BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power transmission, and more particularly, to a power transmission suitable for a drive mechanism of a compressor used in an air conditioning system for vehicles. 
   2. Description of Related Art 
   In a known external signal controlled-type compressor used in an air conditioning system for vehicles such as the compressor shown in Japanese Patent Application No. JP-A-6-34684, the compressor&#39;s displacement is controlled by an external signal, and the compressor is connected directly to and driven by an engine. In the known external signal controlled-type compressor, when the air conditioning system is not in operation, the engine drive the compressor at a minimum displacement. Nevertheless, because the compressor is in operation and the engine drives a rotating member of the compressor when the a/c system is not in operation, the energy of the driving engine of the vehicle is wasted. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a power transmission which may be used in an external signal controlled-type compressor, which engages and disengages torque transmission by an uncomplicated mechanism. 
   To achieve the foregoing or other objects, or both, a power transmission according to an embodiment of the present invention comprises a first rotating member driven by an external drive source (e.g., an engine of a vehicle) The power transmission also may comprise an electromagnetic solenoid provided on the first rotating member, a second rotating member connected to a main shaft of a rotary apparatus (e.g., a compressor, such as, an external signal controlled-type compressor), and a mechanism for engaging a plunger of the electromagnetic solenoid to the second rotating member and for disengaging the plunger from the second rotating member. 
   In the power transmission, the torque transmission between the first and second rotating members may be switched with certainty between ON and OFF conditions by engaging the plunger of the electromagnetic solenoid to the second rotating member and disengaging the plunger from the second rotating member, respectively, by controlling the operation of the electromagnetic solenoid. 
   In another embodiment, the mechanism for engaging and disengaging may comprise internal teeth formed on the plunger of the electromagnetic solenoid and external teeth formed on the second rotating member. In yet another embodiment, the mechanism for engaging and disengaging may comprise external teeth formed on the plunger of the electromagnetic solenoid and internal teeth formed on the second rotating member. In still another embodiment, the mechanism for engaging and disengaging may comprise teeth formed on an end of the plunger of the electromagnetic solenoid and on an end of the second rotating member opposing the end of the plunger, and during switching, the teeth of each end are engaged or disengaged from each other. The mechanism for engaging and disengaging may comprise any of these configurations. Further, the second rotating member may comprise a hub for fixedly connecting the second rotating member to the main shaft and a damper positioned between the hub and the second teeth. 
   In yet another embodiment, a power transmission may comprise a first rotating member driven by an external drive source, an electromagnetic solenoid provided on the first rotating member, a second rotating member connected to a main shaft of a rotary apparatus; and a mechanism for engaging a plunger of the electromagnetic solenoid to and for disengaging the plunger from the second rotating member. The mechanism may comprise a motivator manufactured from a magnetic material, such that when the electromagnetic solenoid is activated, said plunger engages the second rotating member, and a biasing means, such that when the electromagnetic solenoid is deactivated, the biasing means disengages the plunger from the second rotating member. 
   Further, the power transmission may comprise a guide mechanism for guiding the plunger of the electromagnetic solenoid along the first rotating member. The guide mechanism may be positioned between the plunger and the first rotating member. The guide mechanism may make, the movement of the plunger more stable. The guide mechanism also may make the engagement and disengagement between the plunger and the second rotating member more smooth. 
   Moreover, the power transmission may comprise a torque limiter provided to the second rotating member. When the rotary equipment is secured, the torque limiter may reduce or eliminate the likelihood that a belt, band, cable, chain, or the like wound between the external drive source and the first rotating member will brake. 
   Thus, in the power transmission according to the present invention, with an uncomplicated structure, the torque transmission between the first and second rotating members may be switched with certainty between ON and OFF conditions by engaging and disengaging, respectively, the plunger of the electromagnetic solenoid from the second rotating member by controlling the operation of the electromagnetic solenoid. Consequently, when it is not necessary to drive the rotary equipment, the rotation of the second rotating member connected to the main shaft of the rotary equipment may be stopped completely, thereby saving energy of the external drive source and without consuming the energy wastefully. Therefore, the power transmission according to the present invention is particularly suitable for applications in which the drive mechanism of a compressor connected directly to an engine used in an air conditioning system for vehicles. 
   Further objects, features, and advantages of the present invention will be understood from the following detailed description of preferred embodiments of the present invention with reference to the accompanying figures. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention now are described with reference to the accompanying figures, which are given by way of example only, and are not intended to limit the present invention. 
       FIG. 1  is a vertical, cross-sectional view of a power transmission according to an embodiment of the present invention. 
       FIG. 2  is an exploded perspective view of the power transmission depicted in  FIG. 1 . 
       FIG. 3  is a partial, vertical, cross-sectional view of a power transmission according to another embodiment of the present invention. 
       FIG. 4  is a partial, vertical, cross-sectional view of a power transmission according to a further embodiment of the present invention. 
   


   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  depict a power transmission according to an embodiment of the present invention applied to an external signal controlled-type compressor used in an air conditioning system for vehicles. As depicted in  FIGS. 1 and 2 , a power transmission A comprises an annular pulley  1  as a first rotating member, e.g., an annular pulley  1 , which a U-shaped cross section. Pulley  1  may be connected to an engine (not shown) of a vehicle, and the engine may be used as an external drive source via a belt, band, cable, chain, or the like (not shown). Pulley  1  may be supported by a casing (not shown) of an external signal control-type compressor via a bearing  2 . 
   An electromagnetic solenoid  3  may be provided within pulley  1 . Electromagnetic solenoid  3  may have a coil  3   a  and an annular yoke  3   b  having a rectangular, hollow, cross-section which contains the coil  3   a . Yoke  3   b  may be fixed to the casing of the external signal controlled-type compressor. A slit  3   c  may be formed in the outer surface of yoke  3   b  over the entire circumference of yoke  3   b . An annular plunger  3   d  may be mounted onto the outer surface of yoke  3   b  movably in the axial direction and rotatably relative to the yoke  3   b . A plurality of projected portions  3   d ′ may extend from a first end of plunger  3   d . Each portion  3   d ′ may have an arc shape and may be formed to be received through slits  1   a , which slits  1   a  are formed through an end plate of pulley  1 . Internal teeth  3   e  may be formed on the inner surface of respective projected portions  3   d ′. An annular collar portion  3   d ″ may be formed on the outer surface of plunger  3   d  at a position distal to portions  3   d ′. An outer surface of collar portion  3   d ″ may be engaged slidably with the inner surface of pulley  1 . An annular ring  3   f  of a magnetic material, such as iron or a ferromagnetic alloy, may be fixed on the inner surface of a second end of plunger  3   d . A spring  3   g  may be positioned between collar portion  3   d ″ and the end plate of pulley  1 . 
   Power transmission (A) also may comprise a second rotating member e.g., an annular hub  4 . Hub  4  may be fixed to a main shaft (not shown) of the external signal control-type compressor. An annular damper  5  may be fitted and fixed onto a radially outer surface of hub  4 . Damper  5  may be formed of a shock and vibration absorbent material, such as rubber or another suitable polymeric material. An annular connecting member  6  may be fitted and fixed onto the outer surface of damper  5 . External teeth  6   a  may be formed on the outer surface of connecting member  6 , and the external teeth  6   a  may engage internal teeth  3   e  formed in plunger  3   d  of electromagnetic solenoid  3 . 
   Power transmission (A) also may comprise a torque limiter. Torque limiters are known in the art. In one embodiment, hub  4 , damper  5 , and connecting member  6  may be configured to act together to perform the function of a torque limiter. For example, it is known in the art to have damper  5  configured to disconnect from hub  4  or from connecting member  6  when the torque exceeds a predetermined level. Other methods of incorporating a torque limiter into a driving mechanism also are known in the art, and will be apparent to those of ordinary skill in the art upon a consideration of the present specification. 
   In power transmission (A), when the air conditioning system is in operation, electric power may be applied to coil  3   a  of electromagnetic solenoid  3 , annular ring  3   f  may move in the axial direction by the generated magnetic force, plunger  3   d  may compress spring  3   g , projected portions  3   d ′ may penetrate pulley  1  through slits  1   a , and internal teeth  3   e  may engage external teeth  6   a . The engagement between internal teeth  3   e  and external teeth  6   a  may occur when the vehicle engine is stopped or rotated at a low speed. 
   The engine may transmit a torque to pulley  1  via a belt (e.g., an endless belt), band, cable, chain, or the like. The torque is transmitted to connecting member  6  via the engagement between internal teeth  3   e  and external teeth  6   a , transmitted to hub  4  through damper  5 , and then transmitted to the main shaft of the external signal controlled-type compressor. Thus, when the external signal controlled-type compressor is in operation, refrigerant is compressed, and the compressed refrigerant is sent to a refrigerant circuit of the air conditioning system. 
   When the air conditioning system is not in use, for example, in cold weather, electric power is not transmitted to coil  3   a  of electromagnetic solenoid  3 . Consequently, no magnetic force is generated by coil  3   a , plunger  3   d  is pushed back by spring  3   g , projected portions  3   d ′ are retreated through slits  1   a , and internal teeth  3   e  and external teeth  6   a  is disengage. The disengagement of internal teeth  3   e  from external teeth  6   a  may occur at an arbitrary timing. When internal teeth  3   e  end external teeth  6   a  disengage, the transmission of torque from pulley  1  to hub  4  is interrupted. Consequently, the transmission of torque from the vehicle engine to the external signal controlled-type compressor also is interrupted, and the waste of energy of the vehicle engine in operating the compressor may be reduced or eliminated. 
   Further, collar portion  3   d ″ of plunger  3   d  and the inner surface of pulley  1  may form a guide mechanism positioned between plunger  3   d  and pulley  1 . Because collar portion  3   d ″ of plunger  3   d  slidably engages with the inner surface of pulley  1 , and collar portion  3   d ″ is guided along the inner surface of the pulley  1 , the movement of the plunger  3   d  may be made more stable, and the engagement and disengagement between internal teeth  3   e  and external teeth  6   a  may be made more smooth. 
   As depicted  FIG. 3 , the mechanism for engaging and disengaging plunger  3   d  from hub  4  may be constructed, such that external teeth  3   e ′ are formed on an outer surface of projected portions  3   d ′ of plunger  3   d , and internal teeth  6   a ′ are adapted to engage external teeth  3   e′.    
   Further, as depicted in  FIG. 4 , the mechanism for engaging and disengaging plunger  3   d  from hub  4  may be constructed such that teeth  3   e ″ are formed on an end of projected portions  3   d ′ of plunger  3   d , and teeth  6   a ″ are adapted to engage teeth  3   e″.    
   Further, a torque limiter may be provided at a position radially inside of plunger  3   d , for example, at a position of damper  5 . Many known mechanisms may be employed as the mechanism for the torque limiter (for example, torque limiters disclosed in U.S. Pat. No. 6,425,837 B1 and U.S. Pat. No. 6,494,799 B1 and U.S. Patent Application Publication No. US2003/0104890 A1, which are incorporated herein by reference). When the external signal controlled-type compressor is secured, the torque limiter operates, the transmission of torque from pulley  1  to hub  4  is interrupted, and the likelihood that the belt or other connection means wound between the vehicle engine and pulley  1  may brake or be damaged is reduced or eliminated. 
   While the present invention has been described in connection with preferred embodiments, it will be understood by those skilled in the art that various modifications of the preferred embodiments described above may be made without departing from the scope of the invention. Other embodiments will be apparent to those skilled in the art from a consideration of the specification or from a practice of the invention described herein. It is intended that the specification and the described examples are considered exemplary only, with the true scope of the invention indicated by the following claims.
A power transmission includes a first rotating member driven by an external drive source, an electromagnetic solenoid provided to the first rotating member, a second rotating member fixed to a main shaft of a rotary apparatus, and an mechanism for engaging and disengaging a plunger of the electromagnetic solenoid from the second rotating member. When the rotary apparatus is not used, the energy of the external drive source may be prevented from being wasted by interrupting the torque transmission from the first rotating member to the second rotating member by the operation of the mechanism for engaging and disengaging.