CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from Japanese Patent Application Hei 11-125679 filed on May 6, 1999, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electric motor having a worm mechanism, which is used in a power drive system such as a vehicle power window system. 
     2. Description of the Related Art 
     A motor used in a power window system has a speed reduction worm mechanism. The worm mechanism has a worm shaft which is coaxially linked to the armature shaft of the motor. When the motor rotates, the rotation speed of the motor is transmitted to the worm shaft and reduced by the worm mechanism. Thus, torque is increased and transmitted to a window regulator to open or close a window. 
     In the above stated motor, axial movement of the armature shaft and the worm shaft is not effectively restricted, resulting in vibration of the shafts. 
     SUMMARY OF THE INVENTION 
     A main object of the invention is to provide an improved motor having a worm gear mechanism which can prevent the above problem. 
     According to a main feature of the invention, a motor is comprised of a motor section, a clutch, and an output section. The motor section includes an armature having an armature shaft, and the output section includes a worm mechanism having a worm shaft disposed coaxially with the armature shaft, and a spring for biasing the worm shaft to separate from the armature shaft. The clutch is disposed between the armature shaft and the worm shaft and includes a stationary housing, a rotatable driving member and a rotatable driven member in engagement with each other, and rollers disposed between the driven member and the housing. The drive member controls location of the rollers to restrict the driven member when the driven member rotates the drive member and to free the driven member when the drive member rotates the driven member. The spring can urge rollers in the axial direction, and no additional biasing means is necessary. 
     Since the motor section and the output section are connected by the clutch, they can be manufactured separately. 
     According to another feature of the invention, a motor comprising includes a motor housing having a plurality of permanent magnets, an armature having an armature shaft disposed in the motor housing, a worm mechanism having a worm shaft disposed coaxially with the armature shaft, and a one way clutch disposed between the armature shaft and the warm shaft. The clutch includes teeth members in engagement with each other, a plurality of rollers for controlling the teeth members, and a spring for not only holding the rollers but also urging the warm shaft to separate from the armature shaft. 
     The armature shaft can be also urged by permanent magnets to separate from the worm shaft. As a result, there is no interference between the armature shaft and the worm shaft, so that vibration can be eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
     FIG. 1 is an exploded perspective view of a motor according to a first embodiment of the invention; 
     FIG. 2 is a cross-sectional longitudinal view of the motor according to the first embodiment; 
     FIG. 3 is an exploded perspective view of a clutch of the motor according to the first embodiment; 
     FIG. 4 is a cross-sectional plan view of the clutch; 
     FIG. 5 is a cross-sectional side view of the clutch; 
     FIGS. 6A and 6B are fragmentary cross-sectional plan views of the clutch; 
     FIGS. 7A and 7B are fragmentary cross-sectional plan views of the clutch; 
     FIGS. 8A and 8B are fragmentary cross-sectional plan view of the clutch; 
     FIGS. 9A and 9B are fragmentary cross-sectional plan view of the clutch; 
     FIG. 10 is a schematic diagram illustrating a power window system for a vehicle; 
     FIG. 11 is a fragmentary cross-sectional side view of a clutch of a motor according to a second embodiment of the invention; and 
     FIG. 12 is a perspective view of a driven member of a motor according to the second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
     A motor according to a first embodiment of the invention is described with reference to FIGS. 1-10. 
     As illustrated in FIG. 10, motor  1  is mounted in a power window system of a vehicle door  2 . Motor  1  is comprised of motor section  5  and output section  6  which has output shaft  7 . Rotation of motor section  5  is transmitted from gear  7   a , which is formed on output shaft  7 , via gear  8   a  which is formed on an arm of window regulator  8 . Window regulator  8  opens window  9  if motor  1  rotates in one direction, while it closes window  9  if motor  1  rotates in the other direction. 
     As shown in FIG. 2, motor section  5  is comprised of cylindrical motor housing  11 , a pair of permanent magnets  12 , armature shaft  13 , armature  14 , commutator  15 , brush holder  16  and a pair of brushes  17 . Motor housing  11  has a bottom which supports armature shaft  13  via bearing  18  at the center thereof. The pair of permanent magnets  12  is fixed to opposite portions of the inner periphery of motor housing  11  around armature  14 . Armature shaft  13  is supported by thrust bearing  19  at one end, and has D-shaped cut portion  13   a  at the other end thereof. Armature  14  is axially positioned in the magnetic field formed by permanent magnets  12  so that the end of armature shaft  13  can urge thrust bearing  19  downward in FIG.  2 . Commutator  15  is fixed to the upper end of armature  14 . 
     Motor housing  11  has an open end, to which brush holder  16  is force-fitted. Brush holder  16  has bearing  20  at the center thereof to rotatably support the other end of armature shaft  13 . 
     The pair of brushes  17  is held in brush holder  16  to be in contact with commutator  15 . Electric current is supplied to armature  14  by the pair of brushes  17  through commutator  15 . 
     Clutch  21  is disposed at the front end of motor section  5 . As shown in FIGS. 3-5, clutch  21  is comprised of clutch housing  22 , resin-made drive member  23 , steel ball  24 , driven member  25 , three rollers  26 , and brass-made spring washer  27 . 
     Clutch housing  22  has cylindrical outer wall  22   a  and bottom  22   b  having center hole  22   c.    
     Drive member  23  has shaft portion  23   a , female coupler portion  23   b , and center hole  23   c . Center hole  23   c  has D-shaped cut portion  23   d  at its lower half as shown in FIG. 3, so that D-shaped cut portion  13   a  of armature shaft can be fitted thereto. Accordingly, the motor rotation can be transmitted to drive member  23 . 
     Female coupler portion  23   b  has three tooth members  31  and three slots  34  formed between tooth members  31 , forming female coupling surface  32 . Each tooth member  31  has internal tooth  31   a  and semi-cylindrical wall  31   b . Internal tooth  31   a  extends inwardly from the middle of the wall to form first side surface  32   a  and second surface  32   b  on both sides of internal tooth  31   a , as shown in FIG.  4 . Each wall  31   b  has an outer periphery of radius R 1  and an inner periphery of radius R 2 . Circumferential width of each slot  34  is wider than thickness W 1 , which is the difference between radiuses R 1  and R 2 . 
     Driven member  25  has male coupler portion  25   a , cylindrical shaft portion  25   b , as shown in FIG. 5, extending from one end thereof, and generally square shaft  25   c  extending from the other end. Shaft  25   c  can be substituted by a shaft having D-shape or other angular cross-section. 
     Cylindrical shaft portion  25   b  is fitted to center hole  23   c , after steel ball  24  is inserted therein, that is, between shaft portion  25   b  and armature shaft  13 . 
     Male coupler portion  25   a  has fan-shaped three teeth  35  extending radially outward at circumferentially equal intervals. Each tooth  35  has semi-cylindrical outer periphery of radius R 2 , concave control surface  35   c  having the deepest bottom of radius R 5  (which is smaller than R 2 ) formed at the middle of the outer periphery. Each tooth  35  has a circumferential width that is smaller than the distance between teeth  31  of female coupler  23   b , so that male coupler portion  25   a , having first side surface  35   a  and second side surface  35   b , can be loosely fitted into female coupling surface  32  between first side surface  32   a  and second side surface  32   b  thereof, as shown in FIG.  4 . Cylindrical rollers  26  are respectively disposed in slots  34  to be parallel with the axis of center hole  23   c  in spaces between concave surface (or concave control surface)  35   c  of male coupler portion  25   a  and inner periphery of clutch housing  22 . Each roller  26  has outside diameter D 1  which is larger than thickness W 1  of semi-cylindrical wall  31   b  and chamfered edges  26   a , as shown in FIGS. 3 and 4. The width of tooth  35  is wider than the width of slot  34 . 
     Drive member  23 , coupled with driven member  25 , is loosely fitted into clutch housing  22  with shaft portion  23   a  being rotatably inserted into center hole  22   c.    
     Spring washer  27  has lower ring portion  27   a  and upper conical spring portion  27   b . The outside diameter thereof is approximately equal to the inside diameter of clutch housing  22  so that spring washer  27  is force-fitted into clutch housing  22 . Conical spring portion  27   b  has a center hole through which shaft  25   c  passes. Rollers  26  are axially held between bottom  22   b  of clutch housing  22  and spring washer  27 . 
     The operation of clutch  21  will be described later. 
     Output section  6  is comprised of gear housing  41 , gear wheel  42 , rubber cushion  43 , output plate  44 , cover plate  45 , and output shaft  7 . 
     Gear housing  41  has worm housing portion  51 , wheel housing portion  52 , and flange  41   a , which is connected to motor housing  11 . 
     Worm housing portion  51  is a cylindrical member having a bottom at the upper end thereof. Worm housing portion  51  accommodates worm shaft  53  having worm  53   a  and supports the same at the upper and lower ends thereof via cylindrical sliding bearings  54  and  55 . The upper end of worm shaft  53  is also supported by thrust bearing  56 . The lower end of worm shaft  53  has generally square hole  53   b , to which square shaft  25   c  of driven member  25  is fitted. Thus, worm shaft  53  is driven by driven member  25 . Worm housing portion  51  also has cylindrical extension  57  at the lower end thereof near bearing  53 . Cylindrical extension  57  is fitted into stationary clutch housing  22 . The lower end of worm shaft  53  abuts conical spring portion  27   b  so that worm shaft  53  is urged by spring washer  27  toward thrust washer  56 , as shown in FIGS. 2 and 5. 
     Wheel housing portion  52  is also a cylindrical member having a bottom which has cylindrical sliding wall  52   a  at the center thereof. Sliding wall  52   a  has center hole  52   b  to which output shaft  7  is rotatably inserted. 
     Gear wheel  42  has worm wheel portion  42   a  in mesh with worm  53   a , cylindrical wall  42   b  formed at the center thereof, and three U-shaped partitions  42   d  formed around cylindrical wall  42   b  at equal intervals (i.e. 120 degree in angle). Cylindrical wall  42   b  has center hole  42   c , to which sliding wall  52   a  is rotatably inserted. Partitions  42   d  define three compartments X which are circumferentially connected with each other by grooves Y. 
     Rubber cushion  43  has three fan-shaped cushion portions  43   a  and connection ring portion  43   b . Each cushion portion  43   a  has radially extending groove  43   c  and is fitted to one of the compartments X, so that rubber cushion  43  can rotate together with gear wheel  42 . 
     Output plate  44  is a metal plate which has three arms  44   a  cut therefrom to extend downward and center hole  44   b , as shown in FIG.  1 . Each arm  44   a  engages one of grooves  43   c  of rubber cushion  43 , so that gear wheel  42  can rotate output plate  44  via rubber cushion  43 . Center hole  44   b  has four notches formed at equal intervals so that the end of output shaft  7  is fitted thereto, as shown in FIG.  2 . Thereafter, output shaft  7  is rotated by output plate  44 . 
     Wheel housing portion  52  is covered by disk-like plate cover  45  having four claws  45   a  which extend radially outward. Each claw  45   a  is bent radially inward against the outer periphery of wheel housing portion  52 . Accordingly, plate cover  45  restricts axial movement of output shaft  7 . 
     Gear  7   a  of output shaft  7  meshes window regulator  8 , as described previously. 
     The operation of clutch  21  is described with reference to FIGS. 6A-9B. 
     As shown in FIG. 6A, if drive member  23  rotates clockwise, and first side surface  32   a  of drive member  23  presses first side surface  35   a  of driven member  25 , roller  26  abuts side surface  34   a  of the semi-cylindrical wall. If drive member  23  rotates counter-clockwise, and second side surface  32   b  of drive member  23  pushes second side surface  35   b  of driven member  25 , roller  26  abuts side surface  34   b  of the semi-cylindrical wall. Thus, roller  26  is located at the middle of concave surface  35   c . Although the outside diameter D 1  of roller  26  is larger than the thickness W 1  of the semi-cylindrical wall, roller  26  is movably disposed on the concave surface  35   c , so that drive member  23  and driven member  25  can rotate relative to clutch housing  22 . This condition is hereinafter referred to as neutral. 
     On the other hand, if driven member  25  rotates clockwise or counter-clockwise first, roller  26  is left behind the middle of concave surface  35   c  and sandwiched between concave surface  35   c  and the inner periphery of stationary clutch housing  22 , as shown in FIGS. 7A and 7B. As a result, drive member  23  is not driven by driven member  25 . 
     If motor  1  rotates clockwise when roller  26  is caught as shown in FIG. 7A, drive member  23  rotates clockwise so that first side surface  32   a  thereof pushes first side surface  35   a  of driven member  25 , as shown in FIG.  8 A. As a result, driven member  25 , together with drive member  23 , rotates clockwise, and roller  26  is freed. Roller  26  is controlled to stay neutral by side surfaces  34   a  and  34   b  of the semi-cylindrical walls. 
     If motor  1  rotates counter-clockwise while roller  26  is caught as shown in FIG. 7A, drive member  23  rotates counter-clockwise so that side surface  34   b  of the semi-cylindrical wall pushes and frees roller  26 , as shown in FIG.  9 A. Then, second side surface  32   b  of drive member  23  pushes second side surface  35   b  of driven member  25  to rotate driven member  25  counter-clockwise. Thereafter, roller  26  is controlled to stay neutral by side surfaces  34   a  and  34   b  of the semi-cylindrical walls. 
     On the other hand, if motor  1  rotates counter-clockwise while roller  26  is caught as shown in FIG. 7B, drive member  23  rotates counter-clockwise so that second side surface  32   b  thereof pushes second side surface  35   b  of driven member  25 , as shown in FIG.  8 B. As a result, driven member  25 , together with drive member  23 , rotates counter-clockwise, and roller  26  is freed. Roller  26  is controlled to stay neutral by side surfaces  34   a  and  34   b  of the semi-cylindrical walls. 
     If motor  1  rotates clockwise while roller  26  is stopped as shown in FIG. 7B, drive member  23  rotates clockwise so that side surface  34   a  of the semi-cylindrical wall pushes roller  26  to be free, as shown in FIG.  9 B. Then, first side surface  32   a  of drive member  23  pushes first side surface  35   a  of driven member  25  to rotate driven member  25  clockwise. Thereafter, roller  26  is controlled to stay neutral by side surfaces  34   a  and  34   b  of the semi-cylindrical walls. 
     If a force is exerted on a window pane of window  9 , torque is transmitted through output shaft  7 , output plate  44 , rubber cushion  43 , gear wheel  42 , and worm shaft  53  to driven member  25 . However, roller  26  is soon sandwiched between concave surface  35   c  and the inner periphery of clutch housing  22 . As a result, driven member  25  is stopped from further rotating, and window is not opened further. 
     (Second Embodiment) 
     A motor according to a second embodiment of the invention is described with reference to FIGS. 11 and 12. 
     The same reference numeral as the first embodiment corresponds to the same or substantially the same portion or component. 
     Spring washer  27  is substituted by coil spring  62  and resinous retainer plate  63  in this embodiment. Square shaft  25   c  has spring chamber  61 , which accommodates coil spring  62  within square hole  53   b  of worm shaft  53 . Clutch housing  22  and driving member  23  are linked with each other at bottom  22   b  and female coupler  23   b . Clutch housing  22  is fixed to cylindrical housing  57 . 
     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention in this document is to be regarded in an illustrative, rather than restrictive, sense.
A motor includes a motor section, a one-way clutch and an output section. The motor section includes an armature and armature shaft, and the output section includes a worm mechanism having a worm shaft disposed coaxially with the armature shaft. The one-way clutch is disposed between the armature shaft and the warm shaft. The one-way clutch includes a couple of teeth members in engagement with each other, a plurality of rollers for controlling the teeth members, and a spring holding the rollers. The spring urges the worm shaft to separate from the armature shaft.