Abstract:
An interruptible power transmission mechanism couples a drive source to a compressor. The power transmission mechanism has a pulley, which rotates in synchronism with the drive source, and a receiving member, which rotates in synchronism with the compressor. A limit spring couples the pulley and the receiving member such that they rotate together. When the load torque of the compressor exceeds a predetermined value, the diameter of the limit spring is decreased so that the limit spring engages a rib provided on the receiving member. Then, the deformation of the limit spring in the radial direction is locally restricted, causing stress at a specific portion of the limit spring to increase rapidly. As a result, the limit spring is reliably broken at a torque near the desired load torque, thus interrupting power in a desirable manner.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a power transmission mechanism provided between a drive source and a driven machine. More specifically, this invention relates to a power transmission mechanism that interrupts transmission between a drive source and a driven machine when an excess load torque is produced by the driven machine. 
     In general, a power transmission mechanism is provided between a drive source, such as an engine or a motor, and a driven machine, such as a compressor. When an abnormality (e.g., seizure) occurs in the driven machine, the power transmission mechanism positively shuts off power transmission between the drive source and the driven machine to prevent the excess load torque from affecting the drive source. 
     For example, Japanese Unexamined Patent Publication (KOKAI) Hei No. 8-319945 discloses a clutchless compressor in which a pulley, which is fitted over the end portion of the rotary shaft, is driven by an engine. The pulley, or power transmission mechanism, has a plurality of arcuate holes arranged at predetermined intervals on an imaginary circle about the axis of the rotary shaft. The portions between adjacent pairs of holes form break portions. When the rotary shaft is unable to rotate due to an abnormality in the internal mechanism of the compressor and a load torque equal to or greater than a predetermined value acts on the break portion, the break portion breaks. Consequently, the power transmission to the rotary shaft from the engine is cut off. 
     According to the power transmission mechanism of the aforementioned publication, the break portion does not always fully break when the load torque reaches the predetermined value. Specifically, for example, the failure stresses of the individual members, if they are of the same kind or are the same member, are not quite the same and have a certain variation. It is therefore actually very hard to reliably break the break portion in the vicinity of a load torque where breaking is expected in individual power transmission mechanisms that have such individual differences. Accordingly, a simple structure that has a break portion merely provided at a part of the pulley is not practical, and there is no guarantee that breakage will occur as expected. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a power transmission mechanism that reliably breaks in the vicinity of a desired load torque to accomplish suitable power cutoff. 
     To achieve the above object, this invention provides a power transmission mechanism for coupling a drive source to a driven machine in an interruptible manner. The power transmission mechanism includes a first rotary body, which rotates in synchronism with the drive source, and a second rotary body, which rotates in synchronism with the driven machine. Coupling means couples the first rotary body and the second rotary body in a synchronously rotatable manner. Engagement means engages with the coupling means when the load torque of the driven machine exceeds a predetermined value. The engagement means, which is in engagement with the coupling means, increases stress at a specific portion of the coupling means to break the coupling means. 
     A power transmission mechanism provided according to another aspect of this invention includes a first rotary body, which rotates in synchronism with the drive source, and a second rotary body, which rotates in synchronism with the driven machine. Coupling means couples the first rotary body and the second rotary body in a synchronously rotatable manner. As the load torque of the driven machine increases, the stress of the coupling means increases. Engagement means engages with the coupling means to increase the ratio of the change in the stress of the coupling means to the change in the load torque of the driven machine. The engagement means engages with the coupling means to break the coupling means when the load torque of the driven machine exceeds a predetermined value. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a variable displacement compressor according to one embodiment of the present invention; 
     FIG.  2 (A) is a front view of a power transmission mechanism equipped in the compressor in FIG. 1; 
     FIG.  2 (B) is a cross-sectional view taken along the line  2 B— 2 B in FIG.  2 (A); 
     FIG. 3 is a cross-sectional view of a receiving member taken along the line  3 — 3  in FIG.  2 (A); 
     FIG.  4 (A) is a front view of a boss of a pulley; 
     FIG.  4 (B) is a perspective view of the boss of the pulley; 
     FIG. 5 is an explanatory diagram showing the state of a coil spring in a power transmitting state; 
     FIG. 6 is an explanatory diagram showing the state of the coil spring immediately before breaking; and 
     FIG. 7 is a graph illustrating the relationship between the load torque of a compressor and stress which acts on a limit spring. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention, as embodied in a variable displacement compressor of an air-conditioning system for a vehicle, will now be described with reference to FIGS. 1 through 7. The compressor in this embodiment is called a clutchless compressor because it does not require a clutch mechanism, such as an electromagnetic clutch, between itself and an engine, or drive source. A power transmission mechanism according to this invention is used in place of such a clutch mechanism and has two functions, which are power transmission in a normal mode and power cutoff in an emergency mode. 
     As shown in FIG. 1, the vehicular air-conditioning system comprises a rocking swash plate type variable displacement compressor  10 , an external refrigeration circuit  30  and a controller  34 , which performs general control of the air-conditioning system. The external refrigeration circuit  30  has, for example, a condenser  31 , a temperature type expansion valve  32  and an evaporator  33 . The external refrigeration circuit and the compressor  10  constitute a refrigeration cycle. 
     The compressor  10 , or driven machine, has a cylinder block  11 , a front housing  12 , which is connected to the front end face of the cylinder block  11 , a valve plate  14  and a rear housing  13 , which is connected to the rear end face of the cylinder block  11  through the valve plate  14 . The cylinder block  11 , the front housing  12 , the rear housing  13  and the valve plate  14  constitute the housing of the compressor  10 . 
     A crank chamber  15  is defined between the front housing  12  and the cylinder block  11 . A drive shaft  16  is rotatably supported by the front housing  12  and the cylinder block  11 . In the crank chamber  15 , a lug plate  18  is fixed to the drive shaft  16 . The lug plate  16  contacts the inner wall of the front housing  12  via a thrust bearing  17 . A swash plate  19  as a drive plate is supported in the crank chamber  15  by the drive shaft  16  such that the swash plate  19  can tilt and slide in the axial direction. The swash plate  19  is coupled to the lug plate  18  via a hinge mechanism  20 . The lug plate  18  and the hinge mechanism  20  allow the swash plate  19  to slide and tilt with respect to the drive shaft  16  and rotate integrally with the drive shaft  16 . 
     A plurality of cylinder bores  11   a  (only one shown in FIG. 1) are located in the cylinder block  11 . The cylinder bores  11   a  are provided at equal intervals on a circle centered on axial line L of the drive shaft  16 . A one-headed piston  21  is retained in each cylinder bore  11   a  in a reciprocatable manner. One end of each piston  21  is coupled to the peripheral portion of the swash plate via a pair of shoes  22 . In each cylinder bore  11   a , a compression chamber is defined between the end face of the piston  21  and the valve plate  14 . As the drive shaft  16  rotates, the swash plate  19  rotates and each piston  21  reciprocates in the cylinder bore  11   a.    
     A suction chamber  25  and a discharge chamber  26  are defined in the rear housing  13 . The suction chamber  25  and the discharge chamber  26  are connected together by the external refrigeration circuit  30 . The valve plate  14  is constructed by stacking at least three metal plates. The valve plate  14  has suction ports and discharge ports in association with the individual cylinder bores  11   a . The valve plate  14  further has inlet valves  14   a , which are flapper valves, corresponding to the individual suction ports and discharge valves  14   b , which are flapper valves, corresponding to the individual discharge ports. When the piston  21  moves from the top dead center to the bottom dead center, the refrigerant gas in the suction chamber  25  pushes the inlet valve  14   a  open and flows into the cylinder bore  11   a . When the piston  21  moves from the bottom dead center to the top dead center, the refrigerant gas in the cylinder bore  11   a  is compressed to a predetermined pressure and pushes the discharge valve  14   b  open from the discharge port and is discharged into the discharge chamber  26 . 
     A supply passage  23 , which connects the crank chamber  15  to the discharge chamber  26 , is provided in the cylinder block  11 , the valve plate  14  and the rear housing  13 . Located in the supply passage  23  is a displacement control valve  24 , which is incorporated into the rear housing  13 . The displacement control valve  24  is, for example, an electromagnetic valve having a solenoid  24   a , a valve body  24   b  and a port  24   c . The port  24   c  constitutes a part of the supply passage  23 . The controller  34  supplies a current to the solenoid  24   a . When the solenoid  24   a  is excited, the valve body  24   b  closes the port  24   c , and when the solenoid  24   a  is deexcited, the valve body  24   b  opens the port  24   c.    
     A support hole  11   b  which supports the rear end of the drive shaft  16  is formed in nearly the center of the cylinder block  11 . A pressure-release passage  16   a  is formed in the drive shaft  16  to extend along the axis L. The pressure-release passage  16   a  has an inlet, which opens into the crank chamber  15 , and an outlet, which opens into the support hole  11   b . The support hole  11   b  is connected to the suction chamber  25  via a restriction hole  27 , which passes through the cylinder block  11  and the valve plate  14 . The pressure-release passage  16   a , the support hole  11   b  and the restriction hole  27  serve as a bleeding passage for allowing the refrigerant gas in the crank chamber  15  to escape into the suction chamber  25 . 
     The discharge displacement of the compressor  10  is changed by adjusting the pressure in the crank chamber  15  (crank pressure) with the displacement control valve  24 . Specifically, as the controller  34  controls the current supply to the control valve  24 , the position of the control valve  24  is adjusted. As a result, the relationship between the amount of the gas that is supplied into the crank chamber  15  from the discharge chamber  25  via the supply passage  23  and the amount of gas that flows into the suction chamber  26  from the crank chamber  15  via the bleeding passage changes, thus adjusting the crank pressure. 
     When the crank pressure rises, the inclination angle of the swash plate  19  becomes smaller and the stroke of each piston  21  becomes smaller, thus reducing the discharge displacement. When the crank pressure becomes lower, on the other hand, the inclination angle of the swash plate  19  becomes larger and the stroke of each piston  21  becomes larger, thus increasing the discharge displacement. 
     The controller  34  determines the level of the cooling load in a vehicle based on detection information from various sensors (not shown), including a temperature sensor provided on the evaporator  33 , and controls the current supply to the control valve  24  in accordance with the cooling load. Consequently, the angle of the control valve  24  changes and the crank pressure or the inclination angle of the swash plate  19  is determined in accordance with the inclination angle, so that the discharge displacement of the compressor  10  is adjusted to match the cooling load. As apparent from the above, the discharge displacement (compression performance) undergoes feedback control based on the control of the inclination angle of the swash plate  19  according to a change in cooling load. 
     As shown in FIG. 1, the maximum inclination angle of the swash plate  19  is restricted when a stopper  19   a  provided on the swash plate  19  abuts against the lug plate  18 . In addition, the minimum inclination angle of the swash plate  19  is restricted as the swash plate  19  abuts on a restriction ring  28  provided on the drive shaft  16 . In general, the minimum inclination angle is set slightly larger than 0° so that the stroke of the piston  21  does not become zero. 
     The power transmission mechanism provided in the compressor  10  will now be described. As shown in FIGS. 1,  2 (A) and  2 (B), a support cylinder  41  extends from the front end of the front housing  12 . An angular bearing  42  is provided around the support cylinder  41 . A pulley  43 , or a first rotary body, is fixed to the outer race of the angular bearing  42 . Therefore, the pulley  43  is supported to rotate with respect to the support cylinder  41 . The pulley  43  is coupled to a vehicular engine  35 , or a drive source, via a power transmission belt  44 , such as a V belt. The pulley  43  has a boss  43   a , which is attached to the outer race of the angular bearing  42 , an outer ring  43   b , on which the belt  44  is wrapped, a disc portion  43   c , which connects the boss  43   a  to the outer ring  43   b . An annular recess (or an annular groove)  46  is located in the area bounded by the boss  43   a , the outer ring  43   b  and the disc portion  43   c.    
     A receiving member  50  is fixed to the front end of the drive shaft  16  by a bolt  47 . Therefore, the drive shaft  16  and the receiving member  50  rotate together. The drive shaft  16  and the receiving member  50  form a second rotary body. 
     FIG. 3 shows the cross section of the receiving member  50  along the line  3 — 3  in FIG.  2 (A). As shown in FIGS.  2 (A),  2 (B) and  3 , the receiving member  50  has a cylinder portion  51 , which is fitted over the outer surface of the front end of the drive shaft  16 , and a pair of plate arm portions  52 , which extend from the outer end portion of the cylinder portion  51  in the radial direction. The plate arm portions  52  are arranged linearly, on opposite sides of the bolt  47 . That is, the pair of plate arm portions  52  are angularly spaced apart by 180° about the axis of the receiving member  50 . A step portion  52   a  is formed at the distal end of each plate arm portion  52 . 
     The receiving member  50  further has a pair of ribs  53  that extend in the radial direction. The ribs  53  constitute engagement means. The pair of ribs  53  is provided in association with the pair of plate arm portions  52 . The ribs  53  are each provided on the bottom surface of the associated plate arm portion  52 . 
     As shown in FIG.  2 (A), the distal end (the outermost end in the radial direction) of each rib  53  extends to the position of the outer surface of the boss  43   a  of the pulley  43 . In other words, the distance from the axial center of the receiving member  50  to the distal end of the rib  53  coincides with the radius of the outermost periphery of the boss  43   a.    
     As shown in FIGS.  2 (A) and  2 (B), a limit spring  60  as coupling means is placed around the boss  43   a  of the pulley  43 . The limit spring  60  comprises first and second torsion coil springs  601  and  602 . Both coil springs  601  and  602  are made of metal. Each of the coil springs  601 ,  602  has a body portion  61  formed in a helical shape and a first end portion  62  and a second end portion  63 , which are located at the ends of the body portion  61 . In FIG. 5, only one of the coil springs  601  and  602  is shown. 
     As shown in FIGS.  2 (A) and  5 , the first and second end portions  62  and  63  of each torsion coil spring  601 ,  602  are positioned outside the helical cylinder that the body portion  61  defines. As shown in FIGS.  2 (A) and  2 (B), each first end portion  62  is fixed by rivets to the disc portion  43   c  at a corner portion which is formed by the inner surface of the outer ring  43   b  of the pulley  43  and the disc portion  43   c . Each second end portion  63  is fixed to the step portion  52   a  of the plate arm portion  52  of the receiving member  50  by rivets. 
     The body portion  61  of each of the torsion coil springs  601 ,  602  is held between the outer surface of the boss  43   a  and the inner surface of the outer ring  43   b  without contacting them. That is, with the first and second end portions  62  and  63  respectively fixed to the disc portion  43   c  and the plate arm portion  52 , the radius of the helical cylinder defined by the body portion  61  is set in such a way as to be greater than the radius of the outer surface of the boss  43   a  and smaller than the inside diameter of the outer ring  43   b . The outside diameter of the cylindrical boss  43   a  is smaller than at least the diameter of each of the coil springs  601 ,  602  in the normal state. 
     Each body portion  61  is wound around the boss  43   a  approximately two and half helical turns. Note that the portion of the body portion  61  that faces the outer surface of the boss  43   a  ranges from the first end portion  62  to about one and half turns to about two turns, and the remaining portion close to the second end portion  63  (about one turn to about a half turn) is located forward of the distal end of the boss  43   a , as shown in FIG.  2 (B). That is, the limit spring  60  has a first portion (rear half) arranged around the boss  43   a  to face the boss  43   a  in the radial direction and a second portion (front half) which does not face the boss  43   a  in the radial direction. The ribs  53  of the receiving member  50  are also located forward of the distal end of the boss  43   a . In FIGS.  2 (A),  5  and  6 , the annular end face  48  of the distal end of the boss  43   a  has a flecked pattern to help understand the drawings. 
     As shown in FIGS.  4 (A),  4 (B) and  5 , the annular end face  48  is provided with engagement projections  491  and  492  (only one of the engagement projections  491  and  492  is shown in FIG.  5 ). The engagement projections  491  and  492  are formed to extend from the annular end face  48 . The engagement projections  491  and  492  are located at positions of 180° from each other about the axis of the boss  43   a . The first engagement projection  491  is associated with the coil spring  601  and the second engagement projection  492  is associated with the coil spring  602 . 
     For example, the first coil spring  601  has a layout relation with the first engagement projection  491  and one of the ribs  53  as shown in FIG. 5 with the end portions  62  and  63  fixed to the pulley  43  and the receiving member  50 . FIG. 6 shows the state in which the first coil spring  601  is on the verge of breaking as a result of relative rotation between the pulley  43  and the receiving member  50  caused by excess load torque generated in the inner mechanism of the compressor. At this time, the first engagement projection  491  and the rib  53  are arranged opposite to each other (angularly separated by almost 180°). The second coil spring  602 , the second engagement projection  492  and the rib  53 , which works in cooperation with the projection  492 , have a layout relationship similar to that described above. 
     Each of the engagement projections  491  and  492  serves as a hook portion to prevent a part of the spring wound around the outer surface of the boss  43   a  from coming off that outer surface when the diameter of the associated coil spring  601 ,  602  is reduced. 
     As shown in FIG.  2 (A), the end portions  62  and  63  of the torsion coil spring  601  and those of the torsion coil spring  602  are located at angularly separated positions different from each other by approximately 180° about the bolt  47 . The torsion coil springs  601  and  602  are joined to constitute the single limit spring  60 . Therefore, the limit spring  60  serves as a double torsion coil spring having two wires wound to be parallel to each other. 
     As shown in FIG.  2 (B), the rear half of the limit spring  60  is retained in the annular recess  46  of the pulley  43 , and the front half of the limit spring  60  is exposed outside of the annular recess  46 . The limit spring  60  is located, compressed in the axial direction, between the disc portion  43   c  of the pulley and the receiving member  50 . Therefore, the restoring force of the limit spring  60  urges the receiving member  50  and the drive shaft  16  forward. 
     As apparent from the above, the pulley  43  is coupled to the receiving member  50  and the drive shaft  16  in a power transmittable manner via the limit spring  60 , which includes two torsion coil springs  601  and  602 . The limit spring  60  therefore serves as a coupling means that couples the first rotary body and the second rotary body in a synchronously rotatable manner. 
     The operation of this embodiment will now be discussed with reference to FIGS. 5 to  7 . Note that FIGS. 5 and 6 omit the receiving member  50  and show only one of the two coil springs  601  and  602  for easier understanding. 
     The power of the engine  35  is normally transmitted to the drive shaft  16  via the belt  44 , the pulley  43 , the limit spring  60  (torsion coil springs  601  and  602 ) and the receiving member  50 . That is, the supply torque of the engine  35  is balanced with the load torque of the compressor  10 , and the pulley  43  and the drive shaft  16  synchronously rotate with the angular velocity ω 1  of the pulley  43 , which is equal to the angular velocity ω 2  of the receiving member  50 , and the drive shaft  16  as shown in FIG.  5 . In this case, the body portion  61  of each torsion coil spring  601 ,  602  is kept separated from the outer surface of the boss  43   a  of the pulley. 
     In accordance with the power transmission to the drive shaft  16 , the swash plate  19  coupled to the drive shaft  16  causes the individual pistons  21  to reciprocate. The pistons  21  perform suction and compression of the refrigerant gas. In accordance with this work (load condition), a load torque in the opposite direction to the rotational direction of the pulley  43  acts on the drive shaft  16  and the receiving member  50 . If the load torque does not exceed a predetermined limit value and is not large enough to impart an undesirable influence on the engine  35 , however, the power transmission to the receiving member  50  and the drive shaft  16  from the pulley  43  via both coil springs  601  and  602  is maintained. As long as this power transmission is maintained, even if the load torque varies under the predetermined limit value due to a phase shift of the pressure change in each cylinder bore  11   a , a variation in the compression load or the like, such a variation in load torque is sufficiently accommodated by the spring elasticity of the coil springs  601  and  602 . 
     When some kind of problem (e.g., seizure) occurs inside the compressor and the load torque of the compressor  10  exceeds the predetermined limit value, on the other hand, a difference between the angular velocity ω 1  of the pulley  43  and the angular velocity ω 2  of the receiving member  50  and the drive shaft  16  (see FIG. 6; ω 2 ′&lt;ω 1 ) occurs. That is, the pulley  43  and the receiving member  50  and the drive shaft  16  do not rotate synchronously. Specifically, while the first end portions  62  of the coil springs  601  and  602  coupled to the pulley  43  try to stay in synchronous rotation with the pulley  43 , the second end portions  63  coupled to the receiving member  50  strongly resist synchronous rotation with the pulley  43 , producing an angular velocity difference (ω 1 −ω 2 ′) between the ends  62  and  63 . 
     This angular velocity difference deforms each coil spring  601 ,  602  such that its diameter decreases. As a result, as shown in FIG. 6, the rear half of the body portion  61  of the coil spring  601  (or  602 ) is wound around the outer surface of the boss  43   a  of the pulley  43  tightly and a part of the front half of the body portion  61  abuts against the distal end of the rib  53 . When the rear half of the body portion  61  is wound around the outer surface of the boss  43   a , further deformation is restricted. 
     Based on the angular velocity difference between the pulley  43  and the receiving member  50 , the engagement projection  491  (or  492 ) is positioned as shown in FIG. 6 with respect to the rib  53 . As twisting is further applied to each of the coil springs  601 ,  602  in the direction of reducing its diameter, the boundary portion between the front half and the rear half of the body portion  61  is bent inward of the cylinder defined by the outer surface of the boss  43   a  at the engagement projection  491  (or  492 ) and the portion of the body portion  61  that is in contact with the rib  53  is further bent sharply. As a result, stress due to the twisting of each of the coil springs  601 ,  602  concentrates particularly at the portion in contact with the rib  53 , so that the body portion  61  finely breaks at that location. 
     This embodiment uses two coil springs  601  and  602 , and if one coil spring breaks, all the load torque is applied to the remaining coil spring so that the remaining coil spring breaks immediately. When an excess load torque that exceeds a predetermined limit value is produced, both coil springs  601  and  602  break almost simultaneously, so that power transmission to the drive shaft  16  from the engine  35  is positively discontinued. 
     FIG. 7 is a graph illustrating the relationship between the torque applied to the limit spring  60  (coil springs  601 ,  602 ) from the compressor (i.e., load torque) and the stress that acts on the limit spring  60 . In this graph, the solid line indicates the characteristics of the power transmission mechanism according to this embodiment, and the two-dot chain line indicates the characteristics of a comparative example equivalent to the structure of the power transmission mechanism of this embodiment except that the pair of ribs  53  and the pair of engagement projections  491  and  492  are not present. Because the coil springs in use in both cases are the same, a range F from the upper limit to the lower limit of the stress (rupture stress) that is needed to break the body portion  61  is the same in both cases. 
     Since the line representing the characteristics of the comparative example has the same slope over the entire range of the applied torque, a range T 2  of the applied torque corresponding to the rupture stress range F also becomes relatively wide as shown in FIG.  7 . In contrast, the slope of the line representing the characteristics of this embodiment suddenly increases at a transition point B. That is, the transition point B indicates the time when the body portion  61  contacts the distal end of the rib  53 . In the range of the applied torque before the transition point B, the body portion  61  and the rib  53  are not in contact with each other, and the slope of the characteristic line does not differ between this embodiment and the comparative example. 
     In this embodiment, after the body portion  61  contacts the distal end of the rib  53 , however, the stress caused by the load torque concentrates at the point of contact with the rib  53  so that the stress tends to rise sharply from there. Since the aforementioned rupture stress range F corresponds to the torque range after the transition point B where the slope of the characteristic line is large, the range T 1  for the applied torque corresponding to that stress range F is relatively narrow (T 1 &lt;T 2 ). Therefore, that the range of the load torque for breaking the spring is narrower in this embodiment than in the comparative example and power transmission can positively be cut off when the load torque of the compressor approximately reaches the expected limit value (i.e., the break-expected torque). 
     This embodiment has the following effects. 
     The provision of the ribs  53  narrows the range T 1  of the load torque corresponding to the rupture stress range F of both coil springs  601  and  602  so that the coil springs  601  and  602  can be broken with certainly at the load torque at which breaking is expected, thus adequately accomplishing power cutoff. It is therefore possible to guarantee protection of the engine  35  or the like against excess load torque. 
     Until the load torque of the compressor reaches the break-expected torque, the rear half of the twisted coil springs  601 ,  602  are be wound around the outer surface of the boss  43   a . During this period, each of the coil springs  601 ,  602  and the boss  43   a  rotate synchronously, so that there is no abnormal sound produced by the winding, and no wear or the like occurs between the coil springs  601 ,  602  and the boss  43   a.    
     When the coil springs  601 ,  602  are twisted further by the load torque from the state where the rear half of the coil springs  601 ,  602  are wound around the outer surface of the boss  43   a , a part of each coil spring  601 ,  602  is hooked on the associated engagement projection  491 ,  492  of the boss  43   a  and is further bent there. The presence of the engagement projection  491 ,  492  as a hook prevents the rear halves of the coil springs  601 ,  602 , which are wound around the outer surface of the boss  43   a  from coming off the boss  43   a  when twisting is further applied. Therefore, the twisting action caused by the load torque on the front halves of the coil springs  601 ,  602 , which are located in front of the boss  43   a , is concentrated, so that even a slight increase in load torque increases the amount of deformation of the coil spring  601 ,  602  at the point where the rib  53  makes contact. In this sense, the engagement projections  491  and  492  are means for aiding the breaking action of the ribs  53 . 
     The limit spring  60  includes a plurality of coil springs  601 ,  602 . The end portions  62  and  63  of the coil spring  601  and those of the coil spring  602  are coupled to other members at equal angular distances (i.e., angular positions different by 180°). During power transmission from the engine  35 , therefore, a moment that tilts the drive shaft  16  with respect to the axis L is not produced and the rotation of the receiving member  50  and the drive shaft  16  is stable, and torque is transmitted efficiently. Since the two torsion coil springs  601  and  602  support each other, the postures of the coil springs  601  and  602  are stable when the two coil springs  601  and  602  are combined. 
     If a design that allows the first end portion  62  of each coil spring  601 ,  602  to be engageable with and disengageable from the inner surface portion of the outer ring  43   b  of the pulley  43  is employed, wear may occur at the engagement location. Because both end portions  62  and  63  of each coil spring are secured to the respective members in this embodiment, by way of contrast, there is no need to consider wear. 
     Since the metal coil springs  601  and  602  are means that couples the first rotary body and the second rotary body, it is possible to set the spring constant of the limit spring  60  considerably low (more specifically, lower than the spring constant of an ordinary rubber damper). This makes it possible to set the resonance frequency of the power transmission system lower than the minimum frequency of a variation in the load torque that occurs in the compressor  10 , or the driven machine. It is therefore possible to reduce noise and abnormal vibration due to resonance based on the load torque variation and to prevent the inner mechanism of the compressor  10  from being damaged(see Japanese Patent Application No. Hei 9-30075 filed by the present applicant for more details). 
     Unless the load torque produced by the compressor exceeds a predetermined limit value, variation in the torque that acts on the drive shaft  16  can be suppressed by the twisted deformation of the limit spring  60 . That is, the limit spring  60  also serves as a damper. 
     Because the power transmission mechanism of this embodiment does not require a vibration isolating member such as a rubber cushion, it has fewer components and is simple. 
     The intervening limit spring  60 , which is axially compressed between the pulley  43  and the receiving member  50 , also urges the drive shaft  16  together with the receiving member  50  frontward. This suppresses rattling of the drive shaft  16  in the axial direction. It is therefore unnecessary to consider the provision of a special spring member or the like for urging the drive shaft  16  in the axial direction. The limit spring  60  therefore also contributes to simplifying of the compressor  10 . 
     The above-described embodiment may be modified in the following forms. 
     The portion of the body portion  61  of each coil spring  601 ,  602  that contacts the rib  53  and its neighboring portion may be quenched by means such as a laser to make that portion harder than the other portions. The increase in hardness of the coil spring  601 ,  602  makes that portion more susceptible to stress fracture. 
     Although the portion of each of the coil springs  601 ,  602  that is closer to the receiving member  50  (part of the front half) is designed to break in the embodiment in FIGS. 1 through 6, the portion of the coil spring  601 ,  602  that is closer to the pulley (part of the rear half) may be designed to break. 
     Even if the part of the front half of each coil spring  601 ,  602  is intended to break, it is unnecessary to make the ribs  53  as a main cutoff member and make the engagement projections  491  and  492  as a break-aiding member. The shapes of the engagement projections  491  and  492  may be changed so that the engagement projections  491  and  492  themselves become a main cutoff member. 
     The engagement projections  491  and  492  may be omitted. 
     It is not essential to angularly separate the rib  53  and the engagement projection  491  (or  492 ) opposite to each other by 180°, sandwiching the axial center of the boss  43   a  as shown in FIG. 6, when the compressor is overloaded. On the verge of breaking (see FIG.  6 ), they may have such a layout relation that the angle θ that is formed by the rib  53 , the axial center of the boss  43   a  and the engagement projection  491  (or  492 ) is less than 180°. However, note that if the angle θ is too small, the cooperative and synergetic effect of the rib  53  and the engagement projection  491 ,  492  may become low. 
     The shape of the engagement projection  491 ,  492  is not limited to the one shown in FIG. 4, but it may be a hook pin protruding from the annular end face  48  of the boss  43   a . Alternatively, the hook portion may be protrude from the outer surface of the boss  43   a.    
     One of the two torsion coil springs  601  and  602  may be omitted so that the limit spring  60  is comprised of a single coil spring. Alternatively, the limit spring  60  may be constructed by using three or more coil springs. That is, the limit spring  60  includes at least one coil spring. 
     The compressor  10  in FIG. 1 may be additionally provided with an electromagnetic clutch.