Abstract:
A child seat for a vehicle which can be firmly secured to a vehicle seat even with a shoulder belt of a three-point seat belt and which can wind up the seat belt to securely apply a predetermined tension to the seat belt. The child seat includes a child seat and a shaft attached to the child seat. The shaft is rotatable about an axis of the shaft. Additionally, a first end of the shaft is disposed at a higher elevation than a second end of the shaft. The shaft further includes at least one slit configured to accept a seat belt.

Description:
BACKGROUND  
         [0001]    The present invention relates to a child seat which is mounted and secured to a vehicle seat by a seat belt.  
           [0002]    An example of this type of child seat has been disclosed in U.S. Pat. No. 5,839,789, herein incorporated by reference. FIG. 8 is a rear view of a child seat of U.S. Pat. No. 5,839,789, and FIG. 9 is a perspective view showing the structure for an adult seat belt winding mechanism at the bottom of the child seat.  
           [0003]    The child seat  110  is mounted and secured to a vehicle seat (not shown) with an adult seat belt  100  (comprising a lap belt  100   a  and a shoulder belt  100   b ) of a vehicle and comprises a seat squab  112  on which a child is seated, a seat back  114 , and a pair of dogleg-shaped side walls  116 ,  118  disposed on both sides of the combination of the seat squab  112  and the seat back  114 . The dogleg-shaped side walls  116 ,  118  are provided with seat belt through apertures  120 ,  122 , respectively, to allow the adult seat belt  100  to extend in the width direction of the child seat through these apertures  120 ,  122 . The child seat  110  is also provided on the bottom thereof (the backside of the seat squab  112 ) with a wind-up reel  124  for winding the seat belt  100  extending between the seat belt through apertures  120  and  122  to tension the seat belt  100 . The wind-up reel  124  has a slit  124   a , for insertion of the seat belt, formed to penetrate the wind-up reel  124  in a diametrical direction. The slit  124   a  has a deep groove configuration extending in the axial direction of the wind-up reel  124  and exposed at one end of the axial direction.  
           [0004]    The seat belt through apertures  120 ,  122  are formed in lower portions of the dogleg-shaped side walls  116 ,  118  to face each other such that the lap belt  100   a  composing the seat belt  100  extends substantially parallel to the top surface of the seat squab (not shown) of the vehicle seat when the lap belt  100   a  is threaded through one of the through apertures  120 ,  122  and is then threaded through the other.  
           [0005]    The wind-up reel  124  is positioned halfway between the seat belt through apertures  120  and  122  such that the axial direction of the wind-up reel  124  is equal to the front-to-back direction of the child seat  110 . The slit  124   a  is formed in the rear end of the wind-up reel  124 . The rear end of the wind-up reel  124  is exposed at the seat back side of the child seat  110 . In FIGS. 8 and 9, numeral  126  designates guide members for introducing the seat belt  100 , inserted through the seat belt through apertures  120 ,  122 , to the wind-up reel  124 .  
           [0006]    Integrally connected to the wind-up reel  124  is a torsion rod  128  which is disposed to extend along the bottom surface of the child seat  110  in the front-to-back direction. Secured to the front end of the torsion rod  128  is a worm wheel  130  which is meshed with a worm drive  134  described later.  
           [0007]    A worm shaft  132  is disposed adjacent to the worm wheel  130  to extend in a width direction perpendicular to the axial direction of the torsion rod  128  and the wind-up reel  124 . The worm drive  134  is fixed to the worm shaft  132  and is meshed with the worm wheel  130 . The both ends of the worm shaft  132  penetrate the dogleg-shaped side walls  116 ,  118  so as to extend outside the child seat  110  through the right and left side walls  116 ,  118 , respectively. The worm shaft  132  is provided at its both ends with knobs  136 ,  138  for operation of rotating the worm shaft  132  about its axis.  
           [0008]    By the operation of rotating either of the knobs  136 ,  138 , the wind-up reel  124  is rotated through the worm shaft  132 , the worm drive  134 , the worm wheel  130 , and the torsion rod  128 , whereby the seat belt  100  engaged in the slit  124   a  is wound around the wind-up reel  124 . The worm gear composed of the worm wheel  130  and the worm drive  134  has a self-locking function. Therefore, even though the user looses his/her grip of the knob  136  or  138  after the seat belt  100  is wound around the wind-up reel  124 , the wind-up reel  124  is stayed against the tension of the seat belt  100  by the self-locking function not to rotate in a direction opposite to the belt winding direction.  
           [0009]    The child seat  110  is provided with clamps  140 ,  142  for clamping the seat belt  100  which are disposed on both lateral sides of the wind-up reel  124 . Each clamp  140 ,  142  is adapted to allow the seat belt  100  to pass therethrough in a direction for winding up the seat belt  100  around the wind-up reel  124  and not to allow the seat belt  100  to pass therethrough in a direction opposite to the belt winding direction. Since the seat belt  100  is clamped by the clamps  140 ,  142 , the seat belt  100  is prevented from being pulled out through the seat belt through apertures  120 ,  122  even when external force in a direction opposite to the belt winding direction is exerted to the seat belt  100 .  
           [0010]    A release lever  144  for releasing the clamping on the seat belt  100  by the clamps  140 ,  142  is provided at an upper portion of the seat back  114 . The release lever  144  is interconnected to the clamps  140 ,  142  through a cable  146 . As the release lever  144  is lifted, the respective clamps  140 ,  142  open to release the clamping on the seat belt  100 . As the release lever  144  is depressed, the respective clamps  140 ,  142  close so that the seat belt  100  is clamped by the clamps  140 ,  142 . In FIGS. 8 and 9, numeral  148  designates guide members for introducing the cable  146 .  
           [0011]    To secure the child seat  110  having the aforementioned structure to a vehicle seat, the release lever  144  is lifted into its open position to keep the clamps  140 ,  142  open. In this state, the seat belt  100  is threaded through one of the seat belt through apertures  120 ,  122  (in FIG. 8, the through aperture  122 ) and is threaded through the other through aperture  122  or  120  (in FIG. 8, the through aperture  120 ) so as to extend between the through apertures  120  and  122 . Then, a tongue (not shown) is latched into a buckle (not shown).  
           [0012]    The seat belt  100  extending between the through apertures  120  and  122  is inserted into the clamps  140 ,  142  and the halfway of the seat belt  100  is inserted into the slit  124   a  from the rear end of the wind-up reel  124 . Then, the release lever  144  is depressed whereby the seat belt  100  is clamped by the clamps  140 ,  142 .  
           [0013]    After that, either of the knobs  136 ,  138  is turned to rotate the wind-up reel  124  so that the seat belt  100  is wound around the wind-up reel  124 . Therefore, sufficient tension is applied to the seat belt  100 , thereby firmly securing the child seat  110  to the vehicle seat.  
           [0014]    When a three-point seat belt composed of a lap belt  100   a  and a shoulder belt  100   b  is used for securing the child seat  110  disclosed in U.S. Pat. No. 5,839,789, tension is applied to the shoulder belt  100   b  by winding up the shoulder belt  100   b  around the wind-up reel  124  so that the shoulder belt  100   b  tends to straighten between a deflection fitting and the tongue (both are not shown) to apply a raising force to the wind-up reel and the rear portion of the child seat  110  is thus biased upwardly.  
           [0015]    Accordingly, there remains a need for a child seat which can be firmly secured to a vehicle seat even with a shoulder belt of a three-point seat belt.  
           [0016]    In addition, in the child seat  110  disclosed in U.S. Pat. No. 5,839,789, it is difficult to recognize that sufficient tension has been really applied to the seat belt  100  by turning the knob  136 ,  138  to wind up the seat belt  100  around the wind-up reel  124 .  
           [0017]    Accordingly, there remains a need for a child seat which can wind up a seat belt to securely apply a predetermined tension to the seat belt.  
         SUMMARY OF THE INVENTION  
         [0018]    According to an embodiment of the present invention, a child seat is provided which is mounted and secured to a vehicle seat by a seat belt designed primarily for use by an adult occupant (hereinafter, sometimes referred to as “adult seat belt”). More particularly, a child seat which is adapted to be firmly secured to a vehicle seat by winding an adult seat belt around a wind-up shaft to tension the adult seat belt is provided.  
           [0019]    The child seat is adapted to be secured to a seat of a vehicle by a seat belt designed primarily for use by an adult occupant in the vehicle. The child seat includes a wind-up shaft, which has a slit into which the adult seat belt is inserted and which can rotate when the adult seat belt is inserted into the slit so as to wind up the adult seat belt thereby applying tension to the adult seat belt. The wind-up shaft extends in a vertical direction and includes a lap belt slit into which a lap belt of the adult seat belt can be inserted and a shoulder belt slit into which a shoulder belt of the adult seat belt can be inserted. The shoulder belt slit is located above the lap belt slit.  
           [0020]    Since the wind-up shaft extends in a vertical direction and the shoulder belt slit is located above the lap belt slit, the shoulder belt extends substantially straight from a deflective fitting to a tongue when the shoulder belt is inserted in the shoulder belt slit and wound around the wind-up shaft. Therefore, the shoulder belt never applies a lifting force to the child seat, thereby stabilizing the attitude of the child seat.  
           [0021]    According to another embodiment, it is preferable that the shoulder belt slit and the lap belt slit are disposed adjacent to each other and continue into each other via a common belt inlet.  
           [0022]    In this embodiment, even when the lap belt is inserted into the shoulder belt slit for winding up the lap belt, the lap belt moves into the lap belt slit because of the tension applied to the lap belt. Similarly, even when the shoulder belt is inserted into the lap belt slit for winding up the shoulder belt, the shoulder belt moves into the shoulder belt slit because of the tension applied to the shoulder belt.  
           [0023]    According to another embodiment, a child seat is adapted to be secured to a seat of a vehicle by a seat belt designed primarily for use by an adult occupant in the vehicle. The child seat includes a wind-up shaft, which has a slit into which the adult seat belt is inserted and which can rotate when the adult seat belt is inserted into the slit so as to wind up the adult seat belt, thereby applying tension to the adult seat belt. The child seat further includes a winding torque limiting means for preventing winding torque exceeding a predetermined value from being exerted on the wind-up shaft.  
           [0024]    In this embodiment, the seat belt can be wound up to securely apply a predetermined tension to the seat belt. In addition, excessive winding torque is prevented from being exerted on the wind-up shaft.  
           [0025]    According to another embodiment, the child seat preferably includes a knob for rotating the wind-up shaft. It is preferable that the winding torque limiting means forces the knob to idle when a torque exceeding the predetermined value is applied. According to this embodiment, the operator can recognize from the idling of the knob that the seat belt has been wound sufficiently.  
           [0026]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    These and other features, aspects, and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are described briefly below.  
         [0028]    [0028]FIG. 1 is a rear perspective view of a child seat according to an embodiment of the present invention.  
         [0029]    [0029]FIG. 2 is a front perspective view of the child seat in FIG. 1.  
         [0030]    [0030]FIG. 3 is a perspective view showing a rotational mechanism for rotating a wind-up shaft according to an embodiment of the present invention.  
         [0031]    [0031]FIG. 4 is a partial perspective view showing a situation where a seat belt is engaged with a wind-up shaft according to an embodiment of the present invention.  
         [0032]    [0032]FIG. 5 is a partial perspective view showing a situation where a seat belt is engaged with a wind-up shaft according to an embodiment of the present invention.  
         [0033]    [0033]FIG. 6 is a rear perspective view of a child seat according to an embodiment of the present invention in which a seat belt is engaged with a wind-up shaft.  
         [0034]    [0034]FIG. 7 is a rear perspective view of a child seat according to an embodiment of the present invention in which a seat belt is engaged with a wind-up shaft.  
         [0035]    [0035]FIG. 8 is a rear view of a child seat of U.S. Pat. No. 5,839,789, as a conventional example.  
         [0036]    [0036]FIG. 9 is a perspective view of a conventional child seat showing the structure of a wind up shaft rotational mechanism at the bottom of the child seat.  
         [0037]    [0037]FIG. 10 is a perspective view of a knob according to an embodiment of the present invention in which the knob accommodates a finger grip.  
         [0038]    [0038]FIG. 11 is a perspective view of the knob of FIG. 10 where the finger grip is in use.  
         [0039]    [0039]FIG. 12 is an exploded front perspective view of the knob of FIG. 10 showing the knob and torque clutch.  
         [0040]    [0040]FIG. 13 is an exploded rear perspective view of the knob of FIG. 10 showing the knob and torque clutch.  
         [0041]    [0041]FIG. 14( a ) is a plan view of a knob according to an embodiment of the present invention showing operation of a torque clutch when the torque applied to the knob is equal to a predetermined value or less.  
         [0042]    [0042]FIG. 14( b ) is a sectional view of the knob in FIG. 14( a ) taken along the line B-B.  
         [0043]    [0043]FIG. 14( c ) is an enlarged view of portion C in FIG. 14( b ).  
         [0044]    [0044]FIG. 15( a ) is a plan view of a knob according to the present invention showing operation of a torque clutch when the torque applied to the knob exceeds a predetermined value.  
         [0045]    [0045]FIG. 15( b ) is a sectional view of the knob of FIG. 15( a ) taken along the line B-B.  
         [0046]    [0046]FIG. 15( c ) is an enlarged view of portion C in FIG. 15( b ).  
     
    
     DETAILED DESCRIPTION  
       [0047]    Embodiments of the present invention will be described below with reference to the accompanying drawings.  
         [0048]    As shown in FIGS.  1 - 7 , a child seat  10  is adapted to be mounted and secured to a vehicle seat (not shown) with an adult seat belt  1  (comprising a lap belt  1   a  and a shoulder belt  1   b ) of a vehicle.  
         [0049]    The seat belt  1  is a well known seat belt of which a proximal end portion is connected to a retractor such that the seat belt can be wound up by the retractor; a distal end portion is connected to a vehicle body via a lap anchor; and a midway portion passes through a deflection fitting. The seat belt  1  passes through a belt through aperture of a tongue (not shown). A portion of the seat belt between the tongue and the deflection fitting is defined as the shoulder belt  1   b  and a portion of the seat belt between the tongue and the lap anchor is defined as the lap belt  1   a . As well known in the art, in the state that the tongue is latched into the buckle, the shoulder belt  1   b  extends obliquely from the tongue to the deflection fitting across the seat back. The lap belt  1   a  extends substantially horizontally from the tongue to the lap anchor across a corner between the seat squab and the seat back.  
         [0050]    The child seat  10  comprises a seat squab  12  on which a child is seated; a backrest portion  14 ; a pair of ribs  16 ,  18  projecting rearwards from both side edges of the back face of the backrest portion  14 ; adult seat belt through apertures  20 ,  22  which are formed in the ribs  16 ,  18 , respectively; and a wind-up shaft  24  for winding up the adult seat belt which is positioned halfway between the seat belt through apertures  20  and  22 .  
         [0051]    The wind-up shaft  24  extends vertically along the backrest portion  14 . The wind-up shaft  24  has a lap belt slit  24   a  and a shoulder belt slit  24   b  into which the lap belt  1   a  and the shoulder belt  1   b  of the adult seat belt are inserted, respectively. The shoulder belt slit  24   b  is located above the lap belt slit  24   a . The lap belt slit  24   a  and the shoulder belt slit  24   b  are disposed adjacent to each other in a vertical direction and continue into each other via a common belt inlet  24   c.    
         [0052]    As shown in FIGS.  4 - 5 , the lap belt slit  24   a  and the shoulder belt slit  24   b  are formed to penetrate the wind-up shaft  24  in the diametrical direction and to extend in the axial direction (vertical direction) of the wind-up shaft  24 . In this embodiment, the lap belt slit  24   a  and the shoulder belt slit  24   b  are formed linearly in the axial direction of the wind-up shaft  24 . The belt inlet  24   c  is formed at a middle portion between the lap belt slit  24   a  and the shoulder belt slit  24   b  to cut a side peripheral surface of the middle portion of the wind-up shaft  24 .  
         [0053]    The ribs  16 ,  18  extend vertically along the back face of the backrest portion  14 . The seat belt through apertures  20 ,  22  are also formed to extend vertically along the backrest portion  14 . The seat belt through apertures  20 ,  22  have such size and arrangement as to allow the lap belt  1   a  to extend substantially parallel to the upper surface of the seat squab of the vehicle and also allow the shoulder belt  1   b  to extend substantially straight from the deflection fitting to the tongue when the seat belt  1  is threaded through one of the seat belt through apertures  20 ,  22  and is then threaded through the other through aperture  20  or  22  and the tongue is latched into the buckle as shown in FIGS.  6 - 7 .  
         [0054]    The lap belt slit  24   a  is formed at such a level that the lap belt  1   a  can be inserted into the lap belt slit  24   a  while the lap belt  1   a  extends in parallel with the top surface of the seat squab of the vehicle between the seat belt through apertures  20  and  22 . The shoulder belt slit  24   b  is formed at such a level that the shoulder belt  1   b  can be inserted into the shoulder belt slit  24   b  while the shoulder belt  1   b  threaded through the seat belt through apertures  20  and  22  extends substantially straight from the deflection fitting to the tongue.  
         [0055]    The upper end of the wind-up shaft  24  is supported by a bracket  26  attached to the back face of the backrest portion  14  such a manner as to allow the rotation of the wind-up shaft  24  about its axis. The lower end of the wind-up shaft  24  is inserted into a mechanical box  28  arranged at a lower portion of the backrest portion  14 . Within the mechanical box  28 , a worm wheel  30  meshed with a worm drive  34  is fixed to the lower end of the wind-up shaft  24 .  
         [0056]    The mechanical box  28  is arranged between lower end portions of the left and right ribs  16  and  18 . The both side faces of the mechanical box  28  are connected to the opposite faces of the ribs  16 ,  18 , respectively. Through holes (not shown) are formed in the both side faces of the mechanical box  28  and the ribs  16 ,  18 , respectively, so that a worm shaft  32  described later can pass through these through holes.  
         [0057]    As shown in FIG. 3, within the mechanical box  28 , the worm shaft  32  is arranged adjacent to the lower end (worm wheel  30 ) of the wind-up shaft  24  to extend in the width direction perpendicular to the extending direction of the wind-up shaft  24 . The worm drive  34  is fixed to the worm shaft  32  and is meshed with the worm wheel  30 . The both ends of the worm shaft  32  penetrate the both side faces of the mechanical box  28  and the left and right ribs  16 ,  18  so as to extend outside the child seat  10 , respectively. The worm shaft  32  is provided at its both ends with knobs  36 ,  38  for rotating the worm shaft  32  about its axis. The knobs  36 ,  38  are provided with finger grips  40  (see FIGS.  10 - 11  described later) for facilitating the operation of the knobs  36 ,  38 .  
         [0058]    In FIG. 3, numeral  26   a  designates a bearing to which the upper end of the wind-up shaft  24  is rotatably fitted. Numeral  28   a  designates a bracket holding the worm shaft  32 , and numeral  28   b  designates bearings attached to the bracket  28   a  for rotatably supporting the worm shaft  32 .  
         [0059]    By the operation of rotating either of the knobs  36 ,  38 , the wind-up shaft  24  is rotated through the worm shaft  32 , the worm drive  34 , and the worm wheel  30 , whereby the lap belt  1   a  inserted in the lap belt slit  24   a  or the shoulder belt  1   b  inserted in the shoulder belt slit  24   b  is wound around the wind-up shaft  24 . The worm gear composed of the worm wheel  30  and the worm drive  34  has a self-locking function. Therefore, even though the user looses his grip of the knob  36  or  38  after the lap belt  1   a  or the shoulder belt  1   b  is wound around the wind-up shaft  24 , the wind-up shaft  24  is stayed against the tension of the belt  1   a ,  1   b  by the self-locking function so as not to rotate in a direction opposite to the belt winding direction, thereby preventing slack from developing in seat belt  1  after the lap belt  1   a  or the shoulder belt  1   b  is wound up.  
         [0060]    In this embodiment, each knob  36 ,  38  is connected to the worm shaft  32  via a torque clutch  50  with torque limiter for limiting the winding torque. The torque clutch  50  is designed to force the knob  36 ,  38  to idle when the knob  36 ,  38  is rotated in the belt winding direction after the lap belt  1   a  or the shoulder belt  1   b  is sufficiently wound around the wind-up shaft  24  and the preset tension has been exerted on the belt  1   a ,  1   b , thereby preventing winding torque from being further applied to the wind-up shaft  24 .  
         [0061]    The structures of the knob  36 ,  38  and the torque clutch  50  will now be described with reference to FIGS.  10 - 15 ( c ).  
         [0062]    The knob  36  comprises a knob casing  36   a  having a substantial cylindrical container of which the back side is an open end face; a cover  36   b  attached to the open end face of the knob casing  36   a ; and an arm  36   c  for supporting the finger grip  40 , which is disposed on a front end face of the knob casing  36   a . The torque clutch  50  is accommodated within the knob casing  36   a . The worm shaft  32  is inserted into the knob casing  36   a  through an aperture  36   d  of the cover  36   b.    
         [0063]    The knob casing  36   a  has a groove  36   e  formed in the front end face thereof such that the arm  36   c  can be fitted in the groove  36   a  in the diametrical direction of the knob casing  36   a  when the longitudinal direction of the arm  36   c  is equal to the diametrical direction of the knob casing  36   a . One end of the groove  36   e  is exposed to a grip accommodating space  36   f  which is formed by cutting a portion of the periphery of the knob casing  36   a.    
         [0064]    The arm  36   c  has a length substantially equal to the radius of the end face of the knob casing  36   a . An end (proximal end) of the arm  36   c  is pivotally supported within the groove  36   e  at about the center of the end face of the knob casing  36   a . As shown in FIGS.  10 - 11 , the arm  36   c  is pivotable about its proximal end so that the arm  36   c  can be selectively fitted in either of halves of the groove  36   e . The finger grip  40  is rotatably attached to the other end (distal end) of the arm  36   c  such that the finger grip  40  is accommodated in the grip accommodating space  36   f  when the distal end of the arm  36   c  is fitted in the groove  36   e  to face the grip accommodating space  36   f.    
         [0065]    In FIGS.  10 - 11 , numeral  36   g  designates pairs for retaining members for retaining the arm  36   c  in the groove  36   e  in a state that the finger grip  40  is accommodated in the accommodating space  36   f  and in a state that the finger grip  40  is taken out from the accommodating space  36   f  and is in usable condition. In FIG. 12, numeral  40   a  designates a bolt for rotatably attaching the finger grip  40  to the arm  36   c  and numeral  36   h  designates vises for fixing the cover  36   b  to the knob casing  36   a.    
         [0066]    The torque clutch  50  comprises a first clutch disc  52  fixed at the end of the worm shaft  32 , a second clutch disc  54  disposed facing the first clutch disc  52 , and a clutch spring  56  for pressing the second clutch disc  54  against the first clutch disc  52 . The first clutch disc  52  and the second clutch disc  54  have serrations  52   a ,  54   a , composed of triangle convexities and engageable with each other, on the respective opposed surfaces thereof. The serrations  52   a ,  54   a  are arranged on the surfaces of the clutch discs  52 ,  54  to form circles having the same radius coaxially with the worm shaft  32 .  
         [0067]    The first clutch disc  52  is held in a first clutch disc holding space  36   j  formed in the interior side surface of the cover  36   b . The second clutch disc  54  is held in a second clutch disc holding space  36   k  formed in the knob casing  36   a.    
         [0068]    The second clutch disc holding space  36   k  is provided with projections  361  for guiding the second clutch disc  54  in a direction closer to and apart from the first clutch disc  52  and for coupling the second clutch disc  54  and the knob casing  36   a  to rotate together. The projections  361  are slidably engaged with guide grooves  54   b  formed in the periphery of the second clutch disc  54 .  
         [0069]    The clutch spring  56  is compressed and disposed between the second clutch disc  54  and the bottom surface of the second clutch disc holding space  36   k . As the knob casing  36   a  is rotated, the second clutch disc  54  is rotated together. Though the serrations  54   a  of the second clutch disc  54  tend to cross over the serrations  52   a  of the first clutch disc  52  so that the second clutch disc  54  tends to come off the first clutch disc  52 , the clutch spring  56  has such a biasing force (spring constant) as to press the second clutch disc  54  not to come off the first clutch disc  52  so as to prevent the serration  54   a  from crossing over the serrations  52   a  until rotational torque exceeding a preset value (for example, 10 kgf-cm) is applied to the knob  36 .  
         [0070]    In the torque clutch  50  having the aforementioned structure, when the rotational torque applied to the knob  36  is equal to the preset value or less, the serrations  54   a  do not cross over the serrations  52   a  so as to keep the engagement between the first clutch disc  52  and the second clutch disc  54  because of the biasing force of the clutch spring as shown in FIGS.  14 ( b )- 14 ( c ). Therefore, the first clutch disc  52  and the second clutch disc  54  are rotated together so that the rotational torque applied to the knob  36  is transmitted to the worm shaft  32 , whereby the worm shaft  32  is rotated.  
         [0071]    When the rotational torque applied to the knob  36  exceeds the preset value, the second clutch disc  54  comes off the first clutch disc  52  while the serrations  54   a  cross over the serrations  52   a  against the biasing force of the clutch spring  56  as shown in FIGS.  15 ( b )- 15 ( c ). Therefore, the knob  36  idles so as to prevent the rotational torque applied to the knob  36  from being transmitted to the worm shaft  32 .  
         [0072]    The steps of installing the child seat  10  according to an embodiment of the present invention to a vehicle seat will now be described.  
         [0073]    In case using an adult seat belt  1  composed of only a lap belt  1   a , i.e., a so-called two-point seat belt, the child seat  10  is put on the vehicle seat. After that, the lap belt  1   a  is threaded through one of the seat belt through apertures  20 ,  22  (in FIG. 7, the through aperture  22 ) and is threaded through the other through aperture  22  or  20  (in FIG. 7, the through aperture  20 ) so as to extend between the through apertures  20  and  22  as shown in FIG. 7. A tongue connected to the end of the lap belt  1   a  is then latched into a buckle. After that, the halfway of the lap belt  1   b  extending between the through apertures  20  and  22  is inserted into the lap belt slit  24   a  through the belt inlet  24   c.    
         [0074]    After that, either of the knobs  36 ,  38  is turned to rotate the wind-up shaft  24  so that the lap belt  1   a  is wound around the wind-up shaft  24  until sufficient tension is applied to the lap belt  1   a , thereby firmly securing the child seat  10  to the vehicle seat with the lap belt  1   a.    
         [0075]    In case using an adult seat belt  1  composed of a lap belt  1   a  and a shoulder belt  1   b , i.e., a so-called three-point seat belt, the child seat  10  is put on the vehicle seat. After that, the lap belt  1   a  and the shoulder belt  1   b  are threaded through one of the seat belt through apertures  20 ,  22  (in FIG. 6, the through aperture  22 ) and are threaded through the other through aperture  22  or  20  (in FIG. 6, the through aperture  20 ) so as to extend between the through apertures  20  and  22  as shown in FIG. 6. A tongue is then latched into a buckle. After that, the halfway of the shoulder belt  1   b  extending between the through apertures  20  and  22  is inserted into the shoulder belt slit  24   b . In this case, the lap belt  1   a  is not inserted into the lap belt slit  24   a.    
         [0076]    After that, either of the knobs  36 ,  38  is turned to rotate the wind-up shaft  24  so that the shoulder belt  1   b  is wound around the wind-up shaft  24  until sufficient tension is applied to the lap belt  1   a  and the shoulder belt  1   b , thereby firmly securing the child seat  10  to the vehicle seat with the lap belt  1   a  and the shoulder belt  1   b.    
         [0077]    In the child seat  10  in this embodiment, the wind-up shaft  24  extends in a vertical direction and is provided with the lap belt slit  24   a  at such a level that the lap belt  1   a  can be inserted into the lap belt slit  24   a  while the lap belt  1   a  extends in substantially parallel with the seat squab of the vehicle between the seat belt through apertures  20  and  22  and further provided with the shoulder belt slit  24   b  at such a level that the shoulder belt  1   b  can be inserted into the shoulder belt slit  24   b  while the shoulder belt  1   b  threaded through the seat belt through apertures  20  and  22  extends substantially straight from the deflection fitting to the tongue.  
         [0078]    Since the shoulder belt  1   b  thus extends substantially linearly from the deflection fitting to the tongue after the shoulder belt  1   b  is inserted into the shoulder belt slit  24   b  and wound around the wind-up shaft  24 , the shoulder belt  1   b  never applies a lifting force to the child seat  10 , thereby stabilizing the attitude of the child seat  10 .  
         [0079]    In this embodiment, the lap belt slit  24   a  and the shoulder belt slit  24   b  are formed to continue in a vertical direction into each other via the common belt inlet  24   c . Therefore, even when the lap belt  1   a  is inserted into the shoulder belt slit  24   b , the lap belt  1   a  moves into the lap belt slit  24   a  because of the tension by the seat belt retractor. Similarly, even when the shoulder belt  1   b  is inserted into the lap belt slit  24   a , the shoulder belt  1   b  moves into the shoulder belt slit  24   b  because of the tension by the seat belt retractor. After that, the lap belt  1   a  or the shoulder belt  1   b  can be smoothly wound.  
         [0080]    In this embodiment, the knobs  36 ,  38  are connected to the worm shaft  32  via the torque clutches  50  so that the knobs  36 ,  38  are adapted to idle after the lap belt  1   a  or the shoulder belt  1   b  is wound around the wind-up shaft  24  enough and the preset tension has been applied to the lap belt  1   a  and the shoulder belt  1   b , thereby preventing excessive winding torque from being exerted on the wind-up shaft  24 .  
         [0081]    Since the torque clutches  50  are installed in the knobs  36 ,  38 , respectively, the operator can easily recognize from vibration and sound generated by idling of the knob  36 ,  38  that the belt  1   a ,  1   b  has been wound to obtain the preset tension.  
         [0082]    As the aforementioned embodiment is an example of the present invention, it is to be understood that the invention is not limited to the aforementioned embodiment thereof. For example, the torque limiter for limiting the winding torque may be of a type besides such a type as the torque clutches  50  that physically stops the further winding of the seat belt when the tension on the seat belt reaches the preset value. For instance, a torque sensor (strain gauge) is provided for detecting the winding torque exerted on the wind-up shaft, and an indicator is provided which emits light when the torque detected by the torque sensor reaches to a preset value or a monitor is provided which indicates numerically the torque detected by the torque sensor to let the operator know the completion of the winding of the seat belt. In addition, a means for changing the preset value may be provided.  
         [0083]    As described in the above, a child seat according to an embodiment of the present invention can be firmly secured to a vehicle seat even with a shoulder belt of a three-point seat belt.  
         [0084]    In addition, with a child seat according an another embodiment of the present invention, the seat belt can be wound up to securely apply a predetermined tension to the seat belt.  
         [0085]    The priority documents, Japanese Patent Application No. 2002-299201, filed Oct. 11, 2002, and Provisional Application No. 60/430,439, filed Dec. 3, 2002, are hereby incorporated by reference.  
         [0086]    Given the disclosure of the present invention, one versed in the art would appreciate that there may be other embodiments and modifications within the scope and spirit of the invention. Accordingly, all modifications attainable by one versed in the art from the present disclosure within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is to be defined as set forth in the following claims.