Patent Publication Number: US-10766120-B2

Title: Method and device for manufacturing compressor scrolls, compressor scroll, and scroll compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a method and device for manufacturing a scroll for a compressor, a compressor scroll, and a scroll compressor. 
     BACKGROUND ART 
     Conventional scroll compressors include a fixed scroll with a spiral-shaped wall portion provided on a first side of an end plate and an orbiting scroll with a wall portion on a first side of an end plate with essentially the same spiral shape as that of the wall portion of the fixed scroll. The first sides of the end plates of the fixed scroll and the orbiting scroll are brought to face one another to assembly the wall portions together. In this mated state, the orbiting scroll orbits about the fixed scroll to gradually reduce the volume of the compression chamber formed between the wall portions and compress the fluid in the compression chamber. 
     An example of a conventional scroll for a compressor is described in the method of manufacturing a scroll compressor of Patent Document 1. In Patent Document 1, a fixed scroll and/or an orbiting scroll is rendered with a plurality of minute recesses on the side (wrap side) opposite the end plate (end cover) by being jetted with a fluid containing abrasive particles. This is to help with the retention of lubricating oil on the surface. 
     Additionally, for example, in the method of enhancing residual stress of a metallic material described in Patent Document 2, to prevent stress corrosion cracking at a weld portion of the metallic material and nearby, a fluid flow containing cavitation bubbles is impinged on the surface of the metallic material, the cavitation bubbles being generated by cavitation via water jet. The impact force generated by the collapse of the cavitation bubbles imparts compressive residual stress to the metallic material. 
     CITATION LIST 
     Patent Documents 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-074540A 
     Patent Document 2: Japanese Patent No. 3162104B 
     SUMMARY OF INVENTION 
     Technical Problem 
     Scrolls for compressors experience a concentration of stress at the corner portion where the end plate and the wall portion are joined when in operation. Such fatigue is likely to cause cracks. Accordingly, it is desirable to enhance the fatigue strength by imparting compressive residual stress to the target region particularly vulnerable to fatigue-generated cracks. Methods of imparting such residual stress include peening. However, in typical shot peening methods, the steels balls used for peening may not hit the target region. Thus, such methods are not suitable for application to scrolls. A method in which cavitation bubbles generated by a water jet are used is more suitable for application to scrolls than shot peening methods because these cavitation bubbles tend to reach smaller regions such as the target region described above. 
     However, while the application of cavitation bubbles generated by a water jet can be considered suitable, scrolls have a shape different from the plate-like shape of the workpiece in Patent Document 2 in that a wall portion is provided on an end plate. Such a shape can make it difficult for the cavitation bubbles to impinge on the target region to impart compressive residual stress thereto. 
     To solve the problems described above, the present invention provides a method and device for manufacturing a compressor scroll capable of appropriately impinging cavitation bubbles on a target region of the scroll, and a compressor scroll and a scroll compressor resistant to cracks. 
     Solution to Problem 
     To achieve the object described above, an embodiment of the present invention is a method for manufacturing a compressor scroll, the compressor scroll including a first scroll provided on a spiral-shaped first wall portion disposed on a first side of a first end plate, a second scroll provided on a spiral-shaped second wall portion disposed on a first side of a second end plate, with the second wall portion meshed with the first wall portion of the first scroll, wherein the second scroll is supported for orbiting movement and prevented from rotating, a step portion disposed on the first side of each of the end plates where a height transitions from high on a center portion side of the spiral following the respective wall portion to low on an outer end side, and a stepped portion disposed on each of the wall portions where a height transitions from low on the central portion side of the spiral to high on the outer end side, wherein the stepped portions engage with the corresponding step portions, the method comprising the step of water jet peening by jetting cavitation bubbles generated underwater by a water jet at the first side of the end plate of at least one of the scrolls, with a center of the cavitation bubbles being offset from a center of the spiral shape of the wall portion on the end plate and the step portion and the stepped portion positioned at an outer peripheral portion of the cavitation bubbles. 
     According to this method for manufacturing a compressor scroll, the center of the cavitation bubbles is offset from the center of the spiral shape of the wall portion on the end plate. When the step portion and the stepped portion are positioned at the outer peripheral portion of the range of the cavitation bubbles, the position of the center of the cavitation bubbles becomes positions where corner portions of the wall portion near the step portion and the stepped portion are positioned on a straight line through the spiral-shaped channel of the wall portion. This allows the flow of fluid flow containing the cavitation bubbles to not be obstructed by the wall portion, and thus allow the cavitation bubbles to impinge on the corner portions. In other words, the cavitation bubbles can be appropriately impinged on target regions of the scroll, imparting compressive residual stress to the target regions to prevent cracks. 
     In a method for manufacturing a compressor scroll according to another embodiment of the present invention, the water jet peening step includes moving the cavitation bubbles and the scroll relative to one another to intersect with a straight imaginary line that joins the step portion and the stepped portion and the positions of the cavitation bubbles and the scroll. 
     According to this method for manufacturing a compressor scroll, the cavitation bubbles can be appropriately impinged on the target regions (the corner portions) of the scroll, imparting compressive residual stress to the target regions to prevent cracks. 
     In a method for manufacturing a compressor scroll according to another embodiment of the present invention, the water jet peening step includes stopping the movement of the cavitation bubbles and the scroll relative to one another for a predetermined period of time at the positions of the cavitation bubbles and the scroll. 
     According to this method for manufacturing a compressor scroll, the cavitation bubbles can be sufficiently impinged on the target regions (the corner portions) of the scroll, imparting compressive residual stress to the target regions to prevent cracks. 
     In a method for manufacturing a compressor scroll according to another embodiment of the present invention, the water jet peening step is performed before surface treatment of the scroll. 
     According to this method for manufacturing a compressor scroll, the water jet peening step is performed before the surface treatment of the scroll. This facilitates imparting compressive residual stress via impingement of the cavitation bubbles to obtain a significant effect of preventing cracks. 
     In a method for manufacturing a compressor scroll according to another embodiment of the present invention, a cleaning fluid is mixed in with the water where the cavitation bubbles are generated. 
     According to this method for manufacturing a compressor scroll, the scroll can be cleaned by the cleaning fluid at the same time as the water jet peening step. 
     To achieve the object described above, an embodiment of the present invention is a device for manufacturing a compressor scroll, the compressor scroll including a first scroll provided on a spiral-shaped first wall portion disposed on a first side of a first end plate, a second scroll provided on a spiral-shaped second wall portion disposed on a first side of a second end plate with the second wall portion meshed with the first wall portion of the first scroll, wherein the second scroll is supported for orbiting movement and prevented from rotating, a step portion disposed on the first side of each of the end plates where, following the respective wall portion, a height transitions from high on a center portion side of the spiral to low on an outer end side, and a stepped portion disposed on each of the wall portions where a height transitions from low on the central portion side of the spiral to high on the outer end side, wherein the stepped portions engage with the corresponding step portions, the device comprising a vessel containing water; a positioning unit that positions at least one of the scrolls in the vessel; a water jet jetting disposed underwater in the vessel that includes a nozzle that jets a water jet at the scroll; wherein cavitation bubbles generated underwater in the vessel by the water jet of the water jet jetting unit are jet at the first side of the scroll, with a center of the cavitation bubbles being offset from a center of the spiral shape of the wall portion on the end plate and the step portion and the stepped portion positioned at an outer peripheral portion of the cavitation bubbles. 
     According to this device for manufacturing a compressor scroll, the water jet peening step described above in the method for manufacturing a compressor scroll can be performed. 
     In a device for manufacturing a compressor scroll according to another embodiment of the present invention, the positioning unit includes a fixing mechanism that engages with the end plate of the scroll to fix the scroll. 
     According to this device for manufacturing a compressor scroll, by fixing the scroll via this fixing mechanism, the scroll can be supported in place when the cavitation bubbles impinge on the scroll, allowing the cavitation bubbles to be appropriately impinged on the target regions (the corner portions) to impart compressive residual stress to the target regions and prevent cracks. 
     In a device for manufacturing a compressor scroll according to another embodiment of the present invention, the positioning unit includes a movement mechanism that moves the scroll to intersect with a straight imaginary line that joins the step portion and the stepped portion and the positions of the cavitation bubbles and the scroll. 
     According to this device for manufacturing a compressor scroll, the cavitation bubbles can be appropriately impinged on the target regions (the corner portions) of the scroll, imparting compressive residual stress to the target regions to prevent cracks. 
     In a device for manufacturing a compressor scroll according to another embodiment of the present invention, the movement mechanism includes a plurality of fixing mechanisms to move a plurality of the scrolls. 
     According to this device for manufacturing a compressor scroll, the cavitation bubbles can be appropriately impinged in order to target regions (the corner portions) of a plurality of scrolls. As a result, the water jet peening step of the method for manufacturing a compressor scroll described above can be efficiently performed. 
     In a device for manufacturing a compressor scroll according to another embodiment of the present invention, the water jet jetting unit includes a pivot mechanism that pivots the nozzle so that the cavitation bubbles are pivoted with respect to the scroll. 
     According to this device for manufacturing a compressor scroll, the cavitation bubbles can directly impinge on the target regions (the corner portions), which are internal angle portion of the end plate and the wall portion. Thus, the cavitation bubbles can be sufficiently impinged on the target regions of the scroll. 
     To achieve the object described above, an embodiment of the present invention is a compressor scroll made using the device for manufacturing a compressor scroll described above. 
     According to this compressor scroll, cracks can be prevented, and accidents caused by cracks can be reduced. 
     To achieve the object described above, an embodiment of the present invention is a scroll compressor, comprising the compressor scroll described above. 
     According to this scroll compressor, cracks can be prevented, and accidents caused by cracks can be reduced. 
     Advantageous Effects of Invention 
     According to the present invention, cavitation bubbles can be appropriately impinged on target regions of a scroll. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an example of a scroll compressor according to an embodiment of the present invention. 
         FIG. 2  is a perspective view illustrating a fixed scroll and an orbiting scroll according to an embodiment of the present invention. 
         FIG. 3  is a front view illustrating a fixed scroll according to an embodiment of the present invention. 
         FIG. 4  is a front view illustrating an orbiting scroll according to an embodiment of the present invention. 
         FIG. 5  is a schematic view illustrating a method for manufacturing a compressor scroll according to an embodiment of the present embodiment. 
         FIG. 6  is a schematic side view illustrating a device for manufacturing a compressor scroll according to an embodiment of the present embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments according to the present invention will be described below on the basis of the drawings. Note that the present invention is not limited by these embodiments. In addition, the constituent elements in the embodiments described below include those that can be easily replaced by a person skilled in the art or those that are substantially the same. 
       FIG. 1  is a cross-sectional view illustrating an example of a scroll compressor according to the present embodiment.  FIG. 2  is a perspective view of a fixed scroll and an orbiting scroll according to the present embodiment.  FIG. 3  is a front view of the fixed scroll according to the present embodiment.  FIG. 4  is a front view of the orbiting scroll according to the present embodiment. 
     A scroll compressor  10  illustrated in  FIG. 1  is used mainly to compress a refrigerant of a vehicular air conditioning device. The scroll compressor  10  is provided with a scroll compression mechanism including a fixed scroll  12 , or first scroll, and an orbiting scroll  13 , or second scroll, inside a housing  11 . 
     The housing  11  composes a housing main body  11 A and a cover  11 B. The housing main body  11 A is hollow and includes an integrated tubular large diameter portion  11 Aa and small diameter portion  11 Ab. An opening end of the housing main body  11 A on the side where the large diameter portion  11 Aa is located is mated and closed with the cover  11 B fixed via a plurality of bolts  20 . A drive shaft  14  is inserted in the housing main body  11 A on the side where the small diameter portion  11 Ab is located and a shaft seal  11 D seals the space between the drive shaft  14  and the housing main body  11 A. In such a manner, the housing  11  is configured as a sealed container that encloses the entire scroll compressor. 
     The fixed scroll  12 , as illustrated in  FIG. 2 , includes a disk-shaped end plate (disk)  12 A, and a spiral-shaped wall portion (wrap)  12 B provided on a first side of the end plate  12 A. 
     The fixed scroll  12 , as illustrated in  FIGS. 2 and 3 , also includes a step portion  12 Aa on the first side of the end plate  12 A where the wall portion  12 B is provided. Following the spiral direction of the wall portion  12 B, the level of the end plate  12 A transitions from high on the center portion side of the step portion  12 Aa to low on the outer end side. Additionally, the fixed scroll  12  includes a stepped portion  12 Ba where the level of the wall portion  12 B transitions from low on the center portion side to high on the outer end side. Furthermore, the fixed scroll  12  includes a groove formed on the tip of the wall portion  12 B in which a tip seal  12 Bb is provided. Note that in the present embodiment, the fixed scroll  12 , as illustrated in  FIG. 3 , also includes a bypass hole  12 Ab on the end plate  12 A for preventing excessive compression in a compression chamber S 1  described below. 
     The orbiting scroll  13  is similar to the fixed scroll  12  and, as illustrated in  FIG. 2 , includes a disk-shaped end plate (disk)  13 A, and a spiral-shaped wall portion (wrap)  13 B provided on a first side of the end plate  13 A. 
     The orbiting scroll  13  is also similar to the fixed scroll  12  in that, as illustrated in  FIGS. 2 and 4 , it also includes a step portion  13 Aa on the first side of the end plate  13 A where the wall portion  13 B is provided. Following the spiral direction of the wall portion  13 B, the level of the end plate  13 A transitions from high on the center portion side of the step portion  13 Aa to low on the outer end side. Additionally, the orbiting scroll  13  includes a stepped portion  13 Ba where the level of the wall portion  13 B transitions from low on the center portion side to high on the outer end side. Furthermore, the orbiting scroll  13  includes a groove formed on the tip of the wall portion  13 B in which a tip seal  13 Bb is provided. 
     The fixed scroll  12  and the orbiting scroll  13 , as illustrated in  FIG. 1 , are disposed inside the large diameter portion  11 Aa of the housing main body  11 A. The respective first sides of the end plates  12 A,  13 A are brought together in opposition and the wall portion  12 B,  13 B are engaged offset by a phase of 180° with the tips in contact with the first sides of the end plates  12 A,  13 A, thus forming the compression chamber S 1  in the space defined by the end plates  12 A,  13 A and the wall portions  12 B,  13 B. Here, when the fixed scroll  12  and the orbiting scroll  13  are assembled together, the step portions  12 Aa,  13 Aa and the stepped portions  12 Ba,  13 Ba are engaged. Additionally, as illustrated in  FIG. 1 , a suction chamber S 3  that communicates with the compression chamber S 1  is formed inside the housing main body  11 A at the periphery of the wall portions  12 B,  13 B of the fixed scroll  12  and the orbiting scroll  13 . The housing main body  11 A includes a suction port  11 Ac for the suction of a refrigerant gas which opens to the suction chamber S 3 . 
     As illustrated in  FIG. 1 , an outer peripheral portion on a second side of the end plate  12 A of the fixed scroll  12  fits closely and mates with an inner peripheral surface of the cover  11 B. The cover  11 B is fixed to the fixed scroll  12  at a plurality of positions via a plurality of bolts  21 . In such a manner, a discharge chamber S 2  is defined on the other side of the end plate  12 A of the fixed scroll  12  with the cover  11 B of the housing  11 . The fixed scroll  12  is provided with a discharge port  12 C at a central position of the spiral shape of the wall portion  12 B on the end plate  12 A. The discharge port  12 C passes through the fixed scroll  12  connecting the compression chamber S 1  and the discharge chamber S 2 . Additionally, the fixed scroll  12  is provided with a discharge valve  12 D on the end plate  12 A. The discharge valve  12 D includes a flat spring to open the discharge port  12 C when the pressure reaches a predetermined amount. 
     Additionally, a second side of the end plate  13 A of the orbiting scroll  13  is in contact with a wall  11 Ad, which is where the large diameter portion  11 Aa and the small diameter portion  11 Ab inside the housing main body  11 A meet. This restricts movement of the orbiting scroll  13  in the axial direction, which is the extending direction of the drive shaft  14 . 
     The drive shaft  14 , as described above, is inserted in the small diameter portion  11 Ab of the housing main body  11 A. The drive shaft  14  is able to freely rotate with a first end portion  14 A of the drive shaft  14  being supported inside the small diameter portion  11 Ab by a bearing  22 , a large diameter disk portion  14 B disposed in a central portion being supported by a bearing  23 , as illustrated in  FIG. 1 . At a second end portion of the drive shaft  14 , an eccentric shaft  14 C disposed eccentric to the rotation center of the drive shaft  14  is provided integrally with the disk portion  14 B. Rotation of the drive shaft  14  moves the eccentric shaft  14 C in an orbiting manner. 
     The eccentric shaft  14 C mates with a balance bushing  24  disposed on the outer periphery thereof. The balance bushing  24  moves in an orbiting manner integrally with the eccentric shaft  14 C. The balance bushing  24  is integrally provided with a balance weight  24 A to offset the amount of unbalance caused by the orbiting scroll  13 . The portion that mates with the eccentric shaft  14 C of the balance bushing  24  is cylindrical, and an annular drive bushing  25  is mounted on the outer peripheral portion thereof. 
     The orbiting scroll  13  is provided with a protruding boss  13 C in the central portion on the other side of the end plate  13 A. The boss  13 C is provided with a circular recessed portion  13 D with a center corresponding to the position of the center of the spiral shape of the wall portion  12 B. The drive bushing  25  is inserted in the recessed portion  13 D of the orbiting scroll  13 , the two being able to rotate relative to one another via a bearing  26 . The orbiting scroll  13  is provided with a circular rotation-restricting recessed portion  13 E on the outer peripheral portion on the other side of the end plate  13 A. A plurality of the rotation-restricting recessed portions  13 E are provided about the recessed portion  13 D. A rotation stopping pin  11 Ae that is fixed to the housing main body  11 A is inserted in each of the rotation-restricting recessed portions  13 E. By inserting the rotation stopping pins  11 Ae in the rotation-restricting recessed portion  13 E, the rotation of the orbiting scroll  13  is prevented. 
     The drive shaft  14  is driven in rotation by a drive unit  15 . The drive unit  15  includes a pulley  15 A supported for free rotation by a bearing  27  mounted on the outer peripheral portion of the small diameter portion  11 Ab of the housing main body  11 A. The drive unit  15  includes a rotation plate  15 B fixed to the first end portion  14 A of the drive shaft  14  by a nut  28 . The rotation plate  15 B is coupled to a support ring  15 C on the outer peripheral portion thereof. An end surface of the pulley  15 A is fixed to the support ring  15 C. An electromagnetic clutch  15 D is provided inside the pulley  15 A. The pulley  15 A transmits torque from the driving source (engine, for example) via a drive belt (not illustrated). 
     In the scroll compressor  10  configured as such, when the electromagnetic clutch  15 D is disengaged, the driving source torque is transmitted to the pulley  15 A of the drive unit  15  and the drive shaft  14  rotates. The rotation of the drive shaft  14  rotates the eccentric shaft  14 C in an eccentric manner. The rotation of the eccentric shaft  14 C is transmitted to the orbiting scroll  13  via the balance bushing  24  and the drive bushing  25 . The orbiting scroll  13  orbits with its rotation prevented via the engagement of the rotation-restricting recessed portion  13 E and the rotation stopping pin  11 Ae. The refrigerant gas taken in to the suction chamber S 3  inside the housing  11  from the suction port  11 Ac is taken into the compression chamber S 1  by this movement. Then, as the orbiting scroll  13  continues to orbit, the compression chamber S 1  becomes gradually narrower towards the center of the scrolls  12 ,  13  and the volume decreases. Inside the compression chamber S 1 , the refrigerant gas is compressed and it flows toward the central portion of the scrolls  12 ,  13  until reaching the discharge port  12 C. The discharge valve  12 D opens or closes depending on the difference in pressure between the compression chamber S 1  and the discharge chamber S 2 . In other words, the refrigerant gas is compressed in the compression chamber S 1  and when the compression chamber S 1  has a higher pressure than the discharge chamber S 2 , the refrigerant gas pushes open the discharge valve  12 D and flows into the discharge chamber S 2 . Thereafter, the high pressure refrigerant gas is discharged from the discharge chamber S 2  through a discharge port (not illustrated) provided on the cover  11 B and outside of the housing  11  and introduced into an air conditioner mounted in a vehicle. 
     A method and device for manufacturing a compressor scroll according to the present embodiment will be described below.  FIG. 5  is a schematic view illustrating a method for manufacturing a compressor scroll according to the present embodiment.  FIG. 6  is a schematic side view illustrating a device for manufacturing a compressor scroll according to the present embodiment. Note that in the description below, “compressor scroll” includes the fixed scroll  12  and the orbiting scroll  13  described above and is simply referred to as “scroll” below. Additionally, for the sake of convenience, the scroll illustrated in  FIGS. 5 and 6  is the orbiting scroll  13 . 
     In a method and device for manufacturing the compressor scroll according to the present embodiment, to enhance crack resistance at a corner portion of the end plate  13 A and the wall portion  13 B of the scroll  13 , a fluid flow containing cavitation bubbles generated underwater via cavitation by a water jet is impinged on the corner portion. The impact force generated by the collapse of the cavitation bubbles imparts compressive residual stress to the metallic material. 
     Here, target regions for compressive residual stress, which are crack-prone regions, are corner portion A and corner portion B as illustrated in  FIG. 5 . Corner portion A is the base of the spiral wall portion  13 B located near the stepped portion  13 Ba. Corner portion B is the base of the spiral wall portion  13 B located near the step portion  13 Aa. The corner portions A, B have a shape conducive to stress concentration. Additionally, in particular, stress is likely to concentrate at the corner portion B as it is where corner portions meet. Thus, it is desirable for cavitation bubbles to be impinged on the corner portions A, B. 
     Here, in the method for manufacturing a compressor scroll according to the present embodiment, as illustrated in  FIG. 5 , water jet peening is performed. Cavitation bubbles C are jetted toward the first side of the end plate  13 A of the scroll  13 , and the center P of the cavitation bubbles C is offset from the center Oof the spiral shape of the wall portion  13 B on the end plate  13 A so that the step portion  13 Aa and the stepped portion  13 Ba are positioned at the outer peripheral portion of the range of the cavitation bubbles C (the circular range indicated by the long dashed double-short dashed line in  FIG. 5 ). The center P of the cavitation bubbles C, as illustrated in  FIG. 5 , may be located at position P 1  where the corner portions A, B are positioned on a straight line through the spiral-shaped channel of the wall portion  13 B, position P 2  where the corner portion B is positioned on a straight line through the spiral-shaped channel of the wall portion  13 B, and position P 3  where the corner portion B is positioned on a straight line through the spiral-shaped channel of the wall portion  13 B. 
     For example, if the center P of the cavitation bubbles C is located at the center O of the spiral shape of the wall portion  13 B on the end plate  13 A, because the corner portions A, B are not positioned on a straight line through the spiral-shaped channel of the wall portion  13 B, the flow of the fluid flow containing the cavitation bubbles C is inhibited and interrupted by the wall portion  13 B, thus making it difficult for the cavitation bubbles C to impinge on the corner portions A, B. 
     Alternatively, according to a method for manufacturing a compressor scroll according to the present embodiment, as described above, the center P of the cavitation bubbles C is offset from the center O of the spiral shape of the wall portion  13 B on the end plate  13 A. When the step portion  13 Aa and the stepped portion  13 Ba are positioned at the outer peripheral portion of the range of the cavitation bubbles C, the position of the center P of the cavitation bubbles C becomes positions P 1 , P 2 , or P 3  where the corner portions A, B of the wall portion  13 B near the step portion  13 Aa and the stepped portion  13 Ba are positioned on a straight line through the spiral-shaped channel of the wall portion  13 B. This allows the flow of fluid flow containing the cavitation bubbles C to not be obstructed by the wall portion  13 B, and thus allow the cavitation bubbles C to impinge on the corner portions A, B. In other words, the cavitation bubbles C can be appropriately impinged on target regions of the scroll  13 , imparting compressive residual stress to the target regions to prevent cracks. 
     Additionally, a method for manufacturing a compressor scroll according to the present embodiment, as illustrated in  FIG. 5 , may include a water jet peening step. In this step, the cavitation bubbles C and the scroll  13  are moved relative to one another to intersect with a straight imaginary line L that joins the step portion  13 Aa and the stepped portion  13 Ba and the positions P 1 , P 2 , P 3  of the cavitation bubbles C and the scroll  13 . Movement can be performed by moving the cavitation bubbles C, the scroll  13 , or the cavitation bubbles C and the scroll  13 . 
     According to this method for manufacturing a compressor scroll, the cavitation bubbles C can be appropriately impinged on the target regions (the corner portions A, B) of the scroll  13 , imparting compressive residual stress to the target regions to prevent cracks. 
     Additionally, a water jet peening step of a method for manufacturing a compressor scroll according to the present embodiment may include stopping the movement of the cavitation bubbles C and/or the scroll  13  for a predetermined period of time at the positions P 1 , P 2 , P 3  of the cavitation bubbles C and the scroll  13 . 
     According to this method for manufacturing a compressor scroll, the cavitation bubbles C can be sufficiently impinged on the target regions (the corner portions A, B) of the scroll  13 , imparting compressive residual stress to the target regions to prevent cracks. Note that “predetermined period of time” refers to a period of time necessary for a target regions to be imparted with compressive residual stress. 
     Additionally, in a method for manufacturing a compressor scroll according to the present embodiment, a water jet peening step is performed before the surface treatment of the scroll  13 . 
     Surface treatment may be alumite treatment in which the surface is coated with alumite to enhance the corrosion resistance and abrasion resistance in case where the scroll  13  is made of an aluminum alloy. By performing surface treatment, the compressive residual stress imparted via impingement of the cavitation bubbles C may be suppressed and thus the effect of preventing cracks may be reduced. Thus, according to this method for manufacturing a compressor scroll, the water jet peening step is performed before the surface treatment of the scroll  13 . This facilitates imparting compressive residual stress via impingement of the cavitation bubbles C to obtain a significant effect of preventing cracks. 
     Additionally, in a method for manufacturing a compressor scroll according to the present embodiment, a cleaning fluid is mixed in with the water where the cavitation bubbles C are generated. 
     According to this method for manufacturing a compressor scroll, the scroll  13  can be cleaned by the cleaning fluid at the same time as the water jet peening step. 
     A device for manufacturing a compressor scroll used in the method for manufacturing a compressor scroll described above will be explained below. 
     A device  1  for manufacturing a compressor scroll according to the present embodiment, as illustrated in  FIG. 6 , includes a vessel  2  containing water, a positioning unit  3  that positions the scroll  13  in the vessel  2 , and a water jet jetting unit  4  disposed underwater in the vessel  2  that includes a nozzle  4 A that jets a water jet J at the scroll  13 . 
     The vessel  2  has a water depth sufficient for the water jet peening step to be performed, in which cavitation bubbles C generated by the water jet J jetted from the nozzle  4 A are impinged on the scroll  13  positioned by the positioning unit  3 . 
     The positioning unit  3  is capable of positioning the scroll  13  inside the vessel  2  in a manner so that the water jet peening can be performed. The positioning unit  3 , for example, includes a contact portion  3 A that comes into contact with the second side of the end plate  13 A of the scroll  13 , and a chuck portion  3 B that engages at a plurality of positions (three for example) around the periphery of the end plate  13 A of the scroll  13 . 
     The water jet jetting unit  4  includes the nozzle  4 A, a nozzle support portion  4 B that supports the nozzle  4 A, and a high-pressure water pump  4 C that supplies high-pressure water to the nozzle  4 A. 
     The device for manufacturing a compressor scroll jets cavitation bubbles C generated underwater in the vessel  2  by the water jet J of the water jet jetting unit  4  at the first side of the scroll  13  positioned by the positioning unit  3 , and as illustrated in  FIG. 5 , with the center P of the cavitation bubbles C offset from the center O of the spiral shape of the wall portion  13 B on the end plate  13 A, the outer peripheral portion of the cavitation bubbles C is positioned at the step portion  13 Aa and the stepped portion  13 Ba. 
     According to such a device  1  for manufacturing a compressor scroll, the water jet peening step described above in the method for manufacturing a compressor scroll can be performed. 
     Additionally, in the device  1  for manufacturing a compressor scroll according to the present embodiment, the positioning unit  3  includes the contact portion  3 A and the chuck portion  3 B which compose a fixing mechanism that fixes the scroll  13  by engaging with the end plate  13 A of the scroll  13 . 
     According to this device  1  for manufacturing a compressor scroll, by fixing the scroll  13  via this fixing mechanism, the scroll  13  can be supported in place when the cavitation bubbles C impinge on the scroll  13 , allowing the cavitation bubbles C to be appropriately impinged on the target regions (the corner portions A, B) to impart compressive residual stress to the target regions and prevent cracks. 
     In the device  1  for manufacturing a compressor scroll of the present embodiment, the positioning unit  3 , as illustrated in  FIGS. 5 and 6 , includes a movement mechanism  3 C that moves the scroll  13  to intersect with the straight imaginary line L that joins the step portion  13 Aa and the stepped portion  13 Ba and the positions P 1 , P 2 , P 3  of the cavitation bubbles C and the scroll  13 . 
     The movement mechanism  3 C is preferably a belt conveyor or other similar means for moving the fixing mechanism (the contact portion  3 A and the chuck portion  3 B) in a parallel manner while supported. 
     According to the device  1  for manufacturing a compressor scroll, the cavitation bubbles C can be appropriately impinged on the target regions (the corner portions A, B) of the scroll  13  to impart compressive residual stress to the target regions to prevent cracks. 
     In the device  1  for manufacturing a compressor scroll according to the present embodiment, the movement mechanism  3 C includes a plurality of fixing mechanisms to move a plurality of scrolls  13 . 
     According to this device  1  for manufacturing a compressor scroll, the cavitation bubbles C can be appropriately impinged in order on target regions (the corner portions A, B) of a plurality of scrolls  13 . As a result, the water jet peening step of the method for manufacturing a compressor scroll described above can be efficiently performed. 
     In the device  1  for manufacturing a compressor scroll according to the present embodiment, the water jet jetting unit  4  includes a pivot mechanism  4 D that pivots the nozzle  4 A so that the cavitation bubbles C are pivoted with respect to the scroll  13 . 
     The pivot mechanism  4 D is provided on the nozzle support portion  4 B and allows the jet direction of the water jet J from the nozzle  4 A to be inclined with respect to a vertical line V illustrated in  FIG. 6  and rotated about the vertical axis. Such a configuration allows the cavitation bubbles C to directly impinge on the target regions (the corner portions A, B), which are internal angle portion of the end plate  13 A and the wall portion  13 B. Thus, the cavitation bubbles C can be sufficiently impinged on the target regions of the scroll  13 . 
     REFERENCE SIGNS LIST 
     
         
           1  Device for manufacturing a compressor scroll 
           2  Vessel 
           3  Positioning unit 
           3 C Movement mechanism 
           4  Water jet jetting unit 
           4 A Nozzle 
           4 D Pivot mechanism 
           12  Fixed scroll (first scroll) 
           12 A End plate 
           12 Aa Step portion 
           12 B Wall portion 
           12 Ba Stepped portion 
           13  Orbiting scroll (second scroll) 
           13 A End plate 
           13 Aa Step portion 
           13 B Wall portion 
           13 Ba Stepped portion 
         A, B Corner portion 
         C Cavitation bubble 
         J Water jet 
         L Imaginary line 
         O Center 
         P Center 
         P 1 , P 2 , P 3  Position