Abstract:
A thrust ball bearing provided in an open scroll-type compressor can withstand a large compressive load on the bearing and prevents wear of the mating surfaces. The thrust ball bearing has a first thrust plate  9   a  for performing spiral revolution; a second thrust plate  4   a  opposing the first thrust plate  9   a ; and a retaining device  40  disposed between the thrust plates  4   a,    9   a  and provided with retaining cavities  42  for retaining a ball  41  in each cavity where a radius ρ of spiral revolution of the first plate, a diameter d of the ball and a diameter D of the retaining cavity are related by an expression D≧d×ρ.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to open-type scroll compressors, and relates in particular to an open-type scroll compressor that can be operated in the critical region of a cooling medium such as carbon dioxide to provide vapor compression cooling cycles. 
     2. Description of the Background Art 
     From the standpoint of environmental protection, there has been proposals, in recent years, to use carbon dioxide gas as a replacement for freon gas as a working gas (cooling medium) to provide cooling cycles (referred to as CO 2  cooling cycles), for example, in a Japanese Patent Application, First Publication Hei  7-18602 . The operation of the CO 2 -based cooling cycle is similar to that of the conventional vapor compression cooling cycle based on freon. That is, as shown in FIG. 7 (using a Mollier diagram for CO 2 ) by an A-B-C-D-A cycle, the compressor compresses gas-phase CO 2  (A-B), and the compressed high temperature gaseous CO 2  is cooled in a heat dissipater (gas cooler) (B-C). Next, the gas pressure is reduced (C-D) in a pressure reducer, and the condensed liquid-phase CO 2  is vaporized (D-A) so that the latent heat of vaporization is gained from an external fluid medium such as air thus resulting in cooling the external fluid. 
     However, because the critical temperature for CO 2  is 31° C. which is lower than that of freon, which is the conventional cooling medium, so that when the outside temperature is high, such as during the summer season, the temperature of CO 2  in the heat dissipater circuit becomes higher than the critical temperature of CO 2 . In other words, CO 2  does not condense at the exit-side of the heat dissipater (line BC does not cross saturated liquid line SL). Also, because the conditions at the exit-side (point C) of the heat dissipater are determined by the discharge pressure of the compressor and the temperature of CO 2  at the exit-side of the heat dissipater and the temperature of CO 2  at the exit-side of the heat dissipater is determined by the heat releasing capability of the heat dissipater and the outside temperature (not controllable), the temperature at the exit-side of the heat dissipater cannot be controlled in practice. Therefore, it follows that it is possible to control the conditions at the exit-side (point C) of the heat dissipater by controlling the discharge pressure of the compressor (heat dissipater exit-side pressure). In other words, to obtain sufficient cooling capacity (enthalpy difference) when the external temperature is high such as during the summer season, it is necessary to increase the heat dissipater exit-side pressure as shown by a cycle E-F-G-H-E. For this reason, it is necessary to increase the operating pressure of the compressor for CO 2 -based cooling cycle compared with that for conventional freon-based cooling cycle. 
     For example, in an automobile air conditioner, operating pressure required for conventional R134-based (freon-based) compressor is about 3 kg/cm 2  while it is 40 kg/cm 2  for CO 2  based compressor, and the stationary pressure is about 15 kg/cm 2  for R134 (freon) while that for CO 2  is 100 kg/cm 2 . Therefore, it is necessary for the compressor to be built to withstand the pressure of such a high magnitude. 
     An example of the compressor used in the conventional automobile air conditioner is shown in FIG.  8 . As shown in this diagram, a spiraling scroll  52  is provided inside a housing  51 , and a fixed scroll  53  for engaging with the spiraling scroll  52  is situated above the spiraling scroll  52 . 
     Inside a cylindrical boss  54  formed in the center section of the outer surface (lower surface in the diagram) of the end plate  52   a  of the spiraling scroll  52 , an eccentric shaft  55  is freely rotatably supported by a scroll bearing  56 , which also serves as the radial bearing. The eccentric shaft  55  is able to rotate eccentrically with a radius p by means of an eccentric drive, which is omitted from the diagram. 
     Also, between the outer surface periphery of the end plate  52   a  and the fixed frame  57  fixed to the housing  51 , a thrust ball bearing  58  is provided to support the spiraling scroll  52 . 
     This thrust ball bearing  58  is comprised by a pair of ring shaped race members  59  mounted on the fixed frame  57  and the spiraling scroll  52  and balls  60  intervening between the race members  59 . On opposing surfaces of the pair of race members  59 , spiraling race grooves  61  are disposed in several places for providing rolling motion of the balls  60 . These race grooves  61  are formed in an arc shape such that the profile radius of the groove is slightly larger than that of the balls  60 . 
     The operation of the thrust bearing so constructed will be explained below. The spiraling scroll  52  is driven by the eccentric shaft  55  to produce spiral revolution with a scroll radius ρ. During the motion, the fixed frame  57  is coupled to the spiraling scroll  52  by means of the balls  60  intervening between the race members  59 , and, because the rolling range of the balls  60  is restricted by the race grooves  61 , the spiraling scroll  52  is prevented from self-rotating about its own axis. 
     Also, a large axial load is applied to the spiraling scroll  52  by the pressure from the compressed gas, but the axial load is supported by the balls  60  and the race members  59 . 
     The thrust ball bearing  58  described above not only supports the load in the thrust direction but also prevents self-rotation of the spiraling scroll  52 . 
     In other words, because the fixed frame  57  and the spiraling scroll  52  are coupled by means of the balls  60 , the race grooves  61  of the race members  59  on the fixed frame side slide against the balls  60 , and the race grooves  61  of the race members  59  on the spiraling scroll side slide against the balls  60 . 
     Specifically, as shown in FIG. 9, while the ball  60  is under the load Ft in the axial direction generated by the compressed gas acting, it is also under a pressing force Fh acting in the left/right direction resulting from the tendency of the spiraling scroll  52  to self-rotate about its own axis. The ball  60  exerts a reaction force to this pressing force Fh to prevent self-rotation of the spiraling scroll  52 . 
     However, because the ball  60  is rolling on the race groove  61 , the pressing force Fh acting in the left/right direction causes the ball  60  to slide against the race groove  61 , thereby generating friction at the interface. For this reason, lubrication film between the ball  60  and the race groove  61  is lost and the mechanical loss is increased, and the ball  60  and race groove  61  are worn by the friction to lead to shortening the service life of the bearing. 
     Frictional effects become significantly higher the higher the load on the bearing. This effect becomes particularly severe when CO 2  is used as the working gas because the compressed gas pressure is higher compared with freon gas, and presents a problem that the bearing service life is reduced considerably. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a thrust ball bearing that can withstand higher loading than conventional thrust ball bearings while preventing frictional effects between the moving parts, and to provide an open-type scroll compressor incorporating such a thrust ball bearing. 
     The object has been achieved in a thrust ball bearing comprising: a first thrust plate for providing spiral revolution; a second thrust plate opposing the first thrust plate; a retaining device, provided with retaining cavities for retaining a ball in each cavity, disposed between the first thrust plate and the second thrust plate; and balls retained in said retaining cavities; wherein a radius ρ of spiral revolution of the first plate, a diameter d of the balls and a diameter D of the retaining cavity are related by an expression D≧d+ρ. 
     In this thrust ball bearing, because the diameter of each retaining cavity is related by the expression D≧d+ρ, there is no interference with the revolution motion of the ball. 
     In other words, as shown in FIG. 3, ball  41  of the thrust ball bearing is under compression from the first and second thrust plate  4   a ,  9   a , and revolves in-phase with the spiraling scroll (revolving spiral body)  9 . In the condition shown in FIG. 3A, ball  41  is positioned at its uppermost position, and is in contact with the interior surface of the retaining cavity of the retaining device  40 . 
     FIG. 3B shows a condition of the ball  41  when the revolving action of the spiraling scroll  9  has caused the ball  41  to move to a position 180° away from the position shown in FIG.  3 A. In FIG. 3B, the ball  41  is positioned at its lowermost position, and the ball  41  is in contact with the outside surface of the retaining cavity of the retaining device  40 . 
     Because the spiraling scroll  9  is revolving with a scrolling radius of ρ, the ball  41  that is revolving in contact with the spiraling scroll  9  is revolving at the scroll radius ρ/2. As shown in FIG. 3B, the diameter of the moving trace of the ball  41  is d/2+ρ+d/2=d+ρ. 
     Because the diameter D of the retaining cavity  42  satisfies the condition D≧d+ρ, the retaining device does not interfere with the revolution motion of the ball  41 . 
     Therefore, friction is not generated between the ball and the interior surface of the retaining cavity or thrust plate, thereby enabling to prevent the wear of the interior surface of the retaining cavity, thrust plate, and the ball. 
     A second aspect of the thrust ball bearing is that a noise proofing material is disposed at least on interior surfaces of the retaining cavities of the retaining device. 
     When the spiraling scroll starts to spiral, the balls in the thrust ball bearing start rolling from a stationary position to an operating position. When this happens the balls impact the interior surface of the retaining cavity, but because noise proofing material is provided at least on the interior surface of each retaining cavity of the retaining device, it is possible to reduce the noise level generated when the ball impacts the interior surface of each retaining cavity. 
     Possible noise proofing materials should include materials having a low elastic coefficient, and are effective in suppressing the noise generated when the balls hit the respective interior surfaces that are made from a material including resins, synthetic rubber and vibration-controlling alloys etc. 
     When using rubber and resins, the entire retaining device may be made of such materials to be economical, but if a vibration-controlling, alloy is being used, it may be more practical to provide it partially in the interior surface of the retaining cavity because of its high cost. 
     Furthermore, if an open scroll-type compressor is provided with the thrust ball bearing as described above, it is possible to reduce wear of the interior surface of the retaining cavity, thrust plates and the balls. 
     Using the open scroll type compressor provided with a noise proofing material disposed at least on interior surfaces of the retaining cavities of the retaining device, it is possible to reduce the noise level produced when the balls impact the interior surface of the retaining cavity. 
     In another aspect of the open scroll type compressor, the working gas for producing the cooling cycle includes carbon dioxide gas, thereby making the invention applicable to those thrust ball bearings where high operating pressure and loading are required. 
     The beneficial effects of present invention are summarized in the following. 
     According to the present invention, the diameter of the retaining cavity of the retaining device is given by the expression D≧d+ρ. Therefore, friction is not generated between the interior surface of the retaining cavity and the ball so that the mechanical loss can be reduced. Further, because the wear of the interior surface of the retaining cavity, thrust plates and balls can be prevented, the service life of the thrust ball bearing can be increased and the tolerance against the bearing load on the thrust ball bearing can be increased. 
     According to the present invention, because the noise proofing material is provided at least in the retaining cavity of the retaining device, it is possible to reduce the noise generated by impact of the ball with the interior surface of the retaining cavity when the compressor is started up. 
     According to the present invention, because the wear of the retaining cavity, thrust plates and balls is prevented in the present thrust ball bearing, the load on the bearing can be increased. 
     According to the present invention, the noise generated when the balls impact the interior surface of the retaining cavity of the present thrust ball bearing can be reduced. 
     The present invention can be a thrust ball bearing operated in the cooling cycle utilizing carbon dioxide as a working gas and tolerating high working pressures and high bearing loads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of an embodiment of the open scroll-type compressor of the present invention. 
     FIG. 2 is a plan view of a retaining device of the thrust ball bearing used in the open scroll-type compressor. 
     FIG. 3A,  3 B are enlarged views of the vicinity of the thrust ball bearing shown in FIG.  1 . 
     FIG. 4A,  4 B are diagrams to show the rolling directions of the ball in the above thrust ball bearing. 
     FIG. 5 is a cross sectional view of the retaining device of the thrust ball bearing in another embodiment of the present invention. 
     FIG. 6 is a schematic diagram to show a cooling cycle in a vapor compression type compressor. 
     FIG. 7 is a Mollier diagram of CO 2 . 
     FIG. 8 is a cross sectional view of a conventional thrust ball bearing. 
     FIG. 9 is an enlarged view of the vicinity of the ball in the thrust ball bearing shown in FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, preferred embodiments of the present open scroll type compressor will be explained with reference to the drawings. 
     First, a cooling cycle with CO 2  gas in the present scroll type compressor will be explained with reference to FIG.  6 . The CO 2  cycle S in this compressor represents an example of the application to automobile air conditioner comprising open scroll-type compressor  1  (referred to as the compressor  1  hereinbelow) that compresses gas-phase CO 2 . The compressor  1  is operated by a driving device, an engine in this case, which is omitted in the diagram. A gas cooler (heat dissipater)  1   a  cools the compressed CO 2  by heat exchange with the open air, and choke  1   c  controls an exit pressure of the gas cooler  1   a  according to the temperature of CO 2  at the exit-side of the gas cooler  1   a . The pressure of CO 2  gas is reduced by the actions of the pressure control valve  1   b  and a choke  1   c , and the gaseous CO 2  transforms into a two-phase CO 2  comprised by a low-pressure liquid phase and a vapor phase at a low-temperature. A vaporizer (heat absorber)  1   d  serves as a car air cooling device, and the two-phase CO 2  in the vaporizer gassifies (vaporizes) by obtaining the latent heat of vaporization from the car interior air, thereby cooling the interior air. An accumulator  1   e  temporarily stores vapor phase CO 2 . The compressor  1 , gas cooler  1   a , pressure control valve  1   b , choke  1   c , vaporizer  1   d  and an accumulator  1   e  are interconnected with each other by respective piping  1   f  to form a close circuit. 
     An embodiment of the compressor  1  will be explained with reference to FIGS.  1 ˜ 4 . 
     The compressor  1  is provided with a housing  1 A (compressor main body) and the housing  1 A is comprised by a cup-shaped main casing  2  and a front casing  4  linked to the main casing  2  by means of bolts  3 . A crank shaft  5  gets through the front casing  4 , and is freely rotatably supported in the front casing  4  through a main bearing  6  and a sub-bearing  7 . The crank shaft  5  is operated by transmitting the rotational force of the car engine (not shown) by means of a known type of electromagnetic clutch  32 . Here, the reference numerals  32   a ,  32   b  refers to a coil and a pulley of the electromagnetic clutch  32   
     In the interior of the housing  1 A are disposed a fixed scroll  8  and a spiraling scroll (revolving spiral member)  9 . 
     The fixed scroll  8  is provided with an edge plate  10  and a spiral protrusion  11  erected on the inside surface of the edge plate  10 , and a back pressure block  13  is undetachably fixed to the back surface of the edge plate  10  by means of bolts  12 . O-rings  14   a ,  14   b  are embedded respectively on the internal and external peripheries of the back pressure block  13 , and serve to isolate a high pressure chamber  16  (to be described later) from a low pressure chamber  15  disposed within the main casing  2  by intimately contacting the inside surface of the main casing  2 . The high pressure chamber  16  is comprised by an interior space  13   a  of the back pressure block  13  and a depression  10   a  formed on the back surface of the end plate  10  of the fixed scroll  8 . 
     The spiraling scroll  9  is provided with an end plate  17  and a spiral protrusion  18  erected on the inside surface of the end plate  17 , and the shape of the spiral protrusion  18  is essentially the same as that of the spiral protrusion  11  of the fixed scroll  8 . 
     A ring-shaped plate spring  20   a  is disposed between the fixed scroll  8  and the front casing  4 , and is joined around its periphery by means of bolts  20   b , alternately, to fixed scroll  8  and front casing  4 . By this arrangement, the fixed scroll  8  is allowed to move only in the axial direction to a maximum allowable deflection of the plate spring  20   a  (float structure). Also, the ring-shaped plate spring  20   a  and bolts  20   b  constitute a fixed scroll support device  20 . Because of the provision of a space c provided between the protrusion section of the back pressure block  13  and the housing  1 A, the back pressure block  13  is able to move in the axial direction along with the fixed scroll  8 . 
     The fixed scroll  8  and the spiraling scroll  9  are offset by a radius of the spiral revolution and before they are coupled to each other as illustrated, they are rotated with respect to the other so as to be out of phase by 180 degrees, and a chip seal (not shown) imbedded at the tip of the spiral protrusion  11  touches the inside surface of the end plate  17 , and a chip seal (not shown) imbedded at the tip of the spiral protrusion  18  touches the inside surface of the end plate  10 , and the side surface of the spiral protrusions  11  and that of the spiral protrusions  18  touch each other intimately at several locations. This arrangement delineates a plurality of sealed spaces  21   a ,  21   b  that are distributed approximately symmetrically about the center of the spiral protrusions. Between the fixed scroll  8  and the spiraling scroll  9  is provided a self-rotation prevention ring  27  for preventing rotation of the spiraling scroll  9  about its own axis while permitting it to revolve about the eccentric shaft. 
     In the interior of a cylindrical boss  22  formed in the center section of outer surface of the end plate  17 , a drive bushing  23  is freely rotatably housed in a spiral bearing  24  that serves also as a radial bearing, and an eccentric shaft  26  extended to the inner end of the crank shaft  5  is freely rotatably coupled into a throughole  25  bored into the drive bushing  23 . 
     A known mechanical seal  28  is disposed on the outer periphery of the crank shaft  5 , and the mechanical seal  28  is provided on the outer side of the main bearing  6 . 
     Also, between the outer surface of the end plate  17  and the front casing  4 , a thrust ball bearing  19  is disposed for supporting the spiraling scroll  9 . Details of the thrust ball bearing will be explained below. 
     Ring-shaped thrust plates  4   a ,  9   a  are disposed in association with the front casing  4  and the spiraling scroll  9 , respectively, about the eccentric shaft  26 . The thrust plate  4   a  is fastened to the front casing  4  by means of bolts  4   b , and the thrust plate  9   a  is fastened to the back surface of the end plate  17  of the spiraling scroll  9  by means of bolts  9   b.    
     The thrust plate  9   a  associated with the spiraling scroll  9  is made of a flat plate material. The opposing thrust plate  4   a  is attached to a retaining device  40 . The retaining device  40  together with the thrust plate  4   a  is attached as a unit to the front casing  4  by means of bolts  4   b . 
     As shown in FIG. 2, the retaining device  40  is a ring-shaped device made of a polymeric resin to serve as a noise reducer. The material of the retaining device  40  may include nylon, Teflon and epoxy resins. 
     Also, the balls  41  serving as a rolling member are held individually in a plurality of retaining cavities  42  disposed circularly, and the diameter D of the retaining cavity is given by a relation D=d+ρ, as shown in FIG. 3A where ρ represents scroll radius of the spiraling scroll  9  and d represents the diameter of the ball  41 . Also, the thickness of the retaining device  40  is made thinner than the ball diameter d, so that a ball  41  retained in the retaining device  40  extends beyond the wall surface of the retaining device  40  and abuts thrust plates  4   a ,  9   a.    
     Next, the operation of the scroll type compressor  1  will be explained. 
     When the electromagnetic clutch is turned on, the rotational force of the car engine is transmitted to the crank shaft  5 , which transmits the rotational force to the spiraling scroll  9  by means of the spiral drive mechanism comprised by the eccentric shaft  26 , throughole  25 , drive bushing  23 , spiral bearing  24 , and the boss  22 , so that the spiraling scroll  9  is made to undergo a circular trace generated by a spiraling revolution with the revolving scroll radius ρ while the rotation of the spiraling scroll  9  itself is being prevented by the action of the self-rotation prevention ring  27 . 
     When the spiraling scroll  9  revolves, the line contact sections formed by the spiral protrusions  11 ,  18  gradually moves towards the center of the spiral, causing the sealed spaces  21   a ,  21   b  to move towards the center of the spiral while reducing the volume of the sealed spaces  21   a ,  21   b . This movement between the spirals causes that the working gas introduced through the inlet opening into the low pressure chamber  15  (represented by a broad arrow A in FIG. 1) is trapped inside the sealed space  21  a from outer end opening sections of the two spiral protrusions  11 ,  18  and is led to the center section  21   c  while it is being compressed, and the compressed gas passes through a discharge port  34  bored into the end plate  10  of the fixed scroll  8  and pushes open the discharge valve  35  to jet into the high pressure chamber  16  and flows out of the discharge opening  38 . Accordingly, due to the spiraling motion of the spiraling scroll  9 , the fluid introduced from the low pressure chamber  15  is compressed within the sealed spaces  21   a ,  21   b , and the compressed gas is discharged. 
     And, although the spiraling scroll  9  is pressed in the axial direction due to the pressing force of the compressed gas, the thrust ball bearing  19  supports this load. 
     In the following, the function of the thrust ball bearing  19  will be explained. 
     FIG. 3A shows an enlarged view of the thrust ball bearing  19 , when the spiraling scroll  9  is in its uppermost position. 
     A ball  41  in the thrust ball bearing  19  is clamped between the thrust plates  4   a  (a second thrust plate) and  9   a  (first thrust plate), and revolves in-phase with the spiraling scroll  9 . In the condition shown in this diagram, the ball  41  is in its uppermost position, and is in contact with the interior surface of the inside wall of the retaining cavity  42  of the retaining device  40 . 
     FIG. 3B shows a condition of the ball  41  when the revolving action of the spiraling scroll  9  has caused the ball  41  to move to a position 180° away from the position shown in FIG.  3 A. In FIG. 3B, the ball  41  is in its lowest position, and is in contact with the interior surface of the outside wall of the retaining cavity  42  of the retaining device  40 . 
     As described earlier, the spiraling scroll  9  is revolving with a scroll radius ρ so that the ball  41  which rolls in contact with the spiraling scroll  9  revolves with a scroll radius ρ/2. As shown in FIG. 3B, the diameter of the motion trace of the ball  41  is given by d/2+ρ+d/2=d+ρ. 
     Because the diameter D of the retaining cavity  42  is given by a relation D=d+ρ, it means that the ball  41  will always roll in contact with the interior surface of the retaining cavity  42  while it is revolving. 
     In this design of the thrust ball bearing  19 , because the thrust plate  9   a  is made of a flat plate, it does not bind with the ball  41 . Also, because the diameter of the retaining cavity  42  is D=d+ρ, the cavity does not interfere with the rolling action of the ball  41 . 
     Therefore, friction is not generated between the interior surface of the retaining cavity  42  and the ball  41 , so that the efficiency of the compressor can be improved. Also, because the attrition effects among the retaining cavity  42 , ball  41 , thrust plates  4   a ,  9   a  are prevented, the load tolerance on the bearing can be increased. 
     Furthermore, because the retaining device itself is made of a polymeric resin, the following beneficial effects can be expected. 
     During the compression operation, the balls undergo revolving motion within the retaining cavity. During this period, because the revolution phase of the spiraling scroll  9  matches the revolution phase of the ball  41 , as shown in FIG. 4A, the rolling direction “a” of the ball  41  always coincides with the tangential directions “1” of the ball  41  and the retaining cavity  42  so that the ball  41  can move without colliding with the interior surface of the retaining cavity  42 . 
     However, during the startup time of the compressor, the ball  41  may impact the interior surface of the retaining cavity  42 . For example, the cases shown in FIGS. 4A,  4 B relate to such a situation. In both cases, it is assumed that the revolution phases of the spiraling scroll  9  are matching. In this phase, during the operation of the compressor, the ball  41  is always in the location shown in FIG. 4A so that there is no danger of the ball  41  colliding with the interior surface of the retaining cavity  42 . However, during the startup time of the compressor, the location of the ball  41  is indeterminate, so that if it is in a location shown in FIG. 4B, for example, the rolling direction “a” of the ball  41  differs from the tangential direction “1” of the ball  41  and the retaining cavity  42 . Therefore, the ball  41  collides with the interior surface of the retaining cavity  42 . 
     Afterward, when the rolling direction “a” of the ball  41  and the tangential direction “1” of the retaining cavity  42  coincide (that is, revolution phases of the spiraling scroll  9  and that of the ball  41  are matched), the ball begins a revolving motion as shown in FIG.  4 A. 
     As above mentioned, the ball  41  collides with the interior surface of the retaining cavity  42 , but the noise during the startup is reduced because the ball retaining device  40  is made of polymeric resin. 
     Here, in the above embodiment, although the diameter of the retaining cavity  42  provided in the retaining device  40  is given by D=d+ρ, it is only necessary to select the diameter D so as not to interfere with the rolling motion of the ball  41 , and therefore, it is sufficient for the relation to be D≧d+ρ. When D=d+ρ, ball  41  rolls while touching the interior surface of the retaining cavity  42 , and when D≧d+ρ, ball  41  rolls without touching the interior surface of the retaining cavity  42 . 
     Further, the interior surface of the retaining cavity  42  may be shaped in a shape other than a cylindrical shape as described below. 
     That is, as shown in FIG. 5, the interior surface of the retaining cavity  42  is made as a concave shaped to correspond to the curvature of the ball  41 . The diameter D of the retaining cavity  42  is made as large as possible. In this case, the ball  41  and the convex surface of the retaining cavity  42  contact each other over an area, so that the contact area is increased and the contact stress is reduced. 
     Further, in the above embodiment, the retaining device  40  was attached to the front casing  4  together with the thrust plate  4   a , but the installation site of the retaining device  40  is not limited to this location, and it may be attached to either the spiraling scroll  9  or the thrust plate  9   a.    
     Also, although the retaining device  40  itself was made of a resin material, it is only necessary that the interior surface of the ball retaining device  42  be made of a resin material, and a resin ring member inserted in the cavity of a non-resin retaining device  40  may be used, for example. 
     Furthermore, other material for suppressing the noise, for example rubber or vibration-controlling materials may substitute for the resin material. 
     Also, in the above embodiment, the open scroll type compressor was operated using a CO 2  cooling cycle, but it is not necessary to limit to such a system, and it is permissible to use other working gases such as freon for the vapor compression type cooling cycle.