Patent Publication Number: US-6663521-B2

Title: Power transmitting mechanism

Description:
BACKGROUND OF THE INVENTION 
     (A) Field of the Invention 
     The present invention relates to a power transmitting mechanism that disconnects power transmission from a first rotor to a second rotor when an excessive torque (load) is transmitted between the first rotor and the second rotor. 
     (B) Description of the Related Art 
     Japanese Unexamined Patent Publication No. 11-30244 discloses such a power transmitting mechanism, which has a rotor driven by an external drive source and a rotor for a device. The rotors are coupled to each other by a rubber part for transmitting power. When the transmission torque from the external drive source to the device is excessive due to a malfunction of the device, or when the device is locked, the rubber part breaks. Thus, power transmission from one of the rotors to the other is disconnected. Accordingly, the mechanism prevents the external drive source from being affected by an excessive transmission torque. 
     According to the above prior art, even though the rubber part broken out due to the excessive torque, the external drive source and the device are partially engaged by friction at the location of the rubber part. Thus, power transmission between the rotors is not completely disconnected. This results in poor fuel economy when, for example, the external drive source is an engine of a vehicle and the device is a vehicle auxiliary device. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a power transmitting mechanism that reliably disconnects power transmission between a first rotor and a second rotor when the transmission torque between the rotors is excessive. 
     To achieve the foregoing objective, the present invention provides a power transmitting mechanism comprising a first rotor, a second rotor, and a coupler. The second rotor is coaxial to the first rotor and is driven by the first rotor. The coupler connects the first rotor to the second rotor such that the coupler uncouples when the torque transmitted by the coupler exceeds a predetermined value. The coupler includes a first coupling member and a second coupling member. The first coupling member is formed on the first rotor. The second coupling member is formed on the second rotor. One of the coupling members includes an arm. A distal end of the arm engages the other of the coupling members. The arm is disengaged from the other of the coupling members. The distal end moves in a generally radial direction of the rotors to a non-interfering position when the coupler uncouples. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view illustrating a compressor that has a power transmitting mechanism according to a first embodiment of the present invention; 
     FIG. 2 is a front view illustrating the power transmitting mechanism of FIG. 1 without a cover; 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a diagram explaining the operation of the power transmitting mechanism of FIG. 1; 
     FIG. 5 is a diagram explaining the torque limit operation of the power transmitting mechanism of FIG. 1; 
     FIG. 6 is a diagram explaining the torque limit operation of the power transmitting mechanism of FIG. 1; and 
     FIG. 7 is a cross-sectional view illustrating the power transmitting mechanism according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A power transmitting mechanism according to a first embodiment of the present invention will now be described. This embodiment relates to an air-conditioning system for a vehicle. A variable displacement swash plate type compressor is a driven auxiliary device and an engine is used as an external drive source. The power transmitting mechanism is in the power transmission path between the engine and the compressor. 
     Variable Displacement Swash Plate Type Compressor 
     As shown in FIG. 1, the compressor includes a cylinder block  1 , a front housing member  2 , and a rear housing member  4 . The front housing member  2  is secured to the front end of the cylinder block  1 . The rear housing member  4  is secured to the rear end of the cylinder block  1 . A valve plate  3  is secured between the cylinder block  1  and the rear housing member  4 . The cylinder block  1 , the front housing member  2 , and the rear housing member  4  form the housing assembly of the compressor. In FIG. 1, the left side of the figure is defined as the front, and the right side of the figure is defined as the rear. 
     A crank chamber  5  is defined between the cylinder block  1  and the front housing member  2 . A drive shaft  6  is rotatably supported in the crank chamber  5 . A lug plate  11  is located in the crank chamber  5  and is secured to the drive shaft  6  to integrally rotate with the drive shaft  6 . 
     The front end of the drive shaft  6  is coupled to the engine E of a vehicle by means of a power transmitting mechanism PT. In this embodiment, the engine E functions as the external drive source. The power transmitting mechanism PT may be a clutch mechanism (such as an electromagnetic clutch), which selectively transmits and disconnects power by external electrical control. The power transmitting mechanism PT may also be a clutchless type mechanism (such as a combination of a belt and a pulley), which does not have a clutch mechanism and constantly transmits power. The clutchless type power transmitting mechanism PT is employed in the first embodiment. A power transmitting mechanism PT that is used with a clutch will be described in the second embodiment. 
     A swash plate  12  is accommodated in the crank chamber  5 . The swash plate  12  is supported by the drive shaft  6  to slide and to incline. A hinge mechanism  13  is arranged between the lug plate  11  and the swash plate  12 . Accordingly, the swash plate  12  rotates integrally with the lug plate  11  and the drive shaft  6  by means of the hinge mechanism  13 . The swash plate  12  inclines with respect to the drive shaft  6  while sliding along the axis of the drive shaft  6 . 
     Cylinder bores  1   a  (only one of the cylinder bores is shown in FIG. 1) are formed in the cylinder block  1  to encompass the drive shaft  6 . Each cylinder bore  1   a  is formed through the cylinder block  1 . A single-headed piston  20  is housed in each cylinder bore  1   a . The valve plate  3  closes the rear opening of each cylinder bore  1   a  and the piston  20  closes the front opening of each cylinder bore  1   a . A compression chamber is defined in each cylinder bore  1   a . The volume of the compression chamber varies as each piston  20  reciprocates in the corresponding cylinder bore  1   a . Each piston  20  is coupled to the periphery of the swash plate  12  by a pair of shoes  19 . Therefore, when the swash plate  12  rotates integrally with the drive shaft  6 , rotation of the swash plate  12  reciprocates each piston  20  by means of the pair of shoes  19 . 
     A suction chamber  21  and a discharge chamber  22  are respectively defined between the valve plate  3  and the rear housing member  4 . A suction port  23  and a suction valve  24 , which selectively opens and closes the port  23 , are formed in the valve plate  3  for each cylinder bore  1   a.  A discharge port  25  and a discharge valve  26 , which selectively opens and closes the port  25 , are formed in the valve plate  3  for each cylinder bore  1   a . The suction chamber  21  and each cylinder bore  1   a  are connected by the corresponding suction port  23 . Each cylinder bore  1   a  and the discharge chamber  22  are connected by the corresponding discharge port  25 . 
     The movement of each piston  20  from the top dead center to the bottom dead center draws refrigerant gas in the suction chamber  21  into the associated cylinder bore  1   a  through the corresponding suction port  23  and the corresponding suction valve  24 . The movement of each piston  20  from the bottom dead center to the top dead center compresses the refrigerant gas drawn into the associated cylinder bore  1   a , to a predetermined pressure. Then, the compressed refrigerant gas is discharged to the discharge chamber  22  through the corresponding discharge port  25  and the corresponding discharge valve  26 . 
     In the above mentioned compressor, the inclination angle of the swash plate  12  is arbitrarily set between the maximum inclination angle (as shown in FIG. 1) and the minimum inclination angle by adjusting the internal pressure of the crank chamber  5  using an electromagnetic control valve CV. 
     The crank chamber  5  and the suction chamber  21  are connected by a bleed passage  27 . The discharge chamber  22  and the crank chamber  5  are connected by a supply passage  28 , in which the electromagnetic control valve CV is located. The flow rate of highly pressurized discharge gas that is conducted to the crank chamber  5  from the discharge chamber  22  through the supply passage  28  is set by adjusting the opening degree of the electromagnetic control valve CV using a control apparatus, which is not shown in the figures. The internal pressure of the crank chamber  5  is determined by the relationship between the flow rate of gas entering the crank chamber  5  and the flow rate of gas that is flowing from the crank chamber  5  into the suction chamber  21  through the bleed passage  27 . The difference between the internal pressure of the crank chamber  5  and the internal pressure of each cylinder bore  1   a  changes according to the internal pressure of the crank chamber  5 . The inclination angle of the swash plate  12  is determined by this pressure difference. As a result, the stroke of each piston  20 , or the displacement, is adjusted. 
     As shown in FIGS. 2 and 3, the exterior wall of the front housing member  2  protrudes to form a support cylinder that surrounds the front end of the drive shaft  6 . A pulley  32 , which functions as a first rotor, includes a cylindrical belt engaging member  32   a  and an annular support member  32   b.  A belt  33 , which extends from the output axis of the engine E (refer to FIG.  1 ), is wrapped around the cylindrical belt engaging member  32   a.  The annular support member  32   b  is inward of the inner surface of the belt engaging member  32   a.  The support member  32   b  is rotatably supported by the support cylinder  31  through a bearing  34 . The pulley  32  is located around the same axis as the axis L of the drive shaft  6  and rotates relative to the drive shaft  6 . 
     A receiving member  35 , which functions as a second rotor, is secured to the front end of the drive shaft  6  to integrally rotate with the drive shaft  6 . The receiving member  35  includes a cylindrical member  35   a  and a disc-shaped hub  35   b.  The cylindrical member  35   a  is fitted on the front end of the drive shaft  6 . The hub  35   b  is fitted into the front end of the cylindrical member  35   a.    
     Support pins  36  (four support pins are used in this embodiment) are secured to the periphery of the hub  35   b  at equal angular intervals (90 degrees in this embodiment) about the axis L. A cylindrical sleeve  37  is fitted on the periphery of each support pin  36  with an appropriate pressure. When a strong rotational force is applied to one of the sleeves  37 , it can rotate relative to the corresponding support pin  36 . 
     Engaging pins  38  (four engaging pins are applied in this embodiment) are secured to the front surface of the support member  32   b  of the pulley  32  at equal angular intervals (90 degrees in this embodiment) about the axis L. A cylindrical roller  39  is rotatably supported by each engaging pin  38 . The engaging pins  38  are further from the axis L than the support pins  36 . 
     In the pulley  32 , an annular fitting groove  32   c  is formed at the front portion of the belt engaging member  32   a.  The periphery of an annular stopper  40 , which is a flat ring, is fitted in the fitting groove  32   c.  A cylindrical limit ring  41  is connected to the pulley  32  by the inner edge of the stopper  40 . The limit ring  41  is coaxial with the pulley  32  and encompasses the rollers  39 . The middle section of the inner surface of the limit ring  41  bulges inwardly, as shown, and forms a limit surface  41   a.    
     A power transmission arm  42  is formed by a leaf spring and is located between each sleeve  37  and one of the rollers  39 . The proximal end of each power transmission arm  42  is securely wound around the sleeve  37  of the corresponding support pin  36 . Each power transmission arm  42  extends from the corresponding sleeve  37  toward the corresponding roller  39  in a clockwise direction as viewed from the perspective of FIG.  2 . Each power transmission arm  42  is slightly arched toward the periphery of the pulley  32  as shown. 
     The distal end of each power transmission arm  42  is between the corresponding roller  39  and the limit surface  41   a  of the limit ring  41 . In other words, the distal end of each power transmission arm  42  is closer to the periphery of the pulley  32  than the corresponding roller  39 . The distal end of each power transmission arm  42  curves inwardly as shown in FIG.  2 . Therefore, a curved end  43 , which is hooked around the corresponding roller  39 , is formed at the distal end of each power transmission arm  42 . In other words, each power transmission arm  42  of the receiving member  35  is engaged with the corresponding roller  39  by the curved end  43 . The receiving member  35  and the pulley  32  are connected with each other by the arms  42  to transmit power and to rotate relative to one another within a predetermined angular range while transmitting power. 
     According to this embodiment, each roller  39  and the corresponding curved end  43  are located about the axis L of the rotors  32 ,  35 . Each roller  39  is radially inward of the corresponding curved end  43 . Each power transmission arm  42  is supported by the receiving member  35  and the corresponding support pin  36 . The support pins  36  are closer to the axis L than the corresponding curved ends  43 . 
     A fulcrum portion  44  is formed on a back surface  42   a  of each power transmission arm  42  to oppose the limit surface  41   a  of the limit ring  41 . The fulcrum portions are formed by, for example, attaching a piece of vulcanized rubber to each arm  42 . Each fulcrum portion  44  is compressed between the back surface  42   a  of the corresponding power transmission arm  42  and the limit surface  41   a  of the limit ring  41 . Each power transmission arm  42  is pressed against the corresponding roller  39  by the repulsive force of the corresponding fulcrum portion  44 . In this state, the cylindrical surface  39   a  of each roller  39  is pressed against a concave surface  43   a  of the corresponding curved end  43  of each power transmission arm  42 . The radius of curvature of the cylindrical surface  39   a  of each roller  39  is less than the radius of curvature of the concave surface  43   a  inside the corresponding curved end  43 , thus linear contact occurs between each cylindrical surface  39   a  and the corresponding concave surface  43   a.    
     The concave surface  43   a  of each curved end  43  is curved. Thus, the inclination of a tangent to the curve of each arm increases at the distal and proximal ends. In the state shown in FIG. 2, the contact point between the cylindrical surface  39   a  of each roller  39  and the concave surface  43   a  of the corresponding curved end  43  moves toward the distal end or toward the proximal end of the corresponding power transmission arm  42  when one of the rollers  39  and the corresponding power transmission arm  42  move relative to one another. As a result, each roller  39  applies force to the corresponding power transmission arm  42  in an outward direction when the pulley  32  is driven. 
     A cover  45  has a cylindrical shape with a closed end. A flange  45   a,  which is formed at the periphery of the cover  45 , is fitted in the fitting groove  32   c  together with the outer edge of the stopper  40 . The cover  45  is used to cover the front end of the pulley  32 . Each member that transmits power between the pulley  32  and the drive shaft  6  (receiving member  35 , support pins  36 , engaging pins  38 , rollers  39 , limit ring  41 , and power transmission arms  42 ) is accommodated in the space between the cover  45  and the pulley  32 . An annular sealing member  47  is fitted in the fitting groove  32   c  along a side wall surface. The sealing member  47  contacts the flange  45   a  of the cover  45  to seal the space between the cover  45  and the pulley  32 . 
     Operation of the Power transmitting mechanism 
     The engine E transmits power to the pulley  32  via the belt  33 . The power is then transmitted to the receiving member  35  by the rollers  39  and the power transmission arms  42 . The power is then transmitted to the drive shaft  6  of the compressor. Load torque is generated between the receiving member  35  of the compressor and the pulley  32  of the engine E during power transmission. The load torque causes each roller  39  and the corresponding power transmission arm  42  to move relative to one another, which causes the pulley  32  and the receiving member  35  to rotate relative to one another. 
     As shown in FIG. 4, when the pulley  32  rotates clockwise, the load torque tends to rotate the receiving member  35  counter-clockwise with respect to the pulley  32 . Therefore, each roller  39  and the corresponding power transmission arm  42  tend to move relative to one another. The contact points between them move toward the distal ends of the power transmission arms  42 . The location where the fulcrum portion  44  presses against the limit surface  41   a  of the limit ring  41  functions as a fulcrum. Then, the distal end of the power transmission arm  42  is elastically deformed generally outward. That is, the power transmission arm  42  is elastically deformed based on the load torque. Thus, the curved end  43  changes attitude with respect to the receiving member  35 , in other words, the concave surface  43   a  is deformed. 
     When the displacement of the compressor increases and the load torque is increased, the force that elastically deforms the distal end of each power transmission arm  42  generally outward is increased. Therefore, each roller  39  further elastically deforms the corresponding power transmission arm  42  and relatively moves to the distal end of the corresponding power transmission arm  42 . As a result, each roller  39  rotates along the corresponding concave surface  43   a  and the contact point further moves toward the distal end of the corresponding power transmission arm  42 . Accordingly, the relative rotation angle between the pulley  32  and the receiving member  35  is increased. 
     However, when the displacement of the compressor decreases and the load torque is decreased, the force that elastically deforms the distal end of each power transmission arm  42  generally outward is decreased. Therefore, some of the energy that is accumulated in each power transmission arm  42  is released and the roller  39  relatively move to the proximal ends of the corresponding power transmission arms  42 . As a result, each roller  39  rotates along the concave surface  43   a  and the contact point moves to the proximal end of the corresponding power transmission arm  42 . Accordingly, the relative rotation angle of the pulley  32  and the receiving member  35  is decreased. 
     When the compressor is actually driven by the engine E, the output torque of the engine E or the driving torque of the auxiliary equipment, for example, a hydraulic pump of a power steering apparatus, fluctuates. Thus, the power that is transmitted from the pulley  32  to the receiving member  35  varies. In this case, the position of the contact point is changed repeatedly. In other words, the pulley  32  repeats relative rotation in the clockwise and counter-clockwise direction within the predetermined angular range. Thus, the fluctuation of power that is transmitted from the pulley  32  to the receiving member  35  is suppressed. 
     When the amount of the load torque does not adversely affect the engine E, that is, when the load torque is smaller than the maximum allowable torque, the contact point is kept on the concave surface  43   a.  In other words, each roller  39  and the corresponding curved end  43  are kept engaged and the power transmission from the engine E to the drive shaft  6  is continued. 
     However, as shown in FIG. 5, when an abnormality occurs in the compressor, or when the compressor is locked, the load torque becomes equal to or greater than the maximum torque. In this case, the stiffness of each power transmission arm  42  is insufficient to keep the contact point on the concave surface  43   a.  Accordingly, the roller  39  moves beyond the curved end  43  to the distal end of the power transmission arm  42  and separates from the concave surface  43   a.  Thus, each roller  39  and the corresponding power transmission arm  42  are disengaged. Therefore, the power transmission between the pulley  32  and the receiving member  35  is disconnected. This prevents the engine E from being affected by excessive load torque. 
     After each roller  39  and the corresponding power transmission arm  42  are disengaged, a next roller  39  on the pulley  32  contacts the back surface  42   a  of the corresponding power transmission arm  42  due to the free relative rotation of the pulley  32  with respect to the receiving member  35 . This rotates the corresponding power transmission arm  42  about the corresponding support pin  36 , as shown in FIG.  6 . As a result, the corresponding power transmission arms  42  are rotated clockwise with the respective sleeves  37  about the respective support pins  36 . Thus, the power transmission arms  42  change position with respect to the receiving member  35 . 
     The curved end  43  of each power transmission arm  42  is closer to the periphery of the pulley  32  than the roller  39  just after the arm  42  comes off the roller  39 . However, the curved end  43  of each power transmission arm  42  is moved closer to the center of the pulley  32  than the roller  39  after the pulley rotates by a quarter revolution, or in other words, after each roller  39  contacts the corresponding power transmission arm  42  at the back surface  42   a.  Each support pin  36  is inserted in the corresponding sleeve  37  with an appropriate pressure. Thus, even if an external force is applied, for example, by the vehicle vibration, the power transmission arms  42  reliably keeps the rollers  39  from being engaged (as shown in FIG.  6 ). Accordingly, the rollers  39  do not interfere with the power transmission arms  42  (or curved ends  43 ). Thus, power transmission between the pulley  32  and the receiving member  35  is reliably disconnected. Interference between the roller  39  and the power transmission arms  42 , which would apply load against the engine E and would cause a loss of engine power, is prevented. This structure prevents the roller  39  and the power transmission arm  42  from hitting each other repeatedly and thus causing noise and vibration. 
     This embodiment provides the following advantages. 
     The invention minimizes the loss of fuel efficiency by reliably discontinuing power transmission between the pulley  32  and the receiving member  35  when the load torque between the pulley  32  and the receiving member  35  is excessive. 
     The position of each power transmission arm  42  is changed by rotating it about the corresponding support pin  36  when the curved ends  43  and the corresponding rollers  39  are disengaged. Therefore, compared with a structure that changes the position of the power transmission arm  42  by deformation, the change of position is performed more smoothly. 
     The rollers  39  and the engine E are used for changing the position of the power transmission arms  42 . Accordingly, no special member, such as springs, is required for changing the position of the power transmission arms  42 . Thus, the structure of the power transmitting mechanism is simplified. 
     The cylindrical surface  39   a  of each roller  39  rolls along the concave surface  43   a  of the corresponding curved end  43  repeatedly against the friction between the cylindrical surface  39   a  and the concave surface  43   a.  This reduces torque shock applied to the engine. 
     Each roller  39  rotates while sliding along the concave surface  43   a  of the corresponding curved end  43 . Compared with an engaging pin  38 , which does not rotate while directly contacting the concave surface  43   a  of the corresponding curved end  43  (such an engaging pin is also within the concept of the present invention), the likelihood of a malfunction in slidability is reduced. Thus, fluctuation of power transmission is effectively suppressed. 
     Compared with a concave surface  43   a  that is formed by a combination of planar surfaces with different inclination angles (such a concave surface is also within the concept of the present invention), each roller  39  smoothly rolls on the corresponding concave surface  43   a.  This permits smooth relative rotation between the pulley  32  and the receiving member  35 . Thus, smooth power transmission is achieved, and fluctuation of power transmission is effectively suppressed. 
     Each curved end  43  is connected to the hub  35   b  by means of the corresponding power transmission arm  42 , which functions as an elastic member. Thus, each curved end  43  changes position with respect to the hub  35   b  by elastic deformation of the corresponding power transmission arm  42 . In other words, the elastic arms  42  add elasticity to the transmission apparatus. Compared with a case, for example, where separate elastic members are provided in addition to the coupler, the number of power transmission members are reduced. 
     The position of the contact point changes along the concave surface  43   a  repeatedly when the transmitted power varies. Accordingly, the distance between the contact point and the fulcrum of the deformation of the corresponding power transmission arm  42  (contact point between each fulcrum portion  44  and the limit ring  41 ) changes. The modulus of elasticity of the power transmission arm  42  and resonance frequency constantly change accordingly. Thus, the mechanism prevents the resonance from being generated by the vibration of the relative rotation, which is based on the variation of the transmitted power, of the pulley  32  and the receiving member  35 . 
     Each power transmission arm  42  is formed by a leaf spring. Each curved end  43  is formed by curving the corresponding power transmission arm  42 . Therefore, the curved ends  43  are easily formed. 
     Each power transmission arm  42  elastically deforms in the radial direction of the pulley  32  (each curved end  43  changes shape) when the torque is transmitted. Each power transmission arm  42  also rotates to position inwardly in the radial direction of the pulley  32  when the torque transmission is disconnected. Therefore, no space is required in the direction of the axis L for deformation and rotation of each power transmission arm  42 . Thus, the size of the power transmitting mechanism PT, more specifically, the size of the compressor, which has the power transmitting mechanism PT, is miniaturized in the direction of axis L. The space allotted for the compressor in an engine compartment of a vehicle is limited. For an air-conditioning compressor in a vehicle, miniaturization in the direction of the axis L is preferred over miniaturization in the radial direction. Accordingly, the power transmitting mechanism PT in the first embodiment has a suitable structure for a compressor of a vehicle air-conditioning system. The elastic deformation of each power transmission arm  42  does not generate the reaction force in the direction of axis L of the drive shaft  6 . Thus, the mechanism prevents force from acting on the compressor in the direction of axis L, which adversely affects the compressor. 
     The pulley  32  includes the cover  45 . Each member that transmits power (such as the receiving member  35 , the support pins  36 , the engaging pins  38 , the rollers  39 , the limit ring  41 , and the power transmission arms  42 ) is accommodated in the space between the cover  45  and the pulley  32 . This structure prevents foreign objects and water, oil, or dust in the engine compartment of a vehicle from affecting the transmission parts. Thus, wear resulting from the contamination of the members is eliminated. The structure also prevents foreign objects from being caught between the cylindrical surface  39   a  of each roller  39  and the concave surface  43   a  of the corresponding curved end  43 . Accordingly, smooth rotation of the rollers  39  is maintained. 
     Second Embodiment 
     In the second embodiment, only the parts different from the first embodiment are explained. Like members are given like numbers and detailed explanations are omitted. 
     In the second embodiment, a pulley  32  has an electromagnetic clutch, which selectively transmits and disconnects power by external electrical control, as shown in FIG. 7. A cover  45  is supported by a hub  35   b  of a receiving member  35 . A leaf spring  51  is located between the cover  45  and the hub  35   b.  An armature  52  is secured to the cover  45  and is located between the pulley  32  and a limit ring  41 . Engaging pins  38  are secured to the armature  52 . The limit ring  41  is not engaged with the pulley  32  and is fitted on the power transmission arm  42 . A core  53  is located at the rear of the pulley  32  in the front housing member  2 . 
     When the core  53  is excited by the externally applied power, the armature  52  and the cover  45  is drawn towards the pulley  32  with the rollers  39  against the leaf spring  51 . Therefore, a clutch surface  52   a  of the armature  52  is pressed against a clutch surface  32   d  of the pulley  32 . Thus, power is transmitted between the pulley  32  and the engaging pin  38  (or the roller  39 ). 
     In this state, when the core  53  is demagnetized by stopping the current supply, the force of the leaf spring  51  urges the armature  52  and the cover  45  with the roller  39  away from the pulley. Therefore, the clutch surface  32   d  and  52   a  are separated, thus, power transmission between the pulley  32  and the engaging pin  38  is disconnected. 
     In the second embodiment, for example, a compressor may be stopped by an external control when air-conditioning is not required. Thus, loss of power of an engine E is reduced. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     Elasticity need not be provided in the power transmission path. That is, the power transmission arms  42  may be rigid bodies in the above embodiments. Instead, the limit ring  41  may be formed of an elastic material, which elastically deforms to radially expand and contract. Thus, each power transmission arm  42  (curved end  43 ) rotates about the corresponding support pin  36  according to the load torque when the roller  39  and the curved end  43  are engaged. As a result, each curved end  43  changes position with respect to the receiving member  35 . 
     The engaging pins  38  may be closer to the axis L than the pins  36 . 
     In the illustrated embodiments, four pairs of rollers  39  and power transmission arms  42  are provided. The number of pairs is not limited to four, but may be six, five, three, two, or one. If the number of the pairs is reduced, the assembly of the power transmitting mechanism is simplified and the cost is reduced. If the number of the pairs is increased, the amount of transmission torque transmitted by each pair is reduced. Thus the endurance of each roller  39  and the corresponding power transmission arm  42  is improved. In other words, the endurance of the power transmitting mechanism PT is improved. 
     A part of the back surface  42   a  of each power transmission arm  42  may be deformed to integrally form the fulcrum portion  44 . 
     Balls may be used instead of rollers  39  as a rotating element. 
     The rollers may be arranged to change position with respect to the rotor on which the rollers are located, instead of the curved ends. For example, the curved ends  43  may be fixed instead of the engaging pins  38 . The rollers  39  may be provided on the distal ends of the power transmission arms  42  to engage with the corresponding curved ends  43 . 
     Both curved ends  43  and the rollers  39  may be arranged to change position with respect to the rotors  32  and  35 , respectively. 
     A spring, which urges each power transmission arm  42  radially inward, may be provided between each power transmission arm  42  and the corresponding receiving member  35 . Each spring changes the position of the corresponding power transmission arm  42 . Each spring may be arranged to pull the corresponding power transmission arm  42  toward the drive shaft  6 . Each spring may also be provided between one of the support pins  36  and the corresponding sleeve  37  to rotate the sleeve  37 . In this case, when the rollers  39  and the corresponding power transmission arms  42  are disengaged, the power transmission arms  42  rotate to the withdrawn position without contacting the rollers  39 . That is, the corresponding power transmission arms  42  change position with respect to the receiving member  35 . This reliably prevents noise and vibration caused by collision of the arms  42  and the rollers  39 . 
     The second embodiment may be modified to include an electromagnetic clutch structure between the receiving member  35  and the drive shaft  6 . 
     The use of the torque transmitting mechanism of the above embodiments is not limited to power transmission between an engine E and an air-conditioning compressor. The mechanism may be used for power transmission between an engine E and any auxiliary device (such as a hydraulic pump for a power steering apparatus or a cooling fan for a radiator). The application of the power transmitting mechanism of the above embodiments is not limited to a power transmission path of a vehicle. The mechanism may be used for a power transmission path between a drive source and in a machine tool. The power transmitting mechanism of the above embodiments has general versatility and may be applied to any power transmission path. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.