Abstract:
The present invention relates to a power transmission device for a water pump, including a pulley, a magnetic flux generator, a disk assembly, and a hub assembly, wherein the disk assembly has a disk, a friction ring for providing a frictional contact with the hub assembly, and a resilient ring adapted to pressurize the friction ring toward the hub assembly, so that when the attracting magnetic flux of the magnetic flux generator is not applied, the friction ring makes a frictional contact with the hub assembly through the resilient pressurization of the resilient ring, whereas, when it is applied, the disk moves toward the pulley to allow the frictional contact between the friction ring and the hub assembly to be released.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of pending International Patent Application PCT/KR2010/008900 filed on Dec. 13, 2010, which designates the United States and claims priority of Korean Patent Application No. 10-2009-0129985 filed on Dec. 23, 2009, Korean Patent Application No. 10-2009-0129986 filed on Dec. 23, 2009 and Korean Patent Application No. 10-2010-0089584 filed on September 13, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a power transmission device for a water pump, and more particularly, to a power transmission device for a water pump for cooling an automobile engine, wherein power regulation between a disk and a hub in the water pump can be accurately performed, rotation synchronization between them can be stably achieved, and the damage of the parts can be prevented. 
     BACKGROUND OF THE INVENTION 
     A water pump for a vehicle is adapted to pressurize and transmit cooling water to an engine, so that the cooling water is circulated in the engine and the heat of the engine is thus released. In the same manner as a compressor of an air conditioner, the water pump is activated by the rotary power applied from the engine. That is, the water pump is generally operated together with the activation of the engine, thereby supplying the cooling water to the engine. 
       FIG. 1  is an exploded perspective view showing a power transmission device for a water pump in a conventional practice, wherein the conventional power transmission device for a water pump includes a pulley  60  adapted to receive the rotary power of an engine through a belt mounted thereon. 
     The pulley  60  has a field coil assembly-mounting space  64  formed on one side surface (on the right side in the drawing) so as to mount a field coil assembly  50  into which a field coil  51  is embedded thereon. The pulley  60  also has a moving space  69  formed on the opposite side to the field coil assembly-mounting space  64 , in which a disk  70  is movably mounted. 
     If power source is applied to the field coil  51 , the disk  70  is moved to the pulley  60  by means of an attracting magnetic flux generated from the field coil  51  and is thus separated from a hub assembly  80 . Contrarily, if the power source applied to the field coil  51  is removed, the disk  70  is returned to its original position and is thus coupled to the hub assembly  80 . 
     A resilient member  74  attached to the periphery of the disk  70  is fixed to the moving space  69  of the pulley  60  to apply a resilient force to the disk  70 , so that the disk  70  is brought into close contact with the hub assembly  80 . When the disk  70  is moved to the pulley  60  by means of the generation of the attracting magnetic flux, resilient deformation occurs around the periphery of the resilient member  70  fixed to the pulley  60 , so that the disk  70  is separated from the hub assembly  80 . 
     The disk  70  has a plurality of rivets  76  coupled thereto in such a manner as to be protruded toward the hub assembly  80 . The rivets  76  have interlocking grooves  77  formed at the front ends thereof, and the interlocking grooves  77  are lockedly coupled to interlocking protrusions  84  formed on the side surface of an interlocking plate  82  of the hub assembly  80 . Each interlocking groove  77  has a shape corresponding to each interlocking protrusion  84 , and as the interlocking groove  77  is lockedly fitted to the interlocking protrusion  84 , the disk  70  and the hub assembly  80  can be rotated together. 
     The hub assembly  80  has a hub  86  disposed at the center thereof, and the hub  86  has the rotary shaft for driving the water pump press-fitted thereto, so that the hub  86  rotates as the rotary shaft rotates. 
     According to the conventional power transmission device for the water pump under the above-mentioned configuration, if power source is applied to the field coil  51 , the field coil  51  generates the attracting magnetic flux therefrom, which is applied to the disk  70  and exceeds the resilient force of the resilient member  74 , so that the disk  70  is moved to the pulley  60  within the moving space  69 . Accordingly, the lock fitting between the rivets  76  and the interlocking protrusions  84  is released, and the disk  70  is thus separated from the hub assembly  80 . Even though the pulley  60  is rotated by means of the power applied from the engine, therefore, the power is not transmitted to the rotary shaft mounted on the hub assembly  80 , and thus, the cooling water is not supplied to the engine. 
     So as to supply the cooling water to the engine, the application of the power to the field coil  51  shuts off. As a result, the attracting magnetic flux is not generated from the field coil  51 , and the disk  70  is moved to the direction being distant from the pulley  60  within the moving space  69  by means of the resilient force of the resilient member  74 . Accordingly, the interlocking grooves  77  of the rivets  76  are lockedly fitted to the interlocking protrusions  84  of the hub assembly  80 , and thus, the hub assembly  80  is coupled to the disk  70 , thereby being rotated together with the disk  70 . Furthermore, an impeller coupled to one end of the hub assembly is rotated together with the rotary shaft, thereby supplying the cooling water to the engine. 
     Like this, if the power transmitted from the pulley  60  to the disk  70  is regulated through the application of the power source to the field coil  61  or the shut-off of the application of the power source, the power regulation can be performed rapidly and accurately, and at the same time, the rotation synchronization can be achieved at a high speed and stably. 
     According to the conventional power transmission device for a water pump, however, some problems occur when the interlocking grooves  77  of the rivets  76  of the disk  70  are lockedly fitted to the interlocking protrusions  84  on the interlocking plate  82  of the hub assembly  80 . Through the movement of the disk  70  toward the hub assembly  80 , in more detail, when the rivets  76  being rotated are brought into contact with the interlocking plate  82  being in a stop state, the interlocking grooves  77  should be lockedly fitted to the interlocking protrusions  84  in a reliable manner, but the lock fitting is not carried out momentarily and accurately, thereby making the synchronization typically delayed. In this process, the interlocking grooves  77  and the interlocking protrusions  84  have just matching coupling to each other, thereby generating abnormal noises during the lock fitting process and causing the wearing and damage on the interlocking grooves  77  and the interlocking protrusions  84 . These problems undesirably make the reliability in the operation of the power transmission device deteriorated badly. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made in view of the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide a power transmission device for a water pump for cooling an automobile engine, wherein power regulation between a disk and a hub in the water pump can be rapidly and accurately performed, while the wearing and damage of the parts can be prevented. 
     To accomplish the above object, according to the present invention, there is provided a power transmission device for a water pump, the device including: a pulley being rotated by a torque applied from an engine and having a magnetic flux generating means-mounting portion formed on one side surface thereof and a disk assembly-mounting portion formed on the opposite side surface to one side surface thereof; a magnetic flux generating means mounted on the magnetic flux generating means-mounting portion of the pulley to generate an attracting magnetic flux therefrom; a disk assembly mounted on the disk assembly-mounting portion of the pulley and adapted to have frictional contact with a hub assembly; and the hub assembly having a drive shaft mounted thereon and a frictional contact surface adapted to have the frictional contact with the disk assembly, the drive shaft being rotated through the frictional contact with the disk assembly, wherein the disk assembly includes: a disk made of a magnetic material; a friction ring frictionally contacting with the frictional contact surface of the hub assembly; and a resilient ring interposed between the disk and the friction ring in such a manner as to be fixed to the disk assembly-mounting portion of the pulley and adapted to resiliently pressurize the friction ring toward the hub assembly, whereby when the attracting magnetic flux of the magnetic flux generating means is not applied, the friction ring makes frictional contact with the hub assembly through the resilient pressurization of the resilient ring, whereas, when the attracting magnetic flux is applied, the disk moves toward the pulley to allow the frictional contact between the friction ring and the hub assembly to be released. 
     According to the present invention, desirably, the resilient ring includes at least one installing member adapted to fix the disk assembly to the disk assembly-mounting portion of the pulley and at least one resilient member formed on the installing member. 
     According to the present invention, desirably, the installing member includes an outer ring and an inner ring fixed correspondingly to an outside wall and an inner wall of the disk assembly-mounting portion of the pulley, and the resilient member includes resilient arms having a form of an arch-shaped cantilever between the outer ring and the inner ring. 
     According to the present invention, desirably, the resilient ring includes a support ring fixed to an inner side wall of the disk assembly-mounting portion of the pulley and protruding portions extended radially from the support ring in such a manner as to be fixed to the outer side wall of the disk assembly-mounting portion, and the resilient member includes resilient arms having a form of an arch-shaped cantilever extended from the protruding portions in the circumferential direction of the support ring. 
     According to the present invention, desirably, the installing member further includes: a plurality of head rivets adapted to be passed through coupling holes formed on intermediate portions of the resilient arms and to couple the intermediate portions  244   g  of the resilient arms with the disk, each head rivet having a protruding head passed through the friction ring; and a fitting means having a plurality of grooves formed on the frictional contact surface of the hub assembly in the circumferential direction thereof and an interlocking hole formed at the most front portion of each groove, whereby when the friction ring and the hub assembly have the frictional contact therebetween, the protruding heads of the head rivets are lockingly fitted to the grooves formed on the frictional contact surface of the hub assembly and as they are moved along the grooves, they are insertedly fitted to the interlocking holes. 
     According to the present invention, desirably, each resilient arm having a form of an arch-shaped cantilever has a free end portion extended in the circumferential direction toward the opposite direction to the rotating direction of the drive shaft. 
     According to the present invention, desirably, each resilient arm having a form of an arch-shaped cantilever has a free end portion extended in the circumferential direction toward the rotating direction of the drive shaft. 
     According to the present invention, desirably, at least a portion of each of the frictional surface of the friction ring and the frictional contact surface of the hub assembly is formed of a friction material having a friction coefficient of more than 0.3. 
     According to the present invention, desirably, the number of resilient arms is more than three. 
     According to the present invention, desirably, each of the frictional surface of the friction ring and the frictional contact surface of the hub assembly is formed of a flat frictional surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view showing a power transmission device for a water pump in a conventional practice; 
         FIG. 2  is an exploded perspective view showing a power transmission device for a water pump according to a first embodiment of the present invention; 
         FIG. 3  is an exploded perspective view showing a configuration of a disk assembly in the power transmission device for a water pump of  FIG. 2 ; 
         FIG. 4  is an exploded perspective view showing another configuration of a disk assembly in the power transmission device for a water pump of  FIG. 2 ; 
         FIG. 5  is a front view showing a resilient ring of the disk assembly of  FIG. 4 ; 
         FIG. 6  is an exploded perspective view showing a variation of the disk assembly in the power transmission device for a water pump of  FIG. 2 ; 
         FIG. 7  is an exploded perspective view showing a hub assembly in the power transmission device for a water pump of  FIG. 2 ; 
         FIG. 8  is an exploded perspective view showing a power transmission device for a water pump according to a second embodiment of the present invention; 
         FIG. 9  is a perspective view showing a hub assembly in the power transmission device for a water pump of  FIG. 8 ; and 
         FIG. 10  is a perspective view showing a frictional surface of a hub assembly in the power transmission device for a water pump according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an explanation on a power transmission device for a water pump according to preferred embodiments of the present invention will be in detail given with reference to the attached drawings. 
       FIG. 2  is an exploded perspective view showing a power transmission device for a water pump according to a first embodiment of the present invention. The power transmission device for a water pump according to the first embodiment of the present invention has a pulley  100  adapted to receive power applied from an engine through a belt mounted thereon. The power applied to the pulley  100  is transmitted to a hub assembly  160  by means of a disk assembly  140 . Further, a rotary shaft (not shown) mounted at the center of the hub assembly  160  rotates as the hub assembly  160  rotates. An impeller (not shown) is disposed on the rear end portion (on the left end in the drawing) of the rotary shaft rotating along with the hub assembly  160 , and the impeller presses and supplies cooling water to the engine. 
     The pulley  100  is formed of a generally donut-like member open on the center thereof and has a field coil assembly mounting portion  108  concavedly formed on one side surface thereof, that is, on the right side in the drawing, to mount a field coil assembly  120  as a magnetic flux generating means thereinto. As a result, the field coil assembly  120  is insertedly fitted to the field coil assembly-mounting portion  108  concavedly formed on one side surface of the pulley  100 . A field coil embedded in the field coil assembly  120  generates a magnetic flux therefrom through the application of the power thereto, so that a disk  146  in the disk assembly  140  is attracted toward the pulley  100 . 
     The pulley  100  has a disk assembly-mounting portion  102  formed concavedly in a shape of a ring on the opposite side surface to one side surface thereof. The disk assembly-mounting portion  102  is a space formed correspondingly to the field coil assembly-mounting portion  108 , in which the disk assembly  140  is accommodated. The pulley  100  has an intermediate plate  106  defining the disk assembly-mounting portion  102  and the field coil assembly-mounting portion  108  thereby, and the intermediate plate  106  has a plurality of magnetic field slots  104  formed thereon. The magnetic field slots  104  are formed to allow the disk assembly-mounting portion  102  and the field coil assembly-mounting portion  108  to communicate with each other, so that the magnetic flux generated from the field coil  122  embedded in the field coil assembly  120  is applied to the disk assembly-mounting portion  102  of the pulley  100 . 
     The ring-shaped disk assembly  140  is mounted in the interior of the disk assembly-mounting portion  102  of the pulley  100 . The disk assembly  140  includes the disk  146  made of a magnetic material, a friction ring  142  having frictional contact with a frictional contact surface  170  of the hub assembly  160  as will be discussed later, and a resilient ring  144  interposed between the disk  146  and the friction ring  142  in such a manner as to be fixed to the disk assembly-mounting portion  102  of the pulley  100  and adapted to resiliently pressurize the friction ring  142  toward the hub assembly  160 . 
     As shown in  FIG. 3 , the resilient ring  144  has a roughly ring-like shape and includes an outer ring  144   a  and an inner ring  144   b  serving as a installing member, and resilient arms  144   c  serving as a resilient member, which are disposed in a form of an arch-shaped cantilever between the outer ring  144   a  and the inner ring  144   b . The outer ring  144   a  and the inner ring  144   b  serve to fixedly seat the disk assembly  140  into the disk assembly-mounting portion  102  of the pulley  100 . The outer ring  144   a  and the inner ring  144   b  are fixed correspondingly to an outside wall  112  and an inner wall  114  of the disk assembly-mounting portion  102  of the pulley  100  by means of caulking. 
     Moreover, the friction ring  142  and the disk  146  are fixed to each other by means of rivets  148 , and at this time, the rivets  148  are passed through coupling holes  144   e  formed on the free ends of the resilient arms  144   c.    
     The resilient ring  144  of the disk assembly  140  is in a state of being fixed to the disk assembly-mounting portion  102  of the pulley  100 , and the friction ring  142  and the resilient ring  144  are in a state of maintaining resilient support by which the resilient arms  144   c  as the resilient member pressurize the friction ring  142  toward the hub assembly  160 , while having a given distance therebetween by means of the resilient arms  144   c . The disk  146  is in a state of being contacted with the resilient ring  144 . Further, the friction ring  144  is in a state of being supported movably in the left and right directions, while being coupled integrally with the disk  146  by means of the rivets  148 . 
     The friction ring  142  also has a roughly ring-like shape. In the first embodiment of the present invention, the friction ring  142 , which has frictional contact with the frictional contact surface  170  of the hub assembly  160 , has a plurality of convex portions  142   a  relatively more protruded than the frictional surface thereof and a plurality of concave portions  142   b  relatively more depressed therethan, in an alternating manner. In this embodiment, three convex portions  142   a  and three concave portions  142   b  are formed. 
     The convex portions  142   a  and the concave portions  142   b  are coupled respectively to the convex portions and concave portions having the corresponding shapes thereto formed on the frictional contact surface  170  of the hub assembly  160 , so that the rotary power is transmitted from the pulley  100  to the hub assembly  160 . That is, as shown in  FIG. 7 , a plurality of concave portions  172  and a plurality of convex portions  174  are formed on the frictional contact surface  170  of the hub assembly  160  in such a manner as to be lockedly fitted correspondingly to the convex portions  142   a  and the concave portions  142   b  of the friction ring  142 . As a result, if the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  are brought into contact with each other, the convex portions  142   a  of the friction ring  142  is lockedly fitted to the concave portions  172  of the frictional contact surface  170  of the hub assembly  160 , thereby rigidly and reliably achieving the transmission of the rotary power to the hub assembly  160 . 
     According to the present invention, at least a portion of each of the frictional surface of the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  is formed of a friction material. For example, at least a portion of the convex portions  142   a  and the concave portions  142   b  of the frictional surface of the friction ring  142  is formed of a friction material as will be described later. As a result, the frictional contact surface  170  of the hub assembly  160  is brought into contact with the friction ring  142  and thus serves as a frictional surface capable of transmitting the rotary power. In more detail, the rotary force from the engine of a vehicle is transmitted to the disk assembly  140  through the pulley  100 , and next, it is transmitted to the hub assembly  160  through the friction ring  142  and the frictional contact surface  170  of the hub assembly  160 , thereby causing the rotary shaft to rotate. As shown in  FIG. 7 , the frictional contact surface  170  of the hub assembly  160  has the convex portions  174  and the concave portions  172  formed alternatively to each other on a friction portion  176 , and a stepped portion is formed between each convex portion  174  and each concave portion  172 . Further, the friction material is formed on at least a portion of the convex portions  174  and the concave portions  172 . Alternatively, the friction material may be formed selectively on any portions of the frictional contact surface  170  of the hub assembly  160  and the friction ring  142 , and as shown in  FIG. 6 , it may be formed entirely on the contact surfaces between the frictional contact surface  170  and the friction ring  142 . 
       FIG. 4  shows a variation of the resilient arms  144   c  of the resilient ring  144  as described in the first embodiment of the present invention, which are disposed in a form of an arch-shaped cantilever between the outer ring  144   a  and the inner ring  144   b . However, the resilient arms  144   c  are different from those in  FIG. 3  in that they are extended from the attached end portion toward the free end portion in the direction of an arrow A, and such rotating direction corresponds with the direction of the rotation of the pulley  100  by the rotary power received from the engine, which also corresponds with the direction of the rotation of the rotary shaft for the supply of cooling water. 
     As mentioned above, the rivets  148  of the friction ring  142  are passed through the coupling holes  144   e  and are thus coupled with the resilient ring  144  and the disk  146 . In more detail, the rivets  148  are coupled to the concave portions  142   b  of the friction ring  142 , and the coupling holes  144   e  are formed on the free end portions of the resilient arms  144   c.    
     The resilient arms  144   c  resiliently support the friction ring  142  against the hub assembly  160 . The forces of the resilient arms  144   c  resiliently supporting the friction ring  142  should be most strongly applied from the free end portions thereof. Also, when the friction ring  142  is brought into contact with the frictional contact surface  170  of the hub assembly  160 , a strong contact pressure should be applied to the friction ring  142 . That is, the resilient arms  144   c  of the resilient ring  144  have to pressurize the friction ring  142  with sufficiently strong resilient forces, which enables the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  to be firmly contacted to make the rotary force transmitted to the hub assembly  160 . If the forces applied to the friction ring  142  are not sufficient to apply a relatively strong contact pressure between the friction ring  142  and the frictional contact surface  170  of the hub assembly  160 , the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  slide against each other, thereby causing them to idle. 
     The resilient arms  144   c  formed to the shape of the cantilever extended in the rotating direction can pressurize the friction ring  142  with the forces stronger than those caused by the resilient arms formed in the opposite direction thereto, so that when the friction ring  142  is brought into contact with the frictional contact surface  170  of the hub assembly  160 , a sufficient contact pressure can be applied to the friction ring  142 . 
     Referring to  FIG. 5 , the free end portions b of the resilient arms  144   c  pressurize the friction ring  142  more resiliently than the attached end portions a toward the hub assembly  160 . That is, the free end portions b of the resilient arms  144   c  have relatively larger displacement quantity from the outer ring  144   a  or the inner ring  144   b  than the attached end portions a thereof, while being closer to the hub assembly  160 . Accordingly, the free end portions b of the resilient arms  144   c  pressurize the friction ring  142  against the hub assembly  160  with a maximum degree of resilient force generated therefrom. Thus, the friction ring  142  pressurized by the resilient forces of the resilient arms  144   c  is rotated in the rotating direction A and is brought into contact with the frictional contact surface  170  of the hub assembly  160 . 
     Alternated long and short dash lines in  FIG. 5  show examples of the convex portions  142   a  and the concave portions  142   b , and the boundary portions therebetween form given stepped portions  142   m  and  142   n . In this case, the stepped portions  142   m  are formed before the resilient arms  144   c  in the rotating direction, and the stepped portions  142   n  are formed after the resilient arms  144   c  in the rotating direction. 
     In case where the resilient arms  144   c  are extended in the rotating direction, when the convex portions  142   a  and the concave portions  142   b  of the rotating friction ring  142  are coupled with the concave portions  172  and the convex portions  174  of the frictional contact surface  170  of the hub assembly  160 , the stepped portions of the friction ring  142  forming the boundaries between the convex portions  142   a  and the concave portions  142   b,  which are first coupled with the stepped portions of the hub assembly  160 , are the stepped portions  142   m  formed before the free end portions of the resilient arms  144   c  in the rotating direction. 
     Contrarily, in case where the resilient arms  144   c  are extended in the opposite direction to the rotating direction of the disk assembly  140 , the stepped portions of the friction ring  142  forming the boundaries between the convex portions  142   a  and the concave portions  142   b , which are first coupled with the stepped portions forming the boundaries between the concave portions  172  and the convex portions  174  of the frictional contact surface  170  of the hub assembly  160 , are the stepped portions  142   n  formed after the free end portions of the resilient arms  144   c  in the rotating direction. The positions of the stepped portions  142   n  are formed correspondingly to the middle portions of the resilient arms  144   c . The stepped portions  142   n  are weaker in the resilient forces than the free end portions b of the resilient arms  144   c , and that is, they pressurize the friction ring  142  against the hub assembly  160  with relatively small resilient forces. 
     Therefore, in case where the resilient arms  144   c  are extended in the opposite direction to the rotating direction of the disk assembly  140 , they pressurize the portions of the friction ring  142  locked to the frictional contact surface  170  of the hub assembly  160  when the friction ring  142  is brought into contact with the frictional contact surface  170 , thereby failing to apply a sufficient contact pressure with the hub assembly  160  to the friction ring  142 . As a result, when the convex portions  142   a  and the concave portions  142   b  of the friction ring  142  are coupled with the concave portions  172  and the convex portions  174  of the frictional contact surface  170  of the hub assembly  160 , they are not completely coupled to each other, so that the friction ring  142  slides over the frictional contact surface  170  of the hub assembly  160 , thereby causing idling. 
     Most desirably, therefore, the resilient arms  144   c  of the resilient ring  144  are extended in the arch-like shape in the same direction as the rotating direction. That is, the formation of the resilient arms  144   c  in such a manner as to be extended in the same direction as the rotating direction enables a maximum contact pressure to be applied to the friction ring  142  when the friction ring  142  is coupled to the frictional contact surface  170  of the hub assembly  160 , so that the friction ring  142  can be momentarily coupled reliably and firmly to the hub assembly  160 . 
     Under the above-mentioned structure, next, an explanation on the operation of the power transmission device for a water pump according to the first embodiment of the present invention will be given. 
     In case where there is no need for the supply of cooling water to the engine, that is, in case where there is no need for the rotation of the rotary shaft, power source is applied to the field coil  122  to shut power off. If the power source is applied to the field coil  122 , an attracting magnetic flux is generated from the field coil  122  to move the disk assembly  140  to the right side of the drawings, that is, toward the pulley  100 , so that the friction ring  142  coupled to the disk  146  by means of the rivets  148  is released from the frictional contact with the hub assembly  160 . Thus, the rotary shaft is not in a rotating state, and the supply of cooling water is not performed. 
     If the engine is heated to need the supply of cooling water, the power source applied to the field coil  122  shuts off. If no power source is applied to the field coil  122 , the resilient arms  144   c  of the resilient ring  144  fixed to the disk assembly-mounting portion  102  pressurize the friction ring  142  to the left side of the drawings, that is, toward the hub assembly  160 , so that the friction surface of the friction ring  142  has the frictional contact with the frictional contact surface  170  of the hub assembly  160  to allow the rotary shaft mounted on the hub assembly  160  to be rotated. As the rotary shaft is rotated, the cooling water is supplied to the engine through the impeller disposed at the rear end portion of the rotary shaft. 
     Referring to the coupling state between the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  when the friction ring  142  of the disk assembly  140  has the frictional contact with the frictional contact surface  170  of the hub assembly  160  by means of the resilient forces of the resilient arms  144   c , the convex portions  142   a  of the friction ring  142  are coupled with the concave portions  172  of the frictional contact surface  170 , and the concave portions  142   b  of the friction ring  142  are coupled with the convex portions  174  of the frictional contact surface  170  of the hub assembly  160 . As mentioned above, at least a portion of the friction ring  142  or the frictional contact surface  170  of the hub assembly  160  is formed of a friction material (for example, the friction portion  176  as shown in  FIG. 7 ). As such, the friction material may be formed on a portion of, or an entire surface of the friction ring  142  or the frictional contact surface  170  of the hub assembly  160 , and if it is formed on a portion thereof, it is desirable that the corresponding coupling surfaces of the two members should be formed with a friction material. In the state where the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  are coupled to each other, the rotary power can be transmitted from the pulley  100  to the hub assembly  160  in accurate, rapid and firm manners, by means of the sufficient friction force provided from the friction portions as well as the coupling surfaces between the concave portions and the convex portions. 
     Moreover, as shown in  FIGS. 4 and 5 , since the resilient arms  144   c  are extended along the circumferential direction in the rotating direction of the pulley  100 , the stepped portions  142   m  formed before the resilient arms  144   c  in the rotating direction are coupled to the stepped portions of the hub assembly  160 . The coupled positions of the resilient arms  144   c  are formed, on which a maximum contact pressure is applied to the friction ring  142 , in case where the friction ring  142  is really coupled to the frictional contact surface  170  of the hub assembly  160 , so that the idling between the disk assembly  140  and the hub assembly  160  can be prevented to transmit the rotary power reliably and to achieve synchronization. 
     Next, an explanation on a variation of the power transmission device according to the first embodiment of the present invention will be given with reference to  FIG. 6 . 
     The variation as shown in  FIG. 6  has the same configuration as in the first embodiment of the present invention, except that the friction surfaces of the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  are configured in a different manner from those of the first embodiment of the present invention. Therefore, the same components as those in the first embodiment of the present invention are denoted by the same reference numerals as each other, and also, a detailed description on them will be avoided. 
     According to the variation of the first embodiment of the present invention, the friction ring  140  has the friction surface made of a material having a sufficient friction force and a shape of a flat plate with no concave and convex portions formed thereon. 
     The hub assembly  160  has a shaft hole  168  formed at the center thereof, into which the rotary shaft is insertedly fitted, and since the frictional contact surface  170  of the hub assembly  160  entirely has the friction surface made of the friction material having the sufficient friction force, the power can be transmitted by means of the friction force between the frictional contact surface  170  and the friction ring  142  of the disk assembly  140  when they are contacted with each other. 
     Next, the friction materials used for the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  contacted with each other so as to perform the power transmission will be described. According to the present invention, the power transmission can be accurately performed by means of the contact force between the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  and further by means of the mechanical coupling between the convex portions  142   a  and the concave portions  142   b  of the friction portions of the friction ring  142  and the convex portions  174  and the concave portions  172  corresponding to the frictional contact surface  170  of the hub assembly  160  through the lock fitting manner, and during the rotation of the pulley  100  at a high speed, for example, at 9000 rpm, accurate and rapid synchronization can be maintained at the time of the power transmission to the hub assembly  160 , thereby preventing the pulley  100  from idling on the hub assembly  160 . 
     Thus, the friction portion between the friction ring  142  and the frictional contact surface  170  of the hub assembly  160  should have a higher friction force than a steel material generally used as a material thereof. For example, fiber components are added to the steel material to improve the friction force, and therefore, the friction portion can be made of steel fibers. In addition, a variety of materials used for making an automobile brake disk pad may be adopted, and for example, the friction material may be made of a composite material to which resin, rubber, Kevlar, melamine, aramid fibers, potassium titanate, and the like are added. Like this, the friction material used for the parts transmitting power using the friction force is desirably made of a material having a friction coefficient of more than 0.3. If the friction coefficient of the friction material is less than 0.3, the pulley is likely to idle during the high speed rotation at 9000 rpm. 
     Referring next to  FIGS. 8 to 10 , an explanation on a power transmission device for a water pump according to a second embodiment of the present invention will be in detail given. 
     According to the second embodiment of the present invention, as shown in  FIGS. 8 and 9 , a magnetic flux generating means  220  and a pulley  200  have the same configuration as those in the first embodiment of the present invention, except that the disk assembly and the hub assembly are different in configuration from those in the first embodiment of the present invention. Therefore, a detailed description on the same components as those in the first embodiment of the present invention will be avoided. 
     According to the second embodiment of the present invention, as shown in  FIG. 8 , in the same manner as the first embodiment of the present invention, a disk assembly  240  basically includes a disk  246 , a resilient ring  244  disposed at the front surface of the disk  246  and the friction ring  242  disposed in front of the resilient ring  244 , but the detailed configuration and operating effects of the disk assembly  240  are different from those in the first embodiment of the present invention. Hereinafter, they will be discussed. 
     In the same manner as the first embodiment of the present invention, the disk  246  is moved toward the pulley  200  in response to the magnetic flux generated from a field coil  222  of the magnetic flux generating means  220 . The resilient ring  244  includes a support ring  244   a , protruding portions  244   b  extended radially from the support ring  244   a , and resilient arms  244   c  extended from the protruding portions  244   b  in the circumferential direction of the support ring  244   a . The resilient arms  244   c  serve to pressurize the friction ring  242  toward a frictional contact surface  262  of a hub assembly  260 . 
     Each resilient arm  244   c  is formed in such a manner as to have the attached end portion, the intermediate portion  244   g , and the free end portion  244   d  protruded gradually toward the friction ring  242 , when viewed from the plane, so as to pressurize the friction ring  242  toward the frictional contact surface  262  of the hub assembly  260 . Accordingly, the resilient arms  244   c  are entirely inclined toward the friction ring  242 , and the free end portions  244   d  are more protruded than the intermediate portions  244   g  toward the friction ring  242 . That is, the intermediate portions  244   g  of the resilient arms  244   c  are more protruded forwardly than the support ring  244   a  toward the friction ring  242 , and the free end portions  244   d  of the resilient arms  244   c  are more protruded than the intermediate portions  244   g  toward the friction ring  242 . According to the second embodiment of the present invention, the number of resilient arms  244   c  is three, but it is not limited thereto. Further, each free end portion  244   d  has a coupling hole  244   f  formed thereon. 
     The friction ring  242  serves to transmit the rotary force applied from the pulley  200  to the hub assembly  260  through the contact with the hub assembly  260 . Also, if the friction ring  242  is separated from the hub assembly  260 , the rotary force applied from the pulley  200  is not transmitted to the hub assembly  260 . 
     Referring to the coupling relation between the friction ring  242 , the resilient ring  244  and the disk  246 , the friction ring  242  is coupled to the resilient ring  244  by means of rivets R passed through the coupling holes  244   f  formed on the free end portions  244   d  of the resilient arms  244   c  of the resilient ring  244 . Since the free end portions  244   d  of the resilient arms  244   c  are protruded toward the friction ring  242 , a given gap is formed between the friction ring  242  and the resilient ring  244  in the state where the coupling is made by means of the rivets R. 
     On the other hand, the resilient ring  244  and the disk  246  are coupled to each other by means of head rivets Ra. Each head rivet Ra has a protruding head Rb extended toward the friction ring  242  in such a manner as to be protruded toward the friction ring  242  by a given distance in the state where the resilient ring  242  and the disk  246  are coupled to each other. The head rivets Ra are passed through coupling holes  244   e  formed on the intermediate portions  244   g  of the resilient arms  244   c  and serve to couple the intermediate portions  244   g  of the resilient arms  244   c  with the disk  246 . 
     Accordingly, in the state where the friction ring  242  and the resilient ring  244  are coupled to each other by means of the rivets R and the resilient ring  244  and the disk  246  are coupled to each other by means of the head rivets Ra, the resilient ring  244  and the disk  246  are resiliently spaced apart from each other, and the friction ring  242  and the resilient ring  244  are resiliently spaced apart from each other by a longer gap than the gap between the resilient ring  244  and the disk  246 . The protruding heads Rb of the head rivets Ra attached to the intermediate portions  244   g  of the resilient ring  244  are inserted into the through holes  246   a  of the friction ring  242 , while being not protruded from the friction surface of the friction ring  242  contacted with the frictional contact surface  262  of the hub assembly  260 . This is because that the friction ring  242  is coupled to the free end portions  244   d  of the resilient arms  244   c , while having a gap formed from the free end portions  244   d  by means of the rivets R. 
     The disk assembly  240  under the above-mentioned configuration is mounted inside a disk assembly-mounting portion  202  of the pulley  200 . The resilient ring  244  is fixed to the inside of the disk assembly-mounting portion  202  by means of caulking. That is, the support ring  244   a  of the resilient ring  244  is caulked around an inner side wall  214 , and the protruding portions  244   b  is caulked around an outer side wall  212 , so that the resilient ring  244  is fixed to the disk assembly-mounting portion  202  of the pulley  200 . Accordingly, the friction plate  242 , which is coupled by means of the rivets R to the coupling holes  244   f  of the free end portions  244   d  of the resilient arms  244   c  of the resilient ring  244 , is in a state of being movable by a given distance in the axial direction of the rotary shaft by means of the resilient forces of the resilient arms  244   c.    
     The disk assembly  240  transmits the power applied from the pulley  200  to the hub assembly  260  or shuts off the transmission of the power thereto through the contact or separation with/from the hub assembly  260 . The hub assembly  260  has a ring-shaped interlocking plate  261  and a hub  270  coupled to the center of the interlocking plate  261 . 
     The interlocking plate  261  has the frictional contact surface  262  having a given friction coefficient formed on the back surface thereof, and through the contact coupling between the frictional contact surface  262  of the friction ring  242  and the frictional contact surface  262  of the interlocking plate  261 , therefore, the interlocking plate  261  is interlocked with the friction ring  242 . The interlocking plate  261  has a plurality of grooves  264  formed in the circumferential direction thereof. The number of grooves  264  has the corresponding number of head rivets Ra. Each groove  264  has an interlocking hole  266  formed at the most front portion thereof in the rotating direction of the hub assembly  260 . The interlocking hole  266  serves to allow each head rivet Ra to be fully fitted thereto, thereby ensuring accurate synchronization. The interlocking holes  266  may be completely passed through the interlocking plate  261 , or may be formed of grooves having a higher depth than the grooves  264 . 
     Under the above-mentioned structure, next, an explanation on the operation of the power transmission device for a water pump according to the second embodiment of the present invention will be given. 
     In case where there is no need for the activation of the water pump, that is, in case of the initial starting of the engine, there is no need for the activation of the water pump, so that load can be minimized. In this case, power source is applied to the field coil  222 . If the power source is applied to the field coil  222 , an attracting magnetic flux is generated from the field coil  222  to attract the disk  246  of the disk assembly  240  toward the pulley  200 . 
     If the disk  246  is pulled toward the pulley  200 , the intermediate portions  244   g  of the resilient arms  244   c  coupled to the disk  246  by means of the head rivets Ra are also pulled toward the pulley  200 . As a result, the friction ring  242  coupled to the free end portions  244   d  of the resilient arms  244   c  by means of the rivets R are also pulled toward the pulley  200 . Like this, if the friction ring  242  is pulled toward the pulley  200 , the coupling relation with the interlocking plate  261  of the hub assembly  260  being brought into contact with the friction ring  242  is released. Accordingly, the transmission of power from the pulley  200  to the hub assembly  260  shuts off. 
     In case where there is a need for the circulation of cooling water to the engine, the supply of the power to the field coil  222  stops, and thus, if the attracting magnetic flux disappears, the resilient arms  244   c  are resiliently deformed toward the hub assembly  260  by means of their resilient restoring forces. If the resilient arms  244   c  are resiliently deformed toward the hub assembly  260 , the friction ring  242  coupled to the free end portions  244   d  of the resilient arm  244   c  is first moved to the hub assembly  260 . Thus, if the friction ring  242  moving to the hub assembly  260  is brought into contact with the frictional contact surface  262  of the hub assembly  260 , friction between the friction ring  242  and the frictional contact surface  262  occurs. At the initial contact between the friction ring  242  and the frictional contact surface  262 , the friction ring  242  and the interlocking plate  261  start to their interlocking, but their complete synchronization is not performed yet. 
     If the resilient restoring forces of the resilient arms  244   c  are fully exerted, the complete contact between the friction ring  242  and the frictional contact surface  262  is carried out, thereby allowing the protruding heads Rb of the head rivets Ra coupled to the intermediate portions  244   g  of the resilient arms  244   c  to be protruded through the through-holes  246   a  of the friction ring  242 . The protruding heads Rb are inserted into the grooves  264  of the interlocking plate  261 , while the friction ring  242  and the interlocking plate  261  are somewhat rotated after their interlocking, and are finally fitted to the interlocking holes  266 . 
     If the protruding heads Rb are insertedly fitted to the grooves  264 , the complete synchronization between the friction ring  242  and the hub assembly  260  is conducted mechanically as well as through the friction force between the friction ring  242  and the frictional contact surface  262  of the hub assembly  260 . 
     This state means that the rotary power of the pulley  200  is fully transmitted to the hub assembly  260 , so that the rotary shaft coupled to a shaft-supporting hole  272  of the hub  270  is rotated to rotate the impeller, thereby supplying the cooling water to the engine. 
     Like this, in the state where the supply of power to the field coil shuts off, the water pump is activated, and if there is no need for the activation of the water pump, as mentioned above, the power is applied to the field coil to stop the operation of the water pump. 
     According to the second embodiment of the present invention, as described above, if the supply of power to the field coil  222  shuts off to transmit the power applied from the pulley  200  to the rotary shaft of the water pump, the friction ring  242  and the hub assembly  260  start to conduct their interlocking through the friction force between the friction ring and the frictional contact surface  262  of the hub assembly  260  in the initial state, and next, the complete synchronization in their interlocking state is achieved through the mechanical coupling between the head rivets Rb and the grooves  264  of the interlocking plate  261 , thereby reliably and rapidly transmitting the rotary power to the hub assembly  260 . 
     As described above, there is provided the power transmission device for a water pump according to the preferred embodiments of the present invention, wherein when the rotary power is transmitted from the pulley, to which the rotary force from the engine is applied, to the hub assembly rotating the rotary shaft for the water pump, the transmission of the power is conducted primarily through the friction between the friction ring and the hub assembly and is continuously conducted through the locking mechanisms, so that the transmission of power from the pulley can be achieved rapidly, stably and reliably, and the rotating speeds can be also synchronized. 
     Further, since the resilient arms of the resilient ring are extended in the shape of the arch in the rotating direction thereof, the friction ring can be contacted with the hub assembly with a strong contact pressure. When the rotary shaft is rotated to supply cooling water, accordingly, the idling of the friction ring of the disk assembly and the hub assembly can be prevented and at the same time accurate synchronization can be achieved. As a result, the idling of the parts between the friction ring and the hub assembly can be also prevented, thereby improving the reliability of the product and suppressing the damages of the parts. 
     Additionally, the contact surfaces between the friction ring and the hub assembly are formed of the friction material having a sufficient friction coefficient, so that the power regulation can be more accurately performed. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.