BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a power transmission apparatus. More particularly, the invention is suitably used while being assembled in a compressor of a car air conditioner and operated from an external power source, such as an engine, through a belt.  
         [0003]     2. Description of the Related Art  
         [0004]     A refrigerant compressor of a car air conditioner is driven from an external power source such as an engine through a belt and a pulley. To cut off connection between the engine and the compressor, an electromagnetic clutch may be interposed between them. However, the electromagnetic clutch is not interposed in many cases because the production cost can be decreased when an electromagnetic clutch is not disposed. In this case, a torque limiter (power cutoff member) is disposed in the power transmission apparatus of the compressor for the car air conditioner operated through the belt to avoid disadvantage such as belt damage when the compressor seizes.  
         [0005]     The torque limiter uses screw meshing for a part of a power transmission route and utilizes an excessive axial force that occurs at the screw meshing portion, owing to the excessive torque when the compressor seizes (refer to Japanese Unexamined Patent Publication No. 2003-206950, for example). Because this torque limiter system utilizes friction for cutoff, however, it involves the problem that the operation torque of the torque limiter changes with the passage of time because the coefficient of friction changes due to corrosion of the friction surface, as a contact surface, and the adhesion of grease.  
         [0006]     In the power transmission apparatuses of the prior art such as the one described above, another torque limiter is known with sealing the friction surface and which avoids the problem described above. The power transmission apparatus of this type has a structure in which a part of the power transmission portion has screw coupling. The torque limiter system utilizing this screw coupling breaks a part of the power transmission route and cuts off the power transmission route by utilizing an excessive axial force occurring at the screw coupling portion by the excessive torque that occurs when the compressor seizes.  
         [0007]     The requirement for reducing the power losses of the compressor and the power transmission apparatus are high at present. A technology of reducing a diameter of a shaft is known to reduce the loss of a shaft seal device and a bearing as a sliding loss of a shaft of a compressor. In the power transmission apparatus of the prior art having the torque limiter described above, the reduction of the diameter of a rotary shaft on the output side invites another problem. When the diameter of the rotary shaft is decreased in the fastening structure between a hub and the compressor of the power transmission apparatus of the prior art for transmitting power to the compressor, the problem occurs in that the torque limiter mechanism does not operate.  
         [0008]      FIG. 5  is a partial sectional side view of an embodiment of the fastening structure of the prior art. In the case of the power transmission apparatus  50  for fastening the rotary shaft  5  of the compressor having a slide portion  5   d  with the shaft seal device having a relatively large outer diameter, seat faces (bearing surfaces)  8   a  and  8   b  of a washer  8  sufficient for power transmission can be arranged. The power cutoff member  3  is fastened at its screw portion  3   d  to the rotary shaft  5  and the hub  2  mechanically fitted is coupled or meshed with the power cutoff member  3 . It can be appreciated from  FIG. 5  in this construction that a support surface having a sufficient area for the axial force generated as the torque transmitted from the pulley is converted can be formed on the seat faces  8   a  and  8   b  of the washer  8  and on the seat face (bearing surface)  5   a  of the rotary shaft  5 . Therefore, the surface pressure acting on the seat faces  8   a  and  8   b  and the seat face  5   a  can be limited to a low level.  
         [0009]      FIG. 6  shows the power transmission apparatus  60  when the fastening structure shown in  FIG. 5  is extended to a rotary shaft having a small diameter.  FIG. 7  is an enlarged view of the fastening portion in  FIG. 6 . The area of the seat face  5   a  of the rotary shaft  5  becomes small, the axial force supporting area of the seat faces  8   a  and  8   b  (particularly  8   b ) of the washer  8  becomes small, and a high torque occurs in the screw fastening direction in the screw portion  3   d  of the power cutoff member  3  and in the screw portion  5   b  of the rotary shaft  5  during the high torque operation due to the high load operation of the compressor, so that the seat face  5   a  of the rotary shaft  5  and the seat faces  8   a  and  8   b  of the washer  8  undergoes plastic deformation and the continuous operation cannot be made. Also, the seat face  5   a  of the rotary shaft  5  and the seat faces  8   a  and  8   b  of the washer  8  undergo deformation due to the high torque resulting from the seizure of the compressor and the notch portion  3   c  provided to the power cutoff member  3  cannot cut-off power. Incidentally, reference numerals of the constituent portions of the prior art examples shown in FIGS.  5  to  7  correspond to the reference numerals of similar constituent portions in the embodiment shown in  FIGS. 1 and 2 .  
         [0010]     Another prior art technology that makes portions in the proximity of the rotary shaft compact in the power transmission apparatus for the compressor is known (Japanese Unexamined Patent Publication No. 2001-173759, for example) but the reference does not disclose the present invention.  
       SUMMARY OF THE INVENTION  
       [0011]     In view of the circumstances described above, the invention provides a power transmission apparatus for a compressor capable of securing high fastening strength and having a rotary shaft having a small diameter and, eventually, a power transmission apparatus capable of being fitted to a compressor having a low power loss. More specifically, the invention provides a power transmission apparatus for a normal operation type compressor for a car air conditioner operated from an external power source, such as an engine, through a belt and not having an electromagnetic clutch but having a torque limiter, which power transmission apparatus transmits power from the outside to the compressor through a pulley and has a hub fitted to the rotary shaft of the compressor requiring a small diameter shaft by screw fastening means. The power transmission apparatus can operate the torque limiter at a desired torque and can transmit a high torque even by using a rotary shaft having a small diameter.  
         [0012]     According to one aspect of the invention, there is provided a power transmission apparatus ( 10 ) comprising a rotary portion ( 1 ) to which a turning driving force from a driving source is transmitted and which can rotate; a power cutoff member ( 3 ) mechanically connected to the rotary portion ( 1 ) and one of the ends of a rotary shaft ( 5 ) of a driven apparatus, and cutting off transmission of an excessive torque between the rotary portion and the rotary shaft; and a cap ( 4 ) connected to one end of the rotary shaft ( 5 ) on one hand and fastened by screw meshing to the power cutoff member ( 3 ) on the other hand to transmit power from the power cutoff member ( 3 ) to the rotary shaft ( 3 ). The rotary portion ( 1 ), the power cutoff member ( 3 ), the cap ( 4 ) and the rotary shaft ( 5 ) rotate integrally with one another. The cap ( 4 ) has a flange ( 4   h ) protruding in a radial direction with respect to an axis of the rotary shaft ( 5 ). The torque transmitted from the rotary portion ( 1 ) to the rotary shaft ( 5 ) is converted to an axial force in the axial direction of the rotary shaft ( 5 ) as the cap ( 4 ) and the power cutoff member ( 3 ) are fastened with each other through screw meshing, and the flange ( 4   h ) supports the axial force.  
         [0013]     When the power transmission apparatus employs the construction described above, the power cutoff member (torque limiter) and the cap provided to the distal end of the rotary shaft of the driven apparatus are fastened and the torque limiter can be operated at a desired torque. Therefore, the power transmission apparatus can be used for an apparatus having a rotary shaft of a smaller diameter. In the power transmission apparatus having the torque limiter, therefore, the invention can solve the disadvantage such as the collapse of the seat face in the proximity of the rotary shaft due to the axial load and can transmit a high torque. As a result, the invention provides a power transmission apparatus that can be fitted to a compressor having a low power loss.  
         [0014]     In the invention, the power transmission apparatus further comprises a hub ( 2 ) connected to the rotary portion ( 1 ) on one hand and to the power cutoff member ( 3 ) on the other hand, and transmitting power from the rotary portion ( 1 ) to the power cutoff member ( 3 ), and wherein the rotary portion ( 1 ), the hub ( 2 ) and the power cutoff member ( 3 ) rotate integrally with one another.  
         [0015]     This discloses a more concrete construction of the power cutoff apparatus according to the invention.  
         [0016]     The cap ( 4 ) is fastened to the rotary shaft ( 5 ) in such a fashion as to encompass the rotary shaft ( 5 ). This arrangement further embodies the power transmission apparatus of the invention.  
         [0017]     The cap ( 4 ) and the rotary shaft ( 5 ) are fastened to each other through screw meshing. This arrangement further embodies the fastening structure of the cap ( 4 ) and the rotary shaft ( 5 ) in the invention.  
         [0018]     The flange portion ( 4   h ) is formed on the cap ( 4 ) on the side opposing the power cutoff member ( 3 ). This arrangement further embodies the structure of the cap ( 4 ) of the invention.  
         [0019]     The power transmission apparatus is connected to a compressor of a car air conditioner as the driven apparatus.  
         [0020]     The present invention may be more fully understood from the description of preferred embodiments of the invention, as set forth below, together with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     In the drawings:  
         [0022]      FIG. 1  is a longitudinal sectional view showing a schematic construction of a power transmission apparatus according to a first embodiment of the invention;  
         [0023]      FIG. 2  is a partial enlarged view of a portion near a rotary shaft in  FIG. 1 ;  
         [0024]      FIG. 3  is an enlarged sectional view of a portion near a rotary shaft fastening portion of a power transmission apparatus according to a second embodiment of the invention;  
         [0025]      FIG. 4  is an enlarged sectional view of a cap of a modified embodiment of the second embodiment of the invention;  
         [0026]      FIG. 5  is a partial longitudinal sectional view of a fastening structure of a power transmission apparatus according to the prior art;  
         [0027]      FIG. 6  is a partial longitudinal sectional view showing an example when the fastening structure shown in  FIG. 5  is expanded to a rotary shaft having a small diameter; and  
         [0028]      FIG. 7  is a partial enlarged sectional view of the fastening portion in  FIG. 6 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Preferred embodiments of the invention will be hereinafter explained in detail with reference to the accompanying drawings.  FIGS. 1 and 2  schematically show a power transmission apparatus according to the first embodiment of the invention, wherein  FIG. 1  is a sectional side view showing a schematic construction of a power transmission apparatus  10  according to the first embodiment and  FIG. 2  is a partial enlarged view of a fastening portion of a rotary shaft and a cap in the proximity of the rotary shaft. Referring to  FIGS. 1 and 2 , the same reference numeral will be used to identify the same constituent element in  FIGS. 1 and 2  as that of the prior art example shown in FIGS.  5  to  7 .  
         [0030]     The power transmission apparatus  10  according to the first embodiment of the invention is used for a car air conditioner and is an apparatus for transmitting the rotating force of an external driving source such as an engine to a compressor of a car air conditioner. The power transmission apparatus  10  has a power cutoff member (torque limiter)  3 . In the power transmission apparatus  10 , power from the external power source such as the engine is transmitted to a pulley  1  (corresponding to a rotary portion) through a belt not shown in the drawing. Power is transmitted to an inner hub  2   d  of a hub  2  as a fitting portion  2   a  formed of an elastic member provided to an outer periphery of the hub  2  meshes with a fitting portion  1   a  of the pulley. The pulley  1  is supported by a casing  7  of the compressor, not shown, through a bearing  6  in such a manner as to be capable of rotating. Power is further transmitted from the hub  2  to the power cutoff member  3  disposed inside the hub  2 , then from the power cutoff member  3  to the cap  4  meshing with the power cutoff member  3 , and thereafter to the rotary shaft  5  of the compressor meshing with the cap  4 .  
         [0031]      FIG. 2  is an enlarged view of the fastening portion shown in  FIG. 1 . The cap  4  of the metal shown in  FIG. 2  is fitted to the distal end (one of the ends) of the rotary shaft  5  having a small diameter in the construction of the embodiment described above. The inner hub  2   d  of the hub  2  and the power cutoff member  3  are fitted through faucet joint of a hexagonal shape or a rectangular shape at the fitting portion  2   e  of the inner hub  2   d  and the fitting portion  3   a  of the power cutoff member, and transmit power from the hub  2  to the power cutoff member  3  owing to this construction. A first screw portion  4   e  is formed around an outer peripheral portion of the cap  4  and meshes with the screw portion  3   d  of the power cutoff member  3 . A second screw portion  4   d  formed on the inner peripheral side of the cap  4  meshes with a screw portion  5  on the outer peripheral side of the rotary shaft  5 . When the screw portion  5   b  of the rotary shaft  5  is screwed into the second screw portion  4   d,  an end face  5   c  at the distal end of the rotary shaft comes into touch with a seat face (bearing surface)  4   b  formed inside the cap  4  as shown in  FIG. 2 , thereby inhibiting further screw-in of the second screw portion  4   d.    
         [0032]     Next, the operation of the power transmission apparatus  10  when an excessive torque acts thereon will be explained. The torque (power) is transmitted from the hub  2  to the power cutoff member  3  through the faucet joint portion, to the cap  4  through the respective screw portions  3   d  and  4   e  of the power cutoff member  3  and the cap  4  and to the rotary shaft  5  through the respective screw portions  4   d,    5   b  of the cap  4  and the rotary shaft  5 . Power transmission between the cap  4  and the rotary shaft  5  is made through the frictional force due to the axial force between the end face  5   a  and the seat face  4   b  resulting from the contact of the end face  5   a  of the distal end of the rotary shaft  5  and the seat face  4   b  of the cap  4  in addition to the screw portions  4   b,    5   b.    
         [0033]     In the torque transmission between the power cutoff member  3  and the cap  4 , on the other hand, power is converted to the axial force as the seat face  3   f  of the power cutoff member  3  comes into touch with the seat surface (bearing surface)  2   g  of the inner hub  2   d.  The cap  4  has a disc-like flange portion  4   h  that is disposed on the opposing side to the power transmission member, and the flange portion  4   h  protrudes in a radial direction with respect to the axis of the rotary shaft  5 . The axial force acting on the inner hub  2   d  on the seat face  2   g  is borne by the seat face (bearing surface)  4   g  on the inner hub side in the axial direction of the flange portion  4   h  as can be appreciated from  FIG. 2 . In the power transmission apparatus  60  of the prior art described already, this axial force is received by the seat face (bearing surface)  5   a  through the washer  8 . The axial force transmitted from the inner hub  2   d  to the power cutoff member  3  through the seat face (bearing surface)  3   f  is borne by the first screw portion  4   e  of the cap  4  through the screw portion  3   d.  Furthermore, the axial force is transmitted to the rotary shaft  5  through the contact of the seat face  4   b  of the cap  4  with the end face  5   c  of the rotary shaft  5  and through fastening between the second screw portion  4   d  of the cap  4  and the screw portion  5   b  of the rotary shaft  5  and, at the same time, the torque is transmitted.  
         [0034]     The power cutoff member  3  has a notch portion  3   c  having a reduced section and a notch. Therefore, the screw portions  3   d  and  4   e  are fastened by the excessive torque that occurs when the compressor undergoes seizure, and the notch portion  3   c  provided to the power cutoff member  3  undergoes breakage to cut-off power and to avoid the problem that the belt of the car is damaged. In this instance, the contact surface of the seat surface  4   g  as the axial force bearing surface of the flange portion  4   h  of the cap  4  and the inner hub  2   d  can form a sufficient area, as shown in  FIG. 2 , and can suppress the surface pressure acting on the seat face  4   g  to a sufficiently low level. Though the axial force acting on the seat faces  4   b  and  5   c  increases, it is easy to set the strength of the cap  4  and the rotary shaft  5  against this axial force to be greater than the breaking strength of the notch portion  3   c.  According to such a construction, it becomes possible to avoid the problem of the prior art described already in that the seat face  5   a  of the rotary shaft  5  and the seat faces  8   a  and  8   b  of the washer  8  undergo plastic deformation such as sinking and consequently, the torque limiter does not operate. Furthermore, this construction can eliminate the washer  8  that has been necessary in the prior art example.  
         [0035]      FIG. 3  is a partial enlarged sectional view of a portion in the proximity of the rotary shaft fastening portion of a power transmission apparatus according to the second embodiment of the invention. When the cap structure shown in  FIG. 2  is employed in the first embodiment described above, the escape portion  4   c  shown in  FIG. 2  is necessary for processing the screw portion  4   e,  but it is also possible to form a portion (head portion)  4   a  having a tool portion disposed at the cap  4  into a separate component and to couple it with the cap  4  by means such as push-in, welding, etc. The rest of the constructions of this embodiment are the same as those of the first embodiment. Therefore, an explanation will be omitted.  
         [0036]      FIG. 4  shows an enlarged sectional view of a cap in a modified example of the second embodiment. In the cap  4  shown in  FIG. 4 , the first screw portion  4   e  capable of coupling with the screw portion  3   d  of the power cutoff member  3  is formed around the cap outer peripheral portion but means for coupling with the rotary shaft  5  of the compressor inside the cap  4  may be a spline structure  4   k  in place of the screw in this embodiment. The construction of this embodiment may be applied to the first embodiment.  
         [0037]     Referring to  FIGS. 3 and 4 , like reference numerals are used to identify like constituent elements as in the first embodiment shown in  FIGS. 1 and 2 .  
         [0038]     Next, the effects and operations of the embodiments described above will be explained.  
         [0039]     The following effects can be expected by the power transmission apparatus according to the first embodiment of the invention.  
         [0040]     In the power transmission apparatus having the power cutoff member (torque limiter) installed inside the hub, the torque limiter can be operated at a desired torque by arranging the cap having the flange at the distal end portion of the rotary shaft of the compressor.  
         [0041]     Therefore, in the power transmission apparatus having the torque limiter, it becomes possible to eliminate the problems such as sinking of the seat face in the proximity of the rotary shaft owing to the axial load, to transmit the high torque without affecting the power cutoff performance of the power cutoff member and eventually to fit the power transmission apparatus to a compressor having a low power loss.  
         [0042]     The power transmission apparatus according to the second embodiment of the invention can provide the following effect in addition to the effects of the first embodiment.  
         [0043]     Namely, processing of the screw portion of the cap becomes easier.  
         [0044]     The power transmission apparatus of the modified embodiment of the second embodiment of the invention can provide the fastening structure of the cap and the rotary shaft of the compressor that can expect the same effects as those of the first embodiment.  
         [0045]     The embodiments given above represent the example where the invention is used as the power transmission apparatus for the compressor of the car air conditioner but the invention may be used for other applications. In other words, the application of the invention is not limited to the car air conditioner.  
         [0046]     In the embodiments described above or shown in the accompanying drawings, power of the driving source is transmitted through the belt and the pulley but the invention is not limited thereto. Namely, power may be transmitted through other mechanisms such as gears.  
         [0047]     While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto, by those skilled in the art, without departing from the basic concept and scope of the invention.
A power transmission apparatus ( 10) includes a rotary portion ( 1) to which turning driving force from a driving source is transmitted and which can rotate, a power cutoff member ( 3) mechanically connected to the rotary portion ( 1) and one of the ends of a rotary shaft ( 5) of a driven apparatus and cutting off transmission of excessive torque between them, and a cap ( 4) connected to the end of the rotary shaft ( 5) on one hand and fastened by screw meshing to the power cutoff member ( 3) on the other hand, and transmitting power from the power cutoff member ( 3) to the rotary shaft ( 5). The rotary portion ( 1), the power cutoff member ( 3), the cap ( 4) and the rotary shaft ( 5) all rotate integrally with one another. The cap ( 4) has a flange portion ( 4 h) protruding in a radial direction with respect to an axis of the rotary shaft ( 5) and supporting an axial force.