reversely, which drives the reeling wheel to rotate reversely and the part of the lift cord exposed outside is thereby retracted. 
     The weight of the bottom rail and the weight of the portion of the covering that is stacked on the bottom rail collectively form a gravitational force. Without any other external force acting on the bottom rail, the difference between aforesaid spring force and aforesaid gravitational force may form an upward force or a downward force applied to the covering and the bottom rail. Accordingly, the covering and the bottom rail would move upwardly or downwardly. At this moment, a static friction great enough is required for resisting such upward force or such downward force in order to maintain the bottom rail in an expected position. However, the spring force, the gravitational force and the friction are variable while the bottom rail and the covering are moving. As a result, the cordless window coverings usually have the problem that the bottom rail moves upwardly or downwardly after the operation has stopped. For instance, when the user applies a downwardly external force to the bottom rail for pulling it down and extending the covering, the gravitational force may gradually reduce because of reduction of the portion of the covering that is stacked on the bottom rail. After the downwardly external force has been removed, the bottom rail may move upwardly because the spring force is too large and the friction cannot compensate the over large spring force. Or the bottom rail may move downwardly because the spring force is too small to independently support the bottom rail and the covering, while the friction cannot compensate the over large gravitational force. In both situations, the bottom rail fails to stay in a designated position. Such drawback of the conventional cordless window coverings causes inconvenience. 
     SUMMARY OF THE DISCLOSURE 
     In light of the above reasons, one aspect of the present disclosure is to provide a cord retractor applied to the cordless window coverings and can provide resistance in a single direction to resolve the problem of the cordless window coverings that cannot be stationary in a certain position. 
     To achieve the above objective, the cord retractor according to one embodiment of the present disclosure is provided, and the cord retractor comprises a housing, comprising a bottom plate and a shaft situated on a first side of the bottom plate. A driving wheel, situated on the first side of the bottom plate. A spring, received in the housing and comprising a first end coupled with the driving wheel, wherein the spring is wound on the driving wheel or unwound from the driving wheel according to a rotating direction of the driving wheel. A reeling wheel, situated on the first side of the bottom plate and coupled with the driving wheel for rotating with the driving wheel. A friction member in a closed loop shape and surrounding the shaft. A first limiting member, situated outside the friction member and comprising an arm. A second limiting member, situated outside the first limiting member and comprising a plurality of ratchets for engaging with the arm of the first limiting member, wherein the second limiting member is driven by the reeling wheel for rotating with the reeling wheel. When the reeling wheel rotates in a first direction, the second limiting member is driven to rotate in a locking direction with respect to the first limiting member, thereby making the arm of the first limiting member engaged between two neighboring ratchets of the second limiting member, after which the first limiting member rotates together with the second limiting member, and one of a first frictional force and a second frictional force is generated for providing substantially constant resistance to rotation of the reeling wheel. The first frictional force is generated between the first limiting member and the friction member, as the first limiting member rotates with respect to the friction member, and the second frictional force is generated between the friction member and the shaft, as the friction member is driven by the first limiting member to rotate with respect to the shaft. When the reeling wheel rotates in a second direction opposite to the first direction, the second limiting member is driven to rotate in an unlocking direction with respect to the first limiting member, and the arm of the first limiting member skids over the ratchets of the second limiting member, so that the first limiting member is not driven to rotate together with the second limiting member, and the friction member does not rotate with respect to the shaft or the first limiting member, therefore the friction member does not provide resistance to the rotation of the reeling wheel when the reeling wheel rotates in the second direction. 
     To achieve the above objective, the cord retractor according to another embodiment of the present disclosure is provided, and the cord retractor comprises a housing, comprising a bottom plate and a shaft situated on a first side of the bottom plate. A driving wheel, situated on the first side of the bottom plate. A spring, received in the housing and comprising a first end coupled with the driving wheel, wherein the spring is wound on the driving wheel or unwound from the driving wheel according to a rotating direction of the driving wheel. A reeling wheel, situated on the first side of the bottom plate and coupled with the driving wheel for rotating with the driving wheel. A friction member in a closed loop shape and surrounding the shaft. A unidirectional transmission mechanism coupled with the friction member and the reeling wheel and operated in one of a first operation mode and a second operation mode according to a rotating direction of the reeling wheel. When the reeling wheel rotates in a first direction, the unidirectional transmission mechanism is driven to operate in the first operation mode with the friction member to generate a frictional force for providing substantially constant resistance to rotation of the reeling wheel. When the reeling wheel rotates in a second direction opposite to the first direction, the unidirectional transmission mechanism is driven to operate in the second operation mode and restricts the friction member from providing resistance to the rotation of the reeling wheel. 
     To achieve the above objective, the window covering according to one embodiment of the present disclosure is provided, wherein the window covering comprises an upper rail. One or more covering materials, provided below the upper rail. A bottom rail, provided below the one or more covering materials. A lift cord coupled between the upper rail and the bottom rail. A cord retractor for receiving or releasing the lift cord for respectively moving the bottom rail towards or away from the upper rail, wherein the cord retractor comprises a housing, having a bottom plate and a shaft situated on a first side of the bottom plate. A driving wheel, situated on the first side of the bottom plate. A spring, received in the housing and comprising a first end coupled with the driving wheel, wherein the spring is wound on the driving wheel or unwound from the driving wheel according to a rotating direction of the driving wheel. A reeling wheel, situated on the first side of the bottom plate and coupled with the driving wheel for rotating with the driving wheel. A friction member in a closed loop shape and surrounding the shaft. A first limiting member, situated outside the friction member and comprising an arm. A second limiting member, situated outside the first limiting member and comprising a plurality of ratchets for engaging with the arm of the first limiting member, wherein the second limiting member is driven by the reeling wheel to rotate with the reeling wheel. When the reeling wheel rotates in a first direction, the second limiting member is driven to rotate in a locking direction with respect to the first limiting member, thereby making the arm of the first limiting member engaged between the two neighboring ratchets of the plurality of ratchets of the second limiting member, after which the first limiting member rotates together with the second limiting member, and one of a first frictional force and a second frictional force is generated for providing substantially constant resistance to rotation of the reeling wheel. The first frictional force is generated between the first limiting member and the friction member, as the first limiting member rotates with respect to the friction member, and the second frictional force is generated between the friction member and the shaft, as the friction member is driven by the first limiting member to rotate with respect to the shaft. When the reeling wheel rotates in a second direction opposite to the first direction, the second limiting member is driven to rotate in an unlocking direction with respect to the first limiting member, and the arm of the first limiting member skids over the ratchets of the second limiting member, so that the first limiting member is not driven to rotate together with the second limiting member, and the friction member does not rotate with respect to the shaft or the first limiting member, such that the friction member does not provide resistance to the rotation of the reeling wheel while the reeling wheel rotates in the second direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be understood by referring to the following detailed description of some illustrative embodiments in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a perspective view of the window covering having the cord retractor of the present disclosure; 
         FIG.  2    is a perspective view of the cord retractor according to a first preferred embodiment of the present disclosure; 
         FIG.  3    is an exploded view of the cord retractor in  FIG.  2   ; 
         FIG.  4    is a perspective view of the first limiting member in  FIG.  2   ; 
         FIG.  5    is a perspective view of the friction member in  FIG.  2   ; 
         FIG.  6    is a perspective view of the combination of the first limiting member and the friction member in  FIG.  2   ; 
         FIG.  7    is a perspective view of the combination of the driving wheel and the second limiting member in  FIG.  2   ; 
         FIG.  8    is a schematic diagram illustrating the window covering in  FIG.  1    is operated to be extended by lowering the bottom rail; 
         FIG.  9    is a bottom view of the cord retractor in  FIG.  2    without the bottom plate of the housing, illustrating the action of the cord retractor while the window covering is operated to be extended; 
         FIG.  10    is an enlarged view of the unidirectional transmission mechanism and the driving wheel in  FIG.  9   ; 
         FIG.  11    is a schematic diagram illustrating the window covering in  FIG.  1    is operated to be retracted by raising the bottom rail; 
         FIG.  12    is a bottom view of the cord retractor in  FIG.  2    without part of the housing, illustrating the action of the cord retractor while the window covering is operated to be retracted; 
         FIG.  13    is an enlarged view of the unidirectional transmission mechanism and the driving wheel in  FIG.  12   ; 
         FIG.  14    is a perspective view of another exemplified friction member of the present disclose; 
         FIG.  15    is a perspective view of the combination of the first limiting member and the friction member in  FIG.  14   ; 
         FIG.  16    is an exploded view of the cord retractor according to a second preferred embodiment of the preset disclosure; 
         FIG.  17    is a perspective view of the combination of the reeling wheel and the second limiting member in  FIG.  16   ; 
         FIG.  18    is a bottom view of the cord retractor in  FIG.  16    without the bottom plate of the housing, illustrating the action of the cord retractor while the window covering is operated to be extended; 
         FIG.  19    is a bottom view of the cord retractor in  FIG.  16    without the bottom plate of the housing, illustrating the action of the cord retractor while the window covering is operated to be retracted; 
         FIG.  20    is an exploded view of the cord retractor according to a third preferred embodiment of the preset disclosure; 
         FIG.  21    is a perspective view of the combination of the transmission wheel and the second limiting member in  FIG.  20   ; 
         FIG.  22    is a bottom view of the cord retractor in  FIG.  20    without the bottom plate of the housing, illustrating the action of the cord retractor while the window covering is operated to be extended; 
         FIG.  23    is a bottom view of the cord retractor in  FIG.  20    without the bottom plate of the housing, illustrating the action of the cord retractor while the window covering is operated to be retracted; 
         FIG.  24    is an exploded view of the cord retractor according to a fourth preferred embodiment of the preset disclosure; 
         FIG.  25    is a perspective view of the first limiting member in  FIG.  24   ; 
         FIG.  26    is a perspective view of the second limiting member in  FIG.  24   ; 
         FIG.  27    is a cross sectional view of the cord retractor according to the fourth preferred embodiment of the preset disclosure, illustrating the action of the cord retractor while the window covering is operated to be extended; 
         FIG.  28    is an enlarged view of the unidirectional transmission mechanism and the intermediate wheel in  FIG.  27   ; 
         FIG.  29    is a cross sectional view of the cord retractor according to the fourth preferred embodiment of the preset disclosure, illustrating the action of the cord retractor while the window covering is operated to be retracted; 
         FIG.  30    is an enlarged view of the unidirectional transmission mechanism and the intermediate wheel in  FIG.  29   ; 
         FIG.  31    is an exploded view of the cord retractor according to a fifth preferred embodiment of the preset disclosure; 
         FIG.  32    is a perspective view of the combination of the driving wheel and the first limiting member in  FIG.  31   ; 
         FIG.  33    is a perspective view of the swing element of the first limiting member in  FIG.  32   ; 
         FIG.  34    is a perspective view of the second limiting member in  FIG.  31   ; 
         FIG.  35    is a cross sectional view of the cord retractor according to the fifth preferred embodiment of the preset disclosure, illustrating the action of the cord retractor while the window covering is operated to be extended; 
         FIG.  36    is an enlarged view of one of the swing elements in  FIG.  35   ; 
         FIG.  37    is a cross sectional view of the cord retractor according to the fifth preferred embodiment of the preset disclosure, illustrating the action of the cord retractor while the window covering is operated to be retracted; 
         FIG.  38    is an enlarged view of one of the swing elements in  FIG.  37   . 
     
    
    
     DETAILED DESCRIPTION 
     In the following paragraphs and the accompanying drawings, the features and the implementations of several embodiments of the present disclosure are described in more detail along with the accompanying drawings. The features and the implementations described in the following paragraphs can be adopted solely or in combination with each other. In addition, the embodiments can be modified in various forms, as disclosed in the following paragraphs, and should not be limited to the embodiments described in the following paragraphs. Unless specified otherwise, the same reference characters refer to the same components. 
     The technical features provided in the present disclosure are not limited to the specific structures, uses, and applications described in the embodiments. The language used in the descriptions is illustrative and descriptive language which can be understood by the person of ordinary skill in the art. The terms regarding directions mentioned in the specification, including “front”, “rear”, “up”, “down”, “left”, “right”, “top”, “bottom”, “inside”, and “outside”, are illustrative and descriptive terms based on common usage scenarios, and manifests no intent to limit the scope of claims. 
     Furthermore, the definite and indefinite articles “a” and “the” and the numerical term “one” used in the specification referring to components of singular form do not exclude the concept of plural form. Equivalences known by one having ordinary skill in the art should be also included. All conjunctions used in similar situations should be interpreted in the broadest ways. The specific shapes, structural features, and technical terms described in the descriptions should also be interpreted to include equivalent structures and techniques which could achieve the same functionality. 
     Please refer to  FIG.  1   , which shows a cordless window covering  1  having the cord retractor of the present disclosure. The cordless window covering  1  comprises an upper rail  10 , a plurality of covering materials  20 , a bottom rail  30 , two lift cords  40  and a cord retractor  100 . The upper rail  10  has a horizontally extending hollow casing for receiving at least part of the lift cords  40  and the cord retractor  100 . The covering materials  20  are also horizontally extending and arranged one-by-one below the upper rail  10 . The bottom rail  30  also has a horizontally-extending hollow casing and located below the lowest one of the covering materials  20 . 
     The cord retractor  100  is disposed within the upper rail  10 . One end of each lift cord  40  is coupled with the cord retractor  100  as another end of each lift cord  40  is extending out from the upper rail  10 , penetrating through the covering materials  20 , and extending to and coupled with the bottom rail  30 . While the bottom rail  30  is pushed upwardly towards the upper rail  10 , the covering materials  20  are stacked on the bottom rail  30  one by one from the lowest to the highest and moving towards the upper rail  10  together with the bottom rail  30 . Meanwhile, the exposed parts of the lift cords  40  are retracted by the cord retractor  100  for retracting the window covering  1 . On the other hand, while the bottom rail  30  is pulled downwardly away from the upper rail  10 , the covering materials  20  separate from one another sequentially from the highest to the lowest one and moving away from the upper rail  10  together with the bottom rail  30 . Meanwhile, the lift cords  40  are released by the cord retractor  100  for extending the window covering  1 . 
     In a first preferred embodiment of the present disclosure, referring to  FIGS.  2  to  13   , the cord retractor  100  comprises a housing, and a reeling wheel  130 , a driving wheel  140 , a spring wheel  150 , a spring  160 , a friction member  170  and a unidirectional transmission mechanism  180  which are contained in the housing. The housing comprises a bottom plate  110  and an upper cover  120 , as well as a first shaft  111 , a second shaft  112  and a third shaft  113  disposed on a first surface of the bottom plate  110 . As shown in  FIG.  2    and  FIG.  3   , the reeling wheel  130 , the driving wheel  140  and the spring wheel  150  are sleeved on the first shaft  111 , the second shaft  112  and the third shaft  113 , respectively. The reeling wheel  130  comprises a wheel body  131  and a reeling gear  132 , wherein the wheel body  131  is divided into an upper portion and a lower portion, and the reeling gear  132  is disposed around the periphery of an end surface of the wheel body  131  close to the bottom plate  110 . The wheel body  131  is sleeved on the first shaft  111  to make the reeling wheel  130  rotatable about the first shaft  111 . The two lift cords  40  are coupled with and wound on the upper portion and the lower portion of the wheel body  131 , respectively. The two lift cords  40  are wound in the same direction so as to be released or retracted together when the reeling wheel  130  is rotating. 
     The driving wheel  140 , the spring wheel  150  and the spring  160  collaboratively form a powering unit, which provides the window covering  1  with a retracting force. The driving wheel  140  comprises a hollow body  141  and a driving gear  142  disposed around the periphery of an end surface of the hollow body  141  close to the bottom plate  110 . The driving gear  142  meshes with the reeling gear  132  of the reeling wheel  130  such that the reeling wheel  130  and the driving wheel  140  can drive each other to rotate concurrently. The spring wheel  150  is an idler sleeved on the third shaft  113 . The spring  160  is a torsion spring having a first end and a second end, wherein the first end is coupled with and wound on the driving wheel  140 , and the second end is coupled with and wound on the spring wheel  150 . According to a rotating direction of the driving wheel  140 , the spring  160  is wound on the driving wheel  140  and unwound from the spring wheel  150  synchronously, or alternatively, wound on the spring wheel  150  and unwound from the driving wheel  140  synchronously. In view of the action of the spring  160 , the spring wheel  150  can be driven by the driving wheel  140  to rotate. 
     The friction member  170  is in a closed loop shape and sleeved on the second shaft  112 . The friction member  170  may comprise at least one of a rubber component, a silicon component and an elastic plastic component. Moreover, an inside diameter of the friction member  170  does not change while the friction member  170  is sleeved on the second shaft  112  and provides resistance to the rotation of the reeling wheel  130 . In this embodiment, as shown in  FIG.  3   , the friction member  170  comprises three O-shaped rings, which are closely sleeved on the second shaft  112  to form a tight fit or an interference fit therebetween. Such O-shaped ring is a conventional seal component commonly used in industry, which is usually made of nitrile butadiene rubber, fluoroelastomer materials (known as FKM) or silicone. It has elasticity and is capable of bearing frequent friction, and hardly being affected by the change of temperature, which make it a low cost and reliable friction member. The friction member  170  is not limited to the O-shaped rings which have a circular cross section, as the cross section of the friction member  170  can also be square, rectangular or any suitable shape. In some other embodiments, the cross section of the friction member  170  may have a plural-circles-union shape, which is similar to the cross section of plural O-shaped rings stacked together along a longitudinal direction but the plural O-shaped rings are formed integrally. As long as the friction member is formed of an elastic ring, the different cross sections will not affect the use of the friction member. 
     Referring to  FIGS.  3  to  7    and  FIG.  9   , in this embodiment, the unidirectional transmission mechanism  180  is received within the driving wheel  140  and comprises a first limiting member  181  and a second limiting member  182  (shown in  FIG.  9   ). The first limiting member  181  comprises a main body  183  which is hollow, and three arms  184  radially extending out from the periphery of the main body  183 . The arms  184  are spaced apart from each other by the same interval. Each arm  184  is made by an elastic material and is deformable with respect to the main body  183 . Moreover, as shown in  FIG.  6   , there are several bumps  186  on an inner wall  185  of the main body  183 . The bumps  186  are abutting against the outer periphery of the friction member  170  for a tight fit therebetween while the friction member  170  is disposed between the main body  183  and the second shaft  112 . Referring to  FIG.  7   , the second limiting member  182  is exemplified by plural ratchets  187  situated on an inner wall of the hollow body  141  of the driving wheel  140 . As so, the second limiting member  182  can rotate together with the driving wheel  140 . The form of the second limiting member  182  can be the ratchets  187  integrally formed with the driving wheel  140 , or an internal toothed ring gear as an independent component that the hollow body  141  of the driving wheel  140  is sleeved on and fixedly coupled with for rotating together. 
     Please refer to  FIGS.  8  to  13   , which show a status of the cord retractor  100  as all components thereof have been assembled and illustrate the interactions among the components of the cord retractor  100  while the window covering  1  is extending or retracting. As shown, the friction member  170  and the unidirectional transmission mechanism  180  have been installed within the driving wheel  140 . The friction member  170  is sleeved tightly on the second shaft  112 . The main body  183  of the first limiting member  181  is sleeved on and closely engaged with the friction member  170  by the bumps  186  protruding from the inner wall  185  of the main body  183 , thereby forming a tight fit between the friction member  170  and the first limiting member  181 . Meanwhile, the ratchets  187  of the second limiting member  182 , which are situated on the inner wall of the hollow body  141  of the driving wheel  140 , are in contact with the arms  184  of the first limiting member  181 . 
     In this embodiment, no matter where the window covering  1  is moved while extending or retracting, a rewinding force provided by the spring  160  to the lift cords  40  is set to be less than a downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. As a result, if there is no intervention of any other external force, e.g., the friction provided by the friction member, the bottom rail  30  will descend and the lift cords  40  will be correspondingly released until the window covering  1  is completely extended. 
     Please refer to  FIGS.  8  to  10   , as the bottom plate  110  of the housing has been removed in  FIG.  9    and  FIG.  10    for clearly showing the interactions among the components of the cord retractor  100 . When the window covering  1  is operated to be extended, the user drags down the bottom rail  30 , which correspondingly pulls the lift cords  40  in the downward direction. Thereby, the reeling wheel  130  coupled with the lift cords  40  rotates in a first direction for releasing the lift cords  40  wound on the wheel body  131 . While the reeling wheel  130  rotates in the first direction, the driving gear  142  meshed with the reeling gear  132  is driven to rotate with the reeling wheel  130 , which makes the spring  160  wound on the driving wheel  140  for storing the rewinding force. In this embodiment, the first direction is in a counterclockwise direction as designated in  FIG.  9   . When the reeling wheel  130  rotates in the first direction, it drives the driving wheel  140  engaged therewith to rotate reversely (e.g., in a clockwise direction), and the second limiting member  182  rotates in the same direction (e.g., in a clockwise direction) with the driving wheel  140  as it moves together with the driving wheel  140 . 
     Please refer to  FIG.  10   , which is a partial enlarged view of  FIG.  9   . The second limiting member  182  rotates together with the driving wheel  140  in the clockwise direction with respect to the first limiting member  181 . When the second limiting member  182  rotates to a position that each arm  184  of the first limiting member  181  is engaged with the recess between the two neighboring ratchets  187 , each arm  184  is in a first position as shown in  FIG.  10   , and the second limiting member  182  and the first limiting member  181  are therefore inter-locked. In other words, the first limiting member  181  is driven to rotate together with the second limiting member  182  when the second limiting member  182  rotates. This status in which the second limiting member  182  and the first limiting member  181  rotate in unison is defined as a first operation mode hereinafter, and the direction which the second limiting member  182  rotates in is defined as a locking direction. When the second limiting member  182  rotates in the locking direction, the unidirectional transmission mechanism  180  is driven to operate in the first operation mode. In this embodiment, the locking direction of the second limiting member  182  is a clockwise direction. 
     In this embodiment, a first maximum static friction between the first limiting member  181  and the friction member  170  is greater than a second maximum static friction between the friction member  170  and the second shaft  112 . Once a force driving the first limiting member  181  to rotate (equals to the force driving the second limiting member  182  to rotate, or the force driving the driving wheel  140  to rotate, or the force driving the reeling wheel  130  to rotate, or the force exerted by the user on the bottom rail  30  to release the lift cords  40 , or the force driving the window covering  1  to be extended) is greater than the second maximum static friction between the friction member  170  and the second shaft  112 , the second limiting member  182  drives the first limiting member  181  together with the friction member  170  to rotate with respect to the second shaft  112  immediately. While rotating, the force driving the first limiting member  181  to rotate must resist the slip friction between the friction member  170  and the second shaft  112 . While the window covering  1  is operated to be extended, the friction member  170  rubs against the second shaft  112 , as the contact surface between the friction member  170  and the second shaft  112  is in the form of a continuous uninterrupted loop. Moreover, an inside diameter of the friction member  170  does not change while the friction member  170  rubs against the second shaft  112 . Therefore, the friction member  170  continuously and stably provides constant resistance to the cord retractor  100 . Such constant resistance can prevent the bottom rail  30  from falling down uncontrollably and spontaneously, while the user is dragging down the bottom rail  30 . Thus, the user can control the moving speed and moving distance of the bottom rail  30  easily. 
     In some other embodiments, the first maximum static friction between the first limiting member  181  and the friction member  170  is less than the second maximum static friction between the friction member  170  and the second shaft  112 . Once the driving force is greater than the first maximum static friction, the first limiting member  181  rotates with respect to the friction member  170  whereas the friction member  170  remains still and will not rotate with respect to the second shaft  112 . There is also a slip friction existing between the first limiting member  181  and the friction member  170  while the first limiting member  181  rotates. Since the first limiting member  181  continuously rubs against the friction member  170 , the friction member  170  still provides constant resistance to the cord retractor  100 . 
     In more detail, while the user drags down the bottom rail  30 , the lift cords  40  drive the unidirectional transmission mechanism  180  to operate in the first operation mode. In other words, the second limiting member  182  rotates in the locking direction till each arm  184  of the first limiting member  181  is engaged with the recess between two neighboring ratchets  187  of the second limiting member  182 . In this circumstance, even if the user lets go of the bottom rail  30  to stop pulling down the bottom rail  30 , there is still a downward resultant force acting on the cord retractor  100  due to the reason that a downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon is greater than the rewinding force provided by the spring  160 . Thus, the arms  184  of the first limiting member  181  are maintained in the first positions as each arm  184  is engaged with the recess between two neighboring ratchets  187 . Moreover, the maximum static friction could be generated by the friction member  170  is set to be always greater than the difference obtained by subtracting the rewinding force provided by the spring  160  from the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. Therefore, when the user stops dragging down the bottom rail  30 , the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the rewinding force provided by the spring  160 , and the static friction correspondingly generated by the friction member  170  make a balance to maintain the bottom rail  30  in the current position and prevent the bottom rail  30  from descending. 
     Once the user drags down the bottom rail  30  again, the second limiting member  182  directly and immediately drives the first limiting member  181  to rotate. Thus, even if the user drags down the bottom rail  30  in an intermittent way, the friction member  170  accurately provides constant resistance to the cord retractor  100  every time the user starts dragging the bottom rail  30 , which is carried out in view of the collaboration of the first limiting member  181  and the second limiting member  182  of the unidirectional transmission mechanism  180 . 
     Referring to  FIGS.  11  to  13   , as the bottom plate  110  of the housing has been removed in  FIG.  12    and  FIG.  13    for clearly showing the interactions among the components of the cord retractor  100  while the window covering  1  is retracting. When the window covering  1  is operated to be retracted, the user pushes the bottom rail  30  up, in which an upward resultant force is applied to the bottom rail  30  and counteracts the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. Therefore, the window covering  1  is out of balance, and the rewinding force stored in the spring  160  is released, which drives the driving wheel  140  to rotate reversely (in a rotating direction opposite to its rotating direction while the window covering  1  is extending). Moreover, the reeling wheel  130  engaged with the driving wheel  140  is driven to rotate. Thus, while pushing the bottom rail  30  up, the reeling wheel  130  rotates in a second direction which is opposite to the first direction, and keeps retracting the lift cords  40  and winding them on the wheel body  131 . In this embodiment, the second direction is in a clockwise direction, as designated in  FIG.  12   . When the reeling wheel  130  rotates in the second direction to retract the lift cords  40 , it also drives the driving wheel  140  to rotate reversely (e.g., in a counterclockwise direction). Since the second limiting member  182  moves together with the driving wheel  140 , the second limiting member  182  rotates in the same direction with the driving wheel  140  (e.g., in a counterclockwise direction). 
     Please refer to  FIG.  13   , which is a partial enlarged view of  FIG.  12   . The second limiting member  182  rotates together with the driving wheel  140  in the counterclockwise direction with respect to the first limiting member  181 . When the second limiting member  182  rotates to a position that each arm  184  of the first limiting member  181  is about to escape from the recess between the two neighboring ratchets  187 , each arm  184  is pushed by the corresponding ratchet  187  and deformed, being in a second position as shown in  FIG.  13   . When each arm  184  has skidded over the top of the corresponding ratchet  187 , each arm  184  returns to the first position by its elasticity as shown in  FIG.  10   . Since the second limiting member  182  keeps rotating, the arms  184  are about to escape from the recess between the two neighboring ratchets  187  again, then repeating the process of being pushed by the corresponding ratchets  187  and deformed, skidding over the top of the corresponding ratchets  187 , and returning to the first position. In other words, each arm  184  swings between the first position and the second position reciprocatedly. As a result, rather than driving the first limiting member  181  to rotate, the second limiting member  182  only skims over the first limiting member  181  when rotating counterclockwise with respect to the first limiting member  181 . This status in which the second limiting member  182  and the first limiting member  181  do not rotate together is defined as a second operation mode hereinafter, and the direction which the second limiting member  182  rotates in is defined as an unlocking direction. When the second limiting member  182  rotates in the unlocking direction, the unidirectional transmission mechanism  180  is driven to operate in the second operation mode. In this embodiment, the unlocking direction of the second limiting member  182  is a counterclockwise direction. 
     When the unidirectional transmission mechanism  180  is in the second operation mode, the first limiting member  181  is not driven by the rotation of the driving wheel  140  and the second limiting member  182 , which means the main body  183  of the first limiting member  181  has no tendency to rotate with respect to the friction member  170  or the possibility to drive the friction member  170  to rotate with respect to the second shaft  112 . Hence, there is no relative rotation between the first limiting member  181  and the friction member  170 , or between the first limiting member  181  and the second shaft  112 . As a result, the force driving the rotation of the driving wheel  140  and the second limiting member  182  has no need to resist the first maximum static friction between the first limiting member  181  and the friction member  170  or the second maximum static friction between the friction member  170  and the second shaft  112 . In the meantime, there is no slip friction between any two of the first limiting member  181 , the friction member  170  and the second shaft  112 . Therefore, while the window covering  1  is retracting, the friction member  170  does not provide resistance to the cord retractor  100 , which saves labor by reducing the force required for lifting the bottom rail  30 , and also ensures that the rewinding force provided by the spring  160  is effectively applied to the driving wheel  140  and the reeling wheel  130  as they rotate for retracing the lift cords  40 . 
     When the user stops pushing the bottom rail  30  up and lets go of the bottom rail  30 , a downward resultant force acts on the cord retractor  100  again due to the reason that the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon is greater than the rewinding force provided by the spring  160 . Thus, the arms  184  of the first limiting member  181  return to the first position again as being engaged with the recess between two neighboring ratchets  187 , as shown in  FIG.  10   . Meanwhile, the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the rewinding force provided by the spring  160 , and the static friction correspondingly generated by the friction member  170  make a balance such that the bottom rail  30  is maintained in the current position and stays still. 
     Please refer to  FIG.  14    and  FIG.  15   , which show another exemplified friction member of the present disclosure. In this embodiment, the main body  183  of the first limiting member  181  comprises a first engaging portion on the inner wall  185  of the main body  183 , e.g., the bumps  186 . Meanwhile, the friction member  171  is in a closed loop shape and further comprises a second engaging portion, e.g., a plurality of protrusions  171   a.  The protrusions  171   a  are protruding outwardly from an outer wall of the friction member  171  and spaced apart from each other for engaging with the bumps  186  on the inner wall  185  of the first limiting member  181 . Since each protrusion  171   a  is engaged with the recess between the two neighboring bumps  186 , the friction member  171  rotates together or stays still together with the first limiting member  181 . When the friction member  171  is sleeved on the second shaft  112  (as shown in  FIG.  3   ), the contact surface between the friction member  171  and the second shaft  112  is in the form of a continuous uninterrupted loop. Moreover, an inside diameter of the friction member  171  does not change while the friction member  171  rubs against the second shaft  112 . Therefore, the friction member  171  continuously and stably provides constant resistance to the cord retractor  100  while the window covering  1  is operated to be extended. 
     In a second preferred embodiment of the present disclosure, referring to  FIGS.  16  to  19   , the cord retractor  200  comprises a bottom plate  210  and an upper cover  220 , as well as a reeling wheel  230 , a driving wheel  240 , a spring wheel  250 , a spring  260 , a friction member  270  and a unidirectional transmission mechanism  280  which are situated between the bottom plate  210  and the upper cover  220 . The friction member  270  and the unidirectional transmission mechanism  280  are disposed within the reeling wheel  230 . 
     As shown in  FIG.  16   , the reeling wheel  230 , the driving wheel  240  and the spring wheel  250  are sleeved on the first shaft  211 , the second shaft  212  and the third shaft  213 , respectively. The reeling wheel  230  comprises a hollow body  231  and a reeling gear  232  disposed around the periphery of an end surface of the hollow body  231 . The hollow body  231  is divided into an upper portion and a lower portion, and the two lift cords  40  are coupled with and wound on the upper portion and the lower portion, respectively. 
     The driving wheel  240  comprises a wheel body  241  and a driving gear  242 . The wheel body  241  is sleeved on the second shaft  212 . The driving gear  242  is disposed around the periphery of an end surface of the wheel body  241  and meshed with the reeling gear  232 . The spring wheel  250  is sleeved on the third shaft  213 . The spring  260  is a torsion spring having a first end and a second end, wherein the first end is coupled with and wound on the driving wheel  240 , and the second end is coupled with and wound on the spring wheel  250 . By such configurations, the reeling wheel  230  and the driving wheel  240  can drive each other to rotate correspondingly, and the spring wheel  250  can be driven by the driving wheel  240  to rotate by action of the spring  260 . The friction member  270  is identical to the friction member  170  exemplified in  FIG.  3    as comprising plural elastic O-shaped rings. 
     As shown in  FIGS.  16  to  18   , the unidirectional transmission mechanism  280  comprises a first limiting member  281  and a second limiting member  282 . The first limiting member  281  comprises a main body  283 , plural arms  284  extending outwardly from the periphery of the main body  283 , and plural bumps  286  situated on an inner wall  285  of the main body  283  for engaging with the friction member  270  to form a tight fit. As shown in  FIG.  17   , the second limiting member  282  is presented as an internal toothed ring gear having plural ratchets  287  integrally formed with an inner wall of the hollow body  231 . When the reeling wheel  230  rotates, the second limiting member  282  rotates together with the reeling wheel  230 . 
     In this embodiment, no matter where the window covering  1  is moved while extending or retracting, a rewinding force provided by the spring  260  to the lift cords  40  is set to be less than the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. As a result, if there is no intervention of any other external force, the bottom rail  30  will descend and the lift cords  40  will be correspondingly released until the window covering  1  is completely extended. 
     Please refer to  FIG.  18   , in which the bottom plate  210  of the housing has been removed for clearly showing the interactions among the components of the cord retractor  200 . When the window covering  1  is operated to be extended, the lift cords  40  drive the reeling wheel  230  to rotate in a first direction, which is counterclockwise as designated in  FIG.  18   . The second limiting member  282  also rotates counterclockwise as it moves together with the reeling wheel  230 . When the second limiting member  282  rotates to a position that each arm  284  of the first limiting member  281  is engaged with the recess between the two neighboring ratchets  287 , each arm  284  is in a first position as shown in  FIG.  18   , and the second limiting member  282  and the first limiting member  281  are inter-locked. In other words, the first limiting member  281  is driven to rotate together with the second limiting member  282  when the second limiting member  282  rotates. This status is referred to as the first operation mode. In the meantime, the counterclockwise direction of the second limiting member  282  is referred to as the locking direction. 
     When the first limiting member  281  rotates together with the second limiting member  282 , the main body  283  drives the friction member  270  to rotate together, as the bumps  286  on the inner wall  285  are tightly fitted to the friction member  270 . Since the friction member  270  continuously rubs against the first shaft  211 , the friction member  270  continuously provides constant resistance to the cord retractor  200  for assisting the user in determining the moving speed and moving distance of the bottom rail  30  while the window covering  1  is operated to be extended. Furthermore, when the user lets go of the bottom rail  30 , there is still a downward resultant force acting on the cord retractor  200 , which is generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon minus the rewinding force provided by the spring  260 . As so, the arms  284  of the first limiting member  281  are maintained in the first positions as being engaged with the recess of the two neighboring ratchets  287 . Because the maximum static friction could be generated by the friction member  270  is set to be greater than the difference obtained by subtracting the rewinding force provided by the spring  260  from the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the bottom rail  30  is maintained in the current position after the user stops the operation. 
     Once the user drags down the bottom rail  30  again, the second limiting member  282  directly and instantly drives the first limiting member  281  to rotate. Thus, even if the user drags down the bottom rail  30  in an intermittent way, the friction member  270  accurately provides constant resistance to the cord retractor  200  every time the user starts dragging the bottom rail  30 , which is carried out by the collaboration of the first limiting member  281  and the second limiting member  282  of the unidirectional transmission mechanism  280 . 
     Referring to  FIG.  19   , as the bottom plate  210  of the housing has been removed for clearly showing the interactions among the components of the cord retractor  200  while the window covering  1  is operated to be retracted. When the window covering  1  is retracting, the reeling wheel  230  rotates in a second direction to retract the lift cords  40 , which is opposite to the first direction and is clockwise as designated in  FIG.  19   . In the meantime, the second limiting member  282  also rotates clockwise as it moves together with the reeling wheel  230 . When the second limiting member  282  rotates, each arm  284  of the first limiting member  281  swings to a second position as shown in  FIG.  19   , in which each arm  284  is about to escape from the recess between the two neighboring ratchets  287 , being pushed by the corresponding ratchet  287  and deformed. Afterwards, each arm  284  of the first limiting member  281  skids over the top of the corresponding ratchet  287  and returns to the first position by its elasticity. Since the second limiting member  282  keeps rotating, the arms  284  repeat the process of being pushed by the ratchets  287  and deformed, skidding over the top of the corresponding ratchets  287 , and returning to the first position. In other words, each arm  284  swings between the first position and the second position reciprocatedly. As a result, rather than driving the first limiting member  281  to rotate, the second limiting member  282  only skims over the first limiting member  281  when rotating clockwise with respect to the first limiting member  281 . This status in which the second limiting member  282  and the first limiting member  281  do not rotate together is referred to as the second operation mode, and the clockwise direction of the second limiting member  282  is referred to as the unlocking direction. Since the first limiting member  281  does not rotate while the window covering  1  is operated to be retracted, there is no relative rotation between the first limiting member  281  and the friction member  270 , or between the first limiting member  281  and the second shaft  212 . Therefore, while the window covering  1  is operated to be retracted, the friction member  270  does not provide resistance to the cord retractor  200 , which saves labor by reducing the force required for lifting the bottom rail  30 , and also ensures that the rewinding force provided by the spring  260  is effectively applied to the driving wheel  240  and the reeling wheel  230  as they rotate for retracing the lift cords  40 . 
     When the user stops pushing the bottom rail  30  up and lets go of the bottom rail  30 , as described above, a downward resultant force acts on the cord retractor  200  again due to the reason that the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon is greater than the rewinding force provided by the spring  260 . Thus, the arms  284  of the first limiting member  281  return to the first position again as being engaged with the recess between the two neighboring ratchets  287 , as shown in  FIG.  18   . Meanwhile, the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the rewinding force provided by the spring  260 , and the static friction correspondingly generated by the friction member  270  make a balance again to maintain the bottom rail  30  in the current position. 
     In a third preferred embodiment of the present disclosure, referring to  FIGS.  20  to  23   , the cord retractor  300  comprises a bottom plate  310  and an upper cover  320 , and a reeling wheel  330 , a driving wheel  340 , a spring wheel  350 , a spring  360 , a friction member  370  and a unidirectional transmission mechanism  380  which are situated between the bottom plate  310  and the upper cover  320 . There are a first shaft  311 , a second shaft  312 , a third shaft  313  and a fourth shaft  314  disposed on the bottom plate  310 . Furthermore, the cord retractor  300  comprises a transmission wheel  390  sleeved on the fourth shaft  314 , and the friction member  370  and the unidirectional transmission mechanism  380  are disposed within the transmission wheel  390 . 
     As shown in  FIG.  20   , the transmission wheel  390 , the reeling wheel  330 , the driving wheel  340  and the spring wheel  350  are sleeved on the fourth shaft  314 , the first shaft  311 , the second shaft  312  and the third shaft  313 , respectively. The reeling wheel  330  comprises a wheel body  331  and a reeling gear  332 , wherein the wheel body  331  is sleeved on the first shaft  311 , and the reeling gear  332  is disposed around the periphery of an end surface of the wheel body  331 . The wheel body  331  is divided into an upper portion and a lower portion. The lift cords  40  are coupled with and wound on the upper portion and the lower portion, respectively. 
     The driving wheel  340  comprises a wheel body  341  a driving gear  342 . The wheel body  341  is sleeved on the second shaft  312 . The driving gear  342  is disposed around the periphery of an end surface of the wheel body  341  and meshed with the reeling gear  332 . The spring wheel  350  is sleeved on the third shaft  313 . The spring  360  is a torsion spring, the two ends of which is coupled with and wound on the driving wheel  340  and the spring wheel  350 , respectively. The transmission wheel  390  comprises a hollow body  391  and a transmission gear  392 . The transmission gear  392  is disposed around the periphery of an end surface of the hollow body  391  and meshed with the reeling gear  332 . By such configurations, the transmission wheel  390 , the reeling wheel  330  and the driving wheel  340  can rotate correspondingly, and the spring wheel  350  can be driven by the spring  360  to rotate with the driving wheel  340 . The friction member  370  is in a closed loop shape and sleeved on the fourth shaft  314 , and is exemplified by plural elastic O-shaped rings. 
     Referring to  FIGS.  20  to  22   , the unidirectional transmission mechanism  380  comprises a first limiting member  381  and a second limiting member  382 . The first limiting member  381  comprises a main body  383  and plural arms  384 . The arms  384  are extending outwardly from the periphery of the main body  383  and made by an elastic material. Moreover, there are plural bumps  386  protruding from an inner wall  385  of the main body  383  for fitting tightly on and closely engaged with the friction member  370 . The second limiting member  382  is in a form as an internal toothed ring gear having plural ratchets  387  integrally formed with an inner wall of the hollow body  391  of the transmission wheel  390 , as shown in  FIG.  21   . Hence, the second limiting member  382  can rotate together with the transmission wheel  390  when the transmission wheel  390  rotates. 
     Please refer to  FIG.  22   , as the bottom plate  310  of the housing has been removed for clearly showing the interactions among the components of the cord retractor  300  while the window covering  1  is operated to be extended. When the user operates the window covering  1  to be extended, the lift cords  40  drive the reeling wheel  330  to rotate in a first direction, which is the counterclockwise direction in this embodiment, as designated in  FIG.  22   . The transmission wheel  390  engaged with the reeling wheel  330  thereby rotates clockwise, and the second limiting member  382  also rotates clockwise as it moves together with the transmission wheel  390 . When the second limiting member  382  rotates to a position that each arm  384  of the first limiting member  381  is engaged with the recess between the two neighboring ratchets  387 , each arm  384  is in a first position (as shown in  FIG.  22   ), and the second limiting member  382  and the first limiting member  381  are operating in the first operation mode in which the second limiting member  382  and the first limiting member  381  are inter-locked and rotate together. In this embodiment, the clockwise direction of the second limiting member  382  is the locking direction. 
     While the first limiting member  381  rotates together with the second limiting member  382  in the clockwise direction, the main body  383  drives the friction member  370  to rotate together, through the bumps  386  on the inner wall  385 . Therefore, the friction member  370  continuously rubs against the fourth shaft  314  and thereby continuously providing constant resistance to the cord retractor  300  while the window covering  1  is operated to be extended. In this embodiment, no matter where the window covering  1  is moved when being retracted or extended, the rewinding force provided by the spring  360  to the lift cords  40  is set to be less than the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. As a result, if there is no intervention of any other external force, the bottom rail  30  will move downwardly and the lift cords  40  will be correspondingly released until the window covering  1  is completely extended. The constant resistance provided by the friction member  370 , as mentioned above, helps the user to determine and control the moving speed and the moving distance while dragging down the bottom rail  30 . 
     In addition, when the user lets go of the bottom rail  30  to stop dragging down the bottom rail  30 , a force in the same direction with the force that user exerted on the bottom rail  30  while dragging down the bottom rail  30 , is applied to the cord retractor  300 , due to the weight of the bottom rail  30  and the covering materials  20  stacked thereon. Thus, the arms  384  of the first limiting member  381  are maintained in the first positions as being engaged with the recess of the two neighboring ratchets  387 . Meanwhile, since the maximum static friction could be generated by the friction member  370  is set to be greater than the difference obtained by subtracting the rewinding force provided by the spring  360  from the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the bottom rail  30  is maintained in the current position after the user stops the operation. 
     Once the user drags down the bottom rail  30  again, the second limiting member  382  directly and instantly drives the first limiting member  381  to rotate. Thus, even if the user drags down the bottom rail  30  in an intermittent way, the friction member  370  accurately provides constant resistance to the cord retractor  300  every timer the user starts dragging the bottom rail  30 , which is carried out in view of the collaboration of the first limiting member  381  and the second limiting member  382  of the unidirectional transmission mechanism  380 . 
     Please refer to  FIG.  23   , as the bottom plate  310  of the housing has been removed for clearly showing the interactions among the components of the cord retractor  300  while the window covering  1  is operated to be retracted. When the window covering  1  is operated to be retracted, the reeling wheel  330  rotates in a second direction for retracting the lift cords  40 . The second direction is opposite to the first direction. In this embodiment, the second direction is in a clockwise direction, as designated in  FIG.  23   . The transmission wheel  390  engaged with the reeling wheel  330  is driven to rotate counterclockwise, and the second limiting member  382  also rotates counterclockwise as it moves together with the transmission wheel  390 . When the second limiting member  382  rotates, each arm  384  of the first limiting member  381  escapes from the recess between the two neighboring ratchets  387  and is pushed by the corresponding ratchet  387 , being deformed and swinging to a second position, as shown in  FIG.  23   . Afterwards, each arm  384  of the first limiting member  381  skids over the top of the corresponding ratchet  387  and returns to the first position by its elasticity, as shown in  FIG.  22   . Since the second limiting member  382  keeps rotating, the arms  384  repeat the process of escaping from the recess between the two neighboring ratchets  387 , being pushed by the corresponding ratchets  387  and deformed, and skidding over the top of the corresponding ratchets  387 . In other words, each arm  384  reciprocatedly swings between the first position and the second position. As a result, rather than driving the first limiting member  381  to rotate, the second limiting member  382  only skims over the first limiting member  381  when rotating counterclockwise with respect to the first limiting member  381 . This status in which the second limiting member  382  and the first limiting member  381  do not rotate together is referred to as the second operation mode, and the counterclockwise direction of the second limiting member  382  is referred to as the unlocking direction. Since the first limiting member  381  is not driven to rotate, the first limiting member  381  and the friction member  370  which is tightly fitted therewith will not rotate with respect to the fourth shaft  314 . Therefore, while the window covering  1  is operated to be retracted, the friction member  370  does not provide resistance to the cord retractor  300 , which saves labor by reducing the force required for lifting the bottom rail  30 , and also ensures that the rewinding force provided by the spring  360  is effectively applied to the driving wheel  340  and the reeling wheel  330  as they rotate for retracing the lift cords  40 . 
     When the user stops pushing the bottom rail  30  up and lets go of the bottom rail  30 , a downward resultant force acts on the cord retractor  300  again due to the reason that the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon is greater than the rewinding force provided by the spring  360 . Thus, the arms  384  of the first limiting member  381  return to the first positions again, in which the arms  384  are engaged with the recess between the two neighboring ratchets  387  as shown in  FIG.  22   . At this moment, the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the rewinding force provided by the spring  360 , and the static friction correspondingly generated by the friction member  370  make a balance, such that the bottom rail  30  is maintained in the current position. 
     In a fourth preferred embodiment of the present disclosure, referring to  FIGS.  24  to  30   , the cord retractor  400  comprises a bottom plate  410  and an upper cover  420 , and a reeling wheel  430 , a driving wheel  440 , a spring wheel  450 , a spring  460 , a friction member  470 , a unidirectional transmission mechanism  480  and an intermediate wheel  490  which are situated between the bottom plate  410  and the upper cover  420 . A first shaft  411 , a second shaft  412 , a third shaft  413 , a fourth shaft  414  and a groove  415  are disposed on a first surface of the bottom plate  410 . Compared with the unidirectional transmission mechanisms  180 ,  280  of the cord retractors  100 ,  200  in above-mentioned embodiments which are respectively disposed within the hollow body  141  of the driving wheel  140  and the hollow body  231  of the reeling wheel  230 , the unidirectional transmission mechanism  480  in this embodiment is disposed outside of the intermediate wheel  490 . 
     The intermediate wheel  490 , the reeling wheel  430 , the driving wheel  440  and the spring wheel  450  are sleeved on the fourth shaft  414 , the first shaft  411 , the second shaft  412  and the third shaft  413 , respectively. The reeling wheel  430  comprises a wheel body  431  and a reeling gear  432 . The wheel body  431  is sleeved on the first shaft  411  and is divided into an upper portion and a lower portion. The two lift cords  40  are coupled with and wound on the upper portion and the lower portion of the wheel body  431 , respectively. The reeling gear  432  is disposed around the periphery of an end surface of the wheel body  431 . 
     The driving wheel  440  comprises a wheel body  441  and a driving gear  442 . The wheel body  441  is sleeved on the second shaft  412 . The driving gear  442  is disposed around the periphery of an end surface of the wheel body  441  and meshed with the reeling gear  432 . The spring wheel  450  is sleeved on the third shaft  413 . The spring  460  is a torsion spring having a first end and a second end, wherein the first end is coupled with and wound on the driving wheel  440 , and the second end is coupled with and wound on the spring wheel  450 . The intermediate wheel  490  comprises a sleeve  491  and a transmission member  492 . The sleeve  491  is sleeved on the fourth shaft  414 , and the transmission member  492  is disposed around the periphery of the sleeve  491  and meshed with the reeling gear  432  of the reeling wheel  430 . In such meshing configuration, the intermediate wheel  490 , the reeling wheel  430  and the driving wheel  440  are driven to rotate correspondingly, and the spring wheel  450  is driven by the spring  460  to rotate correspondingly with the driving wheel  440 . The friction member  470  is in a closed loop shape and sleeved on the sleeve  491  of the intermediate wheel  490 . In this embodiment, the friction member  470  comprises plural elastic O-shaped rings, which are sleeved on and tightly fitted with the sleeve  491 . 
     Referring to  FIGS.  24  to  28   , the unidirectional transmission mechanism  480  comprises a first limiting member  481  and a second limiting member  482  which act collaboratively. The first limiting member  481  is disposed at the groove  415  on the bottom plate  410 . The second limiting member  482  is disposed on the intermediate wheel  490 . As shown in  FIG.  25   , the first limiting member  481  comprises a rod  483 , an arm  484  and a recovering component  485 . The arm  484  and the recovering component  485  are protruding from the rod  483  at different angles with respect to the rod  483 . In other words, the arm  484  and the recovering component  485  extend towards the second limiting member  482  in the different directions. The recovering component  485  is elastic and deformable with respect to the rod  483 . The rod  483  is rotatably inserted into and installed on the groove  415 . The arm  484  protrudes out from the groove  415  for contact with the second limiting member  482 . The recovering component  485  is abutting against an internal surface of the groove  415 . As shown in  FIG.  26   , the second limiting member  482  comprises a main body  486  and plural ratchets  487  surrounding the outer periphery of the main body  486 . In addition, there are plural bumps  489  protruding from an inner wall  488  of the main body  486 . 
     Please refer to  FIGS.  27  to  30   , which show a completely assembled status of the cord retractor  400  and the interactions among the components of the cord retractor  400  while the window covering  1  is extending or retracting. After the friction member  470 , the unidirectional transmission mechanism  480  and the intermediate wheel  490  have been assembled, the sleeve  491  of the intermediate wheel  490  is sleeved on the fourth shaft  414 , and the friction member  470  is further sleeved on the outer peripheral surface of the sleeve  491 . Furthermore, the main body  486  of the second limiting member  482  is sleeved on the friction member  470  and closely engaged with the friction member  470  by the plural bumps  489  on the inner wall  488  thereof, which forms a tight fit between the second limiting member  482  and the friction member  470 . Meanwhile, the arm  484  of the first limiting member  481  is abutting against one of the ratchets  487  of the second limiting member  482 . 
     In this embodiment, no matter where the window covering  1  is moved while extending or retracting, the rewinding force provided by the spring  460  to the lift cords  40  is set to be less than the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. As a result, if there is no intervention of any other external force, the bottom rail  30  will descend and the lift cords  40  will be correspondingly released until the window covering  1  is completely extended. 
     Please refer to  FIGS.  27  and  28   , which are the cross sectional views along a horizontal plane at the altitude of the recovering component  485  for clearly showing the interactions among the components of the cord retractor  400  while the window covering  1  is extending. When the window covering  1  is operated to be extended, the user drags down the bottom rail  30  and the lift cords  40  are correspondingly pulled, thereby driving the reeling wheel  430  to rotate in a first direction. In this embodiment, the first direction is a clockwise direction as designated in  FIG.  27   . As a result, the intermediate wheel  490  engaged with the reeling wheel  430  is driven to rotate counterclockwise. Since the sleeve  491  of the intermediate wheel  490  is tightly fitted with the friction member  470 , and the friction member  470  is tightly fitted with the main body  486  of the second limiting member  482 , the friction member  470  and the second limiting member  482  tend to rotate together with the intermediate wheel  490  in the counterclockwise direction. 
     Referring to  FIG.  28   , which is a partial enlarged view of  FIG.  27   , when the second limiting member  482  rotates to a position that the arm  484  of the first limiting member  481  is abutting against the recess between the two neighboring ratchets  487 , the arm  484  is in a first position and restrains the second limiting member  482  from rotating. Such status in which the first limiting member  481  and the second limiting member  482  are inter-locked is referred to as the first operation mode. In this embodiment, the counterclockwise direction of the second limiting member  482  is the locking direction. When the second limiting member  482  rotates in the locking direction, the unidirectional transmission mechanism  480  is driven to operate in the first operation mode. 
     When the second limiting member  482  is blocked by the first limiting member  481  and stops rotating, the intermediate wheel  490  still keeps rotating. Thus, there is relative motion between the intermediate wheel  490  and the second limiting member  482 . Because the second limiting member  482  is closely engaged with the friction member  470  through the bumps  489  on the inner wall  488  of the main body  486 , a first maximum static friction between the second limiting member  482  and the friction member  470  is greater than a second maximum static friction between the friction member  470  and the sleeve  491 . Therefore, once the driving force is greater than the second maximum static friction between the friction member  470  and the sleeve  491 , the sleeve  491  of the intermediate wheel  490  starts rotating with respect to the friction member  470 . While the intermediate wheel  490  rotates, the driving force has to resist the slip friction between the friction member  470  and the sleeve  491 . Therefore, when the window covering  1  is extending, the sleeve  491  of the intermediate wheel  490  rubs against the friction member  470 , as the contact surface between the friction member  470  and the sleeve  491  is in the form of a continuous uninterrupted closed loop. The friction member  470  continuously provides constant resistance to the cord retractor  400  for assisting the user in determining the moving speed and moving distance of the bottom rail  30 , thereby preventing the bottom rail  30  from uncontrollably falling down. 
     In some other embodiments, the first maximum static friction between the second limiting member  482  and the friction member  470  is less than the second maximum static friction between the friction member  470  and the sleeve  491 . Under this circumstance, once the driving force is greater than the first maximum static friction between the second limiting member  482  and the friction member  470 , the intermediate wheel  490  drives the friction member  470  to rotate together with respect to the second limiting member  482 . During the rotation, the driving force has to resist the slip friction between the second limiting member  482  and the friction member  470 . Since the friction member  470  continuously rubs against the second limiting member  482 , the friction member  470  continuously provides constant resistance to the cord retractor  400 . 
     In more detail, while dragging down the bottom rail  30 , the lift cords  40  drive the unidirectional transmission mechanism  480  to operate in the first operation mode. In other words, the second limiting member  482  has rotated to the position that the arm  484  of the first limiting member  481  is abutting against the recess between the two neighboring ratchets  487 . In this circumstance, even if the user lets go of the bottom rail  30  to stop pulling down the bottom rail  30 , there is still a downward resultant force acting on the cord retractor  400  due to the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. Thus, the arm  484  of the first limiting member  481  is maintained abutting against the recess between the two neighboring ratchets  487 . Moreover, the maximum static friction could be generated by the friction member  470  is set to be greater than the difference obtained by subtracting the rewinding force provided by the spring  460  from the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, such that the bottom rail  30  is maintained in the current position after the user stops the operation. 
     Once the user drags down the bottom rail  30  again, the first limiting member  481  directly and immediately restrains the second limiting member  482  from rotating. Thus, even if the user drags down the bottom rail  30  in an intermittent way, the friction member  470  accurately provides constant resistance to the cord retractor  400  every time the user starts dragging the bottom rail  30 , which is carried out in view of the collaboration of the first limiting member  481  and the second limiting member  482  of the unidirectional transmission mechanism  480 . 
     Please refer to  FIGS.  29  and  30   , which are the cross sectional views along a horizontal plane at the altitude of the recovering component  485  of the first limiting member  481  for clearly showing the interactions among the components of the cord retractor  400  while the window covering  1  is retracting. When the window covering  1  is operated to be retracted, the user pushes the bottom rail  30  up, and the reeling wheel  430  rotates in a second direction (designated as a counterclockwise direction in  FIG.  29   ) opposite to the first direction for retracting the lift cords  40 . The intermediate wheel  490  engaged with the reeling wheel  430  thereby rotates in a clockwise direction. When the intermediate wheel  490  rotates, both the friction member  470  sleeved on the intermediate wheel  490  and the second limiting member  482  rotate together with the intermediate wheel  490  in the clockwise direction. 
     Referring to  FIG.  30   , which is a partial enlarged view of  FIG.  29   , when the second limiting member  482  rotates to a position that the arm  484  of the first limiting member  481  has escaped from the recess between the two neighboring ratchets  487 , the arm  484  is pushed by the corresponding ratchet  487  towards the recovering component  485  to be in a second position, in which an angle between the arm  484  and the recovering component  485  becomes smaller (compared with the angle therebetween when the arm  484  is in the first position). After the arm  484  skids over the top of the corresponding ratchet  487 , the arm  484  returns to the first position (as shown in  FIG.  28   ) as it is under the effect of an elastic recovery force provided by the recovering component  485 . During the rotation of the second limiting member  482 , the arm  484  repeat being pushed by the corresponding one of the ratchets  487  and deformed, skidding over the top of the corresponding one of the ratchets  487 , and recovered to the previous position that the arm  484  is not being pushed yet. In other words, the arm  484  swings between the first position and the second position reciprocatedly. The status in which the second limiting member  482  rotates with respect to the first limiting member  481  is referred to as the second operation mode, and the clockwise direction of the second limiting member  482  is referred to as the unlocking direction. When the second limiting member  482  rotates in the unlocking direction, the unidirectional transmission mechanism  480  is driven to operate in the second operation mode. 
     In the second operation mode, the second limiting member  482  is not restrained by the first limiting member  481  and thereby rotates together with the intermediate wheel  490 . As so, there is no relative motion among the sleeve  491  of the intermediate wheel  490 , the friction member  470  and the main body  486  of the second limiting member  482 . As a result, the driving force has no need to resist a first maximum static friction between the friction member  470  and the main body  486  of the second limiting member  482 , or a second maximum static friction between the sleeve  491  and the friction member  470 . In addition, the slip friction between any two of the sleeve  491 , the friction member  470  and the main body  486  does not exist at this moment. Therefore, when the window covering  1  is retracting, the friction member  470  does not provide resistance to the cord retractor  400 , which saves labor by reducing the force required for lifting the bottom rail  30 , and also ensures that the rewinding force provided by the spring  460  is effectively applied to the driving wheel  440  and the reeling wheel  430  as they rotate for retracing the lift cords  40 . 
     When the user stops pushing the bottom rail  30  up and lets go of the bottom rail  30 , a downward resultant force acts on the cord retractor  400  again due to the reason that the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon is greater than the rewinding force provided by the spring  460 . Thus, the arm  484  of the first limiting member  481  returns to the first position again, in which the arm  484  is engaged with the recess between two neighboring ratchets  487 , as shown in  FIG.  28   . At this moment, the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the rewinding force provided by the spring  460 , and the static friction correspondingly generated by the friction member  470  make a balance such that the bottom rail  30  is maintained in the current position. 
     In a fifth preferred embodiment of the present disclosure, referring to  FIGS.  31  to  38   , the cord retractor  500  comprises a bottom plate  510  and an upper cover  520 , and a reeling wheel  530 , a driving wheel  540 , a spring wheel  550 , a spring  560 , a friction member  570  and a unidirectional transmission mechanism  580  disposed between the bottom plate  510  and the upper cover  520 . The difference between the cord retractor  500  and the other exemplified cord retractors  100 ,  200 ,  300  and  400  in above-mentioned embodiments is mainly presented on the unidirectional transmission mechanism  580 . 
     Referring to  FIG.  31   , the reeling wheel  530 , the driving wheel  540  and the spring wheel  550  are sleeved on the first shaft  511 , the second shaft  512  and the third shaft  513 , respectively. The reeling wheel  530  comprises a wheel body  531  and a reeling gear  532 . The wheel body  531  is sleeved on the first shaft  511  and is divided into an upper portion and a lower portion. The two lift cords  40  are coupled with and wound on the upper portion and the lower portion of the wheel body  531 , respectively. The reeling gear  532  is disposed around the periphery of an end surface of the wheel body  531 . 
     The driving wheel  540  comprising a hollow body  541  and a driving gear  542 . The hollow body  541  is sleeved on the second shaft  512 . The driving gear  542  is disposed around the periphery of an end surface of the hollow body  541  and meshed with the reeling gear  532 . The spring wheel  550  is sleeved on the third shaft  513 . The spring  560  is a torsion spring having a first end and a second end, wherein the first end is coupled with and wound on the driving wheel  540 , and the second end is coupled with and wound on the spring wheel  550 . By such configuration, the reeling wheel  530  and the driving wheel  540  can drive each other to rotate and rotate synchronously. The spring wheel  550  is driven by the spring  560  to rotate correspondingly with the driving wheel  540 . The friction member  570  is in a closed loop shape and sleeved on the second shaft  512 . In this embodiment, the friction member  570  comprises plural elastic O-shaped rings, which are sleeved on and tightly fitted with the second shaft  512 . 
     Referring to  FIGS.  31  to  35   , the unidirectional transmission mechanism  580  is disposed within the driving wheel  540  and comprises a first limiting member  581  and a second limiting member  582 . The first limiting member  581  has a hollow cylinder  583  and four swing elements, wherein the inner wall of the hollow cylinder  583  is provided with four grooves  584  for containing the four swing elements, respectively. As shown in  FIG.  33   , each swing element comprises a rod  585 , two arms  586  and a recovering component  587 . The two arms  586  are extending outwardly from the rod  585 , and the recovering component  587  is situated in between the two arms  586 . Moreover, the recovering component  587  is elastic and extending in a direction different from the extending direction of the arms  586 . The rod  585  is rotatably inserted into and installed on the groove  584 , the arms  586  protrudes out from the groove  584  for contact with the second limiting member  582 , and one end of the recovering component  587  is abutting against an inner wall of the groove  584 . In this embodiment, as shown in  FIG.  32   , the hollow cylinder  583  of the first limiting member  581  is integrally formed with an inner wall of the hollow body  541  of the driving wheel  540  for rotating together with the driving wheel  540 . In some other embodiments, the first limiting member  581  is an independent component and the hollow body  541  of the driving wheel  540  is sleeved on and fixedly coupled with the first limiting member  581 , by which the first limiting member  581  and the driving wheel  540  also can rotate together. Furthermore, in still some other embodiments, the wheel body  531  of the reeling wheel  530  is hollow, and the hollow cylinder  583  of the first limiting member  581  is fixedly situated on an inner wall of the hollow wheel body  531 , in the manner of being integrally formed or assembled thereto, such that the hollow cylinder  583  of the first limiting member  581  can be driven by the reeling wheel  530  to rotate. The second limiting member  582  is sleeved on and tightly fitted with the friction member  570  by an inner wall  588  thereof. Moreover, the second limiting member  582  comprises plural ribs  589  arranged circularly on an outer peripheral wall thereof and arranged in equally spaced apart. 
     Please refer to  FIGS.  35  to  38   , which illustrate the cord retractor  500  in completely assembled status and the interactions among the components of the cord retractor  500  when the window covering  1  is extending or retracting. After the friction member  570  and the unidirectional transmission mechanism  580  have been installed within the driving wheel  540 , the friction member  570  is sleeved on the second shaft  512 , the second limiting member  582  is sleeved on and tightly fitted with the friction member  570 , and each arm  586  extending out from the groove  584  of the hollow cylinder  583  is abutting against the corresponding one of the ribs  589  of the second limiting member  582 . In addition, in this embodiment, no matter where the window covering  1  is moved while extending or retracting, a rewinding force provided by the spring  560  to the lift cords  40  is set to be less than a downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. As a result, if there is no intervention of any other external force, the bottom rail  30  will descend and the lift cords  40  will be correspondingly released from the cord retractor  500 , until the window covering  1  is completely extended. 
     Please refer to  FIGS.  35  to  36   , which are the cross sectional views along a horizontal plane at the altitude of the recovering component  587  for clearly showing the interactions among the components of the cord retractor  500  while the window covering  1  is extending. When the window covering  1  is operated to be extended, the user drags down the bottom rail  30 , and the lift cords  40  are correspondingly pulled and thereby drive the reeling wheel  530  to rotate in a first direction. In this embodiment, the first direction is a clockwise direction as designated in  FIG.  35   . The driving wheel  540  engaged with the reeling wheel  530  is driven to rotate counterclockwise. The first limiting member  581  also rotates counterclockwise as it moves together with the driving wheel  540 . 
     When the first limiting member  581  rotates, the swing elements disposed at the grooves  584  also rotate counterclockwise. Referring to  FIG.  36   , which is a partial enlarged view of  FIG.  35   , when the first limiting member  581  rotates to a position that each arm  586  has stretched into the recess between the two neighboring ribs  589  and is abutting against the corresponding rib  589 , each arm  586  is in a first position, as shown in  FIG.  36   . At this moment, the first limiting member  581  and the second limiting member  582  are inter-locked, which means the first limiting member  581  is able to drive the second limiting member  582  to rotate together therewith in the counterclockwise direction. The status in which the first limiting member  581  and the second limiting member  582  can rotate together is referred to as the first operation mode, and the counterclockwise direction of the first limiting member  581  is referred to as the locking direction. When the first limiting member  581  rotates in the locking direction, the unidirectional transmission mechanism  580  is driven to operate in the first operation mode. 
     When the second limiting member  582  rotates together with the first limiting member  581 , in a circumstance that a first maximum static friction between the second limiting member  582  and the friction member  570  is greater than a second maximum static friction between the friction member  570  and the second shaft  512 , once the driving force is greater than the second maximum static friction between the friction member  570  and the second shaft  512 , the first limiting member  581  instantly drives the second limiting member  582  together with the friction member  570  to rotate with respect to the second shaft  512 . During the rotation, the driving force has to resist the slip friction between the friction member  570  and the second shaft  512 . Therefore, while the window covering  1  is operated to be extended, the friction member  570  rubs against the second shaft  512 , as the contact surface between the friction member  570  and the second shaft  512  is in the form of a continuous uninterrupted loop. The friction member  570  continuously provides constant resistance to the cord retractor  500  for assisting the user in determining the moving speed and moving distance of the bottom rail  30  as preventing the bottom rail  30  from uncontrollably falling down. 
     In some other embodiments, the first maximum static friction between the second limiting member  582  and the friction member  570  is less than the second maximum static friction between the friction member  570  and the second shaft  512 . Under the circumstance, once the driving force is greater than the first maximum static friction between the second limiting member  582  and the friction member  570 , the second limiting member  582  rotates with respect to the friction member  570 . During the rotation, the driving force has to resist the slip friction between the second limiting member  582  and the friction member  570 . Since the second limiting member  582  continuously rubs against the friction member  570 , the friction member  570  continuously provides constant resistance to the cord retractor  500 . 
     In more detail, while dragging down the bottom rail  30 , the lift cords  40  drive the unidirectional transmission mechanism  580  to operate in the first operation mode. In other words, the first limiting member  581  has rotated to the position (referred to as the first position) that each arm  586  has stretched into the recess between the two neighboring ribs  589  and is abutting against the corresponding rib  589 . At this moment, even if the user lets go of the bottom rail  30  to stop dragging down the bottom rail  30 , there is still a downward resultant force acting on the cord retractor  500  due to the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. Thus, each arm  586  is maintained in the first position as stretching into the recess between the two neighboring ribs  589  and abutting against the corresponding rib  589 . Moreover, since the maximum static friction could be generated by the friction member  570  is set to be greater than the difference obtained by subtracting the rewinding force provided by the spring  560  from the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the bottom rail  30  is maintained in the current position after the user lets go of the bottom rail  30 . 
     Once the user drags down the bottom rail  30  again, the first limiting member  581  directly and immediately drives the second limiting member  582  to rotate. Thus, even if the user drags down the bottom rail  30  in an intermittent way, the friction member  570  accurately provides constant resistance to the cord retractor  500  every time the user starts dragging the bottom rail  30 , which is carried out in view of the collaboration of the first limiting member  581  and the second limiting member  582  of the unidirectional transmission mechanism  580 . 
     Please refer to  FIGS.  37  and  38   , which are the cross sectional views along a horizontal plane at the altitude of the recovering component  587  of the swing element for clearly showing the interactions among the components of the cord retractor  500  while the window covering  1  is retracting. When the window covering  1  is operated to be retracted, the user lifts the bottom rail  30 , and the reeling wheel  530  rotates in a second direction for retracting the lift cords  40 . The second direction is opposite to the first direction. In this embodiment, the second direction is a counterclockwise direction, as designated in  FIG.  37   . The driving wheel  540  engaged with the reeling wheel  530  is driven to rotate clockwise. The first limiting member  581  also rotates clockwise as it moves together with the driving wheel  540 . 
     Please refer to  FIG.  38   , which is a partial enlarged view of  FIG.  37   . When the first limiting member  581  rotates to the position that each arm  586  is pushed by the corresponding rib  589 , each arm  586  moves towards the corresponding recovering component  587  and is located in the second position as shown in  FIG.  38   . At this moment, the angle between the recovering component  587  and the arm  586  becomes smaller (compared with the angle therebetween when the arm  586  is in the first position). Since the first limiting member  581  keeps rotating, each arm  586  skids over the corresponding rib  589  and returns to the first position as being under the effect of an elastic recovery force provided by the recovering component  587 . At this moment, each arm  586  is stretching into the recess between the two neighboring ribs  589  and abutting against the corresponding rib  589 , as shown in  FIG.  36   . During the rotation of the first limiting member  581 , the arms  586  are repeating the process of being pushed by the corresponding ribs  589  and deformed, skidding over the top of the corresponding ribs  589 , and going back to the first position as stretching into the recess between the neighboring ribs  589  and abutting against the corresponding ribs  589 . In other words, each arm  586  keeps swinging between the first position and the second position reciprocatedly. As a result, rather than driving the second limiting member  582  to rotate, the first limiting member  581  only skims over the second limiting member  582  when rotating clockwise with respect to the second limiting member  582 . This status in which the first limiting member  581  rotates with respect to the second limiting member  582  is referred to as the second operation mode, and the clockwise direction of the first limiting member  581  is referred to as the unlocking direction. When the first limiting member  581  rotates in the unlocking direction, the unidirectional transmission mechanism  580  is driven to operate in the second operation mode. 
     When the unidirectional transmission mechanism  580  is in the second operation mode, the second limiting member  582  does not rotate together with the first limiting member  581 , which means there is no relative rotation between the second limiting member  582  and the friction member  570 , or between the second limiting member  582  and the second shaft  512 . The second limiting member  582  does not tend to rotate with respect to the friction member  570 , and does not tend to drive the friction member  570  to rotate with respect to the second shaft  512 . As a result, the driving force has no need to resist the first maximum static friction between the second limiting member  582  and the friction member  570  or the second maximum static friction between the friction member  570  and the second shaft  512 . In the meantime, there is no slip friction between any two of the second limiting member  582 , the friction member  570  and the second shaft  512 . Therefore, while the window covering  1  is retracting, the friction member  570  does not provide resistance to the cord retractor  500 , which saves labor by reducing the force required for lifting the bottom rail  30 , and also ensures that the rewinding force provided by the spring  560  is effectively applied to the driving wheel  540  and the reeling wheel  530  as they rotate for retracing the lift cords  40 . 
     Once the user stops lifting the bottom rail  30  and lets go of the bottom rail  30 , a downward resultant force acts on the cord retractor  500  again due to the reason that the downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon is greater than the rewinding force provided by the spring  560 . Thus, each arm  586  of the first limiting member  581  then returns to the first position as stretching into the recess between the two neighboring ribs  589  and abutting against the corresponding rib  589 , as shown in  FIG.  36   . The downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon, the rewinding force provided by the spring  560 , and the static friction correspondingly generated by the friction member  570  make a balance again, which maintains the bottom rail  30  in the current position. 
     In view of the action of the unidirectional transmission mechanisms  180 ,  280 ,  380 ,  480  and  580  mentioned above, the friction members  170 ,  270 ,  370 ,  470  and  570  provide resistance in only one rotating direction of the reeling wheel, which makes the cord retractors  100 ,  200 ,  300 ,  400  and  500  of the present disclosure capable of providing different resistance to ensure the overall balance of the window covering  1  while extending or retracting the window covering  1 . The resistance provided by the friction members  170 ,  270 ,  370 ,  470  and  570  is constant, and is provided accurately at the time when the user executes dragging-down operation, which is carried out by collaboration of the first limiting members  181 ,  281 ,  381 ,  481  and  581  as well as the second limiting members  182 ,  282 ,  382 ,  482  and  582 . The constant resistance is beneficial to prevent the bottom rail  30  from uncontrollably falling down, which makes the user easier to determine the moving speed and moving distance of the bottom rail  30 . Meanwhile, because the way that the cord retractors  100 ,  200 ,  300 ,  400  and  500  provide resistance is limited to single direction, the user can be easier to move the bottom rail  30  in another direction as the resistance is not provided then. Moreover, the maximum static friction could be generated by the friction members  170 ,  270 ,  370 ,  470  and  570  is set to be always greater than the difference obtained by subtracting a rewinding force provided by the springs  160 ,  260 ,  360 ,  460  and  560  from a downward force generated by the weight of the bottom rail  30  and the covering materials  20  stacked thereon. Therefore, no matter when the user stops dragging down the bottom rail  30  and where the bottom rail  30  is stopped, the bottom rail  30  is stably maintained in the latest position, therefore a shading effect can be provided accurately. 
     In all above-mentioned embodiments, the cord retractor is disposed on the upper rail, and the way of operating the window covering is exemplified by operating the bottom rail. However, in some other embodiments, the window covering may further comprise a middle rail, and the cord retractor can be disposed on the middle rail or the bottom rail other than the upper rail. In addition, the user can operate the window covering by any movable rail other than the one on which the cord retractor is disposed. 
     The embodiments described above are only some exemplary embodiments of the present disclosure. All equivalent structures which employ the concepts disclosed in this specification and the appended claims should fall within the scope of the present disclosure.